A Hierarchical Nano/Sub-nano Hybrid Ion Conduction Channel Induced by Ion-Cluster-Confined Hydrolysis for High Ion Selectivity in Vanadium Redox Flow Batteries

H+/Vn+ ion selectivity of ion-conductive membranes is essential but challenging for vanadium redox flow batteries (VRFBs). Herein, the design of a hierarchical nano/sub-nano hybrid ion conduction channel through an ion-cluster-confined hydrolysis method is proposed to tune the topological and chemical structures in the channel. The ion clusters, co-assembled by sulfonic acid and sulfate ester groups in the precursor polybenzimidazole membrane, evolve into nano/sub-nano-sized pores during the hydrolytic conversion of sulfate ester groups into smaller hydroxyl groups, while the remaining sulfonic acid groups remain in smaller nano-sized ion clusters. The well-formed water-absorbed pores and ion clusters in the conductive channel construct strong hydrogen bond (0.97 Å) networks for fast proton hopping. The sub-nanopores (3.1–5.5 Å) decrease vanadium permeability by the size screening effect. With low area resistance (0.12 Ω cm2) and vanadium permeability (2.38 × 10–10 cm2 s–1), very high H+/Vn+ ion selectivity (1.89 × 1011 S s cm–3) is achieved, which is 3 orders of magnitude higher than that of Nafion 212 (3.32 × 108 S s cm–3). A VRFB containing such a channel presents excellent performance at a high current density of 260 mA cm–2, achieving 80.1% energy efficiency, a discharge capacity decay rate of 0.1%/cycle, and 1000 charge–discharge cycles without chemical structure change.