Classification of Electrodes

No universally adopted general classification of electrodes exists however, when dealing with thermodynamic aspects of the electrode potential notion, we dwell on the 

Electrodes of the first kind contain electronic conductors as the reduced forms, and ions (particularly, complex ions) as the oxidized forms. The equilibrium can be established with respect to cations and anions in the absence of ligands, the cations are more typical. The examples are Cu Cu +. .. or... Au [Au(CN)2]  

This group can be supplemented also by amalgam electrodes (or other liquid electrodes) and electrodes fabricated from nonstoichiometric solids capable of changing their composition reversibly (intercalation compounds based on carbons, oxides, sulfides, and multicomponent salts, particularly, Li-intercalating electrodes of batteries). 

Electrodes of the second kind contain a layer of a poorly soluble compounds (salt, oxide, hydroxide), which is in contact with a solution of the same anion. The equilibrium is always established with respect to anions. The most typical examples are based on poorly soluble compounds of mercury and silver (Table 1). 

The redox polymers, both of organic and inorganic origin (such as polyvinylpyridine modified by redox-active complexes of metals Prussian blue and related materials), can be considered as a version of electrodes of the second kind however, the equilibrium is usually estabhshed with respect to cations. Electron conducting polymers (polyanyline, polypyrrol, and so forth) also pertain, in the first approximation, to the electrodes of the second kind, which maintain equilibrium with respect to anion. Ion exchange polymer films on electrode surfaces form a subgroup of membrane electrodes. 

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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 ... 

On the basis of this and in connection with the electrode performance in practical applications, a more detailed classification of electrodes can be given (see page 7). 

A classification of electrodes has already been given in Section 1.3.1. The function of the indicator electrode is to indicate by means of its potential the concentration of an ion or the ratio of the concentrations of two ions belonging to the same redox system. Under non-faradaic conditions, the relationship between the potential and these concentrations is given by the Nemst or the more extended Nernst-Van t Hoff equation, as explained below. As a single potential between an electrode and a solution cannot be measured in the absolute sense but only in a relative manner, a reference electrode is needed its function is merely to possess preferably a constant potential or at any rate a known potential under the prevailing experimental conditions. Often both electrodes cannot be placed in the same solution, so that a second solution... 

In accordance with the classification of electrodes in Section 1.3.1 one can distinguish between redox electrodes (inert type) and electrodes of the 1st, 2nd and 3rd kinds (active type). 

CLASSIFICATION OF ELECTRODE REACTIONS ACCORDING TO ELECTRODE KINETICS... 

A second mode of classification of electrode reactions is based entirely on the electrode mechanism. Here it is necessary to know the number of chemical and electrochemical steps involved and the order between the different steps. By denoting an electrochemical step by an if and a chemical one by a C and postulating that every electrochemical step involves the transfer of one electron, it is immediately evident that, e.g., an ECEC process consists of ... 

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 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. 

Chemical analysis shall be performed on specimens of the electrode being classified. Any analytical technique may be used but, in cases of dispute, reference shall be made to established publish methods. The referee method shall be ASTM F288. The results of the analysis shall meet the requirements of Table 1 for the classification of electrode under test. 

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