Synthesis of self-assembled nickel cobaltite microspheres and their electrocapacitive behavior in aqueous electrolytes

Self-assembled nickel cobaltite microspheres composed of crystalline nanorods have been synthesized with adjustable morphologies by combining hydrothermal synthesis and subsequent heat treatments. Nickel cobaltite microspheres obtained from the synthesis system containing ethanol and water exhibited the highest specific capacitance (244.3 F g(-1)) at the current density of 0.1 A g(-1) measured using a symmetric supercapacitor with aqueous 2 M KOH solution as the electrolyte, while the capacitance of the electrode increased to 533.3 F g(-1) obtained in measured using a three-electrode system under the otherwise identical conditions. It was also observed that the capacitance of the nickel cobaltite microspheres-based supercapacitors increased first with the KOH concentration changing from 1 M to 2 M and then decreased with the concentration up to 6 M. The energy storage features of nickel cobaltite microspheres in both two-electrode and three-electrode systems appears different according to the electrochemical measurements. Based on these experimental data, the formation mechanism and structure-property relationship of the self-assembled nickel cobaltite microspheres are analysed.