A ferrocyanide-polysulfide redox flow battery with homogeneous catalysis for high-performance energy storage

Aqueous ferrocyanide-polysulfide redox flow batteries (Fe-S RFBs) are promising for grid-scale energy storage yet suffer from slow polysulfide kinetics, membrane crossover, and supporting-electrolyte mismatch, which increase polarization and undermine cycling durability. Although riboflavin sodium phosphate (FMN-Na) has been used as a homogeneous catalyst to promote polysulfide conversion, previous formulations relied on externally added KOH, creating an OH-/Cl- imbalance and placing greater demands on membrane selectivity. Herein, we construct a KOH-free Fe-S RFB using KCl as the common supporting electrolyte and FMN-Na as a homogeneous molecular catalyst. The system utilizes the alkalinity generated by K2S hydrolysis, removing the need for added KOH while providing a suitable pH environment for FMN-Na electrocatalysis. Whereas earlier work focused on long-chain polysulfide reduction, FMN-Na facilitates polysulfide conversion dominated by the S22-/S2- couple through electron shuttling, reducing polarization and improving reversibility. The optimized Fe-S RFB delivers outstanding cyclability with 5,000 stable cycles at 40 mA cm-2 and a peak power density of 197.2 mW cm-2. With a high-concentration catholyte, the cell yields an initial energy efficiency (EE) of 83.07% and retains 77.34% after 440 cycles. Tailoring the electrolyte compositions extends operation from -20 to 60 °C. Furthermore, Prussian blue-mediated redox targeting and increased active-material concentrations yield a catholyte-volume-normalized discharge energy density of 102.12 Wh L-1, corresponding to 51.06 Wh L-1 when normalized to the combined electrolyte volume. This work provides a practical route towards low-cost Fe-S RFBs combining durable cycling, wide-temperature operation, and high electrolyte-level energy density for grid-scale storage.