U.S. patent application number 10/684982 was filed with the patent office on 2004-04-29 for membrane electrode assemblies using ionic composite membranes.
Invention is credited to Fenton, James M., Kunz, H. Russell, Lin, Jung-Chou.
Application Number | 20040081876 10/684982 |
Document ID | / |
Family ID | 24647824 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040081876 |
Kind Code |
A1 |
Fenton, James M. ; et
al. |
April 29, 2004 |
Membrane electrode assemblies using ionic composite membranes
Abstract
A membrane electrode assembly comprising a composite membrane
having a first major surface area and a second major surface area
comprising a porous polymeric matrix containing ionically
conductive solid and ionomeric binder, at least one protective
layer disposed adjacent to the porous polymeric matrix membrane
comprising an ionomeric binder and an ionically conductive solid,
an anode comprising an oxidizing catalyst adjacent said first major
surface area of said composite membrane and a cathode comprising a
reducing catalyst adjacent said second major surface area of said
composite membrane, and a method for manufacturing the same.
Inventors: |
Fenton, James M.; (Tolland,
CT) ; Kunz, H. Russell; (Vernon, CT) ; Lin,
Jung-Chou; (Storrs, CT) |
Correspondence
Address: |
Edwards & Angell LLP
301 Tresser Blvd.
Stamford
CT
06901
US
|
Family ID: |
24647824 |
Appl. No.: |
10/684982 |
Filed: |
October 14, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10684982 |
Oct 14, 2003 |
|
|
|
09660028 |
Sep 12, 2000 |
|
|
|
6638659 |
|
|
|
|
09660028 |
Sep 12, 2000 |
|
|
|
09562235 |
Apr 28, 2000 |
|
|
|
6465136 |
|
|
|
|
60132038 |
Apr 30, 1999 |
|
|
|
Current U.S.
Class: |
429/128 ;
204/296; 427/115; 429/483; 429/535; 502/101 |
Current CPC
Class: |
C08J 5/2281 20130101;
H01M 8/1004 20130101; Y02P 70/50 20151101; H01M 2300/0082 20130101;
H01M 8/1053 20130101; H01M 4/9083 20130101; H01M 4/8605 20130101;
H01M 8/1048 20130101; H01M 6/187 20130101; H01M 2300/0094 20130101;
C08J 2327/12 20130101; H01M 4/921 20130101; C08J 5/2275 20130101;
B01D 69/12 20130101; H01M 4/8828 20130101; H01M 4/8882 20130101;
B01D 69/141 20130101; H01M 8/0289 20130101; H01M 4/886 20130101;
H01M 8/109 20130101; H01M 6/181 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/030 ;
429/042; 429/033; 429/044; 427/115; 502/101; 204/296 |
International
Class: |
H01M 008/10; H01M
004/96; H01M 004/86; B05D 005/12; H01M 004/88; C25B 013/08 |
Claims
What is claimed is:
1. A membrane electrode assembly (MEA) comprising: a) a composite
membrane having a first major surface area and a second major
surface area comprising: 1) a membrane layer comprising an
ionically conductive solid and an ionomeric binder; 2) at least one
protective layer disposed adjacent to the membrane layer comprising
an an ionically conductive solid and ionomeric binder, and
optionally hygroscopic fine powder; b) an anode comprising an
oxidizing catalyst adjacent said first major surface area of said
composite membrane; c) a cathode comprising a reducing catalyst
adjacent said second major surface area of said composite
membrane.
2. The MEA of claim 1 wherein the mebrane layer comprises a porous
polymeric matrix.
3. The MEA of claim 1 further comprising one or more collectors in
contact with said anode and/or cathode.
4. The MEA of claim 1 wherein the anode further comprises an
ionomeric binder.
5. The MEA of claim 4 wherein the anode further comprises an
ionically conductive solid.
6. The MEA of claim 6 wherein the ionomeric binder of the anode
comprises a proton conducting ionomer.
7. The MEA of claim 6 wherein the proton conducting ionomer of the
anode is perfluorosulfonic acid.
8. The MEA of claim 1 wherein the cathode further comprises an
ionomeric binder.
9. The MEA of claim 8 wherein the cathode further comprises an
ionically conductive solid.
10. The MEA of claim 4 wherein the ionomeric binder of the cathode
comprises a proton conducting ionomer.
11. The MEA of claim 10 wherein the proton conducting ionomer is
perfluorosulfonic acid.
12. The MEA of claim 1 wherein the ionomeric binder of the
composite membrane is a proton conducting ionomer.
13. The MEA of claim 12 wherein the proton conducting ionomer of
the composite membrane is perfluorosulfonic acid.
14. The MEA of claim 4 wherein the ionomeric binder content of the
is between about 10% to about 100% of the anode catalyst content by
volume.
15. The MEA of claim 8 wherein the ionomeric binder content of the
cathode is between about 10% to about 100% of the cathode catalyst
content by volume.
16. The MEA of claim 1 wherein the oxidizing catalyst of the anode
is supported on carbon particles.
17. The MEA of claim 16 wherein the percentage of catalyst in the
anode that is supported on carbon is 20% to 60% by weight.
18. The MEA of claim 16 wherein the catalyst loading of the cathode
is between 0.05 and 5 mg/cm.sup.2 frontal area.
19. The MEA of claim 1 wherein the reducing catalyst of the cathode
is supported on carbon particles.
20. The MEA of claim 19 wherein the percentage of catalyst in the
cathode that is supported on carbon is 20% to 60% by weight.
21. The MEA of claim 19 wherein the catalyst loading of the cathode
is between 0.05 and 5 mg/cm.sup.2 frontal area.
22. The MEA of claim 1 wherein the ionically conductive solid of
the cathode is a heteropoly acid.
23. The MEA of claim 22 wherein the heteropoly acid is selected
from the group consisting of: phosphotungstic acid, phosphomolybdic
acid, and zirconium hydrogen phosphate.
24. The MEA of claim 5 wherein the ionically conductive solid of
the anode is a heteropoly acid.
25. The MEA of claim 24 wherein the heteropoly of the anode is
selected from the group consisting of: phosphotungstic acid,
phosphomolybdic acid, and zirconium hydrogen phosphate.
26. The MEA of claim 9 wherein the ionically conductive solid of
the cathode is a heteropoly acid.
27. The MEA of claim 26 wherein the heteropoly acid of the cathode
is selected from the group consisting of: phosphotungstic acid,
phosphomolybdic acid, and zirconium hydrogen phosphate.
28. The MEA of claim 9 wherein the ionically conductive solid of
the cathode is between 20% and 40% of the content of the ionomer by
volume.
29. The MEA of claim 5 wherein the ionically conductive solid of
the anode is between 20% and 40% of the content of the ionomer by
volume.
30. The MEA of claim 3 wherein the one or more collectors in
contact with said anode and/or cathode consists of a porous
material.
31. A fuel cell comprising the MEA of claim 1.
32. An electrolysis cell comprising the MEA of claim 1.
33. A vehicle comprising the fuel cell of claim 30.
34. An electromechanical system comprising the electrolysis cell of
claim 32.
35. A process for fabricating a membrane electrode assembly (MEA)
comprising: a) obtaining a composite membrane having a first major
surface area and a second major surface area comprising: 1) a
membrane layer containing ionically conductive solid and an
ionomeric binder; 2) at least one protective layer disposed
adjacent to the membrane layer comprising an ionomeric binder and
an ionically conductive solid, and optionally a hygroscopic fine
powder; b) spraying a mixture of oxidizing catalyst, ionomeric
binder and ionically conductive solid in a solvent on said first
major surface area; c) spraying a mixture of reducing catalyst,
ionomeric binder and ionically conductive solid in a solvent on
said second major surface area.
36. The process of claim 35 wherein the membrane layer of step
(a)(1) comprises a porous polymeric matrix.
37. The process of claim 35 wherein the composite membrane of step
a) is heat treated from at least about 10 to about 20 minutes at a
temperature above 100.degree. C. prior to steps b) and c).
38. The process of claim 35 wherein the composite membrane of step
a) is heat treated from at least about 10 to about 20 minutes at a
temperature above about 120.degree. C. prior to steps b) and
c).
39. The process of claim 35 wherein the spraying employs a carrier
gas.
40. The process of claim 39 wherein the carrier gas is selected
from the group consisting of: nitrogen, helium, argon, and carbon
dioxide
41. A process for fabricating a membrane electrode assembly (MEA)
comprising: a) obtaining a composite membrane having a first major
surface area and a second major surface area comprising: 1) a
membrane layer containing ionically conductive solid and an
ionomeric binder; 2) at least one protective layer disposed
adjacent to the membrane layer comprising an ionomeric binder and
an ionically conductive solid, and optionally a hygroscopic fine
powder; b) applying a mixture of oxidizing catalyst, ionomeric
binder and ionically conductive solid in a solvent on said first
major surface area; c) applying a mixture of reducing catalyst,
ionomeric binder and ionically conductive solid in a solvent on
said second major surface area.
42. The process of claim 41 wherein the membrane layer of step
(a)(1) comprises a porous polymeric matrix.
43. The process of claim 41 wherein the composite membrane of step
a) is heat treated from at least about 10 to about 20 minutes at a
temperature above 100.degree. C. prior to steps b) and c).
44. The process of claim 41 wherein the composite membrane of step
a) is heat treated from at least about 10 to about 20 minutes at a
temperature above about 120.degree. C. prior to steps b) and
c).
45. The process of claim 41 wherein the application of the mixture
of oxidizing catalyst is performed by coating, transferring screen
printing, brushing, curtain coating, or drip coating.
46. The process of claim 41 wherein the application of the mixture
of reducing catalyst is performed by coating, transferring screen
printing, brushing, curtain coating, or drip coating.
47. A process of fabricating a membrane electrode assembly (MEA)
comprising: a). obtaining a membrane having a first major surface
area and a second major surface area; b) applying a solvent
comprising an oxidizing catalyst, inomeric binder, and ionically
conductive solid in a solvent of said first major surface area; c)
applying a mixture of reducing catalyst, ionomeric binder, and
ionically conductive solid on said second major surface area.
48. The process of claim 47 wherein the membrane obtained in step
(a) further comprises a polymeric matrix.
49. The method of claim 47 wherein the application of the mixture
of oxidizing catalyst is performed by coating, transferring, screen
printing, brushing, curtain coating or drip coating.
50. The method of claim 47 wherein the application of the mixture
of reducing catalyst is performed by coating, transferring, screen
printing, brushing, curtain coating, or drip coating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
U.S. patent application Ser. No. 09/562,235, filed Apr. 28, 2000,
which claims priority from U.S. Provisional Application No.
60/132,038, the disclosures of which are hereby incorporated in
their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to novel membrane electrode
assemblies, improved membranes for use in such membrane electrode
assemblies, and fuel cells employing such membrane electrode
assemblies.
[0004] 2. Brief Description of the Related Art
[0005] Fuel cells are electrochemical cells in which a free energy
change resulting from a fuel oxidation reaction is converted into
electrical energy. Fuel cells are attractive electrical power
sources, due to their higher energy efficiency and environmental
compatibility compared to the internal combustion engine. The most
well-known fuel cells are those using a gaseous fuel (such as
hydrogen) with a gaseous oxidant (usually pure oxygen or
atmospheric oxygen), and those fuel cells using direct feed organic
fuels such as methanol. In contrast to batteries, which must be
recharged, electrical energy from fuel cells can be produced for as
long as the fuels, e.g., methanol or hydrogen, and oxidant, are
supplied. Thus, a considerable interest exists in the design of
improved fuel cells to fill future energy needs.
[0006] While a number of different types of electrochemical cells
have been employed in the manufacture of fuel cells, arguably ion
exchange membrane (IEM) cells have received the most attention. An
IEM cell typically employs a membrane comprising an ion-exchange
polymer. This ion-exchange polymer membrane serves as a physical
separator between the anode and cathode, while also serving as an
electrolyte. IEM cells can be operated as electrolytic cells for
the production of electrochemical products, or operated as fuel
cells for the production of electrical energy.
[0007] In some IEM cells, a cation exchange membrane is used
wherein protons are transported across the membrane as the cell is
operated. Such cells are often referred to as proton exchange
membrane (PEM) cells. For example, in a cell employing the
hydrogen/oxygen couple, hydrogen molecules (fuel) at the anode are
oxidized donating electrons to the anode, while at the cathode the
oxygen (oxidant) is reduced accepting electrons from the cathode.
The H.sup.+ ions (protons) formed at the anode migrate through the
membrane to the cathode and combine with oxygen to form water. In
many fuel cells, the anode and/or cathode comprises a layer of
electrically conductive, catalytically active particles (usually in
a polymeric binder) on the proton exchange membrane. The resulting
structure (sometimes also including current collectors) is referred
to as a membrane electrode assembly (MEA).
[0008] In one approach to the construction of an ion exchange
membrane, perfluorinated sulfonic acid polymers such as Nafion.RTM.
(and other ion exchange materials) are incorporated into films, for
example porous polytetrafluoroethylene (PTFE), to form composite
membranes, as described for example in U.S. Pat. No. 5,082,472, to
Mallouk, et al.; JP Laid-Open Pat. Application Nos. 62-240627,
62-280230, and 62-280231; U.S. Pat. No. 5,094,895 to Branca, U.S.
Pat. No. 5,183,545 to Branca et al.; and U.S. Pat. No. 5,547,551 to
Bahar, et al. (each of the foregoing references being incorporated
herein in their entirety).
[0009] In another approach to construction of an ion exchange
membrane, a composite membrane is prepared, for example, by
precipitation of a water-insoluble, inorganic conductor such as
zirconium hydrogen phosphate into a porous Nafion.RTM. membrane
(See,. e.g., CT/US96/03804 to Grot, et al.). or incorporation of
phosphotungstic acid into a Nafion.RTM. membrane (See, e.g.,., S.
Malhotra, et al., in "Journal of the Electrochemical Society," Vol.
144, No. 2, L23-L26, 1997--although the resulting membrane was said
to demonstrate high conductivity at elevated temperature, the
composite membrane lacked sufficient strength at reduced thickness
for hydrogen fuel cell applications).
[0010] Fuel cells that employ IEMs and direct organic fuels such as
methanol frequently suffer from so-called "crossover" of fuel
through the membrane. The term "crossover" refers to the
undesirable transport of fuel through the membrane from the fuel
electrode, or anode, side to the oxygen electrode, or cathode side
of the fuel cell. After having been transported across the
membrane, the fuel will either evaporate into the circulating
oxygen stream or react with the oxygen at the oxygen electrode.
Fuel crossover diminishes cell performance for two primary reasons.
Firstly, the transported fuel cannot react electrochemically to
produce useful energy, and therefore contributes directly to a loss
of fuel efficiency (effectively a fuel leak). Secondly, the
transported fuel interacts with the cathode, i.e., the oxygen
electrode, and lowers its operating potential and hence the overall
cell voltage. The reduction of cell voltage lowers specific cell
power output, and also reduces the overall efficiency.
[0011] Fuel cells that employ IEMs and hydrogen as a fuel also
suffer from disadvantages. Certainly, the difficulty of on-board
storage and refueling of hydrogen is a major concern in the
application of hydrogen fuel cells in vehicles. One approach for
surmounting this obstacle has been to utilize the hydrogen fuel
obtained through steam reforming of gasoline. Unfortunately,
hydrogen fuel from steam reforming of gasoline usually contains a
trace amount of carbon monoxide, which results in severe poisoning
of anode catalysts. Operating the fuel cell at high temperature can
effectively alleviate the carbon monoxide poisoning of anode
catalysts. However, at elevated temperature, membranes comprising
perfluorinated sulfonic acid polymers such a Nafion.RTM. quickly
lose ionic conductivity at ambient pressure due to dehydration.
Operation at high temperatures with such membranes thus requires
that the cells be pressurized.
[0012] One particularly useful group of cation-exchange membrane
materials for PEM cells is perfluorinated sulfonic acid polymers
such as Nafion.RTM., available from E.I. duPont de Nemours &
Co. Such cation-exchange polymers have good conductivity and
chemical and thermal resistance, which provide long service life
before replacement. However, increased proton conductivity is
desired for some applications, particularly for fuel cells, which
operate at high current densities.
[0013] PEMs must have enough strength, minimum fuel crossover, and
high ionic conductance at elevated temperature to be useful in fuel
cell applications using hydrogen fuel from partial oxidation or
steam reforming of hydrocarbons or other sources. Membrane
thickness has been reduced in an effort to improve conductance.
However, reduction in thickness results in insufficient membrane
strength, necessitating use of additional reinforcing materials,
and an increase in crossover. For example, pure Nafion.RTM.
membranes have not provided sufficient strength at reduced
thicknesses. To increase the strength additional reinforced
materials are needed.
[0014] PEMS also require effective catalysts associated with the
membranes to provide for reactivity with the fuel source and
resulting products of catalysis. Typically, a catalyst layer is
applied to the membrane using, for example, a combination of
temperature, pressure, and perhaps an adhesive. Such layered
structure may be placed between two porous substrates.
[0015] Most recently, an alternative low-platinum-loading catalyst
layer structure has been developed by Wilson at LANL (M. S. Wilson,
U.S. Pat. Nos. 5,211,984 and 5,234,777 (1993)) and Grot (U.S. Pat.
No. 5,330,860) to make membrane electrode assemblies. In this
structure, recast ionomer (Nafion.RTM.) is used instead of PTFE to
bind the catalyst layer structure together, and the low-loading
catalyst layer is applied to the membrane, rather than to the gas
diffusion structure. Such (PTFE-free) layers have been described as
"thin-film" catalyst layers, because the high performance is
obtained with a very low catalyst loading (0.12-0.16 mg
Pt/cm.sup.2) in a thin layer (<10 .mu.m thick). By virtue of
their thinness and the high ionomer contents achievable with these
catalyst layers, high catalyst utilizations are obtained and the
continuity and integrity of the catalyst layer/membrane interface
is greatly improved compared with the structures prepared by hot
pressing catalysts that are bonded to the gas diffusion layers on
to the membrane.
[0016] There accordingly remains a need for a membrane capable of
maintaining high conductivity at elevated temperature, and which
will enable use of PEM fuel cells for vehicle applications. There
remains a further need for a membrane that maintains functionality
in methanol/hydrogen fuel cells in particular when the fuel
contains trace carbon monoxide, as for example, produced during a
steam reforming process. There remains a further need for a
membrane that operates at a temperature high enough to boil water
for use in fuel processing and provide high quality waste heat for
on-site space heating use. There further remains a need for a
membrane exhibiting sufficient strength at reduced thicknesses,
high conductance at elevated temperature, and minimum fuel
crossover for hydrogen fuel cell applications.
SUMMARY OF THE INVENTION
[0017] The present provides a MEA comprising a composite membrane
structure having a porous polymeric matrix, ionically conductive
solid dispersed in the polymeric matrix and an ionomeric binder,
that is flanked by a anode and cathode catalytic layer. A preferred
anode catalytic layer of the present invention comprises an
oxidizing catalyst composition in intimate contact with carbon
powder, an ionically conductive solid, and an ionomeric binder
positioned to bind the ionically conductive solid to the oxidizing
catalyst composition. A preferred cathode catalytic layer of the
present invention comprises a reducing catalyst composition in
intimate contact with carbon powder, an ionically conductive solid,
and an ionomeric binder positioned to bind the ionicaly conductive
solid to the reducing catalyst composition.
[0018] It has been unexpectedly found that the incorporation of
effective amounts of an ionically conductive solid into both the
cathode and anode layers, as well as the composite membrane
structure, may greatly improve the overall performance of the MEA,
in particular with regard to voltage drop across the assembly. It
has further been unexpectedly found that the performance of a MEA
comprising a PTFE membrane imbued with ionically conductive solids,
can be greatly enhanced by heat treatment of the ionically
conductive solid-imbued PTFE membrane. While not be bound thereby,
applicants have hypothesized that such improvement is due to a
marked reduction in cross-over.
[0019] Formulation of the oxidizing and reducing catalyst
compositions found useful in the present invention is dependent on
the type of oxidant utilized and fuel employed. For example, if
hydrogen is used as a fuel, the catalyst composition should be
active as a hydrogen oxidation catalyst (such as a 40 wt %
platinum/ruthenium alloy supported on the surface of carbon
powder), and if oxygen is used as the oxidizing agent on the
cathode portion of the MEA, the cathode catalyst should be active
as an oxygen reduction catalyst (such as platinum/chromium/cobalt
alloy supported on the surface of carbon powder).
[0020] In one embodiment of the present invention there is
disclosed a membrane electrode assembly (MEA) comprising: (a) a
composite membrane having a first major surface area and a second
major surface area comprising: (1) a membrane layer comprising an
ionically conductive solid and an ionomeric binder; and (2) at
least one protective layer disposed adjacent to the membrane layer
comprising an an ionically conductive solid and ionomeric binder,
and optionally hygroscopic fine powder; (b) an anode comprising an
oxidizing catalyst adjacent said first major surface area of said
composite membrane; and (c) a cathode comprising a reducing
catalyst adjacent said second major surface area of said composite
membrane.
[0021] In another embodiment of the present invention there is
disclosed a membrane electrode assembly (MEA) comprising a
composite membrane having a first major surface area and a second
major surface area comprising:
[0022] 1) a porous polymeric matrix containing ionically conductive
solid and ionomeric binder;
[0023] 2) at least one protective layer disposed adjacent to the
porous polymeric matrix composite membrane comprising an ionomeric
binder and an ionically conductive solid;
[0024] and an anode comprising an oxidizing catalyst adjacent said
first major surface area of said composite membrane, and a cathode
comprising a reducing catalyst adjacent said second major surface
area of said composite membrane.
[0025] In another embodiment of the present invention there is
disclosed a process for fabricating a MEA comprising: (a) obtaining
a composite membrane having a first major surface area and a second
major surface area comprising: (1) a porous polymeric matrix
containing ionically conductive solid and an ionomeric binder, and
(2) at least one protective layer disposed adjacent to the porous
polymeric matrix composite membrane comprising an ionomeric binder
and an ionically conductive solid; (b) spraying a mixture of
oxidizing catalyst, ionomeric binder and ionically conductive solid
in a solvent on said first major surface area; and (c) spraying a
mixture of reducing catalyst, ionomeric binder and ionically
conductive solid in a solvent on said second major surface
area.
[0026] And in yet another embodiment of the present invention there
is disclosed a process for fabricating a membrane electrode
assembly (MEA) comprising: (a) obtaining a composite membrane
having a first major surface area and a second major surface area
comprising: (1) a membrane layer containing ionically conductive
solid and an ionomeric binder, (2) at least one protective layer
disposed adjacent to the membrane layer comprising an ionomeric
binder and an ionically conductive solid, and optionally a
hygroscopic fine powder; (b) spraying a mixture of oxidizing
catalyst, ionomeric binder and ionically conductive solid in a
solvent on said first major surface area; (c) spraying a mixture of
reducing catalyst, ionomeric binder and ionically conductive solid
in a solvent on said second major surface area.
[0027] And in yet another embodiment of the present invention there
is disclosed a process for fabricating a MEA comprising: (a)
obtaining a composite membrane having a first major surface area
and a second major surface area comprising: (1) a porous polymeric
matrix containing ionically conductive solid and an ionomeric
binder, and (2) at least one protective layer disposed adjacent to
the porous polymeric matrix composite membrane comprising an
ionomeric binder and an ionically conductive solid; (b) applying a
mixture of oxidizing catalyst, ionomeric binder and ionically
conductive solid in a solvent on said first major surface area; and
(c) applying a mixture of reducing catalyst, ionomeric binder and
ionically conductive solid in a solvent on said second major
surface area.
[0028] And yet in other process embodiment of the present invention
there is disclosed a process of fabricating a membrane electrode
assembly (MEA) comprising: (a). obtaining a membrane having a first
major surface area and a second major surface area; (b) applying a
solvent comprising an oxidizing catalyst, inomeric binder, and
ionically conductive solid in a solvent of said first major surface
area; and (c) applying a mixture of reducing catalyst, ionomeric
binder, and ionically conductive solid on said second major surface
area.
[0029] The above-discussed and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a bar graph of the resistance of
Teflon/Nafion.RTM. membranes before and after heat treatment.
[0031] FIG. 2 is a graph of fuel (hydrogen) cross-over in a MEA
having a Teflon/Nafion.RTM. membrane, both before and after heat
treatment of the membrane.
[0032] FIG. 3 illustrates the performance and internal resistance
of a MEA having a Nafion.RTM./PTA-imbued Teflon.RTM. membrane
coated along its major surface areas with a layer of Nafion.RTM.,
an anode comprising Pt--Ru/C: Nafion.RTM., and a cathode comprising
a platinum black/Nafion.RTM. layer and a Pt/Vulcan catalyst
layer.
[0033] FIG. 4 is a graph of the internal resistance-free (IR-free)
performance data of the MEA of FIG. 3.
[0034] FIG. 5 is a graph of the fuel (hydrogen) cross-over of the
MEA of FIG. 3.
[0035] FIG. 6 is a graph depicting the voltage/resistance across a
MEA comprising a membrane comprising a porous Teflon matrix
impregnated with phosphotungstic acid and Nafion.RTM., and pressed
commercial E-tek.RTM. catalyst electrodes with 0.4 mg/cm.sup.2
(total metal) of 20% Pt/C (cathode) and 20% Pt--Ru/C (1:1 atomic
ratio) (anode), for select current densities.
[0036] FIG. 7 is a graph depicting the voltage/resistance across a
MEA comprising a membrane comprising a porous Teflon matrix
impregnated with phosphotungstic acid and Nafion.RTM. and catalytic
electrodes, comprising 60% Pt/C (cathode) or 45% Pt--Ru/C (anode)
and Nafion.RTM., sprayed onto the each of the two major surfaces of
the membrane, for select current densities.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present invention provides improved MEAs (membrane
electrode assemblies) which may find employment in PEM (proton
exchange membrane) fuel cells.
[0038] I. The Membrane Electrode Assembly
[0039] A membrane electrode assembly of the present invention
comprise three main parts, the membrane, the anode catalyst layer
and the cathode catalyst layer which are necessary for the
production of useable electrochemical power. The membrane separates
the anode from the cathode and provides a path between the same for
ion exchange, thereby allowing current to be drawn from the
assembly.
[0040] The membrane electrode assembly of the present invention can
be fabricated using a number of different approaches. In one case
the catalysts layers are bonded to the membrane using a combination
of temperature, pressure, and perhaps an adhesive. This package is
then placed between two porous substrates. The MEA and substrates
are inserted between the cell separator plates to complete the
entire cell. In another case, the catalyst layers are bonded to the
porous substrates. These catalyst layer/substrate composites are
then bonded to the membrane using a combination of temperature,
pressure, and perhaps an adhesive. It has been discerned by the
present inventors that MEA performance can be improved by spraying
the anode and cathode catalyst layer onto the membrane surface
rather than in pressing electrodes (such as E-TEK electrodes) onto
the membrane itself.
[0041] Improved components of an MEA of the present invention,
i.e., the membrane, anode, and cathode, are described in detail
below:
[0042] The Membrane
[0043] In a preferred embodiment of the present invention the
membrane comprises an ionically conductive solid and an ionomeric
binder which preferably is ionically conductive as well. The
membrane preferably also comprises a porous polymeric matrix. A
membrane according to the present invention may be fabricated by
casting a solution of ionically conductive solid and ionomeric
binder to form a film. If a porous polymeric matrix is employed,
the membrane may be fabricated by dispersing the ionically
conductive solid into the matrix and incorporating an ionomeric
binder into the matrix.
[0044] As disclosed in U.S. patent application Ser. No. ______,
through which priority is claimed in the present application to
U.S. Provisional Application No. 60/132,038 (both references which
are incorporated by reference in their entirety herein), improved
performance of the MEA can be achieved when the membrane preferably
comprises at least one protective layer disposed adjacent to the
composite membrane, and more preferably at least one protective
layer disposed adjacent to each side of the two major surface areas
of the membrane to form a tri-layer membrane. Preferably the anode
and cathode of the MEA are in intimate contact with the two
protective layers. A protective layer preferably comprises
ionomeric binder and ionically conductive solid, hygroscopic fine
powder or a combination thereof. However, even pure ionomeric
protective layers, such as Nafion.RTM., without a ionically
conductive solid incorporated therein, have been found to improve
voltage drop across the membrane at low current densities. Cell
resistance, however, may be excessive when pure ionomeric layers
are used as the protective layers.
[0045] The tri-layer may also comprise protective layers of
different composition. For example, the tri-layer membrane
structure may comprise a center layer comprising ionomeric binder,
porous polymeric matrix and ionically conductive solid, one
protective layer comprising ionomeric binder and ionically
conductive solid and one protective layer comprising ionomeric
binder and fine powder. Optionally a noble metal is dispersed
within the protective layer either in place of, or in addition to,
the ionically conductive solid. The present inventors have
determined that application of a pure ionomeric binder layer to the
protective layers while reducing crossover, in general raises cell
resistance to an excessive level, thereby reducing MEA performance
rather than increasing performance.
[0046] It has also been determined by the present inventors that
the performance of MEAs using membranes comprising ionically
conductive solids in a polymeric matrix may be significantly
improved by heat treating the solid-impregnated membrane. For
solid-impregnated polytetrafluoroethylene ("PTFE") membranes (e.g.,
Teflon), improved performance of the MEA is noted when the membrane
is heat treated at a temperature above about 100.degree. C., more
preferably above about 115.degree. C., and more preferably at about
121.degree. C. A preferred regimen for heat treating such PTFE
membranes impregnated with ionically conductive solids is treat the
membrane under a force of about 2000 lb. at 121.degree. C. for at
least about 10 minutes, more preferably at least about 20 minutes,
prior to the application of the catalysts. As would be understood
by the person of ordinary skill in the art, the particular
temperature of optimal heat treatment, the length of the heat
treatment, and the optimal pressure under which the heat treatment
should be undertaken, will depend both upon the matrix and the
ionically conductive solid employed in the membrane. The
determination of such optimal parameters is believed to be well
within the skill of the ordinary artisan given the present
teachings.
[0047] In one embodiment, the composite membrane comprises a porous
polymeric matrix (which is preferably non-cationically conductive),
an ionically conductive solid dispersed within said matrix, and
preferably, an ionomeric binder. These composite membranes may be
prepared by first dissolving the ionomeric binder and the ionically
conductive solid in an organic solvent. Suitable solvents include
low molecular alcohols such as methanol, ethanol, and isopropanol,
and any other inert organic solvents, which can dissolve both
ionically conductive solids and ion exchange resins. The porous
polymeric matrix is then impregnated with this mixture, and the
impregnated membrane is then dried.
[0048] Impregnation of ionically conductive solid into porous
polymeric matrix can vary depending on the materials. Thus, in an
alternative preparation, the ionomeric binder alone is first
dissolved in an organic solvent, and then impregnated with a second
solution comprising the ionically conductive solid.
[0049] In another embodiment of the composite membrane, the
membrane comprises an ionically conductive solid and an ionomeric
binder. Such membranes are prepared by co-dissolving a suitable
ionomeric binder and an ionically conductive solid, and casting the
solution to form a film. The cast film is then dried. The resulting
membrane can then be thermally treated.
[0050] The individual components of the membranes are described in
detail below.
[0051] 1. The Matrix
[0052] The matrix functions as a host and/or support for the
ionically conductive solid dispersed therein. Suitable matrix
materials are thin, possess a high porosity, fine pore size, and
exhibit sufficient chemical and dimensional stability to allow
fabrication and to function in their intended environments. The
porous polymeric matrix may be cationically conductive, or
essentially non-ionically conductive, so long as the materials have
negligible electronic conductivity In particular, suitable matrix
materials maintain physical and dimensional stability at high
temperatures, preferably above 60.degree. C., more preferably above
about 90.degree. C., and even more preferably up to about
250.degree. C. Most preferably, the matrix maintains integrity at
temperatures in the range form about 90.degree. C. to about
200.degree. C. The matrix material, or at least a portion of the
matrix material, furthermore preferably maintains its integrity in
oxidizing, reducing, acidic, and alkaline environments. Preferred
matrix materials also have negligible electronic conductivity.
[0053] Materials suitable for use as matrices in the present
membranes include, but are not limited to, polytetrafluoroethylene
(PTFE) (e.g., Teflon.RTM. available form E.I. duPont de Nemours
& Co.), polyvinylidene fluoride (PVDF), polyetheretherketone
(PEEK), polyethersulfone (PES), perfluoroalkoxy (PFA), fluorinated
ethylene propylene (FEP), polybenzimidazole, polyethersulfone,
sulfonated polyetheretherketone, poly(phenylene oxide),
polyaniline, polystyrene oxide, poly(methacrylate), and co-polymers
and mixtures thereof. The polymer matrix preferably has a melting
point in the range from about 100.degree. C. to about 300.degree.
C.
[0054] The matrix is preferably as thin as possible to facilitate
impregnation of ion exchange materials (i.e., ionically conductive
solid), and ionomeric binder (when present), while still providing
sufficient strength to the membrane for fuel cell applications.
Preferred matrix thicknesses are in the range from about 0.25 mil
to less than about 4 mil. Preferably, the thickness of the matrix
is in the range from about 0.5 mil to about 3 mil, and most
preferably, in the range from about 0.5 mil (0.00127 cm) to about
1.5 mil (0.0038 cm).
[0055] In addition to reduced thickness, it is preferred that the
matrix possess high porosity (preferably the pores encompassing not
less than about 40% of the surface area and being approximately
evenly distributed along the surface) and extremely fine pore size
(preferably the maximum dimension of a pore is less than about 1
.mu.m). The combination of thinness, high porosity and fine pore
size is important, as membranes having thick host matrices with
large pores behave like pure ion exchange membranes when
impregnated with ion exchange material. The present pore size is
preferably selected to be as fine as possible while being large
enough to accept the ionically conductive solid into the matrix
pores. For example, when Nafion.RTM. is utilized as the ionomeric
binder, the pore size of the host matrix should be larger than
about 0.02 .mu.m (micron), i.e., the size of the polymer.
Preferably, the present matrix has pores possessing a maximum
dimension in the range from about 0.025 .mu.m to about 1 .mu.m, and
most preferably from about 0.025 .mu.m to about 0.2 .mu.m. The
matrix porosity is preferably in the range of from about 40% to
about 95% of the total surface area, more preferably, from about
60% to about 90%.
[0056] 2. The Ionically Conductive Solid
[0057] It is believed that the ionically conductive solid serves to
impart and/or enhance conductivity of the membranes. As defined
herein, "solid" means a material that is solid at fuel cell
fabrication and operating temperatures.
[0058] It is preferred that the ionically conductive solid possess
no unbound water, such that when operating as a membrane in a fuel
cell, the vapor in the fuel stream is unsaturated at the operating
temperature.
[0059] Preferred ionically conductive solids have ionic high
conductivity, preferably in the range from about 10.sup.-4 per ohm
per centimeter (.OMEGA..sup.-1 cm.sup.-1) to about 0.2
.OMEGA..sup.-1 cm.sup.-1, more preferably from about 10.sup.-2
.OMEGA..sup.-1 cm.sup.-1 to about 0.2 .OMEGA..sup.-1 cm.sup.-1, and
most preferably from about 0.1.OMEGA..sup.-1 cm.sup.-1 to about 0.2
.OMEGA..sup.-1 cm.sup.-1.
[0060] The ionically conductive solid may be prepared by reaction
in situ or pre-made and then impregnated into the porous polymeric
matrix, depending on the material used. The ionically conductive
solid and, when present, preferable ionomeric binder material, are
substantially impregnated into the porous structure of the membrane
in order to render an interior volume of the membrane
occlusive.
[0061] Examples of preferred ionically conductive solids include,
but are not limited to, polyoxymetalates, and heteropoly acids such
as phosphotungstic acid, phosphomolybdic acid, silicotungstic acid,
phosphosolicic acid, zirconium hydrogen phosphate, and zeolites.
Heteropoly acids are particularly preferred, being proton
conductive solids having conductivities of up to about 0.17
.OMEGA..sup.-1 cm.sup.-1 at 25.degree. C. Phosphotungstic acid, for
example, in its hydrate from
(H.sub.3PO.sub.4(WO.sub.3).sub.12.nH.sub.2O) has an ionic
conductivity of about 0.17 .OMEGA..sup.-1 cm.sup.-1. Since the
conductivity of heteropoly acids is due to the hydrated water, an
ion exchange composite membrane containing a heteropoly acid is
expected to have higher conductivity than an ion exchange composite
membrane without heteropoly acid, because the interaction between
water and solid acid should be stronger than the interaction
between ion exchange resin and water, resulting in an improved
ability to retain water. The addition of heteropoly acids not only
creates more acid sites for proton transfer, but also provides the
water for the ion exchange resin at elevated temperature. Other
interactions between heteropoly acids and Nafion.RTM. have been
described by Lin, et al., in "High Temperature Nafion.RTM.-based
Composite Membranes for Hydrogen PEM Fuel Cell", which is
incorporated by reference herein in its entirety.
[0062] 3. The Ionomeric Binder
[0063] An ionomeric binder is preferably present in the composite
membrane. The ionomeric binder is believed to aid in preventing
fuel crossover and to enhance proton conductivity. Suitable
ionomeric binders for use in the present membranes may be any
chemically and electrochemically stable ion exchange resins or
other polymers with high ionic conductivity.
[0064] Examples of preferred ionomeric binders include, but are not
limited to, ion exchange resins such as Nafion.RTM., other
perfluorinated sulfonic acids, polystyrene sulfonic acid, and
perfluorinated carboxylic acid resins. Other polymetric acid
resins, which form polymers, may also be used.
[0065] The ratio of ionomeric binder, when present, to ionically
conductive solid should adjusted to as to provide optimal physical,
chemical, and electrical characteristics. Composite membranes
having too high a ratio of ionomeric binder to ionically conductive
solid do not have high conductivity at elevated temperature.
Conversely, composite membranes having too low a ratio of ionomeric
binder to ionically conductive solid will result in a membrane with
increased rates of fuel crossover. When the ionomeric binder is
present, the ratio of ionomeric binder mass to ionically conductive
solid mass is in the range from 1:5 to 5:1.
[0066] B. The Anode Catalyst Layer
[0067] As would be understood by one of ordinary skill in the art,
the form of the anode depends on the type of fuel to be used. If a
mixture of hydrogen, carbon dioxide, water, and carbon monoxide
from a steam reformed hydrocarbon is used, the anode catalyst
should preferentially be very active as a hydrogen oxidation
catalyst and be tolerant to the presence of carbon monoxide that
poisons many catalysts. Since such a catalyst is most likely a
precious metal, it should be supported on a high surface area
electronically conductive material to enhance its surface area and
effectiveness per unit of cost. The concentration of the catalyst
on the support should be an optimum since a high concentration
tends to reduce the surface area whereas a low concentration tends
to thicken the anode. A thick anode restricts the ability of ions
and reactants to migrate through the anode to the surface of the
catalyst. An example of such an anode catalyst is a 40 wt %
platinum/ruthenium alloy supported on the surface of a carbon
powder such as Vulcan XC-72 produced by the Cabot Corporation.
[0068] It has been discovered that in order to provide improved MEA
performance, good ionic conductivity should be maintained within
the anode. The present inventors have discovered that performance
of the MEA can be significantly enhanced when the electrolyte
within the anode incorporates the features as described for the
membrane by using a mixture of ionically conductive solid and
ionomeric binder in an optimal ratio. It has been unexpectedly
found that the ratio of ionically conductive solid to ionomeric
binder in the anode is of significant importance, with too high
ratios leading to adsorption of the ionically conductive solid onto
the catalyst supports (impairing the catalyst and electronic
conductivity), and too low ratios resulting in an poor electrolyte
ionic conductivity.
[0069] C. The Cathode Catalyst Layer
[0070] As would also be understood by one of ordinary skill in the
art, the form of the cathode depends on the type of oxidant to be
used. For example, if air is used, the cathode catalyst should
preferentially be very active as an oxygen reduction catalyst and
be stable in the highly corrosive environment of the cathode. Since
such a catalyst is most likely a precious metal, it should be
supported on a high surface area conductive material to enhance its
surface area and effectiveness per unit of cost. The concentration
of the catalyst on the support should be an optimum since a high
concentration tends to reduce the surface area whereas a low
concentration tends to thicken the cathode. A thick cathode
restricts the ability of ions and reactants to migrate through the
cathode to the surface of the catalyst. An example of such a
cathode catalyst is a 60 wt % platinum/chromium/cobalt alloy
supported on the surface of a carbon powder such as Vulcan XC-72
produced by the Cabot Corporation.
[0071] As with the anode, it has been discovered that in order to
provide improved MEA performance, good ionic conductivity should be
maintained within the cathode. The present inventors have
discovered that performance of the MEA can be significantly
enhanced when the electrolyte within the cathode incorporates the
features as described for the membrane by using a mixture of
ionically conductive solid and ionomeric binder in an optimal
ratio.
[0072] As with the anode, it also unexpectedly has been found that
the ratio of ionically conductive solid to ionomeric binder in the
cathode is of significant importance, with too high ratios leading
to adsorption of the ionically conductive solid onto the catalyst
supports (impairing the catalyst and electronic conductivity), and
too low ratios resulting in an poor electrolyte ionic
conductivity.
[0073] The performance of an MEA cell has further been found to be
optimized by maximizing the catalyst surface area of the cathode
utilized and in improving the ionic conductivity of the catalyst
layer. Application of a catalyst layer onto the cathode-side
membrane surface has also been found to increase MEA performance,
presumably by reducing the path length for ionic conduction. For
example, the coating of a platinum black layer onto a membrane
containing Nafion.RTM. 960 with phosphotungstic acid, followed by
coating the platinum black layer with a Pt/Vulcan catalyst layer
containing Nafion.RTM., was found to produce increased cathode
catalyst activity.
EXAMPLE 1
Improved Performance of MEAs with Heat Treatment of the
Membrane
[0074] A study was undertaken to determine whether heat treatment
of the membrane would improve performance of a MEA by reducing
resistance and crossover.
[0075] Five membranes were prepared using Tetratec.RTM. (Tetratec
Co., Fosterville, Pa.) Teflon.RTM. membranes filled with various
electrolytes. These electrolytes were:
[0076] 1100 Equivalent Weight ("EW") Nafion.RTM.,
[0077] 960 EW Nafion.RTM.,
[0078] 1100 EW Nafion.RTM. containing 3 wt % silica
[0079] The membranes were prepared as trilayers and either sprayed
with catalyst layers without heat treatment or were heated under a
force of 2000 lb. (6.8 cm.sup.2 area) at 121.degree. C. for 20
minutes prior to the application of the catalyst layers. In all
cases, the electrolyte in the electrodes was the same as that in
the membrane but with the absence of the silica. The MEAs were not
heat treated after the application of the catalyst layers.
[0080] FIG. 1 is a graph illustrating the resistance of untreated
and heat-treated Teflon/Nafion.RTM. membranes using saturated
reactants at 80.degree. C. All of the resistances can be seen to be
good with the pure heat-treated 960 EW Nafion.RTM. being the lowest
followed by the pure heat-treated 1100 EW Nafion.RTM.. Since the
heat treatment is thought to introduce some crystallinity into the
Nafion.RTM., the heat-treated samples are labeled as crystalline
(e.g., heat treated Nafion.RTM./Teflon is denoted "Cry NT").
[0081] FIG. 2 is a graph that illustrates that heat-treatment of a
Teflon/Nafion.RTM. membrane significantly reduces fuel (hydrogen)
crossover (as adjudged by crossover current) in a MEA. While all of
the heat-treated membranes showed crossover current at 80.degree.
C. of about 2 ma/cm.sup.2, the value was much higher for the
membranes that had not been heat-treated.
[0082] It may be concluded from this experiment, that heat treating
a membrane imparts a reduction in fuel crossover, and a subsequent
improvement in MEA performance.
EXAMPLE 2
Reduction of Path Length for Ionic Conduction by Use of Platinum
Black Layer
[0083] A study was undertaken to determine whether performance of
the MEA could be enhanced by reduction of the path length for ionic
conduction in the cathode. A platinum black layer on the
cathode-side membrane surface was applied in an attempt to reduce
the path length for ionic conduction.
[0084] A polytetrafluoroethylene membrane support was imbued with
Nafion.RTM. 960 and PTA in an additive weight ratio of 1 part PTA
to 4 parts Nafion.RTM. 960, to form an imbued membrane support
having a total thickness of 0.6 mils. Nafion.RTM. was then applied
along the two major surfaces of the imbued membrane support to form
two protective layers of thickness 2 mils and a tri-layer composite
membrane. Because of the difficulty of adding phosphotungstic acid
to the cathode catalyst layer to enhance its conductivity at high
temperature, a layer containing only platinum and Nafion.RTM. 960
was first sprayed onto the tri-layer composite membrane on the
cathode side to give a metal loading of about 0.4 mg.cm.sup.2. A 40
wt % Pt/Vulcan catalyst layer containing Nafion.RTM. alone was then
applied over the platinum black layer. The anode catalyst layer was
applied to the other major surface of the tri-layer composite
membrane by spraying a Pt--Ru/C: Nafion.RTM. 960 mix in methanol
onto the surface. The ratio of 45 wt % Pt/Ru on carbon (Vulcan
XC-72) to Nafion.RTM. 960 was 3:1 by weight. Metal loading at the
anode catalyst layer was about 0.4 mg/cm.sup.2. Ultrasonic
dispersion of the anode catalyst was performed (80 mg Pt--Ru/C in
2.4 g methanol) for 3 hours.
[0085] The performance and internal resistance of this cell at
atmospheric pressure and 120.degree. C. using hydrogen as the fuel,
and air or oxygen as the oxidant, are shown in FIG. 3. The cell
voltage at 400 ma/cm.sup.2 and 120.degree. C. was very good at
about 0.45 volts on air with a cell resistance of about 0.2
.OMEGA.cm.sup.2.
[0086] The IR-free performance data are shown in FIG. 4. The
semilogarithmic slope of voltage versus current on oxygen (Tafel
slope) was about 85 mv/decade between 20 and 80 ma/cm.sup.2, lower
than anticipated without the platinum black layer. A reaction
controlled Tafel slope of 78 mv/decade would be expected if the
Tafel slope equals 2.3 RT/F (R=gas constant, T=temperature, and
F=Faraday constant), based on the anticipated rate-determining step
for the oxygen reduction reaction.
[0087] As depicted in FIG. 4, the current ratio between the
performance on air and oxygen was below 4.8:1 (expected for a first
order process with oxygen) at all current densities, going from
3.5:1 to 2.5:1 for air current densities of 20 ma/cm.sup.2 and 200
ma/cm.sup.2, respectively. This may be interpreted as suggesting
that either ionic conductance is still poor within the cathode
catalyst layer or that the performance of the cell was changing
with time between the times at which the air and oxygen data were
obtained. Some ionic resistance is probably present.
[0088] As a measure of cathode catalyst activity, the current in
FIG. 4 at 0.8 volts on oxygen was about 80 ma/cm.sup.2. This
compares to 26 ma/cm.sup.2 for a similar cell that had no platinum
black layer. This ratio of activities would be expected to increase
the performance by 38 mv assuming an activation Tafel slope of 78
mv/decade. The cell voltage of a similar cell with no platinum
black layer was 0.41 volts at 400 ma/cm.sup.2 and is just about 38
mv below the 0.45 volts of the present cell. The performance
increase is deemed to be due primarily to the increase in cathode
catalyst activity.
[0089] As seen in FIG. 5, fuel (hydrogen) crossover current of the
cell was fairly low at about 4 ma/cm.sup.2 of hydrogen at
110.degree. C. Voltammetry results (down to 0.1 volts) on the
cathode, also depicted in FIG. 5, results in a platinum area of
about 821 cm.sup.2. The similar area for cell without a platinum
black layer is about 195 cm.sup.2. The ratio of these numbers is
4.2 that is close to the ratio of currents at 0.8 volts of
80/26=3.1. This agreement supports the conclusion that the improved
performance is due to increased cathode catalyst activity.
[0090] The results demonstrate that the introduction of a platinum
black layer on the cathode-side membrane surface is effective in
increasing cell performance.
EXAMPLE 3
Fabrication of an Membrane Electrode Assembly Comprising PTA in the
Membrane and Pressed Catalytic Electrodes with PTA
[0091] A membrane (containing a porous Teflon.RTM. support,
phosphotungstic acid and EW=960 Nafion.RTM.) was prepared as
follows: A 5% Nafion.RTM. solution was evaporated to dryness and
re-dissolved in an equivalent amount of ethanol to obtain a 5%
Nafion.RTM. solution in ethanol. 0.5 g of phosphotungstic acid
(PTA) was dissolved in 15 g of methanol to obtain a solution. The
5% Nafion.RTM. solution (10 g) and the PTA solution (15 g) were
then combined to obtain a solution with 1:1 weight ratio of PTA to
Nafion.RTM.. The porous polymeric matrix was mounted in a holder.
The PTA/Nafion.RTM. solution was applied to both sides of the
matrix by brush to form a composite membrane. The membrane was then
dried at 60.degree. C. for 5 minutes to remove any solvent. The
composite membrane was then mounted vertically, kept warm using a
heat lamp, and sprayed with PTA/Nafion.RTM. solution on both sides
until the total weight of the composite membrane
(Nafion.RTM.-Teflon-Phosphotungsic Acid ("NTPA") was about 0.25
g/70 cm.sup.2. The composite membrane was then heat pressed at
120.degree. C. under 10.sup.6 Pa for 20 minutes.
[0092] The resulting membrane was subsequently used to fabricate
the membrane electrode assembly (MEA). Commercial electrodes with
0.4 mg/cm.sup.2 (total metal) of 20% Pt/C (cathode) and 20%
Pt--Ru/C (1:1 atomic ratio) (anode) obtained from E-Tek company
were first impregnated by Nafion.RTM. solution (EW=960) blended
with phosphotungstic acid and then dried in an oven at 80.degree.
C. for 10 minutes. The ratio for dry Nafion.RTM. to phosphotungstic
was 2:1 by weight. 1.0 mg/cm.sup.2 Nafion.RTM. loading in the
electrodes was obtained after impregnation.
[0093] To obtain intimate contact between membrane and electrodes,
a thin layer of concentrated Nafion.RTM. solution was applied to
the Pt (cathode) and Pt--Ru (anode) electrodes, respectively.
Before the Nafion.RTM. solution dried, the two electrodes were
placed on opposite sides of the membrane, pressed within two
Teflon.RTM. films+silicon rubber sheet gasket+metal plates and then
heated in an oven at 120.degree. C. for 20 minutes with a 5 kg
weight (6.8 cm.sup.2 area) pressing down on the top metal
plate.
[0094] The resulting MEA was tested in a 5 cm.sup.2 single cell
obtained from Electrochem Inc. The performance of MEA-A, a MEA made
as described in this example, at 120.degree. C. is shown in FIG. 6.
The water saturation temperatures and the stoichiometric flow rates
of the hydrogen and air are specified in the caption of FIG. 6.
EXAMPLE 4
Fabrication of an Membrane Electrode Assembly Comprising PTA in the
Membrane and Catalytic Electrodes Layers with PTA Sprayed on the
Membrane
[0095] A membrane (containing a porous Teflon.RTM. support,
phosphotungstic acid and EW=1100 Nafion.RTM.) prepared as described
was used to fabricate the membrane electrode assembly (MEA).
Nafion.RTM.-bonded catalysts were prepared by mixing Nafion.RTM.
solution (EW=1100) with catalyst, stirring, sonifying and then
drying in a convention oven to remove the solvent. Nafion.RTM. and
PTA loadings for the 60% Pt/C (E-Tek) cathode catalysts and 45%
Pt--Ru/C (Johnson Matthey) anode catalysts were 15% by weight and
25% by weight, respectively. The resulting catalysts were
re-dispersed by methanol. The concentration of the catalyst
solutions was about 3%. The MEA was prepared by spraying the
catalysts on each side of the membrane using nitrogen gas.
[0096] The performance of this MEA-S is shown in FIG. 7. The water
saturation temperatures and the stoichiometric flowrates of the
hydrogen and air are specified in the caption of FIG. 7.
[0097] While the invention has been described with respect to
preferred embodiments, those skilled in the art will readily
appreciate that various changes and/or modifications can be made to
the invention without departing from the spirit or scope of the
invention as defined by the appended claims. All documents cited
herein are incorporated in their entirety.
* * * * *