Nickel surface state has been studied by means of electrochemical impedance spectroscopy (EIS) method. The experiments were performed in sodium hydroxide solutions in the concentrations range from 0.001 to 0.1 M at the different levels of the potentiostatic anodic polarization. Two semicircles have been observed on the Nyquist diagram at the potentials below +400 mV (Ag/AgCl). The high-frequency arc is caused by ionic charge transfer from the electrolyte through the barrier oxide-hydroxide layer on the electrode surface. Another low-frequency semicircle on the Nyquist diagram is related to the Faraday oxidation processes at the solid-phase boundary between the oxide and the metal. An equivalent circuit that is adequate to the experimental EIS results was chosen. The correlation of the equivalent circuit element parameters with the variable experimental factors, such as the sodium hydroxide concentration and the anodic polarization level, has been analyzed. It has been established that the ionic charge transfer resistance across the liquid phase and oxide-hydroxide boundary practically does not depend on the potential, but obeys a power dependence on the alkali concentration and can be described in terms of the Freundlich isotherm. The electrical double layer capacity of nickel electrode is a square root function of the alkali concentration, i.e., corresponds to the classical Gouy-Chapman model. The model element parameters corresponding to the Faraday process on the metal, on the contrary, are weakly dependent on the alkali concentration, and are determined by the applied anodic potential.
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