Radiation grafted fuel cell membranes based on co-grafting of α-methylstyrene and methacrylonitrile into a fluoropolymer base film

Proton exchange membranes based on poly(styrenesulfonic acid) obtained through radiation grafting show susceptibility towards oxidative degradation under fuel cell operating conditions. An alternative grafting monomer with higher intrinsic chemical stability is α-methylstyrene (AMS). As AMS by itself does not readily polymerize by the radical mechanism, it is co-grafted with methacrylonitrile (MAN). Proton exchange sites are subsequently introduced by sulfonation of the AMS units. The copolymerization kinetics were found to be similar to solution copolymerization, with a high tendency of AMS and MAN to form an alternating copolymer chain. The most favorable grafting rates and highest ratio of incorporated AMS:MAN units are obtained in a mixture of isopropanol and water as solvent. The ion exchange capacity (IEC) of the resulting membranes can be tuned via the degree of grafting. Samples tested in single cells had an IEC of ∼1.3 mmol g−1 and a proton conductivity between 50 and 100 mS cm−1. The durability of AMS-based membranes in the fuel cell at a temperature of 80 °C is around 10 times that of a styrene-based membrane. Further improvement is obtained through crosslinking of the graft copolymer structure. Locally resolved post mortem analysis indicates that polymer degradation is more pronounced in areas near the hydrogen inlet of the fuel cell.