Room-temperature ionic liquids (RTILs) based on 1-butyl-3-methylimidazolium ([bmim]) with a variety of fluorinated anions were prepared, and the thermal behavior, density, viscosity, self-diffusion coefficients of the cations and anions, and ionic conductivity were measured over a wide temperature range. The temperature dependencies of the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been fitted to the Vogel-Fulcher-Tamman equation, and the best-fit parameters for the self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity have been estimated, together with the linear fitting parameters for the density. The self-diffusion coefficients determined for the individual ions by pulsed-fieldgradient spin-echo NMR method exhibit higher values for the cation compared with the anion over a wide temperature range, even if its radius is larger than that of the anionic radii. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order] at 30 °C, and the order of the diffusion coefficients greatly contrasts to the viscosity data. The ionic association is proposed from the results of the ratios of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst-Einstein equation. The ratio for the ionic liquids follows the order30 °C and provides quantitative information on the active ions contributing to ionic conduction in the diffusion components.

To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity of an ionic liquid. In situ free radical polymerization of compatible vinyl monomers in a room temperature ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI), afforded a novel series of polymer electrolytes. Polymer gels obtained by the polymerization of methyl methacrylate (MMA) in EMITFSI in the presence of a small amount of a cross-linker gave self-standing, flexible, and transparent films. The glass transition temperatures of the gels, which we named "ion gels", decreased with increasing mole fraction of EMITFSI and behaved as a completely compatible binary system of poly(methyl methacrylate) (PMMA) and EMITFSI. The temperature dependence of the ionic conductivity of the ion gels followed the Vogel-Tamman-Fulcher (VTF) equation, and the ionic conductivity at ambient temperature reached a value close to 10(-2) S cm(-1). Similarly to the behavior of the ionic liquid, the cation in the ion gels diffused faster than the anion. The number of carrier ions, calculated from the Nernst-Einstein equation, was found to increase for an ion gel from the corresponding value for the ionic liquid itself. The cation transference number increased with decreasing EMITFSI concentration due to interaction between the PMMA matrix and the TFSI(-) anion, which prohibited the formation of ion clusters or associates, as was the case for the ionic liquid itself.

A new series of Brønsted acid−base ionic liquids were derived from the controlled combination of a monoprotonic acid with an organic base under solvent-free conditions. Appropriate amounts of solid bis(trifluoromethanesulfonyl)amide (HTFSI) and solid imidazole (Im) were mixed at various molar ratios to have compositions varying from an equimolar salt to HTFSI- or Im-rich conditions. The mixture at equivalent molar ratio formed a protic neutral salt with a melting point of 73 °C, which was thermally stable at temperatures even above 300 °C. The melting points of other compositions were lower than those of the equimolar salt and Im or HTFSI, giving eutectics between the equimolar salt and HTFSI or Im. Some of the compositions with certain molar ratios of Im and HTFSI were liquid at room temperature. For Im excess compositions, the conductivity was found to increase with increasing Im mole fraction, and the 1H NMR chemical shift of the proton attached to the nitrogen atom of Im was shifted to a lower magnetic field. On the contrary, the conductivity decreased with increasing HTFSI mole fraction, and the 1H NMR chemical shift of the proton attached to the TFSI imide anion also shifted to a higher magnetic field. Self-diffusion coefficients, measured by pulsed-gradient spin−echo NMR (PGSE-NMR) methods in Im- or HTFSI-rich compositions, indicated that fast proton exchange reactions between the protonated Im cation and Im take place in excess Im. The proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms. Direct current polarization measurements were used for the confirmation of proton conduction in Im-rich compositions. Furthermore, reduction of molecular oxygen could be observed at the interface between a Pt electrode and these ionic liquids. This introduces the Brønsted acid−base ionic liquid system as a new candidate for proton conductor such as a fuel cell electrolyte to operate under anhydrous conditions and at elevated temperature.

A series of room-temperature ionic liquids (RTILs) were prepared with different cationic structures, 1-butyl-3-methylimidazolium ([bmim]), 1-butylpyridinium ([bpy]), N-butyl-N-methylpyrrolidinium, ([bmpro]), and N-butyl-N,N,N-trimethylammonium ([(n-C(4)H(9))(CH(3))(3)N]) combined with an anion, bis(trifluoromethane sulfonyl)imide ([(CF(3)SO(2))(2)N]), and the thermal property, density, self-diffusion coefficients of the cation and anion, viscosity, and ionic conductivity were measured over a wide temperature range. The self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity follow the Vogel-Fulcher-Tamman equation for temperature dependencies, and the best-fit parameters have been estimated, together with the linear fitting parameters for the density. The relative cationic and anionic self-diffusion coefficients for the RTILs, independently determined by the pulsed-field-gradient spin-echo NMR method, appear to be influenced by the shape of the cationic structure. A definite order of the summation of the cationic and anionic diffusion coefficients for the RTILs: [bmim][(CF(3)SO(2))(2)N] > [bpy][(CF(3)SO(2))(2)N] > [bmpro][(CF(3)SO(2))(2)N] > [(n-C(4)H(9))(CH(3))(3)N][(CF(3)SO(2))(2)N], has been observed, which coincides with the reverse order to the viscosity data. The ratio of molar conductivity obtained from the impedance measurements to that calculated by the ionic diffusivity using the Nernst-Einstein equation quantifies the active ions contributing to ionic conduction in the diffusion components and follows the order: [bmpro][(CF(3)SO(2))(2)N] > [(n-C(4)H(9))(CH(3))(3)N][(CF(3)SO(2))(2)N] > [bpy][(CF(3)SO(2))(2)N] > [bmim][(CF(3)SO(2))(2)N] at 30 degrees C.

Novel Brønsted acid-base ionic liquids, derived from a simple combination of a wide variety of organic amines with bis(trifluoromethanesulfonyl) amide are electroactive for H2 oxidation and O2 reduction at a Pt electrode under non-humidifying conditions, which shows the prospect of the use of protic ionic liquids as new materials for anhydrous proton conductors at elevated temperatures.

Neutralization of an organic super-strong base, 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU), with different Brønsted acids affords a novel series of protic ionic liquids (PILs) with wide variations in the ΔpK(a) of the constituent amine and acids. The physicochemical properties of these PILs, such as thermal properties, density, conductivity, viscosity, self-diffusion coefficient, vibrational stretching frequency, and (1)H-chemical shifts of the N-H bond, have been studied in detail. The generated PILs have melting temperatures below 100 °C, and six are liquids at ambient temperatures. Thermogravimetric analyses (TGA) conducted under isothermal and programmed heating conditions have shown that PILs with ΔpK(a)≥ 15 exhibit good thermal stability similar to aprotic ionic liquids. For instance, PILs with ΔpK(a) > 20 show remarkably high short-term thermal stability up to ca. 450 °C under a nitrogen atmosphere. The viscosity, ionic conductivity, and molar conductivity of the PILs fit well with the Vogel-Fulcher-Tamman equation for their dependencies on temperature. The relative cationic and anionic self-diffusion coefficients of the PILs estimated by the pulsed-field gradient spin-echo (PGSE) NMR method appear to be dependent on the structure and strength of the Brønsted acids. Evaluation of the ionicity based on both the Walden plot and PGSE-NMR revealed that it increases until ΔpK(a) becomes 15 for the PILs.

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The temperature dependence of the N-H proton chemical shift in protic ionic liquids (PILs) and FT-IR spectra of the N-H bonds indicated the presence of strong hydrogen bonds between the protonated cation and the anion, depending on the ΔpK(a) of the constituent acid and base, and their successive breaking with temperature, which may explain the characteristic properties of PILs such as relatively low ionicity and its decrease with temperature.