Determination of Electrode Kinetics

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

Chiu SL, Selman JR (1992) Determination of electrode kinetics by corrosion potential measurements zinc corrosion by bromine. J Appl Electrochem 22 28—37. doi 10.1007/ BFO1093008... 

The impedance method for determination of electrode kinetics was found to work for very fast electrode reactions. Measurements of I o up to 10" A m" (1 A cm ) are possible higher values of Iq can also be determined, but with less accuracy. The reason for the diminished accuracy for the extremely fast electrode reactions is that these have considerable diffusion component to the impedance even at fairly high frequencies, thus masking the form of the semicircle. The form of the semi-circle can be distorted also if the capacity of the double layer, C, is very large. It is thus advisable to work with small indicating electrodes. 

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. 

In the majority of methods described thus far, the interfacial kinetics are deduced by measuring concentration changes in the bulk of the solution rather than at the interface, where the reaction occurs. This introduces a time lag, limiting the resolution of the measurement in the determination of interfacial kinetics. A more direct approach is to identify the interfacial flux. This can be achieved in the electrolyte dropping electrode, via the current flow, but this method is only applicable to net charge-transfer processes at externally polarized interfaces. 

The rate constant, k, may then be derived from variation of the plateau current with the rotation rate by means of the popular Koutecky-Levich plots, where the inverse of the plateau current is plotted against the inverse of the square root of the rotation rate (Figure 4.12). The intercept allows the determination of the kinetic constant kr°, and of the rate constant k, if the amount of catalyst on the electrode surface is known. 

Determination of the kinetic parameters by using cyclic voltammetry is conceptually very similar to this t = 0 is taken to be the time at the formation of the intermediate (here Br2), i.e. at the forward current peak Ipa, and the time when it is monitored at t = t, i.e. at the current peak for the reverse electrode process, pc. The time-scale of the reaction, r, is given by the following equation ... 

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)]. 

Equation (6) is valid only if it is justly assumed that the equilibrium values of qM and E are established infinitely quickly. This is not the case at low electrolyte concentrations since then the diffusion of the ions composing the double-layer becomes a rate-determining factor. In other words, mass transport complicates the charging process. For practical reasons, studies of electrode kinetics are usually made in well-conducting solutions, so that this effect can be ignored. 

The determination of the kinetic parameters of simple electrode reactions from the jF vs. t1/2 curve resulting from a potential step, was described in 1954 by Gerischer and Vielstich [44], Originally, it was... 

The secondary current distribution is calculated by including the effects of the ohmic drop in the electrolyte and the effects of sluggish electrode kinetics. While the secondary distribution may be a more realistic approximation, its calculation is more difficult therefore, we need to assess the relative importance of electrode kinetics to determine whether we can neglect them in a simulation. 

Weppner W, Huggins RA. Determination of the kinetic parameters of mixed-conducting electrodes and application to the system Li3Sb. J Electrochem Soc 1977 124 1569-1578. 

Conversely for slow reactions, low rates of mass transport will be required to achieve significant deviations from N fs equalling one. Consequently, it can be appreciated that it is a study of the competition between the rates of mass transport and chemical kinetics that leads to the quantitative determination of electrode reaction mechanisms in hydrodynamic voltammetry. Importantly, for each hydrodynamic technique, there is one assessable convective transport parameter that directly relates to the kinetic time-scale. 

From a phenomenological point of view, the study of electrode kinetics involves the determination of the dependence of current on potential. It is therefore appropriate that we start this book with a general qualitative description of such a relationship, as shown in Fig. lA. 

One of the specificities of the photoelectrochemical processes, involving minority charge carriers photogenerated in the semiconductor electrode, is that the current-potential relationship is of rather limited usefulness with regard to the determination of reaction kinetics. This is connected with the fact that, under such conditions, changes in electrode... 

M. Handschuh, W. Lorenz, C. Adgerter, and T. Katterle, Determination of the kinetics of photoelec-trochemical processes with minority carriers from photocurrent onset potential on semiconductor electrodes, J. Electroanal. Chem. 144, 99, 1983. 

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