Structures and electrochemical performances of pyrolized carbons from graphite oxides for electric double-layer capacitor

The structural features and the electrochemical performances of pyrolized needle cokes from oxidized cokes are examined and compared with those of KOH-activated needle coke. The structure of needle coke is changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an NaClO3/needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized needle coke is expanded to 6.9 angstrom with increasing oxygen content. After heating at 200 degrees C, the oxidized needle coke is reduced to a graphite structure with an inter-layer distance of 3.6 angstrom. By contrast, a change in the inter-layer distance in KOH-activated needle coke is not observed. An intercalation of pyrolized needle coke, observed on first charge, occurs at 1.0 V. This value is lower than that of KOH-activation needle coke. A capacitor using pyrolized needle coke exhibits a lower internal resistance of 0.57 Omega in 1 kHz, and a larger capacitance per weight and volume of 30.3 Fg(-1) and 26.9 F ml(-1), in the two-electrode system over the potential range 0-2.5 V compared with those of a capacitor using KOH-activation of needle coke. This better electrochemical performance is attributed to a distorted graphene layer structure derived from the process of the inter-layer expansion and shrinkage. (C) 2007 Elsevier B.V. All rights reserved.