U.S. patent application number 12/158357 was filed with the patent office on 2010-12-02 for membrane electrode assembly for organic/air fuel cells.
Invention is credited to Mohamed Abdou, Mookkan Periyasamy, Jo-Ann T. Schwartz, Harvey P. Tannenbaum.
Application Number | 20100304266 12/158357 |
Document ID | / |
Family ID | 38179613 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100304266 |
Kind Code |
A1 |
Periyasamy; Mookkan ; et
al. |
December 2, 2010 |
MEMBRANE ELECTRODE ASSEMBLY FOR ORGANIC/AIR FUEL CELLS
Abstract
A membrane electrode assembly for an organic/air fuel cell is
provided comprising a proton exchange membrane, an anode electrode,
and a cathode electrode. The proton exchange membrane is made of a
highly fluorinated ion-exchange polymer. The anode electrode is
comprised of an anode electrocatalyst of platinum and ruthenium
supported on particulate carbon and a highly fluorinated
ion-exchange polymer binder, and the metal loading in the anode
electrode is less than 3 mg/cm.sup.2. The cathode electrode is
comprised of a cathode electrocatalyst of platinum supported on
particulate carbon and a highly fluorinated ion-exchange polymer
binder, and the metal loading in the cathode electrode is less than
3 mg/cm.sup.2. Organic/air fuel cells comprised of such membrane
electrode assemblies are also provided. A process for operating
such membrane electrode assemblies of an organic/air fuel cell is
also provided.
Inventors: |
Periyasamy; Mookkan;
(Wilmington, DE) ; Schwartz; Jo-Ann T.; (Avondale,
PA) ; Abdou; Mohamed; (Chadds Ford, PA) ;
Tannenbaum; Harvey P.; (Wynnewood, PA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
38179613 |
Appl. No.: |
12/158357 |
Filed: |
December 21, 2006 |
PCT Filed: |
December 21, 2006 |
PCT NO: |
PCT/US06/48985 |
371 Date: |
June 20, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60752996 |
Dec 21, 2005 |
|
|
|
Current U.S.
Class: |
429/483 ;
429/535 |
Current CPC
Class: |
D21H 13/20 20130101;
C09K 3/1028 20130101; F16D 69/026 20130101; D21B 1/12 20130101 |
Class at
Publication: |
429/483 ;
429/535 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Claims
1. A membrane electrode assembly for an organic/air fuel cell
comprising: a proton exchange membrane made of a highly fluorinated
ion-exchange polymer, said membrane having opposite first and
second sides; an anode electrode adjacent said first side of the
membrane, said anode electrode comprised of 50 to 90 wt % of an
anode electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, said anode electrocatalyst being
comprised of an anode metal supported on carbon, wherein the anode
metal is comprised of platinum and ruthenium and the carbon is
particulate carbon, said anode electrocatalyst being comprised of
at least 40 wt % platinum, at least 15 wt % ruthenium, and 15 to 50
wt % particulate carbon wherein the total loading of the anode
metal in the anode electrode is less than 3 mg/cm.sup.2; a cathode
electrode adjacent said second side of the membrane, said cathode
electrode comprised of 50 to 90 wt % of a cathode electrocatalyst
and 10 to 50 wt % of a highly fluorinated ion-exchange polymer
binder, said cathode electrocatalyst being comprised of a cathode
metal supported on carbon, wherein the cathode metal is comprised
of platinum and the carbon is particulate carbon, said cathode
electrocatalyst being comprised of at least 50 wt % platinum and 15
to 50 wt % particulate carbon wherein the total loading of the
cathode metal in the cathode electrode is less than 3
mg/cm.sup.2.
2. The membrane electrode assembly of claim 1, wherein the anode
electrocatalyst consists essentially of platinum, ruthenium and
particulate carbon.
3. The membrane electrode assembly of claim 2, wherein the anode
electrocatalyst includes 40 to 60 wt % platinum, 20 to 40 wt %
ruthenium and 20 to 30 wt % particulate carbon.
4. The membrane electrode assembly of claim 1, wherein the cathode
electrocatalyst consists essentially of platinum and particulate
carbon.
5. The membrane electrode assembly of claim 4, wherein the cathode
electrocatalyst includes 60 to 80 wt % platinum and 20 to 40 wt %
particulate carbon.
6. The membrane electrode assembly of claim 1 wherein the total
loading of the anode metal in the anode electrode is less than 2.5
mg/cm.sup.2, and wherein the total loading of the cathode metal in
the cathode electrode is less than 2.5 mg/cm.sup.2.
7. The membrane electrode assembly of claim 1 wherein the total
loading of the anode metal in the anode electrode is less than 2
mg/cm.sup.2, and wherein the total loading of the cathode metal in
the cathode electrode is less than 2 mg/cm.sup.2.
8. The membrane electrode assembly of claim 1 wherein the sum of
the total loading of anode metal in the anode electrode and cathode
metal in the cathode electrode is less than 5 mg/cm.sup.2.
9. The membrane electrode assembly of claim 8 wherein the sum of
the total loading of anode metal in the anode electrode and cathode
metal in the cathode electrode is less than 3.5 mg/cm.sup.2.
10. The membrane electrode assembly of claim 1 wherein the proton
exchange membrane consists essentially of a perfluorinated sulfonic
acid membrane in acid form.
11. The membrane electrode assembly of claim 10 wherein the highly
fluorinated ion-exchange polymer binder in both the anode electrode
and the cathode electrodes consist essentially of a perfluorinated
sulfonic acid membrane in proton form.
12. The membrane electrode assembly of claim 11 wherein the proton
exchange membrane consists essentially of a perfluorinated sulfonic
acid membrane in acid form, and wherein the highly fluorinated
ion-exchange polymer binder in both the anode electrode and the
cathode electrodes consist essentially of a perfluorinated sulfonic
acid membrane in acid form.
13. The membrane electrode assembly of claim 1 wherein the anode
and cathode electrodes are adhered directly to the opposite first
and second sides of the polymer exchange membrane.
14. The membrane electrode assembly of claim 1 wherein the anode
and cathode electrodes are coated on the opposite first and second
sides of the polymer exchange membrane.
15. The membrane electrode assembly of claim 1 further comprising a
first electrically conductive gas diffusion substrate disposed on
the first side of the proton exchange membrane, said anode
electrode being disposed between said first conductive gas
diffusion substrate and the first side of the proton exchange
membrane, wherein said anode electrode is adhered to the first
conductive gas diffusion substrate and is in direct contact with
the first side of the proton exchange membrane, and a second
electrically conductive gas diffusion substrate disposed on the
second side of the proton exchange membrane, said cathode electrode
being disposed between said second conductive gas diffusion
substrate and the second side of the proton exchange membrane,
wherein said cathode electrode is adhered to the second conductive
gas diffusion substrate and is in direct contact with the second
side of the proton exchange membrane.
16. The membrane electrode assembly of claim 15, wherein the
electrically conductive gas diffusion substrate is a carbon-fiber
based paper or cloth.
17. The membrane electrode assembly of claim 1, wherein the
particulate carbon is from the group of turbostratic or graphitic
carbons.
18. The membrane electrode assembly of claim 1 wherein said anode
electrocatalyst is in the form of anode electrocatalyst particles
and said cathode electrocatalyst is in the form of cathode
electrocatalyst particles, and wherein at least 98% of the anode
and cathode electrocatalyst particles have a particle diameter of
less than 10 microns.
19. An organic/air fuel cell comprising the membrane electrode
assembly of claim 1.
20. A process for producing a membrane electrode assembly for an
organic/air fuel cell, comprising (a) providing a proton exchange
membrane made of a highly fluorinated ion-exchange polymer, said
membrane having opposite first and second sides; b) forming an
anode electrode adjacent said first side of the membrane, said
anode electrode comprised of 50 to 90 wt % of an anode
electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, said anode electrocatalyst being
comprised of an anode metal supported on carbon, wherein the anode
metal is comprised of platinum and ruthenium and the carbon is
particulate carbon, said anode electrocatalyst being comprised of
at least 40 wt % platinum, at least 15 wt % ruthenium, and 15 to 50
wt % particulate carbon wherein the total loading of the anode
metal in the anode electrode is less than 3 mg/cm.sup.2; and (c)
forming a cathode electrode adjacent said second side of the
membrane, said cathode electrode comprised of 50 to 90 wt % of a
cathode electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, said cathode electrocatalyst being
comprised of a cathode metal supported on carbon, wherein the
cathode metal is comprised of platinum and the carbon is
particulate carbon, said cathode electrocatalyst being comprised of
at least 50 wt % platinum and 15 to 50 wt % particulate carbon
wherein the total loading of the cathode metal in the cathode
electrode is less than 3 mg/cm.sup.2.
21. The process of claim 20, wherein forming the membrane electrode
assembly for an organic/air fuel cell includes the steps of making
an anode electrocatalyst ink comprised of highly fluorinated
ion-exchange polymer, anode electrocatalyst particles of the
platinum and ruthenium supported on particulate carbon, and a
solvent, wherein at least 98% of such anode electrocatalyst
particles have a diameter of less than 10 microns, forming the
anode electrode by forming a coating of the anode electrocatalyst
ink and removing the solvent from the anode electrocatalyst ink,
making a cathode electrocatalyst ink comprised of highly
fluorinated ion-exchange polymer, cathode electrocatalyst particles
of the platinum supported on particulate carbon, and a solvent,
wherein at least 98% of such cathode electrocatalyst particles have
a diameter of less than 10 microns, and forming the cathode
electrode by forming a coating of the cathode electrocatalyst ink
and removing the solvent from the cathode electrocatalyst ink.
22. The process of claim 20, wherein forming the membrane electrode
assembly for an organic/air fuel cell includes the steps of making
an anode electrocatalyst ink comprised of highly fluorinated
ion-exchange polymer, platinum and ruthenium supported on
particulate carbon, and a fluorinated solvent, the highly
fluorinated ion-exchange polymer being a perfluorinated polymer
having sulfonyl fluoride end groups, forming the anode electrode by
forming a coating of the anode electrocatalyst ink and removing the
solvent from the anode electrocatalyst ink, making a cathode
electrocatalyst ink comprised of highly fluorinated ion-exchange
polymer, the platinum supported on particulate carbon, and a
fluorinated solvent, the highly fluorinated ion-exchange polymer
being a perfluorinated polymer having sulfonyl fluoride end groups,
forming the cathode electrode by forming a coating of the cathode
electrocatalyst ink and removing the solvent from the cathode
electrocatalyst ink, applying the anode and cathode electrodes to
opposite side of the proton exchange membrane, and converting the
sulfonyl fluoride end groups in the ion-exchange polymer of the
anode electrode and cathode electrode to acid end groups by a
hydrolysis treatment followed by an acid exchange step.
23. The process of claim 20, wherein forming the membrane electrode
assembly for an organic/air fuel cell includes the steps of making
an anode electrocatalyst ink comprised of highly fluorinated
ion-exchange polymer, platinum and ruthenium supported on
particulate carbon, and a solvent, the highly fluorinated
ion-exchange polymer being a perfluorinated polymer having sulfonic
acid end groups, forming the anode electrode by forming a coating
of the anode electrocatalyst ink and removing the solvent from the
anode electrocatalyst ink, making a cathode electrocatalyst ink
comprised of highly fluorinated ion-exchange polymer, the platinum
supported on particulate carbon, and a solvent, the highly
fluorinated ion-exchange polymer being a perfluorinated polymer
having sulfonic acid end groups, forming the cathode electrode by
forming a coating of the cathode electrocatalyst ink and removing
the solvent from the cathode electrocatalyst ink, applying the
anode and cathode electrodes to opposite side of the proton
exchange membrane, said proton exchange membrane being comprised of
a highly fluorinated ion-exchange polymer in proton form.
24. A process for operating a membrane electrode assembly of an
organic/air fuel cell, comprising (a) providing a proton exchange
membrane made of a highly fluorinated ion-exchange polymer, said
membrane having opposite first and second sides; (b) forming an
anode electrode adjacent said first side of the membrane, said
anode electrode comprised of 50 to 90 wt % of an anode
electrocatalyst and 20 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, said anode electrocatalyst being
comprised of an anode metal supported on carbon, wherein the anode
metal is comprised of platinum and ruthenium and the carbon is
particulate carbon, said anode electrocatalyst being comprised of
at least 40 wt % platinum, at least 15 wt % ruthenium, and 15 to 50
wt % particulate carbon wherein the total loading of the anode
metal in the anode electrode is less than 3 mg/cm.sup.2; (c)
forming a cathode electrode adjacent said second side of the
membrane, said cathode electrode comprised of 50 to 90 wt % of a
cathode electrocatalyst and 15 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, said cathode electrocatalyst being
comprised of a cathode metal supported on carbon, wherein the
cathode metal is comprised of platinum and the carbon is
particulate carbon, said cathode electrocatalyst being comprised of
at least 50 wt % platinum and 15 to 50 wt % particulate carbon
wherein the total loading of the cathode metal in the cathode
electrode is less than 3 mg/cm.sup.2, (d) forming an electric
circuit between the anode and cathode electrodes, and (e) feeding a
liquid organic fuel to the anode electrode and oxygen to the
cathode electrode so as to generate an electric current in said
electric circuit.
25. The process of claim 24 wherein the liquid organic fuel is
selected from the group of methanol, ethanol, formaldehyde, formic
acid, and combinations thereof.
26. The process of claim 25 wherein the liquid organic fuel is
methanol.
Description
[0001] The present invention relates to membrane electrode
assemblies, the manufacture of such assemblies, and the use of such
assemblies in organic/air fuel cells such a direct methanol fuel
cells.
BACKGROUND
[0002] Electrochemical cells generally include an anode electrode
and a cathode electrode separated by an electrolyte. A well-known
use of electrochemical cells is in a stack for a fuel cell (a cell
that converts fuel and oxidants to electrical energy) that uses a
proton exchange membrane (PEM) as the electrolyte. In such a fuel
cell, a reactant or reducing gas such as hydrogen or methanol is
supplied to the anode electrode, and an oxidant such as oxygen or
air is supplied to the cathode electrode. The reducing gas
electrochemically reacts at a surface of the anode electrode to
produce hydrogen ions and electrons. The electrons are conducted to
an external load circuit and then returned to the cathode
electrode, while hydrogen ions transfer through the electrolyte to
the cathode electrode, where they react with the oxidant and
electrons to produce water and release thermal energy.
[0003] Fuel cells are typically formed as stacks or assemblages of
membrane electrode assemblies (MEAs), which each include a PEM, an
anode electrode and cathode electrode, and other optional
components. The fuel cells typically also comprise a porous
electrically conductive sheet material that is in electrical
contact with each of the electrodes and permits diffusion of the
reactants to the electrodes, and is known as a gas diffusion layer,
gas diffusion substrate or gas diffusion backing. When the
electrocatalyst is coated on the PEM, the MEA is said to include a
catalyst coated membrane (CCM). In other instances, where the
electrocatalyst is coated on the gas diffusion layer, the MEA is
said to include gas diffusion electrode(s) (GDE).
[0004] The most efficient fuel cells use pure hydrogen as the fuel
and oxygen as the oxidant. However, the use of pure hydrogen has
known disadvantages, including relatively high cost and storage
considerations. Consequently, attempts have been made to operate
fuel cells using fuels other than pure hydrogen. In an organic/air
fuel cell, an organic fuel such as methanol, ethanol, formaldehyde,
or formic acid is oxidized to carbon dioxide at an anode, while air
or oxygen is reduced to water at a cathode. Fuel cells employing
organic fuels are attractive for both stationary and portable
applications, in part, because of the high specific energy of the
organic fuels. One such organic/air fuel cell is a "direct
oxidation" fuel cell in which the organic fuel is directly fed into
the anode, where the fuel is oxidized. A direct methanol fuel cell
is one such fuel cell system.
[0005] Materials customarily used as electrocatalysts are metals or
simple alloys (e.g., Pt, Pt/Ru, Pt--Ir). For example, the
state-of-the-art anode catalysts for organic/air fuel cells (e.g.,
direct methanol) may be based on platinum-ruthenium alloys. In
hydrogen fuel cells, metal catalysts have been supported on high
surface area conductive materials such as carbon to reduce the
amount of catalyst required. For direct methanol fuel cell
applications, where relatively large amounts of metal are typically
needed for both the anode and cathode due to sluggish methanol
oxidation kinetics and methanol cross-over to the cathode,
supported catalysts have conventionally not been used due to the
large amount of precious metal required. The use of a supported
catalyst on the anode or cathode of a hydrogen or direct methanol
cell in which the catalyst has a high metal to support ratio is
disclosed in PCT publication WO2005/001978. However, because the
metal catalyst is the most expensive component of fuel cell MEAs
for organic/air fuel cells, it is essential to somehow further
reduce the amount of catalyst metal used in MEAs without
sacrificing performance in order to make such fuel cells more
economical.
BRIEF DESCRIPTION OF THE DRAWING
[0006] FIG. 1 shows the voltage drop vs. time plot for the direct
methanol MEAs described in Comparative Example G and in Example
5.
DETAILED DESCRIPTION
[0007] The present invention provides a membrane electrode assembly
for an organic/air fuel cell comprising a proton exchange membrane,
an anode electrode, and a cathode electrode. The proton exchange
membrane is made of a highly fluorinated ion-exchange polymer, and
it has opposite first and second sides.
[0008] The anode electrode is adjacent to the first side of the
membrane, and is comprised of 50 to 90 wt % of an anode
electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder. The anode electrocatalyst is comprised
of an anode metal supported on carbon, where the anode metal is
comprised of platinum and ruthenium and the carbon is particulate
carbon. The anode electrocatalyst is comprised of at least 40 wt %
platinum, at least 15 wt % ruthenium, and 15 to 50 wt particulate
carbon, and the total loading of the anode metal in the anode
electrode is less than 3 mg/cm.sup.2.
[0009] The cathode electrode is adjacent the second side of the
membrane, and is comprised of 50 to 90 wt % of a cathode
electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder. The cathode electrocatalyst is
comprised of a cathode metal supported on carbon, where the cathode
metal is comprised of platinum and the carbon is particulate
carbon. The cathode electrocatalyst is comprised of at least 50 wt
% platinum and 15 to 50 wt % particulate carbon, and the total
loading of the cathode metal in the cathode electrode is less than
3 mg/cm.sup.2.
[0010] The invention further provides organic/air fuel cells
comprised of the membrane electrode assemblies of the
invention.
[0011] The invention is also directed to a process for operating a
membrane electrode assembly of an organic/air fuel cell. In the
process, a proton exchange membrane made of a highly fluorinated
ion-exchange polymer is provided. An anode electrode, as described
above, is formed on one side of the membrane and a cathode
electrode, as described above, is formed on the opposite side of
the membrane. The metal loading in the anode electrode is less than
3 mg/cm.sup.2 and the metal loading in the cathode electrode is
less than 3 mg/cm.sup.2. An electric circuit is formed between the
anode and cathode electrodes, and an organic fuel is fed to the
anode electrode and oxygen is fed to the cathode electrode so as to
generate an electric current in the electric circuit. The organic
fuel is preferably selected from the group of methanol, ethanol,
formaldehyde, formic acid, and combinations, and is more preferably
liquid methanol.
Proton Exchange Membrane
[0012] The proton exchange membrane for use in organic/air fuel
cell MEAs according to the invention are comprised of ion-exchange
polymers. Ion-exchange polymers suitable for use in making PEMs of
the MEAs according to the present invention are highly fluorinated
ion-exchange polymers. "Highly fluorinated" means that at least 90%
of the total number of univalent atoms in the polymer are fluorine
atoms. Most typically, the polymer is perfluorinated. It is typical
for polymers used in fuel cell membranes to have sulfonate ion
exchange groups. The term "sulfonate ion exchange groups" as used
herein means either sulfonic acid groups or salts of sulfonic acid
groups, typically alkali metal or ammonium salts.
[0013] The ion-exchange polymer employed comprises a polymer
backbone with recurring side chains attached to the backbone with
the side chains carrying the ion-exchange groups. Homopolymers or
copolymers or blends thereof can be used. Copolymers are typically
formed from one monomer that is a nonfunctional monomer and that
provides atoms for the polymer backbone, and a second monomer that
provides atoms for the polymer backbone and also contributes a side
chain carrying a cation exchange group or its precursor, e.g., a
sulfonyl halide group such a sulfonyl fluoride (--SO.sub.2F), which
can be subsequently hydrolyzed to a sulfonate ion exchange group.
For example, copolymers of a first fluorinated vinyl monomer
together with a second fluorinated vinyl monomer having a sulfonyl
fluoride group can be used. The sulfonic acid form of the polymer
may be utilized to avoid post treatment acid exchange steps.
Exemplary first fluorinated vinyl monomers include
tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride,
vinylidine fluoride, trifluoroethylene, chlorotrifluoroethylene,
perfluoro (alkyl vinyl ether), and mixtures of two or more thereof.
Exemplary second monomers include fluorinated vinyl ethers with
sulfonate ion exchange groups or precursor groups that can provide
the desired side chain in the polymer. The first monomer can also
have a side chain that does not interfere with the ion exchange
function of the sulfonate ion exchange group. Additional monomers
can also be incorporated into the polymers if desired.
[0014] Typical polymers for use in the PEMs include polymers having
a highly fluorinated, most typically a perfluorinated, carbon
backbone with a side chain represented by the formula
--(O--CF.sub.2CFRf).sub.a-(O--CF.sub.2).sub.c--(CFR'f).sub.bSO.sub.3M,
where Rf and R'f are independently selected from F, Cl or a
perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or
2, b=0 to 6, and c=0-1, and M is hydrogen, Li, Na, K or
N(R.sub.1)(R.sub.2)(R.sub.3)(R.sub.4) and R.sub.1, R.sub.2,
R.sub.3, and R.sub.4 are the same or different and are H, CH.sub.3
or C.sub.2H.sub.5. Specific examples of suitable polymers include
those disclosed in U.S. Pat. Nos. 3,282,875; 4,358,545; and
4,940,525. One exemplary polymer comprises a perfluorocarbon
backbone and a side chain represented by the formula
--O--CF.sub.2CF(CF.sub.3)--O--CF.sub.2CF.sub.2SO.sub.3H. Such
polymers are disclosed in U.S. Pat. No. 3,282,875 and can be made
by copolymerization of tetrafluoroethylene (TFE) and the
perfluorinated vinyl ether
CF.sub.2.dbd.CF--O--CF.sub.2CF(CF.sub.3)--O--CF.sub.2CF.sub.2SO.sub.2F,
perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF),
followed by conversion to sulfonate groups by hydrolysis of the
sulfonyl fluoride groups and ion exchanging to convert to the acid
form, also known as the proton form. Another ion-exchange polymer
of the type disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525 has
a side chain --O--CF.sub.2CF.sub.2SO.sub.3H. The polymer can be
made by copolymerization of tetrafluoroethylene (TFE) and the
perfluorinated vinyl ether
CF.sub.2.dbd.CF--O--CF.sub.2CF.sub.2SO.sub.2F,
perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed by
hydrolysis and acid exchange. Suitable perfluorinated polymer
ion-exchange membranes in sulfonic acid form are available under
the trademark Nafion.RTM. from E.I. du Pont de Nemours and Company,
Wilmington, Del.
[0015] For perfluorinated polymers of the type described above, the
ion-exchange capacity of a polymer can be expressed in terms of
ion-exchange ratio ("IXR"). Ion-exchange ratio is the number of
carbon atoms in the polymer backbone in relation to the
ion-exchange groups. A wide range of IXR values for the polymer are
possible. Typically, however, the IXR range for perfluorinated
sulfonate polymers is from about 7 to about 33. For perfluorinated
polymers of the type described hereinabove, the cation exchange
capacity of a polymer can be expressed in terms of equivalent
weight (EW). Equivalent weight (EW), as used herein, is the weight
of the polymer in acid form required to neutralize one equivalent
of NaOH. For a sulfonate polymer having a perfluorocarbon backbone
and a side chain
--O--CF.sub.2--CF(CF.sub.3)--O--CF.sub.2--CF.sub.2--SO.sub.3H (or a
salt thereof), the equivalent weight range corresponding to an IXR
of about 7 to about 33 is about 700 EW to about 2000 EW. A
preferred range for IXR for such a polymer is from about 8 to about
23 (750 to 1500 EW), and a more preferred range is from about 9 to
about 15 (800 to 1100 EW).
[0016] The proton exchange membranes can be made by known extrusion
or casting techniques and may have thicknesses that can vary
depending upon the intended application. The membranes typically
have a thickness of 350 .mu.m or less, with some membranes employed
in certain MEAs for organic/air fuel cell applications having a
thickness of 50 .mu.m or less.
[0017] Reinforced perfluorinated ion exchange polymer membranes can
also be utilized in the MEA according to the invention. Reinforced
membranes can be made by impregnating a porous substrate with
ion-exchange polymer. The porous substrate may improve mechanical
properties for some applications and/or decrease costs. The porous
substrate can be made from a wide range of materials, such as but
not limited to non-woven or woven fabrics, using various weaves
such as the plain weave, basket weave, leno weave, or others. The
porous support may be made from glass, hydrocarbon polymers such as
polyolefins, (e.g., polyethylene, polypropylene, polybutylene, and
copolymers), and perhalogenated polymers such as
polychlorotrifluoroethylene. Porous inorganic or ceramic materials
may also be used. For resistance to thermal and chemical
degradation, the support typically is made from a fluoropolymer,
more typically a perfluoropolymer. For example, the
perfluoropolymer of the porous support can be a microporous film of
polytetrafluoroethylene (PTFE) or a copolymer of
tetrafluoroethylene. Microporous PTFE films and sheeting are known
that are suitable for use as a support layer. For example, U.S.
Pat. No. 3,664,915 discloses uniaxially stretched film having at
least 40% voids. U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390
disclose porous PTFE films having at least 70% voids. Impregnation
of expanded PTFE (ePTFE) with perfluorinated sulfonic acid polymer
is disclosed in U.S. Pat. Nos. 5,547,551 and 6,110,333. ePTFE is
available under the trade name "Goretex" from W. L. Gore and
Associates, Inc., Elkton, Md., and under the trade name "Tetratex"
from Tetratec, Feasterville, Pa.
Electrocatalysts
[0018] The MEA of the invention includes an anode electrode
comprised of 50 to 90 wt % of a platinum and ruthenium containing
electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder, and a cathode electrode comprised of
50 to 90 wt % of a platinum containing electrocatalyst and 10 to 50
wt % of a highly fluorinated ion-exchange polymer binder. The anode
electrode electrocatalyst is comprised of at least 40 wt %
platinum, at least 15 wt % ruthenium, and 15 to 50 wt % particulate
carbon wherein the platinum and ruthenium are supported on the
particulate carbon. The cathode electrode electrocatalyst is
comprised of at least 50 wt % platinum and 15 to 50 wt %
particulate carbon wherein the platinum is supported on the
particulate carbon.
[0019] Preferred particulate carbon support materials (prior to any
optional chemical treatment) are turbostratic or graphitic carbons
of varying surface areas. The carbon is preferably a medium to high
surface area powder having a surface area of 100 to 2000 m.sup.2/g.
Examples of such particulate carbons include Cabot Corporation's
Vulcan.RTM. XC72R, Akzo Noble Ketjen.RTM. 600 or 300, Vulcan.RTM.
Black Pearls (Cabot Corporation), acetylene black (Denki Kagku
Kogyo Kabushiki Kaisha), as well as other carbon particle
varieties. Other particulate carbons include acetylene black and
other graphite powders, single or multiwalled carbon nanotubes,
short fibers and other carbon structures (e.g., fullerenes,
nanohorns).
[0020] The electrocatalysts can be made according to those methods
known to those skilled in the art. For example, the supported
catalysts can be made by a colloidal oxide method (Watanabe et al.,
J. Electroanal. Chem., 229-395, 1987) in which platinum sulfite
acid and other oxidizable precursors (e.g., RuCl.sub.3) are reacted
with hydrogen peroxide to create the colloidal oxide particles to
create deposited electrocatalysts. Other known methods, such as
impregnation followed by chemical reduction or reduction with gas
phase hydrogen, can also be used.
[0021] Suitable metals for use in the anode electrocatalyst include
platinum and ruthenium, and may optionally include additional
metals such as palladium, silver, chromium, cobalt, tungsten,
rhodium, iridium, rhenium and molybdenum and combinations (or
alloys) thereof. The preferred metals for the anode electrocatalyst
are platinum/ruthenium alloys or other compositions containing
platinum and ruthenium. The anode metals may be elemental metals,
metal alloys, metal oxides, and hydrated metal oxides or
combinations thereof. The metals can be used in either a zero
valence state or a non-zero valence state. The metal compositions
are described herein by reference to the weight percentage of the
metal components as a percentage of the total electrocatalyst
weight (including the particulate carbon support). The metals
weight percent is based solely on the weight of the elemental
metals, metal alloys and the metal components of metal oxides or
hydrated metal oxides, and does not include the weight of the
carbon support or other non-metal components. The anode
electrocatalyst of the invention is preferably comprised of about
40 to about 80 weight percent platinum and about 15 to about 50
weight percent ruthenium, and the atomic ratio of Pt:Ru preferably
ranges from 1:1 to 4:1. More preferably, the anode electrocatalyst
is comprised of about 40 to about 60 weight percent platinum, about
20 to about 40 weight percent ruthenium, and about 20 to about 30
weight percent particulate carbon. Most preferably, the anode
electrocatalyst is comprised of about 50 weight percent platinum,
about 25 weight percent ruthenium, and about 25 weight percent
particulate carbon.
[0022] A suitable metal for use in the cathode electrocatalyst is
platinum, and may optionally include additional metals such as
palladium, silver, ruthenium, iridium, chromium, cobalt, tungsten,
rhodium and molybdenum and combinations (or alloys) thereof. The
cathode metals may be elemental metals, metal alloys, metal oxides,
and hydrated metal oxides or combinations thereof. The metals can
be used in either a zero valence state or a non-zero valence state.
The metal compositions are described herein by reference to the
weight percentage of the metal element as a percentage of the total
electrocatalyst weight (including the particulate carbon support).
The metals weight percent is based solely on the weight of the
elemental metals, metal alloys and the metal components of metal
oxides or hydrated metal oxides, and does not include the weight of
the carbon support or other non-metal components. The cathode
electrocatalyst of the invention is preferably comprised of about
50 to about 80 weight percent platinum and about 15 to about 50
weight percent particulate carbon. More preferably, the cathode
electrocatalyst is comprised of about 70 weight percent platinum
and about 30 weight percent particulate carbon.
[0023] In a process to produce the anode or cathode
electrocatalyst, an aqueous platinum mixture may be utilized. The
aqueous platinum mixture may contain a soluble platinum precursor
that either is in a lower valence state (below Pt 4+) or can be
reduced to a lower valence state. The platinum in the aqueous
platinum mixture is preferably provided in its +2 oxidation state
for use in making the catalyst. For example, chloroplatinic acid,
H.sub.2PtCl.sub.6 can be reduced with NaHSO.sub.3 to form
H.sub.3Pt(O.sub.3).sub.2OH, platinum sulfite acid, a Pt(II)
reagent, in situ. Chloroplatinic acid contains Pt in a +4 oxidation
state, i.e., Pt(IV). Alternatively, H.sub.3Pt(SO.sub.3).sub.2OH, or
other soluble platinum +2 (Pt(II)) salts such as ammonium
tetrachloroplatinate (II), potassium tetrachloroplatinate (II),
water soluble platinum (II) phosphine complexes (e.g.
chlorotris(2,3,5-triaza-7-phosphoadamantane)platinum (II) chloride,
(TPA).sub.3PtCl.sub.2) or other lower valent water soluble platinum
salts, can be used directly. However, the use of chloroplatinic
acid, followed by reaction with NaHSO.sub.3, or of
H.sub.3Pt(SO.sub.3).sub.2OH directly is preferred. When
electrocatalysts containing platinum are prepared using
chloroplatinic acid, the concentration of the chloroplatinic acid
solution is not critical. However, the concentration of
chloroplatinic acid can generally vary between about 1 and about 20
weight percent platinum, with about 5 to about 15 weight percent
platinum being advantageously used.
[0024] The anode electrocatalyst comprising platinum and ruthenium
can be prepared using a reagent solution also containing ruthenium,
such as ruthenium chloride solution, which is combined with an
aqueous platinum mixture as described above, in the presence of an
oxidant, to form an electrocatalyst mixture. Ruthenium chloride
solutions can be prepared by methods known to those skilled in the
art. Although the concentration of ruthenium chloride in the
solution is not critical, from about 1 weight percent to about 10
weight percent can be advantageously used, and about 2 weight
percent is preferred. The ruthenium chloride solution is preferably
added to the aqueous platinum mixture described above. Also
preferably, when used with a particulate carbon support, the
ruthenium chloride solution is added at a rate greater than 0.3
mmoles Ru/minute, preferably from about 0.7 to about 4.0 mmoles
ruthenium/minute, more preferably, from about 0.9 to about 3.6
mmoles Ru/minute. Other soluble ruthenium precursors can also be
used, such as ruthenium (III) nitrosylnitrate, ruthenium (III)
nitrosylsulfate, and other water soluble ruthenium reagents with a
ruthenium valence less than (IV). Ruthenium chloride is
preferred.
[0025] In a process for making a supported platinum cathode
catalyst, following the generation of, or with the direct use of, a
Pt(II) reagent, and the formation of the electrocatalyst mixture,
an oxidizer, such as hydrogen peroxide, is added to the
electrocatalyst mixture. Other suitable oxidizing agents include
water soluble agents (e.g., hypochlorous acid) or gas phase
oxidizing agents such as ozone. Gas phase oxidizing agents can be
introduced by bubbling into the liquid media. The oxidizing agent
is added to convert the Pt(II) reagent to colloidal PtO.sub.2, in
which platinum is Pt(IV). The introduction of the oxidizing agent
forms a colloid mixture, in which the platinum is present in
colloidal form.
[0026] In a process for making a supported platinum/ruthenium anode
catalyst, the oxidizers described immediately above can also be
used. When ruthenium chloride is present in the electrocatalyst
mixture, and excess oxidizing agent (e.g., excess hydrogen
peroxide) is present, the oxidizing agent can react with the
ruthenium chloride to form ruthenium oxide, which is also present
as a colloid.
[0027] The amount of hydrogen peroxide used in the reaction can be
from about 15:1 to 700:1, based on the mole ratio of H.sub.2O.sub.2
to total moles of metal, preferably 100:1 to 300:1 and more
preferably about 210:1. When a platinum and ruthenium
electrocatalyst mixture is being generated, instead of adding all
of the hydrogen peroxide after the addition of the platinum
solution, a portion of the hydrogen peroxide can be added
simultaneously with the ruthenium chloride.
[0028] A surfactant or dispersant can be added to the
chloroplatinic acid solution, following the addition of NaHSO.sub.3
to generate H.sub.3Pt(SO.sub.3).sub.2OH). Alternatively, if the
Pt(II) reagent is incorporated directly rather than generated in
situ, a surfactant or dispersant can be added directly thereto.
When making the platinum/ruthenium anode electrocatalyst, a
surfactant or dispersant can also be directly added after the
addition of the ruthenium chloride, which may be desirable when
foaming or reaction of the surfactant with hydrogen peroxide is
likely. As yet another alternative, the surfactant or dispersant
can be added to the carbon, which is then added to the colloid
mixture. In one preferred embodiment, the surfactant is added to
the colloid mixture following the RuCl.sub.3 addition, optionally
dispersing the carbon support and adding the surfactant carbon
support slurry to the reaction mixture.
[0029] Surfactants and dispersants known to those skilled in the
art are disclosed in PCT application WO 2004/073090, which is
hereby incorporated by reference. As used herein, "dispersants"
refers to a class of materials that are capable of bringing fine
solid particles into a state of suspension so as to inhibit or
prevent their agglomeration or settling in a fluid medium. The term
"surfactant" (or surface active agent) as used herein refers to
substances with certain characteristic features in structure and
properties, such as amphipathic structure (having groups with
opposing solubility tendencies); solubility in liquid media;
formation of micelles at certain concentrations; formation of
orientated monolayers at phase interfaces--surfactant molecules and
ions form oriented monolayers at phase interfaces (in this case,
liquid-solid interface); and adsorption at interfaces. Thus,
although a surfactant can operate to disperse particles, a
dispersant need not have the properties of a surfactant and can
operate by different mechanisms than would a surfactant.
Accordingly, the terms are not used interchangeably herein.
Surfactants and dispersants suitable for use in the processes for
making the electrocatalysts can be anionic surfactants containing
carboxylate, sulfonate, sulfate or phosphate groups; and nonionic
surfactants such as those derived from ethoxylates, carboxylic acid
esters, carboxylic amines, and polyalkylene oxide block
copolymers.
[0030] The surfactant or dispersant is preferably provided in the
form of a suspension. The suspension contains sufficient surfactant
or dispersant to stabilize the colloid and the particulate carbon.
Preferably, the suspension contains from about 0.0001 weight
percent to about 20 weight percent of surfactant or dispersant
based on the total combined weight of solids. Total combined weight
of solids means the total weight of surfactant/dispersant, metal,
and particulate carbon. More preferably, the suspension contains
from about 0 weight percent to about 10 weight percent of
surfactant or dispersant, even more preferably from about 0.01 to
about 5 weight percent surfactant or dispersant, and still more
preferably from about 1 to about 2 weight percent surfactant or
dispersant, based on the total combined weight of solids. The
concentration of surfactant or dispersant in the suspension is not
critical. However, it has been found that a surfactant or
dispersant concentration of about 10 weight percent can be
advantageously used.
[0031] Sodium hydrogen sulfite, NaHSO.sub.3, which converts
platinum (IV) chloride to a platinum (II) hydrogen sulfite, can
also be present in the suspension and can be provided in this
manner for the conversion of the platinum to the +2 oxidation
state. The concentration of NaHSO.sub.3 can vary, and, expressed in
terms of the mole ratio of NaHSO.sub.3 to platinum, is preferably
from about 3:1 to about 20:1, more preferably from about 5:1 to
about 15:1, and even more preferably from about 7:1 to about
12:1.
[0032] After the platinum reagent has been generated to form a
cathode electrocatalyst mixture or after the platinum and ruthenium
reagents have been generated to form an anode electrocatalyst
mixture, and following addition of hydrogen peroxide, chemically
treated particulate carbon, such as acidified particulate carbon,
is added to the catalyst mixture. The carbon can be provided, for
example, as a slurry or in solid form. Chemically treating the
carbon can be accomplished by methods know to those skilled in the
art. Acidification can be carried out using various oxidizing
acids. For example, carbon particles can be treated with an
oxidizing agent such as oxygen gas, hydrogen peroxide, organic
peroxides, ozone, or they can be oxidized and acidified with
oxidizing acids such as, for example, nitric acid, perchloric acid,
chloric acid, permanganic acid, or chromic acid. In some
embodiments, a slurry of particulate carbon can be made with a
dilute acid solution, and acidification can be effected by heating,
for example, by refluxing the slurry. Optionally, in particular
when the particles are treated with a functionalizing agent such as
oxygen gas, ozone or a volatile organic peroxide, the particles can
be heated, for example, to a temperature of about 175.degree. C.,
preferably no higher than about 100.degree. C. to avoid
decomposition of the carbon.
[0033] After the carbon has been added to the catalyst mixture, the
catalyst mixture and carbon are contacted with a precipitating
agent, which, it is believed, partially reduces the catalyst
mixture and helps precipitate or deposit the metal on the support.
Hydrogen gas is a preferred precipitating agent. Optionally, the
contacting with the reducing agent can be done in a controlled
environment, such as, for example, in the presence of nitrogen. The
use of an inert atmosphere such as a nitrogen atmosphere may be
desirable when hydrogen gas is used as the precipitating agent.
[0034] Controlling the rate of addition of certain components used
in making the electrocatalysts to other components improves the
quality of the electrocatalysts produced according to the processes
disclosed herein. In addition, functionalization of the particulate
carbon support used in the processes, when combined with control of
the rate of addition of components to each other and/or the use of
a surfactant or dispersant, provides improved properties of the
electrocatalyst produced including minimization or elimination of
metal particle agglomeration on the carbon support.
[0035] In a preferred embodiment of the invention, the anode
electrocatalyst is in the form of anode electrocatalyst particles
and the cathode electrocatalyst is in the form of cathode
electrocatalyst particles. These particles may be made up of
aggregates of smaller primary particles. When these electrocatalyst
particles are incorporated into an electrode, it is preferred that
at least 98% of the anode and cathode electrocatalyst particles
have a particle diameter of less than 10 microns. Particles size
may be measured using laser light scattering measurement
techniques. The size of the particles in a liquid catalyst ink is
measured using a Hegman guage.
Electrodes
[0036] The MEA of the invention includes an anode electrode facing
one side of the PEM and a cathode electrode facing the opposite
side of the PEM. The anode and cathode electrodes may be coated on
or adhered directly to the PEM so as to form a CCM, which is
sometimes referred to as an MEA3. Alternatively, one or both of the
electrodes may be coated on or adhered to the PEM-facing side of
gas diffusion layers positioned on opposite sides of the PEM.
[0037] According to the invention, the anode electrode adjacent a
first side of the PEM is comprised of 50 to 90 wt % of an anode
electrocatalyst and 10 to 50 wt % of a highly fluorinated
ion-exchange polymer binder. The anode electrocatalyst is comprised
of at least 40 wt % platinum, at least 15 wt % ruthenium, and 15 to
50 wt % particulate carbon wherein the platinum catalyst and
ruthenium catalyst are supported on the particulate carbon, and
wherein the total loading of the platinum, ruthenium and any other
metal in the anode electrode is less than 3 mg/cm.sup.2.
[0038] According to the invention, the cathode electrode adjacent
the opposite second side of the PEM from the anode electrode is
comprised of 50 to 90 wt % of an electrocatalyst and 10 to 50 wt %
of a highly fluorinated ion-exchanged polymer binder. The cathode
electrocatalyst is comprised of at least 50 wt % platinum and 15 to
50 wt % particulate carbon wherein the platinum is supported on the
particulate carbon, and wherein the total loading of the platinum
and any other metal in the cathode electrode is less than 3
mg/cm.sup.2.
[0039] In the anode electrode, it is preferable to adjust the
amounts of anode electrocatalyst, ion-exchange polymer and other
components, if present, so that the anode electrocatalyst is a
major component by weight of the resulting electrode. More
preferably, the weight ratio of anode electrocatalyst to
ion-exchange polymer binder in the anode electrode is about 1:1 to
about 10:1, and more preferably 2:1 to 5:1.
[0040] In the cathode electrode, it is preferable to adjust the
amounts of cathode electrocatalyst, ion-exchange polymer and other
components, if present, so that the cathode electrocatalyst is a
major component by weight of the resulting electrode. More
preferably, the weight ratio of cathode electrocatalyst to
ion-exchange polymer binder in the cathode electrode is about 1:1
to about 10:1, and more preferably 2:1 to 5:1.
[0041] For the electrodes to function effectively in the fuel
cells, effective anode and cathode electrocatalyst sites must be
provided in the anode and cathode electrodes. In order for the
anode and cathode to be effective: (1) the electrocatalyst sites
must be accessible to the reactant, (2) the electrocatalyst sites
must be electrically connected to the gas diffusion layer, and (3)
the electrocatalyst sites must be ionically connected to the fuel
cell electrolyte. It is believed that the incorporation of a porous
particulate carbon support in the anode and cathode
electrocatalysts helps to make the electrocatalyst sites more
accessible to the reactants while also electrically connecting the
electrocatalyst sites to the diffusion layer of the MEA. The
electrocatalyst sites are ionically connected to the electrolyte
via the ion-exchange polymer binder of the electrode.
[0042] Because the binder employed in the electrode serves not only
as binder for the electrocatalyst particles, but may also assist in
securing the electrode to the membrane, it is preferred that the
ion-exchange polymers in the binder composition be compatible with
the ion-exchange polymer in the membrane. Most typically,
ion-exchange polymers in the binder composition are the same type
as the ion-exchange polymer in the PEM. Where the electrodes are
coated on or otherwise adhered to the membrane, the binder used in
the electrodes is preferably the same ion exchange polymers that
comprise the membranes. Ion-exchange polymers suitable for the
binder of the anode and cathode electrodes of the MEAs of the
invention are the highly fluorinated ion-exchange polymers
discussed above for use in making the proton exchange membranes.
The ion-exchange polymers typically have end groups in sulfonyl
halide form, but may alternatively have end groups in the sulfonic
acid form.
[0043] In order to form the anode or cathode electrodes, the anode
electrocatalyst or the cathode electrocatalyst, as described above
is slurried with a dispersion of a highly fluorinated ion-exchange
polymer, preferably a perfluorinated ionomer in water, alcohol, or,
preferably a water/alcohol mixture to form a catalyst dispersion.
Any additional additives such as are commonly employed in the art
may also be incorporated into the slurry.
[0044] Preferred ionomers are those described above for use in the
proton exchange membrane, such as perfluorinated copolymers of PTFE
and a monomer having pendant groups described by the formula
--(O--CF.sub.2CFR.sub.f).sub.a--O--CF.sub.2(CFR'.sub.f).sub.b
SO.sub.2F,
where R.sub.f and R'.sub.f' are independently selected from F, Cl
or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1
or 2, and b=0 to 6. Preferably, R is trifluoromethyl, R' is F, a=0
or 1 and b=1. Most preferably, R.sub.f is trifluoromethyl,
R'.sub.f' is F, a=1 and b=1. Alternatively, the ion-exchange
polymer can be a copolymer of PTFE and a monomer having pendant
groups described by the formula
--(O--CF.sub.2CFR.sub.f).sub.a--O--CF.sub.2(CFR'.sub.f).sub.bSO.sub.3-M+-
,
where R.sub.f and R'.sub.f' are independently selected from F, Cl
or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1
or 2, b=0 to 6, and M is H or a univalent metal. Preferably,
R.sub.f is trifluoromethyl, R'.sub.f' is F, a=0 or 1, b=1, and M is
H or an alkali metal, and most preferably, a=1 and M is H. When M
is an alkali metal such that the sulfonyl halide form of the
polymer is used (e.g., --SO.sub.2F), an additional ion exchange
step must be introduced at some convenient stage in the process
herein outlined to convert the electrode to the acid form (i.e.,
convert the M to H).
[0045] Anode and cathode electrodes for the MEAs of the invention
can be produced using an ion-exchange polymer binder in the acid
form in the ink. The anode or cathode electrocatalyst may be
combined with the acid form of the ionomer, and electrochemically
active electrodes can fabricated directly from the combination
without additional treatment steps. Alternatively, the cathode and
anode electrodes can be formed from a slurry in which the
perfluorinated ionomer is in sulfonyl fluoride form and later
converted to the acid form after the electrodes have been formed.
Contacting the sulfonyl fluoride form of the ionomer with a mineral
acid in any convenient manner will suffice to convert it to the
acid form required for it to conduct protons. Suitable ion-exchange
polymers have equivalent weights in the range of 700-2000EW.
[0046] An electrocatalyst ink or paste for use in making the anode
or cathode electrode is made by combining the electrocatalyst, the
highly fluorinated ion-exchange polymer, and a suitable liquid
medium. It is advantageous for the medium to have a sufficiently
low boiling point that rapid drying of electrode layers is possible
under the process conditions employed, provided however, that the
composition does not dry so fast that the composition dries before
transfer to the membrane in cases where it is desired for the
electrode to be wet at the time of transfer. When flammable
constituents are to be employed, the medium can be selected to
minimize process risks associated with such constituents, as the
medium is in contact with the electrocatalyst during use. The
medium should also be sufficiently stable in the presence of the
ion-exchange polymer which, in the acid form, has strong acidic
activity. The liquid medium is typically polar for compatibility
with the ion-exchange polymer, and is preferably able to wet the
proton exchange membrane. Preferably, the ion-exchange polymer in
the electrode forms a stable layer upon drying of the liquid medium
and the polymer does not require post treatment steps such as
heating to form a stable electrode layer. Where the liquid medium
is water, it may be used in combination with surfactant, alcohols
or other miscible solvents.
[0047] A wide variety of polar organic liquids and mixtures thereof
can serve as suitable liquid medium for the electrocatalyst coating
ink or paste. Water can be present in the medium if it does not
interfere with the coating process. Although some polar organic
liquids can swell the membrane when present in sufficiently large
quantity, the amount of liquid used in the electrocatalyst coating
is preferably small enough that the adverse effects from swelling
during the process are minor or undetectable. It is believed that
solvents able to swell the ion-exchange membrane can provide better
contact and more secure application of the electrode to the
membrane. A variety of alcohols are well suited for use as the
liquid medium.
[0048] Typical liquid medium include suitable C.sub.4 to C.sub.8
alkyl alcohols such as n-, iso-, sec- and tert-butyl alcohols; the
isomeric 5-carbon alcohols such as 1, 2- and 3-pentanol,
2-methyl-1-butanol, 3-methyl, 1-butanol; the isomeric 6-carbon
alcohols, such as 1-, 2-, and 3-hexanol, 2-methyl-1-pentanol,
3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl, 1-pentanol,
4-methyl-1-pentanol; the isomeric C.sub.7 alcohols and the isomeric
C.sub.8 alcohols. Cyclic alcohols are also suitable. Preferred
alcohols are n-butanol and n-hexanol, and n-hexanol is more
preferred. Other preferred liquid mediums are fluorinated solvents
such as the primarily 12 carbon perfluoro compounds of FC-40 and
FC-70 Fluorinert.TM. brand electronic liquids from 3M Company. The
amount of liquid medium used in the electrocatalyst coating ink or
paste varies and is determined by the type of medium employed, the
constituents of the electrocatalyst coating, the type of coating
equipment employed, the desired electrode thickness, the process
speeds, and other process conditions.
[0049] The size of the particles in the electrocatalyst ink is
reduced by grinding, milling or sonication to obtain a particle
size that results in the best utilization of the electrocatalyst.
The particle size, as measured by a Hegman gauge, is preferably
reduced to less than 10 microns and more preferably to less than 5
microns.
[0050] The resulting electrocatalyst paste or ink may then be
coated onto an appropriate substrate for incorporation into an MEA.
The method by which the coating is applied is not critical to the
practice of the present invention. Known electrocatalyst coating
techniques can be used and produce a wide variety of applied layers
of essentially any thickness ranging from very thick, e.g., 30
.mu.m or more, to very thin, e.g., 1 .mu.m or less. Typical
manufacturing techniques involve the application of the
electrocatalyst ink or paste onto either the polymer exchange
membrane or a gas diffusion substrate. Additionally, electrode
decals can be fabricated and then transferred to the membrane or
gas diffusion backing layers. Methods for applying the
electrocatalyst onto the substrate include spraying, painting,
patch coating and screen printing or flexographic printing.
Preferably, the thickness of the anode and cathode electrodes
ranges from about 0.1 to about 30 microns, more preferably less
than 25 micron. The applied layer thickness is dependent upon
compositional factors as well as the process used to generate the
layer. The compositional factors include the metal loading on the
coated substrate, the void fraction (porosity) of the layer, the
amount of polymer/ionomer used, the density of the polymer/ionomer,
and the density of the carbon support. The process used to generate
the layer (e.g. a hot pressing process versus a painted on coating
or drying conditions) can affect the porosity and thus the
thickness of the layer.
[0051] In a preferred embodiment, a catalyst coated membrane is
formed wherein thin electrode layers are attached directly to
opposite side of the proton exchange membrane. In one method of
preparation, the catalyst film is prepared as a decal by spreading
the catalyst ink on a flat release substrate such as Kapton.RTM.
polyimide film (available from the DuPont, Wilmington, Del.). The
decal is transferred to the surface of the membrane by the
application of pressure and optional heat, followed by removal of
the release substrate to form a CCM with a catalyst layer having a
controlled thickness and catalyst distribution. The membrane is
preferably wet at the time that the electrode decal is transferred
to the membrane. Alternatively, the electrocatalyst ink may be
applied directly to the membrane, such as by printing, after which
the catalyst film is dried at a temperature not greater than
200.degree. C. The CCM, thus formed, is then combined with a gas
diffusion backing substrate to form an MEA.
[0052] Another method is to first combine the catalyst ink of the
invention with a gas diffusion backing substrate, and then, in a
subsequent thermal consolidation step, with the proton exchange
membrane. This consolidation may be performed simultaneously with
consolidation of the MEA at a temperature no greater than
200.degree. C., preferably in the range of 140-160.degree. C. The
gas diffusion backing comprises a porous, conductive sheet material
such as paper or cloth, made from a woven or non-woven carbon
fiber, that can optionally be treated to exhibit hydrophilic or
hydrophobic behavior, and coated on one or both surfaces with a gas
diffusion layer, typically comprising a film of particles and a
binder, for example, fluoropolymers such as PTFE. Gas diffusion
backings for use in accordance with the present invention as well
as the methods for making the gas diffusion backings are those
conventional gas diffusion backings and methods known to those
skilled in the art. Suitable gas diffusion backings are
commercially available, including for example, Zoltek.RTM. carbon
cloth (available from Zoltek Companies, St. Louis, Mo.) and
ELAT.RTM. (available from E-TEK Incorporated, Natick, Mass.).
Membrane Electrode Assemblies for Fuel Cells
[0053] The present invention also contemplates the use of the
membrane electrode assemblies in a fuel cell, wherein the assembly
includes the proton exchange membrane, the anode and cathode
electrodes, and the gas diffusion backings. Bipolar separator
plates, made of a conductive material and providing flow fields for
the reactants, are placed between adjacent MEAs. A number of MEAs
and bipolar plates are assembled in this manner to provide a fuel
cell stack.
[0054] It is desirable to seal reactant fluid stream passages in a
fuel cell stack to prevent leaks or inter-mixing of the fuel and
oxidant fluid streams. Fuel cell stacks typically employ fluid
tight resilient seals, such as elastomeric gaskets between the
separator plates and membranes. Such seals typically circumscribe
the manifolds and the electrochemically active area. Sealing can be
achieved by applying a compressive force to the resilient gasket
seals. Compression enhances both sealing and electrical contact
between the surfaces of the separator plates and the MEAs, and
sealing between adjacent fuel cell stack components. In
conventional fuel cell stacks, the fuel cell stacks are typically
compressed and maintained in their assembled state between a pair
of end plates by one or more metal tie rods or tension members. The
tie rods typically extend through holes formed in the stack end
plates, and have associated nuts or other fastening means to secure
them in the stack assembly. The tie rods can be external, that is,
not extending through the fuel cell plates and MEAs, however,
external tie rods can add significantly to the stack weight and
volume. It is generally preferable to use one or more internal tie
rods that extend between the stack end plates through openings in
the fuel cell plates and MEAs as described in U.S. Pat. No.
5,484,666. Typically resilient members are utilized to cooperate
with the tie rods and end plates to urge the two end plates towards
each other to compress the fuel cell stack.
[0055] The resilient members accommodate changes in stack length
caused by, for example, thermal or pressure induced expansion and
contraction, and/or deformation. That is, the resilient member
expands to maintain a compressive load on the fuel cell assemblies
if the thickness of the fuel cell assemblies shrinks. The resilient
member may also compress to accommodate increases in the thickness
of the fuel cell assemblies. Preferably, the resilient member is
selected to provide a substantially uniform compressive force to
the fuel cell assemblies, within anticipated expansion and
contraction limits for an operating fuel cell. The resilient member
can comprise mechanical springs, or a hydraulic or pneumatic
piston, or spring plates, or pressure pads, or other resilient
compressive devices or mechanisms. For example, one or more spring
plates can be layered in the stack. The resilient member cooperates
with the tension member to urge the end plates toward each other,
thereby applying a compressive load to the fuel cell assemblies and
a tensile load to the tension member.
[0056] Organic/air fuel cells made using the membrane electrode
assemblies as disclosed herein show an unexpected level of
performance at significantly lower catalyst metal loadings than
conventional organic/air fuel cells. A current density of 70
mWcm.sup.2 at a voltage of 400 mVolts is generally the minimum
necessary for use in a direct methanol fuel cell. This level of
performance is exceeded by MEAs according to the invention having
anode and cathode metals loading per area of electrode of less than
3 mg metal/cm.sup.2, and even less than 2 mg metal/cm.sup.2. By
contrast, in conventional MEAs where both cathode and anode
electrocatalysts are unsupported, metal loading per electrode of at
least 4.5 mg metal/cm.sup.2 is needed to achieve the necessary
minimum current density. Even where the cathode or anode catalyst
has been supported on a carbon support, it has not been possible to
achieve minimum necessary current densities with metals loading of
under 4.5 mg metal/cm.sup.2 in both the anode and cathode
electrodes. Surprisingly, with an MEA according to one embodiment
of the present invention, satisfactory current density was obtained
with a total cathode and anode metals loading of less than 2.5 mg
metal/cm.sup.2. Equally significant, MEAs according to the
invention maintain their voltage performance significantly longer
than conventional MEAs with metal loadings much higher than the
MEAs according to the invention. Thus the MEAs of the invention
have the advantage that they lasts longer while at the same time
using significantly less of an expensive catalyst metal to provide
the desired current density.
Examples
[0057] The following specific examples are intended to illustrate
the practice of the invention and should not be considered to be
limiting in any way.
[0058] The following electrodes were prepared for use in the
comparative examples and the examples below.
[0059] Cathode-A: A cathode catalyst dispersion ink was prepared in
an Eiger.RTM. bead mill, (manufactured by Eiger Machinery Inc.,
Greylake, Ill.), containing 70 ml 1.0-1.25 millimeter zirconia
grinding media. 100 grams of platinum black (unsupported) catalyst
powder (fuel cell grade catalyst obtained from Colonial Metals,
Elkton Md.) and 317.5 grams of a 3.5 wt % Nafion.RTM. solution
(DuPont, Wilmington, Del.) in FC-40 Fluorinert.TM. brand electronic
liquid perfluorinated solvent (3M Company, Minneapolis, Minn.) (the
polymer resin had a 830 EW measured by FTIR and was in the sulfonyl
fluoride form) were mixed and charged into the mill and dispersed
for about 2 hours. Material was withdrawn from the mill and
particle size was measured. The ink was tested to ensure that the
particle size was under 10 microns and the percent solids was about
22.5 wt %. The ink was concentrated in a rotovap to 13 wt % solids.
A catalyst electrode decal was prepared by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film (obtained from DuPont, Wilmington,
Del.). A Pt loading of 4.5 mg Pt/cm.sup.2 was achieved by knife
drawdown coating with this ink. The dry coating thickness was about
0.5 mil.
[0060] Cathode-B: The electrocatalyst ink used to produce Cathode-A
(solids content 13 wt %) was used to produce another catalyst
electrode decal by drawing down the catalyst ink to a dimension of
5 cm.times.5 cm (to give a total area of 25 cm.sup.2) on a 10
cm.times.10 cm piece of 3 mil thick Kapton.RTM. polyimide film. A
Pt metal loading of 5.4 mg/cm.sup.2 was achieved by knife drawdown
coating with this ink.
[0061] Cathode-C: The electrocatalyst ink used to produce Cathode-A
(solids content 13 wt %) was used to produce another catalyst
electrode decal by drawing down the catalyst ink to a dimension of
5 cm.times.5 cm (to give a total area of 25 cm.sup.2) on a 10
cm.times.10 cm piece of 3 mil thick Kapton.RTM. polyimide film. A
Pt metal loading of 4.8 mg/cm.sup.2 was achieved by knife drawdown
coating with this ink. The dry coating thickness was about 0.5
mil.
[0062] Anode-A: An anode catalyst dispersion ink was prepared in an
Eiger.RTM. bead mill, containing 70 ml 1.0-1.25 millimeter zirconia
grinding media. 40 grams of platinum/ruthenium (50/50 alloy) black
(unsupported) catalyst powder (Hi-Spec 6000 obtained from Johnson
Mathey, London, England), and 162.9 grams of 3.5 wt % Nafion.RTM.
solution (DuPont, Wilmington, Del.) in FC-40 Fluorinert.TM. brand
electronic liquid perfluorinated solvent (3M Company, Minneapolis,
Minn.) (the polymer resin had a 830 EW measured by FTIR and was in
the sulfonyl fluoride form) were mixed and charged into the mill
and dispersed for about 2 hours. Material was withdrawn from the
mill and particle size measured. The ink was tested to ensure that
the particle size was under 10 microns and the % solids was about
13 wt %. A catalyst electrode decal was prepared by drawing down
the catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 4.5
mg/cm.sup.2 was achieved by knife drawdown coating with the ink.
The dry coating thickness was about 0.5 mil.
[0063] Anode-B: The electrocatalyst ink used to produce Anode-A
(solids content 13 wt %) was used to produce another catalyst
electrode decal by drawing down the catalyst ink to a dimension of
5 cm.times.5 cm (to give a total area of 25 cm.sup.2) on a 10
cm.times.10 cm piece of 3 mil thick Kapton.RTM. polyimide film. A
Pt/Ru loading of 4.3 mg/cm.sup.2 was achieved by knife drawdown
coating with this ink.
[0064] Anode-C: The electrocatalyst ink used to produce Anode-A
(solids content 13 wt %) was used to produce another catalyst
electrode decal by drawing down the catalyst ink to a dimension of
5 cm.times.5 cm (to give a total area of 25 cm.sup.2) on a 10
cm.times.10 cm piece of 3 mil thick Kapton.RTM. polyimide film. A
Pt/Ru loading of 5.1 mg/cm.sup.2 was achieved by knife drawdown
coating with this ink. The dry coating thickness was about 0.6
mil.
[0065] Cathode-1: 380.8 grams of a 3.5 wt % Nafion.RTM. solution in
FC-40 Fluorinert.TM. brand electronic liquid perfluorinated solvent
(the polymer resin had a 830 EW measured by FTIR and was in the
sulfonyl fluoride form) was placed in a 600 gram container. The
container was cooled in an ice bath to bring down the solution
temperature to .about.0.degree. C. while stirring the solution at
350 rpm using a high speed mixer (BDC 2002 mixer made by Caframo)
in a nitrogen atmosphere. After the solution temperature reached
.about.0.degree. C., 30 grams of carbon supported Pt catalyst (67
wt % Pt, 33 wt % particulate carbon) with a BET surface area of 215
m.sup.2/g (TEC10E70TPM catalyst obtained from Tanaka Kikinzoku
Kogyo KK, Kanagawa, Japan) was added slowly to the Nafion.RTM.
solution over a period of about 5-7 minutes while mixing continued.
Stirring was continued for 45 minutes after the addition of all of
the carbon supported Pt. The mixture was then "sonicated" using a
Branson Sonifier 450 at 70% power to break-up the electrocatalyst
particles for 3-5 minutes at a time or until the temperature
reached about 70.degree. C. When the dispersion temperature reached
70.degree. C., the sonication was stopped and the dispersion was
cooled to room temperature in the ice bath before "sonication" was
resumed. Sonication was stopped when the maximum particle size in
the ink dispersion was determined to be less than 5 microns.
Particle size was measured using a Hegman gauge. This ink
dispersion was concentrated using a "rotovap" at about 70.degree.
C. until the solids content of the ink was about 23 wt %. The
maximum particle size in the ink was once again tested. If the
maximum particle size was more than 5 micron, the ink was sonicated
again using the sonication process described above until the
maximum particle size was below 5 microns. The solid content and
the viscosity of the ink were measured and they were 21.4 wt % and
26,510 centipoise respectively.
[0066] A catalyst electrode decal was prepared by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt metal loading of 2.2 mg/cm.sup.2
was achieved by knife drawdown coating with this ink. The dry
coating thickness was about 0.9 mil.
[0067] Cathode-2: The electrocatalyst ink used to produce Cathode-1
(solids content 21.4 wt % and viscosity 26510 centipoise) was used
to produce another catalyst electrode decal by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt metal loading of 2.21 mg/cm.sup.2
was achieved by knife drawdown coating with this ink. The dry
coating thickness was about 0.9 mil.
[0068] Cathode-3: The electrocatalyst ink used to produce Cathode-1
(solids content 21.4 wt % and viscosity of 26,510 centipoise) was
used to produce another catalyst electrode decal by drawing down
the catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt metal loading of 1.12 mg/cm.sup.2
was achieved by knife drawdown coating with this ink. The dry
coating thickness was about 0.45 mil.
[0069] Cathode-4: The electrocatalyst ink used to produce Cathode-1
was used to produce another catalyst electrode decal by drawing
down the catalyst ink to a dimension of 5 cm.times.5 cm (to give a
total area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil
thick Kapton.RTM. polyimide film. A Pt loading was achieved by
knife drawdown coating with this ink. A Pt metal loading of 1.9 mg
Pt/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 2.0 mil.
[0070] Cathode-5: The electrocatalyst ink used to produce Cathode-1
(solids content 21.4 wt % and viscosity of 26,510 centipoise) was
used to produce another catalyst electrode decal by drawing down
the catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt metal loading of 1.0 mg/cm2 was
achieved by knife drawdown coating with this ink. The dry coating
thickness was about 0.4 mil.
[0071] Cathode-6: 109.6 grams of Nafion.RTM. 920 EW dispersion (in
the proton form) (DuPont DE2020, 21.3% solids) was added to a 1000
ml poly beaker which was immersed in an ice bath. The poly beaker
was in a nitrogen purged box in a hood and the dispersion was
chilled to 10.degree. C. while stirring the solution with high
shear mixing. After the dispersion temperature reached
.about.10.degree. C., 81.7 g of an electrocatalyst comprised of
platinum metal supported on particulate carbon (70% wt % Pt, 30 wt
% particulate carbon) (TEC10E70TPM obtained from Tanaka Kikinzoku
Kogyo KK, Kanagawa, Japan) was slowly added to the Nafion.RTM.
dispersion over a period of about 15 minutes with high shear mixing
continued. An ice chilled mixture of n-propanol (130.7 g) and
iso-propanol (178.9 g) was slowly added to the dispersion of
Nafion.RTM. and electrocatalyst over a period of about 15 minutes
while mixing continued. Mixing was stopped and the dispersion was
allowed to warm up to room temperature and equilibrate for 24
hours. After 24 hours, a large sonic horn and a high shear mixer
was introduced into the dispersion. The dispersion was chilled to 9
C..degree. while stirring with the high shear mixer. The chilled
dispersion was sonicated a setting of 9 for three minutes with high
shear mixing. The dispersion was then allowed to cool for 15
minutes while high shear mixing was continued. Five more cycles of
three minutes of sonication followed by 15 minutes of cooling were
carried out. The dispersion was allowed to sit overnight. To this
dispersion 35.8 grams of Diprolyleneglycolmonomethyl ether (DPM)
was added and the dispersion was agitated for 1 minute followed by
10 minutes of equilibration. Two more cycles of agitation and
equilibration were carried out.
[0072] A cathode decal was prepared by casting onto a Kapton.RTM.
polyimide film with a 60 bar rod, drying overnight, and then
casting a second layer with a 70 rod bar applied perpendicular to
the casting direction of the first film layer. The decal was dried
in a vacuum oven with a nitrogen purge for two hours at 150.degree.
F.
[0073] Cathode 7: 33.9 grams of Nafion.RTM. 920 EW dispersion (in
the proton form) (DuPont DE2020, 21.3% solids) was added to a
beaker which was immersed in an ice bath. The beaker was in a
nitrogen purged box in a hood and the dispersion was chilled to
about 10.degree. C. while stirring the solution with high shear
mixing. 41.74 grams of a 1:1:1 mixture of isopropyl alcohol, normal
propyl alcohol and deionized water was added to the dispersion.
After the dispersion temperature reached .about.10.degree. C.,
16.48 grams of an electrocatalyst comprised of platinum metal
supported on particulate carbon (70% wt % Pt, 30 wt % particulate
carbon) (TEC10E70TPM obtained from Tanaka Kikinzoku Kogyo KK,
Kanagawa, Japan) was slowly added to the dispersion over a period
of about 15 minutes with high shear mixing continued and the
dispersion was cooled to about 6.degree. C. Mixing was stopped and
the dispersion was allowed to warm up to ambient temperature. Upon
warming to ambient temperature, the mixture was circulated through
an Eiger.RTM. bead mill, (manufactured by Eiger Machinery Inc.,
Greylake, Ill.), containing 70 ml 1.0-1.25 millimeter zirconia
grinding media for four minutes. The particle size in the
dispersion was measured with a Coulter counter and indicated the
D.sub.50 to be 2.6 microns. To this dispersion 7.88 grams of
diprolyleneglycolmonomethyl ether (DPM) was added and the
dispersion was agitated for 1 minute followed by 10 minutes of
equilibration.
[0074] A cathode decal was prepared by casting onto a Kapton.RTM.
polyimide film with a 60 bar rod, drying overnight, and then
casting a second layer with a 70 rod bar applied perpendicular to
the casting direction of the first film layer. The decal was dried
in a vacuum oven with a nitrogen purge for two hours at 150.degree.
F.
[0075] A cathode catalyst decal was cast on a 2-mil 200LP PFA film
with a 7-mil rod, air-dried for 45 minutes at ambient temperature
followed by 1 hour in a oven at120.degree. F. The resulting cathode
film had a thickness of 0.31 mils and Pt loading of 0.95
mg/cm.sup.2 as measured by XRF.
[0076] Anode-1: A 3.5 wt % Nafion.RTM. solution in FC-40
Fluorinert.TM. brand electronic liquid perfluorinated solvent (the
polymer resin had a 830 EW measured by FTIR and was in the sulfonyl
fluoride form) was concentrated to 7% by placing it in a rotovap
and removing the FC-40 solvent until desired solid of 7 wt % was
obtained. 201.4 grams of 7 wt % Nafion.RTM. solution was then
placed in a hot water bath to keep the Nafion.RTM. in liquid form.
33 grams of an electrocatalyst comprised of platinum/ruthenium
metal supported on particulate carbon (75 wt % metal/25 wt %
carbon) was added to the Nafion.RTM. solution and mixed well by
hand to make a slurry with 20 wt % solids. The electrocatalyst was
comprised of 49.8 wt % platinum, 24.5 wt % ruthenium, and about 25
wt % particulate carbon. The electrocatalyst had a surface area of
217 m.sup.2/g and a pore volume of 0.50 cc/g (both measured
according to standard nitrogen BET). The moisture content of the
electrocatalyst, measured by thermo gravametric analysis using a TA
Instruments TGA, was 1.7% and the initial particle size
distribution, measured by LS 13 320 Laser Diffraction Particle Size
Analyzer, was as follows: d.sub.10=12.1 microns, d.sub.50=75.3
microns, d.sub.90=170.6 microns. The surface area of the
platinum/ruthenium metal in the electrocatalyst was greater than
100 m.sup.2/g of metal.
[0077] The Nafion.RTM./electrocatalyst slurry was then placed into
an Eiger mill containing 40 ml of 1.0-1.25 mm zirconia grinding
media and was milled at 4000 rpm for 2 hours. The Eiger mill was
heated using a circulating bath set at 50.degree. C. to ensure that
the ink flowed easily. Material was withdrawn from the mill and
particle size measured using a Hegman gauge. The largest particle
size detected was less than 4 microns. The ink was then removed
from the Eiger mill. The solid content and the viscosity of the ink
were measured and they were 21.5 wt % and 31,500 centipoise
respectively.
[0078] A catalyst electrode decal was prepared by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 0.94
mg/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 0.92 mil.
[0079] Anode-2: The electrocatalyst ink used to produce Anode-1
(solids content 21.5 wt % and viscosity 31,500 centipoise) was
further concentrated to obtain a solids content and the viscosity
of 19.5 wt % and 19,500 centipoise respectively. A catalyst
electrode decal was prepared by drawing down the catalyst ink to a
dimension of 5 cm.times.5 cm (to give a total area of 25 cm.sup.2)
on a 10 cm.times.10 cm piece of 3 mil thick Kapton.RTM. polyimide
film. A Pt/Ru metal loading of 1.99 mg/cm.sup.2 was achieved by
knife drawdown coating with this ink. The dry coating thickness was
about 2.1 mil.
[0080] Anode-3: The electrocatalyst ink used to produce Anode-2
(solids content 19.5 wt % and viscosity 19,500 centipoise) was used
to produce another catalyst electrode decal by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 2.0
mg/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 2.2 mil.
[0081] Anode-4: The electrocatalyst ink used to produce Anode-2
(solids content 19.5 wt % and viscosity 19,500 centipoise) was used
to produce another catalyst electrode decal by drawing down the
catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 1.18
mg/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 1 mil.
[0082] Anode-5: The electrocatalyst ink used to produce Anode-2 was
used to produce another catalyst electrode decal by drawing down
the catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 1.0
mg/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 0.93 mil.
[0083] Anode-6: The electrocatalyst ink used to produce Anode-2 was
used to produce another catalyst electrode decal by drawing down
the catalyst ink to a dimension of 5 cm.times.5 cm (to give a total
area of 25 cm.sup.2) on a 10 cm.times.10 cm piece of 3 mil thick
Kapton.RTM. polyimide film. A Pt/Ru metal loading of 2.1
mg/cm.sup.2 was achieved by knife drawdown coating with this ink.
The dry coating thickness was about 2.3 mil.
[0084] Anode-7: 83.84 grams of Nafion.RTM. 920 EW dispersion in
proton form (DuPont DE2020, 21.3% solids) and 84.02 grams of a 1:1
solution of n-propyl alcohol/iso-propyl alcohol were added to a 500
ml poly beaker which was immersed in an ice bath. The poly beaker
was in a nitrogen purged box in a hood. The dispersion was chilled
to 6 C. 20 grams of an electrocatalyst comprised of
platinum/ruthenium metal supported on particulate carbon (41 wt %
Pt/32 wt % Ru/27 wt % carbon) (TEC81E81 obtained from Tanaka
Kikinzoku Kogyo KK, Kanagawa, Japan) was slowly added over five
minutes while monitoring temperature with high shear mixing.
Stirring was continued for an additional 10 minutes. After this
time an additional 13.83 grams of the same electrocatalyst was
slowly added over two minutes. The stirring was continued for 30
minutes with a plastic lid over beaker to prevent splashing. After
this time, the stirring was stopped and the poly beaker was removed
from the ice bath. The dispersion was then transferred to a plastic
bottle and allowed to sit overnight. After 24 hours, a large sonic
horn and a high shear mixer was introduced into the dispersion. The
dispersion was chilled to 9 C..degree. while stirring with the high
shear mixer. The chilled dispersion was sonicated a setting of 9
for three minutes with high shear mixing. The dispersion was then
allowed to cool for 15 minutes while high shear mixing was
continued. Four more cycles of three minutes of sonication followed
by 15 minutes of cooling were carried out. The dispersion was
allowed to sit overnight. To this dispersion 11.8 grams of
diprolyleneglycolmonomethyl ether (DPM) was added and the
dispersion was agitated for 1 minute followed by 10 minutes of
equilibration. Two more cycles of agitation and equilibration were
carried out.
[0085] An anode decal was prepared on a 3-mil PFA film with a 15
mil doctor blade. The resulting anode had a total metal loading (Pt
plus Ru) of 2.74 mg/cm.sup.2.
[0086] Anode-8: 32.65 grams of Nafion.RTM. 920 EW dispersion in
proton form (DuPont DE2020, 21.3% solids) and a mixture of 32.86
grams of isopropyl alcohol and 18.50 grams normal propyl alcohol
were added to a beaker which was immersed in an ice bath. The
beaker was in a nitrogen purged box in a hood. The dispersion was
chilled to 6 C. 15.99 grams grams of an electrocatalyst comprised
of platinum/ruthenium metal supported on particulate carbon (75 wt
% metal/25 wt % carbon) was slowly added to the Nafion.RTM.
dispersion and mixed well to make a slurry. The electrocatalyst was
comprised of 49.8 wt % platinum, 24.5 wt % ruthenium, and about 25
wt % particulate carbon. The electrocatalyst had a surface area of
217 m.sup.2/g and a pore volume of 0.50 cc/g (both measured
according to standard nitrogen BET). The moisture content of the
electrocatalyst, measured by thermo gravametric analysis using a TA
Instruments TGA, was 1.7% and the initial particle size
distribution, measured by Beckman Coulter Model LS 13 320 Laser
Diffraction Particle Size Analyzer, was as follows: d.sub.10=12.1
microns, d.sub.50=75.3 microns, d.sub.90=170.6 microns. The surface
area of the platinum/ruthenium metal in the electrocatalyst was
greater than 100 m.sup.2/g of metal. The stirring of the
slurry/dispersion was continued for about 30 minutes. After this
time, the stirring was stopped and the beaker was removed from the
ice bath. The dispersion was allowed to warm to ambient
temperature. The dispersion was sonicated for 20 minutes using a
Branson Sonifier 450 with a 0.5 inch probe at 70% power that give a
35% power output until the D.sub.50 particle size was reduced to
2.5 microns, as measured with the laser diffraction particle size
analyzer referenced above. To this dispersion, 59.36 grams of a
solution (44 grams of isopropyl alcohol, 44 grams of normal propyl
alcohol, 12 grams of water and 11.1 grams of
diprolyleneglycolmonomethyl ether (DPM)) until the solids content
was reduced to 16% by weight.
[0087] An anode decal was prepared on a 2-mil PFA film with a 15
mil rod. The film was air-dried at ambient temperature for 60
minutes. The resulting anode had a thickness of 1.43 mils and a
total metal loading (Pt plus Ru) of 1.94 mg/cm.sup.2 as measured by
XRF.
Membrane Electrode Assembly
[0088] In Comparative Examples A-G and in Examples 1-5, catalyst
coated membranes were produced using a Nafion.RTM. N117 proton
exchange membrane in the sulfonic acid form and having a thickness
of about 7 mil and a size of about 4 inch.times.4 inch. A piece of
wet membrane was used. For each example, a membrane was sandwiched
between one of the anode electrode decals described above on one
side of the membrane and one of the cathode electrode decals
described above on opposite side of the membrane. Care was taken to
ensure that the coatings on the two decals were registered with
each other and were positioned facing the membrane. The entire
assembly was introduced between two preheated (to about 160.degree.
C.) 8 inch.times.8 inch plates of a hydraulic press and the plates
of the press were brought together quickly until a pressure of 5000
lbs was reached. The sandwich assembly was kept under pressure for
approximately 2 minutes and then the press was cooled for
approximately 2 minutes (viz. until it reached a temperature of
<60.degree. C.) under the same pressure. Then the assembly was
removed from the press and the Kapton.RTM. films were slowly peeled
off the electrodes on both sides of the membrane showing that the
anode and cathode electrodes had been transferred to the membrane.
Each catalyst coated membrane was immersed in a tray of water (to
ensure that the membrane was completely wet) and carefully
transferred to a zipper bag for storage and future use.
[0089] In Comparative Examples A-G and in Examples 1-5, the CCMs
were chemically treated in order to convert the ionomer in the
anode electrode and the cathode electrode from the SO.sub.2F form
to the H+ acid form. This required a hydrolysis treatment followed
by an acid exchange procedure. The hydrolysis of the CCMs was
carried out in a 30 wt % NaOH solution at 80.degree. C. for 30
minutes. The CCMs were placed between Teflon.RTM. mesh, (obtained
from DuPont, Wilmington, Del.), and placed in the solution. The
solution was stirred to assure uniform hydrolysis. After 30 minutes
in the bath, the CCMs were removed and rinsed completely with fresh
deionized water to remove all the NaOH.
[0090] Acid exchange of the CCMs that were hydrolyzed in the
previous step was done in 15 wt % nitric acid solution at a bath
temperature of 65.degree. C. for 45 minutes. The solution was
stirred to assure uniform acid exchange. This procedure was
repeated in a second bath containing 15 wt % nitric acid solution
at 65.degree. C. and for 45 minutes. The CCMs were then rinsed in
flowing deionized water for 15 minutes at room temperature to
ensure removal of all the residual acid. They were then packaged
wet and labeled.
[0091] In Examples 6 and 7, the anode and cathode electrodes were
made with a binder in sulfonic acid form, so hydrolysis and acid
exchange steps were not used. Catalyst Coated membranes were
fabricated by anode and decal transfer to wet Nafion.RTM. N115
proton exchange membrane in the sulfonic acid form and having a
thickness of 5 mil. In these two Examples, the membrane was
sandwiched between one of the anode electrode decals described
above on one side of the membrane and one of the cathode electrode
decals described above on opposite side of the membrane. Care was
taken to ensure that the coatings on the two decals were registered
with each other and were positioned facing the membrane. In Example
6, the assembly was introduced between two heated plates at
140.degree. C. for 8 minutes under 10,000 load pounds, and in
Example 7, the assembly was introduced between two heated plates at
125.degree. C. for 5 minutes under 5000 load pounds. The pressure
was maintained while the press was cooled for approximately 3
minutes (until it reached a temperature of <90.degree. C.). The
assembly was removed from the press and the support films were
slowly peeled off the electrodes on both sides of the membrane
showing that the anode and cathode electrodes had been transferred
to the membrane. Each catalyst coated membrane was immersed in a
tray of water (to ensure that the membrane was completely wet) and
carefully transferred to a zipper bag for storage and future
use.
[0092] CCM performance measurements were made employing a single
cell test assembly obtained from Fuel Cell Technologies Inc, New
Mexico. Membrane electrode assemblies were made that comprised one
of the above CCMs sandwiched between two sheets of the gas
diffusion backing (taking care to ensure that the GDB covered the
electrode areas on the CCM). The anode gas diffusion backing was
comprised an 8 mil thick carbon paper coated with a 1.7 mil thick
microporous carbon powder coating. The cathode diffusion backing
comprised an 8 mil thick nonwoven carbon fabric with a PTFE coating
(FCX0026 from Freudenberg). The microporous layer on the anode-side
GDB was disposed toward the anode catalyst. Two 7 mil thick glass
fiber reinforced silicone rubber gaskets (Furan--Type 1007,
obtained from Stockwell Rubber Company) each along with a 1 mil
thick FEP polymer spacer were cut to shape and positioned so as to
surround the electrodes and GDBs on the opposite sides of the
membrane and to cover the exposed edge areas of each side of the
membrane. Care was taken to avoid overlapping of the GDB and the
gasket material. The entire sandwich assembly was assembled between
the anode and cathode flow field graphite plates of a 25 cm.sup.2
standard single cell assembly (obtained from Fuel Cell Technologies
Inc., Los Alamos, N.M.). The test assembly was also equipped with
anode inlet, anode outlet, cathode gas inlet, cathode gas outlet,
aluminum end blocks, tied together with tie rods, electrically
insulating layer and the gold plated current collectors. The bolts
on the outer plates of the single cell assembly were tightened with
a torque wrench to a force of 2 ft.lbs.
[0093] The single cell assembly was then connected to the fuel cell
test station. The components in a test station included a supply of
air for use as cathode gas; a load box to regulate the power output
from the fuel cell; a MeOH solution tank to hold the feed anolyte
solution; a liquid pump to feed the anolyte solution to the fuel
cell anode at the desired flow rate; a condenser to cool the
anolyte exiting the cell from the cell temperature to room
temperature and a collection bottle to collect the spent anolyte
solution.
[0094] With the cell at room temperature, 1M MeOH solution and air
were introduced into the anode and cathode compartments through
inlets of the cell at flow rates of 1.55 cc/min and 202 cc/min,
respectively. The temperature of the single cell was slowly raised
until it reached 70.degree. C. The methanol and air feed rates were
maintained proportional to the current while the resistance in the
circuit was varied in steps so as to increase current. The voltage
at each current step was recorded so as to produce a current vs.
voltage plot for the cell. Using this plot, the current density
(expressed in mW/cm.sup.2) at a voltage of 400 mVolts was
determined.
[0095] In Table 1, the anode and cathode electrodes for each
example are shown along with the current density measured.
TABLE-US-00001 TABLE 1 Anode Cathode Current Metals Metals Density
Anode Loading Cathode Loading (mW/cm.sup.2) Example Electrocatalyst
(mg/cm.sup.2) Electrocatalyst (mg/cm.sup.2) @400 mV Comparative
Anode-A 4.5 Cathode-1 2.2 78 Ex. A (Pt/Ru unsupported) (Pt/C
supported) Comparative Anode-3 2.0 Cathode-A 4.5 80 Ex. B (Pt/Ru/C
(Pt supported) unsupported) Comparative Anode-A 4.5 Cathode-A 4.5
100 Ex. C (Pt/Ru (Pt unsupported) unsupported) Comparative Anode-5
1.0 Cathode-A 4.5 52 Ex. D (Pt/Ru/C (Pt supported) unsupported)
Comparative Anode-A 4.5 Cathode-5 1.0 107 Ex. E (Pt/Ru (Pt/C
unsupported) supported) Comparative Anode-B 4.3 Cathode-B 5.4 66
Ex. F (Pt/Ru (Pt unsupported) unsupported) Comparative Anode-C 5.1
Cathode-C 4.8 86 Ex. G (Pt/Ru (Pt unsupported) unsupported) Ex. 1
Anode-1 0.94 Cathode-1 2.2 78 (Pt/Ru/C (Pt/C supported) supported)
Ex. 2 Anode-2 2.0 Cathode-1 2.2 87 (Pt/Ru/C (Pt/C supported)
supported) Ex. 3 Anode-3 2.0 Cathode-2 2.2 104 (Pt/Ru/C (Pt/C
supported) supported) Ex. 4 Anode-4 1.2 Cathode-3 1.1 78 (Pt/Ru/C
(Pt/C supported) supported) Ex. 5 Anode-6 2.1 Cathode-4 1.9 96
(Pt/Ru/C (Pt/C supported) supported) Ex 6 Anode 7 2.7 Cathode 6 1.3
88 (Pt/Ru/C (Pt/C supported) supported) Ex 7 Anode 8 1.9 Cathode 7
0.95 93 (Pt/Ru/C (Pt/C supported) supported)
[0096] A current density of 70 mW/cm.sup.2 at a voltage of 400
mVolts is generally the minimum considered necessary for use in a
direct methanol fuel cell. It can be seen in the examples, that
this minimum level of performance is surpassed by MEAs according to
the invention having anode and cathode metals loading per electrode
of less than 3 mg metal/cm.sup.2, and even less than 2 mg
metal/cm.sup.2. By contrast, in the conventional MEA of Comparative
Example F where both cathode and anode electrocatalysts were
unsupported, a total metals loading (cathode and anode) of more
than 10 mg metal/cm.sup.2 did not achieve this necessary minimum
current density. In Comparative Example D, a supported Pt/Ru anode
electrode with a relatively low 1.0 mg metal/cm.sup.2 was combined
with an unsupported Pt cathode electrocatalyst at a metals loading
of 4.5 mg metal/cm.sup.2, and the current density again was below
the minimum need for direct methanol fuel cells. Surprisingly, with
an MEA according to one embodiment of the present invention
(Example 4), satisfactory current density was obtained with a total
metals loading of less than 2.5 mg metal/cm.sup.2. Nearly 3 times
as much catalyst metal was needed to achieve a similar performance
in the conventional MEA of Comparative Example A where the cathode
electrocatalyst metal was supported on carbon, but the anode
electrocatalyst metal was not.
[0097] A long-term accelerated durability test was performed on the
catalyst coated membranes of Comparative Example G and Example 5.
The long term durability test was conducted with the same single
cell test assembly described above and the MEAs prepared by the
same procedure. With the cell at room temperature, 1M MeOH solution
and air were introduced into the anode and cathode compartments
through inlets of the cell at flow rates of 1.55 cc/min and 202
cc/min, respectively. The temperature of the single cell was slowly
raised until it reached 70.degree. C. The initial open cell voltage
(no applied load) was first detected. A resistance load was then
applied so as to maintain a current of 3.75 Amps and held for 30
minutes, and the average voltage drop was measured for this period
at this current. The resistance was then removed for 30 seconds
before the start of the next 30 minute test cycle. During each test
cycle, the resistance necessary to maintain a 3.75 Amp current was
applied and the average voltage drop was measured and recorded. The
cycles were continued for an extended period of time.
[0098] A plot of the average voltage drop over time for the MEAs of
Comparative Example G and Example 5 is shown in FIG. 1. It can be
seen that the MEA of Example 5 (curve "a") maintains its ability to
achieve a voltage drop very long after the conventional MEA of
Comparative Example G (curve "b") has degenerated well below the
acceptable level for a direct methanol fuel cell. This improved
durability has great utility in a organic/air fuel cells.
* * * * *