Co₃O₄ nanoparticles-embedded nitrogen-doped porous carbon spheres for high-energy hybrid supercapacitor electrodes

This work synthesizes cobalt oxide (Co₃O₄)-embedded nitrogen-doped porous carbon spheres (Co₃O₄@NPCS) with inherent hierarchical structures through hydrothermal and pyrolysis processes. The synthesized material shows porous spherical carbon with nitrogen functionalities and contains Co₃O₄ nanoparticles on the surface of the carbon framework. The hybrid supercapacitor (HSC) is fabricated by Co₃O₄@NPCS and NPCS as anode and cathode, respectively, using aqueous KOH as electrolyte. The HSC displays exceptional electrochemical performance over the wide voltage window of 1.5 V with a large capacitance of 464 F g⁻¹ at 1 A g⁻¹, a high energy of 98.5 Wh kg⁻¹, and power of 1500 W kg⁻¹. Long charge-discharge cyclic stability demonstrates 87.2 % capacitance retention of the HSC over 10,000 cycles with almost 100 % coulombic efficiency. Additionally, first principles investigations show that the improved hybrid supercapacitors performance is as a consequence of the increased conductivity and the charge transfer. The proposed electrode and hybrid design show a potential for future high-energy storage.