Kinetics of the Electrode Process

This is the Tafel equation (5.2.32) or (5.2.36) for the rate of an irreversible electrode reaction in the absence of transport processes. Clearly, transport to and from the electrode has no effect on the rate of the overall process and on the current density. Under these conditions, the current density is termed the kinetic current density as it is controlled by the kinetics of the electrode process alone. 

The shape and symmetry of the RPV curve give information about the kinetics of the electrode process. This feature can be used for the extraction of kinetic parameters by defining the parameter... 

From the analytical expressions obtained for DDPV (given by Eq. (4.140)), an explicit analytical solution for the ADDPV current is immediately deduced, which is valid for spherical electrodes of any size and whatever the kinetics of the electrode process [55] ... 

Characterization of modified electrodes can be carried out by electrochemical, spectroscopic, and microscopic methods. Of the electrochemical methods we stress cyclic voltammetry, chronocoulometry, and impedance, which combined together measure the number of redox centres, film conductivity, kinetics of the electrode processes, etc. Almost all the non-electrochemical techniques described in Chapter 12 have been employed for the characterization of modified electrodes. 

The use of a potential-step technique such as cyclic staircase voltammetry represents a simple alternative to Ichise s method (j0 of obtaining information on both adsorption and electron transfer kinetics. The current decay immediately after a step is primarily capacitive while current at later times is almost totally due to electron transfer reactions. Thus, by measuring the current at several times during each step and by changing the scan rate, information on both the kinetics of the electrode process and the differential capacity can be obtained with a single sweep. 

Activation control of an overall dissolution rate can, of course, reside in the reduction process, in the oxidation process, in a mixture of both, or in a mixture including some transport control. The reduction process is usually more influential in determining the overall rate. Thus, in the absence of transport control, the kinetics of the electrode process for reduction of hydrated protons, or water molecules, or dissolved molecular oxygen plays the major role in metal dissolution kinetics. Indeed the literature confirms the conclusion that many of the systems seen in experiment or in practice are diffusion controlled that most of the rest are under mixed diffusion and activation control and that those with some activation control... 

Changes of A from one metal to another, for a given process (e.g. the HER), provide the principal basis for dependence of the kinetics of the electrode process on the metal and are recognized as the origin of electrocatalysis associated with a reaction in which the first step is electron transfer, with formation of an adsorbed intermediate. In the case of the HER, this effect is manifested in a dependence of the logarithm of the exchange current density, I o (i.e., the reversible rate of the process, expressed as A cm , at the thermodynamic reversible potential of the reaction) on metal properties such as 0 (Fig. 2) (14-16, 20). However, as was noted earlier, for reasons peculiar to electrochemistry, reaction rate constants cannot depend on under the necessary condition that currents must be experimentally measured at controlled potentials (referred to the potential of some reference... 

Current/Voltage Relationships for Irreversible Reactions Many voltammetric electrode processes, particularly those associated with organic systems, are partially or totally irreversible, which leads to drawn-out and less well defined waves. The quantitative description of such waves requires an additional term (involving the activation energy of the reaction) in Equation 23-11 to account for the kinetics of the electrode process. Although half-wave potentials for irreversible reactions ordinarily show some dependence on concentration, diffusion currents are usually still linearly related to concentration many irreversible processes can, therefore, be adapted to quantitative analysis. 

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... 

When an experimental polarization curve is performed, the overvoltage is applied from the outside between the points A and D of the dipole. Consequently, the true polarization, which coincides with the potential difference between B and D and determines the kinetics of the electrode process, generally differs from the measured value. 

As with any chemical process, it is necessary to consider both the thermodynamics and the kinetics of the electrode process. If we connect the two electrodes and monitor the cell potential while allowing no current to flow, the potential of the working electrode will eventually reach a steady-state value indicating that the cell... 

An understanding of the chemistry and the electrochemistry which occur in the cell, is essential for the successful adoption of an electrochemical process. This understanding is embodied in knowledge of the reaction mechanisms and thermodynamics and kinetics of the electrode processes. Electrode reactions are heterogeneous, multistep processes and can involve several species and phases liquid, solid and gas. The tendency for a parti-... 

The impedances acquired in the case of processes controlled by diffusion and kinetics of the electrode process is described by the Randles circuit (Fig. 4.2). They were analyzed by several different methods. 

Such an analysis gives easy access to the kinetics of the electrode process. 

The CV/potential step analysis was carried out in a series of other solvents. The variation in the reduction potential (measured against a ferrocene standard in each solvent) was shown to be best correlated with the solvent donor number (rather than the dielectric constant), indicating a specific interaction of the solvent with the nitronium ion. The kinetics of the electrode process were much... 

HEDP, indicating that the thickness of surface oxide layer decreases and changes the effect of the oxide layer on the kinetics of the electrode process. 

The kinetics of the electrode processes in vanadium-containing chloride melts was investigated in the middle of the last century [1-5]. Most of the studies were conducted using polarization measurements. Data obtained by different authors are rather fragmentary and often contradictory. For example, there is no information on anodic dissolution of vanadium metal in chloride electrolytes. 

Frumkin AN, Bagotskiy VS, lofa ZA, Kabanov BN (1952) Kinetika elektrodnykh protsessov (The kinetics of the electrode processes). Lomonosov Moscow State University, Moscow... 

We know that thermodynamics is a very powerful tool for the study of systems at equilibrium, but electrode processes are systems not at equilibrium when at equilibrium there is no net flow of current and no net reaction. Therefore electrode reactions should be studied using the concepts and formalities of kinetics. Indeed, the same period that saw the flourishing of solution electrochemistry, also saw the formulation of the fundamental theoretical concepts of electrode kinetics the work of Tafel on the relationship of current and potential was published in 1905 those of Butler and Volmer and Erdey Gruz, which formulated the basic equation for electrode kinetics, were published in 1924 and 1930 respectively. Frumkin in 1933 showed the correlation between the structure of the double layer and the kinetics of the electrode process. The first quantum mechanical approach to electrode kinetics was published by Gurney in 1931. 

The total overpotential at the electrode can be further resolved into two overpotential components, jjs and rjc. The first, rjs, is the surface (or activation ) overpotential, which relates directly to the kinetics of the electrode processes. The second overpotential component, r]c, is the concentration overpotential, accounting for the voltage dissipation associated with transport limitations. 

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