Tafel lines various temperatures

Processes with = 0 and a = 0 may be referred to as activationless. The possibility of such processes has been discussed in the literature over many years. The first mention of such a situation can be found in a paper by lofa and Mikulin, who attributed to the activation energy becoming zero the fact that the extrapolated Tafel lines for different temperatures intersect at a single point. Audubert, who proceeded from the potential curve diagram, also arrived at the conclusion that the activation energy tends to zero at sufficiently high overpotentials. Later, various aspects of this problem were studied by a number of investigators (for a review see Ref. (70)). 

It should be emphasized that for the reaction in Eq. (1), under the restrictions already mentioned, a pair of ultimate parameters (Ju,Vu) foj both directions, can alternatively be obtained from the intersection points of the extrapolated Tafel lines at various temperatures, as first observed by Post and Hiskey and schematically presented in Fig. 4. It may be concluded from Fig. 4 that the ultimate overpotentials possess the singular property of being temperature-independent. 

Figure 4. Schematic representation of Tafel lines for the cathodic and anodic directions of the reaction in Eq. (1), occurring in an isothermal WE-RE cell without transference, at various temperatures. It is assumed that Pc=Pa = 0-5 and that both symmetry factors are independent of temperature. EARO, Experimentally accessible region of overpotential.

Tafel lines at various temperatures. It is well known that the hydrogen electrode reaction in the anodic direction cannot be studied over a wide range of overpotentials because of certain limitations that have not been elucidated completely as yet. Therefore, the anodic branch of the isU ri)-ri dependence in Fig. 5 is missing. 

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