Side-chain vertical imidazole backbone enhances phosphoric acid uptake in Poly(2,5-benzimidazole) membranes for high-temperature PEMFCs

Abstract

PA-doped polybenzimidazole membranes dominate high-temperature proton exchange membranes (HT-PEMs), but the current membranes have poor PA uptake ability, which directly affects the operational efficiency and stability of fuel cells. In this work, it is proposed that a series of grafted polybenzimidazole membranes containing benzimidazole side chains were successfully prepared by grafting 2-chloromethyl benzimidazole to the backbone of poly (2,5-benzimidazole) (ABPBI) via nucleophilic substitution reaction. All these membranes exhibited excellent mechanical properties, oxidative stability, and thermal stability. The introduction of benzimidazole side chains not only increased the free volume of the membranes to store more PA but also added additional basic sites to form hydrogen bonds with PA molecules and constructed a dense hydrogen bonding network, which greatly enhanced the PA uptake of the membranes and accelerated the proton conduction to reduce the activation energy. The proton conductivity of the PA-doped g-ABPBI-50% membrane reached 0.122 S cm−1 at 180 °C under humidity-free conditions, while the peak power density of the PA-doped g-ABPBI-30% membrane was as high as 0.372 W cm−2 at 160 °C, which was 1.75 times higher than that of the pure ABPBI membrane. These results highlight the potential of this grafted membrane for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) applications.