A modified activated carbon material for supercapacitors is prepared via the treatment of gulfweed-based activated carbon with hydrofluoric and nitric acid. The properties of the carbons are characterized by nitrogen adsorption-desorption, scanning electron microscope and Fourier transform infrared spectroscopy. The effect of two-step acid treatment on the electrochemical performance of the activated carbons is investigated by galvanostatic charge-discharge, cyclic voltammetry and AC impedance method. The results show that the hydrofluoric and nitric acid can effectively dissolve the silicon dioxide and other metallic impurities in the carbon materials. The removal of the inorganic impurities contributes to the generation of new pore structure, which provides more transport channels and effective surface area for electrolyte ions and thus improving the electrochemical performance of the carbon materials. Both the capacitance and rate performance improvements of activated carbon are observed after the two-step acid treatment. The gravimetric capacitance of the activated carbons increases from 245 to 310 F g(-1) at a current density 0.1 A g(-1). The capacitance value of the treated carbon material even remains at 239 F g(-1) at a current density of 10 A g(-1), which shows excellent high-rate capacitive performance.