A heteroatom-enriched activated carbon with low surface area is prepared from poly(ethyleneterephthalate) and is used as an active electrode material for supercapacitor in 1.28 g cm−3 H2SO4 solution. Thus prepared heteroatom-enriched activated carbon is characterized by nitrogen adsorption/desorption at 77 K, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical characterizations are performed by cyclic voltammetry and galvanostatic charge/discharge in a three-electrode system. The prepared heteroatom-enriched activated carbon shows an interesting electrochemical activation phenomenon because of which the specific capacitance is significantly increased by charge at high potentials. The activation process involves the insertion of ions into the narrow enclosed free space inside the carbon material to generate new pores. The final results show that the heteroatom-enriched activated carbon electrode has a better discharge rate capability than the commercial activated carbon (Norit AZO) over a wide range of loading current (1–20 A g−1). The highest specific capacitance (201 F g−1) is obtained at 5 mV s−1, which is two fold greater than the one before electrochemical activation. In addition, the specific capacitance of the heteroatom-enriched activated carbon is retained approximately 92% of the initial value after 20,000 cycles, indicating its excellent cycle stability.