Cyclic electrode kinetics

The ability of varying the rate of the mass transport by agitating the solution (or the working electrode) constitutes the basis of hydrodynamic methods (hydrodynamics = liquids in motion), which are a further support to the study of electrode kinetics. Nevertheless we wish to cite them here simply to cover a drawback of cyclic voltammetry. In fact, cyclic voltammetry is unable to discriminate between oxidation and reduction processes, and vice-versa. 

As is well known in the field of electrochemistry in general, electrode kinetics may be conveniently examined by cyclic voltammetry (CV) and by frequency response analysis (ac impedance). The kinetics of the various polymer electrodes considered so far in this chapter will be discussed in terms of results obtained by these two experimental techniques. 

Fig. 8.11. A cyclic voltammogram for a reversible charge-transfer reaction. (Reprinted from V. D. Parker, Linear Sweep and Cyclic Voltammetry, in Comprehensive Chemical Kinetics, Electrode Kinetics, Principles and Methodology, C. H. Bamford and R. C. Compton, eds., copyright 1986, p. 148, with permission from Elsevier Science.)...

As stated earlier, cyclic voltammetry is a good road map when one first comes to examine an electrode reaction. It gives the researcher some idea of the potential near which there is reactivity. Cyclic voltammetry should always be used to get some idea of things at the beginning of an electrode kinetic investigation.24... 

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 . 

G. K. Rowe, M. T. Carter, J. Richardson, and R. W. Murray, Langmuir 11 1797 (1995). Obtaining electrode kinetic parameters from cyclic voltamograms involving proteins. 

Singh and Dutt, using the approximation described in Sect. 2.3, have theoretically predicted, and experimentally verified, the behaviour when very fast scans are applied in both the linear sweep and cyclic voltammetric modes, for reversible [22, 23], quasi-reversible [25], and irreversible [24] electrode kinetics. Very attractive agreement with experiment is typically found, of which Fig. 15 is representative. 

Data collection itself in making measurements in electrode kinetics is relatively straightforward. There are a number of commercial systems that will carry out rudimentary data collection, such as those marketed by Solartron for impedance measurements, PAR for cyclic voltammetry and impedance measurements, BAS for cyclic voltammetry. The difficulty comes in designing a reasonably intelligent system that will deal with carrying out suitable experiments and cope with large amounts of accumulated data. The latter is not trivial and, in operational terms, is more important than the first. 

The combination of the high sensitivity of SEIRAS and a rapid-scan FT-IR spectrometer enables the spectral collection simultaneously with electrochemical measurements such as cyclic voltammetry and potential-step chronoamperome-try. The time-resolved measurement can give some information on electrode kinetics and dynamics, as has been shown in Fig. 8.24. Figures 8.25 and 8.26 represent another example of millisecond time-resolved ATR-SEIRAS study of current oscillations during potentiostatic formic acid oxidation on a Pt electrode [123]. At a constant applied potential F of 1.1 V, the current oscillates as shown in Fig. 8.25 a. Synchronizing with the current oscillations, the band intensities of linear CO and formate oscillate as shown in Fig. 8.26 (and also in Fig. 8.25 c). 

In many cases, some property of the output of the simulation is described by a known analytical or empirically derived expression which can be used to test if the simulation output is correct. The peak current of a cyclic voltammogram is the largest current recorded on the forward sweep. For an electrode process with fully reversible electrode kinetics, the peak current of a voltammogram in amps (A) is given by the Randles-Sevcfk equation [7-9] for a one-electron reversible reduction process ... 

Fig. 4.4. Effect of the electrode kinetics on the cyclic voltammetry of an E mechanism...

In this paper, we had employed binary carbon supports to fabricate thin film electrodes in DMFCs. The roles of binary carbon supports and an optimal mixing ratio will be evaluated and characterized through cyclic voltammetry measurements. It will be shown that with the usage of two carbon supports, electrochemical activities and loading contents of catalysts can be enhanced. This improvement is further exemplified by the enhanced electrode kinetics of methanol oxidation for a binary carbon support-electrode in comparison to a single support-electrode. 

Fast potential step experiments have a great advantage over cyclic vol-tammetric methods with regard to the analysis of the EC mechanism. Analysis by CV is made difficult at high scan rates data are distorted as a result of separations due to IR drop and slow electrode kinetics. DPS has neither of these problems. 

This is our first encounter with the use of simulation to analyze CV results. Through the theory of simulation (Chapters 4-6), a cyclic voltammetric or potential step response can be calculated for any electrochemical mechanism, given the parameters that describe the experiment (scan rate, scan range, electrode area) and the mechanism (reduction potentials, electrode kinetics, chemical reaction kinetics, and diffusion coefficients of all chemical species). The unknown parameters of the electrochemical mechanism can be varied until a simulation is obtained that closely resembles the experimental result. 

Chapter 2 introduces experimental and conceptual aspects of cyclic voltammetry. The relationship between electrode kinetics, chemical kinetics, and diffusion is explored, and the important concept of electrochemical reversibility is discussed. 

Lavagnini I, Pastore P, Magno F (1992) Application of cyclic voltammograms under mixed spherical/semi-infinite linear diffusion at microdisk electrodes for measurement of fast electrode kinetics. J Electroanal Chem 333 1-10... 

Peak potential (in voltammetry) — It is the potential at which the maximum current appears in -> linear scan voltammetry (LSV) and several other techniques peak height). It is one of diagnostic criteria for the estimation of electrode kinetics. If the reaction is simple, fast, and - reversible the peak potential is independent of the scan rate, or frequency, or pulse duration. The condition is that the electrode reaction is not complicated by the -> adsorption, the -> amalgam formation, the precipitation of solid phase, the gas evolution, or the coupled chemical reactions. In LSV and cyclic voltammetry (CV) of a simple, reversible, semi-infinite planar diffusion-controlled reaction Oxaq h- e Redaq the cathodic and anodic peak potentials are p,c = ... 

The measurement of formal potentials allows the determination of the Gibbs free energy of amalgamation (cf. Eq. 1.2.26), acidity constants (pIQ values) (cf. Eq. 1.2.31), stability constants of complexes (cf. Eq. 1.2.33, Chap. 1.2.34), solubility constants, and all other equilibrium constants, provided that there is a definite relationship between the activity of the reactants and the activity of the electrochemical active species, and provided that the electrochemical system is reversible. Today, the most frequently applied technique is cyclic voltammetry. The equations derived for the half-wave potentials in dc polarography can also be used when the mid-peak potentials derived from cyclic voltammograms are used instead. Provided that the mechanism of the electrode system is clear and the same as used for the derivation of the equations in dc polarography, and provided that the electrode kinetics is not fully different in differential pulse or square-wave voltammetry, the latter methods can also be used to measure the formal potentials. However, extreme care is advisable to establish first these prerequisites, as otherwise erroneous results will be obtained. 

All methods for the determination of electrode kinetics discussed here cover a very wide range of exchange current density values. The approximate value for i o can be estimated from cyclic voltammetry and the appropriate technique can then be chosen for further study. However, the choice of technique may not be determined by the value of io alone, the availability of instruments may influence the decision as well as other factors, e.g. extreme conditions of temperature and pressure or electrode configuration. 

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