Investigation of microelectrodes: IX: Study of the edge effects at a microdisk electrode

Abstract

The behaviour of microdisk electrodes has attracted much notice, owing to their strong edge effect [1,2]. The diffusion regions at these electrodes have been studied theoretically [2-91. In order to describe the mass transport rate at a microdisk electrode, some authors [1,8,9] have compared the current density and the mass transport rate at the microdisk electrode with those at a rotating disk electrode, but the results were quite different, even for electrodes of identical radius, However, these data have often been cited in various situations to characterize the mass transfer efficiency of the microdisk electrode, producing a confusion of data. Thus, it is necessary to study further the basic problem and to establish a reasonable kinetic model. Our work describes the relations of the current density and the mass transport rate between these two types of electrode. Furthermore, the edge effect of the microdisk electrode can be illustrated using these relations. At a low scan rate, the cyclic voltammogram of a microdisk electrode is a “sigmoid” curve, owing to the effect of radial diffusion, i.e. the forward and backward sweep curves overlap each other, and only one kind of electrode reaction occurs. However, the two sweeps of the cyclic voltammogram separate gradually with increasing scan rate, and appear as two neighbouring curves, denoted the “retention cycle”. Because the inverse reaction occurs during the backward sweep, the total current is composed of the currents of the positive reaction I, and the inverse reaction I,. Galus et al. [3] determined a relation for the two currents: