U.S. patent application number 14/184702 was filed with the patent office on 2014-06-19 for flow battery and regeneration system with improved safety.
This patent application is currently assigned to Ftorion, Inc.. The applicant listed for this patent is Ftorion, Inc.. Invention is credited to Yuriy Vyacheslalovovich Tolmachev.
Application Number | 20140170511 14/184702 |
Document ID | / |
Family ID | 50931275 |
Filed Date | 2014-06-19 |
United States Patent
Application |
20140170511 |
Kind Code |
A1 |
Tolmachev; Yuriy
Vyacheslalovovich |
June 19, 2014 |
Flow Battery And Regeneration System With Improved Safety
Abstract
A method for producing electric power and regenerating an
aqueous multi-electron oxidant (AMO) and a reducer in an energy
storage cycle is provided. A discharge system includes a discharge
unit, an acidification reactor, and a neutralization reactor. The
acidification reactor converts an oxidant fluid including the AMO
into an acidic oxidant fluid. The discharge unit generates electric
power and a discharge fluid by transferring electrons from a
positive electrode of an electrolyte-electrode assembly (EEA) to
the AMO and from a reducer to a negative electrode of the EEA. The
neutralization reactor neutralizes the discharge fluid to produce a
neutral discharge fluid. The regeneration system splits an alkaline
discharge fluid into a reducer and an intermediate oxidant in a
splitting-disproportionation reactor and releases the reducer and a
base, while producing the AMO by disproportionating the
intermediate oxidant. The regenerated AMO and reducer are supplied
to the discharge unit for power generation.
Inventors: |
Tolmachev; Yuriy
Vyacheslalovovich; (Boston, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ftorion, Inc. |
Boston |
MA |
US |
|
|
Assignee: |
Ftorion, Inc.
Boston
MA
|
Family ID: |
50931275 |
Appl. No.: |
14/184702 |
Filed: |
February 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13969597 |
Aug 18, 2013 |
|
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14184702 |
|
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61684805 |
Aug 19, 2012 |
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Current U.S.
Class: |
429/418 ;
429/416 |
Current CPC
Class: |
H01M 8/188 20130101;
Y02E 60/50 20130101; H01M 8/08 20130101; H01M 8/0656 20130101; H01M
8/22 20130101; Y02E 60/528 20130101; H01M 8/20 20130101 |
Class at
Publication: |
429/418 ;
429/416 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Claims
1. A method for producing electric power from an aqueous
multi-electron oxidant and a reducer and for simultaneously
generating a discharge fluid, said method comprising: providing a
discharge system comprising one or more forms of a reducer fluid,
one or more forms of an oxidant fluid, a discharge unit, and an
acidification reactor, said discharge unit comprising an
electrolytic cell stack, said electrolytic cell stack comprising a
plurality of electrolytic cells, wherein each of said electrolytic
cells comprises an electrolyte-electrode assembly; and facilitating
discharge of said discharge unit for producing said electric power
from a neutral oxidant fluid comprising one or more forms of said
aqueous multi-electron oxidant, and from said reducer fluid
comprising one or more forms of said reducer, said facilitation of
said discharge comprising: lowering pH of said neutral oxidant
fluid in said acidification reactor for generating an acidic
oxidant fluid; transferring electrons from a positive electrode of
said electrolyte-electrode assembly to said aqueous multi-electron
oxidant in said acidic oxidant fluid; and transferring electrons
from said reducer fluid to a negative electrode of said
electrolyte-electrode assembly to produce said electric power in an
external electric circuit operably connected to terminals of said
discharge unit and to generate an acidic discharge fluid on
consumption of said acidic oxidant fluid and said reducer
fluid.
2. The method of claim 1, further comprising optionally
neutralizing said acidic discharge fluid in a neutralization
reactor of said discharge system to produce a neutral discharge
fluid.
3. The method of claim 1, wherein said aqueous multi-electron
oxidant comprises one or more of halogens, halogen oxides, halogen
oxoanions, and salts and acids of said halogen oxoanions.
4. The method of claim 3, wherein said halogen oxoanions comprise
one or more of hypochlorite, chlorite, chlorate, perchlorate,
hypobromite, bromite, perbromate, hypoiodite, iodite, iodate, and
periodate.
5. The method of claim 3, wherein one of said halogen oxoanions is
bromate.
6. The method of claim 1, wherein said acidic oxidant fluid
comprises water, said one or more forms of said aqueous
multi-electron oxidant, an extra acid, and one or more of a
plurality of counter cations.
7. The method of claim 6, wherein said extra acid is one or more of
a phosphoric acid, a 3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, a methanesulfonic acid, a
triflic acid, a substituted sulfonic acid, a substituted phosphonic
acid, a perchloric acid, a sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic moieties, and an acid with a
pKa<2.
8. The method of claim 6, wherein said counter cations comprise
alkali metal cations, alkali earth metal cations, and organic
cations.
9. The method of claim 6, wherein one of said counter cations is
lithium.
10. The method of claim 6, wherein one of said counter cations is
sodium.
11. The method of claim 1, further comprising maintaining stability
of said acidic oxidant fluid by performing an ignition regime in
said discharge system at low acid concentrations in said acidic
oxidant fluid.
12. The method of claim 1, wherein concentration of said one or
more forms of said aqueous multi-electron oxidant in one or more of
said neutral oxidant fluid and said acidic oxidant fluid supplied
to said discharge unit is above one of 1M, 2M, 5M, and 10M.
13. The method of claim 1, wherein concentration of acidic protons
in said acidic oxidant fluid supplied to said discharge unit is
below one of 0.1M, 0.5M, 1M, 2M, 5M, and 10M.
14. The method of claim 1, wherein concentration of acidic protons
in said acidic oxidant fluid stored in said discharge system is
below one of 0.1M, 0.5M, 1M, 2M, and 5M.
15. The method of claim 1, wherein said transfer of said electrons
from said positive electrode of said electrolyte-electrode assembly
to said aqueous multi-electron oxidant in said acidic oxidant fluid
is performed at a high current density and at low flow rates in an
ignition mode of operation of said discharge system.
16. The method of claim 1, wherein said acidic discharge fluid
comprises one or more of water, a halide, a hydroxonium cation, an
extra acid, and one or more counter cations.
17. The method of claim 1, wherein said reducer is hydrogen.
18. The method of claim 1, wherein said reducer is selected from a
group consisting of ammonia, hydrazine, hydroxylamine, phosphine,
methane, a hydrocarbon, an alcohol, an aldehyde, a carbohydrate, a
hydride, an oxide, a sulfide, an organic compound, an inorganic
compound, and any combination thereof, with one of each other,
water, and another solvent.
19. The method of claim 1, wherein said generation of said acidic
oxidant fluid from said neutral oxidant fluid is performed in said
acidification reactor via an electric field driven orthogonal ion
migration across laminar flow process.
20. The method of claim 1, wherein said generation of said acidic
oxidant fluid from said neutral oxidant fluid is performed in said
acidification reactor by one or more of an ion exchange on solids,
an ion exchange in liquids, electrolysis, and adding an extra acid
to said neutral oxidant fluid during said discharge of said
discharge unit.
21. The method of claim 1, wherein said discharge is facilitated on
said positive electrode of said electrolyte-electrode assembly by
one or more of electrocatalysis, a solution-phase chemical
reaction, a solution-phase comproportionation, a solution-phase
redox catalysis, a solution-phase redox mediator, an acid-base
catalysis, and any combination thereof.
22. The method of claim 1, wherein said discharge is facilitated
via a solution-phase comproportionation of said aqueous
multi-electron oxidant with a final product of a reduction of said
aqueous multi-electron oxidant.
23. The method of claim 22, wherein said solution-phase
comproportionation is pH-dependent and said discharge is
facilitated in a presence of an acid.
24. The method of claim 1, further comprising regenerating a
certain amount of an intermediate oxidant and said reducer in said
discharge unit from said acidic discharge fluid by applying an
electric current of a polarity opposite to a polarity of electric
current through said discharge unit during said discharge.
25. A discharge system comprising: one or more forms of an oxidant
fluid comprising one or more forms of an aqueous multi-electron
oxidant; one or more forms of a reducer fluid comprising one or
more forms of a reducer; a discharge unit comprising an
electrolytic cell stack, said electrolytic cell stack comprising a
plurality of electrolytic cells, wherein each of said electrolytic
cells comprises an electrolyte-electrode assembly; an acidification
reactor operably connected to said discharge unit, said
acidification reactor configured to lower pH of a neutral oxidant
fluid for generating an acidic oxidant fluid; and said discharge
unit configured to produce said electric power from said acidic
oxidant fluid and from said reducer fluid by: transferring
electrons from a positive electrode of said electrolyte-electrode
assembly to said aqueous multi-electron oxidant in said acidic
oxidant fluid; and transferring electrons from said reducer fluid
to a negative electrode of said electrolyte-electrode assembly to
produce said electric power in an external electric circuit
operably connected to terminals of said discharge unit and to
generate an acidic discharge fluid on consumption of said acidic
oxidant fluid and said reducer fluid.
26. The discharge system of claim 25, further optionally comprising
a neutralization reactor configured to neutralize said acidic
discharge fluid to produce a neutral discharge fluid.
27. The discharge system of claim 26, wherein said acidification
reactor and said neutralization reactor are functionally combined
as an orthogonal ion migration across laminar flow reactor.
28. The discharge system of claim 27, wherein said orthogonal ion
migration across laminar flow reactor comprises flow cell
assemblies, end plates, and bipolar plates, wherein each of said
flow cell assemblies of said orthogonal ion migration across
laminar flow reactor comprises: ion exchange membranes comprising a
positive side ion exchange membrane and a negative side ion
exchange membrane positioned parallel to each other; a positive
electrode layer and a negative electrode layer flanking outer
surfaces of said ion exchange membranes, wherein said positive
electrode layer is configured for a hydrogen oxidation reaction and
said negative electrode layer is configured for a hydrogen
evolution reaction; an intermembrane flow field comprising a
plurality of flow channels, said intermembrane flow field
interposed between said ion exchange membranes; and porous
diffusion layers flanking said outer surfaces of said ion exchange
membranes and in electric contact with one of said bipolar plates
and said end plates.
29. The discharge system of claim 25 configured to operate in an
electric partial recharge mode for facilitating regenerative
breaking when said discharge system powers an electric vehicle,
wherein said reducer is produced on said negative electrode of said
electrolyte-electrode assembly and an intermediate oxidant is
produced on said positive electrode of said electrolyte-electrode
assembly during said electric partial recharge mode.
30. A method for regenerating an aqueous multi-electron oxidant and
a reducer in stoichiometric amounts from one or more forms of a
neutral discharge fluid using external power, said method
comprising: converting said neutral discharge fluid into an
alkaline discharge fluid by using one or more of an externally
supplied base and a base produced in a splitting-disproportionation
reactor configured for one of an aqueous multi-electron
oxidant-on-negative electrode mode of operation, a no-aqueous
multi-electron oxidant-on-negative electrode mode of operation, and
a combination thereof; splitting said alkaline discharge fluid into
a reducer and an intermediate oxidant in said
splitting-disproportionation reactor, wherein said intermediate
oxidant is converted into one or more forms of said aqueous
multi-electron oxidant via disproportionation of said intermediate
oxidant with said base, and wherein said splitting releases a
stoichiometric amount of said reducer and said base in said
splitting-disproportionation reactor; and continuing said splitting
and said disproportionation in said splitting-disproportionation
reactor in one of a batch mode of operation, a cyclic flow mode of
operation, a cascade flow mode of operation, and a combination
thereof until a desired degree of conversion of a discharge product
of said aqueous multi-electron oxidant into said one or more forms
of said aqueous multi-electron oxidant is achieved.
31. The method of claim 30, wherein said splitting is performed by:
electrolyzing said alkaline discharge fluid into said reducer and
said intermediate oxidant in said splitting-disproportionation
reactor configured as an electrolysis-disproportionation reactor,
wherein said intermediate oxidant produced at one or more positive
electrodes of said electrolysis-disproportionation reactor is
converted into said one or more forms of said aqueous
multi-electron oxidant via said disproportionation of said
intermediate oxidant produced at said one or more positive
electrodes with said base, wherein said electrolysis releases said
stoichiometric amount of said reducer and said base at said one or
more negative electrodes of said electrolysis-disproportionation
reactor; and continuing said electrolysis and said
disproportionation in said electrolysis-disproportionation reactor
in said one of said batch mode of operation, said cyclic flow mode
of operation, said cascade flow mode of operation, and a
combination thereof until a desired degree of conversion of said
discharge product of said aqueous multi-electron oxidant into said
one or more forms of said aqueous multi-electron oxidant is
achieved.
32. The method of claim 30, wherein said discharge fluid comprises
one or more of water, a halide, a hydroxonium cation, a buffer, and
one or more counter cations.
33. The method of claim 30, further comprising optimizing and
stabilizing pH of said alkaline discharge fluid in said
splitting-disproportionation reactor using a buffer present in said
one or more forms of said discharge fluid to facilitate said
disproportionation of said intermediate oxidant into said one or
more forms of said aqueous multi-electron oxidant.
34. The method of claim 33, wherein said pH of said alkaline
discharge fluid is one of between 6 and 10 and between 4 and 9.
35. The method of claim 33, wherein said buffer is configured to
maintain said pH of said alkaline discharge fluid at one of between
6 and 10 and between 4 and 9.
36. The method of claim 33, wherein a base component of said buffer
is selected from a group comprising a hydroxide ion, hydrogen
phosphate, a phosphate ester, a substituted phosphonate,
alkylphosphonate, arylphosphonate, a deprotonated form of one or
more of Good's buffers, an amine, a nitrogen heterocycle, and any
combination thereof.
37. The method of claim 33, wherein a cationic component of said
buffer comprises a cation of lithium.
38. The method of claim 33, wherein a cationic component of said
buffer comprises a cation of sodium.
39. The method of claim 33, wherein an anionic component of said
buffer comprises one or more of
.omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate.
40. The method of claim 33, wherein an anionic component of said
buffer is one or more of .omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate, and wherein a cationic
component of said buffer is lithium.
41. The method of claim 33, wherein an anionic component of said
buffer comprises one or more of an alkylphosphonate and an
arylphosphonate.
42. The method of claim 33, wherein an anionic component of said
buffer comprises one or more of an alkylphosphonate and an
arylphosphonate, and wherein a cationic component of said buffer is
lithium.
43. The method of claim 33, wherein a base component of said buffer
is monohydrogen phosphate, and wherein a cationic component of said
buffer is sodium.
44. The method of claim 30, wherein said splitting of said alkaline
discharge fluid into said reducer and said intermediate oxidant in
said splitting-disproportionation reactor is performed via one of
electrolysis, photolysis, photoelectrolysis, radiolysis,
thermolysis, and any combination thereof.
45. The method of claim 44, wherein said photolysis and said
photoelectrolysis of said alkaline discharge fluid is performed in
one of a presence and an absence of a light adsorbing facilitator,
a semiconductor, a catalyst, and any combination thereof.
46. A regeneration system comprising: a
splitting-disproportionation reactor configured to convert a
neutral discharge fluid into an alkaline discharge fluid by using
one or more of an externally supplied base and a base produced in
said splitting-disproportionation reactor; said
splitting-disproportionation reactor further configured to split
said alkaline discharge fluid into a reducer and an intermediate
oxidant, wherein said splitting releases a stoichiometric amount of
said reducer and said base in said splitting-disproportionation
reactor; said splitting-disproportionation reactor further
configured to convert said intermediate oxidant into one or more
forms of an aqueous multi-electron oxidant via disproportionation
of said intermediate oxidant with said base; and said
splitting-disproportionation reactor further configured to continue
said splitting and said disproportionation in one of a batch mode
of operation, a cyclic flow mode of operation, a cascade flow mode
of operation, and any combination thereof, until a desired degree
of conversion of a discharge product of said aqueous multi-electron
oxidant into said one or more forms of said aqueous multi-electron
oxidant is achieved.
47. The regeneration system of claim 46, further optionally
comprising a concentrating reactor configured to produce a
concentrated solution of a neutral oxidant fluid comprising a salt
form of said aqueous multi-electron oxidant.
48. The regeneration system of claim 46, further comprising one or
more separation reactors configured to separate gases from liquids
during a regeneration process.
49. The regeneration system of claim 46, wherein said
splitting-disproportionation reactor is further configured for an
aqueous multi-electron oxidant-on-negative electrode mode of
operation using a multilayer structure on a negative electrode side
of said splitting-disproportionation reactor.
50. The regeneration system of claim 49, wherein said multilayer
structure on said negative electrode side of said
splitting-disproportionation reactor is configured to minimize
reduction of a regenerated aqueous multi-electron oxidant in a
regenerated oxidant fluid on said negative electrode side while
facilitating hydrogen evolution and an increase in pH of said
regenerated oxidant fluid.
51. The regeneration system of claim 46, wherein said
splitting-disproportionation reactor is further configured for a
no-aqueous multi-electron oxidant-on-negative electrode mode of
operation by transferring a base produced on one or more negative
electrodes of said splitting-disproportionation reactor to a
regenerated oxidant fluid produced at one or more positive
electrodes of said splitting-disproportionation reactor and
comprising said one or more forms of said aqueous multi-electron
oxidant and said intermediate oxidant.
52. A regeneration system comprising: an
electrolysis-disproportionation reactor configured to convert a
neutral discharge fluid into an alkaline discharge fluid by using
one or more of an externally supplied base and a base produced at
one or more negative electrodes of said
electrolysis-disproportionation reactor in one of an aqueous
multi-electron oxidant-on-negative electrode mode of operation, a
no-aqueous multi-electron oxidant-on-negative electrode mode of
operation, and a combination thereof; said
electrolysis-disproportionation reactor further configured to split
said alkaline discharge fluid into a reducer and an intermediate
oxidant via electrolysis, wherein said electrolysis releases a
stoichiometric amount of said reducer and said base at said one or
more negative electrodes of said electrolysis-disproportionation
reactor; said electrolysis-disproportionation reactor further
configured to convert said intermediate oxidant produced at one or
more positive electrodes of said electrolysis-disproportionation
reactor into one or more forms of an aqueous multi-electron oxidant
via disproportionation of said intermediate oxidant produced at
said one or more positive electrodes with said base; and said
electrolysis-disproportionation reactor further configured to
continue said electrolysis and said disproportionation in one of a
batch mode of operation, a cyclic flow mode of operation, a cascade
flow mode of operation, and any combination thereof, until a
desired degree of conversion of a discharge product of said aqueous
multi-electron oxidant into said one or more forms of said aqueous
multi-electron oxidant is achieved.
53. A method for producing electric power and regenerating an
aqueous multi-electron oxidant and a reducer in an energy storage
cycle, said method comprising: providing a discharge system
comprising one or more forms of a reducer fluid, one or more forms
of an oxidant fluid, a discharge unit, an acidification reactor,
and optionally a neutralization reactor, said discharge unit
comprising an electrolytic cell stack, said electrolytic cell stack
comprising a plurality of electrolytic cells, wherein each of said
electrolytic cells comprises an electrolyte-electrode assembly;
facilitating discharge of said discharge unit for producing said
electric power from a neutral oxidant fluid comprising one or more
forms of said aqueous multi-electron oxidant, and from said reducer
fluid comprising one or more forms of said reducer, said
facilitation of said discharge comprising: lowering pH of said
neutral oxidant fluid in said acidification reactor for generating
an acidic oxidant fluid; transferring electrons from a positive
electrode of said electrolyte-electrode assembly to said aqueous
multi-electron oxidant in said acidic oxidant fluid; and
transferring electrons from said reducer fluid to a negative
electrode of said electrolyte-electrode assembly to produce said
electric power in an external electric circuit operably connected
to terminals of said discharge unit and to generate an acidic
discharge fluid on consumption of said acidic oxidant fluid and
said reducer fluid; optionally neutralizing said acidic discharge
fluid in said neutralization reactor of said discharge system to
produce a neutral discharge fluid; regenerating said one or more
forms of oxidant fluid comprising said aqueous multi-electron
oxidant and said reducer fluid comprising said reducer in
stoichiometric amounts from one or more forms of said neutral
discharge fluid in a regeneration system using external power, said
regeneration comprising: converting said neutral discharge fluid
into an alkaline discharge fluid by using one or more of an
externally supplied base and a base produced in a
splitting-disproportionation reactor configured for one of an
aqueous multi-electron oxidant-on-negative electrode mode of
operation, a no-aqueous multi-electron oxidant-on-negative
electrode mode of operation, and a combination thereof; splitting
said alkaline discharge fluid into a reducer and an intermediate
oxidant in said splitting-disproportionation reactor, wherein said
intermediate oxidant is converted into one or more forms of said
aqueous multi-electron oxidant via disproportionation of said
intermediate oxidant with said base, and wherein said splitting
releases a stoichiometric amount of said reducer and said base in
said splitting-disproportionation reactor; and continuing said
splitting and said disproportionation in said
splitting-disproportionation reactor in one of a batch mode of
operation, a cyclic flow mode of operation, a cascade flow mode of
operation, and a combination thereof, until a desired degree of
conversion of a discharge product of said aqueous multi-electron
oxidant into said one or more forms of said aqueous multi-electron
oxidant is achieved; and supplying said regenerated one or more
forms of said oxidant fluid comprising said aqueous multi-electron
oxidant and said regenerated reducer fluid comprising said reducer
to said discharge system for said facilitation of said discharge of
said discharge unit.
54. The method of claim 53, wherein said aqueous multi-electron
oxidant comprises one or more of halogens, halogen oxides, halogen
oxoanions, and salts and acids of said halogen oxoanions.
55. The method of claim 54, wherein said halogen oxoanions comprise
one or more of hypochlorite, chlorite, chlorate, perchlorate,
hypobromite, bromite, perbromate, hypoiodite, iodite, iodate, and
periodate.
56. The method of claim 54, wherein one of said halogen oxoanions
is bromate.
57. The method of claim 53, wherein said acidic oxidant fluid
comprises water, said one or more forms of said aqueous
multi-electron oxidant, optionally an extra acid, and one or more
of a plurality of counter cations.
58. The method of claim 57, wherein said counter cations comprise
alkali metal cations, alkali earth metal cations, and organic
cations.
59. The method of claim 57, wherein one of said counter cations is
lithium.
60. The method of claim 57, wherein one of said counter cations is
sodium.
61. The method of claim 53, wherein concentration of said one or
more forms of said aqueous multi-electron oxidant in one or more of
said neutral oxidant fluid and said acidic oxidant fluid supplied
to said discharge unit of said discharge system is above one of 1M,
2M, 5M, and 10M.
62. The method of claim 53, wherein concentration of acidic protons
in said acidic oxidant fluid supplied to said discharge unit of
said discharge system is below one of 0.1M, 0.5M, 1M, 2M, 5M, and
10M.
63. The method of claim 53, wherein concentration of acidic protons
in said acidic oxidant fluid stored in said discharge system is
below one of 0.1 M, 0.5 M, 1 M, 2 M, and 5 M.
64. The method of claim 53, wherein said transfer of said electrons
from said positive electrode of said electrolyte-electrode assembly
of said discharge system to said aqueous multi-electron oxidant in
said acidic oxidant fluid is performed at a high current density
and at low flow rates in an ignition mode of operation of said
discharge system.
65. The method of claim 64, wherein a limiting current of said
transfer of said electrons from said positive electrode of said
electrolyte-electrode assembly to said aqueous multi-electron
oxidant in said acidic oxidant fluid in an ignition regime is
limited by one of a mass transport of said aqueous multi-electron
oxidant, a mass transport of acidic protons, and a rate of
comproportionation.
66. The method of claim 53, wherein said acidic discharge fluid
comprises one or more of water, a halide, a hydroxonium cation, an
extra acid, and one or more counter cations.
67. The method of claim 53, wherein said reducer is hydrogen.
68. The method of claim 53, wherein said reducer is selected from a
group consisting of ammonia, hydrazine, hydroxylamine, phosphine,
methane, a hydrocarbon, an alcohol, an aldehyde, a carbohydrate, a
hydride, an oxide, a sulfide, an organic compound, an inorganic
compound, and any combination thereof, with one of each other,
water, and another solvent.
69. The method of claim 53, wherein said generation of said acidic
oxidant fluid from said neutral oxidant fluid is performed in said
acidification reactor of said discharge system via an electric
field driven orthogonal ion migration across laminar flow
process.
70. The method of claim 53, wherein said generation of said acidic
oxidant fluid from said neutral oxidant fluid is performed in said
acidification reactor of said discharge system via one or more of
an ion exchange on solids, an ion exchange in liquids,
electrolysis, and adding an extra acid to said neutral oxidant
fluid during said discharge of said discharge unit of said
discharge system.
71. The method of claim 53, wherein said discharge is facilitated
on said positive electrode of said electrolyte-electrode assembly
by one or more of electrocatalysis, a solution-phase chemical
reaction, a solution-phase comproportionation, a solution-phase
redox catalysis, a solution-phase redox mediator, an acid-base
catalysis, and any combination thereof.
72. The method of claim 53, wherein said discharge is facilitated
via a solution-phase comproportionation of said aqueous
multi-electron oxidant with a final product of a reduction of said
aqueous multi-electron oxidant.
73. The method of claim 72, wherein said solution-phase
comproportionation is pH-dependent and said discharge is
facilitated in a presence of an acid.
74. The method of claim 53, further comprising optimizing and
stabilizing pH of said acidic oxidant fluid in said
splitting-disproportionation reactor of said regeneration system
using an extra acid present in said acidic oxidant fluid to
facilitate comproportionation of said aqueous multi-electron
oxidant with a final product of a reduction of said aqueous
multi-electron oxidant into said intermediate oxidant.
75. The method of claim 74, wherein said extra acid is one or more
of a phosphoric acid, a 3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, another
.omega.-(N-morpholino)propanesulfonic acid, a methanesulfonic acid,
triflic acid, a substituted sulfonic acid, a substituted phosphonic
acid, a perchloric acid, a sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic acid moieties, and an acid with a
pKa<2.
76. The method of claim 53, wherein said pH of said acidic
discharge fluid is below one of 0, 1, 2, and 3.
77. The method of claim 53, wherein concentration of acidic protons
in said acidic discharge fluid is below one of 0.1M, 0.5M, 1M, 2M,
5M, and 10M.
78. The method of claim 53, wherein said splitting of said alkaline
discharge fluid into said reducer and said intermediate oxidant in
said splitting-disproportionation reactor of said regeneration
system is performed via one of electrolysis, photolysis,
photoelectrolysis, radiolysis, thermolysis, and any combination
thereof.
79. The method of claim 78, wherein said photolysis and said
photoelectrolysis of said alkaline discharge fluid is performed in
one of a presence and an absence of a light adsorbing facilitator,
a semiconductor, a catalyst, and any combination thereof.
80. The method of claim 53, wherein said
splitting-disproportionation reactor of said regeneration system is
configured as an electrolysis-disproportionation reactor for said
aqueous multi-electron oxidant-on-negative electrode mode of
operation using a multilayer structure on a negative electrode side
of said electrolysis-disproportionation reactor.
81. The method of claim 80, wherein said multilayer structure on
said negative electrode side of said
electrolysis-disproportionation reactor is configured to minimize
reduction of a regenerated aqueous multi-electron oxidant in a
regenerated oxidant fluid on said negative electrode side while
facilitating hydrogen evolution and an increase in pH of said
regenerated oxidant fluid.
82. The method of claim 53, wherein said
splitting-disproportionation reactor of said regeneration system is
configured as an electrolysis-disproportionation reactor for said
no-aqueous multi-electron oxidant-on-negative electrode mode of
operation by transferring a base produced on one or more negative
electrodes of said electrolysis-disproportionation reactor to a
regenerated oxidant fluid produced at one or more positive
electrodes of said electrolysis-disproportionation reactor and
comprising said one or more forms of said aqueous multi-electron
oxidant and said intermediate oxidant.
83. The method of claim 53, wherein said acidification reactor and
said neutralization reactor of said discharge system are
functionally combined as an orthogonal ion migration across laminar
flow reactor.
84. A method for producing electric power and regenerating hydrogen
and an oxidant fluid comprising lithium bromate in an energy
storage cycle, said method comprising: providing a discharge system
comprising a discharge unit, an acidification reactor, and
optionally a neutralization reactor, said discharge system
comprising a neutral oxidant fluid comprising said lithium bromate,
one or more forms of a buffer, and said hydrogen, wherein
concentration of said lithium bromate dissolved in said neutral
oxidant fluid is above one of 1M, 2M, 5M, and 10M; converting said
neutral oxidant fluid into an acidic oxidant fluid in said
acidification reactor, wherein concentration of acidic protons in
said acidic oxidant fluid is below one of 0.1M, 0.5M, 1M, 2M, 5M,
and 10M; facilitating discharge of said discharge unit for
producing said electric power from said acidic oxidant fluid and
from said hydrogen and generating an acidic discharge fluid on
consumption of said acidic oxidant fluid and said hydrogen, wherein
said discharge is facilitated via a pH-dependent solution-phase
comproportionation of bromate with bromide formed via
electroreduction of an intermediate bromine; optionally
neutralizing said acidic discharge fluid in said neutralization
reactor of said discharge system to produce one or more forms of a
neutral discharge fluid; regenerating said hydrogen and one or more
forms of said oxidant fluid comprising said lithium bromate in
stoichiometric amounts from one or more forms of said neutral
discharge fluid in a regeneration system using external power, said
regeneration comprising: splitting said one or more forms of said
neutral discharge fluid into stoichiometric amounts of bromine,
hydrogen, and a base form of said buffer using said external power
in a splitting-disproportionation reactor, and producing said
lithium bromate via disproportionation of said bromine with said
base form of said buffer, wherein said splitting is performed via
one or more of electrolysis, photolysis, photoelectrolysis,
radiolysis, and thermolysis, and wherein said disproportionation of
said bromine is facilitated by a buffer capable of maintaining a
solution pH between 4 and 9; and continuing said splitting and said
disproportionation in said splitting-disproportionation reactor in
one of a no-aqueous multi-electron oxidant-on-negative electrode
mode of operation and an aqueous multi-electron oxidant-on-negative
electrode mode of operation in one of a plurality of modes, until a
desired degree of conversion of said bromide into said bromate is
achieved; and supplying said regenerated one or more forms of said
oxidant fluid comprising said bromate and said regenerated hydrogen
to said discharge system for subsequent generation of electric
power on demand.
85. The method of claim 84, wherein said modes comprise a batch
mode, a cyclic flow mode, a cascade flow mode, and any combination
thereof.
86. The method of claim 84, wherein a cationic component of said
buffer is lithium, and wherein an anionic component of said buffer
is one or more of .omega.-(N-morpholino)alkanesulfonate,
3-(N-morpholino)methanesulfonate, 3-(N-morpholino)ethanesulfonate,
3-(N-morpholino)propanesulfonate, 3-(N-morpholino)butanesulfonate,
methylphosphonate, an alkylphosphonate, an arylphosphonate, and a
molecule comprising phosphonate moieties and sulfonate
moieties.
87. The method of claim 84, wherein a cationic component of said
buffer is sodium, and wherein an anionic component of said buffer
is one or more of .omega.-(N-morpholino)alkanesulfonate,
methylphosphonate, 3-(N-morpholino)ethanesulfonate,
3-(N-morpholino)propanesulfonate, an alkylphosphonate, an
arylphosphonate, and a molecule comprising phosphonate moieties and
sulfonate moieties.
88. The method of claim 84, wherein said discharge system further
comprises a deprotionated form of an extra acid.
89. The method of claim 88, wherein said extra acid comprises one
or more of bromic acid, sulfuric acid, perchloric acid, triflic
acid, a sulfonic acid, molecules comprising phosphonate moieties
and sulfonate moieties, and an acid with a pKa.ltoreq.2.
90. The method of claim 84, wherein said base form of said buffer
is one or more of .omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
4-(N-morpholino)butanesulfonate, a phosphoric acid derivative, an
alkylphosphonate, an arylphosphonate, a molecule comprising
phosphonate moieties and sulfonate moieties, an amine, a nitrogen
heterocycle, and a base with a pKa between 4 and 9.
91. The method of claim 84, wherein one or more forms of said
acidic oxidant fluid comprises said lithium bromate, water, one or
more forms of a buffer, and optionally one or more forms of an
extra acid,
92. The method of claim 84, wherein said buffer is in an acid form
during said discharge with a pH.ltoreq.4, and wherein said acid
form of said buffer comprises one or more of a phosphoric acid
derivative, a phosphoric acid ester, one or more substituted
phosphonic acids, one or more 2-(N-morpholino) alkanesulfonic
acids, molecules comprising both phosphonate and sulfonate
moieties, and buffers capable of maintaining pH between 4 and
9.
93. A system for producing electric power and regenerating an
aqueous multi-electron oxidant and a reducer in an energy storage
cycle, said system comprising: a discharge system comprising: a
neutral oxidant fluid comprising one or more forms of said aqueous
multi-electron oxidant; a reducer fluid comprising one or more
forms of said reducer; a discharge unit comprising an electrolytic
cell stack, said electrolytic cell stack comprising a plurality of
electrolytic cells, wherein each of said electrolytic cells
comprises an electrolyte-electrode assembly; an acidification
reactor operably connected to said discharge unit, said
acidification reactor configured to lower pH of said neutral
oxidant fluid for generating an acidic oxidant fluid; and said
discharge unit configured to produce said electric power from said
acidic oxidant fluid and from said reducer fluid by performing:
transferring electrons from a positive electrode of said
electrolyte-electrode assembly to said aqueous multi-electron
oxidant in said acidic oxidant fluid; and transferring electrons
from said reducer fluid to a negative electrode of said
electrolyte-electrode assembly to produce said electric power in an
external electric circuit operably connected to terminals of said
discharge unit and to generate an acidic discharge fluid on
consumption of said acidic oxidant fluid and said reducer fluid;
and a regeneration system comprising: an
splitting-disproportionation reactor configured to convert a
neutral discharge fluid into an alkaline discharge fluid by using
one or more of an externally supplied base and a base produced in
said splitting-disproportionation reactor configured for one of an
aqueous multi-electron oxidant-on-negative electrode mode of
operation, a no-aqueous multi-electron oxidant-on-negative
electrode mode of operation, and a combination thereof; said
splitting-disproportionation reactor configured to split said
alkaline discharge fluid into a reducer and an intermediate oxidant
via one of electrolysis, photoelectrolysis, photolysis,
thermolysis, and radiolysis, wherein said splitting also releases
stoichiometric amounts of said reducer and said base in said
splitting-disproportionation reactor; said
splitting-disproportionation reactor configured to convert said
intermediate oxidant produced in said splitting-disproportionation
reactor into one or more forms of said aqueous multi-electron
oxidant via disproportionation of said intermediate oxidant with
said base; and said splitting-disproportionation reactor configured
to continue said splitting and said disproportionation in one of a
batch mode of operation, a cyclic flow mode of operation, a cascade
flow mode of operation, and a combination thereof, until a desired
degree of conversion of a discharge product of said aqueous
multi-electron oxidant into one or more forms of said aqueous
multi-electron oxidant is achieved.
94. The system of claim 93, wherein said discharge system further
optionally comprises a neutralization reactor operably connected to
said discharge unit, wherein said neutralization reactor is
configured to raise pH of said acidic discharge fluid for
generating one or more forms of said neutral discharge fluid.
95. The system of claim 94, wherein said acidification reactor and
said neutralization reactor of said discharge system are
functionally combined as an orthogonal ion migration across laminar
flow reactor.
96. The system of claim 95, wherein said orthogonal ion migration
across laminar flow reactor comprises flow cell assemblies, end
plates, and bipolar plates, wherein each of said flow cell
assemblies of said orthogonal ion migration across laminar flow
reactor comprises: ion exchange membranes comprising a positive
side ion exchange membrane and a negative side ion exchange
membrane positioned parallel to each other; a positive electrode
layer and a negative electrode layer flanking outer surfaces of
said ion exchange membranes, wherein said positive electrode layer
is configured for a hydrogen oxidation reaction and said negative
electrode layer is configured for a hydrogen evolution reaction; an
intermembrane flow field comprising a plurality of flow channels,
said intermembrane flow field interposed between said ion exchange
membranes; and porous diffusion layers flanking said outer surfaces
of said ion exchange membranes and in electric contact with one of
said bipolar plates and said end plates.
97. The system of claim 93, wherein said discharge system is
configured to operate in an electric partial recharge mode for
facilitating regenerative breaking when said discharge system
powers an electric vehicle, wherein said reducer is produced on
said negative electrode of said electrolyte-electrode assembly and
an intermediate oxidant is produced on said positive electrode of
said electrolyte-electrode assembly during said electric partial
recharge mode.
98. The system of claim 93, wherein said regeneration system
further optionally comprises a concentrating reactor configured to
produce a concentrated solution of a neutral oxidant fluid
comprising a salt form of said aqueous multi-electron oxidant.
99. The system of claim 93, wherein said regeneration system
further comprises one or more separation reactors configured to
separate gases from liquids during a regeneration process.
100. The system of claim 93, wherein said
splitting-disproportionation reactor of said regeneration system is
further configured for an aqueous multi-electron
oxidant-on-negative electrode mode of operation using a multilayer
structure on a negative electrode side of said
splitting-disproportionation reactor.
101. The system of claim 100, wherein said multilayer structure on
said negative electrode side of said splitting-disproportionation
reactor is configured to minimize reduction of a regenerated
aqueous multi-electron oxidant in a regenerated oxidant fluid on
said negative electrode side while facilitating hydrogen evolution
and an increase in pH of said regenerated oxidant fluid.
102. The system of claim 93, wherein said
splitting-disproportionation reactor is further configured for a
no-aqueous multi-electron oxidant-on-negative electrode mode of
operation by transferring a base produced on one or more negative
electrodes of said splitting-disproportionation reactor to a
regenerated oxidant fluid at one or more positive electrodes of
said splitting-disproportionation reactor and comprising said one
or more forms of said aqueous multi-electron oxidant and said
intermediate oxidant.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
non-provisional patent application Ser. No. 13/969,597 titled "Flow
Battery And Regeneration System", filed in the United States Patent
and Trademark Office on Aug. 18, 2013, which claims priority to and
the benefit of provisional patent application No. 61/684,805 titled
"Fluid Battery With Water-compatible Oxidants", filed in the United
States Patent and Trademark Office on Aug. 19, 2012. The
specifications of the above referenced patent applications are
incorporated herein by reference in their entirety.
BACKGROUND
[0002] The first widely commercialized automobiles at the dawn of
the last century were electric and powered by lead acid batteries.
Lead acid batteries are currently used in cars for starting,
lighting, and ignition purposes. Lead acid batteries cost, for
example, about 170 dollars/kilowatt hour (kWh) and are cheaper than
many other rechargeable batteries known. However, the energy
content of lead acid batteries is rather low. The specific energy
of lead acid batteries is, for example, about 35 watt hour
(Wh)/kilogram (Kg) or about 20% of their theoretical value. This is
notably reflected in the short driving range provided by the lead
acid batteries, for example, of about 30 km in fully electric
vehicles. A long recharge time, for example, of about 2 hours
required for lead acid batteries necessitates in many applications,
a cumbersome mechanical swap of a discharged battery by a charged
battery.
[0003] By the year 1910, improvements in the performance of an
internal combustion engine, the development of mechanical
transmission, combined with a wide availability of liquid
hydrocarbon fossils, resulted in the displacement of electric
vehicles by gasoline vehicles in the terrestrial transportation
market. Gasoline power systems provide high energy content, for
example, about 4,000 Wh/kg at wheels, that is, about 500 kilometres
driving range, and a quick mechanical refill. This provided
gasoline power systems an advantage over batteries with solid
electroactive materials (SEAM). Gasoline cars were widely used even
through the oil crises of the 1970s. The oil crisis provoked a
concern about the availability of hydrocarbon resources and
promoted a short lasting interest in electric battery and hydrogen
vehicles.
[0004] The current interest in electric cars started in 1990 with
the passage of the zero-emissions vehicle mandate by the California
Air Resources Board. Nickel-metal hydride batteries, commercialized
around this time, were considered briefly for automotive
applications. Although nickel-metal hydride batteries provided
better performance than the lead acid batteries, for example, a
driving range of about 60 km, a specific energy of about 60 Wh/Kg
to about 90 Wh/Kg, an energy density of about 200 Wh/L-300 Wh/L, a
specific power of about 200 W/kg, and an electric recharge of about
3 hours, albeit at a higher cost of about $1,000/kWh, the
nickel-metal hydride batteries were not an acceptable replacement
for gasoline from the customer's perspective.
[0005] By the year 1990, hydrogen fuelled polymer electrolyte
membrane fuel cells (PEMFCs), which were originally developed
within American and Soviet space exploration programs, became the
leading contender among power sources for electric vehicles. The
interest in PEMFCs was due to the following factors: the perceived
availability of hydrogen fuel, a high specific energy, for example,
of about 33.39 kWh/Kg for the low heating value of hydrogen
(H.sub.2), a high specific power of PEMFCs, for example, about 0.7
W/cm.sup.2 at about 60% efficiency and about 0.35 kW/Kg and about
0.35 kW/L at the stack level, a competitive system energy density,
for example, about 1,000 Wh/L for a 700 bar gas, and about 1200
Wh/L for 1 atmospheric pressure (atm) liquid H.sub.2 allowing for a
600 km driving range, as well as a good energy efficiency, for
example, about 60% for PEMFCs versus about 13% for an internal
combustion engine.
[0006] In the following 20 years, the idea of hydrogen economy and
automotive fuel cells received a significant political and economic
impetus which was justified by the concerns with the rising
atmospheric carbon dioxide (CO.sub.2) levels and an unstable supply
of liquid hydrocarbons. This was reflected in the statement by
President G. W. Bush in his 2003 State of the Union address: "a
child born today will be driving a car, as his or her first car,
which will be powered by hydrogen and pollution free."." In 2004
General Motors was spending more than a quarter of its research
budget on fuel cell vehicles and Larry Burns, GM's Vice President
for R&D and Planning, said in February 2004 that the company
will have a commercially viable fuel cell vehicle by 2010. In 2004,
the State of California said it would build a hydrogen highway,
with hydrogen fueling stations every 20 miles along major highways
in the next few years. Despite the dedicated work of many
scientists and engineers worldwide, the hydrogen fuelled polymer
electrolyte membrane fuel cell (PEMFC) technology did not result in
a market success of electric vehicles. The reasons are as follows:
to achieve practically useful power density on the positive
electrode, high platinum (Pt) loading is required which increases
the cost of the PEMFCs; the dissolution of a Pt catalyst at
positive potentials makes the positive electrode less durable; the
lack of an inexpensive, sustainable, and a clean hydrogen source;
and the lack of a hydrogen manufacturing and distribution
infrastructure. Hence, there is a need for a technology that avoids
the macro scale infrastructure required for hydrogen production and
distribution and also reduces the amount of Pt required for
on-board electricity generation.
[0007] Several revolutionary developments also occurred in the
field of batteries with solid electroactive materials (SEAM). The
advantages of a lithium (Li) metal anode, for example, a low
equivalent weight, very negative redox potential, and a small
cation size, allowing for an easy intercalation into cathode
materials, were realized in the early 1970s. However, the first
lithium batteries had a poor cycle life since the electronically
insulating surface film formed on metallic lithium leads to
dendritic Li plating during recharge. In 1981, researchers from
Sony Corporation demonstrated a rechargeable lithium ion battery
(LIB) with a graphite intercalation cathode. This lead to the
commercialization of lithium batteries with a carbon anode in
portable applications, within one decade. Since LIBs have a high
energy density when compared to other commercialized room
temperature batteries, LIBs have been used in commercial electric
vehicles since the year 2010 despite a somewhat high cost, for
example, of about $400/kW.
[0008] However, fully electric vehicles, unlike plug-in hybrids,
based on lithium ion batteries (LIBs) did not achieve a widespread
commercial success, primarily due to a low energy content, that is,
a low driving range, and a high total cost of ownership of the
batteries. For example, Nissan Leaf.RTM. of Nissan Jidosha
Kabushiki Kaisha DBA Nissan Motor Co. Ltd., has a battery that
weighs about 20% of the total car weight with about 200 Wh/Kg, that
is, about 53% of the theoretical value, and about 230 Wh/L, and
provides about 60 Km to about 100 Km driving range depending on
whether the air conditioner is on or off. A larger sport utility
vehicle (SUV), for example, Toyota RAV4.RTM. EV of Toyota Jidosha
Kabushiki Kaisha TA Toyota Motor Corporation, also shows a similar
performance. The often quoted statistics that 60% of daily car
trips in the United States are less than 60 Km is apparently not
helping the sales of lithium-ion battery powered cars as most
drivers need the capability to make longer trips. Apart from the
low driving range, the LIBs also have a low electric recharge rate,
for example, the Nissan Leaf.RTM. takes about 30 minutes for a
charge of about 80% of full capacity, and the construction of a
large scale battery swapping infrastructure is not justified due to
the lack of a sizable LIB electric vehicle market, as illustrated
by a recent bankruptcy of Better Place. Also, the capital cost of
the LIBs needs to be reduced in the long term, for example, from
about $500/kWh to $125/kWh and from about $30/kW to $8/kW at 250
Wh/kg, 400 Wh/L, and 2 kW/kg.
[0009] The scientists at General Motors (GM) arrived at the same
conclusion, that is, the battery electric vehicles based on current
and targeted Li ion battery technology will be limited to small
vehicle, low mileage-per-day applications due to relatively low
specific energy and long recharge time constraints, and it is
possible that fundamental physical limitations may prevent pure Li
ion based battery electric vehicles (BEVs) from delivering the
freedom of providing long trips, with intermittent quick refills,
that consumers currently receive from their cars. According to
Toyota spokesman John Hanson "We don't think that lithium-ion
batteries are going to help us get to a point where we can
dramatically increase volume and really call it a mass market.
We're going to have a more significant breakthrough and probably go
into some other area of battery chemistry." MIT's Yet-Ming Chiang
concurs: "It is clear that long-term vehicle
electrification--especially affordable 200 mile all-electric
range--will require batteries with approximately three times
greater energy densities at about one third the cost per kWh than
that of LIBs." Kevin See, analyst for Boston-based Lux Research,
said "It is not realistic or feasible for automakers to
significantly cut the price of lithium-ion batteries. There is
going to be incremental improvement, but we don't believe it will
be enough to spur the huge adjustment everyone was hoping for."
Tesla Motors has conceded that new technologies will eventually be
required. According to Steve Visco, the founder of Polyplus: "What
has happened over the past couple of years is the growing
realization that lithium-ion chemistry will not take EVs to a mass
adoption vehicle. It is just too expensive and they're too
heavy."
[0010] Numerous attempts to commercialize lithium ion batteries
(LIBs) for use in fully electric vehicles in the last 5 years
failed as eloquently illustrated by the mismatch of large
production capacities and negligible sales by all 9 award
recipients of the August 2009 $1.5 billion Department of Energy's
(DOE) "Electric Vehicle Battery and Component Manufacturing
Initiative" who had a primary focus on electric vehicle (EV)
batteries including Dow Kokam, Johnson Controls, A123 Systems,
Compact Power, EnerDel, General Motors, SAFT America, and LG Chem.
The public's lack of appetite for battery-powered cars persuaded
the Obama's administration in January of 2013 to back away from its
aggressive goal to put 1 million electric cars on U.S. roads by
2015. According to Takeshi Uchiyamada, Toyota's Vice Chairman, "the
current capabilities of electric vehicles", based on fuel cells or
lithium ion batteries, "do not meet society's needs, whether it may
be the distance the cars can run, or the costs, or the long time to
charge. Because of its shortcomings, that is, driving range, cost,
and recharging time, the battery or fuel cell electric vehicle is
not a viable replacement for most conventional cars. We need
something entirely new". Thus, there is a need for a solution that
departs from the currently available technologies and differs from
others under investigation in the electric vehicle battery field.
More specifically, there is a need for a power source for electric
vehicles that provides a longer driving range, lower total cost of
ownership, and allows for a quick recharge or refill than
lithium-ion batteries.
[0011] The history of technology teaches that if the show stopping
part in any device is identified and replaced with another part,
then this may change the device from a non-functional device to a
functional device, though the performance in one or more parameters
may have to be sacrificed. In the case of lithium batteries, the
aforementioned abandonment of the metallic lithium electrode in
favor of lithium intercalated into graphite resulted in about 30%
decrease in the theoretical energy density but created a marketable
battery with a long cycle life. Flow systems such as fuel cells
(FCs) and redox flow batteries (RFBs) allow an independent scaling
on energy and power, and are thus better suited for transportation
than batteries with solid electroactive materials (SEAMs). Other
advantages of flow systems, when compared to SEAM batteries, are a
higher system energy density, if the reactants are not too dilute,
a quick refill time, an intrinsic fluid heat management, and a
simple cell balancing. The advantages of redox flow batteries over
fuel cells are: electric regeneration that does not require a
construction of a new fuel distribution infrastructure, for
example, a hydrogen distribution infrastructure, higher efficiency,
and in general, a lower cost. Conventional redox flow batteries
such as vanadium redox flow batteries have a low energy density
that translates into a short driving range, because the components
have low solubilities and a large amount of an otherwise useless
solvent which has to be carried on-board to keep the components in
the fluid state. For this reason, flow batteries have been
considered mostly for stationary storage applications rather than
for electric vehicles.
[0012] A Massachusetts based start-up, 24M, proposed a method that
retains the advantages of flow batteries while overcoming drawbacks
of traditional solution chemistry, by developing a slurry flow
battery based on the C.sub.6--LiFePO.sub.4 chemistry used by A123
Systems for batteries with solid electroactive materials (SEAM) or
SEAM batteries. However, such a battery in an electric vehicle such
as the Nissan Leaf.RTM. or the Toyota RAV4.RTM. would provide only
from about 90 Km to about 150 Km driving range, even if the battery
reaches, for example, about 80% of its theoretical energy density.
Improvements in packing factor, that is the ratio of practical to
theoretical energy density, by using, for example, binder free SEAM
batteries with a soluble mediator or a soluble redox couple or
metal containing ionic liquid flow batteries or protected Li metal
anode, run into the fundamental limitation that the intrinsic
energy densities of known battery chemistries are not sufficiently
high for fully electric vehicle applications. Also, the cost of
such batteries is likely to stay above the mid-term target of about
$100/kWh and about $30/kW, or about $2,250/car with about 100
horsepower. Hence, there is an unmet need for flow batteries with
higher energy content and a lower cost in order to gain market
acceptance of fully electric vehicles.
[0013] Polymer electrolyte membrane (PEM) fuel cells have high
power and energy density at low operating temperatures as well as a
flow design which makes the PEM fuel cells well suited for
automotive applications. Furthermore, fuel cells provide for a very
high system energy density since the oxidant, that is, O.sub.2 is
not carried on-board. However, the fundamental problems related to
the slow kinetics of the oxygen electrode result in high cost and
poor durability of PEM fuel cells due to the necessity of high Pt
loading in the case of near ambient temperature fuel cells. Another
problem with fuel cells, in general, is the source of the fuel, for
example, hydrogen. Hence, there is a need for a discharge flow
battery that ensures a high energy density, high energy efficiency,
generates a high electric power by replacing the free oxygen from
air with a high energy density and kinetically fast on-board fluid
oxidant, and allows for the regeneration of a fuel and an oxidant
from the exhaust products.
[0014] Flow batteries use electrochemical power cells similar to
fuel cells. Flow batteries also use fluid reactants, for example,
liquid, gaseous, or suspended reactants to store energy and to
generate electric power. However, instead of oxygen or air, a
different oxidant or a solution of an oxidant can be employed. Due
to the carrying of an on-board oxidant, the flow battery typically
entails a lower system energy density than a fuel cell. The reasons
for using the on-board oxidant method comprise, for example,
increasing the efficiency of energy conversion, reduction in the
amount of precious metal catalysts, potential to change the
operating temperature of the electrochemical power cell, improved
heat management, the possibilities of electric recharge and of
mechanical refill, etc. When compared to batteries with solid
electroactive materials (SEAM) or SEAM batteries, for example,
lithium ion batteries, flow batteries offer an independent scaling
of energy and power, a higher ratio of practical to theoretical
energy density that is, packing factor for systems with a
sufficiently long discharge time, a possibility of quick mechanical
recharge, intrinsic liquid cooling, etc. Commercialized redox flow
batteries, such as Vanadium Redox Flow Batteries have low energy
densities because of the use of redox couples with low solubilities
and with a low number of redox-active electrons per electroactive
atom. Paul Zigouras, Director of New Business Development at EPC
Corporation, eloquently summarizes the status quo as: "Flow
batteries are a great idea, but unfortunately, no fluid currently
exists that will hold a decent amount of energy. Even the best
experimental fluids have about 1/5.sup.th the energy density of the
required value. I am hopeful, but also doubtful that a fluid will
ever be developed that can effectively do this".
[0015] Hydrogen-halogen flow batteries employ fluid reagents and
products, and thus, may avoid the aforementioned energy density
dilution by a solvent. In the series from fluorine (F.sub.2) to
iodine (I.sub.2), the theoretical energy density decreases while
the efficiency, cathode power, and exchange current increases. As a
result, F.sub.2 has poor cycle efficiency, in addition to material
compatibility issues, whereas I.sub.2 has a low energy density in
addition to solubility problems. Hence, only bromine (Br.sub.2) and
chlorine (Cl.sub.2) may be of interest for transportation
applications. However, the chorine cells use an expensive ruthenium
(Ru)-containing catalyst and provide poor energy efficiency. The
theoretical energy density of hydrogen-bromine cells is only
marginally better than that of lithium-ion batteries. The energy
density becomes even lower if bromine is used as an aqueous
solution with hydrogen bromide (HBr) to reduce the oxidant's cross
over through membrane via the formation of Br.sub.3.sup.- anions
and to lower the pressure of the Br.sub.2 vapour. Hydrogen-bromine
cells are therefore considered at present mostly for grid storage
rather than for electric vehicles.
[0016] There is a need for resolving the aforementioned TRIZ
contradiction between energy density and energy efficiency of
halogens, for example, by introducing a new dimension to the choice
of oxidants, for example, by adding a second dimension of
oxocompounds such as oxides and oxoacids to the one dimensional
space of elements such as halogens. Although hydrogen-oxoacid flow
batteries such as H.sub.2--HNO.sub.3 have been considered in the
past, these flow batteries have poor discharge efficiency and lack
the ability of electrical recharge or regeneration of the reagents.
The direct electroreduction of halogen oxoacids is highly
irreversible under the polymer electrolyte membrane fuel cell
(PEMFC) conditions. There is a need to overcome this problem, for
example, by performing a slow reduction of an oxocompound in a
solution, that is, in three dimensions rather than on an electrode,
that is, two dimensional.
[0017] Transition metal ion catalyzed electroreduction of oxoanions
has been known for over 100 years. However, such reactions did not
find applications in energy storage and conversion, mostly due to
their poor reversibility. A more useful way to facilitate the
electroreduction of halogen oxoanions is to employ a preceding
homogeneous reaction such as comproportionation with a halide
product as exemplary demonstrated for a halate by the equations
below:
XO.sub.3.sup.-+6e.sup.-+6H.sup.+=X.sup.-+3H.sub.2O on the
electrode, slow. (1)
XO.sub.3.sup.-+5X.sup.-+6H.sup.+=3X.sub.2+3H.sub.2O in solution,
fast. (2)
X.sub.2+2e.sup.-=2X.sup.- on the electrode, fast. (3)
[0018] where X.dbd.Cl, Br, I.
[0019] In practice, reaction (3) may precede reaction (1) during
the initial stage of the cycle. Furthermore, at high concentrations
of halogen oxoanion and of an acid and for a thick diffusion layer,
the steady-state limiting current, determined by the balance of the
rate of halogen, that is, X.sub.2 intermediate formation via
comproportionation (2) and by the rate of halogen loss into the
solution bulk, can reach enormous values over 1 A/cm.sup.2.
[0020] The reverse process of oxidation of halides is generally
believed to follow the same pathway. For example, the oxidation of
the halides such as iodide, bromide, and chloride at alkaline pH
shows that the reverse of the chemical reaction indicated by
equation (2) occurs through the formation of an intermediate
hypohalate via a homogeneous disproportionation: Here, R is a
base:
2X.sub.2+2H.sub.2O+2R.sup.-=2HXO+2X.sup.-+2RH (4)
followed by another homogeneous disproportionation:
5HOX(hypohalous acid)=4X.sup.-+XO.sub.3.sup.-+H.sup.++2H.sub.2O
(5)
or (4) and (5) combined
3X.sub.2+3H.sub.2O+6R.sup.-=XO.sub.3.sup.-+5X.sup.-+6RH (6)
[0021] Thus, disproportionation, for example, reaction (6), can be
used to regenerate a halogen oxoanion from a halide present in the
discharge fluid via an intermediate halogen produced by one or
several routes of oxidation of halide.
[0022] The occurrence of homogeneous disproportionation reactions
(4), (5), (6), and a comproportionation reaction (2) facilitates
discharge and regeneration processes respectively in the energy
cycle. The occurrence of these reactions allows for a high power,
high efficiency operation based on a fast electrode reaction
(X.sub.2+2e.sup.-=2X.sup.-) while performing slower steps such as
reduction of the oxoanion with the electro-generated halide in the
three dimensional bulk of the solution which can accommodate a
higher reaction rate than the two dimensional electrode surface.
Although the use of a mediator leads in theory to reduced energy
efficiency compared to a direct electrode reaction, this
thermodynamic loss of energy efficiency is often smaller than the
kinetic loss associated with electrode over-voltage at the same
power using oxidants such as oxygen or using direct
electroreduction of the oxoanions.
[0023] The chemical methods of producing halogen oxoacids are used
on an industrial scale. In the case of bromic acid, this chemical
method consists of solution-phase disproportionation of bromine in
Ba(OH).sub.2, followed by Ba.sup.2+ precipitation with sulfuric
acid and by evaporation of the excess water. However, this process
irreversibly consumes Ba(OH).sub.2, H.sub.2SO.sub.4 and generates
BaSO.sub.4 waste. Also, this process does not co-produce a
stoichiometric amount of hydrogen, which is required for the
complete energy cycle of discharge and regeneration. Thus, this
precipitation route does not meet the application requirements. An
alternative method for preparing up to 40%-50% bromic acid via the
electrooxidation of aqueous bromine solutions uses a lead dioxide
anode at the current density of 10-20 mA/cm.sup.2 and a potential
of +2.1 to +2.2 V versus a normal hydrogen electrode. Although this
method is chemical and waste free, this method has poor energy
efficiency and a low throughput.
[0024] Sunlight is a clean and carbon dioxide (CO.sub.2) free
energy source and the sun's energy can be harvested thermally,
photoelectrically, photochemically, or photoelectrochemically.
While about 120,000 terawatts (TW) of sunlight, year averaged
power, reaches the earth, the current total energy consumption of
human civilization is only about 13 TW. Currently, with a wide
scale utilization of solar technologies, there is a TRIZ
contradiction between cost and efficiency intrinsic to all
commercialized means of sunlight energy conversion. For example,
semiconductor based photovoltaic solar panels, for example,
polycrystalline silicon photovoltaic solar panels, multilayer
photovoltaic solar panels, InxGa (1-x) Se2, etc., are either
inefficient or too expensive. Photoelectrochemical water splitting
into hydrogen (H.sub.2) and oxygen (O.sub.2) using anatase
TiO.sub.2 nanoparticles also suffers from a low efficiency due to
the high over voltage of the oxygen production centers. Hence,
there is a need for a method for converting sunlight energy into
chemical energy or electric energy at low cost and without
producing any chemical waste.
[0025] Hence, there is a long felt but unresolved need for an
electrochemical flow battery that provides for a high energy
density, that is, a long driving range, a high energy efficiency
and power at a low operational and manufacturing cost, and requires
a short refill time. Moreover, there is a need for a method and a
system that regenerates an oxidant and a fuel simultaneously from a
discharge fluid, in stoichiometric amounts, without consumption of
extra chemicals and without generating chemical waste and by using
electric or solar energy as the primary energy source. Furthermore,
there is a need for an electrochemical flow battery that provides
better safety and stability by storing on-board and off-board a
stable form of the oxidant.
SUMMARY OF THE INVENTION
[0026] This summary is provided to introduce a selection of
concepts in a simplified form that are further disclosed in the
detailed description of the invention. This summary is not intended
to identify key or essential inventive concepts of the claimed
subject matter, nor is it intended for determining the scope of the
claimed subject matter.
[0027] The method and the discharge system disclosed herein address
the above stated needs for a mechanically refillable,
electrochemical flow battery that provides a high energy density, a
high energy efficiency, and a high electric power at a low cost,
requires a short refill time, reduces or completely eliminates
usage of platinum and other precious materials in the electrodes,
and reduces the size or completely eliminates the humidification
system. The method and the discharge system disclosed herein
produce electric power from two fluids, namely, a reducer fluid
also referred to as a "fuel", and an oxidant fluid comprising an
aqueous multi-electron oxidant (AMO), and release one or more
discharge fluids. The oxidant is an element or a compound in a
reduction-oxidation reaction that receives one or more electrons
from another species or from an electrode. The aqueous
multi-electron oxidant (AMO) is an oxidant that, in at least one of
its forms such as an acid form, has a high solubility in water, for
example, over 0.5 M, and that transfers in a solution-phase redox
reaction or in an electrochemical reaction more than 1 mole of
electrons per 1 mole of the AMO. The AMO can be present in one or
more of a salt form, an acid form, and other forms, and unless
specified otherwise, the term "AMO" used herein refers to all these
forms. The reducer is an element or a compound in a
reduction-oxidation reaction that donates one or more electrons to
another species or to an electrode. The methods and the systems
disclosed herein use an aqueous multi-electron oxidant selected
from oxides and oxoacids of non-metals such as halogens, for
example, chlorine, bromine, and iodine in the form of gases,
liquids, melts, low melting point solids, liquid solutions or
suspensions.
[0028] Moreover, the method and the regeneration system disclosed
herein regenerate an aqueous multi-electron oxidant in a salt form
or other form and a reducer simultaneously from a discharge fluid
in a salt form or other form simultaneously, in stoichiometric
amounts, without consumption of extra chemicals and without
generating chemical waste. As used herein, the term "discharge
fluid" refers to an exhaust fluid obtained as a result of an
electrochemical discharge process, that is, electric power
generation, in a flow battery or in a discharge system. In an
embodiment, the regeneration process consumes, for example,
electric energy, solar energy, thermal energy, radiolytic energy,
or any combination thereof. In another embodiment, the regeneration
process comprises one or more of an electrochemical process,
photoelectrolysis, photolysis, thermolysis, radiolysis, etc. In
another embodiment, the regeneration process is performed via
chemical processes. In an embodiment, the method and the
regeneration system disclosed herein regenerate a reducer and an
aqueous multi-electron oxidant in one or more forms simultaneously
and in stoichiometric amounts from a discharge fluid by means of,
for example, electrolysis, photoelectrolysis, homogeneous solution
phase reaction, disproportionation, pH change, ion exchange,
heterogeneous ion exchange such as using resins, homogeneous
ion-exchange such as via an orthogonal ion migration across laminar
flow (OIMALF) process, and if desired, concentration performed, for
example, by evaporation or reverse osmosis. As used herein, the
term "laminar flow" refers to a type of fluid flow in which
directions and magnitudes of fluid velocity vectors in different
points within a fluid do not change randomly in time and in space.
Also, as used herein, the term, "migration" refers to a movement of
an electrically charged object, such as an ion, due to the action
of an external electric field. Disproportionation is a redox
reaction in which an element, free or in a compound, is reduced and
oxidized in the same reaction to form different products. For
example, an element with an oxidation state A, not necessarily A=O,
on disproportionation, is distributed between several species with
different oxidation states B, C, etc., which differ from the
element's initial oxidation state A, so that B>A>C. As used
herein, the term "orthogonal" in the phrase "OIMALF", implies that
the vectors of the laminar flow velocity and of the electric field
are not parallel and not anti-parallel. In an embodiment, the
methods and the systems disclosed herein facilitate halogen
oxoanion/halide conversion in both directions by means of
electrochemical reactions or other reactions and pH-dependent
homogeneous reactions. Disclosed herein is also a complete energy
cycle comprising a method for generating electric power and a
discharge fluid from one or more forms of an aqueous multi-electron
oxidant and a reducer using the discharge system, and a
regeneration of the aqueous multi-electron oxidant and the reducer
from the discharge fluid using the regeneration system and electric
or other energy input. In the methods and systems disclosed herein,
multi-electron redox couples with high solubilities of reagents and
products are used to overcome low energy densities of known flow
batteries.
[0029] Disclosed herein is a discharge system comprising an oxidant
fluid stored in an oxidant fluid tank, a reducer fluid stored in a
reducer fluid tank, and a discharge unit. The discharge unit is
also referred to as a "flow battery". The oxidant fluid is a
chemical or a mixture of chemicals that accepts electrons during a
discharge process in a discharge mode of operation of the discharge
unit. As used herein, the term "the discharge mode of operation"
refers to a process of releasing the chemical energy stored in the
discharge system in the form of sustainable electric current and
voltage. The acid form of the oxidant fluid comprises one or more
forms of an aqueous multi-electron oxidant (AMO), water, other
solvents, an extra acid, and a buffer in their acid forms. The
other solvent is, for example, a liquid other than water. The AMO
is one or a combination of an oxide of an element such as a
halogen, an oxoanion of an element such as a halogen, etc. The
buffer in the acid form is, for example, one or more of phosphoric
acid, a dihydrogen phosphate of lithium, a dihydrogen phosphate of
another cation, a substituted phosphonic acid, buffering agents
such as Good's buffers, and any combination thereof, capable of
maintaining pH of the oxidant fluid at a value, for example, below
4. In an embodiment, the buffer is in acid form during discharge
with a pH.ltoreq.7. The extra acid is a strong acid such as
sulfuric acid, triflic acid, another sulfonic acid, halogen
oxoacid, halic acid, etc. In an embodiment, the acid form of the
AMO serves as the extra acid. The AMO can be pre-mixed with the
buffer in the oxidant fluid. In an embodiment, the AMO is an oxide
or an oxoacid of an element, for example, nitrogen, xenon, sulfur,
etc. In another embodiment, the AMO is selected from a group
consisting of a halogen compound such as a halogen oxide, a halogen
oxoacid, etc., an interhalogen compound, a nitrogen compound, an
oxide of nitrogen, a nitrogen oxoacid, an oxide of xenon, an
oxoacid of xenon, an oxide of a chalcogen such as an oxide of
sulfur, an oxide of nitrogen or another pnictogen, an oxoacid of
nitrogen or another pnictogen, a volatile oxide of an element, a
fluid oxide of an element, a soluble oxide of an element, a
volatile oxoacid of an element, a fluid oxoacid of an element, a
soluble oxoacid of an element, etc., any combination thereof.
[0030] The methods and the systems disclosed herein expand the
choice of oxidants from one dimensional series of elements into a
multidimensional matrix of compounds, and more specifically, into
oxides of and oxoacids of a halogen, nitrogen and other pnictogens,
sulfur and other chalcogens, and xenon. That is, the methods and
the systems disclosed herein expand the one dimensional series of
elements such as halogens into a multidimensional matrix of
oxocompounds such as oxides and oxoacids. The oxide is a compound
containing oxygen and another element. The halogen oxoacid is a
compound having a formula H.sub.pX.sub.qO.sub.r, where X is one of
multiple halogens in particular Cl, Br, and I, O is oxygen, and
1.ltoreq.p, q, r.ltoreq.6. In one embodiment, the acid form of the
aqueous multi-electron oxidant (AMO) is halogen oxoacid, for
example, HBrO.sub.3. The reagents, products, and intermediates of
the halogen oxoacid reduction are either gases or liquids or are
soluble in water. If the reagents and products are anions, their
cross over through a cation exchange membrane is minimal. In an
embodiment, the oxoacid is a compound having a formula
H.sub.pXO.sub.r, where X is a halogen (Cl, Br, I), 1.ltoreq.p<6,
and 1.ltoreq.r.ltoreq.6. In an embodiment, the oxoacid is a
compound having a formula HXO.sub.r, where X is a halogen, for
example, Cl, Br, I, and 1.ltoreq.r.ltoreq.4.
[0031] In an embodiment, the aqueous multi-electron oxidant (AMO)
is a nitrogen oxide having a formula N.sub.xO.sub.n, where x=1 or 2
and 1.ltoreq.n.ltoreq.5. In another embodiment, the AMO is a
nitrogen oxoacid having a formula H.sub.kN.sub.lO.sub.m, where
1.ltoreq.k, l, m.ltoreq.3. In another embodiment, the nitrogen
oxoacid is a compound having a formula HNO.sub.n, where
1.ltoreq.n.ltoreq.3. In an embodiment, the oxoacid is a compound
having a formula H.sub.pX.sub.qO.sub.r, where X is one of multiple
halogens, nitrogen, other pnictogens, chalcogens, xenon, or other
element, and where 1.ltoreq.p, q, r.ltoreq.6. In an embodiment, the
acid form of the AMO is chloric acid which forms an aqueous room
temperature solution, for example, up to about 40% w/w. In an
embodiment, the acid form of the AMO is bromic acid which forms an
aqueous room temperature solution, for example, up to about 55%
w/w. In another embodiment, the acid form of the AMO is iodic acid
which forms an aqueous room temperature solution, for example, up
to about 74% w/w. In another embodiment, the acid form of the AMO
is perchloric acid which forms an atmospheric aqueous azeotrope,
for example, about 72.5% w/w. In another embodiment, the AMO is
nitric acid which forms an atmospheric aqueous azeotrope with, for
example, about 68.4% w/w. Halogen oxoacids allow for
energy-efficient and waste-free routes to their regeneration from
the discharge fluid.
[0032] The reducer fluid, also referred herein as a "fuel", is a
chemical that donates electrons during the discharge process. The
reducer is, for example, hydrogen. In an embodiment, the reducer is
selected from a group consisting of ammonia, hydrazine,
hydroxylamine, phosphine, methane, a hydrocarbon, an alcohol such
as methanol, ethanol, etc., an aldehyde, a carbohydrate, a hydride,
an oxide, a sulfide, another organic and inorganic compound, or any
combination thereof, with each other, with water, or with another
solvent. A hydrogen reducer is used herein because the hydrogen
reducer can be regenerated from the discharge fluid along with the
aqueous multi-electron oxidant (AMO) with a high efficiency and
without irreversible consumption of other chemical and without
generating chemical waste.
[0033] The discharge unit of the discharge system comprises a stack
of multiple electrolytic cells also referred to as an "electrolytic
cell stack". Each electrolytic cell comprises a 5-layer
electrolyte-electrode assembly and half of a bipolar plate/1
endplate. The 5-layer electrolyte-electrode assembly is flanked on
each side by a bipolar plate or an endplate. The 5-layer
electrolyte-electrode assembly comprises a 3-layer
electrolyte-electrode assembly flanked by a negative diffusion
layer on the negative electrode side and a positive diffusion layer
on the positive electrode side. The 3-layer electrolyte-electrode
assembly comprises an electrolyte layer interposed between or
flanked by a positive electrode layer and a negative electrode
layer. The 3-layer electrolyte-electrode assembly and/or the
5-layer electrolyte-electrode assembly are herein referred to as an
"electrolyte-electrode assembly".
[0034] In an embodiment, the electrolytic cell stack is configured
as a planar cell stack comprising planar electrolytic cells. The
planar electrolytic cells in the planar cell stack are connected
electrically in series so that the voltage of the electrolytic cell
stack is the sum of the voltages of the electrolytic cells. Each
planar electrolytic cell shares one bipolar plate with an adjacent
planar electrolytic cell. One side of a bipolar plate contacts a
positive side of one planar electrolytic cell and another side of
the bipolar plate contacts a negative side of the adjacent planar
electrolytic cell. The bipolar plates and the endplates are
equipped with channels for delivering reagents, that is, the
oxidant fluid and the reducer fluid to the electrolyte-electrode
assemblies in the electrolytic cell stack and for removing the
products, that is, one or more discharge fluids. The planar cell
stack is further flanked by a pair of endplates. The endplates are
further equipped with ports for the oxidant fluid, the reducer
fluid, and the discharge fluid, and electric connections.
[0035] In an embodiment, the electrolyte layer of the
electrolyte-electrode assembly is composed of a material capable of
ionic conduction, for example, protonic conduction but not
electronic conduction. In another embodiment, the electrolyte layer
of the electrolyte-electrode assembly is composed of an ionomer, a
solid ion conductor, a solid proton conductor, or a liquid under
laminar flow. The electrolyte is compatible with water, the aqueous
multi-electron oxidant (AMO), the reducer, and the products. In
another embodiment, the electrolyte layer of the
electrolyte-electrode assembly is composed of a material comprising
a chemical moiety selected from a group consisting of one or more
proton donor moiety or proton acceptor moiety. In an embodiment,
the electrolyte material is a cation-conductive polymer, for
example, a polyperfluorosulfonic acid such as Nafion.RTM. of E. I.
du Pont de Nemours and Company Corporation, Hyflon Ion of Ausimont
S.R.L. Corporation, Aciplex.RTM. of Asaki Kasei, Flemion.RTM. of
AsahiGlass, Aquivion.RTM. of Solvay-Solexis, etc. In another
embodiment, the electrolyte layer in the electrolyte-electrode
assembly is made of a composite material such as GoreSelect.RTM. of
W. L. Gore and Associates, Inc., or of an ionically conducting
liquid retained in the pores of a solid matrix. In another
embodiment, the electrolyte layer of the electrolyte-electrode
assembly comprises a material with a cationic conduction exceeding
an anionic conduction of the material. Such cation-selective
conductivity of the electrolyte is beneficial for both discharge
and regeneration systems since electrolyte reduces the crossover of
the AMO and of its reduction products and intermediates to the
negative electrode.
[0036] In the discharge unit, during the discharge mode of
operation, the positive electrodes of the electrolyte-electrode
assemblies are supplied with the oxidant fluid containing one or
more forms of the aqueous multi-electron oxidant and the negative
electrodes of the electrolyte-electrode assemblies are supplied
with the reducer fluid containing the reducer during the discharge
mode of operation. The bipolar plate provides an electron pathway
from one electrolytic cell in the electrolytic cell stack to the
next electrolytic cell. The bipolar plates also supply reactants to
the 5-layer electrolyte-electrode assemblies and remove the
products. The endplates flank the electrolytic cell stack. The
inner sides of the endplates operate in a manner similar to the
bipolar plates. The endplates comprise inlet ports for adding
reagents, outlet ports for removing products, and electric
connections to an external electric circuit. The endplates provide
electric connections and flow connections from the electrolytic
cell stack to the other components of the discharge system.
[0037] During the discharge mode of operation, the reagents, that
is, the oxidant fluid and the reducer fluid in the discharge system
are converted into products to produce electric current through the
electrolytic cell stack and through the external electric circuit.
More specifically, the reagents in the oxidant fluid and in the
reducer fluid are converted into products to produce an electric
current through the external circuit and through the bipolar plates
and an ionic current through the electrolyte layers. The oxidant
fluid and the reducer fluid are supplied from their respective
tanks which are periodically filled from an external source, for
example, the regeneration system. The discharge system disclosed
herein operates with an external electric circuit operably
connected to the electrolytic cell stack of the discharge unit.
During the discharge mode of operation, the external electric
circuit comprising, for example, an electric engine connected to
the discharge unit consumes the electric power generated by the
discharge unit. In the discharge unit, the reducer is configured to
donate the electrons to the negative electrodes, and the aqueous
multi-electron oxidant (AMO) is configured to accept the electrons
at the positive electrodes for producing an electric current in the
external electric circuit that connects the positive endplate and
the negative endplate, and for simultaneously producing an ionic
current through the electrolyte layer of an electrolytic cell or
the electrolyte layers of the electrolytic cells of the
electrolytic cell stack of the discharge unit. In an embodiment, a
solution-phase reaction facilitates one or more discharge reactions
on the positive electrode of the electrolyte-electrode assembly. In
an embodiment, the solution-phase reaction disclosed herein is, for
example, a pH-dependent solution-phase comproportionation, a
solution-phase redox mediated catalysis, etc. As used herein, the
term "comproportionation" is a redox reaction in which an element,
free or in compounds, with oxidation states A and C, is converted
into another substance or substances in which the element's
oxidation states are B, such that A>B>C. In an embodiment,
the rate of the solution-phase comproportionation depends on the pH
of the solution.
[0038] The power generation in the discharge unit may benefit from
a catalyst, a redox mediator, etc., for facilitating a charge
transfer between the electrodes of the electrolyte-electrode
assembly and the aqueous multi-electron oxidant (AMO) and the
reducer. In an embodiment, a halide mediator, for example, a
bromide mediator or a chloride mediator facilitates one or more
discharge reactions on the positive electrode of the
electrolyte-electrode assembly. For example, a redox mediator such
as a halogen/halide couple facilitates a charge transfer between
the positive electrode of the electrolyte-electrode assembly and
the AMO. In another embodiment, multiple immobilized heterogeneous
mediators, immobilized heterogeneous catalysts, homogeneous
mediators, or homogeneous catalysts facilitate a charge transfer
between the positive electrode of the electrolyte-electrode
assembly and the AMO. In another embodiment, a catalyst selected
from a group consisting of a homogeneous catalyst, a heterogeneous
catalyst, a redox mediator catalyst, or any combination thereof,
facilitates one or more discharge reactions on the positive
electrode of the electrolyte-electrode assembly. In another
embodiment, one or more forms of a redox mediator, a product of an
electrode reaction, an acid, or any combination thereof accelerates
a rate of discharge of the AMO during one or more discharge
reactions via a solution-phase reaction. In an embodiment, a
product of the discharge reaction facilitates the discharge
reaction via comproportionation. In another embodiment, a catalyst,
for example, ruthenium dioxide (RuO.sub.2), lead dioxide
(PbO.sub.2), or a platinoid electrocatalyst facilitates one or more
electrochemical reactions on the positive electrode of the
electrolyte-electrode assembly. In another embodiment, a platinoid
electrocatalyst facilitates one or more electrochemical reactions
on the negative electrode of the electrolyte-electrode assembly.
The discharge system stores the energy in reducer and oxidant fluid
tanks or containers and produces electric power on demand using the
discharge unit, for stationary, mobile, and portable devices that
require electrical power.
[0039] In an embodiment, the discharge unit disclosed herein
operates in a regenerative mode or electric recharge mode or as a
secondary flow battery. In the regenerative mode of operation, one
or more of reagents and intermediates are regenerated within the
discharge unit, by applying a voltage of the polarity opposite to
the polarity observed during the discharge mode of operation to the
terminals of the external electric circuit. For example, an
intermediate such as bromine can be regenerated from bromide
present in the discharge fluid using the discharge unit, if the
discharge unit is operated under reverse polarity.
[0040] Also, disclosed herein is a regeneration system configured
to regenerate one or more forms of the oxidant fluid and the
reducer in stoichiometric amounts from the discharge fluid produced
by the discharge unit using electric power. The regeneration system
comprises, for example, an electrolysis-disproportionation (ED)
reactor, storage tanks such as a regenerated oxidant fluid tank and
a regenerated reducer fluid tank for storing the regenerated
oxidant fluid and the regenerated reducer fluid respectively,
optionally a neutralization reactor, for example, an ion exchange
reactor such as an orthogonal ion migration across laminar flow
(OIMALF) reactor, one or more separation reactors, and a
concentrating reactor. In an embodiment, the neutralization reactor
comprises a mixing reactor. In an embodiment, the regeneration
system is configured for a batch mode of operation. In another
embodiment, the regeneration system is configured for a flow mode
of operation. In another embodiment, the regeneration system is
configured for a cyclic flow mode of operation. In another
embodiment, the ED reactor is configured for a cascade flow mode of
operation and comprises a stack of regeneration flow cells. The ED
reactor performs either electrolysis or electrolysis and a solution
phase reaction, for example, disproportionation, in one or more
sub-reactors. The sub-reactors are also referred herein as
individual cells of a stack or regeneration flow cells or cells.
The separation reactors of the regeneration system are gas-liquid
separators and are used to separate gases from the liquids during a
regeneration process.
[0041] In an embodiment, the electrolysis-disproportionation (ED)
reactor comprises, for example, an electrolysis unit or an
electrolyzer and a disproportionation unit. In another embodiment,
both electrolysis and disproportionation are performed within a
single ED flow cell which does not comprise separable electrolysis
and disproportionation units. Various configurations of the ED
reactor can be operated in a batch mode, a cascade flow mode, a
cyclic flow mode, and any combination thereof. In an embodiment,
the configuration of the ED reactor is similar to that of a polymer
membrane fuel cell stack with a modification of a graded catalytic
layer on the negative electrode which prevents the electroreduction
of relevant forms of the aqueous multi-electron oxidant (AMO) while
allowing for the hydrogen evolution reaction and alkalization to
proceed and to the electrolytic cell or of the electrolytic cell
stack of the discharge unit. The ED reactor comprises a number of
flow cells connected, for example, electrically in series and
flow-wise in parallel. Such stack-type ED reactor can be operated
in a cyclic flow mode, a cascade flow mode, a batch mode, and any
combination thereof. The ED reactor can be further configured for
an AMO-on-negative electrode mode of operation, also referred to as
an "AMO-on-negative mode of operation", wherein the negative
electrode comprises a multilayer or a graded catalytic layer
configured to prevent the electroreduction of relevant forms of the
AMO while allowing for the hydrogen evolution reaction and
alkalization to proceed, or for a no-AMO-on-negative electrode mode
of operation, also referred to as a "no-AMO-on-negative mode of
operation", wherein the base produced on the negative electrode is
mixed with one or more forms of oxidant fluid or discharge fluid
without bringing the AMO in contact with the negative
electrode.
[0042] In an electrolysis-disproportionation (ED) reactor
configured for the aqueous multi-electron oxidant (AMO)-on-negative
mode of operation in the cascade flow mode of operation, one or a
mixture of a regenerated solution and the discharge fluid passes
through a cascade or a series or stack of
electrolysis-disproportionation (ED) reactors, that is, through the
negative electrode of the first cell with a graded catalytic layer
to allow for the hydrogen evolution reaction and the buffer
alkalization to proceed while suppressing the electroreduction of
all forms of the AMO, to the separator that removes H.sub.2 from
the regenerated fluid, to the positive electrode of the first cell,
wherein the process of electrolysis-disproportionation leading to
one or more forms of the AMO takes place, then to the negative
electrode of the second cell, then to the positive electrode of the
second cell, and so on. The reducer and the base generated at the
negative electrode of each of the ED reactors are separated in the
separation reactor, where the base is returned into the mixing
reactor preceding this ED reactor and the reducer is collected in a
reducer container.
[0043] In the cyclic flow mode, as few as one regeneration flow
cell can be used with an alternating flow between the negative and
positive electrodes through the valves while releasing the produced
H.sub.2 or other fuel through the separator. In another embodiment,
referred herein as an aqueous multi-electron oxidant
(AMO)-on-negative mode of operation, the problem of aqueous
multi-electron oxidant (AMO) reduction on the negative electrode or
electrodes in the electrolysis-disproportionation (ED) reactor is
avoided by generating a base solution by passing a fluid such as
pure water free of the AMO through the negative electrode. In this
case, the base formed at the negative electrode via
H.sub.2O+e.sup.-+M.sup.+=1/2H.sub.2+MOH, where M is a cation, for
example, Li, is mixed in one or more mixing reactors with the fluid
produced at the positive electrode allowing the disproportionation
to occur. This process can be performed in a batch mode of
operation, a cascade flow mode of operation, and a cyclic flow mode
of operation. This process avoids the possibility of AMO reduction
on the negative electrode but requires removal of the excess water
from the regenerated AMO. The water is dragged though the membrane
along with M.sup.+ from the positive electrode to the negative
electrode and causes the dilution of the stock AMO solution such as
LiBrO.sub.3 solution. The water removal process can be performed by
evaporation, reverse osmosis, and other methods. In an embodiment,
the water removal process is performed in a concentrating
reactor.
[0044] In the cyclic flow mode, a regenerated solution or the
discharge fluid is cycled between the mixing reactor and the
electrolysis-disproportionation (ED) reactor until the desired
degree of conversion is achieved. An ED reactor configured for the
cyclic flow mode has a lower capital cost but requires a longer
regeneration time. The ED reactor(s) configured for the cascade
flow mode has a higher capital cost but is capable of a faster
regeneration. Multiple combinations of cyclic and cascade flow
modes are implemented for a hardware combination that involves more
than one series of neutralization reactors, ED reactors, and
separation reactors of one series connected to the neutralization
reactor of the same or the next series. The concentrating reactor
concentrates a solution of the aqueous multi-electron oxidant (AMO)
in a salt form or other forms to remove water or other solvents
from a dilute fluid that enters the concentrating reactor and
releases a concentrated fluid and water or another solvent.
[0045] Also, disclosed herein is a method for producing electric
power from the reducer and the oxidant fluid comprising the aqueous
multi-electron oxidant (AMO) and for simultaneously generating the
discharge fluid. The method disclosed herein provides the discharge
system comprising the oxidant fluid, the reducer fluid, and the
discharge unit. The method for producing electric power facilitates
electrochemical reactions in the discharge unit. Discharge occurs
by transferring electrons, either directly or via a mediator, from
the positive electrode of the 5-layer electrolyte-electrode
assembly to the AMO and from the reducer to the negative electrode
of the 5-layer electrolyte-electrode assembly to produce electric
power, that is, a sustainable electric current and a sustainable
electric voltage in the external electric circuit connected to the
terminals of the discharge unit accompanied by electric current of
ions through the electrolyte layer by electrochemical reactions on
the electrodes. The discharge is facilitated on the positive
electrode of the 5-layer electrolyte-electrode assembly, for
example, by one or more of an electron transfer, electrolysis,
electrocatalysis, a solution-phase chemical reaction, a
solution-phase comproportionation, a solution-phase redox
catalysis, an acid-base catalysis, lowering the solution pH, and
any combination thereof.
[0046] The discharge unit consumes the aqueous multi-electron
oxidant (AMO) and the reducer to produce electric power and to
generate the discharge fluid. The discharge fluid comprises, for
example, one or more of water, one or more forms of the buffer, a
halogen, one or more halogen oxoanions, hydrogen ions, halide ions,
a halogen oxoacid, a salt of halogen oxoacid, an extra acid, a
counter cation, or any combination thereof. Since the discharge
fluid coming out of the discharge unit is not water or not only
water, the discharge fluid is not disposed into the surrounding
environment but collected in a discharge container to be
regenerated later into the reducer fluid and an oxidant fluid
comprising the AMO. In an embodiment, a certain amount of
intermediate oxidant is regenerated on the positive electrode in
the discharge unit, for example, Br.sup.--1e=1/2Br.sub.2 from the
discharge fluid by reversing a polarity of an electric current
flowing through the discharge unit during discharge of the
discharge unit. This process is useful for regenerative
breaking.
[0047] Also, disclosed herein is a method for regenerating the
aqueous multi-electron oxidant (AMO) and the reducer in
stoichiometric amounts from the discharge fluid using an external
energy source. The method disclosed herein reuses all the required
chemicals in the complete discharge-regeneration cycle, does not
consume stoichiometric amounts of external chemicals, and does not
generate stoichiometric amounts of chemical waste. The regeneration
system is capable of performing the required electrochemical and
chemical reactions for the conversion of the discharge fluid from
the discharge unit back into the oxidant fluid and the reducer
fluid. The regeneration system neutralizes, if necessary, the
discharge fluid with an excess of a base form of a buffer in the
neutralization reactor to produce a solution of a salt form of the
discharge fluid. The regeneration system performs decomposition of
one or more forms of the discharge fluid comprising, for example,
water and bromide anion, into a reducer such as H.sub.2 and an
intermediate oxidant such as Br.sub.2. The decomposition can be
performed by means of one or more of the following: electrolysis,
photoelectrolysis, photolysis, thermolysis, radiolysis, etc. In an
embodiment, the regeneration system electrolyzes one or more forms
of the discharge fluid comprising, for example, bromide, yielding
an intermediate oxidant such as bromine at a positive electrode in
the electrolysis-disproportionation (ED) reactor. The decomposition
process also releases the reducer such as H.sub.2 and a base such
as hydroxide MOH of the buffer's cation M.sup.+ or the basic form
of the buffer, for example, M.sub.2HPO.sub.4. In the case where the
decomposition is by electrolysis, the reducer and the base are
released at the negative electrode of the ED reactor. The reducer
and the base are separated in the separation reactor. In the
no-AMO-on-negative mode of operation, the base is sent to the first
mixing reactor or the neutralization reactor to neutralize the
incoming discharge fluid to produce an alkaline discharge fluid. At
the positive electrode of the ED reactor, the electrolysis process
releases an intermediate oxidant, such as Br.sub.2, which reacts
with the excess of the base to produce the salt form of the AMO
such as MBrO.sub.3. The conversion of the intermediate oxidant, for
example, bromine into the original aqueous multi-electron oxidant
(AMO) in the salt form such as bromate at the positive electrode of
the ED reactor can be facilitated not only by disproportionation
but also by a mediated oxidation using a solution phase mediator
such as a chlorine/chloride couple, or electrocatalysts such as
those comprising one or more of the following: lead dioxide,
ruthenium dioxide, dimensionally stable anode materials,
perovskites, graphite, glassy carbon, conductive diamond, other
carbonaceous materials, etc. All these methods of facilitation can
be used together.
[0048] The aqueous multi-electron oxidant (AMO) is regenerated via
an electron transfer at the positive electrode followed by
disproportionation of the intermediate oxidant and the reducer is
regenerated via an electron transfer at the negative electrode of
the electrolysis-disproportionation (ED) reactor. The buffer
maintains the pH of the discharge fluid in the optimal range, for
example, between pH 7 and 9 for disproportionation. The base
component of the buffer is selected, for example, from a group
comprising hydroxide, hydrogen phosphate, one or more forms of one
or more of Good's buffers, an amine, a tertiary amine, a nitrogen
heterocycle, a substituted phosphonate, and any combination
thereof. The cation component of the buffer, if necessary, is
selected, for example, from a group comprising lithium, sodium,
other alkali metal cations, alkali earth metal cations, other
inorganic cations, organic cations, etc.
[0049] In an embodiment, the oxidant fluid produced in the
regeneration system, for example, comprising LiBrO.sub.3, is
further concentrated via the removal of water within the
regeneration system to produce oxidant fluid for future use in the
discharge system. The removal of water from the ionic components of
the oxidant fluid, also referred herein as concentrating, is
performed by one or a combination of the following: evaporation,
pervaporation, reverse osmosis, dialysis, and other methods known
in the art.
[0050] In an embodiment, the regeneration of the aqueous
multi-electron oxidant (AMO) and/or the reducer is facilitated, for
example, by an electrocatalyst, a solution-phase redox mediator, a
pH-dependent solution-phase disproportionation, etc., or any
combination thereof. The conversion of the salt form of the AMO
into the acid form of the AMO in the acidification reactor, also
referred herein as the "ion exchange reactor" is facilitated by an
acid, a buffer, hydrogen electrooxidation, other proton-releasing
electrooxidation, electrochemical hydrogen evolution, ion-exchange
on solids, ion exchange in solution, orthogonal ion migration
across laminar flow, or any combination thereof.
[0051] In an embodiment, a mediator such as chlorine facilitates
regeneration of the aqueous multi-electron oxidant (AMO) from one
or more forms of discharge fluid in the
electrolysis-disproportionation (ED) reactor. The ED reactor or the
ED reactors are configured to operate in one of multiple modes
comprising, for example, a batch mode, a single pass mode, a cyclic
flow mode, and a combination thereof. If an orthogonal ion
migration across laminar flow (OIMALF) reactor is used as ion
exchange reactor or as an acidification-neutralization reactor, the
regeneration system is configured to support the operation of the
OIMALF reactor in a flow through mode, for example, using
additional storage tanks. An OIMALF reactor can work simultaneously
as one or more OIMALF reactors are operated in a single pass
flow-through mode or a cyclic flow-through mode but not in the
batch mode, although an OIMALF reactor working in one of the flow
modes can be used in combination with an ED reactor working in a
batch mode.
[0052] In other embodiments, one or more forms of the aqueous
multi-electron oxidant (AMO) and/or the reducer are regenerated,
for example, using electrolysis, an ion exchange on solids, an ion
exchange in a liquid, ion exchange in the discharge fluid or in an
intermediate regenerated solution, pH-dependent solution-phase
disproportionation, or any combination thereof. Ion exchange in a
liquid such as water with a dissolved salt form of the AMO and the
dissolved salt form of the buffer is performed, for example, by an
electric field driven orthogonal ion migration across laminar flow
(OIMALF) process which is substantially similar to eluent
suppression in anion chromatography. The ion exchange process
occurs before and/or after and outside of any series of the
neutralization-electrolysis-disproportionation loops. The
regeneration of the AMO from the discharge fluid or from the
intermediate regenerated solution comprises neutralizing an acid of
the discharge fluid or the intermediate regenerated solution with a
base to obtain an alkaline discharge fluid. The required base is
produced, for example, at the negative electrode(s) of one or many
electrolysis-disproportionation (ED) reactors. The regeneration
system then converts the alkaline discharge fluid to the neutral
oxidant fluid, that is, a liquid comprising water, the AMO, and one
or more forms of the buffer, for example, via electrolysis, pH
dependent solution phase disproportionation and orthogonal ion
migration across laminar flow processes.
[0053] The reducer, for example, hydrogen, is co-produced in a
stoichiometric amount with one or more forms of the aqueous
multi-electron oxidant (AMO) in the electrolysis-disproportionation
(ED) reactor. The conversion of the salt form of the AMO into the
acid form of the AMO is performed using an acidification reactor
such as the ion exchange reactor. If the ion exchange reactor is,
for example, an orthogonal ion migration across laminar flow
(OIMALF) reactor, the conversion comprises consuming electric power
and recycling the hydrogen released on one or more negative
electrodes of the OIMALF reactor and electro-oxidized on one or
more positive electrodes of the OIMALF reactor. In an embodiment,
the hydrogen produced in an ED reactor is flown through the flow
field of the positive electrode of one or many OIMALF reactors and
combined with the hydrogen produced at a negative electrode of one
or many OIMALF reactors either before or after one or many OIMALF
reactors. The regeneration of the reducer and the oxidant fluid by
the ED-OIMALF method occurs by using an external electric energy
input and without consumption or generation of external chemicals.
Also, disclosed herein is the use of the pH-dependence of the
spontaneous homogeneous disproportionation of a halogen and
comproportionation of a halide and a halogen oxoanion in order to
facilitate the electrode reactions on the positive electrodes
during regeneration and discharge. The method disclosed herein
facilitates the forward and reverse halogen oxoanion/halide
conversion and other redox processes involving oxoanions via
pH-dependent homogeneous reactions.
[0054] Also, disclosed herein is an embodiment of the discharge
system comprising one or more forms of an oxidant fluid comprising
one or more forms of an aqueous multi-electron oxidant (AMO), for
example, an aqueous solution comprising LiBrO.sub.3, stored in an
oxidant fluid tank, one or more forms of a reducer fluid comprising
one or more forms of a reducer such as hydrogen stored in a reducer
fluid tank, an acidification reactor, optionally a neutralization
reactor, a discharge unit, and a discharge fluid tank to collect
the discharge fluid for future regeneration or disposal. In an
embodiment, the acidification reactor and the neutralization
reactor are functionally combined as an orthogonal ion migration
across laminar flow (OIMALF) reactor. In another embodiment, the
neutralization reactor is integrated with the acidification reactor
into the OIMALF reactor. In another embodiment, the neutralization
reactor is an OIMALF reactor. In this embodiment, the acidification
process, for example, an ion exchange process is performed on-board
in the discharge system rather than off-board, in order to improve
the stability and safety of the systems disclosed herein. The
discharge system disclosed herein is configured to operate in an
electric partial recharge mode for facilitating regenerative
breaking when the discharge system powers an electric vehicle.
During the partial recharge mode, the reducer is produced on the
negative electrode of the electrolyte-electrode assembly and an
intermediate oxidant is produced on the positive electrode of the
electrolyte-electrode assembly.
[0055] In the embodiment of the discharge system with improved
safety, a neutral oxidant fluid comprising, for example,
LiBrO.sub.3 is stored in the oxidant fluid tank. The discharge
system initially converts the aqueous multi-electron oxidant (AMO)
in the salt form such as LiBrO.sub.3 in the neutral oxidant fluid
into the AMO in the acid form such as HBrO.sub.3, found in the
acidic oxidant fluid, using the acidification reactor. In an
embodiment, the conversion of the salt form of the AMO into the
acid form of the AMO is performed via an ion exchange process. The
ion exchange process can be performed via a multiphase flow
process, for example, based on ion-exchange resins or via a
single-phase flow process such as an electric field driven
orthogonal ion migration across laminar flow (OIMALF) process in
the OIMALF reactor. In the case where the acidification reactor is
an OIMALF reactor, the acidification of the oxidant fluid is
accompanied by a simultaneous neutralization of the acidic
discharge fluid while recycling the reducer such as H.sub.2
produced at one or more negative electrodes of the OIMALF reactor
and consumed at one or more positive electrodes of the OIMALF
reactor. The OIMALF process is substantially similar to eluent
suppression of ion chromatography. The OIMALF reactor converts the
neutral oxidant fluid into an acidic oxidant and an acidic
discharge fluid into a neutral discharge fluid simultaneously.
[0056] The orthogonal ion migration across laminar flow (OIMALF)
reactor comprises an OIMALF cell stack which is configured similar
to a polymer electrolyte fuel cell (PEFC) stack but with a liquid
electrolyte flowing between two ionically conducting membranes.
Only the outer sides, which are not in contact with the flowing
liquid, of the membranes are coated with catalytic layers. The
OIMALF reactor comprises flow cell assemblies, endplates, and
bipolar plates. Each flow cell assembly of the OIMALF reactor
comprises a couple of ion exchange membranes, an intermembrane flow
field, a positive electrode layer and a negative electrode layer,
and two porous diffusion layers. The ion exchange membranes are
coated with a catalytic layer only on their outer sides which are
not in contact with fluids comprising the aqueous multi-electron
oxidant (AMO). The intermembrane flow field is interposed between
the ion exchange membranes and comprises multiple flow channels.
The positive electrode layer and the negative electrode layer flank
outer surfaces of the ion exchange membranes. The two porous
diffusion layers flank the outer surfaces of the ion exchange
membranes and are in an electric contact with the adjacent bipolar
plates or endplates. The ion exchange membranes comprise a positive
side ion exchange membrane and a negative side ion exchange
membrane positioned parallel to each other. The positive electrode
layer is configured for hydrogen oxidation reaction and the
negative electrode layer is configured for hydrogen evolution
reaction. Further variations of the electrode layers, for example,
additional macro-porous and micro-porous layers are possible and
known in the art of hydrogen polymer electrolyte fuel cell anodes
and hydrogen polymer electrolyte water electrolyzer cathodes.
[0057] The acidic oxidant fluid comprises one or more of water, one
or more forms of the aqueous multi-electron oxidant (AMO), for
example, an acid or a salt form or as a combination thereof, an
extra acid, and one or more of multiple counter cations. The AMO
comprises one or a combination of halogens, halogen oxides, halogen
oxoanions, and salts and acids of the halogen oxoanions. The
halogen oxoanions comprise, for example, one or more of
hypochlorite, chlorite, chlorate, perchlorate, hypobromite,
bromite, perbromate, hypoiodite, iodite, iodate, and periodate. In
an embodiment, the halogen oxoanion is bromate. The counter cations
comprise, for example, alkali metal cations, alkali earth metal
cations, and organic cations. In an embodiment, one of the counter
cations is lithium. In another embodiment, one of the counter
cations is sodium. A buffer may be present in the oxidant fluid if
the buffer is carried over from the regeneration process. In an
embodiment, the buffer functions as the extra acid. The buffer is
in one of its forms, for example, one or more of monohydrogen
phosphate, a 3-(N-morpholino)propanesulfonate, a
3-(N-morpholino)ethanesulfonate, a substituted phosphonate, an
amine, a tertiary amine, a nitrogen heterocycle, other base with an
acid dissociation constant pKa between, for example, 6 and 9. The
extra acid is, for example, one or more of a phosphoric acid, a
3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, a methanesulfonic acid, a
triflic acid, a substituted sulfonic acid, a substituted phosphonic
acid, a perchloric acid, a sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic moieties, and an acid with a
pKa<2. The AMO in one or several forms can be pre-mixed with one
or several components of the buffer in the oxidant fluid in the
storage tank, in the acidification reactor or in both. In an
embodiment, the AMO is selected from a group consisting of a
halogen compound such as a halogen oxide, a halogen oxoacid, a
water-soluble salt of halogen oxoacid, and any combination thereof.
The AMO can be stored on-board and off-board in the acid or in one
or more salt forms on in a combination thereof. The salt forms of
the AMO are considered over the acid form due their better
stabilities, provided that they have high solubilities.
[0058] In an embodiment, the discharge system also performs
complete or partial conversion of a stable form of the aqueous
multi-electron oxidant (AMO), such as LiBrO.sub.3, into an active
form of the AMO, such as HBrO.sub.3, using one or more disclosed
acidification processes, for example, one or any combination of the
following: addition of a stored acid, ion exchange on resins, and
the orthogonal ion migration across laminar flow (OIMALF). The
acidification process is performed either in a dedicated
acidification reactor, which can be an OIMALF reactor, or in a
suitably modified other reactor, such as the discharge unit itself
or in both. In an embodiment, the discharge system also performs
complete or partial conversion of the discharged fluid, such as one
containing HBr or another acid, into a less corrosive form, such as
LiBr, using one or more disclosed neutralization processes, such
addition of a stored base or/and an OIMALF process. The
neutralization process is performed in a dedicated reactor such as
a neutralization reactor, which can be an OIMALF reactor, or in a
suitably modified other reactor, such as discharge unit. In an
embodiment, the neutralization reactor comprises a mixing
reactor.
[0059] Also, disclosed herein is an embodiment of the method for
producing electric power from an aqueous multi-electron oxidant
(AMO) and a reducer and for simultaneously generating a discharge
fluid. The method disclosed herein provides the discharge system
comprising one or more forms of a reducer fluid, one or more forms
of an oxidant fluid, a discharge unit, and an acidification
reactor. The method disclosed herein facilitates discharge of the
discharge unit for producing electric power from a neutral oxidant
fluid comprising one or more forms of the aqueous multi-electron
oxidant, and from the reducer fluid comprising one or more forms of
the reducer. The facilitation of the discharge comprises: lowering
pH of the neutral oxidant fluid in the acidification reactor for
generating an acidic oxidant fluid; transferring electrons from the
positive electrode of the electrolyte-electrode assembly to the
aqueous multi-electron oxidant in the acidic oxidant fluid; and
transferring electrons from the reducer fluid to the negative
electrode of the electrolyte-electrode assembly to produce electric
power in the external electric circuit operably connected to the
terminals of the discharge unit and to generate an acidic discharge
fluid on consumption of the acidic oxidant fluid and the reducer
fluid. The transfer of the electrons from the positive electrode of
the electrolyte-electrode assembly to the aqueous multi-electron
oxidant in the acidic oxidant fluid is performed at a high current
density and at low flow rates in an ignition mode of operation of
the discharge system. A limiting current of the transfer of the
electrons from the positive electrode of the electrolyte-electrode
assembly to the aqueous multi-electron oxidant in the acidic
oxidant fluid in an ignition regime is limited, for example, by a
mass-transport of the aqueous multi-electron oxidant, a
mass-transport of acidic protons, and a rate of comproportionation.
The acidic discharge fluid comprises, for example, one or more of
water, a halide, a hydroxonium cation, an extra acid, and one or
more counter cations. In an embodiment, the stability of the acidic
oxidant fluid is maintained by performing an ignition regime in the
discharge system at low acid concentrations in the acidic oxidant
fluid. In an embodiment, the method disclosed herein further
comprises optionally neutralizing the acidic discharge fluid in the
neutralization reactor of the discharge system to produce a neutral
discharge fluid. The concentration of one or more forms of the
aqueous multi-electron oxidant in the neutral oxidant fluid or the
acidic oxidant fluid supplied to the discharge unit is, for
example, above 1M, 2M, 5M, or 10M. The concentration of acidic
protons in the acidic oxidant fluid supplied to the discharge unit
is, for example, below 0.1M, 0.5M, 1M, 2M, 5M, or 10M. The
concentration of acidic protons in the acidic oxidant fluid stored
in the discharge system is, for example, below 0.1M, 0.5M, 1M, 2M,
or 5M. In an embodiment, the method disclosed herein further
comprises regenerating a certain amount of an intermediate oxidant
and the reducer in the discharge unit from the acidic discharge
fluid by applying an electric current of a polarity opposite to a
polarity of electric current through the discharge unit during
discharge.
[0060] In an embodiment, the generation of the acidic oxidant fluid
from the neutral oxidant fluid is performed in the acidification
reactor via an electric field driven orthogonal ion migration
across laminar flow (OIMALF) process. In another embodiment, the
generation of the acidic oxidant fluid from the neutral oxidant
fluid is performed, for example, by one or more of an ion exchange
on solids, an ion exchange in liquids, electrolysis, and adding an
extra acid to the neutral oxidant fluid during discharge of the
discharge unit. In an embodiment, the discharge is facilitated on
the positive electrode of the electrolyte-electrode assembly, for
example, by one or more of electrocatalysis, a solution-phase
chemical reaction, a solution-phase comproportionation, a
solution-phase redox catalysis, a solution-phase redox mediator, an
acid-base catalysis, and any combination thereof. In another
embodiment, the discharge process is facilitated via a
solution-phase comproportionation of the aqueous multi-electron
oxidant with a final product of a reduction of the aqueous
multi-electron oxidant. In an embodiment, the solution-phase
comproportionation is pH-dependent and the discharge is facilitated
in the presence of an acid.
[0061] Also, disclosed herein is an embodiment of the regeneration
system comprising a splitting-disproportionation (SD) reactor, a
concentrating reactor, multiple separation reactors, and storage
tanks such as a regenerated oxidant fluid tank, a regenerated
reducer fluid tank, a discharge fluid tank, and a water tank. In an
embodiment, the SD reactor is configured as an
electrolysis-disproportionation (ED) reactor comprising
sub-reactors, for example, an electrolysis unit or an electrolyzer
and a disproportionation unit. In an embodiment, the SD reactor is
configured for an aqueous multi-electron oxidant (AMO)-on-negative
mode of operation using a multilayer structure on a negative
electrode side of the SD reactor. The multilayer structure on the
negative electrode side of the SD reactor minimizes reduction of a
regenerated AMO in a regenerated oxidant fluid on the negative
electrode side while facilitating hydrogen evolution and an
increase in pH of the regenerated oxidant fluid. In another
embodiment, the SD reactor is configured for the no-AMO-on-negative
mode of operation by transferring a base produced on one or more
negative electrodes of the SD reactor to a regenerated oxidant
fluid produced at one or more positive electrodes of the SD reactor
and comprising one or more forms of the AMO and the intermediate
oxidant. The SD reactor is configured to operate in multiple modes,
for example, a batch mode, a cycle flow mode, a cascade flow mode,
and any combination thereof.
[0062] The splitting-disproportionation (SD) reactor splits the
alkaline discharge fluid into a reducer and an intermediate
oxidant. The SD reactor converts the intermediate oxidant produced
in the SD reactor into one or more forms of the aqueous
multi-electron oxidant via disproportionation of the intermediate
oxidant with the base. The splitting process releases a
stoichiometric amount of the reducer and the base in the SD
reactor. The SD reactor optimizes and stabilizes the pH of the
alkaline discharge fluid using a buffer present in one or more
forms of the discharge fluid to facilitate disproportionation of
the intermediate oxidant into one or more forms of the aqueous
multi-electron oxidant. The SD reactor continues the splitting and
disproportionation processes in a batch mode of operation, a cyclic
flow mode of operation, a cascade flow mode of operation, or a
combination thereof, until a desired degree of conversion of a
discharge product of the aqueous multi-electron oxidant into one or
more forms of the aqueous multi-electron oxidant is achieved. The
SD reactor splits one or more forms of the alkaline discharge fluid
into the reducer and the intermediate oxidant, for example, via
electrolysis, photolysis, photoelectrolysis, radiolysis,
thermolysis, or any combination thereof. The process of photolysis
and photoelectrolysis of the alkaline discharge fluid is performed
in the presence or absence of a light adsorbing facilitator, a
semiconductor, a catalyst, and any combination thereof.
[0063] In an embodiment, the splitting-disproportionation reactor
is configured as an electrolysis-disproportionation (ED) reactor.
The ED reactor converts a neutral discharge fluid into an alkaline
discharge fluid by using an externally supplied base and/or a base
produced at one or more negative electrodes of the ED reactor in an
aqueous multi-electron oxidant-on-negative mode of operation, a
no-aqueous multi-electron oxidant-on-negative mode of operation, or
a combination thereof. The ED reactor splits the alkaline discharge
fluid into a reducer and an intermediate oxidant via electrolysis.
The process of electrolysis releases a stoichiometric amount of the
reducer and the base at one or more negative electrodes of the ED
reactor. The ED reactor converts the intermediate oxidant produced
at one or more positive electrodes of the ED reactor into one or
more forms of the aqueous multi-electron oxidant via
disproportionation of the intermediate oxidant produced at one or
more positive electrodes with the base. The ED reactor continues
the electrolysis and disproportionation process in a batch mode of
operation, a cyclic flow mode of operation, a cascade flow mode of
operation, or any combination thereof, until a desired degree of
conversion of a discharge product of the aqueous multi-electron
oxidant (AMO) into one or more forms of the AMO is achieved.
[0064] Also, disclosed herein is an embodiment of the method for
regenerating an aqueous multi-electron oxidant (AMO) and a reducer
in stoichiometric amounts from one or more forms of a neutral
discharge fluid using external power. The discharge fluid
comprises, for example, one or more of water, a halide, a
hydroxonium cation, a buffer, and one or more counter cations. The
method disclosed herein comprises converting the neutral discharge
fluid into an alkaline discharge fluid by using an externally
supplied base and/or a base produced in the
splitting-disproportionation (SD) reactor in an aqueous
multi-electron oxidant-on-negative mode of operation, a no-aqueous
multi-electron oxidant-on-negative mode of operation, or a
combination thereof. The pH of the alkaline discharge fluid is, for
example, between 6 and 9 or between 4 and 9. The buffer is
configured to maintain the pH of the alkaline discharge fluid, for
example, between 6 and 9 or between 4 and 9. In an embodiment, the
base component of the buffer is selected from a group comprising,
for example, a hydroxide ion, hydrogen phosphate, a phosphate
ester, a substituted phosphonate, alkylphosphonate,
arylphosphonate, a deprotonated form of one or more of Good's
buffers, an amine, a nitrogen heterocycle, and any combination
thereof. In an embodiment, the cationic component of the buffer
comprises a cation of lithium. In another embodiment, the cationic
component of the buffer comprises a cation of sodium. In another
embodiment, the anionic component of the buffer comprises one or
more of .omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate. In another embodiment, the
anionic component of the buffer is one or more of
.omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate and the cationic component of
the buffer is lithium. In another embodiment, the anionic component
of the buffer comprises one or more of an alkylphosphonate or an
arylphosphonate. In another embodiment, the anionic component of
the buffer comprises one or more of an alkylphosphonate, an
arylphosphonate, and a cationic component of the buffer is lithium.
In an embodiment, the base component of the buffer is monohydrogen
phosphate and a cationic component of the buffer is sodium.
[0065] Also, disclosed herein is a method for producing electric
power and regenerating an aqueous multi-electron oxidant (AMO) and
a reducer in an energy storage cycle. The method disclosed herein
provides the discharge system comprising one or more forms of a
reducer fluid, one or more forms of an oxidant fluid, the discharge
unit, the acidification reactor, optionally the neutralization
reactor, and one or several storage tanks. The oxidant fluid
comprising the AMO is converted into an acidic oxidant fluid. In an
embodiment, the acidification of the oxidant fluid is performed by
adding an acid, for example, sulfuric acid, triflic acid,
phosphoric acid etc., to the oxidant fluid stored in the oxidant
fluid tank before the oxidant fluid enters the discharge unit. In
another embodiment, the acidification is performed using an
orthogonal ion migration across laminar flow (OIMALF) reactor
positioned between the AMO storage tank or the oxidant storage tank
and the discharge unit. The method disclosed herein facilitates
discharge of the discharge unit for producing electric power from
the reducer and the oxidant fluid comprising the AMO, and generates
the discharge fluid.
[0066] In an embodiment, one or more forms of the aqueous
multi-electron oxidant (AMO) undergoes discharge in the ignition
mode, that is, under the condition when the time required for the
product such as a halide to comproportionate with the AMO such as a
halate is shorter than the time required for the product to diffuse
away from the electrode. The ignition mode assures a high power
density of the discharge unit. For a sufficiently high
concentration of the AMO such as provided by a highly soluble
LiBrO.sub.3, the ignition mode can be observed even when the ratio
of the total concentration of acid protons to the total
concentration of the AMO is below the stoichiometric number
required by the chemical equation of the redox half-reaction.
Herein, the total concentration of acid protons is the
concentration of acid determined by titration with a strong aqueous
base, such as NaOH, below the endpoint at pH 7.0. The AMO reduction
can practically proceed in the ignition mode even when the ratio of
the total concentration of acid protons to the total concentration
of the AMO is below one and can be as low as 0.05 when a high
concentration of the AMO, a strong acid, and a thick diffusion
boundary layer are employed at the same time.
[0067] The use of substoichiometric acid concentration for the
electroreduction of the aqueous multi-electron oxidant (AMO)
reduces energy and chemical expenses associated with the
acidification of the oxidant fluid particularly when performed
on-board, reduces system size, and improves safety. Furthermore,
experimental data shows that at least in the case of the AMO being
LiBrO.sub.3, the ignition regime can be observed at low acid
concentrations and acidic oxidant fluid remains stable as evidenced
by very low Br.sub.2 formation for over two weeks. This finding
allows the elimination of the on-board acidification process and of
the on-board acidification reactor.
[0068] The method disclosed herein further comprises optimizing and
stabilizing pH of the acidic oxidant fluid in the
splitting-disproportionation reactor using an extra acid present in
the acidic oxidant fluid to facilitate comproportionation of the
aqueous multi-electron oxidant with a final product of a reduction
of the aqueous multi-electron oxidant into an intermediate oxidant.
The pH of the acidic discharge fluid is, for example, below 0, 1,
2, or 3. The concentration of acidic protons in the acidic
discharge fluid is, for example, below 0.1M, 0.5M, 1M, 2M, 5M, or
10 M. The extra acid is one or a combination of a phosphoric acid,
a 3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, another
.omega.-(N-morpholino)propanesulfonic acid, a methanesulfonic acid,
a triflic acid, a substituted sulfonic acid, a substituted
phosphonic acid, a perchloric acid, a sulfuric acid, a molecule
comprising sulfonic moieties and phosphonic acid moieties, and an
acid with a pKa<2.
[0069] If the acidic oxidant fluid is stored in the discharge
system or produced by the addition of an extra acid, for example,
H.sub.2SO.sub.4, F.sub.3CSO.sub.3H, etc., the discharge fluid
leaving the discharge unit is in an acid including a partially acid
form. In an embodiment, the acidic discharge fluid is neutralized
with a base form of a buffer in the neutralization reactor of the
discharge system to produce a solution of a neutral form of the
discharge fluid. In this scenario, the discharge fluid leaving the
discharge system is in a neutralized form including
partially-neutralized form. The acidic discharge fluid comprises
one or more of hydrogen bromide, hydrogen chloride, hydrogen
iodide, and any combination thereof. In an embodiment, the acidic
discharge fluid comprises one or more of water, a halide, a
hydroxonium cation, and a counter cation. In the orthogonal ion
migration across laminar flow (OIMALF) acidification embodiment,
the discharge fluid comprises one or more of water, an extra acid,
an acid form of the buffer, a discharge acid, a halogen, one or
more forms of the aqueous multi-electron oxidant (AMO) such as
neutral, acidic or alkaline, and any combination thereof. The
OIMALF reactor replaces acidic protons in the outgoing acidic
discharge fluid for another cation such as Li.sup.+ present in the
incoming neutral oxidant fluid, while simultaneously converting an
incoming neutral oxidant fluid into an outgoing acidic oxidant
fluid and recycling H.sub.2 produced on one or more negative
electrodes and consumed on one or more positive electrodes.
[0070] The aqueous multi-electron oxidant (AMO) and the reducer are
regenerated in stoichiometric amounts from the discharge fluid in
the regeneration system. The method and the system disclosed herein
reduces the amount of electric energy utilized by the acidification
reactor, for example, an orthogonal ion migration across laminar
flow (OIMALF) reactor, for converting the salt form of the AMO into
the acid form of the AMO by adding an extra acid, for example, one
or more of triflic acid, sulfuric acid, perchloric acid, nitric
acid, and any combination thereof to the oxidant fluid before or
during the discharge process. The extra acid facilitates a faster
comproportionation, and thus a higher power during discharge, for
example, higher than H.sub.3PO.sub.4 alone can cause, and reduces
the charge required for on board OIMALF. In an embodiment, the acid
form of the buffer comprising, for example, a sulfonic acid group,
is used as the extra acid. In an embodiment, the acid form of the
AMO is used as the extra acid. The regenerated reducer fluid
comprising the reducer and the regenerated one or more forms of the
oxidant fluid comprising one or more forms of the AMO are supplied
to the discharge system for facilitation of the discharge of the
discharge unit. In an embodiment, the heat released during the
discharge process is used to preheat one or more forms of the
oxidant fluid prior to discharge.
[0071] In an embodiment, the regeneration system disclosed herein
performs regeneration of the oxidant and the fuel from the
discharged solution via photolysis, photoelectrolysis, or any
combination thereof. The reagents are regenerated
photoelectrochemically using sunlight and with semiconductor
particles or electrodes. In this embodiment, the
splitting-disproportionation reactor is configured as a
photoelectrolysis-disproportionation reactor. The photolysis and/or
the photoelectrolysis of the alkaline discharge fluid is performed
in the presence or absence of a light adsorbing facilitator, a
catalyst, and any combination thereof, in the
photoelectrolysis-disproportionation reactor. The method disclosed
herein induces a splitting of a discharge product, for example, HBr
in the photoelectrolysis-disproportionation reactor by irradiating
the discharged solution with light. The regeneration system
comprising the photoelectrolysis-disproportionation reactor
regenerates one or more of the oxidant, for example, the aqueous
multi-electron oxidant (AMO) and the fuel from the discharged
solution.
[0072] Also, disclosed herein is a method for producing electric
power and regenerating hydrogen and a neutral oxidant fluid
comprising lithium bromate in an energy storage cycle. The method
disclosed herein provides the discharge system comprising the
discharge unit, the acidification reactor, and optionally the
neutralization reactor. The discharge system comprises a neutral
oxidant fluid comprising lithium bromate, and hydrogen. In an
embodiment, the discharge system comprises one or more forms of a
buffer. In another embodiment, the discharge system further
comprises one or more forms of a base. In an embodiment, the
cationic component of the buffer is lithium and the anionic
component of the based form of the buffer is one or more of
.omega.-(N-morpholino)alkanesulfonate,
3-(N-morpholino)methanesulfonate, 3-(N-morpholino)ethanesulfonate,
3-(N-morpholino)propanesulfonate, 3-(N-morpholino)butanesulfonate,
methylphosphonate, an alkylphosphonate, an arylphosphonate, and a
molecule comprising one or more of phosphonate moieties and
sulfonate moieties. In another embodiment, the cationic component
of the buffer is sodium, and the anionic component of the base form
of the buffer is one or more of
.omega.-(N-morpholino)alkanesulfonate, methylphosphonate,
3-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
an alkylphosphonate, an arylphosphonate, and a molecule comprising
phosphonate moieties and sulfonate moieties. In an embodiment, the
discharge system further comprises a deprotionated form of an extra
acid comprising, for example, one or more of an aqueous
multi-electron oxidant (AMO) in the acid form, bromic acid,
sulfuric acid, perchloric acid, triflic acid, a sulfonic acid,
molecules comprising phosphonate moieties and sulfonate moieties,
and an acid with a pKa.ltoreq.2. The buffer is in an acid form
during discharge with a pH.ltoreq.4, and the acid form of the
buffer comprises one or more of a phosphoric acid derivative, a
phosphoric acid ester, one or more substituted phosphonic acids,
one or more .omega.-(N-morpholino) alkanesulfonic acids, molecules
comprising both phosphonate and sulfonate moieties, and buffers
capable of maintaining pH between 4 and 9.
[0073] The concentration of lithium bromate dissolved in the
neutral oxidant fluid is, for example, above 1M, 2M, 5M, or 10M.
The acidification reactor converts the neutral oxidant fluid into
an acidic oxidant fluid. The concentration of acidic protons in the
acidic oxidant fluid is, for example, below 0.1M, 0.5M, 1M, 2M, 5M,
or 10M. The method disclosed herein facilitates discharge of the
discharge unit for producing electric power from the acidic oxidant
fluid and from hydrogen and generates an acidic discharge fluid on
consumption of the acidic oxidant fluid and hydrogen. The discharge
is facilitated via a pH-dependent solution-phase comproportionation
of bromate with bromide formed via electroreduction of intermediate
bromine. In an embodiment, the neutralization reactor optionally
neutralizes the acidic discharge fluid to produce one or more forms
of a neutral discharge fluid.
[0074] The regeneration system regenerates hydrogen and one or more
forms of the oxidant fluid comprising lithium bromate in
stoichiometric amounts from one or more forms of the neutral
discharge using external power. The regeneration is performed by
splitting one or more forms of the neutral discharge fluid into
stoichiometric amounts of bromine, hydrogen, and a base form of the
buffer using external power in the splitting-disproportionation
reactor, and producing lithium bromate via disproportionation of
bromine with the base form of the buffer. The splitting process is
performed, for example, via electrolysis, photolysis,
photoelectrolysis, radiolysis, thermolysis, and other methods know
to those skilled in the art. The disproportionation reaction is
facilitated by a buffer capable of maintaining a solution pH, for
example, between 4 and 9. The splitting-disproportionation reactor
continues splitting and disproportionation in a no-aqueous
multi-electron oxidant-on-negative mode of operation and an aqueous
multi-electron oxidant-on-negative electrode mode of operation
until a desired degree of conversion of bromide into bromate is
achieved. The splitting-disproportionation reactor is configured
for a batch mode, a cyclic flow mode, a cascade flow mode, and any
combination thereof. The regeneration system supplies the
regenerated one or more forms of the oxidant fluid comprising
bromate and the regenerated hydrogen to the discharge system for
subsequent generation of electric power on demand.
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] The foregoing summary, as well as the following detailed
description of the invention, is better understood when read in
conjunction with the appended drawings. For the purpose of
illustrating the invention, exemplary constructions of the
invention are shown in the drawings. However, the invention is not
limited to the specific methods and components disclosed herein.
The description of a structure or a method step referenced by a
numeral in a drawing carries over to the description of that
structure or method step shown by that same numeral in any
subsequent drawing herein.
[0076] FIG. 1 illustrates a system for generating an electric power
and a discharge fluid from an oxidant fluid and a reducer fluid
using a discharge system and for regenerating an oxidant and/or a
reducer from the discharge fluid using a regeneration system.
[0077] FIG. 2 exemplarily illustrates a perspective view of a
dissembled single electrolytic cell of an electrolytic cell stack
of a discharge unit of the discharge system and of an electrolyzer
of an electrolysis-disproportionation reactor of the regeneration
system.
[0078] FIG. 3 exemplarily illustrates a perspective view of a
planar cell stack of the discharge unit, showing three
multi-layered electrolyte-electrode assemblies, two bipolar plates,
and two endplates.
[0079] FIG. 4 exemplarily illustrates a discharge and regeneration
cycle as flows of energy, materials, and processes, showing the
discharge unit with hydrogen as an example of a reducer, an aqueous
HXO.sub.n as an example of an aqueous multi-electron oxidant, and a
regeneration system using MZ as an example of a buffer in a base
form.
[0080] FIGS. 5A-5B exemplarily illustrate a table showing different
reactions used or considered for electrochemical energy storage and
energy conversion.
[0081] FIG. 6 exemplarily illustrates mass flows in a single
electrolytic cell of an electrolytic cell stack of the discharge
unit during discharge with H.sub.2 as the fuel and HXO.sub.3 as the
oxidant.
[0082] FIG. 7 illustrates a method for producing electric power
from an aqueous multi-electron oxidant and a reducer and for
simultaneously generating a discharge fluid.
[0083] FIG. 8 illustrates a method for regenerating an aqueous
multi-electron oxidant and a reducer in stoichiometric amounts from
a discharge fluid using electric power.
[0084] FIG. 9 exemplarily illustrates a negative-ion electrospray
ionization-mass spectrometry spectrum of a 0.5M sodium phosphate pH
7.0 buffer solution after addition of 50 mM of Br.sub.2.
[0085] FIGS. 10A-10B exemplary illustrate an
electrolysis-disproportionation orthogonal ion migration across
laminar flow method for regenerating a reducer (H.sub.2) and an
oxidant fluid comprising an aqueous multi-electron oxidant
(HXO.sub.3) from a discharge fluid (HX+H.sub.2O) with MOH as a
base.
[0086] FIGS. 11A-11B exemplary illustrate a cyclic operation of a
flow-through electrolysis-disproportionation reactor with bromate
as an aqueous multi-electron oxidant, hydrogen phosphate as a base
form of a buffer, and sodium as a counter cation.
[0087] FIG. 12 exemplarily illustrates calculated and
experimentally measured limiting currents on a rotating disk
electrode in aqueous solutions of bromic acid of various
concentrations.
[0088] FIG. 13 exemplary illustrates a graphical representation of
a power-voltage curve calculated for a H.sub.2-50% w/w HBrO.sub.3
discharge flow battery and measured with a glassy carbon rotating
disk electrode, and with a platinum gauze electrode in a flow cell,
and a corresponding curve for a commercial proton exchange membrane
fuel cell running on hydrogen and air.
[0089] FIGS. 14A-14G exemplarily illustrate graphical
representations showing comparative performances of three on-board
power sources at a nominal power of 130 kW: a gasoline-internal
combustion engine, a lithium ion battery, and an H.sub.2-aqueous
multi-electron oxidant discharge unit as well as the targets of the
Advanced Research Projects Agency-Energy.
[0090] FIG. 15 illustrates an embodiment of the system for
generating electric power and a discharge fluid from an oxidant
fluid and a reducer fluid using a discharge system comprising an
orthogonal ion migration across laminar flow reactor and for
regenerating an oxidant and/or a reducer from the discharge fluid
using a regeneration system.
[0091] FIG. 16 exemplarily illustrates a process flow diagram
showing mass and electricity flows in an energy cycle between the
discharge unit, an acidification reactor, and a neutralization
reactor of the discharge system.
[0092] FIGS. 17A-17B exemplarily illustrate mass flows in a single
cell of an electrolysis-disproportionation reactor configured for
regeneration in an aqueous multi-electron oxidant-on-negative
electrode mode of operation.
[0093] FIG. 18 exemplarily illustrates mass flows in a single cell
of an electrolysis-disproportionation reactor configured for
regeneration in a no-aqueous multi-electron oxidant-on-negative
electrode mode of operation and a batch mode.
[0094] FIG. 19 exemplary illustrates a mass and electricity flow
diagram of a discharge system comprising a single cell discharge
unit and an orthogonal ion migration across laminar flow
reactor.
[0095] FIG. 20A illustrates a method for producing electric power
from an aqueous multi-electron oxidant and a reducer and for
simultaneously generating a discharge fluid.
[0096] FIG. 20B illustrates a method for regenerating an aqueous
multi-electron oxidant and a reducer in stoichiometric amounts from
one or more forms of a neutral discharge fluid using external
power.
[0097] FIG. 20C illustrates a method for producing electric power
and regenerating an aqueous multi-electron oxidant and a reducer in
an energy storage cycle.
[0098] FIG. 20D illustrates a method for producing electric power
and regenerating hydrogen and an oxidant fluid comprising lithium
bromate in an energy storage cycle.
[0099] FIG. 21A exemplary illustrates polarization curves of a
glassy carbon rotating disk electrode in solutions comprising 5M
LiBrO.sub.3+50% w H.sub.3PO.sub.4+1 mM LiBr at different rotation
rates and 20.degree. C.
[0100] FIG. 21B exemplary illustrates polarization curves of a
glassy carbon rotating disk electrode in a solution comprising 30%
H.sub.2SO.sub.4+166 mM LiBrO.sub.3+5 mM NaBr.
[0101] FIG. 22 exemplarily illustrates Pourbaix diagrams for
bromine in aqueous media at pH 0 and pH 14.
[0102] FIG. 23A exemplarily illustrates a solar radiation spectrum
at sea level and the positions of the silicon (Si) band-gap,
bromine and/or bromide, and bromate and/or bromide standard
electrode potentials.
[0103] FIG. 23B exemplarily illustrates a batch mode of a
photoelectrolysis-disproportionation method for regenerating a
halate from a halide.
[0104] FIG. 24 exemplarily illustrates a graphical representation
showing background-subtracted limiting currents of bromide
electrooxidation-disproportionation on a glassy carbon rotating
disk electrode in a 0.5M sodium phosphate buffer at various
rotation rates.
[0105] FIG. 25 exemplarily illustrates a staircase cyclic
voltammetry on a glassy carbon rotating disk electrode in a 2 hour
aged solution containing 2.0 M H.sub.2SO.sub.4 and approximately 5M
LiBrO.sub.3 at various rotation rates.
[0106] FIG. 26 exemplarily illustrates an electrospray
ionization-mass spectrometry (MS) spectrum, showing experimental
data demonstrating the feasibility of a regeneration process.
DETAILED DESCRIPTION OF THE INVENTION
[0107] FIG. 1 illustrates a system 100 for generating an electric
power and a discharge fluid from an oxidant fluid and a reducer
fluid using a discharge system 101 and for regenerating an oxidant
and/or a reducer from the discharge fluid using a regeneration
system 106. The oxidant fluid is a chemical or a mixture of
chemicals that accepts electrons during a discharge process in a
discharge mode of operation of a discharge unit 104 of the
discharge system 101. As used herein, the term "the discharge mode
of operation" refers to a process of releasing chemical energy
stored in the discharge unit 104 in the form of sustainable
electric current and voltage, for example, direct current (DC). The
discharge unit 104 disclosed herein is also referred to as a "flow
battery". The oxidant fluid comprises one or more forms of an
aqueous multi-electron oxidant (AMO), water, other solvents, acids,
bases, catalysts, and one or more forms of a buffer or buffers. The
AMO may be present at various stages in the methods disclosed
herein in one or several forms, for example, acid forms, salt forms
such an Li form, etc., differing in composition, concentration,
etc. The phrase "aqueous multi-electron oxidant" or "AMO" refers
collectively to all such forms and any combination thereof. The
other solvent is, for example, a liquid other than water. The
reducer fluid, also referred herein as a "fuel", is a chemical that
donates electrons during the discharge process. The reducer fluid
is, for example, hydrogen gas. The discharge fluid is an exhaust
fluid obtained as a result of an electrochemical discharge process.
The discharge fluid comprises, for example, water, other solvents,
hydrogen cations, lithium cations, other cations, halide anions,
one of more forms of the buffer, and the unreacted AMO.
[0108] The system 100 disclosed herein comprises the discharge
system 101 and the regeneration system 106. The discharge system
101 disclosed herein comprises an oxidant fluid tank 102 comprising
oxidant fluid comprising aqueous multi-electron oxidant (AMO), a
reducer fluid tank 103 comprising a reducer, a discharge fluid tank
113 for collecting discharge fluid, and a discharge unit 104. The
AMO is a chemical that accepts electrons from an electrode during
the electrochemical discharge process and acts as an oxidizing
agent. The reducer is a chemical that donates electrons to an
electrode during the electrochemical discharge process and acts as
a reducing agent. The discharge system 101 disclosed herein can be
technically classified as a type of a redox flow battery. Unlike
conventional redox flow battery systems, the discharge system 101
disclosed herein carries a minimal amount of a solvent and thus
provides a higher system energy density. Also, unlike conventional
redox flow battery, the discharge unit 104 is not intended for
complete regeneration of oxidant fluid and reducer fluid by
reversing the flow of electric current and of reagents through the
discharge unit 104, although partial regeneration, for example, by
producing intermediate oxidant such as Br.sub.2 is possible and
recommended, for example for regenerative breaking when used in an
electric vehicle such as an electric car. Also, unlike conventional
fuel cell systems that carry a reducer but not oxidant, the
discharge system 101 disclosed herein carries both the reducer and
the AMO in reducer fluid tanks 103 and oxidant fluid tanks 102
respectively. In an embodiment, the AMO and the reducer are stored
in reagent containers or supplied via multiple oxidant fluid tanks
102 and reducer fluid tanks 103 respectively.
[0109] The aqueous multi-electron oxidant (AMO) is an oxidant that,
in at least one of its forms such as an acid form or a salt form,
for example, a Li salt has a high solubility in water, for example,
over 1M, and that transfers in a solution-phase redox reaction or
in an electrochemical reaction more than 1 mole of electrons per 1
mole of the AMO. The AMO comprises one or more of halogens, halogen
oxoacids, halogen oxoanions, and other oxoanions. The AMO is one or
more of an oxide of an element such as a halogen, an oxoacid of an
element such as a halogen oxoacid. The halogen is, for example, one
or more of chlorine, bromine, and iodine. An oxoanion is an anion
comprising one or more oxygen atoms and one or more atoms of
another element. An oxoacid is a compound comprising an oxoanion
and one or more forms of hydrogen cation. In the energy cycle
disclosed herein, the AMO is present in the charged oxidant fluid
along with water and one or more forms of a buffer. The buffer in
the base form is used during regeneration to maintain pH of the AMO
at an appropriate value, for example, greater than 7, while
providing sufficient solubility, for example, >1M for the salt
form of the AMO. The buffer is chemically compatible with the AMO,
the intermediate oxidant, the discharge fluid, the electrolysis
process, etc. The buffer in the base form comprises, for example,
anions such as OH.sup.-, a monohydrogen phosphate, a substituted
phosphonate, an amine, a tertiary amine, one or more of a buffering
agent described as Good's buffers, etc. Good's buffers comprise
about twenty buffering agents for biochemical and biological
research selected and described by Norman Good and others. In
addition to a group defining its buffering property, the buffer
comprises a strong acidic group such as a sulfonate which is
beneficial for the buffer as the strong acidic group reduces its
crossover throughout the cation exchange membrane during discharge
and electrolysis-regeneration.
[0110] The cation component of the buffer is one or more of lithium
(Li.sup.+), other alkali metals, alkali earth elements, other
elements, protonated nitrogen bases, quaternary nitrogen cations,
quaternary phosphorous cations, etc. Li.sup.+ provides a
substantially high solubility for bromate and bromide. Li.sup.+
does create problems with poor solubility of lithium phosphate
which forms upon decomposition and/or precipitation of its base
form Li.sub.2HPO.sub.4 (=1/2Li.sub.3PO.sub.4+1/2LiH.sub.2PO.sub.4),
if hydrogen phosphate is used as the buffer, but since this may
happen only during off-board regeneration and only in no-aqueous
multi-electron oxidant (AMO)-on-negative electrode mode of
operation also referred to as a "no-AMO-on-negative mode of
operation", use of Li.sup.+ will not create a safety problem. The
AMO in one or more forms can be pre-mixed with the buffer. In an
embodiment, the AMO is an oxide or an oxoacid of an element, for
example, nitrogen, xenon, sulfur, etc. In another embodiment, the
AMO is selected from a group consisting of, for example, a halogen
compound such as a halogen oxide, a halogen oxoacid, etc., an
interhalogen compound, an oxide of nitrogen, a nitrogen oxoacid, an
oxide of xenon, an oxoacid of xenon, an oxide of sulfur, an oxoacid
of sulfur, an oxide of a chalcogen, an oxoacid of a chalcogen, an
oxide of a pnictogen, an oxoacid of a pnictogen, a volatile oxide
of an element, a fluid oxide of an element, a soluble oxide of an
element, a volatile oxoacid of an element, a fluid oxoacid of an
element, a soluble oxoacid of an element, and any combination
thereof.
[0111] The oxide is a compound having a formula X.sub.mO.sub.n,
where X is one or more chemical elements, and where O is oxygen,
and m and n are integers. In an embodiment, 1.ltoreq.m.ltoreq.2 and
1.ltoreq.n.ltoreq.7. For example, the aqueous multi-electron
oxidant (AMO) is a halogen oxide having a formula X.sub.mO.sub.n,
where X is one of multiple halogens, O is oxygen, and
1.ltoreq.m.ltoreq.2, and 1.ltoreq.n.ltoreq.7. The oxoacid is a
compound having a formula H.sub.pX.sub.qO.sub.r, where X is one of
multiple halogens, nitrogen, chalcogens, xenon, or other element,
and 1.ltoreq.p, q, r.ltoreq.6. In an example, the halogen oxoacid
is a compound having a formula H.sub.pX.sub.qO.sub.r, where X is
one of multiple halogens, O is oxygen, and 1.ltoreq.p, q,
r.ltoreq.6 such as HBrO.sub.3 or bromic acid. The reagents,
products, and intermediaries of the reduction of halogen oxoacids
are either gases, liquids or are soluble in water. If the reagents,
intermediates, and products are anions, their cross over through a
cation exchange membrane is minimal. In an embodiment, the oxoacid
is a compound having a formula H.sub.pXO.sub.r, where X is a
halogen, H is hydrogen, O is oxygen, 1.ltoreq.p<6, and
1<r<6. In an embodiment, the AMO is a nitrogen oxide having a
formula N.sub.xO.sub.n, where x=1 or 2 and 1.ltoreq.n.ltoreq.5. In
another embodiment, the AMO is a nitrogen oxoacid having a formula
H.sub.kN.sub.lO.sub.m, where H is hydrogen, N is nitrogen, O is
oxygen, and 1.ltoreq.k, l, m.ltoreq.3. In another embodiment, the
AMO is a nitrogen oxoacid having a formula HNO.sub.n, where H is
hydrogen, N is nitrogen, O is oxygen, and 1<n<3. In another
embodiment, the AMO in acid form is chloric acid which forms a
stable aqueous room temperature solution, for example, up to about
40% w/w. Chloric acid can be used, for example, for military and
aerospace applications where high energy density is needed. In
another embodiment, the AMO in acid form is bromic acid
(HBrO.sub.3) which forms a stable aqueous room temperature
solution, for example, up to about 55% w/w. Bromic acid and/or its
salt is convenient for the regeneration part of the energy cycle
and thus used, for example, in automotive applications. In another
embodiment, the AMO in acid form is iodic acid which forms a stable
aqueous room temperature solution, for example, up to about 74%
w/w. In another embodiment, the AMO is nitric acid which forms an
atmospheric aqueous azeotrope with, for example, about 68.4% w/w.
The AMO can be used as an aqueous or non-aqueous solution. Other
examples of the AMO in acid form are hypochlorous acid, hypobromous
acid, perbromic acid, perchloric acid, periodic acid, etc. A
subgroup of the AMO comprising oxoacids (and salts of oxoanions) of
halogens (Cl, Br, I) is of special interest in energy storage
applications since the latter AMOs can be regenerated from
discharge fluid with full recycling of all chemicals.
[0112] In an embodiment, high energy oxidants rather than oxygen or
air are used with the discharge system 101 which is otherwise
similar to a polymer electrolyte membrane fuel cell (PEMFC) system,
except for a difference in the structures of one or more
electrodes. The high energy density aqueous multi-electron oxidant
(AMO) and a mediator are components of the oxidant fluid which is
stored in the oxidant container or the oxidant fluid tank 102. The
reducer is, for example, hydrogen. The use of hydrogen as the
reducer imparts a benefit of an efficient regeneration via electric
energy, solar energy, etc., in a regeneration system 106 or in the
discharge system 101 or in both. In an embodiment, the reducer is
selected from a group consisting of, for example, ammonia,
hydrazine, hydroxylamine, phosphine, methane, a hydrocarbon, an
alcohol such as methanol, ethanol, etc., an aldehyde, a
carbohydrate, a hydride, an oxide, a chalcogenide, another organic
and inorganic compound and any combination thereof. The oxide is,
for example, carbon monoxide (CO), nitrous oxide (N.sub.2O), nitric
oxide (NO), sulfur dioxide (SO.sub.2), etc.
[0113] The discharge unit 104 of the discharge system 101 comprises
an electrolytic cell stack 105. The electrolytic cell stack 105
comprises multiple electrolytic cells 200. Each electrolytic cell
200 comprises a 5-layer electrolyte-electrode assembly 206
exemplarily illustrated in FIG. 2. The 5-layer
electrolyte-electrode assembly 206 comprises a 3-layer
electrolyte-electrode assembly 205 flanked by two diffusion layers
201a and 201b exemplarily illustrated in FIG. 2. The 3-layer
electrolyte-electrode assembly 205 comprises a positive electrode
205a, a negative electrode 205b, and an electrolyte layer 205c
interposed between the positive electrode 205a and the negative
electrode 205b. The positive electrode 205a and the negative
electrode 205b are herein collectively referred to as "electrodes".
The term "electrode" refers to an electronic conductor or a mixed
electronic-ionic conductor, the surface of which is in contact with
an ionically conducting medium. The 3-layer electrolyte-electrode
assembly 205 is flanked by a positive diffusion layer 201a on the
positive side and a negative diffusion layer 201b on the negative
side forming the 5-layer electrolyte-electrode assembly 206. The
5-layer electrolyte-electrode assembly 206 is flanked on each side
by a bipolar plate 202 or an endplate 301, exemplarily illustrated
in FIG. 3. The electrolytic cell stack 105 with the oxidant fluid
tank 102, the reducer fluid tank 103, a discharge fluid tank 113,
and connecting lines form the discharge system 101. In an
embodiment, the discharge unit 104 comprises the electrolytic cell
stack 105, an enclosure, electric leads, gas hoses and/or liquid
hoses. In an embodiment, the electrolytic cell stack 105 is
configured as a planar cell stack 300 exemplarily illustrated in
FIG. 3, comprising electrolytic cells 200 exemplarily illustrated
in FIG. 2.
[0114] The theoretical standard equilibrium single cell voltages
and tanks' energy densities of the discharge system 101 using
various combinations of reducers and aqueous multi-electron
oxidants as well as of other more commonly used battery materials
are exemplarily illustrated in FIGS. 5A-5B. The halogens, the
halogen oxoacids, and discharge products, for example, hydrogen
halides and water are present as liquids, gases, or liquid
solutions, thereby simplifying mass transport processes in the
discharge system 101 and the regeneration system 106.
[0115] The chemistry of the oxides and oxoacids of halogens, of
chalcogens, and of pnictogens may pose problems such as
disproportionation of lower oxides and oxoacids, and precipitation
of solid phases. Disproportionation is a redox reaction in which an
element, free or in a compound, is reduced and oxidized in the same
reaction to form different products. For example, an element with
an oxidation state A, not necessarily A=0, on disproportionation is
distributed between several species with different oxidation states
B, C, etc., which differ from the element's initial oxidation state
A, so that B>A>C. For example, the formation of I.sub.2 may
result in phase-segregation such as pore blocking and manifold
blocking, when the temperature (T) is low, for example, for iodine
below its melting point of about 114.degree. C. To keep all the
compounds, for example, I.sub.2, in a fluid state, T>120.degree.
C. may be desired. The high temperature also benefits ionic
conductivity, reaction kinetics, and the rate of heat rejection.
However, other factors, for example, startup time, materials
corrosion, and pressure limits of the seals may favor a lower
temperature for operation, for example, about 60.degree. C. Since
the discharge system 101 disclosed herein comprising the oxidant
fluid tank 102 and the reducer fluid tank 103, and a discharge unit
104 with the electrolytic cell stack 105 can be enclosed, the
operation of the discharge system 101 at such elevated temperatures
and/or pressures is relatively easier than in the case of regular
fuel cells that use O.sub.2 from air.
[0116] The fast kinetics on the positive electrode 205a such as
bromine-bromide reactions, assures high power density and
efficiency of the discharge unit 104 as well as the possibility of
partial electric recharge which conventional fuel cells lack.
Aqueous multi-electron oxidants (AMOs) with high energy content,
for example, above 400 watt-hour (Wh)/kilogram (kg) and above 200
Wh/litre (L) are used to ensure a driving range of about 200-300
kilometres or more. Although the required energy densities can be
achieved with many highly soluble or fluid in the pure state and
multi-electron redox couples, for example, nitric acid, the
requirements for fast reversible kinetics and high faradaic
efficiency of both electroreduction on the positive electrode 205a
of the discharge unit 104 and electro-oxidation on the positive
electrode of the electrolyzer 107a of the regeneration system 106
rules out most of such oxidants. Suitable AMOs must assure that the
reagents, products, and intermediates of the reduction of the AMOs
are gases, liquids, or are highly soluble and compatible with the
entire group consisting of water, electrolyte layer materials,
electrode materials, hose materials, and all other materials that
come in contact with the oxidant fluid, the discharge fluid, and
the reducer. Also, the reagents and the products of the process of
reduction or electroreduction of the AMOs can be anions which
provide an additional benefit of a reduced crossover if a cation
exchange membrane is used as the electrolyte layer 205c.
[0117] In an embodiment, the discharge unit 104 disclosed herein
operates in the discharge mode. In the discharge mode of operation,
the discharge unit 104 produces the electric power in an external
electric circuit 203, exemplarily illustrated in FIG. 4, when
supplied with the reducer 401 and the aqueous multi-electron
oxidant (AMO) 402 from external reducer fluid tanks 103 and oxidant
fluid tanks 102 respectively, that can be periodically refilled by
pumping the reducer and the AMO from a refueling station or
multiple reagent sources into their respective reagent containers
or tanks 103 and 102.
[0118] In an embodiment, the discharge unit 104 operates in a
regenerative mode, also referred herein as an "electric recharge
mode". In the electric recharge mode of operation, the discharge
unit 104 produces a reducer or an intermediate reducer and an
intermediate oxidant which may or may not be the same as the
reducer and the aqueous multi-electron oxidant (AMO) used during
the discharge. The discharge unit 104 operating in the electric
recharge mode produces an oxidant or an intermediate oxidant, for
example, a halogen or a halogen compound, and the reducer, for
example, hydrogen by consuming a sustainable electric current from
an external power source or external electric circuit 203,
exemplarily illustrated in FIG. 2, and by splitting the discharge
products in the discharge fluid, for example, hydrogen halides. The
method of regeneration uses, in combination with other steps or by
itself, electrolysis, that is, with consumption of electric energy.
In the electric recharge mode or the electric recuperation mode of
operation of the discharge unit 104, the reducer or the
intermediate reducer is produced on the negative electrode 205b,
and the AMO or the intermediate oxidant is generated on the
positive electrode 205a, when the electric current is forced
through the electrodes 205a and 205b of the discharge unit 104
and/or the 5-layer electrolyte-electrode assembly 206, also
referred herein as the discharge cell, in a direction opposite to
the direction of the electric current during the discharge mode of
operation, provided that proper chemicals, for example, the
discharge products are supplied to the respective electrodes 205a
and 205b. The electric recharge mode or the electric recuperation
mode is useful for regenerative breaking when discharge system 101
is used to power a vehicle.
[0119] In an embodiment, a solution-phase reaction facilitates one
or more discharge reactions on the positive electrode 205a of the
electrolyte-electrode assembly 205. In an embodiment, the
solution-phase reaction disclosed herein is, for example, a
pH-dependent solution-phase comproportionation, a solution-phase
redox catalysis, etc. Comproportionation is a redox reaction in
which an element, free or in compounds, with oxidation states A and
C is converted into another substance or substances in which the
element's oxidation states are B, such that A>B>C. In an
embodiment, the rate of the solution-phase comproportionation
depends on the pH of the solution. In an embodiment, an
electrocatalyst, for example, lead oxide, ruthenium oxide
(RuO.sub.2) or a platinoid facilitates one or more discharge
reactions on the positive electrode 205a of the
electrolyte-electrode assembly 205. Such facilitation may occur via
a direct electroreduction of an aqueous multi-electron oxidant
(AMO) such as bromate, or via electroreduction of an intermediate
oxidant such as bromine on the positive electrode 205a. In another
embodiment, a platinoid electrocatalyst facilitates one or more
discharge reactions on the negative electrode 205b of the
electrolyte-electrode assembly 205. In another embodiment, a redox
mediator facilitates a charge transfer between the positive
electrodes 205a of the electrolyte-electrode assemblies 205 and the
AMO. The redox mediator is a halogen/halide couple, for example,
Cl.sub.2/Cl.sup.-. In another embodiment, a chloride mediator
facilitates one or more discharge or regeneration reactions on the
positive electrode 205a of the electrolyte-electrode assembly 205,
for example via a reaction:
BrO.sub.3.sup.-+5Cl.sup.-+6H.sup.+.dbd.BrCl+2Cl.sub.2+3H.sub.2O.
[0120] In another embodiment, one or more of multiple immobilized
heterogeneous mediators, immobilized heterogeneous catalysts,
electrocatalysts, homogeneous mediators, or homogeneous catalysts
facilitate a charge transfer between the positive electrodes 205a
of the electrolyte-electrode assemblies 205 and the oxidant fluid.
In another embodiment, a catalyst selected from a group consisting
of, for example, a homogeneous catalyst, a heterogeneous catalyst,
a redox mediator catalyst, or a combination thereof, facilitates
one or more discharge or charge reactions on the positive
electrodes 205a of the electrolyte-electrode assemblies 205. In
another embodiment, a reduced form of a homogeneous solution-phase
mediator, a product of an electrode reaction or any combination
thereof, accelerates a rate of discharge during one or more
discharge reactions via a solution-phase comproportionation, which
may or may not be pH-dependent. For example, pH-dependent
solution-phase comproportionation of the aqueous multi-electron
oxidant (AMO) such as bromate with a final product of a reduction
of the AMO such as bromide accelerates the rate of discharge of the
discharge unit 104.
[0121] The regeneration system 106 of the system 100 disclosed
herein is configured to regenerate the aqueous multi-electron
oxidant (AMO) and the reducer from the discharge fluid produced by
the discharge unit 104. The regeneration system 106 comprises, for
example, an electrolysis-disproportionation (ED) reactor 107, an
acidification reactor, also referred herein as an "ion exchange
reactor" and referenced by the numeral 108, such as an orthogonal
ion migration across laminar flow (OIMALF) reactor, a
neutralization reactor 109, a concentrating reactor 112, multiple
separation reactors 1006, 1007, and 1010 exemplarily illustrated in
FIG. 10B, storage tanks such as a regenerated oxidant fluid tank
110 and a regenerated reducer fluid tank 111. The ED reactor 107
comprises sub-reactors, for example, an electrolysis unit or an
electrolyzer 107a and a disproportionation unit 107b which can be
configured in one ED reactor 107. The configuration of the
electrolyzer 107a of the ED reactor 107 is similar to that of an
electrolytic cell 200 of the electrolytic cell stack 105 of the
discharge unit 104 exemplarily illustrated in FIG. 2. In an
embodiment, the electrolyzer 107a and the disproportionation unit
107b as well as the neutralization reactor 109 are physically
combined in the same hardware.
[0122] The neutralization reactor 109 is configured to neutralize
the discharge fluid, for example, hydrogen halide produced by the
discharge unit 104 with a base form of a buffer to produce a
solution of a neutral or base form of the discharge fluid. In an
embodiment, the neutralization reactor 109 comprises a mixing
reactor. The neutralization reactor 109 is configured to maintain
an optimal pH during the conversion of the discharge fluid into the
oxidant fluid. For example, in the case of a halate as the aqueous
multi-electron oxidant (AMO), the value of the optimal pH is
limited at the low end by the reverse reaction of
comproportionation between halate and halide, and the upper end by
the stability of the intermediate hypohalate toward further
disproportionation. In the case of bromate, the optimal pH range
is, for example, between 7 and 9. The
electrolysis-disproportionation (ED) reactor 107 is configured to
electrolyze the solution of the salt form of the discharge fluid
into an intermediate oxidant such as a halogen at a positive
electrode of the ED reactor 107 accompanied by a release of the
reducer such as hydrogen and a base form of the buffer at a
negative electrode of the ED reactor 107, while producing a salt
form of the aqueous multi-electron oxidant (AMO) at the positive
electrode via disproportionation of the intermediate oxidant
produced at the positive electrode with an excess of the base form
of the buffer, and simultaneously releasing a stoichiometric amount
of the reducer and the base form of the buffer for neutralization.
The ED reactor 107 can be configured to operate, for example, in a
batch mode, as exemplarily illustrated in FIG. 10A a single pass
flow-through cascade mode, and in a multi-pass cyclic flow mode, as
exemplarily illustrated in FIG. 10B.
[0123] The ED reactor 107 is used in series with the ion exchange
reactor 108. The ion exchange reactor 108 is configured to convert
the aqueous multi-electron oxidant (AMO) in a salt form such as
halate into an acid form of the AMO such as a halic acid. The
storage tanks, for example, the regenerated oxidant fluid tank 110,
the regenerated reducer fluid tank 111, and a buffer tank (not
shown) are used to store the regenerated oxidant, the regenerated
reducer, and the buffer respectively. The separation reactors 1006,
1007, and 1010, exemplarily illustrated in FIG. 10B are gas-liquid
separators and are used to separate gases from the liquids during
the regeneration process.
[0124] The electrolysis-disproportionation (ED) reactor 107 or
reactors can be operated in a cyclic flow mode or in a cascade flow
mode. In the cyclic flow mode, the regenerated solution or the
discharge fluid is cycled between a mixing reactor or the
neutralization reactor 109, a three-way valve 1004, and another
three-way valve 1005 exemplarily illustrated in FIG. 10B, through
the ED reactor 107. In the cascade flow mode, the regenerated
solution flows through a cascade (not shown) of functionally
identical mixing reactors of the neutralization reactor 109 and ED
reactors 107, and three-way valves 1004 and 1005. An ED reactor 107
configured for the cyclic flow mode has a lower upfront cost but
requires a longer regeneration time. The ED reactor 107 configured
for the cascade flow mode has a higher upfront cost but is capable
of a faster regeneration or higher throughput.
[0125] An exemplary operation of the
electrolysis-disproportionation (ED) reactor 107 in the cyclic flow
mode is disclosed in the detailed description of FIGS. 11A-11B. A
loop within the ED step including the ED reactor 107, the ion
exchange reactor 108 such as the orthogonal ion migration across
laminar flow (OIMALF) reactor, and the mixing reactor or the
neutralization reactor 109 is disclosed in the detailed description
of FIG. 10B. As used herein, the term "laminar flow" refers to a
type of fluid flow, for example, a liquid flow or a gas flow, in
which directions and magnitudes of fluid velocity vectors in
different points within a fluid do not change randomly in time and
in space. Also, as used herein, the term "migration" refers to a
movement of an electrically charged object such as an ion due to
the action of an external electric field. In the OIMALF process,
the vectors of the laminar flow velocity and the electric field are
not parallel and not anti-parallel. The concentrating reactor 112
concentrates the acid form of the aqueous multi-electron oxidant
(AMO) to remove the excess water produced on the positive electrode
205a during the discharge and to remove water introduced with the
buffer during electrolysis-disproportionation. The concentrating
reactor 112 removes water or other solvents from a dilute fluid
that enters the concentrating reactor 112 and releases a
concentrated fluid and water or another solvent. The concentrating
reactor 112 performs concentration, for example, by evaporation or
reverse osmosis.
[0126] The discharge system 101 and the regeneration system 106 can
be used together in a complete energy cycle that recycles all the
chemicals, does not consume external chemicals, and does not
generate chemical waste. The complete energy cycle employs the
regeneration system 106 in addition to the discharge system 101.
The discharge products such as LiBr and H.sub.2O produced in the
discharge unit 104 of the discharge system 101 are converted back
to intermediates such as Br.sub.2, and/or stable reactants such as
LiBrO.sub.3 and H.sub.2 of the reactants in the ED reactor 107, and
to the active form such as HBrO.sub.3 in the ion exchange reactor
108, for example, the orthogonal ion migration across laminar flow
(OIMALF) reactor.
[0127] The reverse transformation of the cathodic discharge
product, for example, LiBr, into the aqueous multi-electron oxidant
(AMO), for example, LiBrO.sub.3 in the regeneration system 106 is
accompanied by the release of the reducer, for example, hydrogen in
a stoichiometric amount, as exemplified by equations (18)-(21) for
a particular lithium bromate-phosphate chemistry. As a result, the
regeneration system 106 can produce simultaneously both the AMO and
hydrogen, in stoichiometric amounts, which can be used again as
reactants during the direct mode of operation of the discharge unit
104 of the discharge system 101. In an embodiment, the regeneration
of the AMO from the spent discharge fluid or from the intermediate
oxidant is catalyzed by a homogeneous catalyst such as chlorine,
polyvalent metal ions, etc., or by a heterogeneous electrocatalyst
such as ruthenium dioxide, lead dioxide, and their derivatives. The
energy cycle based on the discharge unit 104 and the process of
on-site regeneration disclosed herein eliminates the need for a
macro scale infrastructure for the production, transportation and
storage of the reducer, for example, hydrogen in contrast to
applications based on fuel cells.
[0128] The discharge unit 104 and the
electrolysis-disproportionation (ED) reactor 107 disclosed herein
are implemented with aqueous multi-electron oxidants (AMOs)
compatible with water and with cation-exchange membranes such as
commercially available polyperfluorosulfonic acids. The aqueous
multi-electron oxidants are, for example, halogens, halogen oxides,
halogen oxoanions, and halogen oxoacids. In an embodiment, the
aqueous multi-electron oxidants are, for example, oxides,
oxoanions, and oxoacids of chalcogens, of pnictogens, of xenon,
etc. The listed compounds can assure a higher theoretical energy
density than the elemental halogens and batteries with solid
electroactive materials such as lithium ion batteries, but at the
expense of lower energy efficiency and lower power density and a
higher cost as an expensive catalyst may be required. In this
embodiment, homogeneous reactions near the positive electrode 205a
are utilized in order to achieve a higher power from the positive
electrode 205a. The discharge system 101 disclosed herein
circumvents the drawback of lower energy efficiency and power
density and of higher cost by using a solution-phase redox
mediator. The solution-phase redox mediator is an
Ox.sub.med/Red.sub.med couple which is subject to a rapid and
reversible transformation at an electrode and is capable of a quick
homogeneous redox reaction with the aqueous multi-electron oxidant.
A solution-phase redox mediator is a redox couple dissolved in a
solution, for example, in the oxidant fluid, that is capable of
relatively fast electron transfer reactions both at the electrode
and with a primary aqueous multi-electron oxidant (AMO), for
example, bromine/bromide couple. At the same time, the reduced form
of the Ox.sub.med/Red.sub.med couple participates in a rapid redox
reaction with the high energy but electrochemically inactive
AMO:
AMO+Red.sub.med.fwdarw.Red+Ox.sub.med
Ox.sub.med+n.sub.mede=Red.sub.med
[0129] The solution-phase redox mediators help to realize the
electrochemical process at a low over-voltage on the electrodes
205a with or without a low amount of platinum (Pt) and other
expensive catalyst. The solution-phase redox mediator is stable
with respect to side reactions and hence allows the discharge unit
104 to be used for many days or cycles. The solution-phase redox
mediator can be present only within the positive electrode space of
the discharge unit 104 with minimal cross-over to the negative
electrode space. The solution-phase redox-mediator helps to realize
a high rate of electron transfer from the principal aqueous
multi-electron oxidant (AMO) to the positive electrode 205a on
discharge. The reduced form of the solution-phase redox mediator
(Red.sub.med) causes a rapid solution-phase chemical reaction
during discharge and can be regenerated from the oxidized form of
the solution-phase redox mediator (Ox.sub.med). In an embodiment,
to mediate AMO reduction in the discharge unit 104, a
solution-phase mediator, for example, polyoxometallates is used to
facilitate the electrode reaction on the positive electrode 205a.
In this embodiment, the regenerating couple is suspended or
immobilized polyoxometallates which do not cross the membrane and
do not discharge at the negative electrode 205b due to their large
size, negative charge or a combination thereof. In an embodiment,
the regeneration process is based on the redox-mediated catalysis
by the redox couple:
AMO+Red.sub.med.fwdarw.Discharge
Product+Ox.sub.medRed.sub.med.revreaction.Ox.sub.med+n.sub.mede
[0130] In an embodiment, the reduced form of the mediator is the
final product of the reduction of the aqueous multi-electron
oxidant (AMO) and the homogeneous reaction facilitating a discharge
of the AMO is a comproportionation reaction.
[0131] In a reduction of aqueous multi-electron oxidants (AMOs), a
large number of protons are consumed. The discharge unit 104
disclosed herein produces protons at the negative electrode 205b by
electro-oxidation of hydrogen or a hydride and transfers the
protons to the positive electrode 205a across the electrolyte layer
205c. The hydrogen reducer is automatically co-regenerated with the
aqueous multi-electron oxidant (AMO), or an intermediate,
Ox.sub.med, during the regeneration process. Thus, the regeneration
system 106 restores back both the components of the oxidant fluid,
that is, the AMO, Ox, or the oxidized intermediate, Ox.sub.med; and
the fuel or the reducer such as H.sub.2. The discharge unit 104
disclosed herein uses AMOs. A homogeneous redox mediator is added
to or generated within the discharge unit 104 to perform the
reduction of the AMO during the discharge process in the bulk of
the solution rather than on the surface of the electrode 205a where
the number of active sites is lower. The homogeneous redox
mediators allow for the use of AMOs in electrochemical power
sources and resolve the issue of the slow and irreversible direct
electrode reactions of the AMO.
[0132] The discharge system 101 disclosed herein therefore provides
a long driving range, a high energy density, a high power, and a
high energy efficiency at a lower cost than proton exchange
membrane fuel cells (PEMFCs). The discharge system 101 requires a
short refill time and can be operationally combined with the
regeneration system 106 to enable an electric energy cycle based on
the H.sub.2-aqueous multi-electron oxidant (AMO) chemical matter
cycle. Other combinations of discharge system 101 with various
regeneration systems 106 can use other types of energy, such as
solar energy as the input in the chemical cycle. The discharge unit
104 disclosed herein avoids the need for a large amount of platinum
or other expensive metals required for the electroreduction of
oxygen. Since the discharge unit 104 does not consume oxygen, the
discharge system 101 can be used in enclosed environments such as
submarines, space ships, etc.
[0133] In an embodiment, the discharge unit 104 employs the
ultimate reduction product as the reduced form of the intermediate,
for example by taking advantage of the homogeneous
comproportionation between an oxoanion and a free halide, leading
to an electrochemically active halogen on discharge in an ignition
type cycle. In the case of bromate (BrO.sub.3.sup.-) as an aqueous
multi-electron oxidant (AMO):
3H.sub.2-6e.sup.-=6H.sup.+, fast On the negative electrode
3Br.sub.2+6e.sup.-=6Br.sup.-, fast On the positive electrode
5Br.sup.-+BrO.sub.3.sup.-+6H.sup.+=3H.sub.2O+3Br.sub.2@pH<4 In
the catholyte.
[0134] The discharge unit 104 allows for a fast reversible reaction
on the 2D surface of an inexpensive electrode such as a
carbon-based electrode while performing the slower
comproportionation step utilizing the actual energy storing
species, for example, the aqueous multi-electron oxidant (AMO) such
as bromate or other halogen oxoanion, in the three-dimensional (3D)
bulk of the solution where a higher reaction rate can be sustained.
The reagent and the product of the discharge are anions which
result in their low crossover from the positive electrode 205a
through a cation-exchange membrane 205c to the negative electrode
205b. Among the bromine oxoacids HBrO.sub.n, 1.ltoreq.n.ltoreq.4,
bromic acid (HBrO.sub.3) presents a useful compromise between the
energy density and the energy efficiency. The theoretical energy
efficiency of a H.sub.2--HBrO.sub.3 discharge unit 104 on discharge
can be estimated as the ratio of the standard equilibrium potential
of the bromine/bromide couple, for example, about 1.07V and the
standard equilibrium potential of the bromate/bromide couple, for
example, at about 1.42V, measured with respect to the standard
hydrogen electrode and is equal to about 75%, which is acceptable
for transportation applications. The bromate/bromide direct
electroreduction is slower than the iodate/iodide direct
electroreduction. At pH 10, the difference between the onset
potentials of bromate reduction to bromide and bromide oxidation to
bromate on Pt amounts to 0.4 V. In an acidic solution, the
reduction of iodate follows the same pathway as the reduction of
bromate, that is, via a homogeneous comproportionation to
bromine.
[0135] The method and the system 100 disclosed herein use halic
acids or halate anions as the aqueous multi-electron oxidant (AMO)
among halogen oxoacids due to a number of reasons and/or factors.
One of the factors is, for example: perhalates are inert
kinetically, both in direct reduction on an electrode 205a and in
homogeneous comproportionation, whereas halites and hypohalites
have lower energy densities. Other factors are considered too. For
example: during the discharge, both the efficiency of the halogen
electrode kinetics, that is, the halogen/halide exchange current
and the ratio of the standard electrode potentials of
halogen/halide to oxohalate/halide are important in the overall
cycle energy efficiency. Due to the first factor, bromine oxoacids
are used instead of chlorine oxoacids and due to the second factor,
bromine oxoacids are used instead of iodine oxoacids. The discharge
system 101 can be used on-board, for example, a vehicle. The
regeneration system 106 can be used on-board or off-board. The
structures of the discharge unit 104 or the electrolytic cell stack
105 are based on the corresponding structures in proton exchange
membrane fuel cells (PEMFCs).
[0136] In an embodiment, the reagent containers, for example, the
reducer fluid tanks 103, and the oxidant fluid tanks 102,
exemplarily illustrated in FIG. 1, are refilled by pumping the
reducer and the oxidant fluid comprising the aqueous multi-electron
oxidant (AMO) from their respective stationary storage facilities
such as an off-road fueling station. In an embodiment, the reagent
containers, for example, 102 and 103 are located outside the
discharge unit 104 and are connected to the electrolytic cell stack
105 via the ports 302 and 303. In another embodiment, the reagent
containers, for example, 102 and 103 are refilled by regenerating
or partially regenerating the intermediate oxidant and the reducer,
for example, by electrolysis, by applying an electric current of a
polarity opposite to the polarity of the electric current that the
discharge unit 104 generates during the discharge mode of
operation, etc. This partially regenerated AMO is useful for
regenerative braking while driving an electric vehicle, load
leveling, etc.
[0137] FIG. 2 exemplarily illustrates a perspective view of a
dissembled single electrolytic cell 200 of an electrolytic cell
stack 105 of the discharge unit 104 of the discharge system 101 and
of the electrolyzer 107a of the electrolysis-disproportionation
(ED) reactor 107 of the regeneration system 106 exemplarily
illustrated in FIG. 1. Each electrolytic cell 200 comprises the
3-layer electrolyte-electrode assembly 205. The 5-layer
electrolyte-electrode assembly 205 of the electrolytic cell stack
105 is flanked by pair of diffusion layers 201a and 201b, where the
pair of diffusion layers 201a and 201b is flanked by a pair of
bipolar plates 202. The diffusion layers 201a and 201b are
electronically conducting and porous. The diffusion layers 201a and
201b are sheets capable of gas transport or liquid transport
through pores of the diffusion layers 201a and 201b or though the
bulk of the diffusion layers 201a and 201b. Moreover, the diffusion
layers 201a and 201b are capable of electronic conductivity through
their bulk. The diffusion layers 201a and 201b are positioned on
either side of the 3-layer electrolyte-electrode assembly 205 in
order to facilitate a uniform distribution of the reactants and
removal of the discharge products over the areas of the electrodes
205a and 205b. The 3-layer electrolyte-electrode assembly 205
flanked by a negative diffusion layer 201b on the negative
electrode side and a positive diffusion layer 201a on the positive
electrode side forms a 5-layer electrolyte-electrode assembly 206.
The 5-layer electrolyte-electrode assembly 206 flanked by two
bipolar plates 202 or a bipolar plate 202 and an endplate 301,
exemplarily illustrated in FIG. 3, forms a single electrolytic cell
200. Multiple electrolytic cells 200 connected electrically in
series and flanked by endplates 301 form the electrolytic cell
stack 105 so that any two adjacent electrolytic cells 200 share a
common bipolar plate 202.
[0138] The diffusion layers 201a and 201b are made of, for example,
porous carbon, composites containing carbon particles and fibers,
and carbon cloths such as those used for hydrogen-air proton
exchange membrane fuel cells (PEMFCs) and for redox flow batteries.
The bipolar plates 202 comprise flow channels 202a exemplarily
illustrated in FIG. 3, for supplying the reducer and the aqueous
multi-electron oxidant (AMO) from the storage tanks 103 and 102
respectively, into the electrolytic cell stack 105 and for removing
the discharge products from the electrolytic cell stack 105. The
bipolar plates 202 are made of, for example, graphite, other
carbonaceous materials, carbon-polymer composites, metals, alloys,
or electrically conductive ceramic. The 3-layer
electrolyte-electrode assembly 205 and/or the 5-layer
electrolyte-electrode assembly 206 are hereafter referred to as
"electrolyte-electrode assembly".
[0139] The 3-layer electrolyte-electrode assembly 205 comprises the
electrolyte layer 205c flanked by the positive electrode layer 205a
and the negative electrode layer 205b as disclosed in the detailed
description of FIG. 2. The positive electrode 205a is supplied with
the oxidant fluid comprising the aqueous multi-electron oxidant
(AMO) and the negative electrode 205b is supplied with the reducer
fluid during the discharge mode of operation of the discharge unit
104. The positive electrode 205a produces the intermediate oxidant
such as Br.sub.2 and the negative electrode 205b produces the
reducer such as H.sub.2 on partial recharge, that is, when electric
current is forced through the discharge unit 104 in a direction
opposite to the direction of the electric current during discharge.
A certain amount of the intermediate oxidant in the discharge unit
104 is regenerated from the discharge fluid by reversing a polarity
of the electric current flowing through the discharge unit 104
during discharge. In an embodiment, the electrodes 205a and 205b
are multiphase systems comprising an electron conducting phase, an
ion conducting phase, an electrocatalyst phase that can be
functionally combined with an electron conductor, and a
reactant/product-transporting porous phase that can be functionally
combined with an ion conductor. The discharge unit 104 is a device
that converts chemical energy of the reducer and the AMO into
electrical energy by means of electrochemical reactions on the two
electrodes 205a and 205b and an ion transport through the
electrolyte layer 205c.
[0140] The electrolyte layer 205c of the electrolyte-electrode
assembly 205 in the discharge unit 104 acts as an ion conductor, as
well as an electronically non-conducting mechanical barrier
separating the negative electrode 205b and the positive electrode
205a of the electrolytic cells 200, thereby precluding an internal
electrical and chemical short circuit from being established
between the positive electrode 205a and the negative electrode 205b
as well as between the aqueous multi-electron oxidant (AMO) and the
reducer. In an embodiment, the electrolyte layer 205c of the
electrolyte-electrode assembly 205 is composed of a material, for
example, a solid, a gel, a liquid, a polymer, an ionomer, a solid
ion conductor, or a solid proton conductor or a combination
thereof, that is capable of protonic conduction or, more generally,
of ionic conduction but not electronic conduction. The electrolyte
layer 205c conducts ions but not electrons. The electrolyte layer
205c with a higher permeability and/or conductivity to cations than
to anions has an additional advantage of reducing the chemical
short-circuiting during discharge via the reduction of the AMO on
the negative electrode 205b. The electrolyte layer 205c is
compatible with water, with the AMO, with the reducer, with the
buffer, and with the discharge products. Furthermore, since durable
fluorinated polymer cation selective fuel cell membranes are
available commercially, the discharge unit 104 disclosed herein
uses such cation-conductive fluorinated polymer electrolytes. In
another embodiment, the electrolyte layer 205c of the
electrolyte-electrode assembly 205 is composed of a material with a
cationic conduction exceeding an anionic conduction of the
material. In an embodiment, the electrolyte layer 205c is composed
of a material that contains one or more proton donor groups or
proton acceptor groups, for example, sulfonic, phosphonic, boronic
or nitrogen-base groups. In an embodiment, the electrolyte layer
205c is a solid in which hydrogen ions are mobile. In another
embodiment, the electrolyte layer 205c is a liquid or a gel in
which hydrogen ions are mobile.
[0141] Examples of the electrolytes 205c used in the
electrolyte-electrode assembly 205 disclosed herein comprise
polymers such as Nafion.RTM. of E. I. du Pont de Nemours and
Company Corporation, Flemion.RTM. series of polymers of Asahi Glass
Company, Aciplex.RTM. of Asahi Kasei Chemicals Corporation,
short-chained trifluorovyniloxy polymers from Dow Chemicals,
Hyflon.RTM.-Ion of Solvay Specialty Polymers, Aquivion.RTM. of
Solvay SA Corporation, a polymer with
--O--(CF.sub.2).sub.4--SO.sub.3H pendant groups developed by 3M
Company, BAM membrane from Ballard Advances Materials Corp.,
sulfonamide based polymers developed by DesMarteau, reinforced
membranes from W. L. Gore & Associates, Inc.,
polybenzimidazole, and other polymers with acidic groups, basic
groups or a combination thereof. The acidic groups comprise, for
example, sulfonic, phosphonic, boronic, and carboxylic groups. In
an embodiment, the electrolyte 205c is a polymer capable of anionic
conduction, for example, polymers with quaternary nitrogen and
phosphorus groups such as polymers employed in alkaline membrane
fuel cells. Another example of an electrolyte 205c employed in the
electrolyte-electrode assembly 205 disclosed herein is an ionically
conducting liquid retained in the pores of a solid matrix. Examples
of such ionically conducting liquid electrolytes comprise
phosphoric acid in a silicon carbide (SiC) matrix, hydroxide melts,
and electrolyte solutions comprising, for example, solid oxide
matrices, polymer matrices, and a combination thereof. Another
example of an electrolyte 205c employed in the
electrolyte-electrode assembly 205 disclosed herein is a solid
proton conductor such as CsH.sub.2PO.sub.4, CsHSO.sub.4, and
related materials, alkaline-earth cerate- and zirconate-based
perovskite materials such as doped SrCeO.sub.3, BaCeO.sub.3, and
BaZrO.sub.3, as well as rare-earth niobates, tantalates, and
tungstates. Polymer electrolytes are considered due to their
mechanical properties. Cation-conductive electrolytes are
considered due to their ability to reduce crossover such as
self-discharge.
[0142] In an embodiment, the electrolyte layer 205c is a porous
solid matrix imbibed with a liquid or gel or solid ion conducting
material. That is, the electrolyte layer 205c is a composite
material comprising an ion conducting liquid or gel or solid within
pores of the porous solid matrix. The liquid in the electrolyte
layer 205c is, for example, phosphoric acid or an aqueous solution
of phosphoric acid, a hydroxide or an aqueous solution of a
hydroxide, molten carbonates, molten hydroxides, a molten salt,
etc. The conducting ion in the electrolyte layer 205c is, for
example, H.sup.+, OH.sup.-, F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
or a combination thereof. The porous solid matrix is, for example,
polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a
dielectric such as silicon carbide, silicon dioxide, a silicate,
other ceramic materials, other polymer materials, etc. The ion
conducting liquid is, for example, water, an acid, a base, a salt,
a molten electrolyte, an organic solvent, or a combination
thereof.
[0143] In another embodiment, the electrolyte layer 205c of the
electrolyte-electrode assembly 205 is composed of a material, for
example, a solid membrane, capable of protonic conduction. The
solid membrane is, for example, Nafion.RTM. of E. I. du Pont de
Nemours and Company Corporation and related sulfonic acid polymers,
a sulfonamide polymer, acid doped polybenzimidazole, alkali
hydrogen sulfates, phosphates such as cesium hydrogen sulfate
(CsHSO.sub.4), other solid proton conductors, etc. In an example,
the material used as the electrolyte layer 205c is
CsH.sub.2PO.sub.4, other solid proton conductors, etc., when the
reducer used in the discharge unit 104 is hydrogen or a hydride.
Although selective ionic conduction of the electrolyte layer 205c
is not required, an H.sup.+ conducting membrane confers the benefit
of a more complete reduction of the aqueous multi-electron oxidant,
and higher solubility of the discharge products, that is, of a
larger energy density of the discharge unit 104 and the
regeneration system 106. In another embodiment, the electrolyte
layer 205c is a liquid under laminar flow.
[0144] In an embodiment, the electrolyte-electrode assembly 205 of
the discharge unit 104 further comprises electrodes or electrode
layers 205a and 205b disposed on each of the electrolyte layers
205c. The electrode layers 205a and 205b comprise, for example,
catalysts, carbon particles or fibers, a binder, a pore-forming
agent, etc. In an embodiment, the catalyst in the electrode layer
disposed on the negative hydrogen electrode 205b is platinum (Pt)
and Pt nanoparticles on carbon microparticles or on carbon
microfibers. In an embodiment, the catalyst in the electrode layer
disposed on the positive electrode 205a is one or more carbonaceous
materials with or without metals, metal oxides, such as RuO.sub.2
and dimensionally stable anodes (DSAs), other metallic and
non-metallic materials, etc.
[0145] The discharge unit 104 disclosed herein produces, in the
discharge mode, an electric power, that is, sustainable electrical
current and electric voltage, via an electrochemical reaction using
two reactants: the reducer and the aqueous multi-electron oxidant
(AMO) on spatially separated electrodes 205a and 205b. During the
discharge mode of operation of the discharge unit 104, the negative
electrodes 205b are supplied with the reducer, such as H.sub.2 and
the positive electrodes 205a are supplied with the AMO, such as
bromate resulting in a sustainable voltage difference and
sustainable electric current between the electrodes 205a and 205b.
The discharge mode of operation of the discharge unit 104 is also
known as a power generating mode of operation or a direct mode of
operation. The discharge unit 104 produces electric potential
difference between the electrodes 205a and 205b, which in turn
produces an electric potential difference between the diffusion
layers 201a and 201b and between the bipolar plates 202, when the
reactants are supplied to their respective electrodes 205a and
205b. The reducer donates electrons to the negative electrode 205b
and produces ions. The external electric circuit 203 conducts or
transfers electrons from the negative electrode 205b to the
positive electrode 205a. The aqueous multi-electron oxidant, either
directly or via an intermediate, accepts the electrons from the
positive electrode 205a for producing the electric current in the
external electric circuit 203. The electrolyte layer 205c provides
for a movement of the ions between the negative electrode 205b and
the positive electrode 205a, thereby maintaining electroneutrality
of the electrolyte layer 205c and conservation of charge in the
discharge unit 104, and producing a sustainable current and
sustainable voltage between the electrodes 205a and 205b and
between the bipolar plates 202.
[0146] When a load 204, for example, a light bulb, is attached
between the terminals of the electrolytic cell 200 or between the
endplate terminals of the discharge unit 104, the electric current
flows for as long as the reactants are supplied to the electrodes
205a and 205b and the discharge products are removed from the
electrodes 205a and 205b. In the discharge mode of operation, the
discharge unit 104 consumes the reducer and the aqueous
multi-electron oxidant that are produced from the discharge fluid
outside the discharge unit 104 or fully or partially regenerated
inside the discharge unit 104. As used herein, the term "partially
regenerated" refers to the number of electrons donated by the
discharge fluid being less the number of electrons lost by the
parent oxidant fluid regardless of how these electrons are
distributed between various chemical species.
[0147] FIG. 3 exemplarily illustrates a perspective view of a
planar cell stack 300 of the discharge unit 104 exemplarily
illustrated in FIG. 1, showing three 5-layer electrolyte-electrode
assemblies 206, two bipolar plates 202, and two endplates 301. The
planar cell stack 300 comprises multiple electrolytic cells 200
combined electrically in a series combination for delivering a
larger electric power than a single electrolytic cell 200
exemplarily illustrated in FIG. 2. When individual electrolytic
cells 200 are connected electrically in series, the planar cell
stack 300 produces more electric power via a large voltage with
about the same current, than the electric power produced by a
single electrolytic cell 200. In an embodiment, each 5-layer
electrolyte-electrode assembly 206 of the planar cell stack 300
comprises a 3-layer electrolyte-electrode assembly 205 flanked by
the diffusion layers 201a and 201b. Each stack in the planar cell
stack 300 comprises the 5-layer electrolyte-electrode assembly 206
positioned between two bipolar plates 202 or between a bipolar
plate 202 and an endplate 301 that conduct electrons.
[0148] The bipolar plates 202 in the planar cell stack 300 comprise
flow channels 202a. The flow channels 202a are grooves which allow
the reactants to be delivered to the electrodes 205b and 205a and
for the discharge products of the electrochemical reaction from the
electrodes 205b and 205a through the diffusion layers 201b and 201a
to be removed. The flow channels 202a of the bipolar plates 202
allow transport of the reagents and products to and from the
electrodes 205b and 205a and to and from the endplates 301. The
planar cell stack 300 terminates with the endplates 301. The
endplates 301 are similar in structure to the bipolar plates 202
but do not comprise the flow channels 202a on the outer surfaces
301b of the endplates 301. The endplates 301 comprise the flow
channels 202a on the inner surfaces 301a of the endplates 301.
Moreover, the endplates 301 comprise connecting ports 302 and 303,
for example, inlet ports and outlet ports on the outer surfaces
301b of the endplates 301 for facilitating movement of the reducer
fluid and the oxidant fluid into the planar cell stack 300 and for
the discharge fluid to be moved out of the planar cell stack 300.
Furthermore, the endplates 301 comprise electric contacts on the
outer surfaces 301b of the endplates 301.
[0149] Each electrolytic cell 200 shares one or two bipolar plates
202 with an adjacent electrolytic cell or cells 200. One side of
each bipolar plate 202 contacts a positive side of one electrolytic
cell 200 and another side of each bipolar plate 202 contacts a
negative side of the adjacent electrolytic cell 200. The
electrolytic cells 200 in the planar cell stack 300 are stacked
electrically in series such that each bipolar plate 202 faces a
diffusion layer 201a of the positive electrode 205a of one
electrolytic cell 200 on one side and the diffusion layer 201b of
the negative electrode 205b of another electrolytic cell 200 on the
other side. The individual electrolytic cells 200 are stacked
together such that each bipolar plate 202 contacts the negative
side of the electrolytic cell 200 at the left of the bipolar plate
202 and contacts the positive side of the electrolytic cell 200 at
the right of the bipolar plate 202. The electrolytic cells 200 in
the planar cell stack 300 are stacked electrically in series such
that each bipolar plate 202 serves as the positive side of one
electrolytic cell 200 and as the negative side of the next
electrolytic cell 200. Moreover, the bipolar plates 202 are
equipped with through channels (not shown) that provide for
transport of the reducer, the aqueous multi-electron oxidant (AMO)
and the discharge products from the electrolytic cell 200 to the
next electrolytic cell 200 in the planar cell stack 300 or to the
connecting ports 302 and 303. The number of repeat units or
electrolytic cells 200 in the planar cell stack 300 can be adjusted
according to the desired power or voltage. The endplates 301 and
the bipolar plates 202 are made of chemically inert electronically
conducting materials, for example, carbon or carbon composite, and
are equipped with flow channels 202a for supplying the reactants
and removing the products.
[0150] The oxidant fluid and the reducer fluid are stored in
reagent containers, for example, the oxidant fluid tanks 102 and
the reducer fluid tanks 103 exemplarily illustrated in FIG. 1. The
reagent containers or tanks 102 and 103 are connected to the
endplates 301 of the planar cell stack 300 via pipes 302 and 303.
In a small planar cell stack 300, the reagent containers or tanks
102 and 103 can be placed above the planar cell stack 300 for
gravity feeding the reactants to the electrolyte-electrode assembly
205. In an embodiment, in order to overcome the friction in the
flowing fluids, pressurized reagent containers are used or pumps
are inserted into the connecting lines. In a large planar cell
stack 300, the reagent containers or tanks 102 and 103 are placed
at some distance from the planar cell stack 300 and may include
heat transfer loops (not shown) for cooling or heating the
reactants and the discharge products. For purposes of illustration,
the detailed description refers to a planar electrolytic cell 200
and planar cell stacks 300; however, the scope of the method and
the system 100 disclosed herein is not limited to the planar
electrolytic cell 200 or planar cell stacks 300 but may be extended
to other configurations of flow batteries and fuel cells known in
the art, for example, a tubular stack.
[0151] FIG. 4 exemplarily illustrates a discharge and regeneration
cycle as flows of energy, materials, and processes, showing the
discharge unit 104 with hydrogen as an example of the reducer 401,
an aqueous HXO.sub.n as an example of an aqueous multi-electron
oxidant (AMO), and the regeneration system 106 using MZ as an
example of a buffer in a base form. In FIG. 4, HXO.sub.n refers to
the AMO in the acid form, MXO.sub.r, refers to the AMO in the salt
form, HZ refers to the buffer in the acid form, and MZ refers to
the buffer in the base form. The flow of materials is represented
using solid arrows and the flow of electric energy is represented
using dotted arrows. Electric power is used during the process of
concentrating 412 by reverse osmosis although other sources, for
example, heat can also be used, for example, for evaporation.
[0152] The discharge unit 104 is similar to the polymer electrolyte
membrane fuel cells (PEMFCs), with a low cost Pt-free porous carbon
positive electrode 205a, hydrophilic positive diffusion layer 201a,
and with the air feed line replaced by an aqueous multi-electron
oxidant (AMO) line, for example, a HBrO.sub.3 line. This
combination may provide about 1,200 Ah/kg.times.1.42 V=1,704 Wh/kg
theoretical energy density, and about 426 Wh/kg system-level energy
density for about 5% w/w compressed H.sub.2, and about 50% w/w
aqueous HBrO.sub.3. The pH-driven disproportionation reactions
allow solution-phase transformation from a high energy bromate to
high power bromine during discharge, for example, at a pH<2. The
discharge unit 104 also allows for a partial recharge via
electrooxidation of bromide into bromine in the discharge fluid
which is useful, for example, for regenerative breaking.
[0153] During the discharge process, the discharge unit 104 is
supplied with the reducer 401, for example, H.sub.2, and the acidic
oxidant fluid comprising the aqueous multi-electron oxidant (AMO)
in acid form HXO.sub.n 402, for example, HBrO.sub.3. In an
embodiment, the AMO, for example, HXO.sub.n 402 is mixed with a
buffer in acid form HZ such as H.sub.3PO.sub.4, carried over from
the regeneration step. The reducer 401 donates electrons to the
negative electrode 205b, also referred to as an "anode", and splits
into ions. The reaction at the negative electrode 205b is, for
example, 3H.sub.2-6e.sup.-=6H.sup.+. The external electric circuit
203 conducts and transfers electrons from the negative electrode
205b to the positive electrode 205a. The reaction at the positive
electrode 205a, also referred to as a "cathode", is, for example,
3Br.sub.2+6e.sup.-=6Br.sup.-, or when combined with the
comproportionation reaction the catholyte, for example,
BrO.sub.3.sup.-+6e.sup.-+6H.sup.+.dbd.Br.sup.-+3H.sub.2O. The
aqueous multi-electron oxidant accepts the electrons at the
positive electrode 205a for producing the electric current in the
external electric circuit 1 203. The discharge unit 104 releases
403 HX, for example, HBr and the buffer HZ in the acidic form, if
the buffer HZ is added initially, and generates electric current in
the external electric circuit 1 203. The electrolyte layer 205c
provides for a movement of the ions between the negative electrode
205b and the positive electrode 205a. At a steady state, the
electric current transferred through the discharge unit 104 is
equal to the electric current through the external electric circuit
1 203.
[0154] The thermodynamics of the discharge process is illustrated
herein using the example of H.sub.2--HBrO.sub.3 reaction. Bromate
is a good aqueous multi-electron oxidant (AMO) since it provides a
good thermodynamic efficiency (Ebromate/Ebromine) and the
corresponding bromine/bromide couple has a fast electrode kinetics
even on inexpensive carbonaceous electrodes. Since bromine reacts
on the electrode 205a and bromate is the energy storing species in
the oxidant fluid tank 102 exemplarily illustrated in FIG. 1, the
fraction of the hydrogen-bromate system energy that can actually be
converted into electrical energy is less than 100%. To estimate the
fraction of energy, that is, the projected energy efficiency, the
standard potentials of the couples of interest are used:
5Br.sub.2+10e.sup.-=10Br.sup.-
E.degree..sub.A=+1.0873 V (7)
5Br.sub.2+5H.sub.2=10HBr
.DELTA.G.degree..sub.A=10F E.degree..sub.A (8)
and
2BrO.sub.3.sup.-+12H.sup.++10e.sup.-=Br.sub.2(1)+6H.sub.2O
E.degree..sub.B=+1.48 V (9)
2HBrO.sub.3+5H.sub.2.dbd.Br.sub.2(1)+6H.sub.2O
.DELTA.G.degree..sub.B=10F E.degree..sub.B (10)
[0155] The energy stored on-board is given by:
3H.sub.2+HBrO.sub.3.dbd.HBr+3H.sub.2O
.DELTA.G.degree..sub.C=.DELTA.G.degree..sub.A/5+.DELTA.G.degree..sub.B
(11)
[0156] The electric power produced by the discharge unit 104 is
given by equation (8). The ratio of the electric power produced in
the discharge unit 104 to the chemical energy of the reagents in
the tanks 102 and 103 exemplarily illustrated in FIG. 1, gives the
projected discharge efficiency:
MDE=.DELTA.G.degree..sub.A/.DELTA.G.degree..sub.C=.DELTA.G.degree..sub.A-
/(.DELTA.G.degree..sub.A/5+.DELTA.G.degree..sub.B)=10F
E.degree..sub.A/(2F E.degree..sub.A+10F
E.degree..sub.B)=E.degree..sub.A/(E.degree..sub.B+E.degree..sub.A/5)=1.08-
73/(1.0873+1.48/5)=78%
[0157] For the homogeneous disproportionation and/or
comproportionation:
HBrO.sub.3+5HBr=3Br.sub.2+6H.sub.2O
.DELTA.G.degree..sub.D=(.DELTA.G.degree..sub.B-.DELTA.G.degree..sub.A)/2-
=5F(E.degree..sub.B-E.degree..sub.A)=5F*0.3927V=379 kJ/mol
K.sub.C=[Br.sub.2].sup.3/[H.sup.+].sup.6[BrO.sub.3.sup.-][Br.sup.-].sup.-
5=exp(-.DELTA.G.degree..sub.D/RT)=exp(-153)=10.sup.-66.4
[0158] The equilibrium constant K.sub.C comprises [H+] and can be
used at any pH.
[0159] For RT=2.479 kJ/mol, the critical pH at which
[Br.sub.2].sup.3/[H.sup.+].sup.6[BrO.sub.3.sup.-][Br.sup.-].sup.5=1,
is 11. Thus, for the comproportionation reaction to occur, the
solution pH can be brought below 11; however due to the formation
of an intermediate hypobromite, which is kinetically stable above
the acid dissociation constant pKa (HBrO)=8.6, and due to a slow
rate of comproportionation at neutral pHs, a lower pH value such as
below 3, is used. In acidic solutions in the discharge unit 104,
the comproportionation reaction is strongly favored.
[0160] Several embodiments of the method of regeneration of the
H.sub.2-aqueous multi-electron oxidant (AMO) chemistry are
disclosed herein. For purposes of illustration, the detailed
description refers to a method of regeneration using HBrO.sub.3 as
the AMO in the acid form, however the scope of the method and the
system 100 disclosed herein is not limited to HBrO.sub.3 but can be
extended to include other AMOs such as HClO.sub.3, HClO.sub.4,
HBrO.sub.4, HIO.sub.3, HIO.sub.4, etc. The regeneration process
starts with neutralization 404 of the acid in the discharge fluid
with a base, for example, HBr with LiOH or another base such as
Li-3-(N-morpholino) propanesulfonic acid (MOPS) in the
neutralization reactor 109 of the regeneration system 106
exemplarily illustrated in FIG. 1. Neutralization 404 is a chemical
reaction in which a base and an acid react to form a salt. The
neutralization 404 of the discharge fluid, HX, with the base, MOH,
is performed in the neutralization reactor 109. In an embodiment,
some process steps of the energy cycle, for example, neutralization
404, and electrolysis and disproportionation 406 can be combined in
a single reactor. The base is regenerated at the negative electrode
of the electrolyzer 107 of the regeneration system 106 during the
electrolysis process.
[0161] The neutralization 404 of the discharge fluid acid with a
base, for example, HBr with LiOH or Li-3-(N-morpholino)
propanesulfonic acid (MOPS) produces 405 a solution of a salt MX
such as LiBr. The solution of a salt such as LiBr and H.sub.2O
undergoes electrooxidation into the intermediate oxidant such as
Br.sub.2 at the positive electrode while H.sub.2 and LiOH or
H.sub.2 and Li-MOPS are produced at the negative electrode. The
process of electrolysis 406 is accompanied by the release of the
reducer 401, for example, hydrogen in stoichiometric amounts which
is used as the reducer 401 in the discharge unit 104. In the case
of Br.sub.2, if the pH at the positive electrode is maintained near
8, a disproportionation 406 to bromate occurs, for example, with a
LiOH base:
3Br.sub.2+6LiOH=5LiBr+LiBrO.sub.3+3H.sub.2O (12)
[0162] Electrolysis 406 of the LiBr+H.sub.2O solution and the
disproportionation 406 reactions proceed in a batch mode, a cascade
flow mode, or a cyclic flow mode till most of the LiBr is converted
into LiBrO.sub.3. The residual LiBr may or may not be removed. In
the latter case, the product LiBrO.sub.3 will have some LiBr
present. In an embodiment, a provision to remove the residual LiBr
is provided. In an embodiment, a buffer is used during the cyclic
process in order to maintain the pH at a near constant value which
is optimal for the disproportionation 406, for example,
6<pH<10 or near 8. In another embodiment, the buffer
comprises hydrogen phosphate and dihydrogen phosphate. In another
embodiment, the buffer comprises one or more of Good's buffers,
other amines, other tertiary amines, and nitrogen heterocycles. In
another embodiment, the buffer comprises a phosphonic acid
derivative. In another embodiment, the buffer comprises a lithium
counter-cation. H.sub.2PO.sub.4.sup.2- has a proper pH for
disproportionation and is chemically compatible with the rest of
the chemistry throughout the whole energy cycle if, for example,
sodium is used as the counter cation.
[0163] In the regeneration process, the electrooxidation step or
electrolysis 406 is followed by the disproportionation 406 of the
intermediate oxidant such as bromine. The disproportionation 406 is
the reverse of the comproportionation of the aqueous multi-electron
oxidant (AMO) discharge and is favored at a higher pH than the
comproportionation of discharge. In the beginning of electrolysis
406 of hydrobromic acid, Br.sub.2 and H.sub.2 are formed in the
equal molar amounts:
HBr=1/2H.sub.2+1/2Br.sub.2 (13)
[0164] If there is no buffer present, the anolyte turns acidic due
to hydrolysis:
Br.sub.2+H.sub.2O.dbd.HBr+HBrO (14)
[0165] In a reactor with a cation-selective membrane, the anolyte
compartment turns into a solution of HBrO through the equations
(13) to (14) route. A further oxidation of HBrO does not proceed on
a carbon electrode at low over-voltages; however, a further
disproportionation 406 of HBrO can occur in the aqueous phase
yielding bromate:
3HOBr=2HBr+HBrO.sub.3 (15)
or combined: 3Br.sub.2+6OH-=5Br.sup.-+BrO.sub.3.sup.-+3H.sub.2O
(16)
[0166] The disproportionation 406 of Br.sub.2 to BrO.sub.3.sup.-
and Br.sup.- is strongly favored thermodynamically at pH above 11,
which is equivalent to 1 mM of OH.sup.-, although this reaction has
the maximal rate at pH near 8 due to the formation of an
intermediate hypobromite which is stable toward further
disproportionation 406 at pH>pKa(HBrO)=8.8. However, even if the
HBr produced in equation (15) is consumed in equation (13), one
proton per BrO.sub.3.sup.- will not get electro-reduced due to the
lack of an anodic counter-process unless both faradaic and voltage
efficiency are sacrificed by running oxygen evolution reactions
(OER) or other parasitic process to make O.sub.2 and OH.sup.-. The
resulting pH drop due to the formation of a strong acid HBrO.sub.3
will cause equations (15) and (16) to proceed to the left thus
ceasing the regeneration when bromine's average oxidation state is
around +1. Thus, a disproportionation 406 of Br.sub.2 to Br (+5)
requires an introduction of an external base. In the case of an
anionic base with a counter cation, this will result in formation
of a bromate salt rather than of bromic acid. The hydroxide
generated during hydrogen evolution reaction (HER) on the negative
counter electrode can be used as the needed base or to make the
needed base. Li.sup.+ can be used as a counter-cation to achieve
high solubilities of the salts involved such as bromide and
bromate. A pH buffer comprising, for example, a dissolved
phosphonate and/or one or more of Good's buffers is used to prevent
spatial and temporal deviations of pH from the value of near 8
within the electrolysis-disproportionation (ED) reactor 107. The
resulting product, for example, LiBrO.sub.3, 407 needs to be
converted or partially converted to the electrochemically active
aqueous multi-electron oxidant (AMO), for example, HBrO.sub.3. This
can be accomplished via a solution-phase cation exchange process in
the ion exchange reactor 108, for example, the orthogonal ion
migration across laminar flow (OIMALF) reactor with a simultaneous
conversion of the input discharge fluid into a salt, for example,
HBr into LiBr. LiBrO.sub.3 is converted into HBrO.sub.3 using the
orthogonal ion migration across laminar flow (OIMALF) or the ion
exchange process 408. The buffer is converted from the acid form
into a base form simultaneously.
[0167] The continuous electrolysis-disproportionation (ED)
406-orthogonal ion migration across laminar flow (OIMALF) process
408 disclosed herein for the regeneration of HBrO.sub.3 from HBr
ends with an ion exchange of the base form of the oxidant fluid
comprising, for example, LiBrO.sub.3 into the acid form of the
oxidant fluid comprising, for example, HBrO.sub.3 in the ion
exchange reactor 108 while realizing hydrogen at the negative
electrode and consuming hydrogen at the positive electrode as
disclosed in the detailed description of FIG. 10B. The principle of
OIMALF 408 is identical to ion suppression in anion chromatography.
In an embodiment, The OIMALF process 408 generates and consumes
H.sub.2 within the OIMALF reactor or the ion exchange reactor 108.
The OIMALF process 408 of converting MXO.sub.n into HXO.sub.n, for
example, LiBrO.sub.3 into HBrO.sub.3 avoids cumbersome chemical
separation and ion exchanger regeneration steps. The net reaction
of the ion exchange or the OIMALF process 408 is, for example,
LiBrO.sub.3+HA=HBrO.sub.3+LiA, where HA is a source of protons, for
example, water, phosphoric acid, dihydrogen phosphate, one or more
of Good's buffers, etc. The regeneration system 106 is connected to
an external electric circuit 2 409 which provides electric power
for the OIMALF process 408. The base, for example, MOH or LiA 410
generated as a result of the OIMALF process 408 is used during the
process of neutralization 404 of the discharge fluid, for example,
HBr. In an embodiment, LiBrO.sub.3 is converted into HBrO.sub.3
using ion exchange on resins. This is followed by electrolysis (E)
406 of LiBr into bromine and, in the same ED reactor 107 or another
reactor, by disproportionation (D) 406 of the halogen into halate
and halide in a suitable buffer, for example, a lithium hydrogen
phosphate buffer, one or more of Good's buffers, or any combination
thereof, near pH 8. The electrolysis-disproportionation 406 cycle
continues in the same flow or batch ED reactor 107 till
[bromide]/[bromate] ratio decreases, for example, below 0.05. The
resulting solution can be concentrated 412, for example, using
reverse osmosis or evaporation. The concentrated solution, for
example, approximately 10M LiBrO.sub.3 solution, the concentration
of which is limited by the solubility of LiBrO.sub.3 at the
operating temperature, for example, of about 20.degree. C., then
goes back into the ion exchange reactor 108 such as the OIMALF
reactor, where Li.sup.+ in LiBrO.sub.3 is exchanged for H.sup.+
from the incoming HBr, thus producing, for example, a solution
comprising 0.5M HBrO.sub.3 and 9.5M LiBrO.sub.3. The hardware
components of the hydrogen-bromate energy cycle disclosed herein
comprise analytical chemical detectors (not shown) used for process
monitoring and control.
[0168] The resulting concentrated HBrO.sub.3 solution is used as
the aqueous multi-electron oxidant (AMO) for the discharge unit
104. The net result of regeneration for an exemplary combination of
the AMO and the buffer LiA is:
HBr+3H.sub.2O=(electricity in two places,LiA
recycled)=3H.sub.2+HBrO.sub.3 (17)
[0169] The electrolysis-disproportionation (ED) 406-orthogonal ion
migration across laminar flow (OIMALF) 408 process has a reasonably
high projected energy efficiency of about 70%. The oxidant fluid
comprising one or more forms of the aqueous multi-electron oxidant
(AMO) may be further concentrated. The commercial process of
concentrating 412 HBrO.sub.3 uses evaporation, with an estimated
energy loss of approximately 10-15% if heat exchangers are used.
The evaporation is likely to lead to the loss of volatile bromine
species and evaporation may be less energy efficient than reverse
osmosis (RO). The reverse osmosis process requires overcoming of
the osmotic pressure, for example, of 536 bars, which is possible
in a cascade flow mode with commercial supported ion exchange
membranes. The minimal energy expense at an infinitely slow
filtration rate is 6.6% of the energy content of the product 50%
w/w HBrO.sub.3 and 3H.sub.2. Due to a finite flow rate, the
regeneration process disclosed herein uses optimization of the unit
size, power, and operating pressure in terms of the energy
efficiency and capital cost.
[0170] Since the kinetics of all the processes involved in the
chemical cycle of the discharge unit 104 and the economic figures
for polymer electrolyte membrane fuel cells (PEMFCs) are well
known, quantitative predictions on the performance of the discharge
unit 104 disclosed herein can be derived. The data for the
discharge unit 104 disclosed herein, also referred to as a flow
battery or a discharge flow cell, is calculated for a
one-dimensional model with a flow-by smooth carbon cathode for a
constant solution composition outside of the diffusion boundary
layer as well as from the experimental data disclosed in the
detailed description of FIG. 13 and using other relevant
performance figures from the PEMFCs literature are compared with
the Advanced Research Projects Agency-Energy (ARPA-E) targets. The
projected performance of the discharge unit 104 and the ARPA-E
targets are shown in Table 1 below.
TABLE-US-00001 TABLE 1 ARPA-E Projected Parameter Units Target
Value Manufacturing cost $/kWh <100-125 140 Effective specific
energy Wh/kg >150 570 Effective energy density Wh/L >230 900
Effective specific power on W/kg >300 690 discharge, 80% DOD/30
s Cycle life at 80% depth of cycles >1000 1000 discharge (DOD)
Calendar life years >10 6 Operating temperature .degree. C.
>-30 -40
[0171] The discharge unit 104 meets the requirements as the primary
power source for electric vehicles (EVs). The one-way discharge
efficiency of about 85% at the target power of about 0.05
W/cm.sup.2 is found using a precious metal free smooth glassy
carbon rotating disk electrode (RDE) as disclosed in the detailed
description of FIG. 12, FIG. 13, FIG. 21, and FIG. 25. The
discharge unit 104 disclosed herein has a short refueling time in
EV applications when combined with off-board regeneration, which is
based on the disproportionation 406 of Br.sub.2 electrochemically
regenerated from the discharged LiBr, HBr, etc.
[0172] In Table 1, the projected temperature refers to a cold-start
up and is limited by the aqueous multi-electron oxidant (AMO)'s
freezing point. The cost figures are calculated based on the design
of modern polymer electrolyte membrane fuel cells (PEMFCs) minus
the cost of the Pt catalyst on the positive electrode 205a. The
cost figures do not account for the economy-of-scale discount. The
parameters refer to the system 100 exemplarily illustrated in FIG.
1, with H.sub.2 storage as a 5% w/w metal hydride and 50%
w/w/HBrO.sub.3 and 78% discharge efficiency at 0.5 W/cm.sup.2
power. The power is calculated for a smooth flow-by carbon cathode
on the basis of kinetic parameters reported in the literature and
by assuming membrane resistance of 0.1 ohm/cm.sup.2 as exemplarily
illustrated in FIG. 13. The durability number is the operational
life and not the calendar life. The projected durability of the
discharge unit 104 is limited by the degradation of Pt on the
hydrogen anode accounting for the aqueous multi-electron oxidant
cross-over at open circuit potential (OCP) on the basis of relevant
data for the PEMFCs. Purging both the electrodes 205b and 205a with
on-board water on shut-downs can increase the projected
durability.
[0173] The results of system level modeling in the Advanced
Research Projects Agency-Energy (APRA-E) metrics show that the most
conservative estimate for the energy density of the 5% H.sub.2-50%
HBrO.sub.3 on-board system is 426 Wh/kg, which is 2.8 times larger
than the ARPA-E target of 150 Wh/kg and 6.5 times larger than the
corresponding number for lithium iron phosphate (LFP) batteries in
Tesla Roadster.RTM. of Tesla Motors, Inc. The estimate of the
specific energy of the discharge system 101 disclosed herein
depends on the type of H.sub.2 storage and varies from 208 Wh/L for
350 bar gas, 339 Wh/L for 5% w/w metal hydride and 400 Wh/L for
liquid H.sub.2. For a 150 kWh sport utility vehicle (SUV), the
system volume is 750, 970, and 2,000 L for liquid, metal hydride
and compressed H.sub.2, respectively, of which only 300 L is the
aqueous multi-electron oxidant (AMO) tank. These values fall in
between the volumes of the combination of a gasoline tank with an
internal combustion engine (ICE) and the combination of a lithium
ion battery (LIB) with an electric engine. Regardless of the
H.sub.2 storage method, the system-level energy density of the
discharge system 101 meets the ARPA-E target of 230 Wh/L.
[0174] The energy and material cycle exemplarily illustrated in
FIG. 4 incorporates an affordable method to regenerate both the
reducer such as hydrogen (H.sub.2) reducer 401 and the aqueous
multi-electron oxidant (AMO), for example, bromate
(BrO.sub.3.sup.-) from the discharge fluid, for example, aqueous
solution comprising bromide (Br.sup.-), without reliance on fossil
fuels, thus resolving the need for a hydrogen source which is
injurious to the development of hydrogen economy. Although the
gravimetric specific energy of hydrogen is high, the volumetric
energy density of hydrogen is low even at the highest practically
achievable pressures and hydrogen storage. Both high pressure
carbon composite cylinder and metal hydrides tank may satisfy the
mass and the volume requirements. In addition, 20% of the required
H.sub.2 can be regenerated on board from the discharge fluid using
metals, which in turn can be regenerated by electrolysis 406 of
MBr.sub.2 off-board:
M(M=Zn,Sn,Fe,etc.)+2HBr.fwdarw.MBr.sub.2+H.sub.2
[0175] The safety of the H.sub.2-aqueous multi-electron oxidant
(AMO) discharge system 101 is also considered. Since the two
reagents, that is, the AMO and hydrogen do not come in contact
under normal operating conditions and only small amounts of H.sub.2
and AMO may contact each other without reaction in an accident
within the discharge unit 104, the safety of H.sub.2 and of the AMO
such as 5-20 M aqueous LiBrO.sub.3, is individually considered.
On-board hydrogen is safer than gasoline systems due to faster
escape in an open space. Bromates are moderately toxic, comparable
to nitrites, and although suspected carcinogens, are widely used as
additives in bread flour in the United States. In an outdoor
environment bromates eventually turn into benign bromides.
Moreover, bromates are listed as oxidants and are corrosive but not
explosive. The intermediate bromic acid, present in the ion
exchange reactor 108 and discharge unit 104, is classified as an
oxidizer, but not as an explosive. Bromic acid can be safely
concentrated 412 by vacuum-distillation at 80.degree. C. up to 50%
w/w. From a practical handling viewpoint, HBrO.sub.3 is similar to
HNO.sub.3 although the former does not stain skin. HBr is a well
known corrosive agent having a long history of safe use in various
applications. The system energy density of the H.sub.2-AMO
discharge system 101 disclosed herein is about 6 times larger when
compared to the Li-ion battery pack of the Tesla Roaster.RTM. and
hence allows for the incorporation of additional safety features
such as a collision and/or spill-proof enclosure without
jeopardizing the driving range and power of the electric vehicle.
The risk of using such a corrosive oxidant, that is, HBrO.sub.3 if
it is present on-board only in a small amount in the discharge unit
104 can be mitigated.
[0176] The H.sub.2-aqueous multi-electron oxidant (AMO) discharge
system 101 disclosed herein has the following advantages: high
system energy content, for example, about 426 Wh/kg and about
200-400 Wh/L, which is 6 times greater than that of a lithium ion
battery (LIB) pack; high power density, for example, of about 690
W/kg, which exceeds the Advanced Research Projects Agency-Energy
(ARPA-E) target more than twice; mechanic refill: can be refilled
at a pump in less than 5 min; long range, for example, of about 300
miles per refill with about 120 L storage; aqueous chemistry which
is intrinsically safer than Li-ion batteries; low materials and
manufacturing cost, for example, of about $120/kWh and about
$115/kW; low total cost of ownership (TOC), for example, of about
$0.15/mile for a 6 year lifetime of the discharge system 101 and
the regeneration system 106; simultaneous stoichiometric
regeneration of H.sub.2 and AMO using electricity as the only input
and without irreversibly consuming other chemicals and without
generating chemical waste.
[0177] FIGS. 5A-5B exemplarily illustrate a table showing different
reactions used or considered for electrochemical energy storage and
energy conversion. The characteristics of the redox reactions
comprise, for example, theoretical charge density in
(ampere*hour)/kilogram (A*h/kg), standard equilibrium cell
potential (Eeq) in volts (V), the reactants' theoretical energy
density in watt-hour per kilogram (Wh/kg), the oxidant's solubility
(weight percentage %), maximum practical energy density (ED) in
Wh/kg, exchange current in milliampere (mA)/square centimeter
(cm.sup.2), energy efficiency in %, and practical energy
efficiency.times.energy efficiency in Wh/kg. As exemplarily
illustrated in FIGS. 5A-5B, some H.sub.2-aqueous multi-electron
oxidant (AMO) chemistries can afford four times higher theoretical
energy densities than batteries with solid electroactive materials,
for example, lithium ion batteries. The practical ratio may be as
much as 10 due to a higher packing ratio in a flow battery or in
the discharge system 101 exemplarily illustrated in FIG. 1, but not
in batteries with solid electroactive materials (SEAM) such as
lithium ion batteries. The practical energy density includes water
in the concentrated aqueous multi-electron oxidant (AMO). The
maximum energy density includes oxidant solubility but not H.sub.2
storage and energy efficiency in %. For oxohalic acids, the
projected energy efficiency is defined as the ratio of the standard
equilibrium potentials of halogen/halide and oxohalate/halide.
[0178] FIG. 6 exemplarily illustrates mass flows in a single
electrolytic cell 200 exemplarily illustrated in FIG. 2, of an
electrolytic cell stack 105 of the discharge unit 104 exemplarily
illustrated in FIG. 1, during discharge with H.sub.2 as the fuel
and HXO.sub.3 as the oxidant. In this example, the reducer is
H.sub.2. The aqueous multi-electron oxidant (AMO) is HBrO.sub.3.
The standard equilibrium potential for this combination is 1.42 V
and the theoretical energy density is, for example, about 1,705
Wh/kg, which is 4.4 times higher than that of lithium iron
phosphate/graphite chemistry. HBrO.sub.3 can be used, for example,
pumped as an aqueous solution which is stable up to, for example,
about 55% w/w concentration (938 Wh/kg). In another embodiment,
HIO.sub.3 can be used as the AMO. The equilibrium voltage is 1.19V
and the energy density is 1,052 Wh/kg and the room temperature
solubility is 74% at 20.degree. C. HIO.sub.3 has faster kinetics,
that is, electrolytic cell power, but the intermediate I.sub.2 is
solid and reduces at a lower potential thus lowering the efficiency
of the energy cycle. The discharge unit 104 is configured similar
to a polymer electrolyte fuel cell but with a hydrophilic liquid
diffusion layer on the positive electrode 205a. A cation exchange
membrane, for example, Nafion.RTM. of E. I. du Pont de Nemours and
Company Corporation is used as the electrolyte 205c. The cation
exchange membrane minimizes the crossover or rejects anionic
species and assures nearly single or exclusive hydrogen ion
conductivity and high power density. The operating temperature of
the discharge unit 104 is maintained above the ambient temperature
to facilitate heat rejection and electrode kinetics. The negative
electrode 205b, for example, the hydrogen side of the
electrolyte-electrode assembly 205 has a standard design and
prepared by standard methods known to those skilled in the art of
polymer electrolyte fuel cells (PEFCs).
[0179] The design of the positive electrode 205a is also similar to
polymer electrolyte fuel cell (PEFC) electrodes but the positive
electrode layer 205a is paired with a liquid diffusion layer on the
back since the reagents and products on the positive electrodes
205a are in the liquid phase in contrast to an air-supplied proton
exchange membrane fuel cell (PEMFC). In an embodiment, a parallel
flow field is used, although numerous other designs, for example,
single, multiple, serpentine, meander, inter-digitated, etc., known
to those skilled in the art are employed. In an embodiment, the
walls of the flow field of the positive electrode 205a are made of
a porous carbon and used without a liquid diffusion layer, or the
whole flow field can be made of a solid material and a liquid
diffusion layer, for example, made of a porous carbon sheet and can
be placed between the flow field and the electrolyte-electrode
assembly 205. The positive side of the membrane and the positive
wall of the bipolar plate 202 exemplarily illustrated in FIGS. 2-3,
can be coated with catalytic layers. The intermediate oxidant, for
example, Br.sub.2 can be regenerated by a direct electrochemical
process on the positive electrode 205a. Suitable positive
electrodes 205a comprise, for example, one or any combination of
carbon, platinum, PbO.sub.2, RuO.sub.2, dimensionally stable anode
(DSA), and other oxides, metals and non-metals, including
conductive diamond.
[0180] FIG. 7 illustrates a method for producing electric power
from an aqueous multi-electron oxidant (AMO) and a reducer and for
simultaneously generating a discharge fluid. The method disclosed
herein provides 701 the discharge system 101 comprising the oxidant
fluid tank 102 comprising the AMO, the reducer fluid tank 103
containing the reducer, and the discharge unit 104 as exemplarily
illustrated and disclosed in the detailed description of FIG. 1.
The method for producing electric power facilitates 702 discharge
of the discharge unit 104. Discharge occurs by transferring 702a
electrons from the positive electrode 205a of the 5-layer
electrolyte-electrode assembly 206 exemplarily illustrated in FIG.
2, to the AMO and transferring 702b electrons from the reducer to
the negative electrode 205b of the 5-layer electrolyte-electrode
assembly 206 exemplarily illustrated in FIG. 2, to produce 702c an
electric power or (I*U.noteq.0) or a sustainable electric current,
that is, a direct current (DC) in an external electric circuit 203
connected to the terminals of the discharge unit 104 and
transferring ions between the positive electrodes 205a and the
negative electrode 205b of the 5-layer electrolyte-electrode
assembly 206, thus conserving the charge. The discharge is
facilitated on the positive electrode 205a of the 5-layer
electrolyte-electrode assembly 206, for example, by one or more of
electrolysis, electrocatalysis, a solution-phase chemical reaction,
a solution-phase comproportionation, a solution-phase redox
catalysis, an acid-base catalysis, and any combination thereof.
[0181] The discharge unit 104 consumes the aqueous multi-electron
oxidant (AMO) and the reducer supplied from their respective
storage tanks 102 and 103 to generate the discharge fluid stored in
a discharge fluid storage tank (not shown) and electric power in
the external electric circuit 203. The discharge fluid comprises,
for example, one or more of water, an acid form of the buffer, a
base form of the buffer, a halogen, a hydrogen halide, a halogen
oxoacid, and any combination thereof. Since the discharge fluid
coming out of the discharge unit 104 is not water or not only
water, the discharge fluid is not disposed into surroundings but
collected in a discharge fluid storage tank or container (not
shown) to be regenerated later into the reducer and the AMO. The
buffer is in the acid form during the discharge with a pH.ltoreq.7.
The acid form of the buffer is, for example, one or more of
phosphoric acid, a dihydrogen phosphate of lithium, Good's buffers,
and any combination thereof.
[0182] Consider an example where the aqueous multi-electron oxidant
(AMO) is bromic acid and the reducer is hydrogen. The processes in
the discharge unit 104 are: oxidation of hydrogen on the negative
electrode 205b, transport of a hydrogen ion with water from the
negative electrode 205b to the positive electrode 205a through a
cation exchange membrane, comproportionation of bromate with
bromide in the fluid near the positive electrode 205a, and
reduction of bromine on the positive electrode 205a. Protons or
other positive ions are transferred through the cation exchange
membrane from the negative electrode 205b to the positive electrode
205a, for example, due to a concentration gradient. Electrons are
transferred from the negative electrode 205b to the positive
electrode 205a, thus producing electric power, that is, current and
voltage in the external electrically conducting connecting circuit,
that is, the external electric circuit 203.
[0183] FIG. 8 illustrates a method for regenerating the aqueous
multi-electron oxidant (AMO) and the reducer in stoichiometric
amounts from a discharge fluid using electric power. The method
disclosed herein provides the regeneration system 106 comprising
the neutralization reactor 109, the electrolysis-disproportionation
(ED) reactor 107, the ion exchange reactor 108, and the
concentrating reactor 112 exemplarily illustrated in FIG. 1. The
neutralization reactor 109 neutralizes 801 the discharge fluid,
produced by the discharge unit 104 exemplarily illustrated in FIG.
1. The electrolysis-disproportionation proceeds in the ED reactor
107 configured to electrolyze 802 the solution of the salt form of
the discharge fluid into an intermediate oxidant, for example,
Br.sub.2, at a positive electrode in the ED reactor 107. The
regeneration system 106 performs electrolytic decomposition of the
discharge fluid, for example, HBr into the reducer, for example,
H.sub.2 and the intermediate oxidant, for example, Br.sub.2. The
electrolysis process releases the reducer and the base form of the
buffer at a negative electrode of the ED reactor 107 while
producing a salt form of the AMO at the positive electrode via a
series of chemical and electrochemical reactions. The ED reactor
107 is further configured to disproportionate 802 the intermediate
oxidant produced at the positive electrode with an excess of the
base form of the buffer to produce the salt form of the AMO, while
simultaneously releasing a stoichiometric amount of the base form
of the buffer for neutralization. The base form of the buffer is,
for example, phosphonate, hydrogen phosphate, an amine, a tertiary
amine, a morpholine derivative, etc. The cation of the buffer is,
for example, lithium, other alkali metal, substituted ammonium,
imidazolium, organic cation, etc. Other examples of the buffer
components are hydroxide, a lithium cation, a magnesium cation,
etc. In an embodiment, the buffer is one or more of the Good's
buffers. The regeneration system 106 continues 803 the cycle of
electrolysis-disproportionation in a single ED reactor 107 of a
cascade of ED reactors till the desired degree of conversion of,
for example, bromide into bromate is achieved.
[0184] The regeneration system 106 also converts the intermediate
oxidant, for example, bromine produced at the positive electrode or
electrodes of the electrolysis-disproportionation (ED) reactor 107
into the aqueous multi-electron oxidant (AMO) in the salt form,
such as bromate, using a chemical process, for example, a
homogeneous chemical reaction such as a disproportionation reaction
driven by a pH change, or a homogeneous oxidation by a mediator.
The electrolysis-disproportionation (ED) reactor 107 of the
regeneration system 106 simultaneously releases on its negative
electrode or electrodes a stoichiometric amount of hydrogen and of
the base form of the buffer for the disproportionation. In an
embodiment, the salt form of the AMO is converted into the acid
form of the AMO in an orthogonal ion migration across laminar flow
(OIMALF) reactor by recycling the reducer, for example, H.sub.2
produced at the negative electrode and consumed at the positive
electrode of the OIMALF reactor. Other embodiments for recycling or
partially recycling H.sub.2 during the regeneration are possible as
disclosed in the detailed description of FIGS. 10A-10B. The ion
exchange process proceeds in the ion exchange reactor 108
configured to convert 804 the AMO in the salt form, for example,
LiBrO.sub.3 into the AMO in the acid form, for example, HBrO.sub.3.
All forms of the AMO is referred herein as the AMO. The conversion
of the salt form of the AMO produced at the positive electrode into
the acid form of the AMO is performed by an ion exchange process,
for example, an electric field driven orthogonal ion migration
across laminar flow (OIMALF) method known to those skilled in ion
chromatography, in the ion exchange reactor 108. In an embodiment,
the conversion of the salt form of the AMO produced at the positive
electrode into the acid form of the AMO is accompanied by a
conversion of the base form of the buffer into the acid form of the
buffer. The AMO and the reducer are stored in the regeneration
system 106 until they are transferred to the discharge system 101
exemplarily illustrated in FIG. 1. The acid or salt form of the AMO
is concentrated 805 in the concentrating reactor 112 to remove
water produced on the positive electrode during the discharge and
to remove water introduced with the buffer during
electrolysis-disproportionation. The AMO is regenerated via an
electron transfer to the positive electrode with or without a
combination with a solution-phase process such as
disproportionation; and the reducer is regenerated at the negative
electrode of the ED reactor 107. The buffer maintains or stabilizes
the pH of the discharge fluid at an optimal level or a constant
value, for example, between 7 and 11 or at pH 8 for
disproportionation in the ED reactor 107. The buffer in the base
form is selected from a group comprising, for example, an alkali
metal hydroxide, an alkali metal hydrogen phosphate, an alkali
metal salt of one of Good's buffers, substituted phosphonic acid,
and any combination thereof. The alkali metal is, for example,
lithium or sodium. The base form of the buffer, if its structure
permits, is converted into an acid form or a neutral form during
ion exchange.
[0185] In an embodiment, the regeneration of the aqueous
multi-electron oxidant (AMO) and/or the reducer is facilitated, for
example, by an electrocatalyst, a solution-phase redox mediator
such as chlorine/chloride, a pH-dependent solution-phase
disproportionation, etc., or any combination thereof. In an
embodiment, the conversion of the intermediate oxidant, for
example, bromine, into one or more forms of the AMO is facilitated
by a buffer in the disproportionation unit 107b. In another
embodiment, a chloride mediator facilitates regeneration of the AMO
from the discharge fluid. In another embodiment, the regeneration
of the AMO and/or the reducer from the discharge fluid is
facilitated by adding a base to the discharge fluid. The
electrolysis-disproportionation (ED) reactor 107 is configured to
operate in one of multiple modes comprising, for example, a batch
mode, a cascade flow mode, and a cyclic flow mode. The regeneration
system 106 is configured for batch, cyclic or cascade flow modes of
operation, or any combination thereof.
[0186] The electrolysis-disproportionation (ED) reactor 107
converts a discharged product such as bromide, into a salt form of
the aqueous multi-electron oxidant, for example, bromate. The ion
exchange reactor 108 converts the aqueous multi-electron oxidant
such as bromate from the salt form into the acid form. The ion
exchange reactor 108 also converts the discharge fluid from the
acid form into a neutral form. The ED reactor 107 adds a base, for
example, HPO.sub.4.sup.2- to the discharge fluid comprising, for
example, bromide as exemplary illustrated for one specific
chemistry in the no-aqueous multi-electron oxidant
(AMO)-on-negative mode of operation in equation (18) below:
Br.sup.-+H.sub.2PO.sub.4.sup.2-+OH.sup.-=Br.sup.-+HPO.sub.4.sup.2-
(18)
and electrolyzes the resulting alkaline discharge fluid to produce
hydrogen (H.sub.2) and the intermediate oxidant such as
Br.sub.2.
anode:Br.sup.--e.sup.-=1/2Br.sub.2; (19)
cathode:H.sub.2O+e.sup.-.dbd.OH.sup.-+1/2H.sub.2 (20)
[0187] The counter cation used in the process shown in equations
(18)-(19) is one or a combination of an alkali metal, for example,
Na.sup.+, an alkali earth metal, quaternary nitrogen or phosphorus
cations, derivatives of nitrogen heterocycles, and other organic
and inorganic cations.
[0188] The base is regenerated in the catholyte in the course of
the hydrogen evolution reaction. The intermediate oxidant such as
bromine further disproportionates via a reaction with the base, for
example, as follows:
3Br.sub.2+6HPO.sub.4.sup.2-+3H.sub.2O=BrO.sub.3.sup.-+5Br.sup.-+6H.sub.2-
PO.sub.4.sup.2- (21)
[0189] The process of electrolysis-disproportionation as shown in
equations (18)-(21) above continues in a cyclic flow mode or a
cascade flow mode until all or almost all the bromide is converted
into bromate. In the next stage, which can be performed either
on-board within the discharge system 101 or off-board within the
regeneration system 106 or in both systems, the bromate is
converted into bromic acid in the ion exchange reactor 108, for
example, an orthogonal ion migration across laminar flow (OIMALF)
reactor. The salt left over from the disproportionation buffer such
as lithium dihydrogen phosphate is, for example, also converted
into an acid such as phosphoric acid and for example, and is used
in the oxidant fluid with the aqueous multi-electron oxidant
without separation.
[0190] In an embodiment, the intermediate oxidant, for example, a
halogen, is regenerated via an electron transfer at the positive
electrode, and the reducer such as hydrogen is regenerated at the
negative electrode of the electrolyzer 107a of the
electrolysis-disproportionation (ED) reactor 107. In another
embodiment, the intermediate oxidant disproportionates during the
process of regeneration by consuming a base and provides the final
aqueous multi-electron oxidant, for example, a halate in the form
of a salt. The base required for the disproportionation of the
intermediate oxidant can be supplied externally or can be produced
in the course of the cathodic counter reaction, such as hydrogen
evolution, in the electrolyzer 107a of the ED reactor 107. A buffer
is added to either the catholyte chamber or the anolyte chamber or
in both the catholyte chamber and the anolyte chamber of
electrolyzer 107a in order to stabilize the pH at the level optimal
for the disproportionation. A suitable pH of the buffer is between,
for example, 7 and 11 depending on the target aqueous
multi-electron oxidant (AMO). A suitable buffer is, for example, a
solution of Na.sub.2HPO.sub.4 and NaH.sub.2PO.sub.4 in various
ratios and concentrations. Another suitable buffer is one or more
of the Good's buffers, other secondary amine, other amine,
substituted phosphonate, and a nitrogen heterocycle. During the
disproportionation reaction, in the presence of a buffer or a base
containing a cation other than hydrogen, a salt form of the aqueous
multi-electron oxidant, for example, NaBrO.sub.3 is produced.
[0191] The intermediate product, that is, the salt of the aqueous
multi-electron oxidant (AMO) is converted into the acid form in the
ion exchange reactor 108, for example, the orthogonal ion migration
across laminar flow (OIMALF) reactor using one or a combination of
electrolysis, ion exchange on solids, ion exchange in a fluid, and
an electric-field driven OIMALF process. The ion exchange occurs
after the electrolysis-disproportionation (ED) loop or cascade as
exemplarily illustrated in FIGS. 10A-10B. The ED loop is a cyclic
process involving oxidation of the salt form or other forms of the
discharged oxidant, for example, bromide, on the positive electrode
of the electrolyzer 107a of the ED reactor 107 into the
intermediate oxidant, for example, bromine; a disproportionation
reaction that converts the intermediate oxidant such as bromine
into the salt form of the AMO such as bromate, and into the salt
form of the discharged oxidant such as bromide; oxidation of the
salt form of the discharged oxidant on the positive electrode of
the electrolyzer 107a into the intermediate oxidant, etc.
[0192] In an embodiment, the regeneration of the aqueous
multi-electron oxidant from the discharge fluid occurs by reverse
transformation of a cathodic discharge product in the discharge
fluid and without oxygen consumption or evolution. In another
embodiment, the regeneration of the aqueous multi-electron oxidant
from the discharge fluid comprises neutralizing an acid of the
discharge fluid, for example, via an ion exchange such as
orthogonal ion migration across laminar flow (OIMALF). The
regeneration system 106 then converts the neutralized discharge
solution into an intermediate oxidant and the reducer by means of
electrolysis. The intermediate oxidant is further converted into
the salt form of the aqueous multi-electron oxidant (AMO) via pH
dependent solution phase disproportionation and the salt form of
the AMO is converted into to the acid form of the AMO via ion
exchange such as orthogonal ion migration across laminar flow
process. The regeneration process on the positive electrode of the
electrolyzer 107a of the electrolysis-disproportionation (ED)
reactor 107 is facilitated by using one or a combination of an
electrocatalyst, a solution-phase catalyst, an ion exchange on
solids, an ion exchange in a liquid, a pH-dependent
disproportionation, and an orthogonal ion migration across laminar
flow in one ED reactor 107 or separate reactors in series and/or in
parallel. For the H.sub.2--HBrO.sub.3 regeneration, different
embodiments of the methods or routes of electrochemical
regeneration of hydrogen and bromic acid from aqueous hydrogen
bromide are disclosed herein. Direct electrolysis such as with
PbO.sub.2 and RuO.sub.2-based anodes and mediated electrolysis such
as with Cl.sub.2-mediator are also implemented.
[0193] FIG. 9 exemplarily illustrates a negative-ion electrospray
ionization-mass spectrometry spectrum of a 0.5M sodium phosphate pH
7.0 buffer solution after addition of 50 mM of Br.sub.2. Bromide
and bromate are the only detectable negative Br species with 2Da
1:1 doublets. These data affirm that bromine disproportionates only
into bromide and bromate in a pH 7 buffer. The labeled signals of
bromide and bromate prove the occurrence of the regeneration
reaction (6) in this buffer. The kinetics of the bromine
disproportionation has been studied mostly in near neutral media
4.ltoreq.pH.ltoreq.8, where the rates of various steps fall in the
range convenient for experimental measurements. The
disproportionation of Br.sub.2 in water may go all the way to
bromate and even to perbromate. The first step occurs at near
neutral 4.ltoreq.pH.ltoreq.8 via the following pathway:
Br.sub.2+H.sub.2O.dbd.HBrO+H.sup.++Br.sup.- (22)
Br.sub.2+OH.sup.-.dbd.HBrO+Br.sup.- (23)
[0194] Herein, bromine disproportionates into bromide and
hypobromite in two parallel reactions with water and with another
base such as hydroxide, that is, via a general base mechanism. The
equilibrium constant at 25.degree. C. for equation (22) at 0.5M
ionic strength is 6.1.times.10.sup.-9 M.sup.2. The first order rate
constant for the forward reaction for equation (22) is 97 s.sup.-1,
while for the reverse comproportionation reaction with H.sup.+ it
is 1.6.times.10.sup.10 M.sup.-2s.sup.-1. The bromine
disproportionation has not been studied computationally, but
molecular dynamics show that the homologous chlorine reaction in
water clusters proceeds as a bimolecular Cl.sup.+ transfer between
Cl.sub.2 and H.sub.2O. The chlorine disproportionation in acidic
solutions also follows a general acid-base catalysis route, first
order in Cl.sub.2 and in the general base, while the reverse
comproportionation reaction is first order in HOCl, Cl.sup.- and in
the general acid.
[0195] The hypobromous acid formed in reactions (22) and (23) above
undergoes a further disproportionation which is strongly pH
dependent. At a low pH bromine and bromate are formed:
5HOBr.revreaction.2Br.sub.2+BrO.sub.3.sup.-+2H.sub.2O+H.sup.+pH.ltoreq.4
(24)
[0196] The bimolecular rate constant with respect to the total
Br(I) is approximately 2.2.times.10.sup.-3 M.sup.-1s.sup.-1 when
extrapolated to pH 0 and increases at higher pH due to the
participation of a deprotonated hypobromite in the rate limiting
step. At a higher pH, bromide and bromate are formed:
3HOBr.revreaction.2Br.sup.-+BrO.sub.3.sup.-+3H.sup.+ pH.gtoreq.4
(25)
and the rate of the reaction decreases with pH above the pK.sub.a
of hypobromous acid of 8.8, although the kinetic equation retains
the second order in total Br(I) and the general base catalysis is
operative. Both reactions (24) and (25) occur in parallel at the
intermediate 4.ltoreq.pH.ltoreq.8 where the formal second-order
rate constant is the highest. Thus, the optimal pH for the
regeneration process
3Br.sub.2+6OH-=BrO.sub.3.sup.-+5Br.sup.-+3H.sub.2O (26)
is 4-9. For example, the optimal pH for the regeneration process is
between 6-8. The reaction is slower at higher pH since an
intermediate hypobromite is more stable, and at a lower pH, the
equilibrium shifts towards Br.sub.2. These considerations based on
a literature analysis are confirmed in ESI-MS data, as exemplarily
illustrated in FIG. 9 and FIG. 26, which prove the feasibility of
the regeneration process as per equation (26).
[0197] FIGS. 10A-10B exemplary illustrate an
electrolysis-disproportionation (ED)-orthogonal ion migration
across laminar flow (OIMALF) method for regenerating a reducer
(H.sub.2) and an oxidant fluid comprising an aqueous multi-electron
oxidant (HXO.sub.3) from a discharge fluid comprising HX and
H.sub.2O with MOH as the base. FIG. 10A illustrates
electrolysis-disproportionation in the batch mode with no-aqueous
multi-electron oxidant (AMO)-on-negative mode of operation. FIG.
10B illustrates electrolysis-disproportionation in the cyclic flow
mode with no-AMO-on-negative mode of operation. FIG. 10A
exemplarily illustrates a method for regenerating halic acid and
hydrogen from discharged hydrogen halide with a batch ED reactor
107. FIG. 10A exemplary illustrates a regeneration system 106
comprising an electrolysis-disproportionation reactor 107 and an
ion exchange reactor 108 in a batch mode of operation for
regenerating reducer (H.sub.2) and acidic oxidant fluid comprising
an aqueous multi-electron oxidant (HXO.sub.3) from the discharge
fluid (HX+H.sub.2O) with MOH as a base configured for the
no-AMO-on-negative mode of operation. FIG. 10B exemplarily
illustrates a method for regenerating halic acid and hydrogen from
discharged hydrogen halide with a flow-through ED reactor 107 in a
cyclic flow mode. FIG. 10B exemplary illustrates a regeneration
system 106 comprising a flow-type electrolysis-disproportionation
reactor 107 configured for the no-AMO-on-negative mode of operation
and an ion exchange reactor 108 for regenerating reducer (H.sub.2)
and the oxidant fluid comprising the aqueous multi-electron oxidant
(HXO.sub.3) from the discharge fluid (HX+H.sub.2O) with MOH as a
base. For purposes of illustration and not of limitation, the
concentrating reactor 112 is exemplarily illustrated after rather
than before the ion exchange reactor 108. The regeneration system
106 is equipped with the ion exchange reactor 108, in addition to
the electrolysis-disproportionation (ED) reactor 107, that converts
salts into acids, for example, an aqueous solution comprising
lithium bromate and a 3-(N-morpholino) propanesulfonic acid (MOPS)
anion into an aqueous solution comprising bromic acid and
protonated MOPS using a flow-through OIMALF reactor, which is
similar to an eluent suppression reactor employed in ion
chromatography The OIMALF process of converting salts into acids
avoids chemical separation and ion exchanger regeneration steps.
The OIMALF reactor can employ hydrogen produced in the ED reactor
107 as exemplarily illustrated in FIGS. 10A and 10B, or hydrogen
stored on board as exemplarily illustrated in FIG. 19, or recycle
the hydrogen produced on the negative electrode and consumed on the
positive electrode of the ED reactor 107.
[0198] The regeneration system 106 converts the discharge fluid
back into the aqueous multi-electron oxidant (AMO) and the reducer
using the ED reactor 107 and depending on the preferred options,
the ion exchange reactor 108, a mixing reactor or the
neutralization reactor 109, and separation reactors, for example,
1006, 1007, and 1010 as exemplarily illustrated in FIG. 10B, if
needed, are not counted as parts of the other devices. In an
embodiment, the ED reactor 107 is configured and operated as a
batch reactor or a flow-through reactor. In the batch ED reactor,
also referred to as a stirred reactor, the liquid in the positive
electrode compartment is stirred to achieve a uniform composition.
The batch ED reactor operates in the start-stop batch regime till
the desired degree of conversion of bromide into bromate is
achieved.
[0199] A series of a single neutralization reactor 109, a single
flow-through ED reactor 107, and a single H.sub.2/base separation
reactor 1006 can operate in the cyclic regime till the desired
degree of conversion of bromide into bromate is achieved. The
feedbacks 1009 and 1003 return the base from the H.sub.2 separation
reactor 1006 and partially regenerated oxidant fluid from the
positive loop valve 1004, respectively. The H.sub.2 is accumulated
in the H.sub.2 container during this cycle.
[0200] A series of a single neutralization reactor 109, a single
flow-through the ED reactor 107, and a single H.sub.2/base
separation reactor 1006 can operate in the cascade regime, wherein
the discharge fluid HX is first neutralized with an excess of a
base generated earlier in the ion exchange reactor 108, for
example, the orthogonal ion migration across laminar flow (OIMALF)
reactor. The first flow-through ED reactor 107 then converts some
Br.sup.- into BrO.sub.3.sup.- on the positive electrode while
releasing H.sub.2 and base on the negative electrode. The H.sub.2
goes into an H.sub.2 storage container (not shown), and the base
from the separation reactors 1006, 1007, and 1010 is returned to
the mixing reactor of the neutralization reactor 109 preceding this
H.sub.2 separation reactor 1006 in the series. The partially
regenerated oxidant fluid, for example, comprising LiBr and
LiBrO.sub.3 in some ratio, instead of going into one preceding
mixing reactor of the neutralization reactor 109 as in the cyclic
flow mode, goes into the second mixing reactor in the cascade,
where LiBr+LiBrO.sub.3 is neutralized by the base produced in the
second mixing reactor and so on. The number of repeated mixing
reactor-ED-separation reactor series in the cascade is, for
example, between 5 and 8, and is determined by desired throughput,
power, cost, degree of conversion, etc.
[0201] The regeneration process comprises the steps of
neutralization in the neutralization reactor 109,
electrolysis-disproportionation in the ED reactor 107, separation
of the reducer, that is, H.sub.2 gas from the aqueous
multi-electron oxidant (AMO) species in water in the separation
reactor 1006, and ion exchange via an orthogonal ion migration
across laminar flow (OIMALF) in the ion exchange reactor 108 as
disclosed in the detailed description of FIG. 8. The regeneration
system 106 takes HX+3H.sub.2O from the discharge fluid and produces
3H.sub.2+HXO.sub.3, while recycling within itself, water, and the
buffer. The separation reactor 1006 separates the liquid solution
with the base from the hydrogen gas reducer. The base component of
the buffer is represented as OH.sup.-.
[0202] The ED reactor 107 has an electrolysis unit or an
electrolyzer 107a with multilayer electrolyte-electrode assemblies
(not shown), a number of bipolar plates, and two endplates. The
discharge fluid from the discharge fluid storage tank (not shown)
is mixed in the neutralization reactor 109, with the solution of
the buffer in the base form coming out of the gas-liquid separation
reactors 1006, 1007, and 1010 through the return lines 1009, 1002,
and 1003, and then sent to the positive compartment of the ED
reactor 107. The neutralized discharge fluid MX is electro-oxidized
into the intermediate oxidant X.sub.2 at the positive electrode of
the ED reactor 107. The intermediate oxidant X.sub.2 reacts with
the base form of the buffer to produce a salt form of the aqueous
multi-electron oxidant (AMO), for example, LiBrO.sub.3. On the
negative electrode of the ED reactor 107, hydrogen gas is produced
upon electrolysis, and a base, for example, an amine, a
phosphonate, or a hydroxide is formed in the negative electrode.
The base from the negative compartment is mixed with the discharge
fluid in the mixing reactor or the neutralization reactor 109 prior
to or directly in the positive compartment of the ED reactor 107.
On the positive electrode, an intermediate oxidant such as bromine
is produced during electrolysis and reacts with the base introduced
from the negative compartment yielding, for example, a bromate and
a bromide via a disproportionation reaction. The remaining halide
is oxidized into halogen on the positive electrode of the ED
reactor 107 and disproportionates in a reaction with the base in
the next ED cycle.
[0203] The electrolysis-disproportionation (ED) process can proceed
as a single pass process with a three-way valve 1004 for a
sufficiently long ED reactor 107 and sufficiently high amount of
the buffer in the base form added in the neutralization reactor
109. In an embodiment, the ED process can run in a cyclic flow mode
with two three-way valves 1004 and 1005 in the loop, which is
useful for a shorter ED reactor 107, which increases the
regeneration time but saves on capital expenses. The three-way
valves 1004 and 1005 are exemplarily illustrated in FIG. 10B in
positions for the single pass mode of operation of the ED reactor
107. The three-way valves 1004 and 1005 send the aqueous
multi-electron oxidant (AMO) in the salt form, that is, MXO.sub.3,
for ion exchange via an orthogonal ion migration across laminar
flow (OIMALF) in the ion exchange reactor 108 to produce the acid
form of the AMO, that is, HXO.sub.3.
[0204] The electrolysis-disproportionation (ED) reactor 107 can be
configured and operated in a batch mode or in a flow-through mode.
The flow-through mode can be a cycle with one or more units or a
cascade with 2 or more units. When a sufficient degree of
conversion, that is, ratio of bromate concentration to the total
concentration of Br in all forms is achieved in the ED reactor 107,
after a certain charge, that is, time or number of cycles passed,
the electrolysis is completed. The liquid from the positive
electrode chamber of the ED reactor 107 goes into the ion exchange
reactor 108 where, in the middle chamber in the exemplary case of
the Li-3-(N-morpholino) propanesulfonic acid (MOPS) base form of
the buffer, the bromate is converted into bromic acid, Li-MOPS
buffer is converted into a protonated MOPS zwitter-ion, and
hydrogen is consumed in the positive chamber, and hydrogen is
produced in the negative chamber. The base, for example, Li-MOPS or
its equivalent is generated in the negative chamber along with
hydrogen. The base solution is used to neutralize the acidic
discharge fluid, for example, comprising HBr, incoming from the
discharge unit 104 exemplarily illustrated in FIG. 1. The
separation reactors 1006, 1007, and 1010 are used to separate the
gases from the liquids.
[0205] Halates are produced by disproportionation of a halogen in
the presence of a base. The process of disproportionation of
halogens is described by the following equation using hydroxide as
an example of a base:
3X.sub.2+6MOH=MXO.sub.3+5MX+3H.sub.2O. (27)
[0206] In the electrolysis-disproportionation (ED) reactor 107,
exemplarily illustrated in FIGS. 10A-10B, if the liquid in the
positive electrode chamber and the liquid in the negative electrode
chamber are allowed to mix, the halogen produced on the positive
electrode can react with the base produced on the negative,
yielding, for example, a halate and a halide. The halide is
oxidized on the anode, thus initiating the new cycle of the
loop:
MX+H.sub.2O+electrolysis=(0.5H.sub.2+MOH) negative electrode
chamber+0.5X.sub.2 positive electrode chamber, (28)
3X.sub.2+6MOH=5MX+MXO.sub.3+H.sub.2O after mixing in the positive
electrode chamber, (29)
where the disproportionation described in equation (29) can be
performed either in a flow-through process or a batch process with
or without assistance of a buffer such as a phosphate buffer. The
net equation of the regeneration process, that is, the ED process
is:
6MX+3H.sub.2O=MXO.sub.3+3H.sub.2 (30)
with the electrolysis-disproportionation loop 109 to 1007 as
exemplarily illustrated in FIGS. 10A-10B. The reduction of
XO.sub.3.sup.- on the negative electrode in the electrolyzer 107a
is prevented, for example, by using a negative electrode with a
multilayer structure with a cation-selective membrane facing the
solution comprising the aqueous multi-electron oxidant (AMO). In an
embodiment, a membraneless reactor can also be used if the negative
electrode comprises, for example, Ni capable of selective reduction
of water into hydrogen without reducing XO.sub.3.sup.-, or if the
electrolyte layer 205c is not a solid membrane but a laminar flow
electrolyte. The optional concentrating reactor 112 removes water
introduced with the buffer during electrolysis-disproportionation.
A portion of the water is transferred to the neutralization reactor
109 via the water return line 1008 exemplarily illustrated in FIG.
10B.
[0207] The choice of the counter-cation in the regeneration schemes
of FIG. 8 and FIGS. 10A-10B depends on the solubility of the
counter-cation's halide, halate, and buffer salts such as a
phosphonate, a Good's buffer, etc., since circulating a small
volume of a liquid and minimizing the energy and capital expenses
of water removal in making a concentrated aqueous multi-electron
oxidant (AMO) solution is beneficial. These considerations are
relaxed in the case of an off-board regeneration system 106 as
compared to an on-board regeneration system 106 exemplarily
illustrated in FIG. 1. Lithium bromide (18.4 m) and bromate (13.3
m) have substantially high solubilities in water at 20.degree. C.
and so does lithium hydroxide (5.3m). Li salts with a suspension of
hydroxide or phosphate or with addition of a complexing agent such
as 15-crown-5 (15C5), benzo-15-crown-5 (B15C5),
dicyclohexano-18-crown-6 (DC18C6),18-crown-6 (18C6), 12-crown-4
(12C4), dibenzo-18-crown-6 (DB18C6), and their more water-soluble
derivatives are also considered. For K.sup.+, bromate solubility is
low, for example, about 0.41m at about 20.degree. C. Na.sup.+ salts
have intermediate solubilities in water, for example, of about 2.4m
for bromate, and about 8.8m for bromide at about 20.degree. C. The
"m" does not have to be a monovalent cation. For example, magnesium
bromate has a good solubility in water (2.5m at 0.degree. C., 5.36m
at 60.degree. C.). Calcium bromate has also a good solubility
(1.86m at 40.degree. C.) that shows only a weak dependence on
temperature. Also, Good's buffers have high solubility often above
2m. The symbol "m" refers to the molal concentration, that is, the
moles of solute per kg of solvent.
[0208] FIGS. 11A-11B exemplary illustrate a cyclic operation of a
flow-through electrolysis-disproportionation (ED) reactor 107 with
bromate as an aqueous multi-electron oxidant (AMO), hydrogen
phosphate as a base form of a buffer, and sodium as the counter
cation configured for no-AMO-on-negative mode of operation. FIGS.
11A-11B exemplarily illustrate a method for regenerating sodium
bromate and hydrogen from sodium bromide and water. The charge of
one electron per bromide is shown in each cycle for the sake of
illustration not of limitation. Theoretically estimated water
transfer numbers are shown for the sake of illustration and not of
limitation. The balance of water dragged with ions is not shown. A
buffer can be used instead of the hydroxide or in addition to the
hydroxide. This method allows for minimizing the spatial and
temporal variations of pH outside of the range between 7 and 9. For
example, a solution comprising Na.sup.+ cation and any of the forms
of deprotonated phosphoric acid can be used as a component of the
buffer. FIGS. 11A-11B exemplarily illustrate a method to execute
the electrolysis-disproportionation (ED) regeneration step based on
a cyclic operation of the ED reactor 107 with a cation exchange
membrane or in a row of, for example, 6 cells connected in series.
For purposes of illustration, the detailed description refers to
the bromate chemistry, the Na.sup.+ cation, and a phosphate buffer;
however the scope of the method and the system 100 disclosed herein
is not limited to the bromate chemistry, the Na.sup.+ cation, and
the phosphate buffer but can be extended to include other aqueous
multi-electron oxidants (AMOs), cations including Li.sup.+, and
buffers including Good's buffers.
[0209] FIG. 12 exemplarily illustrates calculated and
experimentally measured limiting currents on a rotating disk
electrode in aqueous solutions of bromic acid of various
concentrations. FIG. 12 exemplarily illustrates the calculated
kinetic limiting current of bromate comproportionation in a 0.1 M
HBrO.sub.3+1 mM Br.sub.2 and experimentally measured limiting
currents on a glassy carbon rotating disk electrode for the
1.sup.st wave in 50% w HBrO.sub.3 attributed to the
electroreduction of bromine generated via equation (2) and for the
2.sup.nd wave in 50% w HBrO.sub.3 attributed an unidentified
intermediate of equation (2) comprising Br in the oxidation state
0<OS<5. The rational for using a diluted acidic aqueous
multi-electron oxidant (AMO) solution for the 2.sup.nd wave
measurements was the current range limitation of the potentiostat.
The calculated current and experimental current of the first wave
is due to the reduction of bromine and its value is determined by
the rate of the comproportionation of bromate with electrogenerated
bromide near the electrode surface. The current of the second wave
is tentatively attributed to an intermediate in the
comproportionation reaction, such as BrO.sub.x.sup.- with x=1 or 2.
A direct electrochemical reduction of bromate at room temperature
occurs with a significant overvoltage on all electrode materials,
and a bromate reduction on an electrode can be facilitated via a
homogeneous comproportionation with bromide into highly
electrochemically active bromine:
In the catholyte:5HBr+HBrO.sub.3.dbd.3H.sub.2O+3Br.sub.2 (31)
On the cathode:Br.sub.2+6e.sup.-=6Br.sup.- (32)
[0210] The cathode refers to the electrode where the
electroreduction takes place, that is, the positive electrode in
this case.
[0211] The comproportionation reaction (31) is known to be first
order in [BrO.sub.3.sup.-] and [Br.sup.-] and second order in
[H.sup.+] at pH below 2. An additional term with a second order in
bromide is apparent at high bromide concentrations. The actual
mechanism involves several serial and parallel steps that show
general acid catalysis effects, and the mechanism is similar to the
homologous chlorine and iodine processes. Chloride accelerates
reaction (31). The effect of the addition of chloride species on
both the discharge and regeneration processes is also disclosed
herein since the intermediate chlorine increases the energy and
power densities of the system 100 with Br.sub.2 as the intermediate
oxidant due to a complex interplay between the aqueous chemistries
of the two halogens. The electroreduction of Br.sub.2 as per
equation (32) is a first order process with a high exchange current
even on carbon electrodes which are used in Zn-Br.sub.2 and
NaS.sub.x--Br.sub.2 batteries.
[0212] The calculated dependence of the limiting current on the
rotation rate in a 0.1M aqueous multi-electron oxidant (AMO)
solution is represented in FIG. 12 as a solid line, the
experimental data on a glassy carbon rotating disk electrode for
the main wave in approximately 20% AMO solution is represented as a
dotted line, and the prewave in approximately 50% AMO solution is
represented as a dashed line. Currents over 0.5A/cm.sup.2 are
obtained on a smooth carbon electrode. The limiting current shows a
decrease with the rotation rate due to the loss of the intermediate
bromine into the solution, when the thickness of the diffusion
boundary layer is smaller than the thickness of the kinetic
boundary layer. The dependence of the limiting current on a log of
the rotation rate in HBrO.sub.3 solutions is also exemplarily
illustrated in FIG. 12.
[0213] While the theoretical energy density of the
H.sub.2/HBrO.sub.3 system 100 is, for example, about 1,951 Wh/kg,
the limited stability of bromic acid solution with the
concentration above 55% w/w, makes 938 Wh/kg, that is, 3.25 times
higher than the theoretical value for the lithium iron phosphate
(LFP) chemistry, a more realistic estimate. Taking into account the
5% w/w H.sub.2 content for high-pressure storage and the flow
design, about 426 Wh/kg is obtained as a realistic target value at
the system level, which is 6 times larger than the corresponding
number for the LFP battery pack, for example, in Tesla
Roadster.RTM. of Tesla Motors, Inc.
[0214] FIG. 13 exemplary illustrates a graphical representation of
a power-voltage curve calculated for a H.sub.2-50% w/w HBrO.sub.3
discharge unit 104 and measured with a glassy carbon rotating disk
electrode, and with a platinum gauze electrode in a flow cell and a
corresponding curve for a commercial proton exchange membrane fuel
cell running on hydrogen and air. The +1.4 V onset of HBrO.sub.3
electroreduction on Pt implies a direct process rather than
comproportionation-mediated electrode process. The reduction of
bromate on the positive electrode is modeled for one dimensional
(1D) diffusion normal to the electrode and for constant thicknesses
of the kinetic and hydrodynamic boundary layers. As used herein,
the term "diffusion" refers to mass transport due to a
concentration gradient.
[0215] In FIG. 13, which exemplary illustrates the calculated power
versus voltage plots for a hydrogen-bromate discharge unit 104, the
term "Z.sub.0" refers to the ratio of the hydrodynamic boundary
layer thickness to the kinetic boundary layer thickness and under
the conditions of the experiment, the latter is equal to
approximately 1.5 .mu.m. The term "C.sub.0" exemplarily illustrated
in FIG. 13 refers to the bulk concentration of free intermediate
oxidant such as bromine. A typical curve for a H.sub.2-air polymer
electrolyte membrane fuel cell (PEMFC) is also shown in FIG. 13 for
comparison. The membrane resistance for the solid line is equal to
0.1 Ohm/cm.sup.2 and the membrane resistance for the dotted line is
equal to 0.25 Ohm/cm.sup.2. The lines with circles represent
experimental data in 50% HBrO.sub.3 aqueous multi-electron oxidant
(AMO) solution. The solid lines and the dashed lines represent
experimental data with the glassy carbon rotating disk electrode
(GCRDE) at different rotation rates, and the dashed-double dotted
line represents experimental data in a proton exchange membrane
(PEM) type flow cell, for example, a Fuel Cell Store.TM. #1071025
with Pt gauze electrodes on both sides, powered by H.sub.2 and 50%
HBrO.sub.3.
[0216] The experimental data of FIG. 13 with glassy carbon (GC)
electrodes in an aqueous solution of HBrO.sub.3 shows three regions
in the power-voltage curve--a cathodic pre-wave at +1.15 V versus a
standard hydrogen electrode (SHE) with a 42 mV/decade Tafel slope,
a main cathodic wave at +0.7 V versus SHE with a 208 mV/decade
Tafel slope, and an anodic wave. Both cathodic waves show a
decrease in the limiting current at higher rotation rates as
approximately i.about.1/.omega..sup.-1 and at lower aqueous
multi-electron oxidant (AMO) concentrations as
i.about.C.sub.AMO.sup.3. The more positive wave, that is, the
1.sup.st wave on GC is attributed to the predicted reduction of the
intermediate Br.sub.2 since the positive wave on GC occurs at the
appropriate potential and has a low Tafel slope, close to 60 mV/dec
that is usually observed, whereas the more negative wave, that is,
the 2.sup.nd wave is likely due to an intermediate with a lower
exchange current such as BrO.sup.- or BrO.sub.2.sup.-, formed
during the comproportionation before Br.sub.2. The small but
unambiguous anodic wave positive to E.degree. (Br.sub.2/2Br.sup.-)
with a very high formal Tafel slope of 332 mV/dec is likely due to
the oxidation of bromide which is slowly formed via the reversible
disproportionation of bromine present in equilibrium with
HBrO.sub.3. A discharge power of 5 mW/cm.sup.2 at 70% efficiency
with respect to E.degree. of BrO.sub.3.sup.-/Br.sup.- is achieved
with a smooth carbon electrode and dilute 20% AMO.
[0217] The 1 W/cm.sup.2 target can be achieved by using a high area
porous electrode, increasing the concentration of the aqueous
multi-electron oxidant (AMO) and by adding additional proton donors
such as an extra acid, to the AMO stock. Unlike the case of the
glassy carbon rotating disk electrode (GC-RDE), the onset of
bromate reduction on Pt, exemplarily illustrated in FIG. 13, occurs
at 1.42 V versus reversible hydrogen electrode (RHE) at pH.about.0,
which is more positive than the E.degree. (Br.sub.2/Br.sup.-)=1.066
V. This is attributed to the direct electroreduction of bromate on
Pt in acid. Despite the possibility of having a higher efficiency
discharge unit 104 exemplarily illustrated in FIG. 1, the
preliminary economic analysis suggests against the use of Pt at the
0.2 mg/cm.sup.2 loading in the cathode of the discharge unit 104 in
automotive applications due to an increased upfront cost which will
not amortize over 3 years, which is the projected Pt cathode
durability, by the lower operational cost and energy efficiency. A
Pt or another catalyst can be used on the positive electrode 205a
exemplarily illustrated in FIG. 2 in other high-end applications
such as in military applications and aerospace applications. Oxide
catalysts such as RuO.sub.2 and its derivates are suitable for the
use on the positive electrode 205a of the discharge unit 104.
[0218] FIGS. 14A-14G that exemplarily illustrate graphical
representations showing comparative performances of three on-board
power sources at a nominal power of 130 kW: a gasoline-internal
combustion engine, a lithium ion battery, and an H.sub.2-aqueous
multi-electron oxidant discharge unit as well as the targets of the
Advanced Research Projects Agency-Energy are disclosed along with
the examples enumerated later in this description.
[0219] FIG. 15 illustrates an embodiment of the system 100 for
generating electric power and a discharge fluid from an oxidant
fluid and a reducer fluid using a discharge system 101 comprising
an orthogonal ion migration across laminar flow (OIMALF) reactor
1501, and for regenerating an oxidant and/or a reducer from the
discharge fluid using a regeneration system 106. The system 100
disclosed herein cyclically discharges and recharges or refills the
discharge system 101 in an energy storage cycle. In this
embodiment, the acidification process such as the ion exchange
process is performed within the discharge system 101 rather than
within the regeneration system 106 in order to improve the
stability and safety of the systems disclosed herein. The discharge
system 101 comprises the discharge unit 104, an acidification
reactor 1501a, optionally a neutralization reactor 1501b, the
oxidant fluid tank 102, the reducer fluid tank 103, and a discharge
fluid tank 113. The acidification reactor 1501a comprises one or
more of an acid storage tank (not shown) storing H.sub.2SO.sub.4,
TfOH, etc., and an acid mixing tank (not shown) in the OIMALF
reactor 1501. The acidification reactor 1501a converts the neutral
oxidant fluid stored in oxidant fluid tank 102 into an acidic
oxidant fluid by lowering the pH of the acidic oxidant fluid for
facilitating further electroreduction of acidic oxidant fluid at
one or more positive electrodes 205a of the discharge unit 104 via
a pH-dependent comproportionation.
[0220] The acidic oxidant fluid comprises, for example, one or more
of water, one or more forms of the aqueous multi-electron oxidant
(AMO), for example, an acid or a salt form or as a combination
thereof, an extra acid, and one or more of multiple counter
cations. The AMO comprises one or a combination of halogens,
halogen oxides, halogen oxoanions, and salts and acids of the
halogen oxoanions. The extra acid is, for example, one or more of a
phosphoric acid, a 3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, a methanesulfonic acid, a
triflic acid, a substituted sulfonic acid, a substituted phosphonic
acid, a perchloric acid, a sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic moieties, and an acid with a
pKa<2. The halogen oxoanions comprise, for example, one or more
of hypochlorite, chlorite, chlorate, perchlorate, hypobromite,
bromite, perbromate, hypoiodite, iodite, iodate, and periodate. In
an embodiment, the halogen oxoanion is bromate. The counter cations
comprise alkali metal cations, alkali earth metal cations, and
organic cations. In an embodiment, one of the counter cations is
lithium. In another embodiment, one of the counter cations is
sodium. The acidic oxidant fluid has a sufficient chemical
reactivity to cause an ignition regime of electroreduction on the
positive electrodes 205a of the discharge unit 104. The
neutralization reactor 1501b neutralizes the discharge fluid, for
example, hydrogen halide produced by the discharge unit 104 with a
base form of a buffer to produce a solution of a salt form of the
discharge fluid also referred to herein as a "neutral discharge
fluid". In an embodiment, the neutralization reactor 1501b
comprises a mixing reactor. The discharge fluid tank 113 is used to
collect the discharge fluid for future regeneration or
disposal.
[0221] In an embodiment, the acidification reactor 1501a and the
neutralization reactor 1501b are functionally combined as an
orthogonal ion migration across laminar flow (OIMALF) reactor 1501.
In another embodiment, the neutralization reactor 1501b is
integrated with the acidification reactor 1501a into the OIMALF
reactor 1501 as exemplarily illustrated in FIG. 15 and FIG. 19. The
OIMALF reactor 1501 comprises an OIMALF cell stack (not shown)
which is configured similar to a polymer electrolyte fuel cell
(PEFC) stack but with a liquid electrolyte flowing between two
ionically conducting membranes. The OIMALF reactor 1501 comprises
flow cell assemblies, endplates, and bipolar plates. Each flow cell
assembly of the OIMALF reactor 1501 comprises a couple of ion
exchange membranes comprising a positive side ion exchange membrane
and a negative side ion exchange positioned parallel to each other,
an intermembrane flow field interposed between the ion exchange
membranes and comprising multiple flow channels, a positive
electrode layer and a negative electrode layer flanking outer
surfaces of the ion exchange membranes, and porous diffusion layers
flanking the outer surfaces of the positive and negative electrode
layers. The porous diffusion layers are in electric contact with
the adjacent bipolar plates or endplates. The positive electrode
layer is configured for hydrogen oxidation reaction and the
negative electrode layer is configured for hydrogen evolution
reaction. Although, the on-board OIMALF reactor 1501 adds to the
weight of the discharge system 101, this addition can be tolerated
due to the high power density and low energy consumption of the
OIMALF reactor 1501. Moreover, only 10% or less of the electric
power generated by the discharge unit 104 is required to support
the OIMALF reactor 1501. Also, the estimated weight of the OIMALF
reactor 1501 for a 130 kW discharge system 101 is about 54.2 kg
which is only about 50% of the weight of the discharge unit 104 and
30% of the weight of the oxidant and the reducer, and thus adds
only approximately 14% to the weight of the discharge system
101.
[0222] The discharge system 101 disclosed herein is configured to
operate in an electric partial recharge mode for facilitating
regenerative breaking when the discharge system 101 powers an
electric vehicle. During the partial recharge mode, the reactions
on the positive and negative electrode reverse their directions,
that is, the reducer is produced on the negative electrode 205b of
the electrolyte-electrode assembly 205 and an intermediate oxidant
is produced on the positive electrode 205a of the
electrolyte-electrode assembly 205. For example, H.sub.2 is
produced on the negative electrode 205b and Br.sub.2 is produced on
the positive electrode 205a. Since the pH of the oxidant fluid is
acidic during the discharge, the disproportionation does not occur
and the aqueous multi-electron oxidant (AMO), that is, bromate is
not formed. The regeneration stops at the bromine which is the
intermediate oxidant and can be easily consumed to provide power
when the current direction goes back to the discharge mode.
[0223] The discharge unit 104 disclosed herein reduces the
crossover of the anionic oxidants and products from the positive
cathode to the negative hydrogen anode by employing a
cation-exchange membrane between the electrodes. In contrast to a
polymer electrolyte fuel cell, the discharge system 101 reduces or
completely eliminates platinum from the positive electrode 205a,
uses a thicker hydrophilic porous electrode (HPE) instead of a thin
catalytic layer and a hydrophobic gas diffusion layer on the
positive electrode 205a which assures a higher power per
cross-sectional area, reduces the size or completely eliminates the
humidification system due to back diffusion of water from the
aqueous multi-electron oxidant (AMO) solution to the hydrogen
electrode within each electrolytic cell 200, and allows for energy
recuperation by oxidation on the positive electrode 205a of bromide
in the discharge fluid into bromine with simultaneous hydrogen
evolution on the negative electrode 205b.
[0224] The regeneration system 106 of the system 100 disclosed
herein is configured to regenerate the aqueous multi-electron
oxidant (AMO) and the reducer from the discharge fluid produced by
the discharge unit 104. The regeneration system 106 comprises, for
example, a splitting-disproportionation (SD) reactor 1502, a
concentrating reactor 112, multiple separation reactors 1010, and
storage tanks such as a regenerated oxidant fluid tank 110, a
regenerated reducer fluid tank 111, a discharge fluid tank 1503,
and a water tank 1504. An electrolysis-disproportionation reactor
107 is an example of the splitting-disproportionation reactor 1502.
In an embodiment, the SD reactor 1502 is configured as the
electrolysis-disproportionation (ED) reactor 107, exemplarily
illustrated in FIG. 1, comprising sub-reactors, for example, an
electrolysis unit or an electrolyzer 107a and a disproportionation
unit 107b, exemplarily illustrated in FIG. 1. In an embodiment, the
electrolyzer 107a and the disproportionation unit 107b are
physically combined in the same hardware.
[0225] In an embodiment, the splitting-disproportionation (SD)
reactor 1502 uses electrolytic splitting and is configured for flow
modes of operation. The SD reactor 1502 comprises a stack of SD
flow cells configured similar to a conventional polymer electrolyte
fuel cell (PEFC) bipolar stack so that one side of every inner
bipolar plate serves the current collector of a negative electrode
and the other side serves as the current collector of a positive
electrode. Several SD flow cells can be stacked and operated in a
cascade flow mode. Each SD flow cell has a structure similar to a
polymer electrolyte membrane fuel cell (PEMFC) with a 5-layer
membrane-electrode assembly, where the gas diffusion layer on the
positive side is replaced with a hydrophilic porous layer.
Furthermore, the stack and the negative electrodes of the 5-layer
membrane-electrode assembly are configured for either the aqueous
multi-electron oxidant (AMO)-on-negative electrode mode of
operation also referred to as the "AMO-on-negative mode of
operation", or the no-AMO-on-negative electrode mode of operation
also referred to as the "no-AMO-on-negative mode of operation". The
individual SD flow cells in the bipolar stack are connected
electrically in series so that the bipolar stack voltage is the sum
of the individual SD flow cell voltages. The individual SD flow
cells in the bipolar stack are connected flow-wise in parallel or
series, with a parallel connection affording more uniform voltages
in different SD flow cells in the bipolar stack. In an embodiment,
the SD reactor 1502 is configured for the AMO-on-negative mode of
operation using a multilayer structure on a negative electrode side
of the SD reactor 1502. The multilayer structure on the negative
electrode side minimizes reduction of a regenerated aqueous
multi-electron oxidant in a regenerated oxidant fluid on the
negative electrode side while facilitating hydrogen evolution and
increase in pH of the regenerated oxidant fluid. In another
embodiment, the SD reactor 1502 is configured for the
no-AMO-on-negative mode of operation by transferring a base
produced on one or more negative electrodes of the SD reactor 1502
to a regenerated oxidant fluid produced at one or more positive
electrodes of the SD reactor 1502 and comprising one or more forms
of the aqueous multi-electron oxidant and the intermediate
oxidant.
[0226] The splitting-disproportionation (SD) reactor 1502 or
reactors can be configured and operated in a batch mode, in a
cyclic flow mode or in a cascade flow mode. An SD reactor 1502
configured for the cyclic flow mode has a lower upfront cost but
requires a longer regeneration time. Such an SD reactor 1502 may be
utilized for at-home-garage regeneration. The SD reactor 1502
configured for the cascade flow mode has a higher upfront cost but
is capable of a faster regeneration. This SD reactor 1502 may be
utilized at multi-user charging stations. A combination of cyclic
and cascade flow modes in the same regeneration unit allows for an
optimization of the capital cost and throughput and it is
recommended for most applications.
[0227] In an embodiment, the concentrating reactor 112 is placed
between the splitting-disproportionation (SD) reactor 1502 and the
orthogonal ion migration across laminar flow (OIMALF) reactor 1501
whereby the concentrating reactor 112 produces a concentrated
solution of neutral oxidant fluid comprising a salt from of the
aqueous multi-electron oxidant (AMO). The concentrating reactor 112
increases the concentrations of one or more forms of the AMO as
well as the total AMO concentration in the oxidant fluid produced
by the splitting-disproportionation (SD) reactor 1502 before the
AMO is stored in the regenerated oxidant fluid tank 110. The
concentrating reactor 112 removes water or other solvents from a
dilute fluid that enters the concentrating reactor 112 and releases
a concentrated fluid and water or another solvent. The
concentrating reactor 112 performs concentration, for example, by
evaporation, pervaporation, reverse osmosis, and other known
methods. The storage tanks, for example, the regenerated oxidant
fluid tank 110, the regenerated reducer fluid tank 111, the water
tank 1504, and a buffer tank (not shown) are used to store the
regenerated neutral oxidant fluid, the regenerated reducer, water,
and the buffer respectively. The separation reactors 1010 are
gas-liquid separators or separation reactors 1010 and are used to
separate gases from the liquids during the regeneration
process.
[0228] FIG. 16 exemplarily illustrates a process flow diagram
showing mass and electricity flows in an energy cycle between the
discharge unit 104, the acidification reactor 1501a, and the
neutralization reactor 1501b of the discharge system 101. The on
board system is enclosed in a dotted frame with the reducer fluid,
oxidant fluid, and discharge fluid shared by both on-board and
off-board systems. In an embodiment, the discharge system 101
comprises a single reactor such as an orthogonal ion migration
across laminar flow (OIMALF) reactor 1501 which performs both
acidification 1602 and neutralization 1606. In FIG. 16, HXO.sub.n
refers to the aqueous multi-electron oxidant (AMO) in the acid
form, MXO.sub.n refers to the AMO in the salt form, HA refers to
the buffer in the acid form, and MA refers to the buffer in the
base form. The flow of materials is represented using solid arrows
and the flow of electric power is represented using dotted arrows.
The aqueous multi-electron oxidant (AMO) may be present at various
stages in the discharge and regeneration energy cycle in one or
several forms, for example, acid form, salt forms such Li form,
etc., differing in composition, concentration, etc. If not
specified, the term "aqueous-multi-electron oxidant" or "AMO"
refers collectively to all such forms and any combination
thereof.
[0229] Certain salts of both the aqueous multi-electron oxidant
(AMO) and the discharge product of the AMO have high aqueous
solubilities as well as high rates of homogeneous
disproportionation and comproportionation. Such a combination can
be obtained, for example, when the AMO salt is lithium bromate with
a solubility of, for example, over 10 molal at 20.degree. C. and
over 20 molal at 80.degree. C. and the discharge salt is lithium
bromide with the solubility of, for example, over 15 molal at
20.degree. C. and over 25 molal at 80.degree. C. Although a salt
form of the AMO can be used directly in a discharge unit 104 to
produce electric power, the slow kinetics of the direct
electroreduction of the salt form of the AMO requires the use of
expensive platinoid catalysts and, even then, occurs with poor
energy efficiency. The electroreduction of the salt form of the AMO
can process more efficiently, even on a bare carbon electrode, when
it is mediated by a soluble mediator. In an embodiment, the
electroreduction product, for example, bromide, is utilized as the
reduced form of the mediator. In this case, the mediation reaction
is a comproportionation reaction. The reduction of the AMO in
general and the comproportionation reaction in particular requires
proton donors to proceed at a useful rate. Proton donors can be
introduced into a stable stock solution of the salt form of the AMO
in a process referred herein as acidification. Also, in the method
and systems or energy cycle disclosed herein, a pH manipulation
and/or change is used to facilitate the conversion between a stable
but low-power salt form of the AMO and a high-power but poorly
stable acid form of the AMO.
[0230] The neutral oxidant fluid 1601 has a high energy density due
to the high solubility of the aqueous multi-electron oxidant (AMO)
such as LiBrO.sub.3 and due to the multi-electron oxidant property
of the AMO: 6 electrons are transferred during the reduction of one
bromate ion into one bromide ion. Thus, the discharge unit 104 can
store a large amount of energy or charge per unit of weight or
volume and this storage is safe due to a low reactivity of the AMO
at neutral and alkaline pH. However, to achieve a high power, for
example during an on-board discharge process, the AMO needs to be
present in an acidic form that is at a low pH. This can be achieved
by converting the neutral oxidant fluid 1601 into an acidic oxidant
fluid 1603 in the acidification reactor 1501a. The process of
acidification 1602 can be performed via ion exchange on solids, ion
exchange in solution or by any other known acidification method,
and by any combination thereof. In an embodiment, the acidification
is performed via the orthogonal ion migration across laminar flow
(OIMALF) process. The use of the OIMALF process confers an
additional benefit of being free of input and output chemicals, as
well as the benefits of high power density and of high energy
efficiency. In another embodiment, the acidification is performed
by adding an extra acid HA such as phosphoric acid H.sub.3PO.sub.4,
carried over from the regeneration step, sulfuric acid, triflic
acid, other strong acid, etc., to the neutral oxidant fluid. Also,
the on-board storage of a salt form of the AMO is used over an acid
form of the AMO for safety reasons. The use of salts forms rather
than of acid forms puts forward additional requirements for high
solubilities of the AMO and its discharge product(s) in their salt
forms. The complete acidification with 1:1 stoichiometric ratio of
acidic protons to the AMO, for example, bromate, is not necessary
for the ignition regime of the AMO electroreduction to occur, and a
partial acidification suffices. This finding confers the benefits
of improved safety, energy efficiency, and reduced size of the
discharge system 101, which facilitate application of the discharge
system 101 in automotive applications.
[0231] In an embodiment, the stability of the acidic oxidant fluid
is maintained by performing an ignition regime in the discharge
system 101 at low acid concentrations of the acidic oxidant fluid.
The concentration of one or more forms of the aqueous
multi-electron oxidant in the neutral oxidant fluid or the acidic
oxidant fluid supplied to the discharge unit 104 is, for example,
above 1M, 2M, 5M, or 10M. The concentration of acidic protons in
the acidic oxidant fluid supplied to the discharge unit 104 is, for
example, below 0.1M, 0.5M, 1M, 2M, 5M, or 10M. The concentration of
acidic protons in the acidic oxidant fluid stored in the discharge
system 101 is, for example, below 0.1M, 0.5M, 1M, 2M, or 5M. In an
embodiment, the acidification process is performed off-board in the
discharge system 101, yielding a weakly acidic solution that is
capable of ignition-like electro-reduction yet is sufficiently
stable on the week time scale for automotive applications. In the
discharge system 101 disclosed herein, the concentration of acid
that is required to cause ignition with a practically suitable
power in a highly concentrated aqueous multi-electron oxidant
[AMO]>10M is very low about 5 mM. The AMO does not decompose for
over a week. This allows the acidification process such as
orthogonal ion migration across laminar flow (OIMALF) process to be
performed off-board and also allows storage of the acidic oxidant
fluid on-board in the oxidant fluid tank 102 of the discharge
system 101 for almost a week. The stored AMO is a stable solution
capable of ignition. The method and the discharge system 101
disclosed herein allow the storage of a more stable form of the AMO
on-board which is achieved with an acceptable sacrifice in the
system energy density.
[0232] The discharge system 101 uses the acidification reactor
1501a to convert the neutral oxidant fluid 1601 into acidic oxidant
fluid 1603 which has sufficient chemical reactivity to cause an
ignition regime of electroreduction on the positive electrodes 205a
of the discharge unit 104. During the acidification process 1602, a
stable aqueous multi-electron oxidant (AMO) stock stored on board,
such as neutral oxidant fluid 1601 comprising LiBrO.sub.3 is
converted into a chemically reactive form of the AMO, for example,
acidic oxidant fluid 1603 comprising HBrO.sub.3. This can be
accomplished via a solution-phase cation exchange process in the
orthogonal ion migration across laminar flow (OIMALF) reactor 1501
with a simultaneous conversion of the outgoing acidic discharge
fluid 1605 into a neutral form 1607, for example, HBr into LiBr.
LiBrO.sub.3 is converted into HBrO.sub.3 using the OIMALF process
or another ion exchange process or direct addition of an extra
acid. In an embodiment, the OIMALF process generates and consumes
H.sub.2 within the OIMALF reactor 1501. The OIMALF process of
converting, including partially converting, MXO.sub.n into
HXO.sub.n, for example, LiBrO.sub.3 into HBrO.sub.3 avoids
cumbersome chemical separation and ion exchange regeneration steps.
The choice of the acid form of the AMO can be expanded beyond
HBrO.sub.3 to other AMOs comprising, for example, HClO.sub.4,
HClO.sub.3, HClO.sub.2, HClO, HBrO.sub.4, HBrO.sub.2, HBrO, etc.
Phosphoric acid will be present in the oxidant fluid if a phosphate
buffer is used during regeneration. The net reaction of the ion
exchange or the OIMALF process is: LiBrO.sub.3+HA=HBrO.sub.3+LiA,
where HA is a source of protons comprising, for example, one or
more of the following: water, phosphoric acid, dihydrogen
phosphate, one or more of Good's buffers, one or more derivatives
of sulfonic acid, sulfuric acid, triflic acid, perchloric acid,
etc. For on-board operation, the OIMALF reactor 1501 is operably
connected to an on-board power source such as discharge unit 104 or
a battery (not shown) which provides electric power for the OIMALF
process.
[0233] During the discharge process, the discharge unit 104 is
supplied with the reducer 1604, for example, H.sub.2, and the
acidic oxidant fluid 1603 comprising the aqueous multi-electron
oxidant (AMO) in the acid form, HXO.sub.n, for example, HBrO.sub.3.
The reducer 1604 donates electrons to the negative electrode 205b,
and splits into ions. The reaction at the negative electrode 205b
is, for example, 3H.sub.2-6e.sup.-=6H.sup.+. The on-board electric
circuit 203 conducts and transfers electrons from the negative
electrode 205b to the positive electrode 205a. The reaction at the
positive electrode 205a, for example, 3Br.sub.2+6e.sup.-=6Br.sup.-,
or when combined with the comproportionation reaction the
catholyte, for example,
BrO.sub.3.sup.-+6e.sup.-+6H.sup.+=Br.sup.-+3H.sub.2O. The aqueous
multi-electron oxidant accepts the electrons at the positive
electrode 205a for producing the electric current in the on-board
electric circuit 203. The discharge unit 104 releases the acidic
discharge fluid HX 1605 comprising, for example, HBr and the buffer
HA in the acidic form and generates electric current in the
on-board electric circuit 203. The cation-selective electrolyte
layer 205c provides for a movement of cations, such as hydronium
ions, between the negative electrode 205b and the positive
electrode 205a.
[0234] The generation of electric power using the aqueous
multi-electron oxidant (AMO), for example, bromate during the
discharge is accompanied by the following chemical
transformations.
Negative Electrode:3H.sub.2+6e.sup.-=6H.sup.+ (33)
Positive
Electrode:BrO.sub.3.sup.-+6H.sup.+-6e.sup.-=Br.sup.-+3H.sub.2O
(34)
[0235] The latter electrode half-reaction may proceed not by a
direct electroreduction of a bromate species on the electrode but
via the formation of a Br.sub.2 intermediate in a homogeneous
comproportionation reaction between bromate and bromide as shown
below:
Comproportionation:BrO.sub.3.sup.-+5Br.sup.-+6H.sup.+=3Br.sub.2+3H.sub.2-
O (35)
Reduction:3Br.sub.2+6e.sup.-=6Br.sup.- (36)
[0236] An extra acid, for example a strong acid, HA, such as
H.sub.2SO.sub.4, LiHSO.sub.4, HCl, HNO.sub.3, HClO.sub.4,
F.sub.3CSO.sub.3H, F.sub.3CCOOH, etc., can be added in a small
concentrations compared to the total aqueous multi-electron oxidant
(AMO) concentration to accelerate the rate of reaction (20) on
discharge. The use of such an extra acid may be more advantageous
than an increase in the phosphoric acid (H.sub.3PO.sub.4)
concentration, which is a weak acid and which is limited by the
properties of Li.sub.2HPO.sub.4 decomposing in water into a very
soluble LiH.sub.2PO.sub.4 and a poorly soluble Li.sub.3PO.sub.4. In
an embodiment, bromic acid itself is used as the extra acid. The
use of a higher acid concentration, afforded by adding the extra
acid, facilitates the rate of the comproportionation because for a
general acid-catalyzed reaction such as
BrO.sub.3.sup.-+5Br.sup.-+6H.sup.+=3Br.sub.2+3H.sub.2O, the same
rate can be obtained with a lower concentration of a strong extra
acid such as HClO.sub.4 than with a weaker acid such as
H.sub.3PO.sub.4. A smaller concentration of the extra acid,
compared to the concentration of phosphoric acid that shows
comparable rate constant for the comproportionation, requires a
smaller charge in the orthogonal ion migration across laminar flow
(OIMALF) process, thus reducing the energy expenses and the size of
the OIMALF reactor 1501. For purposes of illustration, the detailed
description is described with reference to an OIMALF process for
conversion of the salt form of the AMO into the acid form of the
AMO; however the scope of the method and system disclosed herein is
not limited to the OIMALF process but can be extended to include
other processes such as a ion exchange on resins, a direct addition
of the extra acid, and can be justified in other applications.
[0237] In an embodiment, aqueous multi-electron oxidant (AMO) in a
stable form, for example, LiBrO.sub.3 is converted, at least
partially, into an active form, for example, HBrO.sub.3, using, for
example, ion exchange on resins or ion exchange in solution such as
an orthogonal ion migration across laminar flow (OIMALF) within the
discharge system 101. The resulting acidic oxidant fluid 1603
comprising bromate as the aqueous multi-electron oxidant (AMO) is
used in the discharge unit 104. This is followed by discharge of
hydrogen on negative electrodes 205b of discharge cells and bromate
on the positive electrodes 205a of discharge cells of the
electrolytic cell stack 105, with a release of bromide and water on
the positive electrodes 205a of discharge cells, provided that the
discharge cells are equipped with cation-conductive membranes such
as Nafion.RTM. or its analogues. In an embodiment, the discharge on
the positive electrodes 205a is facilitated by a homogeneous
comproportionation of bromide product with bromate oxidant, or in
general of a halide with a halogen oxoanion. The discharge process
based on the sequence of orthogonal ion migration across laminar
flow (OIMALF), comproportionation, and electroreduction process has
a reasonably high projected energy efficiency of about 70%. For
on-board operation, the OIMALF reactor 1501 is operably connected
to an on-board power source such as the discharge unit 104 or a
battery (not shown) which provides electric power for the OIMALF
process.
[0238] The regeneration process is preceded by raising the pH of
one or more forms of the discharge fluid 1605 with a base, for
example, Na.sub.2HPO.sub.4, LiOH or Li-3-(N-morpholino)
propanesulfonic acid (MOPS) in the neutralization reactor 1501b of
the discharge system 101 exemplarily illustrated in FIG. 15. The
acidic discharge fluid comprises one or more of water, a halide, a
hydroxonium cation, an extra acid, and a counter cation.
Neutralization 1606 is a chemical reaction in which a base and an
acid react to form a salt. The neutralization reactor 1501b
neutralizes 1606 the acidic discharge fluid 1605 into neutral
discharge fluid 1607 which is safe to handle, for example, to
transfer to an off-board regeneration system 106. The base
generated as a result of the orthogonal ion migration across
laminar flow (OIMALF) process is used during the process of
neutralization 1606 of the acidic discharge fluid 1605, for
example, comprising HBr. The neutralization 1606 can be performed
using an OIMALF reactor 1501. In an embodiment, some process steps
of the energy cycle, for example, neutralization 1606 and
acidification 1602 can be combined in a single reactor such as
1501. In another embodiment, the concentration can precede
conversion to acid.
[0239] The aqueous multi-electron oxidant (AMO) and the reducer are
regenerated in stoichiometric amounts from the discharge fluid in
the regeneration system 106. The splitting-disproportionation (SD)
1608 process disclosed herein for the regeneration of the oxidant
fluid comprising the AMO, for example, bromate from the neutral
discharge fluid 1607 comprising, for example, bromide starts with
an optional pH optimization of the discharge fluid for the
disproportionation step. The pH optimization can be performed
within the discharge system 101 or within the regeneration system
106 or in both by adding acid or base to the discharge fluid in
question via electrolysis, ion exchange on solids, ion exchange in
solution such as orthogonal ion migration across laminar flow
(OIMALF), etc. and any combination thereof. A buffer present in one
or more forms of the oxidant fluid and/or the discharge fluid is
used to facilitate the pH optimization. During the regeneration of
the AMO and the reducer, the splitting-disproportionation (SD)
reactor 1502 of the regeneration system 106 splits 1608 the neutral
discharge fluid 1607 comprising halide into an intermediate oxidant
such as a halogen accompanied by a release of the reducer 1604 such
as hydrogen and a base form of the buffer. In the case of splitting
being electrolysis, the intermediate oxidant is produced at the
positive electrode of the SD reactor 1502, and the reducer and the
base are produced at a negative electrode of the SD reactor 1502.
In an embodiment, the SD reactor 1502 is configured as an
electrolysis-disproportionation reactor 107 and is powered by the
off-board electric circuit 409. The neutral discharge fluid 1607
comprising, for example, LiBr and H.sub.2O undergoes electrolysis,
photolysis, photoelectrolysis, radiolysis, or thermolysis to the
intermediate oxidant such as Br.sub.2 at the positive electrode
and, for example, H.sub.2 and LiOH or H.sub.2 and
Li-3-(N-morpholino) propanesulfonic acid (MOPS) at the negative
electrode. The process of splitting 1608 is accompanied by the
release of the reducer 1604, for example, hydrogen in
stoichiometric amounts which is used as the reducer 1604 in the
discharge unit 104. H.sub.2 is produced on the negative electrode,
configured for use with a liquid electrolyte, leaving behind an
aqueous base solution, for example LiOH:
6H.sub.2O+6e.sup.-+6Li.sup.+(aq.)=6LiOH+3H.sub.2 (37)
[0240] The liquid containing the base, such as LiOH, and the
hydrogen gas are separated in separation reactors 1010. The
regenerated hydrogen is collected in fuel storage tank or the
regenerated reducer fluid tank 111, while the base-containing
liquid is pumped into the positive electrode compartment. On the
positive electrode, halogen X.sub.2 is produced:
6X.sup.--6e.sup.-=3X.sub.2(aq.) (38)
[0241] In the presence of the base, provided that the pH of the
liquid in the positive electrode compartment is maintained at a
proper level, for example, between 4 and 9, or between 6 and 8,
using an appropriate buffer, such as monohydrogen phosphate, a
substituted phosphonate, amine, one or more of Good's buffers, or a
combination thereof, the halogen disproportionates producing the
desired aqueous multi-electron oxidant (AMO) such as a halate. For
example, with A.sup.- as the base:
3X.sub.2(aq.)+6LiA+3H.sub.2O.dbd.LiXO.sub.3+5LiX+6HA (39)
[0242] In an embodiment, the base form of the buffer is obtained by
a reaction of the neutral form of the buffer generated in the
disproportionation reaction with the base produced at the negative
electrode: 6LiOH+6HA=6LiA. The water necessary to prevent drying
and LiOH precipitation on the negative electrode in the no-aqueous
multi-electron oxidant (AMO)-on-negative mode of operation is
supplied from the positive electrode compartment via electro
osmotic drag by Li.sup.+ cations, by pressure-driven flow through
the membrane, etc., or from a separate water tank 1504. This excess
water can be removed from the regenerated fluid using the
concentrating reactor 112 using reverse osmosis, evaporation,
pervaporation, etc. and stored in water tank 1504.
[0243] In the case of Br.sub.2, if the pH of the anolyte is
maintained between 6 and 8, or between 4 and 9, a
disproportionation 1608 to bromate occurs, for example, with a LiOH
base: 3Br.sub.2+6LiOH=5LiBr+LiBrO.sub.3+3H.sub.2O. Splitting 1608
of the LiBr+H.sub.2O solution and the disproportionation 1608
reactions proceed in a cyclic fashion or in a cascade, in batches
or continuous modes, till most of the LiBr is converted into
LiBrO.sub.3. The disproportionation of the intermediate oxidant
such as halogen into aqueous multi-electron oxidant (AMO) can be
implemented in AMO-on-negative mode of operation and in
no-AMO-on-negative mode of operation which require different
hardware designs. The base required for the disproportion of
halogen produced on the positive electrode during regeneration is
conveniently produced as a by-product of hydrogen evolution on the
negative electrode. There are two possible methods for introducing
the base into the regenerated solution, that is, the
AMO-on-negative electrode mode of operation and the
no-AMO-on-negative electrode mode of operation as well as multiple
combinations of the two. These are illustrated in FIGS. 17A-17B and
FIG. 18 using XO.sub.3.sup.- or bromate as AMO, M.sup.+ or Li.sup.+
as the counter-cation, and A.sup.- as the base form of the buffer.
The AMO-on-negative mode of operation is exemplarily illustrated in
FIGS. 17A-17B for a cyclic flow mode. The multilayer structure of
the negative electrode configured for this mode and the operation
of the SD reactor 1502 is disclosed in the detailed description of
FIG. 17A-17B. The no-AMO-on-negative mode of operation is
exemplarily illustrated in FIG. 18 for a batch mode. Various modes
of regeneration namely batch, flow-cycle, flow-cascade can be
combined with either the AMO-on-negative and no-AMO-on-negative
modes of operation.
[0244] Li.sup.+ can be used as a counter-cation to achieve high
solubilities of the salts involved, such as bromide and bromate. A
pH buffer comprising, for example, a dissolved base, LiA, such as a
lithium alkylphosphonate or arylphosphonate, an amine or amines
such as one or more of Good's buffers is used to prevent spatial
and temporal deviations of pH from the range between 4 and 9, for
example, between 6 and 8, within the disproportionation reactor.
The resulting product, for example, LiBrO.sub.3, in the off-board
neutral oxidant fluid 1601, if needed or desired, can be
concentrated off-board in the neutral oxidant fluid 1601 using the
concentrating reactor 112 before the neutral oxidant fluid 1601 is
placed on-board. The neutral oxidant fluid is stable, non-corrosive
and safe to handle, thus allowing for it transfer between off-board
and on-board tanks and on-board storage without undue risk and
without extraordinary precautions. Furthermore, the on-board
storage of the neutral oxidant fluid 1601 mitigates the risk of
spillage of the neutral oxidant fluid 1601 in the case of an
accident. The net balanced chemical equation of regeneration for an
exemplary combination of the aqueous multi-electron oxidant (AMO)
and the buffer is:
LiBr+3H.sub.2O=(electricity in two places,LiA
recycled)=3H.sub.2+LiBrO.sub.3 (40)
[0245] The splitting-disproportionation (SD) process converts, for
example, concentrated LiBr in the neutral discharge fluid 1607 into
a concentrated LiBrO.sub.3 in the neutral oxidant fluid 1601.
Nevertheless, upon numerous discharge-regeneration cycles the
solutions get diluted due to accumulation of water. To keep the
energy density of the neutral oxidant fluid 1601 high, a water
removal process is performed occasionally, for example, as part of
the off-board regeneration before placing the neutral oxidant fluid
1601 on board. The commercial process of concentrating salts uses
evaporation, with an estimated energy loss of approximately 10-15%
if heat exchangers are used. The reverse osmosis process requires
overcoming of the osmotic pressure, for example, of about 536 bars,
which is possible in a cascade flow mode with commercial supported
ion exchange membranes. The minimal energy expense at an infinitely
slow filtration rate is, for example, about 7% of the energy
content of the product 10M LiBrO.sub.3 and 3H.sub.2. Due to a
finite flow rate, the regeneration process disclosed herein uses
optimization of the unit size, power, and operating pressure in
terms of the energy efficiency and capital cost.
[0246] The splitting-disproportionation 1608 cycle continues in the
same flow or batch SD reactor 1502 till the [bromide]/[bromate]
concentration ratio decreases to the desired value, for example,
below 0.05. The resulting neutral oxidant fluid 1601, for example,
approximately 5-10 M LiBrO.sub.3, can be further concentrated, for
example, to about 10-20 M, using reverse osmosis, evaporation or
other methods known in the art. The use of evaporation for
concentrating has an additional advantage of producing a hot
solution of LiBrO.sub.3 which has almost twice the solubility of a
cold solution of LiBrO.sub.3. The concentrated solution, for
example, approximately 10M LiBrO.sub.3 solution, the concentration
of which is limited by the solubility of LiBrO.sub.3 at the
operating temperature, for example, about 20.degree. C., then goes
back into the orthogonal ion migration across laminar flow (OIMALF)
reactor 1501, where Li.sup.+ in LiBrO.sub.3 is exchanged for
H.sup.+ from the incoming HBr, thus producing for example, a
solution comprising 0.5M HBrO.sub.3 and 9.5M LiBrO.sub.3. Further
exchange for Li.sup.+ for H.sup.+ is unnecessary since the ignition
regime of electroreduction is already observed at such composition
and may be detrimental due to reduced stability of bromate, which
decomposes with oxygen evolution in highly acidic solutions.
[0247] The hot solution of LiBrO.sub.3 can be pumped to an on-board
oxidant storage tank 102 where it may be allowed to cool with
precipitation of solid LiBrO.sub.3, thus increasing the theoretical
energy density of the on-board discharge system 101. The heat
released during the cooling and precipitation of the hot
concentrated solution of LiBrO.sub.3 can be used to preheat the
neutral discharge fluid 1607 or the neutral oxidant fluid 1601
prior to their use. The neutral oxidant fluid 1601 undergoes
acidification 1602 in the acidification reactor 1501a. The
precipitated LiBrO.sub.3 can be re-dissolved in water or in an
acidic discharge fluid and delivered as the acidic oxidant fluid
1603 to the discharge unit 104 for producing electric energy. The
hardware components of the hydrogen-bromate energy cycle disclosed
herein comprise analytical chemical detectors (not shown) used for
process monitoring and control.
[0248] In an embodiment, in the first step in the scheme of
regeneration, the halogen and a stoichiometric amount of hydrogen
are regenerated by sunlight energy harvesting, that is, via
photolysis or photoelectrolysis of the spent hydrogen halide. In
this embodiment, the splitting-disproportionation reactor 1502 is
configured as a photoelectrolysis-disproportionation reactor (not
shown). A decomposition into H.sub.2 and X.sub.2 is induced in the
discharge fluid in the photoelectrolysis-disproportionation reactor
by irradiating the discharge fluid with sunlight in the presence of
a photocatalyst such as a semiconductor. The regeneration system
106 disclosed herein comprising the
photoelectrolysis-disproportionation reactor, regenerates the
aqueous multi-electron oxidant (AMO) and the reducer during the
induced reverse electrochemical process by consuming solar energy
and the discharge products.
[0249] Since the regeneration system 106 replaces O.sub.2 with the
aqueous multi-electron oxidant (AMO), the sunlight energy
harvesting method acquires a different perspective. A halogen, for
example, bromine, the first intermediate in the regeneration
process is produced from the spent hydrogen halide
photoelectrochemically with a higher efficiency than water
splitting achieves since there is no oxygen evolution over-voltage,
and at a lower cost than photoelectrochemical water splitting as
the Pt catalyst is not required for oxygen evolution. The
photolysis process and the photoelectrolysis process involve
irradiation of the hydrogen halide solution with light or without
the presence of a light adsorbing facilitator, a catalyst, or a
combination thereof. The light adsorbing facilitator is, for
example, a semiconductor, a dye, a transition metal complex or a
combination thereof. A semiconductor is, for example, TiO.sub.2 in
an anatase or rutile form and preferably in the form of particles
suspended in the solution to be oxidized. The particle surface is
also coated by one or several catalysts to facilitate evolution of
hydrogen and/or halogen.
[0250] The projected performance of the H.sub.2-aqueous
multi-electron oxidant (AMO) discharge system 101 versus a 2012
Toyota RAV4EV lithium ion battery pack and the 2013 ARPA-E targets
are shown in Table 2 below.
TABLE-US-00002 TABLE 2 Toyota H.sub.2-AMO Parameter Units Target
RAV4 EV System Manufacturing cost $/kWh <100-125 500 120
Effective specific energy, Wh/kg >150 110 426 system level
Effective energy density, Wh/L >230 <200 200-400 system level
Effective specific power W/kg >300 303 690 on discharge 80% DOD/
30 s Cycle life at 80% depth Cycles >1000 >1000 1000 of
discharge (DOD) Calendar life Years >10 <8 years >10 (6
operational) Operating temperature .degree. C. >-30 >-30
>-40
[0251] In Table 2, the projected low temperature limit refers to a
cold-start up and it is limited by the freezing or precipitation
point of the neutral oxidant fluid. The cost figures are calculated
based on the design of modern proton exchange membrane fuel cells
(PEMFCs) minus the cost of Pt catalyst on the cathode. The cost
figures do not account for the economy-of-scale discount. The
parameters refer to the system 100 with 5% w/w H.sub.2 storage and
50% w/w/aqueous multi-electron oxidant (AMO) at 78% discharge
efficiency corresponding to 0.5 W/cm.sup.2 power. The power is
calculated for a smooth flow-by carbon cathode on the basis of
kinetic parameters reported in the literature and assuming membrane
resistance of 0.1 ohm/cm.sup.2. A five times higher power can be
reasonably expected from a flow-through porous electrode. The
number shown is the operational not calendar life if the discharge
flow battery is limited by the degradation of Pt on the hydrogen
anode accounting for the oxidant cross-over at open circuit
potential (OCP) on the basis of relevant data for PEFCs. Purging
both electrodes with on-board water on shut-downs can increase the
projected durability. System energy density increases for H.sub.2
storage methods in the order of: 350 bar gas<cryo-liquid<5%
metal hydride. Although the gravimetric specific energy of hydrogen
is high, the volumetric energy density of hydrogen is low even at
the highest practically achievable pressures and hydrogen storage.
Both high pressure carbon composite cylinder and metal hydrides
tank may satisfy the mass and the volume requirements.
[0252] FIGS. 17A-17B exemplarily illustrate mass flows in a single
cell 1700 of a splitting-disproportionation reactor 1502, more
specifically, an electrolysis-disproportionation (ED) reactor 107
configured for regeneration in an aqueous multi-electron oxidant
(AMO)-on-negative electrode mode of operation. FIG. 17A exemplarily
illustrates an operation of a single regeneration flow cell 1700.
LiA is the buffer in the base form, for example,
Li-3-(N-morpholino) propanesulfonic acid (MOPS). In an embodiment,
the ED reactor 107 is configured for the AMO-on-negative mode of
operation using a modified membrane-electrode assembly (MEA) 1701.
The negative electrode layer 1702 of the MEA 1701 of the ED cell
1700 when configured for the AMO-on-negative mode of operation has
a graded and/or multilayer structure and/or composition in order to
avoid and/or minimize on the negative electrode 1702, the reduction
of the AMO regenerated on the positive electrode 1703 while
allowing for H.sub.2 evolution and for maintaining the pH in the
optimal basic range. The side or the negative electrode layer 1702a
closer to a cation-conductive polymer electrolyte membrane 1704
which is the inner layer, is a catalytic layer comprising Pt/C
embedded into a cation-conducting polymer electrolyte (CCPE) such
as Nafion.RTM.. A Pt catalyst is supplied to facilitate the
reduction of H.sup.+ into 1/2H.sub.2. Cations, such as Li.sup.+ and
H.sup.+ and neutral species, such as H.sub.2 and H.sub.2O can
permeate through the CCPE. Anions, such as halate and halide,
permeate the CCPE to a much smaller extent. The outer layer of the
MEA catalytic layer comprises CCPE and carbon but not Pt thus
allowing for the transport of electrons and cations but preventing
the reduction of the AMO species on the negative electrode 1702
during the regeneration.
[0253] In the cyclic flow mode under the aqueous multi-electron
oxidant (AMO)-on-negative mode of operation for a single cell
electrolysis-disproportionation (ED) reactor 107, the solution
containing the AMO is cycled between the negative electrode 1702 of
the ED reactor 107 where neutralization and/or alkalization occurs
and the positive electrode 1703 where electrooxidation and
disproportionation occur. In the cascade flow mode under the
AMO-on-negative mode of operation, the solution containing the AMO
moves between the negative electrode 1702 of one ED flow cell 1700
where neutralization occurs to the positive electrode 1703 of an
adjacent ED flow cell 1700 where electrooxidation and
disproportionation occur. In the cascade flow mode, the regenerated
AMO solution flows through a cascade of functionally identical ED
reactors 107 such as positive electrode compartments of individual
cells 1700.
[0254] An operation of the aqueous multi-electron oxidant
(AMO)-on-negative mode of operation is exemplarily illustrated in
FIG. 17A with an electrolysis-disproportionation (ED) reactor 107
represented by a single cell 1700 operating in the cyclic flow
mode. A neutral discharge fluid, for example, from a car's
discharge tank, or a neutral partially regenerated oxidant fluid,
for example, from a previous regeneration cycle, passes through a
negative compartment and a negative electrode 1702 of the
regeneration flow cell or the ED cell 1700 where hydrogen is
produced and the pH of the discharge fluid is raised. The
AMO-on-negative mode of operation is facilitated via the use of the
outer negative electrode layer 1702b to prevent the access of AMO
anions to the surface of electrocatalysts in the negative electrode
layer 1702a. If the discharge fluid is flushed in the second and
subsequent cycles through the negative electrode 1702 to lower the
pH, an undesirable electroreduction of bromate on an
electrocatalyst may occur:
LiBrO.sub.3+6e.sup.-+6H.sup.+=LiBr+3H.sub.2O (41)
[0255] In this aqueous multi-electron oxidant (AMO)-on-negative
mode of operation, the reduction of the AMO anion species on the
electrocatalysts such as Pt in the negative electrode 1702 can be
minimized or prevented by blocking the surface of Pt by a
cation-selective coating, such as Nafion polymer, by using, for
example, a two layer electrode 1702, with only the inner layer
1702a containing a catalyst, for example, Pt capable of hydrogen
evolution reaction; and the outer layer 1702b exposed to the
flowing electrolyte comprising, for example, a Pt-free porous
carbon containing an electron-conducting component such as carbon
particles and fibers, for providing electronic current between the
inner layer 1702a and a current collector 1705, a cation-selective
component such as Nafion polymer, which allows for cation transport
between the flowing liquid 1009 in the negative electrode
compartment and flowing liquid in the positive electrode
compartment. The structure of the inner layer 1702b is similar to
the modern generation of the catalytic layers of the
membrane-electrode assemblies of polymer electrolyte fuel cells.
The inner layer 1702b allows for a transport of electrons, protons,
and other cations to the Pt electrocatalyst but of anions, thereby
selectively allowing hydrogen production and suppressing AMO
reduction.
[0256] A more detailed illustration of the chemistry aspects of the
aqueous multi-electron oxidant (AMO)-on-negative mode of operation
at the membrane-electrode assembly 1701 level is exemplarily
illustrated in FIG. 17B using the first cycle with the charge of
one electron per bromide. FIG. 17B shows an operation of an ED
reactor 107 in the flow modes and the AMO-on-negative mode of
operation showing neutralization performed at the negative
electrode 1702 with a multilayer structure. Water flux through
membrane is not shown. 1 electron per cycle is shown as a means of
illustration not of limitation. HA is, for example,
Li-3-(N-morpholino) propanesulfonic acid (MOPS). The two layer
negative electrode 1702 is exemplarily illustrated in FIG. 17B with
the inner layer 1702a containing Pt on carbon fibers or particles
embedded into a cation-conductive membrane and the outer layer
1702b made of carbon fibers or particles without Pt and embedded
into a cation-conductive membrane. The inner layer 1702a with Pt
allows for hydrogen evolution reaction to occur while preventing
the electroreduction of bromate on Pt.
[0257] FIG. 18 exemplarily illustrates mass flows in a single cell
1700 of a splitting-disproportionation reactor 1502, more
specifically, an electrolysis-disproportionation (ED) reactor 107
configured for regeneration in a no-aqueous multi-electron oxidant
(AMO)-on-negative electrode mode of operation and a batch mode.
Only the first two e.sup.-/X.sup.- cycles are shown. There are two
modes of operation for proceeding with the ED regeneration cycle:
with and without passing AMO through the negative electrode 1702.
Furthermore, each of these two modes of operation can be
implemented in a batch mode or in a flow mode. The flow mode can be
implemented in a cyclic flow mode or in a cascade flow mode.
Furthermore, these different modes of operation can be combined
within one ED cell 1700, within a single ED reactor 107, and within
one regeneration system 106. In an embodiment, the ED reactor 107
is configured for the no-AMO-on-negative mode of operation using an
additional mixing reactor (not shown) to add a base produced on the
negative electrode 1702 to the AMO containing fluid on the positive
electrode 1703. The no-AMO-on-negative mode of operation avoids
exposure of the AMO to the negative electrode(s) 1702 in the ED
reactor 107 and instead relies on the transfer of a base produced
on the negative electrode 1702 during the hydrogen evolution or
generation reaction, for example,
H.sub.2O+e-+M.sup.+=1/2H.sub.2+MOH (42)
to the disproportionation reactor which can be the positive
electrode compartment as exemplarily illustrated in FIG. 18. Only
shown are the first two electrolysis-disproportionation (ED)
cycles. Water fluxes are not shown.
[0258] In this no-aqueous multi-electron oxidant (AMO)-on-negative
mode of operation, the electroreduction of the AMO on the negative
electrode 1702 of the ED cell 1700 can be prevented by preventing
the flow of the AMO-containing fluid through the negative electrode
1702. The base such as MOH shown in FIG. 18, produced on the
negative electrode 1702 in reaction (42) and required for the
disproportionation can be carried over from the negative electrode
1702 to the positive electrode 1703 with a solvent such as water.
During the regeneration ED cycles, this water can be supplied to
the negative electrode 1702 from an external source or from the
positive electrode 1703 through the cation-conductive polymer
electrolyte membrane 1704 by one or a combination of the following:
electro osmotic drag with M.sup.+, by applying pressure to the
positive electrode 1703, by other methods known in the art. This
excess water may be separated from ionic components in the oxidant
fluid, yielding concentrated AMO solution suitable for an on-board
use, produced on the positive electrode 1703, using one or more of
the following: distillation, reverse osmosis, evaporation,
nanofiltration, pervaporation, ion exchange, freezing, other
methods known in the art, and by any combination thereof.
[0259] The no-aqueous multi-electron oxidant (AMO)-on-negative mode
of operation uses a less complicated structure of the positive
electrodes 1703 of the ED reactor 107, and when a LiOH base with
solubility over 5 molal is used, it can provide a practical and
useful system power density which, nevertheless, can be limited by
the maximal sustainable pH gradient across the cation-conductive
polymer electrolyte membrane 1704. On the other hand, the
AMO-on-negative mode of operation does not suffer from poor
solubility of the base transferred and it overcomes a potential
problem of the instability of aqueous Li.sub.2HPO.sub.4 toward
decomposition into Li.sub.3PO.sub.4 (solid) and LiH.sub.2PO.sub.4
(solute) by consuming hydrogen phosphate in the disproportionation
before the aqueous Li.sub.2HPO.sub.4 decomposes.
[0260] FIG. 19 exemplary illustrates a mass and electricity flow
diagram of a discharge system 101 comprising a single cell
discharge unit 104 and an orthogonal ion migration across laminar
flow (OIMALF) reactor 1501, exemplarily illustrated in FIG. 1 and
FIG. 15. The aqueous multi-electron oxidant (AMO)-on-negative mode
of operation is represented using dash-dotted lines and the
no-AMO-on-negative mode of operation is represented using dotted
lines LiBrO.sub.3, H.sub.3PO.sub.4, LiZ chemistry is exemplarily
illustrated for the sake of illustration and not as a limitation.
The discharge system 101 comprises the OIMALF reactor 1501
represented by a single orthogonal ion migration across laminar
flow (OIMALF) cell 1900, the discharge unit 104 represented by a
single discharge cell 104a, connecting electric lines, hoses,
valves, and an electric management system (not shown). The OIMALF
reactor 1501 comprises an OIMALF cell stack (not shown) which is
configured similar to a polymer electrolyte fuel cell (PEFC) stack
but with a liquid electrolyte flowing between two ionically
conducting membranes. The OIMALF reactor 1501 comprises endplates
1902a and 1902b and bipolar plates (not shown), and the OIMALF flow
cell assembly 1901 as disclosed in the detailed description of FIG.
15. Each flow cell assembly 1901 of the OIMALF reactor 1501
comprises an intermembrane flow field (not shown) with multiple
OIMALF flow channels 1903, two layers of an ion exchange membrane
comprising a positive side ion exchange membrane 1904a and a
negative side ion exchange membrane 1904b positioned parallel to
each other on each side of the intermembrane flow field, a positive
electrode layer 1905a and a negative electrode layer 1905b flanking
outer surfaces of the ion exchange membranes, and porous diffusion
layers 1905a and 1905b flanking the outer surfaces of the ion
exchange membranes and in electric contact with the adjacent
bipolar plates or endplates 1902a and 1902b. The positive electrode
layer 1905a is configured for a hydrogen oxidation reaction and the
negative electrode layer 1905b is configured for a hydrogen
evolution reaction. Two modes of neutralizing the discharge fluid
are exemplarily illustrated in FIG. 19: (i) directly at the
negative electrode(s) 1905b in the OIMALF flow cell 1900 which
requires graded/multilayer negative electrode layers 1905b in the
OIMALF reactor 1501 to prevent the reduction of an AMO anion on the
catalyst surface of the negative electrode(s) 1905b, and (ii)
indirectly in a neutralization reactor 1501b, using the base such
as LiOH produced at the negative electrode(s) 1905b of the OIMALF
cell stack.
[0261] The orthogonal ion migration across laminar flow (OIMALF)
reactor 1501 or the OIMALF cell 1900 converts the salt forms of the
aqueous multi-electron oxidant (AMO), for example, aqueous
LiBrO.sub.3 and of the other components of the neutral oxidant
fluid, for example, LiH.sub.2PO.sub.4, and of the extra acid, for
example, LiZ into acidic oxidant fluid which comprises their acid
forms, for example, HBrO.sub.3, HA, etc. A complete conversion of
the salt form of the AMO into the acidic form is not necessary and
a partial conversion is suitable in many applications. An acid
concentration, for example, below 1M may cause an ignition regime
provided that the total concentration of all forms of the aqueous
multi-electron oxidant (AMO) is maintained high, for example, over
1 m and the thickness of the diffusion boundary layer is large, for
example, over 1 micron. The buffer, for example, one or more forms
of phosphate is present in the acidic oxidant fluid because it is
carried over from the splitting-disproportionation (SD) reactor
1502 of the regeneration system 106 where the base form of the
buffer is used in the disproportionation reaction such as the one
shown below:
3Br.sub.2+6LiA+3H.sub.2O=5LiBr+LiBrO.sub.3+6HA (43)
[0262] An extra acid, for example, HA, such as H.sub.2SO.sub.4,
LiHSO.sub.4, HCl, HNO.sub.3, HClO.sub.4, CF.sub.3SO.sub.3H, etc.,
can be added to accelerate the rate of comproportionation as shown
in equation (44) below on discharge. The use of such an extra acid
may be more advantageous than an increase in the phosphoric acid
(H.sub.3PO.sub.4) concentration which is a weak acid. The use of a
higher acid concentration, afforded by adding the extra acid,
facilitates the rate of the comproportionation because for a
general acid-catalyzed reaction such as:
BrO.sub.3.sup.-+5Br.sup.-+6H.sup.+=3Br.sub.2+3H.sub.2O (44)
[0263] The same rate can be obtained with a lower concentration of
a strong extra acid, such as HClO.sub.4 or F.sub.3CSO.sub.3H than
with a weaker acid such as H.sub.3PO.sub.4. A smaller concentration
of the extra acid, compared to the concentration of phosphoric acid
that shows comparable rate constant for the comproportionation,
requires a smaller charge in the orthogonal ion migration across
laminar flow (OIMALF) process, thus reducing the energy expenses
and the size of the OIMALF reactor 1501. For purposes of
illustration, the detailed description is described with reference
to an OIMALF process for conversion of the salt form of the aqueous
multi-electron oxidant (AMO) into the acid form of the AMO; however
the scope of the method and system disclosed herein is not limited
to the OIMALF process but can be extended to include other
processes such as a ion exchange on resins and in other
applications.
[0264] The acidic oxidant fluid travels from a central compartment
1903 of the orthogonal ion migration across laminar flow (OIMALF)
cell 1900 to the positive electrode compartment of the discharge
cell 104a of the discharge unit 104 where the acidic oxidant fluid
undergoes electroreduction and comproportionation as shown
below.
3Br.sub.2+6e.sup.-=6Br.sup.- (45)
BrO.sub.3.sup.-+5Br.sup.-+6H.sup.+=3Br.sub.2+3H.sub.2O (46)
[0265] The reducer, for example, H.sub.2, undergoes
electrooxidation, represented by: 3H.sub.2-6e.sup.-=66H.sup.+, at
the negative electrode 205b of the discharge cell 104a. The
discharge system 101 produces electric power for the consumer and,
if needed, for operating the orthogonal ion migration across
laminar flow (OIMALF) reactor 1501.
[0266] The final step performed by the discharge system 101 is
neutralization of the acidic discharge fluid. For the aqueous
multi-electron oxidant (AMO)-on-negative mode of operation to
neutralization, the acidic discharge fluid comprises, for example,
one or more forms of water, HBr, H.sub.3PO.sub.4, and HA such as
H.sub.2SO.sub.4, F.sub.3CSO.sub.3H, etc., in concentrations
between, for example, about 1 mM and 20 M. In an embodiment, the
acidic discharge fluid comprises, for example, one or more of
water, a halide, a hydroxonium cation, and a counter cation. In an
embodiment, the acidic discharge fluid produced at the positive
electrode compartment of the discharge cell 104 flows through or by
the negative electrode 1905b of an orthogonal ion migration across
laminar flow (OIMALF) cell 1900 where a hydrogen evolution or
production reaction and pH increase occur as shown by the equations
below:
HA+e.sup.-+Li.sup.+.dbd.LiA+1/2H.sub.2 (47)
[0267] The negative electrodes 1905b of the orthogonal ion
migration across laminar flow (OIMALF) reactor 1501 can take
advantage of the Pt-free electron and cation conductive inner layer
similar to the negative electrodes 1702 of the regeneration cells
or the SD cells 1700 configured for the aqueous multi-electron
oxidant (AMO)-on-negative mode of operation, to prevent the
electroreduction of residual AMO in the discharge fluid. Such a
layer, however, is not necessary if the discharge of the AMO in the
discharge unit 104 proceeds to near completion so that little AMO
is present in the acidic discharge fluid.
[0268] In other words, as exemplarily illustrated in FIG. 19, the
acidic discharge fluid is passed over the negative electrode 1905b
of the orthogonal ion migration across laminar flow (OIMALF)
reactor 1501 where the acidity of the discharge fluid is lowered
via a hydrogen evolution or production reaction with a simultaneous
replacement of H.sup.+ with a cation from the salt of the aqueous
multi-electron oxidant (AMO), for example, a Li.sup.+ cation. This
produces a neutralized discharge fluid and avoids the formation and
handling of corrosive and moderately soluble LiOH and is utilized
in on-board applications.
[0269] Alternatively, if the no-aqueous multi-electron oxidant
(AMO)-on-negative mode of operation is implemented in the SD
reactor 1502, a base such as LiOH, produced in the negative
electrode compartment of the SD reactor 1502 is mixed with the
acidic discharge fluid allowing for the following chemical
processes to occur:
HBr+xHA+(1+x)LiOH.dbd.LiBr+(1+x)H.sub.2O+xLiA (48)
[0270] The neutralized discharge fluid passes first through a
negative electrode 1702 of a regeneration flow cell or SD cell
1700, where the neutralized discharge fluid is converted into
alkaline regenerated fluid and H.sub.2 as shown below. The alkaline
regenerated fluid and H.sub.2 are separated in the separation
reactor 1010, exemplarily illustrated in FIG. 10B and FIG. 17A.
LiBr+xLiH.sub.2PO.sub.4+zLiZ+xe.sup.-+xLi.sup.+=LiBr+xLi.sub.2HPO.sub.4+-
zLiZ+x/2H.sub.2 (49)
[0271] During the regeneration process the neutral discharge fluid
passes through the positive electrodes 1703 of the SD reactor 1502
or the regeneration system 106. At the positive electrode 1703 of
the SD cells 1700 also referred herein as the "regeneration flow
cell", bromide is oxidized into bromine and bromine
disproportionates into bromide and bromate by reacting with water
in the presence of an alkaline form of the buffer, for example,
A.sup.-
LiBr+-xe.sup.--xLi.sup.+=(1-x)LiBr+x/2Br.sub.2 (50)
(1-x)LiBr+x/2Br.sub.2+xLiA+x/2H.sub.2O=(1-x/6)LiBr+x/6LiBrO.sub.3+xHA
(51)
[0272] A complete regeneration of the aqueous multi-electron
oxidant (AMO) may not be necessary and a partially regenerated,
that is with LiBrO and LiBr present, neutral or near-neutral
oxidant fluid can be loaded on-board. The cycle of
electrooxidation-disproportionation (ED) can be continued in a
batch mode, cyclic flow mode, cascade flow mode or in any
combination thereof using one or more regeneration systems 106
configured for such a mode. The cycle or cascade of regeneration is
continued till the desired ratio of [Br.sup.-]/[BrO.sub.3.sup.-] is
obtained. The cascade flow mode provides a higher throughput and
the cyclic flow mode provides a lower capital cost. The cascade
flow mode of regeneration is utilized for multi-user facilities and
the cyclic flow mode is utilized for at-home regeneration. Based on
the equations (23)-(24), 6 cycles are needed to convert all bromide
into bromate. However, a smaller or larger number can be used in
practice since a 100% conversion of bromide to bromate is not
necessary either in a single SD cycle or in a complete regeneration
process for the working of the disclosed energy cycle.
[0273] FIG. 20A illustrates a method for producing electric power
from an aqueous multi-electron oxidant and a reducer and for
simultaneously generating a discharge fluid. The method disclosed
herein provides 2001 the discharge system 101 comprising one or
more forms of a reducer fluid, one or more forms of an oxidant
fluid, the discharge unit 104, and the acidification reactor 1501a
exemplarily illustrated in FIG. 15. The method disclosed herein
facilitates 2002 discharge of the discharge unit 104 for producing
electric power from a neutral oxidant fluid comprising one or more
forms of the aqueous multi-electron oxidant, and from the reducer
fluid comprising one or more forms of the reducer, for example,
hydrogen. In an embodiment, the reducer is selected from a group
consisting of ammonia, hydrazine, hydroxylamine, phosphine,
methane, a hydrocarbon, an alcohol such as methanol, ethanol, etc.,
an aldehyde, a carbohydrate, a hydride, an oxide, a sulfide, an
organic compound, an inorganic compound, and any combination
thereof, with each other, with water, or with another solvent. The
facilitation of discharge comprises lowering 2002a pH of the
neutral oxidant fluid in the acidification reactor 1501a for
generating an acidic oxidant fluid, transferring 2002b electrons
from the positive electrode 205a of the electrolyte-electrode
assembly 205 to the aqueous multi-electron oxidant in the acidic
oxidant fluid, and transferring electrons from the reducer fluid to
the negative electrode 205b of the electrolyte-electrode assembly
205 to produce electric power in the external electric circuit
operably connected to the terminals of the discharge unit 104 and
to generate an acidic discharge fluid on consumption of the acidic
oxidant fluid and the reducer fluid. A limiting current of the
transfer of the electrons from the positive electrode 205a of the
electrolyte-electrode assembly 205 to the aqueous multi-electron
oxidant in the acidic oxidant fluid in an ignition regime is
limited, for example, by a mass-transport of the aqueous
multi-electron oxidant, a mass-transport of acidic protons, and a
rate of comproportionation. The transfer of electrons from the
positive electrode 205a of the electrolyte-electrode assembly 205
to the aqueous multi-electron oxidant in the acidic oxidant fluid
is performed at a high current density and at low flow rates in an
ignition mode of operation of the discharge system 101. The acidic
discharge fluid comprises, for example, one or more of water, a
halide, a hydroxonium cation, an extra acid, and one or more
counter cations. In an embodiment, the method disclosed herein
further comprises optionally neutralizing the acidic discharge
fluid in the neutralization reactor 1501b to produce a neutral
discharge fluid. In an embodiment, the method disclosed herein
further comprises regenerating a certain amount of an intermediate
oxidant and the reducer in the discharge unit 104 from the acidic
discharge fluid by applying an electric current of a polarity
opposite to the polarity of electric current through the discharge
unit 104 during discharge.
[0274] In an embodiment, the generation of the acidic oxidant fluid
from the neutral oxidant fluid is performed in the acidification
reactor 1501a via an electric field driven orthogonal ion migration
across laminar flow process. In another embodiment, the generation
of the acidic oxidant fluid from the neutral oxidant fluid is
performed, for example, by one or more of an ion exchange on
solids, an ion exchange in liquids, electrolysis, and adding an
extra acid to the neutral oxidant fluid during discharge of the
discharge unit 104. In an embodiment, the discharge is facilitated
on the positive electrode 205a of the electrolyte-electrode
assembly 205, for example, by one or more of electrocatalysis, a
solution-phase chemical reaction, a solution-phase
comproportionation, a solution-phase redox catalysis, a
solution-phase redox mediator, an acid-base catalysis, and any
combination thereof. In another embodiment, the discharge process
is facilitated via a solution-phase comproportionation of the
aqueous multi-electron oxidant with a final product of a reduction
of the aqueous multi-electron oxidant. In an embodiment, the
solution-phase comproportionation is pH-dependent and the discharge
is facilitated in the presence of an acid.
[0275] FIG. 20B illustrates a method for regenerating an aqueous
multi-electron oxidant and a reducer in stoichiometric amounts from
one or more forms of a neutral discharge fluid using external
power. The discharge fluid comprises, for example, one or more of
water, a halide, a hydroxonium cation, a buffer, and one or more
counter cations. In the method disclosed herein, one or more forms
of a buffer are present in the oxidant fluid and in the discharge
fluid, but the buffer is not essential for the discharge. The
method disclosed herein comprises converting 2003 the neutral
discharge fluid into an alkaline discharge fluid by using an
externally supplied base and/or a base produced in the
splitting-disproportionation reactor 1502 exemplarily illustrated
in FIG. 15, configured for an aqueous multi-electron
oxidant-on-negative mode of operation, a no-aqueous multi-electron
oxidant-on-negative mode of operation, or a combination
thereof.
[0276] The pH of the alkaline discharge fluid is optimized and
stabilized in the splitting-disproportionation reactor 1502 using a
buffer present in one or more forms of the discharge fluid to
facilitate disproportionation of the intermediate oxidant into one
or more forms of the aqueous multi-electron oxidant. The pH of the
alkaline discharge fluid is maintained between 6 and 10, for
example, between 4 and 9. The buffer is configured to maintain the
pH of the alkaline discharge fluid between 6 and 10, for example,
between 4 and 9. In an embodiment, the base component of the buffer
is selected from a group comprising a hydroxide ion, hydrogen
phosphate, a phosphate ester, a substituted phosphonate, an
alkylphosphonate, an arylphosphonate, a deprotonated form of one or
more of Good's buffers, an amine, a nitrogen heterocycle, and any
combination thereof. In an embodiment, the cationic component of
the buffer comprises a cation of lithium. In another embodiment,
the cationic component of the buffer comprises a cation of
sodium.
[0277] In another embodiment, the anionic component of the buffer
comprises one or more of .omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate. In another embodiment, the
anionic component of the buffer is one or more of
.omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
and 4-(N-morpholino)butanesulfonate and the cationic component of
the buffer is lithium. In another embodiment, the anionic component
of the buffer comprises one or more of an alkylphosphonate and an
arylphosphonate. In another embodiment, the anionic component of
the buffer comprises one or more of an alkylphosphonate, an
arylphosphonate, and a cationic component of the buffer is lithium.
In an embodiment, the base component of the buffer is monohydrogen
phosphate and a cationic component of the buffer is sodium.
[0278] The splitting-disproportionation reactor 1502 splits 2004
the alkaline discharge fluid into a reducer and an intermediate
oxidant. The splitting-disproportionation reactor 1502 converts the
intermediate oxidant into one or more forms of the aqueous
multi-electron oxidant via disproportionation of the intermediate
oxidant with the base. The splitting process releases a
stoichiometric amount of the reducer and the base in the
splitting-disproportionation reactor 1502. The
splitting-disproportionation reactor 1502 continues 2005 the
splitting process and disproportionation in a batch mode of
operation, or a cyclic flow mode of operation, or a cascade flow
mode of operation, or a combination thereof, until a desired degree
of conversion of a discharge product of the aqueous multi-electron
oxidant into one or more forms of the aqueous multi-electron
oxidant is achieved. The splitting-disproportionation reactor 1502
splits the alkaline discharge fluid into the reducer and the
intermediate oxidant, for example, via electrolysis, photolysis,
photoelectrolysis, radiolysis, thermolysis, or any combination
thereof. The process of photolysis and photoelectrolysis of the
alkaline discharge fluid is performed in the presence or absence of
a light adsorbing facilitator, a semiconductor, a catalyst, and any
combination thereof.
[0279] In an embodiment, the splitting-disproportionation reactor
1502 is configured as an electrolysis-disproportionation reactor
107. The electrolysis-disproportionation reactor 107 converts a
neutral discharge fluid into an alkaline discharge fluid by using
an externally supplied base and/or a base produced at one or more
negative electrodes of the electrolysis-disproportionation reactor
107 in an aqueous multi-electron oxidant-on-negative mode of
operation, a no-aqueous multi-electron oxidant-on-negative mode of
operation, or a combination thereof. The
electrolysis-disproportionation reactor 107 splits or electrolyzes
the alkaline discharge fluid into a reducer and an intermediate
oxidant via electrolysis. The process of electrolysis releases a
stoichiometric amount of the reducer and the base at one or more
negative electrodes of the electrolysis-disproportionation reactor
107. The electrolysis-disproportionation reactor 107 converts the
intermediate oxidant produced at one or more positive electrodes of
the electrolysis-disproportionation reactor 107 into one or more
forms of the aqueous multi-electron oxidant via disproportionation
of the intermediate oxidant produced at one or more positive
electrodes with the base. The electrolysis-disproportionation
reactor 107 continues the electrolysis and disproportionation
processes in a batch mode of operation, or a cyclic flow mode of
operation, or a cascade flow mode of operation, or a combination
thereof, until a desired degree of conversion of a discharge
product of the aqueous multi-electron oxidant into one or more
forms of the aqueous multi-electron oxidant is achieved.
[0280] FIG. 20C illustrates a method for producing electric power
and regenerating an aqueous multi-electron oxidant and a reducer in
an energy storage cycle. The method disclosed herein provides 2001
the discharge system 101 comprising one or more forms of a reducer
fluid, one or more forms of an oxidant fluid, the discharge unit
104, the acidification reactor 1501a, and optionally a
neutralization reactor 1501b exemplarily illustrated in FIG. 15.
The method disclosed herein facilitates 2002 discharge in the
discharge unit 104 for producing electric power from a neutral
oxidant fluid comprising one or more forms of the aqueous
multi-electron oxidant (AMO) and from the reducer fluid comprising
one or more forms of the reducer. The pH of the oxidant fluid is
lowered 2002a in the acidification reactor 1501a such as the
orthogonal ion migration across laminar flow (OIMALF) reactor 1501.
The oxidant fluid is converted into an acidic oxidant fluid via an
electric field driven OIMALF process. The discharge system 101
converts or partially converts the AMO in the salt form such as
LiBrO.sub.3 into the AMO in the acid form such as HBrO.sub.3 in the
acidification reactor 1501a. When OIMALF process is employed in the
acidification reactor 1501a and the neutralization reactor(s)
1501b, the conversion of the AMO from the salt form to the acid
form is accompanied by a simultaneous release of stoichiometric
amount of the base form of the buffer for neutralization of the
discharge fluid. The conversion of the salt form of the AMO
produced at the positive electrode into the acid form of the AMO is
performed via an addition of an acid, ion exchange on resins, ion
exchange in solution, for example, an electric field driven
orthogonal ion migration across laminar flow (OIMALF) process in
the acidification reactor 1501a. The conversion of the salt form of
the AMO into the acid form of the AMO in the acidification reactor
1501a is facilitated by an acid, a buffer, electrolysis, ion
exchange in solution, ion exchange on surfaces, or any combination
thereof. In an embodiment, the choice of the acid form of the AMO
can be expanded beyond HBrO.sub.3 to other AMOs comprising, for
example, HClO.sub.4, HClO.sub.3, HClO.sub.2, HClO, HBrO.sub.4,
HBrO.sub.2, HBrO, etc. Phosphoric acid will be present in the
oxidant fluid if phosphate buffer is used during the
regeneration.
[0281] In an embodiment, the conversion of the salt form of the
aqueous multi-electron oxidant (AMO) into the acid form of the AMO
occurs within the acidification reactor 1501a which is used to
produce electric power in combination with the discharge unit 104
and located, for example, on-board of a vehicle. If the
acidification reactor 1501a is an orthogonal ion migration across
laminar flow (OIMALF) reactor, the acidification process occurs by
consuming electric power and by recycling the hydrogen released on
the negative electrode of the acidification reactor 1501a and
electro-oxidized on the positive electrode of the acidification
reactor 1501a. In an embodiment, the hydrogen produced at the
negative electrode or electrodes 1702 in the SD reactor 1502 is
combined with the hydrogen produced at the negative electrode 1905b
of one or many OIMALF reactors 1501 either before or after one or
many OIMALF reactors 1501 or at the negative electrode or
electrodes 1905b of the OIMALF reactor 1501, and the hydrogen is
flown through the flow field of the positive electrode 1905a of one
or many OIMALF reactors 1501. The method disclosed herein reduces
the amount of electric charge utilized by the acidification reactor
1501a for converting the salt form of the AMO into the acid form of
the AMO by adding another acid to the AMO during the discharge
process. In order to reduce the electric charge required by the
acidification reactor 1501a and the degree of conversion required
in the OIMALF process to convert the salt form of the AMO into the
acid form of the AMO, another acid or its anion, for example,
H.sub.2SO.sub.4, HClO.sub.4, F.sub.3CSO.sub.3H, another strong
acid, etc., can be co-present with the AMO during the discharge in
all stages of the energy cycle.
[0282] In another embodiment, the process of on-board acidification
does not comprise orthogonal ion migration across laminar flow
(OIMALF) but rather an addition on an acid stored within the
discharge unit 104. Furthermore, the requirement for storing a
stoichiometric amount of H.sub.2 in the discharge system 101 can be
reduced by up to 20% if an extra H.sub.2 is produced from the
acidic discharge fluid using metals stored in the discharge system
101 as shown in the reaction below.
M(M=Zn,Sn,Fe,etc.)+2HBr.fwdarw.MBr.sub.2+H.sub.2
[0283] Such a metal can be used in a complete energy cycle, with
regeneration performed by splitting of MBr.sub.2 off-board:
MBr.sub.2.fwdarw.M+Br.sub.2
[0284] The discharge unit 104 facilitates discharge by
simultaneously transferring 2002b electrons from a positive
electrode 205a of the 5-layer electrolyte-electrode assembly 206
exemplarily illustrated in FIG. 2, to the aqueous multi-electron
oxidant (AMO) in the acidic oxidant fluid, and transferring
electrons from the reducer fluid to a negative electrode 205b of
the 5-layer electrolyte-electrode assembly 206 to produce electric
power in an external electric circuit operably connected to the
terminals of the discharge unit 104 and to generate an acidic
discharge fluid on consumption of the acidic oxidant fluid and the
reducer fluid. The pH of the acidic discharge fluid in the
acidification reactor 1501a is optionally raised 2002c for
generating a neutral discharge fluid. The generation of electric
power using the AMO, for example, bromine during the discharge is
accompanied by the following half-cell electrochemical
reactions:
Negative Electrode:3H.sub.2+6e.sup.-=6H.sup.+ (52)
Positive
Electrode:BrO.sub.3.sup.-+6H.sup.+-6e.sup.-=Br.sup.-+3H.sub.2O
(53)
[0285] The positive electrode half-reaction (53) may proceed not
only by a direct electroreduction of the aqueous multi-electron
oxidant (AMO), for example, bromate species on the electrode but
rather facilitated via the formation of an intermediate, for
example, bromine in a homogeneous comproportionation reaction, for
example, between bromate and bromide (54) as shown below:
Comproportionation:BrO.sub.3.sup.-+5Br.sup.-+6H.sup.+=3Br.sub.2+3H.sub.2-
O (54)
Electroreduction:3Br.sub.2+6e.sup.-=6Br.sup.- (55)
[0286] In an embodiment, the neutralization reactor 1501b
neutralizes the acidic discharge fluid, for example, by using the
orthogonal ion migration across laminar flow (OIMALF) reactor to
raise the pH of the discharge fluid and produce neutral discharge
fluid. The discharge fluid is then collected in the discharge fluid
tank 113 for subsequent regeneration. The aqueous multi-electron
oxidant (AMO) and the reducer are regenerated 2006 in
stoichiometric amounts from the discharge fluid in the regeneration
system 106. In an embodiment, the pH of the discharge fluid is
optimized by adding or generating an acid or a base to the
discharge fluid. During the regeneration of the AMO and the
reducer, the neutral discharge fluid is converted 2003 into an
alkaline discharge fluid by using an externally supplied base
and/or a base produced in the splitting-disproportionation (SD)
reactor 1502 of the regeneration system 106. The SD reactor 1502
splits 2004 the alkaline discharge fluid at the selected pH into a
reducer and an intermediate oxidant in the SD reactor 1502. The
intermediate oxidant is converted into one or more forms of the AMO
via disproportionation of the intermediate oxidant with the base.
The splitting process releases a stoichiometric amount of the
reducer and the base in the SD reactor 1502. The intermediate
oxidant disproportionates when contacted with a base such as the
base form of the buffer produced at the negative electrode 1702.
The disproportion reaction produces the desired AMO in a stable
salt form, for example, LiBrO.sub.3 as well as discharged oxidant,
for example, LiBr which undergoes further cycles of
splitting-disproportionation until the desired degree of
conversion, for example,
[BrO.sub.3.sup.-]/([BrO.sub.3.sup.-]+[BrO.sup.-]+2[Br.sub.2]+[Br-
.sup.-])>0.95 is achieved. The cycle of
splitting-disproportionation is continued 2005 till the desired
degree of conversion of the discharge product of the aqueous
multi-electron oxidant into one or more forms of the aqueous
multi-electron oxidant is achieved. The regenerated one or more
forms of the oxidant fluid comprising the AMO and the regenerated
reducer fluid comprising the reducer are then supplied 2007 to the
discharge system 101 for facilitating discharge of the discharge
unit 104.
[0287] The pH of the acidic oxidant fluid in the
splitting-disproportionation (SD) reactor 1502 of the regeneration
system 106 exemplarily illustrated in FIG. 15, is optimized and
stabilized using an extra acid present in the acidic oxidant fluid
to facilitate comproportionation of the aqueous multi-electron
oxidant with a final product of a reduction of the aqueous
multi-electron oxidant into the intermediate oxidant. The extra
acid is, for example, one or more of a phosphoric acid, a
3-(N-morpholino)propanesulfonic acid, a
3-(N-morpholino)ethanesulfonic acid, another
.omega.-(N-morpholino)propanesulfonic acid, a methanesulfonic acid,
triflic acid, a substituted sulfonic acid, a substituted phosphonic
acid, a perchloric acid, a sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic acid moieties, and an acid with a
pKa<2. The pH of the acidic discharge fluid is, for example,
below 0, 1, 2, or 3. The concentration of acidic protons in the
acidic discharge fluid is, for example, below one of 0.1M, 0.5M,
1M, 2M, 5M, or 10M.
[0288] FIG. 20D illustrates a method for producing electric power
and regenerating hydrogen and an oxidant fluid comprising lithium
bromate in an energy storage cycle. The method disclosed herein
provides 2001a the discharge system 101 comprising the discharge
unit 104, the acidification reactor 1501a, and optionally the
neutralization reactor 1501b as exemplarily illustrated in FIG. 15.
The discharge system 101 comprises a neutral oxidant fluid
comprising lithium bromate, and hydrogen. In an embodiment, the
discharge system 101 comprises one or more forms of a buffer. In
another embodiment, the discharge system 101 further comprises one
or more forms of a base. In an embodiment, the cationic component
of the buffer is lithium and the anionic component of the buffer
is, for example, one or more of
.omega.-(N-morpholino)alkanesulfonate,
3-(N-morpholino)methanesulfonate, 3-(N-morpholino)ethanesulfonate,
3-(N-morpholino)propanesulfonate, 3-(N-morpholino)butanesulfonate,
other amines, monohydrogen phosphate, methylphosphonate, an
alkylphosphonate, an arylphosphonate, and a molecule comprising
phosphonate moieties and sulfonate moieties. In another embodiment,
the cationic component of the buffer is sodium, and the anionic
component of the buffer is, for example, one or more of
.omega.-(N-morpholino)alkanesulfonate, methylphosphonate,
3-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
an alkylphosphonate, an arylphosphonate, and a molecule comprising
phosphonate moieties and sulfonate moieties. In the discharge
process only the extra acid is relevant not the buffer. Some
molecules can function as both the buffer and the extra acid. Those
comprising both phosphonic and sulfonic moieties are utilized here.
The discharge system 101 further comprises a deprotionated form of
an extra acid. The extra acid comprises, for example, one or more
of bromic acid, sulfuric acid, perchloric acid, triflic acid, a
sulfonic acid, molecules comprising phosphonate moieties and
sulfonate moieties, and an acid with a pKa.ltoreq.2. The buffer is
in an acid form during discharge with a pH.ltoreq.4 and the acid
form of the buffer comprises one or more of a phosphoric acid
derivative, substituted phosphonic acids, 2-(N-morpholino)
alkanesulfonic acid(s), molecules comprising both phosphonate and
sulfonate moieties, amines and buffers capable of maintaining pH
between 4 and 9. The base form of the buffer is, for example, one
or more of .omega.-(N-morpholino)alkanesulfonate,
2-(N-morpholino)ethanesulfonate, 3-(N-morpholino)propanesulfonate,
4-(N-morpholino)butanesulfonate, a phosphoric acid derivative, an
alkylphosphonate, an arylphosphonate, a molecule comprising
phosphonate moieties and sulfonate moieties, an amine, a nitrogen
heterocycle, and a base with a pKa between 4 and 9.
[0289] The concentration of lithium bromate dissolved in the
neutral oxidant fluid is, for example, above 1M, 2M, 5M, or 10M.
The acidification reactor 1501a converts 2008 the neutral oxidant
fluid into an acidic oxidant fluid. The concentration of acidic
protons in the acidic oxidant fluid is, for example, below 0.1M,
0.5M, 1M, 2M, 5M, or 10M. The method disclosed herein facilitates
2009 discharge of the discharge unit 104 for producing electric
power from the acidic oxidant fluid and from hydrogen and generates
an acidic discharge fluid on consumption of the acidic oxidant
fluid comprising lithium bromate and hydrogen. The discharge is
facilitated via a pH-dependent solution-phase comproportionation of
bromate with bromide formed via electroreduction of intermediate
bromine. In an embodiment, the neutralization reactor 1501b
neutralizes 2010 the acidic discharge fluid to produce one or more
forms of a neutral discharge fluid.
[0290] The method disclosed herein further comprises optimizing and
stabilizing the pH of the acidic oxidant fluid in the
splitting-disproportionation reactor 1502 using an extra acid
present in the acidic oxidant fluid to facilitate
comproportionation of the aqueous multi-electron oxidant with a
final product of a reduction of the aqueous multi-electron oxidant
into an intermediate oxidant. The pH of the acidic discharge fluid
is below 3, 2, 1 or 0. The extra acid is one or a combination of
bromic acid, another aqueous multi-electron oxidant (AMO) acid,
phosphoric acid, 3-(N-morpholino)propanesulfonic acid,
3-(N-morpholino)ethanesulfonic acid, other
.omega.-(N-morpholino)propanesulfonic acid, methanesulfonic acid,
triflic acid, substituted sulfonic acid, substituted phosphonic
acid, perchloric acid, sulfuric acid, a molecule comprising
sulfonic moieties and phosphonic acid moieties, and an acid with a
pKa<2.
[0291] The regeneration system 106 regenerates 2011 hydrogen and
one or more forms of the oxidant fluid comprising lithium bromate
in stoichiometric amounts from one or more forms of the neutral
discharge using external power. The regeneration is performed by
splitting 2011a one or more forms of the neutral discharge fluid
into stoichiometric amounts of bromine, hydrogen, and a base form
of a buffer using external power in the
splitting-disproportionation reactor 1502, and producing lithium
bromate via disproportionation of bromine with the base form of the
buffer. The splitting process is performed via electrolysis,
photolysis, photoelectrolysis, radiolysis, or thermolysis. In the
case of splitting being electrolysis, bromine is produced on a
positive electrode of the electrolysis-disproportionation reactor
107 and hydrogen and a base are produced at a negative electrode of
the electrolysis-disproportionation reactor 107. The
disproportionation reaction is facilitated by a buffer capable of
maintaining a solution pH between 4 and 9 or between 6 and 8. The
splitting-disproportionation reactor 1502 continues 2011b splitting
and disproportionation in a cyclic manner in the no-aqueous
multi-electron oxidant-on-negative mode of operation or the aqueous
multi-electron oxidant-on-negative mode of operation in one of
multiple modes until a desired degree of conversion of bromide into
bromate is achieved. The modes comprise, for example, a batch mode,
a cyclic flow mode, a cascade flow mode, and any combination
thereof. The regeneration system 106 supplies 2012 the regenerated
one or more forms of the oxidant fluid comprising bromate and the
regenerated hydrogen to the discharge system 101 for subsequent
generation of electric power on demand.
[0292] FIG. 21A exemplary illustrates polarization curves of a
glassy carbon rotating disk electrode in solutions comprising 5M
LiBrO.sub.3+50% w H.sub.3PO.sub.4+1 mM LiBr at different rotation
rates in rpm and 20.degree. C. The decrease of the limiting current
at higher rotation rates is noticeable as exemplarily illustrated
in FIG. 12. Such a regime is observed when the concentration of
acid is sufficiently high so that the limiting current is
controlled by the kinetics of the comproportionation rather than by
the mass-transport of protons. As the concentration and the
dissociation constant of the acids such as H.sub.3PO.sub.4,
H.sub.2SO.sub.4, F.sub.3CSO.sub.3H, etc., in the oxidant fluid is
increased, the limiting current on discharge also increases.
However, the lifetime of the aqueous multi-electron oxidant (AMO)
stock solution decreases. For example, a limiting current of about
50 mA/cm.sup.2 for a 50% w H.sub.3PO.sub.4 in combination with 5M
LiBrO.sub.3 on a smooth carbon electrode is produced while the
lifetime of this AMO stock or acidic oxidant fluid is about 10 days
at 20.degree. C. A shorter lifetime is obtained with 30% w
H.sub.2SO.sub.4. Therefore, when the orthogonal ion migration
across laminar flow (OIMALF) process is performed on-board rather
than off-board, a TRIZ contradiction between the power and
stability of the acidic oxidant fluid can be resolved with an
additional benefit of improved safety of the on-board discharge
system 101. This way only safe and stable AMO salt solution, for
example, LiBrO.sub.3 with a high energy density and room
temperature solubility of about 13.27 molal and charge density of
ca. 660 Ah/kg is stored on-board and off-board, and the reactive
acid form of the AMO (H-AMO) is generated on-board just before it
is consumed by the discharge unit 104. One feature that enables a
practical use of an on-board acidification system is that the
ignition regime of bromate electroreduction can be observed under
low ratios of proton to bromate concentrations, about
[H.sup.+]/[BrO.sub.3.sup.-]<0.1, if the total concentration of
bromate is high, about over 10 molal which is possible with
LiBrO.sub.3. Also, the base, for example, LiOH, Li-3-(N-morpholino)
propanesulfonic acid (MOPS), etc., produced at the negative
electrode during the on-board OIMALF process is used to neutralize
the acid, for example, HBr produced in the discharge unit 104, so
that the amount of dangerous materials, for example, HBrO.sub.3,
HBr, LiOH, etc., present on board at any time is minimized.
[0293] FIG. 21B exemplary illustrates polarization curves of a
glassy carbon rotating disk electrode in a solution comprising 30%
H.sub.2SO.sub.4+166 mM LiBrO.sub.3+5 mM NaBr. Limiting current
similar to that found in phosphoric acid at a much higher bromate
concentration exemplarily illustrated in FIG. 21A, is observed
which is interpreted as evidence of a higher disproportionation
rate constant when a stronger sulfuric acid is used rather than
when a weaker phosphoric acid is used. In both cases, the limiting
current decreases with the rotation rate suggesting that the
limiting current is controlled by the rate of the
disproportionation rather than by mass transport or, for example,
acidic protons.
[0294] FIG. 22 exemplarily illustrates Pourbaix diagrams for
bromine in aqueous media at pH 0 and pH 14. FIG. 22 exemplarily
illustrates the concept of the possibility of shifting from
disproportionation to comproportionation by changing the pH of the
oxidant fluid. The numbers near the lines denote the standard
potentials of the corresponding electrochemical reactions. When the
potential to the left is lower than the potential to the right, the
species is stable, for example, Br.sub.2 in acid. When the
potential to the left is higher than the potential to the right,
the species disproportionates, for example, HBrO in acid or
Br.sub.2 in alkali.
[0295] FIG. 23A exemplarily illustrates a solar radiation spectrum
at sea level and the positions of a silicon (Si) band-gap, bromine
and/or bromide, and bromate and/or bromide standard electrode
potentials. The solar photo electrochemical regeneration of H.sub.2
and Br.sub.2 from HBr followed by a conversion of Br.sub.2 into
HBrO.sub.3 enables the use of sunlight at a cost similar to the
cost involved in traditional methods such as natural gas combustion
and uranium fission. An open-circuit potential photoelectrolysis of
aqueous HBr on semiconductor particles can be performed with a
lower cost and higher efficiency than the photoelectrolysis of
water advocated by the proponents of hydrogen economy. FIG. 23A
exemplarily illustrates that while the Si band-gap cannot provide
enough energy to convert HBr+3H.sub.2O into 3H.sub.2+HBrO.sub.3
directly, the reaction 2HBr=H.sub.2+Br.sub.2 can be driven by the
Si band-gap energy. The further uphill conversion of bromine into
bromate is performed via disproportionation driven by a pH change,
for example, 3Br.sub.2+3OH.sup.-=5Br.sup.-+BrO.sub.3.sup.- with
hydroxide as a base. The uphill pH change, in turn, is driven
electrochemically by the hydrogen evolution or production reaction
of the negative electrode(s) in an electrolyzer (not shown) of the
photoelectrolysis-disproportionation (ED) reactor.
[0296] FIG. 23B exemplarily illustrates a batch mode of a
photoelectrolysis-disproportionation method for regenerating a
halate from a halide.
Example 1
[0297] FIGS. 14A-14G exemplarily illustrate graphical
representations showing comparative performances of three on-board
power sources at a nominal power of 130 kW: a gasoline-internal
combustion engine, a lithium ion battery (LIB), and an
H.sub.2-aqueous multi-electron oxidant (AMO) discharge unit 104 or
flow battery exemplarily illustrated in FIG. 1, as well as the
targets of the Advanced Research Projects Agency-Energy (ARPA-E).
Table 2 exemplarily compares the projected performance of an
H.sub.2-AMO discharge system 101 at a nominal power of 130 kW with
the performance of 2012 Toyota RAV4EV and with 2013 ARPA-E goals
for a battery for a Fully Electric Vehicle. The AMO is 50% w/w
aqueous HBrO.sub.3. The Toyota RAV4.RTM. EV of Toyota Jidosha
Kabushiki Kaisha TA Toyota Motor Corporation is chosen as an
example of a sport utility vehicle, which is or was available in
gasoline and in lithium-ion battery (LIB) versions, to illustrate
the capabilities of the discharge unit 104. A sport utility vehicle
(SUV) is selected because it is a large vehicle that presents a
greater challenge for electrification than a small urban vehicle.
The data are available for Toyota RAV4.RTM. in both gasoline and
electric vehicle lithium ion battery (LIB) versions. All
calculations are based on the rated power of about 130 kW=174 hp.
The size of the storage unit in the vehicle using the discharge
unit 104 of the discharge system 101 exemplarily illustrated in
FIG. 1, is adjusted to give the same driving range as the gasoline
power system, rather than the electric version. H.sub.2 is stored
in the vehicle using the discharge system 101 as a metal hydride to
minimize the H.sub.2 tank volume.
[0298] In the vehicle using the discharge system 101 disclosed
herein, both the reagent, for example, bromate and the product such
as bromide of the discharge are anions, which minimizes their
cross-over through a cation-exchange membrane such as Nafion.RTM.
and other poly perfluorosulfonic acid (pPFSA) membranes and
prevents a parasitic self-discharge. Also, the electrode reaction
of bromine/bromide does not require an expensive catalyst and the
electrode reaction occurs with an acceptable rate even on carbon
electrodes. It is also estimated that the capital cost of the
discharge unit 104 can be as low as 120 $/kW which is less than a
half of the lithium-ion battery cost in the Nissan Leaf.RTM. of
Nissan Jidosha Kabushiki Kaisha DBA Nissan Motor Co. Ltd., and the
Toyota RAV4.RTM.. The lithium ion battery (LIB) takes up about 20%
of the vehicle weight while the discharge system 101, takes about
10% of the vehicle weight, similar to, for example, the internal
combustion engine (ICE)-gas system as exemplarily illustrated in
FIG. 14A. In the Advanced Research Projects Agency-Energy (ARPA-E)
metrics, the energy density of the on-board discharge unit 101 is,
for example, about 426 Wh/kg, which is about 2.8 times larger than
the ARPA-E target of 150 Wh/kg.
[0299] The volume of the discharge system 101 is, for example,
twice the volume of the gas tank including the internal combustion
engine (ICE) and half of the lithium-ion battery (LIB) and the
electric engine as exemplarily illustrated in FIG. 14B. The energy
density of the discharge unit 104 depends on the method of hydrogen
storage and it is, for example, 200-400 Wh/L, which exceeds the
Advanced Research Projects Agency-Energy (ARPA-E) target of 230
Wh/L. Both the gasoline power system and the discharge unit 104 can
provide a driving range of about 300 miles as exemplarily
illustrated in FIG. 14C, while Toyota RAV4 EV has a range of about
92 miles, according to the Environment Protection Agency (EPA)
criteria, which comes from its low battery stack energy of about
41.8 kWh. The manufacturing cost of the discharge unit 104 is about
$15,000 based on the current prices proton exchange membrane fuel
cells (PEMFCs) produced in low volumes accounting for the absence
of Pt on the positive electrode 205a in the H.sub.2-aqueous
multi-electron oxidant (AMO) system, or about 120$/kWh and 115 $/kW
as exemplarily illustrated in FIG. 14D, and is more than the
manufacturing cost for the ICE, which is about $5,000, but is close
to the Advanced Research Projects Agency-Energy (ARPA-E) target of
<140/kWh, and is three times lower than the cost per mile drive
of the LIB system. The projected tank-to-wheel efficiency of the
discharge unit 104 under realistic operating conditions is slightly
lower than that of lithium ion batteries (LIBs) but much higher
than that of internal combustion engines (ICEs) as exemplarily
illustrated in FIG. 14E. Both the gasoline power system and the
discharge unit 104 can be refilled mechanically within minutes,
while Toyota RAV4.RTM. EV needs about 5 hours for electric recharge
as exemplarily illustrated in FIG. 14F. The standard discharge
efficiency of the discharge unit 104 is about 78% and such
efficiency can be a practical target at about 0.5 W/cm.sup.2. For
the power of about 1 W/cm.sup.2, the finite rate of the
comproportionation and the non-negligible membrane resistance make
65% a more realistic target.
[0300] Platinum on the H.sub.2 electrode is used at the same
loading as the PEMFC but the loading is between 1/10 and 1/20 of
what is used on the air electrode in the PEMFCs, and the loading
has been shown to be sustainable economically, and is not a large
contributor to the cost. FIG. 14G exemplarily illustrates the
projected competitive positions of the H.sub.2-aqueous
multi-electron oxidant (AMO) discharge unit 104 on the Advanced
Research Projects Agency-Energy (ARPA-E) price-range plot for
different vehicle power sources. The discharge system 101 disclosed
herein can meet the range, cost, cost and safety targets for fully
electric vehicles (FEVs) defined by the ARPA-E's Robust Affordable
Next Generation Electric Vehicle (RANGE) program as exemplarily
illustrated in FIG. 14G.
Example 2
[0301] The comparison of a gasoline engine, a lithium ion battery,
and two hydrogen-bromate batteries with different methods of
hydrogen storage, that is, 700 bar compressed and 9% w/w metal
hydride is provided in the table below.
TABLE-US-00003 H.sub.2 storage 50% aqueous multi-electron 5%
oxidant (AMO) 350 bar liquid MH 5.74M theoretical limit g/L 25 70
125 1.48 100 kg, 300 kW real g/L 10 26 20 systems theoretical limit
Ah/L 670 1,875 3,350 923 charge per mass of Ah/kg 26,800 26,786
26800 623.7 pure H.sub.2 real system w % 5 5 5 real system Ah/kg
1,340 1,340 1,340 623.7 real system Ah/L 268 697 536 923 vol. % for
storage H2 77.45 57.0 63.3 system wt. % for storage H2 2.28 2.28
2.27 system volume/charge mL/Ah 3.73 1.44 1.87 1.08 mass/charge
g/Ah 0.0373 0.0373 0.0373 1.60 4 h drive kg 388 388 388 834 RAV4 =
520 kWh 4 h drive L 1,940 746 970 563 RAV4 = 520 kWh system energy
Wh/L 208 397 339 density specific energy Wh/kg 426 426 426 5% w/w
H.sub.2
[0302] The parameters used for lithium ion batteries (LIB s) are
230 Wh/L, 128 Wh/kg, and $0.47/Wh. The parameters used for H.sub.2
storage are 50 g/L compressed 125 g/L MH. The LiBrO.sub.3 solution
density is assumed as 1.49 g/cm.sup.3, the same as for 48% w/w
aqueous HBr. The cost of 50% HBr=$2000/ton=$2/kg.
Example 3
[0303] Reactions at a positive electrode during discharge of
bromate using a vanadium redox mediator are provided below:
HBrO.sub.3+5VO.sup.+2+5H.sup.+=1/2Br.sub.2+5VO.sub.2.sup.++3H.sub.2O
in solution
5VO.sub.2.sup.++5e-+10H.sup.+=5VO.sup.+2+5H.sub.2O on the positive
electrode
1/2Br.sub.2+1e.sup.-=Br.sup.- on the positive electrode
Example 4
[0304] A bromine/bromide couple is used as a mediator for a bromate
reduction "r" on discharge as shown below:
HBrO.sub.3+5Br.sup.-+5H.sup.+=3Br.sub.2+3H.sub.2O; in solution;
3Br.sub.2+6e.sup.-=6Br.sup.-; on the positive electrode.
Example 5
[0305] A chlorine/chloride couple is used as the mediator for
bromide/bromate on charge as shown below:
Cl.sup.--1e.sup.-=1/2Cl.sub.2; on the positive electrode; (56)
1/2Br.sub.2+2.5Cl.sub.2+3H.sub.2O.dbd.HBrO.sub.3+5HCl in solution.
(57)
Example 6
[0306] The conversion of hydrobromic acid to bromic acid using a
resin-type ion exchange reactor is shown below, where M refers to a
cation such as an alkali, an alkali earth metal, or organic cation,
and "solid" refers to an ion exchanging material such as a
resin:
M.sup.+(solid)+HBr(spent)=H.sup.+(solid)+MBr resin regeneration
MBr+6MOH-6e.sup.-=MBrO.sub.3+3H.sub.2O+6M.sup.+ positive
electrode
3H.sub.2O+6e.sup.-+6M.sup.+=3H.sub.2+6MOH negative electrode
MBrO.sub.3+H.sup.+(solid)=HBrO.sub.3+M.sup.+(solid) ion exchange on
the resin
[0307] The above method for regenerating the aqueous multi-electron
oxidant from the spent discharge fluid may result in the incomplete
exchange of M.sup.+ for H.sup.+ under stoichiometric conditions,
which results in an overuse of the acid regenerant and of the
energy needed to produce the acid regenerant. However, a complete
exchange of M.sup.+ for H.sup.+ is not required for the ignition to
occur.
Example 7
[0308] H.sub.2-aqueous multi-electron oxidant (AMO) discharge redox
flow battery: In an embodiment, in H.sub.2--LiBrO.sub.3 discharge
flow batteries, modified single and multiple stack type proton
exchange membrane fuel cells (PEMFCs) are employed. The
electrolyte-electrode assemblies are fabricated using a
polyperfluorosulfonic acid (pPFSA) membrane, with a conventional
negative electrode layer 205b exemplarily illustrated in FIG. 2,
comprising Pt, C, and pPFSA, and a conventional gas diffusion layer
(GDL) used for H.sub.2 oxidation on the negative anode side. The
positive cathode design, however, is different from the proton
exchange membrane (PEM) air cathode, since neither bromate nor
bromide are soluble in the pPFSA, which completely surrounds the
Pt/C electrocatalyst in modern thin-film PEMFC catalytic layers. A
porous flow-through or flow-by media, for example, porous
hydrophilic carbon or carbon cloth, is used for the positive
electrode 205a in a H.sub.2--HBrO.sub.3 discharge flow battery.
[0309] Under operating conditions at a high acid concentration, a
slower yet above stoichiometric flow rate of the aqueous
multi-electron oxidant (AMO)-containing acidic oxidant fluid leads
to a higher cell power in contrast with fuel cells and conventional
redox flow batteries. This is due to a larger fraction of the
intermediate such as bromine escaping the kinetic boundary layer
into the solution bulk as the diffusion boundary layer gets
thinner. This finding suggests that the cell operation at high
power does not require significant energy expenses on pumping and
that, in contrast to fuel cells, a near stoichiometric supply of
the aqueous multi-electron oxidant may provide an optimal
performance in terms of the power, energy efficiency, and system
size. Also, a quick depletion of bromate in the ignition regime and
the higher viscosity of the aqueous multi-electron oxidant (AMO)
compared to air implies a preference for short channels, which, in
combination with a parallel-channel flow field and slow flow rates,
also leads to a lower pressure drop. Also, the absence of the gas
phase in the cathode stream, the large heat capacity, and the high
water content of the AMO supply simplify the design, manufacture,
and operation of the cathode side as well as of the discharge unit
104 and of the whole discharge system 101.
Example 8
[0310] Power and efficiency of the hydrogen-bromate discharge unit
104: In order to estimate the power and voltage of the
hydrogen-bromate discharge unit 104 during discharge, the following
model is used: The discharge unit 104 comprises a single
electrolyte-electrode assembly 205 exemplarily illustrated in FIG.
2. Pure humidified hydrogen is supplied to the anode or the
negative electrode 205b. The anode polarization losses and reagent
cross-over are ignored. The cell or membrane ohmic resistance is,
for example, set to 0.1 Ohm/cm.sup.2. The cathode or the positive
electrode 205a is smooth and is supplied with 50% w/w/HBrO.sub.3
containing a few mM of Br.sub.2, Co.dbd.[Br.sub.2].sub.o, to
initiate the electroreduction cycle. Electrochemical polarization
of the cathode is ignored, that is, bromine/bromide exchange
current is large compared to the applied currents.
[0311] The homogeneous kinetics of the comproportionation is
incorporated through the use of kinetic boundary layer thickness,
L.sub.0=(D.sub.bromide/5 k.sub.con C.sub.bromate).sup.1/2=1.5
.mu.m, where k.sub.con is the appropriate rate constant for the
homogeneous comproportionation. At currents above 1 A/cm.sup.2,
further correction becomes important, i.e.
L=Lo/(1-(iz.sub.o/5D.sub.bromateC.sub.bromate)).sup.1/2. The effect
of convection is incorporated through the use of the diffusion
boundary layer thickness, z.sub.o Z.sub.o=Z.sub.o/L. Its value is
selected on the basis of common values of the respective quantities
for the rotating disk and channel electrodes in aqueous
electrolytes, that is, 15 .mu.m and 150 .mu.m. Diffusion
coefficients for bromide and bromine are set to 1.5.times.10.sup.-5
cm.sup.2/s and 1.0.times.10.sup.-5 cm.sup.2/s, respectively.
Activity coefficients of all species are set to 1.
[0312] A more detailed analysis leads to the following formula for
a polarization curve for bromate
comproportionation-electroreduction on a smooth electrode:
Exp{2(E-E.degree.)F/RT}={[Br.sub.2].sub.o+(iL/FD.sub.bromine)(0.1Z.sub.o-
-0.6thZ.sub.o)}(FD.sub.bromide/iLthZ.sub.o).sup.2
[0313] The corresponding plots for power are exemplarily
illustrated in FIG. 13. Although the experimental data that is
reported in FIG. 13 are for much higher rotation rates (low
Z.sub.0), the data in FIGS. 21A-21B for lower rotation rates
support the conclusion that the comproportionation reaction can
sustain large currents in the discharge unit 104. The
hydrogen-bromate discharge unit 104 can achieve under very
realistic conditions, even with a smooth carbon electrode, a power,
for example, about 1 W/cm.sup.2 at around 1.0 V, which corresponds
to the energy efficiency, for example, of about 68% with respect to
the standard electrode potential of bromate/bromide, that is, about
1.48 V. Such performance compares favorably with the performance of
state-of-the-art hydrogen-air fuel cell, yet it can be achieved
with about a 10 times smaller Pt loading and with electric or solar
regeneration of the reducer and the aqueous multi-electron oxidant.
Under operating conditions at a high acid concentration, the
concentration of free bromine has little effect on the cell
performance, whereas a stronger convection decreases the cell power
in contrast with conventional fuel cells. This is due to a faster
escape of intermediate Br.sub.2, homogeneously produced in the
vicinity of the electrode, into the bulk of the solution at smaller
hydrodynamic boundary layer thicknesses. Such an effect is not
observed in conventional fuel cells and flow batteries since the
electroactive reagent is delivered from the bulk of the solution
rather than formed near the electrode. Also, the kinetic layer
thickness, which determines the minimal meaningful pore diameter in
the porous electrode, is L=1.5 .mu.m in 50% w/w HBrO.sub.3, and a
thicker 6L=9 .mu.m hydrodynamic boundary layer is needed for the
ignition to occur.
[0314] The 1D model disclosed herein assumes a constant solution
composition outside the hydrodynamic boundary. The model disclosed
herein shows that a low near-stoichiometric flow rate is
appropriate for the operation of the discharge unit 104 with
reduced energy losses entailed. The parallel flow field with a
channel length longer than the ignition length but shorter than the
depletion length with a flow rate slightly above the stoichiometric
can provide maximal power while simultaneously reducing pumping
losses.
Example 9
[0315] In an embodiment, the regeneration system 106 produces a
dilute aqueous multi-electron oxidant (AMO) solution, for example
0.5M, which needs to be concentrated, for example, to about 3.88M.
Water evaporation, vacuum distillation, pervaporation are suitable
means of concentrating the AMO solution. Heat exchangers are used
to transfer heat from the concentrated product to dilute input
solution if the water removal is performed at an elevated
temperature. The energy expenses of concentrating dilute AMO
produced in the orthogonal ion migration across laminar flow
(OIMALF) step should be compared with the energy of a H.sub.2-AMO
battery. In the case of bromic acid, the stored electric power
is:
1,705 Wh/kg*0.135 kg/mol=230 Wh/mol=(1 Wh=3.6 kJ)=829 kJ/mol
[0316] The evaporation of excess water is also possible, more
efficiently with heat exchangers, but it leads to the loss of
volatile bromine species. In the case of reverse osmosis (RO)
process of concentrating the aqueous multi-electron oxidant (AMO)
solution, the osmotic pressure difference between the dilute and
concentrated solutions of the AMO such as bromic acid can be
estimated via the Morse equation. Molality is assumed the same as
molarity and dissociation is complete:
.PI.=iMRT=2*(3.88-0.5) mol/L*10.sup.3 L/m.sup.3*300K*8.3145 J
K.sup.-1 mol.sup.-1=16.8 MPa=168 bar
[0317] This pressure falls within the range of commercial cascade
reverse osmosis units, thus, such a process is technically
feasible. The minimal energy expense for reverse osmosis (RO)
concentrating can be estimated as 1.742 kg of water per 1 mole of
HBrO.sub.3 needs to be removed. This corresponds to 1.742 10.sup.-3
m.sup.3*16.8 10.sup.6 Pa=2.93 104 J=29.3 kJ/mol HBrO.sub.3. This is
only 3.3% of the battery energy per 1 mole of bromic acid. This
number is the lower limit at the infinitely slow rate of water
permeation and the number will be higher in practice. For example,
sea water desalination requires usually 5 times more energy than
the theoretical value. Using the factor of 5, about 16.5% battery
energy is obtained which is acceptable in practice.
Example 10
[0318] The molal solubilities, that is, moles of solute per kg of
water of some compounds of interest in the
electrolysis-disproportionation (ED)-orthogonal ion migration
across laminar flow (OIMALF) process at 20.degree. C. and
60.degree. C. are provided in the table below:
TABLE-US-00004 moles of solute per kg of solvent, m bromide bromide
bromate bromate Hydroxide Hydroxide cation 20.degree. C. 60 C.
.degree. 20.degree. C. 60.degree. C. 20.degree. C. 60.degree. C.
Li.sup.+ 18.4 25.7 13.3 19.9 5.3 5.8 Na.sup.+ 8.8 11.5 2.4 4.1 27.3
43.5 K.sup.+ 5.5 7.2 0.4 1.4 20 27.4 NMe.sub.4.sup.+ 7.79E-03 n/a
n/a n/a Mp 67 C., 50% = 5H.sub.2O Ba.sup.2+ 3.7 4.1 1.65E-02
5.77E-02 2.27E-01 1.2 Sr.sup.2+ 4.1 6.1 0.9 01.05 1.46E-01 0.7
Ca.sup.2+ 7.2 13.9 7.8 n/a 2.33E-02 n/a Mg.sup.2+ 5.5 6.1 20.7 n/a
1.71E-03 n/a
[0319] The data in the table above suggests that Li.sup.+ cation
provides a high molal solubility for bromide and bromate. The
limited solubility of LiOH is irrelevant since it does not appear
in the laminar flow of the orthogonal ion migration across laminar
flow (OIMALF) reactor 1501 where solids can disrupt the process.
Also, if a buffer such as 3-(N-morpholino) propanesulfonic acid
(MOPS) is used, LiOH will react with the buffer.
Example 11
[0320] In an embodiment, in the case of a redox couple with both
components being anions, for example, halate and halide, the
cross-over of the oxidant couple to the negative electrode 205b
exemplarily illustrated in FIG. 2, can be prevented with a cation
exchange membrane. In the case of lithium bromate, the discharge
process on the positive electrode 205a is as shown below:
Br.sub.2+2e.sup.-+2Li.sup.+=2LiBr on the electrode. (58)
LiBrO.sub.3+5LiBr+6HA=3Br.sub.2+3H.sub.2O+6LiA in solution (59)
where HA represents the acid present in the acidic oxidant fluid
such as bromic acid, phosphoric acid, and/or the extra acid.
[0321] The concentration of the neutral intermediate Br.sub.2 is
maintained sufficiently low, so that its cross-over to the negative
electrode 205b makes a negligible contribution compared to the
current of the electrolytic cell 200. The ratio of the standard
redox potentials of bromate/bromide and bromine/bromide suggests,
for example, only about 25% loss of efficiency when performing
comproportionation mediated rather than direct discharge of bromic
acid at the equilibrium potential. The regeneration of bromate and
hydrogen from bromide and water or, in general, oxidant fluid from
discharge fluid can be performed off-board. Direct electrolytic
regeneration can be performed with an anode such as PbO.sub.2 or a
dimensionally stable anode (DSA).
[0322] In an embodiment, a solution-phase mediator, for example, a
redox couple is used to expedite the rates of an otherwise slow
electrode reaction and thus to increase the system power and
efficiency. A redox couple that undergoes electron exchange with
both an electrode and a reduced or an oxidized form of the aqueous
multi-electron oxidant can be used to accelerate the rates of
charge or discharge, thereby improving efficiency. Different redox
mediators can be employed in the charge and discharge processes. In
an embodiment, Cl.sub.2/2C1.sup.- can be used as a solution-phase
mediator in the electrochemical regeneration process. Since
oxidations, for example, electro-oxidation of a halide to a halate,
are more facile in alkaline solutions, performing regeneration at
high pH and then, for usage in the discharge unit 104, converting
the salt into acid, for example, by means of the orthogonal ion
migration across laminar flow (OIMALF) process are considered.
[0323] In an embodiment, pH-dependent disproportionation and
pH-dependent comproportionation reactions involving halogens and
their compounds are used to facilitate the discharge and
regeneration of the aqueous multi-electron oxidants. The rate(s)
and the equilibrium constant(s) of the disproportionation
reaction(s) in some cases may show a dependence of the solution pH.
The rate(s) and the equilibrium constant(s) of the
comproportionation reaction(s) in some cases may show dependence of
the solution pH.
[0324] In an embodiment, the aqueous multi-electron oxidant (AMO)
can be regenerated by reacting the halide with ozone or by
photolytic oxidation on a suitable semiconductor such as TiO.sub.2.
In another embodiment, a mediator is used for oxidation at the
positive electrode during regeneration. The preferences for a
suitable mediator in the halide oxidation are a standard redox
potential of about 0.1V-0.4V more positive than the standard redox
potential of the halate, the electrode reaction of the mediator
having a high exchange current, the homogeneous reaction between
the mediator and the halide being fast, the mediator couple not
involving cationic species capable of crossing the membrane, etc.
Chlorine is, for example, a mediator for iodate or iodide at all pH
levels but chlorine evolution requires an electrocatalyst, for
example, dimensionally stable anode (DSA) which can make this
process more expensive than electro-oxidation-disproportionation.
Chlorine is a mediator for bromide oxidation into bromate only in
neural and alkaline media.
[0325] Ozone is a suitable mediator for oxidation or a charge
reaction, though with less than 50% energy efficiency for oxidizing
halides into halates and perhalates or corresponding acids. This
regeneration process can be performed in acidic media by
electrolysis using a proton exchange membrane (PEM) electrolyzer or
a similar device. The co-produced H.sub.2 can be used later as a
reducer in the discharge unit 104 exemplarily illustrated in FIG.
1, while the ozone reacts with the spent hydrogen halide in a
separate vessel to yield the halic acid oxidant. In an embodiment,
the ozone for regeneration can be produced by a gas discharge
according to commercialized methods. Other suitable mediators
comprise, for example, transition metal ion and their compounds
such as negatively charged polyoxometallates to prevent their
cross-over through the cation exchange membrane. In an embodiment,
a direct electrolytic oxidation of halides, for example, bromide to
bromate is performed, for example, with a PbO.sub.2, RuO.sub.2,
dimensionally stable anode (DSA) or a conductive diamond
electrode.
Example 12
[0326] In an embodiment, the discharge unit 104 is a modified
version of a polymer electrolyte fuel cell. A membrane electrode
assembly (MEA) uncoated on the positive side is used in the
discharge unit 104. The diffusion layer on the positive side is
replaced with a hydrophilic porous carbon cloth. The flow field on
the positive side of a carbon bipolar plate 202 exemplarily
illustrated in FIG. 2, is of a double serpentine type but other
types known in the arts of fuel cells and flow batteries are also
employed. In another embodiment, the discharge unit 104 is equipped
with an MEA coated on the positive side with a Pt-free and
perfluorosulfonic acid (PFSA) free carbon fiber layer replacing a
catalyst layer in the conventional polymer electrolyte membrane
fuel cell (PEMFC), thereby reducing the ohmic resistance between
the points where the bromate reduction occurs and the hydrogen
electrode. In an embodiment, a grid with interdigitated millimeter
deep channels in one direction and with thinner channels in the
perpendicular direction can be used for the positive electrode flow
field.
[0327] In another embodiment, for the positive electrode 205a
exemplarily illustrated in FIG. 2, a hydrophilic porous electrode
(HPE) replacing the hydrophobic gas diffusion layer in the
conventional 5-layer proton exchange membrane (PEM)-membrane
electrode assembly (MEA) design with or without a carbon-ionomer
layer (CIL) coating on the positive side of the membrane is
designed. Such an HPE can either be used as a flow-through with an
inter-digitated or with a flow-by or with a parallel channel flow
field. A pore diameter above 12 L that is 18 .mu.m is beneficial,
and the layer thickness or pore length does not need to be much
larger. A suitable channel width can be larger than the
inter-channel spacing, and a parallel channel flow field with
relatively short channels is longer than the ignition length, and
shorter than the depletion length with a low pressure drop and a
near stoichiometric flow rate. As used herein, the term "ignition
length" refers to the distance from the opening of the channel
where the current density on the positive electrode reaches 1/2 of
its maximal value. In the case of bromate as the aqueous
multi-electron oxidant (AMO), the current increase along the
channel is due to accumulation of bromide and bromine and the
resulting increase in the rate of the disproportionation. Also, as
used herein, the term "depletion length" refers to the distance
along the channel past the maximum current density point, where the
current density decreases to 1/2 of its maximal value. This
decrease is due to the depletion of the AMO in the bulk of the
solution as well as due to an increase in the diffusion boundary
layer thickness.
[0328] Suitable carbonaceous materials for the porous electrode are
available commercially. One suitable carbon cloth is, for example,
pyrolysed PAN AvCarb 1071 HCB 80045-001 with about 350 .mu.m
thickness, about 7.5 .mu.m fiber diameter, about 19.3/cm warp,
about 18.5/cm weft, and about 10.sup.-3 ohmcm conductivity. A
thinner unidirectional carbon fabric, for example, about 152 .mu.m
thickness is available from Fibre Glast Developments Corporation.
Some suitable carbon cloth are, for example, potential hydrophilic
carbon cloth with approximately 18 .mu.m diameter for the
hydrophilic porous electrode (HPE), commercial carbon cloth as thin
as 700 .mu.m, cloth made of electrospun carbon fibers as thin as 20
nm, Zoflex.RTM. of Xilor, Inc., weaved carbon down to 400 um, etc.
Surface modification such as sulfonation of carbon can be used to
improve the hydrophilicity.
[0329] A conventional bipolar stack polymer electrolyte membrane
fuel cell (PEMFC) with a hydrophilic porous layer modification on
the positive side of the membrane electrode assembly (MEA) and a
Pt-free positive electrode layer is used. Since the aqueous
multi-electron oxidant (AMO), in contrast to air, is ionically
conducting, shunt currents in a bipolar stack have to be
considered. Methods for minimizing shunt currents are known and
include: increasing ionic resistance between the electrolytic cells
200 in a stack 300 exemplarily illustrated in FIG. 3, for example,
by increasing the length and decreasing the width of the flow
channels within the bipolar plates connecting the electrolytic
cells 200, reducing the number of single electrolytic cells 200 in
series, decreasing the resistances of manifold and channel,
increasing the power of single electrolytic cell 200, placing shunt
resistors in the electrolyte paths, and any combination thereof.
The operating temperature of the discharge unit 104 is between
0.degree. C. and 100.degree. C., for example, between 10.degree. C.
and 60.degree. C.
Example 13
[0330] A steady-state one-dimensional model was developed for a
comproportionation-mediated discharge of bromate with a Nernstian
hydrodynamic boundary layer of a fixed thickness. Such a model is
an adequate first-order approximation for the discharge at the
rotating disk and at channel flow electrodes. For a sufficiently
high rate of the comproportionation reaction ensured by high
concentrations of bromate and protons in bulk solution, there are
three different regimes determined by the ratio diffusion to
kinetic boundary layer thicknesses as exemplarily illustrated in
FIG. 12. The latter decreases as the disproportionation rate gets
larger, for example, at lower pH and higher bulk aqueous
multi-electron oxidant (AMO) concentration and it is equal to 1.5
.mu.m in 50% w/w HBrO.sub.3.
Lo=(D.sub.bromide/5k.sub.conC.sub.bromate).sup.1/2=1.5 .mu.m
(60)
[0331] During electroreduction of the aqueous multi-electron
oxidant (AMO) such as bromate mediated by homogeneous
comproportionation when the diffusion boundary layer is thin
compared to the kinetic boundary layer, that is, at high flow or
stiffing rates, the intermediate bromide formed via
electroreduction of the initial bromine escapes the hydrodynamic
boundary layer before the intermediate bromide comproportionates
with bromate to form more bromine near the electrode. In this
non-ignition (normal) regime (not shown), the limiting current is
the same as it would be in a solution with only bromine and no
bromate present. When the diffusion boundary layer is thick
compared to the kinetic boundary layer, that is, at low flow and/or
rotation rates, the intermediate bromide has enough time to react
with bromate near the electrode producing more bromine as
exemplarily illustrated in FIG. 12, resulting in an ignition regime
with the limiting current significantly exceeding the bromine
limiting current found in the non-ignition regime. The limiting
current in the ignition regime can be limited by the rate of
comproportionation as exemplarily illustrated in FIG. 12, 21A-21B
or by the mass-transport of protons as exemplary illustrated in
FIG. 25. The nature of the limiting current depends on the relative
concentrations of acidic protons and bromate. The behaviour when
the limiting current abnormally decreases with the rotation flow
rate as exemplarily illustrated in FIG. 12, 21A-21B contrasts that
of other flow batteries and fuel cells which show a higher current
and power upon increased flow rate, and such a regime is useful for
practical applications since the regime allows for a high power at
low pumping rates.
[0332] An additional confirmation of the comproportionation
mechanism disclosed herein is obtained through a direct observation
of a brown colored bromine in a layer near the rotating disk
electrode (RDE). The brown cloud (not shown) attached to the
electrode is the intermediate bromine formed during the
comproportionation of bromate with electro-generated bromide as in
equation (2). The current is negative that is cathodic. The visible
thickness of the colored layer and the measured current at constant
potential decreases with the electrode rotation rate (not
shown).
[0333] In the intermediate regime, the limiting current decreases
with flow and/or rotation rate as exemplarily illustrated in FIG.
12, due to the escape of the intermediate bromine. The ignition
regime observed at low mass-transport rates is particularly
interesting for practical applications as it affords a high
generated peak electric power even on a smooth carbon electrode,
that is, over 0.1 A/cm.sup.2 and 0.1 W/cm.sup.2, as exemplarily
illustrated in FIG. 13, at low consumed pumping power in contrast
to other fuel cells and redox flow batteries. The fast kinetics of
the bromine/bromide electrode reaction assures that the energy
efficiency of the discharge unit 104 at high power is over 60%.
Example 14
[0334] The power required for an on-board orthogonal ion migration
across laminar flow (OIMALF) is calculated. The balance of charge
in the OIMALF reactor 1501 and the discharge unit 104 is
exemplarily illustrated in FIG. 19. The matching ratio of currents
in charges per unit time through the OIMALF reactor 1501 and the
discharge unit 104 are (1+x+z+y-w)/(6+x+z-y)=1. In the simplest
case, x=y=z=w=1, thus the charge ratio is 1:6. Assuming the single
cell voltage produced in the discharge unit 104 as 1.0 V, the
current density in the OIMALF reactor 1501 as 0.5 A/cm.sup.2, and
the areal cell resistance as 0.15 .OMEGA.cm.sup.2, which is three
times the areal resistance of 60 .mu.m thick Nafion.RTM. 112, we
obtain 0.5.times.0.195/1.0=10%, justifying a small sacrifice in
energy efficiency while making a significant improvement in the
safety on the on-board system and the complete energy cycle.
Example 15
[0335] The energy and power density of the of the on-board
orthogonal ion migration across laminar flow (OIMALF) discharge
system 101: The Toyota RAV4.RTM. EV of Toyota Jidosha Kabushiki
Kaisha TA Toyota Motor Corporation is chosen as an example of a
sport utility vehicle to illustrate the capabilities of the
discharge unit 104 with the on-board OIMALF reactor 1501. In order
to compare H.sub.2-Li aqueous multi-electron oxidant (AMO) on-board
discharge unit 104 with a lithium-ion battery system, the Toyota
RAV4.RTM. with rated power of 174 hp, that is about 130 kW and
target driving range of 311 miles or 500 Km is selected.
Considering an experimental value of 40 mA/cm.sup.2 at 0.9V for a
smooth carbon electrode in about 5M LiBrO.sub.3+50% w
H.sub.3PO.sub.4+1 mM LiBr, and multiplying it by a roughness factor
of 25 for a porous carbon electrode and a factor of 2 for a
near-saturated LiBrO.sub.3 solution and without considering
additional acceleration due to a high proton concentration in the
on-board OIMALF reactor 1501, a current of 2 A/cm.sup.2 for a
smooth carbon electrode in case of 10M bromate and >0.5M acid, a
cell voltage with an IR drop of 0.8 V and 0.05 .OMEGA.cm.sup.2
areal resistance, a cell power of 1.6 W/cm.sup.2, and discharge
energy efficiency of 61% with respect to standard electrode
potential bromate/bromide, that is, 1.48 V are obtained for the
discharge unit 104 with the on-board OIMALF reactor 1501. Using the
same area-to-volume conversion factor as the fuel cell stack in
Ballard's HD6 0.5 W/cm2 to 371 W/kg, a power density of 1.2 kW/kg
and a weight of 108 kg is obtained to ensure the needed 130 kW of
the rated power for the on-board discharge unit 104.
[0336] Since automotive fuel cell stacks are usually designed for
130 V, the required number of cells in the discharge unit 104 is
equal to 130 V/0.9V=144 cells. This translates for the 130
kW/130V=1 kA current into 1 kA/2 A/cm.sup.2=500 cm.sup.2 total area
of all electrodes in the fuel cell stack and to 500
cm.sup.2/144=3.46 cm.sup.2.apprxeq.2.times.2 cm.sup.2 membrane
electrode assembly (MEA), which is reasonable considering the
slower diffusion and the shorter depletion length expected for an
aqueous multi-electron oxidant (AMO) compared to O.sub.2 in gaseous
air.
[0337] The weight of the on-board orthogonal ion migration across
laminar flow (OIMALF) reactor 1501 can be estimated as follows. The
stoichiometry of the OIMALF process requires about 1/6 of the
current produced in the discharge unit 104 which is 1 kA/6=167A.
Assuming that the OIMALF reactor 1501 has one third of the
current-to-weight ratio, for example, 1/3.times.1000 A/108 kg=3.08
A/kg as the discharge flow battery, we obtain 167 A/3.08 A/kg=54.2
kg for the weight of the on-board OIMALF reactor 1501. The weight
of discharge system 101 obtained by combining the weights of the
discharge unit 104 and the OIMALF reactor 1501 is 108+54.2=162.2 kg
and the power-to-weight ratio is 130 kW/162.2 kg=800 W/kg which
compares favorably with Li ion battery with power density, for
example, 100 W/kg at 1C rate and polymer electrolyte membrane fuel
cell (PEMFC) stack with power density, for example, 100 W/kg at 50%
efficiency. The weight of the power-generating discharge system 101
needs to be combined with the weight of the reagents that determine
the on-board energy, for example, the driving range.
[0338] Using the data for Toyota RAV4.RTM. EV with 166 km driving
range and 41.8 kWh battery, a 500 km driving range would require
126 kWh of energy. For a single cell voltage of 0.9V, this
translates into 140 kAh or 5.22 kmoles of electrons. This in turn
requires 2.61 kmole=5.22 kg of H.sub.2 or 104.4 kg of 5% w H.sub.2
storage system. The equivalent amount of LiBrO.sub.3 required is
870 moles or 90.8 kg of solid or 181.6 kg or 50% w solution, that
is 78% of saturated solution at 20.degree. C. The combined weight
of the oxidants and the discharge system 101 for 500 km is
181.6+104.4+162.2=448 kg, that is 0.896 km/kg which compares
favorably at 2.05 times higher at the system level with 380 kg of
Toyota RAV4.RTM. EV's battery pack that provides only 166 km range,
that is 0.437 km/kg at a significantly higher upfront cost.
[0339] The high solubility of LiBrO.sub.3 at 64% w at 20.degree. C.
and the multi-electron oxidizing nature lead to equivalent molal
concentration of electrons of 13.27M*6.apprxeq.80 N which is more
than 3 times higher than that of solid LiFePO.sub.4 used in a flow
suspension battery under development by 24M, a Massachusetts based
start-up. At the tank level, the combination of 5% w/w H.sub.2 with
64% LiBrO.sub.3 gives 487 Ah/kg, that is 521 Wh/kg whereas the
LiFePO.sub.4+C.sub.6 battery gives 117 Ah/kg, that is 384 Wh/kg at
the reagent level and 31 Ah/kg, that is 100 Wh/kg at the cell
level. The discharge system 101 with the on-board orthogonal ion
migration across laminar flow (OIMALF) reactor 1501 reduces the
energy density of the discharge system 101 by approximately 10% and
the efficiency of the discharge system 101 to 80% from 90%.
However, in many automotive applications, this new performance
metrics is acceptable and the improved safety fully justifies a
small decrease in the system energy density. Furthermore, the
possibility to use higher acid concentrations during discharge
allows for the discharge flow battery to produce a higher power
thus reducing the system power density dilution and lowering the
system cost.
Example 16
[0340] Lithium bromate chemistry with a 3-(N-morpholino)
propanesulfonic acid (MOPS) buffer: In this example, lithium
bromate chemistry that follows a cyclic or cascade rather than a
batch mode is illustrated. In an embodiment, that is, in the
aqueous multi-electron oxidant (AMO)-on-negative mode of operation,
the regenerated solution or the discharge fluid is cycled between a
negative compartment and a negative electrode 1702 of the SD flow
cell 1700 where hydrogen evolution occurs with a resulting increase
in the pH of the regenerated solution.
Li-MOPS+Br.sub.2+H.sub.2O=5/3LiBr+1/3LiBrO.sub.3+H-MOPS (61)
[0341] Experimental data demonstrating the feasibility of reaction
(61) is exemplarily illustrated in FIG. 26.
[0342] The negative electrode 1702 is configured to support the
hydrogen evolution reaction by employing a hydrogen-evolution
catalyst, for example Pt or other platinoid, using porous carbon
flow-through or flow-by support or any combination thereof, etc. At
the same time the negative electrode 1702 is configured to prevent
the electroreduction of bromate, if the aqueous multi-electron
oxidant (AMO)-on-negative mode of operation is used. The hydrogen
gas produced in (27) is separated from the liquid oxidant solution
in the separation reactor 1010 and collected for future use, for
example, in a discharge system 101. The liquid comprising LiBr and
LiA is further carried over to the positive electrode 1703 where
electrooxidation of bromide followed by bromine disproportionation
occurs:
LiBr+e.sup.-=1/2Br.sub.2+Li.sup.+ (62)
1/2Br.sub.2+LiA+1/2H.sub.2O= LiBr+1/6LiBrO.sub.3+HA (63)
[0343] Upon the completion of the first SD cycle only up to 1/6 of
the original bromide can be converted to bromate. Thus, further
cycles or cascade of splitting-disproportionation (SD) is used.
Example 17
[0344] Electric energy cycle with a LiBrO.sub.3 regeneration using
an anionic buffer base and the aqueous multi-electron oxidant
(AMO)-on-negative mode of operation. Lithium bromate and bromide
are well suited for the energy cycle disclosed herein due to their
high aqueous solubilities. Phosphate buffer is utilized due to the
appropriate pH and chemical compatibility with other ingredients.
However, the intermediate acid form of the phosphate buffer
H.sub.2PO.sub.4.sup.- produced in the disproportionation is not the
final acid form H.sub.3PO.sub.4 used in the discharge unit 104. The
conversion of the intermediate acid form of phosphate into the
final acid form requires extra expenses of chemical or energy which
may not be the preferred mode under on-board acidification
scenarios. Also, the possibility of the formation of a poorly
soluble Li.sub.3PO.sub.4 in the case of phosphate buffer, limits
the flexibility of the design of the regeneration system 106. For
these reasons other buffers are considered.
[0345] For purposes of illustration, this example refers to a
Good's buffer HA, for example, Me.sub.2NCH.sub.2CH.sub.2SO.sub.3H
or 3-(N-morpholino) propanesulfonic acid (MOPS) with pKa=7.2 or
4-(N-morpholino) butanesulfonic acid (MOBS) with pKa=7.6 available
from Sigma-Aldrich. One advantage of such buffers is that in their
acidic form .sup.+HMe.sub.2N--R--SO.sub.3H, they can perform the
function of the strong extra acid in the ignition mode of
discharge, eliminating the need for an additional chemical
component. Another advantage is their anionic state which reduced
their cross-over through a cation-exchange membrane. Two
commercially available compounds are of particular interest. The
propyl version, MOPS, is inexpensive at about 390 $/kg since MOPS
is easily produced by reaction of morpholine and propane sultone,
both being readily available, but MOPS has a pKa of 7.2 which is
within the suitable range. The use of Li-MOPS for bromine
disproportionation is exemplarily illustrated in FIG. 26. The
butane version, MOBS has a higher pKa=7.6, requiring a shorter
regeneration time, but has a significantly higher cost of about
16,000 $/kg due to the higher cost and/or difficult synthesis of
the butane sultone precursor.
[0346] The energy and matter cycle starting with neutral discharge
fluid comprising LiBr and the buffer acid HA is disclosed herein.
In the regeneration system 106, the neutral discharge fluid is
first converted into alkaline discharge by passing thru the
negative electrode compartment of the SD reactor 1502 configured
for the aqueous multi-electron oxidant (AMO)-on-negative mode of
operation, producing H.sub.2 and alkaline discharge fluid
comprising LiBr and LiA.
LiBr+HA+1e.sup.-+Li.sup.+.dbd.LiBr+LiA+1/2H.sub.2 on the negative
electrode (64)
[0347] In the separation reactor 1010 the H.sub.2 is separated from
the alkaline discharge fluid and the latter is pumped into the
positive electrode compartment wherein bromide electrooxidation
(65) and disproportionation (66) take place:
LiBr+LiA-1e.sup.-+=1/2Br.sub.2+LiA+Li.sup.+ (65)
1/2Br.sub.2+LiA+1/2H.sub.2O= LiBr+1/6LiBrO.sub.3+HA (66)
while the counter cation such as Li.sup.+ released at the positive
electrode 1703 in (65) moves through the cation-exchange membrane
to the negative electrode 1702, wherein electroreduction and
neutralization shown in (64) take place.
[0348] The partially regenerated neutral oxidant fluid formed in
(66) at the positive electrode 1703 is transferred again to the
negative electrode compartment where the partially regenerated
neutral oxidant fluid enters a new cycle of alkalization (64),
splitting (65), and disproportionation (66). In this example, the
negative electrode 1702 is configured for the aqueous
multi-electron oxidant (AMO)-on-negative mode of operation using a
cation conductive layer and an electron conductive layer 1702b
which prevents the electroreduction of the AMO such as bromate on
the negative electrode 1702. The cycle is continued until the
desired ratio of bromate to all bromine species in the neutral
oxidant fluid reaches a predetermined value, for example 0.95. This
regenerated neutral oxidant fluid and hydrogen are stored in the
regeneration system 106 until they are transferred into a discharge
system 101 such as in an electric vehicle.
[0349] In the discharge system 101, the neutral oxidant fluid is
converted first into acidic oxidant fluid using, for example, an
orthogonal ion migration across laminar flow (OIMALF) reactor 1501.
The chemical transformations in the OIMALF reactor 1501 can be
illustrated by the following examples:
On the positive electrode:1/2H.sub.2-e.sup.-=H.sup.+ (67)
In the central
channel:LiBrO.sub.3+HA+HBr.dbd.HBrO.sub.3+HA+LiBr.sup.- (68)
On the negative
electrode:HBrO.sub.3+HA+6H.sup.++6e.sup.-=HBr+3H.sub.2O+HA (69)
wherein reaction (68) represents the ion exchange process such as
the orthogonal ion migration across laminar flow (OIMALF). In an
embodiment, the H.sub.2 produced on the negative electrode 205b in
(68) is consumed on the positive electrode 205a in (67).
[0350] The acidic oxidant fluid produced in reaction (68) is
supplied to the positive electrode 205a of the discharge cell 104a
wherein the discharge proceeds via the electroreduction
(70)-comproportionation (71) cycle:
2.5Br.sub.2+e.sup.-=5Br.sup.- (70)
5Br.sup.-+HBrO.sub.3+HA+5H.sup.+-=3Br.sub.2+3H.sub.2O+HA (71)
while hydrogen electrooxidation on the negative electrode 205b
supplies the protons consumed in (71):
2.5H.sub.2-e.sup.-=5H.sup.+ (72)
[0351] The reaction (71) produces more Br.sub.2 than reaction (70)
consumes for the same amount of bromide. This feature leads to the
possibility of the ignition regime where the electrode current
increases as the convection rate decreases. A useful feature of the
ED-cycle (70)-(71) is that the use of high acid concentration is
not necessary for the cycle to proceed in the ignition mode. In the
case of highly soluble LiBrO.sub.3, the ratio
[H.sup.+]/[BrO.sub.3.sup.-] as low as 0.05 may suffice. The low
acid concentration in the acidic discharge fluid is critical for
the practical applications of the disclosed technology since it
assures a low rate of the decomposition reaction (73) which
competes with the desired comproportionation reaction (71):
2HBrO.sub.3.dbd.Br.sub.2+2.5O.sub.2+H.sub.2O (73)
[0352] The gross equation for the chemical process in the discharge
unit 104 is:
HBrO.sub.3+3H.sub.2+HA=HBr+3H.sub.2O+HA (74)
and the gross equation for the chemical process in the discharge
system 101 is:
LiBrO.sub.3+HA+3H.sub.2=LiBr+HA+3H.sub.2O (75)
[0353] The neutral discharge fluid produced in (74) is used to
start a new energy cycle with process (64) in the regeneration
system 106.
Example 18
[0354] Solar regeneration of LiBrO.sub.3 from LiBr using an anionic
buffer base and the aqueous multi-electron oxidant
(AMO)-on-negative mode of operation: Unlike the hydrogen economy
scenario, where the poor efficiency of solar water splitting,
either photoelectrochemically, photothermally or some other way,
prevents a large-scale use of sunlight as the primary energy
source, the energy cycle disclosed herein employs splitting of a
hydrogen halide, for example, HBr as the main input step for
external energy. Energy and cost efficient routes to the reaction
2HBr=H.sub.2+Br.sub.2 using solar power, particularly,
photoelectrolysis, have been reported or are known in the art. For
example, a method for decomposing a solution of HBr using a
platinum cathode and platinum-coated n-type amorphous silicon
photo-anode and a red light and approximately 0.5% conversion
efficiency is known in the art. Also, a system with a higher
efficiency, for example, approximately 8%, that utilizes a
p-GaInP2(Pt)/GaAs photoelectrochemical/photovoltaic device is also
known in the art.
[0355] In the solar regeneration example disclosed herein,
splitting of one or more forms of the discharge fluid into hydrogen
and bromine is performed via photoelectrolysis. In an embodiment,
the neutral discharge fluid comprising LiBr and the buffer acid HA
is first converted into alkaline discharge fluid by passing the
neutral discharge fluid through the negative electrode compartment
of a photoelectrolysis-disproportionation reactor (not shown)
configured for the aqueous multi-electron oxidant (AMO) on-negative
mode of operation, producing H.sub.2 and alkaline discharge fluid
comprising LiBr and LiA.
LiBr+HA+1e.sup.-+Li.sup.+.dbd.LiBr+LiA+1/2H.sub.2 on the negative
electrode (76)
[0356] In the separation reactor 1010 H.sub.2 is separated from the
alkaline discharge fluid and the latter is pumped into the positive
electrode compartment wherein bromide electrooxidation (77) and
disproportionation (78) take place:
LiBr+LiA-1e.sup.-+=1/2Br.sub.2+LiA+Li.sup.+ (77)
1/2Br.sub.2+LiA+1/2H.sub.2O= LiBr+1/6LiBrO.sub.3+HA (78)
while the counter cation such as Li.sup.+ released at the positive
electrode 1703 in (77) moves through the cation-exchange membrane
1704 to the negative electrode 1702, wherein electroreduction and
neutralization shown in (76) take place.
[0357] The partially regenerated neutral oxidant fluid formed in
(78) at the positive electrode 1703 goes again to the negative
electrode compartment where it enters a new cycle of alkalization
(76), splitting (77), and disproportionation (78). In this example,
the negative electrode 1702 is configured for the aqueous
multi-electron oxidant (AMO)-on-negative mode of operation using a
cation- and electron-conductive layer which prevents the
electroreduction of the AMO such as bromate on the negative
electrode 1902. The cycle is continued until the desired ratio of
bromate to all bromine species in the neutral oxidant fluid reaches
a predetermined value, for example 0.95. This regenerated neutral
oxidant fluid and hydrogen are stored in the regeneration system
106 until they are transferred into a discharge system 101 such as
in an electric vehicle.
Example 19
[0358] Decomposition of bromate in acid: The discharge process
disclosed herein faces a TRIZ contradiction between the discharge
cell power and the stability of the acidic aqueous multi-electron
oxidant (AMO) solution, that is, upon increasing the acid
concentration in the acidic oxidant fluid, the electroreduction of
the AMO is facilitated while the stability of the AMO deteriorates.
The existence of a composition meeting both requirements for a high
discharge power and stability cannot be predicted theoretically.
Experimental studies were conducted to find a composition of acidic
discharge fluid which meets both requirements for stability and for
discharge power. Solutions of sulfuric acids of various
compositions were prepared by mixing 98% w/w H.sub.2SO.sub.4 and
water to 5 mL volumes. Noticeable heating was observed in all
cases. While the solutions were still hot an excess of solid
LiBrO.sub.3 was added to each of the solutions. The experimental
observations of decomposition of bromate introduced as an excess of
solid LiBrO.sub.3 in various acidic solutions are summarized in
Table 3 below:
TABLE-US-00005 TABLE 3 H.sub.2O:H.sub.2SO.sub.4 H.sub.2SO.sub.4
H.sub.2SO.sub.4 O.sub.2 Br.sub.2 Final v/v w % density
H.sub.2SO.sub.4 M evolution evolution color discharge 5:5 65 1.55
10.3 noticeable vigorous dark brown 6:4 55 1.45 8.1 starts first
starts later light brown 6.25:3.75 52 1.415 7.5 noticeable slow
dark yellow 7:3 44 1.34 6.0 noticeable slower medium yellow 8:2
31.5 1.22 4.0 slow limited light yellow 9:1 17 1.17 1.9 not
observed very limited light yellow
[0359] Two parallel decomposition pathways were observed: one
leading to oxygen evolution or production and the other leading to
bromine evolution or production. The oxygen evolution pathway
dominates at the lower acidities which are of interest to the
disclosed technology. The data disclosed in Table 3 suggest that
acidic bromate solutions are sufficiently stable to be used in a
discharge system 101 when the concentration of a strong acid is
below 4M. Furthermore, as exemplarily illustrated in FIGS. 12-13,
FIGS. 21A-21B, and FIG. 25 a 2M concentration of acidic protons is
sufficient to provide a practically useful discharge power when
bromate is used as the aqueous multi-electron oxidant (AMO) as
disclosed in Example 13. Hence, the acidification process can be
performed off-board and a week's supply of the acidic oxidant fluid
can be stored on-board. The concentrated HBrO.sub.3 stored on-board
is a stable solution yet still capable of discharge with a high
power.
Example 20
[0360] FIG. 24 exemplarily illustrates a graphical representation
showing background-subtracted limiting currents in mA/cm.sup.2 of
bromide electrooxidation-disproportionation on a glassy carbon
rotating disk electrode in a 0.5M sodium phosphate buffer at
various rotation rates in rpm. The sodium phosphate buffer has a pH
of 8.0 and comprises about 5 mM NaBr. The dotted line in FIG. 24
represents the calculated Levich plot for the diffusion limited
current of bromide.
[0361] Electrooxidation-disproportionation of bromide on a glassy
carbon rotating disk electrode: An experiment to demonstrate the
feasibility of the electrooxidation-disproportionation step in the
regeneration process using a phosphate buffer which has a suitable
pH and to determine the time scale of this process was conducted.
In this experiment, a 3-compartment glass cell equipped with a
glassy carbon rotating disk electrode of Pine Instruments with
about 5.0 mm diameter, a Ag/AgCl reference electrode in 3.0 M NaCl
connected via a Luggin capillary, and a Pt counter electrode were
used. The background electrolyte was 0.5M sodium phosphate buffer
with a Ph of about 8.0 procured from Teknova to which about 5 mM
NaBr was added. The background-subtracted limiting currents at
+1.30 Vv Ag/AgCl obtained in this experiment are exemplarily
illustrated in FIG. 24. At high rotation rates, the limiting
current follows the Levich behavior that is, the limiting current
increases linearly with the square root of the rotation rate, as
expected for a diffusion-limited process. At low rotation rates a
positive deviation from the Levich behavior is observed which
agrees with the occurrence of the disproportionation (16). The time
scale of the disproportionation in this buffer can be estimated as
the diffusion time across the diffusion boundary layer at 900 rpm
which is a characteristic point of deviation. According to the
Levich equation, the thickness of the diffusion boundary layer at
this rotation rate in water is ca. 20 .mu.m, which translates via
Fick's 2.sup.nd Law into the diffusion time of 0.5s. Thus, 0.5s is
the characteristic time of the disproportionation of bromine in 0.5
M sodium phosphate buffer. This time-scale is well-suited for a
commercial regeneration process.
Example 21
[0362] FIG. 25 exemplarily illustrates a staircase cyclic
voltammetry on a glassy carbon rotating disk electrode of about
0.283 cm.sup.2 area in a 2 hour aged solution containing 2.0 M
H.sub.2SO.sub.4 and approximately 5M LiBrO.sub.3. The electrode
rotation rates and scan directions are exemplarily illustrated near
the curves. The reference electrode is Ag/AgCl in 3 M NaCl.
[0363] Electroreduction of bromate in acid on a carbon rotating
disk electrode: An experiment was conducted to determine
practically achievable limits of power per electrode area upon
discharge imposed by the aforementioned TRIZ contradiction between
the stability and the limiting current in the acidic oxidant fluid.
Although numerous compositions were tested, only the data for a 2.0
M H.sub.2SO.sub.4 solution are shown in FIG. 25 since this acid
concentration was found to be near-optimal. In order to minimize
the decomposition of the aqueous multi-electron oxidant (AMO)
before the measurements, a solid LiBrO.sub.3 was added to the acid
solution in the electrochemical cell. As noticed in previous
experiments, in this aged solution the more positive wave
attributed to the electroreduction of bromine produced via the
comproportionation is followed by a more negative wave attributed
to the electroreduction of a bromate decomposition intermediate
tentatively, hypobromite. Only the more positive wave is observed
in fresh solutions. The limiting currents of both waves seem to be
controlled by the concentration of acidic protons rather than that
of bromate since the latter is present in a large stoichiometric
excess. This also explains why the decrease in the limiting current
with the rotation rate similar to the one shown in FIG. 25 is not
observed. The solutions become yellow during such experiments in a
batch cell due to the comproportionation of product bromide with
bromate. As Example 11 shows, in the absence of bromide the
stability of bromate in acids is better. The problem of the
parasitic bromate comproportionation with bromide is not present in
the discharge flow cells disclosed herein elsewhere.
Example 22
[0364] Disproportionation of bromine in Li-3-(N-morpholino)
propanesulfonic acid (MOPS) buffer: 1.0 M Li-MOPS solution was
prepared from H-MOPS and LiOH.times.H.sub.2O. The pH of the
resulting solution is 7.2 and the density is 1.11 g/mL. 2 moles of
this solution (2 mL) was mixed with 1 mmole of Br.sub.2 which is
about 160 mg and about 52 .mu.L. One week later, the solution
composition was analyzed using negative mode electrospray
ionization (ESI)-mass spectrometry (MS). A sample of unreacted
Li-MOPS was used as a control. The expected chemical reaction is
given by:
1/2Br.sub.2+Li-MOPS+1/2H.sub.2O= LiBr+1/6LiBrO.sub.3+H-MOPS
[0365] FIG. 26 exemplarily illustrates an electrospray ionization
(ESI)-mass spectrometry (MS) spectrum, showing experimental data
demonstrating the feasibility of a regeneration process. The ESI-MS
spectrum exemplarily illustrated in FIG. 26 confirms the formation
of bromate and bromide.
TABLE-US-00006 Sample # Composition 1 1/2 Br.sub.2 + Li-MOPS +
1/2H.sub.2O = LiBr + 1/6 LiBrO.sub.3 + H-MOPS 2 Li-MOPS only 3
Li-MOPS + NaBr 4 Li-MOPS + Br2 in excess, red liquid
[0366] Similar experiments were carried out using a
lithium-phosphate buffer. 0.2 mole of LiOH.times.H.sub.2O (8.392 g)
was dissolved in 100 mL of water to which 0.1 mole of
H.sub.3PO.sub.4 of about 6.22 mL of 80% w was added. A white
precipitate was formed due to the following reaction:
2LiOH.times.H.sub.2O+H.sub.3PO.sub.4=3H.sub.2O+Li.sub.2HPO.sub.4(=1/2Li.-
sub.3PO.sub.4.dwnarw.+1/2LiH.sub.2PO.sub.4)
[0367] 10 mL, that is, 0.010 moles of Li.sub.2HPO.sub.4 equivalent
of the resulting white slurry was sampled into a separate vial and
treated with 0.0050 of bromine of about 0.25 mL. The following
reaction:
Li.sub.3PO.sub.4.dwnarw.+LiH.sub.2PO.sub.4+Br.sub.2+H.sub.2O=5/3LiBr+1/3-
LiBrO.sub.3+2LiH.sub.2PO.sub.4
proceeds even at 60.degree. C. which is too slow for practical
applications.
[0368] The foregoing examples have been provided merely for the
purpose of explanation and are in no way to be construed as
limiting of the present invention disclosed herein. While the
invention has been described with reference to various embodiments,
it is understood that the words, which have been used herein, are
words of description and illustration, rather than words of
limitation. Further, although the invention has been described
herein with reference to particular means, materials, and
embodiments, the invention is not intended to be limited to the
particulars disclosed herein; rather, the invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims. Those skilled in the art,
having the benefit of the teachings of this specification, may
affect numerous modifications thereto and changes may be made
without departing from the scope and spirit of the invention in its
aspects.
* * * * *