Relationship between Li+ diffusion and ion conduction for single-crystal and powder garnet-type electrolytes studied by 7Li PGSE NMR spectroscopy.

Ion-conducting garnets are important candidates for use in all-solid Li batteries and numerous materials have been synthesized with high ionic conductivities. For understanding ion conduction mechanisms, knowledge on Li+ diffusion behaviour is essential. The proposed nano-scale lithium pathways are composed of tortuous and narrow Li+ channels. The pulsed gradient spin-echo (PGSE) NMR method provides time-dependent 7Li diffusion on the micrometre space. For powder samples, collision-diffraction echo-attenuation plots were observed in a short observation time, which had not been fully explained. The diffraction patterns were reduced or disappeared for single-crystal garnet samples of Li6.5La3Zr1.5Ta0.5O12 (LLZO-Ta) and Li6.5La3Zr1.5Nb0.5O12 (LLZO-Nb). The inner morphology and grain boundaries affect importantly the collision-diffraction behaviours which is inherent to powder samples. The 7Li diffusion observed by PGSE-NMR depends on the observation time (Δ) and the pulsed field gradient (PFG) strength (g) in both powder and single-crystal samples, and the anomalous effects were reduced in the single-crystal samples. The scattered Li diffusion constants converged to a unique value (DLi) with a long Δ and a large g, which is eventually the smallest value. The DLi activation energy was close to that of the ionic conductivity (σ). The DLi values are plotted versus the σ values measured for four powder and two single-crystal garnet samples. Assuming the Nernst-Einstein (NE) relation which was derived for isolated ions in solution, the carrier numbers (NNE) were estimated from the experimental values of DLi and σ. The NNE values of metal-containing garnets were large (<1023 cm-3) and insensitive to temperature. They were larger than Li atomic numbers in cm3 calculated from the density, molecular formula and Avogadro number for LLZOs except for cubic LLZO (Li7La3Zr2O12, NNE∼ 1020 cm-3).

Toward understanding the anomalous Li diffusion in inorganic solid electrolytes by studying a single-crystal garnet of LLZO-Ta by pulsed-gradient spin-echo nuclear magnetic resonance spectroscopy.

Li diffusion was observed by 7Li nuclear magnetic resonance (NMR) spectroscopy in three single-crystal samples of LLZO-Ta (Li6.5La3Zr1.5Ta0.5O12) grown by the floating zone melting method as well as a crushed sample in this study. Previously, the pulsed-gradient spin-echo 7Li NMR method was applied to Li+ diffusion measurements in inorganic solid electrolyte powder samples. Anomalous Li+ diffusion behaviors were observed such as dependence of the observing time (Δ) and pulsed-field-gradient strength (g), and the diffusive-diffraction patterns in short Δ in the echo-attenuation plots. In the powder samples, it is uncertain that the Li ions diffuse in the bulk within grain, across grains, or both. To date, the origins of the anomalous Li+ diffusion have not yet been clearly understood. From models of atomic-level lithium pathways, the micrometer-space diffusion channels are assumed to be narrow with curvatures. In contrast to the powder samples, a single crystal is supposed to be uniform without grain boundaries and the Li ions in single-crystal samples can diffuse in the bulk with negligible effects from the surface. The single-crystal samples are expected to give us proper answers. We found that the 7Li echo-attenuation plots of the single-crystal samples showed anomalous phenomena in dependence on Δ and g with much reduced manners. We found that the phenomena are inherent characteristics of Li+ diffusion in inorganic solid electrolytes. From the aspects of Li+ carrier numbers, the fast divergent Li+ diffusion constants, observed at short Δ with small g, contribute importantly to the electrochemical high ionic conduction measured by impedance spectroscopy.

Non-uniform lithium-ion migration on micrometre scale for garnet- and NASICON-type solid electrolytes studied by 7Li PGSE-NMR diffusion spectroscopy.

The migration behaviours of Li+ in three garnet- and one NASICON-type solid oxide electrolytes were studied on the micrometre scale by pulsed-gradient spin-echo (PGSE) 7Li NMR diffusion spectroscopy to clarify common and specific characteristics of each electrolyte. In these solid electrolytes, clear evidences of grain boundary effects in the diffusion of Li+ were not observed. The Li+ diffusion constants were dependent on parameters such as observation time (Δ) and pulsed field gradient (PFG) strength (g) for all the studied inorganic solid electrolytes. For low Δ values, Li+ ions underwent collisions and diffractions with diffraction distance Rdiffraction [µm]. The apparent Li+ diffusion constants (Dapparent [m2 s-1]) exhibited distributions in a wide range. In this paper, we introduced the apparent diffusion radius, rradius [µm], and compared it with Rdiffraction and mean square displacement (MSD) [µm]; the lengths of these distances were of the micrometre order (10-6 m). The relations between the values of rradius, Rdiffraction and MSD suggested that the migration behaviours of Li+ on the micrometre scale were complicated. Using high Δ and high g values, we obtained an equilibrated value of DLi. The temperature dependences of the number of carrier ions were estimated from the DLi values and ionic conductivities in the four solid oxide electrolytes. For simple comparison and reference, the data of DLi and ionic conductivity of LiPF6 in 1 M solution of propylene carbonate were added.

Long-range Li ion diffusion in NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) studied by 7Li pulsed-gradient spin-echo NMR.

The long-range Li ion diffusion in Li1.5Al0.5Ge1.5(PO4)3 (LAGP), a NASICON-type glass ceramic conductor with high ionic conductivity, was studied using pulsed-gradient spin-echo (PGSE) 7Li NMR. LAGP is stable in air and water, and can be used for all-solid Li batteries as well as next generation Li-air batteries. The Li ion conduction mechanisms in the micrometer space are important for the design of Li batteries and development of new materials. Our previous studies on sulfide-based and garnet-type solid conductors showed that uniform Li+ ion diffusion is hampered by narrow pathways surrounded by stationary anions. The Li ions are engaged in parameter-dependent diffusion with the parameters being observation time (Δ) and pulsed-field gradient (PFG) strength (g); this is completely different from the Li diffusion in solution and polymer electrolytes, and also from molecular diffusion in neutral porous media. In this study, we observed apparent diffusion constant (Dapparent) values for the LAGP, that were almost continuously distributed within a limited range at a certain Δ. At very long observation times (above 300 ms) and large g (∼13 Tm-1), an equilibrated diffusion constant (close to the tracer diffusion constant) could be observed. The apparent activation energy of Li ion diffusion was about 16 kJ mol-1, which was smaller than the activation energy for the ionic conductivity. The temperature-dependent carrier ion numbers were estimated.

Lithium ion micrometer diffusion in a garnet-type cubic Li7La3Zr2O12 (LLZO) studied using 7Li NMR spectroscopy.

Mobile lithium ions in a cubic garnet Li7La3Zr2O12 (Al-stabilized) were studied using 7Li NMR spectroscopy for membrane and powder samples, the latter of which was ground from the membrane. Lithium diffusion in a micrometer space was measured using the pulsed-gradient spin-echo 7Li NMR method between 70 and 130 °C. When the observation time (Δ) was shorter than 20 ms, the echo attenuation showed diffusive diffraction patterns, indicating that the Li+ diffusing space is not free but restricted. For longer Δ, the values of apparent diffusion constant (Dapparent) became gradually smaller to approach an equilibrated value (close to a tracer diffusion constant). In addition, the Dapparent depends on the pulse field gradient strength (g) and became smaller as g became larger. These experimental results suggest that the lithium ions diffuse through Li+ pathways surrounded by stationary anions and lithium ions, and are affected by collisions and diffractions. One-dimensional profiles of the membrane sample of thickness 0.5 mm were observed from 65 to 110 °C and the area intensity, as well as the lithium occurrence near the surface, increased with the increase in temperature. The temperature-dependent area intensity showed a correspondence to the number of Li+ carrier ions estimated from the ionic conductivity and the equilibrated diffusion constant through the Nernst-Einstein relationship.

Lithium ion diffusion measurements on a garnet-type solid conductor Li6.6La3Zr1.6Ta0.4O12 by using a pulsed-gradient spin-echo NMR method.

The garnet-type solid conductor Li7-xLa3Zr2-xTaxO12 is known to have high ionic conductivity. We synthesized a series of compositions of this conductor and found that cubic Li6.6La3Zr1.6Ta0.4O12 (LLZO-Ta) has a high ionic conductivity of 3.7×10(-4)Scm(-1) at room temperature. The (7)Li NMR spectrum of LLZO-Ta was composed of narrow and broad components, and the linewidth of the narrow component varied from 0.69kHz (300K) to 0.32kHz (400K). We carried out lithium ion diffusion measurements using pulsed-field spin-echo (PGSE) NMR spectroscopy and found that echo signals were observed at T≥313K with reasonable sensitivity. The lithium diffusion behavior was measured by varying the observation time and pulsed-field gradient (PFG) strength between 313 and 384K. We found that lithium diffusion depended significantly on the observation time and strength of the PFG, which is quite different from lithium ion diffusion in liquids. It was shown that lithium ion migration in the solid conductor was distributed widely in both time and space.

Nuclear magnetic resonance studies on the rotational and translational motions of ionic liquids composed of 1-ethyl-3-methylimidazolium cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts.

Room temperature ionic liquids (ILs) are stable liquids composed of anions and cations. 1-ethyl-3-methyl-imidazolium (EMIm, EMI) is a popular and important cation that produces thermally stable ILs with various anions. In this study two amide-type anions, bis(trifluoro-methanesulfonyl)amide [N(SO(2)CF(3))(2), TFSA, TFSI, NTf(2), or Tf(2)N] and bis(fluorosulfonyl)amide [(N(SO(2)F)(2), FSA, or FSI] were investigated by multinuclear NMR spectroscopy. In addition to EMIm-TFSA and EMIm-FSA, lithium-salt-doped binary systems were prepared (EMIm-TFSA-Li and EMIm-FSA-Li). The spin-lattice relaxation times (T(1)) were measured by (1)H, (19)F, and (7)Li NMR spectroscopy and the correlation times of (1)H NMR, τ(c)(EMIm) (8 × 10(-10) to 3 × 10(-11) s) for the librational molecular motion of EMIm and those of (7)Li NMR, τ(c)(Li) (5 × 10(-9) to 2 × 10(-10) s) for a lithium jump were evaluated in the temperature range between 253 and 353 K. We found that the bulk viscosity (η) versus τ(c)(EMIm) and cation diffusion coefficient D(EMIm) versus the rate 1/τ(c)(EMIm) have good relationships. Similarly, linear relations were obtained for the η versus τ(c)(Li) and the lithium diffusion coefficient D(Li) versus the rate 1∕τ(c)(Li). The mean one-jump distances of Li were calculated from τ(c)(Li) and D(Li). The experimental values for the diffusion coefficients, ionic conductivity, viscosity, and density in our previous paper were analyzed by the Stokes-Einstein, Nernst-Einstein, and Stokes-Einstein-Debye equations for the neat and binary ILs to clarify the physicochemical properties and mobility of individual ions. The deviations from the classical equations are discussed.

Temperature-dependent 11B spin-lattice relaxation time for BF4 and CF3BF3 anions in room-temperature ionic liquids.

Temperature-dependent (11)B T(1) values were measured for the BF(4) anion and BF(3) in the CF(3)BF(3) anion in room-temperature ionic liquids (RTILs) composed of the cation N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME). Including the lithium-salt-doped samples, two neat and two binary ionic liquids were studied. Arrhenius plots of the (11)B T(1) showed T(1) minima for BF(4) in the temperature range between 243 (or above freezing) and 373 K. Using the Bloembergen, Pound, and Purcell(BPP) equations for the (11)B quadrupolar and (11)B-(19) F dipolar relaxation mechanisms, the correlation times for motions of BF(4) were calculated. Since the internal rotation of BF(3) is assumed in CF(3)BF(3), T(1) minimum was not observed. The effects of the addition of the lithium salt on the (11)B correlation time and (11)BT(1) for the anions in the ILs are discussed.

Dynamic ionic radius of alkali metal ions in aqueous solution: a pulsed-field gradient NMR study.

The dynamic behavior of alkali metal ions, Li+, Na+, K+, Rb+ and Cs+ in aqueous solutions is one of the most important topics in solution chemistry. Since these alkali metals contain nuclear magnetic resonance (NMR) active nuclei, it is possible to directly measure the diffusion constants of the alkali metal ions using the pulsed field gradient (PFG) NMR method. In this paper, the 7Li, 23Na, 87Rb, 133Cs and 1H resonances are observed for diffusion constants in aqueous solution and the solvent H2O. Until now, the values of the diffusion constant have been lacking when discussing hydration effects around alkali metal ions. It is known that the static ionic radius (R ion) increases with increasing the atomic number, and the experimental diffusion constants also increase with increasing the atomic number, which is opposite to the Stokes-Einstein (SE) relation. It suggests that alkali metal ions diffuse through a space of 10-6 m accompanying the hydrated spheres with a time interval of 10-3 s. For each alkali metal ion, the dynamic ionic radius is evaluated.

Studies on the translational and rotational motions of ionic liquids composed of N-methyl-N-propyl-pyrrolidinium (P13) cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts.

Room-temperature ionic liquids (RTIL, IL) are stable liquids composed of anions and cations. N-methyl-N-propyl-pyrrolidinium (P(13), Py(13), PYR(13), or mppy) is an important cation and produces stable ILs with various anions. In this study two amide-type anions, bis(trifluoromethanesulfonyl)amide [N(SO(2)CF(3))(2), TFSA, TFSI, NTf(2), or Tf(2)N] and bis(fluorosulfonyl)amide [N(SO(2)F)(2), FSA, or FSI], were investigated. In addition to P(13)-TFSA and P(13)-FSA, lithium salt doped samples were prepared (P(13)-TFSA-Li and P(13)-FSA-Li). The individual ion diffusion coefficients (D) and spin-lattice relaxation times (T(1)) were measured by (1)H, (19)F, and (7)Li NMR. At the same time, the ionic conductivity (σ), viscosity (η), and density (ρ) were measured over a wide temperature range. The van der Waals volumes of P(13), TFSA, FSA, Li(TFSA)(2), and Li(FSA)(3) were estimated by molecular orbital calculations. The experimental values obtained in this study were analyzed by the classical Stokes-Einstein, Nernst-Einstein (NE), and Stokes-Einstein-Debye equations and Walden plots were also made for the neat and binary ILs to clarify physical and mobile properties of individual ions. From the temperature-dependent velocity correlation coefficients for neat P(13)-TFSA and P(13)-FSA, the NE parameter 1-ξ was evaluated. The ionicity (electrochemical molar conductivity divided by the NE conductivity from NMR) and the 1-ξ had exactly the same values. The rotational and translational motions of P(13) and jump of a lithium ion are also discussed.

Quaternary ammonium room-temperature ionic liquid including an oxygen atom in side chain/lithium salt binary electrolytes: ab initio molecular orbital calculations of interactions between ions.

Interactions of the lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) complex with N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), 1-ethyl-3-methylimidazolium (EMIM) cations, neutral diethylether (DEE), and the DEMETFSA complex were studied by ab initio molecular orbital calculations. An interaction energy potential calculated for the DEME cation with the LiTFSA complex has a minimum when the Li atom has contact with the oxygen atom of DEME cation, while potentials for the EMIM cation with the LiTFSA complex are always repulsive. The MP2/6-311G**//HF/6-311G** level interaction energy calculated for the DEME cation with the LiTFSA complex was -18.4 kcal/mol. The interaction energy for the neutral DEE with the LiTFSA complex was larger (-21.1 kcal/mol). The interaction energy for the DEMETFSA complex with LiTFSA complex is greater (-23.2 kcal/mol). The electrostatic and induction interactions are the major source of the attraction in the two systems. The substantial attraction between the DEME cation and the LiTFSA complex suggests that the interaction between the Li cation and the oxygen atom of DEME cation plays important roles in determining the mobility of the Li cation in DEME-based room temperature ionic liquids.

Quaternary ammonium room-temperature ionic liquid including an oxygen atom in side chain/lithium salt binary electrolytes: ionic conductivity and 1H, 7Li, and 19F NMR studies on diffusion coefficients and local motions.

A room-temperature ionic liquid (RTIL) of a quaternary ammonium cation having an ether chain, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEME-TFSA), is a candidate for use as an electrolyte of lithium secondary batteries. In this study, the electrochemical ionic conductivity, sigma, of the neat DEME-TFSA and DEME-TFSA-Li doped with five different concentrations of lithium salt (LiTFSA) was measured and correlated with NMR measurements of the diffusion coefficients D and the spin-lattice relaxation times T1 of the individual components DEME (1H), TFSA (19F), and lithium ion (7Li). The ion conduction of charged ions can be activated with less thermal energy than ion diffusion which contains a contribution from paired ions in DEME-TFSA. In the doped DEME-TFSA-Li samples, the sigma and D values decreased with increasing salt concentration, and within the same sample generally DLi

Phase transition and conductive acceleration of phosphonium-cation-based room-temperature ionic liquid.

An unusual ionic conduction phenomenon related to the phase transition of a novel phosphonium-cation-based room-temperature ionic liquid (RTIL) is reported; we found that in the phase change upon cooling, a clear increase in ionic conductivity was seen as the temperature was lowered, which differs from widely known conventional RTILs; clearly, our finding of abnormality of the correlation between temperature change and ionic conduction is the first observation in the electrolyte field.

Molecular motions and ion diffusions of the room-temperature ionic liquid 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPImTFSA) studied by 1H, 13C, and 19F NMR.

The diffusive properties of an imidazolium room-temperature ionic liquid (RTIL), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPImTFSA), are studied from the ionic conductivity and the ion diffusion coefficients measured by pulsed field gradient spin echo NMR. The temperature-dependent (1)H, (19)F, and (13)C NMR spin-lattice relaxation time T(1) values were observed, and the (1)H T(1) for DMPIm showed T(1) minima for various protons. According to the Bloemberger-Purcell-Pound (BPP) equation, the correlation time tau(c) values were directly calculated from (1)H NMR. By using the (1)H tau(c) values, an evaluation of the (13)C T(1) was attempted for the carbons having protons. The tau(c) estimated for molecular motions of DMPIm changes from 1.3 ns at 253 K to 72 ps at 353 K. The Stokes-Einstein-Debye (SED) model suggests that the tau(c) is too short for the overall molecular reorientation near room temperature. Consequently, the possibility of small-angle molecular rotation is proposed and tentative flip angles are calculated by using the translational diffusion coefficient, the bulk viscosity measured in this study, and the tau(c) obtained from (1)H T(1) data in the temperature range between 283 and 353 K. The flip amplitude increases with the temperature. DMPIm has isotropic reorientational motions with temperature-dependent amplitude, in addition to fast intramolecular motions such as methylene segmental motions, methyl rotational motion, and conformational exchange of the imidazolium ring. The existence of fast motions of TFSA is also shown. The translational diffusion of the ions is the slowest dynamic process in the present RTIL. Ab initio molecular orbital calculations are performed to understand the geometries of stable complexes of DMPIm(+) and TFSA(-), and the formation energies from the isolated ions are evaluated. The computed results are important for interpreting the (1)H T(1) behaviors observed for the imidazolium ring protons.

Direct in situ observation of dynamic transport for electrolyte components by NMR combined with electrochemical measurements.

Electrochemical studies provide broad, but not cation- or anion-specific information on the migration of charged ions. However, individual ion diffusion (as a weighted average of charged and neutral ions) can be measured using pulsed-gradient spin-echo (PGSE) NMR. In this paper, the lithium transport in an electrolyte including a lithium salt was measured using electrophoretic NMR (ENMR) with non-blocking electrodes. A propylene carbonate (PC) solution doped with LiN(SO(2)CF(3))(2) (LiTFSI) was inserted in a homemade NMR cell equipped with Li/Li electrodes. The drift migrations of lithium cation ((7)Li), anion ((19)F), and solvent ((1)H) were measured independently under potentials of up to 3.0 V. Greatly enhanced dynamic lithium transport was observed for the first time in the bulk electrolyte under an electric field closely related to real conditions in a rechargeable lithium battery.

Alcohol and proton transport in perfluorinated ionomer membranes for fuel cells.

To clarify the transport mechanisms of alcohols and proton in perfluorosulfonated ionomer (PFSI) membranes for fuel cells, four membranes having different equivalent weight (EW) values were examined. Membranes were immersed in methanol, ethanol, and 2-propanol to prepare a total of 12 samples, and membrane swelling, mass (alcohol and proton) transports, and interactions between alcohols and proton were investigated systematically in the fully penetrated state. The membrane expansion fraction theta and alcohol content lambda increased with decreasing the EW value for all the samples. The self-diffusion coefficients (D's) of the alkyl group and of OH (including protons) were measured separately by the pulsed-gradient spin-echo (PGSE)-NMR method and the D's also increased with decreasing the EW value. These results implied that the alcohols penetrate into the hydrophilic regions of the PFSI membranes and diffuse through the space expanded by the alcohols. The ionic cluster regions formed by the alcohols resemble those induced by water in the water swollen membrane, where protons dissociated from sulfonic acid groups transport through the regions together with water molecules. The D values decreased with increasing the molecular weight of alcohols. This trend was supported by activation energies Ea estimated from the Arrhenius plots of D in the temperature range from 30 to -40 degrees C. The PGSE-NMR measurements also revealed that protons move faster than the alkyl groups in the membranes. The proton transport by the Grotthuss (hopping) mechanism was facilitated by the increase of the alcohol content and the decrease of the molecular weight. This result was also supported by the experimental results of proton conductivity kappa and mobility u(H(+)). Density functional theory (DFT) calculations of the interaction energy DeltaE(int) between proton and alcohol (including OH) showed that the /DeltaE(int)/ increases with increasing the molecular weight of alcohols, which is in a inverse relationship with the kappa and u(H(+)) values. The proton transport depends strongly on the DeltaE(int) in the membranes.

Ion conduction mechanisms and thermal properties of hydrated and anhydrous phosphoric acids studied with 1H, 2H, and 31P NMR.

To understand the behaviors of phosphoric acids in fuel cells, the ion conduction mechanisms of phosphoric acids in condensed states without free water and in a monomer state with water were studied by measuring the ionic conductivity (sigma) using AC impedance, thermal properties, and self-diffusion coefficients (D) and spin-lattice relaxation times (T1) with multinuclear NMR. The self-diffusion coefficient of the protons (H+ or H3O+), H2O, and H located around the phosphate were always larger than the diffusion coefficients of the phosphates and the disparity increased with increasing phosphate concentration. The diffusion coefficients of the samples containing D2O paralleled those in the protonated samples. Since the 1H NMR T1 values exhibited a minimum with temperature, it was possible to determine the correlation times and they were found to be of nanosecond order for a distance of nanometer order for a flip. The agreement of the ionic conductivities measured directly and those calculated from the diffusion coefficients indicates that the ion conduction obeys the Nernst-Einstein equation in the condensed phosphoric acids. The proton diffusion plays a dominant role in the ion conduction, especially in the condensed phosphoric acids.

Physicochemical properties and structures of room-temperature ionic liquids. 3. Variation of cationic structures.

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.

How ionic are room-temperature ionic liquids? An indicator of the physicochemical properties.

Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Lambda(imp)/Lambda(NMR)), calculated from the molar conductivity measured by the electrochemical impedance method (Lambda(imp)) and that estimated by use of pulse-field-gradient spin-echo NMR ionic self-diffusion coefficients and the Nernst-Einstein relation (Lambda(NMR)). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), E(T)(30), hydrogen bond donor acidity (alpha), and dipolarity/polarizability (pi), as well as NMR chemical shifts. The Lambda(imp)/Lambda(NMR) well illustrates the degree of cation-anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and cationic backbone structures with fixed counterparts, and by the inductive and dispersive forces for the various alkyl chain lengths in the cations. As a measure of the electrostatic interaction of the RTILs, the effective ionic concentration (C(eff)), which is a dominant parameter for the electrostatic forces of the RTILs, was introduced as the product of Lambda(imp)/Lambda(NMR) and the molar concentration and was compared with some physical properties, such as reported normal boiling points and distillation rates, glass transition temperature, and viscosity. A decrease in C(eff) of the RTILs is well correlated with the normal boiling point and distillation rate, whereas the liquid-state dynamics is controlled by a subtle balance between the electrostatic and other intermolecular forces.

Temperature dependence of ion and water transport in perfluorinated ionomer membranes for fuel cells.

To clarify the mechanisms of transport of ions and water molecules in perfluorosulfonated ionomer membranes for fuel cells, the temperature dependence of their transport behaviors was investigated in detail. Two types of Flemion membranes having different equivalent weight values (EW) were utilized along with Nafion 117 as the perfluorinated ionomer membranes, and H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, respectively. The ionic conductivity, water self-diffusion coefficient (D(H)(2)(O)), and DSC were measured in the fully hydrated state as a function of temperature. The ionic conductivity of the membranes was reflected by the cation transport through the intermediary of water. Clearly, H(+) transports by the Grotthuss (hopping) mechanism, and Li(+) and Na(+) transport by the vehicle mechanism. The differences of the ion transport mechanisms were observed in the activation energies through the Arrhenius plots. The D(H)(2)(O) in the membranes exhibited a tendency similar to the ionic conductivity for the cation species and the EW value. However, no remarkable difference of D(H)(2)(O) between H- and the other cation-form membranes was observed as compared with the ionic conductivity. It indicates that water in each membrane diffuses almost in a similar way; however, H(+) transports by the Grotthuss mechanism so that conductivity of H(+) is much higher than that of the other cations. Moreover, the D(H)(2)(O) and DSC curves showed that a part of water in the membranes freezes around -20 degrees C, but the nonfreezing water remains and diffuses below that temperature. This fact suggests that completely free water (bulk water) does not exist in the membranes, and water weakly interacting with the cation species and the sulfonic acid groups in secondary and higher hydration shells freezes around -20 degrees C, while strongly binding water in primary hydration shells does not freeze. The ratio of freezing and nonfreezing water was estimated from the DSC curves. The D(H)(2)(O) in the membranes was found to be influenced by the ratio of freezing and nonfreezing water. DFT calculation of the interaction (solvation) energy between the cation species and water molecules suggested that the water content and the ratio of freezing and nonfreezing water depend strongly on the cation species penetrated into the membrane.