U.S. patent application number 12/395900 was filed with the patent office on 2009-12-31 for fuel cell electrolyte membrane with acidic polymer.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Matthew H. Frey, Steven J. Hamrock, Gregory M. Haugen, William M. Lamanna, James M. Larson, Phat T. Pham.
Application Number | 20090325030 12/395900 |
Document ID | / |
Family ID | 38605190 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090325030 |
Kind Code |
A1 |
Hamrock; Steven J. ; et
al. |
December 31, 2009 |
FUEL CELL ELECTROLYTE MEMBRANE WITH ACIDIC POLYMER
Abstract
An electrolyte membrane is formed by an acidic polymer and a
low-volatility acid that is fluorinated, substantially free of
basic groups, and is either oligomeric or non-polymeric.
Inventors: |
Hamrock; Steven J.;
(Stillwater, MN) ; Larson; James M.; (Saint Paul,
MN) ; Pham; Phat T.; (Little Canada, MN) ;
Frey; Matthew H.; (Cottage Grove, MN) ; Haugen;
Gregory M.; (Edina, MN) ; Lamanna; William M.;
(Stillwater, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
38605190 |
Appl. No.: |
12/395900 |
Filed: |
March 2, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11230090 |
Sep 19, 2005 |
7517604 |
|
|
12395900 |
|
|
|
|
Current U.S.
Class: |
429/492 ;
429/413 |
Current CPC
Class: |
H01M 8/1081 20130101;
H01M 8/0289 20130101; Y02P 70/50 20151101; H01M 2008/1095 20130101;
H01M 8/1025 20130101; H01M 8/1039 20130101; H01M 8/1048 20130101;
H01M 8/1058 20130101; C08J 5/2262 20130101; Y02E 60/50 20130101;
H01M 8/1093 20130101; H01M 8/1051 20130101 |
Class at
Publication: |
429/33 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Goverment Interests
[0002] This invention was made with U.S. Government support under
Cooperative Agreement DE-FC36-02AL67621 awarded by the Department
of Energy. The U.S. Government has certain rights in this
invention.
Claims
1. An electrolyte membrane comprising: a polysulfonated
fluoropolymer; and a fluorinated acid selected from the group
consisting of sulfonic acids, imide acids, and combinations
thereof.
2. The electrolyte membrane of claim 1, wherein the acidic polymer
comprises a highly fluorinated backbone and pendant groups, wherein
the pendent groups are selected from the group consisting of
--OCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.3Y,
--O(CF.sub.2).sub.4SO.sub.3Y, and combinations thereof, wherein Y
is selected from the group consisting of a hydrogen ion, a cation,
and combinations thereof.
3. The electrolyte membrane of claim 1, wherein the fluorinated
acid comprises a bis-acid.
4. The electrolyte membrane of claim 1, wherein the electrolyte
membrane is substantially free of phosphoric acid.
5. The electrolyte membrane of claim 1, further comprising
inorganic additives.
6. The electrolyte membrane of claim 1, further comprising a
reinforcement matrix.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. Ser. No.
11/230,090, filed Sep. 19, 2005, now allowed, the disclosure of
which is incorporated by reference in its entirety herein.
FIELD OF THE INVENTION
[0003] The present invention relates to electrolyte membranes in
electrochemical devices, such as fuel cells. In particular, the
present invention relates to electrolyte membranes that preserve
proton conductivity and are stable when operated at high
temperatures.
BACKGROUND OF THE INVENTION
[0004] Fuel cells are electrochemical devices that produce usable
electricity by the catalyzed combination of a fuel such as hydrogen
and an oxidant such as oxygen. In contrast to conventional power
plants, such as internal combustion generators, fuel cells do not
utilize combustion. As such, fuel cells produce little hazardous
effluent. Fuel cells convert hydrogen fuel and oxygen directly into
electricity, and can be operated at higher efficiencies compared to
internal combustion generators.
[0005] A fuel cell such as a proton exchange membrane (PEM) fuel
cell typically contains a membrane electrode assembly (MEA), formed
by a catalyst coated membrane disposed between a pair of gas
diffusion layers. The catalyst coated membrane itself typically
includes an electrolyte membrane disposed between a pair of
catalyst layers. The respective sides of the electrolyte membrane
are referred to as an anode portion and a cathode portion. In a
typical PEM fuel cell, hydrogen fuel is introduced into the anode
portion, where the hydrogen reacts and separates into protons and
electrons. The electrolyte membrane transports the protons to the
cathode portion, while allowing a current of electrons to flow
through an external circuit to the cathode portion to provide
power. Oxygen is introduced into the cathode portion and reacts
with the protons and electrons to form water and heat. The MEA also
desirably retains water to preserve proton conductivity between the
layers, particularly at the electrolyte membrane. A reduction in
proton conductivity between the layers correspondingly reduces the
electrical output of the fuel cell.
[0006] A common problem with fuel cells involves carbon monoxide
poisoning of the catalyst layers, which reduces the effectiveness
of the catalyst layers. To counter the reduction, higher catalyst
concentrations are required to provide effective levels of
electrical output. This correspondingly increases the material
costs for manufacturing fuel cells. One technique for reducing the
carbon monoxide poisoning includes operating the fuel cell at
higher temperatures (e.g., above 100.degree. C.). However, the
elevated temperatures cause the water retained in the MEA to
evaporate, thereby reducing the proton conductivity within and
between the layers. As such, there is a need for an electrochemical
device that preserves proton conductivity while operating at high
temperatures.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention relates to an electrolyte membrane
that includes an acidic polymer and an acid, where the acid is a
low-volatility acid that is fluorinated, substantially free of
basic groups, and is either oligomeric or non-polymeric. As a
result, the electrolyte membrane may be used at high operating
temperatures while preserving proton conductivity. The present
invention further relates to a method of forming the electrolyte
membrane and to an electrochemical device that includes the
electrolyte membrane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic illustration of a membrane electrode
assembly of the present invention in use with an external
electrical circuit.
[0009] FIG. 2 is a graph illustrating polarization curves of an
exemplary electrolyte membrane of the present invention and a
comparative electrolyte membrane.
[0010] While the above-identified drawing figures set forth several
embodiments of the invention, other embodiments are also
contemplated, as noted in the discussion. In all cases, this
disclosure presents the invention by way of representation and not
limitation. It should be understood that numerous other
modifications and embodiments can be devised by those skilled in
the art, which fall within the scope and spirit of the principles
of the invention. The figures may not be drawn to scale. Like
reference numbers have been used throughout the figures to denote
like parts.
DETAILED DESCRIPTION
[0011] FIG. 1 is an illustration of MEA 10 in use with external
electrical circuit 12, where MEA 10 includes electrolyte membrane
14 of the present invention. MEA 10 is suitable for use in
electrochemical cells, such as PEM fuel cells, and further includes
anode portion 16, cathode portion 18, catalyst layers 20 and 22,
and gas diffusion layers 24 and 26. Anode portion 16 and cathode
portion 18 generally refer to the anode and cathode sides of MEA
10.
[0012] Electrolyte membrane 14 is disposed between catalyst layers
20 and 22, where electrolyte membrane 14 and catalyst layers 20 and
22 may be a catalyst coated membrane. Electrolyte membrane 14 is
thermally stable, and may be operated at high temperatures (e.g.,
up to 150.degree. C.) for reducing carbon monoxide poisoning of
catalyst layers 20 and 22, while exhibiting good proton
conductivity.
[0013] Catalyst layer 20 is disposed between electrolyte membrane
14 and gas diffusion layer 24, where gas diffusion layer 24 is
located at anode portion 16 of MEA 10. Similarly, catalyst layer 22
is disposed between electrolyte membrane 14 and gas diffusion layer
26, where gas diffusion layer 26 is located at cathode portion 18
of MEA 10. Gas diffusion layers 24 and 26 may each be any suitable
electrically conductive porous substrate, such as carbon fiber
constructions (e.g., woven and non-woven carbon fiber
constructions). Gas diffusion layers 24 and 26 may also be treated
to increase or impart hydrophobic properties.
[0014] During operation of MEA 10, hydrogen fuel (H.sub.2) is
introduced into gas diffusion layer 24 at anode portion 16. MEA 10
may alternatively use other fuel sources, such as methanol,
ethanol, formic acid, and reformed gases. The fuel passes through
gas diffusion layer 24 and over catalyst layer 20. At catalyst
layer 20, the fuel is separated into hydrogen ions (H.sup.+) and
electrons (e.sup.-). Electrolyte membrane 14 only permits the
hydrogen ions to pass through to reach catalyst layer 22 and gas
diffusion layer 26. The electrons cannot pass through electrolyte
membrane 14. As such, the electrons flow through external
electrical circuit 12 in the form of electric current. This current
can power an electric load, such as an electric motor, or be
directed to an energy storage device, such as a rechargeable
battery. Oxygen (O.sub.2) is introduced into gas diffusion layer 26
at cathode portion 18. The oxygen passes through gas diffusion
layer 26 and over catalyst layer 22. At catalyst layer 22, oxygen,
hydrogen ions, and electrons combine to produce water and heat.
[0015] Electrolyte membrane 14 of the present invention
compositionally includes an acidic polymer and an acid. The terms
"acidic polymer" and "acid" are used herein to define different
components and are not used interchangeably (i.e., the term "acid"
does not refer to the acidic polymer, and the term "acidic polymer"
does not refer to the acid). The acidic polymer is thermally stable
and includes bound-anionic functional groups such that, when the
counter-ions to the bound-anionic functional groups are protons,
the resulting acidic polymer has a pKa of less than about 5.
Examples of suitable acidic polymers for use in electrolyte
membrane 14 include fluoropolymers having pendant groups
terminating in acidic groups. Suitable pendent groups for the
fluoropolymer include sulfonic acid groups having the formula
--R.sup.1--SO.sub.3Y, where R.sup.1 may be a branched or unbranched
perfluoroalkyl, perfluoroalkoxy, or perfluoroether group, which
contains 1-15 carbon atoms and 0-4 oxygen atoms, and where Y is a
hydrogen ion, a cation, or combinations thereof. Examples of
suitable pendant groups include
--OCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.3Y,
--O(CF.sub.2).sub.4SO.sub.3Y, and combinations thereof.
[0016] The fluoropolymer may also include one or more acidic
endgroups, such as sulfonyl endgroups having the formula
--SO.sub.3Y. The backbone chain of the acidic polymer may be
partially or fully fluorinated. Suitable fluorine concentrations in
the backbone chain include about 40% or more by weight, based on
the entire weight of the backbone chain. In one embodiment of the
present invention, the backbone chain of the fluoropolymer is
perfluorinated.
[0017] Examples of suitable concentrations of the acidic polymer in
electrolyte membrane 14 range from about 50% by weight to about 95%
by weight, with particularly suitable concentrations ranging from
about 60% by weight to about 80% by weight. The weight percents of
the acidic polymer are based on the entire weight of electrolyte
membrane 14, not including any reinforcement matrix used in
electrolyte membrane 14 (discussed below).
[0018] The acid is a low-volatility acid that is fluorinated, is
either oligomeric or non-polymeric, and provides additional proton
conductivity. The low volatility of the acid prevents the acid from
evaporating at the high temperatures of MEA 10. Otherwise, the acid
would evaporate and exit MEA 10 with the hydrogen and oxygen gas
streams. A "low-volatility acid" is herein defined as an acid that,
after being heated from 1.degree. C. to 200.degree. C. at a ramp
rate of 10.degree. C./minute, and then cooled to 120.degree. C.
within 5 minutes, exhibits a cumulative mass loss of about 6% or
less, based on an initial weight of the acid, while being
maintained at 120.degree. C. for a 24 hour period, where the
cumulative mass loss is measured during the 24 hour period. The
cumulative mass loss may be measured with a thermal gravimetric
analyzer (TGA). In one embodiment of the present invention, the
acid exhibits a volatility that is lower than a volatility of
concentrated (e.g., 95%-98% by weight) sulfuric acid.
[0019] "Oligomeric", with respect to the acid, is defined herein as
an acid molecule that contains twenty acid-functional groups or
less, and a molecular weight of less than 10,000. The acid
desirably contains ten acid-functional groups or less per molecule,
more desirably five acid-functional groups or less per molecule,
and even more desirably two acid-functional groups per
molecule.
[0020] In addition to having multiple acid-functional groups (i.e.,
multi-functional), the acid may also be perfluorinated to increase
thermal stability, such as a perfluorinated bis-acid. The
combination of the acid being non-polymeric and multi-functional
increases the density of acid functional groups per molecule. This
increases the proton conductivity of electrolyte membrane 14 beyond
a level achievable with the polymeric acid alone.
[0021] In one embodiment of the present invention, the acid is also
substantially free of basic groups, such as aromatic heterocyclic
groups, which may undesirably compromise proton conductivity. For
example, nitrogen heteroatoms are basic, which consume protons that
are otherwise available for proton transport. Acids with aromatic
heterocyclic groups are also expensive materials, which would
increase the material costs for manufacturing electrolyte membrane
14.
[0022] Examples of suitable acids for use in electrolyte membrane
14 include sulfonic acids, imide acids, methide acids, and
combinations thereof. Examples of particularly suitable acids for
use in electrolyte membrane 14 include perfluorinated sulfonic
acids, perfluorinated imide acids, and combinations thereof.
Examples of suitable perfluorinated sulfonic acids include acids
having the formula HO.sub.3S(CF.sub.2).sub.nSO.sub.3H, where "n"
ranges from 1-10 (e.g., a disulfonic acid having the formula
HO.sub.3S(CF.sub.2).sub.4SO.sub.3H, which is herein referred to as
disulfonate or disulfonate acid). Examples of suitable
perfluorinated imide acids include acids having the formula
C.sub.mF.sub.2m+1SO.sub.2NHSO.sub.2(CF.sub.2).sub.nSO.sub.2NHSO.sub.2C.su-
b.mF.sub.2m+1, where "m" ranges from 1-8 (e.g., C.sub.1-bis-imide
having the formula
CF.sub.3SO.sub.2NHSO.sub.2(CF.sub.2).sub.4SO.sub.2NHSO.sub.2CF.sub.3
and C.sub.4-bis-imide having the formula
C.sub.4F.sub.9SO.sub.2NHSO.sub.2(CF.sub.2).sub.4SO.sub.2NHSO.sub.2C.sub.4-
F.sub.9).
[0023] Additional examples of suitable perfluorinated sulfonic
acids and perfluorinated imide acids include the above-discussed
acids, where the (CF.sub.2).sub.n groups and the C.sub.mF.sub.2m+1
groups include heteroatoms, such as nitrogen, oxygen, and
combinations thereof. Additionally, further examples of suitable
perfluorinated sulfonic acids and perfluorinated imide acids
include the above-discussed acids, where the (CF.sub.2).sub.n
groups and the C.sub.mF.sub.2m+1 groups are branched, linear,
cyclic, and combinations thereof.
[0024] Examples of suitable concentrations of the acid in
electrolyte membrane 14 range from about 5% by weight to about 55%
by weight, with particularly suitable concentrations ranging from
about 20% by weight to about 35% by weight. The weight percents of
the acid are based on the entire weight of electrolyte membrane 14,
not including any reinforcement matrix used in electrolyte membrane
14 (discussed below).
[0025] Electrolyte membrane 14 also desirably exhibits low
concentrations of phosphoric acid. Phosphoric acid poisons platinum
catalyst layers of fuel cells, which reduces their effectiveness. A
typical solution to overcome the poisoning when using phosphoric
acid involves increasing the concentration of the platinum catalyst
layers to at least about two milligrams/centimeter.sup.2 of
platinum. This platinum concentration, however, is about 10-20
times greater than the desired platinum concentration, and
substantially increases the raw material costs for manufacturing
fuel cells. Accordingly, electrolyte membrane 14 desirably contains
less than about 60% by weight phosphoric acid. More desirably,
electrolyte membrane 14 contains less than about 25% by weight
phosphoric acid. Even more desirably, electrolyte membrane 14 is
substantially free of phosphoric acid.
[0026] Electrolyte membrane 14 may also include inorganic
additives, such as proton conductive inorganic additives. Such
additives allow electrolyte membrane 14 to exhibit good proton
conductivity with lower a concentration of the acid. This is
beneficial because acid washout is proportional to the
concentration of the acid in electrolyte membrane 14, and because
the inorganic additives further aid in retention of the acid. The
acid also plasticizes the acidic polymer to maintain flexibility,
and, in the case of particulate inorganic additives, provides
conductive bridges between the inorganic additives. This is in
contrast to prior membranes made by mixing polymers and inorganic
additives, which may be brittle at the concentrations required for
the adequate proton conductivity.
[0027] The inorganic additives may be particles or may be
molecularly dispersed or dissolved in electrolyte membrane 14.
Examples of suitable inorganic additives include metal oxide
particles, such as silica (e.g., amorphous fumed silica and silica
gel), zirconia, silica having silane-coupled sulfonic acid groups,
zirconia having silane-coupled sulfonic acid groups, sulfated
zirconia, zirconium phosphates, zirconium phosphonates, zirconium
phosphate sulfophenylenephosphonate, mixed metal-oxide gels (e.g.,
silica-calcia-phosphorous oxide gels), mixed metal-oxide glasses,
superprotonic conductors (e.g., hydrogensulfate and
hydrogenphosphate salts of cesium), heteropolyacids, and
combinations thereof. The particle shapes may be spherical,
acicular, branched, plate-like, or fibrous.
[0028] Examples of suitable commercially available inorganic
additives include amorphous fumed silicas available under the trade
designation "CAB-O-SIL" from Cabot Corp., Tuscola, Ill.; amorphous
fumed silicas and silica gels available from Alfa Aesar, Ward Hill,
Mass. (e.g., Catalog Nos. 42737, 41502, and 42729); and a silica
sol that is acid-stabilized and nominally free of anions, and
available under the trade designation "NALCO 1042" from Nalco,
Naperville, Ill.
[0029] Examples of suitable average particle sizes for the
particulate inorganic additives range about 1 nanometer to about 10
micrometers, with particularly suitable average particle sizes
ranging from about 5 nanometers to about 1 micrometer, and even
more particularly suitable average particle sizes ranging from
about 10 nanometers to 500 nanometers. The particulate inorganic
additives may also mesoporous, such as those provided by surfactant
templated synthesis (STS). Metal oxide sols that are free of
stabilizing counter ions and that are transferred into a solvent
for the acidic polymer, as discussed below, may also be used.
[0030] Examples of suitable concentrations of the inorganic
additives in electrolyte membrane 14 range from about 1% by weight
to about 60% by weight, with particularly suitable concentrations
ranging from about 10% by weight to about 40% by weight. The weight
percents of the inorganic additives are based on the entire weight
of electrolyte membrane 14, not including any reinforcement matrix
used in electrolyte membrane 14 (discussed below).
[0031] Electrolyte membrane 14 may also include oxidation
stabilizers. Examples of suitable oxidation stabilizers for use in
electrolyte membrane 14 include those disclosed in Asukabe et al.,
U.S. Pat. No. 6,335,112; Wessel et al., U.S. Patent Application
Publication No. 2003/0008196; and Cipollini et al., U.S. Patent
Application Publication No. 2004/0043283.
[0032] Electrolyte membrane 14 may also be reinforced mechanically
using a reinforcement matrix, such as a woven cloth or non-woven,
and which is made from materials resistant to acidic and oxidizing
conditions at high temperatures. Examples of suitable resistant
matrix materials include polymers such as polytetrafluoroethylene
(PTFE), polyphenylene sulfide, polysulfones, polyetheretherketone
(PEEK), fluorinated ethylene-propylene (FEP),
polyvinylidenedifluoride, ter-polymers of PTFE,
hexafluoropropylene, and vinylidene fluoride (THV), liquid
crystalline polyesters, and glass and other ceramics stable in
acidic environments. For lower operating temperatures,
reinforcement matrices such as ultra-high-molecular weight
polyethylene may also be used.
[0033] The reinforcement matrix desirably exhibits an average pore
size greater than about 0.01 micrometer. When electrolyte membrane
14 includes inorganic additives, the reinforcement matrix desirably
exhibits a large average pore size to allow the inorganic additives
to pass through without hindrance. Examples of suitable average
pore sizes for the reinforcement matrix include sizes that are at
least ten times greater than the average particle size of the
inorganic additives. Examples of particularly suitable average pore
sizes for the reinforcement matrix include sizes that are at least
twenty times greater than the sizes of the largest inorganic
additives. This allows uniform filling of the reinforcement
matrix.
[0034] Examples of suitable reinforcement matrices with smaller
pore sizes include matrices made from polymers having adequate
thermal and chemical stability under highly acidic, oxidizing
conditions at temperatures up to 150.degree. C., such as expanded
polytetrafluoroethylene, polyethersulfone, and other polymers
having aromatic backbones or fluorinated backbones.
Ultra-high-molecular weight polyethylene may also be used.
[0035] Electrolyte membrane 14 may be formed by initially blending
the acidic polymer, the acid, and optionally the inorganic
additives. Prior to blending, the acidic polymer may be dissolved
or dispersed in a liquid to form an acidic polymer
solution/dispersion, where the liquid used may vary based on the
acidic polymer. Examples of suitable liquids include
1-methyl-2-pyrrolidinone, dimethylacetamide, methanol, methane
sulfonic acid, n-propanol, water, and combinations thereof. Small
quantities of other liquids for the acidic polymer may also be used
to assist dissolving or dispersing other components or maintaining
stable suspensions of inorganic additives. The acid may be
dissolved in the same liquid used for the acidic polymer to form an
acid solution. The acidic polymer solution/dispersion and the acid
solution may then be blended together to form a blended solution or
dispersion, which may be further degassed to remove any entrained
bubbles.
[0036] The inorganic additives may be dispersed with the acidic
polymer solution/dispersion, the acid solution, or with both the
acidic polymer solution/dispersion and the acid solution. The
inorganic additives may be dispersed in the acidic polymer
solution/dispersion using standard dispersion techniques that
provide sufficient shear to disperse the inorganic additives in the
acidic polymer solution/dispersion. Additionally, the dispersion
techniques may also reduce the particle sizes of the inorganic
additives to assist in dispersion process. Examples of suitable
dispersion techniques are disclosed in Temple C. Patton, Paint Flow
and Pigment Dispersion, 2.sup.nd Ed., John Wiley & Sons, 1979.
Adsorption of atmospheric water during the dispersion process is
also desirably minimized, since water is generally a non-solvent
for the acidic polymer.
[0037] Sols of inorganic materials in organic solvents may be
dispersed with either the acidic polymer solution/dispersion or the
acid solution before blending, or dispersed in the blended
solution/dispersion. For example, silica and zirconia sols may be
transferred from their native aqueous solvent to
1-methyl-2-pyrrolidinone by adding 1-methyl-2-pyrrolidinone and
n-propanol to the sol. The sols may then be blended with the acidic
polymer solution/dispersion or the acid solution.
[0038] After blending, the blended mixture may then be applied to a
surface (e.g., a glass plate) and dried to form electrolyte
membrane 14. This may be performed by applying the blended mixture
to the surface and spreading the blended mixture. The coating may
then be dried in an oven to remove the solvent. After removal from
the oven, the resulting electrolyte membrane 14 may be allowed to
stand in open air to cool.
[0039] In one embodiment of the present invention, electrolyte
membrane 14 may also be cross-linked using a variety of
cross-linking techniques, such as photochemical, thermal, and
electron-beam techniques. Examples of suitable cross-linking
techniques include electron-beam, infrared, and ultraviolet
cross-linking. The cross-linking may be performed in the presence
of one or more cross-linking agents. Suitable cross-linking agents
for use with the fluoropolymers of the present invention include
multifunctional compounds, such as multifunctional alkenes and
other unsaturated cross-linkers. The cross-linking agents may be
non-fluorinated, fluorinated to a low level, highly fluorinated, or
more preferably, perfluorinated. The cross-linking agents may
introduced to the composition of electrolyte membrane 14 by any
conventional manner. A suitable technique for introducing the
cross-linking agent includes blending the cross-linking agent with
the composition of electrolyte membrane 14 before forming the
composition into a membrane. Alternatively, the cross-linking agent
may be applied to electrolyte membrane 14, such as by immersing
electrolyte membrane 14 in a solution of the cross-linking
agent.
[0040] Electrolyte membrane 14 may also be inserted into a
reinforcement matrix by pressing, coating, filling, or laminating
(or combinations thereof) electrolyte membrane 14 on one or both
sides of the matrix. When pressing or filling the reinforcement
matrix, the reinforcement matrix desirably exhibits pore sizes
greater than about 25 micrometers. The suitable pore size is
generally dependent on the viscosity of the polymer melt and the
pressing conditions. Examples of suitable pressing conditions
include pressing for about 5 minutes at a pressure ranging from 6.9
megapascals (about 1000 pounds/inch.sup.2) to about 34.5
megapascals (about 5000 pounds/inch.sup.2). When filling a
reinforcement matrix that is not penetrated by a solution of the
components of electrolyte membrane 14 (e.g., expanded PTFE), the
reinforcement matrix may be pre-filled with a solvent suitable for
the basic polymer of electrolyte membrane 14, which completely
penetrates the reinforcement matrix. The reinforcement matrix
increases the structural integrity of electrolyte membrane 14 for
use in MEA 10.
[0041] As discussed above, electrolyte membrane 14 exhibits good
proton conductivity under low levels of humidification. While not
wishing to be bound by theory, it is believed that proton
conductivity through an electrolyte membrane increases as the level
of humidification increases. If the level of humidification is
reduced, such as by evaporation at operating temperatures greater
than 100.degree. C., the proton conductivity diminishes. This
correspondingly reduces the overall electrical output of the
electrochemical device. One common technique to maintain the
desired humidity level within the electrochemical device is to
humidify the inlet gas streams. However, humidification of the
inlet gas streams reduces the concentration of reactive gases,
which also reduces the overall electrical output of the
electrochemical device. Another alternative technique involves
pressurizing the inlet gas streams to increase the relative
humidity within the electrochemical device. However, pressurization
imparts a degree of parasitic power loss, which also decreases the
overall electrical output.
[0042] Electrolyte membrane 14, however, exhibits good proton
conductivity under low levels of humidification. This allows MEA 10
to operate at temperatures greater than 100.degree. C. with low
humidification of the gas stream. In one embodiment of the present
invention, MEA 10 may operate with inlet gas streams having a dew
point of 80.degree. C. or less at atmospheric pressure, which
provides a 0.3% relative humidity at 120.degree. C. This allows a
high concentration of the reactive gases to be used in MEA 10 while
also preserving proton conductance through electrolyte membrane 14
at high operating temperatures.
[0043] The composition of electrolyte membrane 14 is also suitable
for use in a catalyst ink, which may be coated on electrolyte
membrane 14 to form catalyst layers 20 and 22. The membrane
composition may be formed in the same manner as discussed above,
and then dispersed in an aqueous and/or alcohol carrier liquid.
Catalyst particles (e.g., carbon particles and catalyst metals) may
also be combined with the dispersed membrane composition to form
the catalyst ink. The catalyst ink may then be coated on
electrolyte membrane 14, and the carrier liquid may be removed, to
form catalyst layers 20 and 22 on electrolyte membrane 14 (i.e., a
catalyst coated membrane). As a result, catalyst layers 20 and 22
may each include the above-discussed composition of electrolyte
membrane 14, and are correspondingly thermally stable and exhibit
good proton conductivity for use in electrochemical devices.
EXAMPLES
[0044] The present invention is more particularly described in the
following examples that are intended as illustrations only, since
numerous modifications and variations within the scope of the
present invention will be apparent to those skilled in the art.
Unless otherwise noted, all parts, percentages, and ratios reported
in the following examples are on a weight basis, and all reagents
used in the examples were obtained, or are available, from the
chemical suppliers described below, from general chemical suppliers
such as Sigma-Aldrich Company, Saint Louis, Mo., or may be
synthesized by conventional techniques.
[0045] The following compositional abbreviations are used in the
following Examples: [0046] 3M PFSA: A perfluorosulfonic acid
copolymer with a 1000 equivalent weight of gaseous
tetrafluoroethylene comonomer (TFE) having a formula
CF.sub.2.dbd.CF.sub.2 and a molecular weight of 100.02, and a
sulfonyl fluoride comonomer (MV4S) having a formula
CF.sub.2.dbd.CFO(CF.sub.2).sub.4SO.sub.2F and a molecular weight of
378.11, where the MV4S was prepared as described in U.S. Pat. No.
6,624,328 (in a hydrolyzed sulfonic acid form), and where the
perfluorosulfonic acid copolymer was prepared as described in U.S.
Patent Application No. 2004/0121210, and which is manufactured by
3M Corporation, St. Paul, Minn. [0047] NAFION: A 20% acidic polymer
dispersion in 60/40 n-propanol/water, which is commercially
available under the trade designation "NAFION 1000" (SE20092) from
DuPont Chemicals, Wilmington, Del. [0048] Disulfonate: Disulfonate
acid with the formula HO.sub.3S(CF.sub.2).sub.4SO.sub.3H.4H.sub.2O,
which is synthesized as discussed below. [0049] C.sub.1-bis-imide:
A bis-imide acid with the formula
CF.sub.3SO.sub.2NHSO.sub.2(CF.sub.2).sub.4SO.sub.2NHSO.sub.2CF.sub.3.4H.s-
ub.2O, which is synthesized as discussed below. [0050]
C.sub.4-bis-imide: A bis-imide acid with the formula
C.sub.4F.sub.9SO.sub.2NHSO.sub.2(CF.sub.2).sub.4SO.sub.2NHSO.sub.2C.sub.4-
F.sub.9.4H.sub.2O, which is synthesized as discussed below.
[0051] The synthesis of disulfonate acid, C.sub.1-bis-imide acid,
and C.sub.4-bis-imide acid were performed as follows:
Disulfonate Acid
[0052] Disulfonate acid used in the following Examples was
synthesized pursuant to the following procedure. 126.1 grams of
LiOH.H.sub.2O, 130.0 grams of deionized water, and 130.0 grams of
methanol were charged to a 3.0 liter 3-necked flask equipped with a
mechanical stirrer, addition funnel, Claisen adapter, reflux
condenser and thermocouple probe. The mixture was chilled to about
0.degree. C. in an ice bath. Liquid
FSO.sub.2(CF.sub.2).sub.4SO.sub.2F was then gradually added from
the addition funnel while stirring. The addition rate was adjusted
so the temperature from the reaction exotherm was controlled
between 56.degree. C. and 75.degree. C. Once the reaction exotherm
subsided, a heating mantle was installed and the reaction
temperature was held at 60.degree. C. overnight to drive the
hydrolysis to completion.
[0053] After cooling to room temperature, the reaction solution was
treated with dry ice pellets for about one hour while controlling
reaction temperature at 30.degree. C., and while stirring to
convert excess LiOH to lithium carbonate. The reaction solution was
then allowed to cool overnight.
[0054] After the overnight cooling, the reaction solution was
treated with 5.6 grams of Celite at room temperature while
stirring. The reaction solution was then filtered by suction
through a pad of Celite to recover the filtrate. The filtrate was
evaporated to dryness on a rotary evaporator at 20 mmHg and
100.degree. C. to yield a white solid. The white solid was
dissolved in 500 milliliters of pure anhydrous methanol to produce
a cloudy solution that was filtered again by suction to give a
clear filtrate. The clear filtrate was evaporated to dryness on a
rotary evaporator at 20 mmHg and 100.degree. C. to yield 279 grams
of white solid dilithium salt. The white solid was then dissolved
in 840 grams of deionized water and the resulting clear solution
was subjected to proton exchange in eight 140-gram portions on a
freshly prepared 34-cemtimeter.times.4-centimeter column of
Mitsubishi SKT10 proton exchange resin. Deionized water was used as
the eluent. The aqueous solutions of disulfonic acid collected from
the proton exchange column were evaporated to dryness on a rotary
evaporator at 20 mmHG and 100.degree. C., which produced a 92%
yield (272 grams) of HOSO.sub.2(CF.sub.2).sub.4SO.sub.2OH.4H.sub.2O
as a slightly off-white solid. The purity was shown to be better
than 99% according to quantitative .sup.1H and .sup.19F-NMR
analysis in CD.sub.3OD.
C.sub.1-Bis-Imide Acid
[0055] C.sub.1-bis-imide acid used in the following Examples was
synthesized pursuant to the following procedure. 305 grams of
anhydrous C.sub.4F.sub.9SO.sub.2NH.sub.2, 221 grams of anhydrous
triethylamine, and 188 grams of liquid
FSO.sub.2(CF.sub.2).sub.4SO.sub.2F were charged to a 2.0 liter
3-necked flask equipped with a mechanical stirrer, addition funnel,
Claisen adapter, water cooled reflux condenser, heating mantle and
thermocouple probe. A moderate reaction exotherm caused
self-heating of the reaction solution to 80.degree. C. After the
reaction exotherm subsided, the reaction temperature was gradually
ramped to 92.degree. C.-99.degree. C. (mild reflux) while stirring
and held at this temperature for 21 hours. The resulting orange
brown reaction solution was allowed to cool to room temperature
while stirring. The reaction solution was then combined with 716
grams of methylene chloride.
[0056] The methylene chloride solution of crude product was
transferred to a separatory funnel and washed with four
800-milliliter portions of deionized water. After the final water
wash, the lower methylene chloride phase was drained to a 2.0
liter, 3-necked flask and combined with 1.0 liter of deionized
water. The flask was fitted with a short path distillation head and
all methylene chloride was removed by distillation with mechanical
stirring at atmospheric pressure. Once all methylene chloride was
removed, 44.85 grams of LiOH.H.sub.2O was added to the contents
remaining in the distillation pot while stirring. Distillation was
then resumed to remove liberated triethylamine and sufficient water
to concentrate the dilithium salt of bis-imide to approximately 50%
by weight in water. The resulting aqueous solution of the dilithium
bis-imide salt was allowed to cool to room temperature.
[0057] The aqueous solution was then treated with 16 grams of DARCO
G-60 (American Norit Company, Inc., Atlanta, Ga.) decolorizing
carbon while stirring and then filtered by suction through a Celite
pad to remove carbon and other insoluble particulates. The
recovered filtrate was a dark red-brown liquid weighing 892 grams
and containing 48.2% non-volatile solids. This solution was divided
into nine portions of equal mass, and each portion was individually
subjected to proton exchange on a freshly prepared
34-cemtimeter.times.4-centimeter column of Mitsubishi SKT10 proton
exchange resin. Deionized water was used as the eluent. The aqueous
solutions of di-imide acid collected from the proton exchange
column were filtered by suction and then evaporated to dryness on a
rotary evaporator at 20 mmHg at 100.degree. C. to produce about a
90% yield (409 grams) of crude
C.sub.4F.sub.9SO.sub.2NHSO.sub.2(CF.sub.2).sub.4SO.sub.2NHSO.sub.2C.sub.4-
F.sub.9.4H.sub.2O as a light brown solid.
[0058] This crude product was purified by redissolution in water
and neutralization with an excess of aqueous potassium hydroxide to
cause crystallization of the dipotassium di-imide salt. The
suspension of crystals was filtered by suction at 0.degree. C.
through a sintered glass frit and washed with water. The recovered
solid was recrystallized two more times from hot water at about 26%
solids producing a 90% overall yield of dipotassium salt as an
off-white crystalline solid. The purified dipotassium salt was then
converted back to the di-imide acid by dissolution in 50:50
methanol/water at 14.5% solids, and subjecting this solution (in
255-gram portions) to proton exchange chromatography as discussed
above, but this time using 50:50 methanol/water as the eluent. The
eluted methanol/water solution of product was evaporated to dryness
on a rotary evaporator at 20 mmHg at 100.degree. C. producing about
an 80% yield of purified
C.sub.4F.sub.9SO.sub.2N(H)SO.sub.2(CF.sub.2).sub.4SO.sub.2N(H)SO.sub.2C.s-
ub.4F.sub.9.4H.sub.2O as an off-white solid. The purity was shown
to be better than 99% according to quantitative .sup.1H and
.sup.19F-NMR analysis in CD.sub.3OD.
C.sub.4-Bis-Imide Acid
[0059] C.sub.4-bis-imide acid used in the following Examples was
prepared pursuant to the procedure discussed above for the
C.sub.1-bis-imide acid, except that anhydrous
CF.sub.3SO.sub.2NH.sub.2 was used in place of
C.sub.4F.sub.9SO.sub.2NH.sub.2 as the reagent. The purity of the
final di-imide acid was shown to be 94.3% according to quantitative
.sup.1H and .sup.19F-NMR analysis in CD.sub.3OD.
Examples 1-10 and Comparative Examples A and B
[0060] An electrolyte membrane of Example 1 was prepared pursuant
to the following procedure. 0.27 grams of disulfonate was added to
10.00 grams of 3M PFSA, where the 3M PFSA was 20% by weight solids
in a 70/30 n-propanol/water solvent. The mixture was shaken to
dissolve and then degassed to remove bubbles. The clear viscous
solution was then hand-coated on a glass plate using a 25-mil gap
stainless steel applicator (BYK Gardner). The wet coating was then
dried at 80.degree. C. for 10-20 minutes and annealed at
160.degree. C.-200.degree. C. for an additional 5-10 minutes. The
resulting electrolyte membrane had a 10% by weight concentration of
disulfonate, and exhibited a clear/light brown color, and had a
thickness of about 25-76 micrometers (about 1-3 mils).
[0061] Electrolyte membranes of Examples 2 and 3 were prepared
pursuant to the procedure discussed above for Example 1, except
that the amounts of disulfonate added were increased. Similarly,
electrolyte membranes of Examples 4-9 were prepared pursuant to the
procedure discussed above for Example 1, except that
C.sub.1-bis-imide or C.sub.4-bis-imide were used instead of
disulfonate. Comparative Example A included 3M PFSA with no acid
added.
[0062] An electrolyte membrane of Example 10 was also prepared
pursuant to the procedure discussed above for Example 1, except
that NAFION was used instead of 3M PFSA. Comparative Example B
included NAFION with no acid added. Table 1 provides the components
and the concentrations of the acids (based on the entire weight of
the given electrolyte membrane) for the electrolyte membranes of
Examples 1-10 and Comparative Examples A and B.
TABLE-US-00001 TABLE 1 Acidic Percent by Example Polymer Acid
Weight of Acid Comparative Example A 3M PFSA None 0 Example 1 3M
PFSA Disulfonate 10 Example 2 3M PFSA Disulfonate 20 Example 3 3M
PFSA Disulfonate 35 Example 4 3M PFSA C.sub.1-bis-imide 20 Example
5 3M PFSA C.sub.1-bis-imide 30 Example 6 3M PFSA C.sub.1-bis-imide
40 Example 7 3M PFSA C.sub.4-bis-imide 27 Example 8 3M PFSA
C.sub.4-bis-imide 40 Example 9 3M PFSA C.sub.4-bis-imide 48
Comparative Example B NAFION None 0 Example 10 NAFION Disulfonate
26
Conductivity Testing of Examples 1-10 and Comparative Examples A
and B
[0063] The conductivities of the electrolyte membranes of Examples
1-10 and Comparative Examples A and B were quantitatively measured
by the following procedure. AC impedance was measured using a
four-point probe conductivity cell from BekkTech (Loveland, Colo.)
on a 1-centimeter.times.3-centimeter sample of the electrolyte
membrane. The conductivity cell was electrically connected to a
potentiostat (Model 273, Princeton Applied Research) and an
Impedance/Gain Phase Analyzer (SI 1260, Schlumberger). The sample
was first conditioned in the cell for 5 hours at 120.degree. C.
with an 80.degree. C. dew point (less than 0.3% relative humidity).
AC impedance measurements were then performed using Zplot and Zview
software (Scribner Associates).
[0064] AC impedance measurements were then performed at different
temperatures after conditioning for one hour (all at constant
80.degree. C. dew point). The electrolyte membranes of Examples 1-9
and Comparative Example A were measured at 80.degree. C. (100%
relative humidity) and 120.degree. C. (less than 0.3% relative
humidity). The electrolyte membranes of Example 10 and Comparative
Example B were measured at 110.degree. C. (less than 1% relative
humidity), and 120.degree. C. (less than 0.3% relative humidity).
For each sample, the ionic (in this case is proton) conductivity
was calculated from the average AC impedance at high frequency by
following the formula, where "R" is the AC impedance measurement,
"L" is the length of the sample, and "A" is the cross-sectional
area of the sample:
Conductivity = ( 1 R ) ( L A ) ##EQU00001##
Table 2 provides the conductivity results for the electrolyte
membranes of Examples 1-9 and Comparative Example A, Table 3
provides the conductivity results for the electrolyte membranes of
Example 10 and Comparative Example B, where the conductivities are
noted in units of millisiemens/centimeter (mS/cm).
TABLE-US-00002 TABLE 2 Percent by Conductivity Conductivity Example
Weight of Acid (80.degree. C.) (120.degree. C.) Comparative Example
A 0 135 9 Example 1 10 239 8 Example 2 20 262 10 Example 3 35 219 9
Example 4 20 226 9 Example 5 30 226 10 Example 6 40 190 12 Example
7 27 68 17 Example 8 40 67 18 Example 9 48 110 21
TABLE-US-00003 TABLE 3 Percent by Conductivity Conductivity Example
Weight of Acid (110.degree. C.) (120.degree. C.) Comparative
Example B 0 9 4 Example 10 26 20 11
[0065] The data provided in Tables 2 and 3 illustrate the benefit
of adding an acid to the electrolyte membrane. For example, at
80.degree. C. the electrolyte membrane of Example 1 (10% by weight
disulfonate) exhibited a conductivity that was substantially
greater the conductivity of the electrolyte membrane of Comparative
Example A (no acid). Moreover, disulfonate generally provided the
greater conductivities with 3M PFSA compared to C.sub.1-bis-imide
or C.sub.4-bis-imide.
[0066] The data provided in Tables 2 and 3 also show that low
levels of humidification and higher temperatures significantly
reduce conductivities. For example, the electrolyte membranes of
Examples 1-9 and Comparative Example A exhibited significantly
greater conductivities at 80.degree. C. (100% relative humidity)
compared to 120.degree. C. (less than 0.3% relative humidity).
Nonetheless, at 120.degree. C., the electrolyte membranes that
contained the acids generally exhibited greater conductivities than
the electrolyte membranes of Comparative Examples A and B,
particularly the electrolyte membranes of Examples 7-9.
AC Impedance Testing of Example 10 and Comparative Example B
[0067] AC impedances of the electrolyte membranes of Example 10 and
Comparative Example B were quantitatively measured as a function of
time pursuant to the following procedure. AC impedance was measured
using a four-point probe conductivity cell from BekkTech (Loveland,
Colo.) on a 1-centimeter.times.3-centimeter sample of the
electrolyte membrane. The conductivity cell was electrically
connected to a potentiostat (Model 273, Princeton Applied Research)
and an Impedance/Gain Phase Analyzer (SI 1260, Schlumberger). The
sample was first conditioned in the cell for 5 hours at 120.degree.
C. with an 80.degree. C. dew point (less than 0.3% relative
humidity). AC impedance measurements were then performed using
Zplot and Zview software (Scribner Associates).
[0068] AC impedance measurements were then performed at different
temperatures after conditioning for one hour, 10 hours, 15 hours,
and 20 hours (all at constant 80.degree. C. dew point). The
electrolyte membrane of Example 10 was measured at 80.degree. C.
(100% relative humidity), 90.degree. C. (39% relative humidity),
and 100.degree. C. (less than 1% relative humidity). The
electrolyte membrane of Comparative Example B was measured at
110.degree. C. (less than 1% relative humidity). Table 4 provides
the AC impedance results for the electrolyte membranes of Example
10 and Comparative Example B, where the AC impedance results are
noted in units of ohms.
TABLE-US-00004 TABLE 4 Temper- AC AC AC AC ature Impedance
Impedance Impedance Impedance Example (.degree. C.) (1 hour) (10
hours) (15 hours) (20 hours) Comparative 110 2345 1644 1611 1610
Example B Example 10 100 790 780 738 720 Example 10 90 327 434 435
434 Example 10 80 150 204 243 226
[0069] The data in Table 4 illustrate the continued conductivity of
the electrolyte membrane of Example 10 over time. As shown, the
electrolyte membrane of Example 10 generally showed little change
in resistance over the 20 hour period at all temperatures. For the
electrolyte membrane of Example 10 measured at 100% relative
humidity (80.degree. C. temperature), the increase in resistance
over time indicates that the disulfonate was beginning to leach out
of the electrolyte membrane. Nonetheless, the electrolyte membrane
of Example 10 continued to exhibit low resistances over time, which
shows that the electrolyte membranes of the present invention
continue to exhibit good proton conductivity over time.
Fuel Cell Evaluation
[0070] An evaluation of the electrolyte membranes of Example 1 and
Comparative Example A under fuel cell conditions were each
performed pursuant to the following procedure. A 5-layer MEA was
made using the electrolyte membrane, which was disposed between a
pair of catalyst layers and a pair of gas diffusion layers in the
same manner as discussed above in FIG. 1. The MEA had an active
surface area of 50 centimeters.sup.2, and was symmetrically
disposed around the electrolyte membrane. The area of the
electrolyte membrane was cut to be 100 centimeters.sup.2 so that
the electrolyte membrane was configurable over a gasket to form a
gas seal. The electrolyte membrane also had a layer thickness of
30.5 micrometers.
[0071] The catalyst layers and the gas diffusion layers were
provided as one lot of machine coated catalyst layer on a roll of
gas diffusion layer as described in patent application Velamakanni
et al., U.S. Patent Application Publication No. 2004/0107869. The
catalyst was a commercially available as a 50% platinum catalyst on
a high surface area carbon, purchased from Nippon Engelhard
Catalyst Corporation, Japan. The binder of the catalyst consisted
of 1100 equivalent weight NAFION (DuPont, Wilmington, Del.) with an
ionomer to carbon ratio of 0.8. The mass loading of the coating was
0.4 milligrams/centimeter.sup.2 platinum. The MEA was assembled in
a 50-centimeter.sup.2 cell purchased from Fuel Cells Technologies,
NM. The gasket was a PTFE, glass fiber reinforced, gasket
commercially available from Nott Corporation, MN, and had a caliper
70% of the caliper of the catalyst coated layer, which lead to a
30% compression. The MEA was formed by bonding the seven layer
(i.e., the five layers and two gaskets) by pressing the sample at a
total pressure of 907 kilograms (i.e., 1 ton) between platens
heated to 132.degree. C. for ten minutes.
[0072] The flow field used was a standard Fuel Cell Technologies,
NM 50-centimeter.sup.2 quad serpentine. The cell was bolted
together with a torque wrench setting of 110 foot-pounds. The test
station included mass flow controllers (MKS, MA) to regulate flow,
HPLC pumps (Lab Alliance, Pennsylvania) to meter in water that was
vaporized in an ejector to hit the set point gas humidification,
temperature controllers (Love Controls, IN), and electronics
(Agilent, CA) to measure and control the current voltage
performance of the cell. A computer running LabVIEW-based software
(National Instruments, Austin, Tex.) controlled the station and
data collection. Electrochemical impedance measurements were used
to measure the MEA resistance of the sample under test. A fast
Fourier transform method was used in which a square wave signal was
sent across the fuel cell test circuit, which included a shunt
resistor that served as the reference.
[0073] The script used to test the samples consisted of there
different phases: incubation, humidity challenge, and aging. The
fuel cell was heated to 80.degree. C. under dry gas flows for
twenty minutes. At that point the gas streams were humidified to
70.degree. C. dew point and the incubation was begun. The flow
conditions were H.sub.2/air (800/1800 standard cubic centimeters)
with ambient pressure outlets. The incubation lasted for six hours,
and then polarization scans were taken running from 0.9 to 0.3
volts, step 0.05 volts, with a twenty second dwell time. Between
polarization scans the cell was held at 0.5 volts for 5 minutes. A
humidity challenge was used to differentiate performance between
electrolyte membranes. The humidity challenge consisted of a series
of constant current 0.5 amps/centimeter.sup.2 scans under constant
gas flow and constant gas humidification with only the cell
temperature changing. The gas flow conditions were H.sub.2/Air,
constant stoichiometry 1.5/2.5, 80.degree. C. dew point, and with
ambient outlet pressures. The cell temperature was incremented from
85.degree. C. to 100.degree. C. in 3.degree. C. steps. Three
fifteen-minute scans were taken at each temperature of which
measurements were taken during the last scan every minute of that
scan. The value of comparison was the voltage recorded in the last
scan.
[0074] FIG. 2 is a graph illustrating polarization curves of the
electrolyte membranes of Example 1 and Comparative Example A. The
graph shows the voltages recorded at the varying temperatures
during the fuel cell evaluations, and high frequency resistances
(HFR) recorded for each electrolyte membrane, which used the same
Y-axis scale as the voltages, but were recorded in units of
ohms-centimeter.sup.2.
[0075] The data in the FIG. 2 show that as the cell temperature
increased, the proton conductivities of the electrolyte membranes
of Example 1 and Comparative Example A decreased. However, at
temperatures above 94.degree. C., the electrolyte membrane of
Comparative Example A exhibited a greater drop in proton
conductivity compared to the electrolyte membrane of Example 1.
Similarly, at temperatures above 91.degree. C., the electrolyte
membrane of Comparative Example A exhibited a greater increase in
HFR compared to the electrolyte membrane of Example 1. The higher
proton conductivities and lower HFR exhibited by the electrolyte
membrane of Example 1 are believed to be due to the addition of the
disulfonate acid. The combination of the acidic polymer and the
acid allowed the electrolyte membrane of Example 1 to exhibit good
conductivities at elevated temperatures.
[0076] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *