Polybenzimidazoles containing heterocyclic benzo[c]cinnoline structure prepared by sol-gel process and acid doping level adjustment for high temperature PEMFC application

Abstract Polybenzimidazoles containing heterocyclic benzo[c]cinnoline structure are synthesized from 3,8-benzo[c]cinnoline dicarboxylic acid, terephthalic acid and 3,3′-diaminobenzidine. Their membranes are prepared by sol-gel process, involving the conversion of polymer solution in polyphosphoric acid to phosphoric acid. The acid doping levels of the as-prepared membranes increase as the contents of benzo[c]cinnoline increase, indicating good interaction between phosphoric acid and benzo[c]cinnoline structure. The as-prepared membranes with high acid doping levels might lead to the dissolution of membranes in phosphoric acid at temperature higher than 120 °C. A new method is proposed to adjust acid doping levels by immersing the as-prepared membranes in diluted phosphoric acid solutions of various concentrations. The adjusted membranes (acid doping levels around 30 PA RU−1) exhibit enhanced mechanical properties with tensile strength in the range of 4.1–5.2 MPa. The proton conductivity of adjusted membranes maintain at 0.15–0.17 S cm−1 at 160 °C under ambient atmosphere without humidification. The single cells based on the adjusted membranes exhibit open circuit voltages and peak power densities from 0.89 to 0.91 V and 691–1253 mW cm−2 at 160 °C, respectively. Compared to other polybenzimidazole membranes prepared by sol-gel process, the adjusted polybenzimidazoles show higher mechanical strength and better single cell performance.