Mass transport inlaid electrodes

A point electrode resembles a spot. It adopts a spherical-shaped concentration profile and potential distribution in the solution. As a result, such electrodes easily achieve a steady state and yield a steady-state current. This current is expected to be proportional to the characteristic length (radius) of the electrode. A typical point electrode is a disk electrode inlaid on an insulating plane. On the other hand, an ultrathin ring electrode shares characteristics of the point electrode and the line electrode. It appears as a point from a position distal from the electrode, but it resembles a curved line upon closer inspection. It exhibits a steady-state current because of the feature of the point electrode. Next, a plane electrode of interest is a microarray electrode, which is composed of point electrodes and line electrodes on a planar insulator. It is versatile in functionality by designing the geometrical arrangement. A mode of mass transport depends on whether elementary electrodes are a point or a line electrode. 

Most of the initial practical and theoretical work in cyclic voltammetry was based on the use of macroscopic-sized inlaid disc electrodes. For this type of electrode, planar diffusion dominates mass transport to the electrode surface (see Fig. II. 1.13a). However, reducing the radius of the disc electrode to produce a micro disc electrode leads to a situation in which the diffusion layer thickness is of the same dimension as the electrode diameter, and hence the diffusion layer becomes non-planar. This non-linear or radial effect is often referred to as the edge effect or edge diffusion . 

If microelectrodes (hemispherical or inlaid-disc types) are used, the mass transport rate due to radial diffusion to the electrode is enhanced so that the current contribution of chemical steps is decreased relative to the diffusive transport. This statement can be easily verified on any EQrrE sequence at a macroelectrode of radius a 100 /zni the diffusion-controlled two-electron wave is observed when two one-electron electrode processes occurs at the same potential. At microelectrodes in steady-state regime the time scale is too short for the overall reaction and the number of electrons measured on the limiting current gradually declines to unity with decreasing electrode radius. On the other hand, the change in the current as a function of the electrode radius can be used to determine the rate constants of involved chemical steps. 

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