Water electrolysis or photoelectrolysis is an attractive source of green hydrogen. The energy required to generate hydrogen from water should ideally come from a renewable source such as a solar cell that powers potentiostat or direct photoelectrolysis. Either option requires a catalyst. This work focuses on making a water oxidation catalyst using mixed-metal spinel oxides containing Ni and Co. Mixed-metal oxides are stable to aqueous conditions, have strong light absorbers, and are inexpensive. The spinel crystal structure specifically contains tetrahedral and octahedral sub-lattice sites that accommodate mixed metal combinations and valances. Recent work indicates that nickel (Ni) contributes to catalytic activity and cobalt (Co) contributes to higher conductivities. The goal of this work is to synthesize NixCo3-xO4 with different ratios of Ni and Co and characterize their electrochemical properties. Different ratios of Ni(NO3)2 and Co(NO3)2 were employed in a solvothermal synthesis and secondary annealing step to create samples in which the ratio of Ni/Co ranged from 0-2. Powder X-ray diffraction and energy dispersive spectroscopy confirm the incorporation of Ni into the Co3O4 spinel structure for syntheses where the ratio of Ni/Co ≤ 0.5. Cyclic voltammograms in aqueous and non-aqueous conditions show that the materials with Ni in acidic conditions or organic electrolytes show capacitive behavior. Future work with these materials includes testing their water oxidation performance and photoelectrochemical activity.

Synthesis and electrochemical properties of nanostructured NixCo3-xO4 materials

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