Porous poly(2,5-benzimidazole) membranes for vanadium redox battery applications were prepared using supercritical CO2 assisted phase inversion method. Membrane morphology, electrolyte uptake, proton and vanadyl ion transport properties as well as membrane performance in single cells of VRBs are studied. Highly selective ABPBI membranes with fibrous structure and improved ion transport properties are obtained. Significant improvement in proton conductivity is observed (values up to 27 mS cm(-1) are achieved), while VO2+ permeability is preserved at a low level (4.1 x 10(-8) cm(2) min(-1)). Single VRB cells with the modified ABPBI membranes demonstrate improved coulombic efficiency without significant deterioration of the voltage efficiency as compared to Nafion membranes. As a result, the energy efficiency of single VRB cells with the new membranes is significantly improved in a low current density region (20-40 mA cm(-2)) as compared to Nafion 115 based cells (up to 14% improvement).