Voltammetry in a Thin Layer of Redox-Containing Electroactive Species

Cyclic Voltammetry in a Thin Layer of Redox-Containing Electroactive Species 

The next problem concerns sweep voltammetry, which involves applying a slowly varying linear waveform starting at a potential where no electrochemistry occurs. The potential is swept past the formal potential of the couple. Once again we consider the reduction of the species O confined within a thin layer [Eq. (15)]. 

A potential profile as shown in Fig. 6.6 is applied to the electrode for the purpose of examining current potential characteristics. Behavior depends on the slope of the ramp If the rate of change of potential is sufficiently slow then on the forward sweep, it is possible to reduce all O within the layer. Thus the current initially increases as the potential is swept toward E and afterward the current decays to zero, since all the O is reduced. A similar situation occurs during the reverse sweep for the oxidation of R, where a (mirror image) peak occurs for its oxidation. If the sweep rate is faster, then the first scan may not capture all the species O for reduction. In other words the diffusion layer may not extend to the edge of the polymer layer. In this case a slightly asymmetric cyclic voltammogram 

FIGURE 6.6. A potential waveform applied to a redox species confined to a thin layer at a rapid sweep rate and a slow sweep rate v is the sweep rate is the formal potential of the couple. 

The system is assumed to be at equilibrium, and therefore the Nemst equation follows, so the surface concentrations are given by  

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