In recent years, fuel cells have been attracting attention as a next generation energy source. In particular, polymer electrolyte fuel cells (PEFCs) using a proton conducting polymer membrane as the electrolyte have high energy density, and are expected to find a wide range of applications, such as in home cogeneration systems, power sources for mobile devices, and power sources for automobiles. The electrolyte membrane of a PEFC is required to serve not only as an electrolyte for conducting protons between the fuel electrode and the oxidant electrode but also as a partition between a fuel supplied to the fuel electrode and an oxidant supplied to the oxidant electrode. If these functions as the electrolyte and the partition are insufficient, the power generation efficiency of the fuel cell decreases. For this reason, polymer electrolyte membranes are required to have high proton conductivity, electrochemical stability, and mechanical strength, and low permeability to fuels and oxidants.
Currently, perfluorocarbon sulfonic acid having a sulfonic acid group as a proton conductive group (for example, Nafion (registered trademark) manufactured by DuPont) is widely used for electrolyte membranes in PEFCs. Although perfluorocarbon sulfonic acid membranes exhibit excellent electrochemical stability, they are very expensive because fluororesin as a source material is not a general-purpose product and also its synthesis process is complicated. A high cost of such electrolyte membranes is a major obstacle in the practical application of PEFCs. Moreover, perfluorocarbon sulfonic acid membranes allow methanol to permeate easily, so it is difficult to use such a perfluorocarbon sulfonic acid membrane as the electrolyte membrane for a direct methanol fuel cell (DMFC) in which a methanol-containing solution is supplied to the fuel electrode.
Accordingly, as an alternative to such perfluorocarbon sulfonic acid membranes, inexpensive hydrocarbon-based electrolyte membranes with reduced methanol crossover have been developed. For example, Patent Literatures 1 to 4 propose electrolyte membranes made of sulfonated poly(ether ether ketone), sulfonated poly(ether sulfone), sulfonated polysulfone, and sulfonated polyimide, respectively. The resins used as the source materials of these hydrocarbon-based electrolyte membranes are less expensive than fluororesin, so the use of these electrolyte membranes can reduce the cost of PEFCs. However, the performance of the hydrocarbon-based electrolyte membranes proposed in Patent Literatures 1 to 4 is not necessarily high enough, and PEFCs using hydrocarbon-based electrolyte membranes have not yet been put into practical use.