Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes

Abstract

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI−TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small-and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Coordination Behavior of a Confined Ionic Liquid in Carbon Nanotubes from Molecular Dynamics Simulations

“…Liquids at surfaces and in confined spaces behave significantly differently from their bulk phase, with the resulting physical properties being highly dependent on the type of liquid and solid. − This is particularly important for energy devices where the liquid is in contact with an electrode. At the molecular level, these property changes result from liquid–electrode interactions and an imposed change in liquid–liquid interactions near the surface. , Water, − water-based electrolytes, − ionic liquids (ILs), ,− and other electrolytes − are often studied in combination with solids such as carbon materials, ,,,,,,, transition-metal oxides, metal–organic frameworks, ,, or metal surfaces. , ILs have several favorable properties as electrolyte materials, such as low volatility, a wide electrochemical window, and high thermal and chemical stability. However, the viscosities of most ILs are high at room temperature, accompanied by a slow diffusion of ions, and thus neat ILs usually have a relatively low ionic conductivity at ambient conditions .…”