Synergistically Tailored Polyacrylonitrile-Based Stable Proton Exchange Membrane for High-Performance Vanadium Redox...

Long-term stability and capacity retention are important parameters for a redox flow battery. Proton exchange membrane (PEM) is a key component that governs the performance of vanadium redox flow batteries (VRFBs). Here, a crosslinked PEM based on acrylonitrile, 4-styrenesulfonic acid, and 2-acrylamido-2-methyl-1-propanesulfonic acid terpolymer is synthesized via the free radical random copolymerization and subsequently crosslinked by hydrazine hydrate. The synthesized CPAN-PEM-x membranes exhibit a dual-functional architecture, wherein the crosslinked nitrogen-containing amidrazone linkages and hydrophobic PAN backbone serve as a physical barrier to suppress the vanadium ion crossover, while sulfonic acid groups significantly enhance proton conductivity and maintain the trade-off. CPAN-PEM-2.5 membrane demonstrates excellent ion-exchange capacity (IEC) (1.49 meq g^-1), and ionic conductivity (10.49 × 10^-2 S cm^-1) as well as low vanadium crossover (4.09 × 10^-7 cm² min^-1) with high ionic selectivity (2.56 × 10⁵ S min cm^-3). VRFB performance of the CPAN-PEM-2.5 membrane demonstrates superior electrochemical efficiency, with a 98.60% coulombic efficiency, 76.08% voltage efficiency and 75.02% energy efficiency at 120 mA cm^-2 for 350 charge-discharge cycles. Furthermore, the CPAN-PEM-2.5 membrane exhibits superior capacity retention (48.78% up to 175 cycles) and a higher peak power density (449.34 mW cm^-2) compared to Nafion-117. These findings establish the PAN-based PEM as a durable and efficient candidate for a grid-scale VRFB systems.