A solid-state electrochemical double-layer capacitor (EDLC) was assembled with perfluorosulfonic acid based electrolyte in the HK+, Na +, and K+ forms and with carbon as electrodes (surface area: 260 m2 /g). The performance of these EDLCs was studied using cyclic voltammetry, impedance spectroscopy, and charge-discharge profiles. The assembled cells behaved as EDLCs with a maximum operating voltage of 2.0 V. The specific capacitance was 20 F/g for the electrolyte in HK+ form, 40 F/g for the Na+ form, and 55 F/g for the K+ form. The increase in the specific capacitance may be attributed to the increase in the charge carrying capacity as we move from the proton form (HK+) to the K+ form. There is an increase in the charge density from 1.0 C/ cm2 for the electrolyte in the HK+ form to 6.0 C/ cm 2 for the electrolyte in the K+ form. © 2010 The Electrochemical Society.