Influence of Urea on the Synthesis of NiCo₂O₄ Nanostructure: Morphological and Electrochemical Studies

The widespread use of miniature electronic devices calls for energy-dense storage strategies. The supercapacitor-based energy storage devices with high areal capacitance are desired energy storage alternative. It is still a challenge to fabricate supercapacitor-based energy devices with consistent performance. The porous metal oxides with large areal capacitance are desired materials for electrode, but there exists a limited understanding of the influence of synthesis parameters on microstructural properties, which largely govern their electrochemical performance. In the present work, hierarchal spinel nickel cobaltite (NiCo₂O₄) nanostructures were synthesized in the presence of the varying amount of hydrolyzing agent via a simple hydrothermal method coupled with a simple post-annealing process. This work focuses on understanding the influence of hydrolyzing agent in controlling the microstructure and hence ensuing electrochemical properties of the NiCo₂O₄ based electrode. Based on the urea hydrolyzing content, the as synthesized NiCo₂O₄ nanostructure varied from the rod, plate to nanoflower. The mesoporous nanostructures, with urea content 1.49 gm, exhibit a sizeable BJH surface area (79.2 m² g^-1) and high mesopore volume (0.140 cm³ g^-1). Remarkably, the NiCo₂O₄ nanoflower shows high specific capacitance of 3143.451 F/g at 2 mV/s scan rate, 1264.5 F/g at 1 A/g current density, energy density of 56 Wh/kg and power density of 8,400 W/kg in 3 M KOH electrolyte. The capacitance loss after 5000 cycles is 48% at the current density of 10 A/g, indicating their excellent cycling stability. The impressive electrocatalytic activity is largely ascribed to the high intrinsic electronic conductivity, superior mesoporous nanostructures and rich surface Ni active species of the NiCo₂O₄ materials, which can largely boost the interfacial electroactive sites and charge transfer rates indicating promising applications as electrodes in future supercapacitors.