Nanoporous carbons from oxidized green needle coke for use in high performance supercapacitors

Green needle coke was oxidized by a mixture of HNO3 and H2SO4 (3:7 vol/vol) acids. The resultant oxidized coke was separated into water dispersible and non-dispersible fractions, which were then activated by KOH using the same KOH/coke weight ratio to obtain two kinds of nanoporous carbons with different pore structures. The oxidized coke and the nanoporous carbons were characterized by XPS, XRD, TEM, N2 adsorption and electrochemical tests. Results indicate that, at the same KOH/coke ratios, the nanoporous carbons derived from the water-dispersible oxidized coke have higher mass specific capacitances and capacitance retention ratios, and lower resistances than those from the non-dispersible ones when used as an electric double layer capacitor electrode in 6 mol/L KOH, but the former have a lower volumetric specific capacitance than the latter.