Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

The linear thermal expansion coefficients (TECs) of perovskite-type La_0.3Sr_0.7Fe_1−xGa_xO_3−δ (x=0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10⁻⁶ K⁻¹ at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10⁻⁴ to 1 atm, the isothermal chemically induced expansion of La_0.3Sr_0.7Fe(Ga)O_3−δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, (ε_C/Δδ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10⁻². The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La_0.3Sr_0.7Fe_1−xGa_xO_3−δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.     

Microstructural analysis of highly active cathode material La0.7Sr0.3Ti0.15Fe0.65Ni0.2O3-δ (LSTFN) by optimizing different processing parameters

The modified Pechini method was applied to prepare a highly active and novel cathode material La_0.7Sr_0.3Ti_0.15Fe_0.65Ni_0.2O_3-δ (LSTFN). This material was coated on the LGSM electrolyte through a screen-printing technique with variable thicknesses of 28 ± 8, 41 ± 8, and 62 ± 8 μm, respectively. Different fabrication parameters, including sintering temperature, time, coating thickness, and variations in ball-milling, which affect the electrochemical performance of the cathode material, were investigated. X-ray diffraction analysis of the cathode material suggested that it exhibits a cubic crystal structure with a LSTFN single phase. The morphological studies were conducted using scanning electron microscopy (SEM), which confirmed that the electrode material had a highly porous structure. Meanwhile, the electrochemical properties of the material were studied by electrochemical impedance spectroscopy (EIS), which revealed that by varying different parameters, the electrochemical performance of the electrode material was enhanced. The coated cathode materials with variable thicknesses were analyzed at different sintering temperatures and times. Experimental results suggest that the optimum sintering temperature and time were 950 °C and 3 h, respectively, at which LSTFN exhibits the minimum polarization resistance (R_P) of 0.046 Ωcm² when sintered at 800 °C for 3 h.     

Catalytic activity improvement for efficient hydrogen oxidation of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3-δ anode for solid oxide fuel cell

In this paper, a ceramic perovskite anode for solid oxide fuel cells (SOFCs) is prepared by infiltrating La and Fe co-doped strontium titanium, La_0.3Sr_0.7Ti_0.3Fe_0.7O_3-δ (LSTF0.7) into porous backbone of scandia-stabilized zirconia (ScSZ) and tested in pure H₂ at 700–850 °C. LSTF0.7 crystal exhibits high reduction stability and good compatibility with ScSZ electrolyte under reducing atmosphere. In order to improve the electrocatalytic activity, 15 wt% of CeO₂ and 7 wt% of Ni are infiltrated into the backbone pores respectively, thus forming LSTF0.7-CeO₂ and Ni-CeO₂-LSTF0.7 composite anodes. The cell with LSTF0.7 single anode shows a relatively lower maximal power density (MPD) of 401 mW cm⁻² in H₂ at 800 °C. While the maximal power densities of the cells with LSTF0.7-CeO₂ and Ni-CeO₂-LSTF0.7 composite anodes are 612 mW cm⁻² and 698 mW cm⁻² operated at the same conditions, respectively. The three anode polarization resistances (Rp,a) distinguished from the corresponding full cells are 0.176, 0.086 and 0.076 Ω cm² at 800 °C, respectively. The values of the activation energy (Ea) towards H₂ oxidation for the three anodes can be calculated to be 52.2, 46.0, 43.9 kJ mol⁻¹ based on their respective Rp,a. Therefore, the LSTF0.7-based anodes are considered to be promising alternatives for solid oxide fuel cell applications.     

Microwave-assisted synthesis and characterization of new cathodic material for solid oxide fuel cells: La0.3Ca0.7Fe0.7Cr0.3O3−δ

In this work, we examine the benefits of alternative powder processing methods, with a primary focus on microwave-based synthesis, that could both lower material manufacturing costs and further enhance cathode performance for solid oxide fuel cell applications. La_0.3Ca_0.7Fe_0.7Cr_0.3O_3−δ (LCFCr), formed using conventional solid-state methods, has been shown in earlier work to be a very promising catalyst for the oxygen reduction reaction. To further increase its performance, microwave methods were used to increase the surface area of LCFCr and to decrease the synthesis time. It was found that the material could be obtained in crystalline form in only 7 h, with the synthesis temperature lowered by roughly 300 °C as compared to conventional methods.     

Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and SrCo0.8Fe0.2O3−δ membranes

The effect of grain size on oxygen permeation properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) and SrCo_0.8Fe_0.2O_3?δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo_0.8Fe_0.2O_2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

Ce掺杂La0.7Sr0.3Cr0.5Fe0.5O3-δ材料的制备及其电化学性能

近年来化石燃料大量消耗导致环境污染日益严重,固体氧化物电解池(SOEC)能够高效、环境友好地将CO₂转化为CO等高附加值化学品,因此受到广泛关注。开发高效稳定的SOEC需要采用性能优异的电极材料,La_0.7Sr_0.3Cr_0.5Fe_0.5O_3-δ(Sto-LSCrF)钙钛矿氧化物因其优异的氧化还原稳定性受到了高度重视。为进一步提高Sto-LSCrF燃料电极材料电解CO₂的能力,在Sto-LSCrF的A位掺杂Ce来调控Ce_0.08La_0.62Sr_0.3Cr_0.5Fe_0.5O_3-δ(Ce-LSCrF)中可移动氧空穴含量以便提高其对CO₂的吸附/活化能力,进而改善其电化学性能。同时对材料的相结构、氧空穴含量以及其对CO₂的吸附/脱附能力进行详细的表征和分析。此外,我们还探究了Ce-LSCrF的电化学性能,发现与Sto-LSCrF相比,Ce-LSCrF燃料电极表现出较高的电解性能,也显示出较好的恒压稳定性,电解性能的增强归因于Ce-LSCrF晶格中较多的可移动氧空位可有效吸附/活化CO₂,以上试验结果表明Ce-LSCrF是性能优异的CO₂电解材料。     

Preparation and characterization of La0.7Sr0.3Cr1-xFexO3-δ anode catalyst with sulfur tolerance for SOFC

采用尿素燃烧法制备了La0.7Sr0.3Cr1-xFexO3-δ（x=0.2,0.3,0.4,0.5）系列阳极材料。采用TG-DTA、XRD对所制备的材料进行表征,利用直流四探针法测定催化剂在400～850 ℃温度时在空气中的电导率,并测试了电池电化学性能。结果表明：LSCrF系列材料具有很好的热稳定性,钙钛矿相晶型结构完整,与BaCe0.475Zr0.425Y0.1O3-δ电解质之间具有较好的化学相容性。同时,XRD显示LSCrF系列材料在硫化氢气氛中具有很好的化学稳定性。LSCrF7355在850℃的电导率最大为1.18 S/cm,其单电池的开路电压值为0.76 V,最大输出功率为7.16 mW/cm2,满足固体氧化物燃料电池阳极催化剂的要求,是一种新型的耐硫阳极材料。     

A medium-entropy perovskite oxide La0.7Sr0.3Co0.25Fe0.25Ni0.25Mn0.25O3-δ as intermediate temperature solid oxide fuel cells cathode material

In this study, we report a novel medium-entropy perovskite oxide of La_0.7Sr_0.3Co_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM73) with high constitutive entropy (S_config) as the cathode material of intermediate temperature solid oxide fuel cells (IT-SOFCs). The intrinsic properties of phase structure, electrical conductivity, thermal expansion and oxygen adsorption capacity of La_1-xSr_xCo_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM, x = 0, 0.1, 0.2, 0.3) oxides are evaluated in detail. The LSCFNM73 oxide exhibits the maximum electrical conductivity of 464 S cm⁻¹ at 800 °C and a relatively lower thermal expansion coefficient (TEC) of 15.34 × 10⁻⁶ K⁻¹, which is selected as the propriate cathode composition. The B-site of LSCFNM73 contains four elements which can increase the configuration entropy. Additionally, NiO-Yttria stabilized zirconia (YSZ) supported fuel cell is fabricated by tape casting, hot pressing-lamination, co-sintering and screen printing technologies. The fuel cell demonstrates a maximum power density of 1088 mW cm² at 800 °C, and excellent stability at 750 °C under 0.75V in 120 h and 10 times thermal cycling between 750 °C and 400 °C. Therefore, the medium-entropy LSCFNM73 oxide can be applied in IT-SOFCs as a competitive cathode material.     

Ferroelastic deformation of La0.58Sr0.4Co0.2Fe0.8O3−δ under uniaxial compressive loading

The mechanical deformation of lanthanum strontium cobalt ferrite under uniaxial compression was investigated at various temperatures. The material revealed a rather complex mechanical behaviour related to its ferroelasticity and stress–strain curves obtained in the 1st and 2nd loading cycles were completely different. A distinctive ferroelastic creep was observed at 293 K whilst typical ferroelastic stress–strain curves were obtained in the temperature range from 473 K to 873 K. At 1073 K, high-temperature creep deformation was observed instead of the ferroelastic deformation. The apparent Young's modulus was evaluated in various ways; the modulus determined from the last unloading curve ranged between 85 and 120 GPa. The obtained critical stress monotonically decreases from about 80 MPa to zero with increasing temperature, corresponding to the behaviour of the remnant strain. The presented results indicate that the importance of an appropriate consideration of the loading history in the practical application of these ferroelastic materials.     

Methane oxidation on the surface of mixed-conducting La0.3Sr0.7Co0.8Ga0.2O3-δ

Mixed-conducting La_0.3Sr_0.7Co_0.8Ga_0.2O_3-δ (LSCG) possesses substantial oxygen permeability, but exhibits a high activity to complete CH₄ oxidation, thus making it necessary to incorporate reforming catalysts in the membrane reactors for methane conversion. Dominant CO₂ formation is observed for the steady-state conversion of CH₄ by atmospheric oxygen (methane/air ratio of 30:70) in a fixed bed reactor with LSCG as catalyst, and for the oxidation of CH₄ pulses supplied in helium flow over LSCG powder. The conversion of dry CH₄ by oxygen permeating through dense LSCG ceramics, stable operation of which under the air/CH₄ gradient is possible due to the surface-limited oxygen transport, yields CO₂ concentrations higher than 90%. The prevailing mechanism of total methane combustion is probably associated with weak Co–O bonding in the perovskite-related LSCG lattice, in correlation with data on oxygen desorption, phase stability and ionic transport.     

Compatibility and conductivity of La2Ni1−xFexO4+δ and LaNi0·6Fe0·4O3−δ with GDC electrolyte

The compatibilities and conductivities of K₂NiF₄ typed La₂Ni_0·9Fe_0·1O_4+δ (L₂NF91) and LaNi_0·6Fe_0·4O_3?δ (LNF64) perovskites, promising cathode materials for solid oxide fuel cell, with Gd_0·1Ce_0·9O_1·95 (GDC) electrolyte were investigated. L₂NF91 and LNF64 were synthesised using citrate and modified citrate methods with the calcination temperature of 1000°C for 5 h. The single phased oxides with the average particle sizes of L₂NF91 and LNF64 ～0·2 μm were obtained. The thermal expansion coefficients of L₂NF91 and LNF64 were 12·7×10^?6 and 13·2×10^?6 K^?1 respectively. The mixtures of cathode materials and the electrolytes were heated between 800 and 1200°C to observe the formation of secondary phases at the operation temperatures of solid oxide fuel cell. The X-ray diffraction and scanning electron microscopy–energy dispersive X-ray results indicated that L₂NF91 and LNF64 had good chemical compatibility with GDC from room temperature up to 900°C. Both L₂NF91 and LNF64 showed higher conductivities when in contact with GDC electrolyte than with Zr_0·92Y_0·08O_1·96 electrolyte.     

Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode by microwave-assisted combustion method

Thermogravimetry, phase formation, microstructural evolution, specific surface area, and electrical properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode were studied as functions of its preparation technique. The pure perovskite LSCF cathode powder was synthesized through glycine–nitrate process (GNP) using microwave heating technique. Compared with conventional heating technique, microwave heating allows the rapid combustion to occur simultaneously between the nitrates and glycine in a controllable manner. The resulting powder is a single-phase nanocrystallite with a mean particle size of 113 nm and a high specific surface area of 12.2 m²/g, after calcination at 800 °C. Impedance analysis indicates that microwave heating has significantly reduced the polarization resistance of LSCF cathode. The area specific resistance (ASR) value of 0.059 and 0.097 Ω cm² at 800 °C and 750 °C, respectively, were observed. These values were twofold lower than the corresponding ASR of the cathode (0.133 and 0.259 Ω cm² at 800 °C and 750 °C, respectively) prepared through conventional heating. Results suggest that the microwave heating GNP strongly contributes to the enhancement of the LSCF cathode performance for intermediate temperature solid oxide fuel cells.     

Towards the fabrication of La0.98−xSrxCo0.2Fe0.8O3−δ perovskite-type oxygen transport membranes

La_0.98−xSr_xCo_0.2Fe_0.8O_3−δ (LSCF) is a candidate material for use as an oxygen transport membrane (OTM). In this work, fabrication-relevant properties (sintering behaviour, thermal and chemical expansion) of LSCF (x = 0.2, 0.4, 0.6, 0.8) were investigated in order to select the preferred composition for fabricating a thin-film supported membrane able to withstand the thermochemical stresses encountered during manufacturing and operation with simultaneously high oxygen permeation flux.Partial substitution of La by Sr ions in LSCF is beneficial for increasing the oxygen permeation rate, but it causes drawbacks regarding manufacturing and operation. A Sr content of x ≥ 0.6 results in a swelling of the material during sintering, which complicates the manufacturing of thin, leak-free membranes. This swelling is related to oxygen release during heating, combined with the formation of a liquid phase above 1200 °C. Furthermore, an increase in total strain with Sr content is observed. This is caused by the chemical expansion, while there is no significant change in thermal expansion with increasing Sr content.The compositions x = 0.4 and x = 0.6 showed tolerable expansion coefficients as well as adequate sintering behaviour and were therefore selected for the fabrication of thin supported membranes. These supported membranes with a thickness of 30 μm were manufactured by sequential tape casting and characterised regarding microstructure and oxygen flux.     

La0.7Sr0.3Ga0.6Fe0.4O3-δ氧离子导体材料的制备与表征

以相应的金属氧化物和盐为原料,通过甘氨酸-硝酸盐法(GNP)合成出La0.7Sr0.3Ga06Fe04O3-δ(LSGF)粉末,经压制、烧结后,得到LSGF烧结体试样.采用X射线衍射仪(XRD)对所得前驱体及其煅烧后粉体的相组成进行分析;采用扫描电镜(SEM)及X射线冠电子能谱仪(EDS)对烧结体的微观组织和成分进行了观察,还对烧结体的致密度、电导率和热膨胀系数等进行了测试分析.实验结果表明,甘氨酸-硝酸盐法所制备的前驱体粉末在700℃煅烧5h后可获得具有单一钙钛矿结构的LSGF粉末,其晶粒尺寸在45 nm左右,所制备的LSGF烧结体在400～900℃范围的电导率在3.97～4.42 S·cm-1之间,热膨胀系数在9.2 ×10-6 ～11.12 ×10-6 K-1之间.     

Symmetric and asymmetric membranes based on La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite with high oxygen semipermeation flux performances and identification of the rate-determining step of oxygen transport

This work focuses on identifying the rate-determining step of oxygen transport through La_0.5Sr_0.5Fe_0.7Ga_0.3O_3-δ membranes with symmetric and asymmetric architectures. The best oxygen semipermeation fluxes are 3.4 10⁻³ mol. m^-2.s^-1 and 6.3 10⁻³ mol. m^-2.s^-1 at 900 °C for the symmetric membrane and asymmetric membrane with a modified surface. The asymmetric membrane with a modified surface leads to an increase of approximately 7 times the oxygen flux compared to that obtained with the La_0.5Sr_0.5Fe_0.7Ga_0.3O_3-δ dense membrane without surface modification. This work also shows that the oxygen flux is mainly governed by gaseous oxygen diffusion through the porous support of asymmetric La_0.5Sr_0.5Fe_0.7Ga_0.3O_3-δ membranes.     

Mechanical behavior of ferroelastic porous La0.6Sr0.4Co0.2Fe0.8O3−δ prepared with different pore formers

The present work investigated the mechanical behavior of porous La_0.6Sr_0.4Co_0.2 Fe_0.8O_3−δ LSCF under uniaxial compression. The porous (LSCF) samples with the same grain size but different porous structures with 1.5–41% of porosity were prepared using three different pore formers. All the samples had ferroelastic domains and exhibited ferroelastic mechanical behaviors under uniaxial compression. Initial and loading moduli as well as critical stress monotonically decreased and remnant strain increased with increasing the porosity. The initial modulus can be determined by the actual porosity regardless of porous structure or grain size, whereas the other properties were more sensitive to experimental condition such as loading rate and maximum applied stress. Compressive fracture strength could be significantly influenced by porous structure.     

Electrical properties of (La0.9Ca0.1)(Co1−xNix)O3−δ cathode materials for SOFCs

Modified perovskite ceramics (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ (x = 0–0.3) cathodes for solid oxide fuel cells (SOFCs) were synthesized by solid state reaction. The lattice parameters, electrical conductivity, activation energy, and microstructures of these specimens were investigated systematically in this study. The results exhibited that all specimens are rhombohedron structures and their tolerance factors were greater than 0.97, indicating that the perovskite was not distorted by Ni²⁺ cation substitution for the B site of (La_0.9Ca_0.1)CoO_3?δ. The microstructures of the (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ specimens showed good densification, and were well-sintered, with few pores. The electrical conductivity behavior conformed to the nature of a semiconductor, for all specimens. As x = 0.1, the electrical conductivity reached the maximum value of 750.3 S/cm at 800 °C, and the activation energy calculated from the Arrhenius plot of the electrical conductivity versus the reciprocal of temperature is 7.1 kJ/mol.The novelty of this study is its introduction of the concept of defect chemistry to explain the relationship between compensation mechanisms and electrical conductivity. The information gleaned regarding charge compensation mechanisms and defect formation may be valuable for a better understanding of the cathode of (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ ceramics used for SOFCs. Moreover, the information about oxygen content versus temperature is useful for expressing the relationship between electrical conductivity and composition. Therefore, we also used thermogravimetric analysis combined with the room-temperature oxygen content which was determined by iodometric titration to investigate the oxygen content from room temperature to high temperature, in air. Based on the experimental results, the (La_0.9Ca_0.1)(Co_0.9Ni_0.1)O_3?δ specimen shows high electrical conductivity. Consequently, it is identified as a promising candidate for cathode SOFC applications.     

La0.7Sr0.3CrO3粉体粒度影响因素的研究

采用溶胶－凝胶法合成了Ｌａ１－ｘＳｒｘＣｒＯ３粉体。对于合成的Ｌａ０．７Ｓｒ０．３ＣｒＯ３粉体,着重从混合溶剂柠檬酸与乙二醇的质量比和煅烧温度等方面,讨论了影响高温燃料电池联接材料粉体粒度的主要因素。最终得出如下结论：柠檬酸与乙二醇的质量比为１：１．２,煅烧温度为８００℃时粉体粒度相对最小,平均为３２ｎｍ。