Preparation of proton exchange membranes based on crosslinked polytetrafluoroethylene for fuel cell applications

Yamaki, Tetsuya; Kobayashi, Kazuhiro; Asano, Masaharu; Kubota, Hitoshi*; Yoshida, Masaru

We prepared proton exchange membranes by the -ray-induced post grafting of styrene into crosslinked polytetrafluoroethylene (PTFE) films and subsequent sulfonation. The degree of grafting was controlled in the range of 7-75% by the crosslinking density of the PTFE matrix as well as the grafting conditions. Under our preparation conditions, the films at the grafting yield of 30% were found to produce ion exchange membranes with a homogeneous distribution of sulfonic acid groups. The resulting membranes showed a large ion exchange capacity up to 2.9 meq g, which exceeded the performance of commercially-available perfluorosulfonic acid films such as Nafion; nevertheless, they appeared to be dimensionally stable in water. These should undoubtedly result from the use of the crosslinked PTFE films as graft substrates and make our ion exchange membranes promising for applications to polymer electrolyte fuel cells.