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

Abstract

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 H₄L^SCH2(AP/AP) ligand comprising two redox-active (non-innocent) 2-aminophenol moieties. The metal ion-coordinated deprotonated ligand, which was in {[L^SCH2(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 %.