Highly proton conductive, dense polybenzimidazole membranes with low permeability to vanadium and enhanced H 2 SO 4 absorption capability for use in vanadium redox flow batteries

H2SO4-doped dense polybenzimidazole (PBI) membranes based on poly[2,2′-(2-benzimidazole-p-phenylene)-5,5′-bibenzimidazole] (BIpPBI) containing benzimidazole side groups were developed for use in vanadium redox flow batteries (VRFBs). BIpPBI was prepared by the polycondensation of 3,3′-diaminobenzidine with 2-benzimidazole terephthalic acid (BITA) in polyphosphoric acid. Many basic properties of BIpPBI were characterized and compared with conventional PBI (mPBI). The BIpPBI membrane has an entirely amorphous structure because of the presence of additional benzimidazole side groups, and the absorption of both H2SO4 (65 wt%) and water (46 wt%) was significantly improved compared to the mPBI membrane in a 4 M H2SO4 solution. The H2SO4-doped BIpPBI membrane has a low area resistance of 0.17 Ω cm2 and significantly lowered permeability to vanadium ions (3.45 × 10−8 cm2 min−1). Finally, the VRF…