U.S. patent application number 16/132914 was filed with the patent office on 2019-05-02 for electrolyzer for gaseous carbon dioxide.
The applicant listed for this patent is The Penn State Research Foundation. Invention is credited to Yuguang C. Li, Thomas E. Mallouk, Zhifei Yan.
Application Number | 20190127865 16/132914 |
Document ID | / |
Family ID | 66245195 |
Filed Date | 2019-05-02 |
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United States Patent
Application |
20190127865 |
Kind Code |
A1 |
Li; Yuguang C. ; et
al. |
May 2, 2019 |
ELECTROLYZER FOR GASEOUS CARBON DIOXIDE
Abstract
An electrochemical device and method can include techniques
involving bipolar membrane electrolysis to transform an input
product into an output product. Some embodiments can include a
gas-diffusion electrode as a cathode, a bipolar membrane configured
to facilitate autodissociation, and an anode that can be configured
as a liquid-electrolyte style electrode or a gas-diffusion
electrode. In some embodiments the electrochemical device can be
configured as a CO.sub.2 electrolyzer that is designed to utilize
input product including carbon dioxide gas and water to generate
output products that can include gaseous carbon monoxide or other
reduction products of carbon dioxide and gaseous oxygen or the
oxidation products of a depolarizer such as hydrogen, methane, or
methanol. Embodiments can be utilized in the production of fuels or
feedstocks for fuels and carbon-containing chemicals, in air
purification systems, flue gas treatment devices, and other
machines and facilities.
Inventors: |
Li; Yuguang C.; (State
College, PA) ; Yan; Zhifei; (State College, PA)
; Mallouk; Thomas E.; (State College, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Penn State Research Foundation |
University Park |
PA |
US |
|
|
Family ID: |
66245195 |
Appl. No.: |
16/132914 |
Filed: |
September 17, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62577357 |
Oct 26, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/00 20130101; C25B
9/08 20130101; C25B 9/10 20130101; C25B 13/08 20130101; C25B 3/04
20130101; C25B 1/10 20130101 |
International
Class: |
C25B 9/10 20060101
C25B009/10; C25B 1/10 20060101 C25B001/10; C25B 3/04 20060101
C25B003/04; C25B 13/08 20060101 C25B013/08 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND
DEVELOPMENT
[0002] This invention was made with government support under Grant
No. DE-FG02-07ER15911 awarded by the Department of Energy. The
Government has certain rights in the invention.
Claims
1. An electrochemical device, comprising: an electrochemical cell
comprising a cathode, an anode, and a membrane; wherein: at least a
portion of the cathode is separated from at least a portion of the
anode by the membrane; the cathode comprises a gas-diffusion
electrode; the anode comprises at least one of a liquid-electrolyte
style electrode and a gas-diffusion electrode; and the membrane is
a bipolar membrane, the bipolar membrane being configured to
maintain a flux of protons to the cathode and also maintain a flux
of hydroxide ions to the anode, wherein the electrochemical cell is
configured to receive carbon dioxide gas and water and output
reduction products of carbon dioxide at the cathode and oxygen or
other oxidized products of a depolarizer at the anode.
2. The electrochemical device recited in claim 1, wherein the
bipolar membrane comprises a cation exchange membrane and an anion
exchange membrane.
3. The electrochemical device recited in claim 1, wherein the
bipolar membrane is configured to promote autodissociation of
water.
4. The electrochemical device recited in claim 1, wherein the
bipolar membrane further comprises a membrane catalyst.
5. The electrochemical device recited in claim 4, wherein the
membrane catalyst comprises at least one of a silicate, an amine
polymer, graphite oxide, and an anolyte solution.
6. The electrochemical device recited in claim 2, wherein the anion
exchange membrane comprises a cation-exchange polymer film.
7. The electrochemical device recited in recited in claim 1,
wherein the electrochemical cell has a cell first end and a cell
second end, the electrochemical device also comprising: a cathode
flow medium positioned between the bipolar membrane and the
cathode; and an anode flow medium positioned between the bipolar
membrane and the anode.
8. The electrochemical device recited in claim 7, wherein: the
cathode flow medium has at least one cell inlet and at least one
cell outlet; and the anode flow medium has at least one cell inlet
and at least one cell outlet.
9. The electrochemical device recited in claim 8, wherein the
cathode flow medium comprises carbon and the anode flow medium
comprises carbon.
10. The electrochemical device recited in claim 8, wherein: the
electrochemical device is configured as a carbon dioxide
electrolyzer, the cathode comprises a cathode catalysts configured
as a carbon dioxide reduction catalyst; and the anode comprises an
anode catalyst configured as a water oxidation catalyst or as a
catalyst for oxidation of the depolarizer, the depolarizer
comprising hydrogen, methane, or methanol.
11. The electrochemical device recited in claim 10, wherein the
electrochemical cell is configured to receive carbon dioxide gas
and generate reduction products of carbon dioxide that include any
one or combination of formic acid, methanol, methane, formaldehyde,
acetaldehyde, acetic acid, glyoxal, ethanol, ethene, ethane,
ethylene glycol, dimethyl ether, methyl formate, propene, propane,
n-propanol, isopropanol, and isomers of butanol, and hydrogen.
12. A method of reducing product crossover in an electrochemical
cell of an electrochemical device, the method comprising:
configuring a bipolar membrane of an electrochemical cell that is
positioned between an anode and a cathode to cause ions to travel
towards an anode electrode and a cathode electrode of the
electrochemical cell when the electrochemical cell is under an
applied current condition; operating the electrochemical cell so
that the bipolar membrane facilitates a supply of protons (H.sup.+)
to the cathode to cause water (H.sub.2O) to self-ionize via
autodissociation to generate hydroxide ions (OH.sup.-) and protons
H.sup.+ to supply a flux of the OH.sup.- to the anode and supply a
flux of the H.sup.+ to the cathode.
13. The method recited in claim 12, wherein the flux of H.sup.+
provided by the bipolar membrane opposes product crossover in the
electrochemical cell.
14. The method recited in claim 12, wherein the bipolar membrane
has an anion exchange layer and a cation exchange layer joined
together at an interfacial layer, the interfacial layer configured
to catalyze autodissociation of H.sub.2O.
15. The method recited in claim 14, further comprising depositing
at least one catalyst layer on the interfacial layer.
16. The method recited in claim 15, further comprising tuning water
dissociation reactions at the interfacial layer via adjusting a
type of the catalyst and/or an amount of the catalyst.
17. The method recited in claim 15, wherein the at least one
catalyst layer comprises graphite oxide.
18. The method recited in claim 15, wherein the cation exchange
layer and the anion exchange layer define a cation-anion exchange
junction region; and wherein the cation-anion exchange junction is
configured so that the cation exchange layer interpenetrates the
anion exchange layer and/or the anion exchange layer
interpenetrates the cation exchange layer.
19. The method recited in claim 18, further comprising: generating
a plurality of transport pathways for water dissociation products
H.sup.+ and OH.sup.- to flow via the interpenetrating cation
exchange layer and anion exchange layer.
20. The method recited in claim 19, wherein: the electrochemical
device is a carbon dioxide electrolyzer, the cathode comprises a
cathode catalyst configured as a carbon dioxide reduction catalyst;
the anode comprises an anode catalyst configured as a water
oxidation catalyst or as a catalyst for a depolarizer, the
depolarizer comprising hydrogen, methane, or methanol; and the
electrochemical cell includes: a cathode flow medium between the
cathode and the bipolar membrane, the cathode flow medium
comprising carbon, at least one cell inlet of the cathode flow
medium is configured to receive carbon dioxide, and at least one
cell outlet of the cathode flow medium is configured to output
carbon monoxide gas and/or water; an anode flow medium between the
anode and the bipolar membrane, the anode flow medium comprising
carbon, at least one cell inlet of the anode flow medium configured
to receive water and/or an electrolyte and/or the depolarizer, and
at least one cell outlet of the anode flow medium configured to
output oxygen or the oxidized product of the depolarizer; and
wherein the operating of the electrochemical cell comprises:
feeding water and/or an electrolyte and/or the depolarizer to the
anode flow medium; feeding a flow of carbon dioxide and water to
the cathode flow medium; outputting oxygen and/or the oxidation
products of the depolarizer from the anode flow medium; and
outputting carbon monoxide and/or other reduction products of
carbon dioxide from the cathode flow medium.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is related to and claims the benefit
of priority of U.S. Provisional Patent Application Ser. No.
62/577,357 filed on Oct. 26, 2017, the entire contents of which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] Embodiments can relate to an electrochemical device capable
of gas phase electrolysis and bipolar membrane electrolysis.
BACKGROUND OF THE INVENTION
[0004] Conventional electrochemical reduction of carbon dioxide
systems and methods can be appreciated from U.S. Pat. No.
9,481,939, U.S. Pat. No. 9,181,625, U.S. Pat. No. 9,085,827, U.S.
Pat. Publ. No. 2017/0183789, U.S. Pat. Publ. No. 2013/0118911, and
Pat. Publ. No. CN 102912374. Conventional systems may be
inefficient, have poor stability, and/or have difficulty in
separating reaction products from the electrolytes. These and other
disadvantages may limit the use of conventional electrochemical
reduction systems.
BRIEF SUMMARY OF THE INVENTION
[0005] Embodiments can be related to an electrochemical device that
may include techniques involving gas phase electrolysis and bipolar
membrane electrolysis to transform an input product into an output
product. Some embodiments can include an electrochemical device
having at least one electrochemical cell, each electrochemical cell
having a cathode, a membrane, and an anode. In some embodiments,
input product can be introduced into the electrochemical device at
the cathode. This can include introducing an input product in a gas
phase. Reactions at the cathode can transform the input product
into reduced chemical products. Some of these reduced chemical
products can be caused to exit the electrochemical device as output
product. Some of these reduced chemical products can be caused to
react with the membrane to generate additional chemical products.
The additional chemical products can be caused to react with the
anode. This can generate additional output product.
[0006] As a non-limiting example, carbon dioxide gas and water may
be introduced into the electrochemical device at the cathode.
Reactions at the cathode can transform the carbon dioxide gas into
reduction products of carbon dioxide and oxygen as output products.
The reduction products of carbon dioxide and the oxygen may be
directed out from the electrochemical device. In some embodiments,
water can be introduced into the electrochemical device at the
anode. Liquid electrolyte and/or the anode can electrochemically
drive the oxidation of the water to oxygen as an output product.
The oxygen can then be directed out from the electrochemical
device. In some embodiments, a depolarizer such as methane,
hydrogen, or methanol can be introduced to the anode of the cell
and its oxidation products may be directed out from the
electrochemical device. Some embodiments can include a
gas-diffusion anode. This may be used to generate an
electrochemical device without a liquid electrolyte.
[0007] With some embodiments, introduction of input product as a
gas can allow for reaction products to be generated in the gas
phase. This may also allow for collection of output product in the
gas phase. These gas phase products can eliminate the need to
provide product separation techniques, as no product is being
dissolved in a liquid electrolyte solution. As no reactant is being
dissolved in a liquid electrolyte solution, the reactants are not
caused to transport through a liquid, which can improve upon the
transport rate of chemical species within the electrochemical
device.
[0008] Some embodiments can include use of a bipolar membrane.
Embodiments of the bipolar membrane can be used to separate the
cathode and the anode, as well as isolate the reactants associated
with the cathode and isolate the reactants associated with the
anode. Embodiments of the bipolar membrane can also be configured
to manage flux of chemical species from the bipolar membrane to the
cathode and/or to the anode. For example, the bipolar membrane can
be used to provide a flux of protons to the cathode and a flux of
hydroxide ions to the anode. This may generate an electrochemical
device that can eliminate or reduce undesired crossover of chemical
product between the cathode and anode. This can also allow the
electrochemical device to operate with a stable electrolyte pH,
even under long-term operation.
[0009] While various embodiments may describe an electrochemical
device configured for carbon dioxide electrolysis into carbon
monoxide and oxygen, other forms of output product can be
generated. For example, it is contemplated for embodiments of the
electrochemical device to be used for carbon dioxide electrolysis
into syngas (carbon monoxide+hydrogen) and oxygen. Syngas can be
used as a precursor to hydrocarbon fuels, other fuels, and other
high value chemicals (e.g., propane, gasoline, methanol,
dlmethylether (DME), formate, methane, methanol, ethylene glycol,
butanol, etc. It is also contemplated for embodiments of the
electrochemical device that different cathode catalysts may be
chosen to reduce carbon dioxide directly to other carbon-containing
products, such as formic acid, acetic acid, ethylene, propylene,
methanol, ethanol, propanol and ethylene glycol.
[0010] In one embodiment, an electrochemical device can include an
electrochemical cell comprising a cathode, an anode, and a
membrane. At least a portion of the cathode can be separated from
at least a portion of the anode by the membrane. The cathode can
have a gas-diffusion electrode. The anode can have at least one of
a liquid-electrolyte style electrode and a gas-diffusion electrode.
The membrane can be a bipolar membrane. The bipolar membrane can be
configured to maintain a flux of protons to the cathode and also
maintain a flux of hydroxide ions to the anode. The electrochemical
cell can be configured to receive carbon dioxide gas and water and
output reduction products of carbon dioxide and oxygen.
[0011] In some embodiments, the bipolar membrane can include a
cation exchange membrane and an anion exchange membrane. In some
embodiments, the bipolar membrane can be configured to promote
autodissociation. In some embodiments, the bipolar membrane further
can have a membrane catalyst. In some embodiments, the membrane
catalyst can be at least one of a silicate, an amine polymer, a
graphite oxide, and an anolyte solution. In some embodiments, the
anion exchange membrane can be laminated by a cation-exchange
polymer film. In some embodiments, the cation-exchange polymer film
can be a sulfonated tetrafluoroethylene based
fluoropolymer-copolymer. In some embodiments, the cation-exchange
polymer film can be a sulfonated poly(ether ether ketone) polymer.
In some embodiments, the cation-exchange polymer film can be a
polymeric weak acid, such as poly(acrylic acid). In some
embodiments, the cation-exchange film can contain an inorganic
cation exchanger such as a clay, a layered transition metal oxide,
or graphite oxide, either alone or as a polymer composite. In some
embodiments, a surface of the cation exchange membrane can be
patterned and/or a surface of the anion exchange membrane can be
patterned. In some embodiments, the cathode can be a cathode
catalyst. In some embodiments, the cathode catalyst can be gold,
silver, copper, indium, bismuth, lead, tin, tellurium, and/or
germanium. In some embodiments, the cathode catalyst can be mixed
with a binder, a polymeric electrolyte coating, and/or an ionic
liquid. In some embodiments, the anode can be an anode catalyst. In
some embodiments, the anode catalyst can be at least one of iridium
oxide, ruthenium alloys, mixed oxides of ruthenium containing
iridium and/or platinum, mixed metal oxides containing cobalt,
nickel, iron, manganese, lanthanum, cerium, copper, nickel borate,
cobalt phosphate, NiFeOx.
[0012] In one embodiment, an electrochemical device can include an
electrochemical cell having a cell first end and a cell second end.
The electrochemical device can have a cathode with a gas-diffusion
electrode. The electrochemical device can have an anode with at
least one of a liquid-electrolyte style electrode and a
gas-diffusion electrode. The electrochemical device can have
bipolar membrane separating at least a portion of the cathode from
at least a portion of the anode. The electrochemical device can
have a cathode flow medium comprising carbon. The electrochemical
device can have an anode flow medium comprising carbon. The
electrochemical device can have a frame configured to hold the
cathode flow medium, the cathode, the bipolar membrane, the anode,
and the anode flow medium together.
[0013] In some embodiments, at least one of the cathode flow
mediums and the anode flow medium has at least one of a cell inlet
and a cell outlet. In some embodiments, the frame has at least one
pass-through region corresponding with at least one of the cell
inlets and the cell outlet. In some embodiments, the frame seals
the electrochemical cell except at the at least one pass-through
region. In some embodiments, the cathode has a cathode catalyst
configured as a reduction catalyst. In some embodiments, the anode
has an anode catalyst configured as an oxidation catalyst.
[0014] In one embodiment, a carbon dioxide electrolyzer can include
an electrochemical cell comprising a cathode, an anode, and a
membrane. At least a portion of the cathode can be separated from
at least a portion of the anode by the membrane. The cathode can
have a gas-diffusion electrode. The anode can have at least one of
a liquid-electrolyte style electrode and a gas-diffusion electrode.
The membrane can be a bipolar membrane. The cathode can be a
cathode catalyst configured as a carbon dioxide reduction catalyst.
The anode can be an anode catalyst configured as a water oxidation
catalyst.
[0015] In one embodiment, an electrochemical device can include an
electrochemical cell having a cell first end and a cell second end.
The electrochemical device can have a cathode comprising a
gas-diffusion electrode. The electrochemical device can have an
anode comprising at least one of a liquid-electrolyte style
electrode and a gas-diffusion electrode. The electrochemical device
can have a bipolar membrane separating at least a portion of the
cathode from at least a portion of the anode. The electrochemical
device can have a cathode flow medium comprising carbon. In some
embodiments, the cathode flow medium can be located between the
cell first end and the cathode. In some embodiments, at least one
cell inlet can be formed in the cathode flow medium configured to
receive carbon dioxide gas. In some embodiments, at least one cell
outlet can be formed in the cathode flow medium configured to
output carbon monoxide gas and/or water. In some embodiments, the
device can have an anode flow medium comprising carbon. The anode
flow medium can be located between the cell second end and the
anode. At least one cell inlet can be formed in the anode flow
medium configured to receive water and/or electrolyte. At least one
cell outlet can be formed in the anode flow medium configured to
output oxygen. The bipolar membrane can be configured to maintain a
flux of protons to the cathode and a flux of hydroxide ions to the
anode.
[0016] In one embodiment, a method of reducing product crossover in
an electrochemical cell can involve configuring a bipolar membrane
of an electrochemical to cause ions to travel towards an anode
electrode and a cathode electrode of the electrochemical cell when
the electrochemical cell is under an applied current condition.
[0017] In some embodiments, the method can involve the bipolar
membrane being configured to supply protons (H.sup.+) to the
cathode and to cause water (H.sub.2O) to self-ionize via
autodissociation to generate hydroxide ions (OH.sup.-). In some
embodiments, the method can involve the bipolar membrane being
configured to supply the OH.sup.- to the anode. In some
embodiments, the method can involve generating a reverse bias to
provide a flux of H.sup.+ to the cathode. In some embodiments, the
method can involve the flux of H.sup.+ opposing the direction of
product crossover in the electrochemical cell. In some embodiments,
the method can involve configuring the bipolar membrane to have an
anion exchange layer and a cation exchange layer joined together at
an interfacial layer, the interfacial layer configured to catalyze
the autodissociation of H.sub.2O. In some embodiments, the method
can involve depositing at least one catalyst layer on the
interfacial layer. In some embodiments, the method can involve
tuning the water dissociation reaction at the interfacial layer via
adjusting a type of the catalyst and/or an amount of the
catalyst.
[0018] In one embodiment, a bipolar membrane can include a cation
exchange layer and an anion exchange layer, the cation exchange
layer being adjacent the anion exchange layer to form a
cation-anion exchange junction region. The bipolar membrane can
include at least one catalyst layer formed within the cation-anion
exchange junction region. In some embodiments, at least one
catalyst layer can be configured to decrease the electric field
intensity applied across the cation-anion exchange junction region.
In some embodiments, at least one catalyst layer is graphite
oxide.
[0019] In at least one embodiment, a bipolar membrane can include a
cation exchange layer and an anion exchange layer, the cation
exchange layer being adjacent the anion exchange layer to form a
cation-anion exchange junction region. In some embodiments, the
cation-anion exchange junction can be configured so that the cation
exchange layer interpenetrates the anion exchange layer and/or the
anion exchange layer interpenetrates the cation exchange layer. In
some embodiments, the interpenetrating cation exchange layer and
anion exchange layer can generate a plurality of transport pathways
for water dissociation products H.sup.+ and OH.sup.- to flow.
[0020] A method of reducing product crossover in an electrochemical
cell of an electrochemical device can include various steps. These
steps can include, for example, configuring a bipolar membrane of
an electrochemical cell that is positioned between an anode and a
cathode to cause ions to travel towards an anode electrode and a
cathode electrode of the electrochemical cell when the
electrochemical cell is under an applied current condition and
operating the electrochemical cell so that the bipolar membrane
facilitates a supply of protons (H.sup.+) to the cathode, to cause
water (H.sub.2O) to self-ionize via autodissociation to generate
hydroxide ions (OH.sup.-) and protons H.sup.+ to supply a flux of
the OH.sup.- to the anode and supply a flux of the H.sup.+ to the
cathode.
[0021] In some embodiments of the method of reducing product
crossover in the electrochemical cell of an electrochemical device,
the electrochemical device can be a carbon dioxide electrolyzer,
the cathode can include a cathode catalyst configured as a carbon
dioxide reduction catalyst, the anode can include an anode catalyst
configured as a water oxidation catalyst, and the electrochemical
cell can include: (i) a cathode flow medium between the cathode and
the bipolar membrane and there is at least one cell inlet of the
cathode flow medium configured to receive carbon dioxide and at
least one cell outlet of the cathode flow medium configured to
output carbon monoxide gas and/or water; and (ii) an anode flow
medium between the anode and the bipolar membrane, at least one
cell inlet of the anode flow medium is configured to receive water
and/or an electrolyte, and at least one cell outlet of the anode
flow medium is configured to output oxygen. For such embodiments,
the operating of the electrochemical cell can include: feeding
water and/or an electrolyte to the anode flow medium; feeding a
flow of carbon dioxide to the cathode flow medium; outputting
gaseous oxygen from the anode flow medium; and outputting carbon
monoxide and/or water from the cathode flow medium.
[0022] Further features, aspects, objects, advantages, and possible
applications of the present invention will become apparent from a
study of the exemplary embodiments and examples described below, in
combination with the Figures, and the appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0023] The above and other objects, aspects, features, advantages
and possible applications of the present invention will be more
apparent from the following more particular description thereof,
presented in conjunction with the following drawings, in which:
[0024] FIG. 1 shows a first exemplary embodiment of an
electrochemical device.
[0025] FIG. 2 shows an exploded view of the first exemplary
embodiment of an electrochemical device.
[0026] FIG. 3 shows an embodiment of an electrochemical cell that
may be used in the first exemplary embodiment of an electrochemical
device.
[0027] FIG. 4 shows a cut-away view of the first exemplary
embodiment of an electrochemical device.
[0028] FIG. 5 is another view of the first exemplary embodiment of
an electrochemical device.
[0029] FIG. 6 is a graph shows stability data for an embodiment of
an electrochemical device.
[0030] FIG. 7 is a graph comparing cell potential over time for an
embodiment of the electrochemical device and a conventional
electrochemical device using a nafion cation exchange membrane.
[0031] FIG. 8 is a graph showing a current-voltage curve for an
embodiment of the electrochemical device operating at high current
density.
[0032] FIGS. 9 and 10 each shows a faradaic efficiency plot for a
conventional device having a bipolar membrane electrolyzer with an
aqueous bicarbonate catholyte. These graphs demonstrate examples of
degradation of electrode selectivity that often occurs in
conventional devices.
[0033] FIG. 11 shows a current density plot of a conventional
bipolar membrane compared to an embodiment of a bipolar membrane
that may be used with an embodiment of the electrochemical
device.
[0034] FIG. 12 is an exemplary block diagram showing the transport
of methanol by electroosmosis through a conventional anion-exchange
membrane.
[0035] FIG. 13 is an exemplary block diagram showing outward flux
of H.sup.+ and OH.sup.- that can occur in an embodiment of a
bipolar membrane.
[0036] FIG. 14 is graphs showing crossover of formate, methanol,
and ethanol versus time in exemplary electrochemical cells having
an anion exchange membrane and an embodiment of the bipolar
membrane.
[0037] FIG. 15 shows graphs illustrating crossover of formate and
methanol at different applied currents after 2 hours in exemplary
electrochemical cells having an anion exchange membrane and an
embodiment of the bipolar membrane.
[0038] FIG. 16 shows graphs illustrating crossover of formate and
methanol at zero current density with 0.5 M KHCO.sub.3 used as
electrolyte on both the cathode and anode sides of exemplary
electrochemical cells having an anion exchange membrane and an
embodiment of the bipolar membrane.
[0039] FIG. 17 shows a schematic of the preparation of an
embodiment of the bipolar membrane having an exemplary interfacial
catalyst layer and a scanning electron microscope (SEM) image of
the bipolar membrane.
[0040] FIG. 18 is a plot showing J-E curves of embodiments of the
bipolar membrane having an exemplary interfacial catalyst layer
prepared by an exemplary layer-by-layer technique.
[0041] FIG. 19 is a plot showing the water dissociation rate
constant kd measured for embodiments of the bipolar membrane having
an exemplary interfacial catalyst layer.
[0042] FIG. 20 is a plot showing water dissociation reaction
resistance Rw measured for embodiments of the bipolar membrane
having an exemplary interfacial catalyst layer.
[0043] FIG. 21 is a plot showing depletion region thickness as a
function of reverse bias voltage for embodiments of the bipolar
membrane having an exemplary interfacial catalyst layer.
[0044] FIG. 22 is a graph showing J-E curves for embodiments of the
bipolar membrane having an exemplary interfacial catalyst
layer.
[0045] FIG. 23 is a graph showing potential distribution profiles
for embodiments of the bipolar membrane having an exemplary
interfacial catalyst layer.
[0046] FIG. 24 is a graph showing concentration profiles of the
water dissociation products H.sup.+ and OH.sup.- for embodiments of
the bipolar membrane having an exemplary interfacial catalyst
layer.
[0047] FIG. 25 is a graph showing electrolyte KNO.sub.3 ion
distributions for embodiments of the bipolar membrane having an
exemplary interfacial catalyst layer.
[0048] FIG. 26 shows schematic drawings of the depletion region for
embodiments of the bipolar membrane having an exemplary interfacial
catalyst layer and embodiments without an exemplary interfacial
catalyst layer, along with enlarged views of the cation-anion
exchange junction for each. The thickness of the black arrows
indicate the higher electric field in the bipolar membrane without
the exemplary interfacial catalyst layer.
[0049] FIG. 27 shows a graph of electric field intensity at a
cation-anion exchange junction for embodiments of the bipolar
membrane having an exemplary interfacial catalyst layer and
embodiments without an exemplary interfacial catalyst layer.
[0050] FIG. 28 shows a graph of electric field intensity at
cation-anion exchange junction for embodiments of the bipolar
membrane having an exemplary interfacial catalyst layer and
embodiments without an exemplary interfacial catalyst layer.
[0051] FIG. 29 shows a scanning electron microscope image and a
schematic of a cation-anion exchange junction of an embodiment of a
3D bipolar membrane with intertwined anion exchange layer-cation
exchange layer fibers.
[0052] FIG. 30 is a graph showing J-E curves for a cation-anion
exchange junction of an embodiment of a 3D bipolar membrane.
[0053] FIG. 31 is a graph showing the water dissociation rate
constant kd for a cation-anion exchange junction of an embodiment
of a 3D bipolar membrane.
[0054] FIG. 32 is a graph showing the water dissociation reaction
resistance Rw for a cation-anion exchange junction of an embodiment
of a 3D bipolar membrane.
[0055] FIG. 33 is a graph showing depletion region thickness d as a
function of reverse bias voltage for a cation-anion exchange
junction of an embodiment of a 3D bipolar membrane.
[0056] FIG. 34 is a graph showing J-E curves for embodiment of a
bipolar membrane.
[0057] FIG. 35 is a graph showing steady-state performance of
embodiment of a bipolar membrane.
DETAILED DESCRIPTION OF THE INVENTION
[0058] The following description is of an embodiment presently
contemplated for carrying out the present invention. This
description is not to be taken in a limiting sense, but is made
merely for the purpose of describing the general principles and
features of the present invention. The scope of the present
invention should be determined with reference to the claims.
[0059] Referring to FIGS. 1-4, various embodiments of the apparatus
disclosed herein can include an electrochemical device 10 capable
of gas phase electrolysis and bipolar membrane electrolysis.
Embodiments of the electrochemical device 10 can be configured to
generate an output product from an input product. The input product
can be a gas, a liquid, a solid or combinations thereof e.g. a
slurry, gas having solid particulates entrained therein, a liquid
having solid particles entrained therein, etc.). The output product
can be a gas, a liquid, a solid, or a combination thereof (e.g. a
slurry, a gas having solid particulates entrained therein, etc.).
In some embodiments, both the input product and the output product
include a gas. In some embodiments, the output product can be a
reduced chemical product of the input product, an oxidized product
of the input product, and/or a combination of both.
[0060] Some embodiments of the electrochemical device 10 can be
configured as an electrolyzer. For example, embodiments of the
electrochemical device 10 can be configured to use electric current
to drive chemical reactions that may facilitate generating the
output product from the input product. In some embodiments, the
electrochemical device 10 can be configured as a carbon dioxide
(CO.sub.2) electrolyzer. As a non-limiting example, the
electrochemical device 10 can be configured to receive carbon
dioxide (CO.sub.2) gas as an input product at the cathode 14.
Reactions within the electrochemical device 10 can generate carbon
monoxide (CO), water (H.sub.2O), and/or hydrogen (H.sub.2) as an
output product. The CO, the H.sub.2O, and/or the H.sub.2 may be
caused to exit the electrochemical device 10 for capture or further
processing. In some embodiments, the H.sub.2O can be caused to
self-ionize at the membrane 22 via autodissociation to generate
protons (H.sup.+) and hydroxide ions (OH.sup.-). Some of the
H.sub.2O generated at the cathode 14 can be caused to move to the
anode 18. Additional H.sub.2O can be introduced into the
electrochemical device 10 as input product at the anode 18. The
additional H.sub.2O can be in the form of a liquid or a vapor. If
the additional H.sub.2O is in the form of a liquid, the OH.sup.-
may be used to react with the anode 18 via electrolyte of the
electrochemical device 10 to generate oxygen (O.sub.2) and/or
H.sub.2O as additional output product. If the additional H.sub.2O
is in the form of a vapor, the OH.sup.- may be used to react
directly with the anode 18 of electrochemical device 10 to generate
oxygen (O.sub.2) and/or H.sub.2O as additional output product. The
O.sub.2 and/or the H.sub.2O may be caused to exit the
electrochemical device 10 for capture or further processing.
[0061] As explained herein, other input products can be used, such
as humidified CO.sub.2 gas, for example. The input product can also
be a mixture of gases that may include gases other than CO.sub.2
gas. In addition, other output products can be generated, such as
formic acid, formate, methane, methanol, ethylene, ethylene glycol,
butanol, etc. For example, in the situation in which the device 10
is used for CO.sub.2 reduction to CO, the device 10 can be
configured to receive CO.sub.2 gas as an input and generate
CO.sub.2 and O.sub.2. In the situation in which the device 10 is
used for CO.sub.2 reduction to products other than CO (e.g.,
formate, methanol, ethylene, etc.), the device 10 can be configured
to receive CO.sub.2 gas and H.sub.2O as inputs and generate
reduction products of and CO.sub.2 and O.sub.2. The reduction
products of CO.sub.2 can include but are not limited to formic
acid, methanol, methane, formaldehyde, acetaldehyde, acetic acid,
glyoxal, ethanol, ethene, ethane, ethylene glycol, dimethyl ether,
methyl formate, propene, propane, n-propanol, isopropanol, isomers
of butanol, as well as mixtures of these products and hydrogen.
[0062] Some embodiments of the electrochemical device 10 can
include an electrochemical cell 12 structure. Embodiments of the
electrochemical cell 12 structure can include a cathode 14 within a
cathode flow medium 16 and an anode 18 within an anode flow medium
20. The cathode flow medium 16 and/or cathode 14 can be separated
from the anode flow medium 20 and/or anode 18 by a membrane 22. The
electrochemical cell 12 can be configured to facilitate intake of
an input product. The input product can enter the electrochemical
cell 12 at a cell inlet 26. The electrochemical cell 12 can be
configured to generate an output product from the input product.
The electrochemical cell 12 can be configured to transform the
input product at the cathode 14 via a reduction reaction. The
electrochemical cell 12 can be configured to transform the input
product at the anode 18 via an oxidation reaction. The output
product can exit the electrochemical cell 12 at a cell outlet
28.
[0063] The electrochemical cell 12 may include a cathode 14. The
cathode 14 may be positioned adjacent or within the cathode flow
medium 16. The cathode flow medium 16 can be positioned at a cell
first end 30 of the electrochemical cell 12. The electrochemical
cell 12 can include an anode 18. The anode 18 may be positioned
adjacent or within the anode flow medium 20. The anode flow medium
20 can be positioned at a cell second end 32 of the electrochemical
cell 12. The electrochemical cell 12 can include a membrane 22. The
membrane 22 may be disposed between the cell first end 30 and the
cell second end 32. This can include being disposed between the
cathode 14 and the anode 18. A volume of space between the cathode
14 and the cell first end 30 of the electrochemical cell 12 can be
referred to as a cathode flow medium 16. The cathode flow medium 16
can include a carbon material filled within the volume of space
between the cathode 14 and the cell first end 30 of the
electrochemical cell. A volume of space between the anode 18 and
the cell second end 32 can be referred to as an anode flow medium
20. The anode flow medium 20 can include a carbon material or a
graphite oxide material filled within the volume of space between
the anode 18 and the cell second end 32. In some embodiments, the
membrane 22 can separate at least a portion of the cathode 14 from
at least a portion of the anode 18. This can include a physical
separation, a chemical separation (e.g., chemical isolation), an
electrical separation (e.g., electrical isolation), etc.
[0064] The electrochemical cell 12 can include any number of
cathodes 14, anodes 18, and/or membranes 22. For example, the
electrochemical cell 12 can include a single cathode 14 or a
plurality of cathodes 14. With a plurality of cathodes 14, each
cathode 14 may be stacked against each other in a serial formation,
in a staggered formation, or in any other type of formation. The
electrochemical cell 12 can include a single anode 18 or a
plurality of anodes 18. With a plurality of anodes 18, each anode
18 may be stacked against each other in a serial formation, in a
staggered formation, or in any other type of formation. The
membrane 22 can include a single membrane 22 or a plurality of
membranes 22. With a plurality of membranes 22, each membrane 22
may be stacked against each other in a serial formation, in a
staggered formation, or in any other type of formation. The
electrochemical cells 12 may also be stacked in series to create a
multi-cell electrolyzer.
[0065] The cathode flow medium 16 can be configured as a flow
compartment. This can include allowing flow of input product and/or
output product. The cathode flow medium 16 can include a cell inlet
26 to allow for introduction of input product. The cathode flow
medium 16 can include a cell outlet 28 to allow for removal of
output product. The anode flow medium 20 can be configured as a
flow compartment. This can include allowing flow of input product,
electrolyte, and/or output product. The anode flow medium 20 can
include a cell inlet 26 to allow for introduction of input product
and/or electrolyte. The anode flow medium 20 can include a cell
outlet 28 to allow for removal of electrolyte and/or output
product.
[0066] Any one or both of the cathode flow medium 16 and/or anode
flow medium 20 can include a desired shape or path. For example, a
portion of the cathode flow medium 16 can have a pathway 24 formed
into a portion thereof or onto a surface thereof. A portion the
anode flow medium 20 can have a pathway 24 formed into a portion
thereof or onto a surface thereof. The pathway 24 can facilitate
flow of fluid (e.g., input product, output product, electrolyte,
etc.) through the electrochemical cell 12. In some embodiments, the
pathway 24 can be configured to maximize the amount of contact the
fluid has with an electrode of the electrochemical cell 12. For
example, the cathode flow medium 16 can include a pathway 24 at an
interface between the cathode flow medium 16 and the cathode 14.
The pathway 24 can direct flow of the input product and/or output
product to maximize the amount to contact (e.g., surface area,
time, etc.) the input product and/or output product has with a
surface of the cathode 16 while the input product and/or output
product is within the electrochemical cell 12. The anode flow
medium 20 can include a pathway 24 at an interface between the
anode flow medium 20 and the anode 18. The pathway 24 can direct
flow of the input product, output product, and/or electrolyte to
maximize the amount to contact (e.g., surface area, time, etc.) the
input product, output product, and/or electrolyte has with a
surface of the anode 18 while the input product, the output
product, and/or electrolyte is within the electrochemical cell 12.
In addition, or in the alternative, the pathway 24 can be
configured to minimize the amount of contact or provide another
predetermined amount of contact of fluid with an electrode of the
system.
[0067] The pathway 24 can be straight, serpentine, zigzagged,
spiraled, etc. The shape and size of any pathway 24 can be the same
as or different from another pathway 24. The number, shape,
dimension, and size of any pathway 24 of the cathode flow medium 16
can be the same as or different from the number, shape, dimension,
and size of any pathway 24 of the anode flow medium 20. The shape,
size, dimension, and path direction can be used to influence
kinetics, fluid dynamics, etc. In some embodiments, any of the
pathways 24 can be in fluid communication with any one of the cell
inlets 26 and/or cell outlets 28.
[0068] The cathode 14 can include an electrical contact 34
configured to transport electrical charge. The anode 18 can include
an electrical contact 34 configured to transport electrical charge.
In some embodiments, the electrical contact 34 of the cathode 14
and the electrical contact 34 of the anode 18 can be placed into
electrical connection with a load 36 for transmission of electrical
current.
[0069] The electrochemical device 10 can be operated in a
galvanostatic mode, in which the anode 18 can be maintained at a
constant current. The electrochemical device 10 can be operated in
a potentiostatic mode, in which the potential difference between
the cathode 14 and the anode 18 can be held constant. For example,
the product selectivity can exhibit a voltage dependence behavior
(e.g., at different voltages, the ratio of products is different).
This can be used as a control parameter since the required ratio of
hydrogen and carbon monoxide (for a CO.sub.2 gas input) is
different for different subsequent reactions. Thus, one can control
the ratio of the products by simply controlling the voltage of the
reaction in potentiostatic mode. This may be suitable for
applications where a dynamic response is required. For
galvanostatic mode, a constant flow rate of the products can be
generated. This may be more suitable for a stationary system (e.g.,
where one single desired mix of products may be required for larger
scale operation).
[0070] In some embodiments, the electrochemical device 10 can
include a frame 38. The frame 38 can be a structure that holds the
electrochemical cell 12 together and/or seals the electrochemical
cell 12. Sealing can include forming a fluid (e.g., gas and/or
liquid) seal so as to prevent any fluid from entering and/or
exiting the electrochemical cell 12 except at a selected
pass-through region 40. For example, the frame 38 can be structured
so that is generates a fluid seal around the electrochemical cell
12, but includes a non-sealed portion to allow fluid to pass
there-through. The non-sealed portion can be the pass-through
region 40. The pass-through region 40 can be an opening in the
frame 38, a permeable portion of the frame 38, a semi-permeable
portion of the frame 38, etc. The frame 38 can be made from metal,
polymer, rubber, etc. The frame 38 can also have any of a number of
different shapes and sizes (e.g. cubical in shape, disc in shape,
polygonal in shape, elliptical in shape, etc.) to meet a particular
set of design criteria. The frame 38 can be configured so that the
electrochemical cell 12 can be incorporated into a machine, a
facility, or other type of device (e.g. conduit of an electricity
generation plant, conduit of an exhaust conduit for an engine,
incorporated into a gas turbine arrangement, incorporation into a
flue gas treatment apparatus, incorporation into a heating,
ventilation, and air conditioning (HVAC) system of a building,
inclusion into an air purification system of a vehicle, etc.).
[0071] In at least one embodiment, the electrochemical device 10
can include an electrochemical cell 12 having a frame 38 that holds
the electrochemical cell 12 together. For example, the frame 38 can
be a structure that holds the cathode 14, the membrane 22, and the
anode 18 of the electrochemical cell 12 in a serial configuration.
The electrochemical device 10 can have a plurality of sides. For
example, the electrochemical device 10 may have a cubic structure
with a device first side 42a, a device second side 42b, a device
third side 42c, a device fourth side 42d, a device fifth side 42e,
and a device sixth side 42f The electrochemical cell 12 can be
configured such that the cell first end 30 is adjacent the device
first side 42a. The cell second end 32 can be adjacent the device
second side 42b. The device third side 42c can be the top. The
device fourth side 42d can be the bottom. The device fifth side 42e
can be the front. The device sixth side 42f can be the rear. While
the various embodiment describe and illustrate the device 10 as
having a cubic structure, other shapes and number of sides can be
used to form the device 10.
[0072] The frame 38 can form a seal around the electrochemical
device 10 except for at a pass-through region 40. For example, a
first pass-through region 40 can be formed into the frame 38 to
facilitate introduction of input product into the electrochemical
cell 12. This can include facilitating introduction of input
product to a cell inlet 26 of the cathode flow medium 16. A second
pass-through region 40 can be formed into the frame 38 to
facilitate removal of output product from the electrochemical cell
12. This can include facilitating removal of output product from a
cell outlet 28 of the cathode flow medium 16.
[0073] A third pass-through region 40 can be formed into the frame
38 to facilitate introduction of electrolyte into the
electrochemical cell 12. This can include facilitating introduction
of electrolyte to a cell inlet 26 of the anode flow medium 20. A
fourth pass-through region 40 can be formed into the frame 38 to
facilitate removal of output product from the electrochemical cell
12. This can include facilitating removal of output product from a
cell outlet 28 of the anode flow medium 20. Some embodiments can
include introduction of input product into a cell inlet of the
anode flow medium 20. A fifth pass-through region 40 can be formed
into the frame 38 to facilitate introduction of input product into
a cell inlet 26 of the anode flow medium 20. Some embodiments can
include removal of electrolyte for processing and re-introduction
back into the electrochemical cell 12. This can facilitate
recycling of the electrolyte. The electrolyte can be removed
through the third pass-through region 40. Alternatively, a sixth
pass-through region 40 can be formed into the frame 38 to
facilitate removal of electrolyte from a cell outlet 28 of the
anode flow medium 20.
[0074] More or fewer cell inlets 26, cell outlets 28, and/or
pass-through regions 40 can be used. The portion(s) of the frame 38
that do generate a seal can prevent and/or inhibit introduction or
removal of input product, output product, electrolyte, and/or other
fluids. The flow rates for the fluid passed into the cell inlets
26, out of the cell outlets 28, or conveyed via the pass-through
regions 40 can be affected or driven by one or more flow control
mechanisms in fluid communication with the electrochemical cell 12.
Such flow control mechanisms can include valves in addition to
pumps or fans. Other devices (e.g. a compressor or a combustor)
that are in fluid communication with the electrochemical cell can
also be controlled to affect the flow rate of the fluid passed into
and out of the electrochemical cell 12. For example, the
electrolyte may be fed via an electrolyte source that is in fluid
communication with an electrolyte cell inlet 26 and the input
product can be fed into the cell via at least one input product
cell inlet 26 that is in fluid communication with at least one
source for the input product (e.g. an engine, a combustor, etc.).
The output product can exit the electrochemical cell via at least
one cell outlet 28. The pass-through regions 40 may be one or more
defined conduits within the frame 38 of the electrochemical cell 12
that facilitate the flow of fluid within the cell. There may be
packing material within the conduits or other elements therein as
well to help facilitate a desired flow rate, a desired residence
time, provide a catalytic effect, or other operational parameter of
the electrochemical cell.
[0075] Fluids (e.g., input product, output product, and/or
electrolyte) can be introduced into the electrochemical cell 12
and/or removed from the electrochemical cell 12 via a pump
(peristaltic pump, rotary pump, impulse pump, etc.). The pump can
be configured to force fluid into the electrochemical cell 12 or
draw fluid from the electrochemical cell 12. Any number or
combination of pumps can be in fluid communication with ay number
or combination of pass-through regions 40.
[0076] Some embodiments of the frame 38 can provide a single
pass-through region 40 in any device side or a plurality of
pass-through regions 40 in any device side. The shape and size of
any pass-through region 40 can be the same as or different from
another pass-through region 40. The number, shape, dimension, and
size of any pass-through region 40 on one device side can be the
same as or different from the number, shape, dimension, and size of
any pass-through region 40 on another device side. The number,
shape, dimensions, and size of the pass-through regions 40 can be
selected to influence kinetics, fluid dynamics, etc.
[0077] Embodiments of the cathode 14 can be an electrode configured
to generate a reduced chemical product from the input product. For
example, the cathode 14 can be configured to reduce the input
product by a reduction reaction. The reduced chemical product can
be used as output product and/or used to interact with the membrane
22. In some embodiments, the cathode 14 can be configured as a
gas-diffusion electrode. This may be done to facilitate transport
of an input product that includes a gas. In some embodiments, the
cathode 14 can include a porous substrate, such as carbon paper,
carbon cloth, electronically conducting metal oxide,
polyelectrolyte, ionic liquid, etc. The cathode 14 can have a first
cathode side 14a. The cathode 14 can have a second cathode side
14b. Some embodiments can include a cathode catalyst 44 disposed on
at least a portion of any one of the first cathode side 14a and the
second cathode side 14b. In some embodiments, the cathode catalyst
44 can be a metal, metal alloy, conductive metal oxide, carbon, or
any combination thereof. Examples of cathode catalysts 44 can
include, but are not limited to gold, silver, copper, indium,
bismuth, lead, tin, tellurium, germanium, zinc, or alloys of two or
more of these elements, etc. The cathode catalyst 44 can be
configured as a reduction catalyst. For example, the cathode
catalyst 44 may be configured as a CO.sub.2 reduction catalyst. A
non-limiting example of a CO.sub.2 reduction catalyst can be silver
nanoparticles.
[0078] In some embodiments, the cathode catalyst 44 can be mixed
with a binder, a polymeric electrolyte coating, and/or an ionic
liquid. This may be done in to increase cathode catalyst 44
utilization. For example, this may provide an ionically conducting
pathway to the membrane 22 and/or an electronically conducting
pathway to the cathode 14. For example, during the carbon dioxide
reduction reaction, protons can be supplied from the cationic side
of the membrane 22. When a proton exits the membrane 22, it can be
transported to the cathode catalyst 44 through the binder material.
The binder material can be a proton conducting material, such as a
sulfonated fluoropolymer, sulfonated polyether, or a polymeric weak
acid, for example. With CO.sub.2 being used as input product, the
carbon dioxide reduction reaction can occur at the interface
between the catalyst surface, the binder surface, and CO.sub.2 gas.
This interface may be referred to as a three-phase boundary.
Maximizing the area of the three-phase boundary can be done to
improve the current density of the electrochemical device 10. In
some embodiments, an additive (e.g., polytetrafluoroethylene) can
be added to the cathode catalyst 44 to control wettability of the
cathode catalyst layer 44 and/or prevent flooding of the cathode
14. The additive can provide hydrophobicity to the cathode catalyst
44 surface. This may prevent or inhibit H.sub.2O from crossing past
the cathode catalyst 44. H.sub.2O crossing past the cathode
catalyst 44 may result in water flooding. If water flooding occurs,
it can affect the transport of CO.sub.2 gas to the cathode catalyst
44. This may cause the current density to decrease. Conventional
electrochemical devices use aqueous catholytes. Under normal
operating conditions, the wettability of the binder materials
increases with prolonged interaction with the electrolyte. Thus,
after prolonged operation, the cathode can fail due to flooding
issues. However, use of a gas phase input product (e.g., using
CO.sub.2 gas as the reactant) can reduce flooding concerns. The
flooding concerns can be further reduced by the addition of the
additive to cathode catalyst 44 to control wettability of the
cathode catalyst layer 44.
[0079] Embodiments of the anode 18 can be an electrode configured
to oxidize an input product. For example, the anode 18 can be
configured to oxidize the input product by an oxidation reaction.
In some embodiments, the anode 18 can be configured as a
liquid-electrolyte style electrode. For example, the
electrochemical cell 12 can be configured to operate by transfer of
electrical charge via liquid electrolyte. The electrolyte can be
contained within the anode flow medium 20. For example, the anode
18, the membrane 22, and the frame 38 can be configured to contain
the electrolyte within the anode flow medium 20. This can include
preventing and/or inhibiting the electrolyte from exiting the anode
flow medium 20. Embodiments of the electrolyte can include an
acidic electrolyte having a pH less than 7, a basic electrolyte
having pH greater than 7, or a buffered electrolyte having a pH at
or near 7. Embodiments of the liquid electrolyte can include an
alkali hydroxide solution such as potassium hydroxide solution
(KOH) for example. Other alkali hydroxide solutions (NaOH, RbOH,
etc.) can be used. The liquid electrolyte can also include alkali
bicarbonate solutions (KHCO3, NaHCO3, etc.)
[0080] In some embodiments, the anode 18 can include a porous
substrate, such as carbon paper, carbon cloth, electronically
conducting metal oxide, polyelectrolyte, ionic liquid, etc. The
anode 18 can have a first anode side 18a. The anode 18 can have a
second anode side 18b. Some embodiments can include an anode
catalyst 46 disposed on at least a portion of any one of the first
anode side 18a and the second anode side 18b. The anode catalyst 46
can be a metal, metal alloy, conductive metal oxide, carbon, or any
combination thereof. Examples of anode catalysts 46 can include,
but are not limited to, iridium oxide, ruthenium alloys or mixed
oxides of ruthenium containing iridium and/or platinum, mixed metal
oxides containing cobalt, nickel, iron, manganese, lanthanum,
cerium, copper, nickel borate, cobalt phosphate, NiFeOx, etc. The
anode catalyst 46 can be configured as an oxidation catalyst. For
example, the anode catalyst 46 may be configured as a H.sub.2O
oxidation or evolution catalyst. A non-limiting example of a
H.sub.2O oxidation catalyst can be NiFeOx. Other oxidation
catalysts can be used. For example oxidation catalysts for any
general oxidation reaction, such as oxygen evolution reaction,
hydrogen oxidation, chloride oxidation, alcohol oxidation, etc. can
be used.
[0081] It may be preferred in some embodiments to use a basic
electrolyte. This may be done so that non-precious metals (e.g.,
nickel, cobalt, iron, manganese, lanthanum, cerium, copper, etc.)
can be used as anode catalysts 46.
[0082] In some embodiments, the anode 18 can be configured as a
gas-diffusion electrode. This may be done to allow the
electrochemical device 10 to operate without a liquid electrolyte.
With this embodiment, H.sub.2O in the form of water vapor or steam
can be introduced into the anode flow medium 20 as the input
product.
[0083] Embodiments of the membrane 22 can include a structure that
separates the cathode flow medium 16 and/or the cathode 14 from the
anode flow medium 20 and/or the anode 18. The separation can
include a physical separation, a chemical separation (e.g.,
chemical isolation), an electrical separation (e.g., electrical
isolation), etc. In some embodiments, the membrane 22 can include a
plurality of membranes. This can include forming a bipolar
membrane. The bipolar membrane can be structured as an ion exchange
membrane that includes at least one anion exchange layer and at
least one cation exchange layer. For example, the membrane 22 can
include a cation exchange membrane 22a and an anion exchange
membrane 22b. The anion exchange membrane 22b and the cation
exchange membrane 22a may be placed adjacent each other to form an
interface 23, which can also be referred to as a cation-anion
exchange junction or an interface layer. In some embodiments, the
anion exchange membrane 22b can be adjacent the anode flow medium
20, the anode 18, and/or the anode catalyst 46. In some embodiment,
the cation exchange membrane 22a can be adjacent the cathode flow
medium 16, the cathode 14, and/or the cathode catalyst 44. Some
embodiments can include a unitary bipolar membrane structure having
the anion exchange membrane 22b joined with the cation exchange
membrane 22a. Some embodiments can include a separate anion
exchange membrane 22b attached to the cation exchange membrane 22a.
This can include a bipolar membrane 22 having a laminate structure
of an anion exchange membrane 22b and a cation exchange membrane
22a.
[0084] In some embodiments, at least a portion of an interface
between the anion exchange membrane 22b and the cation exchange
membrane 22a can include a membrane catalyst. The membrane catalyst
can be configured to promote autodissociation of a product. For
example, the membrane catalyst can promote autodissociation of
H.sub.2O to cause the H.sub.2O to deprotonate into a proton
(H.sup.+) and a hydroxide ion (OH.sup.-). Examples of membrane
catalysts can include silicates, amine polymers, graphite oxides,
anolyte solutions (e.g., alkali metal hydroxides or alkali metal
carbonate solutions), etc.
[0085] Some embodiments can include providing a cation-exchange
polymer film on at least a portion of the anion exchange membrane
22b. This can include coating at least a portion of the anion
exchange membrane 22b with Nafion (e.g., sulfonated
tetrafluoroethylene based fluoropolymer-copolymer), SPEEK
(sulfonated poly(ether ether ketone)), or poly(acrylic acid), for
example. This can be done to improve the performance of the bipolar
membrane 22. For example, conventional bipolar membranes can be
thick and resistive. The resistance of the membrane can be an
impediment to the performance of the electrolysis cell at high
current densities. Thus, the cell performance can be improved by
optimizing the bipolar membrane 22 (e.g. adjusting polymer
materials and fabrication methods) to lower the thickness and
resistivity).
[0086] In some embodiment, the surface areas at the interface 23
between the anion exchange membrane 22b and the cation exchange
membrane 22a can be increased. For example, the surface of any one
or both at least a portion of the anion exchange membrane 22b and
at least a portion of the cation exchange membrane 22a at the
interface 23 of the two can include patterns or other surface
features to increase the surface area of any one or both of
them.
[0087] Referring to FIG. 5, in at least one embodiment, the
electrochemical device 10 can include an electrochemical cell 12.
The electrochemical cell 12 can include a cell first end 30. The
electrochemical cell 12 can include a cell second end 32. The
electrochemical cell 12 may include a cathode flow medium 16 at or
adjacent the cell first end 30. The electrochemical cell 12 may
include an anode flow medium 20 at or adjacent the cell second end
32. The electrochemical cell 12 may include a cathode 14 adjacent
or within the cathode flow medium 16. The cathode 14 can have a
first cathode side 14a and a second cathode side 14b. The first
cathode side 14a can be adjacent or within the cathode flow medium
16. The second cathode side 14b can include a cathode catalyst 44.
The electrochemical cell 12 may include an anode 18 adjacent or
within the anode flow medium 20. The anode 18 can have a first
anode side 18a and a second anode side 18b. The first anode side
18a can be adjacent or within the anode flow medium 20. The second
anode side 18b can include an anode catalyst 46. The
electrochemical cell 12 can include a membrane 22. The membrane 22
can be positioned between the cathode 14 and the anode 18. The
membrane 22 may be configured as a bipolar membrane. For example,
the membrane can include anion exchange membrane 22b and a cation
exchange membrane 22a. The cathode flow medium 16 can be defined as
a volume of space between the cathode 14 and the cell first end 30.
The cathode flow medium 16 can be a carbon material. The cation
exchange membrane 22a can be adjacent the second cathode side 14b,
the cathode flow medium 16, and/or the cathode catalyst 44. The
anode flow medium 20 can be defined as a volume of space between
the anode 18 and the cell second end 32. The anode flow medium 20
can be a carbon material. The anion exchange membrane 22b can be
adjacent the second anode side 18b, the anode flow medium 20,
and/or the anode catalyst 46. An interface 23 between the cation
exchange membrane 22a and the anion exchange membrane 22b can
include a membrane catalyst. In some embodiments, the membrane 22
can be configured to separate the cathode 14 and/or cathode flow
medium 16 from the anode 18 and/or anode flow medium 20.
[0088] Embodiments of cathode flow medium 16 can include at least
one pathway 24. The pathway 24 of the cathode flow medium 16 can be
at the interface between the cathode flow medium 16 and the first
cathode side 14a. Embodiments of the anode flow medium can include
at least one pathway 24. The pathway 24 of the anode flow medium 20
can be at the interface between the anode flow medium 20 and the
first anode side 18a.
[0089] Embodiments of the electrochemical cell 12 can include a
frame 38. The frame 38 can be a structure that holds the cathode
flow medium 16, the cathode 14, the membrane 22, the anode 18, and
the anode flow medium 20 of the electrochemical cell 12 together.
This can include holding the cathode flow medium 16, the cathode
14, the membrane 22, the anode 18, and the anode flow medium 20 in
a serial configuration. The frame 38 can also be configured to seal
at least a portion of the electrochemical cell 12. The frame 38 can
include at least one pass-through region 40. The frame 38 can be
configured to seal the electrochemical cell 12 except at a
pass-through region 40.
[0090] The cathode flow medium 16 can include a cell inlet 26 to
facilitate introduction of input product. This can include
introduction of input product in a gas phase. The cathode flow
medium 16 can include a cell outlet 28 to facilitate removal of
output product. The frame 38 can include a first pass-through
region 40 facilitating introduction of input product to a cell
inlet 26 of the cathode flow medium 16. The frame 38 can include a
second pass-through region 40 facilitating removal of output
product from a cell outlet 28 of the cathode flow medium 16. The
anode flow medium 20 can include a cell inlet 26 to facilitate
introduction of electrolyte. The anode flow medium 20 can include a
cell outlet 28 to facilitate removal of output product. The frame
38 can include a third pass-through region 40 facilitating
introduction of electrolyte to a cell inlet 26 of the anode flow
medium 20. The frame 38 can include a fourth pass-through region 40
facilitating removal of output product from a cell outlet 28 of the
anode flow medium 20. The anode flow medium 20 can include another
cell inlet 26 to facilitate introduction of input product into the
anode flow medium 20. The frame 38 can include a fifth pass-through
region 40 facilitating introduction of input product to a cell
inlet 26 of the anode flow medium 20. The anode flow medium 20 can
include another cell outlet 28 to facilitate removal of electrolyte
from the anode flow medium 20. The frame 38 can include a sixth
pass-through region 40 facilitating removal of electrolyte from a
cell outlet 28 of the anode flow medium 20.
[0091] At least one pump can be connected to the pass-through
regions 40. For example, a first pump can be connected to the first
pass-through region 40 to facilitate introduction of input product.
For example, the first pump can be configured to introduce CO.sub.2
into the electrochemical device 10. A second pump can be connected
to the second pass-through region 40 to facilitate removal of
output product. For example, the second pump can be configured to
remove CO and/or H.sub.2O from the electrochemical device 10. A
third pump can be connected to the third pass-through region 40
facilitate introduction of electrolyte. For example, the third pump
can be configured to introduce electrolyte into the electrochemical
device 10. A fourth pump can be connected to the fourth
pass-through region 40 to facilitate removal of output product. For
example, the second pump can be configured to O.sub.2 from the
electrochemical device 10. A fifth pump can be connected to the
fifth pass-through region 40 to facilitate introduction of input
product. For example, the second pump can be configured to
introduce H.sub.2O into the electrochemical device 10. A sixth pump
can be connected to the sixth pass-through region 40 to facilitate
removal of electrolyte. For example, the second pump can be
configured to remove electrolyte from the electrochemical device
10. Other configurations and number of pumps can be used. For
example, some embodiments can use a pump for introduction or
removal of multiple fluids, thereby reducing the number of pumps
used.
[0092] In some embodiments, the input product into the cathode flow
medium 16 can be CO.sub.2 and/or humidified CO.sub.2. The input
product can be transformed into a reduced chemical product. This
can occur within the cathode flow medium 16. For example, the
reaction within the cathode flow medium 16 can include a reduction
reaction. The reduction reaction can include, for example:
CO.sub.2+2H.sup.++2e.sup.-.fwdarw.CO+H.sub.2O. In some embodiments,
hydrogen may also be generated at the cathode 14. This may be due
to competing reduction of protons. CO, H.sub.2O, and/or hydrogen
can be caused to exit the electrochemical cell 12. This can include
causing the CO, the H.sub.2O, and/or the hydrogen to exit through a
cell outlet 28 and corresponding second pass-through region 40 as
an output product. The CO, the H.sub.2O, and/or the hydrogen can be
captured and/or further processed.
[0093] The membrane 22 can supply H.sup.+ to the cathode flow
medium 16 to cause H.sub.2O to self-ionize via autodissociation to
generate hydroxide ions (OH.sup.-). The membrane 22 can supply the
OH.sup.- to the anode flow medium 20. The flux of H.sup.+ to the
cathode flow medium 16 and OH.sup.- to the anode flow medium 20 can
be achieved by generating reverse bias conditions for the bipolar
membrane electrolysis reaction. A constant or stable pH can be
maintained due to the selective transport of H.sup.- to the cathode
14 and/or cathode flow medium 16 and OH.sup.- to the anode 18
and/or anode flow medium 20. A constant or stable pH can be
maintained even for extended periods of time (e.g., 24+ hours). The
pH value can be the initial value of the electrolyte at the
beginning of the reaction. The pH level may or may not be the same
for the anode 18 and for the cathode 14. In some embodiments, the
pH can be selected by a user depending on the desired application
of the electrochemical device 10. Once that value is set, however,
the electrochemical device 10 can allow the pH value(s) to be
constant throughout the reaction. For example, the pH for the
cathode 14 can be set to a first pH level. The pH for the anode 18
can be set to a second pH level. The first pH level can be the same
as or different from the second pH level. The electrochemical
device 10 can be operated while maintaining the first pH level for
the cathode 14 and the second pH level for the anode 18. Such a
configuration can also minimize undesired crossover of reduced
chemical products from the cathode 14 to the anode 18. For example,
the bipolar membrane 22 can generate H.sup.+ and OH.sup.- during
the reaction. The flux of H.sup.+ and OH.sup.-, which can be
created at the interface 23 between the anion exchanger and cation
exchanger layers 22b, 22a, is outward towards the electrodes 14,
18. The outward flux of H.sup.+ and OH.sup.- can prevent ionic and
neutral products from crossing over from the cathode 14 to the
anode 18. In a conventional electrochemical cell incorporating a
monopolar membrane, the flux of ions goes from one electrode to the
other. This can cause electrodialysis of product anions and
electroosmotic drag of neutral molecules. Electrodialysis of
product anions and electroosmotic drag of neutral molecules from
the cathode to the anode can aggravate the crossover problem.
[0094] In some embodiments, H.sub.2O can be introduced into the
anode flow medium 20 as an input product. OH.sup.- can also be
supplied to the anode flow medium by the membrane 22. The H.sub.2O
and the OH.sup.- can be used to generate O.sub.2 as an output
product. This can occur within the anode flow medium 20 due to
interactions with the electrolyte. For example, the reaction within
the anode flow medium 20 can include an oxidation reaction. The
oxidation reaction can include, for example:
2OH.sup.-=1/2O.sub.2+H.sub.2O+2e.sup.-. The O.sub.2 and/or H.sub.2O
can be caused to exit the electrochemical cell 12. This can include
causing the O.sub.2 and/or the H.sub.2O to exit through a cell
outlet 28 and corresponding fourth pass-through region 40 as an
output product. The O.sub.2 and/or the H.sub.2O can be captured
and/or further processed.
[0095] The e.sup.- generated at the anode 18 of the electrochemical
cell 12 can be delivered to the cathode 14 to complete the circuit.
(See FIG. 4).
[0096] The electrolyte can be introduced into a cell inlet 26 and a
corresponding third pass-through region 40. The electrolyte can
include an aqueous KOH solution, for example. In some embodiments,
the electrolyte can be removed from the electrochemical cell 12 for
processing and re-introduction back into the electrochemical cell
12. The electrolyte can be removed through a cell outlet 28 and a
corresponding sixth pass-through region 40. The electrolyte can be
processed to extract O.sub.2 therefrom. The electrolyte can then be
directed back into the electrochemical cell 12 via a cell inlet 26
and corresponding third pass-through region 40. The cycling of
electrolyte can occur on a continuous or semi-continuous basis.
[0097] Embodiments of the electrochemical device 10 can facilitate
use of an input product in a gas phase. For example, CO.sub.2 can
be introduced into the electrochemical cell 12 as a gas and be
further used as a reactant instead of it being dissolved in the
electrolyte, as would be the case with conventional electrochemical
devices. This can reduce or eliminate the need to provide product
separation techniques, as no product is being dissolved in an
aqueous solution (e.g., the electrolyte). This can further minimize
or eliminate introduction of contaminants into the electrolyte.
Additionally, because the CO.sub.2 need not be dissolved in the
electrolyte, there is no solubility limitation or slow mass
transport issues associated with dissolved CO.sub.2 in liquid
electrolyte to act as an operational constraint, as would be the
case with conventional electrochemical devices. For example,
CO.sub.2 has low solubility in liquid electrolytes suitable for
CO.sub.2 electrolyzer devices, which may lead to reduced mass
transport of CO.sub.2 molecules in the liquid electrolyte.
[0098] Embodiments of the electrochemical device 10 can facilitate
stable operation of the electrochemical cell 12 at high current
density and high faradaic efficiency. For example, embodiments of
the electrochemical device 10 can operate at current densities
within a range from 100 mili-Ampere per square centimeter
(mA/cm.sup.2) to 1 A/cm.sup.2 and with at least 80% faradaic
efficiency. Such current densities and faradaic efficiencies can be
sustained with electrolyte selectivity (e.g., passage of ions) of
at least 40%. Embodiments of the electrochemical device 10 can
operate in a stable manner for over 24 hours (e.g., no significant
decay in current density and faradaic efficiency).
[0099] In some embodiments, the current densities can be improved
by optimizing the thickness and composition of the bipolar membrane
22 and the composition and dispersion of the catalysts 44, 46. FIG.
11 shows the performance improvement (e.g., current density) of an
embodiment of the bipolar membrane 22 via optimization methods, as
compared to a conventional bipolar membrane.
[0100] In some embodiments, product selectivity can be tuned by
selecting different cathode catalysts 44. This can be done to
generate high value chemicals (e.g., methanol, ethylene, DME,
formate, methane, methanol, ethylene glycol, butanol, etc.) in
addition to or in the alternative to generating carbon monoxide.
For example, gold and/or silver can be used as cathode catalysts 44
to generate carbon monoxide. Lead, bismuth, and/or tin can be used
as cathode catalysts 44 to generate formate. Copper-based cathode
catalysts 44 can be used to generate methanol, methane, ethylene,
ethylene glycol, butanol, etc. Generally, a copper-based cathode
catalyst 44 can be in a specific nanostructure or display certain
crystal facets to facilitate tailoring it towards a particular
product.
[0101] Embodiments of the electrochemical device 10 can be used as
part of an air purification unit. For example, the air purification
unit can include at least one electrochemical device 10 configured
as a CO.sub.2 electrolyzer. The air purification unit can be
configured to consume CO.sub.2 at the cathode 14 and generate
O.sub.2 at the anode 18. When used in a confined space (e.g.,
submarine, space vehicles, energy-efficient office buildings,
etc.), the air purification unit can be used to replace CO.sub.2
with O.sub.2, thereby purifying air.
[0102] It is contemplated for the operating temperate of an
embodiment of the electrochemical device 10 to range from 20
degrees Celsius to 130 degrees Celsius. In at least one embodiment,
the electrochemical device 10 can operate under ambient conditions
(25.degree. C. and 1 atmospheric pressure). In some embodiments,
the operating temperature can vary from room temperature
(approximately 25 degrees Celsius) to up to 80 degrees Celsius. In
some embodiments, the operating temperature can be higher than 80
degrees Celsius, and even up to 130 degrees Celsius. For example,
inorganic additives may be used in polymer membranes of the bipolar
membrane 22, which can extend the useful range up to 130
Celsius.
[0103] It is contemplated for the electrochemical device 10 to have
better performance as the operating temperature increases. For
example, higher operating temperatures can improve the kinetics of
ion transport. Higher operating temperatures can improve catalytic
activity of the anode catalyst 46 and/or the cathode catalyst 44.
Operating temperatures at which the membrane 22 begins to
dehydrate, however, may degrade performance.
[0104] In addition, the pressure of the CO.sub.2 gas may be
increased to increase the current density and selectivity for
CO.sub.2 reduction. For example, the operating pressure can range
from 1 atmosphere pressure to 100 atmospheres pressure. Some
embodiments can use an operating pressure greater than 100
atmospheres (e.g., as high as the mechanical structure of the
electrochemical device 10 will hold). Generally, the higher the
pressure, the better is the performance.
[0105] Other operating parameters can include flow rate. For
example, the flow rate of the input product can be determined
through the current density required for the specific application.
For CO.sub.2 gas, for example, the flow rate can be estimated to be
within a range from 0 to 100 liters/minute.
[0106] In some embodiments, the anode catalyst 46 and/or the
cathode catalyst 44 may be hot-pressed together with the bipolar
membrane 22 to create a unitary membrane-electrode assembly.
Hot-pressing the cathode catalyst 44 and/or the anode catalyst 46
with the bipolar membrane 22 can create a more intimate contact
between the surfaces. This may facilitate the transport of protons.
It may also be more beneficial for processes involved with
fabricating membrane-electrode assemblies.
EXAMPLES
[0107] In a non-limiting example, an embodiment of the
electrochemical device 10 was created using a gas diffusion cathode
14. The cathode 14 included a piece of Toray carbon paper
(Toray.RTM. TGP-H-120). Silver nanoparticles (100 nm diameter,
Sigma Aldrich.RTM.) were use as the CO.sub.2 reduction catalyst 44.
Cathode catalyst ink 44 was made by mixing 8 miligrams of silver
nanoparticles with 200 microliters of isopropyl alcohol, 200
microliters of deionized water (18.2 Me), and 15 microliters of 5%
Nafion (sulfonated tetrafluoroethylene based
fluoropolymer-copolymer solution), which was sonicated for 10
minutes. The cathode catalyst ink 44 was then painted onto the
carbon paper at a typical loading of about 5 miligrams per square
centimeter (mg/cm.sup.2). The anode 18 included a piece of Toray
carbon paper (Toray.RTM. TGP-H-120). NiFeOx was used as the anode
catalyst 46. NiFeOx was electrodeposited onto the carbon paper.
[0108] The anode 18 and cathode 14 were assembled together with a
bipolar membrane 22. KOH solution (0.1 M) was delivered to the
anode flow medium 20 via a peristaltic pump as the anolyte. Gaseous
CO.sub.2 was humidified through a water bubbler and then flowed
into the cathode flow medium 16 at 20 standard cubic centimeters
per minute (sccm). FIG. 6 shows stability data for an embodiment of
the electrochemical device operating under constant 2.8 Volts for
24+ hours. FIG. 6 indicates that both the current and the faradaic
efficiency of CO production were stable for 24+ hours. These data
demonstrate the increased stability of CO.sub.2 electrolysis in an
embodiment of the electrochemical device 10. Both current density
and faradaic efficiency may be improved through further engineering
optimization.
[0109] FIG. 7 compares cell potential over time for a conventional
electrochemical device using a nafion cation exchange membrane and
an embodiment of the electrochemical device 10. Both devices were
equipped with the same cathode and anode catalysts. Both were
operated a constant current of 50 mA/cm.sup.2. Both were supplied
with humidified CO.sub.2. The conventional electrochemical device
failed after 8 hours, whereas an embodiment of inventive
electrochemical device 10 showed stable current density for the
entire duration of the test. Performance degradation for the
conventional electrochemical device may be due to the changes in pH
at the anode and cathode. It is contemplated that an
electrochemical device 10 using a bipolar membrane 22 should
operate indefinitely as long as the membrane 22 does not
degrade.
[0110] As noted herein, use of gaseous CO.sub.2 as the reactant
instead of CO.sub.2 dissolved in liquid electrolyte can provide an
advantage. For example, the solubility of CO.sub.2 is about 34
miliMolar in water at 1 atmosphere. Such solutions may support a
maximum current density of about 20 mA/cm.sup.2 for conventional
devices. The use of gaseous CO.sub.2, as with an embodiment of the
inventive electrochemical device 10 however, can obviate the
solubility limit of CO.sub.2 in water. FIG. 8 shows a
current-voltage curve for an embodiment of the electrochemical
device 10 operating at high current density and at 60 degrees
Celsius with the introduction of humidified gas-phase CO.sub.2 at
the cathode 14. NiFeOx was used as the anode catalyst 46. A 0.1 M
aqueous KOH solution was used as the liquid electrolyte. A
bismuth/1-Butyl-3-methylimidazolium trifluoromethanesulfonate
(BMIM+OTf-) was used as the cathode catalyst 44. The cathode 14 was
gas-fed with humidified CO.sub.2. The gradual leveling off of the
current at high cell potential suggests that the mass transport
limit with the bipolar membrane 22 is in excess of 200 mA/cm.sup.2.
As noted herein, other bipolar membrane designs can support current
densities as high as 1 A/cm.sup.2.
[0111] Additionally, trace amounts of impurities in liquid
electrolytes can cause degradation of electrode selectivity. This
can be due to catalytic metals present in the catholyte being
deposited on the cathode 14, which may promote undesired hydrogen
evolution. FIGS. 9 and 10 show faradaic efficiency plots for a
conventional device using a silver catalyst in 0.5 M of KHCO.sub.3
for the cathode electrolyte and 0.1M KOH for the anode electrolyte.
The device was configured as a bipolar membrane electrolyzer with
an aqueous bicarbonate catholyte. These data demonstrate the
degradation of electrode selectivity. For example, the graphs show
a decline in faradaic efficiency for CO production with an increase
for hydrogen production. The most likely cause for this is the
deposition of impurity ions from the electrolyte onto the catalyst
surface and their promotion of the competing hydrogen evolution
reaction. However, as noted herein, use of embodiments of
electrochemical device 10 can reduce or eliminate introduction of
impurities in the liquid electrolyte. For example, use of CO.sub.2
as the gas reactant can allow for the elimination of dissolution of
CO.sub.2 in the electrolyte.
[0112] As noted herein embodiments of the device 10 can be
configured to reduce, inhibit, and/or eliminate product crossover.
For example, embodiments of the bipolar membrane 22 can configured
to provide a flux of H.sup.+ to the cathode 14 and a flux of
OH.sup.- to the anode 18. With conventional designs, chemicals can
crossover from one electrode to another even though they are
separated by an ion exchange membrane. The crossover can be driven
by electrokinetic effects (e.g., when ions pass through an ion
exchange membrane under applied current, they drag along other
molecules). An example of this is illustrated in FIG. 12. In
contrast, conventional designs result in desirable chemical
products that are generated from CO.sub.2 reduction at the cathode
being dragged to the anode and oxidized, which lowers the overall
energy efficiency of the device. Embodiments of the device 10
having an embodiment of the bipolar membrane 22 can be configured
to generate ionic movements that are different from those of
conventional devices. With an embodiment of the device 10 under
applied current conditions, ions move outward toward the electrodes
14, 18. An example of this is illustrated in FIG. 13.
[0113] Due to the bipolar membrane 22 design, the electrokinetic
effects can be used to push product chemicals outward (e.g. away
from the membrane 22 and interface 23), and thus reduce, inhibit,
and/or prevent these products from crossing over the membrane 22.
This can not only prevent products crossover, but also improve the
stability of a device 10 configured as an electrochemical cell 12.
This can be a significant difference as compared to conventional
designs, that result in the electrokinetic effects of the design
causing the product chemicals to move from one electrode 14, 18 to
the other,
[0114] Embodiments of the device 10 can include a bipolar membrane
22 configured to generate ionic movements that are different from
those of conventional devices. For example, an embodiment of the
device 10 can be configured so that, under applied current
conditions, ions move outward toward the electrodes 14, 18. As
noted herein, embodiments of the bipolar membrane 22 can be
configured to supply H+ to the cathode flow medium 16 to cause
H.sub.2O to self-ionize via autodissociation to generate hydroxide
ions (OH.sup.-). The bipolar membrane 22 can supply the OH.sup.- to
the anode flow medium 20. The flux of H.sup.+ to the cathode flow
medium 16 and OH.sup.- to the anode flow medium 20 can be achieved
by generating reverse bias conditions for the bipolar membrane 22
electrolysis reaction. Due to the bipolar membrane 22 design, the
electrokinetic effects, instead of carrying product chemicals from
one electrode 14, 18 to the other, push product chemicals outward,
and thus reduce, inhibit, and/or prevent products from crossing
over the membrane 22.
[0115] In at least one embodiment, the bipolar membrane 22 can
include at least one anion exchange layer 22b and at least one
cation exchange layer 22a joined together at the interface, which
may also be considered an interfacial layer. The interface 23 can
be configured to catalyze the autodissociation of H.sub.2O. Under
reverse bias conditions, H.sup.+ and OH.sup.- ions can be generated
in the catalytic layers 22a, 22b and be driven outward. The flux of
H.sup.+ in the bipolar membrane 22 can oppose the direction of
product crossover from the cathode 14 to the anode 18 of an
electrolytic cell 12. The outward fluxes of H.sup.+ and OH.sup.-
generated in embodiments of the bipolar membrane 22 can inhibit the
crossover of both anionic and neutral products, even with membranes
22 that contain high surface area junctions. In some embodiments,
devices 10 configured as an electrochemical cell 12 having an
embodiment of the bipolar membrane 22 can operate continuously with
high faradaic and energy efficiency, and with current densities in
the 1-2 A cm.sup.-2 range.
[0116] A comparison of the chemical crossover between a
conventional anion exchange membrane (AEM) and an embodiment of the
bipolar membrane 22 for different output products (e.g., formate
methanol, and ethanol) can be seen in FIG. 14. FIG. 14 shows
crossover of formate, methanol, and ethanol versus time in
electrochemical cells having an AEM membrane and an embodiment of
the bipolar membrane 22. 0.5 M KHCO3 was used as the electrolyte on
both the cathode and anode sides of the electrochemical cell 12.
0.15 M formate, methanol, or ethanol was added to the catholyte,
and 50 mA constant current was applied. Concentrations on the
cathode and anode sides are plotted as percentages, normalized to
the initial concentration.
[0117] It can be seen that an approximate 15% decrease of
concentration of formate in the cathode occurred with the AEM
device. In addition, the AEM device utilized a thicker AEM.
Embodiments of the device 10 having a bipolar membrane 22, however,
experienced almost no change in concentration. Similar behaviors
were observed with methanol and ethanol forms of electrolyte.
Details of the crossover experiments are provided below.
[0118] The crossover rates of formate, methanol, and ethanol, which
can be desirable CO.sub.2 reduction products, were compared in
devices containing AEMs and bipolar membranes 22. The crossover of
formate, an anionic CO.sub.2 reduction product, occurs by
electromigration through AEMs, and its rate increases linearly with
current density. Crossover of electroneutral methanol or ethanol
through AEMs occurs to a lesser extent through both diffusion and
electroosmotic drag, the latter increasing with current density in
AEMs. In contrast, the outward fluxes of H.sup.+ and OH.sup.-
generated in embodiments of the bipolar membranes 22 can inhibit
the crossover of both anionic and neutral products, even with
membranes 22 that contain high surface area junctions. Calculated
electroosmotic drag coefficients for each of the neutral products
confirm the better performance of bipolar membranes 22 in terms of
product losses.
[0119] Embodiments of the device 10 can be configured to achieve
operating parameters that are conducive to provide effective
electrolyzer units (e.g., operate continuously with high faradaic
and energy efficiency, and with current densities in the 1-2 A
cm.sup.-2 range). At such high current densities, product crossover
should be considered as a loss mechanism. Because crossover is
primarily driven by electrokinetic effects, its rate increases with
increasing current density. Thus, it is contemplated for
embodiments of the bipolar membrane 22 to be used to generate
devices 10 that can meet desired operating requirements of CO.sub.2
electrolysis cells while minimizing product crossover.
[0120] The bipolar membrane can also be referred to herein as
"BPM". Embodiments of the bipolar membrane 22 can include anion
exchange layers 22b and cation exchange layers 22a joined together
at an interface 23. The interface 23 can be an interfacial layer
that is configured to catalyze the autodissociation of H.sub.2O.
Under reverse bias conditions, H+ and OH- ions can be generated in
the catalytic layers 22a, 22b and be driven outward. The flux of
H.sup.+ in the BPM 22 opposes the direction of product crossover
from the cathode 14 to the anode 18 of an electrolytic cell 12.
This electromigration of anionic products, as well as transport of
neutral molecules by electroosmotic drag, can be minimized in an
electrolytic cell 12 having an embodiment of the BPM 22.
[0121] Experiments were conducted to compare crossover through
conventional electrochemical cells having AEMs and BPMs 22.
Cation-exchange membranes such as Nafion were eliminated from this
study because earlier experiments have shown that they are a poor
choice for maintaining pH balance and minimizing crossover in
CO.sub.2 electrolysis. AEMs are more typically the monopolar
membrane of choice in CO.sub.2 reuction reaction (RR) studies
because of the high solubility of CO.sub.2 (as bicarbonate,
HCO.sub.3.sup.-) under neutral and mildly basic conditions. In
addition to electroneutral products, formate, acetate, and oxalate
are anionic products that can be generated by the electrolysis of
HCO.sub.3.sup.-. Experimental results show that the crossover of
formate is significant when AEMs are used, especially at high
current densities, and that neutral molecules such as methanol and
ethanol exhibit crossover to a lesser extent. In contrast, with
embodiments of the BPM 22, substantially less crossover of both
anionic and neutral molecules were observed. (See FIG. 14).
[0122] The rates of anionic and neutral molecule crossover were
measured in a hydrogen cell with 80 mL of 0.5 m KHCO.sub.3 as the
electrolyte in both the cathode 14 and anode 18 compartments. In
addition, crossover rates were measured with a BPM 22 containing a
two-dimensional junction structure (can be referred to as a 2D BPM
22) and a BPM 22 containing a three-dimensional junction structure
(can be referred to as a 3D BPM 22). Embodiments of the 2D BPM 22
can have a 2D planar junction interface 23 between the anion
exchange layer 22b and the cation exchange layer 22a. Embodiments
of the 3D BPM 22 can contain a network of interpenetrating anion-
and cation-exchange layers 22b, 22a. Embodiments of the 3D BPM 22
can be configured as an extended water dissociation junction so as
to sustain water electrolysis at current densities up to 1 A
cm.sup.-2. Formate, methanol, or ethanol was added at an initial
concentration of 0.15 m to the catholyte of the cell in order to
simulate conversion of .apprxeq.1/4 of the HCO.sub.3-- reactant to
products, and the concentration of these molecules was monitored
periodically by 1H NMR on both the cathode 14 and anode 18 sides of
the cell 12 during electrolysis at constant current.
[0123] FIG. 14 shows the change in concentration versus time,
normalized to the initial concentration of 0.15 m, as a 50 mA
constant current was applied to the electrochemical cell 12. The
exposed area of the membrane was about 1.1 cm.sup.2 in all
experiments. For formate, a linear decrease in concentration with
time was observed at the cathode, up to .apprxeq.15% in 4 hours of
electrolysis with the AEM. A corresponding increase in
concentration was found at the anode, indicating that formate
passed through the membrane to the anode. This is expected since
formate is similar in size to bicarbonate and can pass through the
AEM by electromigration. The concentration of formate in the
anolyte leveled out at .apprxeq.0.015 m (10% of the initial
catholyte concentration), presumably because it was oxidized back
to bicarbonate as it reached the anode. Under these conditions, the
transference number for formate in the AEM is about 0.24 and the
formate to total anion ratio is 0.23, meaning that the AEM is
unselective for formate versus HCO.sub.3.sup.- ions. In contrast,
the crossover rate of formate was about 17 times lower when BPM 22
or 3D BPM 22 was used under the same conditions.
[0124] Methanol and ethanol are neutral molecules that do not
electromigrate, but are susceptible to transport across membranes
by both simple diffusion and electroosmotic drag (see FIG. 12). A
loss of about 1% of methanol and 0.8% of ethanol were observed at
the cathode, and corresponding increases in their concentrations
were found at the anode after 4 hours of electrolysis in the AEM
cell. Under these conditions, the permeation rates for methanol and
ethanol were 0.025 and 0.017 mmol h.sup.-1, respectively, at an
applied current of 50 mA. The flux ratio (.chi.) of neutral
molecules (methanol and ethanol) relative to ions can be calculated
according to:
x = n chem n ion ##EQU00001##
[0125] where n.sub.chem and n.sub.ion are the number of moles of
the neutral molecule and the number of moles of ions that pass
through the membrane, respectively. Table 1 shows these flux ratios
for methanol and ethanol in AEM- and BPM-based cells.
TABLE-US-00001 TABLE 1 Flux ratios for methanol and ethanol in
AEM-and BPM-based cells AEM-based cell BPM-based cell 12 Methanol
0.014 0.007 Ethanol 0.009 0.006
[0126] As noted above, x can contain contributions from both
diffusion and electroosmotic drag. Typical .chi. values for
methanol in Nafion-based direct methanol fuel cells range from
about 1 to 10. The much lower values observed here are likely due
to the fact that the ion-molecule interaction of methanol or
ethanol with protons in Nafion is much stronger than it is with
bicarbonate in the AEM or BPM 22. The electroosmotic drag
coefficient as well as the diffusion constant of methanol is known
to be much smaller in AEMs than in Nafion membranes. For both
methanol and ethanol cases, the flux ratios are significantly lower
in the BPM 22 than in the AEM. The difference appears to be largely
a consequence of electroosmosis in the case of the AEM, because in
that case the crossover rate increases linearly with current
density (see FIG. 15).
[0127] While only about 1.5% of the methanol crosses over in 2
hours at a current of 200 mA (.apprxeq.180 mA cm.sup.-2), with
AEM-based electrolyzers that operate at current densities of 1-2 A
cm.sup.-2, electroosmosis would result in significant crossover
losses. In contrast, the methanol crossover rate increases only
slightly with increasing current density in the BPM-based cell 12,
where the ion flux is directed outward rather than across the
membrane.
[0128] FIG. 15 shows that the crossover rate of formate increases
linearly in the AEM cell, as expected for a transport process that
is dominated by anion electromigration. In contrast, the crossover
flux of formate remains low in the BPM-based cell 12, even at high
current density. The thicknesses of the AEM and BPM 22 used in this
study were 170 and 140 .mu.m, respectively. Given that the
thicknesses are similar, and the crossover flux is much higher
under higher applied current conditions, it can be concluded that
electrokinetic transport mechanisms are much more important than
permeation.
[0129] Embodiments of the 2D BPM 22 have a 2D planar junction
interface 23 between the anion exchange layer 22b and the cation
exchange layer 22a. In contrast, the 3D BPM 22, prepared by
electrospinning, contains a network of interpenetrating anion- and
cation-exchange polymer layers 22b, 22a at the catalytic junction.
Because the favorable I-V characteristics of the 3D BPM 22 depend
on the high surface area of the junction, it may be beneficial to
know whether crossover rates are higher in the 3D BPM 22 than in 2D
BPMs 22. Despite having lower overall thickness and larger
interfacial layer, or interface 23, FIG. 14 shows that the 3D BPM
22 has very similar rates of formate and methanol crossover as the
2D BPM 22. The crossover rates, which are most accurately measured
by the appearance of products on the anode 18 side of the cell 12,
are the same within one standard deviation for the two types of
membranes (the 2D and the 3D BPMs 22). This indicates that the
crossover rate does not depend on the interfacial area of any
embodiment of the BPM 22. Instead, the rate is limited by the
permeation of molecules through the relatively thick anion- and
cation exchange layers 22b, 22a in both kinds of BPM membranes
22.
[0130] Referring to FIG. 16, crossover rates of formate and
methanol at zero current density were also measured in order to
eliminate electrokinetic effects and quantify the flux of molecules
that can permeate the AEM or BPM 22 by diffusion. FIG. 16 shows
that there is a very low rate of methanol crossover through either
the AEM or BPM 22. Formate does show measurable crossover rate
through the AEM, as indicated by a concentration that increases
linearly with time on the anode 18 side of the cell 12. At the pH
of the catholyte (pH=7.3) and anolyte solutions (pH=8.2), formate
(pKa=3.75) exists almost entirely as the formate anion, and neutral
formic acid should not contribute significantly to the flux of
formate across the membrane. At the solution concentrations used
(0.15 m formate and 0.5 m HCO.sub.3.sup.-), formate will occupy a
significant fraction of the anion-exchange sites in the AEM, and
its diffusion across the membrane (as a neutral ion pair) will be
limited by the concentration and diffusion coefficient of the
K.sup.+ co-ion in the AEM. A similar diffusion mechanism can occur
in the anion-exchange layer 22b of the BPM 22, which will contain
both formate and K+ cations under zero current conditions. However,
the formate ion concentration should be low in the cation-exchange
layer 22a of the BPM 22, and the concentrations of both formate and
K.sup.+ should be low in the electroneutral interfacial layer. The
result is that diffusion of formate as a neutral ion pair with
K.sup.+ is significantly slower across the BPM 22 than it is across
the AEM. This observation however is not terribly relevant to the
BPM 22 under electrolytic conditions, where the cation- and
anion-exchange polymer layers 22b, 22a are charge-compensated
predominantly by H.sup.+ and OH.sup.- ions.
[0131] Earlier studies have shown that the use of either AEMs and
Nafion membranes can be problematic with gas diffusion cathodes
that are fed by gaseous CO.sub.2, because the pH of the cathode and
anode shift under continuous operation. In CO.sub.2 electrolyzers
that employ an aqueous catholyte, bicarbonate salts are typically
the electrolyte of choice because of the high solubility of
CO.sub.2, and AEMs are used because the pH can be balanced in
continuous operation by recycling CO.sub.2 (liberated by oxidation
of HCO3.sup.-) from the anode to the cathode. However, the
experiments demonstrated show that in such AEM-based electrolyzers
the crossover of anionic products such as formate occurs even at
relatively low current density, and the crossover of neutral
products such as methanol can become problematic at high current
densities currently employed in water electrolyzers. These
experiments further demonstrate that embodiments of the BPM 22 can
sustain high current densities and can inhibit crossover of both
anionic and neutral products of CO.sub.2 electrolysis.
[0132] In some embodiments, the water dissociation reaction at the
interface 23 of an embodiment of the BPM 22 can be tuned. This can
be done by adding at least one catalyst layer 25 to an embodiment
of the bipolar membrane 22. For example, an embodiment of the
bipolar member 22 can include a cation exchange layer 22a and an
anion exchange layer 22b. In some embodiments, the cation exchange
layer 22a can be adjacent the anion exchange layer 22b to form a
cation-anion exchange junction region at the interface 23. Within
the cation-anion exchange junction region, a catalyst 25 can be
deposited on at least a portion of the anion exchange layer 22b to
form an interfacial catalyst layer 25. Some embodiments can have a
plurality of interfacial catalyst layers 25. Embodiments of the
catalyst 25 can be graphite oxide (GO), polymeric amines, clay
platelets, transition metal phophate particles, or transition metal
oxide particles, for example. Some embodiments can involve
depositing the catalyst 25 via a layer-by-layer assembly technique
(e.g., via lamination). A layer-by-layer assembly technique can
involve serial exposure of the membrane to solutions of polycations
and polyanions, which can facilitate precise control of layer
thicknesses. Other assembly techniques can be spin-coating and dip
coating. These may be advantageous because they can require fewer
processing steps. The cation exchange layer 22a can be deposited on
the catalyst 25. Adding a catalyst layer 25 can be done to balance
the effects of an applied electric field and the interfacial
catalysis 25. For example, embodiments of the bipolar membrane 22
having at least one interfacial catalyst layers 25 can decrease the
electric field intensity across the interface 23. Damping of the
electric field in can be the result of a higher water dissociation
product (H.sup.+/OH.sup.-) flux, which can neutralizes the net
charge density of the cation exchange layer 22a and anion exchange
layer 22b. Thus, the amount and type of catalyst 25 added or the
number of catalyst layers 25 in the cation-anion exchange junction
at the interface 23 can be optimized to tune the performance of an
embodiments of the bipolar membrane 22.
[0133] In some embodiments, the water dissociation reaction at the
interface 23 of an embodiment of the BPM 22 can be tuned. This can
be done by incorporating different layers of graphene oxide (GO)
catalyst 25 to balance the role of electric field and the
interfacial catalysis. For example, an embodiment of the BPM 22 can
be formed by a lamination of a cation exchange layer 22a and an
anion exchange layer 22b. Upon application of a reverse bias, the
ordinarily slow water dissociation reaction at the cation-anion
exchange junction at the interface 23 of the BPM 22 can be
dramatically accelerated by the large electric field at the
interface 23 and by the presence of catalyst.
[0134] Experiments using electrochemical impedance spectroscopy
(EIS) have confirmed that a counterbalanced role of the electric
field and the junction catalyst in accelerating water dissociation
in an embodiment of the BPM 22 can be achieved. Experimental
embodiment of BPMs 22 were prepared from a crosslinked anion
exchange layer 22b and a Nafion cation exchange layer 22a, with a
graphite oxide (GO) catalyst 25 deposited at the cation-anion
exchange junction at interface 23 using layer-by-layer (LBL)
assembly techniques. BPMs 22 with an interfacial catalyst layer 25
were found to have smaller electric fields at the interface
compared to samples with no added catalyst 25. A comprehensive
numerical simulation model showed that the damping of the electric
field in BPMs 22 with a catalyst layer 25 is a result of a higher
water dissociation product (H.sup.+/OH.sup.-) flux, which
neutralizes the net charge density of the cation exchange layer 22a
and anion exchange layer 22b. This conclusion is further
substantiated by EIS studies of a high-performance 3D BPM 22 that
shows a low electric field due to the facile catalytic generation
and transport of H.sup.+ and OH.sup.-. Numerical modeling of these
effects in the BPM 22 provides a prescription for designing
membranes that function at lower overpotential.
[0135] It is contemplated that the rate of water dissociation at
the cation-anion exchange junction at interface 23 can limit the
energy efficiency of BPM-based electrolysis devices 10. This rate
is dramatically increased by the high electric field and the
presence of catalysts 25 in the cation-anion exchange junction
region at interface 23. The combined electrochemical impedance and
simulation study reveals that the electric field across the
cation-anion exchange junction is weakened by the H.sup.+/OH.sup.-
flux from catalyzed water dissociation, which partially neutralizes
the unbalanced fixed charges on the anion exchange layer 22b and
the cation exchange layer 22a. The amount of catalyst 25 in the
cation-anion exchange junction at the interface 23 can be optimized
to tune the performance of embodiments of the BPM 22.
[0136] It is contemplated for proton transport to be a vital
process in embodiments of the electrochemical cell 12 because
cathodic electron transfer is accompanied by the consumption of
protons. Membrane separators are typically incorporated into the
electrolysis system to allow for selective passage of electrolyte
ions and the separation of the cathodic and anodic products. Mass
transfer in membrane separators can induce additional resistance
and can result in a transmembrane pH gradient, compromising the
energy efficiency of the system. Although conventional
electrolyzers normally operate under strongly acidic or basic
conditions to minimize series resistance, pH neutral electrolytes
are advantageous for some oxygen evolution reaction (OER) catalysts
that contain only earth-abundant elements. Previous studies of
electrolytic cells with buffer-based electrolytes and conventional
anion- and cation exchange membranes (A/CEM) have suggested that a
4300 mV pH gradient develops across conventional A/CEM separators
under DC polarization, which is only partially mitigated by back
diffusion if electroneutral buffers are used.
[0137] Embodiment of the BPM 22 having oppositely charged anion
exchange and cation exchange layers 22b, 22a can allow for the
separation of acidic and basic solutions in the cathode 14 and
anode 18 compartments, respectively, thus providing optimal pH
conditions catalysts. In addition, under reverse bias, i.e., with
the cation exchange layer 22a facing the cathode 14, the water
dissociation reaction that occurs in the membrane 22 replenishes
the cathode 14 and anode 18 with H.sup.+ and OH.sup.-,
respectively, minimizing electrolyte adjustments. Moreover, the pH
gradient at the BPM/electrolyte interface is mitigated due to the
predominance of H.sup.+/OH.sup.- species inside the anion exchange
and cation exchange layers 22b, 22a, which match the principal
charge carriers in the electrolyte. As a result, most of the
cross-membrane potential drop occurs at the cation-anion exchange
junction positioned at the interface 23. Thus, it can be beneficial
to tailor the structure of the BPM 22 at the cation-anion exchange
junction.
[0138] Referring to FIG. 17, the large electric field created under
reverse bias and the catalyst 25 in the cation-anion exchange
junction region of the interface 23 of the BPM 22 can dramatically
enhance the rate of water dissociation at the cation-anion exchange
junction of the interface 23. Experiments were conducted to explore
the correlation between the electric field and the junction
catalyst 25 in promoting the water dissociation reaction in an
embodiment of the BPM 22. To systematically and controllably adjust
the structure at the cation-anion exchange junction of the
interface 23, a BPM 22 with a lightly crosslinked anion exchange
layer 22b with a flat surface was generated. A catalyst 25 was then
deposited on at least a portion of the anion exchange layer 22b
with a flat surface. The catalyst 25 was graphite oxide (GO). The
deposition techniques involved layer-by-layer (LBL) assembly
methods. A thin film of Nafion from a solution in dimethylformamide
(DMF) was deposited on the catalyst 25 as the cation exchange layer
22a.
[0139] Exemplary BPMs 22 with one layer of GO and four layers of GO
as the junction catalyst 25 were tested by using electrochemical
impedance spectroscopy (EIS) and compared against a BPM 22 without
a GO catalyst 25. Results demonstrate that incorporating the
catalyst 25 decreases the electric field intensity across the BPM
cation-anion exchange junction of the interface 23. A numerical
simulation model taking into account the ionic transport,
electrostatics, and electric field-dependent dissociation reaction
confirmed the experimental findings. Furthermore, EIS measurements
on a BPM 22 with a 3D junction substantiated the conclusions from
the numerical model. The BPM 22 with a GO catalyst 25 showed a
significantly lower cross-membrane potential drop than the BPM
without a GO catalyst 25 at 4100 mA cm.sup.2 current density and
had comparable stability over a 10 hour test.
[0140] In making the samples, Nafion dissolved in DMF was deposited
at 120.degree. C. The relatively high processing temperature and
the DMF solvent alleviated the rod-like aggregation that can occur
in lower temperature alcohol/water Nafion dispersions. GO layers 25
were deposited as the junction catalyst with
poly-dialkyldimethylammonium (PDDA) as the polycation using
layer-by-layer assembly techniques to allow for precise control
over the interfacial structure of the interface 23. From the
cross-sectional scanning electron microscope (SEM) images of the
BPM 22, an anion exchange layer 22b of about 100 mm and a cation
exchange layer 22a of about 40 mm thickness can be clearly
distinguished, whereas the GO interfacial layer 25 was too thin to
be imaged by this technique.
[0141] EIS measurements were performed while systematically varying
the reverse bias on the 1GO layer 25 samples (samples having only
one catalyst layer 25) and the 4GO layer 25 samples (samples having
four catalyst layers 25) and compared with results from a BPM 22
fabricated without a catalyst 25 0GO layer (samples having no
catalyst layer 25) as a control. EIS measurements were carried out
in a four-electrode cell in which current was applied through outer
working and counter electrodes and the potential was measured
between Ag/AgCl (3 M NaCl) reference electrodes (RE) positioned
close to the faces of the BPM 22 via Haber-Luggin capillaries. This
arrangement minimized the effects of solution resistance and
eliminated the overpotentials for the HER and OER, as well as the
electrode double-layer capacitance at the working (WE) and counter
electrodes (CE) in the EIS measurements and the J-E curves. An AEM
was placed between the CE and the other REs for the same reason. A
DC current was initially applied via the working and counter
electrodes to reach steady-state conditions, and EIS data were then
acquired by applying a small amplitude AC signal.
[0142] An equivalent circuit developed from the neutral layer model
was used to fit all the experimental spectra as it was contemplated
for (1) the incorporation of the GO catalyst layer 25 into the BPM
22 to have been better described by a model that treated the BPM
interfacial layer 25 explicitly and (2) an abrupt junction was
unlikely to exist in BPMs 22 that were prepared. The overall
impedance was then modeled by the series connection of a Gerischer
element, an Ohmic resistor representing the membrane and bulk
electrolyte, and a block consisting of a resistor and a capacitor.
The quality of the EIS fitting to the equivalent circuit was
confirmed by noting the parameter w2, showing a value of B0.01 and
B0.003 for the 4GO and 0GO BPM, respectively.
[0143] FIG. 18 compares J-E curves of the BPMs 22 under reverse
bias ranging from 0.6 V to 1.5 V. All BPMs 22 had similar co-ion
leakage current density, which is below 0.5 mA cm.sup.2, as
indicated by the flat portion of the J-E curve between 0.6 and 0.7
V. Above 0.75 V, the current increases significantly as the
dominant current-carrying ions in the CEL and AEL become H.sup.+
and OH.sup.-, respectively. The dissociation of water can be
described by the following reaction.
H.sub.2O(l)=H.sup.+(aq)+OH.sup.-(aq)
[0144] The dissociation of water has a formal potential (at unit
activity of H.sup.+ and OH.sup.-) of 0.83 V at room temperature,
close to observed onset bias. Beyond the onset potential, the 4GO
BPM 22 has the lowest potential at a given current density,
followed by the 1GO BPM, which has much higher current density than
the 0GO BPM within the studied reverse bias range. The water
dissociation rate constants, kd, of all BPMs 22 increase with
increasing bias (See FIG. 19). This suggests that water
dissociation is enhanced by the electric field, irrespective of the
presence of a catalyst 25. To be specific, the 0GO and 1GO BPMs 22
show appreciable increases in kd at voltages between 0.8 to 0.9 V
(see FIG. 19). In contrast, kd for the 4GO BPM 22 is relatively
large at low reverse bias and increases with increasing
voltage.
[0145] FIG. 20 shows the reaction resistance, Rw, as extracted from
the electric double layer (EDL) in the EIS equivalent circuit. The
BPMs 22 fabricated from the anion exchange layer 22b plus Nafion
share the same trend in Rw, i.e., that it decreases as voltage
increases and gradually converges to a plateau. The flat portion of
the curve corresponds to a quasi-equilibrium region for the water
dissociation reaction, where the forward dissociation and backward
neutralization reaction rates cancel each other and are equal to
the exchange current. Before reaching the quasi-equilibrium region,
the dissociation reaction is largely suppressed because of the fast
backward acid--base neutralization reaction. An increase in reverse
bias helps promote the dissociation reaction, thus decreasing Rw.
The 4GO BPM 22 exhibited the lowest Rw within the studied voltage
range, compared to the 0GO and 1GO BPMs 22, indicating that the
forward dissociation reaction is promoted more efficiently with
more added catalyst 25.
[0146] The dependence of Rw on voltage is much weaker for the
1GO/4GO BPMs, suggesting a lower electric field in BPMs 22 that
contain catalyst layers 25. It is noteworthy that the different
reaction resistances, Rw, between these synthetic BPMs 22 cannot be
simply attributed to the co-ion leakage effect, since all BPMs 22
had similar leakage current density (ses FIG. 18), but very
different values of Rw. In the neutral layer model, a reaction
layer in which the water dissociation reaction becomes prevalent
and produces nearly the total amount of H.sup.+ and OH.sup.-
required for a given current density is sandwiched by an EDL formed
from the unbalanced fixed charge density on the cation exchange
layer 22a and anion exchange layer 22b sides. This unbalanced
charge is a consequence of the depletion of ions under reverse bias
conditions, resulting in the formation of a depletion region. The
depletion layer thickness can be calculated from the following
equation:
d = 0 r A C ##EQU00002##
where .sub.0 and .sub.r are respectively the vacuum electric
permittivity and the dielectric constant in the reaction layer (80
was taken for pure water), and C and A are the capacitance and
active membrane area (1 cm.sup.2).
[0147] A depletion layer thickness, d, on the scale of hundreds of
nanometers for the 0GO BPM 22 and tens of nanometers for the 1GO
and 4GO BPMs 22 can be obtained. (See FIG. 21). As shown in FIG.
21, the depletion thickness d is much smaller for the 1GO and 4GO
BPMs 22 than it is for the 0GO BPM 22. The key findings from this
analysis are the thinner depletion region and weaker dependence on
electric potential with increasing catalyst loading. This indicates
that there is a smaller electric field acting on the reaction layer
in BPMs 22 that contain water dissociation catalysts, which is be
evident from the results of the numerical modeling.
[0148] To gain further insight into the experimental data, a
numerical model was constructed that took into account ionic
transport, electrostatics, and the rates of the water
dissociation/recombination reactions. In order to fully address the
characteristics of BPMs 22, it was beneficial to identify the
mechanisms that participate in the ionic transport of both the
electrolyte and water dissociation products under bias as well as
the water dissociation/recombination reaction. A large body of
previous work devoted to the theoretical understanding of various
phenomena in ion exchange membranes is based on the
Nernst-Planck-Poisson equations (NPP), where the Nernst-Planck
equation describes ionic transport and maintains species continuity
and the Poisson equation describes the fixed charge and the ion
permselectivity. Incorporating the water dissociation reaction is
achieved by adding a flux term in the transport equation for
H.sup.+ and OH.sup.-, which also affects the whole system
electrostatically by modifying the bulk charge density. The
reaction rate can be obtained from a kinetic model for the
dissociation and recombination of H.sup.+ and OH.sup.-, with a
forward rate constant that depends on the electric field and a
field-independent recombination rate constant. Two diffusion
boundary layers were added at the two faces of the BPM 22 in order
to better match the experimental conditions, and this turns out to
be important in-modeling the BPM 22 under higher reverse bias.
[0149] The analysis of the model begins with predictions of the J-E
curve and potential distribution profile at equilibrium. FIG. 22
shows the current density at a given reverse bias and its
comparison with experiment. The agreement of the overall current
density between experiment and simulation is satisfactory at low
reverse bias, whereas the deviation increases under higher bias.
This deviation could be caused by the static boundary conditions
employed in the model, which result in unrealistic concentration
profiles, as will be discussed below. The overall current density
is decomposed into the contributions from water dissociation
products H.sup.+/OH.sup.- and from supporting electrolyte
K.sup.+/NO.sub.3. As expected, the H.sup.+/OH.sup.- flux surpasses
that of K.sup.+/NO.sub.3 only after a certain reverse bias
threshold, 2V, after which the water dissociation reaction is
enhanced dramatically by the electric field according to the second
Wien effect. It has been shown that hysteresis develops in the J-E
curve of BPMs 22 that are subjected to a time-periodic reverse
voltage due to the incomplete depletion of mobile ions at the
junction, and the magnitude of the hysteresis depends on the scan
rate. The absence of hysteresis in the J-E curve, is consistent
with these observations as the current model simulates the
steady-state response.
[0150] Under a reverse bias less than 5 V, more than 90% of the
potential drop occurs across the BPM junction. (See FIG. 23). At
larger reverse bias of 5 V, there is an appreciable potential drop
in the region of electrolyte close to the boundary. This potential
drop can be attributed to the low H.sup.+/OH.sup.- concentration at
the two boundaries, which limits the achievable H.sup.+/OH.sup.-
flux under higher reverse bias. (See FIG. 24). This also gives rise
to an underestimate of the H.sup.+/OH.sup.- concentration to the
overall current density. Improvement of the model may be possible
by using dynamic boundary conditions.
[0151] FIGS. 24 and 25 show the concentration profiles of
H.sup.+/OH.sup.- and K.sup.+/NO.sub.3 at reverse biases of 0.3 V,
2.5 V and 5 V. These concentration profiles were found to be
representative of the overall concentration distributions as the
reverse bias varies. At lower reverse bias, the supporting
K.sup.+/NO.sub.3 ions are the major charge carriers inside both the
BPM 22 and diffusion layers. In contrast, water dissociation
products H.sup.+ and OH.sup.- become the dominant ionic species
under higher reverse bias, expelling K.sup.+ and NO.sub.3 from the
bulk of the membrane and accumulating in the diffusion boundary
layers. The insets in FIGS. 24 and 25 illustrate the formation of a
depletion region at the cation-anion exchange junction at an
interface 23, the thickness of which increases with increasing
reverse bias. In order to assess the effectiveness of the catalyst
25, results from the model without the catalytic effect, were
compared with the BPM 22 having a catalyst layer 25, which enhances
the dissociation rate constant by two orders of magnitude. Lower
current density is observed at a given reverse bias compared with
the BPM 22 having a catalyst layer 25. As expected, the
H.sup.+/OH.sup.- flux is also smaller than that of the BPM 22
having a catalyst layer 25 due to the lower reaction rate constant.
The onset reverse bias at which the H.sup.+/OH.sup.- flux start to
dominate over that of K.sup.+/NO.sub.3 is lower for the BPM 22
having a catalyst layer 25, i.e. 2 V vs. 4.5 V.
[0152] The potential and concentration distribution profiles for
BPMs 22 with and without a catalyst layer 25 resemble each other.
Furthermore, to check the consistency of the model, results for the
BPM 22 without a catalyst layer 25 were subjected to forward bias
conditions. Current contributed from H.sup.+/OH.sup.- flux is
marginally small for the studied voltage range, and the BPM 22
shows typical Ohmic resistance. The predominant charge carriers in
the BPM 22 and diffusion boundary layers are those from the
supporting electrolyte at all voltages. One striking difference
from BPMs 22 under reverse bias is the absence of the depletion
region, which is replaced by a smooth transition of one type of
charge carrier to another. These results are in good agreement with
recent theoretical reports on BPMs in fuel cell applications where
forward bias and the backward recombination reaction are more
relevant. Interestingly, most of the potential drop under forward
bias conditions happens across the diffusion layer, rather than at
the cation-anion exchange junction of the interface 23, due to the
high concentration of ions present. Having established the validity
of the numerical model, the electric field intensity was extracted
at the cation-anion exchange junction \and calculated the depletion
region thickness.
[0153] FIGS. 26-28 compare the electric field intensity and
depletion layer thickness for BPMs 22 with and without an
interfacial catalyst layer 25. Consistent with the experimental
observations, a thinner depletion region is found for the BPM 22
having a catalyst layer 25, leading to a smaller electric field
across the cation-anion exchange junction of the interface 23. The
difference can be understood as a result of the counterbalanced
roles of electric field and catalyst 25 in promoting water
dissociation. Under reverse bias, mobile ions in the BPM 22 are
driven out so that a depletion region forms due to the unbalanced
fixed charge on the anion exchange layer 22b and cation exchange
layer 22a. The resulting electric field enhances water dissociation
and produces overwhelmingly a flux of H.sup.+ and OH.sup.- ions
towards the cation exchange layer 22a and anion exchange layer 22b
of the BPM 22, respectively. As such, the unbalanced fixed charge
density is partially neutralized by the respective counter ions,
i.e. H.sup.+ for the cation exchange layer 22a and OH.sup.- for the
anion exchange layer 22b, hence shrinking the depletion region.
Since the H.sup.-/OH.sup.- flux for the BPM 22 having a catalyst
layer 25 is much larger than BPM 22 without a catalyst layer 25, a
larger portion of the fixed charge is rebalanced, causing the
electric field across the reaction layer to decrease.
[0154] In some embodiments the bipolar membrane 22 can include a
cation exchange layer 22a and an anion exchange layer 22b. The
cation exchange layer 22a can be adjacent the anion exchange layer
22b to form a cation-anion exchange junction region of the
interface 23. In some embodiments, the cation-anion exchange
junction 23 region can be configured as a planar junction
interface. This may be referred to as a two-dimensional junction
structure or a 2D bipolar membrane 22 structure. In some
embodiments, the cation-anion exchange junction region can be
configured as a network of interpenetrating anion- and
cation-exchange layers 22b, 22a. This may be referred to as a
three-dimensional junction structure or a 3D bipolar membrane 22
structure. The intimate contact between the anion exchange layer
22b and cation exchange layer 22a fibers of the 3D bipolar membrane
22 can provide multiple transport pathways for water dissociation
products H.sup.+ and OH.sup.- and greatly facilitate their removal
from the cation-anion exchange junction of the interface 23.
Consequently, a large H.sup.+/OH.sup.- flux and faster water
dissociation can be achieved with an embodiment of the 3D bipolar
membrane 22. With embodiments of the 2D bipolar membrane 22 an
electric field can be applied perpendicular to the depletion layer
plane. With embodiments of the 3D bipolar membrane 22, the electric
field is forced to span a range of angles relative to the membrane
plane. This effect can lower the local electric field across the
dispersed anion exchange layer 22b-cation exchange layer 22a fiber
interface 23, and thus further reduces the overall electric field.
FIG. 29 shows an SEM image of a cation-anion exchange junction 23
of a 3D BPM 22 with intertwined anion exchange layer 22b and cation
exchange layer 22a fibers, and a schematic of the 3D BPM 22. The
intimate contact between the anion exchange layer 22b and cation
exchange layer 22a fibers can be formed by DMF vapor treatment and
hot pressing in the junction provides multiple transport pathways
for water dissociation products H.sup.+ and OH.sup.- and greatly
facilitates their removal from the cation-anion exchange junction
23. Consequently, a large H.sup.-/OH.sup.- flux and faster water
dissociation are expected in the 3D cation-anion exchange junction
23 of the BPM 22, which was experimentally confirmed. As shown
above, a large ion flux from water dissociation should compensate
for the unbalanced fixed charge in the anion exchange layer 22b and
cation exchange layer 22a and decrease the electric field across
the cation-anion exchange junction 23.
[0155] In addition, unlike in the planar junction BPM 22, in which
the electric field is applied perpendicular to the depletion layer
plane, the 3D junctions span a range of angles relative to the
membrane plane, as evidenced in the SEM image of the cation-anion
exchange junction 23. This effect lowers the local electric field
across the dispersed anion exchange layer 22b-cation exchange layer
22a fiber interfaces and thus further reduces the overall electric
field. It is the intimate local contact between the anion exchange
layer 22b and the cation exchange layer 22a that distinguishes the
3D interface from its 2D counterpart.
[0156] Referring to FIGS. 30-33, under steady-state galvanostatic
polarization, the 3D junction BPM 22 exhibits a similar co-ion
leakage current as the 4GO BPM 22 at lower reverse bias in a pH
neutral electrolyte. The 3D junction BPM 22 shows a nearly constant
kd up to a reverse bias of B0.9 V. It is noteworthy that the
observed lower overpotential of the 3D junction BPM 22 does not
stem from catalysis 25 of the water dissociation reaction because
the 4GO BPM 22 exhibits a relatively larger rate constant kd. In
stark contrast to the 4GO BPM 22, the reaction resistance, Rw, for
the 3D junction BPM 22 does not show obvious convergence to a
plateau but rather remains almost constant, and is much smaller
within the studied voltage. The lower reaction resistance for the
3D junction BPM 22 is attributed to facilitated water dissociation
made possible by the rapid removal of H.sup.+/OH.sup.- through the
interpenetrating anion exchange layer 22b and cation exchange layer
22a fibers, which results in a larger H.sup.+/OH.sup.- flux in the
cation-anion exchange junction 23 relative to flat interface BPMs
22. The independence of Rw on the transmembrane voltage indicates a
small electric field in the 3D junction BPM 22 as a result of the
large H.sup.+ and OH.sup.- flux. The smaller electric field is also
verified by the thinner depletion thickness d compared with that of
the 4GO BPM 22.
[0157] Referring to FIGS. 34-34 operating parameters of embodiments
of the BPMs 22 under normal operating conditions were studied to
gain insight into the mechanism of water autodissociation and the
effects of electric field and catalysis. FIGS. 34-35 compare BPMs
22 with no GO/four layers of GO (0GO/4GOBPM), and the 3D junction
BPM 22 with a commercial Fumatech BPM in terms of the potential
drop across the membrane at a given reverse bias current density.
At low current density, the cross-membrane potential is similar for
all membranes except the 0GO BPM, whereas at current densities
greater than 100 mA cm.sup.2, and the 4GO and 3D junction BPMs 22
show significantly lower potential drop than the Fumatech BPM.
Galvanostatic measurements at a reverse-bias current density of 100
mA cm.sup.2 were performed and the results suggest that both
membranes were stable for at least 10 hours of continuous
operation. The moderate increase in the cross-membrane potential
for the 4GO BPM 22 may be associated with degradation of GO in the
interfacial layer 23 during operation. Compared with the BPM 22
having no catalyst layer 25, 0GO BPM 22, the cross-membrane
potential of the 4GO BPM 22 is much lower at all studied reverse
bias values due to the smaller water dissociation reaction
resistance, Rw, of the latter. The depletion layer thickness and
thus the electric field in the 0GO BPM 22 are larger than those of
the BPM 22 having a catalyst layer 25, 4GO BPM 22, and show a clear
dependence of increasing as the reverse bias increases. For the 0GO
BPM 22, a wider depletion region gives rise to a stronger electric
field, which promotes water dissociation to a larger extent so that
the produced H.sup.+/OH.sup.- flux matches the higher current
density at an increased reverse bias. However, for the 4GO BPM 22,
the catalyst 25 provides an alternative means of enhancing the rate
of water dissociation. As such, the depletion region and electric
field do not need to be as enlarged in order to achieve the same
current density. Similarly, the electric field in the 3D junction
BPM 22 is also shown to be small. Two origins for the small
electric field in the 3D junction BPM 22 are: (1) the large
H.sup.+/OH.sup.- flux due to the facile transport of the charged
species because of the interpenetrating anion exchange layer
22b-cation exchange layer 22a dual fiber structure, compared to the
incorporation of an effective catalyst 25 as considered in the 4GO
BPM 22 and (2) the wide range of angles spanned by the anion
exchange layer 22b-cation exchange layer 22a interfaces with
respect to the overall electric field. Because of this effect,
improving the membrane fabrication process so that the anion
exchange layer 22b-cation exchange layer 22a interfaces are more
perpendicular to the membrane plane would be expected to impart a
larger role to the electric field in 3D junction BPMs 22.
[0158] Experiments demonstrate that BPMs 22 can be prepared from a
crosslinked anion exchange layer 22b and Nafion cation exchange
layer 22a with a GO catalyst 25 deposited in between by
layer-by-layer assembly technique, allowing for precise control of
the interfacial 23 structure. By adjusting the GO catalyst layers
25, a balance between the second Wien effect and the catalytic
effect in promoting water dissociation has been discovered. A
comprehensive numerical simulation model elucidated that the
electric field enhancement for water dissociation may be
compromised by incorporating catalysts into the BPM cation-anion
exchange junction 23, as that produces a larger H.sup.+/OH.sup.-
flux that partially mitigates the net fixed charge on the anion
exchange layer 22b and the cation exchange layer 22a of the BPMs
22. This conclusion is further corroborated by testing a 3D
junction BPM 22, which exhibits a large H.sup.+/OH.sup.- flux
because of facilitated ionic transport through the interpenetrating
junction.
[0159] It should be understood that the disclosure of a range of
values is a disclosure of every numerical value within that range,
including the end points. It should also be appreciated that some
components, features, and/or configurations may be described in
connection with only one particular embodiment, but these same
components, features, and/or configurations can be applied or used
with many other embodiments and should be considered applicable to
the other embodiments, unless stated otherwise or unless such a
component, feature, and/or configuration is technically impossible
to use with the other embodiment. Thus, the components, features,
and/or configurations of the various embodiments can be combined
together in any manner and such combinations are expressly
contemplated and disclosed by this statement.
[0160] It will be apparent to those skilled in the art that
numerous modifications and variations of the described examples and
embodiments are possible in light of the above teachings of the
disclosure. The disclosed examples and embodiments are presented
for purposes of illustration only. Other alternate embodiments may
include some or all of the features disclosed herein. Therefore, it
is the intent to cover all such modifications and alternate
embodiments as may come within the true scope of this invention,
which is to be given the full breadth thereof.
[0161] It should be understood that modifications to the
embodiments disclosed herein can be made to meet a particular set
of design criteria. For instance, any of the electrochemical cells
12, cathodes 14, anodes 18, membranes 22, catalysts 44, 46 or any
other component of the device 10 can be any suitable number or type
of each to meet a particular objective. Therefore, while certain
exemplary embodiments of the device 10 and methods of using the
same disclosed herein have been discussed and illustrated, it is to
be distinctly understood that the invention is not limited thereto
but may be otherwise variously embodied and practiced within the
scope of the following claims.
* * * * *