Diffusion potential step, high overpotential

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

Prior to the potential step from Ej to E2, the concentration of the reactant at the surface of the electrode is the bulk concentration C. After the potential is stepped to E2, the surface concentration of the reactant decreases to zero because it reacts immediately under the high overpotential TI2. As time proceeds, the reaction zone depletes, the diffusion distance from the bulk to the eleetrode surface increases, and thus the concentration gradient deereases. So, the current declines with time. On a planar electrode, the eurrent i(t) at any time t is given by the Cottrell equation ... 

If a substance is reduced or oxidized reversibly, then its half-wave potential wiU be near the standard potential for the redox reaction. If it is reduced or oxidized irreversibly, the mechanism of electron transfer at the electrode surface involves a slow step with a high energy of activation. Therefore, extra energy must be applied to the electrode for the electrolysis to occur at an appreciable rate. This is in the form of increased applied potential and is called the activation overpotential. Therefore, Exa will be more negative than the standard potential in the case of a reduction, or if will be more positive in the case of an oxidation. An irreversible wave is more drawn out than a reversible wave. Nevertheless, an S-shaped wave is still obtained, and its diffusion current will be the same as if it were... 

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