Electrode reactions interface

The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

Transport of a species in solution to and from an electrode/solution interface may occur by migration, diffusion and convection although in any specific system they will not necessarily be of equal importance. However, at the steady state all steps involved in the electrode reaction must proceed at the same rate, irrespective of whether the rate is controlled by a slow step in the charge transfer process or by the rate of transport to or from the electrode surface. It follows that the rate of transport must equal the rate of charge transfer ... 

Concentration (diffusion or transport) Overpotential change of potential of an electrode caused by concentration changes near the electrode/solution interface produced by an electrode reaction. 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

The activation overpotential Tiac,w is due to slow charge transfer reactions at the electrode-electrolyte interface and is related to current via the Butler-Volmer equation (4.7). A slow chemical reaction (e.g. adsorption, desorption, spillover) preceding or following the charge-transfer step can also contribute to the development of activation overpotential. 

In addition to the exchange current density the transfer coefficient a is needed to describe the relationship between the electrode potential and the current flowing across the electrode/solution interface. From a formal point of view a can be obtained by calculating the partial current densities with respect to the electrode potential for the anodic reaction ... 

SO sharply defined that they are called surfaces. Well-defined surfaces occur between solids and either gases or liquids and thus are commonly found in catalytic and electrode reactions. More diffuse interfaces may occur between solids, as in microelectronic devices, and between fluids or semifluids, as in many polymeric and colloidal systems. 

In all cases the electrode reaction secures continuity of current flow across the interface, a relay type of transfer of charges (current) from the carriers in one phase to the carriers in the other phase. In the reaction, the interface as a rule is crossed by species of one type electrons [e.g., in reaction (1.22)] or ions [e.g., in reaction (1.21)]. 

Deviations from Faraday s laws can be observed in the case of transient currents, when charge, aside from being involved in the electrode reactions, accumulates in certain parts of the circuit (near interfaces). Such transient currents are also known as nonfaradaic. An apparent departure from the laws of Faraday can be observed at polyfunctional electrodes when the set of reactions taking place is not fully accounted for. 

The general way in which a Galvani potential is established is similar in all cases, but special features are observed at the metal-electrolyte interface. The transition of charged species (electrons or ions) across the interface is possible only in connection with an electrode reaction in which other species may also be involved. Therefore, equilibrium for the particles crossing the interface [Eq. (2.5)] can also be written as an equilibrium for the overall reaction involving all other reaction components. In this case the chemical potentials of aU reaction components appear in Eq. (2.6) (for further details, see Chapter 3). 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

It is typical that in Eq. (3.23) for the EMF, all terms containing the chemical potential of electrons in the electrodes cancel in pairs, since they are contained in the expressions for the Galvani potentials, both at the interface with the electrolyte and at the interface with the other electrode. This is due to the fact that the overall current-producing reaction comprises the transfer of electrons across the interface between two metals in addition to the electrode reactions. 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

Electrode reactions are heterogeneous since they occur at interfaces between dissimilar phases. During current flow the surface concentrations Cg j of the substances involved in the reaction change relative to the initial (bulk) concentrations Cy p Hence, the value of the equilibrium potential is defined by the Nemst equation changes, and a special type of polarization arises where the shift of electrode potential is due to a change in equilibrium potential of the electrode. The surface concentrations that are established are determined by the balance between electrode reaction rates and the supply or elimination of each substance by diffusion [Eq. (4.9)]. Hence, this type of polarization, is called diffusional concentration polarization or simply concentration polarization. (Here we must take into account that another type of concentration polarization exists which is not tied to diffusion processes see Section 13.5.)... 

In situ infrared spectroscopy allows one to obtain stracture-specific information at the electrode-solution interface. It is particularly useful in the study of electrocat-alytic reactions, molecular adsorption, and the adsorption of ions at metal surfaces. 

Many important processes such as electrochemical reactions, biological processes and corrosion take place at solid/liquid interfaces. To understand precisely the mechanism of these processes at solid/liquid interfaces, information on the structures of molecules at the electrode/electrolyte interface, including short-lived intermediates and solvent, is essential. Determination of the interfacial structures of the intermediate and solvent is, however, difficult by conventional surface vibrational techniques because the number of molecules at the interfaces is far less than the number of bulk molecules. 

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