U.S. patent application number 17/734808 was filed with the patent office on 2022-08-18 for membrane electrode assembly with improved cohesion.
The applicant listed for this patent is Ballard Power Systems Inc.. Invention is credited to Rajesh BASHYAM, Alan YOUNG.
Application Number | 20220263096 17/734808 |
Document ID | / |
Family ID | 1000006303844 |
Filed Date | 2022-08-18 |
United States Patent
Application |
20220263096 |
Kind Code |
A1 |
BASHYAM; Rajesh ; et
al. |
August 18, 2022 |
MEMBRANE ELECTRODE ASSEMBLY WITH IMPROVED COHESION
Abstract
A membrane electrode assembly comprises an anode electrode
comprising an anode catalyst layer; a cathode electrode comprising
a cathode catalyst layer; and a polymer electrolyte membrane
interposed between the anode electrode and the cathode electrode;
wherein at least one of the anode and cathode catalyst layers
comprises a block co-polymer comprising poly(ethylene oxide) and
poly(propylene oxide).
Inventors: |
BASHYAM; Rajesh; (Richmond,
CA) ; YOUNG; Alan; (Surrey, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ballard Power Systems Inc. |
Burnaby |
|
CA |
|
|
Family ID: |
1000006303844 |
Appl. No.: |
17/734808 |
Filed: |
May 2, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16604537 |
Oct 10, 2019 |
11355759 |
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PCT/US2018/027616 |
Apr 13, 2018 |
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17734808 |
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62485325 |
Apr 13, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/1023 20130101;
H01M 4/926 20130101; H01M 8/1044 20130101; H01M 4/8668 20130101;
H01M 4/8828 20130101; H01M 2008/1095 20130101; H01M 4/8657
20130101; H01M 4/881 20130101; H01M 8/1004 20130101; H01M 8/1039
20130101 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 4/88 20060101 H01M004/88; H01M 4/92 20060101
H01M004/92; H01M 8/1004 20060101 H01M008/1004; H01M 8/1023 20060101
H01M008/1023; H01M 8/1039 20060101 H01M008/1039; H01M 8/1044
20060101 H01M008/1044 |
Claims
1.-10. (canceled)
11. A method of making a catalyst-coated membrane comprising the
steps of: a) dissolving a non-proton-conducting block co-polymer
comprising poly(ethylene glycol) and poly(propylene glycol) in an
aqueous solution to form a block co-polymer solution; b) mixing the
block co-polymer solution with an ionomer and a catalyst to form a
catalyst ink; c) coating the catalyst ink on one side of an
ion-exchange membrane to form a coated ion-exchange membrane; and
d) drying the coated ion-exchange membrane to form the
catalyst-coated membrane.
12. The method of claim 11, wherein the block co-polymer comprises
poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene
oxide).
13. The method of claim 11, wherein the block co-polymer comprises
poly(propylene oxide)-poly(ethylene oxide)-poly(propylene
oxide).
14. The method of claim 11, wherein the catalyst is selected from
the group consisting of platinum, gold, ruthenium, iridium, cobalt,
nickel, molybdenum, palladium, iron, tin, titanium, manganese,
cerium, chromium, copper, and tungsten, and alloys, solid
solutions, and intermetallic compounds thereof.
15. The method of claim 11, wherein the catalyst comprises a
non-precious metal.
16. The method of claim 11, wherein drying the coated ion-exchange
membrane comprises heating the coated ion-exchange membrane at an
elevated temperature above room temperature.
17. The method of claim 11, further comprising compacting the
coated ion-exchange membrane.
18. An electrode comprising a catalyst layer, the catalyst layer
comprising a catalyst and a binder comprising an ionomer and a
block co-polymer comprising poly(ethylene oxide) and poly(propylene
oxide).
19. The electrode of claim 18, wherein the electrode is an
anode.
20. The electrode of claim 18, wherein the electrode is a cathode.
Description
BACKGROUND
Technical Field
[0001] The present disclosure relates to membrane electrode
assemblies for electrochemical cells, in particular, catalyst
layers with improved cohesion.
Description of the Related Art
[0002] Electrochemical fuel cells convert fuel and oxidant into
electricity. Solid polymer electrochemical fuel cells generally
employ a membrane electrode assembly that includes a solid polymer
electrolyte membrane disposed between two electrodes. The membrane
electrode assembly is typically interposed between two electrically
conductive flow field plates to form a fuel cell. These flow field
plates act as current collectors, provide support for the
electrodes, and provide passages for the reactants and products.
Such flow field plates typically include fluid flow channels to
direct the flow of the fuel and oxidant reactant fluids to an anode
and a cathode of each of the membrane electrode assemblies,
respectively, and to remove excess reactant fluids and reaction
products. In operation, the electrodes are electrically coupled for
conducting electrons between the electrodes through an external
circuit. Typically, a number of fuel cells are electrically coupled
in series to form a fuel cell stack having a desired power
output.
[0003] The anode and the cathode each contain a layer of anode
catalyst and cathode catalyst, respectively. The catalyst may be a
metal, an alloy or a supported metal/alloy catalyst, for example,
platinum supported on carbon black. The catalyst layer may contain
an ion conductive material, such as NAFION.RTM. (provided by E. I.
du Pont de Nemours and Co.) and/or a binder, such as
polytetrafluoroethylene (PTFE). Each electrode further includes an
electrically conductive porous substrate, such as carbon fiber
paper or carbon cloth, for reactant distribution and/or mechanical
support. The thickness of the porous substrate typically ranges
from about 50 to about 250 microns. Optionally, the electrodes may
include a porous sublayer disposed between the catalyst layer and
the substrate. The sublayer usually contains electrically
conductive particles, such as carbon particles, and, optionally, a
water repellent material for modifying its properties, such as gas
diffusion and water management. The catalyst may be coated onto the
membrane to form a catalyst-coated membrane (CCM) or coated onto
the sublayer or the substrate to form an electrode.
[0004] The catalyst is one of the most expensive components in a
fuel cell due to the noble metals that are typically used. Such
noble metals include platinum and gold, which are often mixed with
or alloyed with other metals, such as ruthenium, iridium, cobalt,
nickel, molybdenum, palladium, iron, tin, titanium, manganese,
cerium, chromium, copper, and tungsten, to enhance preferred
reactions and mitigate unwanted side reactions, which are different
for the anode and the cathode.
[0005] The anode and cathode half-cell reactions in hydrogen gas
fuel cells are shown in the following equations:
H.sub.2.fwdarw.2H.sup.++2e.sup.- (1)
1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O (2)
[0006] On the anode, the primary function is to oxidize hydrogen
fuel to form protons and electrons. Depending on the fuel source,
the anode catalyst may need to be tolerant to impurities. For
example, carbon monoxide poisoning of the anode catalyst often
occurs when operating on a reformate-based fuel. To mitigate carbon
monoxide poisoning, a platinum alloy catalyst, such as
platinum-ruthenium, is preferable on the anode.
[0007] On the cathode, the primary function is to reduce oxygen and
form water. This reaction is inherently much slower than the anode
reaction and, thus, the cathode catalyst loading is typically
higher than the anode catalyst loading. One way of enhancing the
cathode half-cell reaction is to improve the electrochemical
activity and catalyst utilization of the catalyst layer, thereby
reducing voltage losses related to catalytic kinetics.
[0008] Catalysts also need to be able to withstand degradation that
may occur during fuel cell operation and fuel cell start-up and
shutdown. Typical catalyst degradation modes include corrosion of
the catalyst support material and platinum dissolution and
agglomeration, which leads to a decrease in fuel cell performance
due to the decreased platinum surface area. Conventional supported
platinum catalysts on high surface area supports, such as platinum
supported on carbon black, have high activity but are more prone to
degradation. Catalyst degradation is an important issue because it
has a detrimental impact on fuel cell lifetime and overall costs.
To mitigate corrosion, graphitized carbon supports are preferable
over carbon black supports because graphitized carbon supports are
more stable and less susceptible to corrosion. However, graphitized
carbon supports also have a lower surface area, which makes it
difficult to homogeneously disperse noble metal catalysts onto
graphitized carbon supports. Therefore, catalysts having noble
metals dispersed on graphitized carbon supports typically show a
lower electrochemical activity and fuel cell performance than
catalysts having noble metals dispersed on high surface area
supports, but better durability.
[0009] As a result, there still exists a need to improve MEA
performance without sacrificing durability. The present description
addresses these issues and provides further related advantages.
BRIEF SUMMARY
[0010] Briefly, the present disclosure relates to membrane
electrode assemblies for electrochemical fuel cells.
[0011] In one embodiment, a membrane electrode assembly comprises
an anode electrode comprising an anode catalyst layer, the anode
catalyst layer comprising an anode catalyst and a first binder; a
cathode electrode comprising a cathode catalyst layer, the cathode
catalyst layer comprising a cathode catalyst and a second binder;
and a polymer electrolyte membrane interposed between the anode
electrode and the cathode electrode; wherein at least one of the
first and second binders comprises an ionomer and a block
co-polymer comprising poly(ethylene oxide) (PEO) and poly(propylene
oxide) (PPO). As used herein, a block copolymer comprising PEO and
PPO may be referred to as a "PEO-PPO" block copolymer. A block
copolymer comprising PEO, PPO and a further block of PEO may be
referred to as a "PEO-PPO-PEO" block copolymer, and a block
copolymer comprising PPO, PEO and a further block of PPO may be
referred to as a "PPO-PEO-PPO" block copolymer
[0012] In specific embodiments, the at least one of the anode and
cathode catalyst layers comprises about 1 wt % to about 10 wt % of
the block co-polymer.
[0013] In another embodiment, a method of making a catalyst-coated
membrane comprising the steps of: a) dissolving a
non-proton-conducting (F108 block co-polymer comprising
poly(ethylene oxide) and poly(propylene oxide) in an aqueous
solution to form a block co-polymer solution; b) mixing the block
co-polymer solution with an ionomer and a catalyst to form a
catalyst ink; c) coating the catalyst ink on one side of an
ion-exchange membrane to form a coated ion-exchange membrane; and
d) drying the coated ion-exchange membrane to form the
catalyst-coated membrane.
[0014] One embodiment provides an electrode comprising a catalyst
layer, the catalyst layer comprising a catalyst and a binder
comprising an ionomer and a block co-polymer comprising
poly(ethylene oxide) and poly(propylene oxide).
[0015] These and other aspects will be evident upon reference to
the attached drawings and following detailed description.
BRIEF DESCRIPTION OF THE FIGURES
[0016] In the figures, identical reference numbers identify similar
elements or acts. The sizes and relative positions of elements in
the figures are not necessarily drawn to scale. For example, the
shapes of various elements and angles are not drawn to scale, and
some of these elements are enlarged and positioned to improve
figure legibility. Further, the particular shapes of the elements,
as drawn, are not intended to convey any information regarding the
actual shape of the particular elements, and have been solely
selected for ease of recognition in the figures.
[0017] FIG. 1 shows a cross-section of an exemplary fuel cell
according to one embodiment of the present description.
[0018] FIG. 2 shows a cross-section of an exemplary fuel cell
according to another embodiment of the present description.
[0019] FIGS. 3a and 3b show pictures of catalyst-coated membranes
with and without the PEO-PPO-PEO block co-polymer in the catalyst
layer.
[0020] FIGS. 4a and 4b show the fuel cell testing results of four
MEAs with varying amounts of the PEO-PPO-PEO block co-polymer in
the cathode catalyst layer at 100% RH and 60% RH, respectively.
DETAILED DESCRIPTION
[0021] Unless the context requires otherwise, throughout the
specification and claims which follow, the word "comprise" and
variations thereof, such as "comprises" and "comprising" are to be
construed in an open, inclusive sense, that is, as "including but
not limited to".
[0022] As discussed in the foregoing, an electrochemical fuel cell
2 includes a solid electrolyte 4 interposed between an anode
electrode 6 and a cathode electrode 8, an anode catalyst layer 10
between electrolyte membrane 4 and anode gas diffusion layer 12,
and a cathode catalyst layer 14 between solid electrolyte 4 and
cathode gas diffusion layer 16, as shown in FIG. 1. The inventors
have surprisingly discovered that catalyst flaking was
significantly reduced while cohesion of the cathode catalyst layer
and adhesion to the membrane were significantly increased by
incorporating a block co-polymer comprising poly(ethylene oxide)
and poly(propylene oxide) into the binder of the catalyst layer.
Without being bound by theory, it is suspected that the PEO-PPO
block co-polymer improves cohesion and adhesion of the catalyst
layer by increasing polymer wet density, thereby improving or
increasing contact between the catalyst particles and the ionomer,
and may improve fuel cell performance and durability.
[0023] In some embodiments, a PEO-PPO block co-polymer in the first
and/or second binder of the catalyst layer(s) has a molecular
weight ranging from about 3500 g/mol to about 14000 g/mol. In some
embodiments, the molecular weight ranges from about 4000 g/mol to
about 13500 g/mol, from about 5000 g/mol to about 12500 g/mol, from
about 6000 g/mol to about 11500 g/mol, from about 7000 g/mol to
about 10500 g/mol, from about 8000 g/mol to about 9500 g/mol, from
about 4000 g/mol to about 10000 g/mol, from about 3500 g/mol to
about 9500 g/mol, from about 3500 g/mol to about 8500 g/mol, or
from about 9000 g/mol to about 14000 g/mol.
[0024] Such PEO-PPO block co-polymers are non-proton-conductive
when being incorporated into the catalyst layer, that is, they do
not contain any proton conducting groups, such as sulfonic acids,
phosphonic acids and phosphoric acids. The PEO-PPO block co-polymer
may be, for example, PEO-PPO-PEO, PPO-PEO-PPO, and combinations
thereof. Exemplary PEO-PPO block co-polymers may include, but are
not limited to, those sold under the Pluronic.RTM. tradename, such
as Pluronic.RTM. F108, F127, R25, and P123. In some embodiments,
the block co-polymer comprises the following structure:
##STR00001##
wherein:
[0025] n and m are each independently an integer greater than zero.
In some embodiments, n ranges from about 1 to about 320. In certain
embodiments, m ranges from about 1 to about 250.
[0026] In certain more specific embodiments, the block co-polymer
comprises the following structure:
##STR00002##
wherein:
[0027] n, m and p are each independently an integer greater than
zero. In some embodiments, n ranges from about 1 to about 320. In
certain embodiments, m ranges from about 1 to about 250. In some
embodiments, p ranges from about 1 to about 320.
[0028] In some embodiments, the block co-polymer comprises the
following structure:
##STR00003##
wherein:
[0029] n, m and p are each independently an integer greater than
zero. In some embodiments, n ranges from about 1 to about 320. In
certain embodiments, m ranges from about 1 to about 250. In some
embodiments, q ranges from about 1 to about 250.
[0030] In some of the foregoing embodiments, n ranges from about 1
to about 300, about 10 to about 250, about 10 to about 200, about
10 to about 150, about 10 to about 100, about 10 to about 50, about
5 to about 35, about 5 to about 25, about 5 to about 50 or about 5
to about 35.
[0031] In some of the foregoing embodiments, p ranges from about 1
to about 300, about 10 to about 250, about 10 to about 200, about
10 to about 150, about 10 to about 100, about 10 to about 50, about
5 to about 35, about 5 to about 25, about 5 to about 50 or about 5
to about 35.
[0032] In some of the foregoing embodiments, m ranges from about 1
to about 200, about 10 to about 150, about 10 to about 100, about
10 to about 50, about 10 to about 35, about 10 to about 25, about 5
to about 75, about 5 to about 50, about 5 to about 25 or about 5 to
about 20.
[0033] In some of the foregoing embodiments, q ranges from about 1
to about 200, about 10 to about 150, about 10 to about 100, about
10 to about 50, about 10 to about 35, about 10 to about 25, about 5
to about 75, about 5 to about 50, about 5 to about 25 or about 5 to
about 20.
[0034] In some embodiments, the electrode is an anode. In some
related embodiments the electrode comprises a anode catalyst layer
and an anode catalyst according to the embodiments described
herein.
[0035] In some embodiments, the electrode is a cathode. In some
related embodiments the electrode comprises a cathode catalyst
layer and a cathode catalyst according to the embodiments described
herein.
[0036] In one embodiment, the catalyst in the anode and cathode
catalyst layers is a noble metal or noble metal alloy. In specific
embodiments, catalyst in the anode and cathode catalyst layers may
be platinum, gold, ruthenium, iridium, cobalt, nickel, molybdenum,
palladium, iron, tin, titanium, manganese, cerium, chromium,
copper, and tungsten, and alloys, solid solutions, and
intermetallic compounds thereof. The noble metal loading of the
anode and cathode electrode should be low to minimize cost. For
example, the platinum loading of the anode electrode may range from
about 0.01 mg Pt/cm.sup.2 to about 0.15 mg Pt/cm.sup.2 while the
platinum loading of the cathode electrode may range from about 0.1
mg Pt/cm.sup.2 to about 0.6 mg Pt/cm.sup.2.
[0037] The catalyst in the anode and cathode catalyst layers may be
supported on a carbonaceous support, such as activated carbon,
carbon black, carbon that is at least partially graphitized, and
graphite. Other suitable carbonaceous support materials include
carbon nanofibers and carbon nanotubes. In specific embodiments,
the carbonaceous support may have a specific surface area of at
least about 150-2000 m.sup.2/g. In other examples, non-carbonaceous
supports include oxide and nitride supports. These include, but are
not limited to, TiO.sub.2, Ti.sub.4O.sub.7, TiRuO.sub.2,
Ta.sub.2O.sub.5, Nb.sub.2O.sub.5.
[0038] In some embodiments, the anode and/or cathode catalyst
layers may contain a mixture of catalysts and/or supported
catalysts. For example, the cathode catalyst layer may contain a
mix of a supported platinum alloy catalyst and a supported platinum
catalyst.
[0039] In some embodiments, the anode and cathode catalyst layers
further comprise an additional binder component, for example, an
ionomer. In some embodiments, the ionomer is perfluorinated,
partially fluorinated, or hydrocarbon-based. For example, the
ionomer may be a sulfonic-acid based perfluorinated ionomer, such
as those that are sold under the Nafion.RTM. (DuPont), Aciplex.RTM.
(Asahi Kasei Corporation), and Aquivion.RTM. (Solvay Plastics)
tradenames, as well as ionomers from 3M.
[0040] As discussed, the PEO-PPO-PEO block co-polymer showed
improved cohesion and adhesion of the catalyst layer when it was
mixed with the binder of the catalyst layer. Therefore, the
PEO-PPO-PEO block co-polymer may be suitable for catalyst layers
that are prone to cracking and/or flaking so that manufacturability
is improved. For example, catalyst layers employing catalysts with
very high surface area and/or too little ionomer in the binder are
typically prone to catalyst layer cracking or flaking. Furthermore,
without being bound by theory, improved adhesion and cohesion of
the anode and cathode catalyst layers may improve durability, for
example, reducing degradation associated with catalyst layer
delamination due to aggressive fuel cell operating conditions and
environment.
[0041] PEO-PPO-PEO has been shown to improve cohesion and adhesion
of the catalyst layer and PPO-PEO-PPO may exhibit enhanced
properties with respect to water management in the catalyst layer
as PPO is more hydrophobic than PEO. Therefore, the PPO-PEO-PPO
block co-polymer may result in a more hydrophobic catalyst layer
than the PEO-PPO-PEO block co-polymer. In some embodiments, the
first and/or second binder(s) comprise a mixture of PEO-PPO-PEO and
PPO-PEO-PPO block co-polymers.
[0042] Block polymers may be formed as blocks of two or more
polymeric segments (e.g., PEO or PPO blocks) via ring opening
polymerization. Typical Pluronic.RTM.-type triblock copolymers are
formed from anionic ring opening polymerization of ethylene oxide
and propylene oxide using an activator such as potassium hydroxide.
The central poly propylene oxide is synthesized as precursor
followed by chain extension through polymerization of ethylene
oxide. A block copolymer is preferred over individual polymeric
segments, such as PEO and PPO on their own, as block polymers
typically exhibit amphiphilic properties with enhanced surface
active properties as compared to individual polymeric segments.
[0043] In certain embodiments, the thickness of the anode and
cathode catalyst layers with noble metals ranges from about 1
micron to about 12 microns. In some embodiments, the catalyst layer
has a thickness ranging from about 1 micron to about 20 microns,
from about 1 micron to about 15 microns, from about 1 micron to
about 10 microns, from about 5 micron to about 20 microns, from
about 5 micron to about 15 microns, or from about 5 micron to about
10 microns.
[0044] A catalyst layer including a PEO-PPO block co-polymer may be
especially useful for thicker catalyst layers (such as about 20
microns and above), which are usually susceptible to cracking, such
as catalyst layers that utilize non-precious metal catalysts. In
certain embodiments, the catalyst layer comprises a non-precious
metal catalyst. In certain more specific embodiments, the catalyst
layer has a thickness greater than about 20 microns. In certain
embodiments, the catalyst layer thickness ranges from about 20
microns to about 30 microns, from about 20 microns to about 40
microns, from about 20 microns to about 50 microns, from about 20
microns to about 60 microns, from about 20 microns to about 70
microns, from about 80 microns to about 30 microns, from about 20
microns to about 90 microns, or from about 20 microns to about 100
microns.
[0045] As the name indicates, non-precious metal catalysts do not
include a precious metal. Non-precious metal catalysts include, but
are not limited to, transition metal nitrogen-containing complexes,
conductive polymer-based catalysts, transition metal chalcogenides,
metal oxides, metal carbides, metal nitrides, metal oxynitrides,
metal carbonitrides and enzyme compounds. In some embodiments, the
non-precious metal catalyst comprises a metal selected from the
group consisting of iron, cobalt, and nickel. In some embodiments,
the non-precious metal comprises a transition metal. In certain
specific embodiments, the non-precious metal catalyst comprises
carbon or nitrogen and a metal selected from the group consisting
of iron and cobalt. Other non-precious metal catalysts are known in
the art, including those described in Banham, Journal of Power
Sources, 285, (2015) 334-348, which is incorporated herein in its
entirety.
[0046] In some embodiments, the cathode catalyst layer may be
divided into two or more cathode catalyst sublayers 18, 20, such as
that shown in FIG. 2. In this situation, any one or all of the
cathode catalyst sublayers may be treated with the block
co-polymer.
[0047] The anode gas diffusion layer and cathode gas diffusion
layer should be electrically conductive, thermally conductive,
adequately stiff for mechanical support of the catalyst layer and
membrane, sufficiently porous to allow for gas diffusion, and thin
and lightweight for high power density. Thus, conventional gas
diffusion layer materials are typically chosen from commercially
available woven and non-woven porous carbonaceous substrates,
including carbon fiber paper and carbon fabrics, such as carbonized
or graphitized carbon fiber non-woven mats. Suitable porous
substrates include, but are not limited to, TGP-H-060 and TGP-H-090
(Toray Industries Inc., Tokyo, Japan); AvCarb.RTM. P50 and EP-40
(Ballard Material Products Inc., Lowell, Mass.); and GDL 24 and 25
series material (SGL Carbon Corp., Charlotte, N.C.). In some
embodiments, the porous substrate may be hydrophobized, and may
optionally include at least one gas diffusion sublayer having
carbon and/or graphite in fibrous and/or particulate form.
[0048] The polymer electrolyte membrane may be any suitable
proton-conducting material or ionomer, such as, but not limited to,
Nafion.RTM. (DuPont), Flemion.RTM. (Asahi Glass, Japan),
Aquivion.RTM. (Solvay Plastics), GORE-SELECT.RTM. (W.L. Gore &
Associates), and Aciplex.RTM. (Asahi Kasei, Japan).
[0049] The MEA and catalyst layers and sublayers can be made via
methods known in the art. For example, the catalyst ink may be
directly applied to the gas diffusion layer or membrane by
screen-printing, knife-coating, spraying or gravure coating, or
decal-transferred to the gas diffusion layer or membrane. The
catalyst ink may be applied in a single application or in multiple
thin coatings to achieve the desired catalyst loading and/or
catalyst layer structure.
[0050] In one method to make a catalyst-coated membrane, the block
co-polymer comprising PEO-PPO is dissolved in an aqueous solution
to form a block co-polymer solution, then mixed with an ionomer to
form an ionomer-block solution, and then mixed with catalyst to
form a catalyst ink. The catalyst ink is coated on one or both
sides of an ion-exchange membrane to form a coated membrane (or
coated on a release sheet to decal transfer the catalyst layer to
the membrane to form a coated membrane), or coated on one side of a
gas diffusion layer to form an electrode. The coated ion-exchange
membrane or electrode is then dried to form a catalyst-coated
membrane or gas diffusion electrode. The resulting catalyst layer
may contain from about 1 wt % to about 10 wt % of the block
co-polymer, of which a portion may be removed from the
catalyst-coated membrane or electrode during manufacturing and/or
operation in a fuel cell. The portion that is removed may range
from about 30 wt % to about 60 wt %.
[0051] In some embodiments, the coated ion-exchange membrane or
electrode may be dried or annealed at an elevated temperature
compared to room temperature, for example, at about 40 degrees
Celsius to about 80 degrees Celsius. In some embodiments, the
coated ion-exchange membrane or electrode may be compacted at an
elevated pressured, for example, at about 5 bar to about 25 bar. In
further embodiments, the coated ion-exchange membrane or electrode
may be simultaneously heated and compacted at an elevated
temperature and pressure.
EXAMPLES
Example 1: Adhesion Test
[0052] About 10 wt % of a representative PEO-PPO block co-polymer
(Pluronic.RTM. F108 from Sigma-Aldrich) was mixed with 1100 EW
Nafion.RTM. ionomer and stirred for one hour at room temperature. A
catalyst having about 47 wt % platinum supported on a low surface
area carbon support (TKK, Japan) was added to the mixture under
stirring, followed by shear mixing for several minutes and then
microfluidized to form a cathode catalyst ink. The cathode catalyst
ink was then coated onto a Nafion.RTM. membrane (DuPont) and air
dried for form a half catalyst-coated membrane. A control half
catalyst-coated membrane was also made without the block
co-polymer.
[0053] To test for adhesion properties, both half catalyst-coated
membranes were immersed in 750 mL of water at about 80 degrees
Celsius for about 2 to 6 hours. As shown in FIGS. 3a and 3b, the
catalyst layer in the control half catalyst-coated membrane had
flaked off after being immersed in hot water for only 2 hours (FIG.
3a) while catalyst layer in the half catalyst-coated membrane with
the block co-polymer was still intact after being immersed in hot
water for 6 hours (FIG. 3b). Therefore, the block co-polymer seemed
to improve adhesion and cohesion of the catalyst layer.
Example 2: Polymer Dissolution
[0054] To determine the amount of block co-polymer that may
dissolve out of the catalyst layer, two half catalyst-coated
membranes were made with 10 wt % of the representative PEO-PPO
block co-polymer using the method above (Example 1). One of the
half catalyst-coated membranes was also annealed at about 150
degrees Celsius and compacted at 15 bar pressure for about 3
minutes to recreate decal transfer conditions of the catalyst layer
to the membrane.
[0055] Both of the half catalyst-coated membranes were washed in
water at about 75 degrees Celsius for about 6 hours and the liquid
was analyzed for total organic carbon (TOC). It was then back
calculated to determine the amount of washout of the PEO-PPO block
co-polymer, which was about 60% for non-annealed sample and about
35% for annealed sample. Without being bound by theory, it is
suspected that at least one of the annealing and compaction process
decreased dissolution of the block co-polymer.
Example 3: MEA Testing
[0056] Four MEAs with 45 cm.sup.2 active area were made with
varying amounts of the representative PEO-PPO block co-polymer as
described in Example 1 in the binder of the cathode catalyst layer
(at 0 wt %, 2.5 wt %, 5 wt % and 10 wt %). The PEO-PPO block
co-polymer was mixed with 1100 EW Nafion.RTM. ionomer and stirred
for one hour at room temperature. A catalyst having 40 wt %
platinum supported on a high surface area carbon support (TKK,
Japan) was added to the mixture under stirring, followed by shear
mixing for several minutes and then microfluidized to form a
cathode catalyst ink. The cathode catalyst ink was then coated onto
a reinforced perfluorosulfonic acid membrane (W.L. Gore &
Associates). An anode catalyst ink having platinum supported on a
high surface area carbon and 1100 EW Nafion.RTM. ionomer was coated
onto a release sheet and decal transferred to the membrane at 150
degrees Celsius to form a catalyst-coated membrane. The anode and
cathode catalyst loadings for all of the MEAs were about 0.1 mg
Pt/cm.sup.2 and about 0.4 mg Pt/cm.sup.2, respectively. The MEAs
also had hydrophobicized carbon fibre paper with microporous layers
(AvCarb.RTM.) as the gas diffusion layers, which were bonded
together with the catalyst-coated membranes. The MEAs that were
made had the following cathode composition: MEA 1 with 0 wt % block
co-polymer/33 wt % Nafion.RTM., MEA 2 with 2.5 wt % block
co-polymer/33 wt % Nafion.RTM., MEA 3 with 5 wt % block
co-polymer/33 wt % Nafion.RTM., MEA 4 with 10 wt % block
co-polymer/33 wt % Nafion.RTM. (all weight percentages indicated of
the block co-polymer are at the starting weight percentage).
[0057] For fuel cell testing, the MEAs were placed between graphite
plates and conditioned overnight at 70 degrees Celsius, 5 PSIG and
100% RH. An air polarization was performed at 60 degrees Celsius
and 5 PSIG, at both 100% and 60% RH. The results are shown in FIGS.
4a and 4b, respectively. It is clear that at 60% RH, the addition
of the PEO-PPO block co-polymer had minimal impact on performance
at all current densities. However, at 100% RH, the addition of the
PEO-PPO block co-polymer at 10 wt % had more substantial
performance impact at high current density.
[0058] All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet, are
incorporated herein by reference, in their entirety. This
application also claims the benefit of U.S. Provisional Patent
Application No. 62/485,325, filed Apr. 13, 2017, and is
incorporated herein by reference in its entirety.
[0059] While particular elements, embodiments, and applications of
the present disclosure have been shown and described, it will be
understood that the disclosure is not limited thereto since
modifications may be made by those skilled in the art without
departing from the spirit and scope of the present disclosure,
particularly in light of the foregoing teachings.
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