Nafion Hybrid Membranes with Enhanced Ion Selectivity via Supramolecular Complexation for Vanadium Redox Flow Batteries

Nafion is considered to be the benchmark among commercial polymer proton exchange membranes. However, the significant size difference between its ionic domains and vanadium ions leads to severe vanadium crossover in vanadium redox flow batteries (VRFBs). Current modification strategies for Nafion can suppress vanadium crossover but at the expense of decreased conductivity. In this work, we employ a supramolecular complexation strategy to precisely modify Nafion ionic domains using poly(2-vinylpyridine) (P2VP), achieving efficient vanadium screening. The excellent compatibility between P2VP and the ionic domains allows Nafion to maintain continuous proton transport channels. Additionally, the substantial steric hindrance and Donnan exclusion effect of P2VP lead to a 30.9% increase in ion selectivity (10.6 × 10⁴ S min cm^–3) compared to Nafion 212 membranes (8.1 × 10⁴ S min cm^–3). The hybrid membranes significantly enhance VRFB performance, demonstrated by superior Coulombic efficiency (98.1%) and energy efficiency (88.8%) at 100 mA cm^–2, compared to Nafion 212 membranes (96.9% and 87.4%). Moreover, after 100 cycles, the capacity retention of VRFBs with hybrid membranes (90.8%) was 1.3 times higher than that of Nafion 212 membranes (67.3%). These results highlight the unique advantages of the supramolecular complexation strategy in finely tuning the structure and ionic conduction of polymer electrolyte membranes.