Electrochemical studies. The porous carbon (1C, 2C, or 3C) electrodes were prepared by mixing the carbon material with polymeric binder polytetrafluoroethylene (PTFE) and the conductive additive carbon black (Super P) to form a homogeneous slurry in ethanol with a weight percentage of 70/10/20. When the ethanol was evaporated, the slurry became a dough and it was cut into small disks (diameter: 8-mm). The electrode disks were dried in a vacuum oven for 12 hours at 80 o C.