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Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids

Matsumoto, Michio, Shimizu, Sunao, Sotoike, Rina, Watanabe, Masayoshi, Iwasa, Yoshihiro, Itoh, Yoshimitsu, Aida, Takuzo
Journal of the American Chemical Society 2017 v.139 no.45 pp. 16072-16075
capacitance, electric power, electrochemistry, electrodes, electrolytes, energy, gold, impedance, ionic liquids, transistors
Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4ᵀFSᴵ with four imidazolium ion units in its structure provides a wide electrochemical window of ∼5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4ᵀFSᴵ exhibits an exceptionally high EDL capacitance of ∼66 μF/cm², which is ∼6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4ᵀFSᴵ as a gate dielectric material and SrTiO₃ as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ∼64 μF/cm², as determined by Hall-effect measurements.