Current-overpotential relationship regions

Figure 6.9. Four regions in the general current-overpotential relationship 1, linear 2, exponential 3, mixed control 4, limiting current density region.

Consider the case of low overpotential, often referred to as the micropolarization region or the linear current-potential region. The exponential terms in Eq. 35E can be linearized, using the relationship exp X = 1 -t- X, which is valid for x 1. This yields a linear relationship between the current density and the overpotential, namely ... 

They obtained a logarithm of current and potential relationship composed of two linear parts, and rationalized the polarization data. They determined the transfer coefficient in tire higher overpotential region 0.25 confinned later by Hori and Suzuki in their measurements of the partial current densities of HCOO formation at a Hg pool electrode.Tliis value indicates that tire rate determining step is tire first electron transfer to form CO2-" anion radical. The transfer coefficient in the lower overvoltage region is 0.67. 

Valuable information on the mechanism of a process can be obtained when the kinetics of the reaction are examined near the equilibrium potential and compared with the characteristics of the Tafel region(s). We have already given some formulas for particular mechanisms [Eqs. (60) and (66)] indicative of the linear relationship between the current and overpotential near the equilibrium, the proportionality factor (effective conductance di/drj )t,=0) being determined by the value of the exchange current. 

C RT/F), the exponents can be expanded into a series. Using the first two terms of the series, one can obtain a linear relationship between the overpotential r] and current density in the low overpotential region (Butler equation) ... 

A closely related experiment attempted to determine the effect of hydroxide ion concentration on the rate of anodic processes in the absence of polarization (78). This was done with a series of Tafel plots (79) (l-V relationship near the region where the OCP) in varying hydroxide ion concentrations. The results indicated that the anodic current representative of dissolution rate in the absence of overpotential is independent of hydroxide concentration. 

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