Ionic liquids (ILs) show promise as next-generation electrolytes in organic and polymeric devices due to, e.g., their high ionic conductivity, negligible vapor pressure, and thermal stability. These properties are sensitively controlled by the specific chemical structure of the bulky cations and anions. Transport behaviors of both cations and anions, regulated by a wealth of non-covalent interactions, drive the properties of ILs as electrolytes, which in turn affect the performance of IL-based electrochemical devices. Electrophoretic NMR (ENMR) is a unique technique that can directly measure the driven electrophoretic mobilities of cations and anions separately using spectral separation. ENMR measurement of ILs with high conductivity and slow diffusion presents a challenge, as it is complicated by nonelectrophoretic flow effects such as electro-osmosis and thermal convection. For the first time, we have accurately measured electrophoretic mobilities (see figure) of cations and anions in pure ILs based on our carefully designed sample cell and using a convection-compensated pulse sequence. We use ENMR to measure transference numbers, thus quantifying the contributions of individual charged species to the total ionic conductivity as well as reflecting subtle ion clustering effects. We will also present temperature and concentration dependencies of these conduction-relevant quantities and discuss our further understanding of conduction mechanisms in ILs and related ion-dense organic conductors.

Direct measurement of ion mobilities in ionic liquids by electrophoretic NMR

245th National Meeting (2013)

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