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.