Sulfonated branched poly(arylene ether ketone sulfone) proton exchange membranes: Effects of degree of branching and ion exchange capacity

Sulfonated branched polymers exhibit substantial potential for application as proton exchange membranes (PEMs). Most studies develop branched polymer PEMs by regulating the degree of branching (DB), without considering another important factor, i.e., the ion exchange capacity (IEC). Herein, we report on a systematical comparison regarding the effects of DB and IEC on the branched polymer PEMs and a solution to advance the properties of PEMs. To accomplish this, two series of branched poly(arylene ether ketone sulfone)s containing densely sulfonated tetraarylmethane units were synthesized. The DB and IEC were controlled in the range of 5-10 % and 1.41-1.94 meq g-1, respectively. It was found that both increasing DB and IEC of membranes promoted water absorption, proton conductivity, single-cell performance, denser distribution of microscopic hydrophilic phase, and also reduced the mechanical property. Furthermore, the strengthening via increased DB maintained dimensional stability and improved oxidative stability, while the impact of IEC was the opposite. Ultimately, a polymer membrane bearing optimal DB (7.5 %) and IEC (1.94 meq g-1) exhibited the highest proton conductivity (134.5 mS cm-1) and maximum power density (426.8 mW cm-2), satisfactory dimensional change (18.8 % in-plane, 27.2 % through-plane, and 79.5 % volume) at 80 celcius, and sufficient thermal, oxidative, and mechanical stability. This work is expected to serve as a guide for designing and synthesizing the DB and IEC of sulfonated branched polymer PEMs.