An electrochemical capacitor using an ionic plastic crystal of N,N-dimethylpyrrolidinium fluorohydrogenate (DMPyr(FH)(2)F) as a solid electrolyte has been successfully operated with activated carbon electrodes at 298 K. Typical square-shapes for electric double-layer capacitors are observed in the cyclic voltammogram within the narrow potential regions. In the wide potential regions, the redox reactions involving fluorohydrogenate anions occur on both the positive and negative electrodes. The galvanostatic charge-discharge test confirms stable operation of the electrochemical capacitor using the present ionic plastic crystal. The capacitances of the positive and negative electrodes are 263 and 221 F g(-1) (capacitance per gram of electrode weight), respectively, at the 300th cycle when the cell is charged up to the voltage of 2.5 V with a current density of 238 mA g(-1). The ac impedance test shows that ions in the ionic plastic crystal form double-layer between electrolyte and internal surface of the pore of activated carbon. The open-circuit voltage decreases proportionally to the square root of time within the first 10 s after galvanostatic charging, which confirms that the ions inhomogeneously distributed in the pore of activated carbon migrate to be homogeneous under a diffusion-limited process. (C) 2013 Elsevier B.V. All rights reserved.