Electrochemical characterization of supercapacitors based on carbons derived from coffee shells

Carbons derived by pyrolysis of coffee shells treated with ZnCl2 were used as electrode materials in symmetric electrochemical supercapacitors. Scanning electron microscopy showed that the carbon from the porogen-free shells show a flake-like structure, while those from the ZnCl2-treated coffee shells have a loose, disjointed structure with no definite shape. X-ray diffraction studies indicated the presence of small domains of coherent and parallel stacking of the graphene sheets. The average surface area of the carbon was 842 m2 g−1, with an average micropore area of 400 m2 g−1. Cyclic voltammetric studies suggested a specific capacitance of about 150 F g−1. Self-discharge studies on the devices showed a large retention time.