U.S. patent application number 14/829981 was filed with the patent office on 2015-12-10 for electrochemical process and apparatus therefor.
This patent application is currently assigned to DET NORSKE VERITAS (U.S.A.), INC.. The applicant listed for this patent is Arun S. Agarwal, Shan Guan, Narasi Sridhar. Invention is credited to Arun S. Agarwal, Shan Guan, Narasi Sridhar, Yumei Zhai.
Application Number | 20150354070 14/829981 |
Document ID | / |
Family ID | 45874391 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150354070 |
Kind Code |
A1 |
Zhai; Yumei ; et
al. |
December 10, 2015 |
Electrochemical Process and Apparatus Therefor
Abstract
A process and apparatus are provided for the electrochemical
reduction of carbon dioxide to formate. According one embodiment,
an electrochemical process includes the catalytic electro-chemical
reduction of carbon dioxide to formate utilizing a cathodic
catalyst comprising tin and zinc, the zinc comprising between three
and six weight percent of the catalyst. In a further embodiment, an
electrochemical reactor for the electrochemical reduction of carbon
dioxide to formate comprises a cathodic catalyst comprising tin and
zinc, the zinc comprising between three and six weight percent of
the catalyst.
Inventors: |
Zhai; Yumei; (Dublin,
OH) ; Guan; Shan; (Dublin, OH) ; Sridhar;
Narasi; (Dublin, OH) ; Agarwal; Arun S.;
(Dublin, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Guan; Shan
Sridhar; Narasi
Agarwal; Arun S. |
Dublin
Dublin
Dublin |
OH
OH
OH |
US
US
US |
|
|
Assignee: |
DET NORSKE VERITAS (U.S.A.),
INC.
Katy
TX
|
Family ID: |
45874391 |
Appl. No.: |
14/829981 |
Filed: |
August 19, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13825822 |
Mar 24, 2013 |
9145615 |
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PCT/US2011/052820 |
Sep 22, 2011 |
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14829981 |
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61386121 |
Sep 24, 2010 |
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61414932 |
Nov 18, 2010 |
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Current U.S.
Class: |
205/441 ;
204/265 |
Current CPC
Class: |
Y02E 60/366 20130101;
C25B 11/035 20130101; C25B 15/08 20130101; C25B 9/08 20130101; C25B
11/0405 20130101; Y02P 20/142 20151101; C25B 11/0478 20130101; C25B
11/0447 20130101; C25B 15/02 20130101; Y02P 20/141 20151101; C25B
3/04 20130101; C25B 11/0484 20130101 |
International
Class: |
C25B 3/04 20060101
C25B003/04; C25B 11/04 20060101 C25B011/04; C25B 9/08 20060101
C25B009/08 |
Claims
1. A process, comprising: (a) introducing an anolyte into an
anolyte compartment of an electrochemical reactor, the anolyte
compartment at least partially containing an anode; (b) introducing
a catholyte into a catholyte compartment of the electrochemical
reactor, the catholyte compartment separated from the anolyte
compartment by a membrane, the catholyte compartment at least
partially containing a cathode, the cathode comprising a cathodic
catalyst, the catalyst comprising: tin; and zinc, wherein the zinc
comprises between three weight percent and six weight percent of
the cathodic catalyst; (c) introducing carbon dioxide into the
catholyte compartment of the electrochemical reactor; and (d)
impressing a DC voltage across the anode and the cathode, whereby
at least a portion of the carbon dioxide is converted to
formate.
2. The process of claim 1, the conversion of carbon dioxide to
formate accomplished at a Faradaic Efficiency of between 65 percent
and 68 percent.
3. The process of claim 2, the Faradaic Efficiency accomplished at
a potential of tween -1.70 V.sub.SCE and -1.90 V.sub.SCE.
4. The process of claim 1, wherein the catholyte is formulated for
the electrochemical reduction of carbon dioxide to formate.
5. The process of claim 4, wherein the catholyte comprises a
solution selected from the group consisting of: 2M potassium
chloride at a pH of 5.5; 2M sodium chloride at a pH of 5.3; 0.5M
potassium bicarbonate at a pH of 7.5; 2M potassium chloride at a pH
of 2.5; 0.5M potassium sulfate at a pH of 6.5; and 0.5M sodium
sulfate at a pH of 5.5.
6. The process of claim 1, wherein the anolyte is formulated for
the electrochemical reduction of carbon dioxide to formate.
7. The process of claim 6, wherein the anolyte comprises a solution
selected from the group consisting of: 1M sodium hydroxide; and
0.5M sulfuric acid.
8. The process of claim 1, wherein the cathode is porous, and
further comprising: (a) introducing carbon dioxide from a gas
compartment in fluid communication with the cathode into the
cathode prior to introducing the carbon dioxide into the
catholyte.
9. The process of claim 8, the catholyte compartment containing a
porous, absorbent nonconductive pad, the pad adjacent to the
membrane side of the cathode.
10. The process of claim 9, further comprising: (a) introducing at
least a portion of the catholyte into the porous, absorbent
nonconductive pad.
11. The process of claim 8, further comprising: (a) introducing
carbon dioxide from the gas compartment into a diffuser prior to
introducing the carbon dioxide gas into the cathode, the diffuser
adjacent to the gas compartment side of the cathode.
12. The process of claim 8, wherein the catholyte is formulated for
the electrochemical reduction of carbon dioxide to formate.
13. The process of claim 12, wherein the catholyte comprises a
solution selected from the group consisting of: 2M potassium
chloride at a pH of 5.5; 2M sodium chloride at a pH of 5.3; 0.5M
potassium bicarbonate at a pH of 7.5; 2M potassium chloride at a pH
of 2.5; 0.5M potassium sulfate at a pH of 6.5; and 0.5M sodium
sulfate at a pH of 5.5.
14. The process of claim 8, wherein the anolyte is formulated for
the electrochemical reduction of carbon dioxide to formate.
15. The process of claim 14 wherein the anolyte comprises a
solution selected from the group consisting of: 1M sodium
hydroxide; and 0.5M sulfuric acid.
16. An apparatus, comprising: a container; a membrane, the membrane
positioned within the container, the membrane dividing the
container into a catholyte compartment and an anolyte compartment;
a cathode, the cathode positioned at least partially within the
catholyte compartment, the cathode comprising a cathodic catalyst,
the cathodic catalyst comprising: tin; and zinc, wherein the zinc
comprises between three weight percent and six weight percent of
the cathodic catalyst; and an anode, the anode positioned at least
partially within the anolyte compartment.
17. The apparatus of claim 16, wherein the cathode is porous, and
further comprising a gas compartment, the gas compartment in fluid
communication with the cathode.
18. The apparatus of claim 17, further comprising a porous,
absorbent nonconductive pad, the pad adjacent to the membrane side
of the cathode.
19. The apparatus of claim 17, further comprising a diffuser
adjacent to the gas compartment side of the cathode.
20. A process, comprising: (a) introducing a catholyte into the
catholyte compartment of claim 16; (b) introducing an anolyte into
the anolyte compartment of claim 16; (c) impressing a DC voltage
across the anode and the cathode of claim 16; and (d) withdrawing
formate from the cathode compartment of claim 16.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. application Ser.
No. 13/825,822, filed Mar. 24, 2013, entitled "Method and Apparatus
for the Electrochemical Reduction of Carbon Dioxide", currently
pending, which U.S. application Ser. No. 13/825,822 is a National
Stage entry under 35 U.S.C. .sctn.371, and claimed priority, to
International Application No. PCT/US2011/052820, filed Sep. 22,
2011, entitled "Method and Apparatus for the Electrochemical
Reduction of Carbon Dioxide", expired, which International
Application No. PCT/US2011/052820 claimed priority to U.S.
Provisional Application No. 61/386,321, filed Sep. 24, 2010,
entitled "Cathodic Catalyst, Catholyte, and Apparatus for the
Electrochemical Reduction of Carbon Dioxide", expired, and U.S.
Provisional Application No. 61/414,932, filed Nov. 18, 2010,
entitled "Anodic Catalyst, Anolyte, and Electrochemical Reduction
of Carbon Dioxide", expired, which applications are incorporated
herein by reference.
TECHNICAL FIELD
[0002] This description relates to electrochemical methods and
apparatus, particularly to the electrochemical reduction of carbon
dioxide, and more particularly to the electrochemical reduction of
carbon dioxide to formate and formic acid.
BACKGROUND
[0003] The buildup of carbon dioxide (CO.sub.2) in the earth's
atmosphere is currently causing concern among many scientists and
others interested in potentially adverse global climate change, and
the primary source of this additional CO.sub.2 is the result of the
combustion of carbon-containing materials, such as fossil fuels. It
is desirable, therefore, to find and develop improved methods of
reducing the discharge of CO.sub.2 into the atmosphere. One method
involves sequestering, or storing, the CO.sub.2 to keep it out of
the atmosphere. There are many natural, biological processes which
effect sequestering, as well as artificial process such as those
involving capture and underground storage of CO.sub.2 and chemical
techniques such as conversion to carbonate mineral forms. In
addition, however, CO.sub.2 may be electrochemically converted into
useful and marketable commodities, including, for example, methane,
ethylene, cyclic carbonates, methanol, and formic acid or formate.
One CO.sub.2 conversion process involves the electrochemical
reduction of CO.sub.2 (ECRC) to formic acid. Formic acid, for
example, finds uses in replacing HCl in steel pickling, traditional
tanning of leather, formic acid-based fuel cells, as a storage
medium for hydrogen (H.sub.2) and carbon monoxide (CO) that are
then used as fuels and chemical feedstocks, conversion to sodium
formate for airport runway deicing, preservatives and antibacterial
agents, as an ingredient in bleaching pulp and paper, and as a
precursor for organic synthesis, including pharmaceuticals. ECRC to
formate and formic acid, however, exhibits the lowest energy
requirements.
[0004] The electrochemical CO.sub.2 conversion process, however,
could benefit from reduced energy requirements and improved
catalysts, CO.sub.2 fixation, and reactor design as well as
better-integrated, more efficient process configurations.
[0005] ECRC involves a highly interrelated process. The entire
process is based upon the balanced flow of electrons, ions in
solution, and gas, as well as current density, and voltage, across
the entire electrochemical reactor as well as across the individual
reactor elements. CO.sub.2 is reduced to end products at a cathode
to which electrons are supplied via an external electrical
connection by the oxidation of water or other compounds at an
anode. The anode and cathode are separated by a selective-ion
membrane which allows certain ions to migrate from the anode to the
cathode, thus completing the electrical circuit. Thus, changes in
one part of the cell can affect other parts. Changes to the anode,
for example, can effect changes in V.sub.CELL (voltage across the
entire reactor), current density, and the efficiency of conversion
of CO.sub.2 to products. Likewise, different cations and different
anions, present in the catholyte and the anolyte, can have
different effects on catalytic behavior. For example, when an acid
is added to the anolyte, the hydrogen ions (protons, H.sup.+) from
the anode migrate through the selective ion membrane to the
catholyte, effecting the formation of formic acid at the cathode.
If, on the other hand, sodium hydroxide (NaOH) is added to the
anolyte, the sodium ions move across, forming sodium formate
(HCOONa) as the end product. In addition, reactor configuration can
impact various other elements of the process, such as electrical
resistance, which can impact energy requirements. Runtime life of
the entire process can also be an important consideration as
shorter runtimes can require more frequent regeneration of
catalysts and repair of reactor components. Finally, it is very
important, and a measure of the performance of the overall process,
to maintain a high Faradaic Efficiency (FE) (expressed as a
percentage or as a fraction) throughout the process and over time.
As used herein, FE indicates the fraction (or percent) of the total
current that passes the electrochemical cell that is used to
produce the desired product (e.g., formate). The higher the FE, the
better, and the maximum FE is 1.0.
[0006] Thus, there is a need for an electrochemical process that
offers improved overall engineering and overall performance.
SUMMARY
[0007] Various embodiments of the present invention include a
tin-based (Sn-based) cathode and methods of preparation of such
cathode, tin-zinc (Sn--Zn) alloy-based cathodes, high surface area
cathode fabrication, improved catholyte compositions, mixed metal
oxide-catalyzed anodes in acidic anolytes, pulsed polarization of
the anode and the cathode, and various configurations of
three-compartment reactors, each of which provides improved
performance of ECRC, particularly ECRC to formate and formic
acid.
[0008] In one embodiment, a process includes, in part, introducing
CO.sub.2 gas into a catholyte compartment of an electrochemical
reactor where the catholyte compartment at least partially contains
a cathode comprising a catalyst comprising a Sn--Zn alloy, the Zn
comprising between about three weight percent and about six weight
percent of the catalyst. In a further embodiment, an apparatus
includes, in part, a cathode comprising a Sn--Zn alloy as just
described and the catholyte compartment includes a mixture
comprising a catholyte, gaseous carbon dioxide, and formate and
formic acid.
[0009] In a further embodiment, a solution formulated to deposit Sn
onto a porous, electrically conductive substrate includes a Sn
salt, at least one complexing agent, optionally, an antioxidant,
and a non-ionic surfactant. In a further embodiment, a process
includes immersing a porous, electrically conductive substrate into
the solution just described, maintaining a temperature of the
solution, and effecting a current density on the substrate.
[0010] In a further embodiment, a manufacture includes a porous,
electrically conductive substrate with a Sn catalyst deposited
thereon. The Sn catalyst comprises Sn deposits having a grain size
of between about 0.5 microns and about five microns, the deposits
substantially covering an outer surface of the substrate. In a
further embodiment, a manufacture is formed by process for
depositing Sn onto a substrate using a solution comprising a Sn
salt, at least one complexing agent, optionally, an anti-oxidant,
and a non-ionic surfactant. In a further embodiment, a manufacture
comprises a substrate comprising metallic foam, metallic felt,
carbon fiber paper, or reticulated vitreous carbon.
[0011] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment that contains an anode which
comprises a substrate and a mixed metal oxide catalyst and also
introducing CO.sub.2 gas from a gas compartment into a catholyte
compartment, where the catholyte compartment contains a catholyte
and a cathode. Further, impressing a DC voltage across the anode
and the cathode converts at least a portion of the CO.sub.2. In a
further embodiment, the anolyte is acidic and the CO.sub.2 is
converted to formic acid. In a further embodiment the anolyte is
alkaline and at least a portion of the CO.sub.2 is converted to
formate. In a further embodiment, the anode comprises a substrate
comprising titanium (Ti), and the metal oxide comprises tantalum
oxide (Ta.sub.2O.sub.5) deposited onto the substrate, and iridium
oxide (IrO.sub.2) deposited onto the tantalum oxide. In a further
embodiment, the cathode comprises a porous, electrically conductive
substrate and a Sn-based catalyst. In a further embodiment, an
apparatus includes a container, a membrane positioned within the
container dividing the container into a catholyte compartment
containing a cathode, and an anolyte compartment containing an
anode, the anode comprising a substrate and a mixed metal oxide
catalyst deposited upon the substrate. A mixture comprising a
catholyte and CO.sub.2 gas is at least partially contained within
the catholyte chamber A voltage source is provided configured to
impress a DC voltage across the anode and the cathode.
[0012] In a further embodiment, a process configured for ECRC to
formate includes introducing into an anolyte compartment an
anolyte, introducing into a catholyte compartment a catholyte
comprising an aqueous solution of about 0.1M to about 2M of a
compound selected from the following: potassium sulfate
(K.sub.2SO.sub.4) having a pH of about 6.5 and sodium sulfate
(Na.sub.2SO.sub.4) having a pH of 5.5. CO2 is also introduced into
the catholyte compartment. A DC voltage is impressed across the
anode and the cathode, whereby at least a portion of the CO.sub.2
is converted to formate.
[0013] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment of an electrochemical reactor,
where the anolyte compartment at least partially contains an anode;
introducing CO.sub.2 gas from a gas compartment into a catholyte
compartment of the electrochemical reactor, the gas compartment in
fluid communication with the catholyte compartment, the catholyte
compartment separated from the anolyte compartment by a membrane,
the catholyte compartment further containing a cathode, the cathode
comprising a substrate and a cathodic catalyst which contains Sn or
a Sn--Zn alloy; introducing a catholyte into the catholyte
compartment; impressing a DC voltage across the anode and the
cathode, whereby at least a portion of the CO.sub.2 is converted;
and removing a deposit from the cathode by periodically applying
anodic polarization, deep cathodic polarization, and a combination
thereof. In a further embodiment, the periodic deep polarization is
superimposed on top of the DC voltage and the total application is
between about 0.3 percent and about 1.7 percent of a maximum time
interval between applications of less than 24 hours. In a further
embodiment, the current density of the anodic polarization is a
positive current density about ten times the current density
effected by the impressed DC voltage across the anode and the
cathode. In a further embodiment, the current density of the deep
cathodic polarization is a negative current density about ten times
the current density effected by the impressed DC voltage across the
anode and the cathode. In a further embodiment, the anodic
polarization and the deep cathode polarization are applied
sequentially.
[0014] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment of an electrochemical reactor,
which anolyte compartment at least partially contains an anode;
introducing a catholyte into a catholyte compartment of the
electrochemical reactor, the catholyte separated from the anolyte
compartment by a membrane, the catholyte compartment further
separated from a gas compartment by a porous cathode, the cathode
and the membrane at least partially defining the catholyte
compartment, the cathode comprising a Sn-based catalyst as
described herein above; introducing CO.sub.2 gas into the gas
compartment; introducing the CO.sub.2 gas from the cathode into the
catholyte; and impressing a DC voltage across the anode and the
cathode, whereby at least a portion of the CO.sub.2 is converted.
In a further embodiment, a porous, absorbent, nonconductive pad,
such as felt or foam, is adjacent to a membrane side of the
cathode. In a further embodiment, the process further includes
introducing at least a portion of the catholyte into the porous,
absorbent, nonconductive pad. In a further embodiment, the process
includes introducing the CO.sub.2 gas from the gas compartment into
a diffuser prior to introducing the CO.sub.2 gas into the cathode,
the diffuser adjacent to the cathode.
[0015] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment of an electrochemical reactor,
the anolyte compartment at least partially containing an anode;
introducing a catholyte into a catholyte compartment of the
electrochemical reactor, the catholyte compartment separated from
the anolyte compartment by a membrane, the catholyte compartment
further separated from a gas compartment by a porous, catalytically
coated cathode, the cathode and the membrane at least partially
defining the cathode compartment; introducing CO.sub.2 gas into the
gas compartment; introducing the CO.sub.2 gas from the gas
compartment into the cathode; introducing the CO.sub.2 gas from the
cathode into the catholyte; and impressing a DC voltage across the
anode and the cathode, whereby at least a portion of the carbon
dioxide is converted to formate. In a further embodiment, the
catholyte compartment contains a porous, absorbent, nonconductive
pad, such as felt or foam, the porous, absorbent, nonconductive pad
adjacent to a membrane side of the cathode. In a further
embodiment, the process further includes introducing at least a
portion of the catholyte into the porous, absorbent, nonconductive
pad. In a further embodiment, the process further includes
introducing the CO.sub.2 gas from the gas compartment into a
diffuser prior to introducing the CO.sub.2 gas into the
cathode.
[0016] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment of an electrochemical reactor,
the anolyte compartment at least partially containing an anode;
introducing a catholyte into a porous, catalytically coated
cathode, the cathode contained within a catholyte compartment, the
catholyte compartment separated from the anolyte compartment by a
membrane, the cathode adjacent to the membrane; introducing
CO.sub.2 gas from a gas compartment into the cathode; and
impressing a DC voltage across the anode and the cathode, whereby
at least a portion of the CO.sub.2 is converted to formate. In a
further embodiment, the process includes introducing the CO.sub.2
gas from the gas compartment into a diffuser prior to introducing
the CO.sub.2 gas into the cathode, the diffuser adjacent to the
cathode.
[0017] In a further embodiment, an apparatus includes a container;
a membrane, the membrane positioned within the container, the
membrane dividing the container into a catholyte compartment and an
anolyte compartment; an anode, the anode positioned at least
partially within the anolyte compartment; a porous cathode, the
cathode separating the catholyte compartment from a gas
compartment, the cathode comprising a Sn-based catalyst; a
catholyte, the catholyte at least partially contained within the
catholyte compartment, the catholyte in fluid communication with
the cathode, the cathode configured to enable the introduction of
CO.sub.2 gas from the gas compartment into the cathode, and then
into the catholyte; a voltage source, the voltage source in
electrical communication with the anode and with the cathode, the
voltage source configured to impress a DC voltage across the anode
and the cathode. In a further embodiment, the apparatus further
comprises a porous, absorbent, nonconductive pad, such as felt or
foam, the porous, absorbent, nonconductive pad positioned within
the catholyte compartment, the porous, absorbent, nonconductive pad
further positioned adjacent to a membrane side of the catalyst. In
a further embodiment, the apparatus further comprises a catholyte
feed in fluid communication with the porous, absorbent,
nonconductive pad, the porous, absorbent, nonconductive pad
containing at least a portion of the catholyte. In a further
embodiment, the apparatus further includes a diffuser, the diffuser
positioned between the gas compartment and the cathode, the
diffuser adjacent to the cathode; the diffuser in fluid
communication with the gas compartment, and the diffuser in fluid
communication with the cathode; the gas compartment, the diffuser,
and the cathode configured to enable CO.sub.2 gas to flow from the
gas compartment, through the diffuser, then through the cathode,
and then into the catholyte.
[0018] In a further embodiment, an apparatus includes a container;
a membrane, the membrane positioned within the container, the
membrane dividing the container into a catholyte compartment and an
anolyte compartment; an anode, the anode positioned at least
partially within the anolyte compartment; a porous, catalytically
coated cathode, the cathode at least partially contained within the
catholyte chamber; a gas compartment, the gas compartment in fluid
communication with the cathode; a voltage source, the voltage
source in electrical communication with the anode and with the
cathode, the voltage source configured to impress a DC voltage
across the anode and the cathode. In a further embodiment, the
apparatus further includes a porous, absorbent, nonconductive pad,
such as felt or foam, the porous, absorbent, nonconductive pad at
least partially contained within the catholyte chamber, the porous,
absorbent, nonconductive pad adjacent to a membrane side of the
cathode. In a further embodiment, the apparatus further includes a
catholyte feed in fluid communication with the porous, absorbent,
nonconductive pad, the porous, absorbent, nonconductive pad
containing at least a portion of catholyte. In a further
embodiment, the apparatus further includes a diffuser, the diffuser
positioned between the gas compartment and the cathode, the
diffuser adjacent to the cathode; the diffuser in fluid
communication with the gas compartment, and the diffuser in fluid
communication with the cathode; the gas compartment, the diffuser,
and the cathode configured to enable CO.sub.2 gas to flow from the
gas compartment, through the diffuser, then through the cathode,
and then into the catholyte.
[0019] In a further embodiment, an apparatus includes a container;
a membrane, the membrane positioned within the container, the
membrane dividing the container into a catholyte compartment and an
anolyte compartment; an anode, the anode positioned at least
partially within the anolyte compartment; a porous, catalytically
coated cathode, the cathode contained within the catholyte chamber;
a catholyte feed, the catholyte feed in fluid communication with
the cathode, the cathode containing at least a portion of the
catholyte; a gas compartment, the gas compartment in fluid
communication with the cathode; and a voltage source, the voltage
source in electrical communication with the anode and with the
cathode, the voltage source configured to impress a DC voltage
across the anode and the cathode. In a further embodiment, the
apparatus further includes a diffuser, the diffuser positioned
between the gas compartment and the cathode, the diffuser adjacent
to the cathode; the diffuser in fluid communication with the gas
compartment, and the diffuser in fluid communication with the
cathode; the gas compartment, the diffuser, and the cathode
configured to enable CO.sub.2 gas to flow from the gas compartment,
through the diffuser, then into the cathode, and then into the
catholyte. In a further embodiment, the apparatus further includes
comprising a porous, absorbent, nonconductive pad, the porous,
absorbent, nonconductive pad at least partially contained within
the catholyte chamber, the porous, absorbent, nonconductive pad
adjacent to a membrane side of the cathode.
[0020] In a further embodiment, a process includes introducing an
acidic anolyte into an anolyte compartment of an electrochemical
reactor, the anolyte compartment containing an anode which anode
comprises a Ti substrate, Ta.sub.2O.sub.5 deposited onto the
substrate, and IrO.sub.2 deposited onto the Ta.sub.2O.sub.5. A
further step includes introducing a catholyte into a catholyte
compartment of the reactor where the catholyte compartment is
separated from the anolyte compartment by a membrane. The catholyte
compartment contains a porous, absorbent, nonconductive pad, such
as felt or foam, adjacent to a catholyte compartment side of the
membrane. The catholyte compartment also contains a cathode which
separates the cathode compartment from a gas compartment. The
cathode comprises a carbon fiber substrate and a Sn catalyst
deposited onto the substrate. The Sn deposits have a grain size of
between about 0.5 microns and about 5 microns, and substantially
cover the outer surface of the substrate. A further step includes
introducing CO.sub.2 gas from the gas compartment into a diffuser
which is adjacent to a gas compartment side of the cathode; the gas
compartment in fluid communication with the cathode and in fluid
communication with the catholyte. A further step includes
introducing the CO.sub.2 gas from the diffuser into the cathode,
introducing the CO.sub.2 from the cathode to the catholyte, and
impressing a DC voltage across the anode and the cathode, whereby
at least a portion of the CO.sub.2 is converted to formic acid.
[0021] In a further embodiment, an apparatus includes a container;
a membrane positioned within the container and dividing the
container into an anolyte compartment and a catholyte compartment.
In addition, an acidic anolyte is at least partially contained
within the anolyte compartment as is an anode. The anode comprises
a Ti substrate, Ta.sub.2O.sub.5 deposited onto the substrate and
IrO.sub.2 deposited onto the Ta.sub.2O.sub.5. A cathode comprising
a carbon fiber paper substrate and a Sn catalyst deposited onto the
substrate is positioned within the catholyte compartment. The Sn
catalyst comprises Sn deposits having a grain size of between about
0.5 microns and about 5 microns which substantially cover the outer
surface of the substrate. The cathode separates the cathode
compartment from a gas compartment. A porous, absorbent,
nonconductive pad is adjacent to a catholyte compartment side of
the membrane and a diffuser is adjacent to a gas compartment side
of the cathode. A mixture comprising a catholyte, CO2 gas, and
formic acid is at least partially contained within the catholyte
compartment. A voltage source, in electrical communication with the
anode and with the cathode, is configured to impress a DC voltage
across the anode and the cathode.
[0022] In a further embodiment, a process includes introducing an
anolyte into an anolyte compartment of an electrochemical reactor
where the anolyte compartment at least partially contains an anode.
The process further includes introducing a catholyte into a
catholyte compartment of the reactor where the catholyte
compartment is separated from the anolyte compartment by a
membrane, the catholyte compartment at least partially containing a
cathode, the cathode comprising a cathodic catalyst comprising tin
and zinc, the zinc comprising between three and six weight percent
of the cathodic catalyst. The process further includes introducing
carbon dioxide into the catholyte compartment and impressing a DC
voltage across the anode and the cathode, converting at least a
portion of the carbon dioxide to formate.
[0023] In a further embodiment, the conversion of carbon dioxide to
formate is accomplished at a Faradaic Efficiency ("FE") of between
65 and 68 percent.
[0024] In a further embodiment, the FE is accomplished at a
potential of between -1.70 V.sub.SCE and -1.90 V.sub.SCE.
[0025] In a further embodiment, the catholyte is formulated for the
electrochemical reduction of carbon dioxide to formate.
[0026] In a further embodiment, the catholyte comprises a solution
selected from the group consisting of (i) 2M potassium chloride at
a pH of 5.5; (ii) 2M sodium chloride at a pH of 5.3; (iii) 0.5M
potassium bicarbonate at a pH of 7.5; (iv) 2M potassium chloride at
a pH of 2.5; (v) 0.5M potassium sulfate at a pH of 6.5; and (vi)
0.5M sodium sulfate at a pH of 5.5.
[0027] In a further embodiment, the anolyte is formulated for the
electrochemical reduction of carbon dioxide to formate.
[0028] In a further embodiment, the anolyte comprises a solution
selected from the group consisting of (i) 1M sodium hydroxide; and
(ii) 0.5M sulfuric acid.
[0029] In a further embodiment, the cathode is porous, and the
process further comprises introducing carbon dioxide from a gas
compartment in fluid communication with the cathode into the
cathode prior to introducing the carbon dioxide into the
catholyte.
[0030] In a further embodiment, the catholyte compartment contains
a porous, absorbent nonconductive pad, the pad adjacent to the
membrane side of the cathode.
[0031] In a further embodiment, the process further comprises
introducing at least a portion of the catholyte into the porous,
absorbent nonconductive pad.
[0032] In a further embodiment, the process further comprises
introducing carbon dioxide from the gas compartment into a diffuser
prior to introducing the carbon dioxide gas into the cathode, the
diffuser adjacent to the gas compartment side of the cathode.
[0033] In a further embodiment, the catholyte is formulated for the
electrochemical reduction of carbon dioxide to formate.
[0034] In a further embodiment, the catholyte comprises a solution
selected from the group consisting of (i) 2M potassium chloride at
a pH of 5.5; (ii) 2M sodium chloride at a pH of 5.3; (iii) 0.5M
potassium bicarbonate at a pH of 7.5; (iv) 2M potassium chloride at
a pH of 2.5; (v) 0.5M potassium sulfate at a pH of 6.5; and (vi)
0.5M sodium sulfate at a pH of 5.5.
[0035] In a further embodiment, the anolyte is formulated for the
electrochemical reduction of carbon dioxide to formate.
[0036] In a further embodiment, the anolyte comprises a solution
selected from the group consisting of (i) 1M sodium hydroxide; and
(ii) 0.5M sulfuric acid.
[0037] In a further embodiment, an apparatus, comprises a
container, a membrane positioned within the container and dividing
the container into a catholyte compartment and an anolyte
compartment, a cathode positioned at least partially within the
catholyte compartment and comprising a cathodic catalyst, the
cathodic catalyst comprising tin and zinc, the zinc comprises
between three weight percent and six weight percent of the cathodic
catalyst, and an anode, the anode positioned at least partially
within the anolyte compartment.
[0038] In a further embodiment, the cathode is porous, and the
apparatus further comprises a gas compartment, the gas compartment
in fluid communication with the cathode.
[0039] In a further embodiment the apparatus includes a porous,
absorbent nonconductive pad adjacent to the membrane side of the
cathode.
[0040] In a further embodiment the apparatus further comprises a
diffuser adjacent to the gas compartment side of the cathode.
[0041] In a further embodiment, a process includes introducing a
catholyte into a catholyte compartment, introducing an anolyte into
an anolyte compartment, impressing a DC voltage across an anode and
a cathode, withdrawing formate from the cathode compartment
[0042] Additional objects, features, and advantages of the
invention will become apparent to those skilled in the relevant art
upon consideration of the following detailed description of
preferred embodiments, the drawings, and the claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0043] The invention will be more readily understood by reference
to the accompanying drawings, wherein like reference numerals
indicate like elements. The drawings are incorporated in, and
constitute a part of, this specification, illustrate several
embodiments consistent with the invention and, together with the
description serve to explain the principles of the invention. For
purposes of illustration, drawings may not be to scale.
[0044] FIG. 1 is a schematic of an overall view of an
electrochemical reactor.
[0045] FIG. 2 is a schematic of a two-compartment electrochemical
reactor illustrating a catholyte mix of catholyte and gas.
[0046] FIG. 3 is a schematic of a vertical electrochemical
half-cell testing apparatus.
[0047] FIG. 4 is a schematic of a horizontal electrochemical
half-cell testing apparatus.
[0048] FIG. 5 is a graph of Faradaic Efficiency (FE) fraction
versus cathode potential for several Sn-based cathodic
catalysts.
[0049] FIG. 6 is a graph of Faradaic Efficiency Percent (FE %)
normalized to pure Sn versus cathode potential for several Sn-based
cathodic catalysts.
[0050] FIG. 7 is a schematic of a three-compartment electrochemical
reactor illustrating CO.sub.2 gas feed through a porous cathode
into the catholyte.
[0051] FIG. 8 is a photomicrograph showing the surface of carbon
fiber paper untreated by metallic electrodeposition.
[0052] FIG. 9 is a photomicrograph showing a closeup of the fibers
of carbon fiber paper untreated by metallic electrodeposition.
[0053] FIG. 10 is a schematic of an electrochemical reactor
apparatus for metallic deposition.
[0054] FIG. 11 is a photomicrograph showing the surface of carbon
fiber paper treated by electrodeposition of Sn by a first method
using the apparatus shown in FIG. 10.
[0055] FIG. 12 is a photomicrograph showing a closeup of the fibers
of carbon fiber paper treated by electrodeposition of Sn by the
first method using the apparatus shown in FIG. 11.
[0056] FIG. 13 is a photomicrograph showing the surface of carbon
fiber paper treated by electrodeposition of Sn by a second method
using the apparatus shown in FIG. 10.
[0057] FIG. 14 is a photomicrograph showing a closeup of the fibers
of carbon fiber paper treated by electrodeposition of Sn by the
second method using the apparatus shown in FIG. 10.
[0058] FIG. 15 is a photomicrograph showing a closeup of nickel
(Ni) foam treated by electrodeposition of Sn by the second method
using the apparatus shown in FIG. 10 and focused on the interior of
the foam.
[0059] FIG. 16 is a graph of FE % and Current Density (mA/cm.sup.2)
versus Time (hr) for a carbon fiber paper treated by
electrodeposition of Sn by the first method using the apparatus
shown in FIG. 10.
[0060] FIG. 17 is a graph of Current Density versus cell potential
for Sn deposited onto carbon fiber paper by the first method and
the second method using the apparatus shown in FIG. 10.
[0061] FIG. 18 is a graph of FE % versus Applied Potential (V, v.
SCE) for tests performed with various catholytes.
[0062] FIG. 19 is a graph of Current Density versus V.sub.CELL for
various anodes in acidic anolyte.
[0063] FIG. 20 is a graph of Current Density versus V.sub.CELL for
a mixed metal oxide (Ti/Ta.sub.2O.sub.5/IrO.sub.2) anode and a
platinum-niobium (Pt--Nb) mesh anode in acidic anolyte.
[0064] FIG. 21 is a graph of Current Density versus V.sub.CELL for
various anodes in the three-compartment reactor shown in FIG.
7.
[0065] FIG. 22 is a graph of FE % versus V.sub.CELL for a
Ti/Ta.sub.2O.sub.5/IrO.sub.2 anode and a Pt--Nb anode in acidic
anolyte.
[0066] FIG. 23 is a graph of FE % for a study of several
polarization pulse combinations.
[0067] FIG. 24 is a schematic of the three-compartment
electrochemical reactor shown in FIG. 7 with a porous, absorbent,
nonconductive pad adjacent to the cathode.
[0068] FIG. 25 is a schematic of the three-compartment
electrochemical reactor shown in FIG. 25 with catholyte feed
directly into the pad.
[0069] FIG. 26 is a schematic of the three-compartment
electrochemical reactor shown in
[0070] FIG. 26 with a diffuser adjacent to the cathode.
[0071] FIG. 27 is as schematic of the three-compartment
electrochemical reactor shown in FIG. 25 with a diffuser adjacent
to the cathode.
[0072] FIG. 28 is a schematic of the three-compartment
electrochemical reactor shown in FIG. 7 with a diffuser adjacent to
the cathode.
[0073] FIG. 29 is a schematic of the three-compartment
electrochemical reactor shown in FIG. 7 with catholyte feed
directly into the cathode.
[0074] FIG. 30 is a schematic of the three-compartment
electrochemical reactor shown in FIG. 30 with a diffuser adjacent
to the cathode.
DETAILED DESCRIPTION
[0075] Looking first at FIG. 1, a general schematic of an
electrochemical reactor, generally 10 illustrates the interrelated
nature of the process. Within the container 12 are an anolyte
compartment 18 and a catholyte compartment 20 which are separated
by a membrane 22 (e.g., a cation exchange membrane, such as
Nafion.RTM. 117, duPont, Wilmington, Del.). Included within the
anolyte compartment 18 is an anolyte 24, formulated for the desired
reaction(s), and an anode 14. Included within the catholyte
compartment 20 is a catholyte 25, also formulated for the desired
reaction(s), and a cathode 16. Note that while the anode 14 and the
cathode 16 are shown adjacent the membrane 22, this need not be the
configuration. Completing the reactor is a DC voltage source 38 in
electrical communication with the anode 14 and with the cathode 16.
In continuous operation, the electrochemical reactor of FIG. 1 also
includes an anolyte feed 36, an anolyte withdrawal 44, which may
contain spent anolyte as well as reaction product(s), a catholyte
feed 34, and a catholyte withdrawal, which may likewise contain
spent catholyte as well as reaction product(s).
[0076] Also illustrated in FIG. 1 are the various voltage drops and
resistances according to the formula:
V.sub.CELL=V.sub.CATHODE+I*R+V.sub.ANODE,
where I=current, R=internal and external resistance, and their
product (I*R) refers to the voltage loss in the cell. Thus, changes
to any of the components in the reactor configuration can
potentially change the operation of the entire reactor. For
example, by employing an improved anodic catalyst, V.sub.ANODE may
be decreased, which decreases V.sub.CELL, thus reducing the energy
consumption of the whole process, since total energy consumption,
E, can be defined as:
E=V.sub.CELL*I*T,
where I is based upon the amount of feed (e.g., CO.sub.2) to be
converted to products (e.g., formate/formic acid), and T is the
time of operation (e.g., one year). An improved anodic catalyst
that also maintains or enhances current density (mA/cm.sup.2) can
provide additional benefits.
[0077] In the ECRC to formate context, the anolyte withdrawal 44
contains anolyte plus oxygen (O.sub.2), the catholyte feed 34, in
addition to containing catholyte contains CO.sub.2, and withdrawn
from the catholyte compartment 20, a catholyte mixture withdrawal
43 contains catholyte, formate or formic acid, H.sub.2, CO, and
unreacted CO.sub.2. The reactions at the cathode 25 include:
2CO.sub.2(aq)+4H.sup.++4e.sup.-.fwdarw.2HCOOH(aq);
2CO.sub.2(aq)+4H.sup.++4e.sup.-.fwdarw.2CO(g)+2H.sub.2O; and
4H.sup.++4e.sup.-.fwdarw.2H.sub.2(g)
The reactions at the anode 14 includes:
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4 e.sup.-; and
4OH.sup.-.fwdarw.O.sub.2+2H.sub.2O+4 e-.
[0078] In one embodiment, a Sn--Zn alloy is employed as a cathodic
catalyst for ECRC to formate where the Zn comprises between about
three weight percent and about six weight percent of the cathodic
catalyst. In a further embodiment, the Zn comprises about three
weight percent and in a further embodiment, the Zn comprises about
six weight percent of the cathodic catalyst.
Sn--Zn Alloy Cathodic Catalyst Tests
[0079] FIG. 3 is a schematic of a vertical electrochemical
half-cell testing apparatus 200 used to test Sn--Zn alloys for
cathodic catalysts. CO.sub.2 gas 30 was passed into a flask 202 via
a CO.sub.2 gas feed 32. The CO.sub.2 gas 30 then rises and passes
through a glass frit 52 and into the catholyte 25 contained in a
container 204 which container 204 also acts as a catholyte
compartment 20. The Sn--Zn alloys of interest were positioned
closely to the catholyte side of the glass filter 52 is a coil
cathode 16b, or working electrode, all in the shape of a coil, as
shown. Thus, the CO.sub.2 30 bubbled by and through the test alloy
creating a three-phase condition, also containing formate/formic
acid, or catholyte compartment mixture 26 at the coil cathode 16b.
The remainder of the apparatus included a reference electrode,
saturated calomel electrode (SCE) 50, an anolyte compartment 18
containing anolyte 24 and a counter electrode acting as an anode
14. A Nafion.RTM. membrane 22 separated the catholyte compartment
20 from the anolyte compartment 18. A potentiostat (not shown) was
used as a voltage source.
[0080] FIG. 4 is a schematic of an analogous, but horizontal,
electrochemical half-cell testing apparatus 300 with a half-cell
horizontal container 304 used to test Sn--Zn alloys for cathodic
catalysts. Similar elements with the vertical apparatus 200 are
referenced similarly in the horizontal apparatus 300.
[0081] The alloys were acquired from Sophisticated Alloys, Inc.,
Butler, Pa. and were fabricated from pure (99.99 percent) element
metals. Potentiostatic polarization at each potential was applied
for two hours and formate concentrations analyzed with a
Dionex.RTM. ion chromatograph (Dionex Corp., Sunnyvale, Calif.).
Pure CO.sub.2 gas 30 was continuously purged through a coil-form
cathode 16b at 75 ml/min. The coils themselves were 40 cm long and
1 mm in diameter. The catholyte 25 was 400 ml 2M KCl and the anode
14 was Pt wire immersed in 1 M NaOH anolyte 24.
[0082] FIG. 5 shows the results of evaluating FE for several Sn--Zn
alloy cathodic catalysts as well a pure Sn at values of -1.7
V.sub.SCE, -1.8 V.sub.SCE, and -1.9 V.sub.SCE. The apparatus used
for these tests was the vertical electrochemical half-cell 200
illustrated in FIG. 3 and described above. The V.sub.SCE range was
chosen to give the best FE, or selectivity of CO.sub.2 to formate
salts. Values below and above that range tend to decrease FE
significantly. As shown in FIG. 5, SnZn3 (Sn with three percent
Zn), SnZn6 (Sn with six percent Zn), SnZn9 (Sn with nine percent
Zn), and SnZn20 (Sn with 20 percent Zn) are all well behaved at the
V.sub.SCE values of interest. That is, FE for each alloy changes
little between -1.7 V.sub.SCE and -1.9 V.sub.SCE, thus allowing for
more robust reactor operation over a broader V.sub.SCE range. In
addition, SnZn3 and SnZn6 exhibit better FE than either SnZn9 or
SnZn20, and SnZn9 shows better FE than SnZn20. SnZn3 and SnZn6 also
exhibit better FE than pure Sn for -1.7 V.sub.SCE and -1.8
V.sub.SCE. There is, therefore, an unexpected benefit to operating
at lower Zn levels and FE does not improve with increasing Zn
content. In large reactors, with electrodes in the range of 1
m.sup.2, there exists a potential variation on the surface, but if
the FE remains constant, the overall reactor FE will remain
constant with the resultant more uniform product selectivity.
[0083] In FIG. 6, the FE values for the various Sn-based cathodic
catalysts have been normalized versus pure Sn:
% Normalized FE = ( FE Sn - Zn Alloy - FE Pure Sn ) FE Pure Sn *
100 ##EQU00001##
Thus, normalized values of SnZn3 and SnZn6, although lower at -1.9
V.sub.SCE, are higher than that of pure Sn at all other potential
values.
[0084] In further embodiments, the Sn--Zn alloys as cathodic
catalysts are combined with a catholyte formulated for ECRC to
formate, which catholyte comprises 2M KCl. In a further embodiment,
the Sn--Zn alloys as cathodic catalysts are combined with anolytes
formulated for ECRC to formate, which anolytes comprise 1M sodium
hydroxide (NaOH) or 0.5M sulfuric acid (H.sub.2SO.sub.4).
[0085] In a further embodiment, the Sn--Zn alloy cathodic catalysts
are employed in a two-compartment electrochemical reactor, as shown
in FIG. 2, and three-compartment electrochemical reactors, as shown
in FIG. 7, and variations thereof as described herein below.
[0086] Turning to FIG. 2, a two-compartment electrochemical reactor
100 is shown in which a container 12 encloses an anolyte
compartment 18, an anode 14, during operation, anolyte 24, a
membrane 22, a catholyte compartment 20, a cathode 16, and, during
operation, a catholyte compartment mixture 26. During operation, a
two-phase catholyte/CO.sub.2 feed (from catholyte feed 34 and CO2
gas feed 32) is introduced into the catholyte compartment 20. The
catholyte compartment 20 contains the cathode 16, the anolyte
compartment 18 contains the anode 14, and the catholyte compartment
20 and the anolyte compartment 18 are separated by the membrane 22.
Either the anode 14, the cathode 16, or both may also include a
catalyst formulated for ECRC to formate. An anolyte feed 36, the
catholyte feed 34, which joins with the with the CO.sub.2 feed 32
into the catholyte compartment 20, an anolyte withdrawal 44, which
withdraws anolyte 24 as well as other anode reaction products, and
a catholyte mixture withdrawal 42 are further included. Contained
within the catholyte compartment 20, during operation, is the
catholyte compartment mixture 26, which mixture comprises
catholyte, CO.sub.2, and ECRC products such as formate and formic
acid.
[0087] Turning now to FIG. 7, a three-compartment electrochemical
reactor 350 is shown in which a three-compartment container 12a
encloses an anolyte compartment 18, an anode 14, and, during
operation, anolyte 24 contained within the anolyte compartment 18;
a membrane 22; a catholyte compartment 20, a porous cathode 16a,
and, during operation, catholyte compartment mixture 26 contained
within the catholyte compartment 20; and a gas compartment 28, the
gas compartment 28 containing, during operation, CO.sub.2 gas 30.
The membrane 22 separates the anolyte compartment 18 and the
catholyte compartment 20 and a porous cathode 16a separates the
catholyte compartment from the gas compartment 28. Also during
operation, an anolyte feed 36 introduces anolyte 24 into the
anolyte compartment 18, an anolyte withdrawal 44 removes anolyte 24
as well as other anode reaction products, a catholyte feed 34
introduces catholyte 25 into the catholyte compartment 20, a
catholyte compartment mixture withdrawal 42 removes catholyte
compartment mixture 26, and a CO.sub.2 gas feed 32 introduces
CO.sub.2 gas 30 into the gas compartment 28. During operation, the
CO.sub.2 gas 30 in the gas compartment 28, under a pressure
differential across the porous cathode 16a, is distributed
(indicated by arrows 40 and flows through the porous cathode 16a
and into the catholyte mixture 26. The porous cathode 16a may
comprise a suitable cathodic catalyst, for example, one based upon
Sn or a Sn-based alloy as described herein. The porous cathode 16a
may further comprise substrates and cathodic catalysts deposited as
described herein.
[0088] In a further embodiment, a porous, electrically conductive
material forms a substrate for the cathode 16a. In one embodiment,
the substrate comprises carbon fiber paper (CFP). Good results have
been obtained with CFP TGP-H-120 Toray (Fuel Cell Earth, LLC,
Stoneham, Mass.). FIG. 8 is a photomicrograph of a sample of CFP
showing the fiber pattern and FIG. 9 shows the same CFP at a higher
magnification, as indicated. The nominal diameter of the fibers is
about 7.5 microns (.mu.m) to about 10 .mu.m and their length is in
the hundreds of .mu.m. The nominal thickness is about 370 .mu.m and
the porosity (.PHI.) about 78. It will be appreciated by those
skilled in the relevant art that these dimensions and properties
are specific to Toray CFP, and that other like materials with
varying configurations will also be suitable. CFP provides a
flow-through structure whereby CO.sub.2 gas 30 in the gaseous feed
stream 32 (see, e.g., FIG. 7) to the electrochemical reactor 350
(FIG. 7) flows through the thickness of the porous cathode 16a to
the catholyte 25 where CO.sub.2 is converted to formate/formic
acid. Such a porous material may be classified as three-dimensional
(3D). That is, in addition to a length and width that define a
two-dimensional superficial geometric area, the material also has
more than nominal thickness. Thus, a porous 3D cathode upon which a
cathodic catalyst is deposited beyond the superficial geometric
area, will be capable of catalytically converting reactants to
products in the interior of the cathode, interior to the
superficial geometric area. For example, the CFP cathode discussed
above herein, with a nominal thickness of about 370 .mu.m, has
shown deposits at a depth of about 25 .mu.m to about 35 .mu.m from
each side into the interior of the superficial geometric area when
fabricated as discussed herein below.
Deposition Method One
[0089] In a further embodiment, using the deposition apparatus 400
illustrated in FIG. 10, and the deposition process described herein
below, pure Sn was deposited onto CFP in a uniform coating as shown
in FIGS. 11 and 12. As above, with photomicrographs of the plain
CFP, FIG. 11 shows the fiber pattern generally with the Sn
deposited thereon. FIG. 12 shows the same deposition, but at a
higher magnification, as indicated.
[0090] Turning now to FIG. 10, the deposition apparatus 400
includes a fluid bath jacket 66 for temperature control and
contains a deposition bath 72. A magnetic stirring bar 68 mixes the
bath 70 and a temperature probe 64 is inserted into the bath 72.
The apparatus 400 includes a reference electrode 58 and two counter
electrodes 56 of Pt-coated Nb mesh, which, to avoid edge effects,
are sized slightly larger than a cathode substrate material 70
attached to a working electrode 60, the onto which the Sn is be
deposited. The counter electrodes 56 are positioned in parallel to
the working electrode 60. A potentiostat 54 supplies constant
current density between the working electrode 60 and the counter
electrodes 56, where the working electrode 60 is negatively charged
during the deposition. For Toray CFP, a current density of about
2.5 mA/cm.sup.2 was used based upon the superficial geometric area.
For example, a rectangular substrate material 70 having a length of
about 3 cm and a width of about 3.3 cm would have a superficial
geometric area of about 10 cm.sup.2. A circular substrate material
70 would have a diameter of 2.54 cm for a 5 cm.sup.2 superficial
geometric area. The temperature of the bath 72 was maintained
between about 50 deg. C. and about 55 deg. C. Time of deposition
ranged from 30 min. to about 1.5 hr.
[0091] In a further embodiment, the deposition bath 72 was as
follows:
TABLE-US-00001 stannous Chloride SnCl.sub.2.cndot.2H.sub.2O 0.19
mol/L (0.08-0.20 mol/L) (dihydrate) tetrapotassium
K.sub.4P.sub.2O.sub.7 0.56 mol/L (0.40-0.60 mol/L) Pyrophosphate
glycine C.sub.2H.sub.5NO.sub.2 0.17 mol/L (0.10-0.20 mol/L)
ammonium hydroxide NH.sub.4OH 0.04 mol/L pH 8-9
The parenthetical values above are ranges for the indicated
compounds.
[0092] The entire process included wetting the cathode substrate
material 70 by immersion in ethanol for about 30 sec. to improve
wettability, electrodepositing Sn onto the substrate 70 as
described herein above, removing the substrate 70 from the bath 72
and immersing the substrate 70 into a large quantity of distilled
water, immersing the substrate 70 into deionized (DI) water,
cleaning in an ultrasonic cleaner for about three min., rinsing
with DI water and slowly blow drying with nitrogen (N.sub.2) gas.
(See FIGS. 11 and 12.)
CFP-Sn Cathodic Catalyst Cathode Tests--Deposition Method One
[0093] Turning now to FIG. 16, a cathode prepared as above under
Method One was tested in a three-compartment reactor as shown in
FIG. 7. The cathode comprised a CFP substrate and Sn deposited as
described herein above under Method One and had a superficial
geometric area of 10 cm.sup.2. CO.sub.2 gas flowrates were between
226 ml/min and 550 ml/min. The catholyte was CO.sub.2 pre-saturated
2M KCl, with flowrates in the range of between 2.25 ml/min and 2.5
ml/min. The anolyte was 1M KOH with flowrates of between 40 ml/min
and 80 ml/min. Pressures inside the catholyte and anolyte
compartments were similar, with catholyte compartment values in the
range of between 0.295 psig and 0.395 psig, and for the anolyte
compartment, between 0.28 psig and 0.528 psig. The pressure in the
gas compartment was between 0.45 psig and 2.01 psig. The pressure
in the gas compartment was always higher that the catholyte
compartment pressure, the difference being between about 0.055 psi
and about 1.646 psi. Anodes of both Ni plate and Ti
plate/Ta.sub.2O.sub.5/IrO.sub.2 (discussed herein below) were used.
V.sub.CELL was 3 V. FIG. 16 illustrates performance over time of
the Sn-deposited CFP cathode in the three-compartment flow cell
shown in FIG. 7. As shown in FIG. 16, both the FE and Current
Density remained high and relatively constant over the four-day
experiment.
Deposition Method Two
[0094] In a further embodiment, Sn was deposited onto CFP using the
deposition apparatus 400 shown in FIG. 10 and as described herein
above. Further, the operating parameters were the same, except the
temperature was between 55 and 65 deg. C., and the time of
deposition was between one and two hours. The deposition bath may
comprise a tin salt, at least one complexing agent, an
anti-oxidant, and a non-ionic surfactant. Specifically, the
deposition bath comprised:
TABLE-US-00002 stannous Chloride (dihydrate)
SnCl.sub.2.cndot.2H.sub.2O 0.2 mol/L tetrapotassium Pyrophosphate
K.sub.4P.sub.2O.sub.7 0.5 mol/L glycine C.sub.2H.sub.5NO.sub.2 0.2
mol/L L-ascorbic acid C.sub.6H.sub.8O.sub.6 10 g/L polyoxyethylene
(12) nonylphenyl ether 0.1 g/L pH 4.5
The results are shown in FIGS. 13, 14, and 15 which show a dense,
uniform coating. FIG. 13 shows the fiber pattern generally with the
Sn deposited thereon. FIG. 14 shows the same deposition, but at a
higher magnification as indicated showing the individual Sn grains.
Finally, FIG. 15 shows Sn deposited onto a Ni foam. The Ni foam
used was Incofoam.RTM. (Novamet Specialty Products, Wyckoff, N.J.).
The photomicrograph was focused on the interior of the foam, so the
grains in the background are more defined.
CFP-Sn Cathodic catalyst Cathode Tests--Deposition Method Two
[0095] A cathode prepared as above under Method Two was also tested
in a three-compartment reactor as shown in FIG. 7. The cathode
comprised a CFP substrate and Sn deposited as described herein
above under Method Two and had a superficial geometric area of 10
cm.sup.2. CO.sub.2 gas flowrate was 70 ml/min. The catholyte was
CO.sub.2 pre-saturated 2M KCl, with flowrates between 7 and 8
ml/min and the anolyte was 1M KOH. The anode was Pt-coated Nb mesh.
V.sub.CELL was between 2.6 V and 4.0 V. The CO.sub.2 gas and
catholyte flowrates were controlled by a gas/liquid (G/L) ratio. As
shown in FIG. 17, Method Two appears to provide superior results
compared with Method One. At the same V.sub.CELL of 3.75 V, for
example, the cathode prepared by Method One achieved a current
density of 45 mA/cm.sup.2, while the cathode prepared by Method Two
achieved a current density of 75 mA/cm.sup.2. While not wishing to
be bound by any particular theory, it is believed that the
increased coverage of Sn, and a more uniform deposit of finer
grains (see, FIGS. 11 and 12 versus 13 and 14), contributed to the
observed improvement.
[0096] In a further embodiment, a process is provided which
includes improved catholyte compositions for ECRC to formate. FIG.
18 illustrates the short-time (2 hours) performance of a pure Sn
coil electrode in selected catholytes in a vertical half-cell as
shown in FIG. 3. Pure CO2 gas was continuously purged through a 40
cm long, 1 mm diameter coil cathode at 75 ml/min. The anode was Pt
wire which was immersed in an anolyte of 1M NaOH.
[0097] As shown in FIG. 18, high FE was obtained using a pure Sn
cathode in 2M KCl at a pH of 5.5, 2M NaCl at a pH of 5.3, and 0.5M
Na.sub.2SO.sub.4 at a pH of 5.5. Other catholytes tested include
0.5M KHCO.sub.3 at a pH of 7.5, 2M KCl at a pH of 2.5, and
K.sub.2SO.sub.4 at a pH of 6.5
[0098] In a further embodiment, a process is provided which
includes an ECRC process comprising introducing an anolyte into an
anolyte compartment, the anolyte compartment containing a metal
oxide catalyst. Particularly, the anolyte is acidic and the anode
comprises a mixed metal oxide catalyst. FIG. 19 shows current
density (mA/cm.sup.2) for a mixed metal oxide anode in an acidic
anolyte. The apparatus shown in FIG. 7 was used to develop the data
in FIG. 19. The anolyte used was 0.5M H.sub.2SO.sub.4 at a flowrate
of 45 ml/min. The catholyte was 2M KCl pre-saturated with CO.sub.2
before introduction into the reactor at a flowrate of between 5 and
10 ml/min. The CO.sub.2 flowrate was maintained at between 69 and
84 ml/min. The cathode used for all of the experiments was Sn
deposited onto CFP as described herein above under Deposition
Method One. V.sub.CELL applied between the anode and the cathode
was varied between 3.25 V and 4.25 V and the current densities
measured. Catholyte samples were collected periodically and
evaluated for the concentration of formic acid. For this analysis,
the pH of the catholyte samples were first increased to 7 from the
original catholyte values of between 2 and 3 using NaOH in order to
convert all formic acid to formate ions. An Ion Chromatograph was
used to measure the concentration of formate ions. Unlike the case
where alkaline solutions (e.g., KOH and NaOH) are used as an
anolyte, in the acidic anolyte case, H ions are formed and
transported across the cation exchange membrane. This allows for
the direct formation of formic acid on the cathode side rather than
formate ions as in the alkaline anolyte case. This leads to a
decrease in the cost of chemical consumption in converting formate
ions to formic acid in later post processing if formic acid is the
desired product. Also, in the alkaline anolyte case, the hydroxide
species (4OH.sup.-.fwdarw.O.sub.2+2H.sub.2O+4 e.sup.-) are consumed
and need to be replaced. This adds the cost of alkaline hydroxides
during operation. In the case of acidic anolytes, only water is
consumed from the acidic electrolyte, which can be recycled after
addition of water, thus reducing the chemical operating
expenditures.
[0099] Turning now to FIG. 19, current density (mA/cm.sup.2) is
plotted against total cell voltage (V.sub.CELL) for several anodes
in an acidic environment as discussed herein above. Mixed metal
oxide (MMO) is an electrode with a Ti plate substrate, a
Ta.sub.2O.sub.5 layer deposited upon the substrate, and an
IrO.sub.2 layer deposited upon the Ta.sub.2O.sub.5 layer
(Ti/Ta.sub.2O.sub.5/IrO.sub.2). The material was acquired from NMT
Electrodes Pty Ltd, Ashwood, ZA. Pt-coated Nb mesh is an electrode
acquired from Anomet Products, Shrewsbury, Mass. The Ni and 316
stainless steel (SS316) were flat metal plates. Both the Ni and
SS316 anodes exhibited pitting corrosion and dissolution during
ECRC. As shown in FIG. 20, MMO exhibited improved performance over
Pt--Nb mesh and reduced V.sub.CELL at constant current density.
[0100] Turning now to FIG. 21, Current Density versus V.sub.CELL is
shown for various mixed metal oxides and several other anode
materials. The apparatus used for these tests is the
three-compartment reactor shown in FIG. 7 under continuous
operation. The indicated PTA mesh is an OPTIMA.TM. PTA Series
platinum-plated, clad, and thermally deposited anode. The substrate
is titanium. (Siemens AS, Water Technologies, Warrendale, Pa.) The
IrO.sub.2 on Ti plate was also acquired from NMT Electrodes Pty
Ltd. The IrO.sub.2, Ta.sub.2O.sub.5 bilayer on Ti plate was also
acquired from NMT.
[0101] Turning now to FIG. 22, it was surprisingly found that the
selectivity (Faradaic Efficiency) of Ti/Ta.sub.2O.sub.5/IrO.sub.2
of ECRC to formate is higher in an acidic anolyte than Pt--Nb,
especially in the V.sub.CELL range of 3.30 V to 3.75 V, even though
the same O.sub.2 evolution reaction is taking place at the anode
for both. While not wishing to be bound by any particular theory,
it is believed that corrosion or dissolution of unsuitable anode
materials, such as Pt, Ni, and iron (Fe) (from stainless steel), to
their respective positive ions could occur in acidic anolyte
solutions at high applied anodic potentials. These ions can then
electrodeposit and coat the surface of the cathode catalyst,
thereby reducing the Sn and Sn alloy catalyst area, thus reducing
the rate of generation of formate salts/formic acid and hence
lowering FE, as Ni, Fe, and Pt do not act a good electrochemical
catalysts for formate generation.
[0102] In a further embodiment, momentary pulsed polarization
restores FE that has degraded over time. High FE at high current
densities at the cathode over extended times is desirable for ECRC.
Under such conditions, at long runtimes black deposits can form on
metal electrodes, including pure Sn. For example, with the pure Sn
and Sn-alloy coil cathodes discussed herein above, in runtimes up
to 20 hours in a half-cell, no deposits were found. However, black
deposits with differing amounts (spots or fully covered coating)
were generally found on electrodes of pure Sn and Sn-based alloys
after more than 20 hours at a current density of about 10
mA/cm.sup.2. Energy-dispersive X-ray (EDX) in scanning electron
microscopy and Raman microscopy analysis of the deposits indicate
they are likely graphite. The partial or full coverage of graphite
on the cathode surface can be the cause of FE decrease because
graphite is not a good catalyst for ECRC to formate. While not
wishing to be bound by any particular theory, it is believed the
carbon formation is the result of further reduction of formate on
the surface of the catalysts if the formate product stays in
contact with the catalyst for too long.
[0103] Different polarization treatment methods, including deep
cathodic polarization (DCP), anodic polarization (AP), and
combinations thereof, were utilized to remove the black deposits
from long runtime pure Sn wire. Polarization was carried out in 2M
KCl at different pHs (adjusted with HCl and/or CO.sub.2). The
visual results indicated that, after the polarization treatment,
both AP, DCP, and combinations of AP and DCP can remove the black
deposits that coated the cathode surface after a long time run of
electrolysis. As shown in FIG. 23, AP was 100 mA/cm.sup.2 for 30
seconds, DCP was -100 mA/cm.sup.2 for 30 seconds, and the combined
AP/DCP was for 100 mA/cm.sup.2 and -100 mA/cm.sup.2, respectively,
each for 30 seconds. FIG. 23 illustrates the FE of used pure Sn
electrode before and after these different polarization treatments.
Tests were performed in the vertical half-cell testing apparatus
shown in FIG. 3. The cathode was in coil form. The catholyte was 2M
KCL and the anolyte was 1M NaOH. The top dashed line is the average
high FE of the original pure Sn cathode before FE degradation due
to the coverage of black deposits. The bottom dash-dot line is the
average low FE of the original pure Sn cathode before polarization
treatment. The square symbols identify the FEs of the
polarization-treated Sn cathode, which were similar to the top
dashed line, an indication that full recovery of FE is achieved by
polarization treatments.
[0104] The cathodes used in the above experiments were relatively
small compared with a large-scale commercial unit, where baseline
current densities on the order of 100 mA/cm.sup.2 to 500
mA/cm.sup.2 would be found. In that case, AP pulses on the order of
500 mA/cm.sup.2 and 5000 mA/cm.sup.2, respectively, and that of DCP
pulses would be on the order of -500 mA/cm.sup.2 and -5000
mA/cm.sup.2, respectively. Thus, the current density of AP pulses
is a positive current density about ten times the current density
effected by the impressed DC voltage across the anode and the
cathode. Likewise, the current density of DCP pulses is a negative
current density about ten times the current density effected by the
impressed DC voltage across the anode and the cathode. AP pulses
and DCP pulses may also be applied sequentially.
[0105] The pulses are infrequent and of short duration, typically
between about five and about 120 seconds every two to ten hours,
or, generally, between about 0.3 percent and 1.7 percent of the
interval between applications. While not wishing to be bound by any
particular theory, it is believed that these polarization methods
may dissolve small amounts of Sn for forming Sn-hydride, which is
unstable. In the process, the black deposits are removed from the
cathode.
[0106] FIG. 24 shows a three-compartment reactor 350, and is a
modification of FIG. 7 as discussed herein above. Further included
is a porous, absorbent, nonconductive pad 46, such as foam or felt,
adjacent to the porous cathode 16a. The porous, absorbent,
nonconductive pad 46 in contact with the porous cathode 16a
prevents the formation of a gas layer near the porous cathode 16a
which can break the continuity in the electrolyte. Good results
have been obtained with plain-backed 1/8-inch-thick wool felt sheet
(Item 8341K31, McMaster-Carr, Aurora, Ohio).
[0107] FIG. 25 shows a three-compartment reactor 360, and is a
modification of FIG. 24, further including a catholyte feed 34 into
the porous, absorbent, nonconductive pad 46. This enables intimate
contact between the catholyte 25 and the membrane 22.
[0108] FIG. 26 shows a three-compartment reactor 370, is a
modification of FIG. 25, further including a diffuser 48 adjacent
the porous cathode 16a on the gas compartment side of the porous
cathode 16a. Good results were obtained with a PTFE-coated porous
material which facilitates the distribution of the CO.sub.2 gas 30.
In a further embodiment, the diffuser 48 is fabricated to allow not
only CO.sub.2 gas 30 traversing the thickness of the diffuser 48,
but to allow CO.sub.2 gas 30 to move laterally along the length or
width of the diffuser 48 as it traverses the thickness of the
diffuser 48.
[0109] FIG. 27 shows a three-compartment reactor 380, and is a
modification of FIG. 24, further including a diffuser 48 adjacent
the cathode 16a on the gas compartment side of the cathode 16a.
Good results were obtained with a PTFE-coated porous material which
facilitates the distribution of the CO.sub.2 gas 30. In a further
embodiment, the diffuser 48 is fabricated to allow not only
CO.sub.2 gas 30 traversing the thickness of the diffuser 48, but to
allow CO.sub.2 gas 30 to move laterally along the length or width
of the diffuser 48 as it traverses the thickness of the diffuser
48.
[0110] FIG. 28 shows a three-compartment reactor 390, and is a
modification of FIG. 7, further including a diffuser 48 adjacent
the cathode 16a on the gas compartment side of the cathode 16a.
Good results were obtained with a PTFE-coated porous material which
facilitates the distribution of the CO.sub.2 gas 30. In a further
embodiment, the diffuser 48 is fabricated to allow not only
CO.sub.2 gas 30 traversing the thickness of the diffuser 48, but to
allow CO.sub.2 gas 30 to move laterally along the length or width
of the diffuser 48 as it traverses the thickness of the diffuser
48.
[0111] Turning now to FIG. 29, a three-compartment electrochemical
reactor 410 is shown in which a three-compartment container 12a
encloses an anolyte compartment 18, and anode 14, and, during
operation, anolyte 24 contained within the anolyte compartment 18;
a membrane 22; a catholyte compartment 20, a porous cathode 16a,
and, during operation, catholyte compartment mixture 26 contained
within the catholyte compartment 20; and a gas compartment 28, the
gas compartment 28 containing, during operation, CO.sub.2 gas 30.
The membrane 22 separates the anolyte compartment 18 and the
catholyte compartment 20. The porous cathode 16a is adjacent the
membrane 22 and virtually occupies the catholyte compartment 20 as
shown. The gas compartment 28 is adjacent the catholyte compartment
20 and the porous cathode 16a. Also during operation, an anolyte
feed 36 introduces anolyte 24 into the anolyte compartment 18, an
anolyte withdrawal 44 removes anolyte 24 as well as other anode
reaction products, a catholyte feed 34 introduces catholyte 25 into
the catholyte compartment 20 and the porous cathode 16a, a
catholyte compartment mixture withdrawal 42 removes catholyte
compartment mixture 26, and a CO.sub.2 gas feed 32 introduces
CO.sub.2 gas 30 into the gas compartment 28. During operation, the
CO.sub.2 gas 30 in the gas compartment 28, under a pressure
differential with the porous cathode 16a, is distributed (indicated
by arrows 40 and flows into the porous cathode 16a and into the
catholyte 16 and catholyte mixture 26. The porous cathode 16a may
comprise a suitable cathodic catalyst, for example, one based upon
Sn or a Sn-based alloy as described herein. The porous cathode 16a
may further comprise substrates and cathodic catalysts deposited as
described herein.
[0112] FIG. 30 shows a three-compartment reactor 420, and is a
modification of FIG. 29, further including a diffuser 48 adjacent
the cathode 16a on the gas compartment side of the cathode 16a.
Good results were obtained with a PTFE-coated porous material which
facilitates the distribution of the CO.sub.2 gas 30. In a further
embodiment, the diffuser 48 is fabricated to allow not only
CO.sub.2 gas 30 traversing the thickness of the diffuser 48, but to
allow CO.sub.2 gas 30 to move laterally along the length or width
of the diffuser 48 as it traverses the thickness of the diffuser
48.
[0113] While certain preferred embodiments of the present invention
have been disclosed in detail, it is to be understood that various
modifications may be adopted without departing from the spirit of
the invention or scope of the following claims.
* * * * *