Electrochemical Growth of Surface Oxides on Nickel. Part 1: Formation of α-Ni(OH)2 in Relation to the Polarization Potential, Polarization Time, and Temperature

Electro-oxidation of Ni(poly) in 0.5 M aqueous KOH solution at various polarization potentials (E-p) up to 0.5 V vs. reversible hydrogen electrode, for polarization times (t(p)) up to 2 h, and at 285 <= T <= 318 K leads to the formation of a thin layer of alpha-Ni(OH)(2). Interfacial capacitance measurements show that the Ni(poly) electrode covered with a layer of alpha-Ni(OH)(2) can be completely reduced back to its metallic state by applying a negative-going potential scan with a lower potential limit of -0.2 V. An increase of E-p, t(p), and/or T results in an increase of the thickness of the alpha-Ni(OH)(2) layer, which, however, never exceeds two monolayers. The electrochemical formation of alpha-Ni(OH)(2) follows a direct logarithmic growth kinetic law. The results reported in this contribution and their interpretation imply that other oxide growth theories, such as the Langmuir-type adsorption, the point defect model, the electron tunneling, or the nucleation-and-growth model, are not applicable to the growth of alpha-Ni(OH)(2). The potentiostatic growth of alpha-Ni(OH)(2) on Ni(poly) is successfully treated by applying the interfacial place-exchange mechanism and the associated kinetic law.