Graphene is considered prospective electrode materials for the electrochemical supercapacitor due to their higher surface area, good electronic conductivity, high chemical stability, and wide operating temperature range. The hetero-atom doping and surface functionalization of graphene are also advantageous to enhance the electrochemical active surface area, thermal and electronic conductivity, wettability and ion transport, and stability. The edge-carboxylated graphene nanoplatelets have been used as oxygen-rich efficient counter electrodes for dye-sensitized solar cells. The catalytic activity in graphene may be due to the oxygen containing functional groups (e.g., carboxyls, hydroxyls, epoxides, and carbonyls), achieved by controlling the amount of oxygen functional groups via thermal annealing or acid oxidation. The surface functional groups or heteroatoms can help the adsorption of ions and then improve the hydrophilicity/lipophilicity of the carbon materials, leading to enhanced wettability and facilitated rapid electrolyte ion transport within the micropores. At the same time, the presence of functional groups (e.g., oxygenated groups) on the surface of carbonaceous materials may induce faradaic redox reactions, leading to a 5–10% increase in the total capacitance. Thus, introducing surface functional groups or heteroatoms on the surface of carbon materials appears to be an effective way of improving an electrode’s capacitance. In the present study, we control the degree of carboxylation in the surface of graphene nanoplatelets by thermal and acidic oxidation processes and studied it effects on the performance of electrochemical supercapacitor.