Preparation of Oxygen-enriched Activated Carbons from Coal-based Humic Acids by Zinc Chloride Activation

Oxygen-enriched activated carbons (ACs) for electrochemical capacitors (ECs) were prepared from coal-based humic acids (HAs) by zinc chloride (ZnCl2) activation. The effects of ZnCl2/HAs ratio and activation temperature on the characteristics of ACs were investigated by nitrogen adsorption at 77K, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The electrochemical performances of ECs with ACs as electrode in a 3M KOH aqueous solution were evaluated by galvanostatic charge-discharge, cyclic voltammetry and Nyquist impedance spectroscopy. The results indicate that the optimum conditions for preparing ACs from coal-based HAs are ZnCl2/HAs ratio of 2.0 and activation temperature of 500 °C. The AC as electrode for ECs prepared under these conditions presents a high specific capacitance of 226 F/g at a high current density of 5 A/g, and exhibits good cycling stability and minimal resistance. The excellent electrochemical performances of ACs are attributed to three aspects: the well-developed porosity with moderate specific surface area, three-dimensional interconnected pore structure and the oxygen-enriched functional groups in carbon network.