Tafel slopes anomalous

It can be seen that it was again difficult to obtain results from specimens where no stable rest potential could be measured. The harmonic currents in all cases were low and for certain specimens were of the same order as the distortion resulting from the input sine wave. The Tafel slopes obtained were in general anomalously high and the corrosion rates varied over several orders of magnitude. 

In some cases, however, anomalous Tafel slopes have been observed for strongly catalytic processes [40—42] if the transfer coefficient is temperature-dependent an extra term, d a/d(l/T), must be considered in eqn. (101). 

Since hydrogen evolution is an activated reaction, an increase in temperature can only be kinetically beneficial. However, in many cases the Tafel slope has been observed not to increase linearly with T (i.e., R T/aF) but to increase at a lower rate or even to remain constant [229-232]. This has stimulated much discussion [233-240] about the fundamental significance of such an anomalous behaviour which has been interpreted in terms of a partly potential dependent entropy of activation as op-... 

Sometimes very low Tafel slopes are claimed (<15mV) [99,253]. It seems difficult to interpret such an observation in terms of a specific mechanism. It is more probable that anomalously low Tafel slopes are the result of a combined thermal activation of the reaction and of the electrode surface state, resulting in a reaction rate limited by the diffusion of molecular hydrogen away from the electrode. Supersaturation of the electrode ad-layer by the evolved gas can also play a decisive role [254,255]. This phenomenon has been amply discussed in the case of Cl2 evolution on oxide electrodes [256], but the same idea can be applied to the case of H2 evolution [257,258],... 

It has already been mentioned that one of most used forms of Ni is Raney Ni which is obtained from Ni-Al or Ni-Zn alloys by leaching A1 or Zn in alkaline solution. However, the properties of the resulting electrocatalyst appear to depend on the nature of the precursor [135], Methods of application of the alloys are various [135]. A particularly convenient one is the so-called LPPS (low pressure plasma spray) [146]. Raney Ni prepared in this way has shown that lower Ihfel slopes can be obtained, thus suggesting a real electrocatalytic effect (Fig. 11). On such highly porous Ni it is possible that the proportion of particularly active sites (at the edges and peaks of crystallites [262] increases considerably. However, the effect of temperature on the Tafel slope is more than anomalous [248] suggesting indeed some temperature-induced surface modifications. In fact, recrystallization phenomena are observed which can be minimized by means of small additions of Ti, Mo or Zr. The... 

It is now recognized that anomalously low Tafel slopes can be observed for the chlorine evolution reaction due to rate-limiting transport of gas away from the electrode surface [471, 474], e.g. in concentrated chloride solutions at high temperatures. 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

Of special interest for the topic of the present chapter is the observation of Weaver that while the double-layer-corrected AS quantities are ligand sensitive, they are found to be independent of potential. This is not the case for the atom and electron transfer process involved in the hydrogen evolution reaction at Hg studied by Conway, et where an appreciable potential dependence of AS is observed, corresponding to conventionally anomalous variation of the Tafel slope with temperature. Unfortunately, in the work with the ionic redox reactions, as studied by Weaver, it is only possible to evaluate the variation of the transfer coefficient or symmetry factor with temperature with a limited variety of redox pairs since Tafel slopes, corresponding to any appreciable logarithmic range of current densities, are not always easily measurable. Alternatively, evaluation of a or /3 from reaction-order determination requires detailed double-layer studies over a range of temperatures. 

Figure 83 Different combinations of partial current potential diagrams to explain anomalous codeposition with preferential deposition of the less noble component in the gray areas the less noble component is preferentially deposited, (a) A and B kinetically controlled, different Tafel slopes, similar exchange current densities, (b) A and B kinetically controlled, different Tafel slopes and different exchange current densities (i q > Jq ), (c) A kinetically controlled, B diffusion controlled, different exchange current densities (Iq > Jq ), (d) A diffusion controlled, B kinetically controlled, similar exchange current densities. (Reproduaxl with permission from Ref. [6], 1994, Elsevier.)...

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