Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1999-12-07
2004-05-04
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C502S101000
Reexamination Certificate
active
06730427
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode for a fuel cell and a method for manufacturing the same.
2. Description of the Related Art
A solid polymer electrolyte fuel cell is composed of an electrolyte of an ion exchange membrane such as a perfluorosulfonic acid membrane and electrodes of an anode and a cathode bonded to both surfaces thereof. The electrolyte fuel cell generates a power under an electrochemical reaction by supplying a reducing agent (e.g. hydrogen, methanol, hydrazine, etc.) to the anode and an oxidizing agent (e.g. air, oxygen, etc.) to the cathode. The electrochemical reaction occurring in each electrode using oxygen as the oxidizing agent and hydrogen as the reducing agent is as follows.
Anode: H
2
→2H
+
+2e
−
Cathode: 1/2O
2
+2H
+
+2e
−
→H
2
O
Entire reaction: H
2
+1/2O
2
→H
2
O
As understood from this reaction, the reaction in each electrode proceeds only in a three-phase boundary where a gas (hydrogen or oxygen) that is an active material, a proton (H
+
) and an electron (e
−
) are simultaneously transferred.
An example of the electrode having such a function is a composite electrode of a solid polymer electrolyte-catalyst containing carbon particles and catalyst material as well as a solid polymer electrolyte. The macroscopic state of this electrode is shown conceptually in FIG.
2
. In
FIG. 2
, reference numeral
21
denotes carbon particle,
22
a solid polymer electrolyte,
23
one of pores, and
24
an ion exchange membrane. As seen from
FIG. 2
, the composite electrode is a porous electrode in which carbon particles
21
supporting the catalyst material and solid polymer electrolyte
22
are mixed with each other so that they are distributed three-dimensionally, and plural pores are formed. In this composite electrode, the carbon supporting the catalyst constitutes an electron conductive channel, the solid polymer electrolyte constitutes a proton conductive passage and the pore constitutes a channel of supplying/discharging oxygen or hydrogen and water which is a product. Within the electrode, these three channels are distributed three-dimensionally so that an infinite number of three-phase boundaries capable of transferring gas, protons (H
+
) and electrons (e
−
) simultaneously are formed to provide a field of the electrode reaction.
Conventionally, the electrode having such a structure has been manufactured by a method comprising the steps of preparing a paste composed of a catalyst supported on carbon particles (in which platinum group metal particles such as platinum particles are supported on carbon particle with highly dispersion) and a PTFE (polytetrafluoroetylene) particle dispersed solution, dispersing the paste on the polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (generally having a thickness of 3-30 &mgr;m), heating/drying the film, and applying a solid polymer electrolyte solution onto the film so that the film is impregnated with the solution. The electrode has been also manufactured by the method comprising a steps of preparing a paste composed of the above catalyst supported on carbon particles, PTFE particles and a solid polymer electrolyte solution, dispersing the paste on the polymer film or the carbon electrode substrate of electro-conductive porous material to make a film (generally having a thickness of 3-30 &mgr;m), and thereafter heating/drying the film. Incidentally, the solid polymer electrolyte solution was prepared by dissolving the material having the same composition as the ion exchange membrane described above in alcohol to provide a solution. The PTFE particles dispersed solution was prepared as a solution dispersed with PTFE particles having a particle diameter of about 0.23 &mgr;m.
The solid polymer electrolyte fuel cell, which has advantages of capable of being actuated at room temperature and being compact and light as well as having a high power, has been developed for use in an application of an electric vehicle. Such a type fuel cell generally uses a gaseous fuel such as hydrogen as a reducing agent, or otherwise a liquid fuel such as methanol, hydrazine, etc.
The case of using hydrogen as the reducing agent includes systems of storing hydrogen in a highly compressed bomb, and storing hydrocarbon fuel such as methanol or natural gas as a raw material and reforming it into hydrogen for use by a reforming device. The latter system is predominant in view of the total cost and official infrastructure for circulation. The reforming reaction using methanol is as follows.
CH
3
OH+H
2
O→3H
2
+CO
2
+(CO)
The system using this reaction has a disadvantage that a very small quantity of CO created as well as CO
2
poisons the catalyst material such as platinum for the anode of the fuel cell and hence reduces the power.
The fuel cell which directly uses methanol of the liquid fuel as the reducing agent is called a direct methanol fuel cell. This fuel cell has advantages that it is easy to deal with since the liquid fuel is used in the low temperature directly without using the reforming device, and it makes the entire system simple and compact because of unnecessity of the reforming device. However, this fuel cell has a disadvantage that it provides a higher overvoltage due to oxidation of the fuel than the fuel using the gaseous fuel since methanol is oxidized at a low speed, and a large amount of noble metallic catalyst is required in the anode, thus increasing the production cost of the fuel cell.
Nowadays, these problems have been improved greatly by compositely using plural metals as the catalyst.
For example, as a catalyst having a CO tolerance characteristic, an alloy catalyst of Pt—Ru, Pt—Sn, Pt—Pd, Pt—Mn or Pt—Co has been proposed. As a catalyst which is active for the electrochemical oxidation reaction of the liquid fuel such as methanol, an alloy catalyst of Pt—Ru, Sn—Ir, Ru—Ir, Pt—Au or Pd—Ag has been proposed. The reason why activity of the catalyst in the form of an alloy is improved can be understood by some explanations, for example, alloying of platinum shortens the distance among platinum atoms, and the catalyst which is greatly meshed to have a highly active surface is solved away from the secondary metal (e.g. Ru, Sn, Pb, Rh).
The technique of alloying the catalyst has been also applied to the cathode catalyst. For example, it has been reported that the catalyst of Pt—Fe or Pt—Ni exhibits a higher activity for the reduction of oxygen than the catalyst of only platinum (see Masahiro Watanabe, the 38-th Battery Symposium in Japan NO.1I13, P29 (1997)).
A carbon supporting such an alloy catalyst can be acquired by impregnating carbon particles with two or more kinds of metal elements and reducing them. For example, a Ru—Ir alloy supported on carbon can be prepared by impregnating the carbon particles with a mixed water or alcohol solution of mixture of Ru and Ir compounds, drying them and thereafter reducing them by a hydrogen gas. In this case, the carbon particles are directly given the catalyst of mixed Ru and Ir in an atom level.
A Pt—Ru alloy supported on carbon can be prepared by impregnating the carbon particles with a water or alcohol solution of a platinum compound, drying them and thereafter reducing them by a hydrogen gas to provide a platinum supported on carbon, and further impregnating the carbon particles with a water or alcohol solution of a Ru compound, drying them and thereafter reducing them by the hydrogen gas. In this case, the carbon particle supports platinum fine particle covered with the layer of Ru. When the carbon particles are treated by hydrogen at a high temperature (500° C.), the surface is deformed from the Ru layer into the Pt layer.
As described above, attempts of giving the activity which cannot be acquired using the catalyst consisting of a sole metal, by alloying two or kinds of metals have attained a preferable result when the carbon particles support a large
Dove Tracy
Japan Storage Battery Co., Ltd.
Ryan Patrick
LandOfFree
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