Mechanism Underlying Electron-Transfer Reactions Involving TEMPO and 4-Hydroxy-TEMPO in Deep Eutectic Solvents

Abstract

Deep eutectic solvents (DES) such as ethaline are considered low-cost, environmentally-benign and non-toxic electrolytes which possess wide electrochemical stability windows. Therefore, these non-aqueous DES electrolytes are attractive for electrochemical applications such as redox flow batteries. However, key drawbacks in applying DES electrolytes include their high viscosities (manifesting in sluggish transport) and sluggish electron-transfer reaction rates [1]. Additionally, mechanistic insights into electron-transfer reactions involving redox-active organic species added to a DES medium are presently unavailable. In this presentation, we report experimental and modeling investigations into the electrochemical oxidation of TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] and 4-Hydroxy-TEMPO in ethaline DES. Specifically, we present evidence that electrochemical oxidation of TEMPO or 4-Hydroxy-TEMPO involves a two-stage process in which electron transfer is followed by adsorption of the oxidized molecules which either block the electrode surface or gradually desorb and diffuse away into the electrolyte. Mathematical modeling of this two-stage process allows quantitative analysis of experimental polarization curves and cyclic voltammograms. Furthermore, electron-transfer associated with aforementioned organics in DES is compared to and contrasted with analogous processes in aqueous electrolytes to gain additional insights into the proposed ‘adsorption-reaction-desorption’ mechanism. Reference: [1]. Shen. D., Vukmirovic. M. B. and Akolkar. R. 2019 J. Electrochem. Soc. 166 E526.