Unlocking Three-State Aqueous Redox Cycling in an Iron Clathrochelate

Redox flow batteries (RFBs) are prominent candidates for energy storage because of the independent relationship between energy and power outputs. Transition-metal complexes are attractive platforms for innovative energy materials since their redox chemistry can be tuned via coordination chemistry. In recent years, iron has captured attention due to its abundance and cost-effective sourcing. However, most iron-based aqueous electrolytes are limited to a single redox pair (Fe^II/Fe^III), requiring asymmetric battery designs that may suffer from cross-contamination and pH/osmotic imbalances. Therefore, a symmetric design, i.e., the same electrolyte in both tanks, remains an important chemical challenge. In this work, the aqueous self-assembly and redox chemistry of a literature-known high-valent iron clathrochelate scaffold are revisited. Electrochemical characterization reveals reversible three-state redox cycling, including a fully new accessible reversible redox process. This multivalent behavior enables the same complex to act as posolyte and negolyte, yielding an aqueous symmetric all-iron RFB.