Showing papers on "Ionic conductivity published in 2009"

Enhanced Sulfur and Coking Tolerance of a Mixed Ion Conductor for SOFCs: BaZr0.1Ce0.7Y0.2–xYbxO3–δ

Abstract: The anode materials that have been developed for solid oxide fuel cells (SOFCs) are vulnerable to deactivation by carbon buildup (coking) from hydrocarbon fuels or by sulfur contamination (poisoning). We report on a mixed ion conductor, BaZr(0.1)Ce(0.7)Y(0.2-)(x)Yb(x)O(3-delta), that allows rapid transport of both protons and oxide ion vacancies. It exhibits high ionic conductivity at relatively low temperatures (500 degrees to 700 degrees C). Its ability to resist deactivation by sulfur and coking appears linked to the mixed conductor's enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking and reforming, as well as enhanced water adsorption capability.

Activation of Carbon Nitride Solids by Protonation: Morphology Changes, Enhanced Ionic Conductivity, and Photoconduction Experiments

Abstract: Covalently bonded carbon nitride materials (e.g., g-C(3)N(4)) have numerous potential applications ranging from semiconductors to fuel cells. But their solubility is poor, which makes characterization and processing difficult. Moreover, the chemistry of the as-synthesized carbon nitrides has been widely neglected. Here we report that some of these handicaps might be overcome by a controllable and reversible protonation. It was found that protonation not only provides better dispersion and exposes a high surface area for g-C(3)N(4) but also enables an adjustment of electronic band gaps and higher ionic conductivity. Recovery or deprotonation toward the original g-C(3)N(4) could be obtained by simple heating, which enables improved sintering but also a potential preservation of the higher surface area of the protonated material. This proton-enhanced sintering process allowed for the first time direct measurement of the photoconductivity of the material. By aid of protonation, other promising g-C(3)N(4) based hybrid composites could also be facilely obtained by simple counteranion exchange.

Electrochemical Performance of MnO2 Nanorods in Neutral Aqueous Electrolytes as a Cathode for Asymmetric Supercapacitors

Abstract: The electrochemical performance of MnO2 nanorods prepared by a precipitation reaction was investigated in 0.5 mol/L Li2SO4, Na2SO4, and K2SO4 aqueous electrolyte solutions. Results show that at the slow scan rates, the nanorods show the largest capacitance (201 F/g) in Li2SO4 electrolyte since the reversible intercalation/deintercalation of Li+ in the solid phase produces an additional capacitance besides the capacitance based on the absorption/desorption reaction. At fast scan rates they show the largest capacitance in the K2SO4 electrolyte due to the smallest hydration radius of K+, highest ionic conductivity, and lowest equivalent series resistance (ESR). An asymmetric activated carbon (AC)/K2SO4/MnO2 supercapacitor could be cycled reversibly between 0 and 1.8 V with an energy density of 17 Wh/kg at 2 kW/kg, much higher than those of the AC/K2SO4/AC supercapacitor and AC/Li2SO4/LiMn2O4 hybrid supercapacitor. Moreover, this supercapacitor exhibits excellent cycling behavior with no more than 6% capacita...

Are there stable ion-pairs in room-temperature ionic liquids? Molecular dynamics simulations of 1-n-butyl-3-methylimidazolium hexafluorophosphate.

Abstract: Molecular dynamics simulations with an all-atom model were carried out to study the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Analysis was carried out to characterize a number of structural and dynamic properties. It is found that the hydrogen bonds are weaker than expected, as indicated by their short lifetimes, which is due to the fast rotational motion of anions. Transport properties such as ion diffusion coefficients and ionic conductivity were also measured on the basis of long trajectories, and good agreement was obtained with experimental results. The phenomenon that electrical conductivity of ionic liquids deviates from the Nernst−Einstein relation was well reproduced in our work. On the basis of our analysis, we suggest that this deviation results from the correlated motion of cations and anions over time scales up to nanoseconds. In contrast, we find no evidence for long-lived ion-pairs migrating together.

Ionic conductivity by correlated barrier hopping in NH4I doped chitosan solid electrolyte

Abstract: Chitosan–NH 4 I and chitosan–NH 4 I–EC films have been prepared by solution cast technique. The sample containing 45 wt% ammonium iodide (NH 4 I) exhibited the highest room temperature conductivity of 3.7×10 −7 S cm −1 . The conductivity of the sample increased to 7.6×10 −6 S cm −1 when 40 wt% ethylene carbonate (EC) was added to the 55 wt% chitosan-45 wt% NH 4 I sample. The conductivity–temperature relationship is Arrhenian. From dielectric loss variation with frequency, the power law exponent was obtained. The temperature dependence of the power law exponent for chitosan–NH 4 I system follows the correlated barrier hopping (CBH) model while conduction mechanism of the plasticized system can be represented by the small polaron hopping (SPH) model.

First-principles study on defect chemistry and migration of oxide ions in ceria doped with rare-earth cations

Abstract: Oxygen transport in rare-earth oxide (RE2O3) doped CeO2 with fluorite structure has attracted considerable attention owing to both the range of practical usage (e.g., fuel cells, sensors, etc.) and the fundamental fascination of fast oxide ion transport in crystalline solids. Using density-functional theory, we have calculated the formation energies of point defects and their migration properties in RE2O3 doped CeO2(RE = Sc, Y, La, Nd, Sm, Gd, Dy, and Lu). The calculated results show that oxygen vacancies are the dominant defect species obtained by RE3+ doping. They form associates with the RE3+ ions, and the corresponding defect association energy is a strong function of the ionic radii of the RE3+ dopants. The migration of an oxygen vacancy was investigated using the nudged elastic band method. The lowest activation energy for oxygen vacancy hopping is obtained for a straightforward migration path between two adjacent oxygen sites. The migration energy of an oxygen vacancy also strongly depends on the ionic radii of the neighbouring dopant cations. Accordingly, we have identified two factors that affect the oxygen vacancy migration; (1) trapping (or repelling) of an oxygen vacancy at the NN site of the RE3+ dopant, and (2) reduction (or enlargement) of the migration barrier by RE3+ doping. These findings provide insight for atomistic level understanding of ionic conductivity in doped ceria and would be beneficial for optimizing ionic conductivity.

Elastic strain at interfaces and its influence on ionic conductivity in nanoscaled solid electrolyte thin films--theoretical considerations and experimental studies.

Abstract: Ionic transport in solids parallel to grain or phase boundaries is usually strongly enhanced compared to the bulk. Transport perpendicular to an interface (across an interface) is often much slower. Therefore in modern micro- and nanoscaled devices, a severe influence on the ionic/atomic transport properties can be expected due to the high density of interfaces. Transport processes in boundaries of ionic materials are still not understood on an atomic scale. In most of the studies on ionic materials the interfacial transport properties are explained by the influence of space charge regions. Here we discuss the influence of interfacial strain at semicoherent or coherent heterophase boundaries on ionic transport along these interfaces in ionic materials. A qualitative model is introduced for (untilted and untwisted) hetero phase boundaries. For experimental verification, the interfacial oxygen ionic conductivity of different multilayer systems consisting of cubic ZrO2 stabilised by aliovalent dopands (YSZ, CSZ) and an insulating oxide is investigated as a function of structural mismatch. Recent results on extremely fast ionic conduction in YSZ/SrTiO3 thin film systems (“colossal ionic concuctivity at interfaces”) is discussed from the viewpoint of strain effects.

Polymerized Ionic Liquids: The Effect of Random Copolymer Composition on Ion Conduction

Abstract: Ionic conductivity in new polymerized ionic liquids is of great interest as it applies to solid-state electrolytes for electrochemical and electromechanical applications. In this study, an ionic liquid monomer was synthesized and polymerized into random copolymers and their ionic conductivity and structure were investigated as a function of copolymer composition. Both nonionic−ionic and ionic−ionic copolymers were synthesized, where the nonionic and ionic monomers were hexyl methacrylate (HMA) and a methacrylate-based imidizolium neutralized with tetrafluoroborate (BF4) or bis(trifluoromethane sulfonyl)imide (TFSI). In the nonionic−ionic copolymer, the ionic conductivity increased by over an order of magnitude with increasing HMA composition, even though the overall charge content decreased, because the addition of HMA significantly lowered the glass transition temperature. The ionic conductivity also increased by more than an order of magnitude in the ionic−ionic copolymer with increasing TFSI content, e...

Molecular mobility and Li(+) conduction in polyester copolymer ionomers based on poly(ethylene oxide).

Abstract: We investigate the segmental and local dynamics as well as the transport of Li(+) cations in a series of model poly(ethylene oxide)-based single-ion conductors with varying ion content, using dielectric relaxation spectroscopy. We observe a slowing down of segmental dynamics and an increase in glass transition temperature above a critical ion content, as well as the appearance of an additional relaxation process associated with rotation of ion pairs. Conductivity is strongly coupled to segmental relaxation. For a fixed segmental relaxation frequency, molar conductivity increases with increasing ion content. A physical model of electrode polarization is used to separate ionic conductivity into the contributions of mobile ion concentration and ion mobility, and a model for the conduction mechanism involving transient triple ions is proposed to rationalize the behavior of these quantities as a function of ion content and the measured dielectric constant.

Ionic Conductivity of Block Copolymer Electrolytes in the Vicinity of Order−Disorder and Order−Order Transitions

Abstract: Order−order and order−disorder phase transitions in mixtures of poly(styrene-block-ethylene oxide) (SEO) copolymers and lithium bis(trifluoromethylsulfonimide) (LiTFSI), a common lithium salt used in polymer electrolytes, were studied using a combination of small-angle X-ray scattering (SAXS), birefringence, and ac impedance spectroscopy. The SEO/LiTFSI mixtures exhibit lamellar, hexagonally packed cylinders, and gyroid microphases. The molecular weight of the blocks and the salt concentration was adjusted to obtain order−order and order−disorder transition temperatures within the available experimental window. The ionic conductivities of the mixtures, normalized by the ionic conductivity of a 20 kg/mol homopolymer PEO sample at the salt concentration and temperature of interest, were independent of temperature, in spite of the presence of the above-mentioned phase transitions.

Crystal Structure–Ionic Conductivity Relationships in Doped Ceria Systems

Abstract: In the past, it has been suggested that the maximum ionic conductivity is achieved in ceria, when doped with an acceptor cation that causes minimum distortion in the cubic fluorite crystal lattice. In the present work, this hypothesis is tested by measuring both the ionic conductivity and elastic lattice strain of 10 mol% trivalent cation-doped ceria systems at the same temperatures. A consistent set of ionic conductivity data is developed, where the samples are synthesized under similar experimental conditions. On comparing the grain ionic conductivity, Nd 0.10 Ce 0.90 O 2-δ exhibits the highest ionic conductivity among other doped ceria systems. The grain ionic conductivity is around 17% higher than that of Gd 0.10 Ce 0.90 O 2-δ at 500°C, in air. X-ray diffraction profiles are collected on the sintered powder of all the compositions, from room temperature to 600°C, in air. From the lattice expansion data at high temperatures, the minimal elastic strain due to the presence of dopant is observed in Dy 0.10 Ce 0.90 O 2-δ . Nd 0.10 Ce 0.90 O 2-δ exhibits larger elastic lattice strain than Dy 0.10 Ce 0.90 O 2-δ with better ionic conductivity at intermediate temperatures. Therefore, it is shown that the previously proposed crystal structure-ionic conductivity relationship based on minimum elastic strain is not sufficient to explain the ionic conductivity behavior in ceria-based system.

FTIR, XRD and ac impedance spectroscopic study on PVA based polymer electrolyte doped with NH4X (X = Cl, Br, I)

Abstract: The present study focuses on characterizing PVA: NH4X (X = Cl, Br, I) proton conducting polymer electrolyte prepared by solution casting technique using XRD, FTIR and ac impedance spectroscopic studies. The XRD patterns of all the prepared polymer electrolytes reveal the amorphous nature of the films. The FTIR spectroscopic study indicates the detailed interaction of PVA with proton. From ac impedance spectroscopic studies, it has been found that PVA doped with NH4I have high ionic conductivity (2.5 × 10−3S cm−1) than PVA doped with NH4Br (5.7 × 10−4S cm−1) and NH4Cl (1.0 × 10−5S cm−1) polymer electrolytes. This is due to the large anionic size and low lattice energy of NH4I (in comparison with NH4Br and NH4Cl).The temperature dependence of ionic conductivity for all the PVA: NH4X (X = Cl, Br, I) polymer films obey Arrhenius equation. Ionic transference number measured has been found to be in the range of 0.93–0.96 for all the polymer electrolytes proving that the total conductivity is mainly due to ions.

Alkali metal crystalline polymer electrolytes

Abstract: The transport and mechanical properties of polymer electrolytes make them important materials for all-solid-state electrochemical devices such as batteries or electrochromic displays. Crystalline polymer electrolytes containing alkali metal salts are now found to exhibit ionic conductivity 1.5 orders of magnitude higher than the best conductor reported so far.

Size-controlled stabilization of the superionic phase to room temperature in polymer-coated AgI nanoparticles.

Abstract: Silver iodide is a well-known ionic conductor. However, it shows superionic conductivity only in its high-temperature phase (above∼150 ∘C). It is now demonstrated that various sizes of nanoparticles can be synthesized for which the superionic phase is stable down to ∼30 ∘C. The results suggest promising applications in silver-ion-based electrochemical devices. Solid-state ionic conductors are actively studied for their large application potential in batteries1 and sensors. From the view of future nanodevices2,3,4,5, nanoscaled ionic conductors are attracting much interest. Silver iodide (AgI) is a well-known ionic conductor for which the high-temperature α-phase shows a superionic conductivity greater than 1 Ω−1 cm−1 (ref. 6). Below 147 ∘C, α-AgI undergoes a phase transition into the poorly conducting β- and γ-polymorphs, thereby limiting its applications. Here, we report the facile synthesis of variable-size AgI nanoparticles coated with poly-N-vinyl-2-pyrrolidone (PVP) and the controllable tuning of the α- to β-/γ-phase transition temperature (Tc↓). Tc↓ shifts considerably to lower temperatures with decreasing nanoparticle size, leading to a progressively enlarged thermal hysteresis. Specifically, when the size approaches 10–11 nm, the α-phase survives down to 30 ∘C—the lowest temperature for any AgI family material. We attribute the suppression of the phase transition not only to the increase of the surface energy, but also to the presence of defects and the accompanying charge imbalance induced by PVP. Moreover, the conductivity of as-prepared 11 nm β-/γ-AgI nanoparticles at 24 ∘C is ∼1.5×10−2 Ω −1 cm−1—the highest ionic conductivity for a binary solid at room temperature. The stabilized superionic phase and the remarkable transport properties at a practical temperature reported here suggest promising applications in silver-ion-based electrochemical devices.