A H3PO4 preswelling strategy to enhance the proton conductivity of a H2SO4-doped polybenzimidazole membrane for vanadium flow batteries

Abstract

A H₃PO₄ preswelling strategy is proposed to prepare H₂SO₄-doped polybenzimidazole (PBI) membranes for vanadium flow batteries (VFB). Before being immersed in 3.0 M H₂SO₄, PBI membranes are preswelled by immersion in concentrated H₃PO₄, which leads to a higher H₂SO₄ doping level, thereby dramatically reducing the area resistance of the PBI membrane to 0.43 Ω cm², which is close to that of Nafion 212 (0.35 Ω cm²) and much lower than that of Fumasep®FAP-450 (0.64 Ω cm²). Meanwhile, the substantially high selectivity is maintained. The VFB assembled with the H₃PO₄ preswelled PBI membrane displays high energy efficiencies (EE: 80.9–89.2%) over a current density range of 20–80 mA cm⁻², much higher than those of the non-preswelled PBI membrane (EE: 66.8–84.5%), Nafion 212 (EE: 63.1–75.6%) and Fumasep®FAP-450 (EE: 75.5–82.6%). The stable performance over 50 charge–discharge cycles demonstrates the good physicochemical stability of the preswelled PBI membrane. Considering the above results, the H₃PO₄ preswelling strategy proposed herein is facile and efficient for fabricating high-performance PBI membranes for VFB.