Abstract
Novel alkaline anion-exchange membranes (AAEMs) were successfully synthesised via radiation grafting of electron beamed films of poly(ethylene-co-tetrafluoroethylene), EB-ETFE. The resultant membranes were characterised, compared with previously developed AAEMs and evaluated in H2/O2 fuel cells. Limited statistically rigorous prior research, into the synthesis of previously developed (standard) trimethylamine (TMA) quaternised membranes, prompted an empirical investigation into the effects that the numerous synthesis parameters (variables) had on the final properties of the resultant AAEMs. Importantly, it was shown that EB-ETFE can be cold stored for at least 16 months at -36±2°C and still produce AAEMs exhibiting fuel cell relevant ion-exchange capacities of 1.0 - 1.8 mmol g-1 and ionic conductivities (through plane, fully hydrated) of 20 - 40 mS cm-1 at ambient temperature. In addition, quaternary head-groups that are alternative to the previously studied benzyltrimethylammonium were evaluated. This required the development of a new titration method for the determination of the chemical composition of the AAEMs (quaternary ammonium and tertiary amine contents). The alternative synthesis was achieved by replacement of the TMA quaternisation agent with the diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). A 15 week investigation into the ex situ chemical stability of the novel AAEMs was conducted; stable ion-exchange capacities of 1.3±0.3 mmol g-1 were observed for both TMA and DABCO quaternised AAEMs when submerged at 60°C in water. The TMA and DABCO quaternised AAEMs (of similar hydrated thickness) exhibited hydrogen fuel cell performances of 120 mW cm-2 with the use of a previously developed alkaline ionomer (designated SION1). The use of a novel DABCO-based ionomer (SION1.1) yielded improved performances of 200 mW cm-2 and decreased the membrane electrode assembly (MEA) internal ohmic resistances. This demonstrates the necessity of further alkaline ionomer optimisation and development. Through a greater understanding the amine head-group chemistry, the membrane synthesis process, and the ionomer interface, the economic outlay associated with AAEM synthesis may be reduced and the subsequent MEA performance and operational lifetime within a hydrogen fuel cell may be improved.