Comb-shaped cationic bridges in phosphoric acid-doped PBI gel membranes: A pathway to flexible and efficient fuel cell operation from −20 °C to 200 °C

Phosphoric acid (PA)-doped high-temperature proton exchange membranes (HT-PEMs) face significant challenges from PA leaching, particularly under low temperatures, which severely compromises their operational durability and flexibility. Herein, we developed innovative polybenzimidazole (PBI)-based PEMs featuring a comb-shaped cationic bridge structure, synthesized via a combined polyphosphoric acid process and heterogeneous free radical polymerization. The resulting membranes exhibited a distinctive porous microstructure characterized by aligned nanosheet arrangements and double dense skin layers, effectively suppressing PA leaching. Strong ion-pair interactions between imidazolium cations and biphosphate enhanced PA retention and promoted PA dissociation, substantially improving proton conductivity. The optimized PA/PBI-AMim-2 membrane achieved exceptional proton conductivity, exceeding 10 mS/cm at −20 °C and reaching 0.34 S/cm at 220 °C, enabling efficient fuel cell operation across a wide temperature range. It demonstrated outstanding single-cell performance, delivering peak power densities of 820 mW/cm 2 at 80 °C and 1605 mW/cm 2 at 160 °C, along with cold start-up capability of 46 mW/cm 2 at −20 °C. Its superior PA retention ensured stable performance with a minimal decay rate of 23.7 μV/h at 40 °C and enabled reliable functionality through 100 frequent start-up/shutdown cycles. This breakthrough bridges the gap between conventional low-temperature and high-temperature PEMFCs, paving the way for robust, flexible and durable systems under real-world operating conditions.