U.S. patent application number 11/544439 was filed with the patent office on 2007-04-12 for metal-polymer composite catalysts.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Rajesh Bashyam, Piotr Zelenay.
Application Number | 20070082253 11/544439 |
Document ID | / |
Family ID | 38309675 |
Filed Date | 2007-04-12 |
United States Patent
Application |
20070082253 |
Kind Code |
A1 |
Zelenay; Piotr ; et
al. |
April 12, 2007 |
Metal-polymer composite catalysts
Abstract
A metal-polymer-carbon composite catalyst for use as a cathode
electrocatalyst in fuel cells. The catalyst includes a heteroatomic
polymer; a transition metal linked to the heteroatomic polymer by
one of nitrogen, sulfur, and phosphorus, and a recast ionomer
dispersed throughout the heteroatomic polymer-carbon composite. A
membrane electrode assembly for fuel cells is also described.
Inventors: |
Zelenay; Piotr; (Los Alamos,
NM) ; Bashyam; Rajesh; (Los Alamos, NM) |
Correspondence
Address: |
LOS ALAMOS NATIONAL SECURITY, LLC
LOS ALAMOS NATIONAL LABORATORY
PPO. BOX 1663, LC/IP, MS A187
LOS ALAMOS
NM
87545
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
38309675 |
Appl. No.: |
11/544439 |
Filed: |
October 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60724552 |
Oct 6, 2005 |
|
|
|
Current U.S.
Class: |
429/483 ;
429/494; 429/525; 429/530; 429/532 |
Current CPC
Class: |
H01M 2008/1095 20130101;
H01M 4/8814 20130101; H01M 2004/8689 20130101; H01M 8/1011
20130101; H01M 4/8605 20130101; Y02E 60/50 20130101; H01M 4/8835
20130101; H01M 4/90 20130101; H01M 4/886 20130101; H01M 2004/8684
20130101; H01M 4/8652 20130101; H01M 2300/0082 20130101; H01M 4/881
20130101; H01M 4/8807 20130101; H01M 4/8828 20130101; Y02P 70/50
20151101; H01M 4/9008 20130101; H01M 4/8839 20130101; H01M 8/0289
20130101; H01M 4/921 20130101; H01M 8/1004 20130101 |
Class at
Publication: |
429/042 ;
429/044; 429/030; 429/033 |
International
Class: |
H01M 4/96 20060101
H01M004/96; H01M 8/10 20060101 H01M008/10 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0002] This invention was made with government support under
Contract No. DE-AC 52-06 NA 25396, awarded by the U.S. Department
of Energy. The government has certain rights in the invention.
Claims
1. A membrane electrode assembly for a fuel cell, the membrane
electrode assembly comprising: a. an ionomeric membrane; b. an
anode catalyst disposed on a first surface of the ionomeric
membrane; and c. a cathode catalyst disposed on a second surface of
the ionomeric membrane, the cathode catalyst comprising: i. a
heteroatomic polymer-carbon composite, wherein the heteroatomic
polymer comprises one of nitrogen, sulfur, phosphorus, and oxygen;
ii. a transition metal linked to the heteroatomic polymer by one of
nitrogen, sulfur, phosphorus, and oxygen, with the proviso that the
transition metal is not platinum; and iii. a recast ionomer
dispersed throughout the heteroatomic polymer-carbon composite.
2. The membrane electrode assembly according to claim 1, wherein
the transition metal is selected from the group consisting of
cobalt, nickel, chromium, molybdenum, ruthenium, iron, manganese,
palladium, vanadium, and combinations thereof.
3. The membrane electrode assembly according to claim 2, wherein
the transition metal is cobalt.
4. The membrane electrode assembly according to claim 1, wherein
the heteroatomic polymer is selected from the group consisting of
polypyrrole, polyaniline, polythiophene, polyethylene
dioxythiophene, polyfuran, poly(vinylpyridine), polyimide,
derivatives thereof, and combinations thereof.
5. The membrane electrode assembly according to claim 4, wherein
the heteroatomic polymer is polypyrrole.
6. The membrane electrode assembly according to claim 1, wherein
the recast ionomer is selected from the group consisting of
poly(perflourosulphonic acid), sulfonated
styrene-ethylene-butylene-styrene, polystyrene-graft-poly(styrene
sulfonic acid), poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid), poly(arylene ether), polyphosphazene, and
combinations thereof.
7. The membrane electrode assembly according to claim 6, wherein
the recast ionomer is poly(perflourosulphonic acid).
8. The membrane electrode assembly according to claim 1, wherein
the ionomeric membrane is selected from the group consisting of
poly(perflourosulphonic acid), sulfonated
styrene-ethylene-butylene-styrene, polystyrene-graft-poly(styrene
sulfonic acid), poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid), poly(arylene ether), polyphosphazene, and
combinations thereof.
9. The membrane electrode assembly according to claim 1, further
comprising a diffusion layer, wherein the diffusion layer is
disposed between a first portion and a second portion of the
cathode catalyst.
10. The membrane electrode assembly according to claim 1, further
comprising a diffusion layer, wherein the diffusion layer is
disposed between a first portion and a second portion of the anode
catalyst.
11. The membrane electrode assembly according to claim 1, further
comprising a diffusion layer disposed between the anode catalyst
and the ionomeric membrane.
12. The membrane electrode assembly according to claim 1, further
comprising a diffusion layer disposed between the cathode catalyst
and the ionomeric membrane.
13. A cathode catalyst, the cathode catalyst comprising: a. a
heteroatomic polymer-carbon composite, wherein the heteroatomic
polymer comprises one of nitrogen, sulfur, phosphorus, and oxygen;
b. a transition metal linked to the heteroatomic polymer-carbon
composite by one of nitrogen, sulfur, phosphorus, and oxygen, with
the proviso that the transition metal is not platinum; and c. a
recast ionomer dispersed throughout the heteroatomic polymer-carbon
composite.
14. The cathode catalyst according to claim 13, wherein the
transition metal is selected from the group consisting of cobalt,
nickel, chromium, molybdenum, ruthenium, iron, manganese,
palladium, vanadium, and combinations thereof.
15. The cathode catalyst according to claim 14, wherein the
transition metal is cobalt.
16. The cathode catalyst according to claim 13, wherein the
heteroatomic polymer is selected from the group consisting of
polypyrrole, polyaniline, polythiophene, polyethylene
dioxythiophene, polyfuran, poly(vinylpyridine), polyimide,
derivatives thereof, and combinations thereof.
17. The cathode catalyst according to claim 16, wherein the
heteroatomic polymer is polypyrrole.
18. The cathode catalyst according to claim 13, wherein the recast
ionomer is selected from the group consisting of
poly(perflourosulphonic acid), sulfonated
styrene-ethylene-butylene-styrene, polystyrene-graft-poly(styrene
sulfonic acid), poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid), poly(arylene ether), polyphosphazene, and
combinations thereof.
19. The cathode catalyst according to claim 18, wherein the recast
ionomer is poly(perflourosulphonic acid).
20. A membrane electrode assembly for a fuel cell, the membrane
electrode assembly comprising: a. an ionomeric membrane; b. an
anode catalyst disposed on a first surface of the ionomeric
membrane; and c. a cathode catalyst disposed on a second surface of
the ionomeric membrane, the cathode catalyst comprising: i. a
heteroatomic polymer-carbon composite, wherein the heteroatomic
polymer comprises one of nitrogen, sulfur, phosphorus, and oxygen;
ii. a transition metal linked to the heteroatomic polymer by one of
nitrogen, sulfur, phosphorus, and oxygen, wherein the transition
metal is selected from the group consisting of cobalt, nickel,
chromium, molybdenum, ruthenium, iron, manganese, palladium,
vanadium, and combinations thereof; and iii. a recast ionomer
dispersed throughout the heteroatomic polymer-carbon composite.
21. The membrane electrode assembly according to claim 1, wherein
the transition metal is selected from the group consisting of
cobalt, nickel, chromium, molybdenum, ruthenium, iron, manganese,
palladium, vanadium, and combinations thereof.
Description
REFERENCE TO PRIOR APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/724,552, filed Oct. 6, 2005.
BACKGROUND OF INVENTION
[0003] The invention relates to fuel cells. More particularly, the
invention relates to fuel cell electrodes. Even more particularly,
the invention relates to cathode materials for fuel cell
electrodes.
[0004] Polymer electrolyte fuel cells (PEFCs) have long been viewed
as the power source of the future and the centerpiece of a hydrogen
economy. However, in order to deliver on the long-standing promise
of commercially viable PEFCs, the barriers of cost and performance
durability must be overcome.
[0005] Presently, platinum is the material of choice for use as a
catalyst in fuel cell electrodes. The high cost of platinum,
however, presents a major obstacle to widespread use of PEFCs. In
addition, platinum electrodes performance degrades in the presence
of methanol, which is a fuel in direct methanol fuel cells
(DFMCs).
[0006] Lowering fuel cell cost will only be possible by replacing
today's platinum-based catalyst with other metal electrode
materials of sufficient activity and stability. Therefore, what is
needed is a fuel cell electrode that utilizes a metal other than
platinum as an electrode catalyst.
SUMMARY OF INVENTION
[0007] The present invention meets these and other needs by
providing a metal-polymer-carbon composite catalyst comprising a
transition metal other than platinum. The catalyst is intended for
use as a cathode electrocatalyst in fuel cells, including polymer
electrolyte fuel cells, hydrogen-oxygen fuel cells, hydrogen-air
fuel cells, and direct methanol fuel cells. The invention also
provides a membrane electrode assembly for such fuel cells.
[0008] Accordingly, one aspect of the invention is to provide a
membrane electrode assembly for a fuel cell. The membrane electrode
assembly comprises: an ionomeric membrane; an anode catalyst
disposed on a first surface of the ionomeric membrane; and a
cathode catalyst disposed on a second surface of the ionomeric
membrane. The cathode catalyst comprises: a heteroatomic
polymer-carbon composite, wherein the heteroatomic polymer
comprises one of nitrogen, sulfur, phosphorus, and oxygen; a
transition metal linked to the heteroatomic polymer by one of
nitrogen, sulfur, phosphorus, and oxygen, with the proviso that the
transition metal is not platinum; and a recast ionomer dispersed
throughout the heteroatomic polymer-carbon composite.
[0009] A second aspect of the invention is to provide a cathode
catalyst. The cathode catalyst comprises: a heteroatomic
polymer-carbon composite, wherein the heteroatomic polymer
comprises one of nitrogen, sulfur, phosphorus, and oxygen; a
transition metal linked to the heteroatomic polymer-carbon
composite by one of nitrogen, sulfur, phosphorus, and oxygen; and a
recast ionomer dispersed throughout the heteroatomic composite,
with the proviso that the transition metal is not platinum.
[0010] A third aspect of the invention is to provide a membrane
electrode assembly for a fuel cell. The membrane electrode assembly
comprises: an ionomeric membrane; an anode catalyst disposed on a
first surface of the ionomeric membrane; and a cathode catalyst
disposed on a second surface of the ionomeric membrane. The cathode
catalyst comprises: a heteroatomic polymer-carbon composite,
wherein the heteroatomic polymer comprises one of nitrogen, sulfur,
phosphorus, and oxygen; a transition metal linked to the
heteroatomic polymer by one of nitrogen, sulfur, phosphorus, and
oxygen, wherein the transition metal is selected from the group
consisting of cobalt, nickel, chromium, molybdenum, ruthenium,
iron, manganese, palladium, vanadium, and combinations thereof; and
a recast ionomer dispersed throughout the heteroatomic
polymer-carbon composite.
[0011] These and other aspects, advantages, and salient features of
the present invention will become apparent from the following
detailed description, the accompanying drawings, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic cross-sectional representation of a
membrane electrode assembly of the present invention;
[0013] FIG. 2 is a schematic cross-sectional representation of a
second embodiment of the membrane electrode assembly;
[0014] FIG. 3 is an equation representing the formation of a
linkage of cobalt to polypyrrole by the reduction of cobalt;
[0015] FIG. 4 is a flow chart for a method of making a cathode
catalyst;
[0016] FIG. 5 is a flow chart for a method of making a membrane
electrode assembly;
[0017] FIG. 6 is a plot of steady-state polarization curves
obtained in H.sub.2/air and H.sub.2/O.sub.2 fuel cells having a
cobalt-polypyrrole-carbon composite cathode (0.06 mg/cm.sup.2 Co)
and a commercially available platinum-ruthenium black anode (6.0
mg/cm.sup.2Pt--Ru);
[0018] FIG. 7 is a plot of hydrogen-oxygen fuel cell polarization
measured at different cell temperatures for a membrane electrode
assembly having a cobalt-polypyrrole-carbon composite cathode
catalyst;
[0019] FIG. 8 is a plot of a life test at 80.degree. C. for a
H.sub.2/O.sub.2 fuel cell having a cobalt-polypyrrole-carbon
composite cathode and a platinum-ruthenium black anode;
[0020] FIG. 9 is a plot of hydrogen-air fuel cell polarization
measured at different cell temperatures for a membrane electrode
assembly having a cobalt-polypyrrole-carbon composite cathode
catalyst;
[0021] FIG. 10 is an H.sub.2/air fuel cell life test at 80.degree.
C. using a cobalt-composite cathode and a platinum-ruthenium black
anode;
[0022] FIG. 11 is a plot of direct methanol fuel cell
(methanol-oxygen) polarization at 80.degree. C. obtained with
different anode concentrations of methanol, a
cobalt-polypyrrole-carbon composite cathode (0.06 mg/cm.sup.2 Co),
and a commercially available platinum-ruthenium black anode (6.0
mg/cm.sup.2 Pt--Ru);
[0023] FIG. 12 is a plot of direct methanol fuel cell
(methanol-air) polarization at 80.degree. C. obtained with
different anode concentrations of methanol, a
cobalt-polypyrrole-carbon composite cathode (0.2 mg/cm.sup.2 Co),
and a commercially available platinum-ruthenium black anode (6.0
mg/cm.sup.2 Pt--Ru);
[0024] FIG. 13 is a scanning electron microscope (SEM) image of a
membrane electrode assembly having a cobalt-polypyrrole-carbon
composite cathode catalyst layer;
[0025] FIG. 14 is a plot of x-ray absorption near edge structure
(XANES) spectra of a cobalt-polypyrrole-carbon composite before and
after fuel cell operation in comparison to Co(0) and CoO
standards;
[0026] FIG. 15 is a plot of x-ray absorption fine structure (XAFS)
spectra of a cobalt-polypyrrole-carbon composite before and after
fuel cell operation in comparison to a CoO standard;
[0027] FIG. 16 is a plot of hydrogen-oxygen fuel cell polarization
with different cathodes: (i) non-pyrolyzed
cobalt-polypyrrole-carbon (Co-PPY-C) composite; (ii) pyrolyzed
cobalt-polypyrrole-carbon composite; (iii) cobalt deposited on
carbon (Co--C); and (iv) polypyrrole-carbon (PPY-C);
[0028] FIG. 17 is a plot of 100-hour and 50-hour life tests at 50
0.5 V and 0.7 V, respectively, life test, both performed at
80.degree. C., on H.sub.2/O.sub.2 fuel cells having a
cobalt-polypyrrole-carbon composite cathode and a
platinum-ruthenium black anode;
[0029] FIG. 18 is a plot of 100-hour life tests at 0.5 V and 0.6 V,
both performed at 80.degree. C., for H.sub.2/air fuel cells having
a cobalt-polypyrrole-carbon composite cathode and a
platinum-ruthenium black anode;
[0030] FIG. 19 is a plot of a 300-hour life test at 0.4 V and
80.degree. C. for a H.sub.2/air fuel cell having a
cobalt-polypyrrole-carbon composite cathode and a
platinum-ruthenium black anode; and
[0031] FIG. 20 is a plot of H.sub.2/O.sub.2 fuel cell polarization
at 80.degree. C. with iron-polypyrrole-carbon composite cathode in
reference to H.sub.2/O.sub.2 fuel cell polarization with a
cobalt-polypyrrole-carbon composite cathode.
DETAILED DESCRIPTION
[0032] In the following description, like reference characters
designate like or corresponding parts throughout the several views
shown in the figures. It is also understood that terms such as
"top," "bottom," "outward," "inward," and the like are words of
convenience and are not to be construed as limiting terms. In
addition, whenever a group is described as either comprising or
consisting of at least one of a group of elements and combinations
thereof, it is understood that the group may comprise or consist of
any number of those elements recited, either individually or in
combination with each other.
[0033] The present invention provides a metal-polymer-carbon
composite catalyst comprising a transition metal other than
platinum. The catalyst is intended for use as a cathode
electrocatalyst in fuel cells, including polymer electrolyte fuel
cells (also referred to herein as "PEFCs"), hydrogen-oxygen fuel
cells, hydrogen-air fuel cells, and direct methanol fuel cells
(also referred to herein as "DMFCs"). The invention also provides a
membrane electrode assembly (also referred to herein as "MEA") for
such fuel cells.
[0034] Referring to the drawings in general and to FIG. 1 in
particular, it will be understood that the illustrations are for
the purpose of describing a particular embodiment of the invention
and are not intended to limit the invention thereto. FIG. 1 is a
schematic cross-sectional representation of a membrane electrode
assembly of the present invention. MEA 100 comprises an ionomeric
membrane 110, a cathode catalyst composite 120 disposed on a first
surface of ionomeric membrane 110, and an anode catalyst 130
disposed on a second surface of ionomeric membrane 110.
[0035] In one embodiment, ionomeric membrane 110 is Nafion.RTM. 117
(poly (perfluorosulphonic acid), also commercially available as
Aciplex.RTM. or Flemion.RTM.). Other ionomeric membrane materials
known in the art, such as sulfonated
styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene
sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid); poly(arylene ether), such as poly(arylene ether
ether ketone) and poly(arylene ether sulfone); polybenzimidazole;
polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy)
phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and
combinations thereof, may also be used. Anode catalyst 130
comprises at least one metal. The at least one metal is selected
from those metals, such as, but not limited to, platinum,
ruthenium, palladium, and combinations thereof, that are known and
used in the art as fuel cell anode materials. Anode catalyst 130 is
typically deposited on ionomeric membrane 110 by preparing an ink
containing the at least one metal and applying the ink to a surface
of ionomeric membrane 110. In one embodiment, anode catalyst 130
comprises a mixture of platinum and ruthenium, such as, for
example, platinum-ruthenium black.
[0036] In another embodiment, shown in FIG. 2, a portion of anode
catalyst 132 is additionally deposited on a surface of a first gas
diffusion layer or "backing" 114. Gas diffusion layer 114 is then
placed in contact with anode catalyst 130, which has been deposited
on the first surface of ionomeric membrane 110. Similarly, a
portion of cathode catalyst 122 may be deposited on a surface of a
second gas diffusion layer 112, which in turn contacts cathode
catalyst 120 deposited on the second surface of ionomeric membrane
110.
[0037] Alternatively, either cathode catalyst composite 120 and
anode catalyst 130 may be disposed on a surface of gas diffusion
layer 112, 114 that is in contact with a surface of ionomeric
membrane 110. The gas diffusion layer may comprise carbon cloth,
carbon paper, or other such materials that are known in the art.
Contact between gas diffusion layers 112, 114 and ionomeric
membrane 110 may be established, for example, by compression or
hot-pressing.
[0038] Cathode catalyst 120 comprises a heteroatomic polymer-carbon
composite, a transition metal linked to the heteroatomic
polymer-carbon composite, and a recast ionomer dispersed throughout
the heteroatomic polymer-carbon composite. As used herein, the term
"heteroatomic polymer" refers to a polymer that includes atoms
other than carbon and hydrogen. The heteroatomic polymer comprises
one of nitrogen, sulfur, phosphorus, and oxygen. In one embodiment,
the heteroatomic polymer is selected from the group consisting of
polypyrrole, polyaniline, polythiophene, polyethylene
dioxythiophene, polyfuran, poly(vinylpyridine), polyimide, and
derivatives or combinations thereof. In a preferred embodiment, the
heteroatomic polymer is polypyrrole.
[0039] The transition metal is linked to the heteroatomic polymer
by one of nitrogen, sulfur, phosphorus, and oxygen. The transition
metal is a metal other than platinum. In one embodiment, the
transition metal is selected from the group consisting of cobalt,
nickel, chromium, molybdenum, ruthenium, iron, manganese,
palladium, vanadium, tungsten, and combinations thereof. In a
preferred embodiment, the transition metal is cobalt. The linkage
between polypyrrole and cobalt, formed by the reduction of cobalt
(Co.sup.2+) to Co, is shown in FIG. 3.
[0040] The recast ionomer is an ionic conductor such as, but not
limited to; poly(perflourosulphonic acid), such as Nafion(.RTM.,
Aciplex.RTM., or Flemion.RTM.; sulfonated
styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene
sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid); poly(arylene ether), such as poly(arylene ether
ether ketone) and poly(arylene ether sulfone); polybenzimidazole;
polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy)
phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and
combinations thereof. In a preferred embodiment, the recast ionomer
is Nafion.RTM..
[0041] A method of making the cathode catalyst 120, described
herein, is also provided. A flow chart showing the method 200 is
shown in FIG. 4. In step 210, a heteroatomic polymer-carbon
composite is formed. In one embodiment, polymerization takes place
by combining a dispersion comprising carbon black and a
heteroatomic monomer with an oxidizing agent, such as hydrogen
peroxide, potassium persulfate, ammonium persulfate, or the
like.
[0042] A transition metal-heteroatomic polymer-carbon composite
(also referred to herein as a "transition metal composite") is
formed by loading the transition metal on the heteroatomic
polymer-carbon composite in step 220 by combining a transition
metal precursor, such as an inorganic salt or a metal-organic
precursor of the transition metal, with the heteroatomic
polymer-carbon composite and reducing the metal. The bond--or
linkage--between polypyrrole and cobalt, formed by the reduction of
cobalt (Co.sup.2+), is shown in FIG. 3. In step 230, the transition
metal composite is combined with the recast ionomer to form a
cathode catalyst ink comprising the cathode catalyst 120.
[0043] A method of making MEA 100 comprising cathode catalyst 120
and anode catalyst 130, described herein, is also provided. FIG. 5
is a flow chart showing method 300. A heteroatomic polymer-carbon
composite is first formed (step 310) and the transition metal is
loaded on the heteroatomic polymer-carbon composite (step 320). In
step 330, the transition metal composite is combined with the
recast ionomer to form a cathode catalyst ink comprising the
cathode catalyst. The anode catalyst is combined with the recast
ionomer to form an anode catalyst ink. The cathode catalyst ink
containing cathode catalyst is then applied to a surface of
ionomeric membrane 110 in step 340, and the anode catalyst ink
containing anode catalyst 130 is applied to another surface of
ionomeric membrane 110 in step 350 to form MEA 100. In another
embodiment, a portion of the anode catalyst ink is also applied to
a surface of a first gas diffusion layer, and the gas diffusion
layer is then placed in contact with a portion of anode catalyst
ink that has been deposited on the first surface of ionomeric
membrane 110. Similarly, a portion of the cathode catalyst ink may
be applied to a surface of a second gas diffusion layer, which in
turn contacts a portion of cathode catalyst ink that has been
applied the second surface of ionomeric membrane 110. The anode
catalyst and cathode catalyst inks may be applied to ionomeric
membrane 110 or, alternatively, to gas diffusion layers 112, 114,
using techniques such as, but not limited to, brush painting,
doctor-blading, ultrasonic spraying, air spraying, screen-printing,
decal transfer, and the like.
[0044] The cathode catalyst 120 described herein is stable and
exhibits extraordinary selectivity for oxygen reduction reactions
in the presence of methanol. Due to their methanol tolerance, the
transition metal-polymer composite catalysts of the present
invention are capable of operating in highly concentrated methanol
solutions and are able to outperform Pt-based cathode catalysts at
relatively low concentrations of methanol in the anode feed stream,
in both regular separated flow fuel cells and mixed-reactant flow
fuel cells.
[0045] The following examples illustrate the various features and
advantages of the invention and are not intended to limit the
invention thereto. While the examples refer to a cathode catalyst
and MEA comprising cobalt and polypyrrole, it is understood that
these materials represent a preferred embodiment of the invention,
and that other metals and polymers described herein may also be
used.
Example 1
Preparation of a Heteroatomic Polymer-carbon Composite
[0046] A carbon dispersion was formed by adding 20 g of carbon
black (Vulcan XC 72) and 5 mL of glacial acetic acid to 150 mL of
deionized water and stirring for 20 minutes at room temperature. To
the carbon dispersion, 4 g of pyrrole (ACROS) was added and stirred
for 5 min. Other monomers that may be used include, but are not
limited to, aniline, thiophene, 3-methyl thiophene, ethylene
dioxythiophene, and the like. Next, 20 mL of 10% H.sub.2O.sub.2
were added as an oxidant and the mixture was stirred at room
temperature for 1-3 hours. The polypyrrole-loaded carbon dispersion
was filtered, washed with warm deionized water, and dried at
90.degree. C. under vacuum for 6 hours. Other oxidants that may be
used for polymerization include, but are not limited to,
FeCl.sub.3, K.sub.2S.sub.2O.sub.8, (NH.sub.4).sub.2S.sub.2O.sub.8,
and the like. The monomer concentration can be varied between about
20 weight percent and 60 weight percent.
Example 2
Cobalt Loading of Polypyrrole-carbon Composite
[0047] The polypyrrole-carbon composite (4.5 g) prepared in Example
1 was placed in a three-necked round bottom flask and intimately
mixed with 100 mL of deionized water. The resultant dispersion was
heated under reflux for 30 minutes with constant stirring, after
which a solution of Co(NO.sub.3).sub.2.xH.sub.2O (2.47 g in 25 mL
of distilled water) was added. The obtained mixture was stirred
between 75.degree. C. and 80.degree. C. for 30 minutes with
vigorous stirring. A reducing agent for reducing the Co(II) ions
was then added at 80.degree. C. The reducing agent comprised
NaBH.sub.4 (5.23 g) and NaOH (0.37 g) dissolved in 500 mL of
distilled water (pH=11.4). Other reducing agents, such as HCHO,
HCO.sub.2H, and the like, may also be used. The reducing agent was
added at a rate of 20 mL per minute using a peristaltic pump. The
pH during the reduction process was 11.1. The pH remained constant
for 30 minutes, indicating that the reduction process was
completed. The resulting catalyst was then filtered and washed
repeatedly with warm de-ionized water until the pH of the filtrate
reached 7.0. The catalyst was then dried overnight at 90.degree. C.
under vacuum. The cobalt loading under the experimental conditions
described above was found to be 10%. The concentration of cobalt
precursor can be varied to obtain a Co loading on the composite
between about 10 weight percent and about 50 weight percent.
Example 3
Preparation of Cathode Catalyst Electrode
[0048] Cathode catalyst ink was prepared by thoroughly blending the
cobalt composite powder prepared in Example 2 with water and recast
Nafion.RTM. ionomer. The cathode catalyst powder was combined with
de-ionized water to achieve a 1:10 ratio by weight. An appropriate
quantity of 5% Nafion.RTM. solution (1100 equivalent, Solution
Technology, Inc.) needed to obtain a 1:1 volume ratio of the
catalyst to Nafion.RTM. in the cathode catalyst layer 120 was added
to the water-wetted catalyst. The solution was placed in an ice
bath to prevent overheating and minimize evaporation of solvents
and ultrasonically mixed for 90 seconds.
Example 4
Preparation of Anode Catalyst Electrode
[0049] The procedure for making anode catalyst ink was similar to
that used for cathode catalyst ink. The anode catalyst was a Pt/Ru
black powder (Johnson Matthey), which was combined with de-ionized
water to obtain a 1:10 ratio by weight. An appropriate quantity of
5% Nafion.RTM. solution (1100 equivalent, Solution Technology,
Inc.) was added to the water-wetted catalyst to obtain a 1:1 volume
ratio of the catalyst to Nafion.RTM. in the anode catalyst layer
130. The solution was placed in an ice bath to prevent overheating
and minimize evaporation of solvents and ultrasonically mixed for
90 seconds.
Example 5
Preparation of MEA
[0050] The anode catalyst and cathode catalyst inks prepared in
Examples 3 and 4 were applied to an ionomeric membrane using brush
painting. Other application methods, such as, but not limited to,
doctor-blading, ultrasonic spraying, air spraying, screen-printing,
and the like may also be used. While the ionomeric membrane
Nafion.RTM. 117 was used in this example, other membranes and gas
diffusion media may be used as well. A piece of an ionomeric
membrane was placed on the top of a vacuum table that had been
preheated to 75.degree. C. The vacuum table was used to hold the
membrane in place and avoid wrinkling while painting. Either of the
anode ink or cathode ink was then applied to one side of the
membrane using a camel hair brush. After the painting of the first
electrode had been completed, the membrane was turned over and the
electrode was painted on the other side. The vials containing the
anode and cathode inks were cooled in an ice bath during painting
and capped whenever possible to minimize evaporation of solvents.
Upon completion of painting, the MEA was left on the heated vacuum
table for an additional 30 minutes to allow the anode and cathode
catalyst layers to cure. The MEA was then removed from the table
and placed in a sealed plastic bag for future use. The anode and
cathode catalyst loadings were approximately 6.0 mg/cm.sup.2 Pt--Ru
and 0.0.06 mg/cm.sup.2 Co, respectively. A scanning electron
microscope image of the MEA 100 is shown in FIG. 13. The thickness
of cathode catalyst layer 120 was determined to be 15 .mu.m.
[0051] The MEA was then assembled in standard 5 cm.sup.2 fuel cell
hardware. Hydrophobic double-sided and single-sided carbon-cloth
gas diffusion layers (backings) from De Nora USA (E-TEK, Inc.) were
used on the cathode and the anode sides of the cell,
respectively.
Example 6
Fuel Cell Performance
[0052] The performance of the MEAs and cathode catalysts described
in the preceding examples was tested in hydrogen-oxygen and
hydrogen-air fuel cells. Performance was tested at 30.degree. C.,
50.degree. C., 70.degree. C., and 80.degree. C. The flow rates of
hydrogen and air were 300 sccm and 466 sccm, respectively. The
gases were humidified at 90.degree. C. and 80.degree. C. for the
anode and cathode, respectively. The backpressure on both sides of
the cell was 30 psig.
[0053] Steady state polarization curves obtained at 80.degree. C.
with a cobalt-polypyrrole-carbon cathode (0.06 mg/cm.sup.2 Co) and
a commercially available Pt--Ru black anode (6.0 mg/cm.sup.2
Pt--Ru) are shown in FIG. 6. The polarization curves in FIG. 6
demonstrate that fuel cell performance in H.sub.2/O.sub.2 and
H.sub.2/air fuel cells is dramatically increased by conditioning in
oxygen.
[0054] Hydrogen-oxygen fuel cell polarization plots obtained for an
MEA having a Co-polypyrrole-carbon composite cathode at different
cell temperatures are shown in FIG. 7. Fuel cell performance
improves with increasing cell temperature, and the
Co-polypyrrole-carbon composite cathode catalyst demonstrates high
catalytic activity.
[0055] The life test performance of the Co-polypyrrole-carbon
composite cathode catalyst in H.sub.2/O.sub.2 fuel cells is shown
in FIG. 8. The performance of the fuel cell increases during fuel
cell operation, possibly indicating that catalytically inactive
species that remain after the synthesis step are gradually removed
from the catalyst surface during fuel cell operations.
[0056] FIG. 9 shows hydrogen-air fuel cell polarization plots
obtained at different cell temperatures for the MEA having a
Co-polypyrrole-carbon composite cathode catalyst. The results show
that fuel cell performance improves with increasing
temperature.
[0057] The life-test activity of the MEA having a
Co-polypyrrole-carbon composite cathode catalyst in an H.sub.2/air
fuel cell at 0.4 V is shown in FIG. 10. The MEA showed no decline
in the current output after more than 110 hours of continuous
operation--a major achievement for a fuel cell operating with a
cathode that does not include a precious metal such as
platinum.
[0058] The composite catalysts were characterized before and after
fuel cell operation using x-ray absorption near-edge structure
(XANES) and x-ray absorption fine structure (XAFS) spectroscopy.
FIG. 14 shows XANES spectra obtained for a
cobalt-polypyrrole-carbon composite before and after fuel cell
operation, as well as spectra for Co(0) and CoO standards. The
spectra indicate that the as-synthesized catalyst contains both
Co(0) and Co(II) states, the ratio of which decreases during
catalyst break-in. A plot of XAFS spectra of a
cobalt-polypyrrole-carbon composite before and after fuel cell
operation is comparison to a CoO standard is shown in FIG. 15. The
XAFS data shown in FIG. 15 reveals that during the break-in, most
of the Co(0) is transformed into Co(II). XAFS data analysis reveals
that Co(II) formed during catalyst break-in is likely to be
stabilized in the polymer composite via a linkage either to
nitrogen or, less likely, oxygen.
[0059] Polarization plots (FIG. 16) provide performance comparison
of different cathode materials in H.sub.2-O.sub.2 fuel cells.
Non-pyrolyzed composite cobalt-polypyrrole-carbon catalyst
("Co-PPY-C" plot in FIG. 16) allows a fuel-cell current density of
0.35 A cm.sup.-2 at 0.40 V to be achieved. However, the same
catalyst shows significantly diminished performance after having
been pyrolyzed at 800.degree. C. in nitrogen atmosphere for two
hours ("pyrolyzed Co-PPY-C" plot in FIG. 16, 0.08 A cm.sup.-2
current density at 0.40 V). This decrease in performance indicates
that the catalyst loses most of the active oxygen reduction
reaction (ORR) sites upon pyrolysis. Even lower oxygen-reduction
activity can be seen with a catalyst obtained by merely depositing
cobalt on carbon ("Co--C" plot in FIG. 16, 0.02 A cm.sup.-2 current
density at 0.40 V). The superior performance of non-pyrolyzed
Co-PPY-C catalyst with respect to pyrolyzed Co-PPY-C and PPY-free
Co--C indicates that ORR activity is gained from entrapment of Co
sites in the polypyrrole matrix, possibly accompanied by the
formation of CoN.sub.x active sites. Catalyst material obtained by
reducing PPY-C--i.e., without any cobalt present--also exhibits a
measurable catalytic activity (.about.0.05 A cm.sup.-2 current
density at 0.40 V), similar to that obtained with the pyrolyzed
composite. In general, however, the test data clearly indicate that
formation of the most active ORR sites require both the presence of
a transition metal (e.g., Co, Fe, etc.) and the heteroatomic
polymer.
[0060] The stability of the cobalt composite catalyst has been
evaluated both in H.sub.2-O.sub.2 (FIG. 17) and H.sub.2-air (FIG.
18) fuel cells at voltage voltages between 0.5 and 0.7 V. These
voltages are higher than that used (0.4 V) in the life test shown
in FIG. 10. It is evident from the data shown in FIGS. 17 and 18
that the cobalt composite catalyst is also stable at higher
voltages.
[0061] A plot obtained for a 300-hour life test at 0.4 V for an
H.sub.2-O.sub.2 fuel cell life having a Co-PPY-C is shown in FIG.
19. The life test provides evidence of very good catalyst stability
at 0.4 V over an operating time of several hundred hours.
[0062] An iron-polypyrrole-carbon (Fe-PPY-C) composite was also
evaluated as a cathode catalyst in H.sub.2-O.sub.2 fuel cells. Fuel
cell polarization data obtained at 80.degree. C. for Fe-PPY-C and
Co-PPY-C cathode catalysts are shown in FIG. 20. As seen in FIG.
20, the Co-PPY-C and Fe-PPY-C composites exhibit similar
performance at low fuel cell current densities. Less effective
oxygen mass transport in the Fe-PPY-C catalyst limits performance
at higher current densities.
Example 7
Methanol Tolerance
[0063] Methanol was supplied to the DFMC anode at a rate of 1.8
mL/min, and oxygen was supplied to the DFMC Co-polypyrrole-carbon
composite cathode. The oxygen was provided at ambient pressure
(unlike the hydrogen-oxygen experiments described above) to the
cathode and was humidified at 80.degree. C. Methanol was provided
at concentrations of 0.5 M, 1.0 M, 5.0 M, and 12 M. The DMFC was
tested at 80.degree. C.
[0064] Polarization plots for a methanol-oxygen DMFC having a
Co-polypyrrole-carbon composite cathode are shown in FIG. 11. The
polarization plots were obtained at 80.degree. C. with different
anode concentrations of methanol. The performance of the
Co-polypyrrole-carbon composite cathode was independent of methanol
concentration. The Co-polypyrrole-carbon composite cathode exhibits
slightly lower performance at a methanol concentration of 12 M
compared to that observed at other methanol concentrations. This
may be due to reduced activity of the DMFC anode at this extreme
methanol concentration, rather than methanol intolerance of the
Co-polypyrrole-carbon composite cathode.
[0065] In a separate experiment, methanol was supplied to the DFMC
anode at a rate of 1.8 mL/min, and air was supplied to the DFMC
cathode. The air provided to the cathode was humidified at
80.degree. C. and supplied at an ambient pressure. Methanol was
provided at concentrations of 0.5 M, 1.0 M, 5.0 M, and 12 M. The
DMFC was tested at 80.degree. C.
[0066] FIG. 12 shows polarization plots for the methanol-air DMFC
having a Co-polypyrrole-carbon composite cathode. The polarization
plots were obtained at 80.degree. C. with different anode
concentrations of methanol. At higher methanol concentrations, the
Co-polypyrrole-carbon composite cathode performance exceeds that of
platinum cathodes. The Co-polypyrrole-carbon composite cell shows
similar performance at methanol of concentrations 5 M and 12 M, but
the performance decrease at 5 M methanol was found to be slightly
greater in methanol-air than methanol-oxygen.
[0067] While typical embodiments have been set forth for the
purpose of illustration, the foregoing description should not be
deemed to be a limitation on the scope of the invention.
Accordingly, various modifications, adaptations, and alternatives
may occur to one skilled in the art without departing from the
spirit and scope of the present invention.
* * * * *