Reactive Deposition of Cobalt Electrodes: VIII . Effect of Oxygen Reduction on the Deposition of Cobalt in Co(II) Chloride DMF Solution

Abstract

The following conditions were studied to determine their effect on the deposition of cobalt from a DMF media in the presence of dissolved oxygen: the reduction of oxygen in the presence of CoCl2; the reduction of oxygen in the absence of CoCl2, and the addition of water. In a 0.5M LiCl + 0.5M LiClO4 DMF solution, the first step of oxygen reduction is a one-electron process to produce superoxide ions, O2-, which are stable in a DMF solution. Oxygen reduction is irreversible on a nickel disk electrode. In a DMF media containing CoCl2, superoxide ion is disproportionated by cobalt ion to produce cobalt oxide CoO on the electrode surface. CoO is highly nonconductive, resulting in a cathodic shift of about 700 mV in the nucleation potential of cobalt as compared to that in the absence of oxygen. Precipitation of CoO on the electrode surface interrupts the normal dense deposition of cobalt and inhibits both nucleation and crystal growth processes of Co2+ ions. As a result, reactive deposition is also applicable in the DMF media. When water is added to the DMF media, O2- ions are quickly decomposed by water and the oxygen reduction is mainly via a four-electron process which forms hydroxide ions. On the other hand, the hydrolysis of the DMF is accelerated by the formation of OH- to produce the weakly acidic and basic products, which have buffering and chemisorption effects. In a CoCl2 DMF-water mixture solution, due to the buffering effect of the hydrolyzed products of DMF, the inhibiting effect of oxygen reduction on the deposition process of cobalt is not as significant as that from the aqueous CoCl2 solution. The critical role of water content in the CoCl2 DMF-water mixture solutions on the oxygen reduction, as well as on the change of the inhibiting factor with the increase of water content, has been demonstrated.