Molecular branching as a simple approach to improving polymer electrolyte membranes

Abstract

A series of branched, sulfonated, phenylated poly(phenylene)s were synthesized by introduction of varying molar ratios of trifunctional monomer into the polymerization mixture. The branched polymers, containing between 0.25 and 2.00 mol% molecular branching, were cast into membranes and comprehensively characterized as polymer electrolyte membranes. Comparison to a linear, unbranched polymer analogue showed universally improved membrane properties as a result of branching. Branched membranes possessed greater tensile strength in the dry state and minor reductions in elongation at break. In the wet state, branched membranes showed improvements in both tensile strength and elongation at break, up to 47 and 43%, respectively. Water sorption decreased with increasing branching content, from 119% water uptake and 145% dimensional swelling, to as low as 45 and 61%, respectively. Notable increases in both thermal and chemical stability were observed. When assessed electrochemically, these trends were further highlighted: ex-situ proton conductivity showed a stepwise increase in membrane performance with increasing degrees of branching, up to 212 mS cm−1 at 80 °C and 95% RH. Finally, in-situ characterizations of membranes integrated into hydrogen fuel cells showed state-of-the-art hydrocarbon membrane performance, comparing favorably to the archetypal Nafion 211 under H2/O2 at both 100% and 50% RH, and outperforming it by as much as 18% in maximum power density under H2/Air at 100% RH.