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Non-aqueous redox flow batteries constitute a promising solution for grid-scale energy storage due to the ability to achieve larger cell voltages than can be readily accessed in water. 

Emergent, flowable electrochemical energy storage technologies suitable for grid-scale applications are often limited by sluggish electron transfer kinetics that impede overall energy conversion efficiencies. To improve our understanding of these kinetic limitations in heterometallic charge carriers, we study the role of solvent in influencing the rates of heterogeneous electron transfer, demonstrating its impact on the kinetics of di-titanium substituted polyoxovanadate-alkoxide cluster, [Ti 2 V 4 O 5 (OMe) 14 ]. Our studies also illustrate that the one electron reduction and oxidation processes exhibit characteristically different rates, suggesting that different mechanisms of electron transfer are operative. We report that a 1 : 4 v/v mixture of propylene carbonate and acetonitrile can lead to a three-fold increase in the rate of electron transfer for one electron oxidation, and a two-fold increase in the one electron reduction process as compared to pure acetonitrile. We attribute this behavior to solvent–solvent interactions that lead to a deviation from ideal solution behavior. Coulombic efficiencies ≥90% are maintained in MeCN–PC mixtures over 20 charge/discharge cycles, greater than the efficiencies that are obtained for individual solvents. The results provide insight into the role of solvent in improving the rate of charge transfer and paves a way to systematically tune solvent composition to yield faster electron transfer kinetics.