Preparation and Characterization of Single-Phase Perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ

Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ has been successfully prepared by using citrate-EDTA complexation method at relatively low calcination temperature. The structure and thermal decomposition process of the complex precursor have been investigated by means of differential scanning calorimetry-thermal gravimetric analysis (DSC/TGA), X-ray diffraction (XRD), and Fourier transform infrared spectroscopic (FT-IR) measurements. The precursor decomposed completely and started to form perovskite-type oxide above 420℃ according to the differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) results. Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ obtained has been confirmed from the XRD pattern, and no peak of SrCO3 was found by XR.D of the oxides synthesized at a relatively low temperature of 800 ℃. The reducibility of La0.6Sr0.4Co0.8Fe0.2O3-δ was also characterized by the temperature programmed reduction (TPR) technique. Disk shaped dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was prepared by the isostatical pressing method. The oxygen flux rate of dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was (2.8-18)×10-8 mol/(cm2·s) in the temperature range of 800-1 000℃.     

The ionic conductivity, thermal expansion behavior, and chemical compatibility of La0.54Sr0.44Co0.2Fe0.8O3-δ as SOFC cathode material

In this paper, the ionic conductivities of La_0.54Sr_0.44Co_0.2Fe_0.8O_3-δ and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ were measured by electron-blocked alternating current impedance analysis technique. The results show that the oxygen ion conductivity of La_0.54Sr_0.44Co_0.2Fe_0.8O_3-δ is nearly five times higher than that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, which makes La_0.54Sr_0.44Co_0.2Fe_0.8O_3-δ cathode more conductive than YSZ electrolyte. Consequently, the electrochemical reaction region is extended from the interface between the cathode and the electrolyte to the whole surface of the cathode grains, with a result of the cathode polarization overpotential being decreased and the cell electrical performance being improved. Besides, the XRD results show that both La_0.54Sr_0.44Co_0.2Fe_0.8O_3-δ and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ begin to react with 8YSZ([Y₂O₃]_0.08·[ZrO₂]_0.92) at 850 °C, but La_0.54Sr_0.44Co_0.2Fe_0.8O_3-δ with a faster reaction rate. The thermal expansion experiments manifest that the two LSCFs have approximate thermal expansion coefficients, being about 14 × 10⁻⁶–15 × 10⁻⁶ K⁻¹ from 500 °C to 700 °C, which is moderately higher than that of 8YSZ.     

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ and (La_0.8Sr_0.2)_0.95MnO_3±δ, using gas‐phase isotope‐exchange with a 1:1 ¹⁶O₂:¹⁸O₂ ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ is better at oxygen dissociation than (La_0.8Sr_0.2)_0.95MnO_3±δ. Heteroexchange, involving lattice oxygen, dominates on La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ. In contrast, (La_0.8Sr_0.2)_0.95MnO_3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.     

Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane

Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9CO0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane in the absence of gaseous oxygen was investigated by continuous flow reaction and sequential redox reaction, Methane was oxidized to syngas with high selectivity by oxygen species of perovskite oxides in the absence of gaseous oxygen. The sequential redox reaction revealed that the structural stability and continuous oxygen supply in redox reaction decreased over La0.8Sr0.2Fe0.9Co0. 1O3 oxide, while LaFeO3 and La0.8Sr0.2FeO3 exhibited excellent structural stability and continuous oxygen supply.     

Electrochemical evaluation of La2NiO4+δ -based composite electrodes screen-printed on Ce0.8Sm0.2O1.9 electrolyte

La₂NiO_4+δ , 60 wt.% La₂NiO_4+δ –40 wt.% La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ , and 60 wt.% La₂NiO_4+δ –40 wt.% Ce_0.8Sm_0.2O_1.9 electrodes were prepared from fine powders on dense Ce_0.8Sm_0.2O_1.9 electrolyte substrates by screen-printing technique. Electrochemical impedance spectroscopy and chronopotentiometry techniques were employed to evaluate the electrochemical properties of the composite electrodes in comparison with the La₂NiO_4+δ electrode. For the three electrodes, main electrode processes were resolved to be charge-transfer at the electrode/electrolyte interface and oxygen exchange on the electrode surface. The contribution of the surface oxygen exchange process was detected to be dominant for the overall electrode polarization. The addition of Ce_0.8Sm_0.2O_1.9 into La₂NiO_4+δ was favorable for the charge transfer process whereas it was undesired for the surface oxygen exchange process. On comparison, adding La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ into La₂NiO_4+δ was found to benefit both the two electrode processes. The La₂NiO_4+δ -La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ composite electrode showed optimum electrochemical properties among the three electrodes. At 800 °C, the composite electrode achieved a polarization resistance of 0.20 Ω cm², an overpotential of 45 mV at a current density of 200 mA cm^?2, together with an exchange current density of ～200 mA cm^?2.     

Preparation of an asymmetric perovskite-type membrane and its oxygen permeability

A crack-free asymmetric membrane of perovskite-type oxide (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) was successfully prepared by coating a slurry containing La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ powders directly on the surface of a green support of the same composition, followed by sintering. It was found that crack-free asymmetric membranes could be obtained by controlling the powder concentration of the slurry in the range of 15–25 wt.%. After sintering, the crystal phase of the top layer of asymmetric membranes prepared was the same as that of powders, which were of the cubic perovskite phase. The nitrogen permeability and SEM photograph of the support showed that the support was porous, and the gas-tight test and SEM demonstrated that the top layer of asymmetric membrane was dense and crack-free. The asymmetric membrane prepared, whose dense top layer was 200 μm thick, exhibited about three to four times as high an oxygen flux as a 2 mm dense sintered disc.     

La0.8Sr0.2Fe1-xScxO3-δ催化剂的制备、表征及甲烷催化燃烧性能

采用甘氨酸-硝酸盐溶液燃烧法制备了钙钛矿型氧化物催化剂La_0.8Sr_0.2Fe_1-xSc_xO_3-δ (LSFS, x=0, 0.3,0.4, 0.5, 0.6, 0.8, 1), 利用X射线衍射(XRD)、H₂程序升温还原(H₂-TPR)、扫描电子显微镜(SEM)和比表面积测试等手段对催化剂进行了系统表征, 并在常压微型固定床反应器上考察了催化剂对甲烷燃烧的催化性能. 结果表明, 经空气气氛下900 °C煅烧5 h制备的LSFS均具有单一的钙钛矿结构, 在La_0.8Sr_0.2FeO_3-δ (LSF)中掺杂Sc有助于改善催化剂的抗烧结性能, 提高催化剂的比表面积. 当LSF 中的Sc 掺杂量为0.4-0.6 时, 所形成的LSFS表现出良好的甲烷催化燃烧活性, 其中Sc 掺杂量为0.5 时, 其起燃温度(T₁₀)和完全转化温度(T₉₀)分别为406和563 °C, 与La_0.8Sr_0.2FeO_3-δ和La_0.8Sr_0.2ScO_3-δ相比, T₁₀分别降低了14和87 °C; T₉₀分别降低了59和95 °C.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol–gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Composite electrodes for proton conducting electrolyte of CaZr<Subscript>0.95</Subscript>Sc<Subscript>0.05</Subscript>O<Subscript>3 – δ</Subscript>

A new method of obtaining gastight ceramic based on CaZr_0.95Sc_0.05O_{3 – δ} is presented. The microstructure and electric properties of the obtained samples, same as the behavior of composite electrodes in contact with this electrolyte, are studied for application of the obtained results in the technology of formation of electrochemical devices. The design of bilayer electrodes is suggested, in which the materials tested as the functional layer were layered lanthanum nickelate La2NiO_{4 + δ} and substituted lanthanum nickelate La_1.7Ca(Sr,Ba)_0.3NiO_{4 + δ} in combination with the electrolyte components of Ce_0.8Sm_0.2O_{2 – δ} and BaCe_0.89Gd_0.1Cu_0.01O_{3 – δ}. The collector layer used was lanthanum nickelate–ferrite LaNi_0.6Fe_0.4O_{3 – δ} and manganite La_0.6Sr_0.4MnO_{3 – δ} that are characterized by high electron conductivity, low layer resistance and are close by their values of coefficient of linear thermal expansion to the materials of functional layers. Electrochemical activity of the obtained electrodes are compared with the characteristics of composite electrodes based on lanthanum ferrite–cobaltite La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 – δ} and deficient lanthanum manganite La_0.75Sr_0.2MnO_{3 – δ}.     

还原剂对La0.7Sr0.3Co0.8Fe0.2O3钙钛矿催化剂NOx储存性能的影响

采用溶胶-凝胶法制备了La0.7Sr0.3Co0.8Fe0.2O3钙钛矿催化剂,考察了还原剂种类(CO,C3H6,H2)对催化剂在氮氧化物储存还原(NSR)循环前后的氮氧化物储存量(NSC)和NO-to-NO2转化率的影响.O2程序升温脱附(O2-TPD)实验结果表明,CO还原后的钙钛矿催化剂上形成了较多的氧空位,而氧空位则是一种有效的NOx储存活性中心.活性测试和傅里叶红外变换(FTIR)光谱表征结果显示:在NSR循环中,以CO为还原剂时催化剂显示了最佳的氮氧化物(NOx)储存效果.进一步的研究结果显示,当采用CO作为还原剂时,经过三次NSR循环后,催化剂中出现了Sr3Fe2O7新物相,而该物相可能具有比La0.7Sr0.3Co0.8Fe0.2O3钙钛矿更佳的NOx储存性能.综上所述,CO作为还原剂时可能使钙钛矿催化剂产生更多的氧空位以及更易于储存NOx的Sr3Fe2O7物相,这些原因使其NOx储存性能得到了大幅度改善.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Characteristics of the La0.6Sr0.4Fe0.8Co0.2O3 − δ electrode in contact with the lanthanum gallate-based electrolyte

Lowering the working temperature of solid oxide fuel cells (SOFCs) is the main trend in their development, which requires selection of materials for electrolyte and electrodes. A highly conducting lanthanum gallate-based electrolyte is a promising material for creating medium-temperature SOFCs. The electrochemical characteristics of the La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 ? δ} cathode that contacted with the La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 electrolyte subject to electrode formation temperatures have been investigated. It was found that at optimum bake-on temperatures of 1200–1250°C, the cathode polarization resistance at 800°C was ～0.08 Ohm cm², which is comparable to the world’s best achievements.     

Study of transition metal oxide doped LaGaO3 as electrode materials for LSGM-based solid oxide fuel cells

Transition metal oxide doped lanthanum gallates, La_0.9Sr_0.1Ga_0.8M_0.2O₃ (where M=Co, Mn, Cr, Fe, or V), are studied as mixed ionic-electronic conductors (MIECs) for electrode applications. The electrochemical properties of these materials in air and in H₂ are characterized using impedance spectroscopy, open cell voltage measurement, and gas permeation measurement. Three single cells based on La_0.9Sr_0.1Ga_0.8 Mg_0.2O₃ (LSGM) electrolyte (1.13 to 1.65 mm thick) but with different electrode materials are studied under identical conditions to characterize the effectiveness of the lanthanum gallate-based MIECs for electrode applications. At 800 °C, a single cell using La_0.9Sr_0.1- Ga_0.8Co_0.2O₃ as the cathode and La_0.9Sr_0.1Ga_0.8Mn_0.2O₃ as the anode shows a maximum power density of 88 mW/cm², which is better than that of a cell using Pt as both electrodes (20 mW/cm²) and that of a cell using La_0.6Sr_0.4CoO₃ (LSC) as the cathode and CeO₂-Ni as the anode (61 mW/cm²) under identical conditions. The performance of LSGM-based fuel cells with MIEC electrodes may be further improved by reducing the electrolyte thickness and by optimizing the microstructures of the electrodes through processing. Received: 9 January 1998 / Accepted: 1 May 1998     

High Temperature Electrochemical Oxidation of Ethanol Over Perovskite-Type Oxides

Ethanol oxidation was investigated in a solid electrolyte electrochemical reactor using a perovskite-type oxide La_0.6Sr_0.2Co_0.8Fe_0.2O₃ as anode. Experiments were conducted between 300–750°C at atmospheric pressure in an ytrria-stabilized-zirconia continuous stirred tank reactor using fuel-rich reactant mixtures.     

Synthesis and characterization of La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>O<Subscript>3±δ</Subscript> nanopowders by microwave assisted sol–gel route

Nano-crystalline La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ powder has been successfully synthesized by microwave assisted sol–gel (MWSG) method. The decomposition and crystallization behavior of the gel-precursor was studied by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis. From the result of FT-IR and X-ray diffraction patterns, it is found that a perovskite La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ was formed by irradiating the precursor at 700 W for 3 min, but the well-crystalline perovskite La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ was obtained at 700 W for 35 min. Morphological and specific area analysis of the powder were done by transmission electron microscopy (TEM), scanning electron microscope (SEM) and Brunauer–Emmett–Teller (BET). The surface areas measured was 38.9 m²/g and the grain size was ∼23 nm. Electrochemical properties of pure LSCF cathode on YSZ electrolyte at intermediate temperatures were investigated by using AC impedance analyzer, which shows a low area specific resistance (0.077 Ω cm² at 1073 K and 0.672 Ω cm² at 953 K). Moreover, the synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.     

Synthesis of Novel Organosiliconsulfur-Containing Tetrasubstituted Imidazoles Sonocatalyzed by LaxSr1−xFeyCo1−yO3 Nanoperovskites

Abstract

The one-pot synthesis of tetrasubstituted imidazoles by use of a series of La_xSr_1 ? xFe_yCo_1 ? yO₃ perovskites as catalysts is described. The La_0.8Sr_0.2Fe_0.34Co_0.66O₃ nanocatalyst had the greatest activity in the heterogeneous cyclocondensation of an aldehyde, benzil, ammonium acetate, and a primary aromatic amine in water under ultrasonic irradiation. Some of the derivatives generated during this work were utilized as substrates for the synthesis in good yields of novel multifunctional tetrasubstituted imidazoles with Me₃Si, C=S, and SH groups, via nucleophilic attack of tris(trimethylsilyl)methyllithium (TsiLi) at the carbon of carbon disulphide.     

固-溶法制备中温固体氧化物燃料电池高性能La0.8Sr0.2MnO3-Ba0.5Sr0.5Co0.8Fe0.2O3阴极

研究了新型固溶法合成La_0.8Sr_0.2MnO₃（LSM）包覆Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃（BSCF）复合粉体（LSM-BSCF）,并探讨了其作为中温固体氧化物燃料电池阴极材料的电化学性能。LSM-BSCF阴极结合了LSM和BSCF阴极的优点,不仅增大了三相界面,而且稳定了微观结构。当温度为600-750℃时,其极化阻抗为0.61-0.09 Ω·cm²。与溶液注入法制备的高性能电极相比,极大地提高了性能稳定性。     

Improvement in stability of La0.4Ba0.6CoO3 cathode by combination with La0.6Sr0.4Co0.2Fe0.8O3 for intermediate temperature-solid oxide fuel cells

Improvement in long-term stability and cathodic activity of La_0.4Ba_0.6CoO₃ (BLC) was studied by mixing with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). LSCF exhibits good long-term stability; however, surface activity is not high like Co-based perovskite. On the other hand, the cathodic activity of BLC is high; however, long-term stability was not so good and large degradation at initial period is observed. Combination of the two oxides shows small overpotential as well as improved long-term stability. Effects of BLC/LSCF ratio on stability and overpotential were studied and it was found that BLC–LSCF (7:3) showed the most stable and small cathodic overpotential among the examined compositions. Although the power density was still slightly decreased over 24 h at 0.5 V terminal voltage, the maximum powder density of the cell using BLC–LSCF composite oxides for cathode shows 2.5 times larger than that of the cell using LSCF cathode and 1.06 times larger than that of BLC. Degradation rate is smaller than 4 % from 5 to 24 h on this BLC–LSCF cathode at current density as high as 682 mA/cm² after 24 h operation.     

Interfacial Enhancement by γ‐Al2O3 of Electrochemical Oxidative Dehydrogenation of Ethane to Ethylene in Solid Oxide Electrolysis Cells

Oxidative dehydrogenation of ethane (ODE) is limited by the facile deep oxidation and potential safety hazards. Now, electrochemical ODE reaction is incorporated into the anode of a solid oxide electrolysis cell, utilizing the oxygen species generated at anode to catalytically convert ethane. By infiltrating γ‐Al₂O₃ onto the surface of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ‐Sm_0.2Ce_0.8O_2‐δ (LSCF‐SDC) anode, the ethylene selectivity reaches as high as 92.5 %, while the highest ethane conversion is up to 29.1 % at 600 °C with optimized current and ethane flow rate. Density functional theory calculations and in situ X‐ray photoelectron spectroscopy characterizations reveal that the Al₂O₃/LSCF interfaces effectively reduce the amount of adsorbed oxygen species, leading to improved ethylene selectivity and stability, and that the formation of Al‐O‐Fe alters the electronic structure of interfacial Fe center with increased density of state around Fermi level and downshift of the empty band, which enhances ethane adsorption and conversion.     

Two types of diffusions at the cathode/electrolyte interface in IT-SOFCs

Analytical transmission electron microscopy, in particular with the combination of energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS), has been performed to investigate the microstructure and microchemistry of the interfacial region between the cathode (La_0.6Sr_0.4Co_0.8Fe_0.2O₃, LSCF) and the electrolyte (Gd-doped ceria, GDC). Two types of diffusions, mutual diffusion between cathode and electrolyte as well as the diffusion along grain boundaries, have been clarified. These diffusions suggest that the chemical stability of LSCF and GDC are not as good as previously reported. The results are more noteworthy if we take into consideration the fact that such interdiffusions occur even during the sintering process of cell preparation.