Characterisation of Redox Species in Pure IL and Its Mixtures with a Molecular Solvent By Electrochemical Impedance Spectroscopy

Abstract

Ionic liquid (IL) are molten salt at room temperature. They are of ever increasing interest due to their outstanding properties and appear to be candidates for many electrochemical applications. Thus, there are more and more works concerning their use as electrolyte for storage or conversion energy1. For this application, the addition of redox species is needed. In such case, the electrochemical properties of these compounds in IL must be determined and especially the determination of their diffusion coefficient (D) and heterogeneous electron-transfer rate constant (k°). One of the drawbacks of IL is their important viscosity. To reduce it, a molecular solvent can be added2.In this work, we studied the electrochemical properties of some redox compounds (Alizarine, Tempo, Quinone, iron chloride) in an aprotic IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, C2mimTFSI, and its mixtures with γ-butyrolactone (GBL) as molecular solvent. The addition of this solvent decreases drastically the viscosity of the medium, at 298K, from 30 mPa.s to 10 mPa.s for a molar fraction of IL about xIL = 0.5. The diffusion coefficient and heterogeneous electron-transfer rate constant of the four redox compounds in IL-GBL mixtures of different composition were determined by cyclic voltammetry and electrochemical impedance spectroscopy. For all these species the diffusion coefficients were about 10-7 to 10-8 cm2 s-1 and their value decreases when the IL content increases as expected, due to the viscosity of the medium. The same trend is observed for the heterogeneous electron-transfer rate constant values. Martins, V. L.; Torresi, R. M., Ionic liquids in electrochemical energy storage. Curr. Opin. Electrochem. 2018, 9, 26-32. Papović, S.; Bešter-Rogač, M.; Vraneš, M.; Gadžurić, S., The effect of the alkyl chain length on physicochemical features of (ionic liquids+γ-butyrolactone) binary mixtures. The Journal of Chemical Thermodynamics 2016, 99, 1-10.