Nano‐Sponge in Concomitant Swelling Membrane for Redox Flow Batteries with High Operational Stability

Abstract Swelling‐induced imbalance between ion transport and sieving severely limits the long‐term stability of ion‐exchange membranes (IEMs) in redox flow batteries (RFBs). Effectively managing membrane swelling during RFB operation remains a significant challenge. Here, a concomitant swelling membrane (CSM) is introduced that fundamentally redefines swelling management by incorporating a 2D nano‐sponge as a co‐expanding medium. Through synergistic expansion with bentonite, the sulfonated poly(ether ether ketone) (SPEEK) matrix achieves dynamic swelling control, enabling enhanced ion sieving without compromising conductivity. Density functional theory simulations reveal that strong electronegative interactions between bentonite oxygen sites and active ions facilitate proton and cation transport, ensuring robust ionic conductivity. Consequently, the CSM exhibits outstanding operational stability across diverse pH environments, retaining 95.88% capacity over 1300 cycles in alkaline zinc/ferricyanide RFBs, sustaining nearly 100% coulombic efficiency over 1200 cycles in acidic vanadium RFBs, and enabling stable operation for ≈80 days in neutral polysulfide/ferricyanide systems. Moreover, a pilot‐scale CSM demonstrates a competitive production cost of $12.72 m −2 , positioning CSM as a scalable and economically viable IEM for large‐scale energy storage applications.