Electrode metals classification

Classification of Electrode Metals in Accordance with Product Selectivity in CO2 Reduction in Aqueous and Nonaqueous Electrolyte (PC). Reproduced from Ref. 23, Copyright (1994) with permission from Elsevier... 

The identity of the cathode materials is essential for the outcome of CO2 electroreduction. While an earlier electrode classification was based on whether the cathode metal belonged to the sp- or the d-metal group [50], Hori considered that the performance of various metals is loosely related to the periodic table. For aqueous electrolytes, Hori [82,83] suggested regrouping the electrode metals into two categories (1) CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni and Ft) and (2) metals that yield formate (Hg, Pb, Zn, In, Sn, Cd and Tl). As discussed in the previous section, copper represents a very special electrode material, enabling the formation of various hydrocarbons, such as methane and ethylene [82]. 

In an aqueous solution, the electrode potentials of CO2 reduction correlate with the heats of fusion (HoF) of the electrode metals low-HoF metals (Hg, Tl, Pb, In, Cd and Zn) yield formate, while high-HoF metals (Pt, Pd, Ni, Au, Cu, Ag, Zn, Sn and Ga) form CO [77,87]. The above classification is far from being perfect, and does not cover for all possible scenarios of CO2 electroreduction. As shown later, in the section on the S5mthesis of organic carbonate, when used in an ionic liquid, indium cathodes are efficient in the preparation of dimethyl carbonate. Also, copper-based bimetallic electrodes may exhibit an improved catalytic activity in reducing CO2 to hydrocarbons. Examples include Cu-Ni, Cu-Sn and Cu-Pb alloys. By contrast, for Cu-Ag and Cu-Cd alloy electrodes, the catalytic activity is diluted [81]. 

Figure 2.1 (Plate 2.1) shows a classification of the processes that we consider they aU involve interaction of the reactants both with the solvent and with the metal electrode. In simple outer sphere electron transfer, the reactant is separated from the electrode by at least one layer of solvent hence, the interaction with the metal is comparatively weak. This is the realm of the classical theories of Marcus [1956], Hush [1958], Levich [1970], and German and Dogonadze [1974]. Outer sphere transfer can also involve the breaking of a bond (Fig. 2. lb), although the reactant is not in direct contact with the metal. In inner sphere processes (Fig. 2. Ic, d) the reactant is in contact with the electrode depending on the electronic structure of the system, the electronic interaction can be weak or strong. Naturally, catalysis involves a strong...

Figure 2.1 Classification of electrochemical electron transfer reaction on metal electrodes. (See color insert.)...

The classification of electrodes is based upon the chemical nature of the substances participating in the electrochemical process [75]. Electrodes of the first type are systems in which the reduced forms are metals of electrodes and oxidized forms are ions of the same metal. Electrodes of second type are systems in which the metal is covered by a layer of low soluble salts (or oxide), and the solution contains anions of these salts (for oxide-OH ions). The Nernst equation for electrodes of the second type can be written as ... 

To form an electrode concentration cell the electrode material must have a variable concentration. Amalgam and gaseous electrodes frill into this classification. An example of electrode concentration cells is the one in which two amalgam electrodes of different concentrations dip into a solution containing the solute metal ions. 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Randles (13) has determined the activation energies for the metal-metal ion reaction for Tl, Cd, Fb, Zn, and Cu as amalgamated electrodes. He found values of about 6 to 10 kcal/ mol, in general agreement with Piontelli s classification given above. These low values indicate that the potential energy curves for Y+ and W+ must be flat and cross at a low point as shown schematically in Fig. 6(a). 

A general classification of most of the metal-free batteries is based on the electrode categories defined in Section 1.2 along with Eqs. (14) and (15). The terminology donor , acceptor (D, A) is used again, in spite of its shortcomings [10]. The other nomenclature, e.g. p-type instead of A-type and n-type instead of D-type, is omitted, however, for it suggests a propinquity to semiconductor physics which does not exist. 

Metal electrodes are divided into 4 groups in accordance with the product selectivity indicated in Table 3. Pb, Hg, In. Sn, Cd, Tl, and Bi give formate ion as the major product. Au. Ag, Zn. Pd, and Ga, the 2nd group metals, form CO as the major product. Cu electrode produces CH4, C2H4 and alcohols in quantitatively reproducible amounts. The 4th metals, Ni, Fe, Pt, and Ti. do not practically give product from CO2 reduction continuously, but hydrogen evolution occurs. The classification of metals appears loosely related with that in the periodic table. However, the correlation is not very strong, and the classification such as d metals and sp metals does not appear relevant. More details of the electrocatalytic properties of individual metal electrodes will be discussed later. 

It is obvious that this reaction can only lead to metal dissolution, if the metal electrode potential is negative from the hydrogen electrode potential. This is the reason for the classification of metals into noble metals (the equilibrium potential is more positive than the standard hydrogen potential) and non-noble metals (the equilibrium potential is more negative than the standard hydrogen potential). The kinetic of the total process can be described by the Butler-Volmer equation for the two partial reactions. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

The classification of electrode film systems is proposed based on the above ideas, and main qualitative regularities of the electrolytic processes in the film systems of different kind are envisaged in Chap. 4. In particular, the mechanism of formation of cathode deposits is considered. It is shown that the deposition of metal-salt carrots or compact metal layers depends on the properties of the cathode film system (prevailing type and ratio of the electronic and ionic conductivity of the film). The nature of crisis phenomena at the electrodes is also analysed (anode effect in fluoride melts, complications at the electrolytic production of Al-Si alloys in industrial-scale electrolytic cells), the mechanisms are elaborated and the means to escape the crises situations are developed. 

In arc lamps, the emission is obtained by the activation of a gas by collision with accelerated electrons generated by an electric discharge between two electrodes, typically tungsten-made. The type of lamp is often denoted by the gas contained in the bulb including neon, argon, xenon, krypton, sodium, metal halide and mercury. In particular, for mercury lamps, the following classification, based on the Hg pressure, is done ... 

According to recent developments in the field, the term molten salts can advantageously be widened in scope to encompass many molten media which may not be wholly ionic or derived from simple salts. Thus, many systems studied within this broad classification may change their ionicity and hence conductivity according to temperature, pressure, or composition, e.g., silicates, group IIB chlorides, and chloroaluminates, respectively. Nevertheless, the majority of melts that have been studied are substantially dissociated in the liquid state, and all processes conducted in these are ipso facto electrochemical. Many of the processes considered here, therefore, involve charge transfer systems, particularly between solids (mainly metals) and melts, viz., electrode processes. ... 

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