Transformations in cobalt amalgam — from a homogeneous Co amalgam to nanocrystallites

The properties of a cobalt amalgam formed during electrochemical reduction of Co(II) ions at mercury electrodes were investigated by cyclic voltammetric and chronocoulometric methods. Experiments were performed in a range of temperatures from −20 to 75°C, and a wide range of Co(II) concentrations, in NaClO4 solutions of various concentrations. At lower temperatures and low cathodic charges of Co(II) reduction, a homogeneous cobalt amalgam is formed, which undergoes a rather fast deactivation, leading probably to a Co3 cluster. The cluster is not electroactive in the accessible range of potentials. At higher concentrations of Co(II) and at higher temperatures, on anodic waves corresponding to oxidation of cobalt amalgam and recorded in unbuffered solutions, up to four oxidation steps are observed. On the other hand, in acetic buffer solution (pH 3.6), under chronocoulometric conditions, only one oxidation peak appears. Thus, other oxidation steps could be ascribed to the oxidation of Co(II) to Co(III) hydroxide. The formal potential, the rate constant at this potential, and the Tafel slopes (anodic-αn, and cathodic-βn) corresponding to the electrode process Co(II)+2e−+Hg=Co(Hg) were determined at 253 K. Determination of these parameters at higher temperatures was impossible, due to fast deactivation of the cobalt amalgam.