U.S. patent application number 13/181124 was filed with the patent office on 2011-12-22 for gas diffusion anode and co2 cathode electrolyte system.
Invention is credited to Bryan Boggs, Valentin Decker, Kasra Farsad, RYAN J. GILLIAM, Nigel Antony Knott, Michael Kostowskyj.
Application Number | 20110308964 13/181124 |
Document ID | / |
Family ID | 43450291 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308964 |
Kind Code |
A1 |
GILLIAM; RYAN J. ; et
al. |
December 22, 2011 |
GAS DIFFUSION ANODE AND CO2 CATHODE ELECTROLYTE SYSTEM
Abstract
A low-voltage, low-energy electrochemical system and method of
removing protons and/or producing a base solution using a gas
diffusion anode and a cathode electrolyte comprising dissolved
carbon dioxide, while applying 2V or less across the anode and
cathode.
Inventors: |
GILLIAM; RYAN J.; (San Jose,
CA) ; Decker; Valentin; (San Jose, CA) ;
Knott; Nigel Antony; (Los Gatos, CA) ; Kostowskyj;
Michael; (Los Gatos, CA) ; Boggs; Bryan;
(Campbell, CA) ; Farsad; Kasra; (San Jose,
CA) |
Family ID: |
43450291 |
Appl. No.: |
13/181124 |
Filed: |
July 12, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12541055 |
Aug 13, 2009 |
7993500 |
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13181124 |
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12503557 |
Jul 15, 2009 |
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12541055 |
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Current U.S.
Class: |
205/555 ;
204/265; 204/277; 205/464; 205/638 |
Current CPC
Class: |
C25B 1/00 20130101; C02F
1/46109 20130101; C25B 1/02 20130101; Y02E 60/366 20130101; C25B
1/14 20130101; Y02E 60/36 20130101; C02F 2201/46185 20130101; C02F
2001/46166 20130101; C02F 2201/46115 20130101; C25B 1/26 20130101;
C02F 2209/06 20130101; C02F 2201/4613 20130101; C02F 2301/046
20130101; C25B 1/04 20130101; C25B 1/16 20130101; C02F 1/68
20130101; C02F 2201/46135 20130101 |
Class at
Publication: |
205/555 ;
204/277; 204/265; 205/638; 205/464 |
International
Class: |
C25B 9/08 20060101
C25B009/08; C25B 1/02 20060101 C25B001/02; C25B 1/22 20060101
C25B001/22; C25B 9/06 20060101 C25B009/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2009 |
US |
PCT/US09/32301 |
Jun 24, 2009 |
US |
PCT/US09/48511 |
Dec 23, 2009 |
US |
PCT/US08/88242 |
Claims
1. An electrochemical system comprising: a gas diffusion anode; and
a cathode contacting a cathode electrolyte comprising dissolved
carbon dioxide.
2. The system of claim 1, further comprising a gas delivery system
configured to deliver hydrogen gas to the anode.
3. The system of claim 2, wherein the hydrogen gas is produced at
the cathode.
4. The system of claim 2, wherein the anode is configured to
produce protons, and the cathode is configured to produce hydrogen
gas and hydroxide ions on application of a voltage across the anode
and the cathode.
5. The system of claim 4, wherein the voltage is less than 2V.
6. The system of claim 4, wherein the system is configured not to
produce a gas at the anode.
7. The system of claim 4, further comprising a first cation
exchange membrane positioned between the cathode electrolyte and a
salt solution, and an anion exchange membrane positioned between
the salt solution and an anode electrolyte.
8. The system of claim 7, wherein the anode contacts the anode
electrolyte.
9. The system of claim 7, wherein a second cation exchange membrane
is positioned between the anode and the anode electrolyte.
10. The system of claim 9, wherein the system is configured to
migrate anions to the anode electrolyte from the salt solution
through the anion exchange membrane.
11. The system of claim 9, wherein the system is configured to
migrate chloride ions to the anode electrolyte from the salt
solution through the anion exchange membrane.
12. The system of claim 11, wherein the system is configured to
migrate cations to the cathode electrolyte from the salt solution
through the first cation exchange membrane.
13. The system of claim 12, wherein the system is configured to
migrate sodium ions to the cathode electrolyte from the salt
solution through the first cation exchange membrane.
14. The system of claim 9, wherein the system is configured to
migrate protons to the anode electrolyte from the anode.
15. The system of claim 14, wherein the system is configured to
migrate hydroxide ions to the cathode electrolyte from the
cathode.
16. The system of claim 15, wherein the system is configured to
produce sodium hydroxide and/or sodium bicarbonate and/or sodium
carbonate in the cathode electrolyte.
17. The system of claim 14, wherein the system is configured to
produce an acid in the anode electrolyte.
18. The system of claim 17, wherein the system is configured to
produce hydrochloric acid in the anode electrolyte.
19. The system of claim 9, further comprising a partition that
partitions the cathode electrolyte into a first cathode electrolyte
portion and a second cathode electrolyte portion, wherein the
second cathode electrolyte portion contacts the cathode and
comprises dissolved carbon dioxide.
20. The system of claim 19, wherein the first cathode electrolyte
portion comprises gaseous carbon dioxide.
21. The system of claim 19, wherein the partition is positioned to
isolate gaseous carbon dioxide gas in the first cathode electrolyte
portion from cathode electrolyte in the second cathode electrolyte
portion.
22. The system of claim 21, wherein the system is configured to
produce hydroxide ions and hydrogen gas at the cathode.
23. The system of claim 22, wherein the system is configured to
produce hydroxide ions in the cathode electrolyte.
24. The system of claim 23, wherein the system is configured to
migrate cations to the cathode electrolyte through the first cation
exchange membrane; migrate anions to the anode electrolyte through
the anion exchange membrane; and migrate protons to the anode
electrolyte from the anode.
25. The system of claim 24, wherein the system is configured to
produce cations, hydroxide ions and/or carbonic acid and/or
carbonate ions and/or bicarbonate ions in the cathode
electrolyte.
26. The system of claim 25, wherein the system is configured to
produce sodium hydroxide and/or sodium carbonate and/or sodium
bicarbonate in the cathode electrolyte.
27. The system of claim 26, wherein the cathode electrolyte is
operatively connected to a carbon dioxide gas/liquid contactor
configured to dissolve carbon dioxide in the cathode
electrolyte.
28. The system of claim 27, wherein the cathode electrolyte is
operatively connected to a system configured to produce carbonates
and/or bicarbonates and/or hydroxides from a solution comprising
carbon dioxide and divalent cations.
29. An electrochemical method comprising: applying a voltage across
a cathode and a gas diffusion anode in an electrochemical system,
wherein the cathode contacts a cathode electrolyte comprising
dissolved carbon dioxide.
30. The method of claim 29, comprising oxidizing hydrogen gas at
the anode.
31. The method of claim 30, comprising producing protons at the
anode.
32. The method of claim 30, comprising producing hydroxide ions and
hydrogen gas at the cathode.
33. The method of claim 32, wherein a gas is not produced at the
anode.
34. The method of claim 32, wherein the voltage is less than
2V.
35. The method of claim 32, comprising directing hydrogen gas from
the cathode to the anode.
36. The method of claim 31, comprising migrating protons from the
anode to an anode electrolyte.
37. The method of claim 34, comprising interposing a cation
exchange membrane between the anode and an anode electrolyte.
38. The method of claim 37, comprising interposing an anion
exchange membrane between the anode electrolyte and a salt
solution.
39. The method of claim 38, comprising interposing a first cation
exchange membrane between the cathode electrolyte and the salt
solution, and wherein the salt solution is disposed between the
anion exchange membrane and the first cation exchange membrane.
40. The method of claim 39, comprising migrating anions from the
salt solution to the anode electrolyte through the anion exchange
membrane, and migrating cations from the salt solution to the
cathode electrolyte through the first cation exchange membrane.
41. The system of claim 40, comprising producing hydroxide ions
and/or carbonate ions and/or bicarbonate ions in the cathode
electrolyte; and an acid in the anode electrolyte.
42. The method of claim 40, comprising producing sodium hydroxide
and/or sodium carbonate and/or sodium bicarbonate in the cathode
electrolyte; and hydrochloric acid in the anode electrolyte.
43. The method of claim 42, comprising contacting the cathode
electrolyte with a divalent cation solution comprising calcium
and/or magnesium ions.
44. The method of claim 42, comprising producing partially
desalinated water in the salt solution.
Description
CROSS-REFERENCE
[0001] This application is continuation-in-part of U.S. patent
application Ser. No. 12/503557 filed on Jul. 16, 2009, titled: "CO2
Utilization In Electrochemical Systems", herein incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] In many chemical processes a base solution is required to
achieve a chemical reaction, e.g., to neutralize an acid, or buffer
the pH of a solution, or precipitate an insoluble hydroxide and/or
carbonate and/or bicarbonate from a solution. One method by which
the base solution is produced is by an electrochemical system as
disclosed in the above-referenced US patent application, herein
incorporated by reference in its entirety. In producing a base
solution electrochemically, a large amount of energy, salt and
water may be used; consequently, lowering the energy and material
used is highly desired.
SUMMARY OF THE INVENTION
[0003] This invention pertains to a low-voltage, low-energy
electrochemical system and method of producing a base solution,
utilizing a cathode electrolyte comprising dissolved carbon dioxide
gas in contact with a cathode, and a gas diffusion electrode. In
one embodiment, the system comprises a gas diffusion anode and a
cathode in contact with a cathode electrolyte comprising dissolved
carbon dioxide. In another embodiment, the method comprises
applying a voltage across a gas diffusion anode and a cathode
wherein the cathode contacts a cathode electrolyte comprising
dissolved carbon dioxide gas. In various embodiments, the system
includes a gas delivery system configured to deliver hydrogen gas
to the anode; the hydrogen gas is produced at the cathode; the
anode is configured to produce protons, and the cathode is
configured to produce hydrogen gas and hydroxide ions on
application of a voltage across the anode and the cathode; the
voltage is less than 2V; a gas is not produced at the anode; the
system includes a first cation exchange membrane positioned between
the cathode electrolyte and a salt solution, and an anion exchange
membrane positioned between the salt solution and an anode
electrolyte; the anode contacts the anode electrolyte; a second
cation exchange membrane is positioned between the anode and the
anode electrolyte; the system is configured to migrate anions to
the anode electrolyte from the salt solution through the anion
exchange membrane when the voltage is applied across the anode and
cathode; the system is configured to migrate chloride ions to the
anode electrolyte from the salt solution through the anion exchange
membrane; the system is configured to migrate cations to the
cathode electrolyte from the salt solution through the first cation
exchange membrane; the system is configured to migrate sodium ions
to the cathode electrolyte from the salt solution through the first
cation exchange membrane; the system is configured to migrate
protons to the anode electrolyte from the anode; the system is
configured to migrate hydroxide ions to the cathode electrolyte
from the cathode; the system is configured to produce sodium
hydroxide and/or sodium bicarbonate and/or sodium carbonate in the
cathode electrolyte; the system is configured to produce an acid in
the anode electrolyte; the system is configured to produce
hydrochloric acid in the anode electrolyte; the system comprises a
partition that partitions the cathode electrolyte into a first
cathode electrolyte portion and a second cathode electrolyte
portion, wherein the second cathode electrolyte portion contacts
the cathode and comprises dissolved carbon dioxide; the first
cathode electrolyte portion comprises gaseous carbon dioxide; in
the system, the partition is positioned to isolate gaseous carbon
dioxide gas in the first cathode electrolyte portion from cathode
electrolyte in the second cathode electrolyte portion; the system
is configured to produce hydroxide ions and hydrogen gas at the
cathode; the system is configured to produce hydroxide ions in the
cathode electrolyte; the system is configured to migrate cations to
the cathode electrolyte through the first cation exchange membrane;
migrate anions to the anode electrolyte through the anion exchange
membrane; and migrate protons to the anode electrolyte from the
anode; the system is configured to produce cations, hydroxide ions
and/or carbonic acid and/or carbonate ions and/or bicarbonate ions
in the cathode electrolyte; the system is configured to produce
sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate
in the cathode electrolyte; the cathode electrolyte is operatively
connected to a carbon dioxide gas/liquid contactor configured to
dissolve carbon dioxide in the cathode electrolyte; and the cathode
electrolyte is operatively connected to a system configured to
produce carbonates and/or bicarbonates and/or hydroxides from a
solution comprising carbon dioxide and divalent cations.
[0004] In various embodiments, the method includes oxidizing
hydrogen gas at the anode; producing protons at the anode;
producing hydroxide ions and hydrogen gas at the cathode; not
producing a gas at the anode; applying a voltage of 2V or less
across the anode and cathode; directing hydrogen gas from the
cathode to the anode; migrating protons from the anode to an anode
electrolyte; interposing a cation exchange membrane between the
anode and an anode electrolyte; interposing an anion exchange
membrane between the anode electrolyte and a salt solution;
interposing a first cation exchange membrane between the cathode
electrolyte and the salt solution, and wherein the salt solution is
disposed between the anion exchange membrane and the first cation
exchange membrane; migrating anions from the salt solution to the
anode electrolyte through the anion exchange membrane, and
migrating cations from the salt solution to the cathode electrolyte
through the first cation exchange membrane; producing hydroxide
ions and/or carbonate ions and/or bicarbonate ions in the cathode
electrolyte; and an acid in the anode electrolyte; producing sodium
hydroxide and/or sodium carbonate and/or sodium bicarbonate in the
cathode electrolyte; and hydrochloric acid in the anode
electrolyte; contacting the cathode electrolyte with a divalent
cation solution comprising calcium and/or magnesium ions; and
producing partially desalinated water in the salt solution.
[0005] In the system, applying a relatively low voltage across the
anode and cathode, e.g., 2V or less, produces hydroxide ions and
hydrogen gas at the cathode, and protons at the anode. In the
system, under the applied voltage, the hydroxide ions produced at
the cathode migrate into the cathode electrolyte to produce the
base solution, and protons produced at the anode migrate to the
anode electrolyte to produce an acid. In various embodiments,
hydrogen gas produced at the cathode is directed to the anode where
it is oxidized to protons. In the system, dissolving carbon dioxide
in the cathode electrolyte alters the pH of the electrolyte in such
a manner that the voltage required across the anode and cathode to
produce the base solution is lowered. In the system, the dissolved
carbon dioxide also produces carbonic acid and/or carbonate ions
and/or bicarbonate ions in the cathode electrolyte, depending on
the pH of the electrolyte. In various configurations, cation
exchange membrane and anion exchange membranes are used in the
system to separate a salt solution, e.g., a solution of sodium
chloride, from the cathode electrolyte and anode electrolyte. In
the system, under the applied voltage, cations in the salt solution
migrate to the cathode electrolyte through the cation exchange
membrane, and anions in the salt solution migrate to the anode
electrolyte through an anion exchange membrane. Consequently, in
the system, a base solution comprising hydroxide ions and/or
dissolved carbon dioxide and/or carbonate ions and/or bicarbonate
ions and/or cations from the salt solution, may be produced in the
cathode electrolyte. Similarly, the anode electrolyte may produce
an acid comprising protons that migrate from the anode and anions
that migrate from the salt solution. In the system, a gas, e.g.,
chlorine or oxygen is not produced at the anode.
[0006] In various embodiments, the gas diffusion anode comprises a
conductive substrate infused with a catalyst that catalyzes the
oxidation of hydrogen to protons. In various embodiments, the
substrate is configured such that on a first side the substrate
interfaces with hydrogen, and on an opposed side the substrate
interfaces with the anode electrolyte. In the system, on applying
the voltages as disclosed herein across the anode and cathode,
protons are produced at the substrate from oxidization of hydrogen
gas. Under the applied voltages, the protons migrate to the anode
electrolyte where they produce an acid.
[0007] Advantageously, with the system and method, since the
voltage across the anode and cathode required to produce the
hydroxide ions is lowered, the energy required to produce the base
solution is lowered.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The following drawings illustrate by way of examples and not
by limitation embodiments of the present system and method.
[0009] FIG. 1 is an illustration of an embodiment of the present
system.
[0010] FIG. 2 is an illustration of an embodiment of the present
anode system.
[0011] FIG. 3 is a flow chart of an embodiment of the present
method.
[0012] FIG. 4 is an illustration of an embodiment of the present
system.
[0013] FIG. 5 is an illustration of Carbonate ion/Bicarbonate ion
speciation in water.
[0014] FIG. 6 is an illustration of Bicarbonate Ion generation in
the cathode electrolyte.
DETAILED DESCRIPTION OF THE INVENTION
[0015] In the following detailed description, a salt solution
comprising sodium chloride is utilized in the system to produce a
base solution in the cathode electrolyte, and an acid in the anode
electrolyte. In this exemplarary embodiment, sodium ions migrate
from the salt solution to produce sodium hydroxide and/or sodium
carbonate and/or sodium bicarbonate in the cathode electrolyte; and
chloride ions migrate from the salt solution to produce
hydrochloric acid in the anode electrolyte. However, as can be
appreciated by one ordinarily skilled in the art, since the system
can be configured to utilize an equivalent salt solution, e.g., a
solution of potassium sulfate and the like, to produce an
equivalent result, e.g., potassium hydroxide and/or potassium
carbonate and/or potassium bicarbonate in the cathode electrolyte,
and sulfuric acid in the anode electrolyte, the system is not
limited to using sodium chloride solution but can be configured to
utilize an equivalent salt solution. Therefore, to the extent that
equivalent salts can be used, these equivalents are within the
scope of the appended claims.
[0016] As disclosed in U.S. patent application Ser. No. 12/503557
filed on Jul. 16, 2009, titled: "CO2 Utilization In Electrochemical
Systems", herein incorporated by reference in its entirety, in
various embodiments, carbon dioxide is absorbed into the cathode
electrolyte utilizing a gas mixer/gas absorber. In one embodiment,
the gas mixer/gas absorber comprises a series of spray nozzles that
produces a flat sheet or curtain of liquid into which the gas is
absorbed; in another embodiment, the gas mixer/gas absorber
comprises a spray absorber that creates a mist and into which the
gas is absorbed; in other embodiments, other commercially available
gas/liquid absorber, e.g., an absorber available from Neumann
Systems, Colorado, USA is used.
[0017] The carbon dioxide used in the system is obtained from
various sources including carbon dioxide from combustion gases of
fossil fuelled electrical power generating plants, cement plants,
ore processing plants and the like. In some embodiments, the carbon
dioxide may comprise other gases, e.g., nitrogen, oxides of
nitrogen (nitrous oxide, nitric oxide), sulfur and sulfur gases
(sulfur dioxide, hydrogen sulfide), and vaporized materials. In
some embodiments, the system includes a gas treatment system that
removes constituents in the carbon dioxide gas stream before the
gas is utilized in the cathode electrolyte. In some embodiments, a
portion of, or the entire amount of, cathode electrolyte comprising
bicarbonate ions and/or carbonate ions/and or hydroxide ions is
withdrawn from the system and is contacted with carbon dioxide gas
in an exogenous carbon dioxide gas/liquid contactor to increase the
absorbed carbon dioxide content in the solution. In some
embodiments, the solution enriched with carbon dioxide is returned
to the cathode compartment; in other embodiments, the solution
enriched with carbon dioxide is reacted with a solution comprising
divalent cations to produce divalent cation hydroxides, carbonates
and/or bicarbonates. In some embodiments, the pH of the cathode
electrolyte is adjusted upwards by hydroxide ions that migrate from
the cathode, and/or downwards by dissolving carbon dioxide gas in
the cathode electrolyte to produce carbonic acid and carbonic ions
that react with and remove hydroxide ions. Thus it can be
appreciated that the pH of the cathode electrolyte is determined,
at least in part, by the balance of these two processes.
[0018] Referring to FIG. 1 herein, the system 100 in one embodiment
comprises a gas diffusion anode 102 and a cathode 106 in contact
with a cathode electrolyte 108, 108A, 108B comprising dissolved
carbon dioxide 107A. The system in various embodiments includes a
gas delivery system 112 configured to deliver hydrogen gas to the
anode 102; in some embodiments, the hydrogen gas is obtained from
the cathode 106. In the system, the anode 102 is configured to
produce protons, and the cathode 106 is configured to produce
hydroxide ions and hydrogen gas when a low voltage 114, e.g., less
than 2V is applied across the anode and the cathode. In the system,
a gas is not produced at the anode 102.
[0019] In the system as illustrated in FIG. 1, first cation
exchange membrane 116 is positioned between the cathode electrolyte
108, 108A, 108B and a salt solution 118; and an anion exchange
membrane 120 is positioned between the salt solution 118 and the
anode electrolyte 104 in a configuration where the anode
electrolyte 104 is separated from the anode 102 by second cation
exchange membrane 122. In the system, the second cation exchange
membrane 122 is positioned between the anode 102 and the anode
electrolyte 104 such that anions may migrate from the salt solution
118 to the anode electrolyte 104 through the anion exchange
membrane 120; however, anions are prevented from contacting the
anode 102 by the second cation exchange membrane 122 adjacent to
the anode 102.
[0020] In various embodiments, the system is configurable to
migrate anions, e.g., chloride ions, from the salt solution 118 to
the anode electrolyte 104 through the anion exchange membrane 120;
migrate cations, e.g., sodium ions from the salt solution 118 to
the cathode electrolyte 108, 108A, 108B through the first cation
exchange membrane 116; migrate protons from the anode 102 to the
anode electrolyte 104; and migrate hydroxide ions from the cathode
106 to the cathode electrolyte 108, 108A, 108B. Thus, in various
embodiments, the system can be configured to produce sodium
hydroxide and/or sodium bicarbonate and/or sodium carbonate in the
cathode electrolyte 108, 108A, 108B; and produce an acid e.g.,
hydrochloric acid 124 in the anode electrolyte.
[0021] In various embodiments as illustrated in FIG. 1, the system
comprises a partition 126 that partitions the cathode electrolyte
108 into a first cathode electrolyte portion 108A and a second
cathode electrolyte portion 108B, wherein the second cathode
electrolyte portion 108B, comprising dissolved carbon dioxide,
contacts the cathode 106; and wherein the first cathode electrolyte
portion 108A comprising dissolved carbon dioxide and gaseous carbon
dioxide is in contact with the second cathode electrolyte portion
108B under the partition 126. In the system, the partition is
positioned in the cathode electrolyte such that gaseous carbon
dioxide in the first cathode electrolyte portion 108A is isolated
from cathode electrolyte in the second cathode electrolyte portion
108B.
[0022] Thus, as can be appreciated, in various embodiments, on
applying the present voltage across the anode and cathode, the
system can be configured to produce hydroxide ions and hydrogen gas
at the cathode 106; migrate hydroxide ions from the cathode into
the cathode electrolyte 108, 108B, 108A; migrate cations from the
salt solution 118 to the cathode electrolyte through the first
cation exchange membrane 116; migrate chloride ions from the salt
solution 118 to the anode electrolyte 104 through the anion
exchange membrane 120; and migrate protons from the anode 102 to
the anode electrolyte 104. Hence, depending on the salt solution
118 used, the system can be configured to produce a base solution,
e.g., sodium hydroxide in the cathode electrolyte.
[0023] In some embodiments, the system is operatively connected to
a carbon dioxide gas/liquid contactor 128 configured to remove
cathode electrolyte from the system and dissolve carbon dioxide in
the cathode electrolyte in the gas/liquid contactor before the
cathode electrolyte is returned to the system.
[0024] In other embodiments, the cathode electrolyte is operatively
connected to a system (not shown) that is configured to precipitate
divalent cation carbonates and/or divalent cation bicarbonates
and/or divalent cation hydroxides from a solution comprising carbon
dioxide gas and divalent cations.
[0025] FIG. 2 illustrates a schematic of a suitable gas diffusion
anode that can be used in the system. In various embodiments, the
gas diffusion anode comprises a conductive substrate 130 infused
with a catalyst 136 that is capable of catalyzing the oxidation of
hydrogen gas to protons when the present voltages are applied
across the anode and cathode. In some embodiments, the anode
comprises a first side that interfaces with hydrogen gas provided
to the anode, and an opposed second side 134 that interfaces with
the anode electrolyte 104. In some embodiments, the portion of the
substrate 132 that interfaces with the hydrogen gas is hydrophobic
and is relatively dry; and the portion of the substrate 134 that
interfaces with the anode electrolyte 104 is hydrophilic and may be
wet, which facilitates migration of protons from the anode to the
anode electrolyte. Preferably, the substrate 130 may be selected
such that an appropriate side is hydrophilic or hydrophobic as
described herein, as well as for a low ohmic resistance for
electron conduction from the anode, and good porosity for proton
migration to the anode electrolyte 116. In various embodiments, the
catalyst may comprise platinum, ruthenium, iridium, rhodium,
manganese, silver or alloys thereof. Suitable gas diffusion anodes
are available commercially, e.g., from E-TEK (USA) and other
suppliers
[0026] As is illustrated in FIG. 1, the system includes a salt
solution 118 located between the anode electrolyte 104 and the
cathode electrolyte 108, 108A, 108B. In various embodiments, the
cathode electrolyte is separated from the salt solution by a first
cation exchange membrane 116 that is allows migration of cations,
e.g., sodium ions, from the salt solution to the cathode
electrolyte. The first cation exchange membrane 116 is also capable
of blocking the migration of anions from the cathode electrolyte
108, 108A, 108B to the salt solution 118. In various embodiments,
the anode electrolyte 104 is separated from the salt solution 118
by an anion exchange membrane 108 that will allow migration of
anions, e.g., chloride ions, from the salt solution 118 to the
anode electrolyte 104. The anion exchange membrane, however, will
block the migration of cations, e.g., protons from the anode
electrolyte 104 to the salt solution 118.
[0027] With reference to FIGS. 1 and 2, the system includes a
hydrogen gas supply system 112 configured to provide hydrogen gas
to the anode 102. The hydrogen may be obtained from the cathode 106
or may be obtained from external source, e.g., from a commercial
hydrogen gas supplier, e.g., at start-up of the system when the
hydrogen supply from the cathode is insufficient. In the system,
the hydrogen gas is oxidized to protons and electrons; un-reacted
hydrogen gas is recovered and circulated 140 at the anode.
[0028] Referring to FIG. 1, in operation, the cathode electrolyte
108, 108A, 108B is initially charged with a base electrolyte, e.g.,
sodium hydroxide solution, and the anode electrolyte 104 is
initially charged with an acidic electrolyte, e.g., dilute
hydrochloric acid. The cathode electrolyte is also initially
charged with carbon dioxide gas 107A, 128, and hydrogen gas is
provided to the anode. In the system, on applying a voltage across
the anode and cathode, protons produced at the anode will enter
into the anode electrolyte and attempt to migrate from the anode
electrolyte 104 to the cathode 106 via the salt solution 118
between the cathode and anode. However, since the anion exchange
membrane will block the migration of protons to the salt solution,
the protons will accumulate in the anode electrolyte 104.
[0029] Simultaneously at the cathode 106, the voltage across the
anode and cathode will produce hydroxide ions and hydrogen gas at
the cathode. In some embodiments, the hydrogen produced at the
cathode is recovered and directed to the anode 102 where it is
oxidized to protons. In the system, hydroxide ions produced at the
cathode 106 will enter into the cathode electrolyte 108, 108A, 108B
from where they will attempt to migrate to the anode 102 via the
salt solution 118 between the cathode and anode. However, since the
cathode electrolyte 108, 108A, 108B is separated from the salt
solution electrolyte by the first cation exchange membrane 116
which will block the passage of anions, the first cation exchange
membrane will block the migration of hydroxide ions from the
cathode electrolyte to the salt solution; consequently, the
hydroxide ions will accumulate in the cathode electrolyte 108,
108A, 108B.
[0030] In the system as illustrated in FIG. 1, with the voltage
across the anode and cathode, since the salt solution is separated
from the cathode electrolyte by the first cation exchange membrane
116, cations in the salt solution, e.g., sodium ions, will migrate
through the first cation exchange membrane 116 to the cathode
electrolyte 108, 108A, 108B, and anions, e.g., chloride ions, will
migrate to the anode electrolyte through the anion exchange
membrane 120. Consequently, in the system, as illustrated in FIG.
1, an acid, e.g., hydrochloric acid 124 will be produced in the
anode electrolyte 104, and base solution, e.g., sodium hydroxide
will be produced in the cathode electrolyte. As can be appreciated,
with the migration of cations and anions from the salt solution,
the system in some embodiments can be configured to produce a
partly de-ionized salt solution from the salt solution 118. In
various embodiments, this partially de-ionized salt solution can be
used as feed-water to a desalination facility (not shown) where it
can be further processed to produce desalinated water as described
in commonly assigned U.S. patent application Ser. No. 12/163,205
filed on Jun. 27, 2008, herein incorporated by reference in its
entirety; alternatively, the solution can be used in industrial and
agricultural applications where its salinity is acceptable.
[0031] With reference to FIG. 1, the system in some embodiments
includes a second cation exchange membrane 124, attached to the
anode substrate 105, such that it separates the anode 102 from the
anode electrolyte. In this configuration, as the second cation
exchange membrane 122 is permeable to cations, protons formed at
the anode will migrate to the anode electrolyte as described
herein; however, as the second cation exchange membrane 122 is
impermeable to anions, anions, e.g., chloride ions, in the anode
electrolyte will be blocked from migrating to the anode 102,
thereby avoiding interaction between the anode and the anions that
may interact with the anode, e.g., by corrosion.
[0032] With reference to FIG. 1, in some embodiments, the system
includes a partition 128 configured into J-shape structure and
positioned in the cathode electrolyte 108, 108A, 108B to define an
upward-tapering channel 144 in the upper portion of the cathode
electrolyte compartment. The partition also defines a
downward-tapering channel 146 in lower portion of the cathode
electrolyte. Thus, with the partition in the place, the cathode
electrolyte 108 is partitioned into the first cathode electrolyte
portion 108A and a second cathode electrolyte portion 108B. As is
illustrated in FIG. 1, cathode electrolyte in the first cathode
electrolyte portion 108A is in contact with cathode electrolyte in
the second cathode electrolyte portion 108B; however, a gas in the
first electrolyte portion 108A, e.g., carbon dioxide, is prevented
from mixing with cathode electrolyte in the second cathode
electrolyte 108B.
[0033] With reference to FIG. 1, the system in various embodiments
includes a cathode electrolyte circulating system 142 adapted for
withdrawing and circulating cathode electrolyte in the system. In
one embodiment, the cathode electrolyte circulating system
comprises a carbon dioxide gas/liquid contactor 128 that is adapted
for dissolving carbon dioxide in the circulating cathode
electrolyte, and for circulating the electrolyte in the system. As
can be appreciated, since the pH of the cathode electrolyte can be
adjusted by withdrawing and/or circulating cathode electrolyte from
the system, the pH of the cathode electrolyte compartment can be by
regulated by regulating an amount of cathode electrolyte removed
from the system through the carbon dioxide gas/liquid contactor
128.
[0034] In an alternative as illustrated in FIG. 4, the system
comprises a cathode 106 in contact with a cathode electrolyte 108
and an anode 102 in contact with an anode electrolyte 104. In this
system, the cathode electrolyte comprises a salt solution that
functions as the cathode electrolyte as well as a source of
chloride and sodium ions for the base and acid solution produced in
the system. In this system, the cathode electrolyte is separated
from the anode electrolyte by an anion exchange membrane 120 that
allows migration of anions, e.g., chloride ions, from the salt
solution to the anode electrolyte. As is illustrated in FIG. 4, the
system includes a hydrogen gas delivery system 112 configured to
provide hydrogen gas to the anode. The hydrogen may be obtained
from the cathode and/or obtained from an external source, e.g., a
commercial hydrogen gas supplier e.g., at start-up of operations
when the hydrogen supply from the cathode is insufficient. In
various embodiments, the hydrogen delivery system is configured to
deliver gas to the anode where oxidation of the gas is catalyzed to
protons and electrons. In some embodiments, un-reacted hydrogen gas
in the system is recovered and re-circulated to the anode.
[0035] Referring to FIG. 4, as with the system of FIG. 1, on
applying a voltage across the anode and cathode, protons produced
at the anode from oxidation of hydrogen will enter into the anode
electrolyte from where they will attempt to migrate to the cathode
electrolyte across the anion exchange membrane 120. However, since
the anion exchange membrane 120 will block the passage of cations,
the protons will accumulate in the anode electrolyte. At the same
time, however, the anion exchange membrane 120 being pervious to
anions will allow the migration of anions, e.g., chloride ions from
the cathode electrolyte to the anode, thus in this embodiment,
chloride ions will migrate to the anode electrolyte to produce
hydrochloric acid in the anode electrolyte. In this system, the
voltage across the anode and cathode is adjusted to a level such
that hydroxide ions and hydrogen gas are produced at the cathode
without producing a gas, e.g., chlorine or oxygen, at the anode. In
this system, since cations will not migrate from the cathode
electrolyte across the anion exchange membrane 116, sodium ions
will accumulate in the cathode electrolyte 108 to produce a base
solution with hydroxide ions produced at the cathode. In
embodiments where carbon dioxide gas is dissolved in the cathode
electrolyte, sodium ions may also produce sodium bicarbonate and or
sodium carbonate in the cathode electrolyte as described herein
with reference to FIG. 1.
[0036] With reference to FIG. 1, depending on the pH of the cathode
electrolyte, carbon dioxide gas introduced into the first cathode
electrolyte portion 108A will dissolve in the cathode electrolyte
and reversibly dissociate and equilibrate to produce carbonic acid,
protons, carbonate and/or bicarbonate ions in the first cathode
electrolyte compartment as follows:
CO.sub.2+H.sub.2O<==>H.sub.2CO.sub.3<==>H.sup.++HCO.sub.3.su-
p.-<==>H.sup.++CO.sub.3.sup.2-
In the system, as cathode electrolyte in the first cathode
electrolyte portion 108A may mix with second cathode electrolyte
portion 108B, the carbonic acid, bicarbonate and carbonate ions
formed in the first cathode electrolyte portion 108A by absorption
of carbon dioxide in the cathode electrolyte may migrate and
equilibrate with cathode electrolyte in the second cathode
electrolyte portion 108B. Thus, in various embodiments, first
cathode electrolyte portion 108A may comprise dissolved and
un-dissolved carbon dioxide gas, and/or carbonic acid, and/ or
bicarbonate ions and/or carbonate ions; while second cathode
electrolyte portion 108B may comprise dissolved carbon dioxide,
and/or carbonic acid, and/ or bicarbonate ions and/or carbonate
ions.
[0037] With reference to FIG. 1, on applying a voltage across anode
102 and cathode 108, the system 100 may produce hydroxide ions and
hydrogen gas at the cathode from water, as follows:
2H.sub.2O+2e.sup.-=H.sub.2+2OH.sup.-
As cathode electrolyte in first cathode electrolyte portion 108A
can intermix with cathode electrolyte in second cathode electrolyte
portion 108B, hydroxide ions formed in the second cathode
electrolyte portion may migrate and equilibrate with carbonate and
bicarbonate ions in the first cathode electrolyte portion 108A.
Thus, in various embodiments, the cathode electrolyte in the system
may comprise hydroxide ions and dissolved and/or un-dissolved
carbon dioxide gas, and/or carbonic acid, and/ or bicarbonate ions
and/or carbonate ions. In the system, as the solubility of carbon
dioxide and the concentration of bicarbonate and carbonate ions in
the cathode electrolyte are dependent on the pH of the electrolyte,
the overall reaction in the cathode electrolyte 104 is either:
2H.sub.2O+2CO.sub.2+2e.sup.-=H.sub.2+2HCO.sub.3.sup.- Scenario
1
or
H.sub.2O+CO.sub.2+2e.sup.-=H.sub.2+CO.sub.3.sup.2- Scenario 2
or a combination of both, depending on the pH of the cathode
electrolyte. This is illustrated in the carbonate speciation
diagram of FIG. 5:
[0038] For either scenario, the overall cell potential of the
system can be determined through the Gibbs energy change of the
reaction by the formula:
E.sub.cell=.DELTA.G/nF
Or, at standard temperature and pressure conditions:
E.degree..sub.cell=-.DELTA.G.degree./nF
where, E.sub.cell is the cell voltage, .DELTA.G is the Gibbs energy
of reaction, n is the number of electrons transferred, and F is the
Faraday constant (96485 JNmol). The E.sub.cell of each of these
reactions is pH dependent based on the Nernst equestion as
demonstrated in FIG. 6 for Scenario 1, as discussed below.
[0039] Also, for either scenario, the overall cell potential can be
determined through the combination of Nernst equations for each
half cell reaction:
E=E.degree.-RT In(Q)/n F
where, E.degree. is the standard reduction potential, R is the
universal gas constant, (8.314 J/mol K) T is the absolute
temperature, n is the number of electrons involved in the half cell
reaction, F is Faraday's constant (96485 JN mol), and Q is the
reaction quotient such that:
E.sub.total=E.sub.cathode+E.sub.anode.
When hydrogen is oxidized to protons at the anode as follows:
H.sub.2=2H.sup.++2e.sup.-,
E.degree. is 0.00 V, n is 2, and Q is the square of the activity of
H.sup.-so that:
E.sub.anode=+0.059pH.sub.a,
where pH.sub.a is the pH of the anode electrolyte. When water is
reduced to hydroxide ions and hydrogen gas at the cathode as
follows:
2H.sub.2O+2e.sup.-=H.sub.2+2OH.sup.-,
E.degree. is -0.83 V, n is 2, and Q is the square of the activity
of OH.sup.-so that:
E.sub.cathode=-0.059pH.sub.C,
where pH.sub.C is the pH of the cathode electrolyte.
[0040] For either Scenario, the E for the cathode and anode
reactions varies with the pH of the anode and cathode electrolytes.
Thus, for Scenario 1 if the anode reaction, which is occurring in
an acidic environment, is at a pH of 0, then the E of the reaction
is 0V for the half cell reaction. For the cathode reaction, if the
generation of bicarbonate ions occur at a pH of 7, then the
theoretical E is 7.times.(-0.059 V)=-0.413V for the half cell
reaction where a negative E means energy is needed to be input into
the half cell or full cell for the reaction to proceed. Thus, if
the anode pH is 0 and the cathode pH is 7 then the overall cell
potential would be -0.413V, where:
E.sub.total=-0.059 (pH.sub.a-pH.sub.C)=-0.059.DELTA.pH.
[0041] For Scenario 2 in which carbonate ions are produced, if the
anode pH is 0 and the cathode pH is 10, this would represent an E
of 0.59 V.
[0042] Thus, in various embodiments, directing CO.sub.2 gas into
the cathode electrolyte may lower the pH of the cathode electrolyte
by producing bicarbonate ions and/or carbonate ions in the cathode
electrolyte, which consequently may lower the voltage across the
anode and cathode in producing hydroxide, carbonate and/or
bicarbonate in the cathode electrolyte.
[0043] Thus, as can be appreciated, if the cathode electrolyte is
allowed to increase to a pH of 14 or greater, the difference
between the anode half-cell potential (represented as the thin
dashed horizontal line, Scenario 1, above) and the cathode half
cell potential (represented as the thick solid sloping line in
Scenario 1, above) will increase to 0.83V. With increased duration
of cell operation without CO.sub.2 addition or other intervention,
e.g., diluting with water, the required cell potential will
continue to increase. The cell potential may also increase due to
ohmic resistance loses across the membranes in the electrolyte and
the cell's overvoltage potential.
[0044] Herein, an overvoltage potential refers to the voltage
difference between a thermodynamically determined half-cell
reduction potential, and the experimentally observed potential at
which the redox reaction occurs. The term is related to a cell
voltage efficiency as the overvoltage potential requires more
energy than is thermodynamically required to drive a reaction. In
each case, the extra energy is lost as heat. Overvoltage potential
is specific to each cell design and will vary between cells and
operational conditions even for the same reaction.
[0045] In embodiments wherein it is desired to produce bicarbonate
and/or carbonate ions in the cathode electrolyte, the system as
illustrated in FIGS. 1-2, and as described above with reference to
production of hydroxide ions in the cathode electrolyte, can be
configured to produce bicarbonate ions and/or carbonate ions in the
first cathode electrolyte by dissolving carbon dioxide in the first
cathode electrolyte and applying a voltage of less than 3V, or less
than 2.5 V, or less than 2V, or less than 1.5V such as less than
1.0V, or even less than 0.8 V or 0.6V across the cathode and
anode.
[0046] In various embodiments, hydroxide ions, carbonate ions
and/or bicarbonate ions produced in the cathode electrolyte, and
hydrochloric acid produced in the anode electrolyte are removed
from the system, while sodium chloride in the salt solution
electrolyte is replenished to maintain continuous operation of the
system. As can be appreciated, in various embodiments, the system
can be configured to operate in various production modes including
batch mode, semi-batch mode, continuous flow mode, with or without
the option to withdraw portions of the hydroxide solution produced
in the cathode electrolyte, or withdraw all or a portions of the
acid produced in the anode electrolyte, or direct the hydrogen gas
produced at the cathode to the anode where it may be oxidized.
[0047] In various embodiments, hydroxide ions, bicarbonate ions
and/or carbonate ion solutions are produced in the cathode
electrolyte when the voltage applied across the anode and cathode
is less than 3V, 2.9V or less, 2.8V or less, 2.7V or less, 2.6V or
less, 2.5V or less, 2.4V or less, 2.3V or less, 2.2V or less, 2.1V
or less, 2.0V or less, 1.9V or less, 1.8V or less, 1.7V or less,
1.6V, or less 1.5V or less, 1.4V or less, 1.3V or less, 1.2V or
less, 1.1V or less, 1.0V or less, 0.9V or less or less, 0.8V or
less, 0.7V or less, 0.6V or less, 0.5V or less, 0.4V or less, 0.3V
or less, 0.2V or less, or 0.1 V or less.
[0048] In another embodiment, the voltage across the anode and
cathode can be adjusted such that gas will form at the anode, e.g.,
oxygen or chlorine, while hydroxide ions, carbonate ions and
bicarbonate ions are produced in the cathode electrolyte and
hydrogen gas is generated at the cathode. However, in this
embodiment, hydrogen gas is not supplied to the anode. As can be
appreciated by one ordinarily skilled in the art, in this
embodiment, the voltage across the anode and cathode will be higher
compared to the embodiment when a gas does not form at the
anode.
[0049] With reference to FIGS. 1-2, in various embodiments, the
invention provides for a system comprising one or more anion
exchange membrane 120, and cation exchange membranes 116, 122
located between the gas diffusion anode 102 and the cathode 106. In
various embodiments, the membranes should be selected such that
they can function in an acidic and/or basic electrolytic solution
as appropriate. Other desirable characteristics of the membranes
include high ion selectivity, low ionic resistance, high burst
strength, and high stability in an acidic electrolytic solution in
a temperature range of 0.degree. C. to 100.degree. C. or higher, or
a base solution in similar temperature range may be used. In some
embodiments, a membrane that is stable in the range of 0.degree. C.
to 80.degree. C., or 0.degree. C. to 90.degree. C., but not stable
above these ranges may be used. For other embodiments, it may be
useful to utilize an ion-specific ion exchange membranes that
allows migration of one type of cation but not another; or
migration of one type of anion and not another, to achieve a
desired product or products in an electrolyte. In some embodiments,
the membrane should be stable and functional for a desirable length
of time in the system, e.g., several days, weeks or months or years
at temperatures in the range of 0.degree. C. to 80.degree. C., or
0.degree. C. to 90.degree. C. and higher and/or lower. In some
embodiments, for example, the membranes should be stable and
functional for at least 5 days, 10 days, 15 days, 20 days, 100
days, 1000 days or more in electrolyte temperatures at 80.degree.
C., 70.degree. C., 60.degree. C., 50.degree. C., 40.degree. C.,
30.degree. C., 20.degree. C., 10.degree. C., 5.degree. C. and more
or less.
[0050] As can be appreciated, the ohmic resistance of the membranes
will affect the voltage drop across the anode and cathode, e.g., as
the ohmic resistance of the membranes increase, the voltage drop
across the anode and cathode will increase, and vice versa.
Membranes currently available can be used and they include
membranes with relatively ohmic resistance and relatively high
ionic mobility; similarly, membranes currently available with
relatively high hydration characteristics that increases with
temperatures, and thus decreasing the ohmic resistance can be used.
Consequently, as can be appreciated, by selecting currently
available membranes with lower ohmic resistance, the voltage drop
across the anode and cathode at a specified temperature can be
lowered.
[0051] Scattered through currently available membrane are ionic
channels consisting of acid groups. These ionic channels may extend
from the internal surface of the matrix to the external surface and
the acid groups may readily bind water in a reversible reaction as
water-of-hydration. This binding of water as water-of-hydration
follows first order reaction kinetics, such that the rate of
reaction is proportional to temperature. Consequently, currently
available membranes can be selected to provide a relatively low
ohmic and ionic resistance while providing for improved strength
and resistance in the system for a range of operating temperatures.
Suitable membranes are commercially available from Asahi Kasei of
Tokyo, Japan; or from Membrane International of Glen Rock, N.J.,
and USA.
[0052] In various embodiments, the cathode electrolyte 108, 108A,
108B is operatively connected to a waste gas treatment system (not
illustrated) where the base solution produced in the cathode
electrolyte is utilized, e.g., to sequester carbon dioxide
contained in the waste gas by contacting the waste gas and the
cathode electrolyte with a solution of divalent cations to
precipitate hydroxides, carbonates and/or bicarbonates as described
in commonly assigned U.S. patent application Ser. No. 12/344,019
filed on Dec. 24, 2008, herein incorporated by reference in its
entirety. The precipitates, comprising, e.g., calcium and magnesium
hydroxides, carbonates and bicarbonates in various embodiments may
be utilized as building materials, e.g., as cements and aggregates,
as described in commonly assigned U.S. patent application Ser. No.
12/126,776 filed on May 23, 2008, supra, herein incorporated by
reference in its entirety. In some embodiments, some or all of the
carbonates and/or bicarbonates are allowed to remain in an aqueous
medium, e.g., a slurry or a suspension, and are disposed of in an
aqueous medium, e.g., in the ocean depths or a subterranean
site.
[0053] In various embodiments, the cathode and anode are also
operatively connected to an off-peak electrical power-supply system
114 that supplies off-peak voltage to the electrodes. Since the
cost of off-peak power is lower than the cost of power supplied
during peak power-supply times, the system can utilize off-peak
power to produce a base solution in the cathode electrolyte at a
relatively lower cost.
[0054] In another embodiment, the system produces an acid, e.g.,
hydrochloric acid 124 in the anode electrolyte 104. In various
embodiments, the anode compartment is operably connected to a
system for dissolving minerals and/or waste materials comprising
divalent cations to produce a solution of divalent cations, e.g.,
Ca++ and Mg++. In various embodiments, the divalent cation solution
is utilized to precipitate hydroxides, carbonates and/or
bicarbonates by contacting the divalent cation solution with the
present base solution and a source of carbon dioxide gas as
described in U.S. patent application Ser. No. 12/344,019 filed on
Dec. 24, 2008, supra, herein incorporated by reference in its
entirety. In various embodiments, the precipitates are used as
building materials e.g., cement and aggregates as described in
commonly assigned U.S. patent application Ser. No. 12/126,776,
supra, herein incorporated by reference in its entirety.
[0055] With reference to FIG. 1, on applying a voltage across the
anode 102 and cathode 106, protons will form at the anode from
oxidation of hydrogen gas supplied to the anode, while hydroxide
ions and hydrogen gas will form at the cathode electrolyte from the
reduction of water, as follows:
H.sub.2=2H.sup.++2e.sup.- (anode, oxidation reaction)
2H.sub.2O+2e.sup.-=H.sub.2+2OH.sup.- (cathode, reduction
reaction)
[0056] Since protons are formed at the anode from hydrogen gas
provided to the anode; and since a gas such as oxygen does not form
at the anode; and since water in the cathode electrolyte forms
hydroxide ions and hydrogen gas at the cathode, the system will
produce hydroxide ions in the cathode electrolyte and protons in
the anode electrolyte when a voltage is applied across the anode
and cathode. Further, as can be appreciated, in the present system
since a gas does not form at the anode, the system will produce
hydroxide ions in the cathode electrolyte and hydrogen gas at the
cathode and hydrogen ions at the anode when less than 2V is applied
across the anode and cathode, in contrast to the higher voltage
that is required when a gas is generated at the anode, e.g.,
chlorine or oxygen. For example, in various embodiments, hydroxide
ions are produced when less than 2.0V, 1.5V, 1.4V, 1.3V, 1.2V,
1.1V, 1.0V, 0.9V, 0.8V, 0.7V, 0.6V, 0.5V, 0.4V, 0.3V, 0.2V, 0.1 V
or less is applied across the anode and cathode.
[0057] As discussed above, in the system, on applying a voltage
across the anode 102 and cathode 106, the positively charged
protons formed at the anode will attempt to migrate to the cathode
through the anode electrolyte 104, while the negatively charged
hydroxide ions formed at the cathode will attempt to migrate to the
anode through the cathode electrolyte 108, 108A, 108B. As is
illustrated in FIG. 1 and with reference to hydroxide ions in the
cathode electrolyte 108, 108A, 108B, since the first cation
exchange membrane 116 will restrict the migration of anions from
the cathode electrolyte 108, 108A, 108B, and since the anion
exchange membrane 120 will prevent the migration of anions from the
anode electrolyte 104 to the salt solution 118, the hydroxide ions
generated in the cathode electrolyte will be prevented from
migrating out of the cathode electrolyte through the cation
exchange membrane. Consequently, on applying the voltage across the
anode and cathode, the hydroxide ions produced at the cathode will
be contained in the cathode electrolyte. Thus, depending on the
flow rate of fluids into and out of the cathode electrolyte and the
rate of carbon dioxide dissolution in the cathode electrolyte, the
pH of the cathode electrolyte will adjust, e.g., the pH may
increase, decrease or remain the same.
[0058] In various embodiments, depending on the ionic species
desired in cathode electrolyte 108, 108A, 108B and/or the anode
electolyte 104 and/or the salt solution 118, alternative reactants
can be utilized. Thus, for example, if a potassium salt such as
potassium hydroxide or potassium carbonate is desired in the
cathode electrolyte 1108, 108A, 108B, then a potassium salt such as
potassium chloride can be utilized in the salt solution 118.
Similarly, if sulfuric acid is desired in the anode electrolyte,
then a sulfate such as sodium sulfate can be utilized in the salt
solution 118. As described in various embodiments herein, carbon
dioxide gas is absorbed in the cathode electrolyte; however, it
will be appreciated that other gases, including volatile vapors,
can be absorbed in the electrolyte, e.g., sulfur dioxide, or
organic vapors to produce a desired result. As can be appreciated,
the gas can be added to the electrolyte in various ways, e.g., by
bubbling it directly into the electrolyte, or dissolving the gas in
a separate compartment connected to the cathode compartment and
then directed to the cathode electrolyte as described herein.
[0059] With reference to FIGS. 1 and 3, method 300 in various
embodiments comprises a step 302 of applying a voltage across a
cathode 106 and a gas diffusion anode 102 in an electrochemical
system 100, wherein the cathode contacts a cathode electrolyte
comprising dissolved carbon dioxide. In some embodiments, the
method includes a step of providing hydrogen to the gas diffusion
anode 102; a step of contacting the cathode 106 with a cathode
electrolyte 108, 108A, 108B comprising dissolved carbon dioxide gas
107A; and a step of applying a voltage 114 across the anode and
cathode; a step whereby protons are produced at the anode and
hydroxide ions and hydrogen gas produced at the cathode; a step
whereby a gas is not produced at the anode when the voltage is
applied across the anode and cathode; a step wherein the voltage
applied across the anode and cathode is less than 2V; a step
comprising directing hydrogen gas from the cathode to the anode; a
step comprising whereby protons are migrated from the anode to an
anode electrolyte; a step comprising interposing an anion exchange
membrane between the anode electrolyte and the salt solution; a
step comprising interposing a first cation exchange membrane
between the cathode electrolyte and the salt solution, wherein the
salt solution is contained between the anion exchange membrane and
the first cation exchange membrane; a step comprising whereby
anions migrate from the salt solution to the anode electrolyte
through the anion exchange membrane, and cations migrate from the
salt solution to the cathode electrolyte through the first cation
exchange membrane; a step comprising producing hydroxide ions
and/or carbonate ions and/or bicarbonate ions in the cathode
electrolyte; a step comprising producing an acid in the anode
electrolyte; a step comprising producing sodium hydroxide and/or
sodium carbonate and/or sodium bicarbonate in the cathode
electrolyte; a step whereby hydrochloric acid is produced in the
anode electrolyte; a step comprising contacting the cathode
electrolyte with a divalent cation solution, wherein the divalent
cations comprise calcium and magnesium ions; a step comprising
producing partially desalinated water from the salt solution; a
step comprising withdrawing a first portion of the cathode
electrolyte and contacting the first portion of cathode electrolyte
with carbon dioxide; and a step comprising contacting the first
portion of cathode electrolyte with a divalent cation solution.
[0060] In various embodiments, hydroxide ions are formed at the
cathode 106 and in the cathode electrolyte 108, 108A, 108B by
applying a voltage of less than 2V across the anode and cathode
without forming a gas at the anode, while providing hydrogen gas at
the anode for oxidation at the anode. In various embodiments,
method 300 does not form a gas at the anode when the voltage
applied across the anode and cathode is less than 3V or less, 2.9V
or less, 2.8V or less, 2.7V or less, 2.6V or less, 2.5V or less,
2.4V or less, 2.3V or less, 2.2V or less, 2.1V or less, 2.0V or
less, 1.9V or less, 1.8V or less, 1.7V or less, 1.6V or less, 1.5V
or less, 1.4V or less, 1.3V or less, 1.2V or less, 1.1V or less,
1.0V or less, 0.9V or less, 0.8V or less, 0.7V or less, 0.6V or
less, 0.5V or less, 0.4V or less, 0.3V or less, 0.2V or less, or
0.1 V or less, while hydrogen gas is provided to the anode where it
is oxidized to protons. As will be appreciated by one ordinarily
skilled in the art, by not forming a gas at the anode and by
providing hydrogen gas to the anode for oxidation at the anode, and
by otherwise controlling the resistance in the system for example
by decreasing the electrolyte path lengths and by selecting ionic
membranes with low resistance and any other method know in the art,
hydroxide ions can be produced in the cathode electrolyte with the
present lower voltages.
[0061] In various embodiments, hydroxide ions, bicarbonate ions and
carbonate ions are produced in the cathode electrolyte where the
voltage applied across the anode and cathode is less than 3.0V,
2.9V, 2.8V, 2.7V, 2.6V, 2.5V, 2.4V, 2.3V, 2.2V, 2.1V, 2.0V, 1.9V,
1.8V, 1.7V, 1.6V, 1.5V, 1.4V, 1.3V, 1.2V, 1.1V, 1.0V, 0.9V, 0.8V,
0.7V, 0.6V, 0.5V, 0.4V, 0.3V, 0.2V, 0.1V or less without forming a
gas at the anode. In various embodiments, the method is adapted to
withdraw and replenish at least a portion of the cathode
electrolyte and the acid in the anode electrolyte back into the
system in either a batch, semi-batch or continuous mode of
operation.
[0062] In an exemplarary embodiment, a system configured
substantially as illustrated in FIGS. 1 and 2 was operated with a
constant current density applied across the electrodes at steady
state conditions while carbon dioxide gas was continuously
dissolved into the cathode electrolyte, at various temperatures and
voltages. In the system, a platinum catalyst, gas diffusion anode
obtained from E-TEK Corporation, (USA) was used as the anode. A
Raney nickel deposited onto a nickel gauze substrate was used as
the cathode. In the system, the initial acid concentration in the
anode electrolyte was 1 M; the initial sodium chloride salt
solution was 5 M; and the initial concentration of the sodium
hydroxide solution in the cathode compartment was 1 M. In the
system, the pH of the cathode compartment was maintained at either
8 or 10 by regulating the amount of carbon dioxide dissolved in the
cathode electrolyte.
TABLE-US-00001 TABLE 1 Experimental Current Density, Temperature
and Voltage Characteristics of the System T Potential Current
density (.degree. C.) (V) pH (mA/cm.sup.2) 25 0.8 10 8.6 8 11.2 1.2
10 28.3 8 29.2 1.6 10 50.2 8 50.6 75 0.8 10 13.3 8 17.8 1.2 10 45.3
8 49.8 1.6 10 80.8 8 84.7
[0063] As is illustrated in Table 1, a range of current densities
was achieved across the electrode in the system. As will be
appreciated by one ordinarily skilled in the art, the current
density that can be achieved with other configurations of the
system may vary, depending on several factors including the
cumulative electrical resistance losses in the cell, environmental
test conditions, the over-potential associated with the anodic and
cathodic reactions, and other factors.
[0064] It will also be appreciated that the current densities
achieved in the present configuration and as set forth in Table 1
are correlated with the production of hydroxide ions at the
cathode, and thus are correlated with the production of sodium
hydroxide and/or sodium carbonate and/or sodium bicarbonate in the
cathode electrolyte, as follows. With reference to Table 1, at
75.degree. C., 0.8 V and a pH of 10, each cm.sup.2 of electrode
passed 13.3 mA of current, where current is a measure of charge
passed (Coulomb) per time (second). Based on Faraday's Laws, the
amount of product, e.g., hydroxide ions, produced at an electrode
is proportional to the total electrical charge passed through the
electrode as follows:
n=(I*t)/(F*z)
where n is moles of product, I is a current, t is time, F is
Faraday's constant, and z is the electrons transferred per product
ionic species (or reagent ionic species). Thus, based on the
present example, 1.38.times.10.sup.-4 moles of hydroxide ions are
produced per second per cm.sup.2 of electrode, which is correlated
with the production of sodium hydroxide in the cathode electrolyte.
In the system the production rate of NaOH dictates the production
rate of NaHCO.sub.3 and Na.sub.2CO.sub.3 through Le Chatelier's
principle following the net chemical equilibria equations of
H.sub.2CO.sub.3+OH.sup.-=H.sub.2O+HCO.sub.3.sup.-
and
HCO.sub.3.sup.-+OH.sup.-=H.sub.2O+CO.sub.3.sup.2-,
where an increase in concentration of one species in equilibria
will change the concentration of all species so that the
equilibrium product maintains the equilibrium constant. Thus, in
the system, the equilibrium concentrations of H.sub.2CO.sub.3,
HCO.sub.3.sup.-, and CO.sub.3.sup.2-vs. pH in the electrolyte will
follow the carbonate speciation diagram as discussed above.
[0065] In the system as illustrated in FIG. 1 and as discussed with
reference to the carbonate speciation graph, supra, the solubility
of carbon dioxide in the cathode electrolyte is dependent on the pH
of the electrolyte. Also in the system, the voltage across the
cathode and anode is dependent on several factors including the pH
difference between the anode electrolyte and cathode electrolyte.
Thus, in some embodiments the system can be configured to operate
at a specified pH and voltage to absorb carbon dioxide and produce
carbonic acid, carbonate ions and/or bicarbonate ions in the
cathode electrolyte. In embodiments where carbon dioxide gas is
dissolved in the cathode electrolyte, as protons are removed from
the cathode electrolyte more carbon dioxide may be dissolved to
form carbonic acid, bicarbonate ions and/or carbonate ions.
Depending on the pH of the cathode electrolyte the balance is
shifted toward bicarbonate ions or toward carbonate ions, as is
well understood in the art and as is illustrated in the carbonate
speciation diagram, above. In these embodiments the pH of the
cathode electrolyte solution may decrease, remain the same, or
increase, depending on the rate of removal of protons compared to
rate of introduction of carbon dioxide. It will be appreciated that
no carbonic acid, hydroxide ions, carbonate ions or bicarbonate
ions are formed in these embodiments, or that carbonic acid,
hydroxide ions, carbonate ions, bicarbonate ions may not form
during one period but form during another period.
[0066] In another embodiment, the present system and method are
integrated with a carbonate and/or bicarbonate precipitation system
(not illustrated) wherein a solution of divalent cations, when
added to the present cathode electrolyte, causes formation of
precipitates of divalent carbonate and/or bicarbonate compounds,
e.g., calcium carbonate or magnesium carbonate and/or their
bicarbonates. In various embodiments, the precipitated divalent
carbonate and/or bicarbonate compounds may be utilized as building
materials, e.g., cements and aggregates as described for example in
commonly assigned U.S. patent application Ser. No. 12/126,776 filed
on May 23, 2008, herein incorporated by reference in its
entirety.
[0067] In an alternative embodiment, the present system and method
are integrated with a mineral and/or material dissolution and
recovery system (not illustrated) wherein the acidic anode
electrolyte solution 104 or the basic cathode electrolyte 108 is
utilized to dissolve calcium and/or magnesium-rich minerals e.g.,
serpentine or olivine, or waste materials, e.g., fly ash, red mud
and the like, to form divalent cation solutions that may be
utilized, e.g., to precipitate carbonates and/or bicarbonates as
described herein. In various embodiments, the precipitated divalent
carbonate and/or bicarbonate compounds may be utilized as building
materials, e.g., cements and aggregates as described for example in
commonly assigned U.S. patent application Ser. No. 12/126,776 filed
on May 23, 2008, herein incorporated by reference in its
entirety.
[0068] In an alternative embodiment, the present system and method
are integrated with an industrial waste gas treatment system (not
illustrated) for sequestering carbon dioxide and other constituents
of industrial waste gases, e.g., sulfur gases, nitrogen oxide
gases, metal and particulates, wherein by contacting the flue gas
with a solution comprising divalent cations and the present cathode
electrolyte comprising hydroxide, bicarbonate and/or carbonate
ions, divalent cation carbonates and/or bicarbonates are
precipitated as described in commonly assigned U.S. patent
application Ser. No. 12/344,019 filed on Dec. 24, 2008, herein
incorporated by reference in its entirety. The precipitates,
comprising, e.g., calcium and/or magnesium carbonates and
bicarbonates in various embodiments may be utilized as building
materials, e.g., as cements and aggregates, as described in
commonly assigned U.S. patent application Ser. No. 12/126,776 filed
on May 23, 2008, herein incorporated by reference in its
entirety.
[0069] In another embodiment, the present system and method are
integrated with an aqueous desalination system (not illustrated)
wherein the partially desalinated water of the third electrolyte of
the present system is used as feed-water for the desalination
system, as described in commonly assigned U.S. patent application
Ser. No. 12/163,205 filed on Jun. 27, 2008, herein incorporated by
reference in its entirety.
[0070] In an alternative embodiment, the present system and method
are integrated with a carbonate and/or bicarbonate solution
disposal system (not illustrated) wherein, rather than producing
precipitates by contacting a solution of divalent cations with the
first electrolyte solution to form precipitates, the system
produces a solution, slurry or suspension comprising carbonates
and/or bicarbonates. In various embodiments, the solution, slurry
or suspension is disposed of in a location where it is held stable
for an extended periods of time, e.g., the
solution/slurry/suspension is disposed in an ocean at a depth where
the temperature and pressure are sufficient to keep the slurry
stable indefinitely, as described in U.S. patent application Ser.
No. 12/344,019 filed on Dec. 24, 2008, herein incorporated by
reference in its entirety; or in a subterranean site.
* * * * *