(Invited) Extracting Kinetics from Scanning Electrochemical Microscopy Images through Least-Squares Fitting of Reactive Discs

Abstract

The strength of scanning electrochemical microscopy measurements (SECM) is its ability to give insights into the kinetics of an underlying surface. Up to now there was no established way to extract kinetics from an SECM image. Our study fills this gap by proposing a method that fits the active area and kinetics of reactive sites from SECM images using the Levenberg-Marquardt algorithm and finite element method simulations.Currently, the kinetics of reactions at a surface are quantified through SECM by performing approach curves: measuring the SECM current as the microelectrode approaches a surface vertically.[1] The theory used to fit approach curves assumes that the kinetics at the surface are uniform across an infinitely large surface. Realistically, sites at which reactions occur in SECM imaging are not infinitely large, leading to an under-estimation of kinetics at finite reactive sites. In some cases, approach curve theory can misidentify diffusion-limited surface reactions at small sites as being electron-transfer-limited. Our new method instead assumes that reactive sites are uniformly reacting discs. High agreement between the fitted and true areas of reactive sites are shown for both simulated and experimental SECM images. Less than 10% error was common for nearly-circular reactive sites. Diffusion-limited processes are not mislabelled as electron-transfer-limited by this method. The rate constants for electron-transfer-limited reactions were usually fit to within 15% error, much lower than the error experienced by the approach-curve method when applied to finite reactive sites.[1] C. Lefrou, R. Cornut, Analytical Expressions for Quantitative Scanning Electrochemical Microscopy (SECM). ChemPhysChem 2010, 11, 547-556.