Surface-engineered Nafion membranes with an ion-sieving layer for high-performance vanadium redox flow battery

Excessive swelling of commercial Nafion membranes in vanadium redox flow battery (VRFB) induces severe vanadium ion permeation, which remains a key challenge limiting battery performance. To address this, we propose a straightforward strategy to construct a selective layer on Nafion membranes. This selective layer is fabricated through the sequential layer-by-layer (LbL) assembly of polyethyleneimine (PEI) and sodium lignosulfonate (SL) on Nafion 115, followed by chemical cross-linking with glutaraldehyde (GA). This layer synergistically combines size-sieving with Donnan exclusion effects to block vanadium ions, while the abundant sulfonic acid groups in SL enable efficient proton conduction via the Grotthuss mechanism. Benefiting from these synergistic effects, the modified N115-(PEI/SL)₂ membrane exhibits remarkably improved performance: vanadium ion permeation decreases by 92.8% with 80% proton conductivity retained compared to Nafion 115 membrane. At 80 mA cm^-2, the energy efficiency reaches 85.14%, substantially higher than the pristine Nafion membrane (81.77%). Furthermore, over 350 consecutive cycles (>400 h) at 200 mA cm^-2, the energy efficiency decays by only ∼2%, demonstrating excellent long-term stability. This work thus provides a simple, scalable, and commercially promising surface modification strategy to resolve the conductivity-selectivity trade-off in VRFB membranes.