Self-assembly of covalent organic frameworks within sulfonated polysulfone membranes via supramolecular interaction for vanadium redox flow batteries

For the purpose of large-scale and long-duration energy storage, vanadium redox flow batteries (VRFBs) have emerged as a highly promising electrochemical solution. These systems require membranes exhibiting high stability and selectivity. Conventional microphase-separated membranes, however, often face a trade-off between achieving high ionic conductivity and minimizing active species crossover, compounded by long-term chemical stability challenges. To address this, we designed a supramolecular interaction-mediated strategy for the in situ synthesis of covalent organic frameworks (COFs) within ion transport channels. Highly sulfonated polymer chains provide SO 3 H groups that serve as supramolecular interaction sites for COF precursors, offering ordered templates for crystallization. The resulting optimized membrane exhibits a molecular sieving effect within its ion channels, demonstrating superior ion selectivity (VO 2+ permeability, P: 3.24×10 -8 cm 2 ·s -1 ; selectivity, S: 5.70×10 5 ) and stable flow battery performance (coulombic efficiency, CE > 99.6%; energy efficiency, EE > 78.89% over 500 cycles). The performance surpasses that of Nafion 212 and several previously reported membranes. This work presents a simple method for modifying ion transport channels—leveraging supramolecular interactions without damaging the polymer backbone—and offers new perspectives on applying COFs in ion exchange membranes (IEMs).