Polymer Electrolyte Membranes for Fuel Cells by Radiation Induced Grafting: State of the Art at Paul Scherrer Institut

Radiation induced grafting is a well established method for the preparation of polymer electrolyte membranes for fuel cells. This method allows the use of a wide variety of base films and monomers which may be tailored to the desired end-use. Since the method can be performed with low-cost starting materials, it offers the promise of cost-competitive membranes for the polymer electrolyte fuel cell (PEFC). Paul Scherrer Institut has been committed to developing fuel cell membranes by radiation grafting since 1992. Styrene based membranes using poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP) and poly (ethylene-alt-tetrafluoroethylene) (ETFE) films as base material have been prepared by radiation grafting followed by sulfonation. Grafting parameters such as irradiation dose, reaction medium, temperature and time have been investigated in detail. The resulting grafted films and membranes have been characterized ex-situ for their thermal behaviour, composition, microstructure, fuel cell relevant properties, and membranes have been characterized in-situ and tested in low temperature PEFC. In addition, the use of α-methylstyrene (AMS), a substituted styrene monomer with protected α-position, for the preparation of alternative membranes of higher chemical stability has been investigated. The parameters of grafting were identified to have significant effect on the degree of grafting and were subsequently optimized [2]. The preparation steps and crosslinking affected thermal properties of the films. Small angle neutron scattering experiments revealed that the overall domain structure made up of crystalline and amorphous regimes of pristine polymer film is preserved. FTIR /ATR measurements indicated that radiation grafted films were more highly crosslinked in their near surface regions. The fuel cell performance of FEP based membranes was comparable to commercially available Nafion112 membranes of similar thickness and durability of several thousand hours. ETFE based membranes exhibited encouraging fuel cell performance and there is room for improvement through optimization of that type of membranes. Replacing styrene with AMS resulted in longer fuel cell lifetime as well as a lower rate of degradation