Electrode for fuel cell and process for producing the same

Details Electrode for fuel cell and process for producing the same Electrode for fuel cell and process for producing the same

2000-09-27

2003-03-04

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C502S101000

Reexamination Certificate

active

06528201

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode for fuel cell and a process for the production thereof.
2. Description of the Related Art
A solid polymer electrolyte type fuel cell (PEFC) is composed of as an electrolyte of a cation-exchange membrane which is a solid polymer electrolyte such as perfluorocarbonsulfonic acid membrane and an anode and cathode connected to the ion-exchange membrane on the respective side thereof. In operation, hydrogen is supplied into the anode while oxygen is supplied into the cathode so that an electrochemical reaction occurs to generate electricity. The electrochemical reaction occurring on these electrodes will be shown below.
Anode: H
2
→2H
+
+2e
−
Cathode: 1/2O
2
+2H++2e
−
→H
2
O
Total reaction: H
2
+1/2O
2
→H
2
O
As can be seen in these reaction formulae, the reaction on the electrodes proceed only on the three-phase boundary site where gas of the active materials, i.e., (hydrogen or oxygen), proton (H
+
) and electron (e
−
) can be received and released at the same time.
An example of the electrode for fuel cell having such a function is a solid polymer electrolyte-catalyst composite electrode made of a cation-exchange resin as a solid polymer electrolyte, carbon particles and a catalyst metal. An example of the structure of a cation-exchange resin-catalyst composite electrode made of a cation-exchange resin as a solid polymer electrolyte, carbon particles supporting a catalyst metal showing a high catalytic activity for the reduction reaction of oxygen and the oxidation reaction of hydrogen is shown in FIG.
9
. In
FIG. 9
, the reference numeral la indicates a carbon particle, the reference numeral
2
a
indicates a cation-exchange resin, the reference numeral
3
a
indicates an ion-exchange membrane, and the reference numeral
4
a
indicates a pore. As can be seen in
FIG. 9
, the carbon particles la supporting a catalyst metal and the cation-exchange resin
2
a
are three-dimensionally distributed and a plurality of pores
4
a
are formed in the porous electrode. The carbon as a support of catalyst metal forms an electron-conductive channel. The cation-exchange resin forms a proton-conductive channel. The pores form a channel for supplying oxygen or hydrogen and discharging water as a product. Further, these three channels are three-dimensionally spread in the electrode to form numerous three-phase boundaries at which gas, proton (H
+
) and electron (e
−
) can be received and released at the same time, thereby providing a site for electrode reaction.
The electrode having such a structure has heretofore been prepared by a process including applying a paste made of a carbon particle supporting highly dispersed a catalyst metal particles such as platinum and suspension of PTFE (polytetrafluoroethylene) particles to a polymer film or electrically-conductive porous carbon substrate to form a film of the paste (normally to a thickness of from 3 &mgr;m to 30 &mgr;m), heating and drying the film, followed by applying a cation-exchange resin solution to the film so that the film is impregnated with the cation-exchange resin solution, and then drying the film or a process including applying a paste made of the foregoing carbon particle supporting catalyst, a cation-exchange resin solution and optionally PTFE particles to a polymer film or electrically-conductive porous carbon electrode substrate to form a film of the paste (normally to a thickness of from 3 &mgr;m to 30 &mgr;m), and then drying the film. As the cation-exchange resin solution there is used one obtained by dissolving a material having the same composition as the previously mentioned ion-exchange membrane in an organic solvent such as alcohol or a mixture of an organic solvent and water to form a liquid like solution. As the suspension of PTFE particles there is used a suspension of PTFE particles having a diameter of about 0.23 &mgr;m.
PEFC is expensive. This prevents PEFC from being put in practical use. In particular, metals belonging to the platinum group which are used as catalyst are expensive. This is a major factor causing the rise of the cost of PEFC. Therefore, how the amount of platinum group metal as catalyst metal to be supported on the electrode can be reduced is the key to technical development in the art.
The conventional electrode used a catalyst metal particle belonging to a platinum group metal supported on carbon. The activity of the electrode depends greatly on the surface of the platinum group metal particle. Therefore, it is an ordinary practice to reduce the particle diameter of the platinum group metal and hence increase the surface area of the platinum group metal per unit weight thereof, enhancing the catalytic activity per unit weight of the platinum group metal. At present, carbon supporting a platinum group metal having an average particle diameter of about 4 nm is used as a catalyst metal. However, it is necessary to support a platinum group metal both on the cathode and anode in an amount as great as 0.4 mg/cm
2
or more in order to obtain sufficient characteristics for practical use. Further, the conventional electrode prepared by the production processes described above shows a reduced percent utilization of the catalyst metal supported on carbon, e.g., only about 10%, further lowering the activity against the total electrode reactions (see Edson A. Tisianelli, “J. Electroanal. Chem.”, 251, 275, 1988). This is attributed to the fact that the conventional production processes involve the mixing of carbon particles supporting a catalyst metal particle such as platinum supported thereon with a cation-exchange resin. In other words, the carbon particle as a support has a particle diameter as small as 30 nm for example. The carbon particle to be mixed with the cation-exchange resin solution is composed of aggregates of some carbon particles having considerably dense roughness formed on the surface thereof. On the other hand, the cation-exchange resin solution has a certain viscosity and thus cannot penetrate deep into the central portions of the aggregate of carbon particles even by a process including impregnating the dispersion film layer made of carbon particles and PTFE particles with a cation-exchange resin solution or a process including the use of a paste obtained by mixing carbon particles, PTFE particles and a cation-exchange resin solution. This phenomenon makes it impossible to form a three-phase boundary in the deep portion in the aggregate of carbon particles. Therefore, the catalyst metal particle positioned at this portion takes no part in electrode reaction, causing a drop of the percent utilization of the catalyst metal. The structure of such an electrode is shown in FIG.
10
. As shown in
FIG. 10
, carbon particles
3
b
supporting catalyst particles
1
b
and
2
b
gather together to form an aggregate of carbon particles (4 piece of carbon particles constituting an aggregate in this figure). In this arrangement, since cation-exchange resin
4
b
doesn't penetrate into a deep portion
5
b
of the aggregate of carbon particles, the catalyst particles
1
b
which are positioned on the contact area of carbon particle with the cation-exchange resin effectively act on the electrode reaction and a catalyst particle
2
b
which has no area in contact with the cation-exchange resin doesn't effectively act on the electrode reaction.
In order to enhance the percent utilization of catalyst metal, studies have been made of supporting of a catalyst metal on the portion where the surface of carbon particle contacts a cation-exchange resin (the state shown in
FIG. 10
excluding the catalyst particles
2
b
). However, the mere study of how the carbon supporting catalyst and the cation-exchange resin are three-dimensionally arranged in the electrode on the basis of the conventional macroscopic consideration of the structure of three-phase boundary in the electrode is limited sufficient for the drastic enhancem

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