Electrochemical properties of novel ionic liquids for electric double layer capacitor applications

Abstract An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF 4 − ) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF 3 SO 2 ) 2 N − ] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N , N -diethyl- N -methyl- N -(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF 4 ) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF 4 is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm −1 at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF 4 as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge–discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.

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