Charge Transport Properties of a Protic Ionic Liquid studied by Dielectric Spectroscopy

Abstract

Three simple models including the Debye type model, Cole-Cole type model and Einstein-Smoluchowski model are used to determine the number density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> , charge carrier density) and the mobilities (μ) of the charge carriers separately from the dielectric spectroscopy. By comparing them, the Cole-Cole type model appears to evaluate charge transport properties very well while the Debye type model doesn’t work for a complex ionic system. The temperature dependence of the mobility exhibits a Vogel-Fulcher-Tammann (VFT) behavior, while the number density of mobile charges is almost insensitive to the temperature. However, the fraction of available (mobile) charge carriers is found to be surprisingly low (~6 % at 25 °C), which can be attributed to charge immobilization in molecular aggregates. A subtle step in the mobility and (mobile) charge density around -10 °C is discussed in terms of a “hidden” phase transition possibly affecting the molecular organization in such aggregates. Increasing the fraction of mobile ionic charges by preventing aggregation would be an effective strategy to enhance σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DC</sub> of the protic ionic liquid trioctylammonium methanesulfonate ([TOA][OMs]).