Ionic liquids containing the tetrafluoroborate anion have the best performance and stability for electric double layer capacitor applications

Fourteen kinds of ammonium salt, including nine ionic liquids, were evaluated for use as the electrolyte (as a solute) of an electric double layer capacitor (EDLC) from the viewpoint of practical performance and thermal stability. For this study, three kinds of anion species were selected for the counter anion: tetrafluoroborate (BF4), hexafluorophosphate (PF6) and bis(trifluoromethylsulfonyl)imide (TFSI) anions. A series of EDLCs with only 0.4% deviation in the capacitance of each cell, based on activated carbon and various salts in propylene carbonate (PC), were strictly prepared. The capacitance at room temperature, and the resistance value of the EDLCs at a relatively large direct current at low temperatures were measured. We also examined the durability of these systems by continuous charging at 70 °C and 3.0 V. We report that N-(2-methoxyethyl)-N-methylpyrrolidinium-BF4, a cyclic aliphatic quaternary ammonium with a methoxyethyl functional group, is the preferred ionic liquid for an EDLC electrolyte with respect to capacitance, power density, even at low temperatures and thermal durability.