Novel sulfonated polybenzimidazole membranes with flexible sulfoalkyl pendants for application in vanadium flow battery

The polybenzimidazole membrane has a low proton conductivity, and its trade-off effect between proton conductivity and vanadium ions blocking has not been addressed. Therefore, improving the proton conductivity and proton selectivity of polybenzimidazole membrane is crucial for its development in vanadium flow battery (VFB). In this work, two types of sulfonated polybenzimidazole (SPBI and HSPBI) membranes with flexible sulfoalkyl pendants are prepared by ring-opening and substitution reactions for application in VFBs. Compared with poly(4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) membrane, SPBI and HSPBI membranes exhibit remarkable proton conductivities. However, the vanadium ions blocking capabilities of SPBI and HSPBI membranes show a slight decline. Excitingly, the HSPBI membrane achieves the highest proton selectivity (1.54 × 10 6  S min cm −3 ) compared with OPBI (6.28 × 10 5  S min cm −3 ), SPBI (1.30 × 10 6  S min cm −3 ) and commercial Nafion 212 (0.43 × 10 5  S min cm −3 ) membranes. Lastly, all membranes are separately assembled into VFBs to verify their battery performances. The HSPBI membrane shows higher Coulomb efficiencies (94.74–99.41%) and energy efficiencies (67.49–83.55%) compared of OPBI, SPBI and Nafion 212 membranes at 80–280 mA cm −2 . Remarkably, the 500 VFB cycles of HSPBI membrane at 140 mA cm −2 are also stably executed, which shows excellent structural stability. Therefore, the HSPBI membrane has a promising potential for VFB application.