Electrochemical reactions electrode-reaction mechanisms

This chapter concerns the study of electrode reaction mechanisms of inorganic and organometallic complexes. The emphasis is on proper use of experimental measurables from cyclic voltammetry for diagnosis of common mechanisms such as E, EC, CE, and ECE reactions. We employ the standard designation of electron transfer (et) reactions as E, and other chemical reactions as C. In practice, mechanistic studies make use of an array of electrochemical and other physical and chemical methods, but space limitations restrict our attention to the powerful and versatile technique of cyclic voltammetry (CV). If necessary, the reader may review the fundamentals of this technique in Chapter 3. 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

For references on electrochemical reaction kinetics and mechanism, see, e.g., Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004 Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001 Bethune and Swendeman, Table of Electrode Potentials and Temperature Coefficients, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, New York 1964, pp. 414-424 and Bethune and Swendeman, Standard Aqueous Electrode Potentials and Temperature Coefficients, C. A. Hampel Publisher, 1964. 

That double channel electrodes are ideally suited to the study of electrode reaction mechanisms involving following chemical reactions is illustrated by reference to studies on the electrochemical oxidation of 4-amino-iV, N,-di-methylaniline (ADMA) in basic solution at a platinum electrode [125], This reaction is thought to proceed via the scheme... 

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). 

Cell Design Albery and coworkers [9-14] used tubular electrodes for ex situ electrochemical EPR experiments. The tubular electrode is equivalent to the channel electrode in all respects, except that the cross section is circular rather than rectangular [82, 137]. Like the later-developed channel flow cell, this setup (shown in Fig. 23) permits the interrogation of electrode reaction mechanisms of relatively long-lived radical species, [9-14] since the convective-diffusion equations are mathematically well defined, which at steady state are given by Eq. (37)... 

Cell Design Waller and Compton [85] effectively mimicked the Allendo-erfer coaxial design vide supra), whilst simultaneously maintaining the mathematically well-defined laminar flow of the channel flow cell. This improved cell for electrochemical EPR [85] allowed an improvement in the channel cell regarding lifetimes of radicals amenable to study, whilst retaining the hydrodynamic flow that is essential for the investigation of electrode reaction mechanisms. 

Schichlein H, Muller AC, Voigts M, Kriigel A, Ivers-Tiffee E. Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells. J Appl Electrochem 2002 32(8) 875-882. 

Without the knowledge of the composition of the intermediates, most of the description of the electrode reaction mechanism is merely a guess, even if qualified, as regards the chemical identity of individual products and intermediates. This drawback was early realized and led to an increased effort to couple the electrochemical techniques with other methods which would make it possible to identify independently various intermediates and products formed in the course of electrode process. These attempts seem to underlay the development of molecular electrochemistry into its present shape and represent one of the major driving forces of its further development. 

Laevers P, Hubin A, Terryn H, Vereecken J (1995) A wall-jet electrode reactor and its application to the study of electrode reaction mechanisms. Part I design and construction. J Appl Electrochem 25 1017-1022... 

Compton RG, Fisher A, Tyley GP (1990) The waU-jet electrode and the study of electrode reaction mechanisms the EC reaction. J Appl Electrochem 20 912-915... 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

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