Phosphonic acid grafted polybenzimidazoles containing pyridine for stable high-temperature proton exchange membrane fuel cells

Abstract

The phosphoric acid (PA) leakage problem has been a major constraint in the practical utilization of PA-doped polybenzimidazole for high-temperature proton exchange membranes (HT-PEMs). Increasing the retaining ability of the proton exchange membranes for free PA while decreasing the amount of PA doping seems to be a viable approach to reduce the possibility of PA leakage. In this work, the mild ester exchange reaction was introduced for the first time to prepare phosphonic acid grafted polybenzimidazoles containing pyridine (g-p-X). The phosphonic acid groups act as transport sites for proton conduction, effectively reducing the need for free PA in g-p-X membranes. Meanwhile, the pyridine immobilizes a portion of the PA through strong ionic interactions, and the membranes exhibit a high PA retention capacity. Among them, the g-p-20 membrane reaches 84.6% PA retention after the 120 h test. Thanks to the dual effects of the phosphonic acid groups and pyridine, the g-p-20 membrane shows a proton conductivity of 42 mS cm⁻¹ at 180 °C without humidification. Moreover, with only 175.3 wt% PA doping, its highest single cell power density achieves 951 mW cm⁻² (180 °C). Notably, after 120 h of steady operation at 140 °C, the voltage decay rate of the g-p-20 is only 10 μV h⁻¹. This work presents a viable concept for addressing the problem of PA leakage from HT-PEMs in long-term operation.