Electrocatalysis Tafel parameter

Sufficiently far from equilibrium, which is the only interesting regime for current generation, the Tafel law (129), (130) with effective parameters is used in the simulations. Measurements performed on model electrocatalysts suggest different values for these parameters depending on the particular catalysts used or just their different structure. The mesoscopic effects in electrocatalysis are the hot topic nowadays [127, 187-189] and they are probable candidates for one of the reasons of this variance. The main cause of this variance may be associated with the complicated interplay between adsorption and reaction stages, and a modeler interested in fuel... 

So far in this chapter we have looked at a few things about electrode surfaces. Except for Tafel s law, reaction rate as an exponential function of potential, all had some relation to electrocatalysis. I started with Tafel s law because it is the basis of much of the rest for example, when one measures rates, current densities, as a function of potential, d//dx or dlny/dr vary with the mechanism of the reaction, and although several mechanisms have the same d In j/dr (i.e., the coefficient is not diagnostic), several do give values of that parameter, which indicates a specific r.d.s. 

The most important parameters in electrocatalysis are the overpotentials, which arise from the losses due to the kinetics at the electrodes and transport losses in the electrolyte. The goal is to have low overpotentials r/i at high currents. In electrocatalysis the current is referred to the electrode surface thus obtaining the current density j. The dependence between overpotentials t] and current density j is described by the Tafel equation (Eq. 9-1)... 

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). 

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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