Abstract: Abstract:Carbon aerogel (CAG), carbon black (CB), and graphite (G) were added to a high surface area activated carbon (HSAC), prepared by KOH activation, to form composites for use as electrodes of electrochemical capacitors (ECs). The performance of the ECs using these electrodes in (C2H5)4NBF4/propylene carbonate electrolyte were analyzed and compared by an electrochemical impedance spectrum (EIS). An equivalent circuit model with mixed kinetic and charge transfer control was assumed and a Marquardt fit procedure was applied to the EIS data, from which model parameters, such as, series solution resistance, Rs, polarization resistance, Rp, Nernst diffusion layer thickness, δ, and average value of the diffusion coefficients of electrolyte ions, D, were extracted and compared for CAG-, CB-, and G-based composites. It was found that the ECs using CAG had the highest D values and a comparable Rp value with CB. The CAG-based composite had the lowest resistance and highest capacitance at the same frequencies among the three types of composites investigated.