Characterization of NiCo 2O 4 electrodes for O 2 evolution : Part II. Non-electrochemical characterization of NiCo 2O 4 electrodes

Abstract

NiCo 2O 4 spinel oxide was characterized by a number of ex situ non-electrochemical techniques. Thermogravimetric analysis was applied to study the course of the decomposition of the metal nitrates in order to determine the temperature range of the spinel oxide phase. The surface morphology of NiCo 2O 4 was investigated using BET measurements and scanning electron microscopy. X-Ray diffraction, temperature-programmed reduction (TPR) and X-ray and Auger photoelectron spectroscopy (XPS and Auger) were employed to study the surface and bulk composition of NiCo 2O 4. On the basis of the TPR data, NiCo 2O 4 can be presented as M 2+M 3+ 2O 4, with an average oxidation state of the metal cation M (M = Ni or Co) equal to 2.67 +. The presence of divalent nickel is proposed on the basis of the binding energy of the Ni2 p 3 2 photoelectron line, and of cobalt as mainly diamagnetic Co 3+ in a low-spin state, and paramagnetic divalent high-spin cobalt based on the satellite structure in the XPS spectrum.