Electrode kinetics diffusion overvoltage

The difference between surface and bulk concentrations depends on the regularities of mass transport and manifests itself in the fact that the certain diffusion overvoltage arises in the system. Use of the rotating disc electrode (RDE) makes it possible to carry out the experiments under different intensities of forced convection and to control the intensity of diffusive mass transport. This provides the means to eliminate the diffusive part of the overvoltage promoting the determination of kinetic parameters. 

In an earlier note (p. 9) we mentioned the occurrence of overvoltage in an electrolytic cell (and overpotentials at single electrodes), which means that often the breakthrough of current requires an Uappl = Eiecomp r] V higher than Ehack calculated by the Nernst equation as this phenomenon is connected with activation energy and/or sluggishness of diffusion we shall treat the subject under the kinetic treatment of the theory of electrolysis (Section 3.2). 

If an electrode reaction is not entirely controlled by the kinetics but also by diffusion we can express the overvoltage r/ by two terms, namely... 

Electrode potentials measure only the relative thermodynamic likelihood for various halfreactions. In practice, kinetic factors can comphcate matters. For instance, sometimes the electrode process is limited by the rate of diffusion of dissolved species to or from the electrode surface. At some cathodes, the rate of electron transfer from the electrode to a reactant is the rate-limiting step, and a higher voltage (called overvoltage) must be apphed to accom-... 

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