Oxalic acid assisted rapid synthesis of mesoporous NiCo₂O₄ nanorods as electrode materials with higher energy density and cycle stability for high-performance asymmetric hybrid supercapacitor applications

In this study, mesoporous nickel cobaltite (NiCo₂O₄) nanorods as electrode materials for high-performance hybrid supercapacitor were fabricated onto Ni foam by a simple and cost effective oxalic acid (OA) assisted rapid co-precipitation method. The effects of different metal precursors (NCO-Nitrate, NCO-Chloride and NCO-Acetate) on the electrochemical capacitive properties were studied. FE-SEM analysis confirmed that all samples exhibited highly dense mesoporous NiCo₂O₄ nanorods vertically grown on the surface of Ni foam with excess accessible surfaces and unique sizes and morphologies. The resultant NiCo₂O₄ nanorod electrodes (for NCO-Nitrate, NCO-Chloride and NCO-Acetate) delivered the maximum specific capacitances of 790, 784, 776 F g⁻¹ at the current density of 1 A g⁻¹ with ultra-high capacitance retention of 82.27, 81.63 and 81.71% even at 20 A g⁻¹ and excellent cyclic stability of 84.25, 83.33 and 83.24% capacitance retention at 5 A g⁻¹ after 5000 cycles. The asymmetric supercapacitor (ASC) device was also sandwiched by using NCO-Nitrate as positive electrode and N-doped graphene hydrogel (NGH) as negative electrode. The fabricated ASC device delivered superior energy density (42.5 W h kg⁻¹) at high power density (746.34 W kg⁻¹) with excellent long cyclic stability (90% initial capacitance retention after 5000 cycles at 5 A g⁻¹).