Self‐Standing Covalent Organic Polymer Membrane with High Stability and Enhanced Ion‐Sieving Effect for Flow Battery

Fabrication of ion‐conducting membranes with continuous sub‐nanometer channels holds fundamental importance for flow batteries in achieving safe integration of renewable energy into grids. Self‐standing covalent organic polymer (COP) membranes provide feasibility due to their rapid and selective ion transport. However, the development of a scale‐up possible, mechanically robust and chemically stable membranes remains a significant challenge. Herein, using irreversible strong secondary amine linkage, we propose a self‐standing COP membrane with sub‐nanometer pores ranging from 4.5 to 6.4 Å, by a simple and efficient in‐situ polymerization approach. This membrane exhibits enhanced selectivity for proton and vanadium ions, especially excellent electrochemical stability, delivering an energy efficiency of over 80% at the current density of 200 mA cm−2 over 1000 cycles for an all‐vanadium redox flow battery (VFB). This study provides novel insights for COP‐based ion‐sieving membranes in sustainable energy fields.