Mechanisms of Proton Transport in Ionic Liquids: Grotthuss vs Vehicular

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Professors Sokolov and Paddison from the University of Tennessee combine experimental and computational studies of ionic liquids to unravel important requirements for elucidating highly efficient proton transport mechanisms. Protons are hydrogen atoms that are missing their single electron, and therefore positively charged. Proton motion is vital for the function of various biological systems (for example, energy production and nerve function) and is important in many technological applications, such as energy conversion and storage. Ionic liquids are substances in which the constituent molecules are themselves charged. For example, a neutral molecule can release a proton (+1 electrical charge) from its structure, thus rendering itself a negatively charged (-1) ion. Ionic liquids are a promising class of substances for energy technology applications because they can facilitate very fast transport of protons (in other words they can be highly electrically conductive). Profs. Sokolov and Paddison are seeking to distinguish the various mechanisms of proton transport. One these, the "Grotthuss mechanism" leads to high conductivity because it arises from the cooperative motion of several protons at a time. Experimental identification of this mechanism and understanding its occurrence relative to different kinds of molecular structures would greatly advance our ability to design molecules for ionic liquid applications.The overarching goal of this proposal is to develop a fundamental molecular level understanding of the chemical and structural aspects that control proton transport in protic ionic liquids. The PIs expect to reveal systems with extremely low energy barriers for proton transfer by choosing the right combination of proton acceptors and proton donors. In addition, they focus on analysis of correlated proton dynamics in search for experimental evidences of the collective Grotthuss-like proton transfer mechanism. To achieve these goals the PIs employ broadband dielectric and light scattering spectroscopies, neutron scattering, rheology, and NMR studies, all in tight connection with density function theory (DFT) based electronic structure calculations and ab-initio molecular dynamics simulations. The broader impact of this work includes significant societal benefits from a predictive understanding of proton transport mechanisms, as well as opportunity for training students and postdoctoral associates in advanced experiments and complex simulations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.