(Keynote) Electrode Stimulation: Ohmic Microscopy and the Monopolar Electrode

Abstract

In a recent communication (1), we described a novel method of controlling the electrostatic potential in the solution neighboring a working electrode, we have dubbed the monopolar electrode , ME, polarized at a constant potential, EME, using a potentiostat. This tactic involved passing current between a second or stimulating electrode , SE, placed in front, and at a relatively small distance from the ME, and a distant auxiliary electrode, AE. Depending on the polarity of the current flowing through the SE, the electrostatic potential just outside the ME could be modified so as to increase or decrease the surface overpotential, and, thus modulate the rates of redox reactions at the ME|electrolyte interface. This presentation will review critical aspects of ohmic microscopy, and illustrate with two examples the electrode stimulation effect, using a ring disk electrode, RRDE, in which current flowing through the disk acting as the SE, is employed to induce reactions on the ring, acting as the ME, namely: i. the oxidation of adsorbed carbon monoxide on Pt using a Pt|glassy carbon RRDE and ii. the reduction of selenite to elemental Se on Au in aqueous 0.1 M HClO4 using a Au|Au RRDE. In both these cases, the stimulation current was generated by scanning the potential of SE within its double layer region, so as to minimize any changes in the actual composition of the solution adjacent to the ME, while monitoring the current flowing through the ME, IME . Under these conditions, the electrostatic potential in solution follows the primary current density. The efficiency of the stimulation process was determined by coulometric analysis of the CO oxidation current and in the case of selenite via a coulometric analysis of the current due to the oxidation of elemental Se produced by the reduction of selenite. The stimulation efficiency, in this latter case, could be accurately determined from a coulometric analysis of the peak for Se oxidation observed by subsequently scanning the ME linearly toward positive potentials to yield selenite. Excellent quantitative agreement was obtained between IME and EME as a function of Idisk and theoretical simulations employing COMSOL using parameters extracted from independent measurements performed under otherwise identical experimental conditions. This overall approach will open new prospects for gaining insight into the rates of surface diffusion and other interfacial dynamics phenomena.AcknowledgementsThis work was supported by a grant from NSF, CHE-1412060 References Han, Q.; Georgescu, N. S.; Gibbons, J.; Scherson, D. Acta 2019, 325, 134957.