Long Alkylene Spacers Promote Structural Ordering and Proton Transport in Phosphonic Acid–Based Polymer Electrolyte Membranes

Next-generation polymer electrolyte fuel cells (PEFCs) require polymer electrolyte membranes (PEMs) capable of operating at temperatures above the boiling point of water (100 °C) and under low-humidity below 40% RH. In this study, we synthesized poly-(8-(<i>p</i>-styryl)-1-octanephosphonic acid) (soPA), a polymer bearing phosphonic acid groups on the side chains connected via eight-carbon alkylene spacers. soPA was insoluble in water and formed a highly oriented lamellar phase-separated nanostructure with a 2.9 nm domain spacing, consisting of a hydrophobic phase formed by the alkylene spacers and the polystyrene backbone, and a hydrophilic phase containing the phosphonic acid groups. Despite its lower acid group density compared with poly-(4-(<i>p</i>-styryl)-1-butanephosphonic acid) (sbPA) with shorter four-carbon spacers, the soPA membrane exhibited higher conductivities than sbPA. For instance, soPA achieved conductivities of 4.4 and 7.5 mS cm^-1 at 120 °C under 20% and 40% RH, respectively, values which are approximately 4 and 2.9 times higher than those of sbPA. These enhanced conductivities of soPA can be attributed to the higher morphological and molecular ordering induced by the nanophase separation as well as the greater freedom of motion of the phosphonic acid groups provided by the flexibility of the longer alkylene spacers, which facilitate more efficient proton transfer between them.