Polysulfonation of PBI-based membranes for HT-PEMFCs: a possible way to maintain high proton transport at a low H3PO4 doping level

Polybenzimidazoles are promising materials to replace Nafion™ as the electrolyte in HT-PEMFCs. One of their problems is striking the proper balance between the H3PO4 doping level, which controls the proton conductivity, and the long-term stability properties of the membrane. Monomer modification is a promising way to maintain high conductivity levels with reduced doping. Here, we reported a novel and facile approach to obtaining an easy modular and reproducible sulfonation degree. Some aryloxy-based polybenzimidazoles were synthesized and sulfonated with different amounts of -SO3H. We prepared many electrolyte membranes by doping the pristine polymers in solutions with different H3PO4 concentrations. The sulfonation degree greatly affected both acid uptake and conductivity. In particular, the membranes holding more protogenic groups absorbed less acid than the monosulfonated ones. However, polysulfonation was particularly efficient in improving proton conductivity at low relative humidity and doping level. We performed MEAs tests at 150 °C using H2 and air as the reactant gases, without any external humidification. We obtained power densities higher than 320 mW cm-2, with fuel cell performances of approximately 580 mV at 0.2 A cm-2, independent of the number of sulfonic groups. Preliminary durability tests did not show any membrane degradation over a 190 hour period. The reported membranes are therefore suitable for use in HT-PEMFCs.