Diffusion overpotential chronoamperometry

In this section, both approaches will be compared in chronoamperometry under limiting current conditions at spherical electrodes and microelectrodes. As is well known, for spherical electrodes and taking into account the Butler-Volmer model, the value of the diffusion-controlled reduction current at large overpotentials, e B is given by the following expression (see Eq. (2.147) of Sect. 2.5.2) ... 

Elimination of diffusion contribution to the overpotential in chronoamperometry and chronopotentiometry... 

In chronoamperometry, after switching on an overpotential, the time dependence of the current is monitored. For purely diffusion-controlled processes, the current depends on time according to the Cottrell equation in Chapter 5, Eq. (5.20). The current decreases proportional to 1/Vt. The combination with charge transfer control leads to the following... 

The Cottrell equation is derived from Pick s second law of diffusion (Section 1.5) and predicts the variation of the current in time, when a potential step is applied under conditions of large overpotential. For this equation to be valid the current must be limited by diffusion of the analyte to the electrode surface, and thus the solution has to be unstirred. The overpotential at which the reaction is driven must be large enough to ensure the rapid depletion of the electroactive species (O) at the electrode surface, such that the process would be controlled by the diffusion to the electrode. This equation is most often applied to potential step methods (e.g., chronoamperometry see Chapter 11) ... 

In linear chronoamperometry the potential is made to vary linearly with time, at a rate of 0.1-2 Vmin" A curve such as that shown (figure C.2) is then obtained. The reaction begins when the decomposition potential is reached, and its rate, and therefore the current, rise rapidly with increasing overpotential. The concentration of the reactant at the electrode surface becomes depleted as the result of this, however, and can only be replaced by diffusion. The current cannot, therefore, increase indefinitely, but after a certain time reaches a... 

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