Vanadium ion recognition by ortho-phenol side chain to improve ion selective conduction for vanadium redox flow battery

High selective conduction of ion conductive membrane is essential to efficiency and cycling stability of vanadium redox flow battery, however, disentanglement of the trade-off between proton conductivity and vanadium ions permeability is challenging. In this work, the ortho-phenol side chain with vanadium ion recognition effect is proposed, which can effectively reduce vanadium ions permeability through coordinated chelation of vanadium ions. The ortho-phenol hydroxyl side chain, together with the sulfonic acid group, can also construct a microphase separation proton conductive channel to promote proton conduction. As the ion exchange capacity of ortho-phenol hydroxyl group (IEC OH ) in sulfonated polybenzimidazole reaches 2.89 mmol g −1 (SPBIOH-2.89), low area resistance (0.21 Ω cm 2 ) and vanadium ions permeability (1.71 × 10 −9  cm 2  s −1 ) are achieved. Accordingly, the H + /V n +  selectivity presents 1.81 × 10 7  S s cm −3 , around 72.8 folds that of commercialized Nafion 212 membrane (2.49 × 10 5  S s cm −3 ). The VRFB assembled with SPBIPhOH-2.89 membrane presents excellent performance (energy efficiency 80.1 % at 200 mA cm −2 and discharge capacity decay rate 0.26 % per cycle at 100 mA cm −2 ), superior to that of Nafion 212 (66.7 % at 200 mA cm −2 , 0.71 % per cycle at 100 mA cm −2 ). The membrane performance is in top level compared with most reported amphoteric ion conductive membranes.