Electrocatalysis Tafel slope

Lead dioxide has been the subject of study as an anode material from the early days of electrocatalysis due, in large part, to its importance in the lead-acid battery. Its good corrosion resistance at high anodic potentials has also resulted in its use in a number of other electrochemical processes, e.g. organic synthesis (see Sect. 8). Aspects of the anodic behavior of Pb02 have been relatively recently reviewed by Randle and Kuhn [320], In acid solution, / -Pb02 has been shown to exhibit a Tafel slope of ca. 120 mV decade -1... 

VII. Tafel Slope Factor in Electrocatalysis and Its Relation to Chemisorption of Intermediates... 

Fig. 12. Illustrating better electrocatalysis for a process (I) with a low Tafel slope, b, value in relation to another process (II) with a higher logt o value but also larger b. HI, III are consecutive processes giving a change of b at t = rj,.

Thus, it is seen that in practical evaluation of electrocatalysis at various materials, the relative Tafel slope b values, and associated conditions of coverage by intermediates, are as important as the material dependence of logi o values, as discussed in Ref. 131. 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

When the electroactive species or an intermediate adsorbs on the electrode surface, the adsorption process usually becomes an integral part of the charge transfer process and therefore cannot be studied without the interference of a faradaic current. In this situation, surface coverages cannot be measured directly and the role of an adsorbate must be inferred from a kinetic investigation. Tafel slopes and reaction orders will deviate substantially from those for a simple electron transfer process when an adsorbed intermediate is involved. Moreover the kinetic parameters, exchange current or standard rate constant, are likely to become functions of the electrode material and even the final products may change. These factors will be discussed further in the section on electrocatalysis (Section 1.4). 

Addendum The Tafel Slope and Reaction Mechanism in Electrocatalysis... 

The kinetic parameters are slightly dilferent for iron N4-macrocyclic complexes, compared to cobalt complexes. In previous investigation of the electrooxidation of hydrazine catalyzed by FeN4 macrocyclics, the proposed mechanism involved adduct formation between Fe and the hydrazine molecule, prior to the rate determining step [46]. It is evident that the formation of a bond between the metal active site and the hydrazine molecule is a crucial step in electrocatalysis phenomena [47-50]. The electrooxidation of hydrazine on iron N4 macrocyclic complexes results in a Tafel plot with slope of around 0.040 V/decade, instead of 0.060 V/decade. The order in hydrazine is still one, but the order with respect to OH is two, so a reaction mechanism was proposed as follows [44, 45] ... 

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