Electrochemical impedance spectroscopy at an ultramicroelectrode

Abstract

The use of microelectrodes represents one of the most important advances in electrochemistry in the last twenty years: ultramicroelectrodes in particular (the dimensions of which are typically a few micrometers) are now used for a wide range of electrochemical studies [l-6]. As a result of their small size, the IR drop in the electrolyte and the charging current are minimised, while a high flux of electroactive species to and from the electrode is ensured as a result of the radial nature of the diffusion profile which is rapidly established following commencement of electrolysis. This last feature is of importance in studies of fast electron transfer processes where it is essential to maximise mass transport in order to permit the heterogeneous rate constant, k, to be determined. AC impedance spectroscopy offers one of the best methods for characterising an electrochemical system since both Faradaic and non-Faradaic processes may be readily measured. This technique has been used widely for studying such processes at electrodes of normal dimensions [7]. By combining ac impedance spectroscopy with ultramicroelectrodes, a powerful tool is created which may be used to probe many and diverse electrochemical systems. Only a limited number of studies have been carried out using ac voltammetry at a microdisk [g-13], and one article [141 has reported ac impedance measurements at a microelectrode array. Here we discuss the measurements and analysis of ac impedance data obtained at ultramicrodisk electrodes with radii ranging from 1 to 12.5 pm. Ultramicroelectrodes may be produced in a number of different shapes, but the easiest to fabricate and hence the most common is the microdisk. Several authors have attempted to derive appropriate equations for the ac impedance at a microdisk [2,13,15-181, but the most comprehensive treatment is that of Fleisch-