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 ( α a ) corresponding to the electro–oxidation of 4–hydroxy–TEMPO in an aqueous medium is ∼0.9, i.e., α 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.
- Research Organization:
- Case Western Reserve Univ., Cleveland, OH (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- SC0019409
- OSTI ID:
- 1870192
- Alternate ID(s):
- OSTI ID: 1868121; OSTI ID: 1869289
- Journal Information:
- Journal of the Electrochemical Society, Journal Name: Journal of the Electrochemical Society Vol. 169 Journal Issue: 5; ISSN 0013-4651
- Publisher:
- The Electrochemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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