Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Making catalytic electrode – process only
Reexamination Certificate
2002-11-20
2004-05-11
Sergent, Rabon (Department: 1711)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Making catalytic electrode, process only
C429S047000, C429S006000
Reexamination Certificate
active
06734132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell, an electrode for it and a method for producing it.
2. Discussion of Background
Attention has been drawn to a hydrogen-oxygen fuel cell as a power generation system which gives no adverse effect to the global environment, since the reaction product is only water in principle. A very high output is expected at a low operation temperature of from room temperature to about 150° C., which is being studied recently. In such a case, it is assumed to use, as a fuel, hydrogen gas obtained by reforming a hydrocarbon such as methane, methanol or gasoline and containing e.g. carbon dioxide.
On the other hand, polymer electrolyte fuel cells have a low operation temperature. Accordingly, exhaust heat can hardly be utilized, for example, as an auxiliary power, and it is utilized only for hot water at best. To offset such a drawback, it is necessary for the polymer electrolyte fuel cell to secure a high output density. Further, for practical application, it is required to secure performance of a high energy efficiency and a high output density even under an operation condition where the fuel and air utilization ratios are high.
As the electrolyte for the polymer electrolyte fuel cell, a perfluorocarbon sulfonic acid type cation exchange membrane, which is an ultrastrong acid, is mainly used, in view of the chemical stability and electric conductivity. When such an acid electrolyte is used, the following reaction occurs at an air electrode, whereby water will be formed.
1/2O
2
+2H
+
+2e
−
→H
2
O
Therefore, under such an operation condition as a low operation temperature, a high current density and a high gas utilization ratio, clogging (flooding) at the pores of the electrode body is likely to take place due to condensation of steam, at the air electrode where water is formed. Accordingly, in order to obtain a stable performance of the fuel cell for a long period of time, it is necessary to secure water repellency of the electrode so as to prevent such flooding. This is particularly important in the case of a polymer electrolyte fuel cell whereby a high output density at a low temperature is desired.
To impart water repellency to the electrode, it has been studied to incorporate a fluorine-containing material to the electrode. Specifically, for example, the following methods (1) to (3) have been proposed. (1) A method in which a catalyst carrier is subjected to a fluorination treatment (JP-A-7-192738). (2) A method in which a fluorine-containing polymer is incorporated in the electrode (JP-A-5-36418). (3) A method in which a fluorinated pitch is incorporated in the electrode (JP-A-7-211324).
Among these, a fluorination treatment such as the method (1) requires a special equipment or technique, and thus it is unsuitable as a means to directly reform the surface of the carrier of the catalyst.
A fluorine-containing polymer which is insoluble in a solvent is used for the method (2). Specific examples include a tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter referred to as FEP), a tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (hereinafter referred to as PFA) and a polytetrafluoroethylene (hereinafter referred to as PTFE). In the present specification, a A-B copolymer means a copolymer having polymer units based on A and polymer units based on B.
To incorporate such a fluorine-containing polymer in the electrode as a water repellent, it is used in the form of a powder or a dispersion of the powder. The method of forming an electrode layer containing the fluorine-containing polymer, may, for example, be a method in which the dispersion of the fluorine-containing polymer is permitted to penetrate into pores of the electrode after the electrode layer is formed (2-1), or a method in which a powder or a dispersion of the fluorine-containing polymer is mixed with the rest of material which forms the electrode and then the electrode layer is formed (2-2).
The pore size of the gas diffusion electrode for a fuel cell depends on the method of producing the electrode. However, in general, the pore size distributes from about 0.01 to about several hundreds &mgr;m. If the water repellency in pores is inadequate, clogging is likely to start from pores having a small pore size due to capillary phenomenon, in general. Accordingly, it is considered that if water repellency is imparted to the inside of pores having a pore size of at least 0.05 &mgr;m, clogging of pores due to condensed water decreases, a quick electrode reaction can be made possible, and fuel cell properties will improve.
However, the primary particle size of the solvent-insoluble fluorine-containing polymer is about 0.1 &mgr;m at smallest. Further, in the case where it is supplied in the form of a powder, it is usually granulated, and thus the average particle size is from about several &mgr;m to about 500 &mgr;m. Accordingly, it was difficult to let the solvent-insoluble fluorine-containing polymer penetrate to the inside of pores having a pore size of about 0.05 &mgr;m, by means of impregnation, spray or filtration, after the electrode layer is formed. Namely, it was difficult to adopt the method (2-1), and the electrode layer was formed by the method (2-2).
On the other hand, in the method (2-2), a catalyst, a catalyst carrier and a conductive agent powder are mixed with the solvent-insoluble fluorine-containing polymer to prepare an electrode. Generally, carbon black is used for the catalyst carrier or the conductive agent, and the particle size is from 0.02 to 0.05 &mgr;m. Namely, the particle size of the fluorine-containing polymer is larger than the particle size of carbon black. Accordingly, if the electrode is prepared by this method, the solvent-insoluble fluorine-containing polymer as a water repellent exists ununiformly in the electrode layer in the form of particles. Therefore, with this method, the inside of pores could not necessarily be made water repellent uniformly, although the pore size could be made large. Further, the solvent-insoluble fluorine-containing polymer is non-electroconductive. Accordingly, if the amount of the water repellent is increased in order to improve water repellency of the electrode, the resistance of the electrode may increase.
In the method (3), a polymerization reaction may take place in the process for producing the fluorinated pitch, and the fluorinated pitch is a polymer in a broad sense. In a case where the fluorinated pitch is insoluble in a solvent, there will be the same problems as in the method (2). Some fluorinate pitches are soluble in a fluorine-type solvent. In this case, the fluorinated pitch has a relatively low molecular weight at a level of from about 1,000 to about 3,000, and a six-membered plane structure, whereby the bonding force among molecules is weak, the film-forming property after drying is inadequate, and the fluorinated pitch is likely to fall off, so that durability is inadequate. Further, bonding between a carbon atom and a fluorine atom in the fluorinated pitch is likely to be cut in the presence of an alkali, and it is not sufficiently stable also in this respect.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrode for a polymer electrolyte fuel cell which is capable of keeping an adequate water repellency for a long period of time, and to provide a fuel cell having a stable performance for a long period of time, by using the electrode.
The present invention provides an electrode for a polymer electrolyte fuel cell, which is a porous gas diffusion electrode comprising a catalyst powder and an ion exchange resin, wherein a solvent-soluble fluorine-containing polymer having substantially no ion exchange groups exists at least at a part of the inner surface of pores of the electrode, and a polymer electrolyte fuel cell having the electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The electrode for a polymer electrolyte fuel cell of the present invention co
Ishizaki Toyoaki
Terazono Shinji
Yoshida Naoki
Yoshitake Masaru
Asahi Glass Company Ltd.
Sergent Rabon
Tran Thao
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