Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1998-11-25
2002-02-05
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C204S283000, C204S290010
Reexamination Certificate
active
06344291
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid polymer electrolyte-catalyst composite electrode, an electrode for a fuel cell and a process for producing these electrodes.
2. Description of the Related Art
As an electrochemical apparatus having an ion-exchange membrane as a solid electrolyte, for example, there are a solid polymer type water electrolysis cell and a solid polymer type fuel cell.
The solid polymer electrolyte type water electrolysis cell is an apparatus having an ion-exchange membrane, for example, such as perfluorosulfonic acid membrane as an electrolyte and an anode and a cathode connected to the respective side of the ion-exchange membrane which supplies oxygen from the anode and hydrogen from the cathode when D.C. voltage is applied across the both electrode while the anode is being supplied with water.
The electrochemical reactions which take place on the two electrodes will be described below.
Anode: H
2
O→1/20
2
30 2H
+
+2e
−
Cathode: 2H
+
+2e
−
→H
2
Total reaction: H
2
O→H
2
+1/20
2
It can be seen in these reaction formulae that the anode reaction proceeds only on a three-phase interface which allows the reception of water as an active material and the delivery of oxygen as a product, proton (H
+
) and electron (e
−
) at the same time while the cathode reaction proceeds only on a three-phase interface which allows the reception of proton (H
+
) and electron (e
−
) and the delivery of hydrogen at the same time.
On the other hand, the solid polymer electrolyte type fuel cell is an apparatus having an ion-exchange membrane, for example, such as perfluorosulfonic acid membrane as an electrolyte and an anode and a cathode connected to the respective side of the ion-exchange membrane which generates electricity due to electrochemical reaction developed by the supply of hydrogen to the anode and oxygen to the cathode.
The electrochemical reactions which take place on the two electrodes will be described below.
Anode: H
2
→2H
+
+2e
−
Cathode: 1/20
2
+2H
+
+2e
−
→H
2
O
Total reaction: H
2
+1/20
2
→H
2
O
It can be seen in these reaction formulae that the both electrode reactions proceed only on a three-phase interface which allows the reception of gas (hydrogen or oxygen) and the delivery or reception of proton (H
+
) and electron (e
−
) at the same time.
An example of the electrode, used in the apparatus, having such a function is a solid polymer electrolyte-catalyst composite electrode comprising a solid polymer electrolyte and catalyst particles. The structure of this electrode with a fuel cell as an example will be explained.
FIG. 12
is an explanation view showing the structure of this electrode. This electrode is a porous electrode comprising catalyst particles
121
and a solid polymer electrolyte
122
three-dimensionally distributed in admixture and having a plurality of pores
123
formed thereinside. The catalyst particles form an electron-conductive channel, the solid electrolyte forms a proton-conductive channel, and the pore forms a channel for the supply and discharge of oxygen, hydrogen or water as product. The three channels are three-dimensionally distributed and numerous three-phase interfaces which allow the reception or delivery of gas, proton (H
+
) and electron (e
−
) at the same time are formed in the electrode, providing sites for electrode reaction. Incidentally, reference numeral
124
represents an ion-exchange membrane.
The preparation of an electrode having such a structure has heretofore been accomplished by the following process. There is a process which comprises applying a paste made of catalyst particles and a solution having PTFE particles (polytetrafluoro ethylene) dispersed therein to a polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (normally having a thickness of from 3 to 30 &mgr;m), heating and drying the film, and then applying a solid polymer electrolyte solution to the film so that the film is impregnated with the solution. Alternatively, there is a process which comprises applying a paste made of catalyst particles thereon, PTFE particles and a solid polymer electrolyte solution to a polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (normally having a thickness of from 3 to 30 &mgr;m), and then heating and drying the film. As the solid polymer electrolyte solution, there is used a solution obtained by dissolving the same composition as the aforementioned ion-exchange membrane in an alcohol. As the solution having PTFE particles dispersed therein, there is used a solution having PTFE particles having a particle diameter of about 0.23 &mgr;m dispersed therein.
The solid polymer electrolyte-catalyst composite electrode comprising metal particles of the platinum group or oxide particles of metal of the platinum group as a catalyst is used in a water electrolysis cell or a fuel cell. On the other hand, the solid polymer electrolyte-catalyst composite electrode comprising platinum group metal supported on carbon as a catalyst is used in a fuel cell.
The aforementioned solid polymer electrolyte-catalyst composite electrode has the following two disadvantages.
One of the two disadvantages is that the solid polymer electrolyte-catalyst composite electrode has a high resistivity. The reason of this disadvantage is as follows.
When catalyst particles are mixed with solid polymer electrolyte solution to prepare a paste, the catalyst particles are covered with solid polymer electrolyte film having no electronic conduction and a pore (void)
132
and a solid polymer electrolyte
133
exist between catalyst particles
131
even after film-making process to prepare an electrode. The formation of a continuous catalyst particle passage (electro-conductive channel) is inhibited, though forming a continuous solid electrolyte passage (proton-conductive channel), as shown in the sectional view of electrode of FIG.
13
.
The other disadvantage is that if a solid polymer electrolyte-catalyst composite electrode comprising the platinum group metal supported on carbon as a catalyst is used in an electrode for a fuel cell. The resulting percent utilization of catalyst supported on carbon is as low as about 10% as reported in Edson A. Tichianlli, “J. Electroanal. Chem.”, 251, 275 (1998).
This is caused by the fact that the preparation process of supporting catalyst such as platinum on carbon particle, and then mixing the carbon particle with a solid polymer electrolyte.
In other words, the carbon particles as a support has a particle diameter as small as 30 nm. Thus, the carbon particle to be mixed with the solid polymer electrolyte has an aggregation of a few carbon particles that gives a carbon particle aggregate having a dense unevenness formed on the surface thereof. On the other hand, the solid electrolyte solution is viscous. Thus, regardless of which is used the process which comprises impregnating the layer having carbon particles and PTFE particles dispersed therein with a solid polymer electrolyte solution or the process which comprises the use of a paste obtained by mixing carbon particles, PTFE particles and a solid polymer electrolyte solution, the solid polymer electrolyte solution cannot penetrate deep into the central portion of the carbon particle aggregate. As a result, it is impossible to form a three-phase interface in the deep portion of the carbon particle aggregate. Accordingly, the catalyst particles disposed in the deep portion of the carbon particle aggregate does not take part in the electrode reaction to thereby cause decrease of percent utilization of catalyst.
The structure of such an electrode is shown in FIG.
14
. As shown in
FIG. 14
, carbon particles
143
having catalyst particles
141
,
142
supported thereon are aggregated to form a carbon particle aggregate (four of the carbon particles are shown forming
Japan Storage Battery Co., Ltd.
Tran Thao
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