Linear high-overpotential region

The method of extrapolation of a linear section of the Tafel line, which is realized over the high overpotential region, to zero overpotential is much simpler, but it introduces a degree of uncertainty as the method relies upon the assumption of a constant Tafel slope, independent of the overpotential. Experiments, however, often indicate some concave curvature in a linear ... 

This is usually called the "totally irreversible case", but we consider the term to be misleading because, as seen in Fig. JL, the reaction can, in fact, be reversed. Referring to it as the "linear Tafel region" implies that the reaction occurs at high overpotentials, where the rate of the reverse reaction can be neglected. 

In Fig. 26, plots of log vs ij are given for three values of (up to one tenth of the limiting current ij, the one-dimensional treatment is in complete agreement with the two-dimensional one between and the solutions of the two-dimensional equation were used), which show clearly that electrocatalytic effects of the substrate are dominant at all overpotentials, d log I d log has values of 1 and at low and high overpotentials, respectively. Similar equations set up for the thin film model show that, in this case, a linear dependence of 7j on % occurs over an even more extended region of tj values. 

Table 7.2 summarises the conclusions for the mechansims which have been considered. While the mechanisms and different rate determining steps lead to several values for the reaction order and Tafel slope, the experimental observations will not be totally diagnostic because different mechanisms can lead to the same values. Moreover, there are many more mechanistic possibilities than have been discussed here. On the other hand, the behaviour described by the data in Table 7.2 is often observed. Platinum is an example of a cathode material where two Tafel regions are observed, see Fig. 7.3. Close to the equilibrium potential, a Tafel line, slope (30 mV)" is observed, while at high overpotentials a further linear region, slope (120 mV) , can be seen. 

Whereas the charge-injection method is a small-amplitude perturbation method in which measurement is conducted during open-circuit decay, we now discuss a different open-circuit measurement, in which the initial overpotential is high, in the linear Tafel region. The equations we need to solve are similar to Eqs. 9K and lOK, except that the value of the current in Eq. lOK is that corresponding to the linear Tafel region, namely... 

As a first approximation, one might expect that the composition of an electroplated alloy would be related to the current observed for each of the elements, when measured alone in the same solution at the same potential. Assume, for simplicity, that both metals are deposited at high negative overpotentials, within the linear Tafel region (where rj/h > 1). Then, one could write the... 

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