Separator for a fuel cell and a method of producing the same

Details Separator for a fuel cell and a method of producing the same Separator for a fuel cell and a method of producing the same

2000-09-12

2003-12-09

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S047000, C429S253000

Reexamination Certificate

active

06660420

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 vehicle, and also to a method of producing the separator, and more particularly to a separator for a fuel cell of the 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 is 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 electrolyte type or the phosphoric acid type in which such energy conversion is conducted is desirably gas-impermeable, and also is made of an electrically conductive material. Conventionally, it is known that, as a material meeting the requirements, 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 60 wt. % of the thermosetting resin is used as an adequate content in consideration of fluidity, moldability, and gas-impermeability of the bondcarbon compound, and in order to ensure the strength (compression and bending) sufficient for preventing the separator from suffering damage such as a breakage due to vibrations or the like which may be produced during a handling operation in an assembling step of a unit cell of a fuel cell, or a use in an automobile.
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 s configured by using a bondcarbon compound, it has been contemplated that the content of a thermosetting resin be reduced as far as possible. When the content of a thermosetting resin is reduced, however, elongation and fluidity of the bondcarbon compound during a molding process are lowered to impair moldability, and the strength is low. When the resin content is 10 wt. % or less, particularly the strength of a separator becomes insufficient, and therefor the separator easily suffers damage such as a breakage or a crack due to vibrations or the like which are continuously applied to the separator in the case where the separator is used in an automobile.
By contrast, in the case where the resin content is set to the above-mentioned adequate range (25 to 60 wt. %), elongation and fluidity of a bondcarbon compound are excellent and moldability is higher, and strength sufficient for preventing a separator from suffering damage such as a breakage or a crack due to vibrations or the like can be ensured. However, the contact resistance with respect to an electrode and serving as the primary factor which largely affects the performance of a fuel cell becomes higher, as the resin content is larger. When the resin content is larger than 40 wt. %, particularly, the contact resistance is suddenly increased, and the performance of the fuel cell is extremely lowered.
The contact resistance serving as the primary factor which largely affects the performance of a fuel cell will be considered. Even when a fuel cell is used in an automobile in which vibrations are always applied to the fuel cell, it is desirable that the contact resistance be stably maintained to 10 m&OHgr;·cm
2
or lower. When the contact resistance is to be stably maintained to such a requested value, a countermeasure in which only the composition ratios of a thermosetting resin and graphite powder are considered cannot satisfy both the requirements on fluidity and moldability of a compound and the strength of a molded member (separator), and the contact resistance, as described above. Development of a separator for a fuel cell which is excellent in moldability and strength, and which can be stably maintained to a low contact resistance of 10 m&OHgr;·cm
2
or lower is strongly requested. At present, however, there exists no separator which can satisfy the noted desirability.
SUMMARY OF THE INVENTION
The present invention has been conducted in order to satisfy the noted desirability. It is an object of the invention to provide a separator for a fuel cell which is excellent in fluidity and moldability, and in which, while ensuring strength sufficient for preventing the separator from suffering damage such as a breakage due to vibrations or the like, the contact resistance can be set to a value lower than a requested value, and the low contact resistance can be stably maintained.
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 of low fluidity is used, a separator which has a uniform and correct shape, and in which a low contact resistance can be stably maintained 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 60 to 90 wt. %, a composition ratio of the thermosetting resin is set to 10 to 40 wt. %, and an average particle diameter of the graphite powder is set to a range of 15 to 125 &mgr;m.
In the complex, preferably, the composition ratio of the graphite powder is set to 70 to 87 wt. %, and the composition ratio of the thermosetting resin is set to 13 to 30 wt. %. Preferably, the average particle diameter of the graphite powder is set to a range of 40 to 100 &mgr;m.
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 have been conducted, and finally found that the contact resistance serving as the primary factor which largely affects the performance of a fuel cell is determined not only by the composition ratios of a resin and graphite powder, the average diameter of the graphite powder closely affects the performance at the highest degree, the contact resistance is largely varied depending on the size of the average diameter, and the average diameter of the graphite powder is closely related also to fluidity, moldability, and strength of the compound. Based on this finding, the composition ratios of a resin and graphite powder, and the average diameter of th

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