Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Organic compound containing
Reexamination Certificate
2001-03-15
2002-12-10
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Organic compound containing
C502S101000, C502S173000, C502S185000, C502S439000, C429S047000, C429S047000, C428S407000, C264S050000, C264S068000, C264S068000, C264S241000, C264S331160
Reexamination Certificate
active
06492295
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composite catalyst suitable for use in solid polymer electrolyte type fuel cells and to processes for producing the catalyst.
DESCRIPTION OF THE RELATED ART
A solid polymer electrolyte type fuel cell (PEFC) is an apparatus which employs a solid ion-exchange membrane as an electrolyte and in which a fuel (e.g., hydrogen gas) and an oxidizing agent (e.g., oxygen gas) are supplied respectively to the anode and the cathode to electrochemically react these feed materials on the catalyst surfaces, thereby obtaining an electric power.
For example, in the case of using hydrogen gas and oxygen gas as a fuel and an oxidizing agent, respectively, the electrochemical reactions occurring at the electrodes are as follows.
Anode: H
2
→2H
+
+2e
−
Cathode: 1/20
2
+2H
+
+2e
−
→H
2
O
Overall reaction: H
2
+1/20
2
→H
2
O
As the formulae given above show, the reactions at the anode and cathode necessitate the feed of oxygen and hydrogen gases and the transfer of protons (H
+
) and electrons (e
−
). Namely, all these reactions proceed only at sites where the feed and transfer are satisfied simultaneously.
An electrode for fuel cells is diagrammatically shown in FIG.
25
. This electrode has a catalyst layer
261
and a gas diffusion layer
263
. The catalyst layer
261
is constituted, for example, of a mixture of carbon particles supporting catalyst metal
265
and a solid polymer electrolyte
271
. The particles
265
and the electrolyte
271
are three-dimensionally distributed so that the layer has pores
267
in inner parts thereof, i.e., is porous. The gas diffusion layer
263
is constituted of a porous electro-conductive material
269
, which comprises, e.g., a porous carbon paper. This electrode is bonded to a cation-exchange membrane
275
to thereby fabricate a fuel cell. This gas diffusion layer
263
provides not only to secure passageways for transferring the oxygen gas and hydrogen gas fed externally as reactants to a surface of the catalyst layer
261
but also to provides passageways for discharging the water yielded in the catalyst layer of the cathode from a surface of the catalyst layer
261
to the outside of the cell. On the other hand, in the catalyst layer
261
, the carbon supporting catalyst metal
265
forms electro-conductive channels and the solid polymer electrolyte
271
forms proton-conductive channels. The pores
267
function not only as feed channels through which the oxygen or hydrogen gas transferred to a surface of the catalyst layer
261
is supplied to inner parts of the catalyst layer but also as gas diffusion channels for discharging the water yielded in inner parts of the catalyst layer (cathode) to a surface of its layer. These three kinds of channels are three-dimensionally distributed in the catalyst layer
261
to form innumerable sites where gas transfer can occur simultaneously with the transfer of protons (H
+
) and electrons (e
−
). Thus, sites for the electrode reactions are provided.
Incidentally, the solid polymer electrolyte
271
comprising a cation-exchange resin used as a proton conductor shows satisfactory proton conductivity only when it is in a hydrous state. Consequently, for preventing the solid polymer electrolyte
271
from drying, a technique is being used in which the gases to be supplied to the anode and cathode are humidified before being supplied. However, this technique has aroused a problem that when the solid polymer electrolyte type fuel cell is operated at a high current density, water floods on the surface of the catalyst layer
261
and in the pores
267
to inhibit gas diffusion, resulting in a considerably reduced output. Especially, this problem is tend to be occurred because the reaction yields water in it.
A technique generally employed for avoiding the water flooding caused by water generation and gas humidification is to impart water repellency to an electrode by incorporating polytetrafluoroethylene (PTFE) particles
273
, which exhibits excellent hydrophobic property, together with catalyst particles
265
in the formation of a catalyst layer or by applying PTFE particles
273
to the surface of an porous electro-conductive material
269
. However, in order to prevent water flooding enough in the electrode during high-current-density operation, it is necessary to incorporate PTFE particles
273
in an even larger amount to thereby enhance water repellency. Although highly water-repellent, the PTFE particles
273
do not have gas-diffusing properties, not to mention electron conductivity and proton conductivity. Because of this, mixing of large amount of the PTFE particles block electron-conductive channels, proton-conductive channels, and gas diffusion channels, arousing a problem that the output of the fuel cell is reduced rather than increased. In addition, the pores
267
formed among the catalyst particles
265
are partly clogged by the cation-exchange resin and, as a result, the gas diffusion channels are partly blocked to prevent a reactant gas from being supplied to the whole catalyst layer
261
including minute regions thereof. Namely, there has been a problem that the degree of catalyst utilization is low and the fuel cell has a high concentration overvoltage and hence a low cell voltage.
SUMMARY OF THE INVENTION
A first object of the invention is to provide a composite catalyst capable of high electron conductivity, proton conductivity, and gas-diffusing properties while preventing water flooding. A second object of the invention is-to provide gas-diffusing properties while improving proton conductivity by adhering a cation-exchange resin to the surface of a catalyst to thereby improve the degree of catalyst utilization.
The invention provides a composite catalyst characterized by comprising catalyst particles and, adherent to the surface thereof, a porous or net-form cation-exchange resin and/or a porous or net-form hydrophobic polymer.
The adhesion of a cation-exchange resin to the surface of catalyst particles improves the proton conductivity of the catalyst surface. Since the porous or net-form cation-exchange resin is adherent to the surface of the catalyst particles so as to leave the catalyst surface partly exposed, reactant gases can reach the surface of the catalyst particles through the pores or net openings of the resin, whereby the degree of catalyst utilization can be improved. The adhesion of a hydrophobic polymer to the surface of catalyst particles is effective in preventing water flooding in an electrode. Furthermore, since the resin or polymer is adherent to the surface of the catalyst Particles so as to leave the catalyst surface partly exposed, electron conductivity, proton conductivity, and gas-diffusing properties can be secured.
In the case of the composite catalyst having both a porous or net-form cation-exchange resin and a porous or net-form hydrophobic polymer which are adherent to the surface of the catalyst particles, it is a matter of course that the effects brought about when the resin and polymer are used alone can be obtained simultaneously. Furthermore, in the invention, the catalyst particles preferably have a porous or net-form cation-exchange resin adherent to the surface thereof and further have a hydrophobic polymer adherent thereto in the pores or net openings of the cation-exchange resin so as to leave the surface of the catalyst particles partly exposed. Consequently, this hydrophobic polymer is in close contact with the cation-exchange resin and the catalyst particles. This promotes to form site where transfer of protons (H
+
) and transfer of electrons (e
−
) can occur simultaneously, whereby the output of a fuel cell can be further improved.
The composite catalyst of the invention can be produced by a process which comprises the steps of: adhering a solution (a) prepared by dissolving a cation-exchange resin and/or a hydrophobic polymer in a solvent to the surface of catalyst particles; and subsequently undergoi
Hitomi Shuji
Mizutani Shunsuke
Tsumura Naohiro
Bell Mark L.
Hailey Patricia L.
Japan Storage Battery Co., Ltd.
Sughrue & Mion, PLLC
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