Acetylacetonate Complexes for Non-Aqueous Redox Flow Battery Applications

An ester functionalized vanadium acetylacetonate (acac) complex was synthesized and characterized for use in redox flow batteries (RFBs). The resulting complex has a solubility of 1.32M, a significant improvement over other metal complexes that have been considered for non-aqueous RFB applications, and a theoretical energy density of 37 Wh/L. While short term flow cell cycling of the complex shows high coulombic efficiencies (95%), capacity fade limits long term cycling. To better understand the causes of capacity fade, a stability study utilizing bulk electrolysis was conducted on both the parent vanadium acetylacetonate, and the ester functionalized complex. The results were used to determine the shelf life, solubility at each oxidation state, and cycle life for each half reaction. The shelf life is greater than the cycling experiment timelines and should not result in a loss of capacity. The solubilities at each oxidation state do not change by more than 25% of the solubility of the parent species. Vanadyl formation is evident during oxidation, and ligand detachment occurrs during reduction. The cycle life for the vanadium acetylacetonate is limited by vanadyl formation and reaches 80% capacity within ten cycles. The ester functionalized system shows rapid vanadyl formation and reaches 80% capacity after just the first cycle. Work is ongoing to better characterize side products and better understand the mechanism of the reactions.