Quinone convertible phenolic hydroxyl group side chain to construct high selective ion conductive channel for vanadium redox flow battery

The trade-off between proton conduction and vanadium permeability is a great challenge for ion conductive membrane to achieve high vanadium redox flow battery (VRFB) performance. Herein, the quinone convertible phenolic hydroxyl group side chain is proposed to endow Donnan effect and continuously narrow ion conductive channel to improve H⁺/V^n + selectivity. Uniquely, phenol can be chemically converted into quinone of lower electronegativity in-situ in the strong acidic VRFB environment, which greatly increases the Zata potential (from 2.6 to 10.2 mV) and Donnan effect to repel vanadium ions. A high phenol hydroxyl group capacity with 5.43 mmol g⁻¹ can be achieved owing to the swelling restriction by the hydrogen bonding crosslinking between quinone and benzimidazole, which constructs small but densely distributed ion clusters and continuously narrow ion conductive channels for selective proton conduction. The quinone type polybenzimidazole membrane (PBIPhQ-5.43) presents low area resistance and vanadium permeability (0.25 Ω cm² and 1.3 × 10⁻⁹ cm² s⁻¹, 13.8 % and 99.6 % decreased, respectively, compared with that of Nafion 212). The VRFB exhibits excellent balance between energy efficiency (83.1 %) and discharge capacity decay (0.29 %/cycle) at 100 mA cm⁻², far superior to that of Nafion 212 membrane (75.1 %, 0.71 %/cycle).