Improving the Ohmic polarization of high temperature proton exchange membrane fuel cells using crosslinked polybenzimidazole membranes containing acidophilic quaternary ammonium groups synthesized by one-step strategy

Abstract

Ingenious crosslinked network structure in phosphoric acid doped polybenzimidazole membranes can mitigate the mutual restriction of proton conductivity and mechanical properties. However, the complicated synthesis of tailored macromolecular crosslinker and the time-consuming post-treatment hinder their practical application as high temperature proton exchange membranes. Herein, crosslinked polybenzimidazole membranes are synthesized using small molecular crosslinker containing acidophilic quaternary ammonium groups through a one-step crosslinking strategy. After doping with phosphoric acid, the quaternary ammonium-biphosphate ion-pair coordination and the crosslinked structure result in the improved anhydrous proton conductivity, oxidation stability, and mechanical strength of the formed membranes compared to sample without crosslinking structure. Membrane with the optimized degree of crosslinking exhibits an anhydrous conductivity of 72.27 mS cm^-1 at 160 °C with a tensile strength of 12.14 MPa. Benefiting from the crosslinked structure and high proton conductivity, the accordingly formed membrane electrode assembly possesses a high open circuit voltage of 1.01 V and the improved Ohmic polarization, delivering a peak power density of 0.66 W cm^-2 using hydrogen as fuel and air as oxidant.