High performance supercapacitor electrodes based on spinel NiCo₂O₄@MWCNT composite with insights from density functional theory simulations

In this work, we demonstrated the supercapacitor performance of pristine and composites of spinel NiCo₂O₄ with a multi-walled carbon nanotube (MWCNT) assembled in a two-electrode cell configuration. Spinel NiCo₂O₄ and NiCo₂O₄@MWCNT composites were synthesized via a facile hydrothermal method. The supercapacitive performance of as-synthesized NiCo₂O₄ and NiCo₂O₄@MWCNT fabricated on Ni-foam was studied in a 0.5M K₂SO₄ electrolyte using electrochemical measurement techniques. The symmetric cell configuration of NiCo₂O₄@MWCNT delivers high specific capacitance (374 F/g at 2 A/g) with high energy density and power density (95 Wh/kg and 3 964 W/kg, respectively) compared to that of pristine NiCo₂O₄ electrodes (137 F/g at 0.6 A/g). Furthermore, the energy storage performance of the asymmetric cells of NiCo₂O₄//MWCNT and NiCo₂O₄@MWCNT//MWCNT was studied to enhance cycling stability (retention of 74.85% over 3000 cycles). We have also theoretically studied the supercapacitance performance of pristine NiCo₂O₄ and NiCo₂O₄@SWCNT hybrid structures through its structural and electronic properties using density functional theory predictions. The higher specific capacitance of the NiCo₂O₄@SWCNT hybrid system with high power density and energy density is supported by the enhanced density of states near the Fermi level and increased quantum capacitance of the hybrid structure. We have theoretically computed the diffusion energy barrier of K⁺ ions of the K₂SO₄ electrolyte in the NiCo₂O₄ layer and compared it with the diffusion barrier for Na⁺ ions. The lesser diffusion energy barrier for K⁺ ions in the NiCo₂O₄ layer contributes toward higher energy storage capacity. Thus, owing to superior electrochemical performance of NiCo₂O₄ composites with MWCNTs, it can serve as a high-performance electrode material for supercapacitor applications.