Synergistic fluorine-imidazolium interactions enhance phosphoric acid retention in ion-pair membranes for high-temperature proton exchange membrane fuel cells

Phosphoric acid (PA)-doped high-temperature proton exchange membrane fuel cells (HT-PEMFCs) are promising for long-distance transportation due to their simplified water and thermal management. However, rapid PA leaching under dynamic operating conditions remains a critical barrier to membrane durability. Here, we introduce a novel ion-pair polymer incorporating spatially adjacent fluorobenzene groups and imidazolium groups, which leverages a newly discovered synergistic electronic interaction between fluorine and imidazolium. This unique electronic coupling, unveiled for the first time through liquid-state nuclear magnetic resonance (NMR) spectroscopy and molecular electrostatic potential (ESP) calculations, significantly strengthens polymer-PA binding while fostering highly interconnected proton channels and nanoconfined PA aggregates within the membrane. In addition, small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulations further reveals that the formation of well-defined microphase-separated structures, which significantly boost proton transport and PA retention. The resulting membrane achieves a peak power density of 1.13 W cm −2 in H 2 /O 2 fuel cells at 160 °C and exhibits a lifetime nearly two orders of magnitude longer than conventional PBI/PA membrane under harsh conditions. This work demonstrates a novel design strategy for the next generation of HT-PEMFCs, bridging molecular-level insights to transformative performance gains.