Zeolitic imidazolate framework (ZIF-8) with a unique morphology of ultrathin hexagonal sheets has been synthesized in the present study. To enhance the energy storage capability of this highly porous framework as supercapacitor electrode active material, it has been functionalized with an amine functional group using two different substances; ammonium hydroxide and ethylenediamine (EDA). Then the synthesized active materials have been characterized with X-ray powder diffraction, field emission scanning electron microscope, fourier-transform infrared spectroscopy, and nitrogen adsorption-desorption isotherms analysis. Then to evaluate the electrochemical behavior of the synthesized materials, cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy tests were conducted. Ammonium hydroxide-functionalized ZIF-8 (NH4OH/ZIF) provided the best results (116.7 C/g) compared to the EDA-functionalized sample. In order to investigate the practical energy storage behavior of these substances, the asymmetric supercapacitor (ASC) was assembled with NH4OH/ZIF as the cathode and a graphene oxide electrode as the anode. This ASC has performed a maximum power density of 20 kW/kg at the energy density of 4.16 Wh/kg. Furthermore, the ASC showed good capacitance retention of 80% after 2700 charge-discharge cycles.