Boron-Formazanate Complexes as Tunable Redox-Active Materials for Non-Aqueous Redox Flow Batteries.

As a broad-scale energy storage solution, redox flow batteries (RFBs) offer high efficiency and tunable design. However, conventional RFBs rely on transition-metal ion couples, (e.g., vanadium or iron), whose implementation is limited by low energy densities, high cost, and environmental leaching. Main-group compounds, comprising earth-abundant, p-block elements, represent highly promising, yet underexplored candidates for RFBs. Herein, we evaluate three boron-formazanate complexes as negolyte and symmetric electrolytes in nonaqueous organic redox flow batteries (NAORFBs). Detailed electrochemical characterization of these complexes reveals two sequential reduction processes with the first being exceptionally stable (<3% capacity fade after charge/discharge cycling in a static H-cell for 3 days). In contrast, cycling that includes the two-electron reduced state results in rapid degradation (>59% capacity fade over 2.5 days in a static H-cell), most likely due to fluoride elimination from the BF2 moiety. Guided by these insights, a B(Ph)2 unit was introduced to mitigate this degradation pathway. The elimination of labile B─F bonds as well as steric protection conferred by two phenyl groups led to improved cycling performance (>85% capacity retention after charge/discharge cycling in a flow battery for 15 days). These findings guide the rational design of inexpensive main-group electrolytes for application in energy storage.