Template-free synthesis of NiO skeleton crystal octahedron and effect of surface depression on electrochemical performance

Abstract

In this work, we were committed to building a nickel oxide (NiO) octahedron with skeleton crystal structure as a capacitor electrode for supercapacitance through a template-free and efficient one-step process. Initially, nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and anhydrous ethanol mixtures were used as a material. The final samples were prepared by calcining the precursor at different temperatures. The mechanism of crystal recrystallization at different temperatures during calcination was discussed. Generally speaking, in the process of rapid growth with only diffusion mechanism, crystal imperfections such as crystal plane depression and skeleton crystal will be formed. At the lower temperature calcination, small depressions are produced due to the effect of crystal face Ostwald ripening. At higher temperatures, the crystal edge growth rate is faster than the surface growth rate during Ostwald ripening and recrystallization, resulting in the formation of NiO octahedron with a large surface depression skeleton crystal structure. The electrochemical test results of the samples showed that the surface depression NiO octahedron has fine supercapacitive behaviors and specific capacitance values (640 F g−1) at the discharging current of 0.5 A g−1 in the 3 mol L−1 KOH electrolyte and maintain excellent cycling stability, remaining constant after 2000 cycles. Electrochemical impedance measurements confirmed the capacitance performance of NiO electrodes.