Showing papers on "Ionic conductivity published in 2004"

Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species

Abstract: 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-field-gradient 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 RT...

Conductivity of PBI Membranes for High-Temperature Polymer Electrolyte Fuel Cells

Abstract: Polybenzimidazole (PB1) film, a candidate polymer electrolyte membrane (PEM) for high-temperature (120-200°C) fuel cells, was cast from PBI/trifluoacetyl/H 3 PO 4 solution with constant molecular weight PBI powder and various acid doping levels. Conductivity measurements on these membranes were performed using an ac method under controlled temperature and relative humidity (RH). A complete set of conductivity data for H 3 PO 4 acid-doped PBI is presented as a function of temperature (60-200°C), RH (5-30%), and acid doping level (300-600 mol %). A mechanism of conductivity is proposed for the proton migration in this PBI/acid system based on this and previous work. Proton transfer in this system appears to occur along different paths for different doping levels, RHs, and temperatures. Hydrogen bonds immobilize the anions and form a network for proton transfer by a Grotthuss mechanism. The rate of proton transfer involving H 2 O is faster, leading to higher conductivity at higher RH. The order of the rate of proton transfer between various species is H 3 PO 4 (H 2 PO 4 -)...H-O-H> H 3 PO 4 ...H 2 PO - 4 > N-H + ...H 2 PO 4 - + N-H + ...H-O-H > N-H + ...N-H. The upper limit of proton conductivity is given by the conductivity of the liquid state H 3 PO 4 .

Li-Storage via Heterogeneous Reaction in Selected Binary Metal Fluorides and Oxides

Abstract: A novel room-temperature molten salt (RTMS) electrolyte based on lithium bis(trifluoromethane sulfone) imide [LiN(SO2CF3)(2), LiTFSI] and acetamide has been prepared and investigated by ac impedance, nuclear magnetic resonance (NMR), Fourier transform infrared, and Raman spectroscopy. The ionic conductivity of the (acetamide)(n) . LiTFSI complex for n = 6 reaches 1.20 x 10(-3) S/cm at 25 degreesC and rises to 5.73 x 10(-3) S/cm at 60 degreesC. The behavior of ion transport obeys well the empirical Vogel-Tamman-Fulcher type relationship for the RTMS electrolyte. Strong cation-solvent interaction has been clarified by the variation of Li-7 NMR chemical shifts and the spectral evolution of the C = O group in the (acetamide)(n) . LiTFSI complex system. The effect of salt concentration and temperature upon the ionic association has also been investigated. It is found that ionic association increases with the increases of salt concentration and temperature in the RTMS electrolyte. (C) 2004 The Electrochemical Society.

Superionics: crystal structures and conduction processes

Abstract: Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

Electrical Conductivity of the High-Temperature Proton Conductor BaZr0.9Y0.1O2.95

Abstract: The impedance of the cubic perovskite BaZr0.9Y0.1O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor.

One-dimensional ion transport in self-organized columnar ionic liquids.

Abstract: New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotrop...

High performance dye-sensitized solar cells using ionic liquids as their electrolytes

Abstract: Room temperature ionic liquids have been used as electrolytes to investigate the performance and the characteristics in dye-sensitized solar cells (DSSCs). The ionic liquids used are 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (EMImTFSI), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6), 1-ethyl-3-methylimidazolium dicyanamide (EMImDCA), and 1-butylpyridinium bis(trifluoromethane sulfonyl)imide (BPTFSI), in which 1-ethyl-3-methylimidazolium iodide (EMImI) and I2 are dissolved as a redox couple. The structure and the property greatly influence the DSSC performances. Especially, the photocurrents are affected by the ionic conductivity (viscosity) and charge transport by the exchange reaction between the iodide/tri-iodide redox couple. EMImDCA is specific in terms of enhancement of the open-circuit voltages. The photo-energy conversion efficiency of DSSCs with EMImDCA under 100 mW cm−2 can be optimized up to 5.5%, when [I−] + [I3−] = 2 M and [I−]:[I2]=10:1 with the addition of 4-t-butylpyridine and LiI.

Ionic Conduction and Ion Diffusion in Binary Room-Temperature Ionic Liquids Composed of [emim][BF4] and LiBF4

Abstract: Binary room-temperature ionic liquid (RTIL) samples including a lithium salt were prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with LiBF4. The ionic conductivity, viscosity, thermal properties, and ion self-diffusion coefficients in [emim][BF4] and the binary [Li][emim][BF4] at six concentrations of LiBF4 ranging from 0.25 to 1.50 M were measured at various temperatures. The self-diffusion coefficients of the individual components, [emim], BF4, and Li, were measured by using 1H, 19F, and 7Li pulsed gradient spin−echo NMR, respectively. Since the Walden product holds similar to typical solution electrolytes, the ion conduction mechanism is interpreted using a flux basis electrolyte theory. The ions form associated structures and diffuse under the influence of the counterions in the binary IL systems. An attempt to correlate the ion diffusion with the ionic conduction was made in the framework of the Nernst−Einstein relationship. The Li net transference number and the appar...

Electrochemical and bioelectrochemistry properties of room-temperature ionic liquids and carbon composite materials.

Abstract: Room-temperature ionic liquids (RTILs) are liquids at room temperature and represent a new class of nonaqueous but polar solvents with high ionic conductivity. The conductivity property of carbon nanotubes/RTILs and carbon microbeads/RTILs composite materials has been studied using ac impedance technology. Enzyme coated by RTILs-modified gold and glassy carbon electrodes allow efficient electron transfer between the electrode and the protein and also catalyze the reduction of O2 and H2O2.

The zwitterion effect in high-conductivity polyelectrolyte materials

Abstract: The future of lithium metal batteries as a widespread, safe and reliable form of high-energy-density rechargeable battery depends on a significant advancement in the electrolyte material used in these devices. Molecular solvent-based electrolytes have been superceded by polymer electrolytes in some prototype devices, primarily in a drive to overcome leakage and flammability problems, but these often exhibit low ionic conductivity and prohibitively poor lithium-ion transport. To overcome this, it is necessary to encourage dissociation of the lithium ion from the anionic polymer backbone, ideally without the introduction of competing, mobile ionic species. Here we demonstrate the effect of zwitterionic compounds, where the cationic and anionic charges are immobilized on the same molecule, as extremely effective lithium ion 'dissociation enhancers'. The zwitterion produces electrolyte materials with conductivities up to seven times larger than the pure polyelectrolyte gels, a phenomenon that appears to be common to a number of different copolymer and solvent systems.

1-ethyl-3-methylimidazolium based ionic liquids containing cyano groups: Synthesis, characterization, and crystal structure

Abstract: New 1-ethyl-3-methylimidazolium (EMI) salts [EMI][C(CN)3] and [EMI][Ag(CN)2] were prepared and characterized. The C(CN)3 salt has a melting point at −11 °C and shows a low viscosity (18 cP) and a high ionic conductivity (1.8 × 10-2 S cm-1) at room temperature. This conductivity is less than that of [EMI][N(CN)2] salt (2.7 × 10-2 S cm-1), possibly due to the larger molecular weight of the anion. The first EMI salt containing Ag(I) complexes [EMI][Ag(CN)2] has a higher melting point of 73 °C. In the crystal, the C−H···π interionic interactions between cations construct zigzag chains in the cationic two-dimensional layer. Close Ag··Ag interionic contacts of 3.226(1) A were observed in the one-dimensional anionic chain, and the relatively high melting point among the EMI salts with a monoanion appears to be governed essentially by these direct Ag···Ag interactions.

Mechanisms of Ion and Water Transport in Perfluorosulfonated Ionomer Membranes for Fuel Cells

Abstract: To clarify transport mechanisms of ions and water molecules in perfluorosulfonated ionomer membranes, various membranes, such as one Nafion, two Aciplex, and four Flemion types, having different equivalent weight values (EW) were examined. H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, and their properties in the fully hydrated state were investigated systematically. The water content of the membranes showed the tendency that the size and/or the number of ionic cluster region increases with decreasing EW value and the Li-form membranes have the most largely expanded ionic cluster regions. The ionic conductivity of the H-form membranes was considerably higher than that of the Li- and Na-form membranes. It was suggested that the proton in the membranes transports by the hopping mechanism and the Li+ and Na+ ions by the vehicle mechanism. In addition, the ionic conductivity of all membranes increased with increasing water content wit...

A Liquid Derivative of 12-Tungstophosphoric Acid with Unusually High Conductivity

Abstract: Surface functionalization of the solid heteropolyacid H3PW12O40 with a bulky PEG-containing quaternary ammonium cation through partial proton exchange leads to a polyoxometalate-based liquid salt with high-temperature proton conductivity ( approximately 10-3 S cm-1 at 140 degrees C) under dry conditions. The proton conductivity of the liquid salt is 4 orders of magnitude higher than that of the solid analogue under identical conditions and shows super ionic behavior as defined by Walden plot.

Highly ion conductive poly(ethylene oxide)-based solid polymer electrolytes from hydrogen bonding layer-by-layer assembly.

Abstract: We report the development of a solid polymer electrolyte film from hydrogen bonding layer-by-layer (LBL) assembly that outperforms previously reported LBL assembled films and approaches battery integration capability. Films were fabricated by alternating deposition of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) layers from aqueous solutions. Film quality benefits from increasing PEO molecular weight even into the 10(6) range due to the intrinsically low PEO/PAA cross-link density. Assembly is disrupted at pH near the PAA ionization onset, and a potential mechanism for modulating PEO:PAA ratio within assembled films by manipulating pH is discussed. Ionic conductivity of 5 x 10(-5) S/cm is achievable after short exposure to 100% relative humidity (RH) for plasticization. Adding free ions by exposing PEO/ PAA films to lithium salt solutions enhanced conductivity to greater than 10(-5) S/cm at only 52% RH and tentatively greater than 10(-4) S/cm at 100% RH. The excellent stability of PEO/PAA films even when exposed to 1.0 M salt solutions led to an exploration of LBL assembly with added electrolyte present in the adsorption step. Fortuitously, the modulation of PEO/PAA assembly by ionic strength is analogous to that of electrostatic LBL assembly and can be attributed to electrolyte interactions with PEO and PAA. Dry ionic conductivity was enhanced in films assembled in the presence of salt as compared to films that were merely exposed to salt after assembly, implying different morphologies. These results reveal clear directions for the evolution of these promising solid polymer electrolytes into elements appropriate for electrochemical power storage and generation applications.

Porous Polybenzimidazole Membranes Doped with Phosphoric Acid: Highly Proton-Conducting Solid Electrolytes

Abstract: New proton-conducting polymer electrolytes based on new porous films of polybenzimidazole doped with phosphoric acid have been obtained. The porous polybenzimidazole films were prepared by leaching out a low-molecular-weight compound using a selective solvent of the porogen from polymer/porogen mixtures. This method allows control over the porosity level up to porosities of 75%. The pore size and morphology strongly depend on the porogen/PBI ratio as observed by SEM. The pore size varies from dimethyl phthalate>diphenyl phthalate>triphenyl phosphate. The acid uptake of the membranes, and therefore the ionic conductivity of the films, increases with the porosity. Ionic conductivity as high as 5 × 10-2 S/cm and mechanically stable membranes are easily obtained by soaking the highly porous films...

Nanocomposite Polymer Electrolytes Based on Poly(oxyethylene) and Cellulose Nanocrystals

Abstract: Lithium-conducting nanocomposite polymer electrolytes based on high molecular weight poly(oxyethylene) (POE) were prepared from high aspect ratio cellulosic whiskers and lithium imide LiTFSI salt. The thermomechanical behavior of the resulting films was investigated by differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. The ionic conductivity and the electrochemical stability of the nanocomposite polymer electrolytes are quite consistent with the specifications of lithium batteries. The ionic mobilities were determined by pulsed magnetic field gradient NMR, and it was shown that the reinforcement does not affect the lithium transference number. High performance nanocomposite electrolytes based on tunicin whiskers were obtained. Indeed, the filler provides a high reinforcing effect, while a high level of ionic conductivity is retained with respect to unfilled polymer electrolytes.

The polarization of mixed conducting SOFC cathodes: Effects of surface reaction coefficient, ionic conductivity and geometry

Abstract: Multi-dimensional finite element simulations of current distributions in mixed ionic and electronic conducting cathodes (MIEC) are presented for the case that the cathodic oxygen incorporation into an electrolyte takes place through the bulk of the electrode. The effects of the ionic conductivity and the surface reaction coefficient on the overall process are analyzed. Depending on these material parameters different parts of the cathode are involved in the oxide ion transport to the electrolyte (from a very small region close to the three phase boundary for a fast surface reaction up to the entire cathode for a very slow surface reaction). The calculations also reveal which combinations of ionic conductivity and surface reaction coefficient are appropriate to achieve acceptable polarization resistances. The influence of the particle size is discussed and interpolation formulae are given to estimate the cathodic polarization in porous MIECs.