Reactions at Nonconsumable Electrodes

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

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In accordance with Faraday s Law, the operation of an IDDS requires redox reactions at the electrodes in proportion to the amount of charge passed. For nonconsumable electrodes, contacting an essentially aqueous electrolyte solution, electrolysis of water is the likely redox reaction. Therefore, the reaction at the anode is ... 

In the other examples, the electrode materials are not involved in the reactions chemically, but constitute the source [sink] of electrons. Such electrodes are called nonconsumable. The term inert electrodes sometimes used is unfortunate insofar as the electrode itself is by no means inert rather, it has a strong catalytic effect on the electrode reaction. For reactions occurring at such electrodes, the terms oxidation- reduction... 

In the case of redox reactions, polarization also depends on the natme of the nonconsumable electrode at which a given reaction occms (for the equilibrium potential, to the contrary, no such dependence exists). Hence, the term reaction will be understood as reaction occurring at a specified efectrode. ... 

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction... 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

Many papers have been published regarding HTSCs used as inert, nonconsumable electrodes for kinetic and mechanistic studies of various electrode reactions occurring at them. Most of these studies were performed at room temperature when the materials were not in their state of superconductivity. Unfortunately, to date a given reaction has rarely been studied at similar temperatures just above and below r , that is, at temperatures where the same material is once in its normal state and once in its superconducting state. The electronic stracture of materials differs sharply between these two states, and quantitative studies under these conditions might provide valuable information as to the mechanism of the elementary act of charge transfer from the electrode to a reacting species, and vice versa. 

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

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