Quinone convertible sulfated ion conductive side chain for highly selective vanadium redox flow batteries

Abstract

The design of ion conductive functional side chains is critical to tune the selectivity of protons to vanadium ions in ion conductive membranes for vanadium redox flow batteries (VRFBs); however, it is far from satisfactory. Herein, the concept of a quinone convertible sulfated ion conductive side chain is proposed with tunable Donnan effect and strong proton conducting ability. The electrically neutral phenol group in the side chain will be chemically converted to quinone of lower electronegativity in situ in the highly acidic VRFB operating environment, which leads to about a 5 fold increase in zeta potential to repel vanadium ions. Meanwhile the sulfate ester cationic exchange group in the side chain dominates the well-formed ion clusters (7.85 nm) and hydrogen bonding for fast proton conduction. With a quinone group capacity (IEC_Q) of 2.43 mmol g⁻¹ in the polybenzimidazole (PBIPhOSO₃-q-2.43), the low area resistance (0.23 Ω cm²) and vanadium permeability (3.2 × 10⁻⁹ s cm⁻²) contribute to about a 16.5 fold increase in H⁺/Vⁿ⁺ selectivity as compared with the Nafion 212 membrane. The corresponding VRFB shows a low discharge capacity decay rate (0.31% per cycle) and high energy efficiency (87.7%) at 100 mA cm⁻² and remains stable during the 500 cycle test. The performance is superior to that of the most recently reported amphiprotic ion conductive membranes.