Spectroscopic and Electrochemical Investigation of Adsorbed Nitroxide Radicals

Abstract

Deep eutectic solvents (DESs) have gained interest as electrolytes for grid-scale energy storage applications such as flow batteries. Here, we report on processes that govern the charge-transfer kinetics involving a redox-active organic (4-hydroxy-TEMPO, HT) species in chloride containing aqueous and DES electrolytes. Using polarization measurements, we show that the apparent anodic charge transfer coefficient α for a one–electron oxidation of HT approaches 1.1 This is a significant deviation from the expected value of α ≈ 0.5 for a symmetric one–electron transfer reaction involving a soluble – soluble redox transition. To explain this unusual value of α, a two–step adsorption – desorption mechanism is proposed, wherein HT undergoes fast charge transfer and adsorption at the electrode surface, followed by slow desorption of the adsorbed oxoammonium cation. Evidence collected using in situ surface enhanced Raman spectroscopy, combined with density functional theory simulations, suggests the presence of the adsorbed oxoammonium on the electrode surface. Additional evidence of adsorption, collected using a novel electrochemical approach, is presented.[1] N. A. Shaheen, M. Ijjada, M. B. Vukmirovic, and R. Akolkar, J. Electrochem. Soc., 167, 143505 (2020).