Anionic conductive group tunable amphoteric polybenzimidazole ion conductive membrane for vanadium redox flow battery

Anionic conductive group repels vanadium ions but is difficult to be incorporated into the selectively ionic conductive channels of the amphoteric ion conductive membrane with tunable ratio. Herein, the novel hydroxyl bromopropane anionic precursor side chain is proposed, in which the hydrophilic hydroxyl groups aggregate into ionic clusters with the sulfate ester cationic conductive side chains of polybenzimidazole via hydrogen bond during membrane casting. The subsequent conversion of the bromopropane precursor to quaternary ammonium can easily tune the anionic exchange capacity and integrate cations into the hydroxyl involved ionic conductive channels for selectively repelling vanadium ions. Compared with Nafion 212 membrane, the obtained amphoteric membrane exhibits much lower vanadium ion permeability (1.9 × 10−9 cm2s−1) and slightly lower area resistance (0.25 Ω cm2). Excellent performance of vanadium redox flow battery (coulombic efficiency, 99.2%, energy efficiency, 85.9% at 100 mA cm−2 and 0.31% discharge decay rate) is achieved, and efficiencies keep stable during the 500 cycles test. The performance of VRFB is surpass than that of most reported amphoteric ion conductive and Nafion 212 membranes.