EQCM, SEC and voltammetric study of kinetics and mechanism of hexaamminecobalt(III) electro-reduction onto gold electrode

The work comprises results of the studies on the mechanism and kinetics of hexaamminecobalt(III) reduction in ammonia-chloride solutions at pH 9.5. The studies were preceded by the thermodynamic analysis of the electrolyte. Electrolyte stability was determined by UV–vis absorbance measurements. Electrochemical examinations were performed by applying cyclic voltammetry (CV) combined with Electrochemical Quartz Crystal Microbalance (EQCM) as well as with spectrophotometric measurements. The results indicated that reduction process of [Co(NH3)6]3+ consists of two steps. In the first step [Co(NH3)6]3+ is reduced to [Co(NH3)4]2+ and [Co(NH3)5]2+, and then ammonium complexes of Co2+ are reduced to Co0. Deposition of Co takes place only in the overpotential deposition range. Values of electrochemical equivalents M/z within this range indicate formation of Co(OH)2 phase. The examined reactions are diffusion controlled. Diffusion coefficients of complexes participating in particular reactions were determined with the use of Randles–Sevcik and Berzins–Delahay equations. In anodic part M/z values indicate 2-electrons reaction of cobalt oxidation connected with chemical dissolution of cobalt hydroxide formed during cathodic sweep.

► Thermodynamic analysis of cobalt(III) and cobalt(II) complexes solution.
► The cobalt deposition from hexaamminecobalt(III) solution was indicated as a two-step process.
► The mechanism of hexaamminecobalt(III) complexes to metallic cobalt was proposed.
► Using the voltammetry techniques the diffusion coefficients of tetra-, pentaamminecobalt(II) and hexaamminecobalt(III) complexes were determined.