Fuel cell separator and method of manufacture

#The present invention relates to a fuel cell separator which requires no machining whatsoever after it has been molded, which has a significantly improved electrical conductivity, heat resistance and mechanical strength, and which is not subject to breakage during molding and cell assembly even when the thin areas therein have a thickness of only 0.2 to 0.5 mm. The invention also relates to a method of manufacturing such fuel cell separators.

#Fuel cells are devices which convert chemical energy directly into electrical energy by placing a pair of electrodes in mutual contact through an intervening electrolyte, feeding a fuel to one of the electrodes and an oxidant to the other electrode, and carrying out oxidation of the fuel electrochemically within the cell. There are several types of fuel cells, depending on the electrolyte used. Solid polymer fuel cells in which the electrolyte is a solid polymer electrolyte membrane have attracted considerable attention recently for their ability to achieve a high energy output.

#As shown in FIG. 1, such solid polymer fuel cells are composed of a stack of from several tens to several hundreds of unit cells. Each unit cell has a pair of fuel cell separators 1, 1 with a plurality of ribs 1a on either side thereof. Between the separators 1 are disposed a solid polymer electrolyte membrane 2 and a pair of gas diffusing electrodes (a fuel electrode and an oxidant electrode) 3, 3.

#FIG. 1 is a perspective view showing an example of a fuel cell.

#As noted above, the fuel cell separator of the invention is formed by molding a resin composition which includes 65 to 90 wt % of graphite, 10 to 35 wt % of a thermosetting resin and 0.1 to 2 wt % of an internal release agent.

#The graphite used herein is a synthetic graphite powder (bulk graphite) prepared by subjecting lump coke to a high degree of graphitization. Such synthetic graphite prepared from lump coke does not readily orient during molding of the resin composition, which is conducive to achieving good strength. Use of this type of synthetic graphite makes it possible to obtain a fuel cell separator endowed with excellent mechanical strength, good electrical conductivity in the thickness direction, and minimal variation in the resistance values.

#If the above graphite powder has too small an average particle size, the resin component is adsorbed thereon, which may lower the flow properties of the overall composition. On the other hand, an average particle size which is too large may lower the strength of the separator obtained therefrom. Hence, it is desirable for the graphite powder to have an average particle size of 10 to 200 μm, preferably 30 to 200 μm, and most preferably 30 to 100 μm.