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:H2→2H−+2e−  (1)(1/2)O2+2H−+2e−→H2O  (2)occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:H2+(1/2)O2→H2O  (3)proceeds. The chemical energy of the fuel is directly converted into an electrical energy, with the result that the cell can exert predetermined cell performance.
A fuel cell separator of the solid polymer electrolyte type or the phosphoric acid type in which such energy conversion is conducted is requested to be gas-impermeable, and also to be made of a material of high electrical conductivity in order to improve the energy conversion efficiency. 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 technique is conventionally employed in which a fuel cell separator is produced by loading the bondcarbon compound into a mold, and resin-molding into a predetermined shape in which ribs for forming fuel gas passages, oxidant gas passages, or coolant water passages are formed integrally on at least one face of a separator molded member.
In such a fuel cell separator which is resin-molded into the predetermined shape by using a bondcarbon compound, when the thermosetting resin is softened by heating during the resin molding process, part of the thermosetting resin oozes to the surface layer to form a thin resin layer on the surface of the separator molded member. The thin resin layer is naturally formed also on the surfaces of the ribs for forming passages and functioning as a contact surface with an electrode in a product (separator).
The thin resin layer which is formed on the surface of the separator molded member in this way is an electrical insulating layer, and does not exhibit conductivity. As a whole of the separator, therefore, the conductivity is lowered, and the specific resistance is increased. Moreover, also the contact resistance with an electrode is increased by the presence of the thin resin layer which is formed on the top surfaces of the ribs. The contact resistance with an electrode which is increased by the formation of the thin resin layer is larger by one digit than the specific resistance of the whole separator which is similarly increased by the formation of the thin resin layer. The increase of the contact resistance more strongly affects the internal resistance of the fuel cell which is the sum of the specific resistance and the contact resistance. In order to improve the performance and efficiency of the fuel cell, therefore, it is requested to reduce the contact resistance of the top surfaces of the ribs with an electrode, as much as possible.
As means for satisfying such a request, conventionally, the following means have been proposed. For example, Japanese Patent Application Laying-Open No. 11-204120 discloses means for polishing or grinding away the surfaces of ribs to physically remove a thin resin layer, and Japanese Patent Application Laying-Open No. 11-297338 discloses means for immersing a separator in, for example, a strongly acidic solution into which one or two or more of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and the like are mixed, to acid-treat the surface, whereby the surface roughness of the surfaces of the ribs is adjusted to Ra=0.1 μm to 10 μm so as to reduce the contact resistance.
In the case of the former one of the means which have been conventionally proposed, i.e., the resin layer physically removing means based on polish or grinding removal of the surfaces of the ribs, it is technically very difficult to remove only the thin resin layer, and hence the contact resistance cannot be sufficiently lowered. This will be described in detail. As shown in a diagrammatic section view of FIG. 11, when the top surfaces of ribs 51 are polished or ground to remove a thin resin layer, also graphite particles 52 which contribute to the conductivity are removed away together to reduce the amount of graphite particles in the surfaces of the ribs 51. Moreover, a contact surface 51a with an electrode is formed by: recesses 53 which are traces of removed graphite particles; a resin portion 54; and exposed graphite particles 52. As a result, the graphite density of the contact surface 51a with an electrode is lowering, and the contact surface 51a with an electrode is formed as a rough face which is greatly uneven, to reduce the contact area with an electrode. Therefore, the contact resistance is not sufficiently lowered although the thin resin layer is removed away.
In the case of the latter means, i.e., the means for immersing a separator in an acidic solution to acid-treat the surface, the acidic solution erodes even the inside of graphite particles to form graphite oxide, and, when the graphite oxide is formed, free electrons join the reaction. Therefore, the conductivity inherent in graphite particles is impaired, so that the specific resistance of the whole separator is increased and the contact resistance with an electrode cannot be sufficiently lowered. This will be described in detail. As shown in a diagrammatic section view of FIG. 12, the thin resin layer on the top surfaces of the ribs 51 can be removed away by the erosion function of the acidic solution to expose the flat graphite particles 52 from the surface. In the case of acid-resistant phenol resin, however, the selective removal of the resin by the acidic solution does not advance, and both the graphite particles 52 which are exposed from the surfaces of the ribs 51 by the erosion function of the acidic solution, and the resin portions 54 between adjacent particles are removed away only by approximately equal amounts. The contact surface 51a with an electrode of low surface roughness is formed by the graphite particles 52 and the resin portions 54, and no gap is formed between adjacent exposed graphite particles 52, so that the areas between adjacent graphite particles 52 remain to be filled with the resin portions 54. Even when an electrode 55 is strongly pressed against the contact surface 51a by a fastening force for forming a stack configuration, therefore, the graphite particles 52 cannot be deformed. Depending on the status of the erosion by the acidic solution, flat faces 52a of the graphite particles 52 sometimes remain to be inclined with respect to the contact face of the electrode 55. Even when the thin resin layer can be removed away by the acid treatment, therefore, the contact area with the electrode 55 cannot be enlarged, and the adaptability is poor. Consequently, there is a problem in that, although the treatment requires a very sophisticated technique of adjusting the surface roughness to the above-mentioned specific range and much labor, the contact resistance with an electrode cannot be sufficiently lowered.