Template-free synthesis of hierarchical mixed-metal cobaltites: Electrocapacitive and Theoretical study

Abstract

The present work elucidates synthesis and electrocapacitive performance of cobaltites viz. Co3O4, NiCo2O4, ZnCo2O4 and MnCo2O4 nanostructured particles synthesized using template free urea assisted hydrothermal method. Morphology and size of the synthesized cobaltites were examined by using scanning electron microscopy. Hierarchical structures namely fibrous and urchin-like microsphere were observed for the cobaltites. Electrochemical measurements were performed using standard three electrode system with 3M KOH electrolyte via cyclic voltammetry and galvanostatic charge-discharge methods. Amongst the cobaltite studied, NiCo2O4 displayed high surface area (46.05m2/g), high specific capacitance (173F/g at 5mV/s) and energy density (7Wh/kg). The specific capacitance of all the cobaltites decreased with the increase in scan rate. Furthermore, the specific capacitance showed invariance with the current density up to 5A/g. The cyclic stability of NiCo2O4 was studied up to 5,000 cycles and about 76% retention in charge storage capacity was observed. Among all cobaltites, the EIS measurements confirm superior performance of NiCo2O4 as it displays minimum impedance at all frequency. Furthermore, specific capacitance was compared for two and three electrode cells with NiCo2O4 as an electrode material. The hybrid density functional theory (DFT) calculations of the electronic density of states of Co3O4, NiCo2O4, ZnCo2O4 and MnCo2O4 was performed. The DFT calculation predicted narrow band gap for NiCo2O4 (∼1.4eV) which is smallest among all cobaltites studied here in. The superior electrocapacitive behavior of the NiCo2O4 is attributed to high surface area urchin like structure and its high electrical conductivity due to its narrow band gap amongst studied cobaltites.