electrochemical conversions that occur at an electrode-electrolyte interface where a charge is transferred through the interface and an electric current flows. Depending on the direction of the electron flow (from the electrode to the material or vice versa) a distinction is made between cathodic and anodic processes, which result in the reduction and oxidation, respectively, of the material. The spatial separation of the oxidation and reduction processes is used in chemical sources of electric current and for electrolysis.
The current density (in amperes per cm2) serves as a precise measure of the rate of an electrode process, which depends on the electrode potential, the structure of the electric double layer, and the presence of adsorbed particles at the phase interface. The rate increases as the overvoltage rises. At the equilibrium potential a dynamic equilibrium is achieved: current does not flow through the electrode, but there is a continuous exchange of charge carriers, that is, ions and electrons, across the phase interface; this exchange current is one of the principal kinetic parameters of electrode processes. The rate may vary within extremely wide limits depending on the nature of the electrode. Thus, the exchange current in the electrochemical process of evolving hydrogen from aqueous solutions of acids ranges from 10–12 ampere per cm2 for a mercury electrode to 0.1 ampere per cm2 for a platinum electrode. The concentration of reactive particles and the temperature also affect the rate.
The simplest electrode processes are electron-transfer reactions of the type Fe2+ → Fe3+ + e. The electron transfer may be accompanied by the breaking of chemical bonds and the transfer of atoms from the original material to the product of the reaction, as in the case C6H5NO2 + 6H+ + 6e → C6H5NH2 + 2H2O. In more complex electrode processes a new phase is formed. Such processes include cathodic deposition and anodic dissolution of metals (such as Λg+ + e → Ag) and the evolution and ionization of gases (for example, 2H+ + 2e ⇄ H2). One of the stages of an electrode process is always the discharge-ionization stage—the transfer of a charged particle across the phase interface—which constitutes the elementary electrochemical event of the overall process. Electrode processes also include a stage in which a reactive material is delivered to the surface of an electrode and a stage in which the products of the reaction are distributed throughout the solution. They may also include chemical stages that precede or follow the discharge-ionization stage. (SeeELECTROPLATING TECHNOLOGY, , and for descriptions of electrode processes widely used in technology.)
V. V. LOSEV