Freestanding covalent organic framework membranes with enhanced proton perm-selectivity for flow batteries

Aqueous organic redox flow batteries (AORFBs) are attractive for energy storage applications, benefiting from the high safety and low cost. Covalent organic frameworks (COFs) with uniformly arranged rigid nanochannels are suitable for fabricating membranes implemented into AORFBs. However, most freestanding COF membranes are challenging to apply directly to flow batteries due to their insufficient mechanical strength. This work proposes a mechanochemistry-based method for fabricating freestanding COF membranes and a corresponding macromolecular suturing strategy to prepare membranes with excellent mechanical properties and enhanced proton conductivity. Through the steric hindrance effect of the introduced sulfonic acid group (-SO₃H) functionalized chains, the ability of the membrane to block the crossover of redox couples is strengthened. Meanwhile, the –SO₃H groups provide additional active sites, constructing a more continuous proton pathway. The optimized membrane exhibits a high voltage efficiency of 79.06% at 40 mA cm⁻² and retains nearly 100% of its discharge capacity even after 100 cycles at 80 mA cm⁻², outperforming the TpAzo membrane. This work offers a novel strategy to promote the utilization of COF membranes in flow battery applications.