Electrode kinetics fundamentals

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

The majority of fundamental measurements in electrode kinetics are made in one of the types of transients described below. It is meaningless to speak of a best transient method, because each electrochemical reaction has its own characteristics, and the large variation in the properties of the reactions concerned means there will be variety in the most appropriate transient method. Some general points follow here. All of them must be considered before picking the best technique to use. Further, it must be recalled that the biggest distinction between electrode reactions is in respect to those (few) that occur without adsorbed intermediates and... 

The symmetry factor P is obviously a central entity in electrodics and a fundamental quantity in the theoretical treatment of charge transfer at surfaces, particularly in relating electrode kinetics to solid-state physics. 

The scope of this first chapter is to present the fundamental concepts of electrode kinetics in relation to the subsequent chapters where more specific and detailed aspects connected with electrode reactions will be treated. 

There are two fundamentally different approaches to utilizing mass transport relationships to determine electrode kinetics. By determining electrode kinetics , we mean elucidating the dependence of the rate of the electrode reaction on the variables that affect it. For the reaction... 

As this chapter is primarily dedicated to the study of electrode kinetics, we wish to deal only briefly with the fundamental consequences of reactant adsorption for the methodology of the relaxation techniques, again confined to the potential step and the impedance methods. In addition, we will review briefly the potentialities of these methods with regard to the study of adsorption itself in the case of the reversible electrode reaction. 

Chapter 1 serves as an introduction to both volumes and is a survey of the fundamental principles of electrode kinetics. Chapter 2 deals with mass transport — how material gets to and from an electrode. Chapter 3 provides a review of linear sweep and cyclic voltammetry which constitutes an extensively used experimental technique in the field. Chapter 4 discusses a.c. and pulse methods which are a rich source of electrochemical information. Finally, Chapter 5 discusses the use of electrodes in which there is forced convection, the so-called hydrodynamic electrodes . 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

Fleig reviews fundamental aspects of solid state ionics, and illustrates many similarities between the field of solid state electrochemistry and liquid electrochemistry. These include the consideration of mass and charge transport, electrochemical reactions at electrode/solid interfaces, and impedance spectroscopy. Recent advances in microelectrodes based on solid state ionics are reviewed, along with their application to measuring inhomogeneous bulk conductivities, grain boundary properties, and electrode kinetics of reactions on anion conductors. 

Refs. [i] Bockris JO M, Reddy AKN, Gamboa-Aldeco M (2000) Modern electrochemistry, fundamentals of electrodics, 2nd ed, vol. 2A. Kluwer, New York, p 1048 [ii] Mayneord WV (1979) Biographical memoirs of fellows of the Royal Society 25 144 [Hi] Butler JAV (1924) Trans Faraday Soc 19 729 [iv] Butler JAV (1924) Trans Faraday Soc 19 734 [v] Erdey-Gruz T, Volmer M (1930) Zphys Chem 150A 203 [vi] Butler JAV (1935) The fundamentals of chemical thermodynamics elementary theory and electrochemistry, 2nd edn. Macmillan, London [vii] Delahay P (1965) Double layer and electrode kinetics. Interscience, New York, pp 154-159 [viii] Butler JAV (1951) (ed) Electrical phenomena at interfaces, in chemistry, physics and biology. Methuen, London... 

Butler-Volmer equation — The Butler-Volmer or -> Erdey-Gruz-Volmer or Butler-Erdey-Gruz-Volmer equation is the fundamental equation of -> electrode kinetics that describes the exponential relationship between the -> current density and the -> electrode potential. Based on this model the -> equilibrium electrode potential (or the reversible electrode potential) can also be interpreted. 

This equation (Tafel equation) is of fundamental importance in studies of electrode kinetics. It is actually an approximation of the - Butler-Volmer equation at... 

July 13,1916 in Berlin, Germany - Dec. 12,1974 in Berlin, Germany) Study of chemistry and physics in Gottingen and Berlin from 1955 professor of physical chemistry at the Free University of Berlin. Cooperation with K. F. Bonhoeffer, - Gerischer, and J. W. Schultze, almost 100 publications (fundamentals of electrode kinetics, overpotential, redox reactions, ion transfer, passivity and corrosion, especially of iron), most important is his book [i] ... 

To discuss the enthalpy of activation in electrode kinetics, we make use of the fundamental rate equation... 

In the preceding sections of this chapter, much attention was devoted to the essential act of electron transfer. Yet although being, for fundamental or analytic purposes, the subject of a large body of the electrochemical literature, pure electron transfer reactions generally are of less interest in terms of preparative electrochemistry. In the opinion of this author, electron transfer at an electrode must be considered as a particular class of activation of molecules to enhance their chemical reactivity. As such, electrode kinetics must be understood (as for other methods of chemical activation) in order to control and eventually direct the overall process to the selected target (compare Chapter 3). 

A number of excellent texts are available that provide a thorough discussion of electrochemical principles. Newman provides a comprehensive and mathematically detailed treatment of electrochemical engineering. Prentice provides slightly greater emphasis on applications. Bard and Faulkner emphasize analytical methods, and Bockris and Reddy provide a very approachable introduction to electrochemical processes. Gileadi provides an excellent treatment of electrode kinetics, and Brett and Brett provide a treatment that includes fundamentals as well as applications, including impedance spectroscopy. 

The net rate of electrochemical current generation in an electrode section is given by the Butler-Volmer equation, a fundamental relation in electrode kinetics [109],... 

It is fair to say that the effect of ultrasound upon the fundamental electron transfer processes at an electrode have been less widely studied than the effects upon mass transport phenomena. Electrode kinetics is defined by the Butler—Volmer equation, which by a series of practical assumptions reduces to the Tafel equation [44],... 

For example, see (a) P. Delahay, Double Layer and Electrode Kinetics. Interscience, New York, 1965, Ch. 7 A rigorous description of the phenomenology of electrochemical kinetics, (b) J. O M. Bockris, A. K. N. Reddy. Modern Electrochemistry, Vol. 2, Plenum Press, New York, 1970, Ch. 8 An explanative description of fundamental concepts. 

In suitable cases, pulse techniques such as chronocoulometry or rapid linear-sweep voltammetry also can be employed to monitor the electrode kinetics within the precursor state "i.e., to evaluate directly the first-order rate constant, k, [Eq. (a) in 12.3.7.2] rather than k. Such measurements are analogous to the determination of rate parameters for intramolecular electron transfer within homeogeneous binuclear complexes ( 12.2.2.3.2). Evaluation of k is of particular fundamental interest because it yields direct information on the energetics of the elementary electron-transfer step (also see 12.3.7.5). 

Conventional heterogeneous catalysis and empiricism could provide a starting point in the selection of electrocatalysts for new unexplored processes for chemical production, energy generation or conservation, and environmental control. However, a fundamental understanding of adsorption characteristics, electrode kinetics, mechanisms, adsorbate-support interactions, and deactivation processes are needed for improved electrocatalyst... 

The opponents of fundamental studies with idealized electrocatalysts and reactions cannot deny the unique insight into surface molecular and electronic or energetic interactions that new surface and mechanistic techniques generate. A combination of surface spectrometries, isotopic reactions, and conventional electrode kinetics could help unravel some of the surface mysteries. The application of such methods in electrocatalysis is limited at present to hydrogen and oxygen reactants on simple catalytic surfaces. Extension to a variety of model and complex reactions should be attempted soon. The prospective explorer, however, should strive and attend with care the standardization of analytical methods for meaningful interpretations and comparisons. 

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