U.S. patent application number 10/317654 was filed with the patent office on 2006-03-23 for methanol-tolerant cathode catalyst composite for direct methanol fuel cells.
Invention is credited to Piotr Zelenay, Yimin Zhu.
Application Number | 20060063061 10/317654 |
Document ID | / |
Family ID | 36045491 |
Filed Date | 2006-03-23 |
United States Patent
Application |
20060063061 |
Kind Code |
A1 |
Zhu; Yimin ; et al. |
March 23, 2006 |
METHANOL-TOLERANT CATHODE CATALYST COMPOSITE FOR DIRECT METHANOL
FUEL CELLS
Abstract
A direct methanol fuel cell (DMFC) having a methanol fuel
supply, oxidant supply, and its membrane electrode assembly (MEA)
formed of an anode electrode and a cathode electrode with a
membrane therebetween, a methanol oxidation catalyst adjacent the
anode electrode and the membrane, an oxidant reduction catalyst
adjacent the cathode electrode and the membrane, comprises an
oxidant reduction catalyst layer of a platinum-chromium alloy so
that oxidation at the cathode of methanol that crosses from the
anode through the membrane to the cathode is reduced with a
concomitant increase of net electrical potential at the cathode
electrode.
Inventors: |
Zhu; Yimin; (Los Alamos,
NM) ; Zelenay; Piotr; (Los Alamos, NM) |
Correspondence
Address: |
UNIVERSITY OF CALIFORNIA;LOS ALAMOS NATIONAL LABORATORY
P.O. BOX 1663, MS A187
LOS ALAMOS
NM
87545
US
|
Family ID: |
36045491 |
Appl. No.: |
10/317654 |
Filed: |
December 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10260780 |
Sep 27, 2002 |
|
|
|
10317654 |
Dec 12, 2002 |
|
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Current U.S.
Class: |
429/492 ;
429/506; 429/524 |
Current CPC
Class: |
Y02E 60/523 20130101;
H01M 4/8605 20130101; H01M 4/921 20130101; Y02E 60/50 20130101;
H01M 4/926 20130101; H01M 8/1011 20130101; H01M 4/928 20130101;
H01M 8/04197 20160201 |
Class at
Publication: |
429/040 ;
429/033; 429/012 |
International
Class: |
H01M 8/00 20060101
H01M008/00; H01M 8/10 20060101 H01M008/10; H01M 4/86 20060101
H01M004/86 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0002] This invention was made with government support under
Contract No. W-7405-ENG-36 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. A direct methanol fuel cell (DMFC) having a methanol fuel
supply, oxidant supply, an anode electrode and a cathode electrode
with a solid polymer electrolyte membrane therebetween forming a
membrane electrode assembly, a methanol oxidation catalyst adjacent
the anode electrode and the membrane, an oxidant reduction catalyst
adjacent the cathode electrode and the membrane, wherein the
improvement comprises an oxidant reduction catalyst layer of
platinum-chromium alloy catalyst that includes a perfluorinated ion
exchange polymer at about 35 to 55 volume percent of the layer, so
that oxidation at the cathode of methanol that crosses from the
anode through the membrane to the cathode is reduced with a
concomitant increase of net electrical potential at the cathode
electrode.
2. The DMFC of claim 1, where the platinum-chromium catalyst is
provided at a loading less than about 1.0 mg cm.sup.-2.
3. (canceled)
4. (canceled)
5. The DMFC of claim 1, wherein the platinum-chromium alloy
catalyst is carbon-supported.
6. The DMFC of claim 1, wherein the platinum-chromium alloy
catalyst is unsupported.
7. (canceled)
8. A membrane electrode assembly (MEA), having an anode electrode,
a cathode electrode with a solid polymer electrolyte membrane
therebetween, for use in a direct methanol fuel cell (DMFC),
further including a methanol oxidation catalyst adjacent the anode
electrode and the membrane, an oxidant reduction catalyst adjacent
the cathode electrode and the membrane, wherein the improvement
comprises an oxidant reduction catalyst layer of platinum-chromium
alloy that includes a perfluorinated ion exchange polymer at about
35 to 55 volume percent of the layer, so that oxidation at the
cathode of methanol that crosses from the anode through the
membrane to the cathode is reduced with a concomitant increase of
net electrical potential at the cathode electrode.
9. The MEA of claim 8, where the platinum-chromium alloy is
provided at a loading less than about 1.0 mg cm.sup.-2.
10. (canceled)
11. (canceled)
12. The MEA of claim 8, wherein the platinum-chromium alloy
catalyst is carbon-supported.
13. The MEA of claim 8, wherein the platinum-chromium alloy
catalyst is unsupported.
14. (canceled)
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. patent
application Ser. No. 10/260,780, filed Sep. 27, 2002.
FIELD OF THE INVENTION
[0003] The present invention relates generally to direct methanol
fuel cells, and, more particularly, to cathode catalyst
compositions for direct methanol fuel cells.
BACKGROUND OF THE INVENTION
[0004] Fuel cells are considered a possible alternative to direct
combustion engines to power transportation vehicles and to possibly
furnish electrical energy to a power distribution grid for home and
business use. In a fuel cell, fuels are chemically reacted with an
oxidant whereby a direct current is produced at a low voltage
across individual cells and stacks of cells produce useful power.
Catalyst materials promote the chemical reactions of the fuels
(typically hydrogen or methanol) and oxidant (typically pure oxygen
or air).
[0005] In a generic embodiment shown in FIG. 1, a fuel cell 10
includes an anode electrode 14 for the fuel oxidation, a cathode
electrode 16 for the oxidant reduction, and a solid-state polymer
electrolyte membrane 18 therebetween to provide an ionic conduction
path. The combination of anode electrode 14, cathode electrode 16,
and membrane 18 is conventionally called a membrane electrode
assembly (MEA) 12. A suitable catalyst is disposed adjacent the
interfaces of electrode/membrane surfaces 14/18 and 16/18 so that
the fuel is oxidized at the anode/membrane interface 14/18 to
produce ions that traverse the membrane to complete the oxidant
reduction at the cathode/membrane interface 16/18. Fuel 24 is
distributed over anode 14 of MEA 12 by fuel distribution plate 22
and unreacted fuel and reaction products 26 are exhausted. Oxidant
32 is distributed over cathode 16 of MEA 12 by oxidant distribution
plate 28 and excess oxidant and reaction products 34 are exhausted.
As a result, electrons generated at anode 14 travel through an
external circuit (not shown) back to cathode 16. The electrons
constitute the flow of electrical current that provides energy to
components connected to the external circuit.
[0006] The most common fuel used in the development of polymer
electrolyte membrane fuel cells has been hydrogen, either in a pure
form or furnished as a reformate from hydrocarbon products. Yet
another approach is to directly use a liquid methanol solution in
direct methanol fuel cells (DMFCs) to avoid the complications
associated with supplying pure hydrogen or providing a separate
system for reforming hydrocarbons to provide reformed hydrogen.
DMFCs with a solid polymer electrolyte can provide high current
density at low temperature and have a relatively simple fuel cell
construction. Methanol is a renewable fuel material and can be
readily transported and supplied with existing transportation and
distribution infrastructure for liquid fuels. Both hydrogen fuel
cells and DMFCs have the generic structure shown in FIG. 1.
[0007] In a DMFC, catalysts promote electrode reactions at the
cathode and the anode, where a metric of performance is the
catalytic activity per unit mass of catalytic metal. This metric is
directly related to the efficiency and output power of the cells
and to the manufactured cost of the cells. Platinum black was an
early cathode catalyst in an ion-exchange MEA for hydrogen fuel
cells, typically a gas diffusion electrode substrate with one
surface coated with platinum black in an amount of 4 to 10 mg
cm.sup.-2 of the MEA.
[0008] To improve the utilization efficiency of platinum, a
catalyst was developed with a platinum alloy supported on
conductive carbon or an unsupported platinum alloy, which was mixed
with an ion-exchange polymer, coated on an electrode substrate, and
joined to an ion-exchange membrane by painting, hot pressing, or
the like, to form the MEA. This process permitted the thickness and
composition of the catalyst layer to be controlled so that the
catalyst was more effectively utilized in the electrode reaction. A
loading of platinum or platinum alloy of only 0.1 to 1.0 mg
cm.sup.-2 was needed to produce a performance equivalent to prior
art hydrogen fuel cells.
[0009] This reduced loading that has been demonstrated for hydrogen
fuel cells has not been achieved for DMFCs. In a DMFC, some
methanol crosses through the membrane from the anode and reacts at
the cathode, competing with the oxygen reduction reaction for
active catalyst surface sites. Reducing the catalyst loading
results in fewer active sites available for the oxygen reduction
reaction, as well as limiting the ability of the catalyst to handle
methanol crossover, with a concomitant reduction in the potential
of the DMFC cathode. Thus, methanol crossover to the cathode not
only lowers fuel utilization, but also adversely affects the oxygen
cathode with overall lower cell performance.
[0010] One way to reduce the effect of methanol crossover on DMFC
performance is to simply reduce methanol crossover by developing a
membrane that is less permeable to methanol. However, this has not
yet been fully realized in the art. Other ways to reduce methanol
crossover include lower methanol feed concentration and optimized
cell design.
[0011] The present invention recognizes that performance losses
associated with methanol crossover arise from the fact that most
Pt-based cathode systems are catalytically active to methanol
oxidation under normal cell operating conditions with a resulting
net cathode potential from oxygen reduction reaction potential
reduced by the methanol oxidation reaction. In accordance with the
present invention, a Pt-alloy catalyst has been identified that is
less catalytically active for methanol oxidation, while having
equal or increased catalytic activity for oxygen reduction.
[0012] Various objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
[0013] The present invention is a direct methanol fuel cell (DMFC)
having a methanol fuel supply, oxidant supply, and a membrane
electrode assembly (MEA) formed of an anode electrode and a cathode
electrode with a solid polymer electrolyte membrane therebetween, a
methanol oxidation catalyst adjacent the anode electrode and the
membrane, an oxidant reduction catalyst adjacent the cathode
electrode and the membrane, wherein the improvement comprises an
oxidant reduction catalyst layer containing platinum-chromium so
that oxidation at the cathode of methanol that crosses from the
anode through the membrane to the cathode is reduced with a
concomitant increase of net electrical potential at the cathode
electrode.
[0014] In one embodiment, the platinum-chromium catalyst is
supplied at a loading less than 1 mg cm.sup.-2 and the cathode
catalyst layer includes a perfluorinated ion exchange polymer in a
volume percent between 35% and 55%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0016] FIG. 1 pictorially depicts the component parts of a fuel
cell having functions described herein.
[0017] FIG. 2 graphically depicts CO stripping voltammograms for
platinum-chromium/C and Pt/C catalysts to illustrate the low
catalytic activity of platinum-chromium/C for methanol oxidation
relative to Pt/C.
[0018] FIG. 3 graphically depicts polarization plots representing
electro-oxidation of crossover methanol at platinum-chromium/C and
Pt/C electrodes to further demonstrate the relatively low catalytic
activity of platinum-chromium/C for methanol oxidation.
[0019] FIG. 4 graphically compares the performance of two DMFCs,
one with a platinum-chromium/C cathode and the other with a Pt/C
cathode.
[0020] FIG. 5 graphically compares the performance of three (3)
DMFCs with MEAs using platinum-chromium/C with one MEA using Pt/C
to illustrate performance improvement consistency.
[0021] FIG. 6 graphically compares the performance effect of
Nafion.RTM. content in the catalyst layer for platinum-chromium/C
and for Pt/C.
DETAILED DESCRIPTION
[0022] In accordance with the present invention, a cathode catalyst
of a platinum-chromium alloy provides reduced catalytic activity
for methanol reaction at a DMFC cathode while maintaining or
increasing catalytic activity for the oxygen reduction reaction at
the cathode. The net potential at the cathode from the methanol
oxidation and oxygen reduction is increased over the net potential
obtained from a conventional carbon supported Pt (Pt/C) cathode
catalyst. The platinum-chromium alloy may be unsupported or carbon
supported.
[0023] A suitable DMFC has the functional fuel cell structure shown
in FIG. 1. It will be appreciated that each structural element
shown in FIG. 1 can be implemented in a variety of structures known
to those skilled in the art, except as specifically described
herein to incorporate the platinum-chromium cathode catalyst.
[0024] As shown below, we have demonstrated that improved DMFC
performance is obtained with a platinum-chromium cathode catalyst
compared to a Pt/C cathode catalyst. The exemplary
platinum-chromium catalyst was a carbon-supported Pt.sub.3C alloy.
In these comparative experiments, the platinum-chromium/C catalyst
was about 44 wt % platinum-chromium and the Pt/C catalyst was about
40 wt % Pt. The anode catalyst was a PtRu black or PtRu/C (45 wt
%). The anode and cathode catalysts were dispersed in appropriate
amounts in water, with an added perfluorinated ion exchange polymer
for ionic conduction adjacent the catalysts (e.g., 5% Nafion.RTM.
solution (1100 EW, Solution Technology, Inc., USA). Exemplary
cathode ink compositions were 65 wt % Pt/C and 35 wt % Nafion.RTM.
and 66 wt % Pt.sub.3Cr/C and 34 wt % Nafion.RTM.; anode ink
compositions were 85 wt % PtRu and 15 wt % Nation.RTM. or 70 wt %
PtRu/C and 30 wt % Nafion.RTM.. MEAs were prepared by painting the
catalytic inks on membranes of Nation 117.RTM.. The desired
catalyst loadings are less than about 1.0 mg cm.sup.-2 so the
cathode catalyst inks were applied to obtain an experimental
loading of about 0.6 mg cm.sup.-1. In all cases the geometric
active area of the MEA was 5 cm.sup.2. TABLE-US-00001 TABLE 1
Preferred composition of catalyst layers Catalyst Catalyst (wt %)
Nafion .RTM. (wt %) PtRu black 85 15 PtRu/C 70 30 Pt/C 65 35
Pt.sub.3Cr/C 66 34
[0025] The reduced catalytic activity of platinum-chromium/C for
methanol oxidation compared to Pt/C is demonstrated by the CO
stripping voltammograms shown in FIG. 2 for a Pt loading of 0.6 mg
cm.sup.-2 in both cases. To obtain these results, DMFCs with
platinum-chromium/C cathode catalyst and with Pt/C cathode catalyst
were operated in a "driven mode", with methanol being oxidized at
the fuel cell cathode and hydrogen evolving at the fuel cell anode,
which acted as a counter/quasi-reference electrode (a dynamic
hydrogen electrode, DHE). CO produced during the methanol oxidation
was adsorbed onto the electrode surface and then stripped to
determine the surface charge density as a measure of catalytic
activity. The charge density for surface CO stripping on the
platinum-chromium/C cathode was 33 mC cm.sup.-2 compared with
.about.67 mC cm.sup.-2 for the Pt/C cathode. These results clearly
indicate a reduced catalytic activity of platinum-chromium/C for
methanol oxidation compared with Pt/C.
[0026] Another indication of different activity of methanol towards
platinum-chromium/C and Pt/C was obtained in direct measurements of
methanol crossover. In these experiments, the cells were again
operated in a driven mode, with methanol oxidized at the fuel cell
cathode and hydrogen evolved at the fuel cell anode, thereby
serving as a hydrogen counter/quasi-reference electrode. After
stabilizing the cell at open voltage, a single voltammetric scan
was applied to the fuel cell cathode and current response recorded,
typically in the range of 0.1-0.6 V. In addition to allowing the
magnitude of crossover to be directly examined, the activity of the
catalyst towards methanol could also be determined in such an
experiment from the kinetic part of the current-potential
plots.
[0027] As shown by the plots in FIG. 3, the regular Pt/C cathode
catalyst is significantly more active towards methanol crossing
through the Nafion 117.RTM. membrane than the platinum-chromium/C
catalyst. For example, at an anode potential of 0.35 V, a typical
DMFC operating potential, the current density of methanol oxidation
with a platiunum-chromium/C catalyst is about 20 mA cm.sup.-2, much
lower than that of 81 mA cm.sup.-2 obtained with the Pt/C catalyst.
Not surprisingly, the differences in the rate of methanol oxidation
disappear once limiting-current conditions are reached on both
electrodes, i.e. at a potential higher than 0.5 V. The same current
density of .about.130 mA cm.sup.-2 is measured in either case, thus
attesting to the expected similar permeation rates of methanol
through the Nafion 117.RTM. membrane used with both
platinum-chromium/C and Pt/C cathode catalysts. However, at lower
potentials, the cathode using the platinum-chromium/C catalyst that
is less sensitive to methanol is expected to remain at a higher net
potential than the Pt/C cathode, which is more susceptible to
becoming depolarized by methanol.
[0028] FIG. 4 graphically compares the performance of a DMFC with
an exemplary carbon-supported Pt.sub.3Cr alloy (Pt.sub.3Cr/C)
cathode catalyst and a DMFC with Pt/C cathode catalyst. The cathode
catalyst loadings were 0.6 mg-cm.sup.-2, the anode catalyst
loadings were 9.6 mg cm.sup.-2 of PtRu black. The cells were
operated at 80.degree. C., cathode pressure of 2.7 atm, with an
anode feed of 0.5 M methanol. Except for the lowest current density
range, below about 20 mA cm.sup.-2, where performance of both
catalyst composites is dominated by the high flux of methanol
through the Nafion membrane, the Pt.sub.3Cr/C catalyst consistently
showed a 70-80 mV voltage advantage over the reference Pt/C
catalyst. This suprisingly large voltage advantage of the
Pt.sub.3Cr/C catalyst has not been observed in hydrogen fuel cells,
although a small voltage advantage is realized by Pt.sub.3Cr/C
because of a higher activity of the catalyst in oxygen reduction at
a cathode.
[0029] Relative performance of the two cathode catalysts was also
tested versus anodes prepared by using carbon-supported PtRu
catalyst at a low loading of .about.1.0 mg cm.sup.-2. Three
different MEAs using Pt.sub.3Cr/C cathode and PtRu/C anode were
made to test reproducibility of the results obtained with the novel
cathode catalyst formulation. Hydrogen-air cell polarization plots
were then recorded as initial tests of cathode activity at a cell
temperature of 80.degree. C., showing very good and reproducible
cathode performance. In particular, a cell current density of 0.2 A
cm.sup.-2 was reached at 0.83-0.84 V, i.e., at a cell voltage
similar to that measured with highly loaded (9.6 mg cm.sup.-2)
unsupported PtRu anode.
[0030] Following operation in hydrogen-air mode, the above cells
underwent regular DMFC testing. Polarization plots for the cells
with Pt.sub.3Cr/C and Pt/C cathodes and PtRu/C anodes at 80.degree.
C. are shown in FIG. 5. All plots represent MEAs having the same
catalyst loading and operated under the same fuel cell operating
conditions. FIG. 5 shows again the very significant performance
advantage of the platinum-chromium/C catalyst over the reference
Pt/C catalyst. As in the testing performed with highly loaded PtRu
anodes, the voltage advantage offered by Pt.sub.3Cr/C catalyst over
regular Pt/C catalyst with low PtRu/C anode loadings (.about.1.0 mg
cm.sup.-2) was 60-80 mV in the entire range of investigated cell
current densities. The results obtained with three different
platinum-chromium/C cells were highly reproducible, with current
densities remaining within just a few percent from one another, as
further shown in FIG. 5.
[0031] To further verify the repeatable nature of the results,
different preparation batches (OMG) of the cathode catalyst and a
catalyst from another manufacturer (E-TEK) were prepared and
tested. Methanol-air polarization plots indicate that there is no
significant difference between these catalysts over the total
current density region.
[0032] In the cathode catalyst layer, an optimal ion-exchange
polymer content should minimize both ohmic and mass transport
limitations, maximize electrochemical activity and Pt utilization.
The influence of polymer content in the catalyst layer with Pt/C or
platinum-chromium/C on performance of methanol/air fuel cell is
shown in FIG. 6. An increase in Nafion.RTM. content improves the
performance up to 35% or 40% for Pt/C or platinum-chromium/C,
respectively, but platinum-chromium/C catalyst shows a
significantly greater performance improvement than does Pt/C.
However, when too much polymer (beyond ca. 52%) is introduced the
current density at 0.45 V decreases because the ohmic and mass
transport limitations appear. This is a reasonable result because
the ca. 52% Nafion.RTM. content means that the volume of
Nafion.RTM. in the catalyst layer is almost the same as that of the
catalyst. It is obvious that platinum-chromium/C is sensitive to
the polymer content.
[0033] The foregoing description of the invention has been
presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching.
[0034] The embodiments were chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *