Electrode kinetics charge transfer process

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. 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

Two points may be made at this stage. First, the quantity of charge transferred between phases in order to establish an equilibrium potential difference is normally so small that the actual change in composition of the solution is negligible. For example, one can show that when a 1 cm2 platinum electrode is immersed in a Fe2+/Fe3+ solution, a net reduction of between 10-9 and 10-,° moles of Fe3+ takes place. Second, and as will be stressed later, the kinetics of the charge transfer process are very important, since if rates are slow, it may not be possible for a true equilibrium to be established. 

The changes in the potential profile of the interfacial region because specific adsorption do indeed affect the electrode kinetics of charge transfer processes, particularly when these have an inner sphere character [13, 26] (see Fig. 1.12). When this influence leads to an improvement of the current response of these processes, the global effect is called electrocatalysis. ... 

The concentration profiles are very sensitive to the kinetics of the electrode reaction. In this context, the determination of the diffusion layer thickness is of great importance in the study of non-reversible charge transfer processes. This magnitude can be defined as the thickness of the region adjacent to the electrode surface where the concentration of electro-active species differs from its bulk value, and it can be accurately calculated from the concentration profiles. In the previous chapter, the extensively used concept of Nemst diffusion layer (8), defined as the distance at which the linear concentration profile (obtained from the straight line tangent to the concentration profile curve at the electrode surface) takes its bulk value, has been explained. In this chapter, we will refer to it as linear diffusion layer since the term Nemst can be misunderstood when non-reversible processes... 

Eq. (3.66). These conditions relate the electrode radius not only to JnDt, like for a reversible charge transfer process (see Sect. 2.7), but also to the kinetic parameters k° and a, and the applied potential E. 

When the rate determining step of the electrode reaction is the charge transfer process (kinetic control), the faradic impedance ZF in Figure 1.18 can be described as RCJ, the charge transfer resistance [7,8], The impedance plot in the Nyquist plane describes a semicircle, as shown in Figure 1.19. 

Equation (16.40), though rather complex in form, is of remarkable importance because it describes the overall charge transfer process via the valence band at a n-type semiconductor electrode for both anodic and cathodic polarizations. As mentioned earlier, jo represents the generation/recombination rate of holes in the bulk of the semiconductor and jo represents the rate of hole transfer at the interface. The ratio jo/ jy indicates whether the generation/recombination or the surface kinetics of the hole transfer is rate determining. If j0/yv° 1, i.e., the rate is controlled by surface kinetics due to slow hole injection, then... 

In the present article, various fundamental photoelectrochemical effects are quantitatively described and discussed, with the main emphasis on the kinetics of charge transfer processes. Although in principle the same reaction mechanisms are valid for extended semiconductor electrodes and particles, different factors govern the reaction rate, as will be discussed in detail. Finally, a brief overview of various applications will be given. 

Mass transport limitation is more often encountered in electrode kinetics than in any other field of chemical kinetics because the activation-controlled charge-transfer rate can be accelerated (by applying a suitable potential) to the point that it is much faster than the consecutive step of mass transport, and therefore no longer controls the observed current. From the laboratory research point of view, mass transport is an added complication to be either avoided or corrected for quantitatively, in order to obtain the true kinetic parameters for the charge-transfer process. 

It is a point peculiar to electrochemical reaction kinetics (77), however, that the rates of charge-transfer processes at electrodes measured, as they have to be, at some well-defined potential relative to that of a reference electrode, are independent of the work function of the electrocatalyst metal surface. This is due to cancellation of electron-transfer energies, O, at interfaces around the measuring circuit. In electrochemistry, this is a well-understood matter, and its detailed origin and a description of the effect may be found, among other places, in the monograph by Conway (77). 

A theoretical current-potential curve (/7/q vs. fj) is given in Fig. 7.3 for r] = 0.5. It should be emphasized here that Eq. (7.11) is only valid in this simple form if the current is really kinetically controlled, i.e. if diffusion of the redox species toward the electrode surface is sufficiently fast. According to the Butler-Volmer equation (Eq. 7.11) the current increases exponentially with potential in both directions. In this aspect charge transfer processes at metal electrodes differ completely from those at semiconductors. When the overpotential is sufficiently large, erj/kT 1. one of the exponential terms in Eq. (7.11) can be neglected compared to the other. In this case we have either... 

Experimentally measured ac current or total admittances are functions of the electrode potential. Figure 17 presents the dependence of the total admittances of a process limited by the diffusion of electroactive species to and from the electrode and the kinetics of the charge-transfer process, on the electrode potential. Information on the kinetics of the electrode process is included in the faradaic impedance. It may be simply... 

Published data on the rates of charge-transfer processes in organic solvent systems are remarkably scarce. This is particularly true of comparative data pertaining to diferent solvents or mixtures and contrasts noticeably with the extensive literature on polarographic data in organic solvents. Nevertheless the role of solvent in electrode kinetics is... 

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