Current hemi spherical electrodes

Once the concentration profiles have been calculated, the current is obtained from Eq. (5.13). Figure 5.3 shows the variation of the cyclic voltam-mograms corresponding to a first-order catalytic mechanism where i = 0 and the electrode reaction is fully reversible. As the chemical reaction is faster and so the regeneration of species A, the reduction current increases and the oxidation peak in the reverse scan disappears. Eventually, for fast reactions (i.e., large Ki values) a sigmoidal, steady-state response is obtained that in the case of (hemi)spherical electrodes when Da = Db = D is described by the following expression [10] ... 

Diffusion of electroactive species to the surface of conventional disk (macro-) electrodes is mainly planar. When the electrode diameter is decreased the edge effects of hemi-spherical diffusion become significant. In 1964 Lingane derived the corrective term bearing in mind the edge effects for the Cotrell equation [129, 130], confirmed later on analytically and by numerical calculation [131,132], In the case of ultramicroelectrodes this term becomes dominant, which makes steady-state current proportional to the electrode radius [133-135], Since capacitive and other diffusion-unrelated currents are proportional to the square of electrode radius, the signal-to-noise ratio is increased as the electrode radius is decreased. 

As discussed for the case of (hemi)spherical microelectrodes in Chapter 4, the response in cyclic voltammetry at microdiscs varies from a transient, peaked shape to a steady-state, sigmoidal one as the electrode radius and/or the scan rate are decreased, that is, as the dimensionless scan rate, a = Y r lv/TZTD, is decreased. The following empirical expression describes the value of the peak current of the forward peak for electrochemically reversible processes [11] ... 

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