U.S. patent application number 17/745472 was filed with the patent office on 2022-09-01 for membrane electrode assembly with improved electrode.
The applicant listed for this patent is BALLARD POWER SYSTEMS INC.. Invention is credited to Rajesh BASHYAM, Ping HE, Siyu YE.
Application Number | 20220278334 17/745472 |
Document ID | / |
Family ID | 1000006348584 |
Filed Date | 2022-09-01 |
United States Patent
Application |
20220278334 |
Kind Code |
A1 |
BASHYAM; Rajesh ; et
al. |
September 1, 2022 |
MEMBRANE ELECTRODE ASSEMBLY WITH IMPROVED ELECTRODE
Abstract
A membrane electrode assembly comprises a polymer electrolyte
interposed between an anode electrode and a cathode electrode, the
anode electrode comprising an anode catalyst layer adjacent at
least a portion of a first major surface of the polymer
electrolyte, the cathode electrode comprising a cathode catalyst
layer adjacent at least a portion of a second major surface of the
polymer electrolyte; at least one of the anode and cathode catalyst
layers comprising: a first catalyst composition comprising a noble
metal; and a second composition comprising a metal oxide; wherein
the second composition has been treated with a fluoro-phosphonic
acid compound.
Inventors: |
BASHYAM; Rajesh; (Richmond,
CA) ; HE; Ping; (Surrey, CA) ; YE; Siyu;
(Burnaby, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BALLARD POWER SYSTEMS INC. |
Burnaby |
|
CA |
|
|
Family ID: |
1000006348584 |
Appl. No.: |
17/745472 |
Filed: |
May 16, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16309090 |
Dec 11, 2018 |
11367878 |
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PCT/US2017/044591 |
Jul 31, 2017 |
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17745472 |
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62370144 |
Aug 2, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/92 20130101; H01M
4/9016 20130101; H01M 2004/8684 20130101; H01M 4/8647 20130101;
H01M 4/8652 20130101; H01M 8/1004 20130101; H01M 4/9041
20130101 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 8/1004 20060101 H01M008/1004; H01M 4/90 20060101
H01M004/90; H01M 4/92 20060101 H01M004/92 |
Claims
1. A membrane electrode assembly, comprising: a polymer electrolyte
interposed between an anode electrode and a cathode electrode, the
anode electrode comprising an anode catalyst layer adjacent at
least a portion of a first surface of the polymer electrolyte, the
cathode electrode comprising a cathode catalyst layer adjacent at
least a portion of a second surface of the polymer electrolyte, at
least one of the anode and cathode catalyst layers comprising: a
first catalyst composition comprising a noble metal; and a second
composition comprising a metal oxide, the second composition has
been treated with a fluoro-phosphonic acid compound.
2. The membrane electrode assembly of claim 1, wherein the noble
metal of the first catalyst composition comprises platinum or a
platinum alloy.
3. The membrane electrode assembly of claim 1, wherein the noble
metal of the first catalyst composition is selected from the group
consisting of platinum, gold, ruthenium, iridium, osmium,
palladium, silver; and compounds, alloys, solid solutions, and
mixtures thereof.
4. The membrane electrode assembly of claim 1, wherein the metal
oxide of the second composition is a single-phase solid solution
comprising ruthenium.
5. The membrane electrode assembly of claim 1, wherein the metal
oxide of the second composition is a single-phase solid solution
comprising iridium.
6. The membrane electrode assembly of claim 1, wherein the metal
oxide of the second composition is selected from the group
consisting of ruthenium oxide, iridium oxide, titanium oxide,
cerium oxide, and their mixtures, solid solutions and composites
thereof.
7. The membrane electrode assembly of claim 1, wherein the metal
oxide of the second composition is supported on a support
material.
8. The membrane electrode assembly of claim 1, wherein the
fluoro-phosphonic acid compound is a fluoroalkyl-phosphonic acid
compound.
9. The membrane electrode assembly of claim 8, wherein the
fluoro-phosphonic acid compound is 2-perfluorohexyl ethyl
phosphonic acid.
10. The membrane electrode assembly of claim 8, wherein the
fluoro-phosphonic acid compound is
(1H,1H,2H,2H-heptadecafluorodec-1-yl) phosphonic acid.
11. The membrane electrode assembly of claim 1, wherein the second
composition comprises about 1.0 wt % to about 20.0 wt % of the
fluoro-phosphonic acid compound.
12. The membrane electrode assembly of claim 1, wherein the
fluoro-phosphonic acid compound has a molecular weight of at least
200.
13. The membrane electrode assembly of claim 1, wherein the first
catalyst composition is in a first discrete layer and the second
composition is in a second discrete layer in the at least one of
the anode and cathode catalyst layers.
14.-24. (canceled)
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a membrane electrode
assembly with an improved electrode for use in PEM fuel cells, and
to catalyst-coated membranes and fuel cells comprising the improved
electrode.
Description of the Related Art
[0002] Fuel cell systems are currently being developed for use as
power supplies in numerous applications, such as automobiles and
stationary power plants. Such systems offer promise of delivering
power economically and with environmental and other benefits. To be
commercially viable, however, fuel cell systems should exhibit
adequate reliability in operation, even when the fuel cells are
subjected to conditions outside their preferred operating
ranges.
[0003] Fuel cells convert reactants, namely, fuel and oxidant, to
generate electric power and reaction products. Polymer electrolyte
membrane fuel cells ("PEM fuel cell") employ a membrane electrode
assembly ("MEA"), which comprises a polymer electrolyte or
ion-exchange membrane disposed between the two electrodes, namely a
cathode and an anode. A catalyst typically induces the desired
electrochemical reactions at the electrodes. Separator plates, or
flow field plates for directing the reactants across one surface of
each electrode substrate, are disposed on each side of the MEA.
[0004] In operation, the output voltage of an individual fuel cell
under load is generally below one volt. Therefore, in order to
provide greater output voltage, multiple cells are usually stacked
together and are connected in series to create a higher voltage
fuel cell stack. (End plate assemblies are placed at each end of
the stack to hold the stack together and to compress the stack
components together. Compressive force provides sealing and
adequate electrical contact between various stack components.) Fuel
cell stacks can then be further connected in series and/or parallel
combinations to form larger arrays for delivering higher voltages
and/or currents.
[0005] In practice, fuel cells need to be robust to varying
operating conditions, especially in applications that impose
numerous on-off cycles and/or require dynamic, load-following power
output, such as automotive applications. For example, fuel cell
anode catalysts are also preferably tolerant to cell voltage
reversals and carbon monoxide poisoning; carbon-supported catalysts
are also preferably resistant to corrosion during start up and
shutdown procedures.
[0006] PEM fuel cells typically employ noble metal catalysts, and
it is well known that such catalysts, particularly platinum, are
very sensitive to carbon monoxide poisoning. This is a particular
concern for the anode catalyst of fuel cells operating on
reformate, but it also a concern for fuel cells operating on
hydrogen, as carbon monoxide (CO) is sometimes present in the
hydrogen supply as a fuel contaminant. As described by, e.g.,
Niedrach et al. in Electrochemical Technology, Vol. 5, 1967, p.
318, the use of a bimetallic anode catalyst comprising
platinum/ruthenium, rather than monometallic platinum, shows a
reduction in the poisoning effect of the CO at typical PEM fuel
cell operating temperatures. Hence, Pt--Ru catalysts are typically
employed as PEM fuel cell anode catalysts.
[0007] Ruthenium-based fuel cell catalysts are also useful for
mitigating voltage reversals. Voltage reversals occur when a fuel
cell in a series stack cannot generate sufficient current to keep
up with the rest of the cells in the series stack. Several
conditions can lead to voltage reversal in a PEM fuel cell, for
example, including insufficient oxidant, insufficient fuel, and
certain problems with cell components or construction. Reversal
generally occurs when one or more cells experience a more extreme
level of one of these conditions compared to other cells in the
stack. While each of these conditions can result in negative fuel
cell voltages, the mechanisms and consequences of such a reversal
may differ depending on which condition caused the reversal. Groups
of cells within a stack can also undergo voltage reversal and even
entire stacks can be driven into voltage reversal by other stacks
in an array. Aside from the loss of power associated with one or
more cells going into voltage reversal, this situation poses
reliability concerns. Undesirable electrochemical reactions may
occur, which may detrimentally affect fuel cell components.
Component degradation reduces the reliability and performance of
the affected fuel cell, and in turn, its associated stack and
array.
[0008] As described in U.S. Pat. No. 6,936,370, fuel cells can also
be made more tolerant to cell reversal by promoting water
electrolysis over anode component oxidation at the anode. This can
be accomplished by incorporating an additional catalyst composition
at the anode to promote the water electrolysis reaction. As a
result, more of the current forced through the fuel cell during
voltage reversal can be consumed in the electrolysis of water
rather than the oxidation of anode components. Among the catalyst
compositions disclosed were Pt--Ru alloys, RuO.sub.2 and other
metal oxide mixtures and/or solid solutions including Ru.
[0009] U.S. Patent Application No. 2004/0013935 also describes an
approach to improving cell voltage reversal tolerance by using
anodes employing both a higher catalyst loading (at least 60 wt %
catalyst) on an optional corrosion-resistant support, and
incorporating certain unsupported catalyst compositions to promote
the water electrolysis reaction. Disclosed preferred compositions
include oxides characterized by the chemical formulae RuOx and
IrOx, where x is greater than 1 and particularly about 2, and
wherein the atomic ratio of Ru to Ir is greater than about 70:30,
and particularly about 90:10.
[0010] However, ruthenium has been shown to be unstable under
certain fuel cell operating conditions. For example, Piela et al.
(J. Electrochem. Soc., 151 (12), A2053-A2059 (2004)), describe
ruthenium crossover from Pt--Ru black catalyst and redeposition at
the Pt cathode catalyst in direct methanol fuel cells (DMFC) and
hydrogen/air fuel cells under abnormal conditions, such as cell
reversal resulting in very high anode potentials (and under normal
DMFC operating conditions). Piela et al. theorized that the Pt--Ru
alloy should likely remain stable under DMFC operating conditions,
and that the source of the ruthenium contamination was neutral
hydrous RuO.sub.2. Taniguchi et al. (J. Power Sources, 130, 42-49
(2004)) observed ruthenium dissolution from a carbon supported
Pt--Ru anode catalyst as a result of high anode potentials
experienced by the fuel cell under cell reversal conditions.
[0011] It has also been shown that Pt--Ru catalysts are prone to
ruthenium dissolution at higher relative humidity operation and
cathode carbon corrosion. For example, P. He et al. (ECS
Transactions, 33 (1) 1273-1279 (2010)) found that relative humidity
(RH) significantly impacted the degree of ruthenium dissolution and
crossover, which subsequently affected the cell performance and CO
tolerance. Lower operating RH during testing resulted in less
ruthenium contamination on the cathode and lower performance
losses. In addition, T. Cheng et al. (Journal of The
Electrochemical Society, 157 (5) B714-B718 (2010)) investigated
anode catalysts with different elemental compositions to cause
various degrees of ruthenium crossover. It was found that after
anode accelerated stress test cycles, ruthenium crossover and
subsequent deposition on the cathode occurred, which resulted in
significant fuel cell performance loss.
[0012] To mitigate ruthenium dissolution of Pt--Ru-based catalysts,
U.S. Pat. No. 7,608,358 discloses an electrode assembly for a fuel
cell comprising an anode catalyst layer, the anode catalyst layer
comprising a first catalyst composition comprising a noble metal,
other than ruthenium, on a corrosion resistant support material; a
second catalyst composition comprising a single-phase solid
solution of a metal oxide containing ruthenium; and a hydrophobic
binder; wherein a through-plane concentration of an ionomer in the
catalyst layer decreases as a function of distance from the
electrolyte. '358 discloses that MEAS and fuel cells with such an
electrode assembly demonstrated markedly improved cell reversal
tolerance and performance in start/stop cycling tests, while
retaining baseline performance and performance in the presence of
CO. '358 also discloses the mechanisms for ruthenium crossover from
the anode and the cathode, particularly due to start/stop
cycling.
[0013] However, ruthenium oxide-based catalysts, such as those
described in '358, are still be prone to ruthenium dissolution. As
a result, there exists a need for membrane electrode assemblies and
fuel cells that are more robust to operating conditions that impose
numerous on-off cycles and/or require dynamic, load-following power
output; are tolerant to cell voltage reversals; and resistant to
carbon monoxide poisoning and corrosion during start up and
shutdown procedures. The present invention addresses this need and
provides associated benefits.
BRIEF SUMMARY OF THE INVENTION
[0014] In brief, a membrane electrode assembly comprises a polymer
electrolyte interposed between an anode electrode and a cathode
electrode, the anode electrode comprising an anode catalyst layer
adjacent at least a portion of a first major surface of the polymer
electrolyte, the cathode electrode comprising a cathode catalyst
layer adjacent at least a portion of a second major surface of the
polymer electrolyte; at least one of the anode and cathode catalyst
layers comprising: a first catalyst composition comprising a noble
metal; and a second composition comprising a metal oxide; wherein
the second composition has been treated with a fluoro-phosphonic
acid compound. In further embodiments, the metal oxide of the
second composition is selected from the group consisting of
ruthenium oxide, iridium oxide, ruthenium iridium oxide, titanium
oxide, cerium oxide, and their mixtures, solid solutions and
composites thereof.
[0015] In specific embodiments, the fluoro-phosphonic acid compound
is a fluoroalkyl-phosphonic acid compound. In further embodiments,
the fluoro-phosphonic acid compound is 2-perfluorohexyl ethyl
phosphonic acid. In other embodiments, the fluoro-phosphonic acid
compound is (1H,1H,2H,2H-heptadecafluorodec-1-yl) phosphonic
acid.
[0016] In further embodiments, the first catalyst composition is in
a first discrete layer and the second composition is in a second
discrete layer in the at least one of the anode and cathode
catalyst layers.
[0017] In another embodiment, a method of making a membrane
electrode assembly, the membrane electrode assembly comprising a
polymer electrolyte interposed between an anode electrode and a
cathode electrode, the anode electrode comprising an anode catalyst
layer adjacent at least a portion of a first major surface of the
polymer electrolyte, the cathode electrode comprising a cathode
catalyst layer adjacent at least a portion of a second major
surface of the polymer electrolyte; at least one of the anode and
cathode catalyst layers comprising a first catalyst composition
comprising a noble metal, other than ruthenium; and a second
composition comprising a metal oxide, wherein the second
composition has been treated with a fluoro-phosphonic acid
compound; the method comprising the steps of: dissolving the
fluoro-phosphonic acid compound in a solvent to form a
fluoro-phosphonic acid compound dispersion; dispersing the
fluoro-phosphonic acid compound dispersion with a metal oxide;
removing the solvent after dispersing the fluoro-phosphonic acid
compound dispersion with the metal oxide to form the treated second
composition; and providing the treated second composition to at
least one of the anode catalyst layer and the cathode catalyst
layer.
[0018] These and other aspects of the invention are evident upon
reference in the attached drawings and following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1a shows the dispersion properties of the untreated
RuIrOx powder in water.
[0020] FIG. 1b shows the dispersion properties of the treated
RuIrOx powder in water.
[0021] FIG. 2 shows the cell reversal tolerance test results of the
fuel cells with the treated and untreated catalyst compositions on
the anode.
DETAILED DESCRIPTION
[0022] In the following description, certain specific details are
set forth in order to provide a thorough understanding of the
various embodiments of the invention. However, one skilled in the
art will understand that the invention may be practiced without
these details. In other instances, well-known structures associated
with fuel cells, fuel cell stacks, batteries and fuel cell systems
have not been shown or described in detail to avoid unnecessarily
obscuring descriptions of the embodiments of the invention.
[0023] 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."
[0024] A "corrosion resistant support material" is at least as
resistant to oxidative corrosion as Shawinigan acetylene black
(Chevron Chemical Company, TX, USA).
[0025] An electrochemical fuel cell includes a polymer electrolyte
interposed between an anode electrode and a cathode electrode, a
cathode catalyst layer between the polymer electrolyte and the
cathode electrode, and an anode catalyst layer between the polymer
electrolyte and the anode electrode. In one embodiment, the anode
catalyst layer includes a first catalyst composition comprising a
noble metal; and a second catalyst composition comprising a metal
oxide; wherein the second catalyst composition has been treated
with a fluoro-phosphonic acid compound. In specific embodiments,
the metal oxide may be a ruthenium-containing metal oxide and/or an
iridium-containing metal oxide.
[0026] The inventors surprisingly discovered that by treating the
ruthenium-containing metal oxide with a fluoro-phosphonic acid
compound, performance degradation due to ruthenium dissolution
under voltage cycling conditions (i.e., start-up/shutdown) was
reduced, thus indicating the ruthenium dissolution was reduced.
Without being bound by theory, the inventors suspect that covalent
bonding of fluoroalkyl phosphonic acid with metal oxides through a
self-assembly condensation process, which increased its
hydrophobicity and reduced degradation.
[0027] However, the inventors expected that by treating the
ruthenium-containing metal oxide with a fluoro-phosphonic acid
compound, cell reversal tolerance would be reduced because water
would be pushed away from the ruthenium-containing metal oxide (due
to the hydrophobic fluorinated alkyl groups), which would reduce
the capability of the metal oxide to electrolyze water. It was
surprisingly discovered that cell reversal tolerance was generally
unaffected with the treatment. Furthermore, the inventors
discovered that cell reversal tolerance was also improved for
iridium-containing metal oxides. Without being bound by theory, it
is suspected that the treatment forms a thin layer of
fluoro-phosphonic acid at the surface of the ruthenium-containing
metal oxide that renders it hydrophobic through the self-assembled
surface via covalent bonding, without significantly affecting the
reaction sites (or surface area) for water electrolysis. In other
words, it is suspected that the treatment renders a change in the
local relative humidity of the catalyst layer without negatively
affecting cell reversal tolerance.
[0028] In some embodiments, the average molecular weight of the
fluoro-phosphonic acid compound ranges from about 200 to 1200. In
specific embodiments, the average molecular weight of the
fluoro-phosphonic acid compound ranges from about 300 to 1000.
[0029] In some embodiments, the fluoro-phosphonic acid compound has
a chain length of six to twelve carbons in its backbone.
[0030] In one embodiment, the fluoro-phosphonic acid compound is a
fluoroalkyl-phosphonic acid compound.
[0031] In specific embodiments, the fluoroalkyl-phosphonic acid
compound is a perfluoro-phosphonic acid compound, such as
2-perfluorohexyl ethyl phosphonic acid and 1H, 1H, 2h,
2H-heptadecaflurorodec-1-yl phosphoric acid (C10-PFPA).
[0032] The first catalyst composition comprises at least one noble
metal. The noble metal may comprise Pt or an alloy of Pt. In
embodiments where a Pt alloy catalyst is employed, the alloy may
include another noble metal, such as gold, ruthenium, iridium,
osmium, palladium, silver; and compounds, alloys, solid solutions,
and mixtures thereof. In some embodiments, the first catalyst
composition comprises a mixture of a noble metal and non-noble
metal, such as cobalt, iron, molybdenum, nickel, tantalum, tin,
tungsten; and compounds, alloys, solid solutions, and mixtures
thereof. While noble metals are described for the first catalyst
composition, it is expected that non-noble metals, such as those
described above, can also be used as the first catalyst composition
in some applications.
[0033] The first catalyst composition may either be unsupported or
supported in dispersed form on a suitable electrically conducting
particulate support. In some embodiments, the support used is
itself tolerant to voltage reversal. Thus, it is desirable to
consider using supports that are more corrosion resistant.
[0034] The corrosion resistant support material may comprise
carbon, if desired. High surface area carbons, such as acetylene or
furnace blacks, are commonly used as supports for such catalysts.
Generally, the corrosion resistance of a carbon support material is
related to its graphitic nature: the more graphitic the carbon
support, the more corrosion resistant it is. Graphitized carbon BA
(TKK, Tokyo, JP) has a similar BET surface area to Shawinigan
acetylene carbon and is a suitable carbon support material in some
embodiments. In other embodiments suitable carbon support materials
may include nitrogen-, boron-, sulfur-, and/or phosphorous-doped
carbons, carbon nanofibres, carbon nanotubes, carbon nanohorns,
graphenes, and aerogels.
[0035] Instead of carbon, carbides or electrically conductive metal
oxides may be considered as a suitable high surface area support
for the corrosion resistant support material. For instance,
tantalum, titanium and niobium oxides may serve as a corrosion
resistant support material in some embodiments. In this regard,
other valve metal oxides might be considered as well if they have
acceptable electronic conductivity when acting as catalyst
supports.
[0036] In embodiments where the first catalyst composition is
supported, the loading of the first catalyst composition on the
support material is from about 20 to about 80% by weight, typically
about 20 to about 50% by weight. For a noble metal catalyst, a
lower catalyst loading on the support is typically preferred in
terms of electrochemical surface area per gram of platinum (ECA),
but a higher catalyst loading and coverage of the support appears
preferable in terms of reducing corrosion of the support and in
reducing catalyst loss during fuel cell operation.
[0037] In some embodiments, the amount of the first catalyst
composition that is desirably incorporated will depend on such
factors as the fuel cell stack construction and operating
conditions (for example, current that may be expected in reversal),
cost, desired lifetime, and so on. For example, the catalyst
loading of the first catalyst composition may range from about 0.05
mg Pt/cm.sup.2 on the low end for the anode electrode to about 0.8
mg Pt/cm.sup.2 on the high end for the cathode electrode. The
second composition comprises a metal oxide, wherein the metal oxide
may be, such as, but not limited to, ruthenium oxide, iridium
oxide, titanium oxide, cerium oxide, and their mixtures, solid
solutions and composites thereof. In addition, the metal oxide
loading of the second composition may range from about 0.001
mg/cm.sup.2 to about 0.10 mg/cm.sup.2. It is expected that some
empirical trials will determine an optimum amount for a given
application. In some embodiments, the metal oxide may be supported
on another metal oxide support.
[0038] In one embodiment, the metal oxide of the second composition
comprises ruthenium. In specific embodiments, the metal oxide is a
single-phase solid solution comprising ruthenium. In further
embodiments, the second composition comprises a single-phase solid
solution of ruthenium oxide (90:10 mole ratio of Ru:Ir). For
example, the metal oxide of the second composition is, but not
limited to, RuO.sub.2 and RuIrO.sub.2. As mentioned, the metal
oxide may be supported on a catalyst support, such as RuO.sub.2
supported on tantalum oxide, titanium oxide or niobium oxide.
[0039] In other embodiments, the second catalyst composition
comprises a metal oxide comprising iridium, such as a single-phase
solid solution of iridium oxide. As mentioned, the metal oxide may
be supported on a catalyst support, such as IrO.sub.2 supported on
tantalum oxide, titanium oxide or niobium oxide.
[0040] In further embodiments, a mixture of treated metal oxides
may be used, depending on the application.
[0041] In other embodiments, the first catalyst composition and the
second composition may be in separate, discrete layers in the anode
catalyst layer and/or cathode catalyst layer. For example, the
first discrete layer with the first catalyst composition is
adjacent the membrane and the second discrete layer with the second
composition is adjacent the gas diffusion layer.
[0042] In one general method to treat the metal oxide of the second
composition, a fluoro-phosphonic acid compound is dissolved in a
first solvent, a metal oxide is dispersed in another solvent that
may be the same or different from the first solvent, and then the
dispersions are dispersed together via conventional methods. The
solvents are then removed from the dispersion, such as by
evaporating the solvent or centrifuging the dispersion or other
methods known in the art, and then further heat-treated at an
elevated temperature for an adequate amount of time to form a
powder of the fluoro-phosphonic acid-treated metal oxide. The
fluoro-phosphonic acid-treated metal oxide may contain about 1.0 wt
% to about 20.0 wt % of the fluoro-phosphonic acid compound.
Without being bound by theory, the phosphonic acid covalently bonds
to the surface oxygen at the surface of the metal oxide with this
treatment.
[0043] The second composition may be incorporated in the catalyst
layer in various ways known in the art. In some embodiments, the
first catalyst and second compositions may be mixed together and
the mixture applied in a uniformly distributed common layer or
layers on a suitable gas diffusion layer (GDL), polymer electrolyte
membrane, or decal transfer sheet. With decal transfer, the
catalyst layer may be decal transferred from the decal transfer
sheet to a GDL to form a gas diffusion electrode, or may be decal
transferred to a polymer electrolyte membrane to form a
catalyst-coated membrane. As mentioned previously, the second
composition may be supported on the same support material as the
first catalyst composition, and thus both compositions are already
"mixed" for application in one or more layers on an anode
substrate, cathode substrate and membrane.
[0044] In further embodiments, the first catalyst composition and
the second composition may instead be applied in discrete, separate
layers on a GDL, polymer electrolyte membrane, or decal transfer
sheet, thereby making a bilayer or multilayer structure. By
applying the first catalyst composition in one discrete layer and a
second composition in a second discrete layer, one may use
different ionomers, solvents and processing steps for each of the
first and second compositions.
[0045] In another embodiment, the second composition may be
non-uniformly distributed, for example, located where degradation
is expected to occur. Persons of ordinary skill in the art can
readily select an appropriate manner of incorporation for a given
application.
[0046] The anode and cathode catalyst layers typically further
include a binder, such as an ionomer and/or hydrophobic agent.
[0047] In some embodiments, the through-plane concentration of
ionomer in the present catalyst layer decreases as a function of
distance from the polymer electrolyte interface. The presence of
ionomer in the catalyst layer effectively increases the
electrochemically active surface area of the catalyst, which
requires an ionically conductive pathway to the cathode catalyst to
generate electric current.
[0048] As previously mentioned, the anode and cathode catalyst
layers may be applied to a GDL to form anode and cathode
electrodes, or to a decal transfer sheet which is then decal
transferred to a surface of the GDL or polymer electrolyte, or
applied directly to the surface of the polymer electrolyte to form
a catalyst-coated membrane (CCM). The electrodes or CCM can then be
bonded under heat and/or pressure with other components to form an
MEA. Alternatively, the application of the catalyst layer on the
desired substrate may occur at the same time the remaining MEA
components are bonded together.
[0049] The present catalyst layers may be applied according to
known methods. For example, the catalyst may be applied as a
catalyst ink or slurry, or as a dry mixture. Catalyst inks may be
applied using a variety of suitable techniques (e.g., hand and
machine methods, including hand brushing, notch bar coating, fluid
bearing die coating, wire-wound rod coating, fluid bearing coating,
slot-fed knife coating, three-roll coating, screen-printing and
decal transfer) to the surface of the polymer electrolyte or GDL.
Examples of dry deposition methods include spraying, vacuum
deposition and electrostatic powder deposition techniques.
[0050] Catalyst inks typically incorporate the catalysts and binder
in a solvent/dispersant to form a solution, dispersion or colloidal
mixture. Suitable solvents/dispersants include water, organic
solvents such as alcohols and polar aprotic solvents (e.g.,
N-methylpyrrolidinone, dimethylsulfoxide, and
N,N-dimethylacetamide), and mixtures thereof. Depending on the
amount of water, one can distinguish water-based inks, wherein
water forms the major part of the solvents used, from inks wherein
organic solvents form the major part. Catalyst inks may further
include surfactants and/or pore forming agents, if desired.
Suitable pore formers include methyl cellulose; sublimating
pore-forming agents such as durene, camphene, camphor and
naphthalene; and pore-forming solvents that are immiscible with the
catalyst ink solvent/dispersant, such as n-butyl acetate in polar
aprotic solvent/dispersant systems.
[0051] The selection of additional components for the catalyst
mixture and the choice of application method and GDL to which it is
applied are not essential to the present invention, and will depend
on the physical characteristics of the mixture and the substrate to
which it will be applied, the application method and desired
structure of the catalyst layer. Persons of ordinary skill in the
art can readily select suitable catalyst mixtures and application
methods for a given application.
EXAMPLES
Example 1
[0052] Five grams of ruthenium iridium oxide (RuIrO.sub.2) powder
purchased from Johnson Matthey (Reading, UK) was dispersed in 60 mL
of ethanol. 0.56 g of 2-perfluorohexyl ethyl phosphonic acid
precursor (Unimatech, Japan) was separately dissolved in 5 mL of
ethanol. Subsequently, the dispersion containing the RuIrO.sub.2
was probe ultasonicated. The solution containing the dissolved
precursor was added to the sonicated dispersion in a dropwise
manner (at about a flow rate of 1 mL/min) while stirring at room
temperature for about 60 minutes. The dispersion was then
transferred to a petri dish and evaporated at about 60 degrees
Celsius overnight. The free flowing powder was transferred to an
oven and heated to 150 degrees Celsius for 30 minutes, resulting in
the final treated RuIrO.sub.2 powder with about 10 wt % of
perfluorohexyl ethyl phosphonic acid.
[0053] FIG. 1 shows the picture of treated RuIrO.sub.2 powder
(right) in comparison to non-treated RuIrO.sub.2 (left) dispersed
in water. As it can be seen, the treated RuIrO.sub.2 powder is
completely hydrophobic and water could not wet the surface and,
hence, it floats on water, even though the density of RuIrO.sub.2
is higher than water. On the other hand, non-treated RuIrO.sub.2
powder disperses well in water as it is highly hydrophilic.
Example 2
[0054] Four grams of ruthenium iridium oxide (RuIrO.sub.2) powder
purchased from Johnson Matthey (Reading, UK) was dispersed in 50 mL
of isopropanol. 0.4 g of (1H,1H,2H,2H-heptadecafluorodec-1-YL)
phosphonic acid (C10-PFPA) precursor (Advanced Technology and
Industrial CO., LTD, Hong Kong) was separately dissolved in 50 mL
of isopropanol. Subsequently, the dispersion containing the
RuIrO.sub.2 was probe ultrasonicated. The solution containing the
dissolved precursor was added to the sonicated dispersion in a
dropwise manner (at about a flow rate of 1 mL/min) while stirring
at room temperature overnight. The dispersion was then centrifuged
to separate the catalyst from the solution. The powder at the
bottom of the centrifuge tube was dried in a vacuum oven at 80
degrees Celsius for overnight, resulting in the final treated
RuIrO.sub.2 powder with about 2 wt % of C10-PFPA.
Example 3
[0055] Five grams of iridium oxide (IrO.sub.2) powder purchased
from Tanaka Kikinzoku International, Inc. (USA) was dispersed in 50
mL of 1-propanol. 0.35 g of (1H,1H,2H,2H-heptadecafluorodec-1-YL)
phosphonic acid (C10-PFPA) precursor was separately dissolved in 30
mL of 1-propanol. Subsequently, the dispersion containing the
IrO.sub.2 was probe ultrasonicated. The solution containing the
dissolved precursor was added to the sonicated dispersion in a
dropwise manner (at about a flow rate of 1 mL/min) while stirring
at room temperature. The dispersion was then heated up to 100
degrees Celsius for approximately 3 hours to evaporate all the
solvent, resulting in the final treated IrO.sub.2 powder with about
7 wt % of C10-PFPA.
[0056] The treated RuIrO.sub.2 powder of Examples 1 and 2, as well
as the IrO.sub.2 powder of Example 3, were then dispersed in a
Nafion.RTM./alcohol mixture with a platinum-based catalyst, then
applied to a decal transfer film and subsequently decal transferred
via heat and pressure to a half CCM (Nafion.RTM. 211 membrane with
cathode catalyst on one side) to form a full CCM. Untreated
RuIrO.sub.2 powder and untreated IrO.sub.2 powder were also
incorporated in a similar fashion to form full CCMs.
[0057] The MEAs were made with the following electrode structures
as listed in Table 1, with the CCM sandwiched between two
AvCarb.RTM. GDLs (AvCarb Materials Solutions, Lowell, Mass.). The
GDLs were bonded to the CCM via heat and pressure. The active area
of each of the MEAs was 45 cm.sup.2.
TABLE-US-00001 TABLE 1 Anode and cathode catalyst structures for
MEAs MEA Example Anode Cathode Comparative 50% Pt supported on
graphitized 50% Pt supported on graphitized MEA 1 carbon black at a
catalyst loading of carbon black at ~0.4 mg Pt/cm.sup.2 ~0.1 mg
Pt/cm.sup.2; Ionomer (23%): Nafion .RTM. ~0.06 mg/cm.sup.2
untreated RuIrO.sub.2 (single-phase solid solution (90:10 mole
ratio Ru/Ir); Johnson Matthey Plc, Reading, UK); Ionomer (23%):
Nafion .RTM. MEA 2 50% Pt supported on graphitized 50% Pt supported
on graphitized (RuIrO.sub.2 of carbon black at a catalyst loading
of carbon black at ~0.4 mg Pt/cm.sup.2 Example 1) ~0.1 mg
Pt/cm.sup.2; Ionomer (23%): Nafion .RTM. ~0.06 mg/cm.sup.2
RuIrO.sub.2 (single-phase solid solution (90:10 mole ratio Ru/Ir);
Johnson Matthey Plc, Reading, UK) treated with 2- perfluorohexyl
ethyl phosphonic acid; Ionomer (23%): Nafion .RTM. MEA 3 50% Pt
supported on graphitized 50% Pt supported on graphitized
(RuIrO.sub.2 of carbon black at a catalyst loading of carbon black
at ~0.4 mg Pt/cm.sup.2 Example 2) ~0.1 mg Pt/cm.sup.2; Ionomer
(23%): Nafion .RTM. ~0.06 mg/cm.sup.2 RuIrO.sub.2 (single-phase
solid solution (90:10 mole ratio Ru/Ir); Johnson Matthey Plc,
Reading, UK) treated with C10 PFPA; Ionomer (23%): Nafion .RTM.
Comparative 50% Pt supported on graphitized 50% Pt supported on
graphitized MEA 4 carbon black at a catalyst loading of carbon
black at ~0.4 mg Pt/cm.sup.2 ~0.05 mg Pt/cm.sup.2; Ionomer (23%):
Nafion .RTM. ~0.026 mg/cm.sup.2 IrO.sub.2 (TKK); Ionomer (20%):
Nafion .RTM. MEA 5 50% Pt supported on graphitized 50% Pt supported
on graphitized (IrO.sub.2 of carbon black at a catalyst loading of
carbon black at ~0.4 mg Pt/cm.sup.2 Example 3) ~0.05 mg
Pt/cm.sup.2; Ionomer (23%): Nafion .RTM. ~0.026 mg/cm.sup.2
IrO.sub.2 (TKK); treated with C10 PFPA; Ionomer (20%): Nafion
.RTM.
[0058] The MEAs were then tested in a Ballard Standard Test Cell
(STC) test fixture with graphite plates. The fuel cells were first
conditioned overnight under the following conditions at 1.3
A/cm.sup.2:
TABLE-US-00002 TABLE 2 Conditioning parameters Temperature
75.degree. C. (coolant) Inlet Dew Point 75.degree. C. (fuel and
oxidant) Fuel 100% hydrogen Oxidant air Reactant inlet pressure 5
psig (fuel and oxidant) Reactant flow 4.5 (fuel), 9.0 (oxidant)
slpm
Cell Reversal Testing
[0059] The fuel cells were conditioned overnight at 1.3 A/cm.sup.2
at the conditions listed in Table 2. The fuel supply was then
switched to humidified nitrogen and the cell was supplied with 200
mA/cm.sup.2 of current through an external power supply under
current control mode. The cell reversal tolerance time was
monitored until the cell voltage reached -2.0 V
[0060] FIG. 2 shows that the first three fuel cells exhibited
almost exactly the same behavior with almost the same cell reversal
times (85 minutes for MEA 1 and
[0061] MEA 1, and 89 minutes for MEA 3). Therefore, the treatment
of RuIrO.sub.2 with either perfluorohexyl ethyl phosphonic acid or
C10-PFPA did not significantly impact cell reversal tolerance,
which, as discussed above, is a surprising and unexpected result
due to the expected lower surface area of the RuIrO.sub.2 after
treatment.
[0062] For iridium oxide, it was shown that MEA 5 with a
C10-PFPA-treated iridium oxide had a significantly improved cell
reversal time in comparison to Comparative MEA 4, which did not
have the C10-PFPA treatment. Comparative MEA 4 had a cell reversal
time of less than 300 minutes while MEA 5 had a cell reversal time
of greater than 450 minutes. Thus, the treatment of IrO.sub.2 with
C10-PFPA had a positive effect on cell reversal tolerance.
Anode Accelerated Stress Test
[0063] Anode accelerated stress tests (ASTs) were used to simulate
potential spikes that occur during fuel cell start-ups and
shutdowns to induce ruthenium dissolution and crossover.
[0064] The fuel cells were operated at 75.degree. C., 5 psig (136
kPa) pressure, and 100% inlet RHs, 70% H.sub.2/30% N.sub.2 for the
fuel, and a beginning of life (BOL) polarization was obtained.
During the AST, the anode potential cycled between .about.0 and 0.9
V by switching the fuel between 70% H.sub.2/30% N.sub.2 for 1
minute and 100% N2 for 30 seconds, while the cathode potential was
kept below 1.0 V to minimize cathode degradation. Unless otherwise
stated, all half-cell potentials reported here are relative to the
dynamic hydrogen reference electrode (DHE). After the AST, the fuel
cells were again operated at the same conditions as the BOL
polarization, and an end of life (EOL) polarization was
obtained.
[0065] At 1 A/cm.sup.2, it was shown that MEA 2 had a performance
loss of 38 mV less than the baseline MEA 1, and MEA 3 had a
performance loss of 67 mV less than the baseline MEA 1. Therefore,
by treating the RuIrO.sub.2 catalyst with either perfluorohexyl
ethyl phosphonic acid or C10-PFPA, the MEAs with the treated
RuIrO.sub.2 catalyst exhibited significantly less performance loss
at 1 A/cm.sup.2 than the untreated MEA.
[0066] While the treated metal oxides have been described for the
anode electrode in the preceding description, it is contemplated
that such treated metal oxides may, additionally or alternatively,
be used on the cathode electrode. Without being bound by theory,
such treated metal oxides are beneficial for improved durability by
mitigating carbon corrosion at high cathode potentials by acting as
a water electrolysis catalyst.
[0067] While the present electrodes have been described for use in
PEM fuel cells, it is anticipated that they may be useful in other
fuel cells having an operating temperature below about 250.degree.
C. They are particularly suited for acid electrolyte fuel cells,
including phosphoric acid, PEM and liquid feed fuel cells. It is
also contemplated that this treatment may also be useful for other
metal oxides comprising ruthenium. All of the above U.S. patents,
U.S. patent application publications, U.S.
[0068] 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.
[0069] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art,
particularly in light of the foregoing teachings. It is therefore
contemplated by the appended claims to cover such modifications
that incorporate those features coming within the scope of the
invention.
[0070] This application also claims the benefit of U.S. Provisional
Patent Application No. 62/370,144, filed Aug. 2, 2016, and is
incorporated herein by reference in its entirety.
* * * * *