2-Amidophenolate and 2-Iminosemiquinone Radical-Coordinated Vanadyl Complex as Catholyte for Non-Aqueous Redox-Flow Batteries.

Fulfilment of energy demand by utilizing renewable energy sources that do not contribute to the production of greenhouse gases is a step forward in mitigating global warming. However, with the energy sources being intermittent in nature, renewable energy needs to be stored effectively on a grid scale. In this context, the development of redox-flow batteries has emerged as a promising technology where charging and discharging processes are accomplished by the redox shuttling of the electrolytes, namely anolytes and catholytes. This report describes the synthesis and characterization of a six-coordinate vanadyl complex (complex 1) with H4LSCH2(AP/AP) ligand comprising two redox-active (non-innocent) 2-aminophenol moieties. The metal ion-coordinated deprotonated ligand, which was in {[LSCH2(AP/ISQ)]3-} form, encountered one-electron reversible oxidation at 0.46 V vs Ag/AgCl where the 2-amidophenolate ([AP]2-) unit was being oxidized to [ISQ]•- radical. Using this redox phenomenon, a static H-cell was constructed in a 1:1 CH3CN:CH2Cl2 solvent mixture where galvanostatic charge/discharge cycles, in the potential range 0 to +0.8 V vs Ag/AgCl, were performed for 54 hours without noticeable degradation of the complex. GCD experiment provided a columbic efficiency (CE) of 97% and energy efficiency (EE) of 71%.