Engineered Microdefects in Nano‐Membranes for Enhanced Ion Selectivity and Membrane Durability in Vanadium Redox Flow Batteries

In vanadium redox flow batteries (VRFBs), ultrathin perfluorinated sulfonic acid (PFSA) membranes with a highly ordered morphology are proposed to enhance proton/vanadium ion selectivity, overcoming the limitations of conventional membranes. However, dense structures hinder proton transport and cause progressive deformation during cell operation, reducing overall performance. In this study, sub-25 nm ultrathin PFSA membranes are demonstrated by harnessing engineered microdefects as size-exclusive pores, promoting proton transport while maintaining structural integrity. To achieve this, only 14 molecularly thin PFSA Langmuir monolayers are stacked with adjusted packing density, and proton-conducting pathways are sufficiently established via pre-swelling. Unlike conventional strategies for robust dense membranes, this approach suppresses deformation of the channel morphology during cell operation, with higher ion selectivity than Nafion 211. Finally, the optimized ultrathin membrane exhibits superior cyclic and rate performance compared to the commercial membrane, delivering ≈76% energy efficiency and long-term stability at 200 mA cm^-2 without capacity decay.