Basic electrode kinetics

For a metal at equilibrium in an aqueous solution, the rates of the oxidizing (anodic) and reducing (cathodic) half-reactions are equal. These rates may be expressed as current densities /a and /c, whose algebraic sums are equal  

The magnitude of / or is a measure of the balanced Faradaic activity and is called the exchange current density. If,  

The potential of the metal at which this occurs is called the equilibrium potential, If the system deviates from equilibrium, say through active dissolution, a new potential is established, which is related to the equilibrium potential by a term called the overpotential  

the Tat el slope, is equal to nFloRT, where F is Faraday s constant and a is the transfer coefficient, Near x] = 0, the expression can be approximated by the linear form 

The potential-current curve in this region can thus be considered linear, with slope equal to RTH nF. This slope takes on the fomt of a resistance, called the charge transfer resistance  

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It is easier to see what experiments should be done in the electrode kinetic regime. Some basic electrode kinetics at polymer-solution interfaces must be measured - and this has not been successfully done. Thus, with the typical redox reactions of the ferrous/ferric type, it is important to establish Tafel parameters, particularly the parameter beta, and exchange current densities and rate constants. 

Experimental studies of electrode kinetics resulted in the formnlation of the basic empirical relationship, the Volmer-Butler equation, (6.10) or (6.13), describing the dependence of the electric current on the electrode potential. This eqnation involves the potential E, the rate constants, and the concentrations. 

This is the basic relationship of electrode kinetics including the concentration overpotential. Equations (5.4.40) and (5.4.41) are valid for both steady-state and time-dependent currents. 

In general, the electrochemical performance of carbon materials is basically determined by the electronic properties, and given its interfacial character, by the surface structure and surface chemistry (i.e. surface terminal functional groups or adsorption processes) [1,2]. Such features will affect the electrode kinetics, potential limits, background currents and the interaction with molecules in solution [2]. From the point of view of electroanalysis, the remarkable benefits of CNT-modified electrodes have been widely praised, including low detection limits, increased sensitivity, decreased overpotentials and resistance to surface fouling [5, 9, 11, 17]. 

Tafel s law is the primary law of electrode kinetics, in the sense that Arrhenius law is the basic law of thermal reaction. It applies universally to all processes that are controlled in rate by the interfacial transfer of electrons or by a rate-determining surface reaction that may be coupled to the interfacial electron [Fig. 9.25(a)]. Redox reactions without surface intermediates demonstrate Tafel s law well [Fig. 9.25(b)]. 

Volumes 26 and 27 are both concerned with reactions occurring at electrodes arising through the passage of current. They provide an introduction to the study of electrode kinetics. The basic ideas and experimental methodology are presented in Volume 26 whilst Volume 27 deals with reactions at particular types of electrode. 

Another important feature of mass transfer processes is related to the very physical nature of the phenomenon. As such it is easily quantifiable and predictable. Thus the rate of mass transfer to and from an electrode may be determined a priori for a given electrochemical system. As a result this rate may be used as natural built-in clock by which the rate of other electrochemical processes may be measured. Such a property was apparent in our earlier discussions related to electrode kinetics (electron transfer and coupled chemical reactions). Basically it proceeds from the same idea as that frequently used in organic chemistry for relative rate constant determinations, when opposing a chemical reaction of known (or taken as the reference in a series of experiments) rate constant against a chemical reaction whose rate constant (or relative rate constant) is to be determined. Many such examples exist in the organic literature, among which are the famous radical-clocks ... 

All the basic laws and concepts of electrode kinetics were developed and verified with the hydrogen electrode. Unfortunately, the Hz electrode must be considered to be extremely complicated. This may well have been the reason for the relatively slow development of electrode kinetics [140]. 

In view of this need, we discuss here a variety of electrocatalytic topics, ranging from basic and microscopic concepts to phenomenological principles. Thus, the origin of electrodic reactions, electrosorption, and electrode kinetics are introduced briefly for the benefit of the nonelectrochemist. Since electrocatalytic reactions take place at the electrode surface, attention is given to recent efforts to link catalyst activity with microscopic surface properties. These include surface crystallographic orientation, crystallite size and distribution, adsorbate-adsorbent-support synergism, multiple adsorption states, identification of surface intermediates, and electrocatalytic surface reaction mechanisms. 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

An overview of the different concepts involved in electrochemistry, spanning (torn basic theory (thermodynamics, transport and electrode kinetics, etc.) to the main applications (batteries corrosion, electrosynthesis and sensors). In addition, it covers the research methods used in electrochemistry, as well as giving an insight into organic electrochemistry. For master s degree level and engineering schools. A dozen practical experiments are described in brief, with a few problems to solve (corrections provided on the website). 

Electrochemical (or electrode) reaction kinetics is one kind of the chemical reaction kinetics. To obtain a better understanding of the theory of electrode reaction kinetics, understanding the basic knowledge of chemical reaction kinetics is necessary. In this section, the general chemical reaction kinetics will be presented first for facilitating the fundamental understanding of electrode kinetics and mechanism, particularly, for oxygen reduction reaction (ORR). 

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