A series of novel room-temperature ionic liquid (RTIL) salts based on C n (MIm) i A and C n (TMA) i A (i =1, 2 as single- and double-cation form, respectively; n =4, 5, 6; A=BF4 −, TFSI−), where MIm, TMA, and TFSI indicate 3-methylimidazolium, trimethylammonium, and bis(trifluoromethanesulfonyl)imide, respectively, are synthesized varying the number of alkyl chains, e.g., butane (n =4), pentane (n =5), and hexane (n =6) in the main backbone. Ionic conductivities of each RTIL salt are measured after dissolving with propylene carbonate (PC) or acetonitrile (ACN) at the salt concentration of 0.5M. A supercapacitor containing activated carbon as an active material in the organic electrolyte containing ionic liquid salts is fabricated to evaluate the supercapacitive properties using cyclic voltammetry. It is found that the effects of counter-anions (BF4 − and TFSI−) and the number of cations (single- and double-cation) in RTIL salts on the supercapacitive property are of importance in determining their potential applications for energy storage devices.