High-performance activated carbons for use in the electrodes of electric double-layer capacitors

Activated carbons (ACs) were prepared by simultaneous physical-chemical activation of coconut shells using ZnCl2 as activating agent. Their specific surface area and porosity parameters were calculated from the nitrogen adsorption-desorption isotherms at 77 K. The electrochemical properties of the electric double layer capacitors using these ACs as electrode materials were studied by a constant current charge-discharge technique and cyclic voltammetry. Results indicated that the specific capacitance of the carbon material could be as high as 360 F/g at a low discharge current of 5 mA, and it remained above 200 F/g at a larger discharge current of 50 mA. The influence of specific surface area and pore size on the electrochemical property was investigated. No special relationship was found between specific surface area and specific capacitance. However, the pore size had a significant influence on the specific capacitance. In a discharge at small current, the contribution of mesopore surface to the electric double-layer capacitance was larger than that of micropores. With increasing discharge current, the capacitance of mesopore surface decreased faster than that of micorpore surface. For discharge at a large current, supermicropores of 1.5-2 nm played an important role for the electrical energy storage.