To increase the capacitance of an electric double layer capacitor (EDLC) containing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) as an ionic liquid electrolyte, barium titanium oxide (BaTiO3) as a ferroelectric material is incorporated into activated carbon electrodes. An EDLC composed of BaTiO3 powder/activated carbon composite electrodes and the ionic liquid electrolyte EMIBF4 exhibits a large capacitance when compared to the corresponding EDLC without BaTiO3. Moreover, the rate performance of the EDLC containing BaTiO3 is improved when the electrodes are prepared by an improved method, i.e., vacuum penetration of BaTiO3-dispersed EMIBF4 into the activated carbon electrodes. No positive addition effect of BaTiO3 is observed for the corresponding EDLC with a typical organic electrolyte composed of a tetraalkylammonium salt and propylene carbonate as a solvent. These results suggest that BaTiO3 incorporated into the electrodes would improve the dissociation of the ionic liquid EMIBF4, and hence increase the carrier concentration available for the formation of an electric double layer. (c) 2007 Elsevier B.V. All rights reserved.