Synergistically Tailored Polyacrylonitrile‐Based Stable Proton Exchange Membrane for High‐Performance Vanadium Redox Flow Batteries

ABSTRACT 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 cm ² min ⁻¹ ) with high ionic selectivity (2.56 × 10 ⁵ 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.