Electrical and electrochemical properties of nanostructured Ni and Zn substituted Co3O4 spinels for thermistors and supercapacitor applications

Abstract

Cobalt oxide-based spinels in the form of Co3O4, NiCo2O4 and ZnCo2O4 are synthesized by the co-precipitation technique. Powder X-ray diffraction studies confirm the arrangement of ions in spinel structure with minor secondary phases of native oxides. Scanning electron micrographs of spinels show the formation of fine nanostructures with an overall grain size below 90 nm for Co3O4 and ZnCo2O4. However, the grain size of NiCo2O4 varied from 100 to 250 nm. Dielectric measurements of all spinels exhibit low dielectric loss in lower temperature regions indicating superior dielectric behavior. Almost stable dielectric loss at lower temperatures confirms the usability of these spinels in supercapacitor applications. The dielectric loss increases with temperature due to the dominance of conductive grains and thereby demonstrates improved electrical conductivity. Temperature-dependent dielectric constant studies show the dominance of space charge polarization in lower temperature regions and orientation polarization in higher temperature regions, respectively. The temperature-dependent dc-conductivity of all spinels exhibits NTCR behavior and the same is confirmed by the Cole-Cole plots. The cyclic voltammettry measurement shows highest specific capacitance of 767 F/g for NiCo2O4 spinel followed 600 F/g for ZnCo2O4 and 439 F/g for Co3O4 coated electrodes. Electrochemical impedance measurement shows no sign of a semicircle along with low internal resistance for Ni and Zn-substituted Co3O4 spinels. This endorses the significance of substitution in improving the electrochemical behavior of Co3O4 spinel and their use in supercapacitor applications.