Spectro-electrochemistry combines spectroscopic and electrochemical techniques, enabling time-resolved and in-situ measurements of phenomena occurring at charged electrode surfaces. Surface-enhanced Raman spectroscopy (SERS) yields highly specific information about the structure and composition of molecules. Coupling SERS with electrochemical experiments gives insight into changes near the electrode surface under polarization. In-situ SERS and electrochemical measurements are demonstrated to provide information on the electrolyte-electrode interface in three different applications: (1) specific ion adsorption in hydrogen-bonded concentrated electrolytes under polarization, (2) adsorption and charge-transfer mechanisms of redox-active organic in aqueous electrolytes, and (3) charge-transfer reactions of redox-active species in hydrogen-bonded electrolytes.

Differential capacitance measurements suggest that surface adsorption is prevalent in deep eutectic solvents (DESs) and more broadly concentrated hydrogen-bonded electrolytes, however, these measurements do not directly probe the surface layer. Therefore, in this study, SERS was employed to investigate potential-dependent changes in the surface layer of the electrode-electrolyte interface where surface adsorption was spectroscopically confirmed. However, this behavior was observed to depend on the charge density and the strength of hydrogen bonding.

In situ SERS coupled with electrochemical measurements was also used to probe redox reactions of redox-active molecules at the electrode-electrolyte interface. One study elucidated an adsorption and charge-transfer mechanism for a potential redox-active organic molecule, 4–hydroxy–2,2,6,6–tetramethylpiperidine–1–oxyl (4–hydroxy–TEMPO).¹ Nitroxide radicals, such as 4–hydroxy–TEMPO, are of interest for application in redox flow batteries due to their relative stability, and tunability through functionalization. In situ SERS combined with density functional theory (DFT) simulations confirmed the presence of surface–adsorbed species on an Au electrode during the oxidation of 4–hydroxy–TEMPO in an aqueous electrolyte. Direct spectroscopic evidence shows the oxidation of 4–hydroxy–TEMPO leads to the adsorption of the oxidation product, which then undergoes slow rate-limiting desorption from the electrode surface. This information is important for the design of redox flow battery systems that employ nitroxide radical redox-active species, as surface adsorption can affect battery performance.

In an H-bonded electrolyte containing redox-active species (methyl viologen and ferrocenedimethanol in ChCl:EG), SERS, electrochemical, and spectrochemical UV-vis techniques were used to study their charge-transfer reactions. UV-vis measurements provide molecular information related to the electronic levels of molecules. UV-vis coupled with SERS allows for the study of changes in the oxidation state of redox-active molecules under electrode polarization.

This work demonstrates the importance of spectrochemical measurements in determining phenomena at the electrode-electrolyte interface, particularly in a complex solvent system such as DESs. Emphasis is placed on the coupling spectroscopic techniques with other techniques to determine interfacial structuring and charge-transfer reactions relevant to energy storage applications.

(1) Shaheen, N. A.; Ijjada, M.; Vukmirovic, M. B.; Akolkar, R. Mechanism of Electrochemical Oxidation of Nitroxide Radicals in Ethaline Deep Eutectic Solvent. Journal of The Electrochemical Society 2020, 167 (14), 143505–143505. https://doi.org/10.1149/1945-7111/abc439.