Charge-transfer polarization curves overpotentials

Fig. 3.1 1 Experimental charge-transfer polarization curves, E vs. log liexl, for positive and negative overpotentials...

Equation (50) forms the basis upon which v can be evaluated (e.g. (1) by the radioactive tracer method to evaluate simultaneously and ), (2) by comparing i values at appropriate potentials for different reactant activities (3) coupling information from high and low overpotential regions of steady-state polarization curves " (extrapolated io and charge-transfer resistance, Rcr, respectively) (4) or by back-reaction correction analysis. 2 qqie first two methods involve determination of v at any single potential while the latter two procedures must assume that the same mechanism (and hence v) applies at different potentials (at which individual measurements are required) and that the reverse reaction occurs by the same path and has the same transition state and thus rate-determining step [for both forward (cathodic) and reverse reactions]. 

The Eq. (2.78) describes the dependence of the overpotential on the deposition time from point b to point c. The overpotential changes due to the change of the surface concentration of adatoms from Co,a at the equilibrium potential to some critical value Ccr.a at the critical overpotential, rj, at which the new phase is formed. Hence, the concentration of adatoms increases above the equilibrium concentration during the cathode reaction, meaning that at potentials from point b to point c there is some supersaturation. The concentration of adatoms increases to the extent to which the boundary of the equilibrium existence of adatoms and crystals has been assumed to enable the formation of crystal nuclei. On the other hand, the polarization curve can be expressed by the equation of the charge transfer reaction, modified in relation to the crystallization process, if diffusion and the reaction overpotential are negligible, as given by Klapka [48] ... 

We will consider only the influence of activation overpotential or overvoltage on secondary current distribution. It is useful to regard the slope of the polarization curve dE /di (if any effect of concentration overpotential can be ignored) as a polarization resistance R. This represents the slowness of charge transfer across the interface and is based on the electrode kinetics of the reaction. If acts in series with R, the resistance of the electrolyte, we can distinguish between two situations. If R R, then the kinetics of charge transfer and not electrolyte resistance determine the current distribution, i.e., secondary current distribution dominates. Conversely, if R R, primary current distribution dominates. Secondary current distributions tend to smooth out the severe nonlinear variations of current associated with primary distributions and they eliminate infinite currents associated with electrode edges. 

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