Non-stationary electrochemical response of polycrystalline cadmium in sodium hydroxide solutions

Abstract

The anodization of cadmium in NaOH solutions produces a hydrous cadmium hydroxide-cadmium oxide layer and soluble cadmium species in solution. These processes are investigated through conventional voltammetry, potentiostatic, rotating ring disc electrode and complex voltammetric techniques by using NaOH solutions in the 0.01-1 M range at 25°C. The threshold potential of the anodic reaction is very close to the reversible potential of the Cd/Cd(OH)₂ electrode. The overall anodic and cathodic reactions can be interpreted through a complex reaction model comprising a number of parallel reactions following the initial formation of CdOH_ad species at the electrode surface level. The relative contribution of each parallel reaction is determined by the concentration of OH− ion at the interface. The reaction model implies a composite structure of layers starting from CdOH_ad at the electrode surface, and subsequent layers with an increase in the water content on approaching the outmost oxide layer. The water content of anodic layer should increase according to the pH and layer thickness and decrease on increasing the applied potential. Under certain conditions the electrochemical reactions become ohmic resistance controlled. This fact presumably involves the formation of aged species and the onset of passivity.