Polymer Electrolyte Membranes of Polystyrene with Directly Bonded Alkylenephosphonate Groups on the Side Chains

There is a pressing need for polymer electrolyte membranes (PEMs) for next-generation polymer electrolyte fuel cells that exhibit good conductivities even at temperatures >100 °C (the boiling point of water) under low humidity. Disadvantages of conventional PEMs made of sulfonic acid polymers generally include low conductivities and easy degradation at temperatures >100 °C under low humidity because of decomposition of the sulfonic acid groups. In this study, diethyl 4-(p-styryl)-1-butanephosphonate and poly(4-(p-styryl)-1-butanephosphonic acid) (sbPA) with phosphonic acid groups on the side chains through a hydrophobic alkylene spacer were synthesized by polymerization of the monomer and deprotection of the phosphonate esters. The sbPA membrane formed a phase-separated structure with a domain spacing of ∼2.5 nm between the hydrophobic part, consisting of phenylene groups and alkylene spacers in the polymer side chains, and the hydrophilic part of the phosphonic acid groups. The sbPA membrane did not dissolve even when immersed in water at 60 °C for 3 h. Despite a lower acid group density than the poly(p-styrenephosphonic acid) (sPA) membrane as a control sample, in which phosphonic acid groups were directly and covalently bonded to the polystyrene backbone, the sbPA membrane exhibited higher conductivities at 80 and 120 °C, especially under low humidity. For example, the sbPA membrane showed a moderate conductivity of 1.1 mS cm–1 at 120 °C and 20% relative humidity, approximately 2 orders of magnitude higher than the conductivity of the sPA membrane. The superior conductivity of the sbPA membrane could be explained by the ease of proton transport between the phosphonic acid groups even under low humidity due to the result of a high freedom of movement made possible by their connection to the polymer backbone directly through the alkylene spacer.