Electrochemical charge-transfer processes

The main act of the electrochemical process, charge transfer, is localized in a very thin double electric layer. This process can take place continuously only when electrically active particles, that is, particles that participate in the charge transfer step are transferred toward the electrode, and the products formed move in the opposite direction - from the surface of the metal phase of the electrode to the solution volume. When electrochemical deposition of metal takes place in simple (noncomplex) salt solutions, there may be no transport of the product, because the metal atoms formed do not participate in the diffusion process, and they form a new solid phase - a crystal lattice. 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

In the first part, Chapters 2-6, some fundamentals of electrode processes and of electrochemical and charge transfer phenomena are described. Thermodynamics of electrochemical cells and ion transport through solution and through membrane phases are discussed in Chapter 2. In Chapter 3 the thermodynamics and properties of the interfacial region at electrodes are addressed, together with electrical properties of colloids. Chapters 4-6 treat the rates of electrode processes, Chapter 4 looking at fundamentals of kinetics, Chapter 5 at mass transport in solution, and Chapter 6 at their combined effect in leading to the observed rate of electrode processes. 

Characteristic potential — means the potential of a certain singular point in electrochemical response that is characteristic of the particular charge-transfer process (- charge-transfer kinetics) or interfacial phenomenon. Any characteristic p. is related to given experimental conditions (such as the temperature, the nature of the solvent, and supporting -> electrolyte and its concentration). The nature and type of characteristic p. depends on the technique that is employed. Typical characteristic potentials are the -> half-wave potential in -> po-larography and the -> peak potentials in -> voltammetry, similar maximum/minimum of inflection points can be listed for other techniques. 

The performance of PEMFC is often presented by the polarization curve that shows the voltage output as a function of current density. Fig. 8 shows a typical polarization curve of PEMFC. As the PEMFC processes charge-transfer reactions and the diffusion of the reactants to and products from the electrochemical interface, the transport and kinetics within the cell determine the polarization characteristics of PEMFC. In the practical PEMFC, the terminal cell potential V... 

Such a heterogeneous charge transfer across the interface followed by a chemical reaction of the reactants formed by the preceding electrochemical process is known as the E Q mechanism in electrode reactions, that is, the electrochemically reversible charge transfer followed by an irreversible chemical reaction [42]. 

Electrochemical systems convert chemical and electrical energy through charge-transfer reactions. These reactions occur at the interface between two phases. Consequendy, an electrochemical ceU contains multiple phases, and surface phenomena are important. Electrochemical processes are sometimes divided into two categories electrolytic, where energy is supplied to the system, eg, the electrolysis of water and the production of aluminum and galvanic, where electrical energy is obtained from the system, eg, batteries (qv) and fuel cells (qv). 

The essential features of the electrochemical mechanism of corrosion were outlined at the beginning of the section, and it is now necessary to consider the factors that control the rate of corrosion of a single metal in more detail. However, before doing so it is helpful to examine the charge transfer processes that occur at the two separable electrodes of a well-defined electrochemical cell in order to show that since the two half reactions constituting the overall reaction are interdependent, their rates and extents will be equal. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Presumably the most important kinetie parameter used in the deseription of the kineties of an eleetrode is the exchange current density or the almost equivalent rate constant. It indicates the speed of the heterogeneous process of charging or discharging species at the phase boundary, i.e. the charge transfer process. Its value is influenced by numerous factors of the investigated system. For both applied and fundamental aspects of electrochemical research a list of reported values should be helpful. It concludes this volume. 

So far, uncatalysed electrochemical processes have had to compete with catalytic organic processes. There is considerable scope for a specific catalyst to be developed for specific organic electrochemical reactions. This implies reduced overpotential and acceleration of slow chemical rather than relatively fast charge-transfer steps (Jansson, 1984). Electrocatalysis... 

However, under working conditions, with a current density j, the cell voltage E(j) decreases greatly as the result of three limiting factors the charge transfer overpotentials r]a,act and Pc,act at the two electrodes due to slow kinetics of the electrochemical processes (p, is defined as the difference between the working electrode potential ( j), and the equilibrium potential eq,i). the ohmic drop Rf. j, with the ohmic resistance of the electrolyte and interface, and the mass transfer limitations for reactants and products. The cell voltage can thus be expressed as... 

Many of the electrochemical techniques described in this book fulfill all of these criteria. By using an external potential to drive a charge transfer process (electron or ion transfer), mass transport (typically by diffusion) is well-defined and calculable, and the current provides a direct measurement of the interfacial reaction rate [8]. However, there is a whole class of spontaneous reactions, which do not involve net interfacial charge transfer, where these criteria are more difficult to implement. For this type of process, hydro-dynamic techniques become important, where mass transport is controlled by convection as well as diffusion. 

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