Redox characteristics of iron ions in different deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) have been used as electrolytes in the redox flow battery due to their unique advantages including non-toxicity, biodegradability, and low cost. The most commonly used DESs are the reline DES system (a DES formed from a molar 1:2 mixture of choline chloride and urea) and the ethaline DES system (a DES formed from a molar 1:2 mixture of choline chloride and ethylene glycol). The difference in the component of DES will result in different electrochemical characteristics of the redox couples dissolved therein. In this work, the electrochemical properties of iron ions in ethaline and reline DESs, as well as those of iron ions with FeCl3 and FeCl3·6H2O dissolved in ethaline DES, were experimentally studied. The cyclic voltammetry (CV) results showed that the diffusion coefficient of iron ions in the ethaline DES is greater than that of the reline DES; the redox reversibility of iron ions with FeCl3·6H2O dissolved in ethaline is higher than that of FeCl3 in the same DES, and the reaction rate constant is larger than that of the former, while the diffusion coefficient of the former is slightly smaller than that of the latter. These differences stem from the intrinsic valence discrepancy of FeCl3·6H2O and FeCl3, which is confirmed by the FT-IR spectra analysis. Electrochemical impedance spectroscopy (EIS) also showed that the total resistance in the ethaline solution with FeCl3·6H2O was significantly lower than that with FeCl3. The results of this work can provide key parameters for electrochemical process research and numerical simulation studies on DES.