Studies of the activated carbons used in double-layer supercapacitors

The specific capacitance of activated carbons is determined by both the ratio of edge/basal orientation and the nature of functional group on the surface. The difference between the edge and the basal layers results from the semiconductive properties of basal layer. The ratio of edge/basal orientation can be estimated by X-ray diffraction (XRD). The wettability of activated carbons is determined by the nature of functional groups on the surface. Most of the surface groups are electrochemically active. The impact of the surface groups on electrochemical performance of the activated carbon electrodes was investigated by means of various surfactant treatments. The ac impedance and constant current discharge techniques were used. Two types of surface groups which had “capacitor-like” or “battery-like” behaviors, respectively, were revealed and discussed in detail. The surface groups with “battery-like” behavior should be avoided. “Non-symmetric” electrode arrangement should be considered for a double-layer supercapacitor in order to take the advantages of pseudo-capacitance of the surface groups with “capacitor-like” behaviors.