Oxygen permeation study of perovskite hollow fiber membranes

The first oxygen permeation data of a dense hollow fiber perovskite membrane based on BaCo_xFe_yZr_zO_3 − δ are reported. The hollow fiber was prepared by a phase inversion process. Dense fibers were obtained with the following typical geometries: outer diameter, 800–900 μm; inner diameter, 500–600 μm; length, 30 cm. The O₂-permeation through the hollow fiber perovskite membrane was studied in a high-temperature gas permeation cell under different operation conditions. The increase of the helium gas flow rate reduces the oxygen partial pressure (p_O2) on the core side and a higher oxygen permeation flux is observed. High oxygen flux of 0.73 m³ (O₂)/(m² (membrane) h) was achieved at 850 °C under the operation parameters F_air (shell side) = 150 ml/min and F_He (core side) = 30 ml/min. The oxygen partial pressure dependence of the O₂ permeation flux indicated an interplay of both surface reaction and bulk diffusion as rate limiting steps. During 5 days of permeation a high and stable oxygen flux was observed. X-ray diffraction patterns of fresh and spent membranes after the permeation measurements revealed that no degradation after oxygen permeation appears.     

Cobalt-free SrFe1−xMoxO3-δ perovskite hollow fiber membranes for oxygen separation

SrFe_0.95Mo_0.05O_3-δ (SFM5) perovskite hollow fiber (HF) membranes with a finger-like structure were fabricated by a phase inversion technique. The oxygen flux through SFM5 hollow fiber membrane was evaluated and reached 0.64 μmol/cm² *s at T = 880 °C, which is 5 times higher than that of a disk SFM5 membrane (0.12 μmol/cm² *s). A further increase in oxygen fluxes was attained by Ag deposition on the inner surface of SFM5 hollow fiber membrane. The oxygen flux of SFM5 HF membranes is governed by surface-exchange reactions on the permeate side. The equilibrium "3 − δ − lg pO₂ − T" diagrams showed that doping of SF by molybdenum leads to a broadening of the cubic perovskite phase stability region.     

Effects of preparation methods on the oxygen nonstoichiometry, B-site cation valences and catalytic efficiency of perovskite La0.6Sr0.4Co0.2Fe0.8O3−δ

La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) powders were synthesized respectively by an EDTA (ethylenediaminetetraacetic acid)–Citrate sol–gel process and a low-temperature auto-combustion process. The samples were characterized by XRD, SEM, BET, TGA and instant temperature analysis. The iodometric titration was used to determine the average valence of Co and Fe ions and the oxygen nonstoichiometry of the prepare powders. The catalytic properties of the synthesized powders were investigated by the hydrogen peroxide catalytic decomposition. Pure-perovskite structure was formed by both synthesis methods. The oxygen nonstoichiometry of the samples prepared by the auto-combustion process is larger than that by the sol–gel process. The catalytic activities of the powders from two synthesis processes also differed largely due to the different oxygen nonstoichiometry, surface area and crystalline sizes.     

Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor

Ionic-electronic mixed-conducting perovskite-type oxide La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the p_O2-range of 10⁻³−1 bar at 1073–1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m⁻² s⁻¹ at 1173 K with a corresponding activation energy of 130–140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1–3% at 1073–1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30–35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized.     

Catalytic performance of La0.66Sr0.34Co0.2Fe0.8O3 perovskite in propane combustion: Effect of preparation and specific surface area

Several compositions in a system of La_1-x SrxCo_1-y FeyO_3-δ perovskites are known as very good electronic and ionic conductors, as well as excellent catalysts for hydrocarbon oxidation. In this study La_0.66Sr_0.34Co_0.2Fe_0.8O₃ was selected as possibly the optimum composition. To assess the effect of preparation and calcination conditions on the activity in propane combustion, a series of different samples was prepared by a method based on slurry of reactive component precursors processed by freeze-drying. Three different materials were used as source of iron and the samples were aged at successively higher temperatures (1,153–1,343 K). The specific surface areas varied between 5.9 and 1 m²/g, depending on iron precursor and/or aging. The activity was determined in an integral U-shape reactor, typically for 1 and 2 vol% propane in air, with 1 g catalyst and 200 or 100 ml/min flowrate. Kinetics determined on the basis of a wider range of concentrations (1–4.3 vol% propane; 10 vol%-pure oxygen) for a selected, the least aged sample indicated that the propane catalytic combustion is best represented by a Mars van Krevelen model with 0.5 order in oxygen and the two kinetic constants having E _app of 83 and 81 kJ/mol, respectively. For the aged samples, the pseudofirst order E _app varied from 85 to 98 kJ/mol.     

Electrochemical promotion of catalysis controlled by chemical potential difference across a mixed ionic-electronic conducting ceramic membrane – an example of wireless NEMCA

A La_0.6Sr_0.4Co_0.2F_0.8O₃ mixed ionic electronic conducting (MIEC) membrane was used in a dual chamber reactor for the promotion of the catalytic activity of a platinum catalyst for ethylene oxidation. By controlling the oxygen chemical potential difference across the membrane, a driving force for oxygen ions to migrate across the membrane and backspillover onto the catalyst surface is established. The reaction is then promoted by the formation of a double layer of oxide anions on the catalyst surface. The electronic conductivity of the membrane material eliminates the need for an external circuit to pump the promoting oxide ion species through the membrane and onto the catalyst surface. This renders this “wireless” system simpler and more amenable for large-scale practical application. Preliminary experiments show that the reaction rate of ethylene oxidation can indeed be promoted by almost one order of magnitude upon exposure to an oxygen atmosphere on the sweep side of the membrane reactor, and thus inducing an oxygen chemical potential difference across the membrane, as compared to the rate under an inert sweep gas. Moreover, the rate does not return to its initial unpromoted value upon cessation of the oxygen flow on the sweep side, but remains permanently promoted. A number of comparisons are drawn between the classical electrochemical promotion that utilises an external circuit and the “wireless” system that utilises chemical potential differences. In addition a ‚surface oxygen capture’ model is proposed to explain the permanent promotion of the catalyst activity.     

A bi-layered composite cathode of La0.8Sr0.2MnO3-YSZ and La0.8Sr0.2MnO3-La0.4Ce0.6O1.8 for IT-SOFCs

A bi-layered composite cathode of La_0.8Sr_0.2MnO₃ (LSM)-YSZ and LSM-La_0.4Ce_0.6O_1.8 (LDC) was fabricated for anode-supported solid oxide fuel cells with a thin YSZ electrolyte film. The cell with the bi-layered composite cathode displayed better performance than the cell with the corresponding single-layered composite cathode of LSM-LDC or LSM-YSZ. At 650 °C, the cell with the bi-layered composite cathode gave a higher maximum power density than the cells with the single-layered LSM-LDC and LSM-YSZ composite cathodes, by 52% and 175%, respectively. The impedance spectra results show that the thin LSM-YSZ interlayer not only improves the cathode/electrolyte interface but also reduces the polarization resistance of the cathode. The activation energy for oxygen reduction on the bi-layered composite cathode is much smaller than that on LSM-YSZ composite cathode, and it is suggested that the special redox property of Ce⁴⁺/Ce³⁺ in LDC facilitates the oxygen reduction process on the bi-layered composite cathode. The cell with the bi-layered composite cathode operated quite stably during a 100 h run.     

Enhanced Microwave Absorption Properties of Intrinsically Core/shell Structured La0.6Sr0.4MnO3 Nanoparticles

The intrinsically core/shell structured La_0.6Sr_0.4MnO₃nanoparticles with amorphous shells and ferromagnetic cores have been prepared. The magnetic, dielectric and microwave absorption properties are investigated in the frequency range from 1 to 12 GHz. An optimal reflection loss of −41.1 dB is reached at 8.2 GHz with a matching thickness of 2.2 mm, the bandwidth with a reflection loss less than −10 dB is obtained in the 5.5–11.3 GHz range for absorber thicknesses of 1.5–2.5 mm. The excellent microwave absorption properties are a consequence of the better electromagnetic matching due to the existence of the protective amorphous shells, the ferromagnetic cores, as well as the particular core/shell microstructure. As a result, the La_0.6Sr_0.4MnO₃nanoparticles with amorphous shells and ferromagnetic cores may become attractive candidates for the new types of electromagnetic wave absorption materials.     

High performance electrode for electrochemical oxygen generator cell based on solid electrolyte ion transport membrane

A double-layer composite electrode based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 (BSCF + SDC) and BSCF + SDC + Ag was investigated to be a promising cathode and also anode for the electrochemical oxygen generator based on samaria doped ceria electrolyte. The Ag particles in the second layer were not only the current collector but also the improver for the oxygen adsorption at the electrode. a.c. impedance results indicated that the electrode polarization resistance, as low as 0.0058 Ω cm² was reached at 800 °C under air. In oxygen generator cell performance test, the electrode resistance dropped to half of the value at zero current density under an applied current density of 2.34 A cm⁻² at 700 °C, and on the same conditions the oxygen generator cell was continual working for more than 900 min with a Faradic efficiency of ∼100%.     

X-ray absorption and diffraction studies of La0.6Ca0.4CoO3 perovskite, a catalyst for bifunctional oxygen electrodes

The chemical stability of La_0.6Ca_0.4CoO₃ perovskite as an electrocatalyst in bifunctional oxygen electrodes has been studied by ex-situ and in-situ X-ray absorption spectroscopy and X-ray diffraction measurements. The catalyst was investigated ex-situ as a powder mixed with BN, and in-situ on carbon supports in bilayer air electrodes at different potentials in an electrochemical cell with an alkaline electrolyte. It was found to be stable for at least 1300 h in electrodes operated under the conditions of rechargeable Zn/air cells.     

Synthesis and electrochemical properties of submicron LiNi0.8Co0.2O2 by a polymer-pyrolysis method

A polymer-pyrolysis method was used to synthesize LiNi_0.8Co_0.2O₂, which has potential application in lithium ion batteries. The effect of calcination temperature and time on the structure and electrochemical performance of the material was investigated. XRD analysis showed that the powders obtained by calcination at 750 °C for 3 h had the best-ordered hexagonal layer structure. SEM image showed these powders were fine, narrowly distributed with platelet morphology. The charge-discharge tests demonstrated these powders had the best electrochemical properties, with an initial discharge capacity of 189 mAh/g and capacity retention of 95.2% after 50 cycles when cycled at 50 mA/g between 3.0 and 4.3 V. Besides, these powders also had exhibited excellent rate capability.     

A novel kojic acid amperometric sensor based on hollow CuO/Fe2O3 hybrid microspheres immobilized in chitosan matrix

Hollow CuO/Fe₂O₃ hybrid microspheres with small uniform holes were synthesized using a convenient hydrothermal method and were applied to fabricated an amperometric sensor for kojic acid. The resulting materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) and then were immobilized into the chitosan (Chi) matrix onto a glassy carbon electrode to obtain CuO/Fe₂O₃–Chi/GCE. The potential utility of the constructed electrodes were demonstrated by applying them to the analytical determination of kojic acid concentration. The electrochemical behavior of kojic acid on CuO/Fe₂O₃–Chi/GCE was explored. The modified electrode displayed excellent amperometric response for kojic acid with a linear range from 0.2 μM to 674 μM with a detection limit of 0.08 μM at a signal-to-noise ratio of 3. In order to validate feasibility, the CuO/Fe₂O₃–Chi/GCE has been used for quantitative detecting kojic acid in real samples.     

Evaluation of some cobalt and nickel based perovskites prepared by freeze-drying as combustion catalysts

A series of cobalt and nickel based perovskite type catalysts with high specific surface area (8–20 m² /g) was prepared by spray-freezing/freeze-drying method. The catalytic activity of all samples in methane combustion was evaluated by measuring the light-off temperature, the conversion at 823 K and the temperature of the end of the reaction. The experimental data suggest higher activity than reported in literature for similar or other perovskites, and confirm its strong dependence on the specific surface area. Among eleven tested catalysts, including seven new compositions four of which showed excellent activity, La_0.66Sr_0.34Ni_0.3Co_0.7O₃ was the best performing.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Metal oxides nanoparticles on SBA-15: Efficient catalyst for methane combustion

Catalysts based on crystalline nanoparticles of Mn and Co metal oxides supported on mesoporous silica SBA-15 have been developed. These materials were characterized by XRD, BET and transmission electron microscopy (TEM) techniques. SBA-15 mesoporous silica was synthesized by a conventional sol–gel method using a tri-block copolymer as surfactant. Supported Mn₃O₄ and Co₃O₄ nanoparticles were obtained after calcination of as-impregnated SBA-15 by a metal salt precursor. The catalytic activity was evaluated in the combustion of methane at low concentration.Co₃O₄/SBA-15 (7 wt.%) exhibits the highest performance among the different oxides. Furthermore, this novel generation of catalysts appeared as active as conventional LaCoO₃ perovskite, usually taken as reference for this reaction. Thanks to its organized meso-structures, SBA-15 material creates peculiar diffusion conditions for reactants and/or products.     

SrCo0.8Fe0.2O3−δ sorbent for high-temperature production of oxygen-enriched carbon dioxide stream

Perovskite-type SrCo_0.8Fe_0.2O_3−δ (SCF) has been prepared by a liquid citrate method and used to produce O₂-CO₂ gas mixture for oxyfuel combustion. Oxygen is desorbed and an oxygen-enriched carbon dioxide stream is obtained when SCF is exposed in a carbon dioxide stream at high temperature. Oxygen is adsorbed when SCF is regenerated in an air stream. A carbonation-reaction mechanism for O₂-desorption has been identified with the evidences of XRD and TGA analysis. It is found that the theoretical oxygen sorption capacity decreases with the increase of temperature. The sorption kinetics over a temperature range of 700-900 °C has been examined by TGA experiment. Both desorption and sorption processes exhibit a high reaction rate in an initial stage followed by a slower rate in a second stage. It is difficult to reach the theoretical oxygen sorption capacity during the whole temperature range due to the slow oxygen desorption rate. Optimal temperatures for oxygen sorption and desorption processes are determined to be 900 and 850 °C, respectively. Multiple sorption and desorption cycles indicate that SCF sorbent has high reactivity and cyclic stability. Comparison with the reference La_0.1Sr_0.9Co_0.5Fe_0.5O_2.6 (LSCF) and Sr_0.5Ca_0.5Co_0.5Fe_0.5O_2.47 (SCCF) sorbents shows that SCF has faster carbonation reaction at high temperature, i.e., 850 and 900 °C, and much higher theoretical oxygen sorption capacities.     

热处理对 Ba0.6Sr0.4TiO3 厚膜介电性能的影响

采用柠檬酸盐法制备了Ba_(0.6)Sr_(0.4)TiO_3粉体,通过丝网印刷法制备了Ba_(0.6)Sr_(0.4)TiO_3厚膜,研究了在空气气氛中进行热处理前后厚膜样品的介电性能。研究结果表明,在空气气氛中进行热处理可以有效地提高厚膜样品的介电性能。经过1000°C热处理,厚膜样品在10 kH z下的介电损耗由1.7%降为1.1%,其优质系数由33提高到55。     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.     

Transient and isotopic studies of the oxygen transport and exchange during oxidative coupling of methane on Sr promoted La2O3

Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La₂O₃ catalysts. Results of the¹⁸O₂/¹⁶O₂ isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La₂O₃. Steps of¹⁸O₂ in the presence of a steady flow of methane over Sr/La₂O₃ catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase¹⁸O². Steps of CO₂ over catalysts in which lattice oxygen has been exchanged with¹⁸O₂, show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen.     

The effect of zirconium oxide coating on the lithium nickel cobalt oxide for lithium secondary batteries

The electrochemical properties of LiNi_0.8Co_0.2O₂ coated with ZrO₂ by three different coating processes (ball-milling, sol-gel method, simple grinding) were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Results showed that the ZrO₂ coating significantly improved the capacity retention of the cathode by suppressing the impedance growth at the interface between electrodes and electrolyte and the best cyclability was obtained in the case of employing the simple grinding for the ZrO₂ coating. On the other hand, the initial capacities of the ZrO₂-coated LiNi_0.8Co_0.2O₂ cathode were slightly decreased.