Membrane contamination-driven sulfonate structuring for enhanced stability in all-iron redox flow batteries

All-soluble all-iron redox flow batteries are considered a promising long-duration, large-scale energy storage technology due to their fully decoupled energy and power design and low-cost active materials. However, the anode requires chelating ligands with electron-donating capabilities to establish a potential difference with ferrocyanide, but ligand crossover results in their oxidation, which subsequently causes failure and restricts stable operation. This work proposes a method based on K⁺-induced aggregation of sulfonate groups in perfluorosulfonic acid membranes, eliminating the need for additional additives. By cycling the membrane through the flow battery process, the barrier effect against anode ligands is enhanced, enabling stable cycling for 3227 cycles at 80 mA/cm² and ensuring long-term stability. Molecular dynamics simulations and cross-membrane diffusion experiments reveal the impact of different metal cations on the morphology of water channels in the membranes. A Na⁺-exchange membrane treated with K⁺ from the electrolyte achieves a high average energy efficiency of 73.4 %, maintaining battery stability. This work provides a simple yet effective strategy to improve the stability of all-soluble all-iron redox flow batteries. By uncovering the underlying microscopic mechanisms through simulations, it paves the way for the practical implementation of this technology in large-scale energy storage systems.