Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids

Abstract

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 IL4^TFSI 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 IL4^TFSI 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 IL4^TFSI 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.