Polymer-assisted synthesis of Co₃O₄/CoO microballs decorated N-doped carbon for symmetric supercapacitor

Cobalt oxide (Co) and cobalt oxide/N-doped carbon composites (Co-NC) were synthesized and applied as electrode materials for supercapacitors. The pristine cobalt oxide was prepared hydrothermally at 160 °C (CoH160) and further calcined at three different temperatures of 300, 400, and 500 °C (CoC300, CoC400, and CoC500). The cobalt oxide prepared at 300 °C was composited with N-doped carbon prepared from g-C₃N₄ at four different weight ratios 1 : 0.03, 1 : 0.06, 1 : 0.15, and 1 : 0.30 (Co-NC1, Co-NC2, Co-NC3, and Co-NC4). X-ray diffraction analysis (XRD) confirms the phase and product formation. Among all the composites, Co-NC2 showed the microball structure decorated on N-doped carbon with an average size of 4.2 μm. The X-ray photoelectron spectroscopy (XPS) of Co-NC2 confirms the presence of Co²⁺, Co³⁺, C, N, and O. The Brunauer-Emmett-Teller analysis (BET) of Co-NC2 showed a surface area of 73.06 m² g^-1 with a pore diameter of 3.39 nm. The energy storage performance of Co-NC2 exhibits a high specific capacitance of 774.38 F g^-1 at a current density of 1 A g^-1 in 1 M KOH electrolyte. The fabricated symmetric device showed a specific capacitance of 84.60 F g^-1 at a current density of 1 A g^-1. The fabricated device showed good cyclic stability with a coulombic efficiency of 92.55% and capacitance retention of 97.75% up to 4000 cycles.