Electrode Stimulation

Abstract

According to Newman[1], the rates of heterogeneous redox reactions can be written in terms of the surface overpotential, η s , i = io{exp(aanFηs/RT)- exp(-acnFηs/RT)}, where η s is defined as “the potential of the working electrode, relative to a reference electrode of the same kind placed in the solution adjacent to the surface of the working electrode.” On this basis, η s can significantly alter the current flowing across the electrode-electrolyte interface. Due to its intrinsic resistive character, the flow of electricity through the electrolyte will elicit changes in the electrostatic potential within that phase, which can be conveniently measured using reference electrodes, as has been implemented in this[2-4] and other laboratories[5]. To be described in this presentation is a novel means of inducing local changes in η s by passing current between a stimulating electrode, SE, and a distant counter electrode in the same solution. The overall tactic was demonstrated using a redox active self-assembled monolayer of N-ethyl-N’-octadecyl-4,4’-bipyridinium dibromide (EOB) adsorbed on a Au(poly) disk electrode as the WE and a Au(poly) disk of larger diameter placed parallel to the WE with its normal axis aligned along the corresponding axis of the WE, as the SE. The potential of the WE, EWE, was set at EWE= -0.35 V, i.e. the layer was polarized in its fully oxidized state (See the cyclic voltammogram in the Insert, Fig 1), while the SE placed at a distance d = 0.5 mm from the WE was scanned from 0.65 V up to 0.8 V, and back at ν = 1 V/s, yielding a iSE vs time curve shown in red dots in Fig. 1. The response of the WE is shown in blue in Fig. 1. As clearly indicated, iSE > 0 elicits a corresponding iWE < 0 due to the change in the electrostatic potential in the vicinity of the WE. Once iSE is reversed, the previously reduced fraction of EOB is oxidized returning the redox layer to its original fully oxidized state. This explanation is also consistent with the increase in iWE induced by a decrease in d (See black, green and magenta curves in Fig. 1). Implementation of this tactic will enable application of a local potential in a region next to a working electrode and thus promote electrochemical processes in that region without the need of a mask. Acknowledgements This work was supported by NSF, CHE-1808592.