Exceptionally\nHigh Electric Double Layer Capacitances\nof Oligomeric Ionic Liquids

Abstract

Electric double layer\n(EDL) capacitors are promising as next-generation\nenergy accumulators if their capacitances and operation voltages are\nboth high. However, only few electrolytes can simultaneously fulfill\nthese two requisites. Here we report that an oligomeric ionic liquid\nsuch as <b>IL4</b>^<b>TFSI</b> with four imidazolium\nion units in its structure provides a wide electrochemical window\nof ∼5.0 V, similar to monomeric ionic liquids. Furthermore,\nelectrochemical impedance measurements using Au working electrodes\ndemonstrated that <b>IL4</b>^<b>TFSI</b> exhibits\nan exceptionally high EDL capacitance of ∼66 μF/cm², which is ∼6 times as high as those of monomeric ionic\nliquids so far reported. We also found that an EDL-based field effect\ntransistor (FET) using <b>IL4</b>^<b>TFSI</b> as a gate dielectric material and SrTiO₃ as a channel\nmaterial displays a very sharp transfer curve with an enhanced carrier\naccumulation capability of ∼64 μF/cm², as\ndetermined by Hall-effect measurements.