Transition Metal Oxides for Supercapacitors

Abstract

Transition metal oxides have been extensively studied for last few years for its vast potentials in energy storage and conversion applications due to its remarkable electrochemical properties, low cost and environmental friendliness. In this direction, ternary nickel cobaltite (NiCo2O4) nanocrystals have become a promising energy storage materials for supercapacitor application due to its prolific electroactive sites and higher electrical conductivity compared to the corresponding binary metal oxides. In the present project work, a simple and generally applicable strategy was developed for the efficient synthesis of nanostructured nickel cobaltite of varying morphologies simply by changing metal to urea ratio. In this direction, sustained and dedicated research efforts were devoted with an aim to achieve binderless hierarchical nanostructures directly on the nickel foam substrate. Cyclic voltammetry and galvanostatic charge-discharge measurements indicated an extremely high areal capacitance of 4349 mF/cm2 for nickel cobalt oxide nanowires compared to other morphologies such as nanoneedles (2100 mF/cm2), nanosheets with wires (2445 mF/cm2) and platelets (968 mF/cm2). Nickel cobaltite nanowire electrode demonstrated extended cyclability at 4 mA/cm2 current density with more than 90% capacity retention at the end of 13000 cycles. To understand the intrinsic charge injection capability of different morphologies, the capacitance values are normalized with respect to electrochemically accessible area. The resulting true area normalized charge injection capability is more than doubled in nickel cobalt oxide nanowires compared to other morphologies. This indicates the exposure of more electroactive cationic sites at nickel cobalt oxide nanowire electrode/electrolyte interface towards tuning the interfacial charge storage mechanism. Investigation of supercapacitive failure mechanism by leakage current and selfdischarge rate suggest the most probable failure pathway in nanowire morpho