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 degrees 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 H-2/O-2 at both 100% and 50% RH, and outperforming it by as much as 18% in maximum power density under H-2/Air at 100% RH.

We systematically investigated the effect of incorporating a sterically hindered pyridyl group into a sulfo-phenylated polyphenylene to control the polymer’s physicochemical properties through acid–base interactions. Homopolymers with similar molecular weights and comparable structures that vary by only one atom (N– vs C−) per repeat unit along the polymer chain were prepared. Compared to a non-pyridyl reference membrane, incorporation of a pyridyl group improves the oxidative stability against free radicals, increases the elongation at break to 55% (from 37%), and enhances the thermal stability to 326 °C (from 246 °C). In an accelerated fuel cell degradation test, polymeric membranes containing the sterically encumbered pyridyl unit exhibited exceptional stability (0.16 mV h–1 degradation rate over 1000 h) and retained ∼80% of their peak power density over this time.

A 24-fold reduction in reaction time is achieved in the preparation of sulfonated poly(polyphenylene)s using microwave synthesis.

A sulfonated poly(arylene ether sulfone) multiblock copolymer bearing perfluorosulfonic acid side chains has been studied as both the membrane and cathode catalyst layer ionomer in fuel cell membrane-electrode assemblies. The multi-block, hydrocarbon-backbone polymer was designed to possess good conductivity with low ion exchange capacity and low water sorption to mimic perfluorosulfonic acid ionomers, but without the synthetic difficulty. A H-2/O-2-fuel cell power density of 1080 mW . cm(-2) was achieved at 80 degrees C, 100% RH using membrane-electrode assemblies incorporating the sulfonated poly(arylene ether sulfone) membrane and traditional PFSA ionomer in the catalyst layer. An accelerated stress test using 30% RH, 90 degrees C, H-2/Air demonstrated durability to open circuit potentials of more than 400 h, which is 4 times longer than for Nafion 211. No membrane thinning was observed over the same period, which is attributed to low gas crossover and inhibition of membrane degradation reactions, despite the rate of water transport across the membrane remaining high. Cathode catalyst layers fabricated using 20 wt% sulfonated poly(arylene ether sulfone) as the ionomer exhibited comparable power densities to benchmark PFSA MEAs, under high current densities, over a wide range of operational relative humidities, demonstrating that hydrocarbon-backbone, solid polymer electrolytes hold technological promise as both the membrane and catalyst layer ionomer in fuel cell membrane-electrode-assemblies. (C) 2018 The Electrochemical Society.