Observation of separate cation and anion electrophoretic mobilities in pure ionic liquids

Abstract

Ionic liquids (ILs) continue to show relevance in many fields, from battery electrolytes, to carbon capture, to advanced separations. These highly ion-dense fluids present unique challenges in understanding their electrochemical properties due to deviations in behavior from existing electrolyte theories. Here we present a novel characterization of ILs using electrophoretic NMR (ENMR) to determine separate cation and anion mobilities. This method uses an applied electric field coincident with a pulsed magnetic field gradient to encode the E-field driven flow into NMR signals for cations ((1)H) and anions ((19)F). We describe the detailed design of these experiments, including quantitative analysis of artifact mitigation and necessary control experiments. We then explore mobilities and diffusion coefficients for two representative ILs: 1-ethyl-3-methyl imidazolium tetrafluoroborate ([C2mim][BF4]) and 1-ethyl-3-methyl imidazolium trifluoromethanesulfonate ([C2mim][TfO]). We further use the individual ion mobilities to calculate the bulk net conductivity, which closely agrees with bulk conductivity measurements obtained using impedance spectroscopy. These observations represent the first reliable measurements of cation and anion mobilities in pure ILs, with errors of ±7%. We discuss this advanced experimental methodology in detail, as well as implications of these sensitive measurements for understanding conduction mechanisms in ion-dense electrolytes.