Porous branched polybenzimidazole membranes with high ion conductivity and selectivity for vanadium flow battery

Developing a high-performance membrane is essential for advancing the commercialization of vanadium flow battery (VFB). Although polybenzimidazole membrane exhibits multiple beneficial characteristics, its practical implementation in VFB is constrained by insufficient ion conductivity and the trade-off effect between ion conductivity and selectivity. Herein, the porous branched polybenzimidazole (PBPBI) membranes were fabricated through the combination of molecular structure regulation and non-solvent induced phase separation strategies. The PBPBI membrane can achieve high ion conductivity and selectivity, which effectively enhanced the efficiencies of VFB. Exhilaratingly, the optimized PBPBI-4 membrane exhibited an ion conductivity of 36.8 mS cm⁻¹, surpassing that of Nafion 212 membrane (30.3 mS cm⁻¹). Additionally, the ion selectivity of PBPBI-4 membrane (5.99 × 10⁶ S min cm⁻³) was substantially superior to Nafion 212 membrane (5.64 × 10⁴ S min cm⁻³). Both ion conductivity and selectivity values of PBPBI-4 membrane remain a top level compared with polybenzimidazole membranes developed in recent five years for VFB applications. The PBPBI-4 membrane exhibited superior coulomb efficiencies (96.09 %–98.18 %), voltage efficiencies (72.11 %–91.07 %) and energy efficiencies (70.80 %–87.51 %) to Nafion 212 membrane under current densities ranged from 80 to 300 mA cm⁻². Moreover, the 700 VFB cycles of PBPBI-4 membrane at 140 mA cm⁻² were also stably executed, which confirmed outstanding structural and mechanical stabilities. Therefore, the PBPBI-4 membrane demonstrates a significant potential for implementation in VFB system.