Chemically Cross-Linked Polybenzimidazole Membranes with Ion-Conductive Sub-Nanometer Channels for Zinc-Iron Flow Batteries.

Ion-selective membranes with sub-nanometer micropores are essential in various separation processes and energy-related devices. However, the absence of molecular-level insights into ion transport behavior in sub-nanochannels challenges the accurate construction of fit-for-purpose membranes. Herein, we design cross-linked polybenzimidazole membranes with varying angstrom-scale pores and functional group densities by in situ crosslinking reaction during dual-coagulation bath-induced phase separation process. The modulation of pore architecture and pore chemistry enables precise control of ion transport under confined channels. Molecular dynamics simulations and experimental results reveal that ion dehydration and ion-pore wall interactions are the two dominant mechanisms governing fast and selective ion transport within charged sub-nanometer channels. Based on "dehydration-diffusion" mechanism, membranes with low steric hindrance and weak ion-pore wall interactions facilitate low-energy-barrier ion transport. We demonstrate their applications in alkalescent zinc-iron flow batteries, achieving a high peak power density of 607.8 mW cm-2 and energy efficiency exceeding 80% at a current density of 200 mA cm-2. Our study advances the understanding of ion transport in membranes with sub-nanometer pores and provides guidelines for designing next-generation ion-selective membranes by regulating channel size and channel chemistry.