Main‐Chain Ion‐Pair Polybenzimidazole Membranes Enabling Reduced‐Temperature HT‐PEMFC Operation (Down to 120°C)

ABSTRACT Proton exchange membrane (PEM) fuel cells are promising for clean and efficient energy conversion across diverse applications. High‐temperature PEM fuel cells (HT‐PEMFCs) using phosphoric acid (PA)‐doped polybenzimidazole (PBI) can operate up to 200°C, but at lower temperatures, water‐induced PA loss often leads to performance degradation and reduced durability. In this work, we present a new class of main‐chain ion‐pair PBI membranes by integrating the imidazolium‐biphosphate ion‐pair units directly into the PBI backbone. This design results in membranes with high acid content, stronger acid‐polymer interactions and stabilized acid retention. In single‐cell tests, the Im + ‐PBI 15 ‐3 wt.% exhibits peak power densities of 0.79 W cm −2 at 160°C and 0.57 W cm −2 at 120°C, with excellent durability and minimal acid loss over prolonged operation. These results demonstrate that main‐chain ion‐pair PBIs offer a robust, high‐performance solution for HT‐PEMFC membranes capable of efficient operation down to 120°C.