The Tafel-Slope Parameter

While the form of the Tafel equation with regard to the potential dependence of i is of major general interest and has been discussed previously both in terms of the role of linear and quadratic terms in 77 and the dependence of the form of the Tafel equation on reaction mechanisms/ the temperature dependence of Tafel slopes for various processes is of equal, if not greater general, significance, as this is a critical matter for the whole basis of ideas of activation and reorganization processes in the kinetics of electrode reactions. 

The conventional form of b, which follows from the recent lUPAC recommendation for the so-called transfer coefficientt a, 

In various places in the text, where generality is implied and the process concerned is not necessarily ale, single-step reaction, the transfer coefficient a is written. In the latter case, e.g., when charge transfer is involved in a rate-controlling desorption step, a factor is included in the relevant value of a (see p. 115). t Introduction of the stoichiometric number p, with a and n, in b is sometimes confusing. Some discussion of this matter is to be found in Ref. 13 and in Gileadi s contribution in Chapter 8 of the present author s monograph, Theory and Principles of Electrode Processes, Ref. 25. 

The main purpose of this paper is to examine the real temperature dependence of the Tafel-slope parameter b for various processes and to discuss the mechanisms of activation in electrode processes in the light of the observed behavior of b as a function of temperature. 

Before proceeding to direct attention to the real temperature dependence of Tafel slopes as found experimentally for a number of systems, it will be necessary to review the conventional behavior usually assumed and describe its theoretical and historical origins. The remarkable contrast of the behavior actually observed, to be described in Section III, to that conventionally assumed will then be apparent and thus the present major gap in our understanding of the fundamental aspect of potential dependence of electrode reaction rates will be better perceived. 

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Chapter 2, by B. E. Conway, deals with a curious fundamental but hitherto little-examined problem in electrode kinetics the real form of the Tafel equation with regard to the temperature dependence of the Tafel-slope parameter 6, conventionally written as fe = RT/ aF where a is a transfer coefficient. He shows, extending his 1970 paper and earlier works of others, that this form of the relation for b rarely represents the experimental behavior for a variety of reactions over any appreciable temperature range. Rather, b is of the form RT/(aH + ctsT)F or RT/a F + X, where and as are enthalpy and entropy components of the transfer coefficient (or symmetry factor for a one-step electron transfer reaction), and X is a temperature-independent parameter, the apparent limiting... 

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