Characterisation of the passive layer and of hydroxide deposits of nickel by impedance spectroscopy

Abstract

The capacitance of the nickel passive layer and hydroxide deposits, studied in 1 M KOH, depends on frequency and potential. Close to the Ni(OH) 2 NiOOH equilibrium potential, Nyquist and Bode diagrams show a characteristic frequency which can be used to determine the hydrogen diffusion coefficient in the solid phase for one of the studied oxides. Outside this domain and on all electrodes, capacitance only involves surface processes depending both on the oxidation state and the nature of the layer. For NiOOH, high capacitances are obtained, similar to those on metals. They are proportional to the layer thickness, and certainly related to the large real area. Conversely, Ni(OH) 2 capacitance decreases with increasing thickness and always exhibits the semiconductive p-type behaviour of the passive layer. This is explained by a difference in the solid state stoichiometry of the deposited layers and of the passive layer. When NiOOH is reduced to Ni(OH) 2 with electronic transfer coupled with proton insertion, the layer composition changes with simultaneous diminution of electron sites (Ni III). When the number of these sites is higher than the number of proton sites, which is assumed to be the case for passive layers, the remaining Ni III act as electronic acceptors and p-type semiconductivity is observed. For deposited layers the number of proton sites is assumed to be equal to that of the electron sites. They therefore exhibit high impedance intrinsic semiconductivity. The Mott-Schottky plot maximums are explained by a reversible change variation of surface states over a limited potential range. Passive layer reduction occurs in the lower potential range explored.