Tafel Slopes for Reactions Proceeding in Multistep Pathways

Tafel Slopes for Reactions Proceeding in Multistep Pathways 

It is well known that when an electrochemical reaction proceeds in a multistep pathway with the steps in series, the Tafel slope observed for the log i vs. rj relation depends on the partial reaction sequence, and the state and potential dependence of adsorption of intermediates [see Eqs. (12) and (13)]. Various values of b for the reaction, RT/0.5F, can arise depending on (1) which step determines the rate of the overall reaction and (2) the conditions of coverage of the electroactive intermediate (H in the h.e.r.) in the reaction and the potential dependence of that coverage. Values RT/0.5F (j8 = 0.5) sometimes arise with reac- 

When steps other than simple electron transfer are rate controlling, the forms of the overall rate equation that then can be deduced are assumed to arise on account of potential dependence of coverage by a reaction intermediate (X) in the main reaction sequence given by 

In the case when discharge is not rate controlling, so that, for example, in the h.e.r., atom-ion desorption step characterizes the mechanism, we have 

This relation gives the regular Tafel slope for this mechanism of ft = RT/ + )8)F only when XxCx exp( VF/RT) 1. Under other conditions it is seen that b = (dr]/d n i) will not be simply propor- 

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