Mechanism of Electrochemical Reaction

Two main contributions to the overpotential will be discussed in this book in some details. The first one is the charge (electron) transfer overpotential, which is due to a particular rate of the electrochemical reaction and takes place just at the electrodesolution interface. The second one is the mass transfer overpotential, which is due to delivering reactants to the electrochemical reaction interface or due to transporting products to the bulk solution. Other physicochemical processes taking place in the Nernst diffusion layer (e.g., chemical reactions and adsorption/desorption) can also contribute to the electrode overpotential, but they will not be discussed in this book. Note that chemical reactions occurring in the bulk solution should be taken into account to correctly estimate the concentration of the reduced, / buik nd oxidized, Obuik. species. 

Electrode overpotential is generally an additive function of all processes mentioned earlier, and each of them is described by its own overpotential, i),. The total overpotential of a single electrode is a sum of all contributions  

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Experimental studies in electrochemistry deal with the bulk properties of electrolytes (conductivity, etc.) equilibrium and nonequilibrium electrode potentials the structure, properties, and condition of interfaces between different phases (electrolytes and electronic conductors, other electrolytes, or insulators) and the namre, kinetics, and mechanism of electrochemical reactions. 

Great promise exists in the use of graphitic carbons in the electrochemical synthesis of hydrogen peroxide [reaction (15.21)] and in the electrochemical reduction of carbon dioxide to various organic products. Considering the diversity in structures and surface forms of carbonaceous materials, it is difficult to formulate generalizations as to the influence of their chemical and electron structure on the kinetics and mechanism of electrochemical reactions occurring at carbon electrodes. 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

It has been shown in this paper particularly that the FTIR spectroscopy can identify radicals and chemical reactions, and by their potential and concentration dependence give considerable information upon the mechanism of reactions and the detailed mechanism of electrochemical reactions, including their ratedetermining step. The analysis of intermediate radicals has always been a need in electrochemical research, and is clearly now here. 

Strong conformational changes may accompany electron transfer. This issue has been discussed in Section 1.5 and illustrated by an experimental example in Section 1.5.5, in the case where conformational change and electron transfer are concerted and the second electron transfer becomes easier than the first. Conformational changes do not necessarily cause the second electron transfer to be easier than the first. In all cases, their influence on the kinetics and mechanisms of electrochemical reactions should be analyzed. 

Selectivity in homogeneous reactions is treated in Refs. [2-6], while the selectivi-ties that are influenced in heterogeneous reactions by diffusion and adsorption are dealt with in Refs. [7, 8]. Kinetics and mechanisms of electrochemical reactions are covered in Chap. 1 of this volume and in Refs. [9-11]. 

Complex mechanisms of electrochemical reactions of PCMs Frequently it can... 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

A calculation of these effects was on the frontier of research in the 1930s, not so much because it helped obtain evidence for mechanisms of electrochemical reactions, but because it was an indirect way of probing the structure of the interface. However, in modern times, many direct methods of probing the interface are available, and the effects of the variation of the potential of the outer Helmholtz plane with the electrode potential can be left to exercises in the problems section of this chapter. 

There is nothing unique about the determination of the mechanism of electrochemical reactions. Electrochemical kinetics is a parallel field to that of heterogeneous chemical kinetics and basically the mechanism tasks in the two related fields are the same. There are three goals that must be reached consecutively. 

A classical approach72 to unravelling the mechanism of electrochemical reactions and to identifying the rate-determining step (RDS) is based on testing the validity of possible sequences of reaction stages according to the elementary theory of electron transfer. As opposed to disc voltammetry, one does not look for direct evidence such as the presence of intermediate components. As a consequence, more reaction sequences appear to be theoretically possible, which in the ideal case can be dismissed, all but one on the basis of experimental evidence. It should be pointed out that neither does the identification of intermediates by means of electrochemical or non-electrochemical techniques automatically lead to the true reaction mechanism. It is only an aid in the sense that identified intermediates must occur in a postulated reaction mechanism and that hence the number of possible mechanisms can be reduced. 

Then appears linear sweep rate voltammetry in which the electrode potential is a linear function of time. The current-potential curve shows a peak whose intensity is directly proportional to the concentration of electroactive species. If the potential sweep takes place in two directions, the method is named cyclic voltammetry. This method is one of the most frequently used electrochemical methods for more than three decades. The reason is its relative simplicity and its high information content. It is very useful in elucidating the mechanisms of electrochemical reactions in the case where electron transfer is coupled... 

In studies of the kinetics and mechanisms of electrochemical reactions it is important to know, at least approximately, the oxidation or reduction potential of the substrate and... 

Similar to chemical kinetics, the mechanism of electrochemical reactions regularly requires a series of physical, chemical, and electrochemical steps, comprising charge-transfer and charge-transport reactions. The velocity of these individual steps controls the kinetics of the electrode reactions and, thus, of the cell reaction. In this sense, three diverse kinetics effects for polarization must be taken into account ... 

The technique of cyclic voltammetry or, more precisely, linear potential sweep chronoamperometry, is used routinely in aqueous electrochemistry to study the mechanisms of electrochemical reactions. Currently, cyclic voltammetry has become a very popular technique for initial electrochemical studies of new systems and has proven very useful in obtaining information about fairly complicated electrochemical reactions. There have been some reported applications of cyclic voltammetry for solid electrochemical systems. It is worth pointing out that, although the theory of cyclic voltammetry originally developed by Sevick, ° Randles, Delahay, ° and Srinivasan and Gileadi" and lucidly presented by Bard and Faulkner, is very well established and understood in aqueous electrochemistry, one must be cautious when applying this theory to solid electrolyte systems of the type described here, as some non-trivial refinements may be necessary. 

The deliberate modification of electrode surfaces by coating with one or more layers of electroactive material has been used for a variety of purposes. Solar energy conversion, electrochromism, corrosion protection, and electrocatalysis are but a few of the applications which are currently of interest. The use of in situ Raman spectroscopic studies can help to determine the structural characteristics of electrode coatings at the molecular level and can provide information on the mechanisms of electrochemical reactions occurring at modified electrode surfaces. 

In situ FTIR spectroscopy has proved to be very useful for the investigation of the reaction mechanisms of electrochemical reactions and structural changes of substances involved in these reactions. Two different methods are used external reflection absorption spectroscopy and internal reflection spectroscopy. The application of these methods to the study of the electrochemical doping process of polypyrrole, and the comparison of the results are described in this contribution. 

Yeager, E. (1976). Mechanism of electrochemical reactions on non-metallic surfaces. Electrocatalysis of Non Metallic Surfaces, NBS Publications, Vol. 455, pp. 203-219. 

Modified electrodes with ad-atoms (usually deposited in the underpotential region) exhibit enhanced electrocatalytic activity for several categories of electrochemical reactions. The most extensively studied reactions are those related to the development of low-temperature fuel cell technology, namely, the reduction of oxygen and the oxidation of organic fuels. The ad-atoms may influence the rate and the mechanism of electrochemical reactions through [26-29]... 

Many partially lithiated (i.e., when x < 1) active materials in lithium-ion cells can be considered as solid solutions of mobile lithium (guest) in the host material—Z. Such host-guest material can be described by the similar set of thermodynamic equations as these used to characterize an amalgam. However, the mechanism of electrochemical reactions proceeding in a real ceU is much more complex than in the case of the reaction of silver amalgam formation. There are many factors complicating the overall process ... 

Photoelectrochemical CO2 Reduction, Fig. 1 Reaction mechanism of electrochemical reaction (a) and PEC... 

Mechanisms of electrochemical reactions of different systems, including transition metal complexes, were examined with a special attention paid to double layer effects and problems of generation and decay of intermediates which arise in such reactions. Electrodes modified with thin films of transition metal hexacyanoferrates and conducting polymers were investigated, also solid state electrochemistry in the absence of external supporting electrolyte were developed. Charge propagation rate in such mixed-valent solid systems and their electrocatalytic properties were studied. 

EQCM [172] significantly contributed to the elucidation of mechanisms of electrochemical reactions of fullerene films. The principle of the method is simple and based on frequency measurement of a piezoelectric quartz crystal. The frequency is sensitive to mass and viscosity changes on a thin metal film (electrode) deposited on the quartz crystal. Most commonly any contribution from shear forces is neglected and the observed fi-equency changes are regarded as being... 

Macagno VA, Giordano MC, Arvia AJ (1969) Kinetics and mechanisms of electrochemical reactions on platinum with solutions of iodine-sodium iodide in acetonitrile. Electrochim Acta 14(4) 335-357. doi 10.1016/0013-4686(69)85005-x... 

E. Yeager in "Mechanisms of Electrochemical Reactions on Nonmetallic Surfaces", NBS special publication 455., (1976), p. 203. 

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