Activated carbon electrodes were synthetized from African palm shell with KOH (CK) at 2/1 KOH/biomass mass ratio and treated thermically at 700°C (1056 m 2 /g). Then CK was modified at different conditions by surface treatment using HNO 3 /CK impregnation mass ratios 1 and 12, temperatures between 25 – 80°C, impregnation time 1 – 4 h. The performance of electrodes as supercapacitors was evaluated in 1 M H 2 SO 4 using electrical impedance spectroscopy (EIS) and charge/discharge technique (CP). The results showed that the specific capacitance increases upon oxidation (57 F/g up to 89.7 F/g). The highest capacitance was obtained for sample prepared at impregnation ratio 1, 4 h and 25°C (606 m 2 /g) without increasing the equivalent resistance series significantly. These results were influenced mainly by the increased of nitrogen functional groups rather than specific surface area, which confirm that the surface composition has a high influence on the electrochemical performance.