KINETICS OF ELECTRODE REACTIONS

In Chapter 1, we established a proportionality between the current and the net rate of an electrode reaction, v. Specifically, v = i/nFA, We also know that for a given electrode process, current does not flow in some potential regions, yet it flows to a variable degree in others. The reaction rate is a strong function of potential thus, we require potential-dependent rate constants for an accurate description of interfacial charge-transfer dynamics. 

Consider two substances, A and B, that are linked by simple unimolecular elementary reactions.  

Both elementary reactions are active at all times, and the rate of the forward process, Vf (M/s), is 

The rate constants, kf and have dimensions of s and one can easily show that they are the reciprocals of the mean lifetimes of A and B, respectively (Problem 3.8). The net conversion rate of A to B is 

The kinetic theory therefore predicts a constant concentration ratio at equilibrium, just as thermodynamics does. 

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In the present chapter we consider the electrochemical methods used to measure OCV and electrode potentials and to study the kinetics of electrode reactions. These methods are also described in great detail in the book by Bard and Eaulkner (2001). 

Frumkin, A. N., V. S. Bagotsky, Z. A. lofa, and B. N. Kabanov. Kinetics of Electrode Reactions [in Russian], Moscow University PubUshers, Moscow, 1952. 

S. Qureshi, Study of kinetics of electrode reaction. Ph.D. thesis, Bhopal University, 1974. 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

Galus, Z., in Advances in Electrochemical Science and Engineering, Vol. 2 (Eds H. Gerischer, C.W. Tobias), VCH, Weinheim, 1994, pp. 217-295. Thermodynamics and kinetics of electrode reactions in non-aqueous and mixed solvents. 

Cyclic voltammetry is used to characterize the redox behavior of compounds such as Q> in Figure 17-24 and to elucidate the kinetics of electrode reactions.25... 

Fawcet has recently applied a simple molecular theory of solute adsorption at electrodes to the kinetics of electrode reactions under the effect of SAS [125]. 

Only two general reviews [38, 39] entirely devoted to the hydrogen evolution reaction have appeared after the start of the development of cathode activation [40]. In several other cases, hydrogen evolution has been discussed within the general frame of electrocatalysis [4, 41-47] or kinetics of electrode reactions [48, 49]. However, only one of the two reviews mentioned above discusses electrocatalytic aspects with literature coverage up to the late 70 s, when the field of cathode activation was at the beginning of its development. 

There are two advantages of the coulostatic method in the study of kinetics of electrode reactions. First, the ohmic drop is not of importance, therefore the measurements can be carried out in highly resistive media. Second, since Ic = IF, Q does not interfere in the measurement. By the help of this technique jo values up to about 0.1 A cm-2 and - standard rate constants up to 0.4cms 1 can be determined. A detailed discussion of coulostatic techniques can be found in Ref. [vi]. 

For more information on kinetics of electrode reactions and on transport phenomena in electrolyte systems see [32] and [33]. 

Studies of the effects of varying the double-layer structure upon the kinetics of electrode reactions have long been an active research topic, especially at the mercury-aqueous interface for which there exists a large and reliable body of data concerning the double-layer composition and structure. Work in this area prior to 1965 is reviewed in Delahay s well-known monograph [58] unfortunately, more recent reviews are conspicuous by their absence (but see, for example, ref. 16a). 

We saw earlier (cf. Section 19) that the potential dependence of adsorption of intermediates formed by charge transfer affects the kinetics of electrode reactions. We have worked out the kinetic parameters for a few mechanisms under so-called Langmuir and Temkin conditions (i.e., when the Langmuir and the Temkin isotherms are applicable, respectively). Here we shall derive the appropriate kinetic equations for the combined adsorption isotherm. 

Kinetics of Electrode Reactions in Pure Non-Aqueous Solvents. 238... 

We would like to present somewhat more extensively the results of the work of Dzhavakhidze et al. [Ill], who studied the role of the spatial dispersion of the solvent dielectric permittivity and field penetration into a metal in determining the kinetics of electrode reactions. Considering the particular case of the field penetration effect on the reorganization energy, they found [111] that the AG value obtained is greater than predicted by the Marcus theory. Moreover, under some eonditions the dependence of AG on the reactant-electrode distance d) exhibits an anti-Mar-cusian behavior. 

It is by now evident that there is nothing unusual in the electrochemical behavior of HTSC oxides at ambient temperatures [49,453]. The kinetics of electrode reactions on HTSC materials can be complicated by numerous side-processes. HTSC investigations under ordinary conditions are complicated by the low stability of cuprates to solvents and/or electrolyte ions, which is closely related to the principal problem of HTSC chemistry the problem of degradation. 

Coating the surface of silicon electrodes with a polymer coating can also be an effective method of stabilizing the electrodes and improving the photovoltage and kinetics of electrode reactions " The polymer film effectively insulates the semiconductor from the superoxide ion and prevents chemical reaction and deterioration. At the same time, the polymer behaves like a surface-bond redox couple to mediate the charge transfer between the semiconductor and the redox species in the solution. Various types of polymers can be used to coat silicon electrodes as shown in Table 6.6. 

The presence of a derivatized surface layer can affect the energetics as well as the kinetics of electrode reactions. It has been found that the flatband potentials of -Si and p-Si electrodes coated with conducting polypyrrole films are shifted by 300 and 500 mV in CH3CN solution." The reaction kinetics on polymer derivatized surface can further be enhanced by impregnation of noble metals such as Pt particles into the... 

The intensive electrochemical studies of polycyclic systems, especially cyclic volta-metry (CV) are now at a stage which justifies naming cyclic voltametry an electrochemical spectroscopy as was suggested by Heinze 65). Early electrochemical studies referred only to the thermodynamic parameters while CV studies provide direct insight into the kinetics of electrode reactions. These include both heterogeneous and homogeneous electron-transfer steps, as well as chemical reactions which are coupled with the electrochemical process. The kinetic analysis enables the determination of reactive intermediates in the same sense as spectroscopic methods do. As already mentioned, electron transfer processes occur in both the electrochemical and metal reduction reactions. 

The presence of the work terms Wp and w. in Eq. (n) of 12.3.7.2 indicates that the structure of the metal-solution double-layer region can exert an important influence upon the kinetics of electrode reactions. The effect of varying the double-layer structure upon electrode kinetics has long been an active research topic, esp>ecially at the Hg-HjO interface for which much thermodynamic information exists concerning double-layer composition and structure. ... 

Electrochemical generation of solvated electrons was first observed in 1897 by Cady who found that when sodium solutions in liquid ammonia are electrolysed the blue coloration intensity increases at the cathode. All information on cathode generation of solvated electrons remained at this qualitative level for over half a century until Laitinen and Nyman made the first attempt to quantitatively investigate the kinetics of this process. This work, however, remained isolated for a long time and only after 20 years, with the awakening of interest in the chemistry of solvated electrons, were systematic studies into the kinetics of electrode reactions of solvated electrons started, almost simultaneously by three groups of researchers in Southampton Tokyo and Moscow In Moscow these studies... 

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. 

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