A Universal Organo-Ferrocyanide Positive Electrolyte for Aqueous Flow Batteries

Aqueous organic redox flow batteries (AORFBs) offer promise for large-scale energy storage due to inherent safety and abundant sourcing. While significant advances have produced stable, soluble negative electrolytes (negolytes), cost-effective positive electrolytes (posolytes) simultaneously achieving high solubility, redox potential, and stability remain limited, restricting the overall energy density and lifetime of the battery system. Herein, we report a highperformance organo-ferrocyanide posolyte, sodium pentacyano(pyrazine-2-carboxylate)iron(II) (FePz), for AORFBs. Derived from commercial nitroprusside through organic ligand engineering, FePz exhibits a high redox potential of 0.62 V and a record 2.3 M aqueous solubility. FePz demonstrates universal pairing capability and unprecedented cycling stability across diverse negolytes like viologen, phenazine derivatives, and zinc anode. Notably, when coupled with a phenazine-based negolyte at 2.0 M FePz concentration, the system maintains steady operation with a super low capacity fade (0.025% per day) over 71 days. Integration with a Zn-based configuration achieves a 1.4 V cell voltage, delivering a maximum power density of 0.22 W m−2 and a high energy density of 38 Wh L−1. Furthermore, modular ligand engineering of organo-ferrocyanides unlocks vast electrolyte design possibilities for energy storage and conversion applications.