Polyfunctional electrodes

Monofunctional and Polyfunctional Electrodes At monofunctional electrodes, one sole electrode reaction occurs under the conditions specified when current flows. At polyfunctional electrodes, two or more reactions occur simultaneously an example is the zinc electrode in acidic zinc sulfate solution. When the current is cathodic, metallic zinc is deposited at the electrode [reaction (1.21)] and at the same time, hydrogen is evolved [reaction (1.27)]. The relative strengths of the partial currents corresponding to these two reactions depend on the conditions (e.g., the temperature, pH, solution purity). Conditions may change so that a monofunctional electrode becomes polyfunctional, and vice versa. In the case of polyfunctional electrodes secondary (or side) reactions are distinguished from the principal (for the given purpose) reaction (e.g., zinc deposition). In the electrolytic production of substances and in other practical applications, one usually tries to suppress all side reactions so that the principal (desired) reaction will occur with the highest possible efficiency. 

Deviations from Faraday s laws can be observed in the case of transient currents, when charge, aside from being involved in the electrode reactions, accumulates in certain parts of the circuit (near interfaces). Such transient currents are also known as nonfaradaic. An apparent departure from the laws of Faraday can be observed at polyfunctional electrodes when the set of reactions taking place is not fully accounted for. 

When such a polyfunctional electrode is polarized, the net current, i, will be given by ii - 4. When the potential is made more negative, the rate of cathodic hydrogen evolution will increase (Fig. 13.2b, point B), and the rate of anodic metal dissolution will decrease (point B ). This effect is known as cathodic protection of the metal. At potentials more negative than the metaTs equilibrium potential, its dissolution ceases completely. When the potential is made more positive, the rate of anodic dissolution will increase (point D). However, at the same time the rate of cathodic hydrogen evolution will decrease (point D ), and the rate of spontaneous metal dissolution (the share of anodic dissolution not associated with the net current but with hydrogen evolution) will also decrease. This phenomenon is known as the difference effect. 

Silyl anions, electrogenerated by the cathodic reduction of polyfunctional chlorosilanes, react with remaining Si—Cl bonds of the starting molecules or of intermediately formed oligomers to give linear polysilanes. The molecular mass of the resulting polysilanes depends on the electrolysis conditions (solvent, electrode material, supporting salt, etc.) [102,... 

Oxidation pathways involve anion insertion coupled to electron transfer process. In experiments performed on Cu-MOF modified electrodes such as those shown in Figure 5.4, additional anodic peaks located in the 4-0.60 to 4-1.80 V potential region are attributable to the oxidation of polyfunctional organic groups. Such oxidation... 

We have seen that the idea of an electrode film system is useful for electrochemistry of molten salts including low-temperature ionic liquids. It is not restricted, however, to this field only. As an example, the protective layer on lithium metal in aprotic organic electrolytes could be mentioned. This layer, so-called solid electrolyte interphase (SEl), exhibits properties of a polyfunctional conductor with high ionic conductivity (Li ions are the carriers) and low electronic conductivity of semiconductive nature. Some peculiarities of film systems with semiconductive character of electronic conductivity are considered below. 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

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