U.S. patent application number 09/827894 was filed with the patent office on 2001-10-25 for fuel cell membrane electrode assemblies with improved power outputs and poison resistance.
Invention is credited to Arps, James H., Cavalca, Carlos A., Murthy, Mahesh.
Application Number | 20010033960 09/827894 |
Document ID | / |
Family ID | 23312977 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033960 |
Kind Code |
A1 |
Cavalca, Carlos A. ; et
al. |
October 25, 2001 |
Fuel cell membrane electrode assemblies with improved power outputs
and poison resistance
Abstract
An electrode-membrane combination for use in a fuel cell and
providing improved power outputs and resistance to poisoning.
Multiple embodiments are described which generally involve use of a
vapor deposited zone or layer of one or more catalytically active
metals. Vapor deposition can be carried out by, for example,
sputtering or physical vapor deposition.
Inventors: |
Cavalca, Carlos A.; (Newark,
DE) ; Arps, James H.; (San Antonio, TX) ;
Murthy, Mahesh; (Elkton, MD) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
345 Park Avenue
New York
NY
10154
US
|
Family ID: |
23312977 |
Appl. No.: |
09/827894 |
Filed: |
April 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09827894 |
Apr 9, 2001 |
|
|
|
09335718 |
Jun 18, 1999 |
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Current U.S.
Class: |
429/431 ;
429/412; 429/483; 429/492; 429/534 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/1004 20130101; H01M 2300/0082 20130101; H01M 4/8642
20130101 |
Class at
Publication: |
429/40 ; 429/42;
429/30 |
International
Class: |
H01M 004/94; H01M
008/10 |
Claims
What is claimed is:
1. An electrode-membrane combination comprising: at least one
reactant diffusive, electronically conductive electrode, wherein
the electrode is substantially free of first catalytically active
metal; and at least one ionically conductive membrane contacting
the electrode to form an electrode-membrane interfacial region,
wherein the interfacial region comprises at least one zone
consisting essentially of at least two second catalytically active
metals, different from each other, and having a zone thickness of
about 3 angstroms to about 5,000 angstroms.
2. A combination according to claim 1, wherein the electrode is
free of first catalytically active metal.
3. A combination according to claim 1, wherein the electrode
comprises at least one tonically conductive polymer.
4. A combination according to claim 1, wherein the electrode is
free of first catalytically active metal and comprises ionically
conductive polymer.
5. A combination according to claim 1, wherein the zone thickness
is about 50 angstroms to about 1500 angstroms.
6. A combination according to claim 4, wherein the zone thickness
is about 50 angstroms to about 1500 angstroms.
7. A combination according to claim 1, wherein the zone thickness
is about 150 angstroms to about 500 angstroms.
8. A combination according to claim 4, wherein the zone thickness
is about 150 angstroms to about 500 angstroms.
9. A combination according to claim 1, wherein the electrode
comprises conductive carbon and at least one hydrophobic binder
polymer for the carbon.
10. A combination according to claim 1, wherein at least two of the
second catalytically active metals are noble metals.
11. A combination according to claim 1, wherein at least two of the
second catalytically active metals are alloyed with each other.
12. A combination according to claim 1, wherein the combination
further comprises at least one catalyzed gas diffusion medium.
13. A combination according to claim 1, wherein the zone consists
essentially of at least three second catalytically active
metals.
14. A combination according to claim 1, wherein the zone consists
essentially of at least four second catalytically active
metals.
15. A combination according to claim 1, wherein the electrode
comprises at least one ionically conductive polymer, and the
ionically conductive membrane comprises an ionically conductive
polymer which is substantially the same as the ionically conductive
polymer of the electrode.
16. A combination according to claim 1, wherein the electrode
further comprises at least one solvent.
17. A combination according to claim 1, wherein the membrane has a
thickness of about 3 microns to about 75 microns.
18. A combination according to claim 1, wherein the membrane has a
thickness less than about 30 microns and a Gurley number greater
than 10,000 seconds, and the membrane comprises a microporous
polymer impregnated with at least one ionically conductive
polymer.
19. A combination according to claim 18, wherein the microporous
polymer is expanded polytetrafluoroethylene and the ionically
conductive polymer is a perfluoroionomer.
20. A combination according to claim 19, wherein the electrode is
free of first catalytically active metal and comprises ionically
conductive polymer, and wherein the zone thickness is about 50
angstroms to about 1,500 angstroms.
21. An electrode-membrane combination comprising: at least one
reactant diffusive, electronically conductive electrode, wherein
the electrode is substantially free of first catalytically active
metal; and at least one ionically conductive membrane contacting
the electrode to form an electrode-membrane interfacial region,
wherein the interfacial region consists essentially of at least one
zone comprising at least two second catalytically active metals,
different from each other, and having a zone loading of about 0.001
mg metal/cm.sup.2 to about 0.7 mg metal/cm.sup.2.
22. The combination according to claim 21, wherein the zone is a
vapor deposited zone and the zone loading is 0.01 mg/cm.sup.2 to
0.4 mg/cm.sup.2.
23. The combination according to claim 21, wherein the zone is
vapor deposited by physical vapor deposition.
24. The combination according to claim 21, wherein the zone is
vapor deposited by electron beam physical vapor deposition.
25. A combination according to claim 21, wherein the zone is vapor
deposited with ion beam assisted deposition.
26. A combination according to claim 21, wherein the zone consists
essentially of sequentially deposited zones of catalytic metal.
27. The combination according to claim 21, wherein the zone is
vapor deposited by sputtering.
28. The combination according to claim 21, wherein the zone is
vapor deposited by magnetron sputtering with use of multiple
independent sputtering targets.
29. The combination according to claim 21, wherein the zone is
deposited by chemical vapor deposition, physical vapor deposition,
thermal deposition, cathodic arc deposition, ion sputtering, ion
beam assisted deposition, or jet vapor deposition.
30. The combination according to claim 21, wherein the membrane has
a thickness less than 30 microns and a Gurley number greater than
10,000 seconds.
31. The combination according to claim 21, wherein the membrane
comprises at least one microporous polymeric film impregnated with
at least one ion exchange resin.
32. The combination according to claim 21, wherein the electrode
further comprises at least one ionically conductive polymer.
33. The combination according to claim 31, wherein the electrode
further comprises at least one ionically conductive polymer.
34. The combination according to claim 32, wherein the ionically
conductive polymer of the electrode impregnates the electrode at
the membrane-electrode interface.
35. The combination according to claim 33, wherein the ionically
conductive polymer of the electrode impregnates the electrode at
the membrane-electrode interface.
36. The combination according to claim 21, wherein at least two of
the second catalytically active metals are alloyed with each other,
and the zone loading is less than 0.3 mg/cm.sup.2.
37. The combination according to claim 31, wherein at least two of
the second catalytically active metals are alloyed with each
other.
38. The combination according to claim 35, wherein at least two of
the second catalytically active metals are alloyed with each
other.
39. The combination according to claim 38, wherein the zone is a
vapor deposited zone.
40. The combination according to claim 38, wherein the zone is
deposited by chemical vapor deposition, physical vapor deposition,
thermal deposition, cathodic arc deposition, ion sputtering, ion
beam assisted deposition, or jet vapor deposition.
41. An electrode membrane combination in which noble metal in the
combination is substantially concentrated at an electrode membrane
interface for fuel cell catalysis, the combination consisting
essentially of (i) at least one electronically conductive electrode
which allows for fuel cell reactant transport and which is surface
enriched with at least one ionically conductive polymer, and (ii)
at least one ionically conductive membrane which contacts the
electrode at the electrode surface which is enriched with ionically
conductive polymer, thereby forming an electrode membrane
interfacial region, wherein a vapor deposited layer comprising at
least two vapor deposited noble metals is disposed at the
interfacial region.
42. The combination according to claim 41, wherein the layer is
deposited by chemical vapor deposition, physical vapor deposition,
thermal deposition, cathodic arc deposition, ion sputtering, ion
beam assisted deposition, or jet vapor deposition.
43. The combination according to claim 41, wherein the layer
comprises an alloy of metals.
44. The combination according to claim 41, wherein the layer
comprises multiple layers of noble metal.
45. The combination according to claim 41, wherein the layer is
deposited by multiple target sputtering.
46. The combination according to claim 41, wherein the layer has a
layer loading of about 0.001 mg metal/cm.sup.2 to about 0.7 mg
metal/cm.sup.2.
47. The combination according to claim 41, wherein the layer has a
layer thickness of about 3 angstroms to about 5,000 angstroms.
48. The combination according to claim 41, wherein the layer has a
layer thickness of about 50 angstroms to about 1,500 angstroms.
49. The combination according to claim 41, wherein the layer has a
layer thickness of about 50 angstroms to about 500 angstroms.
50. The combination according to claim 41, wherein the electrode is
substantially free of noble metal except for the noble metal of the
layer at the interface.
51. The combination according to claim 41, wherein the membrane has
a thickness less than about 30 microns, has a Gurley number greater
than about 10,000 seconds, and comprises expanded
polytetrafluoroethylene impregnated with ionomer.
52. The combination according to claim 41, wherein the layer
comprises at least three vapor deposited noble metals.
53. The combination according to claim 41, wherein all noble metal
in the combination is at the electrode membrane interface.
54. The combination according to claim 41, wherein the metals of
the layer are at least partially alloyed in situ as the metals are
vapor deposited from separate sputtering targets.
55. The combination according to claim 41, wherein the layer is
vapor deposited onto the electrode before an impregnation of the
ionically conductive polymer into the electrode to provide
enrichment and before formation of the interfacial region between
the membrane and the electrode.
56. A membrane electrode assembly for use in a solid polymer
electrolyte fuel cell which has enhanced resistance to poisoning
comprising two electrodes sandwiching a layer of solid polymer
electrolyte to form two membrane electrode interfacial regions,
wherein the solid polymer electrolyte layer is air impermeable and
has a thickness less than about 75 microns, wherein the electrode
is catalyzed with a vapor deposited zone consisting essentially of
a composition of at least two noble metals which is formulated to
enhance poison resistance during fuel cell operation.
57. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.65 or
less when subjected to hydrogen feed with 5 ppm CO and 0.85 or less
when subjected to hydrogen feed with 50 ppm CO.
58. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.50 or
less when subjected to hydrogen feed with 5 ppm CO and 0.85 or less
when subjected to hydrogen feed with 50 ppm CO.
59. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.25 or
less when subjected to hydrogen feed with 5 ppm CO and 0.60 or less
when subjected to hydrogen feed with 50 ppm CO.
60. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.65 or
less when subjected to hydrogen feed with 5 ppm CO.
61. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.25 or
less when subjected to hydrogen feed with 5 ppm CO.
62. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.85 or
less when subjected to hydrogen feed with 50 ppm CO.
63. The assembly according to claim 56, wherein the assembly
provides a poison resistance lambda parameter at 0.6 V of 0.60 or
less when subjected to hydrogen feed with 50 ppm CO.
64. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 350 mA/cm.sup.2
when subjected to hydrogen feed with 5 ppm CO and at least 150
mA/cm.sup.2 when subjected to hydrogen feed with 50 ppm CO.
65. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 450 mA/cm.sup.2
when subjected to hydrogen feed with 5 ppm CO and at least 200
mA/cm.sup.2 when subjected to hydrogen feed with 50 ppm CO.
66. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 500 mA/cm.sup.2
when subjected to hydrogen feed with 5 ppm CO and at least 175
MA/cm.sup.2 when subjected to hydrogen feed with 50 ppm CO.
67. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 350 mA/cm.sup.2
when subjected to hydrogen feed with 5 ppm CO.
68. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 450 mA/cm.sup.2
when subjected to hydrogen feed with 5 ppm CO.
69. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 150 mA/cm.sup.2
when subjected to hydrogen feed with 50 ppm CO.
70. The assembly according to claim 56, wherein the assembly
provides a current density at 0.6 V of at least 175 mA/cm.sup.2
when subjected to hydrogen feed with 50 ppm CO.
71. A fuel cell comprising a plurality of membrane electrode
assemblies according to claim 56.
72. A transportation vehicle comprising a fuel cell according to
claim 71.
73. A membrane electrode assembly prepared by the combination of
steps comprising: providing assembly elements including (i) at
least one reactant diffusive, electronically conductive electrode
which comprises at least one ionically conductive polymer but which
is substantially free of a first catalytically active metal, and
(ii) at least one ionically conductive membrane; depositing onto at
least one of the assembly elements a zone consisting essentially of
at least two second catalytically active metals having a zone
thickness of about 3 angstroms to about 5,000 angstroms, wherein
the zone deposition is (i) a direct deposition onto the assembly
element, or (ii) an indirect deposition onto the assembly element
wherein the deposited zone is first deposited onto a substrate and
then transferred from the substrate onto the assembly element, and
optionally, assembling the membrane electrode assembly from the
assembly elements.
74. A membrane electrode assembly according to claim 73, further
comprising the assembly step.
75. A membrane electrode assembly according to claim 73, wherein
the assembly element is the membrane.
76. A membrane electrode assembly according to claim 73, wherein
the assembly element is the electrode.
77. A membrane electrode assembly according to claim 73, wherein
the deposition is chemical vapor deposition, physical vapor
deposition, thermal deposition, cathodic arc deposition, ion
sputtering, ion beam assisted deposition, or jet vapor
deposition.
78. A membrane electrode assembly according to claim 73, the
assembly further comprising at least one catalyzed gas diffusion
medium.
79. A combination according to claim 21, wherein the zone consists
essentially of simultaneously deposited catalytic metal.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to fuel cell membrane
electrode assemblies with improved power outputs. More
particularly, these improved assemblies feature a relatively thin
zone of one or more catalytically active metals at the
membrane-electrode interface in addition to the catalytically
active metal which also can be present in the electrode.
BACKGROUND TO THE INVENTION
[0002] Fuel cells continue to show great commercial promise
throughout the world as an alternative to conventional energy
sources. This commercial promise should continue to grow as energy
shortages become more acute, environmental regulations become more
stringent, and new fuel cell applications emerge. See, e.g. "FUEL
CELLS", Encyclopedia of Chemical Technology, 4th Ed., Vol. 11, pp.
1098-1121. Strikingly, numerous automotive manufacturers have
announced and continue to announce plans for mass production and
retail sale of fuel cell-powered cars in the near future.
[0003] Despite improvements in fuel cell technology, however, long
felt needs generally exist to increase power output, reduce initial
cost, improve water management, and lengthen operational lifetime.
Initial cost reduction, which is particularly important, can be
most easily achieved by reducing the precious metal content of the
fuel cell electrode. Such reduction, however, generally results in
power output loss which blocks commercialization efforts. Hence,
new discoveries are needed to resolve these and other difficult
compromises.
[0004] There are different types of fuel cells, but they each
produce electrical energy by means of chemical reaction. One type
of increasing import, the "polymer electrolyte membrane fuel cell"
(PEMFC), comprises a membrane electrode assembly (MEA) typically
made of an ionically conducting polymeric membrane sandwiched
between two electronically conducting electrodes. The electrodes,
besides being conductive, are also associated with electrocatalyst
layers which provide catalysis. For commercial application,
multiple MEAs can be electronically connected to form a fuel cell
stack (i.e., "stacked"). Other components associated with typical
PEMFCs include gas diffusion media and current collectors, the
latter of which can also serve as bipolar separators and flow field
elements. PEMFCs have been reviewed in the literature. See S.
Srinivasan et al.; J. Power Sources; 29 (1990); pp. 367-387; and
Fuel Cell Systems, L.J.M.J. Blomen and M. N. Mugerwa (Ed.); Plenum
Press; 1993; Chapter 11.
[0005] In a typical PEMFC, a fuel such as hydrogen gas is
electrocatalytically oxidized at one electrode (anode). At the
other electrode (cathode), an oxidizer such as oxygen gas is
electrocatalytically reduced. The net reaction, when mediated by
the membrane, results in generation of electromotive force and
external current flow between the electrodes. Elevated temperature
can accelerate this reaction, although one increasingly important
advantage of the PEMFC is that lower temperatures (e.g., 80.degree.
C.) can be used. The fuel cell reactions are generally catalyzed by
precious transition metals, commonly a noble metal such as
platinum, which are present in both anode and cathode as an
electrocatalyst layer. Because the fuel cell is often operated with
use of gaseous reactants, typical electrodes are porous materials
(or more generally, reactant diffusive materials) having the
catalytically active metal at or within the porous surfaces. In
some cases, catalyst may be thinly coated by polymer which allows
gas to diffuse to the underlying catalyst. The metal can be in
different morphological forms, but often it is in particulate or
dispersed form and supported on carbon. Fuel cell performance can
depend on the catalyst form. See Poirier et al.; J. Electrochemical
Society, vol. 141, no. 2, February 1994, pp. 425-430.
[0006] Fuel cell systems are complex because the reaction is
believed localized at a three-phase boundary between ionically
conducting membrane, gas, and carbon supported catalyst. Because of
this localization, addition of ionically conductive material to the
electrode can result in better utilization of catalyst as well as
improved interfacial contact with the membrane. However, the
additional ionic conductor can introduce extra cost, especially
when perfluorinated conductors are used, and can increase the
complexity of electrolyte water management, all important to
commercialization. Hence, discoveries are needed which improve
utilization of ionic conductor.
[0007] To minimize the loading of expensive catalytic metal, one
general approach has been to use smaller catalyst particles.
However, long operational lifetimes are particularly difficult to
achieve with lower catalyst loadings, and catalyst poisoning can
occur. Also, catalyst particle size may be unstable and increase by
agglomeration or sintering. Hence, discoveries are also needed to
improve utilization of catalyst, and the combination of catalyst
and ionic conductor.
[0008] Another generic approach which has been tried without
success has been to concentrate the metal at the membrane-electrode
interface. See, e.g., Ticianelli et al.; Journal of
Electroanalytical Chemistry and Interfacial Electrochemistry; Vol.
251 No. 2, Sep. 23, 1988, pp. 275-295. For example, 500 angstrom
dense layers of single metal catalyst reportedly have been
sputtered onto certain gas diffusion electrodes before sandwiching
the ionically conducting membrane between the electrodes. However,
sputtered layers thinner than 500 angstroms were not reported,
possibly because of the difficulty in making uniform thinner
layers. Moreover, the concentration of catalyst approach may not be
suitable for other types of electrodes and deposition techniques
and may upset water balance. Further, testing often is not carried
out under commercial conditions. Particularly poor performance was
reported for electrodes in which all of the catalyst metal was in
the form of a sputtered film. In sum, it is recognized that mere
vapor depositing an allegedly thin layer of catalyst onto the
electrode does not guarantee a suitable MEA for commercial
applications, and in general, industry has not accepted this
approach as realistic. . According to the Srinivasan article noted
above, for example, sputtering may not be economically feasible
compared with wet chemical deposition methods.
[0009] Additional technology is described in the patent literature
including, for example, U.S. Pat. Nos. 3,274,029; 3,492,163;
3,615,948; 3,730,774; 4,160,856; 4,547,437; 4,686,158; 4,738,904;
4,826,741; 4,876,115; 4,937,152; 5,151,334; 5,208,112; 5,234,777;
5,338,430; 5,340,665; 5,500,292; 5,509,189; 5,624,718; 5,686,199;
and 5,795,672. In addition, deposition technology is described in,
for example, U.S. Pat. Nos. 4,931,152; 5,068,126; 5,192,523; and
5,296,274.
[0010] Although much research has focused on fuel cell electrodes,
particularly significant developments relating to fuel cell
membranes are described in U.S. Pat. Nos. 5,547,551; 5,599,614; and
5,635,041 (Bahar et al.). For commercial applications, membrane
design should be integrated with electrode design in a systemic
approach to maximize fuel cell performance. Combinations of
properties, which are vital for commercialization, can be difficult
to achieve without this integrated approach.
[0011] Finally, another problem which can arise and block
commercialization is catalyst poisoning which is caused by
impurities such as carbon monoxide (CO) in the reactants. For
example, when hydrogen fuel is generated by hydrocarbon reforming,
CO can be co-generated which is expensive to remove, particularly
when the CO level in hydrogen is reduced to below 100 ppm.
Poisoning is especially problematic in PEMFCs which have low
catalyst loadings and which employ the single metal platinum.
Discoveries are needed to solve poisoning problems without
generating other problems and to provide suitable compromises.
[0012] Although attempts to mitigate CO poisoning have been
reported, they generally have been unsuccessful and have resulted
in reduced power. Some of these attempts have focused on mixing
platinum with other transition metals such as ruthenium before
deposition on the electrode. Deposition methods have included, for
example, wet ink and vacuum methods. However, ink methods can be
difficult to control precisely, and some vacuum methods can be
expensive and cumbersome, particularly for thin film
deposition.
[0013] Methods of using transition metal catalyst mixtures are
discussed in, for example, U.S. Pat. Nos. 4,430,391; 4,487,818;
5,296,274; 5,395,704; and 5,786,026. In addition, Morita et al.
describe a gold-platinum bimetallic model catalyst on smooth carbon
support by RF sputtering. See "Anodic Oxidation of Methanol at a
Gold-Modified Platinum Electrocatalyst Prepared By RF Sputtering on
a Glassy Carbon Support,"; Electrochimica Acta, vol. 36, No. 5/6,
pp. 947-951 (1991). However, this article reports problems in
obtaining consistent results and reproducible data with bimetallic
systems. In addition, this article only describes methanol
oxidation which is different mechanistically from other fuel
oxidations. It does not describe hydrogen oxidation and the use of
reactant diffusive (or porous) electrodes. The thicknesses of the
catalytic zones are not reported although thicknesses less than one
micron are noted. Alloys are not described, and deposition methods
other than RF sputtering are not described.
[0014] In addition, U.S. Pat. No. 5,750,013 describes a membrane
electrode assembly based on a vacuum-deposited ionically conducting
membrane which is positioned between vacuum-deposited alternating
layers of microparticle metal layer and porous conducting layer.
The entire membrane electrode assembly is prepared by vacuum
deposition. However, there is no concentration of the catalytically
active metal at the membrane-electrode interface. Rather, the
layered structures described are not concentrated and would be
expected to have relatively poor catalytic efficiency. In addition,
a well-integrated three-phase boundary between the ionic conductor
of the membrane, the electronic conductor of the electrode, and the
catalytically active metal of the electrode is not present.
Moreover, the vacuum-deposited membrane is excessively thin and
would not generally be a suitable fuel cell barrier in practical
applications. Finally, no experiments are reported in this patent
on the performance of the membrane electrode assembly, particularly
under commercially realistic conditions.
[0015] U.S. Pat. Nos. 4,430,391 and 4,487,818 (Ovshinsky et al.)
describe fuel cell electrodes in which a host matrix, which
comprises transition metal, is modified with at least one modifier
element to improve catalytic properties. The modified catalytic
layer can be deposited onto a catalyst-free gas diffusion electrode
by sputtering. The minimum thickness of the catalytic layer is
described as 0.5 microns (5,000 angstroms). According to these
patents, the modifier element increases the amount of disorder in
the host matrix which increases the number of catalytically active
sites in the electrode. Although experimental data are reported in
these patents, no experimental data are reported for a working
membrane electrode assembly or fuel cell. Also, polymer electrolyte
membrane fuel cells are not taught or suggested.
[0016] U.S. Pat. Nos. 5,879,827 and 5,879,828 describe membrane
electrode assemblies which are prepared with use of vacuum
deposition of metallic catalyst onto whisker-like supports held by
a substrate. The supported catalyst is then transferred from the
substrate to the membrane or electrode during assembly of the
membrane electrode assembly. These patents, however, do not teach
or suggest catalyst in a form which is not intimately joined or
bonded to the whisker support, which is generally non-conductive.
Also, these patents teach that use of ionically conductive polymer
in the electrode is undesirable, and that maximum contact between
the catalyst and the ionically conductive material is not
important. Growth of the catalyst appears to be organized as
opposed to random. It would desirable to prepare structures which
consist essentially of elements which do not include and do not
require use of whisker-like structures to support the catalyst, and
which have extensive contact between catalyst and ionically
conductive material. Also, random growth of catalyst structure can
be important.
[0017] In general, therefore, the prior art apparently does not
teach, demonstrate, or even suggest fuel cell technology which is
suitable for the current or next generation commercial demands.
SUMMARY OF THE INVENTION
[0018] Despite the prejudices existing in the art, the inventors
have discovered that surprisingly high improvements in power output
can be achieved for low and ultra-low loading catalyst MEAs. By
introducing a relatively thin zone of catalytic metal at the
interface between selected electrodes and membranes, significantly
more power can be produced for the same amount, or even lesser
amounts, of catalyst. Moreover, by combining selected electrodes
and membranes in a systems approach, superior overall fuel cell
performance can be achieved. The test results, significantly, are
promising even under commercially realistic conditions, and in
particular, poison resistance can be improved.
[0019] In one aspect of this invention, the inventors have
discovered an electrode-membrane combination comprising:
[0020] at least one reactant diffusive, electronically conductive
electrode comprising at least one first catalytically active metal
and at least one ionically conductive polymer; and
[0021] at least one ionically conductive membrane contacting the
electrode to form an electrode-membrane interfacial region,
[0022] wherein the interfacial region comprises at least one zone
comprising at least two different second catalytically active
metals and having a zone thickness of about 3 angstroms to about
5,000 angstroms.
[0023] In another aspect of this invention, the inventors have
discovered an electrode-membrane combination comprising:
[0024] at least one porous, conductive electrode comprising at
least one first catalytically active metal and at least one
ionically conductive polymer; and
[0025] at least one ionically conductive membrane contacting the
electrode to form an electrode-membrane interfacial region,
[0026] wherein the interfacial region comprises at least one zone
comprising at least two different second catalytically active
metals and having a zone loading of about 0.001 mg metal/cm.sup.2
to about 0.7 mg metal/cm.sup.2.
[0027] Still further, another aspect of this invention is an
electrode-membrane combination comprising:
[0028] at least one reactant diffusive, electronically conductive
electrode, wherein the electrode is substantially free of first
catalytically active metal; and
[0029] at least one ionically conductive membrane contacting the
electrode to form an electrode-membrane interfacial region,
[0030] wherein the interfacial region consists essentially of at
least one zone comprising at least two second catalytically active
metals, different from each other, and having a zone thickness of
about 3 angstroms to about 5,000 angstroms.
[0031] Another aspect of this invention involves an electrode-
membrane combination comprising:
[0032] at least one reactant diffusive, electronically conductive
electrode, wherein the electrode is substantially free of first
catalytically active metal; and
[0033] at least one ionically conductive membrane contacting the
electrode to form an electrode-membrane interfacial region,
[0034] wherein the interfacial region comprises at least one zone
consisting essentially of at least two second catalytically active
metals, different from each other, and having a zone loading of
about 0.001 mg metal/cm.sup.2 to about 0.7 mg metal/cm.sup.2.
[0035] The present invention also involves an electrode membrane
combination in which noble metal in the combination is
substantially concentrated at an electrode membrane interface for
fuel cell catalysis, the combination consisting essentially of (i)
at least one electronically conductive electrode which allows for
fuel cell reactant transport and which is surface enriched with at
least one ionically conductive polymer, and (ii) at least one
ionically conductive membrane which contacts the electrode at the
electrode surface which is enriched with ionically conductive
polymer, thereby forming an electrode membrane interfacial region,
wherein a vapor deposited layer comprising at least two vapor
deposited noble metals is disposed at the interfacial region.
[0036] The invention further includes, in one aspect, a membrane
electrode assembly for use in a solid polymer electrolyte fuel cell
which has enhanced resistance to poisoning comprising two
electrodes sandwiching a layer of solid polymer electrolyte to form
two membrane electrode interfacial regions,
[0037] wherein the solid polymer electrolyte layer is air
impermeable and has a thickness less than about 75 microns,
[0038] wherein the electrode is catalyzed with a vapor deposited
zone consisting essentially of a composition of at least two noble
metals which is formulated to enhance poison resistance during fuel
cell operation.
[0039] In still another aspect of this invention, a membrane
electrode assembly is provided which is prepared by the combination
of steps comprising:
[0040] providing assembly elements including (i) at least one
reactant diffusive, electronically conductive electrode which
comprises at least one ionically conductive polymer but which is
substantially free of a first catalytically active metal, and (ii)
at least one ionically conductive membrane;
[0041] depositing onto at least one of the assembly elements a zone
consisting essentially of at least two second catalytically active
metals having a zone thickness of about 3 angstroms to about 5,000
angstroms, wherein the zone deposition is (i) a direct deposition
onto the assembly element, or (ii) an indirect deposition onto the
assembly element wherein the deposited zone is first deposited onto
a substrate and then transferred from the substrate onto the
assembly element, and
[0042] optionally, assembling the membrane electrode assembly from
the assembly elements.
[0043] The advantages of this invention, in its multiple
embodiments, are numerous. In addition to improved power output
with better catalyst utilization and poison resistance, a further
important advantage is that multiple methods can be used to prepare
the structures, and that these multiple methods can be tailored to
different commercial applications. More precise design and control
is now possible. Good integration between the membrane and
electrode, and between membrane, cathode, and anode has been
achieved. Also noteworthy are that the zone of catalyst metal does
not substantially upset the water balance of the fuel cell system,
that the invention can be applied to different fuel cell reactants,
and that process scalability has been demonstrated. Different
deposition methods can be used including electron beam physical
vapor deposition and multi-target sputtering. Surprisingly,
mixtures of catalysts can have both improved poison resistance and
improved power output compared to a single catalyst. Finally,
catalyst zones consist essentially of catalytically active metals
for which organized, whisker-like substrates are not needed to
support the metals, and which provide for good interfacial contact
between catalytically active metal and ion conductive material.
[0044] In sum, the invention responds to the market demands to be
commercially realistic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] FIG. 1 is a cross-sectional view of an MEA according to the
invention comprising an ionically conducting membrane sandwiched
between two electronically conducting electrodes and forming two
interfacial regions.
[0046] FIG. 2 is a cross-sectional view of a half cell according to
the invention including an ionically conductive membrane contacting
an electronically conductive electrode to form a membrane-electrode
interfacial region. A zone of catalytically active metal is also
present.
[0047] FIG. 3 is a representation of the z-gradient of
catalytically active metal for one embodiment of the invention. The
cross-sectional view of the electrode shows catalytically active
metal vacuum deposited directly onto the electrode.
[0048] FIG. 4 further represents the general concept of the
z-gradient, step function according to the invention.
[0049] FIG. 5 shows current-voltage (I-V) analysis (or polarization
curve) of an MEA with a z-gradient cathode (Example 1) compared to
a reference MEA without a z-gradient cathode.
[0050] FIG. 6 is another I-V analysis of an MEA with a z-gradient
cathode (Example 1) compared against a reference MEA.
[0051] FIG. 7 includes normalized I-V analysis of an MEA with a
z-gradient cathode (Example 1) compared against a reference
MEA.
[0052] FIG. 8 shows I-V analyses of both an MEA with a z-gradient
cathode and an MEA with a z-gradient anode (Example 1) each
compared against a reference MEA.
[0053] FIG. 9 shows analyses of an MEA with a z-gradient cathode at
two different loadings (Example 2) compared against a reference
MEA.
[0054] FIG. 10 shows I-V analysis of an MEA with a z-gradient
cathode (Example 2) compared against a reference MEA. A compensated
potential analysis is also provided.
[0055] FIG. 11 is a normalized I-V analysis of an MEA with a
z-gradient cathode (Example 2) compared against a reference
MEA.
[0056] FIG. 12 is an I-V analysis of an MEA with a z-gradient
cathode (Example 3) compared against a reference MEA.
[0057] FIG. 13 is an I-V analysis of an MEA with a z-gradient
cathode (Example 3) compared against a reference MEA.
[0058] FIG. 14 is an I-V analysis of an MEA with a z-gradient
cathode (Example 5) compared against a reference MEA.
[0059] FIG. 15 is a field-emission scanning electron microscope
(FE-SEM) analysis of a reference electrode material having catalyst
but no vacuum-deposited z-gradient zone.
[0060] FIG. 16 is an FE-SEM analysis of an electrode having both
catalyst and a 5 angstrom (0.001 mg Pt/cm.sup.2) loading of the
z-gradient zone.
[0061] FIG. 17 is an FE-SEM analysis of an electrode having both
catalyst and a 50 angstrom (0.01 mg Pt/cm.sup.2) loading of the
z-gradient zone.
[0062] FIG. 18 is an FE-SEM analysis of a 500 angstrom (0.1 mg
Pt/cm.sup.2) loading of the z-gradient zone.
[0063] FIG. 19 is XRD analysis of a sample according to the
invention (Example 6) in which sequential EB-PVD was used to
prepare the sample.
[0064] FIG. 20 is wide scan XPS analysis of a sample according to
the invention (Example 6) in which sequential EB-PVD was used to
prepare the sample.
[0065] FIG. 21 is high resolution XPS analysis (platinum region) of
a sample according to the invention (Example 6) in which sequential
EB-PVD was used to prepare the sample.
[0066] FIG. 22 is high resolution XPS analysis (ruthenium region)
of a sample according to the invention (Example 6) in which
sequential EB-PVD was used to prepare the sample.
[0067] FIG. 23 is XRD analysis of a sample according to the
invention (Example 7) in which sequential EB-PVD with use of IBAD
was used to prepare the sample.
[0068] FIG. 24 illustrates the experimental set-up for multiple
target sputtering according to the invention (Example 8).
[0069] FIG. 25 is EDAX analysis of a sample according to the
invention (Example 8) in which sputtering was used to prepare the
sample.
[0070] FIG. 26 is XRD analysis of a sample according to the
invention (Example 8) in which sputtering was used to the prepare
the sample.
[0071] FIGS. 27 and 28 are wide scan XPS analyses of samples
according to the invention (Example 8) in which sputtering was used
to prepare the sample.
[0072] FIGS. 29 and 30 are XPS analyses (platinum region) of
samples according to the invention (Example 8) in which sputtering
was used to prepare the sample.
[0073] FIGS. 31 and 32 are XPS analyses (ruthenium region) of
samples according to the invention (Example 8) in which sputtering
was used to prepare the sample.
[0074] FIGS. 33 and 34 show polarization performance for membrane
electrode assemblies described in Example 8 which comprise a
deposited zone of platinum for reference.
[0075] FIGS. 35 and 36 show polarization performances for membrane
electrode assemblies with bimetallic deposited layer according to
the invention (Example 8).
[0076] FIGS. 37, 38, and 39 are EDAX analyses for samples according
to the invention prepared by sputtering (Example 9).
[0077] FIGS. 40, 41, and 42 are XRD analyses for samples according
to the invention prepared by sputtering (Example 9).
[0078] FIG. 43 is EDAX analysis for a sample according to the
invention prepared by sputtering (Example 10).
[0079] FIG. 44 is XRD analysis for a sample according to the
invention prepared by sputtering (Example 10).
[0080] FIG. 45 is EDAX analysis for a sample according to the
invention prepared by sputtering (Example 11).
[0081] FIG. 46 is XRD analysis for a sample according to the
invention prepared by sputtering (Example 11).
[0082] FIG. 47 is XRD analysis for a sample according to the
invention prepared by sequential EB-PVD (Example 12).
[0083] FIG. 48 is wide-scan XPS analysis for a sample according to
the invention prepared by sequential EB-PVD (Example 12).
[0084] FIG. 49 is XPS analysis (platinum region) for a sample
according to the invention prepared by sequential EB-PVD (Example
12).
[0085] FIG. 50 is XPS analysis (ruthenium region) for a sample
according to the invention prepared by sequential EB-PVD (Example
12).
[0086] FIG. 51 is XRD analysis for a sample according to the
invention prepared by sequential EB-PVD (Example 13).
[0087] FIG. 52 is wide scan XPS analysis for a sample according to
the invention prepared by sequential EB-PVD (Example 13).
[0088] FIG. 53 is XPS analysis (platinum region) for a sample
according to the invention prepared by sequential EB-PVD (Example
13).
[0089] FIG. 54 is XPS analysis (ruthenium region) for a sample
according to the invention prepared by sequential EB-PVD (Example
13).
[0090] FIGS. 55 and 56 show polarization performances for membrane
electrode assemblies with bimetallic deposited layer according to
the invention (Example 13).
[0091] FIG. 57 is XRD analysis for a sample according to the
invention prepared by sequential EB-PVD with use of IBAD (Example
14).
[0092] FIG. 58 is wide scan XPS analysis for a sample according to
the invention prepared by sequential EB-PVD with use of IBAD
(Example 14).
[0093] FIG. 59 is XPS analysis (platinum region) for a sample
according to the invention prepared by sequential EB-PVD with use
of IBAD (Example 14).
[0094] FIG. 60 is XPS analysis (ruthenium region) for a sample
according to the invention prepared by sequential EB-PVD with use
of IBAD (Example 14).
[0095] FIGS. 61 and 62 show polarization performances for membrane
electrode assemblies with bimetallic deposited layer according to
the invention (Example 14).
DETAILED DESCRIPTION OF THE INVENTION
[0096] GENERAL ASPECTS OF MEAs AND HALF CELLS
[0097] FIG. 1 illustrates a cross-section of a planar geometry MEA
according to this invention. The z-direction is shown coplanar with
the page and perpendicular to the plane of the MEA. Components 1
and 3 represent electronically conductive electrodes (first and
second electrodes) which each contact and together sandwich an
ionically conductive polymeric membrane 2. Regions 4 and 5
represent first and second interfacial regions. The regions
separate the membrane 2 from the first and second electrodes (1 and
3). The electrodes can include catalytically active metal including
metal at the interfacial regions as an electrocatalyst region. The
MEA comprises two half cells formed by combination of electrode 1
and membrane 2 (without electrode 3) or by combination of electrode
3 and membrane 2 (without electrode 1).
[0098] FIG. 2 illustrates a half cell according to this invention
comprising the first electrode 1 and the ionically conductive
membrane 2 which together contact and form interfacial region 4.
The extent of the interfacial region can depend on, for example,
(i) the method by which the membrane and electrode are brought into
contact, (ii) the surface roughness and porosity of the membrane
and electrode, and (iii) the possible use of similar materials
(e.g., similar polymeric ionomers) in both electrode and membrane
which merge upon assembly of electrode and membrane. Irrespective
of how the half cell is formed, however, this interfacial region
comprises a zone 6 of catalytically active metal(s) which
optionally is the same catalytically active metal present in the
electrode (first metal). However, the catalytically active metal(s)
of zone 6 (second metal) can be deposited in a separate step from
catalytically active metal in electrode 1. The second metal can be
a different metal entirely from the first metal, or it can be the
same metal but have a different structure or morphology. Mixtures
of metals can be used so that, for example, the zone 6 comprises at
least two different second catalytically active metals or the
electrode comprises at least two different first catalytically
active metals.
[0099] FIG. 3 illustrates by means of a cross-sectional view of an
electrode a preferred embodiment of this invention (see Example 2
below). The electrode, which is porous and allows for reactant
diffusion, comprises ionically conducting perfluorinated ionomer
fused with particles of carbon supported platinum catalyst. In
addition, the electrode comprises a vacuum deposited zone of
platinum which helps form a z-gradient step function of
catalytically active metal at the interface of the electrode and
membrane. Even though the interface can have heterogeneity because
of, for example, porosity and processing effects, the zone can be
viewed singularly as a unitary element of the invention despite the
possibility of zone interruption at particular regions in the
membrane electrode assembly. Hence, for example, the zone can be
characterized by a zone thickness, which is described further
below.
[0100] FIG. 4 further represents the general z-gradient step
function concept of this invention. In this representation,
concentration of catalytically active metal in the electrode is
generally shown as a function of distance from the membrane. The
catalytically active metal can be either metal which is present
originally in the electrode (i.e., first metal) or metal which is
deposited separately (i.e., second metal). Initially, in region A,
the catalytically active metal is entirely or substantially the
second metal, and the electrode is substantially pure metal free
from carbon or ionically conductive polymer. Then, a region B
exists wherein the concentration of second metal drops. The slope
in region B of this general representation can vary depending on,
for example, surface roughness, electrode porosity, homogeneity,
preparation method, and other experimental factors. The slope can
include a linear or substantially linear portion. Finally, a region
C exists wherein the concentration of catalytically active metal is
due to the first metal originally present in the electrode before
deposition of the second. If desired, region C can include a
gradient in concentration of first catalytically active metal with
higher concentrations toward the membrane. In practice,
catalytically active metal can be distributed throughout the
electrode with less than ideal uniformity.
[0101] Although the theory of the present invention is not fully
understood, it is believed that an unexpected synergistic
interaction can occur between the first catalytically active metal
and the zone of deposited second catalytically active metal. As a
result, significant power increases can be observed without
substantial increase in metal loading, particularly when selected
deposition methods are used.
[0102] This invention is widely applicable in fuel cell technology,
particularly PEMFC technology. The fuel is preferably a gas such as
hydrogen, but liquid fuels such as alcohols, including methanol,
also can be used. Hydrocarbons including reformed gasoline or
diesel fuel also can be used to provide fuel to the fuel cell. Air
(oxygen) can be used to oxidize the fuel.
[0103] USE OF MULTIPLE CATALYTICALLY ACTIVE METALS
[0104] Both the first catalytically active metal and the second
catalytically active metal can be present as mixtures of
catalytically active metals without change in this general concept
of a z-gradient step function shown in FIG. 4. If metal mixtures
are present, then the concentrations of each metal would be added
to yield the total concentration.
[0105] In particular, when poisoned fuel such as reformate fuel is
used, a plurality or mixture of catalytically active metals (e.g.,
bimetallic catalysts) can be used to improve performance and reduce
poisoning effects. The mixture can be, but need not be at least
partially alloyed. In particular, carbon monoxide poisoning can be
a problem even at levels as low as 5-100 ppm of carbon monoxide.
For example, in this embodiment, the interfacial region can
comprise at least two of the second catalytically active metals
different from each other. Also, the electrode can comprise at
least two of the first catalytically active metals different from
each other. In this embodiment, the plurality of catalytically
active metals is particularly preferred at the anode, which is
exposed to fuel. The plurality of metals can include three, four,
and even more different metals if desired. Good mixing of the
metals, including metal alloying, can occur. For bimetallic
systems, preferred combinations include Pt-Ru, Pt-Sn, Pt-Co, Pt-Cr,
Pt-Mo, Pt-Al, and Pt-Ni, and the most preferred combination is
Pt-Ru.
[0106] In a bimetallic system, the preferred relative amounts of
the two metals can depend on the particular system. Preferably,
substantially equal amounts (atomic amounts, not weight) of each
metal are present in a Pt-Ru bimetallic system. In general, for a
Pt-X system, wherein X is an additional catalytically active metal,
the amount of X can be, for example, about 5% to about 60%, and
more particularly, about 10% to about 40%, and preferably about 30%
(the balance being platinum). Platinum does not need to be the
element present in the largest amount, but rather, noble metals
such as Pd and Rh also can be used in place of platinum as the
primary metal.
[0107] Terniary catalyst systems are also possible such as, for
example, Pt-Sn-Os, Pt-Ru-Cr, Pt-Ru-Mo, and combinations of these
elements. The additional third catalytically active metal can help
improve the catalyst activity particularly when less catalytically
active metals such as, for example, Ru are present.
[0108] Multi-metallic catalyst systems according to the present
invention are described further below in the description of the
"SECOND EMBODIMENT".
[0109] ELECTRODES
[0110] The reactant diffusive, electronically conductive
electrodes, including cathode and anode, can be prefabricated
before they are contacted with the ionically conductive membrane or
subjected to the deposition of the second catalytically active
metal. In general, conventional gas diffusion electrodes are
commercially available and can be used either directly or with
modification. For example, low platinum loading electrodes, or
catalyst free electrodes, can be obtained from E-TEK, Inc. (Natick,
Mass.) or from Electrochem, Inc.
[0111] Electrodes should comprise components which provide
structural integrity, effective water management, diffusivity to
reactants including porosity or diffusivity to gases, electronic
conductivity, catalytic activity, processability, and good
interfacial contact with the membrane. The structure of the
electrode is not particularly limited provided that these
functional attributes are present. In general, at least one
ionically conductive polymer should be present as part of the
electrode to increase catalyst utilization.
[0112] The electrodes generally can be of substantially planar
geometry. Planar means an article or form made so as to have length
and width dimensions, or radial dimensions, much greater than the
thickness dimension. Examples of such articles include polymeric
films or membranes, paper sheets, and textile fabrics. Once formed,
such planar articles can be used as an essentially flat article, or
wound, folded, or twisted into more complex configurations.
[0113] The electrodes are at least partially porous, wherein porous
means a structure of interconnected pores or voids such that
continuous passages and pathways throughout a material are
provided. More generally, the electrode should allow reactants to
diffuse or be transported through the electrode at commercially
usable rates.
[0114] Electrode preparation, electrocatalyst layers, and other
aspects of fuel cell technology are described in, for example, U.S.
Pat. Nos. 5,211,984 and 5,234,777 to Wilson, which are hereby
incorporated by reference. For example, Wilson teaches use of
catalyst-containing inks and transfer methods to fabricate
electrodes and electrocatalyst layers comprising ionically
conductive polymer and metal catalyst. In these patents, an
uncatalyzed porous electrode is placed against a film of catalyst
during fuel cell assembly to form a gas diffusion backing for the
catalyst film. However, the catalyst films in Wilson, unlike those
of this invention, generally have little if any porosity.
[0115] Preferred electrodes are layers formed of electrically
conductive particulate materials, which may include catalyst
materials, held together by a polymeric binder. If desired,
hydrophobic binders such as polytetrafluoroethylene can be used.
Ion exchange resin also can be used as binder.
[0116] Expanded or porous polytetrafluoroethylene can be used to
support the electrocatalyst. In particular, a preferred electrode
can be prepared by the following procedure ("Procedure A"):
[0117] A dispersion of 5 g of carbon black-platinum (50 wt. %)
particles (from NE Chemcat Co.) in 40 g 2-methyl-1-propyl alcohol
is prepared. To the dispersion is added a liquid composition of
isopropyl alcohol containing 9 wt. % Nafion.RTM. perfluorosulfonic
acid resin (DuPont) and thoroughly mixed, with the aid of
ultrasonic agitation, to form a liquid mixture, having a relative
concentration of 50 wt. % ion exchange resin and 50 wt. % carbon
black supported platinum. The liquid mixture is painted by brush to
impregnate a porous expanded polytetrafluoroethylene
electrode-support film (thickness--16 micrometers; pore volume 94%;
IBP 0.12 kg/cm.sup.2). Solvent is removed by air drying. The
composite structure is heat treated at 120.degree. C. for 24 hours
to complete the procedure.
[0118] This procedure A also can be carried out, for example, with
use of at least 25 wt. % catalyst (carbon black-platinum) with the
balance being perfluorinated ionomer polymer. Preferably, the
electrode in this composite structure has some porosity and is
reactant diffusive.
[0119] For use as an electrode support, the porous or expanded
polytetrafluoroethylene film should be thin and can have, for
example, a thickness of about 3 microns to about 200 microns, and
more particularly, about 3 microns to about 30 microns, and
preferably about five microns to about 20 microns. This relatively
thin catalyst-containing electrode can be contacted with other
electrically conducting components which, for example, do not
contain catalyst and provide passageway for reactants.
[0120] The pore volume of the ePTFE electrode support can be, for
example, about 60% to about 95%, and preferably, about 85% to about
95%. The maximum pore size defined by an isopropanol bubble point
(IBP) can be, for example, about 0.05 kg/cm.sup.2 to about 0.5
kg/cm.sup.2, and preferably, about 0.05 kg/cm.sup.2 to about 0.3
kg/cm.sup.2. The Bubble Point was measured according to the
procedures of ASTM F316-86. Isopropyl alcohol was used as the
wetting fluid to fill the pores of the test specimen. The Bubble
Point is the pressure of air required to displace the isopropyl
alcohol from the largest pores of the test specimen and create the
first continuous stream of bubbles detectable by their rise through
a layer of isopropyl alcohol covering the porous media. This
measurement provides an estimation of maximum pore size.
[0121] Before deposition of the zone of second catalytically active
metal, the electrode preferably has relatively low level of
catalyst loading such as, for example, about 0.01 mg/cm.sup.2 to
about 1 mg/cm.sup.2, and preferably, about 0.02 mg/cm.sup.2 to
about 0.5 mg/cm.sup.2, and more preferably, about 0.05 mg/cm.sup.2
to about 0.4 mg/cm.sup.2. Preferably, it is less than about 0.3
mg/cm.sup.2. Preferably, the total catalyst loading for a single
MEA is less than about 0.65 mg/cm.sup.2, and more preferably, less
than about 0.2 mg/cm.sup.2.
[0122] At least one first catalytically active metal is distributed
throughout the porous surface of the electrodes. Catalytically
active generally means that the metal is in some way helping to
provide catalysis. The first and second catalytically active metals
can be and preferably are the same metals. Both the first and
second catalytically active metals can be, for example, noble
metals or Group VIII metals. Particular examples include Pt, Pd,
Ru, Rh, Ir, Ag, Au, Os, Re, Cu, Ni, Fe, Cr, Mo, Co, W, Mn, Al, Zn,
Sn, with preferred metals being Ni, Pd, Pt, and the most preferred
being Pt. If desired, a plurality of catalytically active metals
(e.g., bimetallic) can also be selected from this list.
Co-catalysts and promoters can also be present such as, for
example, C, Ni, Al, Na, Cr, and Sn. Any conventional agents to
enhance fuel cell performance can be used.
[0123] The first catalytically active metal is preferably in the
form of metal loaded carbon particles. For example, the carbon
particles can be loaded with metal in amounts of at least 10 wt. %
metal, and preferably, at least 20 wt. % metal. Preferably, the
first catalytically active metal is relatively uniformly
distributed and randomly dispersed throughout the electrode.
Electrodes can be, for example, formed from particles of high
surface area carbon, such as Vulcan XC72 (about 200 m.sup.2/g) or
Black Pearls 2000 (about 1000 m.sup.2/g) available from Cabot,
Boston, Mass. which are loaded with particles of platinum of about
20 angstroms to about 50 angstroms in size to an electrode area
loading of about 0.35 mg/cm.sup.2.
[0124] In addition to supported metal catalyst, the electrode
should further comprise ionically conductive polymer to improve the
contact of the electrode to the membrane and increase catalyst
utilization. The ionically conductive polymer of the membrane (a
"first ionically conductive polymer") can be substantially the same
or different than the ionically conductive polymer of the electrode
(a "second ionically conductive polymer"), although they preferably
are substantially the same. Substantially the same means that the
two ionically conductive materials, for example, (i) can be
selected to have different equivalent weights although having the
same general chemical identity, (ii) can be used with different
contents of fillers or additives; or (iii) can have the same
general polymer backbone but different ionic groups.
[0125] The electrode can further comprise at least one hydrophobic
component such as a fluorinated polymer, preferably a
perfluorinated polymer such as polytetrafluoroethylene. If desired,
this hydrophobic component can be concentrated at the
electrode-membrane interface. Other examples include
tetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer (PFA),
or tetrafluoroethylene/hexafluoropropylene copolymer (FEP). This
fluorinated hydrophobic component can help improve water repellency
in the electrode structure.
[0126] A pore-forming agent or sacrificial filler also can be
included in the electrode such as, for example, ammonium
bicarbonate, sodium chloride, or calcium carbonate. This agent can
be removed by, for example, heating or leaching to create voids and
improve gas diffusivity. Gas diffusivity can be tailored to the
application.
[0127] The electrode can further comprise at least one solvent used
during electrode preparation. However, solvent may slowly evaporate
from the electrode. Hence, solvent initially present may not be
present at a later time. Solvents are known in the art for
electrode ink preparations. Exemplary solvents include polar
solvents and alcohols.
[0128] THE MEMBRANE
[0129] The ionically conductive membrane should provide, for
example, strength, high ionic conductance, and good interfacial
contact with the electrode. The structure of the membrane is not
particularly limited provided that these functional attributes are
present. Reinforced composite membranes are preferred.
[0130] The membrane is preferably made largely of one or more
fluorinated polymers, and preferably, mixtures of perfluorinated
polymer and fluorinated ion exchange resin. In a preferred
embodiment, the membrane is prepared from porous or expanded
polytetrafluoroethylene which is impregnated with ion exchange
resin such as a sulfonated perfluorinated ionomer including
NAFION.RTM. (EW can be, for example, 1100). Similar ionomers such
as, for example, FLEMION.RTM. (Asahi Glass) can also be used.
Substantially all (>90%) of the open porous volume can be
impregnated so that a high Gurley number (>10,000 seconds) is
provided.
[0131] Impregnated membranes are described in, for example, U.S.
Pat. Nos. 5,547,551; 5,635,041; and 5,599,614 to Bahar et al.,
which are hereby incorporated by reference. These patents describe
test procedures and characteristics of the membranes.
[0132] Membranes can be prepared with use of a microporous base
material made in accordance with the teachings of U.S. Pat. No.
3,593,566 incorporated herein by reference. Base materials are
available in various forms from W. L. Gore and Associates, Inc.
(Elkton, Md.). Such a base material has a porosity of greater than
35%. Preferably, the porosity is between about 70% and 95%. The
porous microstructure can comprise (i) nodes interconnected by
fibrils, or (ii) fibrils.
[0133] Average pore size for the base material can be, for example,
about 0.05 microns to about 0.4 microns. The pore size distribution
value can be, for example, about 1.05 to about 1.20. Pore size
measurements are made by the Coulter Porometer.TM., manufactured by
Coulter Electronics, Inc. (Hialeah, Fla.). The Coulter Porometer is
an instrument that provides automated measurement of pore size
distributions in porous media using the liquid displacement method
described in ASTM Standard E1298-89. The Porometer determines the
pore size distribution of a sample by increasing air pressure on
the sample and measuring the resulting flow. This distribution is a
measure of the degree of uniformity of the membrane (i.e., a narrow
distribution means there is little difference between the smallest
and largest pore size). The Porometer also calculates the mean flow
pore size. By definition, half of the fluid flow through the filter
occurs through pores that are above or below this size.
[0134] High Gurley numbers are preferred for the membrane. The
Gurley air flow test measures the time in seconds for 100 cc of air
to flow through a one square inch sample at 4.88 inches of water
pressure in a Gurley Densometer (ASTM D726-58). The sample is
placed between the clamp plates. The cylinder is then dropped
gently. The automatic timer (or stopwatch) is used to record the
time (seconds) required for a specific volume recited above to be
displaced by the cylinder. This time is the Gurley number. The
Frazier air flow test is similar but is mostly used for much
thinner or open membranes. The test reports flow in cubic feet per
minute per square foot of material at 0.5 inches water
pressure.
[0135] The composite membrane is preferably thin having a thickness
of, for example, more than about 3 microns, but less than about 75
microns, and more preferably, less than about 50 microns, and even
more preferably, less than about 30 microns. About 20 microns and
less is most preferred. Membrane thickness can be determined with
use of a snap gauge such as, for example, Johannes Kafer Co. Model
No. F1000/302). Measurements are taken in at least four areas in
each specimen.
[0136] In addition, the membranes should have high ionic
conductance, preferably greater than about 8.5 mhos/cm.sup.2, and
more particularly, greater than about 22 mhos/cm.sup.2. Ionic
conductance can be tested using a Palico 9100-2 type test system.
This test system consisted of a bath of 1 molar sulfuric acid
maintained at a constant temperature of 25.degree. C. Submerged in
the bath were four probes used for imposing current and measuring
voltage by a standard "Kelvin" four-terminal measurement technique.
A device capable of holding a separator, such as the sample
membrane to be tested, was located between the probes. First, a
square wave current signal was introduced into the bath, without a
separator in place, and the resulting square wave voltage was
measured. This provided an indication of the resistance of the acid
bath. The sample membrane was then placed in the membrane-holding
device, and a second square wave current signal was introduced into
the bath. The resulting square wave voltage was measured between
the probes. This was a measurement of the resistance due to the
membrane and the bath. By subtracting this number from the first,
the resistance due to the membrane alone was found.
[0137] Impregnated composite membranes can be prepared by
repeatedly contacting one or both sides of the base porous
substrate with a solution of ionically conductive polymer.
Surfactants can be used to impregnate. In each impregnation step,
solvent can be removed and heating carried out to help bind or lock
the ionically conductive polymer in the base substrate.
[0138] Laminated membranes can be used. For example, laminated
membranes can have fewer problems with pinholes. The membranes can
also include catalyst metal such as, for example, platinum, as well
as metallic oxides such as silica. Particularly preferred membranes
include those known as GORE-SELECT.RTM. available from W. L. Gore
and Associates, Inc (Elkton, Md.).
[0139] COMBINING ELECTRODE AND MEMBRANE; THE INTERFACIAL REGION
[0140] An important advantage of this invention is in avoiding
difficulties of combining thin membranes with an electrode by
traditional methods like hot-pressing. Membrane damage can occur
with hot pressing. The electrode-membrane combination should be
mechanically and electrochemically compatible.
[0141] The electrode is brought into contact with the membrane to
form an interfacial region. At the interfacial region, both
membrane and electrode can influence activity occurring at the
region. This interfacial region, like the membrane and the
electrode, generally is substantially planar. At this interfacial
region resides a zone, which preferably is a layer or coating, of
the second catalytically active metal which unexpectedly and
substantially improves the power output of the fuel cell. The
interfacial region may not be perfectly homogeneous because the
mating surfaces can have, for example, softness, inhomogeneity,
porosity, and surface roughness. Also, ion conductive polymer can
be at the surface of the electrode, before the electrode surface is
contacted with the ion conductive polymer of the membrane. Merger
or fusion of the two ion conductive polymers can occur. However, in
general, the zone of second catalytically active metal is
associated more with the electrode side of the interface than the
membrane side because the zone, like the electrode, is
electronically conductive. Nevertheless, it can be possible in some
cases for some of the zone to be associated with the membrane as
well depending on the process used to generate the interface and
the zone of second catalytically active metal.
[0142] The membrane-electrode interfacial region can also be
favorably influenced by impregnating the electrode with ionically
conductive resin after incorporation of the zone of second
catalytically active metal but before combination of the electrode
with the membrane. This impregnation helps improve the three-phase
contact of ion conductive material, catalyst material, and
electronically conductive material.
[0143] MEAs WITH INCREASED POWER OUTPUTS
[0144] The incorporation of the zone of second catalytically active
metal at the interfacial region can result in a large percent
increase in current density (MA/cm.sup.2), and also power output
(p=I.times.V), at a given voltage on a polarization curve (e.g.,
0.6 V) compared to a reference MEA without the zone of second
catalytically active metal. This percentage increase can be as high
as 20% or more, and preferably, 30%, and more preferably, 40% or
more. In some cases, improvements over 90% have been observed.
[0145] Surprisingly, greater percent increases can be found for
thinner zones. Hence, an important advantage of this invention is
that high percent power increases can be observed with introduction
of only a thin catalytic layer, and an R ratio can be defined
as:
[0146] percent increase in current density/zone thickness
(.ANG.)
[0147] wherein current densities are measured at 0.6 V in the
polarization curve under steady-state conditions. Cell temperature
should be between about 60.degree. C. and about 80.degree. C., and
preferably, about 65.degree. C. For example, this R ratio is about
0.7 when a 33% percent increase is found for deposition of a 50
angstrom layer (see working examples). Similarly, this R ratio is
about 0.9 when a 46% increase is found for deposition of a 50
angstrom layer. Surprisingly, R can be greater than 22 (22.6) when
a 113% increase is found for a 5 angstrom coating. Hence,
surprising features of this invention include R values greater than
0.5, preferably greater than 1, more preferably greater than 5,
more preferably greater than 10, and even more preferably greater
than 20. If desired, the R value can be less than 50, and
preferably less than 30, if the system needs to be tailored to a
particular application. Calculation of this R ratio assumes that
some fuel cell reaction occurs in the absence of the zone of second
catalytically active metal.
[0148] ZONE THICKNESSES AND LOADINGS
[0149] The thickness of the zone of second catalytically active
metal, which represents a statistically valid average thickness,
can be determined by statistically sound methods known in the art.
These include direct and indirect (or less direct) measurement
methods. Direct methods using samples of assembled elements can be
used to confirm the thicknesses as measured by indirect methods
employed during production. Direct methods can involve, for
example, cross sectional analysis of structures after combination
of the electrode and membrane. Indirect (or less direct) methods
can involve knowledge of the production method and include use of,
for example, a microbalance together with use of known deposition
rate and deposition time (e.g., 1 .ANG./sec deposition rate for 50
seconds of deposition yields approximately 50 .ANG. average
thickness). Witness slides can be used, and calibration curves can
be established to help determine thickness. Other methods include
scanning electron microscopy, transmission electron microscopy,
atomic absorption, and gravimetric techniques. Multiple methods can
be used, if desired, to confirm that the desired zone thickness is
present, and to determine the effects, if any, of processing on the
thickness for a given system.
[0150] In general, when the zone of second catalytically active
metal comprises, or consists essentially of, only one type of
metal, the zone thickness can be about 3 angstroms to about 475
angstroms, and more particularly, about 5 angstroms to about 250
angstroms, and even more particularly, about 5 angstroms to about
50 angstroms. Thicknesses much greater than about 475 angstroms, in
general, can reduce layer uniformity and possibly block diffusion.
However, the degree to which diffusion is blocked can depend on the
structure of the zone.
[0151] In general, when the zone of second catalytically active
metal comprises, or consists essentially of, at least two types of
metals, the zone thickness can be relatively thicker such as, for
example, about 3 angstroms to about 5,000 angstroms, and more
particularly, about 50 angstroms to about 1,500 angstroms, and even
more particularly, about 150 angstroms to about 500 angstroms.
[0152] Examples of loadings, or surface loadings, of the zone of at
least one second catalytically active metal include about 0.0006
mg/cm.sup.2 to about 0.12 mg/cm.sup.2, and more particularly about
0.0007 mg/cm.sup.2 to about 0.09 mg/cm.sup.2, and more
particularly, 0.001 mg/cm.sup.2 to about 0.05 mg/cm.sup.2, and more
particularly, about 0.005 mg/cm.sup.2 to about 0.02
mg/cm.sup.2.
[0153] In general, when the zone of second catalytically active
metal comprises at least two metals, then relatively larger
thicknesses and higher loadings of second catalytically active
metal can be needed. Ranges for thicknesses can be, for example, 3
.ANG. to 5,000 .ANG., and more particularly, 50 .ANG. to 1,500
.ANG., and even more particularly, 150 .ANG. to 500 .ANG.. Loading
ranges can be, for example, about 0.001 mg/cm.sup.2 to about 0.7
mg/cm.sup.2, and more particularly, 0.01 mg/cm.sup.2 to about 0.4
mg/cm.sup.2, and even more particularly, 0.02 mg/cm.sup.2 to 0.3
mg/cm.sup.2.
[0154] In general, thicknesses and loadings are preferred which
provide optimal combinations of properties, which can depend on the
particular application.
[0155] VACUUM DEPOSITION METHODS
[0156] Typical vacuum deposition methods include chemical vapor
deposition, physical vapor or thermal deposition, cathodic arc
deposition, ion sputtering, and ion beam assisted deposition
(IBAD). A low vacuum/convection-based method which requires less
vacuum is jet vapor deposition (JVD) . Because the materials are
deposited in vacuum (typically less than 13.3 mPa, or
1.times.10.sup.-4 torr), contamination of the films can be
minimized while maintaining good control over film thickness and
uniformity. Deposition over large areas can be achieved via
reel-to-reel or web coating processes. The present invention makes
use of these and other vacuum deposition techniques, particularly
magnetron sputtering and physical vapor deposition.
[0157] Most preferably, electron beam--physical vapor deposition
(EB-PVD) is used. Deposition rates can range, for example, from 0.1
.ANG./sec to 10 .ANG./sec. If necessary, heating of the substrate
can be limited.
[0158] In addition, deposition methods such as, for example,
combustion chemical vapor deposition (CCVD) can be used which do
not require a vacuum. Wet chemical deposition methods can be used
but are not preferred.
[0159] The structure or morphology of the deposited zone of the at
least one second catalytically active metal can depend on, for
example, the substrate, the deposition method, and the loading of
the second catalytically active metal. The structure can be
analyzed by, for example, field-emission scanning electron
microscopy (FE-SEM). This analysis shows that relatively uniform
zones of the second catalytically active metal are formed. This
substantial uniformity is present irrespective of the type of film
morphology present. In general, sputter deposition can provide more
dense zones than thermal evaporation methods such as EB-PVD. In
general, the EB-PVD zones can exhibit a greater degree of surface
texture. Although the theory and detailed structure of the present
invention are not fully understood, the excellent power
improvements found herein may be due to the relatively open surface
texture. This openness may provide, for example, better reactant
transport and more surface area for reaction.
[0160] At relatively thin zone thicknesses of, for example, five
angstroms, the FE-SEM analysis of the electrode can reveal small
but measurable increases in field brightness compared to the
reference electrode without a deposited zone. Surprisingly,
relatively uniform deposition was observed. At thicker thicknesses
of, for example, 50 angstroms, the FE-SEM analysis can reveal
substantially spherical nodules of deposited metal approximately 25
nm to about 100 nm, and in particular, about 30 nm to about 70 nm,
and more particularly, about 50 nm in diameter. At even greater
thicknesses of, for example, 500 angstroms, the FE-SEM analysis can
reveal, in addition to the substantially spherical nodules,
rod-shaped structures in which the rods have diameters of about 20
nm to about 100 nm, and more particularly, about 20 nm to about 60
nm, and even more particularly, about 40 nm. The rod length can
vary. Whisker or hair-like morphology can be produced.
[0161] DESCRIPTION OF A SECOND EMBODIMENT: REDUCED POISONING
[0162] In the embodiments of the present invention described above,
both first and second catalytically active metals were described as
part of the membrane electrode combination. In these embodiments,
the one or more second catalytically active metals are deposited,
preferably by vapor deposition, as a metallic zone at the
membrane-electrode interfacial region. This zone is distinct from
the one or more first catalytically active metals, which are part
of the electrode.
[0163] These embodiments collectively are called for purposes of
this application "THE FIRST EMBODIMENTS" which encompass
embodiments wherein not all of the electrode catalytically active
metal is in the zone of second catalyst.
[0164] In alternative embodiments, however, which are now
described, the electrode of the membrane-electrode combination is
substantially free of first catalytically active metal, and the
zone of second catalytically active metal includes at least two
different catalytically active metals which helps improve
resistance to poisoning. Hence, the primary, and preferably
essentially all, catalytic activity is from the zone. These
alternative embodiments, wherein the electrode is substantially
free of first catalytically active metal, are called collectively
for purposes herein "THE SECOND EMBODIMENT" and are described
further hereinbelow.
[0165] In the second embodiment, preferably, the amount of first
catalytically active metal is minimized so that the electrode is
substantially free of first catalytically active metal, and
preferably, totally free. More specifically, catalytic loading for
the first catalytically active metal can be less than about 0.1
mg/cm.sup.2, and preferably less than about 0.01 mg/cm.sup.2, and
more preferably, less than about 0.001 mg/cm.sup.2 in the second
embodiment. This minimization in the amount of first catalytically
active metal can be particularly desirable when only one first
catalytically active metal is present (e.g., only platinum is
present). In this case, if the amount of first catalytically active
metal is not minimized, then expensive metal can be wasted because
of poisoning.
[0166] In the second embodiment, an essentially uncatalyzed,
electronically conductive gas diffusion electrode can be used as a
substrate for deposition of the zone of at least two second
catalytically active metals. The uncatalyzed electrode can
comprise, for example, electronically conductive carbon particulate
and a hydrophobic binder for the particulate. The hydrophobic
binder can be, for example, a fluorinated polymer, and preferably,
a perfluorinated polymer, such as, for example,
polytetrafluoroethylene. An example of an uncatalyzed gas diffusion
electrode can be found in the uncatalyzed gas diffusion electrodes
or media of the ELAT.TM. series (available from E-TEK, Inc.,
Natick, Mass.) Thickness of the electrode before deposition of the
second catalytically active metal can be, for example, about one
micron (0.001 mm) to about 1,000 microns (1 mm), and more
particularly, about 250 microns (0.250 mm) to about 750 microns
(0.750 microns). Combinations of relatively thin electrode elements
can be used to form thicker electrode structures. As noted above,
fibrillated or expanded polytetrafluoroethylene can be used to
fabricate electrodes.
[0167] In the second embodiment, the thickness of the zone
comprising at least two second catalytically active metals can be,
for example, between about 3 angstroms and about 5,000 angstroms,
and more particularly, between about 50 angstroms and about 1,500
angstroms, and even more particularly, between about 150 angstroms
and about 500 angstroms. For a cathode, the zone thickness should
be at least 500 angstroms, whereas for an anode, the zone thickness
can be less than 500 angstroms.
[0168] In the second embodiment, zone loading can be, for example,
about 0.001 mg/cm.sup.2 to about 0.7 mg/cm.sup.2, and more
particularly, 0.01 mg/cm.sup.2 to 0.4 mg/cm.sup.2, and even more
particularly, 0.02 mg/cm.sup.2 to 0.3 mg/cm.sup.2.
[0169] The zone can comprise at least two catalytically active
metals which can be at least in part alloyed with each other, and
more particularly, substantially alloyed with each other. Multiple
alloy phases can be present, as well as mixtures of alloyed and
unalloyed phases. Alloying can be detected, for example, by x-ray
diffraction (XRD) analysis of the metals. In addition, it can be
detected by x-ray photoelectron spectroscopic (XPS) analysis,
although in general the XPS method is less preferred for detection
of alloying because it reflects surface analysis, whereas the XRD
method reflects bulk analysis. XRD can also be advantageously used
to detect crystallinity in the zone of second catalytically active
metal. Multiple analytical methods can be used to probe the
structure of the deposited zone (e.g., bulk versus surface
structure).
[0170] Deposition methods are noted above in the description of the
first embodiment and include sputtering and thermal evaporation. In
a multi-metallic physical vapor deposition, such as electron beam
physical vapor deposition, separate metal sources of about 99.9 wt.
% platinum and about 99.9 wt. % ruthenium are evaporated.
Preferably, for a bimetallic mixture of these metals, the atomic
ratio of each metal is about 1:1. Simultaneous or sequential
physical vapor deposition methods can be used. In simultaneous
deposition, two different sources are used and simultaneously
evaporated together, and the vapor composition is controlled by
each source's relative evaporation rate. In sequential deposition,
each metal is separately evaporated from its source in a
toggle-like manner and deposited onto the chosen substrate in
alternate thin layers. Alloying of the metals can be encouraged
when the layers are thin enough. In addition, ion bombardment
treatment such as, for example, ion beam assisted deposition (IBAD)
can be used to encourage alloying. In general, simultaneous
deposition is preferred.
[0171] In another embodiment, sputtering processes such as, for
example, multi-target dc sputtering can be used to deposit the zone
of second catalytically active metal. The zone can comprise, for
example, ternary platinum alloys.
[0172] Improvements in membrane electrode assembly performance,
despite the presence of impurities like CO, can be detected by, for
example, (i) the shifting of the onset of substantial electrode
polarization to higher current densities; and (ii) the reduction of
the potential at which the fuel mixture can be oxidized at high
rates. Comparative poison resistance testing can be carried out by
comparing performance between samples in which the only difference
is the composition of the zone of deposited metal. This concludes
the specific description of the second embodiment in this section,
although further description, including performance description, is
given in the Examples section.
[0173] MEA AND FUEL CELL ASSEMBLY METHODS
[0174] Several methods can be used to assemble the half cell or MEA
which incorporates the zone. In describing these methods, assembly
elements include the electrode and the membrane. The zone can be
deposited on assembly elements either directly or indirectly. In
direct deposition, the zone is deposited directly on the electrode,
the membrane, or both as part of MEA assembly. In indirect
deposition, however, the zone is initially deposited onto a
substrate, not an assembly element, and then the zone is
transferred from the substrate to the assembly element, preferably
the membrane. The substrate is not particularly limited but can be,
for example, low surface energy support such as skived
polytetrafluoroethylene which allows for ready transfer and
preservation of the zone.
[0175] Additional components and conventional methods can be used
to assemble fuel cells and stacks. For example, gas diffusion media
include, for example, ELAT.TM. gas diffusion media available from
E-TEK, Inc. (Natick, Mass.) as well as CARBEL.RTM. CL gas diffusion
media available from W. L. Gore and Associates, Inc. (Elkton, Md.).
Gas diffusion media can be catalyzed, if desired, with one or more
metals such as, for example, noble metals including platinum to
improve performance. Catalyzation of gas diffusion media, for
example, can activate heterogeneous catalysis and improve reactions
such as, for example, (i) water-gas shift reaction of carbon
monoxide with water to form carbon dioxide and hydrogen, and (ii)
preferential oxidation reaction of carbon monoxide with oxygen to
form carbon dioxide. Also, the catalyst can provide absorption
sites to capture carbon monoxide coming from poisoned fuels such as
reformate. The gas diffusion media can be catalyzed by vapor
deposition methods such as, for example, physical vapor deposition.
In addition, fuel cell gaskets can be used made of, for example,
GORE-TEX.RTM., also available from W. L. Gore and Associates, Inc.
The present invention is not particularly limited by these
additional components and methods.
[0176] MEAs known as PRIMEA.RTM. (including, for example, the 5000
and 5510 series) are also available from W. L. Gore and Associates,
Inc. (Elkton, Md.).
[0177] REPRESENTATIVE APPLICATIONS AND PERFORMANCE
[0178] The invention is versatile and can be used in a variety of
applications including: (i) transportation vehicles such as cars,
trucks, and buses which have requirements including high power
density and low cost; (ii) stationary power applications, wherein
high efficiency and long life are required; and (iii) portable
power applications such as portable television, fans, and other
consumer products. Methods to use fuel cells in these applications
are known.
[0179] Surprisingly, MEAs according to this invention can provide
catalyst mass activities greater than 2,500 mA/mg of catalytically
active metal, and preferably, greater than 5,000 mA/mg of
catalytically active metal. At this catalyst mass activity level,
commercialization becomes feasible, particularly when poison
resistance is also present. The zone of second catalytically active
metal does not modify important commercial considerations such as
the existing water balance. Hence, MEAs according to the present
invention can be operated under the same temperature and
humidification conditions.
[0180] Finally, additional fuel cell technology is described in,
for example, the references cited in the background as well as the
following references, which are hereby incorporated by reference:
(i) "High performance proton exchange membrane fuel cells with
sputter-deposited Pt layer electrodes"; Hirano et al.;
Electrochimica Acta, vol. 42, No. 10, pp. 1587-1593 (1997); (ii)
"Effect of sputtered film of platinum on low platinum loading
electrodes on electrode kinetics of oxygen reduction in proton
exchange membrane fuel cells"; Mukerjee et al.; Electrochimica
Acta, vol. 38, No. 12, pp. 1661-1669 (1993); (iii) "Sputtered fuel
cell electrodes"; Weber et al.; J. Electrochem. Soc., June 1987,
pp. 1416-1419; and (iv) "Anodic oxidation of methanol at a gold
modified platinum electrocatalyst prepared by RF sputtering on a
glassy carbon support"; Electrochimica Acta, Vol. 36, No. 5/6, pp.
947-951, 1991.
[0181] The invention is further described by means of the following
non-limiting examples.
EXAMPLES
[0182] General Procedures
[0183] In each example, unless otherwise noted, the ionically
conductive membrane (proton exchange membrane, PEM) was
approximately 20 microns thick. The membrane was a fully
impregnated membrane of high Gurley number (>10,000 seconds) and
high ionic conductance prepared by impregnating expanded
polytetrafluoroethylene with a perfluorinated sulfonic acid resin
(FLEMION.RTM., EW 950) as described in U.S. Pat. Nos. 5,547,551;
5,635,041; and 5,599,614 to Bahar et al. The membrane is called
GORE-SELECT.RTM. and is available from W. L. Gore and Associates,
Inc. (Elkton, Md.). These patents are hereby incorporated by
reference in their entirety.
[0184] The electrode comprising the first catalytically active
metal, unless otherwise noted, was prepared as described above for
Procedure A to generate a target metal loading. The electrode
comprises Pt supported on carbon, ionically conductive polymer, and
solvent. The electrodes had platinum loadings which ranged from
0.05 mg Pt/cm.sup.2 to 0.4 mg Pt/cm.sup.2.
[0185] In Examples 2 and 4 below, a zone of second catalytically
active metal was coated or deposited onto a substrate, either
electrode or membrane, by electron beam physical vapor deposition
(EB-PVD). In this procedure, a substrate, typically 6 in..times.6
in., was mounted onto a 4-point holder carrousel in a vacuum
chamber, where each holder was mounted on a rotating axis, each of
which could rotate about the main axis of the carrousel. A platinum
target was prepared by melting 99.95% purity platinum coins in a 2
in. .times.2 in. crucible in the vacuum chamber (1.5 m diameter, 2
m long), followed by recooling. The crucible was also located in
the vacuum chamber. The chamber was then evacuated to less than
10.sup.-4 torr (e.g., 5.times.10.sup.-5 torr) using a diffusion
pump. The platinum target was then evaporated using an electron
beam for heating, and platinum was condensed onto the substrate.
Arial uniformity of the deposited coating was ensured by rotating
the sample about both rotational axes of the holder during
deposition. The amount of platinum zone deposited was measured
using a vibrating crystal microbalance, calibration curves, and
deposition rates and times. Zone thickness and loading amounts were
calculated.
[0186] In Examples 1-3 and 5 below, I-V measurements were obtained
after the MEA had reached steady state.
[0187] In each Example, the area of the cathode and anode
contacting the membrane were substantially the same. In practicing
this invention, however, these areas do not need to be the
same.
[0188] Unless otherwise noted, MEA testing was carried out with: 25
cm.sup.2 electrode active area; ELAT.RTM. gas diffusion media
(available from E-TEK, Inc., Natick, Mass.); clamping at 200 lb
in/bolt torque; and GLOBE TECH.RTM. computer controlled fuel cell
test station. The gas diffusion media was believed to comprise
approximately 70% graphite cloth and 30% polytetrafluoroethylene.
Clamping assured compression of the MEA to the flow field and
diffusers.
[0189] Catalyst and electrode layers were supported on
polytetrafluoroethylene backings and were transferred from the
backing to the membrane by decal methods with hot pressing. Unless
otherwise noted, hot pressing was carried out for 3 minutes at
160.degree. C. with a 15 ton load. The backing was subsequently
peeled off, leaving the coated layer(s) bonded to one side of the
membrane and positioned centrally.
[0190] Reference MEAs, unless otherwise noted, were substantially
the same as the MEA according to the invention except that no
z-gradient zone was present in the reference MEA.
Example 1
[0191] Example 1 illustrates the indirect method wherein the zone
of second catalytically active metal is first deposited onto a
substrate before transfer from the substrate to the membrane or
electrode.
[0192] A 50 .ANG. platinum coating zone (0.01 mg/cm.sup.2) was
deposited at 1 .ANG./sec onto a skived PTFE substrate backing by
EB-PVD. The catalyst zone was then transferred onto the membrane by
the decal method leaving the 50 .ANG. catalyst zone bonded to one
side of the membrane and positioned centrally. The area of the
membrane demarcated by the transferred catalyst is the active area.
A catalyzed electrode (0.3 mg Pt/cm.sup.2) was attached to each
side of the catalyzed membrane also using the decal method, so as
to overlay the active area. Therefore, one side of the MEA had a
z-gradient zone of platinum at the membrane/electrode
interface.
[0193] The prepared MEAs with 25 cm.sup.2 active areas were each
loaded between gaskets in a 25 cm.sup.2 active area fuel cell test
fixture or cell. The electrode containing the z-gradient zone was
placed towards the cathode where it would be in contact with the
oxidant (air). The test fixture was then attached to the fuel cell
test station for acquisition of data.
[0194] MEA performance was evaluated with the cell pressure at 0
psig and at 15 psig. For the 0 psig cell pressure runs, the cell
was operated at 60.degree. C., with hydrogen and air humidified to
dew points of 20.degree. C. and 55.degree. C. respectively. For the
15 psig cell pressure runs, the cell was operated at 75.degree. C.,
with hydrogen and air both supplied at 15 psig and humidified to
dew points of 30.degree. C. and 70.degree. C. Hydrogen and air flow
rates were set to 2 and 3.5 times the stoichiometric value
theoretically needed to produce a given cell current output.
[0195] FIG. 5 shows the fuel cell output voltage at various current
outputs for the MEA at 0 psig. Superior performance was observed in
the MEA according to the invention compared to a reference MEA
which was substantially the same except it did not contain the
z-gradient catalyst layer. For example, at 0.6 V, the MEA according
to the invention produced almost 1200 mA/cm.sup.2 versus only about
820 mA/cm.sup.2 for the reference (a 46% increase).
[0196] Similarly, FIG. 6 shows data for the 15 psig cell. Again,
the polarization analysis showed improved performance over the
entire range of current densities. At 0.6 V, for example, the MEA
containing z-gradient cathode produced almost 1600 mA/cm.sup.2
versus only 1200 mA/cm.sup.2 for the reference MEA (33% increase)
which was substantially the same but did not contain the z-gradient
cathode. Power density is also plotted in FIG. 6 (p=I.times.V), and
improved power density was also evident.
[0197] FIG. 7 shows an electrocatalyst mass activity analysis for
the 15 psig cell. The mass activity is the amount of current
generated (or alternatively power generated) per unit mass of
catalyst metal in the active area. Hence, mass activity units are
mA/mg Pt for current generation (and mW/mg Pt for power
generation). At 0.6 V, the MEA with z-gradient cathode surprisingly
produced over 2,500 mA/mg Pt compared to only 2,000 mA/mg Pt (i.e.,
a 25% increase) for the reference MEA which was substantially
similar but did not contain the z-gradient cathode.
[0198] FIG. 8 shows data for an MEA at 0 psig where the z-gradient
catalyst zone was part of the anode rather than cathode.
Surprisingly, the polarization analysis revealed an improvement in
performance with a z-gradient anode (12% increase at 0.6 V),
although the improvement was not as large as for the MEA with a
z-gradient cathode.
Example 2
[0199] In this Example, direct deposition of the zone on the
electrode was carried out at two zone thicknesses. Deposition was
carried out by EB-PVD. The catalyzed electrodes having the
z-gradient deposited thereon had a loading of 0.1 mg Pt/cm.sup.2
before deposition. For one sample, the deposition rate was 0.2-0.3
.ANG./sec to achieve a 50 .ANG. zone (0.01 mg Pt/cm.sup.2). A
second electrode was coated at a rate of 0.1 .ANG./sec to achieve a
5 .ANG. zone (0.001 mg Pt/cm.sup.2). An electrode (anode)
containing 0.05 mg/cm.sup.2 of platinum was used for both
samples.
[0200] MEA performance was again evaluated with the cell pressure
at 0 psig and at 15 psig. For all runs, the cell was operated at
65.degree. C., with hydrogen and air both supplied at 0 psig, and
humidified to dew points of 60.degree. C. Hydrogen and air flow
rates were set to 1.2 and 3.5 times the stoichiometric value
theoretically needed to produce a given cell current output
respectively.
[0201] FIG. 9 shows the improved power output at 0 psig.
Improvements in current density were observed at 0.6 V from 240
mA/cm.sup.2 for the reference MEA: (i) to 460 mA/cm.sup.2 for the
50 .ANG. deposition (92% increase), and (ii) to 510 mA/cm.sup.2 for
a 5 .ANG. deposition (113% increase). Surprisingly, the lower
loading (thinner deposition) provided a greater percentage increase
at this voltage.
[0202] FIG. 10 shows fuel cell performance at 15 psig cell
pressure, in terms of both current density and power density, for
the 5 A sample. The data indicated an increase in current density
at 0.6 V from 440 to 860 mA/cm.sup.2 (95% increase), with a
substantial increase in peak power density.
[0203] FIG. 10 also shows polarization performance as compensated
cell potential versus current density at 15 psig. When the
polarization curve is expressed in terms of compensated potential,
the electrocatalytic performance of the z-gradient cathode is shown
independent of the effects of other MEA components. By comparison
of compensated potentials, FIG. 10 showed that improved MEA
performance was due to improved cathode performance (resulting from
z-gradient layer), and not from some other spurious secondary
effects.
[0204] FIG. 11 shows the corresponding improvement in
electrocatalyst mass activity and specific power at 15 psig.
[0205] The observed enhancement in electrocatalyst utilization was
proportional to the enhancement in current/ power density.
[0206] Surprisingly, the percent increases in current found at 0.6
V were significantly higher in Example 2 compared to Example 1. In
addition, the MEAs of Example 2 had less precious metal than the
MEAs of Example 1.
Example 3
[0207] This example illustrates DC magnetron sputtering compared to
EB-PVD. An electrode (0.4 mg Pt/cm.sup.2) on a skived PTFE backing
was coated by D.C. magnetron sputtering. A 0.127 mm thickness,
99.9% purity platinum foil served as target, and the vacuum chamber
base pressure was maintained at 8.times.10.sup.-4 torr. More
specifically, a vacuum less than 10.sup.-4 torr was established,
and then high purity argon was bled in so that the pressure rose to
8.times.10.sup.-4 torr. Platinum deposition rate was about 1
.ANG./sec continuous to achieve a platinum loading of 0.01
mg/cm.sup.2 (50 .ANG.). This sputtered electrode was used as
cathode. An unsputtered electrode (0.4 mg Pt/cm.sup.2) served as
anode.
[0208] MEA performance was evaluated with the cell pressure at 0
psig and at 15 psig. For the 0 psig cell pressure runs, the cell
was operated at 70.degree. C., with hydrogen and air both supplied
at 0 psig, and humidified to dew points of 55.degree. C. and
70.degree. C. respectively. The 15 psig runs were performed at a
cell temperature of 80.degree. C., with hydrogen and air both
supplied at 15 psig, and humidified to dew points of 60.degree. C.
and 75.degree. C. respectively. For all runs, hydrogen and air flow
rates were set to 2 and 3.5 times the stoichiometric values
respectively.
[0209] FIG. 12 shows that for 0 psig at 0.6 V there is an
improvement in current density from 820 mA/cm.sup.2 for the
reference MEA to 1050 mA/cm.sup.2 (28% increase) for the sputtered
z-gradient MEA. FIG. 13 shows fuel cell performance at 15 psig cell
pressure. There is an improvement in current density from 1200
mA/cm.sup.2 (reference MEA) to 1360 mA/cm.sup.2 for the sputtered
cathode (13% increase). Hence, the percent increases in Example 3
were not as great as observed in Example 2.
Example 4
[0210] Membranes were coated with platinum using EB-PVD and DC
magnetron sputtering. Loadings for different samples were 0.001,
0.01, 0.05, and 0.1 mg Pt/cm.sup.2. One side of the membrane was
coated. MEAs were prepared from the coated membranes.
Example 5
[0211] A zone of second catalytically active metal (50 .ANG.) was
deposited onto the membrane by the indirect transfer method. The
Pt/skived PTFE was hot pressed against the membrane to bond the Pt
evaporated layer to the membrane by the decal method. The skived
PTFE layer was peeled off, thus leaving a zone of 50 .ANG. Pt layer
bonded to the membrane. Catalyzed electrodes (0.3 mg Pt/cm.sup.2)
were then attached by hot pressing to form a first MEA.
[0212] A second MEA was prepared in which the cathodic active phase
was just the electrode structure formed by a thin 50 A Pt layer
bonded to the membrane. The anode had a loading of 0.2 mg
Pt/cm.sup.2.
[0213] Polarization performance was evaluated at 0 psig cell
pressure. The atmospheric pressure run, having both anode and
cathode at 0/0 psig respectively, was performed at 60.degree. C.
cell temperature with hydrogen and air reactants saturated in
humidification bottles to ca. 100% relative humidity. The anode,
hydrogen, and cathode, air, reactants were then saturated at
20/60.degree. C., respectively. The reactant flow was set to 2/3.5
times the stoichiometric value, for hydrogen and air respectively,
and the stoichiometric flow was maintained throughout the
polarization curve.
[0214] FIG. 14 shows the performance of the first and second MEAs.
The difference in performance observed between the two MEAs
indicates that the 50 A layer presents low activity in itself at
this low loading, but its presence at the interface between the
electrocatalyst layer and membrane produces a power improvement and
improves the electrode current density profile.
[0215] FE-SEM Analysis
[0216] FE-SEM analyses were carried out for one comparative sample
of an electrode with no zone present (FIG. 15) and for 3 samples
with different zone thicknesses deposited onto the electrode (FIGS.
16-18). For FIGS. 15-18, the magnification was 20kX and the
electron beam energy was 2 keV. The analyses showed relatively
uniform zone deposition with FIGS. 15-18 being representative. In
general, the microstructure was represented by a combined spherical
nodular and whisker morphology, with the latter evidenced at
loadings of about 0.1 mg/cm.sup.2 (500 A) (FIG. 18).
[0217] FIG. 15 was taken from a sample of the cathode used in
Example 2 with 0.1 mg/cm.sup.2 Pt loading but without deposition of
the second catalytically active metal. FIG. 15 demonstrates the
electrode porosity, which allows for reactant diffusion, before
deposition of the second catalytically active metal.
[0218] FIG. 16 was taken from a sample of the Example 2 cathode
with a 0.1 mg/cm.sup.2 Pt loading but with a 5 A zone deposition
(0.001 mg/cm.sup.2) by EB-PVD. A small but measurable increase in
field brightness was evident in FIG. 16 compared with the FIG. 15
control. The increased brightness was uniform across the Figure
which suggested an evenly deposited platinum zone. The electrode
remained porous and open to reactant diffusion despite the
deposition.
[0219] FIG. 17 was taken from a sample of the Example 2 cathode
with a 0.1 mg/cm.sup.2 Pt loading but with a 50 A zone deposition
(0.01 mg/cm.sup.2) by EB-PVD. A further increase in field
brightness was observed compared with FIG. 16. Spherical platinum
nodules were present with diameter widths between about 30 and
about 70 nm, and generally about 50 nm. The electrode remained
porous and open to reactant diffusion despite the deposition.
[0220] FIG. 18 was taken from a sample of the electrode similar to
that of Example 2 but with no Pt loading before the deposition. The
electrode was then provided with a 500 A Pt zone by EB-PVD. Again,
spherical platinum nodules were present with diameter widths
between about 25 nm and about 100 nm, and more particularly, about
30 nm and about 70 nm, and generally about 50 nm. In addition,
however, rod-shaped structures were also present. The width
diameter of these rods was about 20 nm to about 60 nm, and
generally, about 40 nm. The electrode remained porous and open to
reactant diffusion despite the deposition.
[0221] Data Summary
[0222] Data from Examples 1-5 are summarized below:
1 Percent increase in current at 0.6 V Example Zone Pressure
compared to number thickness (A) (psig) reference MEA 1 50 0 46 1
50 15 33 2 50 0 92 2 5 0 113 2 5 15 95 3 50 0 28 3 50 15 13
[0223] Examples 1-5 represent the first embodiment of this
invention rather than the second embodiment. In these examples,
electrodes are employed which have catalytically active metal
distinct from the deposited zone. Examples 6 and 7 further
illustrate the first embodiment of this invention but with use of a
bimetallic zone.
Example 6
[0224] A catalyzed electrode (carbon supported on platinum, loading
0.4 mg Pt/cm.sup.2) was used, available from W. L. Gore &
Assoc. under the PRIMEA 5510 name. The electrode included carbon
supported Pt, ionomer, and solvent and was disposed on a substrate
support and subjected to vacuum deposition by sequential Pt/Ru
EB-PVD. The loading of the bimetallic zone was 0.3 mg PtRu/cm.sup.2
(approximately 50% atomic ratio of Pt and Ru), and the deposited
zone thickness was 1,500 .ANG.. Each of the sequentially evaporated
layers of platinum and ruthenium were approximately 250 .ANG.
(equivalent loading per layer: 0.05 mg metal/cm.sup.2) were
deposited at a rate of 1 .ANG./sec as measured with a vibrating
crystal microbalance.
[0225] XRD analysis of the sequentially evaporated deposited zone
is shown in FIG. 19. Matching analyses with diffraction pattern
databases were generally consistent with the presence of unalloyed
Pt and Ru phases.
[0226] XPS analysis was also carried out in which three spots per
sample were analyzed to verify uniformity. The wide scan survey XPS
spectrum is shown in FIG. 20; the higher resolution XPS spectrum
for the platinum region (Pt 4f transition) is shown in FIG. 21; and
the higher resolution XPS spectrum for the ruthenium region (Ru 3d
transition) is shown in FIG. 22. Analysis of the wide scan spectrum
(FIG. 20) confirmed that the main components of the deposited zone
were Pt and Ru, with minor components being C, O, F, and traces of
Cu. Analysis of the higher resolution XPS spectra (FIGS. 21 and
22), however, were consistent with the presence of at least some
alloying between Pt and Ru. The measured binding energy for the Pt
transition (71.7 eV) was significantly shifted from the expected
value of 71.0-71.1 based on the literature. In addition, the
measured binding energy for the Ru transition (280.7 eV) was
significantly shifted from the expected value based on the
literature for metallic Ru (279.9-280.2 eV). The data were also
consistent with the presence of oxides of Ru.
Example 7
[0227] A catalyzed electrode as in Example 6 was subjected to
deposition by sequential PtRu EB-PVD with use of IBAD (ion beam
assisted deposition) to help mix the evaporated layers. The loading
of the evaporated bimetallic zone was 0.1 mg PtRu/cm.sup.2 (50%
a/a), with a total zone thickness of 500 .ANG.. The thickness of
each of the 10 layers within the zone was 50 .ANG. and the loading
per layer was 0.01 mg metal /cm.sup.2) . An Ar.sup.+beam was used,
and the Ar.sup.+ gun was operated at 2 keV, 40 W, and 8 mA beam
current. The deposition rate was 1 .ANG./sec as measured with a
vibrating crystal microbalance.
[0228] The XRD analysis for this sequentially evaporated, ion beam
treated electrocatalyst layer is shown in FIG. 23.
[0229] Hence, Examples 1-7 represent the first embodiment of the
present invention. The following additional Examples 8-14 represent
the second embodiment of the present invention. In these additional
Examples, deposition of bimetallic zones was carried out on
uncatalyzed rather than catalyzed electrode substrates. In Examples
8-11, sputtering was employed, whereas in Examples 12-14, physical
vapor deposition was employed.
Example 8
[0230] ELAT.TM. gas diffusion media (uncatalyzed) was obtained from
E-TEK. The thickness was approximately one-half mm. ELAT.TM. gas
diffusion media (an uncatalyzed electrode) and 6 mil skived PTFE
sheets were placed into a vacuum chamber (1.5 m diameter, 2 m long)
which was pumped down to ca.<10.sup.-4 torr using a diffusion
pump. The mountings and the carousel rotated during the deposition
process, which helped assure catalyst uniformity.
[0231] DC magnetron sputtering was used as the deposition method.
The magnetron unit consisted of a 6" diameter magnetron sputter
source which was loaded with two catalyst target materials
(sources): a Pt foil target (0.127 mm thickness, 99.9% pure) and a
Ru foil target (0.127 mm thickness, 99.9% pure), which are
available from Alfa or Goodfellow. FIG. 24 shows the experimental
setup used for the sputtering runs. Dual (multiple) target
sputtering was used to vaporize the two metals simultaneously,
i.e., the two targets were mounted (spot-welded) on the same
magnetron unit. To achieve the desired atomic ratio of the catalyst
metals on the substrate, calibration runs were carried out. The
relative surface areas of target materials were varied, and the
compositions of the deposited phases were analyzed as a function of
relative surface area.
[0232] In this Example, three different Pt-Ru catalyst loadings
were prepared using dual target magnetron sputtering: 0.1, 0.3, and
0.6 mg PtRu/cm.sup.2 at a nominal composition of 50 atomic percent
Pt and Ru (i.e., one atom of Pt per atom of Ru). Typical conditions
were base pressure =8.times.10.sup.-4 torr, and deposition
rate=1-10 .ANG./sec. Glass witness slides accompanied all runs. The
Pt/Ru catalyst was sputtered in and deposited on (i) the "active"
side of the ELAT substrate, wherein the active side is the side
that normally is placed against a catalyst layer during normal fuel
cell mounting, and (ii) on one side of the skived PTFE material.
The double rotation of the substrates while vacuum catalyzation
took place helped assure coating uniformity. The deposition rate
was on average 6 .ANG./sec as measured with a vibrating crystal
microbalance, with a magnetron power ranging from 250-500 W,
490-570 V dc bias and 450-900 mA Ar.sup.+ flux. The effective zone
thicknesses were 500, 1,500, and 3,000 .ANG. for the 0.1, 0.3 and
0.6 mg PtRu/cm.sup.2 zone loadings, respectively.
[0233] EDAX analysis:
[0234] The sample with zone loading of 0.3 mg PtRu/cm.sup.2 (1,500
.ANG. zone thickness), and the respective glass witness slides,
were analyzed to determine composition and structure of the
deposited zone. The scan (FIG. 25) showed that within the
resolution of the spectrometer only Pt and Ru elements were present
at a ratio of Pt/Ru=53.73%/46.27%, i.e., less than 10% difference
with the target ratio.
[0235] XRD analysis:
[0236] X-ray diffraction (XRD) analysis was carried out with a
Siemens diffractometer using a Cu K.sub..alpha. source. FIG. 26
shows the XRD spectrum. For comparison, the diffraction lines of Pt
(fcc) (from literature, JCPDS 04-0802) is also compared beneath.
The PtRu sputtered electrocatalyst layer exhibited the
characteristic diffraction peaks of the Pt fcc structure. However,
the diffraction lines of the bimetallic sample were shifted to
lower values with respect to the same reflection of Pt in the JCPDS
database. The shift in the bimetallic sample is -0.22 degrees
compared with a pure Pt sample. This evidence is consistent with
the presence of a PtRu alloy in the sputtered zone. This alloying
is reflected as a change in the lattice parameter of the fcc
lattice compared to the lattice parameter of pure Pt. In addition,
no reflection lines that account for tetragonal RuO.sub.2 (JCPDS
21-1172) or Ru (hcp) (JCPDS 6-663) phases were found.
[0237] XPS Analysis:
[0238] The x-ray photoelectron spectroscopic (XPS) measurements
were performed using a Physical Electronics Quantum 2000 scanning
ESCA spectrometer. Samples of the 0.3 mg PtRu/cm.sup.2 (50% a/a
Pt/Ru nominal target ratio) dual target sputtered electrode and of
the sputtered skived PTFE were analyzed. Three spots per sample
were measured to verify uniformity. FIGS. 27 and 28 show XPS
wide-scan spectra for the sputtered gas diffusion media and the
PTFE electrode element, respectively. The wide scan XPS analysis
confirmed that the main components for the electrocatalyst layer
are Pt and Ru, with traces of Zn and Cu also being present. Carbon
also was present. The Pt/Ru atomic ratio for the alloy could be
verified by examining the ratio of the areas of the Pt 4f and Ru
3p3 transitions, which (averaged for the three sampling spots) are
Pt/Ru is approximately equal to Pt4f(area)/Ru3p3(area)=14.17/16.03
(i.e., 0.88) for the sputtered ELAT material and 15.25/14.00 (i.e.,
1.09) for the sputtered PTFE substrate. Both ratios are within
acceptable error from the target 1/1 ratio. Analysis of the binding
energies (BE) of Pt and Ru transitions indicated that Pt and Ru in
the sputtered layer were present in the metallic state with no
evidence of presence of Pt or Ru oxides. Due to the difference in
electronegativities between Pt and Ru (Pt smaller than Ru) a charge
transfer between both elements in the lattice is expected, thus
polarizing the Pt-Ru bond. This charge transfer is then expected to
yield a BE shift. It was found that the Pt 4f.sub.7/2 transition
for the PtRu sputtered phase presented a BE=71.7 e V, while the BE
for elemental Pt found in literature is 71.1-71.2 e V (71.07 e V (2
values), 71.2 e V (13 values), 71.0 e V). This represents a
chemical shift of 0.5-0.6 e V. This shift in BE then suggests
alloying between Pt and Ru in the sputtered phase. The same
analysis was performed for the Ru 3d.sub.5/2 transition (BE=280.1 e
V) but no observable shift was detected from the expected
literature value (279.94, 280.2 (7 values), 280.0 and 280.1 (2
values) e V). Both Pt and Ru transitions were referenced to the
carbon (reference) transition at BE =285.00 e V. FIGS. 29 and 30
show the XPS spectra for the Pt region (Pt 4f transition) and its
deconvoluted spectrum, respectively, for 2 samples (3
spots/sample), for the PtRu sputtered gas diffusion media
electrode. FIGS. 31 and 32 show the XPS spectrum for the Ru region
(Ru 3d transition) and its deconvoluted spectrum, respectively.
[0239] Polarization performance
[0240] MEA preparation:
[0241] The sputtered Pt Ru/ELAT gas diffusion media electrode was
first pretreated with a solution of 4.5% (w/w) mixture of Flemion
(950 EW) proton conducting polymer in isopropanol. The solution was
brushed on the catalyzed side of the gas diffusion electrode and
dried at ca. 80.degree. C. using a heat gun until substantially all
solvent was evaporated. The pretreated electrode was then hot
pressed to a GORE-SELECT.RTM. proton conducting membrane (25 .mu.m
membrane thickness, 950 EW ionomer impregnated) as the anode
half-cell. A standard GORE reference electrode (0.3 mg Pt cm.sup.-2
loading) was used as cathode.
[0242] To provide a reference point for the comparison of the MEA
performance with use of the bimetallic Pt-Ru electrocatalyst zone,
MEAs containing Pt sputtered anodes (loading=0.1 mg Pt/cm.sup.2)
were also prepared. These anodes were prepared using the same
magnetron sputtering unit and at the same vaporization conditions
as the bimetallic dual target sputtered anodes. The MEAs using
these reference anodes (which were ionomer pretreated) were also
prepared by hot pressing this electrode to a GORE-SELECT.RTM.
proton conducting membrane (25 .mu.m, 950 EW) as the anode
half-cell. A standard reference electrode (0.1 mg Pt/cm.sup.2
loading; available from W. L. Gore & Associates under the
PRIMEA 5510 name) was used as cathode.
[0243] MEA testing:
[0244] The MEA, with an electrode active area of 25 cm.sup.2, was
mounted in a standard Fuel Cell Technologies fuel cell fixture
using ELAT as diffusor in the cathode (reference) side. The fixture
was then clamped down to ca. 200 lb in/bolt torque using standard
gasket material and then connected to a GLOBETECH fuel cell test
plant using a Scribner and Assoc. electronic load for conditioning
and testing. Hydrogen/air characterization was done first at
60.degree. C. cell temperature, with anode and cathode saturated at
60.degree. C./60.degree. C., respectively, via sparger bottles at
atmospheric pressure. The characterization was then done at
80.degree. C. cell temperature, with anode and cathode saturated at
90.degree. C./75.degree. C., respectively, at a cell back pressure
of 30/30 psig for anode and cathode respectively. In both cases,
stoichiometric-based flow of 1.2/3.5 was used. Polarization
performance was obtained at steady state after proper conditioning
of the MEA and after at least 1 day on stream. Conditioning of the
MEA was achieved by cycling the cell potential between 0.6 V and
ca. 0.4 V and OCV. The polarization curve was obtained by varying
the cell potential in 50 mV steps and recording the steady state
current density at load based flow (i.e., reactant flow rate
proportional to total cell amperage). Reformate characterization
was done at the higher temperature/pressure testing condition using
two types of CO/H.sub.2 mixtures: 5 ppm and 50 ppm CO. The testing
strategy involved first the characterization of the MEA in
H.sub.2/air feeds upon achieving steady state power output. Then
the 5 ppm CO fuel was injected, and the MEA was allowed to
stabilize overnight (at least 10 hrs.) at 0.6 V with this anode
feed. Then, after steady-state saturation for the anode with the
poisoned feed was achieved, the polarization curve was taken.
Afterwards, the 50 ppm CO/H.sub.2 fuel was injected, and the MEA
was again allowed to stabilize overnight at 0.6 V. The steady-state
polarization curve was taken after that period of time.
[0245] FIGS. 33 and 34 show polarization performance (cell
potential and power density vs. current density) for the Pt
sputtered anode in pure H.sub.2 and in H.sub.2/CO anode feeds,
respectively. FIGS. 35 and 36 show the corresponding evaluation,
but for the MEA with the PtRu sputtered anode, for pure H.sub.2 and
for H.sub.2/CO anode feeds, respectively. Table 1 summarizes the
performance, expressed as current density at 0.6 V in H.sub.2 and
H.sub.2/CO mixtures (5 and 50 ppm CO concentration). The results
indicated that surprisingly the implementation of the PtRu
bimetallic sputtered anodes in pure H.sub.2 anode feed produced an
improvement of ca. 20% in current density/power output when
compared to a sputtered Pt (single metal) electrode. In the
presence of 5 ppm CO/H.sub.2 anode feed, the bimetallic sputtered
anode yielded 84% improved power output at 0.6 V, and at the 50 ppm
level the improvement was ca. 82%, all compared to the sputtered Pt
(reference) electrode (see Table 1).
[0246] To further characterize the poison resistance of the anode
taking also in consideration the performance in clean
(H.sub.2-only) reactant feeds, a poison resistance-related lambda
parameter (.LAMBDA.), defined as
(.LAMBDA.)=(i.sub.H2--i.sub.CO/H2O/i.sub.h2 at a fixed voltage
reference (e.g., 0.6 V) can be used. In this equation, an electrode
with 100% poison resistance (i.e., i.sub.H2.apprxeq.i.sub.CO/H2)
will yield .LAMBDA..fwdarw.0. An electrode that is then totally
poisoned and "shuts down" in poisoned feeds, i.e.,
i.sub.CO/H2.fwdarw.0, will yield .LAMBDA..apprxeq.1. An
electrocatalyst with improved poison resistance will yield smaller
.LAMBDA.s. Table 1 depicts the results for this lambda parameter.
The data demonstrate that while the Pt sputtered anode suffers a
67% drop in performance at 5 ppm CO level and 88% at 50 ppm level
CO, compared to the performance with pure hydrogen feed, the PtRu
sputtered anode decays only 49% (i.e., 18% less) at 5 ppm CO, and
82% (6% less) at 50 ppm CO, in relation to the performance of the
bimetallic anode in pure hydrogen.
[0247] Poison resistance lambda parameters are noted further below.
The MEAs of the present invention, when subjected to CO/hydrogen
feeds, are able to provide excellent lambda parameters of:
[0248] when subjected to hydrogen feed with 5 ppm CO, 0.65 or less,
and more preferably, 0.50 or less, and more preferably, 0.25 or
less; and/or
[0249] when subjected to hydrogen feed with 50 ppm CO, 0.85 or
less, and more preferably, 0.60 or less.
[0250] Current densities for the present invention are also noted
herein. The MEAs of the present invention, when subjected to
CO/hydrogen feeds, are able to provide current densities at 0.6 V
of at least:
[0251] 350 mA/cm.sup.2, and preferably, 450 mA/cm.sup.2, and more
preferably, at least 500 mA/cm.sup.2, when subjected to hydrogen
feed with 5 ppm CO; and/or
[0252] 150 mA/cm.sup.2, and preferably, 175 mA/cm.sup.2, and more
preferably, 200 mA/cm.sup.2, when subjected to hydrogen feed with
50 ppm CO.
Example 9
[0253] Electrode element preparation
[0254] Electrode elements including ELAT gas diffusion media and 6
mil skived PTFE sheets were placed into a vacuum chamber (1.5 m
diameter, 2 m long) which can be pumped down to ca.<10 torr
using a diffusion pump. The 6".times.6" gas diffusion media
substrates were mounted in a 4 point-holder carrousel. Each of the
mountings and the carousel rotated, thus helping to assure uniform
coating. DC magnetron sputtering was used. Bimetallic
electrocatalyst zone comprising Pt and tin (Sn) were prepared using
dual target sputtering.
[0255] The unit consisted of a 6" diameter magnetron, which was
loaded with two catalyst-material targets. The specifications of
the targets were: Pt foil (50 mm.times.50 mm, 0.127 mm thickness,
99.9% pure, from Alfa) and Sn foil (0.1 mm thickness, Puratronic,
99.998% pure).
[0256] Calibration runs helped establish the relative amount of
target material used to provide the desired composition. The
subsequent composition of the deposited zone was analyzed for
calibration purposes. This calibration step was necessary because
each metal had a different sputtering yield, a property that is
intrinsic to the element being sputtered, and the target area
surface area ratio does not necessarily directly correlate with the
vaporized phase composition ratio.
[0257] Three different PtSn catalyst compositions, with different
Pt/Sn atomic ratios, were prepared using dual target magnetron
sputtering: Pt layers with 40, 20, and 15% Sn were produced with
loadings of ca. 0.3 mg PtSn/cm.sup.2. Typical conditions were base
pressure=8.times.10.sup.-4 torr, and deposition rate=1-10 A/sec.
Glass witness slides were used with all runs. The Pt and Sn were
sputtered and deposited on the active side of the ELAT electrode,
wherein the active side is the side that normally is placed against
a catalyst layer during normal fuel cell mounting, as well as onto
one side of the skived PTFE material. Double rotation of the
substrates while vacuum catalyzation took place helped assure
coating uniformity. The electrocatalyst layer was then coated at a
deposition rate averaging 6 A/sec as measured with a vibrating
crystal microbalance to the specified target loading, with a
magnetron power ranging of 250-500 W, 490-570 V dc bias and 450-900
mA Ar.sup.+ flux.
[0258] Electrode element characterization
[0259] EDAX analysis:
[0260] The PtSn deposited zones were prepared by dual target
sputtering and were analyzed to determine composition and structure
of the deposition. FIGS. 37, 38, and 39 show Energy Dispersive
X-Ray (EDAX) spectra of the 0.3 mg/cm.sup.2 loading PtSn deposition
for, respectively, the desired amounts of Sn: 40%, 20%, and 15% Sn
(atomic percent). The scans showed (within the sensitivity of the
spectrometer) that only these two elements (Pt and Sn) were present
and that the elements were present at an approximate ratio of
Pt/Sn=62%/38%, 78%/22% and 84%/16% for the 40, 20 and 15% Sn
targeted atomic ratios.
[0261] XRD Analysis:
[0262] FIGS. 40, 41, and 42 show the X-ray diffraction (XRD)
signatures for, respectively, the 40, 20, and 15% Sn targeted
atomic ratio electrodes. Analysis of the diffractograms indicated
that in all the cases phase change shifts from the (pure) Pt fcc
diffraction lines were observed. These changes are consistent with
the presence of alloy phases.
[0263] Matching analysis with diffraction pattern databases also
were consistent with the presence of multiple alloy phases. For
example, the 40% composition had diffraction peaks that may
indicate the presence of PtSn and PtSn.sub.3 phases. The 20%
system, for example, showed the presence of convoluted diffraction
bands of SnPt.sub.3, Sn, and PtSn. The 15% Sn system showed a
diffraction pattern that suggested the presence of SnPt.sub.3 and
Sn phases.
Example 10
[0264] Electrode element preparation
[0265] Electrode elements including ELAT gas diffusion media and 6
mil skived PTFE sheets were placed into a vacuum chamber (1.5 m
diameter, 2 m long) which were pumped down to ca.<10.sup.-4 torr
using a diffusion pump. The 6".times.6" gas diffusion media
substrates were mounted in a 4 point-holder carrousel. Each of the
mountings and the carousel rotated. DC magnetron sputtering was
used. A bimetallic PtCr zone was deposited using dual target
sputtering.
[0266] The sputtering unit consisted of a 6" diameter magnetron,
which was loaded with two catalyst-material targets. The
specifications of the targets were Pt foil (50 mm.times.50 mm 0.127
mm thickness, 99.95 pure, from Alfa) and Cr sputtering target (50.8
mm.times.31.8 mm, 99.98% pure). Calibration runs were carried out
as noted above.
[0267] Electrodes with 25% atomic percent Cr at a nominal loading
of ca. 0.3 mg PtCr/cm.sup.2 were prepared. Typical conditions were
base pressure=8.times.10.sup.-4 torr, and deposition rate=1-10
A/sec. Glass witness slides were used in all runs. Pt and Cr were
sputtered deposited on the active side of the ELAT substrate as
well as onto one side of skived PTFE. Double rotation of the
substrate was used. The electrocatalyst zone was coated at a
deposition rate averaging 6 .ANG./sec as measured with a vibrating
crystal microbalance to the specified target loading, with a
magnetron power ranging of 250-500 W, 490-570 V dc bias, and
450-900 mA Ar.sup.+ flux.
[0268] Electrode element characterization
[0269] EDAX analysis:
[0270] The PtCr electrocatalysts prepared by dual target sputtering
were analyzed to determine composition and structure of the
deposition. FIG. 43 shows an EDAX spectrum of the 0.3 mg/cm.sup.2
loading PtCr deposition for the desired (targeted) 25% Cr (atomic
percent). The scans showed that mainly these two elements (Pt and
Cr) were present (within the resolution of the spectrometer) and
that the elements were present at an atomic ratio of
Pt/Cr=74%/26%.
[0271] XRD analysis:
[0272] FIG. 44 shows the XRD spectrum of the sputtered sample.
Matching analysis with diffraction pattern databases were
consistent with the presence of a Pt.sub.3Cr phase in addition to
an unalloyed Pt phase.
Example 11
[0273] Electrode element preparation
[0274] Electrode elements such as ELAT gas diffusion media and 6
mil skived PTFE sheets were placed into a vacuum chamber (1.5 m
diameter, 2 m long) which was pumped down to ca.<10.sup.-4 torr
using a diffusion pump. The 6".times.6" gas diffusion media
substrates were mounted in a 4 point-holder carrousel. Each of the
mountings and the carousel rotated. DC magnetron sputtering was
used. In this example, a bimetallic PtMo deposited zone was
prepared using dual target sputtering.
[0275] The unit consists of a 6" diameter magnetron, which is
loaded with two catalyst-material targets. The specifications of
the targets were: Pt foil (50 mm.times.50 mm 0.127 mm thickness,
99.9% pure, from Alfa) and Mo foil (150 mm.times.300 mm, 0.1 mm
thickness 99.95% pure). Calibration runs were carried out in which
the relative amount of target material was varied.
[0276] Electrodes with 25 atomic percent Mo at a nominal loading of
ca. 0.3 mg PtMo/cm.sup.2 were prepared. Typical conditions were
base pressure=8.times.10.sup.-4 torr and deposition rate=1-10
A/sec. Glass witness slides were used. The Pt and Mo were then
sputtered on the active side of the ELAT substrate as well as onto
one side of the skived PTFE material. Double rotation of the
substrates was used while vacuum catalyzation took place. The
electrocatalyst zone was then deposited at a rate averaging 6
.ANG./sec as measured with a vibrating crystal microbalance to the
specified target loading, with a magnetron power ranging of 250-500
W, 490-570 V dc bias, and 450-900 mA Ar.sup.+ flux.
[0277] Electrode element characterization
[0278] EDAX analysis:
[0279] The PtMo electrocatalysts prepared by dual target sputtering
were analyzed to determine composition and structure. FIG. 45 shows
an EDAX spectrum of the 0.3 mg/cm.sup.2 loading PtMo deposition for
the desired (targeted) 25% Mo (atomic percent). The scans show that
these two elements (Pt and Mo) were the main elements present
within the resolution of the spectrometer and that the elements
were present at the ratio of Pt/Mo=74%/26%.
[0280] XRD analysis:
[0281] FIG. 46 shows the XRD spectrum of the sputtered sample.
Matching analysis with diffraction pattern databases were
consistent with the presence of a Pt.sub.3Mo phase in addition to
an unalloyed Pt phase.
[0282] Examples 12-14 illustrate bimetallic systems wherein
sequential EB-PVD was used to deposit the zone. General methods are
first described before the specific examples are described.
[0283] General Methods
[0284] Electrode elements such as ELAT gas diffusion media and 6
mil skived PTFE sheets were placed in a vacuum chamber.(1.5 m
diameter, 2 m long) which was pumped down to ca.<10-4 torr using
a diffusion pump. The 6".times.6" gas diffusion media substrates
were mounted in a 4 point-holder carrousel. Each of the mountings
and the carousel rotated, thus helping to assure substantial
uniformity during coating. As means to vaporize the catalyst,
electron beam-physical vapor deposition (EB-PVD) was used.
[0285] The evaporator consisted of two 2".times.2" crucibles, each
one loaded either with Pt or Ru coins (99.95% purity). The metal
source into each crucible was first remelted and then evaporated
one source at a time, using a single electron beam source. A
switching mechanism allows for placement of the selected crucible
(Pt or Ru) under the electron beam, thus allowing for evaporation
of different metals using one single electron beam source. This
selected crucible loaded with the metal source of choice was then
re-melted and evaporated. The alternate switching of each crucible
using the same electron beam was then used to evaporate
sequentially two different catalyst metals. Typical coating
conditions were base pressure=5.times.10-5 torr and deposition
rate=1-0.1 .ANG./sec. Bimetallic electrocatalyst of the type PtRu
were prepared using sequential EB-PVD of two (Pt and Ru) metal
evaporation sources. Calibration runs were carried out so that the
desired ratio of the elements in the deposited zone could be
achieved.
[0286] Electrodes with a targeted 50% atomic percent PtRu (ie., one
atom of Pt per atom of Ru) and nominal loadings of 0.3 and 0.1 mg
PtRu/cm.sup.2 were prepared using sequential EB-PVD. During
different runs, the individual layer thickness were varied and, in
some runs, the Ion Beam Assisted Deposition (IBAD) technique was
also carried out to further induce mixing between layers. For all
the runs, typical conditions were base pressure=8.times.10-4 torr,
and deposition rate=1-10 .ANG./sec. Glass witness slides were used
for all runs. The Pt and Ru metals were evaporated and deposited on
the "active" side of the ELAT substrate as well as onto one side of
the skived PTFE material. Double rotation of the substrates while
vacuum catalyzation took place helped assure uniformity in the
coating.
Example 12
[0287] ELAT gas diffusion media and skived PTFE electrode elements
were subjected to sequential PtRu EB-PVD. The desired final loading
of the electrode was 0.3 mg PtRu/cm.sup.2, therefore forming a
total electrode effective thickness of 1500 .ANG. (ca. 750 .ANG.
total Pt equivalent effective thickness and 750 .ANG. total Ru
equivalent effective thickness). In this present embodiment,
sequential (Pt-Ru-Pt-Ru etc.) evaporated layers of 250 .ANG. each
(equivalent loading per layer 0.05 mg metal/cm.sup.2) were
deposited to form the electrocatalyst layer. Each layer was
deposited at a rate of 1 .ANG./sec as measured with a vibrating
crystal microbalance.
[0288] Electrode element characterization
[0289] XRD analysis:
[0290] FIG. 47 is the XRD spectrum of the sequentially evaporated
sample. Matching analysis with diffraction patent databases were
consistent with the presence of unalloyed Pt and Ru phases.
[0291] XPS analysis:
[0292] Samples of the 0.3 mg PtRu/cm.sup.2 (50% a/a Pt/Ru nominal
target ratio) sequential (250 .ANG./layer) ELAT gas diffusion media
catalyzed electrode were analyzed using XPS. As before, three spots
per sample were measured to verify uniformity and composition of
the coating. FIG. 48 shows XPS wide-scan spectra for the 250 .ANG.
sequentially evaporated electrode. The wide scan confirmed that the
main components of the electrocatalyst layer were Pt and Ru, with
the presence of minor amounts of N and F. Carbon was also present,
and it can be noted that the gas diffusion media support contained
carbon. The surface Pt-Ru atomic ratio on the evaporated
electrocatalyst layer could be measured by examining the ratio of
the areas of the Pt 4f and Ru 3p3 transitions, which (averaged for
the three sampling spots) were Pt/Ru approximately equals to
Pt4f(area)/Ru3p3(area) =40.23/0.893 (i.e., 45). The magnitude of
this ratio versus the expected value for a perfectly mixed 50% a/a
layer (ca. ratio of 1), taking also in consideration that XPS is a
surface-sensitive technique, suggested that the sequential
evaporated layers (ca. 250 .ANG./layer) were not well mixed between
layers. The XPS-measured ratio, Pt/Ru=45/1, then reflects the
surface composition of the last evaporated layer (i.e., Pt). This
Pt layer is therefore not well mixed with the Ru evaporated layer
immediately underneath.
[0293] Analysis of the binding energy (BE) position of Pt and Ru
elements was done using high-resolution scans. It was found that
the Pt 4f.sub.7/2 transition for the PtRu sequentially EB-PVD
phase, presented a binding energy of BE=71.7 eV, while the BE for
elemental Pt found in literature is ca. 71.0-71.1 eV, thus
presenting a chemical shift of ca. 0.6 eV. This shift in the Pt
binding energy is consistent with alloying between Pt and Ru
elements.
[0294] The XRD (bulk) and XPS (surface sensitive) data are
consistent with the electrocatalyst layer comprising bulk unalloyed
Pt and Ru phases with a surface Pt/Ru alloyed phase.
[0295] The same analysis performed for the Ru 3d.sub.5/2 transition
revealed a binding energy of BE=280.8 eV, thus presenting an
observable shift from the expected literature value for metallic
ruthenium (ca. 279.9-280.0 eV). This result indicates that the
electronic state of the surface Ru is not consistent with metallic
Ru (ie. Ru.sup.0), but more like with RuO.sub.2. Both Pt and Ru
transitions were referenced to the carbon (reference) transition
(C1s) at BE=285.00 eV.
[0296] FIG. 49 shows the XPS spectrum for the Pt region (Pt 4f
transition) and its deconvoluted spectrum for the sequentially
deposited EB-PVD electrode. FIG. 50 shows the spectrum for the Ru
region (Ru 3d transition).
Example 13
[0297] ELAT gas diffusion media and skived PTFE electrode elements
were prepared by sequential PtRu EB-PVD. The desired final loading
of the electrode was 0.1 mg PtRu/cm2 (50% a/a), therefore forming a
total electrode effective thickness of 500 .ANG. (ca. 250 .ANG.
total Pt equivalent effective thickness and 250 .ANG. total Ru
equivalent effective thickness). In this present embodiment,
sequential (Pt-Ru-Pt-Ru etc.) "thinner" evaporated layers (10
layers total, 5 layers of 50 .ANG. each metal at an equivalent
loading per layer of 0.01 mg metal/cm.sup.2) were deposited to form
the electrocatalyst layer. Each layer was deposited at a rate of 1
.ANG./sec as measured with a vibrating crystal microbalance.
Although this strategy was more time consuming, it allowed for the
deposition of thinner catalyst layers, thus producing a more
homogeneous electrocatalyst layer and favoring mixing between the
sequentially evaporated phases.
[0298] Electrode element characterization:
[0299] XRD analysis:
[0300] FIG. 51 shows the XRD spectrum of the sequentially
evaporated sample (at 50 .ANG./layer). Inspection of the spectrum
indicates a more complex structure than the one obtained by
physical vapor deposition of 250 .ANG. metal layers (Example 12).
It can be seen that the phase related to unalloyed Ru is now less
pronounced and the Pt fcc (face cube centered) diffraction appears
shifted, thus suggesting alloying. In addition, a shoulder
diffraction that corresponds to a Pt (unalloyed) phase is also
visible. The XRD analysis (which is a bulk characterization
technique) then is consistent with the presence of an alloyed PtRu
phase in conjunction with unalloyed Pt and Ru phases.
[0301] XPS Analysis
[0302] Samples of the 0.1 mg PtRu/cm.sup.2 (50% a/a Pt/Ru nominal
target ratio) sequential PtRu EB-PVD (50 .ANG./layer) ELAT gas
diffusion media catalyzed electrode were analyzed using XPS. Two
spots per sample were analyzed. Survey scans were run to determine
the surface composition of the deposition. FIG. 52 shows the survey
scan for the 50 .ANG. sequentially evaporated electrode. XPS (ESCA)
scans confirm that the main components of the electrocatalyst layer
were Pt and Ru, with additional presence of N, O, and F. Carbon is
again also found to be present. As before, the surface Pt-Ru atomic
ratio on the evaporated electrocatalyst layer could be measured by
examining the ratio of the areas of the Pt 4f and Ru 3p3
transitions, which (averaged for the two sampling spots) are
Pt/Ru=Pt4f(area)/Ru3p3(area)=29.6.+-.1.8/2.7.+-.0.3, i.e., ratio of
11.0.+-.1.8. The magnitude of this ratio, compared to the one
obtained by physical vapor deposition of relatively thicker 250
.ANG. layers (measured Pt/Ru ratio of ca. 45; Example 12) then
indicates that the sequential evaporation of thinner layers is more
effective in forming a more homogeneous electrode and allowing
interlayer mixing. Analysis of the binding energy (BE) position of
Pt and Ru elements was done using high-resolution scans. A piece of
sputter-cleaned Pt foil was also analyzed to provide a reference
spectrum for Pt. FIG. 53 showed the XPS spectrum for the Pt region
(Pt 4f transition) and its deconvoluted spectrum for this
sequentially deposited EB-PVD electrode. FIG. 54 shows the spectrum
for the Ru region (Ru 3d transition) for the same system.
[0303] It was found that the Pt 4f.sub.7/2 transition for this
sequentially EB-PVD electrocatalyst structure presented a binding
energy of BE=72.0.+-.0.1 eV (value obtained as average of two
measurements, referenced to C(1s) binding energy of 285.0 eV). The
measured equivalent BE for the sputtered Pt foil (reference
material) was 71.0.+-.0.0 eV (referenced to spectrometer Fermi
level). This observed shift in the binding energy is consistent
with PtRu alloying.
[0304] The same analysis was performed for the Ru 3d.sub.5/2
transition which revealed a binding energy of BE=281.0.+-.0.1 eV.
This result was consistent with the presence of RuO.sub.2.
[0305] Polarization Performance
[0306] MEA Preparation
[0307] The 0.1 mg Pt Ru/cm.sup.2 EB-PVD catalyzed ELAT gas
diffusion media electrode was first pretreated with a solution of
4.5% (w/w) of Flemion (950 EW) proton conducting polymer in
isopropanol. The solution was brushed on the catalyzed side of the
gas diffusion electrode and dried out at ca. 80.degree. C. using a
heat gun until solvent was evaporated. The pretreated electrode was
then hot pressed to a GORE-SELECT.RTM. proton conducting membrane
(25 microns, 950 EW) as the anode half cell. As cathode, a standard
reference electrode (0.3 mg Pt/cm.sup.2 loading) was used (PRIMEA
5510 available from W. L. Gore & Associates).
[0308] MEA testing:
[0309] The MEA, with an electrode active area of 25 cm.sup.2 was
mounted in a standard Fuel Cell Technologies fuel cell fixture
using ELAT as diffusor in the cathode (reference) side. The fixture
was then clamped down to ca. 200 lb in/bolt torque using standard
gasket material and then connected to a GLOBETECH fuel cell test
plant using a Scribner and Assoc. electronic load for conditioning
and testing.
[0310] Hydrogen/air characterization was done first at 60.degree.
C. cell temperature, with anode and cathode saturated at
60/60.degree. C. via sparger bottles at atmospheric pressure. The
second evaluation condition was at 80.degree. C. cell temperature,
with anode and cathode saturated at 85/75.degree. C., respectively,
at a cell backpressure of 30/30 psig for anode and cathode
respectively. In both cases, stoichiometric-based flow of 1.2/3.5
was used. Polarization performance was obtained at steady state
after proper conditioning of the MEA and after at least one day on
stream. Conditioning of the MEA was achieved by cycling the cell
potential between 0.6 V and ca. 0.4 V and OCV. The polarization
curve was obtained by varying the cell potential in 50 mV steps and
recording the steady state current density at load based flow
(i.e., reactant flow rate proportional to total cell amperage).
[0311] Reformate characterization was done at the higher
temperature/pressure testing condition using two types of
CO/H.sub.2 mixtures: 5 and 50 ppm. The testing strategy involved
first the characterization of the MEA in H.sub.2/air feeds upon
achieving steady state power output. Then the 5 ppm CO fuel was
injected, and the MEA was allowed to stabilize overnight (at least
10 hours) at 0.6 V with this anode feed. Then, after a steady state
saturation of the anode with the poisoned feed was achieved, the
polarization curve was taken. Afterwards, the 50 ppm CO/H.sub.2
fuel was injected, and the MEA was again allowed to stabilize
overnight at 0.6 V, with a steady-state polarization curve taken
after that period of time. FIGS. 55 and 56 show polarization
performance for this sequential Pt Ru EB-PVD catalyzed anode, for
pure H.sub.2 and for reformate feeds, respectively. Table 2
summarizes the performance, expressed as current density at 0.6 V
in H.sub.2 and H.sub.2/CO mixtures (5 and 50 ppm CO concentration).
Performance of a Pt sputtered anode (0.1 mg/cm.sup.2) is included
as reference.
[0312] The results indicate that the implimentation of a PtRu
EB-PVD anode in the presence of 5 ppm CO/H.sub.2 anode feed yields
81% improved power output at 0.6 V, and at 50 ppm level the
improvement is ca. 152%, all compared to the sputtered Pt
(reference) electrode. When used in H.sub.2 feeds, this bimetallic
anode shows in some respects less desirable performance compared to
a pure Pt system.
[0313] To further characterize the poison resistance of the anode
taking also in consideration the performance in clean (H.sub.2
only) reactant feeds, a poison resistance-related lambda parameter
was calculated from the data as described above, and the results
are given in Table 2.
[0314] The data indicate that the PtRu EB-PVD anode decays only 21%
at 5 ppm CO, and only 59% at 50 ppm CO, in relation to its
performance in pure hydrogen. This decay is considerably smaller
than the one measured for the sputtered bimetallic electrocatalyst
(49% and 82% for 5 and 50 ppm CO, respectively). As reference, the
Pt sputtered anode suffers a 67% drop in performance at 5 ppm CO
level and 88% drop at 50 ppm level CO.
Example 14
[0315] ELAT gas diffusion media and skived PTFE electrode elements
were prepared by sequential Pt Ru EB-PVD with IBAD as means to
encourage mixing of the evaporated layers. The desired final
loading of the electrode was 0.1 mg PtRu/cm.sup.2 (50% a/a),
therefore forming a total electrode effective thickness of 500
angstroms (ca. 250 .ANG. total Pt equivalent effective thickness
and 250 .ANG. total Ru equivalent effective thickness). In this
present embodiment, sequential (Pt-Ru-Pt-Ru etc.) evaporated layers
(10 layers total, 5 layers of 50 .ANG. each metal at an equivalent
loading per layer of 0.01 mg metal/cm.sup.2) were deposited and
mixed using an Ar.sup.+ beam to form the electrocatalyst layer.
Each layer was deposited at a rate of 1 .ANG./sec as measured with
a vibrating crystal microbalance. This strategy allowed for the
deposition and atomic-level mixing of thin catalyst layers while
using sequential evaporation, thus producing a more homogeneous and
uniform (and mixed) electrocatalyst layer.
[0316] Electrode element characterization:
[0317] XRD analysis:
[0318] FIG. 57 shows the XRD spectrum of the sequentially (50
.ANG./layer) EB-PVD/IBAD prepared sample. Inspection of the
spectrum indicates again a more complex structure than the one
obtained by PVD of 250 .ANG. metal layers (Example 12). The
spectrum for this system presents similarities with the one
obtained by evaporation only (thin layers) (Example 13): the phase
related to unalloyed Ru is less pronounced, and the Pt fcc
diffraction appears more shifted (more than that for Example 13),
which was consistent with alloying. In addition, a shoulder
diffraction which corresponds to a Pt (unalloyed) phase was also
visible. The XRD analysis of the electrocatalyst phase (which is a
bulk characterization technique) then again was consistent with the
presence of an alloyed PtRu phase in conjunction with unalloyed Pt
and Ru phases.
[0319] XPS analysis:
[0320] Samples of the 0.1 mg PtRu/cm.sup.2 (50% a/a Pt/Ru nominal
target ratio) sequential Pt Ru EB-PVD/IBAD (50 .ANG./layer) ELAT
gas diffusion media catalyzed electrode were analyzed using XPS. As
before, two spots per sample were analyzed. FIG. 58 shows the
survey scan for the 50 .ANG. sequentially evaporated/ion bombarded
electrode. The XPS scans confirm that the main components of the
electrocatalyst layer were Pt and Ru, with the presence of N, O and
C. As before, the surface Pt-Ru atomic ratio on the evaporated
electrocatalyst layer could be measured by examining the ratio of
the areas of the Pt 4f and Ru 3p3 transitions, which (averaged for
the two sampling spots) are Pt/Ru=Pt4f(area)/Ru3p3(ar- ea)
=34.7.+-.0.4/3.6.+-.0.1, i.e., a ratio of 9.6.+-.0.4. The magnitude
of this ratio, compared to the one obtained by PVD alone of "thin"
50 .ANG. layers (measured Pt/Ru ratio of ca. 11) suggested that
sequential evaporation of thin layers in conjunction with IBAD
allows for improved mixing of the deposited electrocatalyst
layer.
[0321] High resolution scans were used to provide bonding
information. A piece of sputter-cleaned Pt foil was also analyzed
to provide a reference spectrum for Pt. It was found that the Pt
4f.sub.7/2 transition for this sequentially EB-PVD/IBAD
electrocatalyst structure presented a binding energy of
BE=72.0.+-.0.0 eV (value again obtained as average of two
measurements, referenced to C(1s) binding energy of 285.0 eV). The
measured equivalent BE for the sputtered Pt foil (reference
material) was 71.0.+-.0.0 eV (referenced to spectrometer Fermi
level). The observed shift in this binding energy then was
consistent with PtRu alloying.
[0322] The same analysis was performed for the Ru 3ds.sub.5/2
transition which showed a binding energy of BE=280.8.+-.0.0 eV.
This result was consistent with RuO.sub.2.
[0323] FIG. 59 show the XPS spectrum for the Pt region (Pt4f
transition) and its deconvoluted spectrum for this sequentially
deposited EB-PVD electrode. FIG. 60 shows the spectrum for the Ru
region (Ru3d transition) for the same system.
[0324] Polarization Performance
[0325] MEA Preparation
[0326] The 0.1 mg Pt Ru/cm.sup.2 EB-PVD/IBAD catalyzed ELAT gas
diffusion media electrode was first pretreated with the same
procedure described in the prior examples. The pretreated electrode
was then hot pressed to a GORE-SELECT.RTM. proton conducting
membrane (25 microns, 950 EW) as the anode half-cell. As cathode, a
standard reference electrode (0.3 mg Pt/cm.sup.2 loading) was
used.
[0327] MEA testing
[0328] The experimental MEA, with an electrode active area of 25
cm.sup.2 was tested at the same conditions and with the same
protocols used during fuel cell polarization characterization of
experimental MEAs described in the previous examples. FIGS. 61 and
62 show polarization performance for this sequential Pt Ru
EB-PVD/IBAD catalyzed anode, for pure H.sub.2, and for reformate
feeds respectively. Table 3 summarizes the performance, expressed
as current density at 0.6 V in hydrogen and H.sub.2/CO mixtures (5
and 50 ppm CO concentration). Performance of a Pt sputtered anode
(0.1 mg/cm.sup.2) was again included as reference.
[0329] The results indicate that this PtRu EB-PVD/IBAD catalyzed
anode in pure hydrogen anode feed produced an improvement of ca.
29% in current density/power output when compared to a sputtered Pt
(single metal) electrode, an improvement of 7% when compared to a
PtRu catalyzed electrode. In the presence of 5 ppm CO/H.sub.2 anode
feed, this electrode yields 44% improved power output at 0.6 V, and
at 50 ppm level the improvement is ca. 84%, all compared to the
sputtered Pt (reference) electrode.
[0330] Poison resistance-related lambda parameters are also shown
in FIG. 3.
[0331] The data indicate that the MEA containing the PtRu
EB-PVD/IBAD anode decays 63% at 5 ppm CO anode fuel, and 82% at 50
ppm CO, in relation to the power output of the bimetallic anode in
pure hydrogen. These lambda values are 4% and 6% smaller than the
ones observed with a pure sputtered Pt anode when exposed to 5 and
50 ppm, respectively (i.e., 67% and 88%).
[0332] TABLES:
2TABLE 1 current current current at 0.6 V at 0.6 V at 0.6 V with
H.sub.2 with H.sub.2/ lambda with lambda Temper- feed 5 ppm CO
para- H.sub.2/50 ppm para- MEA ature OCV (no CO) feed meter CO feed
meter Anode (.degree. C.) (V) (mA/cm.sup.2) (mA/cm.sup.2) (5 ppm)
(mA/cm.sup.2) (50 ppm) Pt 80 0.945 822 272 0.67 101 0.88 (ref.) 60
0.920 525 -- -- -- -- PtRu 80 0.940 990 500 0.49 183 0.82 60 0.900
622 -- -- -- --
[0333]
3TABLE 2 current current current at 0.6 V at 0.6 V at 0.6 V with
H.sub.2 with H.sub.2/ lambda with lambda Temper- feed (no 5 ppm CO
para- H.sub.2/50 ppm para- MEA ature OCV CO) feed meter CO feed
meter Anode (.degree. C.) (V) (mA/cm.sup.2) (mA/cm.sup.2) (5 ppm)
(mA/cm.sup.2) (50 ppm) Pt 80 0.945 822 272 0.67 101 0.88 (ref.) 60
0.920 525 -- -- -- -- PtRu 80 0.922 625 492 0.21 255 059 60 0.890
426 -- -- -- --
[0334]
4TABLE 3 current current current at 0.6 V at 0.6 V at 0.6 V with
H.sub.2/ lambda with lambda Temper- with H.sub.2 5 ppm CO para-
H.sub.2/50 ppm para- MEA ature OCV feed feed meter CO feed meter
Anode (.degree. C.) (V) (mA/cm.sup.2) (mA/cm.sup.2) (5 ppm)
(mA/cm.sup.2) (50 ppm) Pt 80 0.945 822 272 0.67 101 0.88 (ref.) 60
0.920 525 -- -- -- -- PtRu 80 0.940 1060 391 0.63 186 0.82 60 0.903
680 -- -- -- --
[0335] The foregoing description of preferred embodiments of the
invention have been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Hence, many modifications
and variations are possible in light of the above teaching.
* * * * *