Synthesis of porous carbon nano-onions derived from rice husk for high-performance supercapacitors

Porous carbon nano-onions (CNO) are prepared by a facile nickel assisted graphitization, followed by an activation procedure with renewable and sustainable rice husk as carbon precursor. The nitrogen adsorption/desorption results demonstrate that the CNO-700 possesses a porous structure and a high specific surface area of 3831 m² g⁻¹. The high graphitization degree and carbon-oxygen functional groups of CNO-700 have been characterized by X-ray diffraction, Raman spectroscopy, Transmission electron microscopy and X-ray photoelectron spectroscopy. Further, the electrochemical properties are studied by galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy. CNO-700 shows a high specific capacitance of 350 F g⁻¹ at the current density of 0.1 A g⁻¹, good rate performance (288 F g⁻¹ at 20 A g⁻¹), and superior cycling performance with 99% of the capacitance retention over continuous 10,000 cycles at a current density of 20 A g⁻¹ in the aqueous electrolyte of 6 M KOH. This suggests that the porous carbon nano-onions derived from rice husk is a promising electrode material for supercapacitors.