Proton conducting polymer membrane, method for production thereof and fuel cell therewith

#This is a U.S. national phase application of PCT/JP2006/310858 filed 31 May 2006, which claims priority of Japanese Patent Applications No. 2005-163962 and No. 2005-163982, both filed 3 Jun. 2005, the contents of which are incorporated herein by reference.

#The present invention relates to a proton-conducting polymer membrane that includes an acidic group-containing polybenzimidazole compound and an acidic compound as components and is a composition useful as a polymer electrolyte membrane operable under high-temperature, non-humidified conditions and also relates to a method for production thereof and a fuel cell therewith.

#In recent years, solid polymer-type fuel cell membranes are attracting attention for new energy sources, and polymer membranes for use as cation exchange membranes in fuel cells are being developed. Such polymer membranes have to show good proton conductivity and to be sufficiently stable chemically, thermally, electrochemically, and mechanically. Therefore, perfluorocarbonsulfonic acid membranes, typified by Nafion® manufactured by DuPont (U.S.A.), have been generally used as long-life practical polymer membranes. In the case that perfluorocarbonsulfonic acid membranes are used, however, there is a problem in which the water content of the polymer membranes can rapidly decrease under fuel cell operation conditions at a temperature higher than 100° C., and the softening of the membranes can be significant, so that fuel cells cannot deliver sufficient performance.

#FIG. 1 is a graph showing the temperature dependence of the ion conductivity in Example 1 and Comparative Example 1.

#FIG. 2 is a graph showing the relationship between the current density and the cell voltage at the initial stage of the measurement in Example 1 and Comparative Example 1.

#FIG. 3 is a graph showing the relationship between the open-circuit voltage and the cell voltage at a current density of 0.3 A/cm2 and the time of operation of a fuel cell in Example 1 and Comparative Example 1.

#In the proton-conducting polymer membrane of the present invention, a polymer membrane including a polybenzimidazole compound having a sulfonic acid group and/or a phosphonic acid group as an acidic group or groups is impregnated with vinylphosphonic acid. In the present invention, the sulfonic acid group includes both a group present in the form of free sulfonic acid and a group present in the form of a sulfonate salt, unless otherwise stated, and the phosphonic acid group includes both a group present in the form of free phosphonic acid and a group present in the form of a phosphonate salt, unless otherwise stated. In the present invention, the polybenzimidazole compound refers to a polymer compound containing a benzimidazole ring in a component unit forming the polymer chain.

#In general, polybenzimidazole is synthesized by a polymerization reaction between two types of monomers: an aromatic tetramine or a derivative thereof and a dicarboxylic acid or a derivative thereof. Alternatively, polybenzimidazole may be synthesized by self-condensation of a compound having two amino groups or derivatives thereof and one carboxyl group in a single molecule. Polybenzimidazole may also be synthesized from a mixed system thereof. In the present invention, while there is no particular restriction on the structure of the polybenzimidazole compound having a sulfonic acid group and/or a phosphonic acid group, it is preferably synthesized using a dicarboxylic acid monomer containing a sulfonic acid group or a derivative thereof and/or a phosphonic acid group or a derivative thereof in the polymerization reaction thereof. This is because the sulfonic acid group or the phosphonic acid group may be introduced through a side chain attached to a nitrogen atom on an imidazole ring, but the polymer produced through such an introduction generally tends to have reduced solvent resistance. The sulfonic acid group may also be introduced onto a benzimidazole ring in a polybenzimidazole polymer by a sulfonation reaction or the like. In this case, however, such a polymer tends to have reduced heat resistance.

#Specifically, the polybenzimidazole compound for use in the present invention having a sulfonic acid group and/or a phosphonic acid group preferably includes a component represented by Structural Formula (1):