U.S. patent application number 10/212520 was filed with the patent office on 2003-03-06 for membrane electrode assemblies.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Balsimo, William V., Debe, Mark K., Larson, James M., Steinbach, Andrew J., Ziegler, Raymond J..
Application Number | 20030041444 10/212520 |
Document ID | / |
Family ID | 25488071 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030041444 |
Kind Code |
A1 |
Debe, Mark K. ; et
al. |
March 6, 2003 |
Membrane electrode assemblies
Abstract
Membrane electrode assemblies are described that include an ion
conductive membrane a catalyst adjacent to the major surfaces of
the ion conductive membrane and a porous particle filled polymer
membrane adjacent to the ion conductive membrane. The catalyst can
be disposed on the major surfaces of the ion conductive membrane.
Preferably, the catalyst is disposed in nanostructures. The polymer
film serving as the electrode backing layer preferably is processed
by heating the particle loaded porous film to a temperature within
about 20 degrees of the melting point of the polymer to decrease
the Gurley value and the electrical resistivity. The MEAs can be
produced in a continuous roll process. The MEAs can be used to
produce fuel cells, electrolyzers and electrochemical reactors.
Inventors: |
Debe, Mark K.; (Stillwater,
MN) ; Larson, James M.; (Saint Paul, MN) ;
Balsimo, William V.; (Afton, MN) ; Steinbach, Andrew
J.; (Saint Paul, MN) ; Ziegler, Raymond J.;
(Glenwood City, WI) |
Correspondence
Address: |
Office of Intellectual Property Counsel
3M Innovative Properties Company
P.O. Box 33427
St. Paul
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
25488071 |
Appl. No.: |
10/212520 |
Filed: |
August 5, 2002 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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10212520 |
Aug 5, 2002 |
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09711569 |
Nov 13, 2000 |
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6428584 |
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09711569 |
Nov 13, 2000 |
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09208695 |
Dec 10, 1998 |
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6183668 |
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09208695 |
Dec 10, 1998 |
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08948627 |
Oct 10, 1997 |
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5910378 |
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Current U.S.
Class: |
29/623.1 ;
427/115; 429/483; 429/492; 429/532 |
Current CPC
Class: |
H01M 8/1004 20130101;
Y10T 428/249979 20150401; Y10T 29/4998 20150115; Y10T 29/53135
20150115; Y10T 29/53204 20150115; H01M 8/0297 20130101; Y10T
29/49989 20150115; H01M 4/8825 20130101; H01M 4/8875 20130101; Y02P
70/50 20151101; Y10T 29/49108 20150115; Y10T 29/49885 20150115;
Y10T 428/249953 20150401; H01M 8/0221 20130101; H01M 8/1051
20130101; Y10T 29/532 20150115; H01M 4/8605 20130101; H01M 8/241
20130101; Y10T 428/249962 20150401; Y10T 29/49988 20150115; Y10T
156/1084 20150115; H01M 8/1025 20130101; H01M 8/1039 20130101; H01M
2300/0082 20130101; H01M 8/0206 20130101; Y10T 29/49112 20150115;
H01M 8/0239 20130101; Y02E 60/50 20130101; C25B 9/23 20210101; H01M
8/0271 20130101; H01M 8/1023 20130101; Y10T 29/49114 20150115; Y10T
428/249954 20150401; H01M 8/0273 20130101; Y10T 29/49982 20150115;
H01M 8/0226 20130101; Y10T 428/249955 20150401; H01M 8/109
20130101; Y10T 29/49115 20150115; H01M 8/0247 20130101; Y10T
29/49984 20150115; Y10T 428/249958 20150401; H01M 4/8878 20130101;
H01M 8/0243 20130101; Y10T 29/53139 20150115 |
Class at
Publication: |
29/623.1 ;
427/115; 429/40; 429/44 |
International
Class: |
H01M 006/00; H01M
004/86; H01M 004/90; H01M 004/96; B05D 005/12 |
Claims
What is claimed is:
1. An electrochemical MEA comprising: an ion conductive membrane,
said membrane having a first and second major surface; catalyst
adjacent to said first and second major surfaces; and a porous,
electrically conductive polymer film adjacent to said ion
conductive membrane, said film comprising a polymer matrix and
about 45 to about 98 percent by weight electrically conductive
particles embedded within said polymer matrix.
2. The electrochemical MEA of claim 1, wherein the Gurley value of
said polymer film is less than about 50 s/50 cc.
3. The electrochemical MEA of claim 1, wherein said polymer matrix
comprises a polymer selected from the group consisting of
polyethylene, polypropylene, polyvinylidene fluoride,
polytetrafluoroethylene,
poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and
mixtures thereof.
4. The electrochemical MEA of claim 1, wherein said electrically
conductive particles comprise carbon.
5. The electrochemical MEA of claim 1, wherein said porous polymer
film has an electrical resistivity of less than about 20
ohm-cm.
6. The electrochemical MEA of claim 1, wherein said catalytic
material is disposed at an interface between said ion conductive
membrane and said porous, electrically conductive polymer film.
7. The electrochemical MEA of claim 1, wherein said catalytic
material is disposed upon at least one of said surfaces of said ion
conductive membrane.
8. The electrochemical MEA of claim 1, wherein said catalyst is
disposed in nanostructured elements.
9. An electrochemical MEA comprising: an ion conductive membrane,
said membrane having a first and second major surface; catalyst
adjacent to said first and second major surfaces; and a porous,
electrically conductive polymer film adjacent to said ion
conductive membrane, said film comprising electrically conductive
particles and a porous matrix of fibrillated PTFE fibrils.
10. The electrochemical MEA of claim 9, wherein said catalytic
material is disposed at an interface between said ion conductive
membrane and said porous, electrically conductive polymer film.
11. The electrochemical MEA of claim 9, wherein said catalytic
material is disposed upon at least one major surface of said ion
conductive membrane.
12. The electrochemical MEA of claim 9, wherein said conductive
particles comprise carbon.
13. The electrochemical MEA of claim 9, wherein said porous polymer
film has a Gurley value of less than 50 s/50 cc.
14. The electrochemical MEA of claim 9, wherein said porous polymer
film has an electrical resistivity of less than 20 ohm-cm.
15. A method of producing an electrically conductive polymer film
comprising the step of heating a porous, polymer film comprising a
polymer matrix and about 45 to about 98 percent by weight
electrically conductive particles to a temperature within
20.degree. C. of the melting point of said polymer matrix for
sufficient time to decrease the Gurley value of said film by at
least about 25 percent and decrease the electrical resistivity of
said film by at least about 25 percent while substantially
maintaining the physical integrity and mechanical properties of
said film upon cooling.
16. The method of claim 15, wherein said polymer matrix comprises a
polymer selected from the group consisting of polyethylene,
polypropylene, polyvinylidene fluoride,
poly(tetrafluoroethylene-co-perfl- uoro-(propyl vinyl ether)) and
mixtures thereof.
17. The method of claim 15, wherein said conductive particles
comprise carbon.
18. The method of claim 15, wherein said conductive particles
comprise one or more conductive metals.
19. The method of claim 15, wherein said porous film comprises
between about 80 and about 98 percent by weight conductive
particles.
20. The method of claim 15, wherein said temperature is about 5 to
about 20 degree centigrade above said melting temperature.
21. The method of claim 15, wherein said Gurley value of said film
following heating is less than 50 s/50 cc .
22. The method of claim 15, further comprising the step of
differential cooling for quenching the film to create an asymmetric
film.
23. A method of forming an electrode backing layer for an
electrochemical is MEA comprising the steps of: (a) forming a
polymeric film comprising a crystallizable polyolefin polymer
matrix, conductive particles and a diluent for said polymer, (b)
applying surface texture to said polymeric film; and (c) removing
said oil before or after applying said surface texture.
24. A method of forming an electrochemical MEA comprising the step
of placing an electrode backing layer on both sides of a polymeric
ion conductive membrane, said electrode backing layers each
comprising a gas permeable, electrically conductive porous film
prepared according to the method of claim 1, wherein a catalyst
layer is disposed between each of said ion conductive membrane and
said electrode backing layers.
25. A method of forming an electrochemical MEA comprising the step
of placing an electrode backing layer on both sides of a polymeric
ion conductive membrane, said electrode backing layers each
comprising a gas permeable, electrically conductive porous
fibrillated PTFE film and conductive particles embedded in said
film, wherein a catalyst layer is disposed between each of said ion
conductive membrane and said electrode backing layers.
26. A method of producing a plurality of 5-layer MEAs, comprising
the step of applying catalyst layers and electrode backing layers
at suitable locations along a web of ion conduction membrane such
that a plurality of 5-layer MEAs can be cut from said web of ion
conduction membrane.
27. A film comprising greater than about 45 percent by weight
conducting particles, said film having a surface exhibiting under
contact with water a receding and advancing contact angles greater
than 90.degree., wherein said advancing contact angle is no more
than 50.degree. greater than said receding contact angle.
28. The film of claim 27, wherein said advancing contact angle is
no more than 30.degree. greater than said receding contact
angle.
29. The film of claim 27, wherein said advancing contact angle is
no more than 20.degree. greater than said receding contact
angle.
30. A method of producing a film comprising a polymer and greater
than about 45 percent by weight conducting particles, said method
comprising the steps of heating to a temperature from about the
melting point to about 20 degrees C. above the melting point and
then stretching the film from about 25 percent to about 150 percent
of their original length.
31. A polymer web comprising a plurality of MEA elements.
32. The polymer web of claim 31, wherein said MEA elements are
disposed along a continuous web of ion conducting polymeric
material.
33. The polymer web of claim 32, further comprising suitably
located seal material.
34. The polymer web of claim 31, further comprising nanostructured
catalyst layers.
Description
FIELD OF THE INVENTION
[0001] The invention relates to membrane electrode assemblies and
electrochemical cells such as fuel cells, electrolyzers and
electrochemical reactors.
BACKGROUND OF THE INVENTION
[0002] Fuel cells involve the electrochemical oxidation of a fuel
and reduction of an oxidizing agent to produce an electrical
current. The two chemical reactants, i.e., the fuel and the
oxidizing agent, undergo redox reaction at two isolated electrodes,
each containing a catalyst in contact with an electrolyte. An ion
conduction element is located between the electrodes to prevent
direct reaction of the two reactants and to conduct ions. Current
collectors interface with the electrodes. The current collectors
are porous so that reactants can reach the catalyst sites.
[0003] Fuel cells produce current as long as fuel and oxidant are
supplied. If H.sub.2 is the fuel, only heat and water are
byproducts of the redox reactions in the fuel cell. Fuel cells have
application wherever electricity generation is required.
Furthermore, fuel cells are environmentally benign.
[0004] An electrolyzer involves the splitting of water into
hydrogen and oxygen using electricity. Similarly, an
electrochemical reactor, such as a chlor-alkali cell, uses
electricity to produce chlorine from an alkaline brine.
Electrolyzers and electrochemical reactors basically involve a fuel
cell operating in reverse. For example, for an electrolyzer to
produce hydrogen and oxygen from water by passing an electrical
current through the device, an equivalent ion conductive element
appropriate for use in a fuel cell may be located between catalyst
layers and current collector layers.
SUMMARY OF THE INVENTION
[0005] In a first aspect, the invention features an electrochemical
MEA comprising:
[0006] an ion conductive membrane, the membrane having a first and
second major surface;
[0007] catalyst adjacent to the first and second major surfaces;
and
[0008] a porous, electrically conductive polymer film adjacent to
the ion conductive membrane, the film comprising a polymer matrix
and about 45 to about 98 percent by weight electrically conductive
particles embedded within the polymer matrix.
[0009] In a preferred embodiment, the Gurley value of the polymer
film is less than about 50 s/50 cc. The polymer matrix can include
a polymer selected from the group consisting of polyethylene,
polypropylene, polyvinylidene fluoride, polytetrafluoroethylene,
poly(tetrafluoroethylen- e-co-perfluoro-(propyl vinyl ether)) and
mixtures thereof. The electrically conductive particles can
comprise carbon. The porous polymer film preferably has an
electrical resistivity of less than about 20 ohm-cm.
[0010] The catalytic material can be disposed at an interface
between the ion conductive membrane and the porous, electrically
conductive polymer film. The catalytic material can be disposed
upon the surfaces of the ion conductive membrane. In preferred
embodiments, the catalytic material is disposed in nanostructured
elements.
[0011] In another aspect, the invention features an electrochemical
MEA comprising:
[0012] an ion conductive membrane, the membrane having a first and
second major surface;
[0013] catalyst adjacent to the first and second major surfaces;
and
[0014] a porous, electrically conductive polymer film adjacent to
the ion conductive membrane, the film comprising electrically
conductive particles and a porous matrix of fibrillated PTFE
fibrils.
[0015] The catalytic material can be disposed at an interface
between the ion exchange membrane and the porous, electrically
conductive polymer film. The catalytic material can be disposed
upon at least one major surface of the electrically conductive
polymer film. The conductive particles can comprise carbon. The
porous polymer film preferably has a Gurley value of less than 50
s/50 cc and an electrical resistivity of less than 20 ohm-cm.
[0016] In another aspect, the invention features a method of
producing an electrically conductive polymer film comprising the
step of heating a porous, polymer film comprising a polymer matrix
and about 45 to about 98 percent by weight electrically conductive
particles to a temperature within 20.degree. C. of the melting
point of the polymer matrix for sufficient time to decrease the
Gurley value of the film by at least about 25 percent and decrease
the electrical resistivity of the film by at least about 25 percent
while substantially maintaining the physical integrity and
mechanical properties of the film upon cooling. The polymer matrix
can include a polymer selected from the group consisting of
polyethylene, polypropylene, polyvinylidene fluoride,
poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and
mixtures thereof. The conductive particles can comprise carbon
and/or one or more conductive metals. The porous film preferably
includes between about 80 and about 98 percent by weight conductive
particles. The temperature can range between about 5 to about 20
degree centigrade above the melting temperature. The Gurley value
of the film following heating preferably is less than 50 s/50 cc.
The method can further comprise the step of using differential
cooling for quenching the extruded film to create an asymmetric
film with one side being denser and having smaller pores and the
other side being less dense and having larger pores. The
differential cooling can be accomplished through the use of a
casting wheel at a controlled temperature.
[0017] In another aspect, the invention features a method of
forming an electrode backing layer for an electrochemical MEA
comprising the steps of:
[0018] (a) forming a polymeric film comprising a crystallizable
polyolefin polymer matrix, conductive particles and a diluent for
the polymer,
[0019] (b) applying surface texture to the polymeric film; and
[0020] (c) removing the oil before or after applying the surface
texture.
[0021] In another aspect, the invention features a method of
forming an electrochemical MEA comprising the step of placing an
electrode backing layer on both sides of a polymeric ion conductive
membrane, the electrode backing layers each comprising a gas
permeable, electrically conductive porous film prepared as
described in the preceding paragraph, wherein a catalyst layer is
disposed between each of the ion conductive membrane and the
electrode backing layers.
[0022] In another aspect, the invention features a method of
forming an electrochemical MEA comprising the step of placing an
electrode backing layer on both sides of a polymeric ion conductive
membrane, the electrode backing layers each comprising a gas
permeable, electrically conductive porous fibrillated PTFE film and
conductive particles embedded in the film, wherein a catalyst layer
is disposed between each of the ion conductive membrane and the
electrode backing layers.
[0023] In another aspect, the invention features a method of
producing a plurality of 5-layer MEAs, comprising the step of
applying catalyst layers and electrode backing layers at suitable
locations along a web of ion conduction membrane such that a
plurality of 5-layer MEAs can be cut from the web of ion conduction
membrane.
[0024] In another aspect, the invention features a film comprising
greater than about 45 percent by weight conducting particles, the
film having a surface exhibiting under contact with water a
receding and advancing contact angles greater than 90.degree.,
wherein the advancing contact angle is no more than 50.degree.
greater than the receding contact angle. The advancing contact
angle preferably is no more than 30.degree. greater than the
receding contact angle, more preferably no more than 20.degree.
greater than the receding contact angle.
[0025] In another aspect, the invention features a method of
producing a film comprising a polymer and greater than about 45
percent by weight conducting particles, the method comprising the
steps of heating to a temperature from about the melting point to
about 20 degrees C. above the melting point and then stretching the
film from about 25 percent to about 150 percent of their original
length.
[0026] In another aspect, the invention features a polymer web
including a plurality of MEA elements. The MBA elements can be
disposed along a continuous web of ion conducting polymeric
material. The polymer web can further include nanostructured
catalyst layers and/or suitably located seal material.
[0027] Electrode backing layers as described herein have high
electrical conductivity, high gas permeability, good water
management characteristics and significant production advantages.
Membrane electrode assemblies (MEAs) incorporating the electrode
backing layers can have improved performance as determined by the
current produced at a given fuel cell voltage. Advantageously,
films of the present invention exhibit adequate hydrophobicity for
effective water management without incurring the expense or the
need for a fluoropolymer coating, whose properties can change with
use. The porous polymeric, electrode backing layers can be used in
efficient, commercial production methods of multilayer MEAs
including continuous roll processes. Continuous roll processing
allows for the cost effective assembly of hundreds of
electrochemical cell components at a relatively rapid rate.
[0028] Other features and advantages of the invention will be
apparent from the following description of the preferred
embodiments thereof, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic cross section of a five layer MEA.
[0030] FIG. 2 is a schematic cross section of a fuel cell
stack.
[0031] FIG. 3 is a perspective view of a continuous roll of
MEAs.
[0032] FIG. 4 is an exploded, perspective view of a fuel cell stack
with three cells.
[0033] FIG. 5 is a graph depicting the phase behavior of a
crystalline, thermoplastic polymer, useful for evaluating the
proper conditions in the TIPT process.
[0034] FIG. 6 is a graph of cell voltage vs current density to
obtain the resistivity at high current density of carbon-loaded
electrode backing materials obtained using the TIPT process.
[0035] FIG. 7 is a graph of cell voltage versus current density for
two five layer MEAs incorporating electrode backing layers produced
using the TIPT process and, for comparison, a cell produced using
commercial electrode backing material.
[0036] FIG. 8 is a graph of cell voltage vs current density for
additional cells produced with carbon-loaded electrode backing
materials obtained using the TIPT process and, for comparison, a
cell produced using commercial electrode backing materials.
[0037] FIG. 9 is a bar graph showing the Gurley values for
electrode backing layers produced using the PF process and, for
comparison, commercial electrode backing material.
[0038] FIG. 10 is a graph of the applied voltage as a function of
current density for electrode backing layers produced using the PF
process and a commercial electrode backing material, as measured in
a fuel cell test assembly.
[0039] FIG. 11 is a graph of cell voltage versus current density
for fuel cell MEA's incorporating electrode backing layers produced
using the PF process compared to a fuel cell MEA produced with a
commercial electrode backing layer.
[0040] FIG. 12 is a graph of cell voltage versus current density
for fuel cell MEA's incorporating electrode backing layers produced
using the TIPT process along with a control incorporating a
commercial material, each tested with equivalent catalyst coated
ion conduction membranes.
[0041] FIG. 13 is a graph of cell voltage versus current density
for fuel cells incorporating electrode backing layers produced
using the TIPT process using a smooth casting wheel showing the
difference in cell performance for an asymmetric electrode backing
layer in the cell depending on the orientation of the electrode
backing film with respect to side-to-side placement in the
cell.
[0042] FIG. 14A is a SEM micrograph of the casting wheel side of
the carbon-filled HDPE film of Example 16A without heat
treatment.
[0043] FIG. 14B is a SEM micrograph of the air side of the
carbon-filled HDPE film of Example 16A without heat treatment
[0044] FIG. 14C is a SEM micrograph of a cross-section of the
carbon-filled HDPE film of Example 16A without heat treatment.
[0045] FIG. 15A is a SEM micrograph of the casting wheel side of
the carbon-filled HDPE film of Example 16B following heat treatment
at 130.degree. C.
[0046] FIG. 15B is a SEM micrograph of the air side of the
carbon-filled HDPE film of Example 16B following heat treatment at
130.degree. C.
[0047] FIG. 15C is a SEM micrograph of the cross-section of the
carbon-filled HDPE film of Example 16B following heat treatment at
130.degree. C.
[0048] FIG. 16A is a SEM micrograph of a cross-section of the
carbon-filled UHMWPE film of Example 14 before heat treatment
[0049] FIG. 16B is a SEM micrograph of a cross-section of the
carbon-filled UHMWPE film of Example 14 after heat treatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] A. Electrochemical Cell Structure
[0051] Referring to FIG. 1, membrane electrode assembly (MEA) 100
in a five layer embodiment has various layers for the
electrochemical oxidation of a fuel and reduction of an oxidizing
agent to produce electric current. An ion conductive membrane 102
separates the cathode 104 and anode 106 of MEA 100. Each side of
ion conductive membrane 102 contacts a catalyst layer, i.e.,
cathode 104 and anode 106. Catalyst layers 104, 106 each contact an
electrode backing layer 108, 110. Electrode backing layers 108, 110
respectively contact bipolar plates 112, 114. The shape and size of
the components of the electrochemical cell can vary over a wide
range depending on particular design. FIG. 1 depicts the flow of
reactants for a fuel cell. In electrolyzers and electrochemical
reactors, a voltage is applied to the MEA to decompose a
composition flowed to the electrodes, for example for the formation
of Cl.sub.2. The discussion below focuses on fuel cells, although
the analogy to electrolyzers and electrochemical reactors is
straightforward.
[0052] The ion conductive membrane provides ionic conductivity
between the anode and cathode and forms a gaseous barrier blocking
flow of the reactants. In some embodiments, the ion conductive
membrane may be conductive only of ions either of positive charge
or negative charge, i.e., either a cation exchange membrane or an
anion exchange membrane, or only of one type of ion, e.g., a proton
exchange membrane.
[0053] While being conductive of some type of ions, the ion
conductive membrane should be nonconductive with respect to
electrons and gaseous reactants. To prevent the passage of gaseous
reactants, the ion conductive membrane should have sufficient
thickness for mechanical stability and should be effectively
nonpermeable. The conduction of gaseous reactants through the ion
exchange membrane could result in the undesirable direct reaction
of the reactants. Similarly, the conduction of electrons through
the ion conductive membrane could result in an undesirable short
circuit of the cell. Therefore, materials used in producing the ion
conductive membrane should not conduct electrons. In the case of
direct reaction of the reactants or of a short circuit, the energy
released by the reaction of the fuel and oxidizing agent cannot be
used to produce electricity.
[0054] The ion conductive membrane can include a polymer
electrolyte. The polymers should be chemically stable and
compatible with the catalysts so that the catalyst is not poisoned.
Polymer electrolytes can be made from a variety of polymers
including, for example, polyethylene oxide, poly (ethylene
succinate), poly (.beta.-propiolactone), and sulfonated
fluoropolymers such as Nafion.TM. (commercially available from
DuPont Chemicals, Wilmington, Del.). Nafion.TM. is produced by
hydrolyzing a copolymer of polytetrafluoroethylene with
perfluorosulfonylethoxyvinyleth- er and converting its sulfonyl
radical to a sulfonic radical. A suitable cation exchange membrane
is described in U.S. Pat. No. 5,399,184, incorporated herein by
reference.
[0055] Alternatively, the ion conductive membrane can be an
expanded membrane with a porous microstructure where an ion
exchange material impregnates the membrane effectively filling the
interior volume of the membrane. U.S. Pat. No. 5,635,041,
incorporated herein by reference, describes such a membrane formed
from expanded polytetrafluoroethylene (PTFE). The expanded PTFE
membrane has a microstructure of nodes interconnected by fibrils.
Similar structures are described in U.S. Pat. No. 4,849,311,
incorporated herein by reference.
[0056] The half-cell reactions of the fuel and the oxidizing agent
take place at separate catalyst surfaces. The reactant gases, i.e.,
fuel and oxidizing agent, must be able to penetrate to their
respective catalyst layer. A catalyst generally is in the form of
particles disposed in a layer with an ionomer or electrolyte, in
intimate contact with the ion conductive membrane and the electrode
backing layer. The catalyst layer can be applied to the ion
conductive membrane or the electrode backing layer by various
methods. In other words, the catalyst can be applied to the surface
of the ion conductive membrane and/or to a surface of the electrode
backing layer. Alternatively, the catalyst layer can be
encapsulated or embedded in the surface of the ion conductive
membrane.
[0057] For example, the ion conductive membrane can include a
nanostructured catalyst layer such as the membranes described in
U.S. Pat. No. 5,338,430, incorporated herein by reference. The
nanostructured films have a plurality of nanostructured elements
that are either two-component whiskers coated with catalytically
active material or one component structures including catalytically
active material. The nanostructured elements can be embedded in an
encapsulant such as a solid electrolyte, an ion exchange membrane,
or other polymeric matrix. The production of nanostructured
membranes is described in U.S. Pat. No. 5,238,729, incorporated
herein by reference.
[0058] Appropriate catalysts for fuel cells generally depend on the
reactants selected. Suitable catalyst materials for oxidation of
hydrogen or methanol fuels include metals such as, for example, Pd,
Pt, Ru, Rh and alloys thereof. Commonly used catalysts for oxygen
reduction include platinum supported on carbon particles. Different
catalysts may be preferred for use in electrolyzers and
electrochemical reactors. For example, for oxygen evolution in an
electrolyzer, a mixture of Ru and Ir oxides generally show better
performance than Pt.
[0059] The electrode backing layer functions as a current
collector. The electrode backing layer is porous for the passage of
gaseous reactants. To impart electrical conductivity, the electrode
backing layer includes electrically conducting particles. If
desired, the electrode backing layer can be textured. Detailed
features of the electrode backing layer are described below.
[0060] Bipolar plates typically have channels and/or grooves in
their surfaces that distribute fuel and oxidant to their respective
catalyst electrodes. Typically, bipolar plates are highly
electrically conductive and can be made from graphite and metals.
The electrodes and electrode backing layers of the present
invention generally can be used with any standard fuels including
H.sub.2 and reformed hydrocarbons such as methanol and gasoline,
and standard oxidants including O.sub.2 in air or in pure form.
[0061] Generally, a plurality of fuel cells or MEAs 150 are
combined to form a fuel cell stack 152 as depicted in FIG. 2. The
cells within the stacks are connected in series by virtue of the
bipolar plates such that the voltages of the individual fuel cells
are additive. Further details relatin, to formation of a fuel cell
stack are presented below.
[0062] B. Electrode Backing Layer/Electrode.
[0063] The electrode backing layer comprises a porous polymer film
including a polymer binder and conductive particles. In general,
the film should have a high loading of conductive particles held
together by a relatively small portion of polymer matrix. The film
generally has greater than about 45 percent by volume conductive
particles and more preferably between about 65 percent and about 96
percent by volume conductive particles. In addition to the
conductive particles, a catalyst layer (electrode) can be coated on
a surface of the electrode backing layer.
[0064] The porosity of the polymer film forming the electrode
backing layer provides for flow of reactants to the catalyst
particles at the interface of the electrode backing layer and the
ion conductive membrane. Preferred films have a porosity adequate
to provide for an even flow of reactants while maintaining adequate
electrical conductivity and mechanical strength of the film. Also,
the porosity of the polymer film provides for water management
within the cell. The electrode backing layer preferably is
sufficiently porous to pass fuel gas and water vapor through it
without providing a site for water condensation that would block
the pores of the film and prevent vapor transport. The mean pore
size generally ranges from about 0.01 micrometers to about 10.0
micrometers. Alternatively, porosity of the web can be quantified
by the Gurley value of the web, that is, the amount of time needed
for a given volume of gas to pass through a predetermined area of
the web under a specified pressure drop, as described below.
Suitable webs generally have Gurley values less than about 100
seconds per 10 cc.
[0065] To assist with water management, electrode backing layers
with asymmetric porosity can be used. The electrode backing layer
adjacent to the cathode, where water is formed, preferably has
smaller pores adjacent to the cathode and larger pores at the
outside of the MEA adjacent the bipolar plate. The higher pressure
in the small pores tends to push the water away from the cathode.
The formation of electrode backing layers having asymmetric
porosity is described below.
[0066] Conductive particles can include a variety of conductive
materials such as metals and carbon. The conductive particles can
have a variety of shapes and sizes. Preferred conductive particles
include, for example, conductive carbons. The conductive particles
are preferably less than about 10 microns in diameter and more
preferably less than about 1 micron in diameter. Suitable carbon
particles include, for example, carbon black, graphite, carbon
fibers, fullerenes and nanotubules. Preferred carbon particles
include, for example, carbon blacks. Commercially available carbon
blacks include, for example, Vulcan XC72R.TM. (Cabot Corp.,
Bilerica, Mass.), Shawinigan C-55.TM. 50% compressed acetylene
black (Chevron Chemical Co., Houston, Tex.), Norit type SX1.TM.
(Norit Americas Inc., Atlanta, Ga.), Corax L.TM. and Corax P.TM.
(Degussa Corp., Ridgefield Park, N.J.), Conductex 975.TM.
(Colombian Chemical Co., Atlanta, Ga.), Super S.TM. and Super P.TM.
(MMM Carbon Div., MMM nv, Brussels, Belgium), KetJen Black EC
600JD.TM. (Akzo Nobel Chemicals, Inc., Chicago, Ill.). Useful
graphite particles range in size up to about 50 .mu.m in diameter,
preferably from about 1 to about 15 .mu.m. Suitable commercial
graphites include, for example, MCMB 6-28.TM. (Osaka Gas Chemical
Co., Osaka, Japan), and SFG 15.TM. (Alusuisse Lonza America Inc.,
now Timcal, Fair Lawn, N.J.). Conductive carbon black can have
primary particles as small as about 10 nm to about 15 nm, though as
sold they may be present in agglomerates as large as several mm.
After dispersion, these agglomerates are broken down preferably
into particles less than about 0.1 micron (100 nm). Mixtures of
graphite and more conductive carbon blacks are also useful.
Conductive carbon fibers useful in electrode backing materials of
the invention include, e.g., those available from STREM Chemicals,
Inc., Newburyport, Mass., catalog No. 06-0140, having lengths of
approximately 6 mm and diameters of 0.001 cm.
[0067] In general, the polymer matrix can include any polymer that
can be processed appropriately into a porous film loaded with
particles. Suitable types of polymers include, for example,
thermoplastic polymers, thermosensitive polymers and
fluoropolymers. Two preferred processing methods are described
below. These preferred processing methods provide additional
constraints on the characteristics of the corresponding
polymers.
[0068] In addition to the conductive particles, fillers can be used
to alter the physical properties of the polymer films useful in the
invention. Appropriate fillers include, e.g. silica (SiO.sub.2),
powdered polytetrafluoroethylene and graphite fluoride (CF.sub.a).
The polymer films preferably can include up to about 20 percent by
weight fillers, and more preferably from about 2 to about 10
percent by weight fillers. The fillers are generally in the form of
particles.
[0069] Preferably, the electrode backing layers have an electrical
resistivity of less than about 20 Ohm-cm, more preferably less than
about 10 Ohm-cm, and most preferably less than about 0.5 Ohm-cm.
Also, films useful as electrodes in the invention preferably
exhibit advancing and receding contact angles toward water of
greater than about 90.degree., more preferably of greater than
about 110.degree. wherein the advancing contact angle is greater
than the receding contact angle by less than about 50.degree.,
preferably less than about 30.degree., and more preferably less
than about 20.degree.. The measurement of the advancing and
receding contact angles is described below. Receding and advancing
contact angles of water are an important measure of the
hydrophobicity of the film surface and the ability of the film to
function effectively in the water management of the fuel cell. The
contact angles can be different on the two surfaces of the
electrode backing layer. Similarly, the contact angles for the
cathode and anode can be different.
[0070] The resistance to gas flow of a polymer film can be
expressed in terms of the Gurley value. The Gurley value is a
measure of the flow rate of a gas through a standardized area of
the film under controlled pressure conditions, as described in ASTM
D726-58, Method A, as described further below. The electrode
backing layers preferably have a Gurley value of less than about
100 sec/50 cc air and more preferably less than about 50 sec/50 cc
air.
[0071] The surfaces of the electrode backing layers can be
microtextured possibly providing enhanced interfacial electrical
conductivity, water management and flow field performance. For
example, the material can be cast onto a textured casting wheel or
can be embossed using a nip roll wherein one of the rolls is
textured. A surface textured electrode backing layer can facilitate
gas (e.g. fuel, oxygen, and/or water vapor) transport into and out
of the fuel cell and channeling of liquid water away from the
cathode.
[0072] Two processes for the production of preferred polymer films
are described next.
[0073] 1. TIPT Process
[0074] The first preferred process for the production of porous
electrode backing layers involves thermally induced phase
transition (TIPT). The TIPT process is based on the use of a
polymer that is soluble in a diluent at an elevated temperature and
insoluble in the diluent at a relatively lower temperature. The
"phase transition" can involve a solid-liquid phase separation, a
liquid-liquid phase separation or a liquid to gel phase transition.
The "phase transition" need not involve a discontinuity in a
thermodynamic variable.
[0075] Suitable polymers for the TIPT process include thermoplastic
polymers, thermosensitive polymers and mixtures of polymers of
these types, with the mixed polymers being compatible.
Thermosensitive polymers such as ultrahigh molecular weight
polyethylene (UHMWPE) cannot be melt-processed directly but can be
melt processed in the presence of a diluent or plasticizer that
lowers the viscosity sufficiently for melt processing. Suitable
polymers may be either crystallizable or amorphous.
[0076] Suitable polymers include, for example, crystallizable vinyl
polymers, condensation polymers and oxidation polymers.
Representative crystallizable vinyl polymers include, for example,
high and low density polyethylene; polypropylene; polybutadiene;
polyacrylates such as polymethyl methacrylate; fluorine-containing
polymers such as polyvinylidene fluoride; and corresponding
copolymers. Condensation polymers include, for example, polyesters
such as polyethylene terephthalate and polybutylene terethphalate;
polyamides such as nylons; polycarbonates; andpolysulfones.
Oxidation polymers include, for example, polyphenylene oxide and
polyether ketones. Other suitable polymers include the copolymer,
poly(tetafluoroethylene-co-perfluoro-(propyl vinyl ether)) sold as
Teflon.TM. PFA (E. I. DuPont de Nemours Chemical Corp., Wilmington,
Del.). Blends of polymers and copolymers may also be used.
Preferred crystallizable polymers for electrode backing layers
include polyolefins and fluoropolymers, because of their resistance
to hydrolysis and oxidation.
[0077] Suitable diluents are liquids or solids at room temperature
and liquids at the melting temperature of the polymer. Low
molecular weight diluents are preferred since they can be extracted
more readily than higher molecular weight diluents. Low to moderate
molecular weight polymers, however, can be used as diluents if the
diluent polymer and the matrix polymer are miscible in the melt
state. Compounds with boiling points below the melting temperature
of the polymer can be used as diluents by using a superatmospheric
pressure sufficient to produce a liquid at the polymer melting
temperature.
[0078] The compatibility of the diluent with the polymer can be
evaluated by mixing the polymer while heating to determine whether
a single liquid phase is formed, as indicated generally by
existence of a clear homogeneous solution. An appropriate polymer
dissolves or forms a single phase with the diluent at the melting
temperature of the polymer but forms a continuous network on
cooling to a temperature below the melting temperature of the
polymer. The continuous network is either a separate phase from the
diluent or a gel where the diluent acts as a plasticizer swelling
the polymer network. The gel state may be considered a single
phase.
[0079] For non-polar polymers, non-polar organic liquids generally
are preferred as a diluent. Similarly, polar organic liquids
generally are preferred with polar polymers. When blends of
polymers are used, preferred diluents are compatible with each of
the polymers. When the polymer is a block copolymer, the diluent
preferably is compatible with each polymer block Blends of two of
more liquids can be used as the diluent as long as the polymer is
soluble in the liquid blend at the melt temperature of the polymer,
and a phase transition with the formation of a polymer network
occurs upon cooling.
[0080] Various organic compounds are useful as a diluent, including
compounds from the following broad classifications: aliphatic
acids; aromatic acids; aliphatic alcohols; aromatic alcohols;
cyclic alcohols; aldehydes; primary amines; secondary amines;
aromatic amines; ethoxylated amines; diarnines; arnides; esters and
diesters such as sebacates, phthalates, stearates, adipates and
citrates; ethers; ketones; epoxy compounds such as epoxidized
vegetable oils; phosphate esters such as tricresyl phosphate;
various hydrocarbons such as eicosane, coumarin-indene resins and
terpene resins, tall oil, linseed oil and blends such as petroleum
oil including lubricating oils and fuel oils, hydrocarbon resin and
asphalt; and various organic heterocyclic compounds.
[0081] Examples of particular blends of polymers and diluents that
are useful in preparing suitable porous materials include
polypropylene with aliphatic hydrocarbons such as mineral oil and
mineral spirits, esters such as dioctyl phthalate and dibtityl
phthalate, or ethers such as dibenzyl ether, ultrahigh molecular
weight polyethylene with mineral oil or waxes; high density
polyethylene with aliphatic hydrocarbons such as mineral oil,
aliphatic ketones such as methyl nonyl ketone, or an ester such as
dioctyl phthalate; low density polyethylene with aliphatic acids
such as decanoic acid and oleic acid, or primary alcohols such as
decyl alcohol; polypropylene-polyethylene copolymer with mineral
oil; and polyvinylidene fluoride with dibutyl phthalate.
[0082] A particular combination of polymer and diluent may include
more than one polymer and/or more than one diluent. Mineral oil and
mineral spirits are each examples of a diluent being a mixture of
compounds since they are typically blends of hydrocarbon liquids.
Similarly, blends of liquids and solids also can serve as the
diluent.
[0083] For thermoplastic polymers, the melt blend preferably
includes from about 10 parts to about 80 parts by weight of the
thermoplastic polymer and from about 90 to about 20 parts by weight
of the diluent. Appropriate relative amounts of thermoplastic.
polymer and diluent vary with each combination. For UHMWPE
polymers, an example of a thermosensitive polymer, the melt blend
preferably includes from about 2 parts to about 50 parts of polymer
and from about 98 parts to about 50 parts by weight of diluent.
[0084] For crystalline polymers the polymer concentration that can
be used for a solid-liquid or liquid-liquid phase separation in a
given system can be determined by reference to the
temperature-composition graph for a polymer-diluent system, an
example of which is set forth in FIG. 5. Such graphs can be readily
developed as described in Smolders, van Aartsen and Steenbergen,
Kolloid-Zu Z. Polymere, 243:14-20 (1971). Phase transitions can be
located by determining the cloud point for a series of compositions
at a sufficiently slow rate of cooling that the system stays near
equilibrium.
[0085] Referring to FIG. 5, the portion of the curve from gamma to
alpha represents the thermodynamic equilibrium liquid-liquid phase
separation. T.sub.ucst represents the upper critical temperature of
the systems. The portion of the curve from alpha to beta represents
the equilibrium liquid-solid phase separation. The diluent can be
chosen such that the crystallizable polymer and diluent system
exhibits liquid-solid phase separation or liquid-liquid phase
separation over the entire composition range.
[0086] .PHI..sub.ucst represents the critical composition. To form
the desired porous polymers, the polymer concentration utilized for
a particular system preferably is greater than .PHI..sub.ucst. If
the polymer concentration is below the critical concentration
(.PHI..sub.ucst), the phase separation, upon cooling, generally
forms a continuous phase of diluent with dispersed or weakly
associated polymer particles, and the resulting polymer composition
typically lacks sufficient strength to be useful.
[0087] For a given cooling rate, the temperature-concentration
curve of the diluent-polymer blend can be determined by
Differential Scanning Calorimetry (DSC), for example, as indicated
by the dashed line of FIG. 5 for one rate of cooling. The resulting
plot of polymer concentration versus melting temperature shows the
concentration ranges that result in solid-liquid (sloped portion of
the dashed-curve) and. in liquid-liquid (horizontal portion of the
dashed curve) phase separation. From this curve, the concentration
ranges of the polymer and liquid that yield the desired porous
structure can be estimated. The determination of the melting
temperature-concentration curve by DSC is an alternative to
determination of the equilibrium temperature-composition curve for
a crystalline polymer.
[0088] The above discussion of phase diagrams is applicable to
amorphous polymers except that only liquid-liquid phase separation
can be observed. In this case, a cloud point generally is
indicative of the particular phase transition. Similarly, for gel
forming polymers the phase transition of relevance involves a
transition from a homogeneous solution to a gel. With gel forming
polymers, an abrupt increase in viscosity is indicative of a phase
transition from the melt to the gel, although a cloud point may
also occur in some cases.
[0089] For many diluent-polymer systems, when the rate of cooling
of the liquid-polymer solution is slow, liquid-liquid phase
separation occurs at substantially the same time as the formation
of a plurality of liquid droplets of substantially uniform size.
When the cooling rate is slow enough such that the droplets form,
the resultant porous polymer has a cellular microstructure. In
contrast, if the rate of cooling of the liquid-polymer solution is
rapid, the solution undergoes a spontaneous transformation called
spinodal decomposition, and the resultant porous polymer has a
fine, lacy structure with a qualitatively different morphology and
physical properties than obtained following droplet formation,
which can be obtained if the rate of cooling is slow. The fine
porous structure is referred to as a lacy structure
[0090] When liquid-solid phase separation occurs, the material has
an internal structure characterized by a multiplicity of spaced,
randomly disposed, non-uniform shaped, particles of polymer.
Adjacent polymer particles throughout the material are separated
from one another to provide the material with a network of
interconnected micropores and being connected to each other by a
plurality of fibrils consisting of the polymer. The fibrils
elongate upon orientation providing greater spacing between the
polymer particles and increased porosity. The filler particles
reside in or are attached to the thermoplastic polymer of the
formed structure.
[0091] In the case of ultrahigh molecular weight polyethylene
UHMWPE), the article obtained upon cooling may exist in a gel
state. The nature of the underlying polymer network is affected by
the rate of cooling. Fast cooling tends to promote gel formation
while slower cooling tends to allow more crystallization to occur.
Gel formation tends to dominate for compositions having
diluent/UHMWPE weight ratios greater than 80:20, whereas
crystallization dominates increasingly for diluent/UHMWPE weight
ratios less than 80:20. The polymer network in the case of highly
particle filled UHMWPE as determined by SEM, after extraction of
the diluent, tends to be a fairly dense structure having fine
pores. The structure of the network can be changed by the
extraction process. The highly particle filled UHMWPE films are
porous after extraction without need for restrint during extraction
or stretching.
[0092] If desired, the polymer can be blended with certain
additives that are soluble or dispersible in the polymer. When
used, the additives are preferably less than about 10 percent by
weight of the polymer component and more preferably less than about
2 percent by weight. Typical additives include, for example,
antioxidants and viscosity modifiers.
[0093] The melt blend further includes particulates for
incorporation into the electrode. For the resulting filled
compositions, porous polymer films can be obtained by extraction of
the diluent without physical restraint during extraction or
stretching of the film. In some cases, restraint of the film during
extraction may result in larger bubble points and smaller Gurley
values than for a film extracted without restraint. Particles for
the production of an electrode backing layer can include conductive
particles. The particles can be a mixture of materials. The
particles preferably form a dispersion in the diluent and are
insoluble in the melt blend of polymer and diluent. The appropriate
types of materials have been described above, as long as the
materials are appropriately compatible with the polymer and
diluent.
[0094] Some of the particulates, especially small sized carbon
particles, can serve as nucleating agents. The nucleating agent can
be a solid or gel at the crystallization temperature of the
polymer. A wide variety of solid materials can be used as
nucleating agents, depending on their size, crystal form, and other
physical parameters. Smaller solid particles, e.g., in the
submicron range, tend to function better as nucleating agents.
Preferably, nucleating agents range in size from about 0.01 to
about 0.1 .mu.m and more preferably from about 0.01 to about 0.05
.mu.m. Certain polymers such as polypropylene perform better in the
TIPT process with a nucleating agent present.
[0095] In the presence of a nucleating agent, the number of sites
at which crystallization is initiated is increased relative to the
number in the absence of the nucleating agent. The resultant
polymer particles have a reduced size. Moreover, the number of
fibrils connecting the polymer particles per unit volume is
increased. The tensile strength of the material is increased
relative to porous films made without the nucleating agent.
[0096] In the porous networks, preferably the particles are
uniformly distributed in the polymer matrix, and are firmly held in
the polymeric matrix such that they do not wash out on subsequent
extraction of the diluent using solvent. The average particle
spacing depends on the volume loading of the particles in the
polymer, and preferably, in the case of conductive particles, the
particles are in sufficiently close proximity to sustain electrical
conductivity. Processing of particles in the polymer matrix,
particularly conductive carbon particles, requires care, since
undermixing can result in poor dispersion, characterized by lumps
of particles (e.g., knots of carbon), and overmixing can cause the
agglomerates to disperse completely in the polymer. Conductive
particle proximity is important for higher levels of conductivity.
Therefore, both extremes are unfavorable for the conductive
properties of the mixture.
[0097] The melt blend can contain as high as about 40 percent to
about 50 percent by volume dispersed particles. By combining high
diluent concentrations with high volume percent of particles, a
high weight percent of particles can be achieved after the diluent
has been extracted from the phase separated polymer composition.
Preferably, the extracted and dried polymer material includes from
about 50 percent to about 98 percent particles and more preferably
from about 70 percent to about 98 percent by weight particles.
[0098] The diluent eventually is removed from the material to yield
a particle-filled, substantially liquid-free, porous
electrically-conductive polymeric material. The diluent may be
removed by, for example, solvent extraction, sublimation,
volatilization, or any other convenient method. Following removal
of the diluent, the particle phase preferably remains entrapped to
a level of at least about 90 percent, more preferably about 95 to
percent and most preferably about 99 percent, in the porous
structure. In other words, few of the particles are removed when
the diluent is eliminated, as evidenced by lack of particulates in
the solvent washing vessel.
[0099] The process is described below generally and can be varied
based on the teachings herein. In one embodiment of the TIPT
process, the particles are disposed beneath the surface of the
diluent, and entrapped air is removed from the mixture. A standard
high speed shear mixer operating at several hundred RPM to several
thousand RPM for about several minutes to about 60 minutes can be
used to facilitate this step. Appropriate high speed shear mixers
are made, for example, by Premier Mill Corp., Reading, Pennsylvania
and by Shar Inc., Fort Wayne, Ind.
[0100] If more dispersion is needed following the first mixing
step, it can be achieved through milling of the dispersion before
pumping the dispersion into the extruder, or through introduction
of dispersing elements into the extruder. For shear sensitive
polymers such as UHMWPE, most particulate dispersion preferably is
done prior to pumping the dispersion into the extruder to minimize
the shear needed in the extruder. If required, the second step
involves dispersing the particles in the diluent and may include
breaking down particle agglomerates to smaller agglomerates to
eliminate large clumps within the diluent. Complete dispersion to
primary particles generally is not necessary or desirable since
contact or proximity between conducting particles generally
promotes electrical conductivity.
[0101] The degree of preferred dispersion can be determined by
inspection of the final electrode film for surface roughness and by
determining its conductivity. The surface should be generally
smooth and uniform with no protrusions through the surface large
enough to be seen by eye. Insufficient dispersion of particulates
can result in films having rough surfaces with a texture of fine to
coarse sandpaper. In certain instances, no milling is needed since
the shear used simply to wet out the particulates results in
sufficient dispersion. Appropriate selection of components such as
the diluent and the initial particles can greatly facilitate the
dispersing step.
[0102] When additional dispersion is required or desired, the
diluent containing the particulate material can be processed in a
mill. Preferably, particle/diluent milling is carried out at
relatively high viscosity where the milling process is more
effective. Useful mills include, for example, attritors, horizontal
bead mills and sand mills. Typically, a single pass through a
horizontal bead mill at a moderate through-put rate (i.e., moderate
relative to the maximum through-put rate of the mill) is
sufficient. When significant amounts of dispersion are required,
milling times for recirculation of the dispersion through the mill
of less than an hour may be sufficient in some cases, while milling
times of at least about 4 to about 8 hours may be needed in other
cases.
[0103] An example of an appropriate instrument for processing small
batches is an attritor Model 6TSG-1-4, manufactured by Igarachi
Kikai Seizo Co. Ltd., Tokyo, Japan. This attritor has a
water-cooled with about a 1 liter volume which operates at about
1500 RPM with a capacity to process about 500 cc of material. For
larger batches, appropriate instruments include horizontal mills
such as those sold by Premier Mill Corp., Reading, Pa., in a
variety of sizes.
[0104] Milling reduces agglomerates to smaller agglomerates or
primary particles but generally does not break down primary
particles to smaller particles. Filtration of the milled dispersion
may be an optional step, if a greater number of larger particles
are present than desired. An appropriate filter would be, for
example, a model C3B4U 3 micron rope-wound filter made by Brunswick
Technitics (Timonium, Md.) to remove agglomerated particles or
particles larger than 3 microns, for example.
[0105] Filtering results in a more uniform article and allows
metering of the dispersions under pressure by close tolerance gear
pumps during the extrusion process without frequent breakdowns due
to large particles clogging the pump. After filtering, the
concentration of the particles can be determined, for example,
using a Model DMA-4S Mettler/Paar density meter manufactured by
Mettler-Toledo, Inc., Hightstown, N.J.
[0106] A dispersant can be added to the mixture of diluent and
particles to aid in stabilizing the dispersion of particles in the
diluent and in maintaining the particles as unaggregated. If a
dispersant is used, the diluent-particle mixture preferably
contains from about 1 percent to about 100 percent by weight of
dispersant relative to the weight of the particles.
[0107] Anionic, cationic and nonionic dispersants can be used.
Examples of useful dispersants include OLOA 1200.TM., a succinimide
lubricating oil additive, available from Chevron Chemical Co.,
Houston, Tex., or the Hypermer.TM. series of dispersants, available
from ICI Americas, Wilmington, Del.
[0108] The diluent-particle mixture generally is heated to about
150.degree. C. to degas the mixture before pumping the mixture into
an extruder. The mixture can be pumped into the extruder with or
without cooling the mixture to ambient temperature. The polymer is
fed typically into the feed zone of the extruder using a
gravimetric or volumetric feeder. (In an alternative embodiment, at
least some of the carbon is fed with the polymer into the
extruder.) For thermoplastic polymers, feed and melt zone
temperatures preferably are selected so that the polymer is at
least partially melted before contacting diluent. If the particles
are easily dispersed, the particles can be fed at a controlled rate
into the extruder, and the diluent separately metered into the
extruder. Also, a variety of in-line mixers are available that
provide for dispersion of particulates on a continuous in-line
basis from streams of particles and liquids. Alternatively, in
cases where adequate dispersion can be obtained in the extruder,
separate streams of polymer, diluent and conductive particles can
be fed directly into the extruder.
[0109] Then, a melt blend of the diluent-particle mixture is formed
with the polymer in the extruder. Following sufficient mixing in
the extruder, the melt blend is cast into the desired form.
Typically, since a film is desired, the melt blend is extruded onto
a temperature-controlled casting wheel using a drop die. A
twin-screw extruder is preferred.
[0110] Following formation of the desired shape of material, the
material is cooled, preferably rapidly, to induce the phase
transition. Quench conditions depend on film thickness, extrusion
rate, polymer composition, polymer-to-diluent ratio, and desired
film properties. Preferred conditions for a specific film can be
readily determined. For higher quench temperatures, film strength
may be diminished relative to films formed at lower quench
temperatures. Rapid cooling can be accomplished by, for example,
cooling in sufficiently cold air, cooling by contact on one or more
sides with a temperature-controlled casting wheel or immersion of
the material in a temperature-controlled liquid. Following
quenching, the diluent is removed. If solvent is used to remove the
diluent, remaining solvent is removed by evaporation.
[0111] For a given polymer-diluent combination, use of a casting
wheel, especially a smooth casting wheel, can result in an
asymmetric film. As the casting wheel temperature is lowered, it is
increasing likely that the resulting film will be asymmetric.
Typically, the side of the film toward the casting wheel has a
"skin" that is denser and has smaller pores. Alternatively, a
higher casting wheel temperature relative to the air temperature
can result in a denser surface layer on the air side. In general, a
lower casting wheel temperature produces a film that is stronger,
denser on the casting wheel side, and has a smaller bubble point
and higher Gurley value. Asymetric films can be produced by other
asymmetric quenching methods.
[0112] 2. Polymer-Fibrillation (PF) Process
[0113] The second preferred process for the formation of porous
electrode backing layers involves the preparation of a porous web
comprising conductive particles, such as carbon, metals, and the
like, enmeshed in a fibril forming polymer. The process includes
the formation of a mixture of the fibril forming polymer, a
lubricant and insoluble nonswellable particles such as conductive
carbon particles. The particles are approximately evenly
distributed in the composite and are enmeshed in the fibril forming
polymer. This process is adapted from the process outlined in U.S.
Pat. Nos. 4,153,661, 4,460,642, 5,071,610, 5,113,860, and
5,147,539, which are incorporated herein by reference.
[0114] Preferred fibril forming polymers include halogenated vinyl
polymers such as polytetrafluoroethylene (PTFE). Dry powder PTFE
such as Teflon.TM. 6C can be used as the starting material.
Alternatively, the process can be performed starting with a
commercially-available aqueous dispersion of PTFE particles, such
as Teflon 30.TM., Teflon 30b.TM. and Teflon 42.TM. (E.I. DuPont de
Nemours Chemical Corp., Wilmington, Del.), wherein water acts as a
lubricant for subsequent processing. Commercially available PTFE
aqueous dispersions may contain other ingredients such as
surfactants and stabilizers, which promote continued suspension of
the PTFE particles. In some applications, it is advantageous to
remove the surfactant, if present, by extraction at a desired point
in the process.
[0115] The lubricant must be selected such that the polymer is not
soluble in the lubricant. Preferred lubricants include water,
organic solvents and mixtures of water and miscible organic
solvents that can be conveniently removed by washing or drying. In
some circumstances water has a deleterious effect on the added
particles (i.e., causes unacceptable swelling or agglomeration) or
inhibits dispersion of the particles. Suitable organic lubricants
include, for example, alcohols, ketones, esters, ethers, and
fluorinated fluids. Fluorinated fluids include, for example,
perfluorinated compounds such as Fluorinert.TM. (3M, Saint Paul,
Minn.) or other competitive perfluorinated compositions.
"Perfluorinated" is used to indicate that substantially all of the
hydrogen atoms have been replaced by fluorine atoms. Electrode
backing layers containing carbon particles preferably are prepared
using a perfluorinated liquid lubricant. Preferably, the liquid
used is Fluorinert FC-40.TM., although other liquids such as
Fluorinert FC-5312.TM. can also be used. Alternatives also include
Galden.TM. and Fomblin.TM. perfluorinated fluids (Ausimont USA,
Thorofare, N.J.; Ausimont S.p.A., Montedison Group, Milan,
Italy).
[0116] Preferred nonpolymer particles have a solubility of less
than about 1.0 gram in 100 grams of lubricant at the mixing
temperature. The particles can be but do not need to be absorbent
or adsorbent with respect to the lubricant. The absorptive or
adsorptive capability of the particles with respect to lubricant
preferably is less than about 10 percent by weight and more
preferably less than about 1 percent. The particles preferably have
an average diameter less than about 200 microns, more preferably in
the range from about 0.01 microns to about 100.0 microns and more
preferably in the range from about 0.1 microns to about 10.0
microns. Generally, the nonpolymer particles are primarily or
exclusively conductive particles such as conductive carbon
particles. Due to the wetting properties of certain particles
including conductive carbon particles, non-aqueous, organic
lubricants are preferred when the particles are used in large
quantities.
[0117] Small amounts of additives such as various particulate
surface property modifiers can be added. Any additional additives
should be inert under the conditions of operation of the fuel cell.
Suitable additives include synthetic and natural polymers such as
polyethylene and polypropylene.
[0118] For electrode backing layers formed by the FP process, the
weight ratio of particles to polymer can be in the range from about
40:1 to about 1:4, preferably from about 25:1 to about 1:1, and
more preferably from about 20:1 to about 10:1. The lubricant
preferably is added in an amount exceeding the absorptive and
adsorptive capability of the particles by at least about 3 percent
by weight and below an amount at which the polymer mass loses its
integrity, more preferably by at least about 5 weight percent and
less than about 200 percent, even more preferably by at least about
25 percent and less than about 200 percent and yet even more
preferably by at least 40 percent and less than about 150 percent.
In one preferred embodiment, about 95 parts by weight of conductive
particles is used with about 5 parts by weight of PTFE, and the
weight ratio of inert fluid to solids (conductive particles plus
PTFE) is about 8:1.
[0119] The absorptive capacity of the particles is exceeded when
small amounts of lubricant can no longer be incorporated into the
putty-like mass without separation of lubricant. A large viscosity
change takes place corresponding to a transition from a paste to a
slurry. An amount of lubricant exceeding the absorptive and
adsorptive capacity of the particles should be maintained
throughout the entire mixing operation. Since the void volume and
porosity are controlled by the amount of lubricant used, the amount
of lubricant can be varied in order to obtain electrode backing
layers having a desired porosity and void volume. Generally,
increasing the amount of lubricant increases void volume and mean
pore size.
[0120] The mean pore size of the final article generally is in the
range from about 0.01 micrometers to about 10.0 micrometers, and
more preferably from about 0.1 micrometers to about 1.0
micrometers. With respect to distribution of pore size, preferably
at least about 90 percent of the pores have a size less than 1
micrometer. The void volume as measured by Mercury Intrusion
Porosity preferably ranges from about 10 percent to about 50
percent and more preferably from about 25 percent to about 35
percent. Typical Gurley values for webs of the invention range from
about 2 seconds per 10 cc to about 100 seconds per 10 cc.
Preferably, webs useful in the invention exhibit a Gurley values of
less than about 50 seconds per 10 cc and, more preferably less than
about 40 seconds per 10 cc.
[0121] The resistivity of the final article generally is in the
range from about 0.01 ohm-cm to about 10 ohm-cm, and more
preferably from about 0.1 ohm-cm to about 2.0 ohm-cm.
[0122] To practice the PF process, the materials are blended
together to form a soft dough-like mixture. If a solid powdered
polymer is used, a low surface energy solvent, as described above,
can be used to disperse the polymer into the mix. The blend is
mixed at a temperature and for a time sufficient to cause initial
fibrillation of the PTFE particles. The mixing temperature is
selected to maintain the solvent in liquid form. The temperature
preferably is in the range from about 0.degree. C. and about
100.degree. C., preferably from about 20.degree. C. and about
60.degree. C.
[0123] Initial fibrillation can take place simultaneously with the
initial mixing of the ingredients. If additional mixing is needed,
mixing times generally range from about 0.2 minutes to about 2
minutes to obtain initial fibrillation of the fibril forming
polymer. Initial fibrillation generally is optimum within about 90
seconds after the point when all components have been fully
incorporated together into a putty-like consistency. Mixing for
shorter or longer times may produce a composite sheet with inferior
properties. Preferably, mixing is ended after going through or
reaching a viscosity maximum. This initial mixing causes partial
disoriented fibrillation of the fibril forming polymer
particles.
[0124] Devices useful for obtaining the necessary intensive mixing
include commercially available mixing devices that sometimes are
referred to as internal mixers, kneading mixers, double-blade batch
mixers, intensive mixers and twin screw extruder compounding
mixers. Preferred mixers of this type include sigma-blade mixers
and sigma-arm mixers. Commercially available mixers of this type
include those sold under the designations Banbury.TM. mixer (Farrel
Corp., Ansonia, Conn.), Mogul.TM. mixer (Littelford Day Inc.,
Florence, Ky.), Brabender Prep.TM. mixer and Brabender.TM. sigma
blade mixer (C. W. Brabender Instruments, Inc., South Hackensack,
N.J.) and Ross.TM. mixers (AHing-Lander Co., Chesaire, Conn).
[0125] Following mixing, the putty-like mass is transferred to a
calendering device. The blend is subjected to repeated biaxial
calendering between calendering rolls to cause additional
fibrillation of the polymer. For typical lubricant/polymer
combinations, the calendering rolls preferably are maintained at a
temperature less than about 125.degree., more preferably at a
temperature from about 0.degree. C. to about 100.degree. C. and
even more preferably from about 20.degree. C. to about 60.degree.
C. Lubricant lost to evaporation can be replaced between passes
through the calender. During calendering, lubricant levels are
maintained at least at a level exceeding the absorptive capacity of
the solids by at least about 3 percent by weight, until sufficient
fibrillation occurs to produce the desired void volume and
porosity.
[0126] The calendering is repeated to form a self supporting tear
resistant sheet. The gap between the calendering rolls generally is
decreased with each successive pass. The material typically but not
necessarily is folded and rotated 90.degree. between passes through
the calender. The number of folds and gap settings can be adjusted
to yield the desired properties of the resultant sheet. As the
calendering is repeated, the tensile strength reaches a maximum
beyond which additional calendering becomes deleterious.
Calendering generally is stopped after the maximum tensile strength
is reached and before the tensile strength deteriorates below the
minimum acceptable tensile strength. Generally, about 10 to about
20 passes through the calendering rolls are appropriate. Once a web
of the desired thickness has been obtained, it can be air-dried at
room temperature or placed in a convection oven at an appropriate
temperature in order to remove excess inert fluid. Webs preferably
have a final thickness in the range of 0.1 to 1.0 mm, more
preferably 0.2 to 0.5 mm, and even more preferably in the range of
0.25 to 0.4 mm.
[0127] The resultant electrode backing layer preferably has a
tensile strength of at least about 1 megapascals and more
preferably at least about 3 megapascals. The sheets are
substantially uniformly porous with particles generally uniformly
distributed in a polymer fibril matrix. Almost all of the particles
are separated from each other yet the particles remain in
sufficient proximity such that good electrical conductivity is
obtained.
[0128] C. Additional Processing
[0129] It has been discovered that the performance characteristics
of particle-loaded electrode backing layers, especially those
produced with the TIPT process, can be significantly improved by
additional processing once the polymer films are formed. First, the
polymer electrode backing layer can be heated to a temperature near
the melting point of the polymer matrix. The temperature preferably
is in the range from about 20.degree. C. above to about 20.degree.
C. below the melting point of the polymer matrix, more preferably
at a temperature between the melting point and 10.degree. C. above
the melting point.
[0130] Preferably, the heating is performed for a period of time to
heat the polymer electrode up to the target temperature and for
polymer flow to occur. For laboratory evaluation, a period of about
10 minutes is sufficient to ensure that the film has equilibrated
at the temperature of the oven and for polymer flow to occur. This
period of time accommodates the inevitable loss of heat from an
oven and time for the oven to equilibrate at its set point. For
continuous in-line processing, much shorter residence times may be
sufficient to allow enough time for heating to the target
temperature and for polymer flow to occur. Surprisingly, this
heating does not destroy the porosity of the film, even with the
film being unrestrained during heating. This heating step
significantly reduces the electrical resistance in the electrode
backing layer while decreasing the Gurley and increasing the bubble
point value.
[0131] In addition, the electrode backing layers can be stretched.
Depending on the polymer, stretching generally can be carried out
effectively at a temperature from room temperature to about
20.degree. C. below the melting point of the polymer, as determined
by DSC. For highly particle filled films, stretching is preferably
carried out after extraction of the diluent at temperatures within
plus or minus 20 degrees C. of the melting point of the polymer, as
determined by DSC. Temperatures in this range would normally result
in loss of porousity for unfilled films with the diluent extracted.
While films normally are stretched after extraction of the diluent,
it is also possible to stretch the films with the diluent present,
in which case porosity may or may not develop.
[0132] For small scale evaluation work, a machine such as those
made by T. M. Long Co. (Sommerville, N.J.) can be used. The film is
inserted into the machine at the desired temperature and gripped by
all four edges such that the film can be stretched in one direction
(uniaxial) of both directions (biaxial). Biaxial stretching can be
performed sequentially or simultaneously. For in-line processing,
film can be stretched lengthwise using a device having a series of
rollers that can be set to rotate at increasingly higher rpm.
Stretching in the width direction can be accomplished by a device
referred to as a tenter. A tenter can have several zones that can
be heated to a desired temperature. Moving grips that ride on a
rail through the tenter grab the film by the edges. The spacing
between the two sets of grips on either side of the tenter can be
increased as the film moves through the tenter to accomplish the
desired degree of stretching. Available in-line equipment can be
simultaneous biaxial stretching.
[0133] In general, bubble point increases and Gurley value
decreases as the stretch ratio (the ratio of final film dimension
to initial film dimension) increases, although an extremum
frequently is reached such that higher stretch ratios result in a
lower bubble point and higher Gurley value. The thickness of the
film generally is reduced by stretching. In the case of conductive
carbon particle-filled porous films, stretching has similar effects
on bubble point and Gurley value as with unfilled films but also
tends to increase the resistivity of the film. Careful optimization
is needed to balance suitably the bubble point, Gurley value and
resistivity. In contrast, stretching tends to reduce the
resistivity of porous films loaded with metallic particles such as
tungsten. Unstretched films containing high loadings of tungsten
have high resistivity, which decreases as the stretch ratio is
increased.
[0134] D. MEA Formation
[0135] The catalytic, electrode layer generally is formed as an
integral part of either the ion conducting membrane or the
electrode backing layer. In either case, an electrode backing layer
is placed on each side of the ion conduction membrane with a
catalyst layer between each electrode backing layer and ion
conducting membrane to form the 5-layer MEA. The electrode backing
layers and the ion conducting membrane must be held closely
together in order to reduce resistance to ionic and/or electrical
flow between the elements.
[0136] The elements can be held together by stack pressure,
generally with a container ultimately applying the pressure.
Preferably, the elements are laminated together. Lamination
supplies the physical proximity, as an alternative to stack
pressure. Surprisingly, the lamination step can be performed with
particle-filled, porous polymer components without destroying the
porous characteristic or structural integrity of the elements.
[0137] The lamination step should form cohesive association between
the five layers of the MEA. Selection of appropriate conditions for
the lamination is based on the specific materials used. Particular
examples are described below in the Examples. Lamination conditions
should not compromise membrane properties such as porosity, surface
wetting and electrical resistance.
[0138] The objective of the lamination is to eliminate the physical
gap between the layers. Cohesion or self-adhesion of polymers of
the different layers can be promoted by increasing the total area
of contact, thus increasing the probability of diffusional
interlacing of polymer chains at the areas of contact. Some
preferred polymer components described above are more compressible
than typical polymer films. Increased compressibility makes
pressure more effective in increasing contact area. Evidently, the
particulate filler in the polymer, electrode backing layer helps to
inhibit the collapse of the pores during the lamination.
[0139] Lamination can be accomplished in a variety of ways. These
approaches include the use of heat lamination, pressure lamination
or solvent lamination. Heat lamination and solvent lamination also
can involve some addition of pressure. The appropriate methods for
lamination depend on the materials.
[0140] Continuous roll processing of the MEA greatly enhances the
efficiency of fuel cell production. For example, the 5-layer MEA is
fabricated as a continuous web 200 of identical repeating MEAs 202,
i.e., as illustrated in FIG. 3. On the continuous web of MEAs 200,
catalyst electrode areas 204, including catalyst layers 206 and
electrode backing layers 208, can be applied patch-wise or
continuously on each side to ion conduction membrane 210, supplied
in roll form. Similarly, appropriate seals and gaskets 212, defined
by the mating surfaces of the bi-polar plates, can be applied at
the appropriate locations on roll membrane 210 adjacent catalyst
electrode areas 204. Holes 214 are punched at appropriate locations
at the center of seals or gaskets 216. The boundary between
adjacent MEAs can be indicated for cutting or partially perforated
for fast and easy separation during the stack assembly process. In
addition, registration marks can be applied at the appropriate
spots to facilitate robotic pick-up and alignment during the stack
assembly process.
[0141] If catalyst layer 206 is associated with electrode backing
layer 208, the combined layers can be attached or laminated to ion
conduction membrane 210. Alternatively, catalyst layer 206 and
electrode backing layer 208 can be associated with membrane 210
sequentially. Suitable methods for attaching or applying catalyst
layer 206 to ion conduction membrane 210 depends on the type of
catalyst layer 206. For dispersions of carbon particle supported
catalysts, methods such as those taught in U.S. Pat. No. 5,211,984,
incorporated herein by reference, using heat and pressure can be
used. Nanostructured catalyst layers as taught in U.S. Pat. No.
5,338,430 can be embedded in the surface of membrane 210 using
nip-roll calendering or rapid static pressing of a continuous roll
supply of the nanostructured catalyst into a continuous roll supply
of membrane 210. The catalyst can be applied in a patch-wise
fashion from a continuous roll carrier holding the catalyst in the
desired pattern.
[0142] Electrode backing layers 208 then can be applied in registry
with catalyst electrode area 204 of ion conduction membrane 210 in
a patch-wise fashion. Electrode backing layers 208 and catalyst
layer 206 can also be applied in a continuous roll supply rather
than in patch-wise fashion. Various attachment methods can be used
for securing the electrode backing layers 208 prior to stack
assembly. Suitable attachment methods for electrode backing layer
208 include pressure lamination, heated nip-roll lamination,
limited area adhesive attachment (to avoid blocking all pores with
adhesive), ultrasonic welding, microstructured surface mechanical
attachment and the like. A secure bonding of electrode backing
layer 208 with membrane 210 generally is desirable to minimize
electrical and/or ionic resistivities across the interface between
them, or to facilitate water management at the interface,
especially the cathode interface. The parameters of the attachment
process can be adjusted to provide the preferred degree of bonding.
More secure bonding is especially desirable when catalyst layer 206
is applied first to electrode backing layer 208. Important
requirements for the attachment methods are that the gas transport
properties of electrode backing layers 208 are not adversely
affected, that catalyst layers 206 are not poisoned and that ion
conduction properties of the membrane 210 are not degraded
[0143] Seals and gaskets 212, 216 can be fabricated or die-cut from
any suitable laminar web material, such as Teflon.TM. sheeting or
Teflon.TM. coated fiberglass sheeting available from The Furon Co.,
CHR Division (New Haven, Conn.) or other fluoroelastomers. The seal
material can be applied to perimeter seal points 212 or gas port
edges 216 of MEA roll 200. Attachment of the seals and gaskets to
the membrane at those points can be done using attachment methods
similar to those described above for attaching the electrode
backing layers. In addition to the non-adhesive, laminar web seal
materials, appropriate transfer adhesives also can be used. An
example of such a transfer adhesive is #9485 PC adhesive available
from 3M Co. (Saint Paul, Minn.)
[0144] The seals and gaskets materials and corresponding adhesives
should not contain chemicals that can be extracted by the ion
conduction membrane to lower its conductivity or poison the
catalyst. Also, the seals and gaskets materials should be
chemically and thermally inert to withstand the acidic environment
(for proton exchange fuel cells) and operating temperatures of the
fuel cell for thousands of hours. Furthermore, seals and gaskets
112, 116 should have adequate mechanical properties to have high
resistance to creep and extrusion at the maximum operating
temperatures of the stack under stack-applied compressive forces in
the direction normal to the seal areas and under forces acting in
the plane of the seals generated by internal pressure.
[0145] E. Stack Formation
[0146] A typical fuel cell stack may require more than a hundred
cells to be assembled in series to obtain useful voltages. A
hundred cells in series, each operating at a nominal 0.7 volts,
would yield a 70 volt stack. Assembly of the MEAs and
bi-polar/cooling plates with all the attendant gaskets and seals to
produce a leak free, optimally compressed fuel cell stack can be a
critical issue for reducing the cost of the stack Providing the
MEAs, seals and gaskets in a manufacture-ready format to facilitate
cost effective assembly of a stack is an important issue. For
example, to assemble only 10,000 fuel cell stacks per shift per
production line per year requires one stack with hundreds of
associated cell components to be assembled approximately every 10
minutes. Producing and handling such a large number of components,
each sized, cut, oriented and held in proper registry, in such a
short time is a significant consideration
[0147] In a fuel cell stack 300, each individual cell consists of a
5-layer MEA 302 that is sandwiched between bi-polar plates 304, as
shown in FIG. 4. End plates 306 provide for flow of fuel and
oxidizing agent into and out from fuel cell stack 300. The bi-polar
plates function to a) provide the series connection between cells
by conducting the total electrical current produced by an MEA to
the adjacent cell for eventual transmission at the end plates, b)
prevent any gas transport between adjacent cells, c) provide
mechanical rigidity to the assembled stack such that compressive
forces are effective to minimize leakage of gases past the
perimeter of the MEAs, d) provide flow field grooves and gas
manifold ports for introducing the fuel and oxidant to the MEA
catalyst electrodes and for removal of by-products such as water
and e) provide contact with cooling fluids to extract waste heat
from the cell electrode areas.
[0148] Although there can be many possible configurations and
shapes for fuel cell stacks, generally they are rectilinear or
cylindrical in shape so that the individual planar MEAs and
bi-polar plates within each cell have a corresponding rectangular
or circular shape. U.S. Pat. No. 5,252,410, incorporated herein by
reference, teaches many aspects of bi-polar plates and stack
assemblies including specific aspects for the case in which the
catalyst is applied to the electrode backing layer. The active,
catalyzed area of each MEA generally is smaller than the membrane
area and can be centered on the MEA. The perimeter area of the
membrane bordering the electrode area generally is used for sealing
the MEA to the bi-polar plates, to prevent leakage of fuel and
oxidant from the pressurized interior of the cell. Compressive
forces applied from the end plates of the stack should be
sufficient to keep the gaskets or seals from delaminating at the
maximum internal pressures. The region of the MEA adjacent to the
electrode area may also contain holes for transmission of fuel and
oxidant to the cells from the respective gas supply manifolds.
These holes (i.e., gas ports) also may require seals or gaskets to
prevent leakage.
[0149] Above, a process is described for fabricating MEA's and
supplying the MEAs with appropriate seals and gaskets in a
continuous web format. The continuous web format is extremely well
suited for producing and handling the large number of MEA elements
used to construct the fuel cells at a cost effective rate. The
continuous web is not only well suited for relatively rapid
application to a bi-polar plate but also for accurate alignment of
the MEA. Therefore, the electrode backing layers as described
herein when adapted to the production of a 5-layer MEA in a
continuous roll format yield dramatic advances in fuel cell
processing.
EXAMPLES
[0150] Several properties are measured for the various electrode
backing layers produced in the following examples. Bubble point is
the largest pore size in the film as determined according to ASTM
F-316-80. Ethanol was used as the test liquid. The liquid is used
to fill the pores of the film. Pressure is applied until flow as
bubbles takes place through the largest passageway through the
film. The bubbles are observed from a tube that is connected to the
low pressure side of the test cell and that is submerged in water.
The necessary pressure depends on the surface tension of the test
liquid and the size of the largest passageway. Bubble point in
microns, using ethanol as the test liquid, is equal to
9.25/pressure in psi at breakthrough.
[0151] Gurley value is a measure of resistance of air flow through
a film. Specifically, it is a measurement of the time in seconds
for 100 cc (or other selected volume) of air to pass through one
square inch of a film at a pressure of 124 mm of water, according
to ASTM D-726-58, Method As The film sample is clamped between two
plates. Then, a cylinder is released that provides air to the
sample at the specified pressure. The time for a given amount of
air flow is determined from the marks on the cylinder, which are
read electronically. In the Examples, Gurley values are reported
for passage of 50 cc or 10 cc of air.
[0152] The in-plane electrical resistance is measured using two,
1.5 cm wide aluminum bars that are placed parallel to each other on
the surface of the film . Weights were placed on top of the bars to
give a pressure of 300 g/cm.sup.2. The results generally are
pressure dependent. The resistance between the two aluminum bars
was measured using a standard ohm meter. Alternatively, z-axis
electric resistance was measured at high current densities, as
described in Example 6, below. The resistivity in ohm-cm was
calculated using the following equation:
resistivity=(z-axis resistivity.times.area of film/thickness of
film)
or
resistivity=(in-plane resistance.times.width of the film.times.film
thickness)/distance between bars
[0153] The aqueous contact angle measurements described below were
performed essentially as described in WO 96/344697. Briefly, using
a commercial apparatus (Rame'-Hart contact Angle Goniometer, Model
100), an approximately 1 microliter droplet was expressed out a
hypodermic syringe. Carefully raising the sample surface to just
contact the droplet while still suspended from the syringe defined
the "equilibrium contact angle". The droplet was then enlarged or
shrunk while measuring the contact angle to obtain the advancing
and receding contact angles, respectively. Multiple measurements
were made and the mean and rms deviation obtained for both types of
contact angles at multiple points on the surface. Membranes
exhibiting a higher receding contact angle repel water to a greater
extent that those exhibiting a lower receding contact angle.
Without wishing to be bound by theory, it is believed that
membranes that repel water to a greater extent are less likely to
be flooded during the operation of a fuel cell, and will allow
better flow of fuel and oxidant to the membrane/catalyst
interface.
[0154] In the Examples:
[0155] "room temperature" or "ambient temperature" is taken as
approximately 22.degree. C.;
[0156] Vertrel 423.TM. is dichlorotrifluoroethane
(CHCl.sub.2CF.sub.3), from DuPont Chemicals, Inc., Wilmington,
Del.; and
[0157] All other chemicals and reagents were obtained from Aldrich
Chemical Co., Milwaukee, Wis., unless otherwise specified.
[0158] There are a number of basic processes and materials in
common within the examples. These include the preparation of the
nanostructured catalyst support, application of the catalyst to the
support, determination of the catalyst loading, fabrication of the
membrane-electrode assembly, the type of fuel cell apparatus and
testing station, the fuel cell test parameters, and the kinds of
proton exchange membranes used. These are defined in general as
follows:
[0159] a) Nanostructured catalyst support preparation and catalyst
deposition. In the following examples, the nanostructured catalyst
electrodes and the process for making them are as described U.S.
Pat. No. 5,338,430 and other patents referenced therein. The
nanostructured catalyst consists of catalyst materials, e.g. Pt
coated onto the outer surface of nanometer sized whisker-like
supports. The whiskers are produced by vacuum annealing thin films
(1000-1500 Angstroms) of an organic pigment material (C.I. Pigment
Red 149, or PR149) previously vacuum coated onto substrates such as
polyimide. The whisker-like supports, with lengths of 1-2
micrometers, grow with uniform cross-sectional dimensions of 30-60
nanometers, end-oriented on a substrate to form a dense film of
closely spaced supports (30-40 per square micrometer) which can be
transferred to the surface of a polymer electrolyte to form the
catalyst electrode. The nanostructured catalyst electrode has a
very high surface area which is readily accessible to fuel and
oxidant gases.
[0160] b) Measurement of the catalyst loading is done both by
monitoring the thickness of the Pt layer deposited during vacuum
coating using a quartz crystal oscillator, as is well known in the
art of vacuum coating, and by a simple gravimetric method. In the
later case, a sample of the polyimide supported nanostructured film
layer is massed using a digital balance accurate to 1 microgram,
and its area measured. Then the nanostructured layer is wiped off
the polyimide substrate using a paper tissue or linen cloth, and
the substrate is remassed. Because a preferred property of the
catalyst support is that it transfer easily and completely to the
ion exchange membrane, it also can be easily removed by simple
wiping with a cloth. The mass per unit area of the catalyst support
particles, without Pt, can also be measured this way.
[0161] c) The ion exchange membranes used were all of the
perfluorinated sulfonic acid type. Nafion.TM. 117 or 115 membranes
were obtained from DuPont Corp., Wilmington, Del.
[0162] d) The process used for transferring the catalyst coated
support particles into the surface of the PEM or DCC was a static
pressing or a continuous nip-rolling method. To prepare an MEA with
e.g. the catalyst on the PEM, with 5 cm.sup.2 of active area by the
static pressing method, two 5 cm.sup.2 square pieces of the
nanostructured catalyst, coated on a metallized polyimide
substrate, one for the anode, one for the cathode, are placed on
either side of the center of a 7.6 cm.times.7.6 cm proton exchange
membrane. At least one 25 or 50 micrometer thick sheet of
polyimide, of the same size as the PEM, is placed on each side of
the PEM and nanostructured substrate stock to form a stack. For
static pressing, one sheet of 50 micrometers thick Teflon.TM., of
the same size as the PEM, is placed on each side of the PEM,
nanostructured substrate and polyimide stack.
[0163] For static pressing, this assembly is then placed between
two steel shim plates, and pressed at a temperature near
130.degree. C. and pressures approaching 10 tons/cm.sup.2 for up to
two minutes, using a nine inch Carver.TM. press. A low grade vacuum
may be applied to partially remove air (2 Torr) from the stack just
prior to applying the maximum pressure. Before releasing stack
pressure, the stack can be cooled usually for 5 minutes or less to
near room temperature. The original 5 cm.sup.2 polyimide substrates
are then peeled away from the PEM leaving the catalyst attached to
the surface of the PEM. (Alternatively the catalyst support
particles can be transferred to the PEM or electrode backing by
continuous roll processes such as passing the above sandwich
assemblies in continuous or semi-continuous sheet form through the
nip of a mil as in calendering or laminating processes. The two
mill rolls can be heated, both made of steel, or steel and a softer
material such as rubber, have a controlled gap or use controlled
line pressure (kg/cm) to determine the gap of the nip.
[0164] e) The MEA's from step d) were mounted in a fuel cell test
cell purchased from Fuel Cell Technologies, Inc., Albuquerque, N.
Mex., generally a 5 cm.sup.2, but up to 50 cm.sup.2, sized cell.
Two pieces of 0.015" thick ELAT electrode backing material,
obtained from E-tek, Inc., Natick, Mass. was used as control
electrode backing material. Teflon coated fiberglass gaskets,
purchased from CHR Industries, nominally 250 micrometers thick,
with 10 cm.sup.2 square holes cut in the center (for the 10
cm.sup.2 catalyst area), were used to seal the cell. The ELAT
electrode backing material is designated as carbon only, i.e. it
contains no catalyst.
[0165] f) The test parameters for the fuel cell polarization curves
of examples 9-14 and 28, were obtained under the conditions of 207
kPa H.sub.2 and 414 kPa oxygen gauge pressures with a cell
temperature of 80.degree. C., flow rates of approximately one
standard liter per minute. The humidification of the gas streams
was provided by passing the gas through sparge bottles maintained
at 115.degree. C. and 80.degree. C. respectively for the hydrogen
and oxygen.
[0166] For examples 15-17, polarization curves were obtained to
test the low pressure air performance of the electrode backing
materials. The curves in FIG. 12 were obtained under the conditions
of 207 kPa H.sub.2 and 34.5 kPa air gauge pressures. The
H.sub.2/air flow rates were 400/400 sccm (standard cubic
centimeters per minute) for 10 cm.sup.2 MEAs, and 1 standard liters
per minute (slm)/2 slm for the 50 cm.sup.2 MEAs. The humidification
of the gas streams was provided by passing the gas through sparge
bottles maintained at about 115.degree. C. and 65.degree. C.,
respectively, for the hydrogen and air. The cell temperature was
75.degree. C. A membrane produced following the procedures
described in Example 12 was also run under these fuel cell
conditions. The results are plotted in FIG. 12 as Ex. 12 (air).
Example 1
Conductive Carbon in High Density Polyethylene
[0167] A dispersion of conductive carbon in mineral oil was
prepared by wetting out 1032 g of Conductex.TM. 975 conductive
carbon (Colombian Chemicals Co., Atlanta, Ga.) into a mixture of
2054 g of mineral oil (Superla.RTM. White Mineral Oil No. 31,
AMOCO, Chicago, Ill.) and 1032 g of dispersant, OLOA 1200.TM.
(Chevron Oil Co., San Francisco, Calif.) using a model 2500 HV
dispersator (Premier Mill Corp., Reading, Pa.). Portions of the
carbon and OLOA 1200 were added alternately to the mineral oil. As
the carbon was added, the viscosity increased and the dispersator
rpm increased accordingly to a maximum of about 5000 rpm after all
the carbon and OLOA 1200 had been added
[0168] The resultant dispersion was viscous and lumpy. It was then
heated to about 150.degree. C. while continuing to mix with the
dispersator to degas it. The viscosity decreased as the temperature
increased; the dispersator rate was reduced to 1100 rpm as the
temperature increased. The mixture was held at about 150.degree. C.
for 20 min. The dispersion became smoother with continued mixing
and heating. It was then allowed to cool to about 60.degree. C.
while continuing to mix. The resulting mixture was passed through a
1.5 L horizontal mill (Premier Mill Corp.) containing an 80 vol. %
charge of 1.3 mm diameter chrome-steel beads. The horizontal mill
was operated at a peripheral speed of 1800 fpm (54.9 meters/minute)
and at a through put rate of about 0.5 L/min.
[0169] The dispersion discharged from the horizontal mill was
pumped at about 60.degree. C. into an injection port on the third
zone of a Berstorff.TM. co-rotating twin screw extruder (25
mm.times.825 mm, Berstorff Corp., Charlotte, N.C.). HDPE (high
density polyethylene, grade 1285, Fina Oil & Chemical Co.,
Houston, Tex.) was metered into the feed zone (zone 1) at a rate of
0.55 kg(1.20 lb.)/hr. and the above dispersion was pumped in at a
nominal rate of 69.1 cc/min. using a gear pump. The extruder
profile starting from the feed zone was 193, 254, 254, 204, 166,
160, 166.degree. C., the die temperature was 166.degree. C., and
the screw speed was 120 rpm.
[0170] Film was extruded through an 20.32 cm (8 in.) die onto a
patterned casting wheel heated to 52.degree. C. The wheel pattern
had 45.degree., four-sided pyramids that were 0.125 mm (5 mil) high
at a density of 100 per 6.45 sq. cm ( 1 square inch). The resultant
film was 0.25 mm (10 mil) thick and the experimentally determined
total film throughput rate was 4.53 kg (10.0 lb)/hr. Thus, the
actual dispersion feed rate was 3.99 kg (8.8 lb)/hr. From this and
the known dispersion composition, the total carbon content in the
film after extraction of the oil was calculated to be 64.7 wt.
%.
Example 2
Extraction Using Vertrel 423.TM.
[0171] Oil and OLOA 1200 were extracted from the film of Example 1
in three washes by soaking a portion of the film measuring about 18
cm by 30 cm in about 1 L Vertrel 423.TM. solvent per wash for 10
minutes per wash. On drying at room temperature, film thickness was
0.0241 cm. Physical properties of the film are shown in Table
1.
Example 3
Extraction Using Toluene/Xylenes
[0172] Oil and OLOA 1200 were extracted from a portion of the film
of Example 1 as described in Example 2, using a 1:1 v/v mixture of
toluene/xylenes. Physical properties of the dried film, measuring
0.023 cm thick, are shown in Table 1.
Example 4
Post-extraction Heating, Vertrel 423.TM. extraction
[0173] A portion of the film prepared as described in Example 2 was
hung in a circulating air oven for ten minutes at 130.degree. C. On
cooling, the film measured 0.23 mm thick. Physical properties of
the film, labeled "Example 4A", are shown in Table 1. Advancing and
receding contact angles (water) for the film were 158.degree. and
107.degree., respectively.
[0174] Likewise, a portion of the film from Example 2 was heated in
a circulating air oven for 10 minutes at 150.degree. C. Physical
properties of the film, labeled "Example 4B", are shown in Table
1.
Example 5
Post-extraction Heating, Toluene/Xylenes Extraction
[0175] Films prepared as described in Example 3 was hung in a
circulating air oven for ten minutes at 130.degree. C. On cooling,
the film measured 0.23 mm thick. Physical properties of the film,
labeled "Example 5A", are shown in Table 1.
[0176] Likewise, a portion of the film from Example 3 was heated in
a circulating air oven for 10 minutes at 150.degree. C. Physical
properties of the film, labeled "example 5B", are shown in Table
1.
1TABLE 1 X-Y Extraction Heated, Bubble Gurley no., resistivity*
Example Solvent .degree. C. Point, .mu.m sec/50 cc ohm-cm 2 V.sup.1
No 0.10 310 6.7 3 T/X.sup.2 No -- -- 0.97 4A V 130 0.15 180 0.75 4B
V 150 -- -- 0.67 5A T/X 130 0.14 105 0.53 5B T/X 150 -- -- 0.53
.sup.1Vertrel 423 .TM. .sup.2Toluenc/Xylenes (1:1 v/v) *low current
measurement between parallel aluminum bars with 0.5 kg/cm.sup.2
applied pressure
[0177] As shown in Table 1, heating the film above the melting
point of the HDPE binder (126.degree. C., peak temperature by DSC)
resulted in a significant decrease in Gurley, a significant
increase in bubble point, and a significant decrease in
resistivity.
Example 6
Film Impedance
[0178] The ability of carbon-loaded films of the invention to carry
large current densities, suitable for fuel cells, was demonstrated.
Two 5 cm.sup.2 square samples of the film from Example 4A (Vertrel
423.TM. extraction, 130.degree. C. heating) were mounted face
-to-face in direct parallel contact with one another in a fuel cell
test fixture (2.24 cm.times.2.24 cm, Fuel Cell Technologies, Inc.,
Santa Fe, N. Mex.), i.e., there was no intervening ion conductive
membrane between the samples. Masking frames of Teflon.TM.
impregnated fiberglass 0.015 cm thick were used between the test
cell halves as commonly used in actual fuel cell tests, to prevent
crushing the films to be examined. Cell bolts were torqued to 12.4
N-m (110 in-lbs). High current levels were passed through the cell
at various voltages to measure the impedance of the films under
high current density conditions. Results of these measurements are
shown in FIG. 6, trace A. After measurements were taken, the
combined thickness of the films was 0.042 cm. Resistivity of the
films was measured as 0.57 ohm-cm, comparable to the value shown in
Table 1.
Example 7
Film Impedance
[0179] Films prepared in Example 5A (toluene/xylenes extraction,
130.degree. C. heating) were examined as described in Example 6 to
measure their impedance. Results are shown in FIG. 6, trace B.
Measured resistivity of these films, having a combined thickness of
0.042 cm, was 0.52 ohm-cm.
Example 8 (Comparative)
Film Impedance
[0180] The resistivity of a carbon-only material (woven graphite
cloth impregnated/coated with carbon black/PTFE) commercially
available as ELAT.TM. (Etek, Inc., Natick, Mass.) was examined as
described in Example 6. Results are shown in FIG. 6, trace C.
Combined thickness of the ELAT.TM. films was 0.094 cm, giving an
effective bulk resistivity of 0.28 ohm-cm. The Gurley value of the
ELAT.TM. material was measured to be 7.5 sec/50 cc, and the
advancing and receding contact angles of the film were 155.degree.
and 133.degree., respectively.
Example 9
Membrane Electrode Assembly
[0181] A proton exchange membrane electrode assembly (MEA) was
prepared by applying an electrode layer comprising platinum-coated
nanostructured supports, as described in U.S. Pat. No. 5,338,430,
the teachings of which are incorporated herein by reference, to the
central portion of a 7.6 cm.times.7.6 cm square Nafion.TM. 117 ion
exchange membrane (DuPont Chemicals Co., Wilmington, Del.). The
platinum-coated nanostructured supports were applied to both sides
of the ion exchange membrane using a hot platen press as described
in Example 5 of the above-incorporated '430 patent. The centered
electrode area was 5 cm.sup.2. Two 5 cm.sup.2 pieces of the
carbon-filled electrode backing layer formed as described above in
Example 5A (toluene/xylenes extraction, 130.degree. C. heating)
were placed on either side of the electrode assembly, to form a
5-layer MEA. The assembly was mounted in a 5 cm.sup.2 test cell and
tested on a fuel cell test station (Fuel Cell Technologies, Inc.),
using hydrogen/oxygen gas flows applied to respective sides of the
assembly. FIG. 7, trace A, shows a polarization curve of voltage
vs. current density produced with this assembly.
Example 10
Membrane Electrode Assembly
[0182] A membrane electrode assembly was prepared as described in
Example 9 except that an electrode backing layer as described in
Example 3 (toluene/xylene extraction, no heating) was used. In
addition, the entire assembly comprised 50 cm.sup.2 electrodes and
electrode backing membranes, rather than 5 cm.sup.2. FIG. 7, trace
C, shows a polarization curve of voltage vs. current density
produced by this assembly. The improved performance of this cell
can be attributed, in part, to the larger electrode size.
Example 11 (Comparative)
Membrane Electrode Assembly
[0183] A membrane electrode assembly was prepared as described in
Example 9 except that the ELAT.TM. material described in Example 8
was used as the electrode backing layer. FIG. 7, trace B, shows a
polarization curve of voltage vs. current density produced by this
assembly.
[0184] Examples 9-11 show that an effective electrode backing layer
of the invention can be prepared by the TIPT method. A premium
grade commerically-available membrane provided better fuel cell
performance, perhaps due, in part, to a lower Gurley value and a
higher receding contact angle. A lower Gurley value and a higher
receding contact angle may be indicative of higher diffusion of
hydrogen and oxygen to the catalyst/electrolyte interface and
lesser susceptibility to flooding with water produced at the
cathode, which would further limit oxygen transport.
Example 12
Graphite/Conductive Carbon (95/5) in Ultrahigh Molecular Weight
Polyethylene (TIPT)
[0185] A dry blend of 37.11 g MCMB 6-28 graphite (nominally 6.mu.
mean diameter, Osaka Gas Chemical Co., Osaka, Japan) and 1.91 g
Super P conductive carbon (MMM Carbon Div:, MMM nv, Brussels,
Belgium) was prepared using a spatula for mixing. Portions of this
mixture and portions of 32.2 g mineral oil (Superla.RTM. White
Mineral Oil No. 31) were added alternately to the mixing chamber of
a Haake Rheocord.TM. System 9000 (Haake (USA), Paramus, N.J.)
equipped with roller blades. The mixing chamber was at 60.degree.
C., while mixing at 50 rpm. Then, heating to a set point of
150.degree. C. was begun.
[0186] When the mix temperature reached 120.degree. C., 2.06 g of
ultrahigh molecular weight polyethylene (UHMWPE, grade GUR 4132,
Hoechst Celanese Corp., Houston, Tex.) was added in portions with
time allowed between additions for the previous material to be
assimilated. The ratio of UHMWPE/oil was 6/94. After this addition
was completed, the temperature of the chamber was increased to
150.degree. C., and the mixing rate was increased to 80 rpm. Mixing
was continued for 10 min. after the addition of the UHMWPE had been
completed. The mixture was removed from the mixer while still
hot.
[0187] After cooling, 15 g of solidified mixture was placed between
0.175 mm (7 mil) polyester sheets and placed in a Model 2518
Carver.TM. press (Fred S. Carver Co., Wabash, Ind.) at 160.degree.
C. with 0.25 mm (10 mil) shims placed between the polyester sheets.
After heating in the press for 3 min. with no applied pressure, the
mixture was pressed for 10 sec. using 690 kPa (100 psi). The
resultant film with polyester sheets still attached was immersed
into water at ambient temperature to quench it. The oil was
extracted from the film as described in Example 2. A portion of the
film was heated at 130.degree. C. for 10 min. in a circulating air
oven, as described in Example 4. Physical properties of the film
are shown in Table 2. Advancing and receding contact angles (water)
for the film were 154.degree..+-.10 and 101.degree..+-.5,
respectively.
2TABLE 2 After Washing/ Drying, Before After Heating for 10
Parameter Heating min. at 130.degree. C. Caliper, mm 0.215 0.215
Gurley (sec./50 cc) 95 60 Bubble Point (microns) 0.23 --
Resistivity (ohm-cm) 7.3 4.6
[0188] Curve A in FIG. 8 shows a representative polarization curve
from a fuel cell test using a 50 cm.sup.2 cathode backing layer
made after heating, as described in this example. The same catalyst
coated ion conduction membrane was used to obtain the fuel cell
polarization curves for the 5-layer MEAs using different electrode
backing layers of Examples 12-14 and the ELAT.TM. control (curve
"D" in FIG. 8). After testing one sample electrode backing layer,
the test cell was opened, and the electrode backing layer replaced
on the cathode with the next one. The ELAT.TM. anode backing layer
remained unchanged.
Example 13
Graphite/Conductive Carbon (95/5) in Polypropylene (TIPT)
[0189] A mixture of MCMB 6-28 graphite and mineral oil
(Superla.RTM. White Mineral Oil No. 31) was prepared by mixing 83.3
g of graphite into 91.9 g of mineral oil using a dispersator. Super
P conductive carbon, 1.53 g, was poured into the mixing chamber of
a Haake Rheocord.TM. System 9000 mixer equipped with roller blades
at 100.degree. C. Then, while mixing at 50 rpm, 59.73 g of the
graphite/mineral oil mixture was poured into the mixing chamber. As
the viscosity increased during the addition of the graphite/mineral
oil mixture, the mixing rate was increased to 100 rpm. Then, 7.66 g
of polypropylene (grade DS5D45 from Shell Chemicals, Houston, Tex.)
were added. The mixture was heated to 230.degree. C. over a period
of about 10 min. Total mixing time after addition of polypropylene
was about 33 min. The resultant mixture was removed from the mixer
while hot.
[0190] After cooling, 14.2 g of the solidified mixture was placed
between 0.175 mm (7 mil) polyester sheets, which had been coated
with a thin coating of mineral oil to facilitate release, and
placed in a Carver press at 160.degree. C. with 0.25 mm (10 mil)
shims placed between the polyester sheets. After heating in the
press for 3 min. with no applied pressure, the mixture was pressed
for 10 sec. using 345 kPa (50 psi). The resultant film with
polyester sheets still attached was immersed in water at ambient
temperature to quench it. Oil was extracted from the film as
described in Example 2. A portion of the film was heated in a
circulating air oven for 10 min. at 180.degree. C. Physical
properties of the film before and after heating are shown in Table
3. It was noted that the film became somewhat brittle after this
heating procedure. Advancing and receding contact angles (water)
for the film were 155.degree..+-.5 and 100.+-.5.degree.,
respectively.
3TABLE 3 After Washing/ Drying, Before After Heating for 10
Parameter Heating min. at 180.degree. C. Caliper (mm) 0.205 0.200
Gurley (sec./50 cc) 32 -- Bubble Point (microns) 0.66 --
Resistivity (ohm-cm) 8.96 1.5
[0191] Trace B in FIG. 8 shows a representative polarization curve
with the cathode electrode backing layer made from the heated film.
It displays an improved performance relative to the sample from
Example 12.
Example 14
Graphite/Conductive Carbon (95/5) in Ultrahigh Molecular Weight
Polyethylene (TIPT)
[0192] A dry blend of 27.89 g MCMB 6-28 graphite, and 1.47 g Super
P conductive carbon, was prepared using a spatula for mixing.
Portions of this mixture and portions of 37.1 g mineral oil
(Superla.TM. White Mineral Oil No. 31) were added alternately to
the mixing chamber of a Haake Rheocord.TM. System 9000 mixer
equipped with roller blades at 40.degree. C. while mixing at 50
rpm. Then, 1.55 g of UHMWPE (grade GUR 4132, Hoechst Celanese
Corp.) were added. The ratio of UHMWPE/oil was 4/96. After addition
of the polymer was completed, the temperature of the chamber was
increased to 150.degree. C., and the rpm were increased to 80.
Mixing was continued for 10 min. after the addition of the UHMWPE
had been completed. The mixture was removed from the mixer while
still hot.
[0193] After cooling, 13.1 g of the solidified mixture was placed
between 0.175 mm (7 mil) polyester sheets and placed in a Carver
press at 160.degree. C. with 10 mil shims placed between the
polyester sheets. After heating in the press for 3 min. with no
applied pressure, the mixture was pressed for 10 sec. using 345 kPa
(50 psi). The resultant film with polyester sheets still attached
was immersed into water at ambient temperature to quench it. The
oil was extracted from the film as described in Example 2. A
portion of the film was heated at 130.degree. C. for 10 min. in a
circulating air oven, as described in Example 4. The peak melting
point of the UHMWPE was 138.degree. C. as determined by DSC.
Physical properties of the film are shown in Table 4. Advancing and
receding contact angles (water) for the film were 139.degree..+-.10
and 79.degree..+-.9, respectively.
4TABLE 4 After Washing/ Drying, Before After Heating for 10
Parameter Heating min. at 130.degree. C. Caliper (mm) 0.150 0.150
Gurley (sec./50 cc) 36.8 19.6 Bubble Point (microns) 0.93 0.60
Resistivity (ohm-cm) 36 9.7
[0194] Trace C in FIG. 8 displays a representative polarization
curve with a cathode electrode backing layer made from the heated
film. Further improvement is observed relative to Example 13. Trace
D involves the ELAT.TM. control.
[0195] In the following Examples 15A, 15B, 16A, 16B, 17B, 17C, 17D,
17E and 17F, equivalent catalyst coated ion conduction membranes
were used for tests of different types of cathode backing layers.
Commercial ELAT.TM. was used in each case as the anode backing
layer. The fuel cell polarization curves for these examples are
summarized in FIG. 12, and demonstrate the effects of the different
parameters tested under low pressure air operation. The comparative
control curve with ELAT.TM. as the cathode backing layer is also
shown in FIG. 12. Referring to FIG. 12, in the useful voltage range
of 0.6 volts and higher, the electrode backing layer of Example 15A
exceeds the performance of the ELAT.TM. membrane.
Example 15
Graphite/Conductive Carbon in Polyvinylidene Fluoride
[0196] A mixture of 91.37 g of MCMB 6-28 graphite in 96.18 g of
propylene carbonate was prepared by using a dispersator. Then, 1.60
g Super P conductive carbon was added to the mixing chamber of a
Haake Rheocord.TM. System 9000 mixer at 50.degree. C. and 50 rpm,
followed by addition of 63.0 g of the above graphite-propylene
carbonate mixture. While heating the resulting mixture to
150.degree. C., 12.47 g Solef 1010.TM. polyvinylidene fluoride
(PVDF, Solvay America Inc., Houston, Tex.) was added in portions at
a rate such that the added polymer was assimilated into the
mixture. When steady torque had been established (approximately 6
minutes after commencing polymer addition), the temperature set
point was changed to 120.degree. C. and cooling commenced. After
approximately 4 minutes of cooling, stirring was stopped and the
resulting mixture removed while hot.
[0197] After cooling, 12 g of solidified mixture was placed between
two sheets of polyimide film with 0.25 mm (10 mil) shims between
the polyimide film, and placed in a Carver press at 150.degree. C.
After heating for 90 sec. with no applied pressure, the press was
closed for 5 seconds using 1035 kPa (150 psi). The resultant film
with polyimide sheets still attached was placed between two 15 mm
thick steel plates at 20.degree. C. until the film was cool, after
which the polyimide film was removed. The resultant PVDF film was
washed and then dried as described in Example 2, except that
3.times.1 L isopropyl alcohol washes were used to extract the
propylene carbonate to give sample 15A. A portion of the film was
heated at 160.degree. C. for 10 min. in a circulating air oven, as
described in Example 4 to give sample 15B. Physical properties of
the film are shown in Table 5. The fuel cell polarization results
are shown in FIG. 12.
5 TABLE 5 After Washing/ Drying, Before After Heating for 10
Heating (15A) min. at 160.degree. C. (15B) Caliper, mm 0.241 0.230
Gurley (sec./50 cc) 57 39 Resistivity (ohm-cm) 1.20 0.96 Advancing
Contact Angle 143 .+-. 10.degree. 141 .+-. 8.degree. Receeding
Contact Angle 87 .+-. 12.degree. 87 .+-. 8.degree.
Example 16
Graphite/Super S Conductive Carbon (95/5) in High Density
Polyethylene (TIPT)
[0198] This example demonstrates useful performance at a much lower
loading of carbon. This film was made using the extruder described
in Example 1 and was cast onto a smooth casting wheel (32.degree.
C. set point temperature). Film made this way has typically smaller
pores on the wheel side than on the air side.
[0199] A dispersion of SFG 15 graphite (Alusuisse Lonza America
Inc., now Timcal, Fair Lawn, N.J.) was prepared by adding
incrementally 1090 g of SFG 15 to a mixture of 3030 g of mineral
oil (Superla.TM. White Mineral Oil No. 31) and 57.4 g of
dispersant, OLOA 1200 using a Model 89 dispersator from Premier
Mill Corp. Then, 57.4 g of Super S conductive carbon MMM Carbon
Div., MMM nv, Brussels, Belgium) was mixed into the graphite
dispersion. The carbon/oil mixture was heated to 150.degree. C. and
held at 150.degree. C. for 30 min. while continuing to mix with the
dispersator (rpm were lowered as temperature increased). The
mixture was cooled to 70.degree. C. before being transferred to the
feed tank of the extruder.
[0200] The carbon/oil mixture was pumped into an injection port on
the third zone of a Berstorff.TM. co-rotating twin screw extruder
(25 mm.times.825 mm). High density polyethylene (HDPE, grade 1285,
Fina Oil & Chemical Co.) was metered into the feed zone (zone
1) at a rate of 0.61 kg(1.35 lb)/hr., and the above mixture was
pumped in at a nominal rate of 77 cc/min. using a gear pump. The
extruder profile starting from the feed zone was 199, 271, 271,
188, 188, 188, 188.degree. C., the die temperature was 188.degree.
C., and the screw speed was 125 rpm.
[0201] Film was extruded through an 20.32 cm (8 in.) die onto a
smooth casting wheel at 32.degree. C. A 50 micrometer polyester
film was inserted on top of the film after quenching while still on
the casting wheel to aid in film handling by preventing slippage on
the wheel. The resultant extruded film was 0.3 mm (12 mil) thick
and the experimentally determined total film throughput rate was
5.39 kg (11.9 lb.)/hr. Thus, the actual carbon/oil mixture feed
rate was 4.80 kg (10.6 lb.)/hr. From this and the known
compositions, the total carbon content in the film after extraction
of the oil was calculated to be 68.0 wt. %
[0202] The oil and OLOA 1200 were extracted from the film using
three .times.15 min. washes using Vertrel 423. About 1 L of solvent
per wash was used for a piece of film that was about 17.8 cm (7")
wide by 30.5 cm (12") long. The film was then hung in an exhaust
hood to dry to give sample 16A. A piece of this film was hung in a
circulating air oven for 10 min. at 130.degree. C. to give sample
16B. As shown in Table 6, below, heating the film above the melting
point of the HDPE used, 126.degree. C., resulted in a significant
decrease in Gurley value, significant increase in bubble point, and
significant decrease in resistivity. Physical properties of the
film are shown in Table 6. Fuel cell polarization curves using
these membranes are shown in FIG. 12. For both 16A and 16B, the
casting wheel side of the film was toward the MEA. The
photomicrographs of the casting wheel side, air side and cross
sections of films 16A and 16B are shown in FIGS. 14 and 15,
respectively. The SEM results show the differences in pore size
between the casting wheel and air sides of the films, and the
general enlargement of pore sizes throughout film 16B due to
heating, as described.
6 TABLE 6 After Washing/ Drying, Before After Heating for 10
Heating (16A) min. at 130.degree. C. (16B) Caliper, mm 0.285 0.274
Gurley (sec./50 cc) 245 20.8 Bubble Point (microns) 0.38 1.16
Resistivity (ohm-cm) 71 1.45
[0203] Results given in Examples 17 and 18 below show that similar
physical properties were obtained
[0204] 1. by either heating the film above the melting point of the
HDPE and then stretching at a normal stretch temperature for porous
HDPE (usually about 180 to 220.degree. F.), or
[0205] 2. by stretching at a higher temperature that would normally
result in loss of porosity of an unfilled HDPE in the membrane.
Example 17
Effect of Stretching and Heating on TIPT Membranes
[0206] Samples of the film prepared as described in Example 2 were
variously heated and stretched as shown in Table 7. The film was
stretched using a film stretcher from T. M. Long Co., Somerville,
N.J. After inserting the film into the stretcher at the indicated
temperature, the film was heated for about 30 sec. before
stretching. Stretching was performed in one direction or
sequentially in both directions at about 2.54 cm/sec. After
stretching, the films were annealed at the stretching temperature
for about 2 min. before releasing the stretcher grips and removing
the stretched film. In the Table, the degree of stretching is
indicated in terms of the ratio of final dimension divided by
initial dimension: a stretch ratio of 1.25.times.1 means that the
film was stretched uniaxially by 25% (12.7 cm final length, 10.2 cm
initial length). 1.25.times.1.25 means that the film was stretched
by 25% in both directions, sequentially. Simultaneous biaxial
stretching in both directions is also possible.
7TABLE 7 Stretch Stretch Caliper, Bubble Gurley, Resistivity, Ex.
Treatment Ratio Temp., .degree. C. mm Point, .mu.m sec./50 cc
ohm-cm 17A None -- -- 0.216 0.095 426 5.5 17B Heat Only -- -- 0.225
0.13 210 1.0 17C Heat, then 1.25 .times. 1 87 0.218 0.19 116 1.3
Stretch 17D Stretch 1.25 .times. 1 134 0.175 0.40 38 1.1 Only 17E
Stretch 1.25 .times. 1.25 134 0.165 0.47 49 4.2 Only 17F Stretch
1.5 .times. 1 134 0.18 0.41 47 2.4 Only
[0207] In the Table, Example 17A corresponds to a film as prepared
in Example 2, Example 17 B corresponds to a film as prepared in
Example 3, For Example 17C, the film from Example 17B was cooled
from 130.degree. C. to room temperature prior to stretching, and
then heated in the T.M. Long Co. stretcher to 93.degree. C. before
stretching. For Examples 17D, 17E, and 17F, a film prepared as in
Example 2 was heated to the temperature shown in Table 7 in the T.
M. Long Co. film stretcher and then stretched, without first
heating to 130.degree. C., as in the heat-only method.
[0208] In general, stretching increased bubble point, decreased the
Gurley value and increased the resistivity relative to untreated
film (Example 17A). While low resistivity is desirable, Examples
17C-17F demonstrate that gas flow through the film can be enhanced
without unduly increasing resistivity. Example 17C showed that even
a small amount of stretching of a film that had been previously
heated at 130.degree. C. (Example 17B) provided a significant
increase in bubble point and a significant decrease in Gurley value
while not significantly increasing the resistivity. Examples
17D-17F illustrate the effects of a single step process of
stretching the film from Example 17A at a higher temperature than
the melting point of the polymer. Stretching at higher temperature
resulted in an even larger increase in bubble point and even larger
decrease in Gurley value. The change in resistivity varied with the
amount of stretching from almost no change (Example 17D) to a
moderate change (Example 17E), and to a somewhat larger change
(Example 17F). Advancing and receding contact angles (water) for
the film were, respectively, (17D) 148.degree..+-.6.degree. and
95.degree..+-.5.degree., (17E) 153.degree..+-.4.degree. and
98.degree..+-.5.degree., and (17F) 156.degree..+-.8.degree. and
104.degree..+-.4.degree.. The results are unexpected, in that
unfilled porous films heated at or near their melting point
generally would collapse and become a dense, nonporous films.
Polarization curves for Example 17 are given in FIG. 12.
[0209] As shown in Example 18 below, as little as 20 volume %
carbon relative to the volume of HDPE in conjunction with a high
loading of metallic particles is sufficient to hinder densification
of the membrane upon heating at 130.degree. C. The peak melting
temperature of the HDPE was 126.degree. C. as determined by DSC.
Example 18 also shows that useful TIPT films can be prepared using
conductive metal particles in conjunction with conductive carbon
particles.
Example 18
TIPT Films Loaded with Non-carbon Conductive Particles
[0210] Example 18A: A dispersion was prepared by wetting out 11,574
g of tungsten powder having a primary particle size of 0.5 .mu.m
(Teledyne Wah Chang, Huntsville, Ala.) in 2576 g of mineral oil
(Superla.TM. White Mineral Oil No. 31) and 359 g of OLOA 1200 using
a dispersator having a 2 in. sawtooth disc head (Premier Mill
Corp.). The resultant mixture was then milled by recirculating this
mixture through a 0.25 L horizontal mill (Premier Mill Corp.) that
contained a 50 vol. % charge of 1.3 mm steel beads, for 2 hr. The
resultant dispersion was then filtered through a 20 micron
rope-wound filter that had been pre-wet with oil. An iterative
series of density checks followed by oil additions was performed to
adjust the density until the desired target density of 3.6358 was
reached.
[0211] As described in Example 1, this dispersion was pumped at
59.6 ml/min. into an intermediate zone of a 25 mm twin screw
operated at 90 rpm and HDPE (grade GM 9255 from Hoechst Celanese
Corp., now available as grade 1285 from Fina Oil & Chemical
Co.) was gravimetrically metered into the extruder throat at 0.54
kg (1.2 lb)/hr. The film was cast onto a smooth casting wheel
maintained at 32.degree. C. at about 0.225 mm thick. The oil was
extracted using three -15 min. washes of Vertrel 423.TM. and dried
in an exhaust hood. The resultant film was evaluated after
washing/drying and then after heating for 10 min. at 130.degree.
C., as shown in Table 8. The calculated weight percent of tungsten
in the dried film was 95.0.
[0212] Example 18B: A membrane similar to Example 18A was prepared
that had the same volume percent loading of particulate, 48.3 vol.
%, except that the total particulate contained 73 volume % tungsten
and 27 volume % conductive carbon. The total weight percent
particulate in the final membrane was 93.5% of a 96.29/3.71 by
weight mixture of tungsten and Conductex 975.TM. conductive carbon
(Colombian Chemicals Co.).
[0213] The dispersion was prepared by combining 2400 g of mineral
oil (0.863 g/cc) and 300 g of OLOA 1200 (0.92 g/cc). Then, 8880 g
of tungsten (19.35 g/cc) was wetted out into this mixture using a
dispersator equipped with a 2 in. sawtooth disc head (Premier Mill
Corp.). A 341 g quantity of Conductex 975 (2.0 g/cc) was added in
portions. Heating was commenced to lower the dispersion viscosity
to facilitate wetting out of the carbon. The dispersion was then
heated to 150.degree. C. for 20 min. The hot dispersion was
recirculated for one hour through a 0.25 L horizontal mill operated
at 3500 rpm. The mill contained an 80 vol. % charge of 1.3 mm steel
beads. The dispersion density was adjusted by adding more mineral
oil until a final density of 2.8922 g/cc at 25.degree. C. was
reached.
[0214] As described in Example 1, the dispersion was pumped at 59.6
ml/min. into an intermediate zone of a 25 mm twin screw operated at
90 rpm and HDPE (grade GM 9255 from Hoechst Celanese Corp., now
available as grade 1285 from Fina Oil & Chemical Co.) was
gravimetrically metered into the extruder throat at 0.54 kg (1.2
lb)/hr. The film was cast onto a smooth casting wheel maintained at
32.degree. C. at about 0.225 mm thick. The oil was extracted using
three -15 min. washes of Vertrel 423.TM. and dried in an exhaust
hood. The resultant film was evaluated after washing/drying and
then after heating for 10 min. at 130.degree. C., as shown in Table
8.
8TABLE 8 Caliper, Bubble Gurley no., Resistivity, Example Treatment
mm Point, .mu.m sec./50 cc ohm-cm 18A(1) None 0.24 0.27 135
>10.sup.6 18A(2) 130.degree. C./ 0.163 0.071 * >10.sup.6 10
min. 18B(1) None 0.173 0.10 317 101 18B(2) 130.degree. C./ 0.133
0.18 152 3.5 10 min. *Film broke in Gurley instrument
[0215] The data shown in Table 8 indicate that conductive particles
other than carbon can be used to prepare TIPT films useful in the
invention if at least a minor amount of conductive carbon is
included in order to achieve acceptably low resistivity, decreased
Gurley and increased bubble point on heating the film.
Examples 19-27
Carbon-Loaded Porous PTFE Membranes--PF Process
[0216] In examples 19-27, the carbon loaded Teflon.RTM. (PTFE)
media was prepared using the general process taught, e.g., in U.S.
Pat. No. 5,071,610, incorporated herein by reference. In brief, the
porous, conducting Teflon.RTM. based membranes were prepared by
hand mixing carbon particles, a liquid dispersant and PTFE powder
to form a putty-like mass. The material then was passed multiple
times through a heated mill (Model 4037, Reliable Rubber and
Plastic Machinery Co. Inc., North Bergen, N.J.), with repeated
folding and rotating of the sample and reductions of the mill gap
in between passes through the mill. The final membrane sheet was
then heated above the boiling point of the dispersant, in a vented
oven, to remove the dispersant.
[0217] The dispersant used in all the examples was Fluorinert.TM.,
FC-40 (b.p.=155.degree. C.) highly fluorinated electronic liquid,
available from 3M Co., St. Paul, Minn. The use of a fluorinated
dispersant in the PF process is described in U.S. Pat. No.
5,113,860, incorporated herein by reference.
[0218] The Teflon.RTM. binder, provided in dry form, was PTFE type
6-C, (DuPont Chemical Co., Wilmington, Del.). Carbon particles
consisted of carbon black material and/or carbon fibers. The carbon
black material is identified in each example.
[0219] Carbon fibers were obtained from Strem Chemicals Inc.,
Newburyport, Mass., catalog number 06-0140. The approximately 6 mm
long.times.0.001 cm diameter fibers were received bundled randomly
together and had to be physically dispersed prior to use. This was
done by brushing the fiber bundles with a brass bristle brush to
cause separated fibers to fall into a USA Standard Testing Sieve
(W.S. Tyler Inc., Mentor, Ohio), then shaking on a sieve (100 mesh)
shaker (W.S. Tyler Inc., Mentor, Ohio) for one hour. The individual
carbon fibers then were blended with carbon black and added to the
Teflon.RTM. and Fluorinert mixture.
[0220] In the following examples, the Gurley, resistance, contact
angles and fuel cell performance of several carbon/PTFE composite
membranes are compared to the ELAT.TM. PTFE/carbon material
described in previous examples.
Example 19
PTFE/Carbon Black (95%) Membrane
[0221] Five grams of carbon black(Vulcan XC72R, Cabot Corp.,
Waltham, Mass., average particle diameter of 30 nm) were mixed with
0.263 g of PTFE and 40 g of Fluorinert.TM. FC-40. The mixture was
hand-kneaded and formed as described above, into a porous,
conducting membrane 0.38 mm thick. The membrane was dried in a
vented oven at 180.degree. C. for one hour. The resultant membrane,
measuring approximately 37.5 cm.times.30 cm, was approximately 95%
by weight carbon.
[0222] A Gurley value of 37 seconds per 10 cc was measured for the
membrane (FIG. 9).
Example 20
Membrane Comprising PTFE and Carbon Black/Carbon Fiber (89/6)
Mixture
[0223] A 4.7 gram portion of carbon black, type Vulcan XC72R, and
0.3 g of carbon fibers (Strem Chemicals Inc.) were mixed with 0.263
g of PTFE and 40 g of Fluorinert.TM. FC-40. The mixture was hand
kneaded and formed into a porous, conducting membrane 0.38 mm
thick. The membrane was dried in a vented oven at 160.degree. C.
for one hour. It was then folded in half and passed through the
mill rolls to a thickness of 0.30 mm. The resultant membrane,
measuring approximately 37.5 cm.times.30 cm, was approximately 95%
by weight total carbon; 89% carbon black and 6% carbon fibers.
[0224] The measured Gurley was 21.5 seconds per 10 cc (FIG. 9). The
resistance of two 5 cm.sup.2 pieces of the membrane compressed to
0.51 cm thick, measured in the fuel cell test cell as described in
Example 6, was 4.0 milliohms, compared to 5.7 milliohms for two
similar sized pieces of ELAT.TM. reference material. (FIG. 10) This
corresponds to a bulk resistivity of 0.94 ohm-cm.
Example 21
Membrane Comprising PTFE and Carbon Black/Carbon Particle
Mixture
[0225] A 3.0 gram portion of Vulcan XC72R carbon black and 2.0 g of
Norit SX1 carbon particles, average particle size of 32-75 .mu.m
(American Norit Co. Inc., Atlanta, Ga.) were mixed with 0.263 g of
PTFE and 40 g of Fluorinert.TM. FC-40. The mixture was hand kneaded
and formed into a porous, conducting membrane 0.36 mm thick, as
described above. The membrane was dried in a vented oven. The
resultant membrane was approximately 95% by weight total carbon;
57% carbon black and 38% carbon particles.
[0226] The measured Gurley was 35 seconds per 10 cc (FIG. 9). The
resistance of two 5 cm.sup.2 pieces of the membrane compressed to
0.076 cm thick was 4.0 milliohms (FIG. 10). This corresponds to a
bulk resistivity of 0.26 ohm-cm. The advancing and receding contact
angles were measured to be 153.+-.4.degree. and
113.7.+-.1.6.degree., respectively.
[0227] For examples 22, 23, 26 and 27, the same catalyst coated ion
conducting membrane was used to obtain the fuel cell polarization
curves for the different electrode backing material samples. The
testing was done by opening the cell after the completion of one
test, removing the electrode backing layer, and replacing them with
the next electrode backing layer. In FIG. 11 the order in which the
samples were tested by reference to the particular example was:
Example 22, Example 23, Example 26, Example 27 followed by the ELAT
control. Since the full performance of the catalyzed Nafion 115 ion
conduction membrane was obtained with the last sample using the
ELAT control, interchanging the electrode backing layers did not
damage the catalyzed membrane. As seen from the resistance and
Gurley measurements for these examples in FIGS. 9 and 10, the
significant differences in fuel cell performance cannot be due to
resistance or just porosity. The current limited performance of the
sample from Example 26, due to oxygen limited diffusion through a
cathode water flooding layer, is most likely associated with the
lower porosity (higher Gurley value) and much lower receding
contact angle (107.5.degree.) compared to the other examples in the
series. These examples demonstrate that the wetting characteristics
of the type of carbon particle used is very important since it
influences the receding contact angle.
Example 22
Membrane Comprising PTFE and Carbon Black/Carbon Fibers (87/8)
[0228] A 4.6 gram portion of Vulcan XC72R carbon black and 0.4 g of
carbon fibers (Strem Chemicals Inc.) was mixed with 0.263 g of PTFE
and 40 g of Fluorinert.TM. FC-40. The mixture was hand kneaded and
formed into a porous, conducting membrane 0.28 mm thick. The
membrane was dried in a vented oven at 165.degree. C. for two
hours. The resultant membrane was approximately 95% by weight total
carbon, 87% carbon black and approximately 8% carbon fibers.
[0229] The measured Gurley was 2.1 seconds per 10 cc (FIG. 9). The
resistance of two 5 cm.sup.2 pieces of the membrane compressed to
0.058 cm thick was 9.6 milliohms (FIG. 10). This corresponds to a
bulk resistivity of 0.82 ohm-cm. The advancing and receding contact
angles were respectively measured to be 154.+-.7.degree. and
132.+-.4.degree..
[0230] The fuel cell performance of the electrode backing layers
prepared in this example was measured using a Nafion.TM. 115
membrane-based 3-layer MEA with nanostructured electrodes, as
described in Example 9. FIG. 11 shows the performance of this and
other 5-layer MEAs of the invention, as well as that of electrode
backing layers prepared from ELAN.TM. reference material. The
current density of membranes of this Example at 0.5 volts is seen
to be 0.7 A/cm.sup.2. To obtain the fuel cell polarization curves
in FIG. 11, the fuel cell was operated at a temperature of
80.degree. C. with a hydrogen pressure of 207 Kpa, an oxygen
pressure of 414 Kpa, and flow rates of 1 standard liter per minute,
and the anode/cathode humidification temperatures were 115.degree.
C. and 80.degree. C., respectively.
Example 23
Membrane Comprising PTFE and Carbon Black/Carbon Fiber (78/7)
[0231] A 4.6 gram portion of Shawinigan C-55 carbon black, and 0.4
g of carbon fibers (Strem Chemicals Inc.) was mixed with 0.90 g of
PTFE and 45 g of Fluorinert.TM. FC-40. The mixture was hand kneaded
and formed into a porous, conducting membrane 0.41 mm thick. After
drying in a vented oven, the resultant membrane was approximately
85% by weight carbon; 78% carbon black and approximately 7% carbon
fibers.
[0232] The measured Gurley was 6.2 seconds per 10 cc (FIG. 9). The
resistance of two 5 cm.sup.2 pieces 0.058 cm thick of the membrane
was 10.6 milliohms (FIG. 10). This corresponds to a bulk
resistivity of 0.90 ohm-cm. The advancing and receding contact
angles were respectively measured to be 157.+-.5.degree. and
137.+-.9.degree..
[0233] The fuel cell performance of the electrode backing layers
prepared in this example was measured as described above. The
current density at 0.5 volts was 0.95 A/cm.sup.2.
Example 24
Membrane Comprising PTFE and Carbon Black (92%)
[0234] This membrane was prepared as described in Example 19,
except the total carbon loading was 92% by weight Vulcan XC72R. The
membrane thickness was 0.25 mm.
[0235] The measured Gurley was 24 seconds per 10 cc (FIG. 9), and
the membrane resistance was 20.5 milliohms (FIG. 10). This
corresponds to a bulk resistivity of 1.92 ohm-cm. The advancing and
receding contact angles were respectively measured to be
156.+-.8.degree. and 96.+-.5.degree..
Example 25
Membrane Comprising PTFE and Carbon Black (95%)
[0236] This membrane was prepared with the same ingredients as in
Example 19 except a different thickness membrane was formed. The
resultant membrane was approximately 95% by weight carbon black and
0.32 mm thick.
[0237] The measured Gurley was 73 seconds per 10 cc (FIG. 9), and
the membrane resistance was 5.0 milliohms (FIG. 10), giving a bulk
resistivity of 0.39 ohm-cm.
Example 26
Membrane Comprising PTFE and Carbon Black (90%)
[0238] A 90 wt % carbon-containing membrane was prepared as
described in Example 19 using KetJen-600J carbon black. The porous,
conducting membrane was 0.28 mm thick.
[0239] The measured Gurley was 27 seconds per 10 cc (FIG. 9), and
the resistance of two 5 cm.sup.2 pieces of the membrane was 5.0
milliohms (FIG. 10), for a bulk resistivity of 0.48 ohm-cm. The
advancing and receding contact angles were respectively measured to
be 161.+-.8.5.degree. and 107.5.+-.5.degree..
[0240] The fuel cell performance of the electrode backing layers
prepared in this example was measured as described above and shown
in FIG. 11. The current density at 0.5 volts was 0.28
A/cm.sup.2.
Example 27
Membrane Comprising PTFE and Carbon Black (85%)
[0241] An 85 wt % carbon-containing membrane was prepared as
described in Example 19 using Shawingian C-55 carbon black. The
porous, conducting membrane was 0.39 mm thick.
[0242] The measured Gurley was 4.4 seconds per 10 cc (FIG. 9), and
the resistance of two 5 cm.sup.2 pieces of the membrane was 13.4
milliohms (FIG. 10), for a bulk resistivity of 0.88 ohm-cm. The
advancing and receding contact angles were respectively measured to
be 157.+-.7.degree. and 141.+-.12.degree..
[0243] The fuel cell performance of the electrode backing layers
prepared in this example was measured as described above and shown
in FIG. 11.
Example 28
Effect of TIPT Film Asymmetry
[0244] The carbon-filled HDPE membrane described in example 16B
(heat treated) was evaluated in a fuel cell under the same
conditions as described above with respect to Examples 9-14 except
that Nafion.TM. 115 was used for the ion conduction membrane and
the electrode backing layers were extracted with Vertrel 423. In
Example 28A the film was placed with the side of the film that was
against a smooth casting wheel during quenching facing away from
the catalyzed membrane. In Example 28B the same film was placed
with the casting wheel side of the film facing towards the
catalyzed membrane. SEM photomicrographs of the casting wheel and
air sides of film from 16B are shown in FIG. 15, with comparable
films without heat treatment shown in FIG. 14. The fuel cell
results are presented in FIG. 13. The results show significantly
better performance for Example 28B with the casting wheel side of
the film placed against the catalyzed membrane. As evident from the
SEM results in FIG. 15, the better results are obtained with the
film layer next to the catalyzed membrane having smaller pores and
a denser surface layer. FIG. 16 shows SEM micrographs for UHMWPE
films corresponding to Example 14 with and without heat
treatment.
[0245] The embodiments described above are intended to be
representative and not limiting. Additional embodiments of the
invention are within the claims.
* * * * *