Physicochemical properties and plastic crystal structures of phosphonium fluorohydrogenate salts

Abstract

Fluorohydrogenate salts of quaternary phosphonium cations with alkyl and methoxy groups (tetraethylphosphonium (P(2222)(+)), triethyl-n-pentylphosphonium (P(2225)(+)), triethyl-n-octylphosphonium (P(2228)(+)), and triethylmethoxymethylphosphonium (P(222(101))(+))) have been synthesized by the metatheses of anhydrous hydrogen fluoride and the corresponding phosphonium bromide or chloride precursors. The three salts with asymmetric cations, P(222m)(FH)(2.1)F (m = 5, 8, and 101), are room temperature ionic liquids (ILs) and are characterized by differential scanning calorimetry, density, viscosity, and conductivity measurements. Linear sweep voltammetry using a glassy carbon working electrode shows these phosphonium fluorohydrogenate ILs have wide electrochemical windows (>4.9 V) with the lowest viscosity and highest conductivity in the known phosphonium-based ILs. Thermogravimetry shows their thermal stabilities are also improved compared to previously reported alkylammonium cation-based fluorohydrogenate salts. Differential scanning calorimetry and X-ray diffraction revealed that tetraethylphosphonium fluorohydrogenate salt, P(2222)(FH)(2)F, exhibits two plastic crystal phases. The high temperature phase has a hexagonal lattice, which is the first example of a plastic crystal phase with an inverse nickel arsenide-type structure, and the low-temperature phase has an orthorhombic lattice. The high-temperature plastic crystal phase exhibits a conductivity of 5 mS cm(-1) at 50 °C, which is the highest value for the neat plastic crystals.

Conduction Through Viscoelastic Phase in a Redox‐Active Ionic Liquid at Reduced Temperatures

“…One case is that of cubic liquid crystals, which are extremely viscous and more solid-like. [17] For surfactants forming spherical micelles the viscosity increases by several orders of magnitude at a well-defined concentration. This can easily occur at surfactant volume fractions of the order of 40% and is due to strong micelle-micelle interactions leading to the crystallization and formation of a cubic liquid crystalline phase.…”

Fluorohydrogenate Ionic Liquids, Liquid Crystals, and Plastic Crystals

“…The Walden plot (molar conductivities against the viscosities) of selected ionic liquids including fluorohydrogenate ionic liquids is shown in Figure 5.4. The plot roughly exhibits linearity, suggesting that fluorohydrogenate ionic liquids do not have a special conduction mechanism such as proton hopping (see references [34,36,39,41,42,[47][48][49][50][51][52] for the detailed physical property data). The highest ionic conductivity in this series is observed for [S 111 ][(FH) 1.9 F] (131 mS cm −1 , S 111 + = trimethylsulfonium) [41].…”

“…a These data were reported in Refs. [34][35][36][37][38][39][40][41][42]60,89]. See Figure 9.2 for abbreviation of the cations.…”

“…Since the first report of a fluorohydrogenate ionic liquid (1-ethyl-3-methylimidazolium fluorohydrogenate, ([C 2 C 1 im][(FH) 2.3 F])) [33], various combinations of onium cations and (FH) n F − have been attempted to obtain fluorohydrogenate ionic liquids [34][35][36][37][38][39][40][41][42]. The cations which form fluorohydrogenate ionic liquids are summarized in Figure 5 Excess HF is thoroughly removed with the byproduct HCl under vacuum.…”

Nonhumidified Fuel Cells Using N-Ethyl-N-methyl-pyrrolidinium Fluorohydrogenate Ionic Liquid-poly(Vinylidene Fluoride-Hexafluoropropylene) Composite Membranes

“…The general chemical structure of fluorohydrogenate anions is presented in Figure 1a. We have continually developed and studied a variety of FHILs [8][9][10][11][12][13][14][15][16][17][18]; for example, 1-ethyl-3-methylimidazolium fluorohydrogenate (EMIm(FH)2.3F), which was the first reported FHIL, exhibits high conductivity (100 mS·cm 1 at 25 °C) and has a low melting point (−65 °C) and low glass transition temperature (−125 °C) [8,12], making it a promising electrolyte for electrochemical devices. For fuel cell applications, we have already proposed operation mechanisms for nonhumidified fuel cells using FHILs as electrolytes; these cells are referred to as fluorohydrogenate fuel cells (FHFCs) [19].…”