U.S. patent application number 12/294601 was filed with the patent office on 2009-05-07 for fuel cell electrode catalyst with reduced noble metal amount and solid polymer fuel cell comprising the same.
Invention is credited to Susumu Enomoto, Yosuke Horiuchi, Takahiro Nagata, Toshiharu Tabata, Hiroaki Takahashi, Tomoaki Terada.
Application Number | 20090117448 12/294601 |
Document ID | / |
Family ID | 38069053 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090117448 |
Kind Code |
A1 |
Horiuchi; Yosuke ; et
al. |
May 7, 2009 |
Fuel Cell Electrode Catalyst with Reduced Noble Metal Amount and
Solid Polymer Fuel Cell Comprising the Same
Abstract
An object of the present invention is to reduce the amount of
catalytic metal such as Pt in a fuel cell. The present invention
provides a fuel cell electrode catalyst comprising a conductive
carrier and catalytic metal particles, wherein the CO adsorption
amount of the electrode catalyst is at least 30 mL/gPt.
Inventors: |
Horiuchi; Yosuke; (Aichi,
JP) ; Terada; Tomoaki; (Shizuoka, JP) ;
Nagata; Takahiro; (Shizuoka, JP) ; Tabata;
Toshiharu; (Shizuoka, JP) ; Enomoto; Susumu;
(Shizuoka, JP) ; Takahashi; Hiroaki; (Aichi,
JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
38069053 |
Appl. No.: |
12/294601 |
Filed: |
March 29, 2007 |
PCT Filed: |
March 29, 2007 |
PCT NO: |
PCT/JP2007/057629 |
371 Date: |
September 25, 2008 |
Current U.S.
Class: |
429/480 ;
429/481; 429/483; 429/490; 429/493 |
Current CPC
Class: |
H01M 4/926 20130101;
H01M 4/92 20130101; H01M 2004/8684 20130101; H01M 2008/1095
20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/40 |
International
Class: |
H01M 4/86 20060101
H01M004/86 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2006 |
JP |
2006-092755 |
Claims
1. A fuel cell electrode catalyst comprising a conductive carrier
and catalytic metal particles, characterized in that the CO
adsorption amount of the electrode catalyst is at least 30
mL/gPt.
2. The fuel cell electrode catalyst according to claim 1,
characterized in that the catalytic metal is at least one selected
from the group consisting of noble metal, noble metal-rare earth,
noble metal-transition metal, and noble metal-transition metal-rare
earth.
3. The fuel cell electrode catalyst according to claim 1 or 2,
characterized in that the conductive carrier has a specific surface
area of at least 650 m.sup.2/g.
4. The fuel cell electrode catalyst according to any of claims 1 to
3, characterized in that the amount of catalytic metal per 1
cm.sup.2 of the electrode is at most 0.0001 mg.
5. The fuel cell electrode catalyst according to any of claims 1 to
4, characterized by being an anode catalyst.
6. A solid polymer fuel cell having an anode, a cathode, and a
polymer electrolyte membrane located between the anode and the
cathode, characterized comprising by the fuel cell electrode
catalyst according to any of claims 1 to 5 as an electrode catalyst
for the cathode and/or anode.
7. A solid polymer fuel cell characterized by comprising the fuel
cell electrode catalyst according to any of claims 1 to 5 as an
anode electrode catalyst.
8. A method for evaluating a fuel cell electrode catalyst
comprising a conductive carrier and catalytic metal particles, the
method using the CO adsorption amount of the electrode catalyst as
an index.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fuel cell electrode
catalyst with a reduced noble metal amount, and a solid polymer
fuel cell comprising the fuel cell electrode catalyst.
BACKGROUND ART
[0002] The sizes of solid polymer fuel cells, having polymer
electrolyte membranes, can be easily reduced. The solid polymer
fuel cells are thus expected to be applied to mobile vehicles such
as electric cars and power sources for small cogeneration systems.
However, the solid polymer fuel cells operate at relatively low
temperatures. Further, it is difficult to effectively utilize waste
heat from them for auxiliary power or the like. Accordingly, to be
put to practical use, the solid polymer fuel cells need to offer a
high generation efficiency and a high power density under operating
conditions including a high anode reaction gas (pure water or the
like) utilization rate and a high cathode reaction gas (air or the
like) utilization rate.
[0003] An electrode reaction in a catalyst layer in each of the
anode and cathode of the solid polymer fuel cell occurs at a three
phase interface (hereinafter referred to as a reaction site) where
reaction gases, a catalyst, and a fluorine containing ion exchange
resin are simultaneously present. Thus, the reaction in each
electrode occurs only at the three phase interface, where gas
(hydrogen or oxygen) corresponding to an active substance, protons
(H.sup.+), and electrons (e.sup.-) can be simultaneously
transferred to one another.
[0004] An example of an electrode having this function is a solid
polymer electrode-catalyst composite electrode containing a solid
polymer electrolyte, carbon particles, and a catalytic substance.
For example, in this electrode, the carbon particles carrying the
catalytic substance are mixed with the solid polymer electrolyte so
that the carbon particles, catalytic substance, and solid polymer
electrolyte are three-dimensionally distributed. Further, a
plurality of pores are formed inside the electrode, which is thus
porous. The carbon, a carrier of the catalyst, forms an electron
conducting channel. The solid electrolyte forms a proton conducting
channel. The pores form a supply and discharge channel for oxygen,
hydrogen or water. These three channels spread three-dimensionally
in the electrode to form countless three phase interfaces, where
the gas, protons (H.sup.+), and electrons (e.sup.-) can be
simultaneously transferred to one another. This provides a field
for electrode reactions.
[0005] Thus, for the conventional solid polymer fuel cells, a
catalyst such as a metal catalyst or a metal carrying catalyst (for
example, metal carrying carbon comprising a carbon black carrier
with a large specific surface area and a metal catalyst such as
platinum carried by the carbon black carrier) is coated with the
same fluorine containing ion exchange resin as or a fluorine
containing ion change resin different from that contained in the
polymer electrolyte membrane. The catalyst coated with the fluorine
containing ion exchange resin is then used as a component of the
catalyst layer to perform what is called an operation of making the
reaction sites in the catalyst layer three-dimensional. This
increases the number of reaction sites and improves the utilization
efficiency of expensive noble metal such as platinum, corresponding
to the catalytic metal.
[0006] Putting fuel cell cars to practical use requires a drastic
reduction in costs. However, with the conventional fuel cell
catalysts, a reduction in the amount of noble metal in one of the
anode and cathode may disadvantageously sharply reduce resultant
power owing to the very high activity of the noble metal.
[0007] Thus, to reduce the amount of catalyst, JP Patent
Publication (Kokai) No. 8-148151 A (1996) discloses the invention
of a fuel cell electrode comprising a catalyst layer formed on the
gas diffusion layer and containing catalytic particles carrying an
active metal, wherein the catalyst layer comprises multiple layers
of catalytic particles of different carried active metal
amounts.
[0008] Thus, the conventional techniques for reducing the noble
metal amount focus on the improvement of the electrode structure
and few of them take note of the physical properties of the
electrode catalyst itself.
DISCLOSURE OF THE INVENTION
[0009] The present invention has been made in view of the problems
with the conventional art. An object of the present invention is to
provide an electrode catalyst that does not reduce the power of a
fuel cell in spite of a reduction in the used amount of catalytic
metal such as Pt.
[0010] The present inventors have obtained the present invention by
finding that the above object is accomplished by an electrode
catalyst having particular physical properties.
[0011] First, the present invention provides a fuel cell electrode
catalyst comprising a conductive carrier and catalytic metal
particles, wherein the CO adsorption amount of the electrode
catalyst is at least 30 mL/gPt. Regardless of shape or condition of
the catalytic metal particles carried by the conductive carrier,
catalytic performance can be appropriately evaluated using the CO
adsorption amount as an index.
[0012] As the catalytic metal for the fuel cell electrode catalyst
in accordance with the present invention, it is possible to use any
of various catalytic metals each consisting of only a well-known
noble metal or multiple elements including noble metal and other
elements. Specific preferred examples of the catalytic metal
include at least one selected from the group consisting of noble
metal, noble metal-rare earth, noble metal-transition metal, and
noble metal-transition metal-rare earth. In particular, the
preferred example is platinum.
[0013] The conductive carrier for the fuel cell electrode catalyst
in accordance with the present invention may be any of various
well-known catalyst carriers for fuel cells. In particular, a
preferred example of the conductive carrier is any of various
carbon powders or a fibrous carbon material. The conductive carrier
for the fuel cell electrode catalyst in accordance with the present
invention preferably has a specific surface area of at least 650
m.sup.2/g, more preferably at least 800 m.sup.2/g. Further, the
conductive carrier used is preferably anticorrosive.
[0014] By setting the CO adsorption amount of the electrode
catalyst to at least 30 mL/gPt, more preferably at least 38 mL/gPt,
it is possible to set the amount of catalytic metal per 1 cm.sup.2
of the fuel cell electrode to at most 0.0001 mg. That is, the
amount of expensive noble metal can be reduced to enhance the
practicality of the fuel cell.
[0015] The fuel cell electrode catalyst in accordance with the
present invention can be used for both anode and cathode. The fuel
cell electrode catalyst in accordance with the present invention
can be effectively used particularly as an anode catalyst to offer
appropriate cell performance and to reduce the amount of noble
metal used.
[0016] Second, the present invention provides a solid polymer fuel
cell having an anode, a cathode, and a polymer electrolyte membrane
located between the anode and the cathode, the solid polymer fuel
cell comprising the fuel cell electrode catalyst as an electrode
catalyst for the cathode and/or anode.
[0017] The fuel cell electrode catalyst in accordance with the
present invention can be used for both anode and cathode and can be
effectively used particularly as an anode catalyst as described
above.
[0018] In spite of a successful reduction in the amount of noble
metal used, the electrode catalyst in accordance with the present
invention enables the provision of a solid polymer fuel cell in no
way inferior to the conventional ones in cell power.
[0019] Third, the present invention provides a method for
evaluating a fuel cell electrode catalyst comprising a conductive
carrier and catalytic metal particles, the method using the CO
adsorption amount of the electrode catalyst as an index. Using the
CO adsorption amount as an index enables the fuel cell performance
to be appropriately evaluated regardless of the amount of noble
metal used. Specifically, catalytic performance can be evaluated on
the basis of whether or not the CO adsorption amount of the
electrode catalyst is at least 30 mL/gPt.
[0020] The present invention has enabled fuel cell electrode
catalysts to be appropriately evaluated. The present invention has
also enabled a high fuel cell performance to be maintained in spite
of a successful reduction in the amount of noble metal used. The
reduced amount of catalyst metal such as Pt contributes directly to
a reduction in fuel cell costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows the relationship between anode Pt amount and
cell voltage; and
[0022] FIG. 2 shows the relationship between anode Pt amount and CO
absorption amount in connection with an initial performance of 0.6
V.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] A detailed description will be given of a preferred
embodiment of a fuel cell electrode catalyst and solid polymer fuel
cell comprising the fuel cell electrode catalyst in accordance with
the present invention.
[0024] A metal catalyst contained in the fuel cell electrode
catalyst in accordance with the present invention is not
particularly limited but is preferably platinum or a platinum
alloy. A metal catalyst carried by a conductive carrier is
preferably carried by a conductive carrier. The conductive carrier
is not particularly limited but is preferably a carbon black or an
activated carbon.
[0025] A polymer electrolyte used with the fuel cell electrode
catalyst in accordance with the present invention is preferably a
fluorine containing ion exchange resin, particularly preferably a
sulfonic perfluorocarbon polymer. The sulfonic perfluorocarbon
polymer remains chemically stable in a cathode over a long period
of time and enables quick proton conduction.
[0026] The layer thickness of a catalyst layer in the fuel cell
electrode catalyst in accordance with the present invention may be
equivalent to that of a normal gas diffusion electrode and is
preferably 1 to 100 .mu.m, more preferably 3 to 50 .mu.m.
[0027] A polymer electrolyte membrane for the solid polymer fuel
cell in accordance with the present invention is not particularly
limited and may be any ion exchange membrane exhibiting a high ion
conductivity in a wet condition. A solid polymer material
constituting the polymer electrolyte membrane may be, for example,
a perfluorocarbon polymer having a sulfonic group, a polysulfone
resin, or a perfluorocarbon polymer having a phosphoric group or a
carboxylic group. In particular, the sulfonic perfluorocarbon
polymer is preferred. The polymer electrolyte membrane may be
composed of the same fluorine containing ion exchange resin as or a
fluorine containing ion exchange resin different from that
contained in the catalyst layer.
[0028] The fuel cell electrode catalyst in accordance with the
present invention can be produced by using a coating liquid
obtained by dissolving or dispersing a metal catalyst-containing
conductive carrier and a polymer electrolyte in a solvent or a
dispersive medium. Alternatively, the fuel cell electrode catalyst
may be produced by using a coating liquid obtained by dissolving or
dispersing a catalyst-carrying conductive carrier and a polymer
electrolyte in a solvent or a dispersive medium. Examples of the
solvent or dispersive medium used herein include alcohol, fluorine
containing alcohol, and fluorine containing ether. A catalyst layer
is formed by coating the coating liquid on a carbon cloth or the
like constituting an ion exchange membrane or a gas diffusion
layer. Alternatively, a catalyst layer may be formed on an ion
exchange membrane by coating the coating liquid on a separately
prepared base to form a coating layer and transferring the coating
layer to the ion exchange membrane.
[0029] Here, if the fuel cell electrode catalyst layer is formed on
the gas diffusion layer, the catalyst layer and the ion exchange
membrane are preferably joined together by an adhesive process or
hot press process. If the catalyst layer is formed on the ion
exchange membrane, the cathode may be composed only of the catalyst
layer or of the catalyst layer and the gas diffusion layer placed
adjacent to the catalyst layer.
[0030] A separator with a gas channel formed therein is normally
located outside the cathode. The channel is supplied with hydrogen
containing gas for the anode and oxygen containing gas for the
cathode. The solid polymer fuel cell is configured as described
above.
[0031] In the present invention, the reason why the CO adsorption
amount of the electrode catalyst constitutes an index for a
reduction in noble metal amount is not very clear. However, a
possible reason is that the CO adsorption amount is not very
sensitive to the shape, particle size, or carrying condition of the
noble metal such as Pt on the conductive carrier and that the CO
adsorption amount has a strong correlation with the surface area of
surface of the noble metal which adsorbs CO.
EXAMPLES
[0032] The cathode and solid polymer fuel cell in accordance with
the present invention will be described below in detail with
reference to examples and comparative examples.
Example 1
[0033] First, 3.5 g of Ketjen EC (specific surface area: 800
m.sup.2/g) was added to and dispersed in 0.4 L of pure water. A
hexahydroxo platinum nitrate solution containing 1.5 g of platinum
was dropped into the fluid dispersion, which was sufficiently
blended with the carbon. About 10 mL of 0.075 N ammonia was added
to the fluid dispersion and the fluid dispersion had prepared a pH
of about 12. A hydroxide was thus formed and precipitated on the
carbon. The fluid dispersion was washed, and a powder obtained was
dried in a vacuum at 100.degree. C. for 24 hours. The platinum
carrying catalytic powder obtained had a platinum carrying density
of 30.0 wt %. XRD measurements as physical property examinations
showed only a Pt peak. The average particle size was calculated to
be 1.7 nm on the basis of a peak position on a Pt (111) surface
near 39.degree. and a half-value width. The CO adsorption amount as
an index for the specific surface area of Pt measured 40.1
mL/gPt.
Examples 2, 3, and 5 and Comparative Examples 1 to 3
[0034] In Examples 2, 3, and 5 and Comparative Examples 1 to 3,
conditions were set so as to obtain the physical properties
described below, as in the case of Example 1, in order to examine
the relationship between a reduction in the particle size of noble
metal and a reduction in noble metal amount in the anode
catalyst.
Example 4
[0035] First, 4.5 g of Ketjen EC (specific surface area: 800
m.sup.2/g) was added to and dispersed in 0.8 L of pure water. A
hexahydroxo platinum nitrate solution containing 0.5 g of platinum
was dropped into the fluid dispersion, which was sufficiently
blended with the carbon. About 30 mL of 0.075 N ammonia was added
to the fluid dispersion and the fluid dispersion had prepared a pH
of about 12. A hydroxide was thus formed and precipitated on the
carbon. The fluid dispersion was washed, and a powder obtained was
dried at 100.degree. C. for 24 hours.
[0036] In this example, the components were suspended in water the
amount of which was larger than that in the method of preparing a
catalyst in Example 1. This improves the dispersion of the carbon,
allowing the platinum to be carried more dispersively.
[0037] The platinum carrying catalytic powder obtained had a
platinum carrying density of 5.0 wt %. XRD measurements as physical
property examinations showed only a Pt peak. The average particle
size was calculated to be 1.3 nm on the basis of the peak position
on the Pt (111) surface near 390 and the half-value width. The CO
adsorption amount as an index for the specific surface area of Pt
measured 67.6 mL/gPt.
[0038] Table 1 shows the physical properties of the platinum
carrying catalytic powder obtained.
TABLE-US-00001 TABLE 1 Pt CO Average Reference numerals carrying
adsorption particle in the figure amount amount size of the
separate (%) (mL/g Pt) (nm) sheet Example 1 30 40.1 1.7 (1) Example
2 30 51.4 1.3 (2) Example 3 30 62.6 1.1 (3) Example 4 5 67.6 1.0
(4) Example 5 30 38.3 1.9 (5) Comparative 30 16.3 3.1 (6) Example 1
Comparative 30 22.4 2.6 (7) Example 2 Comparative 30 20.7 4.0 (8)
Example 3
[Performance Evaluations]
[0039] The noble metal carrying catalytic powders obtained in
Examples 1 to 5 and Comparative Examples 1 to 3 were used to form
unit cells for the solid polymer fuel cell as described below.
Electrodes were formed by dispersing each metal carrying catalytic
powder in a mixed solution of an organic solvent and a conductive
material and spray-coating the fluid dispersion on an electrolyte
membrane so that the amount of Pt catalyst per 1 cm.sup.2 of
electrode area was 0.00001, 0.0001, 0.001, 0.01, or 0.1
mg/cm.sup.2. Diffusion layers were installed on the respective
sides of each electrode to form unit cell electrodes. The cathode
of the cell was supplied with 1 L/min of humidified air passed
through a bubbler heated to 70.degree. C. The anode of the cell was
supplied with 0.5 L/min of humidified hydrogen passed through the
bubbler heated to 85.degree. C.
[0040] FIG. 1 shows the relationship between anode Pt amount and
cell voltage. The figure indicates that the relationship between
the dependence on the anode Pt amount and the cell voltage is such
that a catalyst with a large CO adsorption amount such as the one
in Example 1 is unlikely to be degraded in spite of a reduction in
the amount of the noble metal in the anode. For example, a high
generation performance can be maintained even when the amount of Pt
per 1 cm.sup.2 of the anode is at most 0.0001 or 0.00001 mg.
[0041] FIG. 2 shows the relationship between the anode Pt amount
and CO adsorption amount in connection with an initial performance
of 0.6 V. The figure indicates that the initial cell performance of
0.6 V can be sufficiently ensured in spite of a reduction in the
amount of the noble metal in the anode by using, as the anode, any
of the noble metal carrying catalysts in Examples 1 to 5, for which
the corresponding electrode catalysts exhibited a CO adsorption
amount of at least 30 mL/gPt. In contrast, the initial cell
performance of 0.6 V cannot be ensured by using, as the anode, any
of the noble metal carrying catalysts in Comparative Examples 1 to
3, for which the corresponding electrode catalysts exhibited a
catalyst CO adsorption amount of less than 30 mL/gPt, unless the
noble metal amount is increased.
[0042] This is because the oxidizing reaction of hydrogen in the
anode proceeds very fast, so that with an increase in CO absorption
amount resulting in a sufficient number of reaction sites, the
anode did not exhibit a rate-determining compared with the cathode.
Carbon carriers with extremely small specific surface areas such as
the one in Comparative Example 1 are limited in the enhancement of
dispersion. In this case, the CO adsorption amount cannot be
increased, making it difficult to reduce the noble metal amount.
The reduction of the noble metal amount is also difficult with a
carbon carrier with a large specific surface area and an
intentionally significantly increased particle size of noble metal
such as the one in Comparative Example 3.
INDUSTRIAL APPLICABILITY
[0043] Using the CO adsorption amount of the electrode catalyst as
an index, the present invention has enabled a reduction in the
amount of noble metal carried and thus in fuel cell costs. The fuel
cell electrode catalyst in accordance with the present invention
contributes to the practical application and prevalence of fuel
cells.
* * * * *