High‐Performance Zinc–Iron Flow Batteries Enabled by Programmable Sulfonation of Poly(phenylene sulfone) Membranes

The development of high‐performance, cost‐effective ion exchange membranes (IEMs) is crucial for advancing sustainable energy storage technologies. In this study, we focus on sulfonated poly(phenylene sulfone) (SPPSU) membranes, which were fabricated via a copolymerization route with pre‐sulfonated monomers to systematically control their sulfonation degree. Through a targeted screening process, an optimized membrane composition—specifically the 0.5‐SPPSU variant was identified, exhibiting a well‐balanced combination of moderate water uptake, limited swelling, high ionic conductivity, and excellent mechanical and thermal stability. When implemented in alkaline zinc‐iron flow batteries (AZIFBs), this optimized membrane delivered impressive electrochemical performance, achieving an energy efficiency of 80.61%, a Coulombic efficiency of 98.73%, and stable operation over 1000 cycles (380 h) at 200 mA cm −2 . These results highlight the potential of tailored SPPSU membranes as durable and efficient IEMs for long‐cycling AZIFB systems.