Facile wick-and-oil flame synthesis of high-quality hydrophilic onion-like carbon nanoparticles

Because of their unique 0-D structure, small (<10 nm) diameter, high electrical conductivity and relatively easy dispersion, compared to 1-D nanotubes and 2-D graphene, onion-like carbon (OLC) has been shown to be ideal as an active material for supercapacitor electrodes. However, its implementation is fraught with lack of convenient methods for their high yield preparation and purification. Here we report a facile scalable and one-step “wick-and-oil” flame synthesis of OLC nanopowder from 'clarified butter' without contaminated by any other forms of carbon; thus eliminating the post processing purification procedure. Brunauer-Emmett-Teller (BET) specific surface area of as-prepared OLC was 218 m2/g, which is higher as compared to other reported flame synthesis methods. The as-obtained OLC powder is highly dispersible and the suspension is stable for months indicating the presence of surface functional groups which is confirmed by Fourier transform infrared spectroscopy (FTIR). It is demonstrated that by suitable choice of a precursor material, very pure and hydrophilic OLCs can be prepared in bulk quantities by flame synthesis method. An effective and simple strategy was developed to prepare flexible, binder-free OLC electrodes on cotton fabrics and demonstrated their application in a flexible supercapacitor device. The capacitance of OLC is 102 F/g with a measured power density of 1224 W/kg and an energy density of 3.5 W h/kg in 1 M Na2SO4 as the electrolyte.