Electrochemical‐Ellipsometric Studies of Oxide Film Formed on Nickel during Oxygen Evolution

Abstract

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 1 N 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 potentiostatically at 1.5V, are more active than untreated nickel for this reaction. The time variation of current density for oxygen evolution at constant potentials (above 1.56 V) is essentially due to the gradual conversion of Ni3+ to Ni4+ ions in the oxide film on the surface of the electrode. 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 te 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 1.5V exhibit only one linear region with b ~ 40 mV, while dual Tafel regions are observed on nickel prepolarized at 1.8 or 2.0 V: b ~ 40 mV at low overpotential and b ~ 170 mV at high overpotential. Incomparison 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.