In this study, activated carbon was produced from waste tea by H3PO4 activation. Acidic surface modification process was applied to the produced samples at different concentrations (v/v, 3.33 %, 10 %, 20 % and 40 %) using HNO3. The samples were tested as supercapacitor electrode material using aqueous acidic electrolyte (1 M H2SO4). The surface characterization of the untreated and modified activated carbon samples was carried out by various chemical and physical methods and the effect of the type and amount of surface oxygenated groups on the electrochemical performance was investigated. As a result, it has been determined that the acid concentration used in the modification process significantly affects the surface area and porosity of the carbon sample. The specific surface area of the modified samples dramatically decreased when the acid concentration exceeded 3.33 %. The concentration of oxygenated groups on the surface increases, while the type of functional group differs. The electrode prepared from the sample modified with 3.33 % of HNO3 exhibited better performance than the electrode prepared from the untreated activated carbon. As the acid concentration increased, the performance of the electrodes decreased significantly due to the increase in internal resistance of the electrodes.