Synergistic experimental and theoretical visions into spinel Co3O4 via controlled annealing for high-energy-high-power supercapacitors

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The spinel cobalt oxide (Co 3 O 4 ) nanoparticles were prepared by using a simple, low-cost hydrothermal technique followed by well-controlled air annealing at 350, 450, and 550 °C to investigate the influence of annealing temperature on their structural, surface, textural, and electrochemical characteristics. Increasing the annealing temperature improved crystallite growth and enhanced structural arrangement while decreasing defect density. However, annealing at 550 °C produced particle coalescence, resulting in a reduction in electrolyte-accessible surface area and pore volume. In contrast, Co-450 exhibited an optimum balance between enhanced crystallinity and interconnected surface architecture, providing the highest specific surface area and pore volume among the investigated samples. This optimized microstructure facilitated electrolyte penetration, ion diffusion, and effective utilization of electrochemically active sites, resulting in a specific capacitance of 726 F/g at 5 mA/cm 2 and a low interfacial charge-transfer resistance of 0.0079 Ω. XPS confirmed the coexistence of Co 2+ and Co 3+ redox-active states, while DFT calculations provided insights into the electronic structure underlying the capacitive behavior. Furthermore, an asymmetric Co 3 O 4 //AC supercapacitor delivered 196 F/g, an energy density of 18.4 Wh/kg at 1500 W/kg and retained 5.5 Wh/kg at 3000 W/kg.