Epoxides cross-linked hexafluoropropylidene polybenzimidazole membranes for application as high temperature proton exchange membranes

Covalently cross-linked hexafluoropropylidene polybenzimidazole (F6PBI) was prepared and used to fabricate high temperature proton exchange membranes with enhanced mechanical strength against thermoplastic distortion. Three different epoxides, i.e. bisphenol A diglycidyl ether (R1), bisphenol A propoxylate diglycidyl ether (R2) and poly(ethylene glycol) diglycidyl ether (R3), were chosen as the cross-linkers to investigate the influence of their structures on the properties of the cross-linked F6PBI membranes. All the cross-linked F6PBI membranes displayed excellent stability towards the radical oxidation. Comparing with the pure F6PBI membrane, the cross-linked F6PBI membranes showed high acid doping level but less swelling after doping phosphoric acid at elevated temperatures. The mechanical strength at 130 °C was improved from 0.4 MPa for F6PBI membrane to a range of 0.8–2.0 MPa for the cross-linked F6PBI membranes with an acid doping level as high as around 14, especially for that crosslinking with the epoxide (R3), which has a long linear structure of alkyl ether. The proton conductivity of the cross-linked membranes was increased accordingly due to the high acid doping levels. Fuel cell tests demonstrated the technical feasibility of the acid doped cross-linked F6PBI membranes for high temperature proton exchange membrane fuel cells.