Electro-Oxidation of Nitroxide Radicals: Adsorption-Mediated Charge Transfer Probed Using SERS and Potentiometry

Organic compounds containing nitroxide radicals such as 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) are redox-active and are of interest for potential applications in redox flow batteries. The mechanisms governing charge-transfer reactions of such compounds are not well understood. Specifically, the anodic charge transfer coefficient (alpha (a)) corresponding to the electro-oxidation of 4-hydroxy-TEMPO in an aqueous medium is similar to 0.9, i.e., alpha (a) deviates considerably from the expected value (0.5) for a symmetric single-step one-electron transfer redox reaction. In a previous publication (J. Electrochem. Soc., 2020, 167, 143505), we have proposed a reaction mechanism to explain such asymmetric behavior by invoking adsorption-desorption processes. In the proposed mechanism, reversible oxidation of 4-hydroxy-TEMPO leads to the adsorption of the oxidation product, which then undergoes slow rate-limiting desorption from the electrode surface. In the present contribution, supporting evidence is provided for this mechanism. In situ surface-enhanced Raman spectroscopy combined with density functional theory simulations are employed to confirm the presence of surface-adsorbed species at a Au electrode during electro-oxidation of 4-hydroxy-TEMPO. Furthermore, we employ chronopotentiometry to track the gradual re-equilibration of the electrode-electrolyte interface following the electro-oxidation of 4-hydroxy-TEMPO. Analysis of the chronopotentiometry data further suggests the presence of adsorbed species, which were previously proposed and are now confirmed by direct spectroscopic evidence.