Classes of Electrodes

During electrolytic behavior, we experimentally find that copper is dissolved and that dihydrogen evolves on the zinc electrode. Then the cell reaction is 

Obviously, this reaction differs from the preceding one. The cell is irreversible. 

A power supply may generate a potential difference that perfectly counterbalances that existing between the two electrodes. In these conditions, no current flows. The situation is equivalent to that encountered when the two electrodes are not connected. We say that the system is at equilibrium. Sometimes, the potential difference E determined at equilibrium (the zero-current cell potential) is still called the electromotive force of the cell. In the case of a chemically reversible cell, it is endowed with a very well-determined meaning (see Chap. 2 and later in this chapter) It permits us to obtain the standard electrode potentials. Establishing the latter is in the realm of the application of Nernst s law. 

Several classes of electrodes or, more exactly, of classes of cell compartments exist, as follows  

Apart from the role it plays in the redox phenomenon, the metal also plays that of an electrical contactor. The electrodes of the first kind are written conventionally as M M +. In this notation a vertical bar represents a phase boundary class II electrodes a metal in equilibrium with a saturated solution of a slightly soluble salt. They consist of a metal covered by a porous coat of one of its insoluble salts, the entire object dipping into a solution containing the anion giving the 

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Oxides of various metals are a broad class of electrode materials useful in many electrochemical processes (Trasatti, 1980-1981). The surfaces of practically all metals (both base and noble) become covered by layers of chemisorbed oxygen upon anodic polarization. The composition and properties of these layers depend on potential, on the electrolyte, and on the electrolysis conditions. They are often rather thick and have a distinct phase character, so that the metal electrode is converted to a typical oxide electrode. One can also make electrodes directly from oxides deposited in some way or other on various conducting substrates. 

The final class of electrodes we encounter are amalgam electrodes, formed by dissolving a metal in elemental (liquid) mercury, generally to yield a solid. We denote an amalgam with brackets, so the amalgam of sodium in mercury is written as Na(Hg). The properties of such amalgams can be surprisingly different from their... 

There is a class of electrode reaction in which the first step involves an electron-transfer reaction (E), the second a chemical reaction (C), and the third another electrode in a transfer reaction different from the first (E) ... 

Sometimes, we shall address an even simpler class of electrode reaction in which there is only a single electroactive species of a varible activity. The simplest instance of this class is the reduction of metal ions on a cathode composed of that metal, for example... 

Electrodes classified in the second group of electrode systems are those in which the metal electrode is coated with a layer of a sparingly soluble salt of the electroactive species and the metal ion of the metal electrode, such that the potentiometric response is indicative of the concentration of the inactive anion species. Thus the silver/silver-chloride electrode system, which is representative of this class of electrodes, gives a potential response that is directly related to the logarithm of the chloride ion activity (see also Chapter 1, section 1.5), even though it is not the electroactive species ... 

In this section, some general aspects emerging from the survey of the literature, common to all classes of electrode materials and essential to properly evaluate the performance of cathodes, will be illustrated and discussed. 

The following steps depend on the nature of the electrode materials, and make it possible to distinguish between two limiting classes of electrodes, defined as active and nonactive anodes ... 

The working electrode is typically stationary however, there is a class of electrodes, the rotating disk... 

Figure 22. Classes of electrode geometry a) crossed-wire electrodes b) planar base electrode with pad top electrode.

Two classes of electrode are the non-consumable or inert type and the consumable or sacrificial type. Nonconsumable electrodes are made from non-reactive materials, whereas consumable electrodes are electro-chemically active and are structurally altered by the passage of current during treatment. 

The electrochemical and chemical reactivities of the adsorbed hydroxyl radicals strongly depend on the nature of electrode material used. Two extreme classes of electrodes can be defined active and nonactive electrodes ... 

Chapter 2, by Strbac and Wieckowski, gives an impressive overview of a new class of electrode materials that deserve an advanced focus in the future. This chapter is related to the electrochemical or spontaneous deposition of Ru and Os on Au( 111) and Pt( 111) single crystal surfaces. The contribution of this chapter is particularly important for the understanding of the principles of reactivity... 

The transition metal chalcogenides such as n-WSe2 are a particular class of electrode materials, and their photoelectrochemical behaviour is of interest from the fundamental point of view. If the basal planar surfaces (perpendicular to the c-axis) with a low density of steps are contacting the electrolyte, these layered materials are relatively stable. Since the corrosion rate is very small, the anodic photocurrent occurs at a high overvoltage with respect to the flatband potential in the dark. As discussed in Section 2.3.1 (Fig. 2.15), the flatband potential t/fb is shifted on illumination because holes accumulate at the surface. On addition of a redox couple such as [Fe(phen)3] ... 

Semiconductor electrodes form a class of electrode materials that exhibit highly se-... 

A particular class of electrode materials is represented by the transition metal chal-cogenides, such as n-WSe2, n-MoSc2 and others, which form layer crystals. As already... 

A number of commonly used reference electrodes based on mercury belong to this class of electrodes. 

Membrane Electrodes. Membrane electrodes are a class of electrodes that respond selectively to ions by the development of a potential difference (a type of junction potential) across a membrane that separates the analyte solution from a reference solution. The potential difference is related to the concentration difference in the specific ion measured on either side of the membrane. Remember that electrodes really respond to... 

A class of electrode called the electrode of the second kind, which forms from a metal and its sparingly soluble metal salt, finds use as the reference electrode. The most common electrode of this type includes the calomel electrode, Hg/HgClj and the silver-silver chloride electrode, Ag/AgCl. In biomedical applications, particularly in in vivo applications, Ag/AgCl is more suitable as a reference electrode. 

Membrane electrodes are a class of electrodes that respond selectively to ions by the development of a potential difference (a type of junction potential) across a membrane that separates the analyte solution from a reference solution. The potential difference is related to the concentration difference in the specific ion measured on either side of the membrane. These electrodes do not involve a redox reaction at the surface of the electrode as do metallic electrodes. Because these electrodes respond to ions, they are often referred to as ion selective electrodes (ISEs). The ideal membrane allows the transport of only one kind of ion across it that is, it would be specific for the measurement of one ionic species only. As of this writing, there are no specific ISEs, but there are some highly selective ones. Each electrode is more or less selective for one ion therefore, a separate electrode is needed for each species to be measured. In recent years, many different types of membrane electrodes have been developed for a wide variety of measurements. 

A different class of electrode materials is based not upon insertion processes but on the displacement reactions of binary metal compounds [260, 261] (Eq. 2.4) ... 

However, just synthetic issues, namely those relative to the monomer from which to start, constitute a major drawback to the widespread use of similar materials as electrode modifiers. A recent review [80] collects a few results of efforts made in the field and new perspectives are envisioned, encouraging high consideration for such a potentially powerful class of electrode systems. The authors claim Recent developments in the field of synthesis and potential applications of metal-functionalized polymers obtained via electropolymerization are presented, highlighting the significant advances in this field of research. At present, this class of conducting polymers is far from being exhaustively studied, both as to a general definition of the specific properties in respect to electrode electrochemistiy and, even very much less, as to electroanalytical applications. 

For example, the crosslinked PVP composite electrode described above has the same electrochemical properties as a coated electrode, but is polishable, and not susceptible to dissolution in flowing streams, even in the presence of organic solvents. This new class of electrodes and their miniaturized descendants (Shaw, Wang, and Creasy, work in progress) will serve well in electrochemical detectors and sensors of the future. 

The term sensor is really a misnomer, since electrochemical response of this class of electrodes demands the presence of the so-called sensor, e.g. valinomycin, as well as the appropriate solvent mediator. 

The conception of the three-dimensicMial electrode occurred almost simultaneously in France and England around 1966 as a necessity for precious metal recovery from dilute solutions occurring in industrial processes and for the removal of heavy metals from wastewaters. In this class of electrodes, the reaction stiU occurs at the liquid-solid interface, but the surface area is provided by the electrode volume. Porous or three-dimensional electrodes have been... 

In Chapter 1, we described, in a rather broad sense, various classes of electrodes. In the study of voltammetry a more specific definition is required. Voltammetry is the study or measurement of the current-voltage relations which exist at the face of an electrode immersed in a solution which contains electroactive species. For our purposes, the solution will be an aqueous electrolyte. 

This class of electrodes is generally categorized as reversible or non-polarizable. By this, we mean simply that the electrode passes electric current without changing the chemical environment in the region of the electrode. The basic thermodynamic consideration for electrochemical reversibility is expressed by the Gibbs free-energy-loss relation... 

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