Theory and Experiment for the Substrate Generation/Tip Collection Mode of the Scanning Electrochemical Microscope: Application as an Approach for Measuring the Diffusion Coefficient Ratio of a Redox Couple

Abstract

A numerical model for the substrate generation/tip collection (SG/TC) mode of the scanning electrochemical microscope (SECM), with amperometric electrodes, is developed and examined experimentally. Under the conditions of interest, this mode involves the diffusion-limited electrolysis of a solution species, R, to produce O, at a macroscopic substrate (generator) electrode. As O diffuses away, a fraction of the species is intercepted by a tip (collector) ultramicroelectrode (UME), positioned above the substrate, where the reverse reaction occurs, leading to the formation and diffusional feedback of R to the substrate. The time-dependent tip current response is shown to be sensitive to the ratio of diffusion coefficients of the O/R couple. Moreover, when the long-time (steady-state) tip current, determined under these conditions, is combined with measurements of the diffusion-limited current for the electrolysis of R at a tip positioned far from the substrate, differences in the diffusion coefficients of th...