Authors : , H. Tissaoui , M. Bonneau , J. Bouteillon , R. Delimi and M. Molteni
Abstract: The electrochemical oxidation of cobalt and the cobalt electrode open circuit potential have been studied in the complexing aqueous cobalt chloride solutions, when the pH of the solution, the CoCl2 concentration and the temperature were changed on a large scale. The open circuit potential of an immobile cobalt disk electrode did not obey NERNST`s law. It was 0.17 to 0.2 V more positive than the Co/Co(II) equilibrium potential and close to the hydrogen electrode equilibrium potential; it increased linearly with log[CoCl2]/M and decreased with pH. The hydrogen evolution kinetics was rather slow and controlled by convective diffusion when the potential was set at a more negative potential than �1 V/SCE. CoCl2 additions did not affect this kinetics significantly for pH = 3, whereas they slowed it down for pH 3. The kinetics of the cobalt dissolution was controlled by the slow transfer of the Co(II) sp. to the solution. It was accelerated by CoCl2 or NaCl additions to the electrolyte, which increased the free Cl- sp. concentration. The dissolution of cobalt in complex aqueous chloride solutions was assumed to result either in the formation of Co(H2O)62+ or of CoCl+ sp. at the electrode surface, according to either of the two mechanisms: Co + 6 H2O => Co(H2O)62+ + 2 e, Co + Cl- => CoCl+ + 2 e. The effects of either high CoCl2 and NaCl concentrations or of the most anodic overpotentials were linked to the evolution of conductivity and viscosity of the electrolyte.
, H. Tissaoui , M. Bonneau , J. Bouteillon , R. Delimi and M. Molteni , 2007. Cobalt Electrochemistry in Aqueous Chloride Solutions: Study of the Cobalt Oxidation. Journal of Engineering and Applied Sciences, 2: 191-198.