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

Abstract

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 previouslymore » proposed and are now confirmed by direct spectroscopic evidence.« less

Authors:

Shaheen, Nora A. ;

• Search DOE PAGES for author "Shaheen, Nora A."
• Search DOE PAGES for ORCID "0000-0001-7260-6034"

Dean, William; Penley, Drace; Kersten, Bethany; Rintamaki, Jacob; Vukmirovic, Miomir B.; Gurkan, Burcu E. ;

• Search DOE PAGES for author "Gurkan, Burcu E."
• Search DOE PAGES for ORCID "0000-0003-4886-3350"

Akolkar, Rohan

• Search DOE PAGES for author "Akolkar, Rohan"
• Search DOE PAGES for ORCID "0000-0002-9865-5704"

Publication Date:

Fri May 27 00:00:00 EDT 2022

Research Org.:

Case Western Reserve Univ., Cleveland, OH (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1870192

Alternate Identifier(s):

OSTI ID: 1868121; OSTI ID: 1869289

Grant/Contract Number:  

SC0019409

Resource Type:

Published Article

Journal Name:

Journal of the Electrochemical Society

Additional Journal Information:

Journal Name: Journal of the Electrochemical Society Journal Volume: 169 Journal Issue: 5; Journal ID: ISSN 0013-4651

Publisher:

The Electrochemical Society

Country of Publication:

United States

Language:

English

Subject:

25 ENERGY STORAGE; adsorption; electrochemical SERS; charge transfer reactions