Electrode-potential

In general, an electronic conductor which is used to introduce an electric field or electric current is called an electrode. In electrochemistry, an electronic conductor immersed in an electrolyte of ionic conductor is conventionally called the electrode. Since the function of an electrode to provide electric current does not work in isolation but requires the presence of an electrolyte in contact with the electrode, the term of electrode is defined as a combination of an electronic conductor and an ionic electrolyte. Usually, an electrode is used in the form of its partial inunersion in an electrolyte as shown in Fig. 4-1 (a). It is, however, more common to define the electrode in the form of complete immersion in the electrolyte as shown in Fig. 4—1 (b). 

The electronic conductor of an electrode may be either a metal or a semiconductor, and the electrol3de may be either an aqueous solution, fused salt, solid electrolyte, or gaseous electrol34e (gaseous plasma). In this textbook, we deal mainly with metal and semiconductor electrodes in aqueous electrolytes. 

One possible reason for the reluctance of non-electrochemists to venture into this field is that in contrast to the electrochemists claim that controlled potential electrolysis offers a method for the selective introduction of energy into molecules, many electrode reactions carried out at a controlled potential have still been reported to give low yields and a diversity of products. The electrode potential is, however, only one of several variables and the lack of selectivity in the electrode process may be attributed to a failure to understand and to control all the parameters of the overall electrode reaction. 

This chapter is intended to outline the present state of our knowledge about the factors which affect the mechanism, rate and products of an electrode reaction and to show, in turn, how the various parameters affect the individual steps in the overall reaction. In addition it will be shown how many of these parameters, once fully understood, may be turned to an advantage and may be used to design further novel electrosynthetic processes. 

The potential of the working electrode is important to the overall electrode process largely because of its effect on the electron transfer 

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We now consider briefly the matter of electrode potentials. The familiar Nemst equation was at one time treated in terms of the solution pressure of the metal in the electrode, but it is better to consider directly the net chemical change accompanying the flow of 1 faraday (7 ), and to equate the electrical work to the free energy change. Thus, for the cell... 

It is now assumed that consists of a chemical component and an electrical component and that it is only the latter that is affected by changing the electrode potential. The specific assumption is that... 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

The fact that more than one molecule of water may be displaced for each anion adsorbed, and that the adsorption energy of these water molecules will show a complex dependence on the electrode potential. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

The great advantage of the RDE over other teclmiques, such as cyclic voltannnetry or potential-step, is the possibility of varying the rate of mass transport to the electrode surface over a large range and in a controlled way, without the need for rapid changes in electrode potential, which lead to double-layer charging current contributions. 

For example, for iron in aqueous electrolytes, tlie tliennodynamic warning of tlie likelihood of corrosion is given by comparing tlie standard electrode potential of tlie metal oxidation, witli tlie potential of possible reduction reactions. 

Gibbs values and the effective electrode potential follows the Nemst equation (see section C2.11). For the oxidation (anodic) reaction, the potential (E ) of the Nemst equation can be written as ... 

Salaita G N, Lu F, Laguren-Davidson L and Flubbard A T 1987 Structure and composition of the Ag(111) surface as a function of electrode potential in aqueous halide solutions J. Electroanal. Chem. 229 1-17... 

Figure C3.6.4(a) shows an experimental chaotic attractor reconstmcted from tire Br electrode potential, i.e. tire logaritlim of tire Br ion concentration, in tlie BZ reaction [F7]. Such reconstmction is defined, in principle, for continuous time t. However, in practice, data are recorded as a discrete time series of measurements (A (tj) / = 1,...

Since it is not possible to measure a single electrode potential, one electrode system must be taken as a standard and all others measured relative to it. By international agreement the hydrogen electrode has been chosen as the reference ... 

For many purposes the hydrogen electrode is not convenient and it can be replaced by another cell of known standard electrode potential. A well-known example is the calomel cell shown in Figure 4.5. 

The redox (electrode) potential for ion-ion redox systems at any concentration and temperature is given by the Nernst equation in the form... 

At equilibrium at 298 K the electrode potential of the half-reaction for copper, given approximately by... 

The electrode potential for the iron(ir)-iTon(III) system is given by... 

In view of the ionisation energies the electrode potentials for lithium and beryllium might be expected to be higher than for sodium and magnesium. In fact... 

The electrode potential of aluminium would lead us to expect attack by water. The inertness to water is due to the formation of an unreactive layer of oxide on the metal surface. In the presence of mercury, aluminium readily forms an amalgam (destroying the original surface) which is. therefore, rapidly attacked by water. Since mercury can be readily displaced from its soluble salts by aluminium, contact with such salts must be avoided if rapid corrosion and weakening of aluminium structures is to be prevented. 

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