Interpretation of scanning reference electrode measurements for characterization of plasma-liquid systems

Plasma-liquid systems have been used over the past several years for the purpose of promoting chemical transformations in the liquid phase, but methods of characterizing the reaction environment in the solvent are still in nascent development. We have been using a technique that involves measuring the electrostatic potential difference between two nominally identical Ag/AgCl reference electrodes, wherein one electrode is placed near the plasma-liquid interface (PLI), and another is placed far away in the bulk solution.^1-3 The electrode at the PLI can be scanned to reveal the spatial distribution of the reaction environment, and we have identified distinct zones that promote reduction and oxidation half-reactions.² The scanning reference electrode technique has been useful to reveal the qualitative electrochemical structure of the PLI, but quantitative interpretation in plasma-liquid systems has not yet been elucidated. In this presentation, I will elucidate the fundamental meaning of the measurement by derivation from the local electrochemical equilibrium expressions and clarify the criteria that must be met for the measurement to reveal the spatial distribution of reduction potential at the PLI. Examples from well-defined electrochemical experiments involving dissimilar metals, in electrical contact though an external circuit, and ionic contact through an electrolyte, will be used to illustrate the argument.

References.

(1) Oldham, T.; Yatom, S.; Thimsen, E. Plasma parameters and the reduction potential at a plasma–liquid interface. Physical Chemistry Chemical Physics 2022, 24 (23), 14257.

(2) Oldham, T.; Thimsen, E. Electrochemical Structure of the Plasma–Liquid Interface. J. Phys. Chem. C 2022, 126 (2), 1222.

(3) Oldham, T.; Chen, M.; Sharkey, S.; Parker, K. M.; Thimsen, E. Electrochemical characterization of the plasma-water interface. J. Phys. D 2020, 53 (16), 165202.