Chapter 9 - Electrode reactions in ion transfer

Abstract

Anodic transfer of metallic ions from the state of metallic bonding in metal electrodes to the state of hydrated ions in aqueous solutions (anodic metal dissolution) and its reverse reaction (cathodic metal deposition) belong to the category of ionic processes. In transfer equilibrium (η = 0), the anodic current equals the cathodic current. For some metals the transfer of metallic ions involves a reaction intermediate of an adsorbed metallic ion complex, hence, the overall reaction occurs in a series of two elementary steps rather than one. In covalent semiconductors, the accumulation of holes in the valence band or the accumulation of electrons in the conduction band at the electrode interface partially breaks the covalent bonding of the surface atom. For germanium with a small band gap (0.67 eV), both the valence band and the conduction band have been found to participate in anodic dissolution. In the anodic dissolution of covalent semiconductors, the transfer of surface ions across the compact layer (Helmholtz layer) occurs following the ionization of surface atoms Sa. for ionic semiconductors surface ions exist initially and they can transfer across the electrode interface without requiring any currents of holes and electrons in the electrode; thus, no dissolution current can occur in the circuit of polarization cell. Semiconductor electrodes of ionic compounds can also dissolve with the oxidation of surface anions or with the reduction of surface cations.