Electrochemical--Ellipsometric Studies of Oxide Films Formed on Nickel During Oxygen Evolution

摘要

The time variation of current density at constant potentials for oxygen evolution on metals or alloys is one of the most difficult problems needing a solution in commercial water electrolyzers. The mechanism of this phenomenon on nickel electrodes was studied in IN KOH using ellipsometry to analyze the nature of anodic oxide films. Effects of electrochemical pretreatment of oxide films on the kinetics of the oxygen evolution reaction were investigated. Nickel oxide films, formed electrochemically at 1.5 V, are more active than untreated nickel for this reaction. The time variation of current density for oxygen evolution at potentials above 1.56 V is due to the gradual conversion of active sites, Ni exp 3+ ions, on the film surface to inert species, Ni exp 4+ ions, for the oxygen evolution reaction. The electrocatalytic activities of aged electrodes are regained by ''rejuvenating'' the electrodes at 1.5 V. The ''rejuvenation'' of aged oxide films is essentially attributed to the recovery of active sites on the very top layers of the films, rather than the diminution of the film thickness. The higher the electrolyte temperature the shorter the period of time required for approaching a stable current density. In addition, with increasing temperature, there is a more significant improvement of the electrocatalytic activity by ''rejuvenation'' on aged electrodes. Tafel plots for oxygen evolution on nickel preanodized or ''rejuvenated'' at 1.5 V exhibit only one linear region with b = approximately 40 mV, while dual Tafel regions are observed on nickel prepolarized at 1.8 or 2.0 V: b = approximately 40 mV at low eta and b = approximately 170 mV at high eta. In comparison with the thickness of oxide films, the chemical identity of the very top layers of oxide films plays a more significant role in determining the kinetics of the oxygen evolution reaction. (ERA citation 03:004409)