Redox Properties of Solid Booster and Solution Phase Mediator in a Non-Aqueous Redox Targeting Flow Battery System

Application of Non-aqueous redox flow battery systems are hindered by low energy density. To mitigate the issue, the concept of using solid phase ‘Booster’ in the electrolyte tank containing solution phase active material ‘Mediator’ is used in a Redox Targeting Flow Battery (RTFB) System. The solid phase booster, with tuned reduction potential, increases the energy density of the solution phase mediator and obviates the need for high concentration which leads to increased viscosity and decreased performance. Here, we investigate the redox reaction between the RTFB mediator tetrabutylammonium vanadium(iv)(bis)-hydroxyiminodiacetate (TBA₂VBH), which is soluble in a non-aqueous solvent, with Cobalt (II) Hexacyanoferrate (III) (CoHCF), a solid booster. An ex-situ analysis has been carried out to measure the concentration of the mediator in both oxidation states via UV-vis spectroscopy, while independently monitoring the oxidation state of the booster using FTIR spectroscopy. We provide evidence of the importance of the nature of the cations present in the electrolyte, which (de)intercalate within the lattice of the booster during redox cycling. Further, we demonstrate that this property can be used to tune the redox reaction between the RTFB booster and mediator, which must precisely match to have a reversible RTFB system. Cyclic voltammetry analysis further confirms the redox behavior of CoHCF in the present of various cations in the electrolyte system.