Electrochemistry, aqueous

P. Winget, E. J. Weber, C. J. Cramer, and D. G. Truhlar, Computational electrochemistry Aqueous one-electron oxidation potentials for substituted anilines, Phys. Chem. Chem. Phys. 2 1231 (2000). 

Electrochemistry is concerned with the study of the interface between an electronic and an ionic conductor and, traditionally, has concentrated on (i) the nature of the ionic conductor, which is usually an aqueous or (more rarely) a non-aqueous solution, polymer or superionic solid containing mobile ions (ii) the structure of the electrified interface that fonns on inunersion of an electronic conductor into an ionic conductor and (iii) the electron-transfer processes that can take place at this interface and the limitations on the rates of such processes. 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Combination silver—silver salt electrodes have been used in electrochemistry. The potential of the common Ag/AgCl (saturated)—KCl (saturated) reference electrode is +0.199 V. Silver phosphate is suitable for the preparation of a reference electrode for the measurement of aqueous phosphate solutions (54). The silver—silver sulfate—sodium sulfate reference electrode has also been described (55). 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

The theory of rate measurements by electrochemistry is mathematically quite difficult, although the experimental measurements are straightforward. The techniques are widely applicable, because conditions can be found for which most compounds are electroactive. However, many questionable kinetic results have been reported, and some of these may be a consequence of unsuitable approximations in applying theory. Another consideration is that these methods are mainly applicable to aqueous solutions at high ionic strengths and that the reactions being observed are not bulk phase reactions but are taking place in a layer of molecular dimensions near the electrode surface. Despite such limitations, useful kinetic results have been obtained. 

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. 

G. H. Newmann, Proc. Workshop on Lithium Non-Aqueous Battery Electrochemistry, Publication 80-7, The Electrochemical Society, NJ, 1980, p. 143. 

A comprehensive review which discusses the surface properties and their role in the electrochemistry of carbon surfaces was written by Leon and Radovic [26]. This review provides a useful complement to the following discussion on the role of carbon in aqueous batteries. Four key parameters that are important for carbonaceous materials in batteries, which were identified by Fischer and Wissler [24], are ... 

Electrochemistry of metal chelates in non-aqueous media. G. K. Budnikov and T. V. Troepol -skaya, Russ. Chem. Rev. (Engl. Transl.), 1979, 48, 449-462 (100). 

DespiC, A. Electrochemistry of Aluminum in Aqueous Solutions and Physics of Its Anodic Oxide 20... 

MacDonald, D. D. The Electrochemistry of Metals in Aqueous Systems at Elevated Temperatures 11... 

It must be emphasized that Equations (5.24) and (5.25) stem from the definitions of Fermi level, work function and Volta potential and are generally valid for any electrochemical cell, solid state or aqueous. We can now compare these equations with the corresponding experimental equations (5.18) and (5.19) found to hold, under rather broad temperature, gaseous composition and overpotential conditions (Figs. 5.8 to 5.16), in solid state electrochemistry ... 

The technique of AC Impedance Spectroscopy is one of the most commonly used techniques in electrochemistry, both aqueous and solid.49 A small amplitude AC voltage of frequency f is applied between the working and reference electrode, superimposed to the catalyst potential Uwr, and both the real (ZRe) and imaginary (Zim) part of the impedance Z (=dUwR/dI=ZRc+iZim)9 10 are obtained as a function of f (Bode plot, Fig. 5.29a). Upon crossplotting Z m vs ZRe, a Nyquist plot is obtained (Fig. 5.29b). One can also obtain Nyquist plots for various imposed Uwr values as shown in subsequent figures. 

The second approach, followed by Vayenas et al39 is direct measurement of Ntpb and N n using cyclic voltammetry, as in aqueous electrochemistry,49 and measuring the height, Ip, or the area fldt of the cathodic oxygen reduction peak (Fig. 5.28a). Then Ntpb can be estimated from ... 

One basic reason which made the absolute electron potential problem so complicated to solve in aqueous electrochemistry is the experimental difficulty of measuring work functions on metal surfaces covered with liquid films or in contact with liquids and their vapours. 

Trasatti14 16 has done a very thorough and lucid work in clarifying the concept of absolute electrode potentials in aqueous electrochemistry. He has pointed out that at least four different absolute, or single , electron potentials can be defined, depending on the choice of the reference state of electrons. 

Figure 7.1. Definition of absolute electron potential in aqueous electrochemistry according to Trasatti16 in a classical (a) and liquid covered (b) electrode geometry. Point C corresponds to the zero energy level. O0 is the work function of the bare electrode surface and AC>(=eA P) is the work function modification induced by the presence of the electrolyte layer (b). Reprinted with permission from Elsevier Science.

Figure 7.2. Schematic of a normal (a) and an emersed (b) electrode in aqueous electrochemistry showing the conceptual similarity of case (b) with Fig. 7.1b (adapted from Trasatti16). Reprinted with permission from Elsevier Science.

This, at first perhaps surprising fact, is important to remember as the same situation arises in solid state electrochemistry. To understand its validity it suffices to remember that the definition of the reference (zero) energy level of electrons for the she scale is simply the state of an electron at the Fermi level of any metal in equilibrium with an aqueous solution of pH=0 and pH2=l atm at 25°C. 

Similarly to aqueous electrochemistry, potentials in solid state electrochemistry utilizing YSZ are expressed in terms of the potential of a reference metal electrode exposed to P02 = 1 atm at the temperature T of interest. Thus a standard oxygen electrode scale (soe) can be defined. Similarly to equation (7.2) one has ... 

Before discussing the experimental results, which by themselves suggest a unique choice of the reference (zero) state of electrons in solid state electrochemistry, which is the same with the choice of Trasatti for aqueous electrochemistry,14 16 it is useful to discuss some of the similarities and differences between aqueous and solid electrochemistry (Fig. 7.3). 

In aqueous electrochemistry electrochemical (charge transfer) reactions take place over the entire metal/electrolyte interface. 

It must be emphasized that the effective double layer is overall neutral, as the backspillover species (O6, Na6+) are accompanied by their compensating (screening) charge in the metal.32,3,35,36 It must also be clarified that this backspillover formed effective double layer is not in general at its pzc (point of zero charge). This happens only at a specific value of the electrode potential, as in aqueous electrochemistry.37... 

The presence of this backspillover formed effective double layer is important not only for interpreting the effect of electrochemical promotion, but also for understanding the similarity of solid state electrochemistry depicted in Fig. 7.3 with the case of emersed electrodes in aqueous electrochemistry (Fig. 7.2) and with the gedanken experiment of Trasatti (Fig. 7.1) where one may consider that H2O spillovers on the metal surface. This conceptual similarity also becomes apparent from the experimental results. 

The positive charge is on the electrode with the lower work function. Thus under conditions of negligible ion spillover equations (7.11) and (7.12), are not valid. This is the case in aqueous electrochemistry and can also be the case in solid state electrochemistry when the temperature is... 

The same conceptional approach used in aqueous electrochemistry to define "absolute electron potentials can be used in solid state electrochemistry. Thus if one chooses as the zero level an electron just outside the solid electrolyte surface, which has been shown14-16 by Trasatti to be the most realistic choice in aqueous electrochemistry, one has ... 

---