Electrochemical Growth of Surface Oxides on Nickel. Part 3: Formation of β-NiOOH in Relation to the Polarization Potential, Polarization Time, and Temperature

Abstract

Electro-oxidation of Ni(poly) in 0.5 M aqueous KOH solution is examined at various polarization potentials (Ep), polarization times (tp), and temperatures (T) in the 0.7 ≤ Ep ≤ 1.2 V versus RHE, 60 ≤ tp ≤ 10,800 s, and 288 ≤ T ≤ 318 K ranges. The oxide chemical composition is determined using X-ray photoelectron spectroscopy (XPS) and its thickness using XPS depth profiling. XPS measurements demonstrate that such formed oxide layers are multilayer in nature and contain both β-Ni(OH)2 and NiO, with β-Ni(OH)2 being the predominant, outermost species and NiO forming a thin layer sandwiches between metallic Ni and β-Ni(OH)2. An increase of Ep, tp, and/or T results in an increase of the oxide thickness. XPS depth profiling measurements reveal that the thickness (d) of the β-Ni(OH)2 plus NiO layers is in the 2 ≤ d ≤ 11 nm range, depending on Ep, tp, and T. The electrochemical formation of β-Ni(OH)2 plus NiO follows an inverse-logarithmic growth kinetic law with the escape of metal cation from the metal into the oxide at the inner metal/oxide interface being the rate-determining step. The oxide growth is treated mathematically using the Mott–Cabrera oxide growth theory.