Four-electrode process

It is not appropriate here to consider the kinetics of the various electrode reactions, which in the case of the oxygenated NaCl solution will depend upon the potentials of the electrodes, the pH of the solution, activity of chloride ions, etc. The significant points to note are that (a) an anode or cathode can support more than one electrode process and b) the sum of the rates of the partial cathodic reactions must equal the sum of the rates of the partial anodic reactions. Since there are four exchange processes (equations 1.39-1.42) there will be eight partial reactions, but if the reverse reactions are regarded as occurring at an insignificant rate then... 

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that ir E curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin et al., 1966). These facts were interpreted in terms of the mechanism... 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

The study of processes at ITIES and in membrane electrochemistry requires elimination of two ohmic potential differences, achieved with a four-electrode potentiostat, voltage-clamp (Fig. 5.17). 

In an analysis of an electrode process, it is useful to obtain the impedance spectrum —the dependence of the impedance on the frequency in the complex plane, or the dependence of Z" on Z, and to analyse it by using suitable equivalent circuits for the given electrode system and electrode process. Figure 5.21 depicts four basic types of impedance spectra and the corresponding equivalent circuits for the capacity of the electrical double layer alone (A), for the capacity of the electrical double layer when the electrolytic cell has an ohmic resistance RB (B), for an electrode with a double-layer capacity CD and simultaneous electrode reaction with polarization resistance Rp(C) and for the same case as C where the ohmic resistance of the cell RB is also included (D). It is obvious from the diagram that the impedance for case A is... 

Obtained in acetonitrile solution containing 0.2 mol dm-3 Bu NBF4 as supporting electrolyte. Solutions were 1 x 10 3 mol dm-3 in receptor and potentials were determined with reference to an Ag/Ag+ electrode. Two-electron process. Four-electron process. Cathodic shift in redox wave produced by the presence of anions (up to 5.0 equiv) added as their tetrabutyl-ammonium salts. 

Let us consider a semiconductor electrode, at which a redox reaction of type (1) occurs. Electrons of both the conduction band and valence band may take part in the electrode process. As a result, the reversible reaction considered is characterized by four different types of electron transitions (see Fig. 6a). Transitions in which electrons leave the semiconductor and holes come in contribute to the cathodic current, and those where electrons come in and holes escape contribute to the anodic current. Thus, the resultant current is a sum of four currents i p, i >p (when referring to currents we shall always mean current densities). 

Voltammetric current-potential curves are important in elucidating electrode processes. However, if the electrode process is complicated, they cannot provide enough information to interpret the process definitely. Moreover, they cannot give direct insight into what is happening on a microscopic or molecular level at the electrode surface. In order to overcome these problems, many characterization methods that combine voltammetry and non-electrochemical techniques have appeared in the last 20 years. Many review articles are available on combined characterization methods [10]. Only four examples are described below. For applications of these combined methods in non-aqueous solutions, see Chapter 9. 

Conductimetry is a method of obtaining analytical and physicochemical information by measuring the conductivities of electrolyte solutions [25]. Conductivity cells have two or four electrodes, but the processes that occur at or near the electrodes are not directly related to the information obtained by conductimetric measurements. 

According to the yield of C8H8 and the concentrations of the four components, the equilibrium constant of the reaction was determined as 4 X 10 2 The value calculated from the potential difference was 4.4 X 10 2. Consequently, there is coincidence between the calculated equilibrium constant based on the electrode potentials and the equilibrium constant determined from the liquid-phase experiment. The liquid-phase and electrode processes are similar. 

Because the potential is more positive than E i, the formation of only one wave is observed. In acidic and alkaline media where the rate of the acid-base catalysed reaction (35 b) is fast and during the reaction all the phenylhydroxylamine derivative is transformed to quinoneimine, the height of the single wave corresponds to a transfer of six electrons [(35 a) plus (35 c)]. Because the life-time of the quinoneimine intermediate is short, its hydrolysis to form quinone does not affect the electrode process. In the medium pH range where the rate of dehydration is slow, the wave-height corresponds to a four-electron process. A theoretical... 

Joaquin Gonzalez is a Lecturer at the University of Murcia, Spain. He follows studies of Chemistry at this University and got his Ph.D. in 1997. He has been part of the Theoretical and Applied Electrochemistry group directed by Professor Molina since 1994. He is author of more than 80 research papers. Between 1997 and 1999, he also collaborated with Prof. Ms Luisa Abrantes of the University of Lisboa. He is the coauthor of four chapters, including Ultramicroelectrodes in Characterization of Materials second Ed (Kaufmann, Ed). He has taught in undergraduate and specialist postgraduate courses and has supervised three Ph.D. theses. His working areas are physical electrochemistry, the development of new electrochemical techniques, and the modelization, analytical treatment, and study of electrode processes at the solution and at the electrode surface (especially those related to electrocatalysis). 

Fig. 18. SEM image of a 70-nm bismuth nanowire with four electrodes attached to the nanowire. The circle on the large left electrode is a reference point used to find the nanowire and to attach electrodes to it by a lithographic process (Cronin et al., 1999).

In general, organic photovoltaic devices are constructed in a sandwich structure where the organic layer(s) are found between two highly conducting electrodes. One of them is transparent to let the light in, normally indium tin oxide (ITO), and the other mirror reflective, usually aluminum. The mechanism of photocurrent generation includes four basic processes ... 

If catalysts are not good, the hydrogen peroxide is generated at some stages of electrode process. Hydrogen peroxide is very strong oxidizer and destroys the construction of the fuel cell. Therefore, the catalyst must provide four-electron mechanism of reaction. Such catalysts are showed in the Tables 3, 4. 

Collman, and co-workers (346-352) also demonstrated that pyrolitic graphite electrodes modified with cofacial bis(porphyrinato cobalt) complex (CoFTF4) can promote the four-electron reduction of O2 to H2O. According to the proposed model, the dioxygen molecule is activated by the simultaneous coordination to two cobalt porphyrin sites, but electronic factors and/or the lack of electrons can also limit the four-electron process. 

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