Abstract: The electrochemical impedance spectroscopy of an activated carbon/activated carbon supercapacitor and an asymmetric supercapacitor before and after activation process was studied. The asymmetric supercapacitor consisted of spinal LiMn₂O₄(LMO) as a positive electrode and activated carbon (AC) as a negative electrode. After activation process, its resistance at high frequency (10 kHz) has almost no change, but its capacitance at low frequency is greatly increased; its frequency behavior is quite different from that of the AC/AC supercapacitor as a result of the LMO positive electrode. The impedance spectroscopy of the asymmetric supercapacitor before and after the activation process was studied in terms of complex capacitance and complex power, and the relaxation time constant was determined. The impedance behaviors of the AC/AC supercapacitor and the LMO/AC asymmetric supercapacitor were compared, showing the effect of the LMO positive electrode on the frequency response characteristics of a supercapacitor.