Synthesis and crosslinking of hydroxyl-functionalized sulfonated poly(ether ether ketone) copolymer as candidates for proton exchange membranes

Partially hydroxyl-functionalized sulfonated poly(ether ether ketone) (SPEEK) was synthesized and further reacted with tetrafunctional MY721 epoxy resin to produce crosslinked networks. The crosslinked membranes were characterized in terms of gel fraction, solubility, ion exchange capacity (IEC), water uptake, swelling ratio in aqueous methanol solution, proton conductivity and methanol permeability coefficient. The result shows that all the crosslinked membranes exhibited lower water uptake and methanol swelling ratio compared with pristine SPEEK. Moreover, the methanol permeability coefficient dramatically decreased with the increase of epoxy resin content. The membrane crosslinked with 1 equivalent of epoxy group per hydroxyl group on SPEEK showed 2.43 times lower methanol crossover than pristine SPEEK. Despite the fact that the proton conductivity of crosslinked membranes was decreased to some extent as a result of the introduction of epoxy resin and crosslinking effect, the membrane crosslinked with 1 equivalent of epoxy group still showed a proton conductivity of 0.096 S/cm at 30 °C which could actually meet the requirement for proton exchange membrane fuel cell (PEMFC) applications. Furthermore, the membranes crosslinked with 1–2 equivalents of epoxy resin showed better selectivity than pristine SPEEK and Nafion117 taking into consideration the methanol crossover and proton conductivity comprehensively.