Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-09-12
2004-11-09
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S047000, C429S253000
Reexamination Certificate
active
06815111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a separator for a fuel cell which is mainly used as a cell for an electric vechicle, and also to a method of producing the separator, and more particularly to a separator for a fuel cell of the solid polymer electrolyte type or the phosphoric acid type, and also to a method of producing the separator. In a fuel cell of such a type, a unit cell which is a unit constituting the cell is configured by: sandwiching a gas diffusion electrode having a sandwich structure wherein an electrolyte membrane configured by an ion exchange membrane, between an anode and a cathode; sandwiching the gas diffusion electrode between separators; and forming fuel gas passages and oxidant gas passages between the separators, and the anode and the cathode.
2. Description of the Prior Art
In a fuel cell, a fuel gas containing hydrogen is supplied to an anode, and an oxidant gas containing oxygen is supplied to a cathode, so that, in the anode and the cathode, electrochemical reactions indicated by the formulae:
H
2
→2H
−
+2e
−
(1)
(½)O
2
+2H
−
+2e
−
→H
2
O (2)
occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:
H
2
+(½)O
2
→H
2
O (3)
proceeds. The chemical energy of the fuel is directly converted into electrical energy, with the result that the cell can exert a predetermined performance.
A separator for a fuel cell of the solid polymer electrolyte type or the phosphoric acid type that is a kind of fuel cell in which such energy conversion is conducted is desirably gas-impermeable, and also is made of an electrically conductive material. As a material meeting the requirements, conventionally, an electrically conductive resin is used. An electrically conductive resin is a complex which is configured by bonding graphite (carbon) powder by means of a thermosetting resin such as phenol resin, or a so-called bondcarbon (resin-bonded carbon) compound. A separator for a fuel cell is configured by forming such a bondcarbon compound into a predetermined shape.
Conventionally, a separator for a fuel cell having a predetermined shape is formed by using such a bondcarbon compound in the following manner. With respect to the composition ratio of a thermosetting resin such as phenol resin and graphite powder, 25 to 40 wt. % of the thermosetting resin is used as an adequate content in consideration of moldability and workability of the bondcarbon compound.
In a conventional separator for a fuel cell which is configured by using a bondcarbon compound of such composition ratios, the content of a thermosetting resin serving as an electrically insulating material is large, and hence the conductivity of the separator itself is lowered so that the electrical resistance is increased. This is not preferable from the viewpoint of the performance of a fuel cell.
In order to improve the conductivity of a separator for a fuel cell which is configured by using a bondcarbon compound, it is contemplated that the content of a thermosetting resin be reduced as far as possible, and graphite powder of excellent crystallinity and having less impurities such as ash is selectively used. When the content of a thermosetting resin is reduced, however, elongation and fluidity of the bondcarbon compound during a molding process are lowered, and moldability is impaired. In order to improve the conductivity of a separator, furthermore, it is required to use graphite powder of excellent crystallinity. However, graphite powder of excellent crystallinity is usually poor in wettability and bondability to a resin. When graphite powder of excellent crystallinity is used, therefore, a larger resin content is required. As a result, a uniform separator is obtained more hardly as the resin content is smaller.
This will be described in more detail. With respect to a volume resistivity serving as an index of the conductivity of a separator which largely affects the performance of a fuel cell, it is requested to develop a separator having a volume resistivity of 1×10
−2
&OHgr;·cm or lower. In order to obtain a separator of a volume resistivity of such a degree, the resin content in the composition ratio with respect to graphite must be reduced to 15 wt. % or less. When the resin content is reduced to 15 wt. % or less, however, elongation and fluidity of the bondcarbon compound are lowered, and, when the resin content is smaller than 3 wt. %, the bondcarbon compound has substantially no fluidity. In the case where a bondcarbon compound of a resin content of 15 wt. % or less is used, even when the compound is charged into a heated mold, therefore, the compound hardly extends to every corners of the mold because the lack of the resin content causes the molten compound to exert insufficient fluidity. As a result, even when a molding pressure higher than a usual pressure or 5 to 10 MPa is applied, uneven molding easily occurs depending on the charging state of the compound, and a molded member (separator) which is normal in shape cannot be sometimes obtained.
SUMMARY OF THE INVENTION
The present invention has been conducted in view of the above-mentioned circumstances. It is an object of the invention to provide a separator for a fuel cell which can be formed into a uniform and predetermined shape while good conductivity is ensured by reducing the resin content so as to suppress the volume resistivity to 1×10
−2
&OHgr;·cm or lower.
It is another object of the invention to provide a method of producing a separator for a fuel cell wherein, even when a molding material in which the resin content is small, and which is therefore low in elongation and fludity is used, the molding material can extend to every corner of a mold so that the separator having a uniform and correct shape that is free from molding unevenness, and good conductivity can be surely produced.
In order to attain the objects, the separator for a fuel cell of the invention is a separator for a fuel cell consisting of a complex which is configured by bonding graphite powder by means of a thermosetting resin, and characterized in that, in the complex, a composition ratio of the graphite powder is set to 85 to 97 wt. %, a composition ratio of the thermosetting resin is set to 3 to 15 wt. %, an average particle diameter of the graphite powder is set to a range of 15 to 125 &mgr;m, and the complex is molded at a pressure of 10 to 100 MPa.
Preferably, the composition ratio of the thermosetting resin in the complex is set to a range of 4 to 9 wt. %, and the average particle diameter of the graphite powder is set to a range of 40 to 100 &mgr;m. Preferably, the molding pressure of the complex is set to a range of 20 to 50 MPa.
In order to meet the above-mentioned demands for development, intensive studies on a separator for a fuel cell which is configured by using a bondcarbon compound, and finally found that the volume resistivity serving as an element which largely affects the performance of a fuel cell is determined not only by the composition ratios of a resin and graphite powder, but also by the average diameter of the graphite powder, and the molding pressure, and that the size of the average diameter of the graphite powder is closely related not only to the volume resistivity, but also to the fluidity, moldability, and strength of the compound. Based on this finding, the composition ratios of a resin and graphite powder, the average diameter of the graphite powder, and the molding pressure have been respectively set to the above-mentioned ranges, thereby completing the invention.
According to the thus configured invention, as the graphite powder which is the one composition of the complex and which largely affects the volume resistivity, graphite powder in which the average diameter is set to a range of 15 to 125 &mgr;m, preferably, 40 to 100 &mgr;m is used, the thermosetting resin which is the other composition of the complex, and which lar
Nippon Pillar Packing Co. Ltd.
Weiner Laura
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