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.