REACTIVE DEPOSITION OF COBALT ELECTRODES .6. MECHANISTIC STUDIES IN THE PRESENCE OF DISSOLVED-OXYGEN - THE CO(OH)2 COLLOID LAYER MODEL

The mechanism and kinetics of cobalt deposition from unbuffered neutral solutions were studied at a nickel disk electrode in the presence of dissolved oxygen. It was found that in the presence of oxygen the deposition process of cobalt is characterized by localized nucleation and limited crystal growth processes. The formation of such characteristics during the cobalt deposition process is due to dynamic states, i.e., the formation and breaking processes in the Co(OH)2 colloid layer formed at the electrode surface in the presence of oxygen. The dynamic states of the Co(OH)2 colloid layer are very much dependent on the oxidation reaction of Co(OH)2 by H2O2 formed as an intermediate in O2 reduction, and particularly on the sparging effect in the case of bubbling oxygen. Such unique deposition processes are directly responsible for the formation of a highly porous structure produced by reactive deposition. More significantly, it has been shown that in the region where the deposition process is mutually controlled by diffusion transfer and electron transfer processes, the dynamic states of the Co(OH)2 colloid layer are most effective in promoting the localized nucleation and limited crystal growth process of cobalt. The relationship between the dynamic states of the Co(OH)2 colloid layer and the chemical and electrochemical properties of the system has been discussed. In practice, in the presence of bubbling oxygen, reactive deposition is most effective in producing high surface-area and highly porous cobalt electrodes in the conditions of the presence of Cl- ions and 0.25M Co2+ solution.