U.S. patent application number 10/553922 was filed with the patent office on 2009-02-26 for fuel cell, components and systems.
Invention is credited to Benedetto Anthony Iacovelli.
Application Number | 20090053565 10/553922 |
Document ID | / |
Family ID | 33313502 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053565 |
Kind Code |
A1 |
Iacovelli; Benedetto
Anthony |
February 26, 2009 |
FUEL CELL, COMPONENTS AND SYSTEMS
Abstract
Alkali fuel cells, systems, and related methods, and
flow-through, high-surface area electrodes, are employed to
generate electricity. The electrode can include a porous substrate
comprising a first side for fluid ingress, a second side for fluid
egress, and a plurality of walls oriented in different directions
between the first and second sides. Voids can be defined between
the walls. The walls can include surfaces and micro-scale pores. A
multi-directional fluid flow path can be defined between the first
and second sides. A thin film comprising a catalytic material can
be disposed on the surfaces. A fuel/electrolyte mixture can be
flowable generally from the first side, through the voids and the
pores of the substrate and in contact with the thin film, and to
the second side. Additives can be included for refreshing the
electrolyte and/or the electrode. A water/thermal/pressure
management system includes a permeable membrane from which water
can be removed from a fluid while retaining fuel and/or electrolyte
in the fluid. The electrolyte can include an additive that cleans
the electrodes. A refresh cycle can be implemented in which one or
more electrodes are operated in a mode that refreshes catalytic
material of the electrode.
Inventors: |
Iacovelli; Benedetto Anthony;
(Leicester, NC) |
Correspondence
Address: |
David P Gloekler;The Eclipse Group
10605 Balboa Blvd., Suite 300
Granada Hills
CA
91344
US
|
Family ID: |
33313502 |
Appl. No.: |
10/553922 |
Filed: |
April 22, 2004 |
PCT Filed: |
April 22, 2004 |
PCT NO: |
PCT/US04/12635 |
371 Date: |
June 7, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60464874 |
Apr 22, 2003 |
|
|
|
60500123 |
Sep 3, 2003 |
|
|
|
Current U.S.
Class: |
429/434 |
Current CPC
Class: |
H01M 8/0232 20130101;
H01M 4/926 20130101; H01M 8/086 20130101; Y02E 60/50 20130101; H01M
4/921 20130101; H01M 8/083 20130101; H01M 4/923 20130101; H01M
4/8626 20130101 |
Class at
Publication: |
429/15 ; 429/40;
429/44; 429/38; 429/25; 429/13 |
International
Class: |
H01M 8/04 20060101
H01M008/04; H01M 4/86 20060101 H01M004/86; H01M 2/14 20060101
H01M002/14 |
Claims
1. A flow-through electrode for use in a fuel cell, the electrode
comprising: a porous substrate comprising a first side for fluid
ingress, a second side for fluid egress, a plurality of walls
oriented in different directions between the first and second sides
and defining voids between the walls, the walls including surfaces
and micro-scale pores, wherein a multi-directional fluid flow path
is defined between the first and second sides; and a thin film
disposed on the surfaces, the thin film comprising a catalytic
material, whereby a fuel and an electrolyte are flowable generally
from the first side, through the voids and the pores of the
substrate and in contact with the thin film, and to the second
side.
2. The electrode according to claim 1 comprising a conductive
component embedded within the substrate for conducting current from
the electrode.
3. The electrode according to claim 1 wherein the substrate
comprises sintered particles.
4. The electrode according to claim 1 wherein the substrate is
formed as a metal sponge.
5. The electrode according to claim 4 wherein the sponge comprises
nickel.
6. The electrode according to claim 1 wherein the substrate
comprises a microstructure selected from the group consisting of
open cellular, reticular, foamed, sintered, sponge, raney,
nanostructure, vitreous, gel, sol-gel, aero-gel, and combinations
thereof.
7. The electrode according to claim 1 wherein the substrate
comprises a material selected from the group consisting of porous
conductive plastics, carbon compounds, ceramics, metals, oxides of
metals, nitrides or metals, alloys of metals, semiconductors, and
combinations thereof.
8. The electrode according to claim 1 comprising microparticles
disposed in the voids.
9. The electrode according to claim 8 wherein the microparticles
comprise a catalytic material.
10. The electrode according to claim 9 wherein the catalytic
material comprises platinum.
11. The electrode according to claim 8 wherein microparticles
comprise an electrically conductive material.
12. The electrode according to claim 8 wherein the microparticles
comprise a matrix of catalytic material supported on a support
material.
13. The electrode according to claim 12 wherein the catalytic
material comprises platinum and the support material comprises
carbon.
14. The electrode according to claim 12 wherein the catalytic
material comprises platinum and the support material comprises
nickel.
15. The electrode according to claim 8 wherein the microparticles
comprise high-surface area flakes.
16. The electrode according to claim 8 wherein the microparticles
comprise filaments.
17. The electrode according to claim 1 comprising three-dimensional
structures protruding from the walls into the voids.
18. The electrode according to claim 17 wherein the
three-dimensional structures comprise nanostructures.
19. The electrode according to claim 1 wherein the walls have
hollow interiors.
20. The electrode according to claim 19 wherein the thin film is
further deposited on inside surfaces of the walls facing the
interiors.
21. The electrode according to claim 1 wherein the thin film
comprises a component selected from the group consisting of
platinum, silver, gold, iridium, nickel, palladium, osmium,
ruthenium, rhodium, rhenium, tungsten, alloys thereof, oxides
thereof, and nitrides thereof.
22. The electrode according to claim 1 wherein the thin film is
substantially continuous.
23. The electrode according to claim 1 wherein the thin film is
discontinuous.
24. The electrode according to claim 1 comprising a semipermeable
membrane disposed at a side of the substrate, the membrane
comprising a material substantially permeable to water and
electrolyte and substantially impermeable to fuel.
25. The electrode according to claim 1 comprising a semipermeable
membrane disposed at a side of the substrate, the membrane
comprising a material substantially permeable to fuel and
electrolyte and substantially impermeable to water.
26. The electrode according to claim 1 comprising a semipermeable
membrane disposed at a side of the substrate, the membrane
comprising a material substantially permeable to fuel and
substantially impermeable to electrolyte and water.
27. A flow-through electrode for use in a fuel cell, the electrode
comprising: (a) a first region and a second region each comprising
a porous substrate for flowing a fuel/electrolyte combination
therethrough and a thin film disposed on the substrate, the thin
film comprising a catalytic material; and (b) a third region
interposed between the first and second regions and fluidly
communicating with the first and second regions, wherein the pore
density of the third region is less than the pore densities of the
first and second regions.
28. The electrode according to claim 27 wherein the first and
second regions are part of a contiguous substrate.
29. The electrode according to claim 27 wherein the third region is
substantially hollow.
30. (canceled)
31. The electrode according to claim 27 wherein each substrate
comprises a plurality of walls oriented in different directions and
a plurality of voids between the walls, the walls include surfaces
and micro-scale pores, and the thin film is disposed on the
surfaces.
32. A flow-through electrode for use in a fuel cell, the electrode
comprising a plurality of regions, each region adjacent to and
fluidly communicating with at least one other region, each region
comprising a porous substrate for flowing a fuel/electrolyte
combination therethrough and a thin film disposed on the substrate,
the thin film comprising a catalytic material, and each region
having a porosity different from the porosities of the other
regions, wherein the plurality of regions are arranged in order of
successively increasing porosity to define a porosity gradient
whereby the fuel/electrolyte combination can be flowed generally
with or against the porosity gradient.
33. (canceled)
34. The electrode according to claim 32 wherein each substrate
comprises a plurality of walls oriented in different directions and
a plurality of voids between the walls, the walls include surfaces
and micro-scale pores, and the thin film is disposed on the
surfaces.
35. A fuel cell comprising: (a) an anode comprising a porous
substrate and a thin film disposed on the substrate, the thin film
comprising a catalytic material; (b) a cathode; (c) a porous
barrier interposed between the anode and cathode; (d) an anode-side
channel defined between the anode and barrier for receiving a
fuel-rich fluid; and (e) a cathode-side channel defined between the
cathode and barrier for receiving a fuel-depleted fluid.
36. (canceled)
37. The fuel cell according to claim 35 wherein the substrate
comprises a plurality of walls oriented in different directions and
a plurality of voids between the walls, the walls include surfaces
and micro-scale pores, and the thin film is disposed on the
surfaces.
38. The fuel cell according to claim 35 comprising a separator
device communicating with the anode-channel and the cathode-side
channel for separating fuel from a fluid processed by the
anode.
39. A fuel cell comprising: (a) an anode comprising a first anode
section, a second anode section and a third anode section, the
first and second anode sections each comprising a porous substrate
for flowing a fuel/electrolyte combination therethrough, and the
third anode section interposed between the first and second anode
sections and fluidly communicating with the first and second anode
sections; (b) a cathode comprising a first cathode section and a
second cathode section; (c) a first channel interposed between the
first anode section and the first cathode section; and (d) a second
channel interposed between the second anode section and the second
cathode section.
40. The fuel cell according to claim 39 wherein the first and
second anode sections are part of a contiguous substrate.
41. The fuel cell according to claim 39 wherein the pore density of
the third anode section is less than the pore densities of the
first and second anode sections.
42. The fuel cell according to claim 39 wherein the anode comprises
a porous substrate and a thin film disposed on the substrate, and
the thin film comprises a catalytic material.
43. The electrode according to claim 42 wherein the substrate
comprises a plurality of walls oriented in different directions and
a plurality of voids between the walls, the walls include surfaces
and micro-scale pores, and the thin film is disposed on the
surfaces.
44. A fuel cell stack comprising: (a) a first side, a second side
opposing the first side, a third side, and a fourth side opposing
the third side; (b) a plurality of substantially planar electrodes
arranged substantially parallel to each other and comprising
respective edges defining the first, second, third, and fourth
sides, the plurality of electrodes defining a plurality of first
channels fluidly communicating with the first and second sides for
conducting an oxygen-containing fluid generally from the first side
to the second side, and defining a plurality of second channels
fluidly communicating with the third and fourth sides for
conducting a fuel/electrolyte combination generally from the third
side to the fourth side; and (c) a device fluidly communicating
with the second channels for removing water from the
fuel/electrolyte combination by a pressure differential.
45. The fuel cell stack according to claim 44 wherein the plurality
of electrodes include anodes and cathodes, and at least one channel
is interposed between and fluidly communicates with at least two
anodes.
46. (canceled)
47. The fuel cell stack according to claim 44 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of water from the conduit and retention of fuel and
electrolyte components within the conduit.
48. The fuel cell stack according to claim 44 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of fuel and electrolyte components from the conduit
and retention of water within the conduit.
49. The fuel cell stack according to claim 44 comprising a device
fluidly communicating with the second channels for removing heat
from the fuel/electrolyte combination.
50. A fuel cell comprising: (a) an anode region comprising a
plurality of anodes and a plurality of anode channels, each anode
channel communicating with at least one anode, the plurality of
anode channels comprising pre-anode channels for supplying a
fuel-rich fluid to one or more of the anodes and post-anode
channels for receiving a fuel-depleted fluid from one or more of
the anodes; and (b) a cathode region comprising a plurality of
cathodes and a plurality of cathode channels, each cathode channel
communicating with at least one cathode and at least one anode
channel.
51. The fuel cell according to claim 50 wherein the number of
anodes is different from the number of cathodes.
52. The fuel cell according to claim 50 comprising a manifold
communicating with the anode channels and the cathode channels for
transferring electrolyte.
53. The fuel cell according to claim 52 comprising a device fluidly
communicating with the manifold for removing water from the
fuel-depleted fluid by a pressure differential.
54. The fuel cell according to claim 53 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of water from the conduit and retention of fuel and
electrolyte components within the conduit.
55. A fuel cell comprising: (a) an anode section comprising a first
anode, a second anode and an anode channel interposed between and
fluidly communicating with the first and second anodes, the first
and second anodes each comprising a porous substrate for flowing a
fuel/electrolyte combination therethrough; and (b) a cathode
section comprising a plurality of cathodes and a plurality of
cathode channels, each cathode channel communicating with at least
one cathode, and the plurality of cathode channels spaced from and
communicating with the anode section.
56. The fuel cell according to claim 55 comprising a device fluidly
communicating with the anode section for removing water from the
fuel/electrolyte combination by a pressure differential.
57. The fuel cell according to claim 56 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of water from the conduit and retention of fuel and
electrolyte components within the conduit.
58. (canceled)
59. A method for operating a fuel cell comprising: (a) providing a
flow-through electrode comprising a porous substrate and a catalyst
disposed on the substrate; and (b) flowing a fluid through the
electrode, the fluid comprising an alkali electrolyte and an
additive for supplying a supplemental source of hydroxyl ions.
60. (canceled)
61. The method according to claim 59 wherein the hydroxyl-supplying
additive is selected from the group consisting of buffers,
polyhydroxyl alkalis, hydroxyl carriers, and combinations
thereof.
62. The method according to claim 59 wherein the fluid further
comprises an additive for cleaning the electrolyte.
63. The method according to claim 59 wherein the alkali electrolyte
comprises a metal hydroxide.
64. The method according to claim 59 wherein flowing the fluid
comprises flowing the fluid in a net forward direction that
includes oscillatory components.
65. A method for operating a fuel cell comprising: (a) operating a
plurality of electrodes comprising anodes and cathodes to collect
electrons from the anodes; (b) switching the operation of at least
one electrode to a refresh cycle whereby catalyst supported by the
electrode is cleaned, wherein switching comprises disconnecting the
at least one electrode from an electron-receiving load; and (c)
applying an electric charge to one or more electrodes including the
disconnected electrode.
66. (canceled)
67. (canceled)
68. (canceled)
69. The method according to claim 65 comprising operating the at
least one electrode in an electron-collecting cycle after cleaning
the at least one electrode, and switching the operation of at least
one other electrode to the refresh cycle.
70. The method according to claim 65 comprising operating an
electrical controller to switch one or more electrodes between an
electron-collecting cycle and a refresh cycle.
71. The method according to claim 65 comprising flowing an
electrolyte-containing fluid through at least one of the electrodes
in a net forward direction that includes oscillatory
components.
72. The method according to claim 65 comprising flowing an
electrolyte-containing fluid through the electrodes including the
at least one electrode being refreshed.
73. A fuel cell comprising: (a) an anode; (b) a cathode; (c) a
porous barrier interposed between the anode and cathode; (d) an
anode-side channel defined between the anode and barrier for
receiving a fuel-rich fluid; and (e) a cathode-side channel defined
between the cathode and barrier for receiving a fuel-depleted
fluid; and (f) a separator device communicating with the
anode-channel and the cathode-side channel for separating fuel from
a fluid processed by the anode.
74. The fuel cell according to claim 73 wherein the anode comprises
a porous substrate and a thin film disposed on the substrate, and
the thin film comprises a catalytic material.
75. The fuel cell according to claim 74 wherein the substrate
comprises a plurality of walls oriented in different directions and
a plurality of voids between the walls, the walls include surfaces
and micro-scale pores, and the thin film is disposed on the
surfaces.
76. A fuel cell stack comprising: (a) a first side, a second side
opposing the first side, a third side, and a fourth side opposing
the third side; (b) a plurality of substantially planar electrodes
arranged substantially parallel to each other and comprising
respective edges defining the first, second, third, and fourth
sides, the plurality of electrodes defining a plurality of first
channels fluidly communicating with the first and second sides for
conducting an oxygen-containing fluid generally from the first side
to the second side, and defining a plurality of second channels
fluidly communicating with the third and fourth sides for
conducting a fuel/electrolyte combination generally from the third
side to the fourth side; and (c) a device fluidly communicating
with the second channels for removing heat from the
fuel/electrolyte combination.
77. The fuel cell stack according to claim 76 wherein the plurality
of electrodes include anodes and cathodes, and at least one channel
is interposed between and fluidly communicates with at least two
anodes.
78. The fuel cell stack according to claim 76 comprising a device
fluidly communicating with the second channels for removing water
from the fuel/electrolyte combination by a pressure
differential.
79. The fuel cell stack according to claim 78 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of water from the conduit and retention of fuel and
electrolyte components within the conduit.
80. The fuel cell stack according to claim 78 wherein the device
comprises a conduit comprising a semipermeable wall for permitting
transpiration of fuel and electrolyte components from the conduit
and retention of water within the conduit.
81. A method for operating a fuel cell comprising: (a) providing a
flow-through electrode comprising a porous substrate and a catalyst
disposed on the substrate; and (b) flowing a fluid through the
electrode, the fluid comprising an alkali electrolyte and an
additive for cleaning the electrolyte.
82. The method according to claim 81 wherein the fluid further
comprises an additive for supplying a supplemental source of
hydroxyl ions.
83. The method according to claim 82 wherein the hydroxyl-supplying
additive is selected from the group consisting of buffers,
polyhydroxyl alkalis, hydroxyl carriers, and combinations
thereof.
84. The method according to claim 81 wherein the alkali electrolyte
comprises a metal hydroxide.
85. The method according to claim 81 wherein flowing the fluid
comprises flowing the fluid in a net forward direction that
includes oscillatory components.
86. A method for operating a fuel cell comprising: (a) providing a
flow-through electrode comprising a porous substrate and a catalyst
disposed on the substrate; and (b) flowing a fluid through the
electrode in a net forward direction that includes oscillatory
components, the fluid comprising an alkali electrolyte.
87. The method according to claim 86 wherein the fluid further
comprises an additive for supplying a supplemental source of
hydroxyl ions.
88. The method according to claim 87 wherein the hydroxyl-supplying
additive is selected from the group consisting of buffers,
polyhydroxyl alkalis, hydroxyl carriers, and combinations
thereof.
89. The method according to claim 86 wherein the fluid further
comprises an additive for cleaning the electrolyte.
90. The method according to claim 86 wherein the alkali electrolyte
comprises a metal hydroxide.
91. A method for operating a fuel cell comprising: (a) operating a
plurality of electrodes comprising anodes and cathodes to collect
electrons from the anodes; (b) switching the operation of at least
one electrode to a refresh cycle whereby catalyst supported by the
electrode is cleaned; and (c) applying an electric charge to one or
more electrodes whereby at least one of these electrodes undergoes
the refresh cycle.
92. The method according to claim 91 wherein switching comprises
disconnecting the at least one electrode from an electron-receiving
load.
93. The method according to claim 92 wherein applying includes
applying an electric charge to the disconnected electrode.
94. The method according to claim 91 comprising operating the at
least one electrode in an electron-collecting cycle after cleaning
the at least one electrode, and switching the operation of at least
one other electrode to the refresh cycle.
95. The method according to claim 91 comprising operating an
electrical controller to switch one or more electrodes between an
electron-collecting cycle and a refresh cycle.
96. The method according to claim 91 comprising flowing an
electrolyte-containing fluid through at least one of the electrodes
in a net forward direction that includes oscillatory
components.
97. The method according to claim 91 comprising flowing an
electrolyte-containing fluid through the electrodes including the
at least one electrode being refreshed.
98. A method for operating a fuel cell comprising: (a) operating a
plurality of electrodes comprising anodes and cathodes to collect
electrons from the anodes; (b) switching the operation of at least
one electrode to a refresh cycle whereby catalyst supported by the
electrode is cleaned; (c) operating the at least one electrode in
an electron-collecting cycle after cleaning the at least one
electrode; and (d) switching the operation of at least one other
electrode to the refresh cycle.
99. The according to claim 98 wherein switching the operation of
the at least one electrode comprises disconnecting the at least one
electrode from an electron-receiving load.
100. The method according to claim 99 comprising applying an
electric charge to one or more electrodes including the
disconnected electrode.
101. The method according to claim 98 comprising applying an
electric charge to one or more electrodes whereby at least one of
these electrodes undergoes the refresh cycle.
102. The method according to claim 98 comprising operating an
electrical controller to switch one or more electrodes between an
electron-collecting cycle and a refresh cycle.
103. The method according to claim 98 comprising flowing an
electrolyte-containing fluid through at least one of the electrodes
in a net forward direction that includes oscillatory
components.
104. The method according to claim 98 comprising flowing an
electrolyte-containing fluid through the electrodes including the
at least one electrode being refreshed.
105. A method for operating a fuel cell comprising: (a) operating a
plurality of electrodes comprising anodes and cathodes to collect
electrons from the anodes; (b) switching the operation of at least
one electrode to a refresh cycle whereby catalyst supported by the
electrode is cleaned; and (c) flowing an electrolyte-containing
fluid through at least one of the electrodes in a net forward
direction that includes oscillatory components.
106. The method according to claim 105 wherein switching comprises
disconnecting the at least one electrode from an electron-receiving
load.
107. The method according to claim 106 comprising applying an
electric charge to one or more electrodes including the
disconnected electrode.
108. The method according to claim 105 comprising applying an
electric charge to one or more electrodes whereby at least one of
these electrodes undergoes the refresh cycle.
109. The method according to claim 105 comprising operating the at
least one electrode in an electron-collecting cycle after cleaning
the at least one electrode, and switching the operation of at least
one other electrode to the refresh cycle.
110. The method according to claim 105 comprising operating an
electrical controller to switch one or more electrodes between an
electron-collecting cycle and a refresh cycle.
111. The method according to claim 105 comprising flowing an
electrolyte-containing fluid through the electrodes including the
at least one electrode being refreshed.
112. A method for operating a fuel cell comprising: (a) operating a
plurality of electrodes comprising anodes and cathodes to collect
electrons from the anodes; (b) switching the operation of at least
one electrode to a refresh cycle whereby catalyst supported by the
electrode is cleaned; and (c) flowing an electrolyte-containing
fluid through the electrodes including the at least one electrode
being refreshed.
113. The method according to claim 112 wherein switching comprises
disconnecting the at least one electrode from an electron-receiving
load.
114. The method according to claim 113 comprising applying an
electric charge to one or more electrodes including the
disconnected electrode.
115. The method according to claim 112 comprising applying an
electric charge to one or more electrode whereby at least one of
these electrodes undergoes the refresh cycle.
116. The method according to claim 112 comprising operating the at
least one electrode in an electron-collecting cycle after cleaning
the at least one electrode, and switching the operation of at least
one other electrode to the refresh cycle.
117. The method according to claim 112 comprising operating an
electrical controller to switch one or more electrodes between an
electron-collecting cycle and a refresh cycle.
118. The method according to claim 112 comprising flowing an
electrolyte-containing fluid through at least one of the electrodes
in a net forward direction that includes oscillatory components.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/464,874 filed Apr. 22, 2003, entitled
"Improved Designs for a Reduced Cost Flexible Fuel Methanol Capable
Fuel Cell and Components"; and U.S. Provisional Patent Application
Ser. No. 60/500,123 filed Sep. 3, 2003, entitled "Reduced Cost
Flexible Fuel Methanol Capable Fuel Cell and Components"; the
contents of each of which are incorporated herein by reference in
their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Disclosure
[0003] The present invention relates generally to fuel cells. More
particularly, the present invention relates to fuel cells utilizing
flow-through electrodes and alkali electrolytes.
[0004] 2. Background of the Disclosure
[0005] In general terms, a fuel cell generates DC electricity by a
chemical reaction or reactions occurring at an anode and a cathode.
The electricity is utilized by an electrical circuit communicating
with the fuel cell. The fuel cell utilizes an electrolyte that
essentially functions to transport electrically charged particles
from one electrode to the other. Liquid electrolytes such as
alkali, molten carbonate, and phosphoric acid, and solid
electrolytes such as proton exchange membrane (PEM) and solid oxide
have been employed. The fuel cell may also utilize one or more
catalysts to promote the proper reactions. A basic fuel for the
reactions is hydrogen, hydrocarbons, alcohols, or the like that can
be oxidized. In addition, oxygen is generally reduced at the
cathode. A primary byproduct is water, but conventional fuel cell
designs inevitably yield several more undesired byproducts such as
carbon monoxide, sulfur, and the like, particularly when employing
a carbon-based fuel. Different types of fuel cells have been
developed and are well-known. Generally, each type has well-known
advantages and disadvantages.
[0006] In the operation of a typical fuel cell, oxygen is applied
at the cathode and the fuel is applied at the anode. In some types
of fuel cells, the oxygen combines with electrons returning from
the electrical circuit and hydrogen ions that have traveled through
the electrolyte from the anode. In other types of fuel cells, the
oxygen reacts with water and electrons to form hydroxyls, which
then travel through the electrolyte to the anode, where it combines
with hydrogen ions. The electrolyte serves as the vehicle by which
appropriate ions are transported between the anode and cathode. In
alkali fuel cells, the electrolyte transports hydroxyl ions from
the cathode to the anode. In molten carbonate fuel cells, the
electrolyte transports carbon trioxide ions from the cathode to the
anode. In solid oxide fuel cells, the electrolyte transports oxygen
ions from the cathode to the anode. In phosphoric acid and PEM fuel
cells, the electrolyte transports hydrogen ions from the anode to
the cathode. At the anode or cathode, depending on design, hydrogen
and oxygen ions combine to form the water. The fuel cell will
continue to generate electricity as long as hydrogen, oxygen, and
electrolyte are available for reaction and the catalyst is not too
degraded.
[0007] FIG. 1A illustrates a common configuration for a membrane
electrode assembly 10 known in the prior art. Membrane electrode
assembly 10 consists of a proton-exchange membrane (PEM) 12 that
serves as the electrolyte and as a support structure for an anode
14 and a cathode 16. The magnified view of FIG. 1B depicts the
micro-scale composition of the electrodes (anode 14 and cathode
16). The composition consists of a mixture of finely divided,
widely dispersed, very small (typically micro-scale) particles of
platinum dust 22 supported on, and adhered to, larger but still
microscopic carbon particles 24. This substrate/catalyst complex is
mixed with an adhesive binder to hold it together, fibers 26
(typically carbon) to increase structural integrity, and
hydrophobic PTFE 28 to help the egress of water. This mixture is
applied to a structural support that, as indicated for the case of
a PEM-based fuel cell, is often the electrolyte membrane 12
itself.
[0008] The current design as exemplified by membrane electrode
assembly 10 illustrated in FIGS. 1A and 1B is functional and an
improvement over earlier designs, but it is still embodies
significant limitations. Widely recognized problems attending this
design include the following. Only a small fraction of the mass of
the catalyst (e.g., platinum dust 22 in FIG. 1B) is available for
catalytic activity. The catalyst molecules on the surface of the
catalyst particle are the only part that touches the fuel. The rest
of the mass of the catalyst is trapped in the interior of the
particle where it cannot touch the reactants. The extent to which
the expensive catalyst is wasted in the above manner can be
estimated as follows. Assuming that the particles are roughly
spherical, the formula for the volume of a sphere is
V=4/3.pi.r.sup.3, and the formula for the surface area of a sphere
is SA=4.pi.r.sup.2. Accordingly, the ratio of surface area to
volume of a sphere is SA/V=3/r. The radii of typical catalyst
particles for PEM cells are 12.5-24.5 .ANG.. Therefore, in PEM
cells, the ratio of surface area-to-volume ranges from 1:4 to 1:8,
with 1:6 as a median. It follows that only about 1/6.sup.th or 17%
of the platinum is potentially available as a catalyst, such that
roughly 83% of the mass is wasted.
[0009] The degree of waste is much worse in direct methanol fuel
cells, which require larger particles and heavier catalyst
loadings. Direct methanol cells generally employ particles with
radii of 44-125 .ANG.. Plugging these numbers into the above
equations reveals that the surface area-to-volume ratios vary from
1:15 to 1:41 with a median of 1:28. Therefore, only 3.6% of the
surface area of catalyst is available for catalytic activity. The
remaining portion of the catalyst is potentially wasted, for
instance, by being locked inside the particle. Moreover, part of
the 3.6-17% of platinum potentially available for catalytic
activity is additionally lost to the reaction because it is in
intimate contact with inert substrate materials such as PTFE,
glues, structural fibers, or the carbon support itself (see, e.g.,
FIG. 1 and accompanying discussion above). This contact area
excludes contact with the reactants and thus further limits the
usable, catalytic surface area. Losses from these contact areas can
be conservatively estimated to be about an additional 5%, but are
usually functionally much higher.
[0010] Additionally, a yet additional loss of catalytic utilization
occurs because the reaction can only take place at the
triple-interface of the fuel, electrolyte, and catalyst. Only a
fraction of the catalytic mass left after the above reductions
encounters the electrolyte/fuel interface. Even in the case of the
particles that do touch the interface of fuel and electrolyte, not
all of the surface area experiences the interface. A significant
amount of the catalyst does not even touch the electrolyte, and
therefore does not participate in generating electricity.
Furthermore, those particles that do experience a favorable
interface produce or use water, thereby changing the fuel/water
ratio in their immediate microenvironment, often decreasing
catalytic efficiency.
[0011] Another problem is that the materials and mixtures utilized
in the prior art are fairly electrically resistant. This internal
resistance substantially decreases electrical production efficiency
and necessitates the use of conductive current-collecting "field
flow" plates that add sizeable cost and volume to the fuel cell or
fuel cell stack. Currently designed field-flow plates also
contribute to considerable internal resistance.
[0012] In addition, the carbon-platinum mixture utilized in prior
art approaches is essentially a brittle composition of dust or a
composite of powders. The mixture is sensitive to vibration and
mechanical, electrical and thermal stresses. Over time the mixture
tends to disintegrate and thereby limit lifespan and
efficiency.
[0013] Moreover, in current fuel cell designs, the fuel flows by
and diffuses into the anode but not though it. Consequently, inert
compounds can build up in the pores and physically block the fuel
from reaching the catalyst, hence further limiting efficiency.
[0014] Still further, the individual parts of the fuel cell stack
of current technology need a uniform, fairly exact degree of
humidification in order to properly function. Water is produced at
one electrode and thus can potentially flood the electrode. Water
is used up at the other electrode, thereby drying that electrode,
and is dragged away by concentration gradients and electro-osmotic
forces. To ameliorate or compensate for these flooding and drying
events, current fuel cell designs must resort to the addition of
extensive, costly and power-robbing balance-of-plant apparatus.
Generally, balance-of-plant apparatus is the ancillary equipment
necessary for supporting operation of a fuel cell and conditioning
its outputs to usable forms. Balance-of-plant apparatus can include
fuel stock scrubbers, controllers, heat exchangers, fuel reformers,
shift reactors, humidifiers, dehumidifiers, pumps, compressors,
regulators, power conditioners, tanks, valves, pipes, hoses,
sensors, thermal regulators, manifolds, filters, and the like. In
conventional fuel systems, the massive balance-of-plant apparatus
can dwarf the actual fuel cell stack. Much of the prohibitive cost
of fuel cell production can be attributed to the balance-of-plant
and not the fuel cell stack itself. Unfortunately, even with the
use of balance-of-plant apparatus, the simultaneous ideal
humidification for each electrode (anode and cathode, as well as
the electrolyte) is never quite uniformly achieved.
[0015] Poisoning and contamination remain a pervasive problem in
many fuel cell designs. Catalysts suffer from poisoning by common
contaminants found in many fuel stocks. They are even poisoned by
intermediary compounds produced from their own reactions. Poisons
can include, for instance, a variety of sulfur- and carbon-based
compounds. Of special significance is carbon monoxide as it is a
common intermediary compound of carbon-based fuels such as
methanol. Over time, these substances adhere to the catalytic
particles of conventional electrodes, degrading their performance
and limiting their lifespan. Sensitivity to poisoning seriously
limits the feasibility and commercial viability of the currently
existing technologies. It is especially a problem in fuel cells
that use currently available fossil fuels and natural gas
derivatives. These fuels have relatively high amounts of sulfur
compounds and complex hydrocarbons that form a variety of toxic
intermediary compounds. To partially atone for this problem,
manufacturers are forced to incorporate expensive, additional
balance-of-plant apparatus such as those noted above, particularly
fuel stock scrubbers, reformers, shift reactors, and advanced
filters, all of which can be bulky and/or expensive and escalate
inefficiency, maintenance requirements, and pollution. The extra
equipment also requires energy to run that is parasitically drawn
from the output of fuel cell. The costs attending this additional
equipment can obviate the advantages they provide such as the
ability to use readily available fuel stocks from the current
infrastructure.
[0016] In view of the foregoing, a widely recognized need exists
for ongoing improvements in fuel cells and fuel cell systems, and
components thereof. In particular, a continuing need exists for
increasing operating efficiencies and extending the useful life of
materials and components, as well as reducing the costs,
complexities, and components required for providing commercially
viable fuel cells and systems.
SUMMARY OF THE DISCLOSURE
[0017] According to one embodiment, a flow-through electrode is
provided for use in a fuel cell. The electrode comprises a porous
substrate having a continuous and/or contiguous porosity through
which a fluid can flow, and a plurality of walls oriented in
different directions between the first and second sides. Voids are
defined between the walls. The walls include surfaces and
micro-scale pores. A multi-directional fluid flow path is defined
between the first and second sides. A thin film comprising a
catalytic material is disposed on the surfaces. A fuel and an
electrolyte are flowable generally from the first side, through the
voids and the pores of the substrate and in contact with the thin
film, and to the second side.
[0018] According to another embodiment, a flow-through electrode is
provided for use in a fuel cell. The electrode comprises a first
region, a second region, and a third region. The first and second
regions each comprise a porous substrate for flowing a
fuel/electrolyte combination therethrough. The third region is
interposed between the first and second regions and fluidly
communicates with the first and second regions. The pore density of
the third region is less than the pore densities of the first and
second regions. In some embodiments, the third region is
substantially hollow.
[0019] According to another embodiment, a flow-through electrode is
provided for use in a fuel cell. The electrode comprises a
plurality of regions, with each region being adjacent to and
fluidly communicating with at least one other region. Each region
comprises a porous substrate for flowing a fuel/electrolyte
combination therethrough. Each region has a porosity different from
the porosities of the other regions. The plurality of regions can
be arranged in order of successively increasing porosity to define
a porosity gradient whereby the fuel/electrolyte combination can be
flowed generally with or against the porosity gradient.
[0020] According to another embodiment, a fuel cell comprises an
anode, a cathode, a porous barrier interposed between the anode and
cathode, an anode-side channel defined between the anode and
barrier for receiving a fuel-rich fluid, and a cathode-side channel
defined between the cathode and barrier for receiving a
fuel-depleted fluid.
[0021] According to another embodiment, fuel cell comprises an
anode, a cathode, a first channel, and a second channel. The anode
comprises a first anode section, a second anode section and a third
anode section. The first and second anode sections each comprise a
porous substrate for flowing a fuel/electrolyte combination
therethrough. The third anode section is interposed between the
first and second anode sections and fluidly communicates with the
first and second anode sections. The cathode comprises a first
cathode section and a second cathode section. The first channel is
interposed between the first anode section and the first cathode
section. The second channel is interposed between the second anode
section and the second cathode section.
[0022] According to another embodiment, a fuel cell comprises a
first side, a second side opposing the first side, a third side,
and a fourth side opposing the third side. A plurality of
substantially planar electrodes are arranged substantially parallel
to each other and comprise respective edges defining the first,
second, third, and fourth sides. The plurality of electrodes define
a plurality of first channels fluidly communicating with the first
and second sides for conducting an oxygen-containing fluid
generally from the first side to the second side, and define a
plurality of second channels fluidly communicating with the third
and fourth sides for conducting a fuel/electrolyte combination
generally from the third side to the fourth side.
[0023] According to another embodiment, a fuel cell comprises an
anode region and a cathode region. The anode region comprises a
plurality of anodes and a plurality of anode channels. Each anode
channel communicates with at least one anode. The plurality of
anode channels comprise pre-anode channels for supplying a
fuel-rich fluid to one or more of the anodes and post-anode
channels for receiving a fuel-depleted fluid from one or more of
the anodes. The cathode region comprises a plurality of cathodes
and a plurality of cathode channels. Each cathode channel
communicates with at least one cathode and at least one anode
channel.
[0024] According to another embodiment, a fuel cell comprises an
anode section and a cathode section. The anode section comprises a
first anode, a second anode and an anode channel interposed between
the first and second anodes. The first and second anodes each
comprise a porous substrate for flowing a fuel/electrolyte
combination therethrough. The anode channel fluidly communicates
with the first and second anodes. The cathode section comprises a
plurality of cathodes and a plurality of cathode channels. Each
cathode channel communicates with at least one cathode. The cathode
channels are spaced from the anode channel and fluidly communicate
with the anode section.
[0025] According to another embodiment, a fuel cell system
comprises a fuel cell and a conduit. The fuel cell comprises a
first fluid inlet and a first fluid outlet. The conduit comprises a
second fluid inlet fluidly communicating with the first fluid
outlet, a second fluid outlet fluidly communicating with the first
fluid inlet, and a semipermeable wall for permitting transpiration
of water from the conduit and retention of fuel and electrolyte
within the conduit, or vice versa.
[0026] In a method for operating a fuel cell, a flow-through
electrode is provided that comprises a porous substrate and a
catalyst disposed on the substrate. A fluid is flowed through the
electrode. The fluid comprises an alkali electrolyte.
[0027] In another method for operating a fuel cell, the fluid
further comprises an additive for supplying a supplemental source
of hydroxyl ions.
[0028] In another method for operating a fuel cell, a plurality of
electrodes comprising anodes and cathodes are operated to collect
electrons from the anodes. The operation of at least one electrode
is switched to a refresh cycle whereby catalyst supported by the
electrode is cleaned.
[0029] Additional embodiments or methods can include one or more
components, features, elements, or steps recited above, or
combinations of one or more components, features, elements or steps
recited above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1A is an elevation view of a known membrane electrode
assembly;
[0031] FIG. 1B is a detailed view of the structure of an electrode
of the fuel cell unit illustrated in FIG. 1A;
[0032] FIG. 2A is a perspective view of an electrode according to
embodiments of the present disclosure;
[0033] FIG. 2B is a detailed view of a cross-section of the
structure of an electrode according to one embodiment of the
present disclosure;
[0034] FIG. 2C is a detailed view of a structure of an electrode
according to one embodiment;
[0035] FIG. 2D is a detailed view of a structure of an electrode
according to another embodiment;
[0036] FIG. 2E is a detailed view of a structure of an electrode
according to another embodiment;
[0037] FIG. 2F is a detailed view of a structure of an electrode
according to another embodiment;
[0038] FIG. 2G is a detailed view of a structure of an electrode
according to another embodiment;
[0039] FIG. 2H is a detailed view of a structure of an electrode
according to another embodiment;
[0040] FIG. 2I is a detailed view of a structure of an electrode
according to another embodiment;
[0041] FIG. 2J is a detailed view of a structure of an electrode
according to another embodiment;
[0042] FIG. 3A is a cross-sectional elevation view of an electrode
according to an embodiment of the present disclosure;
[0043] FIG. 3B is a cross-sectional elevation view of an electrode
according to another embodiment;
[0044] FIG. 4A is a cross-sectional elevation view of an electrode
according to another embodiment;
[0045] FIG. 4B is a cross-sectional elevation view of an electrode
according to another embodiment;
[0046] FIG. 5 is a cross-sectional elevation view of a fuel cell
arrangement according to an embodiment of the present
disclosure;
[0047] FIG. 6 is a cross-sectional elevation view of a fuel cell
arrangement according to another embodiment;
[0048] FIG. 7 is a cross-sectional elevation view of a fuel cell
arrangement according to another embodiment;
[0049] FIG. 8 is a schematic view of a fuel cell system, including
a cross-sectional elevation view of another fuel cell arrangement,
according to an embodiment of the present disclosure;
[0050] FIG. 9 is a cross-sectional elevation view of a fuel cell
stack according to an embodiment of the present disclosure;
[0051] FIG. 10 is a cross-sectional elevation view of a fuel cell
stack according to another embodiment;
[0052] FIG. 11A is a schematic view of a fuel cell system according
to another embodiment;
[0053] FIG. 11B is a cross-sectional elevation view of a conduit
utilized in a water/thermal/pressure management system according to
an embodiment of the present disclosure;
[0054] FIG. 11C is a detailed view of a portion of the fuel cell
system illustrated in FIG. 11A;
[0055] FIG. 12 is a cross-sectional elevation view of a fuel cell
stack according to another embodiment;
[0056] FIG. 13 is a cross-sectional elevation view of a fuel cell
stack according to another embodiment; and
[0057] FIG. 14 is a schematic view of a fuel cell system according
to another embodiment.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0058] In general, the term "communicate" (e.g., a first component
"communicates with" or "is in communication with" a second
component) is used herein to indicate a structural, functional,
mechanical, electrical, magnetic optical, fluidic, or ionic
transporting, relationship between two or more components or
elements. As such, the fact that one component, element, or feature
is said to communicate with a second component, element, or feature
is not intended to exclude the possibility that additional
components may be present between, and/or operatively associated or
engaged with, or otherwise interacting with, the first and second
components.
[0059] As used herein, the term "fluid" generally means any
flowable medium such as a liquid, gas, vapor, supercritical fluid,
or combinations thereof. Moreover, the term "fluid" encompasses
mixtures or combinations of liquids, vapors, gases and
supercritical fluids with solid particulate matter, as in the case
of particles carried in a fluid flow stream.
[0060] As used herein, the term "liquid" can include a liquid
medium in which a gas is dissolved and/or a bubble is present.
[0061] As used herein, the term "vapor" generally means any fluid
that can move or expand without restriction except for a physical
boundary such as a surface or wall, and thus can include a gas
phase, a gas phase in combination with a liquid phase such as a
droplet (e.g., steam), supercritical fluid, and the like.
[0062] For convenience, the term "air" is used herein to indicate
any oxygen-containing fluid such as, for instance, a gas consisting
essentially of diatomic oxygen molecules or an oxygen-containing
compound from which oxygen ions can be derived for oxidation
reactions.
[0063] As used herein in the context of a fluid, the term
"combination" generally means a multi-component material. Unless
otherwise indicated or apparent, the possible relationship among
the components of a combination may include, but is not necessarily
limited to, a mixture, solution, suspension, matrix, colloidal
structure, emulsion, particle-laden fluid, or the like.
[0064] The present subject matter is primarily directed to alkali
electrolyte-type fuel cells. The basic fuel cell includes an anode
and a cathode, which are typically separated by a space suitable
for fluid flow, such as a chamber, channel, conduit, or the like.
An oxidizable fuel such as a hydrogen-containing fuel is supplied
to the anode, and an oxygen-containing fluid is supplied to the
cathode. The fuel is not required to be pure diatomic hydrogen, but
can include hydrocarbon- or alkyl-based fuels. A particularly
advantageous example of a hydrocarbon fuel is methanol (CH.sub.3OH)
or similar low-molecular weight alcohol. Alkanes and alkenes may
also be utilized but are less preferred due to their byproducts. In
other embodiments, it is advantageous to use a borohydride as the
fuel, such as sodium tetraborohydride (NaBH.sub.4). Advantageously,
the electrolyte is a liquid-phase solution of a hydroxide compound
in water. Suitable hydroxide compounds include metal hydroxides
such as, but not limited to, potassium hydroxide (KOH). In
operation, hydroxyl ions (OH.sup.-) migrate from the cathode to the
anode. At the anode, hydrogen reacts with the OH.sup.- ions to
produce water and release electrons. Electrons generated at the
anode supply electrical power to an external circuit, which can be
any suitable electrical load such as a battery or other storage
device or a work-producing electrical device or machine. The
electrons are then returned to the cathode to complete the
electrical circuit. At the cathode, the electrons react with oxygen
and water to produce more hydroxyl ions that diffuse into the
electrolyte. The electrodes are formed as flow-through structures
that permit the transport of liquid and/or gas phases. The
electrodes can be structured to provide an optimized
catalytically-active flow-through bulk matrix enabling maximal
exposure to fluidic reactants. The structure of the substrate of
the electrode can include an interconnected array or network of
pores and voids (which may also be considered to be pores) of
asymmetrical and/or symmetrical shapes and random and/or ordered
sizes and locations. Fluidic flow through the electrode can be
passive (e.g., hydrodynamic, convectional, or the like) or forced
such as by using a pump.
[0065] According to embodiments of the present disclosure, an
improved electrode is provided for use in a fuel cell, and
particularly a flow-through fuel cell in which the electrolyte is a
flowable medium such as a liquid phase combinable with a suitable
fuel. The electrode is particularly useful as an anode from which
electrons can be conducted for providing a source of electrical
energy. The basic structure of the electrode is a porous,
flow-through, high-surface area microstructure or substrate. The
substrate, or a component or components thereof, can be
electrically conductive and catalytically active. The substrate is
structured to enable a fluid such as a fuel/electrolyte or
fuel/electrolyte/water combination to completely flow through the
substrate, rather than merely by or into the substrate, while
contacting a tremendous surface area (as compared with the bulk
volume) of the substrate. In advantageous embodiments, the catalyst
is provided by a thin film or coating disposed throughout the
extensive surface area of the substrate. The electrode exhibits
good intrinsic electrical conduction for integral current
collection. Current collection can be further enhanced by embedding
or otherwise incorporating a mesh of fine wire or other
electrically conductive material within the substrate.
[0066] A variety of configurations and manufacturing processes can
provide the desired porosity characteristics for the flow-through,
high-surface area design. Examples of configurations for the
substrate can include, but are not limited to, open cellular,
reticular, foamed, sintered, sponge, raney, nanostructure,
vitreous, pasted, slurry, gel, sol-gel, and/or aero-gel
substructures. Materials for the substrate can include, but are not
limited to, porous conductive plastics, carbon compounds or
composites, ceramics, metals, and metal-containing compounds, solid
mixtures and alloys. Metals can include, but are not limited to,
stainless steel, nickel, silver, molybdenum, titanium, and alloy or
compounds including one or more of these metals. Nickel has been
found to be particularly advantageous in embodiments described
herein. The microstructure of the material is engineered with
respect to porosity, pore size, pore shape, wall size, wall shape
and wall thickness, as well as density to maximize the surface area
available for catalyst/reactants interaction, while preserving
desired flow-through characteristics. In one advantageous
embodiment, the electrode comprises a substrate structured as an
open-cell, thin-walled metal sponge with a thin-film coating. An
important factor is that the pore volume, shape and wall
configuration be optimized for both flow and maximal
reactant/catalyst interaction. As described further below, in one
embodiment, the walls of the substrate can be hollow.
[0067] In one advantageous method for fabricating a flow-through,
high-surface area electrode, particles (which may be filaments
and/or flakes) of a base material are coated with a thin continuous
or discontinuous layer of catalyst. The catalyst-coated particles
are then sintered to form a porous electrode having flow-through
and extremely high-surface area characteristics. This method avoids
the pore-size limitations associated with more conventional
techniques such as electroless plating, but has the added challenge
of maintaining the uniformity of the coating layer during the
sintering process. Higher pressure, lower temperature techniques
enable this method. As previously indicated, one or more
conductors, such as a mesh of fine wire, can be incorporated into
the bulk of the particles during sintering to promote electrical
conduction in the resulting electrode.
[0068] In another method, a porous reticular metal substrate is
fabricated. Depending on the method of manufacturing employed to
make the structure porous, additional means are usually employed to
increase the available surface area. One such means includes
sintering additional substrate(s) into the voids of the porous
material (see, e.g., FIG. 2E). This is one approach for obtaining
the desired surface area. The sintered material may be coated metal
particles or flakes (see, e.g., FIG. 2F). Traditional carbon
supported catalyst can be retained in the voids as well.
[0069] Surfaces of the substrate can be roughened to further
increase catalytically-active surface area by any suitable
microfabrication/micromachining or surface preparation technique,
such a technique employed in the microelectronic, MEMS, and
microfluidic fabrication industries. One non-limiting example is
wet (chemical) etching.
[0070] In one embodiment, the electrode has a thickness of
approximately 5 mm or less and the catalytic surface area exposed
to a fluid such as a fuel/electrolyte combination is approximately
10 m.sup.2/cc or greater. The pore size, which may be defined as a
diameter or any other characteristic dimension suitable for the
shape of the pores, may vary within a micro-scale or milli-scale
range. In one embodiment, the average pore size is approximately
0.01 mm. The size of the walls defining the microstructure of the
substrate may be optimized for maximal thinness while preserving
structural integrity and electrical conductivity. Generally,
depending on design and application specifics, there can be
considerable variation of practical densities, porosities, pore
sizes, and the like.
[0071] In some embodiments, the electrode is formed from a nickel
or nickel-chromium based reticulated foam substrate with a pore
size of approximately 0.4 mm. The available surface area is
approximately 5600 cm.sup.2/cc. Various methods for augmenting the
surface area described herein can be utilized to boost the
available surface area above 10 m.sup.2/cm.sup.2. Because the
catalyst is utilized so effectively in embodiments disclosed
herein, much less catalyst may be used while achieving much greater
efficiency.
[0072] In another embodiment, an electrically conductive substrate
for a fuel cell comprises open cellular continuous or contiguous
interlacing porosity with micro- and/or nano-scale surface
modifications that increase surface area and non-scale roughness of
the pore walls. The cavernous porosity and pore size are
particularly adapted for flow-through of a fluidic fuel/electrolyte
combination. The flow can be oscillating but with an overall
directional movement. The porous conductive substrate can be coated
with a thin-film catalyst to intrinsically operate with good
conductivity and low impedance, while the conformation of the
substrate allows for an introduced or externally generated electric
potential to act on the catalyst/fluid interface for the purpose of
removing poisons adhered to the catalyst surface. The flow-through
porosity is further engineered so that the detergent/surfactant
compounds added to the electrolyte in some embodiments, as well as
the bulk flow of the electrolyte, serve to wash away byproducts,
prevent concentration gradients, and reduce poisoning species. A
thin film coating of catalytic material can be applied to the
modified surface of the electrode in such a way that a
substantially high proportion of the catalytic molecules are
presented as the active surface area. This active surface area can
be enhanced by the coating conforming to roughened pore walls, and
providing nano-protrusions of catalytic material that present
additional edges or surfaces.
[0073] In another embodiment, an electrically conductive substrate
to be used in a fuel cell as an electrode comprises sintered
amorphous filaments, flakes, and/or nano-powders that create a
random dispersion of asymmetrical and amorphous voids between wall
structures created by the sintered material. The substrate
establishes an interconnected cavernous network tailored
specifically for oscillating, but generally directional, flow of
electrolyte and/or fuel mixtures through the substrate. The inner
surfaces can also contain modified nano-surfaces throughout the
network of inner cavernous surfaces so that nano-protrusions and
crevices create a high internal specific surface area and are part
of the substrate itself, to which a thin film of catalytic material
is applied so that efficiency of the available catalytic surface is
maximized.
[0074] The substrate of the electrode may be directly composed of
conductive/catalytic material (see, e.g., FIG. 2I) or may embody a
different base material with a thin film coating(s) for conductive
and/or catalytic function (see, e.g., FIG. 2C). The catalytic layer
may be mixed with or layered on the substrate to increase
conductive function. The porous substrate is either electrically
conductive itself or has conductive additives or coatings. In
either case, the resulting microstructure acts as a low-resistance
integral current collector.
[0075] The effectiveness of electrodes disclosed herein as current
collectors enables the use of edge-connection conductive components
that require less material and are low-cost. For example, a simple
off-the-shelf busbar can be connected to an edge of the electrode
for conducting current. The walls may include a dielectric layer
between two conductive layers to create a capacitor effect. The
core may be a polymer/ceramic combined with a metallic conductor
and/or catalyst.
[0076] Materials for electrodes can be selected and
blended/alloyed/combined for conductivity (silver, molybdenum,
etc.), catalytic activity (platinum, palladium, ruthenium,
manganese, silver, etc.), cost (nickel, etc.), corrosion resistance
(stainless steel, etc.). Metals utilized herein may include
transitional metals, as well as oxides and nitrides.
[0077] The electrode may embody a porous substrate combined with
pasted, grown, or packed material in the voids for the enhancement
of the surface area. An example of this would be a metal sponge in
which the voids contain a carbon-supported catalyst mixture. In
another example, nanostructures such as nanotubes on nanohorns are
grown onto the porous base metal into the voids for the dispersion
of catalyst over a greater surface area. During manufacture, the
surface may be treated by a micro-etch process and during the
process three-dimensional structures such as nanospires or bumps
can be added. The resulting roughened surface is analogous to
intestinal microvilli (see, e.g., FIG. 2G). The catalyst can coat
or be integral to the nanostructures. In another example, a powder
of high-surface area, low-mass flakes or particles are pasted,
sintered, etc. directly into the voids (see, e.g., FIG. 2F) or
formed as the base structure itself.
[0078] In other embodiments, modified zeolites are utilized to form
the anode body. Zeolites can provide excellent porosity and surface
area-to-volume ratio. The zeolites are strategically modified by
any suitable technique to increase conductivity and to incorporate
catalytic function. These methods include, but are not limited to,
plating, deposition and substitution of conductive and catalytic
compounds. Zeolites may also be formed in the voids of a porous
substrate.
[0079] The electrode can be manufactured in a variety of
application-specific shapes and sizes. Examples of shapes include,
but are not limited to, flat plate, corrugated, tubular, conical,
and cylindrical.
[0080] In embodiments and methods disclosed herein, fluid flows are
established that can include a combination of fuel, electrolyte,
and/or water. In addition, the primary use of liquid phases
facilitates the incorporation of any suitable functional additives
such as surfactants, detergents, solvents, preservatives, buffers,
and the like. For instance, solvents and/or detergents can be added
to assist in cleaning the electrodes of poisons or toxins and
microclimate concentration gradients. Additives can assist in
dissolving compounds that could otherwise precipitate out and cause
a cake or other obstruction that physically clogs the electrodes.
Additives can break up the surface tension of product water
globules to reduce efficiency-sapping microclimate concentration
gradients. In conventional direct methanol cells, which do not
employ a flow-through design and cleaning additives, concentration
gradients limit designers to larger, more wasteful platinum
particles. Additives can also function to oxygenate the electrolyte
to further enhance cleaning. Additive appropriate for such
functions are readily available and known to persons skilled in the
art.
[0081] Many fuel cells of recent design, an example of which is
described above with reference to FIGS. 1A and 1B, rely on the
diffusion of ions through an electrolyte that is a solid medium to
complete a separated redox reaction that creates the electric
charge potential. By necessity, this medium restricts the passage
of parasitic internal electric currents. The mass transport of ions
through the electrolyte is one limiting factor on overall reaction
rates. Because the overall ionic charge balances must remain
neutral, an ion cannot react at one electrode until one is made at
the other.
[0082] By contrast, the improved electrolyte-containing fluid
employed in the embodiments disclosed herein can include buffers,
hydroxyl carriers, and multibasic compounds (e.g., polyhydroxyl
alkalis). These additives can increase mass transport efficiency.
They allow a degree of charge-balanced hydroxyl supply margin that
can hasten focal reactions that are restricted by conventional
approaches. When focal conditions cause relatively
hydroxyl-depleted regions, additional hydroxyls can be locally
drawn from the electrolyte chemical reserve provided by the
additives.
[0083] In a typical fuel cell, the flow of charged ions through the
electrolyte, and of electrons through the circuitry connected to
the fuel cell, in effect creates a closed-loop path. The reaction
rate of any part of the overall process is restricted to the rate
of the slowest step of the process, which typically is the passage
of ions through the electrolyte. However, according to the subject
matter disclosed herein, extra hydroxyls are available from the
electrolyte to promote a faster basal rate at the rate limiting
focal regions. Sections of the electrode experiencing a relative
dearth of hydroxyls can "pull" the needed hydroxyls from the
chemical reserves of the buffers, polyhydroxyl alkalis, and/or
hydroxyl carriers in the electrolyte. Temporary fluctuations in
microconditions do not impact the overall reaction rate as compared
with prior art approaches. A degree of elasticity is thus
introduced into the system.
[0084] With the advantages of overall increase in hydroxyl
transport by polyhydroxyl alkalis, buffers, and/or hydroxyl
carriers, it is possible to reduce the limitation of charge mass
transport and thereby increase the rate of the overall reaction.
Moreover, the anodes and cathodes can be spaced further apart to
reduce crossover electrical currents, as shown in embodiments
described below.
[0085] Examples of embodiments of electrodes having advantageous
microstructures according to the subject matter disclosed herein
will now be described with reference to FIGS. 2A-2J.
[0086] FIG. 2A illustrates an electrode 100 according to an
exemplary embodiment of the subject matter disclosed herein. In the
illustrated example, electrode 100 comprises a microstructure that
generally forms a flat plate. The scope of the present subject
matter, however, is in no way limited to any particular geometry
for electrode 100 as previously indicated. The microstructure that
defines electrode 100 is structured so as to allow a fluid to flow
through the microstructure. For instance, a fluid can flow from a
fluid ingress side 102 of the microstructure as indicated by arrow
104, through the entire thickness of the microstructure, and out
from a fluid egress side 106 of the microstructure as indicated by
arrow 108. Apart from the example specifically illustrated in FIG.
2, the microstructure can be designed such that fluid ingress can
occur at any one or more sides and fluid egress can occur at any
one or more sides. When electrode 100 is employed as an anode, a
fuel-rich fuel/electrolyte combination can be supplied to the
illustrated fluid ingress side and flowed through the
microstructure in a net forward direction or directions. Due to
catalysis-promoted reactions within the microstructure, a resulting
fuel-depleted fuel/electrolyte combination egresses from the
illustrated fluid egress side. A conductive structure such as a
metal screen or mesh 110 can be incorporated into the
microstructure body to enhance the current conducting capabilities
of electrode 100.
[0087] As illustrated in more detail in FIGS. 2B and 2C, the
microstructure of electrode 100 comprises a porous substrate 120.
Porous substrate 120 can include a solid, yet porous, framework
defined by a plurality of thin walls 122 and voids or flow channels
124 between walls 122. In addition, a thin film 126 (FIG. 2C)
composed of a catalytic and/or conductive material is disposed on
walls 122. The thin film 126 is also porous, meaning that the thin
film 126 is at least permeable to a fuel/electrolyte combination.
The thin film 126 can have a rough or three-dimensional profile to
greatly increase surface area, as described by way of further
examples below. Pores 128 are schematically depicted in FIGS. 2B
and 2C. The walls 122 have a multi-directional orientation. In the
illustrated example, some walls 122 are vertically oriented while
others are horizontally oriented. It will be understood, however,
that the terms "vertically" and "horizontally" are used only in
their relative sense and not as any limitation on the possible
orientations of the various walls 122 of substrate 120. A fluid
such as a fuel/electrolyte combination flows through voids 124 and
pores 128 as indicated by arrow 130 in FIG. 2B. Due to the
multi-directional orientation of the microstructure, the fluid flow
can have several flow components in differing directions. The net
flow, however, occurs in a single forward direction as depicted by
arrow 130. The multi-directional orientation of the microstructure
maximizes contact between a fluid such as a fuel/electrolyte
combination and thin film 126, and thus maximizes catalytic
activity. At the same time, however, the resultant flow is not
significantly restricted by the illustrated microstructure.
Moreover, the catalyst is presented as a thin-film coating 126
covering extensive surface areas of channels 124 of the porous
substrate 120, instead of being bound up inside particles and
clogged with binders. Consequently, a much higher percentage of the
catalyst can be presented as usable surface area, as compared to
the configurations of the prior art. The fuel flowing through the
microscopic voids 124 and pores 128 in electrode 100 is presented
with much more catalytically active surface area per gram of
catalyst than is possible with current designs. Therefore, much
less catalyst is needed. In fact, because the catalyst is used with
such higher efficiency, other catalysts besides platinum can be
blended into the coating, further decreasing platinum loading
requirements and attendant cost.
[0088] FIG. 2D illustrates a microstructure 140 in which
microparticles 142 have been sintered to form the porous substrate
of an electrode. Microparticles 142 can have a high-surface area
morphology, and can be coated with a thin-film catalyst as
described above. As previously indicated, the coating can be
substantially continuous or discontinuous. For a greater catalyst
dispersion and greater ease of sintering, microparticles 142 can be
coated in a patchy or splattered/scattered pattern rather than
continuously and/or the catalyst coated particles can be mixed with
uncoated particles. In some embodiments, catalyst particles are
mixed with particles with less or no catalytic activity.
[0089] FIG. 2E illustrates an embodiment in which a porous
substrate 150 comprises walls 152 defining voids 154 that are
filled with particles 156. These particles 156 can comprise a
conductive and/or catalytic material, and/or a support structure on
which conductive and/or catalytic materials are supported or
retained by any suitable means. As further examples, particles 156
can comprise a carbon support/platinum matrix, or catalyst-coated
sintered particles as depicted in FIG. 2D.
[0090] FIG. 2F illustrates an embodiment similar to that shown in
FIG. 2E, but in which a porous substrate 160 comprises walls 162
defining voids 164 that are filled with high-surface area flakes
166. Flakes 166 may have a twisted geometry. The composition of
flakes 166 can comprise a catalytic material, or a non-catalytic or
less-catalytic core that is coated with a catalytic material.
[0091] FIG. 2G illustrates an embodiment in which a porous
substrate 170 comprises walls 172 defining voids 174. Protrusions
or bosses, such as nanostructures or microstructures 176 are grown,
deposited, attached, or otherwise disposed by any suitable
technique on porous substrate 170. Nanostructures 176 can be
nanotubes or nanohorns. The profile of each nanostructure 176 can
be generally conical, acicular, columnar, crystalline, epitaxial,
domed, pellicular, luminal, shaped as a mesa, shaped as a conic
section, or shaped similar to an intestinal microvillus. Such
protrusions 176 extending into voids 174 greatly increase the
surface area available for contact with a fluid and thus catalytic
activity within the electrode. Nanostructures 176 can inherently be
conductive and/or catalytically active, or can include coatings
that are conductive and/or catalytically active.
[0092] FIG. 2H illustrates an embodiment in which a porous
substrate 180 includes walls 182 that are hollow and thus have
interiors 184 distinct from voids 186. In some embodiments, a thin
film 188 is disposed not only on the surfaces of walls 182 facing
voids 186 but also on inner surfaces of walls 182 facing interiors
184. Pores 190 provide communication between interiors 184 of walls
182 and voids 186, which may enable catalytic and/or ionic activity
within interiors 184 of walls 182.
[0093] FIG. 2I illustrates an embodiment in which electrode
comprises a porous, solid core substrate 200. The material of
substrate 200 directly provides catalytic and conductive functions
without the need for coatings or thin films. Fluid flows through
pores 202 of substrate 200 and voids 204 defined between the solid
portions of substrate 200.
[0094] FIG. 2J illustrates an embodiment in which an electrode
comprises a porous, solid core substrate 210. High-surface area
particles 212, which can be of substantially uniform or irregular
shape, are disposed on the walls of substrate 210 and extend into
voids 214. As in other embodiments, particles 212 add a
three-dimensional profile to substrate 210, greatly increasing
surface area. These particles 212 can comprise a material
exhibiting electrically conductive and/or catalytic properties.
[0095] FIG. 3A illustrates an embodiment of an anode 220 that
comprises a plurality of distinct anode sections (or regions,
layers, zones, etc.) as a means for optimizing fluid/catalyst
interaction while reducing the potential for forming
activity-depleted regions within the body of anode 220. First and
second, or outer, anode regions 222 and 224 comprise a porous
substrate and can be structured according to any of the embodiments
described above. A third anode region 226 is interposed between
first and second anode regions 222 and 224. As compared with third
anode region 226, the microstructure of first and second anode
regions 222 and 224 is characterized by small channel or void
dimensions and a large number of pores per length/area/volume for
maximizing surface area. Third anode region 226 can be hollow and
defined by inward-facing sides of first and second anode regions
222 and 224, in which case third anode region 226 does not include
a porous substrate. Alternatively, third anode region 226 can
comprise a porous substrate of greater pore size and relatively
lower density than the porous substrate of first and second anode
regions 222 and 224. In either case, third anode region 226
provides a high-flow core of anode 220 that facilitates the influx
of a fluid at an inlet end 234 and along the length of anode 220,
and first and second anode regions 222 and 224 provide
comparatively lower flow-through characteristics to maximize
contact and interaction of the fluid with the catalyst. After
introduction into third anode region 226, the fluid can then flow
from third anode region 226 transversely into and through first and
second anode regions 222 and 224 where it can contact a large
surface area of catalyst, and egress out from sides of first and
second anode regions 222 and 224 distal to third anode region 226,
as indicated by arrows 236. A boundary 238 of any suitable type can
be disposed at an axial end 240 of anode 220 opposite to inlet end
234 to facilitate conduction of fluid flow in directions transverse
to the axis of third anode region 226. In other embodiments, the
direction of fluid flow can be the reverse of that shown by the
arrows in FIG. 3A. First, second and third anode regions 222, 224,
and 226 can be arranged as adjacent flat plates or chambers.
Alternatively, first and second anode regions 222 and 224 can be
part of a contiguous substrate such as a cylinder surrounding third
anode region 226, in which case FIG. 3A can be considered as
illustrating a cross-section of anode 220.
[0096] FIG. 3B illustrates another embodiment of an anode 250 that
comprises a plurality of distinct anode sections or regions as a
means for optimizing fluid/catalyst interaction while reducing the
potential for forming activity-depleted regions within the body of
anode 250. In this embodiment, a pore-size gradient is built into
anode 250 by providing anode regions of successively decreasing
pore size. While any number of anode regions can be provided, anode
in the illustrated example includes a first anode region 252, a
second anode region 254 adjacent to first anode region 252, and a
third anode region 256 adjacent to second anode region 254. First
anode region 252 is optimized for high-flow rate, low-flow
resistance conditions by having relatively large pore size and less
density. Second anode region 254 is an intermediate zone having a
medium-sized pores, medium surface area and medium resistance to
flow. Third anode region 256 has even smaller pore sizes and
channels and relatively higher resistance to flow for maximizing
surface area available for catalyst-fluid interactions. In one
embodiment as illustrated, the larger pores are on a fluid ingress
side 262 of anode 250 and the fluid flows into progressively
narrower channels in a net direction toward a fluid egress side 264
as indicated by arrows 266. This design increases the available
catalytic surface area while not significantly increasing
resistance to fluid flow, and helps to eliminate fuel-depleted
regions especially in thicker anode designs. In another embodiment,
fluid flow occurs in a net direction opposite to that indicated by
arrows 266. This latter embodiment may be more advantageous in
certain implementations because the highest fuel concentration is
applied to the greatest catalytic surface area provided by anode
250.
[0097] Any of the electrodes (anodes and cathodes) disclosed herein
can include one or more semipermeable membranes on one or more
sides that serve to separate fuel, electrolyte and/or water from a
combined fluid. For example, FIG. 4A illustrates an electrode 280
that includes a semipermeable membrane 282 disposed on a fluid
egress or downstream side 284 of a substrate body 286 of electrode
280. Membrane 282 is constructed of any suitable material that is
substantially permeable to electrolyte and/or water but is
impermeable or substantially impermeable to fuel such that the
passage of fuel from fluid egress side 284 is substantially
prevented. Accordingly, a fluid such as a fuel/electrolyte (and
water) combination can pass into the complex of electrode 280 from
a fluid ingress side 288 and through the thickness of substrate
body 286 as indicated by arrow portion 290. Membrane 282 permits
the electrolyte (and water) to pass through from fluid egress side
284 as indicated by arrow 292, but in effect selectively impedes
the passage of fuel components through fluid egress side 284 and
membrane 282 such that the fuel is distributed or transmigrates
more fully through substrate body 286 as indicated by arrow 294.
When electrode 280 is employed as an anode, membrane 282 assists in
increasing the fuel concentration in the microstructure, thereby
increasing catalytic efficiency. Moreover, the fluid reaching the
accompanying cathode (not shown) of a fuel cell is further
fuel-depleted, thereby decreasing the potential for problems
associated with fuel crossover losses. A membrane 282 positioned in
the location illustrated in FIG. 4A should also allow the passage
of hydroxyl ions.
[0098] FIG. 4B illustrates an electrode 300 that includes a
semipermeable membrane 302 disposed on a fluid ingress side 304 of
a substrate body 306. This membrane 302 is constructed of any
suitable material that is substantially permeable to fuel and
electrolyte but is impermeable or substantially impermeable to
water such that the passage of water into electrode 300 is
substantially prevented. Accordingly, the fuel and electrolyte
components of a fluid 308 can pass through membrane 302 and into
the complex of electrode 300 from fluid ingress side 304 as
indicated by arrow 310, but water is substantially separated from
the fluid flow and prevented from entering electrode 300 as
indicated by arrows 312. Similar to the effect of membrane 282 in
FIG. 4A, membrane 302 in FIG. 4B serves to increase fuel
concentration within the body of electrode, particularly in case
where the fluid combination supplied to fluid ingress side 304 is
diluted due to the presence of water.
[0099] FIG. 5 illustrates a fuel cell 320 having an electrode
arrangement in which an anode 322 similar to that shown in FIG. 4A
is sandwiched between two cathodes or cathode regions 324 and 326.
First and second, or outer, anode regions 328 and 330 comprise a
porous substrate structured according to any of the embodiments
described above. A third anode region 332 is interposed between
first and second anode regions 328 and 330 and is rarified or
hollow relative to first and second anode regions 328 and 330. A
first channel or chamber 334 is interposed between first anode
region 328 and first cathode region 324, and a second channel or
chamber 336 is interposed between second anode region 330 and
second cathode region 326. A first hydrophobic membrane 342 can be
provided at an air ingress side of first cathode region 324 and a
second hydrophobic membrane 344 can be provided at an air ingress
side of second cathode region 326. First and second hydrophobic
membranes 342 and 344 allow air to permeate first and second
cathode regions 324 and 326, respectively, while preventing the
escape of other fluid components such as water. First, second and
third anode regions 328, 330 and 332; first and second cathode
regions 324 and 326; first and second chambers 334 and 336; and
first and second hydrophobic membranes 342 and 344 can be arranged
as generally parallel flat plates or planar chambers.
Alternatively, first and second anode regions 328 and 330 can be
part of a contiguous substrate such as a cylinder surrounding third
anode region 332, first and second chambers 334 and 336 can be part
of a contiguous annular space coaxially disposed about the anode
layers, first and second cathode regions 324 and 326 can be part of
a contiguous substrate coaxially disposed about the annular space
defined by first and second chambers 334 and 336, and first and
second hydrophobic membranes 342 and 344 can be part of a
contiguous membrane surrounding an outer side of the cathode
structure. In such a case, FIG. 5 can be considered as illustrating
a cross-section of fuel cell 320.
[0100] In one embodiment of fuel cell 320, a fuel-rich fluid enters
one or both axial ends of both first and second chambers 334 and
336, flows between the cathode and anode structures, and filters
through first and second anode regions 328 and 330 as indicated by
the arrows. The resulting fuel-depleted fluid enters the high-flow
third anode region 332 and is discharged from fuel cell 320 at one
or both axial ends of third anode region 332. In another
embodiment, the flow direction is reversed, i.e., fluid enters fuel
cell 320 at one or both axial ends of third anode region 332 and
exits fuel cell 320 at one or both ends of first and second
chambers 334 and 336. The fluid flow paths established by either
embodiment can be advantageous in augmenting the mass transport of
hydroxyl species from the cathode structure to the anode
structure.
[0101] FIG. 6 illustrates another fuel cell 360 having a
flow-by/flow-through arrangement in which an anode 362 is
sandwiched between two cathodes 364 and 366. A first channel or
chamber 368 is interposed between anode 362 and first cathode 364,
and a second channel or chamber 370 is interposed between anode 362
and second cathode 366. A first hydrophobic membrane 372 can be
provided at an air ingress side of first cathode 364 and a second
hydrophobic membrane 374 can be provided at an air ingress side of
second cathode 366. In some embodiments, FIG. 6 represents a
cross-section of a fuel cell having a cylindrical or other
three-dimensional geometry. Fuel-rich fluid enters fuel cell 360 at
one axial end and flows through first and second chambers 368 and
370 between anode 362 and first and second cathodes 364 and 366.
Fuel-depleted fluid exits fuel cell 360 at the other axial end.
Fluid permeates the microstructure of anode 362 from both planar
sides of anode 362. The net direction of fluid flow is through the
axial length of anode 362.
[0102] FIG. 7 illustrates another fuel cell arrangement 380 in
which an anode 382 is paired with a cathode 384. Anode 382 is
thicker than cathode 384. Generally, for any embodiment disclosed
herein, the relative sizes of anodes and cathodes can depend on a
number of factors, including operating environment,
application-specific parameters, the type of fuel employed (e.g.,
methanol, sodium borohydride, etc.), and the like. A chamber 386 is
defined between anode 382 and cathode 384. Fuel-rich fluid
ingresses into anode 382 from a side opposite to cathode 384, as
indicated by arrow 388. Fuel-depleted fluid egresses from anode 382
into chamber 386, from which the fluid can be removed from fuel
cell 380. In another embodiment, the fluid flows in an opposite
path, i.e., through anode 382 from chamber 386. FIG. 7 also
illustrates a hydrophobic membrane 390 located at an air ingress
side of cathode 384.
[0103] FIG. 8 illustrates a fuel cell system 400 according to one
embodiment. Fuel cell system 400 generally comprises a fuel cell
402 and balance-of-plant apparatus 404. Fuel cell 402 includes an
anode 412 and a cathode 414 spaced from anode 412. These electrodes
can have any suitable flow-through structure, but in advantageous
embodiments at least anode 412 has the structure of the embodiments
specifically described in the present disclosure. Fuel cell 402 can
be a single fuel cell unit, i.e., an anode/cathode pair, or can be
a fuel cell stack in which an appropriate combination of a
plurality of anodes 412 and cathodes 414 are arranged with
appropriate manifolds for directing the flow of fuel, electrolyte,
water, and oxygen-containing fluid (e.g., air). In the present
example, a porous, non-selective permeable barrier or membrane 416
is interposed between anode 412 and cathode 414. An anode-side
chamber 422 is disposed or defined between barrier 416 and anode
412. Likewise, a cathode-side chamber 424 is disposed or defined
between barrier 416 and cathode 414. A fluid collection chamber or
manifold 426 is disposed on the side of anode 412 opposite to
barrier 416. Relatively fuel-depleted fluid enters cathode-side
chamber 424 at an inlet 428, flows through cathode-side chamber
424, and is directed through barrier 416 and into anode-side
chamber 422 as indicated by the arrows, and carries hydroxyls
produced by cathode 414. Relatively fuel-rich fluid flows through
anode-side chamber 422 at an inlet 430 where it can combine with
fuel-depleted fluid. While barrier 416 is permeable, its presence
assists in directing the fuel-rich fluid to anode 412 rather than
cathode 414. The resulting fuel-rich, hydroxyl-rich fluid flows
through anode 412 for reaction with its catalyst material. Air or
oxygen-containing fluid is supplied to fuel cell 402 by any
suitable means, such as a fan or other type or fluid mover. In the
illustrated embodiment, air is flowed to an air ingress side of
cathode 414 as indicated by arrows 432. A hydrophobic membrane 434
can be provided at the air ingress side that allows air to permeate
cathode 414 while preventing the escape of other fluid components
such as water. Although not shown in FIG. 8 for convenience but
readily understood by persons skilled in the art, anode 412 and
cathode 414 electrically communicate with appropriate electrical
circuitry, such as electrical leads, busbars, or other
interconnects and a desired electrical load such as a battery or a
device, motor or machinery requiring an input of electrical energy.
Electrons liberated and collected by anode 412 are thus routed in a
suitable manner to the load and then supplied to cathode 414 to
complete an electrical circuit.
[0104] Balance-of-plant apparatus 404 includes means for
recirculating fluid back to fuel cell 402, supplying or
replenishing fuel, and maintaining fuel concentration to the supply
side of anode 412. One or more fuel-processing or separating
chambers 440 are provided separately from fuel cell 402.
Fuel-processing chamber 440 includes a semipermeable membrane 442
that selectively impedes water while allowing fuel and electrolyte
to pass, producing a relatively fuel-rich fluid on the downstream
side of this membrane 442. Membrane 442 is just obstructive enough
to prevent the fuel-rich fluid from mixing back into the fluid on
the upstream side of membrane 442. The fuel-rich fluid enters a
conduit and/or manifold 444 at an exit port 446 of fuel-processing
chamber 440 and is routed to inlet 430 of anode-side chamber 422.
In addition, a fuel supply source 450 is provided with an injector
452 or other discharge means communicating with conduit 444 to
supply fresh fuel to fuel cell 402. As indicated by the arrows,
post-anode, relatively fuel-depleted fluid collected in fluid
collection chamber or manifold 426 enters a conduit and/or manifold
462 and is routed to an inlet port 464 of fuel-processing chamber
440 to membrane 442. As previously noted, membrane 442 separates
fuel from the fluid on the upstream side of membrane 442 such that
the fuel becomes concentrated on the opposing downstream side. The
resulting, even further fuel-depleted, water-rich fluid on the
upstream side of membrane 442 is ideal for cathode function, and is
recycled back to fuel cell 402 by flowing through an exit port 466
of fuel-processing chamber 440, through a conduit or manifold 468,
and to inlet 416 of cathode-side chamber 424. Cathode 414 uses the
water to make hydroxyls for the electrolyte. Hydrostatic pressure
and other pressure differentials in fuel cell system 400 can be
sufficient to maintain relatively continuous flows of fluid through
fuel cell system 400. A pump (not shown), however, can be
positioned at an appropriate location within fuel cell system 400
if needed or desired. It can be seen in this embodiment that the
cathode 414 does not encounter much fuel, and thus the well-known,
undesired fuel crossover effect is minimized. In addition, the net
fluid flow is in the direction of the hydroxyl diffusion
(generally, cathode 414 to anode 412), which increases mass
transport.
[0105] FIG. 9 illustrates an example of a fuel cell stack
arrangement 480. Fuel cell stack 480 includes a plurality of pairs
of anodes 412 and cathodes 414. In the illustrated embodiment, each
corresponding anode 412 and cathode 414 is separated by a permeable
barrier 416 as described with reference to FIG. 8. It will be
understood, however, that any suitable electrode arrangement,
including others described herein, can be implemented in fuel cell
stack 480. An, anode-side chamber 422 is defined between each
corresponding anode 412 and barrier 416, and a cathode-side chamber
424 is defined between each corresponding cathode 414 and barrier
416. Fluid collection chambers 426 are provided on the fluid egress
sides of anodes 412 to collect post-anode, fuel-depleted fluid.
Air-permeable, hydrophobic membranes 434 can be provided at the
sides of cathodes 414 through which air passes. High-concentrate
fuel mixture is supplied to anode-side chambers 422 via a manifold
482. The source of the high-concentrate fuel mixture can be the
fuel-processing chamber 440 illustrated in FIG. 8, downstream of
its membrane 442, and/or can be fuel supply source 450 illustrated
in FIG. 8. Fluid comprising a lower concentration of fuel and a
higher concentration of water is supplied to cathode-side chambers
424 via a manifold 484. The source of the low-fuel, high-water
fluid can be the fuel-processing chamber 440 illustrated in FIG. 8,
upstream of its membrane 442. The fuel-depleted fluid-collected in
fluid collection chambers 426 is transported back to
balance-of-plant apparatus (e.g., balance-of-plant apparatus 404 in
FIG. 8) via a manifold 486. It will be noted from FIG. 9 that the
electrode arrangement of fuel cell stack 480 is not limited to
anode-cathode, anode-cathode, etc. As illustrated in FIG. 9, the
electrode arrangement can be anode-cathode, cathode-anode,
anode-cathode, etc. The illustrated electrode arrangement can be
advantageous in that it reduces the size of fuel cell stack 480
while increasing the power density of fuel cell stack 480, as well
as the number of separate chambers or conduits required and volume
of fluid required. For instance, a common collection chamber 426
can be shared by two adjacent anodes 412, and a common air space
488 can be shared by two adjacent cathodes 414.
[0106] FIG. 10 illustrates a fuel cell stack 500 according to
another embodiment. Fuel cell stack 500 can be housed in a box-like
case or housing 502, which can have a cube or other rectilinear
shape and be constructed from an inexpensive material such as
molded plastic. Construction and materials can be similar to those
for the casing of lead acid batteries. Fuel cell stack 500 is
constructed from an arrangement of anodes and cathodes (not
specifically shown). The electrodes can be arranged as a repetition
of the anode-cathode pair illustrated in FIG. 8, or with the
anode-cathode-cathode-anode-anode pattern illustrated in FIG. 9, or
according to any other suitable arrangement. The anodes and
cathodes are edge-sealed to case 502 to define airtight/watertight
interposing channels as previously described. Channels for airflow
have openings such as slots 504 at an air ingress side 506 of fuel
cell stack 500 and opposing openings (not shown) at an air egress
side 508. Input air enters case 502 at air ingress side 506 and
output air is discharged at air egress side 506. Channels for fuel,
electrolyte and/or water flow have openings such as slots 510 at a
fluid ingress side 512 of fuel cell stack 500 and opposing openings
(not shown) at a fluid egress side 514. Input fluid enters case 502
at fluid ingress side 512 and output fluid is discharged at fluid
egress side 514. The net direction of air flow is thus generally
transverse to the net direction of fluid flow. Although not
specifically shown, the edges of the anodes and cathodes are
located on the opposite sides of the portions of case 502 between
slots 504 and 510. An alternative embodiment can be readily
visualized from FIG. 10 in which fluid ingress proceeds in parallel
with fluid egress, i.e., at the same side of case 502, to provide
an intrinsic countercurrent heat exchanger.
[0107] FIG. 11A illustrates one example of a fuel cell system 520
in which the fuel cell stack 500 illustrated in FIG. 10 can
operate. It will be understood, however, that many other fuel cell
stacks could also operate in fuel cell system 520. A fluid ingress
manifold 522 communicates with slots 510 (FIG. 10) at fluid ingress
side 512 of fuel cell stack 500 for distribution of fuel-containing
fluid to appropriate channels within fuel cell stack 500. Likewise,
a fluid egress manifold 524 communicates with slots 510 at fluid
egress side 514 of fuel cell stack 500 for removal of
fuel-containing fluid from fuel cell stack 500. A fuel processing
system (not shown) similar to that illustrated in FIG. 8 (chamber
440) can also be provided in fuel cell system 520 separate from
fuel cell stack 500. Fuel cell system 520 further includes a
water/thermal/pressure management system 530. In advantageous
embodiments, water/thermal/pressure management system 530 includes
a conduit 532, at least a portion of which is arranged into a coil
for enhancing heat exchanging capability. One or more conduits or
manifolds 542 fluidly interconnect conduit 532 with fluid ingress
manifold 522 for routing fuel-rich fluid to fuel cell stack 500.
Likewise, one or more conduits 544 or manifolds fluidly
interconnect conduit 532 with fluid egress manifold 524 for
returning fuel-depleted fluid to water/thermal/pressure management
system 530. An air manifold 550 communicates with slots 504 (FIG.
10) at air ingress side 506 of fuel cell stack 500. A fan 552 or
other fluid mover flows air between the hydrophobic layers of the
cathodes. The air flow egresses through an air egress manifold 554
communicating with air egress side 508 of fuel cell stack 500, from
which the air passes over the coiled section of conduit 532 of
water/thermal/pressure management system 530. It thus can be seen
that fuel cell stack provides a very efficient thermal management
system. The air flow first cools fuel cell system 520 and then the
coiled section of conduit 532. As described further below, water
sweats out from conduit 532 and thus is subjected to the airflow
and evaporates, and heat is transferred from the interior of
conduit 532 by conduction and/or convection, creating a
self-regulating thermal control system. Moreover, the product water
that transmigrates from conduit 532 can be carried away by the
airflow.
[0108] FIG. 11B illustrates a cross-sectional view of a section of
conduit 532 of water/thermal/pressure management system 530 that is
formed by one or more layers (e.g., layers 552 and 554) of material
as needed to realize a semipermeable membrane. Generally, the
layers employed for the membrane are selected based on application
parameters and fuel composition. A combination of hydrophilic and
hydrophobic layers may be used. Examples of layers include, but are
not limited to, activated nylons, cellulose acetate,
nitrocellulose, oleophobic preparations, NAFION.TM., polyvinyl
fluorides, and filtrous materials typically used for water
purification. The membrane selectively allows the passage of water
from the interior of conduit 532 to the exterior for exposure to
airflow while retaining fuel, electrolyte and ionic components
within conduit 532, thereby separating and removing the water from
the fluid flowing through conduit 532 and increasing the
concentration of fuel in the fluid to be returned to fuel cell
stack 500 (FIG. 11A). As fluid flows through conduit 532, internal
pressure causes a reverse osmosis-like phenomenon wherein water is
transpirated through the membrane of conduit 532. This also
controls pressure build-up within the fluid circuitry. It thus can
be seen that water/thermal/pressure management system 530 provides
a simple, compact, elegant and synergistic means for managing water
build-up, temperature, pressure, and electrolyte concentration in a
self-regulating manner that operates without parasitic energy
drains.
[0109] FIG. 11C illustrates a more detailed view of the
balance-of-apparatus side of fuel cell system 520. As shown in FIG.
11C, a pump 562 can be appropriately positioned in the fluid
circuit for assisting in the circulation of fluid through the
system if needed or desired. As further shown, fan 552 can
alternatively be located on the air egress side of fuel cell stack
500 to pull rather than push air 564 through fuel cell stack
500.
[0110] FIG. 12 illustrates a fuel cell 580 having an electrode
stack arrangement according to another embodiment. Fuel cell 580
comprises an anode section 582 and a separate cathode section 584.
Anode section 582 comprises a plurality of anodes 586 and cathode
section 584 comprises a plurality of cathodes 588. Anodes 586 are
grouped together in anode section 582, cathodes 588 are grouped
together in cathode section 584, and the groupings are separated
from each other. Anodes 586 and cathodes 588 can be respectively
arranged as parallel flat plates or as concentric or annular
structures. Anodes 586 are separated by anode channels, with each
anode channel being adjacent to or otherwise in communication with
at least one anode. The anode channels include pre-anode channels
592 and post-anode channels 594. Axial boundaries 596 can span the
ends of two or more anodes 586 to direct fluid flow in a desired
manner. Cathodes 588 are separated by cathode channels 602, with
each cathode channel 602 being adjacent to or otherwise in
communication with at least one cathode 588. One or more anode
channels 592 and 594 can be in fluid communication with one or more
cathode channels 602 by means of manifolds or chambers 604 and 606,
respectively. Fuel-rich fluid flows from manifold 604 into
pre-anode channels 592 and then passes through anodes 586.
Fuel-depleted fluid flows from anodes 586 into post-anode channels
594 and is flowed to cathode channels 602 via manifold 606. One or
more injectors 612 can be provided in communication with manifold
604 to supply or replenish fuel to fuel cell 580. The injected fuel
can mix with the fluid from cathode section 584 such that a
high-concentrate fuel is provided to anode section 582. Air is
circulated among cathodes 588 by any means, such as through cathode
channels 602 or through additional channels (not shown). As further
shown in FIG. 12, a pump 614 can be provided at one or both
manifolds 604 or 606 to maintain fluid flow if needed or desired.
It will be noted that the number of anodes 586 is different from
the number of cathodes 588, such that the total size of anode
material is different from the total size of cathode material, and
the total flow area in anode channels 592 and 594 is different from
the total flow area in cathode channels 602. Varying the number
and/or dimensions of the anode material and associated channels in
anode section 582 and the cathode material and associated channels
in cathode section 584 can be advantageously utilized to tailor the
fluid flow dynamics and/or reaction kinetics of fuel cell 580 for a
number of purposes, such as maximizing either anode or cathode
functionality.
[0111] FIG. 13 illustrates a fuel cell 630 having an electrode
stack arrangement according to another embodiment. Fuel cell 630
comprises an anode section 632 and a separate cathode section 634.
Anode section 632 comprises one or more anodes or anode regions 636
and cathode section 634 comprises a one or more cathodes or cathode
regions 642. Anodes 636 and cathodes 642 can be respectively
arranged as parallel flat plates or as concentric or annular
structures. In the illustrated example, anode section 632 comprises
a relatively low number of anodes 636 that are sized differently
than cathodes 642, such as by being shorter and thicker. Anodes 636
are separated by a central, relatively high-flow anode channel 644.
An outer anode channel or manifold 646 is disposed on a side of
anodes 636 opposite to central anode channel 644. In the
illustrated example, fluid flows from central anode channel 644
into anodes 636 and is collected in outer anode channel 646 for
transport to cathode section 634. In other implementations, the
direction of flow can be the opposite. Similar to the example
illustrated in FIG. 12, cathodes 642 are separated by cathode
channels 652, with each cathode channel 652 being adjacent to or
otherwise in communication with at least one cathode 642. Central
anode channel 644 fluidly communicates with one or more cathode
channels 652 by means of a manifold 654, and outer anode channel
646 fluidly communicates with one or more cathode channels 652 by
means of a manifold 656. A pump 662 and fuel injector 664 can be
provided as shown. Additionally, a means (not shown) is provided
for supplying air to cathode section 634. This embodiment
represents another technique for varying the number and/or
dimensions of anode material and channels in anode section 632 and
cathode material and channels in cathode section 634 to tailor the
fluid flow dynamics and/or reaction kinetics of fuel cell 630.
[0112] FIG. 14 illustrates a fuel cell system 700 according to
another embodiment. A pump 702 circulates both air and the
fuel/electrolyte/water fluid combination. Air is flowed through a
chamber or manifold portion 704 to another chamber or manifold
portion 706, from which the air is flowed through the intra-cathode
space of a fuel stack assembly 710. The air then flows over the
semipermeable membrane of a water/pressure/thermal management
system 720 (see FIGS. 11A-11C and related description). The fluid
combination is routed to a pre-cathode assembly 732 via a conduit
734, and is mixed with fresh fuel in a fuel mixing chamber 736.
Pre-cathode assembly 732 serves as additional an cathode, and
represents another technique for balancing the differential
kinetics between the anode and cathode material in a fuel cell. A
fuel supply source 738 such as a tank and injector 740 are used to
introduce the fresh fuel into fuel mixing chamber 736. The
resulting fuel-rich fluid then flows through a fluid input manifold
742 and into the intra-anode space of fuel cell stack assembly 710
and into the anode material. Fuel-depleted fluid is collected in a
fluid output manifold 744 and routed to water/pressure/thermal
management system 720 via a conduit 746.
[0113] The various embodiments of electrodes, fuel cells, and fuel
cell systems described thus far offer a number of advantages over
conventional designs. These advantages include the following.
[0114] Catalyst utilization and efficiency are significantly
increased. In all fuel cells, in order for the reactions required
for generating electricity to proceed, there needs to be a triple
interface of catalyst, fuel and electrolyte. The reaction only
occurs at the interface of all three components. If one of these
components is not in direct contact with both of the others at the
same time, that area will not contribute to the reaction. In the
prior art, even under theoretical (and unachievable) ideal
conditions, a very significant portion of the catalyst particles
may not experience the triple interaction and are therefore wasted.
These particles simply do not touch both the fuel and the
electrolyte, as is required for the reaction (see, e.g., FIG. 1 and
accompanying discussion above).
[0115] By contrast, the embodiments disclosed herein implement a
flow-through approach that enables the utilization of a fluid-phase
electrolyte mixed with fuel rather than a solid electrolyte
membrane, thereby enabling the three components of the triple
interface to interact universally. The fuel is dissolved into the
electrolyte, so both are in continual and complete interfacing.
Functionally, every molecule of fuel touches a molecule of
electrolyte. The fuel/electrolyte mixture then flows through a
porous electrode over a tremendous surface area of active catalyst.
In effect, only a double-interface is required, i.e., between the
fluid and the catalyst. The reactants are applied to the catalysts
with orders of magnitude greater efficiency as compared to
conventional technology. The area of triple interface is many times
larger than in the prior art, because up to 100% of the surface
area can be utilized as much as 100% of the time during operation.
Much more of the catalyst on a gram per gram basis is actually used
for making electricity. Thus, the subject matter disclosed herein
provides not only a much greater amount of useful catalytic surface
area, but also essentially every molecule of that catalytic surface
area experiences the triple interaction and contributes to the
reaction. This is true even though less catalyst may be used than
in conventional designs. The increased catalyst surface area boosts
the reaction rate, current density, and efficiency of the
electrode. Therefore, it is possible to employ less catalyst to
obtain more power, and achieve or surpass the goals set forth by
the US Department of Energy for fuel cell catalyst loading.
[0116] Concentration gradient losses in the electrode are
decreased. In conventional designs, as the reactants in the fluid
are used and product water is made at an electrode, a relatively
reactant-depleted microenvironment forms around the catalyst. This
decreases efficiency by limiting catalyst/reactant interaction.
This is worsened because relatively dead-end recesses, paired with
the lack of flow-through design of the current art, trap inert
compounds and waste products. These substances can build up,
keeping fuel from the catalyst. The 80% of air that is not oxygen
(e.g., nitrogen), inert products in the fuel stream, and even the
product water formed on the catalyst can get trapped and block fuel
or ions from reaching the catalyst. Collectively this
microclimate-scale "gumming up" of the catalyst is termed
"concentration gradient loss." By contrast, the flow-through design
of embodiments disclosed herein, the surfactant nature of the
fuel/electrolyte mixture (which can be enhanced by suitable
additives), and oscillating fluid flow all serve to break up
concentration gradients before they can form, thereby preserving
maximal reaction rates. The oscillating fluid flow can be realized
by any appropriate fluid pump capable of this function. One
example, although a limitation, is a peristaltic pump. Fluid flow
is net forward but oscillates back and forth in small amplitudes in
an agitating-like fashion as fluid migrates through the substrate
of an electrode. This not only helps mix the fluids but also forces
additional reactants to interact with the catalyst during each
pass. The net effect is that the reactants encounter much more
catalyst area as they pass though the electrode, further increasing
efficiency.
[0117] Ohmic losses are decreased and thus efficiency gains in
electrical conduction are gained over the current art. Conventional
designs typically require poorly conducting carbon and
nonconducting PTFE components, and thus suffer from a very high
internal resistance that saps electrical energy that would
otherwise be available for useful work. To partially atone for
this, conventional designs have employed field flow plates to serve
as current collectors, adding cost and complexity while seriously
dropping the power density of the fuel cell. In the embodiments
disclosed herein, reduced ohmic losses are achieved by a number of
means. First, the base materials and/or coatings employed in the
electrodes are excellent conductors. Second, the electrodes do not
use binders on a discontinuous substrate. Binders are known to be
significant sources of electrical resistance. The internal
resistance of the electrodes disclosed herein is greatly reduced.
Thus, separate current collectors or field flow plates are not
needed, and the electrodes can be edge connected. Incorporating
various compounds directly onto or into the microstructure of the
electrodes as previously described can further increase
conductivity. In general, the embodiments disclosed herein have
much less electrical impedance. For present purposes, impedance is
the efficiency loss that occurs when electricity has to travel
between the interfaces of materials with different conductivities.
Impedance is minimal in the embodiments disclosed herein because
the charge travels through only similar, intimately connected
materials with very large interfaces. It can be further reduced, if
desired, by including a small amount of the counterinterfacial
compound to each layer. Additionally, the embodiments disclosed
herein have far fewer electrical junctures, further decreasing
impedance.
[0118] In addition, the problems attending water balancing are
greatly ameliorated or avoided in the systems disclosed herein.
Conventional systems are very susceptible to water balance
problems. Fuel cell systems typically need to maintain a very
specific degree of humidification at various components or
locations. If a component such as an electrode becomes too dry or
too wet, it can malfunction. This problem is exacerbated because
fuel cells operate in changing ambient humidity conditions and
encounter frequent temperature fluctuations. Furthermore, water is
formed at one electrode, which can flood it, and water is used by
and dragged from the other electrode, which can dry it. The
conventional PEM membrane is subject to both problems. PEM-based
fuel cells seldom function optimally, are quite sensitive to
ambient and operating conditions, and require additional, bulky and
prohibitively expensive water-handling balance-of-plant equipment.
The water-handling equipment electrically parasitizes the fuel
cell, dropping efficiency.
[0119] By contrast, the systems disclosed herein are not affected
by the kinds of water balance problems that plague conventional
designs, and provide an inexpensive, simple, non-parasitic means
for handling and removing product water while eliminating as much
as 70-90% of the balance-of-plant equipment typically required by
conventional systems, thereby greatly reducing cost and maintenance
while increasing efficiency and power density. An example is the
water/thermal/pressure management system 530 described above with
reference to FIGS. 11A-11C. In addition, the flow-through design of
the systems disclosed herein enables the product water and other
by-products such as carbon dioxide to be washed away as it is made.
This advantage can be enhanced by the afore-described oscillatory
fluid flow and the ability to add detergents, surfactants and the
like to the liquid-phase fuel/electrolyte combination.
Additionally, because the water makes up a large percentage of the
electrolyte mixture, the systems disclosed herein are largely
immune from the degree of changes of water concentration that can
be deleterious to prior art designs and thus do not require
stringent control/regulation over humidification changes. As
previously described, excess water can be removed by transpiration,
pervaporation and/or evaporation elsewhere from the fuel cell or
fuel cell stack with a self-regulating system that does not consume
power. Thus, the intrinsic synergies of the systems disclosed
herein provide a self-regulating system and obviate the need for
most water-handling equipment.
[0120] Efficiency losses and dangers attending the well-known fuel
crossover effect are reduced in the fuel cells disclosed herein.
Generally, crossover is the result of the fact that in typical fuel
cell designs some of the fuel can pass the anode without being used
for reaction. Crossover is undesirable for at least two reasons.
First, the unused fuel represents wasted efficiency and, if not
reused or recycled, increased pollution. Second, the crossover fuel
can oxidize at the cathode, further dropping efficiency. The
embodiments disclosed herein can reduce fuel crossover efficiency
losses by a series of features. In embodiments with
anode-to-cathode flow direction, a large percentage of the fuel is
used in the anode in a "first pass effect." Fuel utilization is
augmented by specific flow rates and the previously described
oscillating flow. The fuel mixture is brought back and forth across
the anode, utilizing a greater percentage of the available fuel,
increasing the first pass efficiency. The little fuel that does
make it through the anode becomes substantially diluted by the
product water made at the anode. In the case of methanol, for every
molecule used, five water molecules are made. Any fuel that makes
it through the anode hence becomes greatly diluted. As a result of
the foregoing, the cathode operates only in a relatively
fuel-depleted environment. Additionally, the cathode can include
catalysts with selectivity against the anode reaction, reducing
crossover efficiency loss. The cathode also may include a number of
fuel-excluding technologies described in a separate
application.
[0121] Regarding the dangers associated with crossover, there is a
risk that explosive crossover fuel can mix with oxygen in the
electrically charged and heated environment of a fuel cell. This
dangerous condition plagues all low-temperature fuel cells. The
conventional PEM design employs a large-area membrane to separate
the hydrogen from the oxygen. The membrane is paper thin and
semipermeable. The gases are hot and pressurized, and create
caustic, reactive intermediary compounds that permeate the
apparatus. Over time, the membrane can develop leaks that allow
mixing of hot pressurized hydrogen and oxygen inside a hot,
electrically active environment, potentially creating conditions
for an explosion. By contrast, the amount of fuel crossover mixing
that occurs in the fuel cells disclosed herein is insignificant is
not unsafe or deleterious. Only a small amount of methanol (or
other hydrocarbon fuel, if used) evaporates from the cathode and is
diluted into the comparatively much greater air volume so that
there is minimal combustion risk. Moreover, embodiments disclosed
above can employ multiple means of reducing the amount of fuel that
gets to and into the cathode. For example, as illustrated for
example in FIG. 8, before the fuel mixture enters the fuel cell
stack it can be flowed though one or a series of fuel processing
devices or separators 440 that serve to channel concentrated fuel
to the anode and away from the cathode. In addition, fuel injector
feeds fuel into only the anode stream. The product water further
dilutes any fuel left in the cathode stream. The cathode
experiences a fuel-depleted environment, so very little of the fuel
is available for crossover. The potential for fuel crossover is
still further limited by the fuel-phobic nature of the materials
within the body of the cathode. Very little fuel reaches the
cathode, even less penetrates it, and the little that gets through
is immediately washed away by such a vastly greater volume of air
that it stays orders of magnitude more diluted than its combustible
concentration. Finally, because the fuel is in liquid form, it does
not mix in the same hazardous way that gases do.
[0122] Mass transport losses are decreased in the fuel cells
disclosed herein. In conventional designs, the time required for
ions to cross the electrolyte membrane causes a significant
bottleneck for the electricity-producing reaction. In the fuel cell
disclosed herein, however, the improved microstructure of the
electrodes enables better conductivity. In addition, the
oscillating flow mixes the electrolyte and thereby promotes faster
ionic diffusion. Moreover, because of the flow-through design
enabling the use of a flowable electrolyte in combination with the
fuel, buffers and dibasic compounds can be added to the fluid for
releasing additional ions as needed at the appropriate electrode
and eliminating localized depletion within the microstructure. Due
to the oscillating flow pattern, the net flow washes ions in the
proper direction, the mixing agitation increase diffusion rates,
and the backward component of flow temporarily increases the
concentration of ions and thereby increases diffusion pressure. The
oscillating flow and flow-through functions not only increase
fuel/catalyst interaction for better fuel utilization and increase
the rate of ionic diffusion, but also wash away the product water
and reduce the building up of precipitates.
[0123] Fuel cell arrangements such as illustrated in FIGS. 5-7, 12
and 13 can be utilized to reduce or eliminate efficiency losses
associated with reaction-kinetic mismatches as between anodes and
cathodes. Conventional fuel cell stacks, and particularly
low-temperature designs, employ equal numbers of anodes and
cathodes grouped in functional units of anode/cathode pairs. In
many designs, the anode could function at a much higher kinetic
rate if it were not limited by the cathode, which often functions
at a relatively slower kinetic rate. The differential in kinetics
among these electrodes can be a high as two orders of magnitude.
Traditionally, this has been compensated for by varying the
catalysts loadings as between the anode and cathode.
[0124] In the fuel cell arrangements disclosed herein, differential
kinetics can be partially or wholly balanced further by employing
more than one cathode for every anode (see, e.g., FIGS. 5, 6 and
13) or vice versa (see, e.g., FIG. 12), or more massive cathodes
relative to anodes or vice versa (see, e.g., FIGS. 7 and 13).
Separating the anodes and cathodes from each other by a distance
can facilitate adjusting the relative numbers of anodes and
cathodes or their relative sizes, as shown for example in FIGS. 12
and 13. In these latter embodiments, any disadvantage stemming from
the increased ion transport distance can be reduced by other
advantages provided by the embodiments, such as increased fluid
flow rates, the use of hydroxyl-supplying additives, reduction or
absence of electrical crossover and fuel crossover, increased fuel
and catalyst utilization at the anode, and the ability to maximize
each type of electrode's performance (e.g., the shape and size of
the substrate structure and of the associated adjacent fluid flow
channels) substantially without the need to account for the
performance of the other type. The cathode channels employed for
fluid flow in FIGS. 12 and 13, respectively, can have a greater
total flow volume than the corresponding anode channels. This
feature slows the fluid down at the cathode side to compensate for
slower reaction rates at the cathodes and hence better balance the
system. Because hydroxyls in the systems illustrated in FIGS. 12
and 13 are delivered by fluid flow rather than by diffusion, the
anodes can be designed thicker, allowing for wider pores to
decrease resistance to flow and increased total surface for greater
fuel utilization. A side fluid circuit can be added to recirculate
unused fuel leaving the anode.
[0125] Many of the various ancillary balance-of-plant components
required in conventional systems, especially those employing
pressurized gaseous fuel stock, are eliminated in the systems
disclosed herein. This is particularly due to the fact that the
systems disclosed herein utilize liquid-phase working fluids. There
is no need for reformers, shift reactors, fuel processors,
high-pressure tanks, field flow/bipolar plates, preheaters,
etc.
[0126] The embodiments and methods disclosed herein can include
means for refreshing the electrolyte. In conventional alkaline fuel
cells, the electrolyte slowly reacts with product and ambient
CO.sub.2 to form an inert product. The most common example of this
reaction is:
KOH+CO.sub.2.fwdarw.K.sub.2CO.sub.3+H.sub.20+heat
[0127] This process impedes the proper functioning of a fuel cell
in a number of ways. First, the concentration of KOH is lowered,
reducing its efficiency as an electrolyte. Second, the dissolved
K.sub.2CO.sub.3 further dilutes the electrolyte. Third,
K.sub.2CO.sub.3 precipitates out, potentially gumming up the
electrodes. In order to extend maintenance-free intervals of
operation, conventional alkaline fuel cells depend on additional
balance-of-plant apparatus to "scrub" the CO.sub.2, to filter out
the K.sub.2CO.sub.3, and to replenish the electrolyte.
[0128] On the other hand, the embodiments and methods disclosed
herein can take advantage of numerous intrinsic synergies to reduce
the rate of electrolyte degradation. One way is to not use
carbon-based fuels. The systems of the present disclosure are
flexible with respect to the range of possible fuels, including
fuels that are not carbon-based and therefore do not synthesize
CO.sub.2. Examples include, but are not limited to, hydrogen and
NaBH.sub.4. NaBH.sub.4 is preferred over hydrogen due to well-known
storage, handling and other problems associated with the use of
hydrogen. It will be noted, however, that while methanol is a
carbon-based fuel, it produces relatively low amounts of CO.sub.2.
Accordingly, both NaBH.sub.4 and methanol are considered to be
among the advantageous fuels that can be employed in embodiments
and methods disclosed herein.
[0129] The process of electrolyte degradation is a reversible
reaction that tends toward equilibrium. As the electrolyte goes
through its flow cycle, it experiences a wide variation in local
conditions. Upstream from the anode, it is relatively concentrated,
cool, and has a high fuel load. As it passes through an anode, it
is heated, the fuel is depleted, and for every molecule of methanol
used, five water molecules dilute it. This drastically alters the
concentration, pH, and temperature of the fluid. At the cathode,
there is a relative increase of hydroxyls and the water is used up
increasing pH and concentration. The electrolyte is also subject to
cooling from the mass flow of air at the cathode. In the
water-handling chamber as, for example system illustrated in FIGS.
11A-11C, the mixture is further cooled and concentrated. More fuel
is then added to the electrolyte downstream.
[0130] Some of the local conditions to which the
electrolyte-containing fluid is subject favor, or come close to
favoring, the reactant side of the above equilibrium. One specific
location that can favor the renewal direction of the reaction
equilibrium is just downstream from the anode. Here, there is a
relative excess of heat and water and relatively low pH and KOH
concentrations. At this point, a specific catalyst can be
incorporated. Because conditions favor the return reaction, a
catalyst here would renew the electrolyte. This catalyst can be
included in or near the surface of the egress side of the anode.
Solubilizing agents can be added to further tip the equilibrium
favorably. In addition, if local conditions do not achieve the
ability to favor the renewing direction of the equilibrium, a
fraction of or the entire electrolyte can be caused to flow through
a side path with a system such as a countercurrent multiplier to
augment favorable conditions.
[0131] In addition to providing means for refreshing the
electrolyte, additional embodiments of the present subject matter
incorporate a refresh cycle that cleans the catalysts of poisons.
In the refresh cycle, a specific electric charge is forced back
through an electrode. Electrical conductors or capacitors can be
provided adjacent to or within the structure of the electrodes for
this purpose. At the catalyst, a specific amount of oxygen and/or
hydrogen is formed, depending on the polarity of the charge. The
oxygen physically and chemically frees the catalyst of adherent
toxins and thereby refreshes its function. The gas then dissolves
or bubbles into the electrolyte and can be removed by the
transpiration or venting equipment. The hydrogen can be caused to
form at a specific, separate, electrode incorporated for the
refresh function for appropriate handling, and may also serve as a
cleaning agent. The electricity may be fed to the electrode with
positive, negative, or alternating polarity for maximal
cleaning/refreshing power with minimal gas production. The oxygen
and hydrogen specifically function as cleaning/refreshing agents
can also be utilized by the electrodes fuel cell in their
traditional oxidizing/reducing roles. In advantageous embodiments,
just enough gas is evolved to totally coat the catalyst with minute
bubbles. The bubbles are small enough that buoyancy does not
overcome the surface tension holding them to the catalyst. At the
end of the refresh cycle, the small quantity of gas may dissolve,
bubble off, or be reabsorbed at the electrode utilizing the fuel
cell's own oxidation-reduction reaction as previously noted. Enough
gas can be evolved to purposely cause bubbling of the gas off an
electrode.
[0132] In some embodiments, the electrodes of the fuel cell are
employed to create the hydrogen and oxygen in a unique form of
internal reforming. Using specific borohydride fuel, hydrogen is
made at the anode and oxygen is made at the cathode. Electricity is
used to crack the hydrogen and oxygen out of the fuel stock. Then
the hydrogen is consumed to produce water and heat. The net energy
output is derived from the difference in enthalpies from cracking
the fuel versus that for the hydrogen-to-water reaction.
[0133] In advantageous embodiments, the refresh cycle is
incorporated into the operation cycle. Extra electrodes are
provided in the stack that are rotated through an off/refresh
cycle. Each electrode can be switched between a normal mode of
operation during which the electrode is employed to generate
electricity in its conventional role, and a refresh mode of
operation during which the electrode is refreshed by the
application of electrical energy to its structure or is employed to
apply such energy to another electrode to refresh that other
electrode. If, for example, the fuel cell stack design calls for
100 MEA ("membrane electrode assembly") equivalents, the fuel cell
according to this embodiment can actually include, for example,
103. Two of the extra electrodes produce the power to clean and
refresh the third electrode, while the remaining 100 are operated
normally to produce electricity. Once the refresh cycle is
completed for a particular electrode, that electrode can then be
switched to normal operation while another set of three electrodes
are operated in accordance with the refresh cycle to refresh one of
the electrodes from the newly selected set. A microprocessor or
other suitable electrical controller can be placed in communication
with the electrodes of the fuel cell stack to cycle the roles of
the electrodes between normal and cleaning modes as the fuel cell
operates. This allows the refresh cycle to occur at higher
temperatures and prevents build up, ensuring the greatest operation
efficiency, while decreasing down-cycle maintenance time. The fuel
cell can be caused to run hotter for refresh cycles by limiting the
cooling flow or amount of cooling coils in the flow circuit.
Alternatively or additionally, the refresh cycle can occur as part
of the start-up and/or shut-down procedure or as part of scheduled
off-cycle maintenance. Moreover, unlike conventional fuel cell
designs, the flow-through embodiments disclosed herein are also
amenable to periodically being flushed with specific gas-phase
and/or liquid-phase cleaners to refresh the catalysts.
[0134] An example of one implementation of a refresh cycle is
illustrated in the simplified schematic view of FIG. 15. A fuel
cell 800 comprises a plurality of electrodes E arranged as anodes
and cathodes according to any suitable arrangement. A
microprocessor .mu.P or any other suitable electrical controller,
as well as associated circuitry, is placed in electrical
communication with each electrode E. During normal operation of
fuel cell 800, electrodes E generate a current that is routed to an
appropriate load L in the ordinary manner of fuel cells. During
operation, microprocessor .mu.P can be programmed to switch one or
more electrodes E to initiate a refresh cycle, during which time
the associated circuitry, which can include a DC voltage source
and/or an AC voltage (which may be a waveform generator with
adjustable frequency and amplitude), applies electrical energy to
the one or more electrodes for the purpose of refreshing a selected
electrode E. After the time period for refreshing has elapsed,
microprocessor .mu.P can switch the electrode or electrodes E
involved in the refresh cycle back to the normal mode of operation
and switch another electrode or set of electrodes E to operate the
refresh cycle. In this manner, at any given instant of time during
operation of fuel cell 800, a desired number of electrodes E are
always operating normally to produce the required amount of
electricity, while at least one other electrode E is being
refreshed by electrical means. Microprocessor .mu.P can be placed
in communicating with the load side of the electrode arrangement in
order to cut off or disconnect a selected one or more electrodes E
from load L for a set period of time. In this manner, current
collected by an electrode E can be prevented from flowing to load
L, whereby charge builds up in the electrode E. Allowing the
build-up of charge, even without applying electrical energy to the
electrode E, can be sufficient to effect the refresh cycle. The
building up of electrical charge within the structure of electrode
E can cause poisonous/contaminant components to separate from the
catalytic material and be washed away by the fluid flowing through
electrode E. The build-up of electrical charge can also cause
bubbles to form on the catalyst, physically separating the
poisonous/contaminant components from the catalytic material, and
can promote the formation of oxygen and hydrogen components that
act as cleaning agents.
[0135] It will be understood that various aspects or details of the
invention may be changed without departing from the scope of the
invention. Furthermore, the foregoing description is for the
purpose of illustration only, and not for the purpose of
limitation--the invention being defined by the claims.
* * * * *