H3PO4-impregnated waste cotton was used as precursor to fabricate high porous activated carbon (AC) by the carbonization and activation processes with ultrahigh heating rate. The obtained activated carbon has unique physicochemical properties such as the structure mainly amorphous and ultrahigh specific surface area of 2769.7 m2/g for samples fabricated at carbonization temperature of 600 ◦C. The double-layer supercapacitors with activated carbon electrodes and electrolyte based on 1,1-dimethylpyrrolidinium tetraflu-oroborate solution in acetonitrile of 1 M concentration were fabricated. The specific capacitance of electrode material fabricated from AC obtained at carbonization temperature of 600 ◦C reached 110.8 F/g at the current density of 50 mA/g and 85.1 F/g at 1000 mA/g. At 1000 mA/g, the degradation was less than 25% after 5000 charge/discharge cycles. The carbonization temperature of 600 ◦C is considered as optimum for fabricating activated carbon and the obtained activated carbon can be used for supercapacitor electrode materials.