Preparation and molecular simulation of grafted polybenzimidazoles containing benzimidazole type side pendant as high-temperature proton exchange membranes

Abstract

Phosphoric acid-doped polybenzimidazole membranes have exhibited great potential in high-temperature proton exchange membrane applications, but proton conductivity and stability should be further improved for high performance fuel cells. Herein, a series of grafted polybenzimidazoles containing benzimidazole type side pendant were successfully prepared by facile N-substitution reaction without any catalyst. These grafted polybenzimidazole membranes with side chains of benzimidazole group exhibit excellent properties in the doping level of phosphoric acid, retention stability of phosphoric acid and proton conductivity under certain mechanical properties, oxidation resistance and thermal stability. Importantly, the phosphoric acid (PA) retention stability and proton conductivity can be preciously controlled by adjusting the length of side chain and the number of benzimidazole groups. With the acid doping level of 19.3, the grafted membrane exhibits a high conductivity of 101 mS cm−1 and a great power density of 305 mW cm−2 at 160 °C without humidification. The molecular dynamics simulation show that the improved performance of the grafted membrane is mainly due to the increase in the free volume of the polymer membrane, the enhancement of the binding energy and the densification of the hydrogen bond network structure between the polymer membrane and phosphoric acid.