Oxidative dissolution of NiO in aqueous electrolyte: An impedance study

The present contribution reports on the analysis of the electrochemical properties of screen-printed nickel oxide (NiO) in aqueous electrolyte when NiO is in the configuration of thin film (thickness, l ≤ 7 μm). This type of NiO samples presents mesoscopic morphology. The latter characteristic combined to the intrinsic electroactivity of the oxide leads to the observation of proportionality between NiO thickness and current density. In particular, NiO undergoes two distinct processes of oxidation with reversible features in aqueous media. These redox processes occur in the solid state and, as such, involve mass/charge transfer between the NiO electrode and the electrolyte. On the other hand, the electrochemical oxidation of NiO is accompanied by the progressive chemical dissolution of the oxide when the latter is in the oxidized state. This is a consequence of the elevated content of stored charge on NiO surface when it interfaces a high polarity solvent like water. In the present work we have considered electrochemical impedance spectroscopy (EIS) for the investigation of the charge transfer/transport properties of NiO during the simultaneous occurrence of chemical and electrochemical processes. The study proposes models of electrical circuits for analyzing the interfacial phenomena involved in the aqueous oxidation of NiO.