Showing papers on "Ionic conductivity published in 2000"

Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Abstract: Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications

Designing fast oxide-ion conductors based on La2Mo2O9

Abstract: The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices1,2. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite3,4,5, perovskites6,7, intergrowth perovskite/Bi2O2 layers8,9 and pyrochlores10,11. Here we report a family of solid oxides based on the parent compound12 La2Mo2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors2,13, this material undergoes a structural transition around 580 °C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 × 10-2 S cm-1 at 800 °C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with β-SnWO4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair—and which has a higher oxidation state—could be used as an original way to design other oxide-ion conductors.

Photonic Crystal Chemical Sensors: pH and Ionic Strength

Abstract: Diffraction from a photonic crystal material composed of a hydrolyzed polymerized crystalline colloidal array (PCCA) can be used to sense pH and ionic strength. The PCCA is a polyacrylamide hydrogel which embeds a polystyrene crystalline colloidal array (CCA). The diffracted wavelength of the PCCA changes as the PCCA volume changes due to the alterations in the CCA lattice constant. We examine the pH and ionic strength dependence of the hydrolyzed PCCA volume by monitoring the Bragg diffracted wavelength. We also develop a zero free parameter quantitative model to describe the pH and ionic strength dependence of the hydrogel volume.

Impedance Spectroscopy Study of PEO‐Based Nanocomposite Polymer Electrolytes

Abstract: The addition of nanometric fillers (e.g., , ) to polymer electrolytes induces consistent improvement in the transport properties. The increase in conductivity and in the cation transference number is attributed to the enhancement of the degree of the amorphous phase in the polymer matrix, as well as to some acid‐base Lewis type, ceramic‐electrolyte interactions. This model is confirmed by results obtained from a detailed impedance spectroscopy study carried out on poly(ethylene oxide) [P(EO)]‐based polymer electrolyte samples with and without ceramic fillers. © 2000 The Electrochemical Society. All rights reserved.

The Defect Chemistry of Metal Oxides

Abstract: 1. Introduction Reference 2. A Few Useful Crystal Structures Introduction Close-Packed Structures Structures for Eight-Coordinate Cations Structures for Ternary Compounds Conclusion References Problems 3. Lattice Defects and the Law of Mass Action Introduction Lattice Defects as Part of the Equilibrium State The Law of Mass Action Another View of Mass Action Lattice Disorder in Elemental Solids Summary References 4. Intrinsic Ionic Disorder Lattice Defects and Reference States Conservation Rules Defect Notation Major Types of Intrinsic Ionic Disorder General Comments on Intrinsic Ionic Disorder References Problems 5. Extrinsic Ionic Disorder Introduction The AgCl-CdCl[2 System The CaF[2-CaO System The TiO[2-Nb[2O[5 System Summary of Important Points Schematic Representation of Defect Concetrations Summary of Extrinsic Ionic Disorder References Problems 6. Defect Complexes and Associates Introduction Complexes Containing an Impurity Center and an Ionic Defect Intrinsic Ionic Defect Associates The Effect of Impurities on the Concentrations of Defect Complexes and Associates References 7. Ionic Transport Introduction Basic Concepts of Diffusion Ionic Conduction in Crystalline Solids Intrinsic and Extrinsic Ionic Conduction Fast Ion Conductors References 8. Intrinsic Electronic Disorder Introduction The Development of Energy Bonds The Mass-Action Approach The Fermi Function Holes, Waves, and Effective Masses Electronic Conductivity Hopping Mechanisms The Band Structure of Compounds Chemistry and the Band Gap Summary References 9. Extrinsic Electronic Disorder Introduction Interactions with the Gaseous Ambient The Choice of Compensating Defect The Chemical Consequences of Electronic Compensation The Interactions of Impurity Centers with Electrons and Holes The Situation for Compounds Summary References 10. Intrinsic Nonstoichiometry Introduction Nonstoichiometry in Pure Crystalline Compounds Nonstoichiometry and Equilibrium Defect Concentrations The Hypothetical Compund MX with Schottky Disorder Summary of the Kroeger-Vink Diagram for MX Conclusion of the Discussion of MX A More Complex Kroeger-Vink Diagram Summary of the Kroeger-Vink Diagrams for Intrinsic Nonstoichiometry Enthalpy Relationships Conclusion Reference Problems 11. Extrinsic Nonstoichiometry Introduction A Simple Example: Donor-Doped MX Enthalpy Relationships A More Complex Example: Acceptor-Doped M[2O[3 General Considerations Nonstoichiometric Reactions in the Impurity-Controlled Region Problems 12. Titanium Dioxide Introduction The Amount of Nonstoichiometry The Equilibrium Electrical Conductivity of Undoped TiO[2 The Seebeck Coefficient of Undoped TiO[2 Ionic Conduction in TiO[2 The Effect of Dopants on TiO[2 General Comments on the Defect Chemistry of TiO[2 References Problems 13. Cobalt Oxide and Nickel Oxide Introduction Cobaltous Oxide, CoO Nickelous Oxide, NiO Summary References 14. Barium Titanate Introduction General Expectations The Equilibrium Conductivity of Undoped BaTiO[3 Insulating Properties of BaTiO[3 Acceptor-Doped BaTiO[3 Ionic Conduction in BaTiO[3 Donor-Doped BaTiO[3 Trivalent Dopants in BaTiO[3 Summary References 15. Order versus Disorder Block Structures Summary References Index

High‐Temperature Proton Conducting Membranes Based on Perfluorinated Ionomer Membrane‐Ionic Liquid Composites

Abstract: Composite membranes that exhibit fast proton transport at elevated temperatures are needed for proton‐exchange‐membrane fuel cells and other electrochemical devices operating in the 100 to 200°C range. Traditional water‐swollen proton conducting membranes such as the Nafion membrane suffer from the volatility of water in this temperature range leading to a subsequent drop in conductivity. Here we demonstrate that perfluorinated ionomer membranes such as the Nafion membrane can be swollen with ionic liquids giving composite free‐standing membranes with excellent stability and proton conductivity in this temperature range while retaining the low volatility of the ionic liquid. Ionic conductivities in excess of 0.1 S/cm at 180°C have been demonstrated using the ionic liquid 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate. Comparisons between the ionic‐liquid‐swollen membrane and the neat liquid itself indicate substantial proton mobility in these composites. © 2000 The Electrochemical Society. All rights reserved.

Electrical and Ionic Conductivity of Gd‐Doped Ceria

Abstract: Electrical conductivity of , as a function of temperature and oxygen partial pressure, is measured with a complex impedance method. The increase of electrical conductivity with reducing oxygen partial pressure can be described well by a model that assumes constant mobility of both oxygen vacancies and electrons, based on an ideal‐solution model of non‐stoichiometry of Gd‐doped ceria. Ionic conductivity is calculated, and its activation energy is discussed. Electronic conductivity is discussed also. © 2000 The Electrochemical Society. All rights reserved.

Crystal Structure of La2Mo2O9, a New Fast Oxide−Ion Conductor

Abstract: The crystal structure of the new fast oxide-ion conductor La2Mo2O9 (ionic conductivity of 0.06 S cm-1 at 800 °C) has been studied. This compound presents a reversible phase transformation around 580 °C from a low-temperature form α-La2Mo2O9 to a high-temperature form β-La2Mo2O9. The high-temperature form β-La2Mo2O9 has a cubic structure (at 617 °C, space group P213; a = 7.2014(5) A; Z = 2; RBragg = 5.8%, Rp = 10.9%, Rwp = 6.5%, χ2 = 7.7) which derives from that of β-SnWO4. Partial site occupation by oxygen atoms, strongly anisotropic thermal factors, and short-range order with a distance characteristic of O−O pairs have been evidenced. An original concept is proposed for the origin of oxide−ion conduction in this compound, which could be applied to the design of new oxide−ion conductors. The low-temperature form α-La2Mo2O9 exhibits a slight monoclinic distortion and a large superstructure relative to β-La2Mo2O9 (2 × 3 × 4), most probably due to the localization of oxygen atoms. The large cell (∼8800 A3) d...

Miscibility Behavior of Polybenzimidazole/Sulfonated Polysulfone Blends for Use in Fuel Cell Applications

Abstract: Polybenzimidazole (PBI) and polysulfone (PSF) compose an immiscible polymer pair; the introduction however of functional groups such as sulfonate groups in the polymeric chain of PSF resulted in the formation of miscible blends with PBI. The miscibility behavior of a series of blends of PBI with sulfonated PSF (SPSF) at various sulfonation levels has been studied by dynamic mechanical analysis (DMA), FT-IR, and FT-Raman spectroscopy. DMA has shown that the sulfonation degree as well as blend composition controls the miscibility behavior of the studied system. In that respect, partially miscible or miscible blends were obtained when the sulfonation level is higher than 10 mol %. Since both polymers exhibit functional groups, which could participate in specific interactions, this possibility has been examined by FT-IR analysis. In absorption FT-IR spectra of PBI−SPSF specimens with high sulfonation degree and high PBI content, band shifts associated with the NH and sulfonate groups are accounted for the ind...

Ferroelectric Materials as a Ceramic Filler in Solid Composite Polyethylene Oxide‐Based Electrolytes

Abstract: The electrochemical properties of mixed‐phase composite electrolytes based on poly(ethylene oxide) (PEO), lithium salts , and ferroelectric materials have been studied. The ion‐conduction and lithium‐ion transference numbers of the composite polymer electrolytes were enhanced by the addition of these ferroelectric materials as a ceramic filler. The conductivity behavior of the composite electrolyte depended on the combination of lithium salt and the ferroelectric materials. The conductivity enhancement in the PEO‐LiX composite electrolytes with ferroelectric materials was rationalized by correlating the association tendency of anions with lithium cations and the spontaneous polarization of the ferroelectric ceramics due to their particular crystal structure. The combined addition of rutile type and assured long‐term interfacial stability. All the electrolytes studied here showed decomposition potentials higher than 4V vs. . © 2000 The Electrochemical Society. All rights reserved.

Conductivity Study of Porous Plasticized Polymer Electrolytes Based on Poly(vinylidene fluoride) A Comparison with Polypropylene Separators

Abstract: The novel porous plasticized poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF/HFP)‐based electrolytes, compared with conventional polypropylene (PP) separators (Celgard® 2400), were studied via electrochemical impedance spectroscopy, gas adsorption/desorption method, differential scanning calorimetry, and a simple wettability test. The obtained impedance spectra of the electrolytes and separators are extensively discussed, inclusive of the effect of poor wetting upon them. The average pore diameter and Brunauer‐Emmett‐Teller surface area of PVDF/HFP‐based electrolyte membranes are 16.4 nm and , respectively. Ionic conductivity and electrolyte retention characteristic of these electrolyte membranes are superior to conventional PP separators. Moreover, PVDF/HFP‐based electrolyte membranes are free from the problem of wetting whereas the poor wetting of PP separators in some electrolytes may cause its effective conductivity to decrease by at least one order of magnitude. The enhanced wettability may be achieved by virtue of the swelling phenomenon between the polymer and the electrolytes. However, the activation energy for the conduction of PVDF/HFP‐based electrolytes is still larger than that of their parent neat electrolytes , which may imply that the influence of PVDF/HFP upon ionic mobility still exists even if they have been made nanoporous. © 2000 The Electrochemical Society. All rights reserved.

Preparation of Novel Room‐Temperature Molten Salts by Neutralization of Amines

Abstract: Examples of a new class of onium salts bearing the tetrafluoroborate anion (BF 4 - ) were prepared by neutralization of the corresponding amines. This method greatly simplifies the preparation of many organic molten salts. Some of the resulting neutralized amines displayed an ionic conductivity greater than 10 -2 S cm -1 at room temperature. The largest ionic conductivity was observed in 2-methyl-1-pyrroline neutralized by HBF 4 . In addition, the ionic conductivity above the melting point is well described by the Vogel-Tamman-Fulcher formula. There is a correlation between the ionic conductivity of neutralized amines and of the corresponding alkylated onium salts having the same heteroaromatic rings. These neutralized amines are convenient models of onium salts having high ionic conductivity.

Oxygen Permeability of Ce0.8Gd0.2 O 2 − δ ‐ La0.7Sr0.3MnO3 − δ Composite Membranes

Abstract: (CGO) and (LSM) possess similar thermal expansion coefficients and were thus combined in dual‐phase membranes for oxygen separation. Studies of oxygen permeation through CGO‐LSM composite ceramics, containing similar volume fractions of the phases, showed that the oxygen transfer is limited by the bulk ionic conductivity. The oxygen conduction in the composites depends strongly on processing conditions, decreasing with interdiffusion of the phase components. Blocking oxygen ionic conduction is assumed to be due to formation of layers with low ionic conductivity at the CGO grain boundaries, caused by diffusion of lanthanum and strontium into CGO. The permeation fluxes through CGO‐LSM membranes at high feed‐side oxygen pressures (1–50 atm) exhibit Wagner‐type behavior and exceed significantly the oxygen permeability at lower oxygen pressures. © 2000 The Electrochemical Society. All rights reserved.