Edward QUITEVIS1, Jagdeep KAUR1, Sophia SAGALA1, Connor HICKS1
1Texas Tech University, Lubbock, United States
Understanding the correlated motions of the ions and their role in determining charge transport in ionic liquids (ILs) is of particular importance in electrochemical energy applications where ILs have been impactful because of their chemical inertness, wide electrochemical window, and high intrinsic ionic conductivity, which obviates the need for a supporting electrolyte. Numerous experimental studies have shown that for ILs, molten salts, and electrolyte solutions, the ratio of the impedance conductivity to the conductivity as derived from the Nernst-Einstein (NE) relation using self-diffusion coefficients obtained from pulse field gradient spin echo (PGSE) NMR measurements is less than or equal to one. The deviation of the impedance conductivity from the NE relation is often used as evidence for ion-pairing and to characterize the “ionicity” of an IL. The concept of ionicity however is ill-defined because any estimate of ion association is dependent on the timescale of the experiments used to determine it. Molecular dynamics simulations and phenomenological theory based on non-equilibrium thermodynamics show that deviations from the NE equation are a natural consequence of differences in the cross-correlation functions of ionic velocities. Recently, Margulis and coworkers have developed a theory in which the distinct diffusion coefficients can be derived from experimentally determined quantities such as the impedance conductivity and self-diffusion coefficients obtained from PGSE-NMR. These distinct diffusion coefficients allow the correlated motion of ions to be probed and the extent of ion-pairing to be rigorously determined. The main goal of this project is to apply this theory to charge transport in ILs to obtain a fundamental understanding of the deviation of the impedance conductivity from the NE equation in terms of inter-ionic motional coupling. These studies were done on 1-ethyl-3-methylimidazolium methyl phosphonate ([C2C1im] [(OMe)(H)PO2]) as a function of temperature. The self-diffusion coefficients were measured using PGSE NMR spectroscopy and impedance conductivity was measured using a YSI 3200 conductivity meter and a YSI 3253 dip-type conductivity cell. 2D heteronuclear NOE (HOESY) experiments were done on the IL to provide information on the nature of ion-ion interactions that control charge transport in these systems.