Profiles, concentration-distance voltammetry

Figure 3.37 Concentration-distance profiles for voltammetry in stirred solution. Do = Dr.

Stirred-solution voltammetry utilizes current-voltage relationships that are obtained at a stationary electrode immersed in a stirred solution. In order to understand this aspect of electrochemistry, it is extremely useful to consider a typical current-voltage curve (voltammogram) in terms of the concept of concentration-distance profiles presented in the preceding section. The discussion will consider the potential, rather than the current, as the controlled variable. 

In this mode, a small SECM tip is used to penetrate a microstructure, for example, a submicrometer-thick polymer film containing fixed redox centers or loaded with a redox mediator, and extract spatially resolved information (i.e., a depth profile) about concentrations, kinetic- and mass-transport parameters [33, 34]. With a tip inside the film, relatively far from the underlying conductor or insulator, solid-state voltammetry, at the tip can be carried out similarly to conventional voltammetric experiments in solution. At smaller distances, the tip current either increases or decreases depending on the rate of the mediator regeneration at the substrate. If the film is homogeneous and not very resistive, the current-distance curves are similar to those obtained in solution. 

If charge diffusion is significantly slower so that the distance of charge transport, L, (=2(Dt) ) is clearly smaller than the thickness of the lamina, 5, the electrochemical response will be equivalent to that recorded when reactants freely diffuse from an infinite volume of solution to the electrode. This situation, often termed as thick-layer behavior, corresponds to semi-infinite boundary conditions, and concentration profiles such as that shown in Figure 2.5c are then predicted. Accordingly, Cottrell-type behavior is observed, for instance, in cyclic voltammetry (CV) and chronoamperometry (CA). In this last technique, a constant potential sufficiently cathodic for ensuring diffusion control in the reduction of Ox to Red is applied. The resulting current-time (i-t) curves should verify the Cottrell equation presented in the previous chapter (Equation (1.3)). 

Patching schemes are helpful in situations where there are singularities in the concentration profiles in certain regions, such as the edge of disc microelectrodes (see Chapter 9) or the boundary of the diffusion domain in thin-film voltammetry, amalgamation processes and electrode arrays (see Chapter 10). In the latter case, the domain is confined to a distance d... 

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