Tafel Plots for Redox ECET and ECIT Processes

Tafel Plots for Redox ECET and ECIT Processes 

In a first approximation under non-FC conditions, the Tafel slope may be attributed to the action of the change in the free energy of electrons (i.e., the change in Fermi level of the electrode with respect to the electrostatic energy of the ions in the solution) on the fractional electronic change on the transition state. However, a persuasive case can be made that the experimental Volcano relations and Tafel slopes represent the same phenomenon. 

For redox processes, there is some evidence of Tafel slope curvature for certain processes under certain circumstances.228,229 These may be a partial result of double layer effects.196 In other cases experimental Tafel plots which are close to linear appear.177 230 The controversial question of P possibly varying with temperature231 232 will not be discussed here, although double layer effects196 may often be responsible. 

A relatively constant Tafel slope for reactions not involving adsorption, and those involving adsorption with complete charge transfer across the double layer, distorted by second order effects, may also be explained in terms of a non-Franck-Condon process. Since adsorbed intermediates in charge transfer processes also show adsorption energies depending on potential in the same way as the potential energy barrier maxima, these should also follow the same phenomena. 

The field is zero for the special case where b = 0 and a = r. The general result is  

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