In this study, a simple, fast, and clean oxygen plasma method is presented to increase the surface area of activated carbon from walnut shells (WAC) and to connect functional groups containing oxygen to the carbon sample surface. X-ray photoelectron spectroscopy (XPS), nitrogen adsorption technique, Fourier-transform infrared spectroscopy (FTIR), X-Ray Diffraction (XRD), and scanning electron microscope (SEM) analyses to investigate the properties of the WAC and WACP are performed. Cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS) measurements are used to investigate the capacitance properties of the supercapacitors. A significant increase in the surface area of the WAC was increased from 22 m(2) g(-1) to 78 m2 g(-1) after oxygen plasma treatment. The specific capacitance was improved from 6.98 F g(-1) to 76.89 F g(-1) at 2.5 mV s(-1). The devices with functionalized electrodes exhibited cyclic stability of 78% after 5000 cycles.