Feasibility Studies of Acidic Type III Deep Eutectic Solvents as Supporting Electrolytes for the Posolyte in Vanadium Flow Batteries

Abstract

Deep eutectic solvents (DES) are gaining interest as promising electrolytes for vanadium flow batteries due to their tuneable solvation properties, high solubility for ionic species, and their potential green credentials, but their application has been limited to a few formulations, based on ethylene glycol and urea, which are neutral / alkaline DES. Taking into consideration the vanadium redox mechanism in aqueous media, we hypothesized that exploring acidic formulations could improve the redox performance of V(IV/V) posolytes. Here, for the first time, we investigate the effect of binary and tertiary acidic type III DES formulations on the electrochemical kinetics of the V(IV/V) redox couple. Cyclic voltammetry revealed that acidic DES moderately improved anodic performance, but significantly destabilized the cathodic process compared to a neutral hydrogen bond donors, which exhibited better cathodic reversibility. Hydroxyl groups were found to stabilize the cathodic process, likely through coordination to vanadium species, whereas carboxylic acids improved anodic kinetics, but increased cathodic irreversibility. Furthermore, the aqueous dilution of the DES strategy was not found to be universally beneficial since high water loadings were observed to destabilize the redox system. The best performing ternary mixture, consisting of choline chloride, ethylene glycol and malonic acid in 1:2:2 molar ratio, exhibited a peak-to-peak separation comparable to aqueous acidic electrolytes, but lower operational currents. The improvements were attributed to the balance between pH modulation and viscosity reduction. These new findings highlight the competing effects of viscosity, solvation, speciation and pH on vanadium redox chemistry and provide useful insights on the need to balance these properties to enhance charge transfer and reversibility in the design of DES formulations for flow battery applications.