Dual‐Functional Membranes With Interfacial MXene/PFSA Nanoarchitectures for Low Hydrogen Permeation in Water Electrolysis and Reduced Vanadium Ion Crossover in Flow Batteries

Proton exchange membrane water electrolysis (PEMWE) and vanadium redox flow batteries (VRFBs) suffer from hydrogen and vanadium ion crossover when using conventional perfluorosulfonic acid (PFSA) membranes. Here we report a layer‐by‐layer MXene/PFSA membrane architecture in which MXene nanosheets are integrated into PFSA coating layers to create MXene‐regulated nanointerfaces that decouple proton conductivity from gas and ion permeation. The lamellar MXene framework introduces tortuous transport pathways and selectively perturbs PFSA hydrophilic domain organization while preserving continuous proton‐conducting networks through interfacial hydrogen bonds. As a result, the optimized membrane exhibits simultaneously enhanced H + /H 2 and H + /VO 2+ selectivity, achieving up to 43.0% lower hydrogen permeability and 37.5% reduced vanadium ion diffusion compared to pristine PFSA, without sacrificing proton conductivity. It exhibits stable long‐term operation in PEMWE with lower hydrogen and retains 67.0% capacity in VRFB over 470 cycles, outperforming pristine PFSA more than 40% of capacity retention. Small‐ and wide‐angle X‐ray scattering reveal MXene‐induced disordering and expansion of large‐scale hydrophilic domains, while density functional theory (DFT) calculations reveal that the MXene/PFSA interface selectively increases transport energy barriers for hydrogen and vanadium ions while preserving low barriers for proton migration. These results validate a versatile interfacial strategy toward overcoming selectivity limitations for both PEMWE and VRFB.