Preparation of High-Performance Proton Exchange Membrane Based on Covalent Cross-Linking and Acid–Base Interactions and Application in Vanadium Redox Flow Batteries

Proton exchange membranes based on sulfonated poly(ether ether ketone) (SPEEK) hold broad application prospects in vanadium redox flow batteries (VRFBs) due to their tunable structure, excellent thermal stability, and low cost. Typically, SPEEK membranes with a high sulfonation degree exhibit higher proton conductivity but poorer vanadium barrier properties. To overcome this bottleneck, this study introduced a diethylenetriamine cross-linked structure onto the mainchain of the SPEEK with a high sulfonation degree (HDS-SPEEK) through a covalent-bonding strategy to construct amphoteric functionalized cross-linked membranes (CS-S). The presence of this covalent cross-linking network effectively enhanced the membrane’s mechanical properties and induced acid–base interactions through its amphoteric functional groups, which construct efficient proton transport channels at the interface. This synergistically enhanced the proton conductivity and significantly inhibited vanadium crossover. Among them, the proton conductivity of 33.3 mS·cm–1 and the ion selectivity as high as 21.3 × 103 S·min·cm–3 were exhibited in the CS-S-0.2 membrane with an optimal comprehensive performance. In the VRFB single-cell tests, the CS-S-0.2 membrane achieved an energy efficiency (EE) of over 66% at 200 mA·cm–2, a self-discharge time of 75.2 h, and a stable Coulombic efficiency (CE, 98.0 ± 1%) and EE (76.5 ± 1.5%) under 500 charge–discharge cycles at 150 mA·cm–2. This work confirms that the synergistic regulation of side-chain cross-linking and acid–base interfacial interactions enhances the proton selective transport properties of SPEEK membranes, providing a novel and straightforward strategy for designing and fabricating high-performance SPEEK proton exchange membranes.