Proton Transfer in Protic Ionic Liquids

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Non-flammable ionic liquids – salts that are liquid at room temperature – have the potential to replace electrolytes, however, the high viscosity of most ‘conventional’ ionic liquids makes charge transport too slow for electrochemical applications. This drawback can be circumvented by using protic ionic liquids, for which charge transport originates not only from motion of bulky ions but also from transport of small and light protons. Charge transport can thus be decoupled from mass transport and the conductivity can be increased. It is however difficult to asses how much – if at all – proton transport enhances the electrolyte conductivity. So far this has been mainly estimated from the protons’ equilibrium distribution or from the long-ranged transport of all charge carriers. As none of these measures can isolate proton conduction, these studies have led to unsatisfactory and inconsistent results. Here we propose a detailed investigation of charge transport in protic ionic liquids at all relevant time- and length-scales. This will be achieved by (i) isolating the contribution of proton transfers to broadband dielectric spectra using a combination of experiments and simulations. We will establish reversible proton transfer in polarizable MD simulations such that agreement with experimental spectra from Megahertz to far-infrared frequencies for various pure methylimidazolium based ionic liquids and their mixtures with the polar solvent acetonitrile is achieved. Having established the methodology for these model systems, we aim at (ii) understanding the fundamental mechanisms of proton conductivity in protic ionic liquids. To this end, we will vary the proton donor strength of the ionic liquid anion using fluorinated carboxylic acids. Using the established methodology we will elucidate the effect of the proton equilibrium between the anion and the cation on proton conduction. To further extract long-ranged proton mobilities, we will use photoacids for a triggered release of excess protons and trace their transport in real-time via infrared detection of its arrival at an accepting base.We envision that the combination of experimental results and reactive, polarizable molecular dynamics simulations offers a novel strategy to elucidate and model charge transport in protic ionic liquids with impact on the design of future electrolytes.