Proton transfer in protic ionic liquids

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 obtained 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.

Non-flammable ionic liquids represent a noteworthy avenue for advancing future battery technologies. The deployment of ionic liquids can potentially mitigate reliance on Critical Raw Materials in the battery manufacturing process. This reliance, particularly evident in the production of contemporary lithium batteries, is fraught with uncertainties regarding these materials' secure and cost-effective supply. Furthermore, ionic liquids present a viable solution to the challenges posed by the flammability of currently utilized electrolytes. However, a significant limitation of 'conventional' ionic liquids is their high viscosity, resulting in comparatively low electrical conductivities. This limitation, pertinent to numerous applications, may be addressed using protic ionic liquids. Distinctively, this subclass is characterized by the reversible proton transfer from an acid (proton donor) to a base (proton acceptor). Such a mechanism facilitates the decoupling of charge transport from mass transport, thereby enhancing conductivity due to the low mass of protons, which significantly increases charge mobility. So far, the ionic mobility and distribution of charge carriers have been widely assessed from the average, equilibrium distribution of protons obtained from molecular spectroscopies, or from the long-ranged transport of all charge carriers as measured via the electrolyte conductivity. We developed a program that enables proton transfer reactions in classical molecular dynamics simulations without sacrificing simulation time or system sizes. Thus, we got a deeper understanding of the mechanism for proton dynamics in complex ionic liquid systems.