Electrochemical study of functionalized carbon nano-onions for high-performance supercapacitor electrodes

Carbon nano-onions (NCNOs), concentrically multilayered fullerenes, are among the least studied carbon allotropes. NCNOs have high surface area, high mesoporosity, facile electrical conductivity, and electrochemical stability. NCNOs are chemically oxidized (ONCNOs) to introduce hydrophilic functional groups, improving their wettability for use in supercapacitor electrodes. RuO ₂·xH ₂O/ONCNOs composites are prepared by chemical precipitation of RuO ₂ on ONCNOs, optimized for homogeneous distribution and maximum utilization of the metal oxide nanoparticles. The materials are characterized by high-resolution transmission electron microscopy, N ₂ absorption-desorption isotherm, and thermogravimetric analysis. Electrochemical performance of the electrodes is studied by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance measurements in 1.0 M H ₂SO ₄. While ONCNOs have specific capacitance of 45 F·g ^-1, a composite with 67.5 wt.% RuO ₂ has specific capacitance of 334 F·g ^-1 at a potential sweep rate of 20 mV·s ^-1 with high power (242.8 kW·kg ^-1) and high energy density (11.6 Wh·kg ^-1). Both the ONCNO and the composites show excellent capacitance behaviors even at very high potential scan rates (>8 V·s ^-1). Simple preparation of high-purity NCNOs and excellent electrochemical behavior of functionalized NCNOs make them a promising electrode material in supercapacitor applications.