Phase transition and oxygen permeability of Pr0.6Sr0.4FeO3-δ ceramic membrane at high temperature

Perovskite-type Pr_0.6Sr_0.4FeO_3-δ (PSF) material was prepared by the sol-gel method and systematically evaluated as an oxygen transport membrane (OTM). The material was accompanied by a phase transition with the temperature elevated, and the detailed phase evolution process was accurately detected by the high-temperature in situ X-ray diffraction and the thermogravimetric analysis technologies. The phase transition is related to the oxygen vacancy concentration. The effects of phase transition on structural parameter, oxygen permeability, rate-controlling step, and stability were investigated. The high-temperature cubic phase has a higher thermal expansion coefficient than the orthorhombic phase, which is more favorable for the movement of lattice oxygen. Combining experimental results and oxygen permeation model, demonstrated that the phase transition leads to the rate-controlling step changing from bulk diffusion to oxygen interfacial exchange. Furthermore, the high-temperature cubic phase is beneficial to limit the migration of the Sr ions and ensure higher operation stability.     

CO2-tolerance and oxygen permeability of novel cobalt-free mixed-conductor oxygen-permeable Pr0.6Sr0.4Fe1-xNbxO3-δ membranes

A series of novel cobalt-free dense oxygen-permeable membranes of the type with Pr_0.6Sr_0.4Fe_1-xNb_xO_3-δ (PSFN_x, x = 0–0.1) were synthesized. Subsequently, the effects of Nb-doping on the microstructure, oxygen permeability, and stability of PSFN_x were studied under a pure He or CO₂ atmosphere. The structure of the material did not change in either atmospheres and its stability of the material was enhanced as the level of Nb-doping increased. For the sample with x = 0 and 0.075, carbonates and sulfates were present on the sweep side of the PSF membrane, but no impurities were detected on the sweep side of the PSFN_0.075. In addition, the oxygen-permeation performance exhibited almost no attenuation when the Nb-doping content were 0.075. As revealed by X-ray photoelectron spectroscopy, the CO₂ resistance of the material was enhanced by reducing the basicity of PSFN_x, which was induced by the substitution of Fe with Nb.     

Self-assembled La0.6Sr0.4FeO3-δ-La1.2Sr0.8NiO4+δ composite cathode for protonic ceramic fuel cells

The oxygen reduction reaction at the cathode is an essential process for protonic ceramic fuel cells. Composite cathode materials are commonly used towards the multiple requirements including high surface oxygen activity as well as sufficient electronic and ionic conductivities. In this study, a cobalt-free composite cathode composed of a perovskite La_0.6Sr_0.4FeO_3-δ phase and a Ruddlesden-Popper La_1.2Sr_0.8NiO_4+δ phase is synthesized with a self-assembly technology. The cathode process is mainly controlled by (I) the reduction of adsorbed oxygen atom to O⁻ on the surface and (II) the migration of O⁻ from the surface into the lattice. The former benefits from the high electrical conductivity of La_0.6Sr_0.4FeO_3-δ, and the latter is accelerated by La_1.2Sr_0.8NiO_4+δ attributed to its superior oxygen activity. The one-pot synthesized composite cathode shows an enhanced synergistic effect due to the uniform distribution of the two phases at the nanoscale. The cathode shows the lowest polarization resistances of 0.055 and 0.095 Ω cm² at 700 °C in oxygen and air, respectively. The results show that self-assembled La_0.6Sr_0.4FeO_3-δ-La_1.2Sr_0.8NiO_4+δ nanocomposite is a promising cathode material for protonic ceramic fuel cells.     

Redox stability of La0.6Sr0.4Fe1-xScxO3-δ for tubular solid oxide cells interconnector

Sc-substituted La_0.6Sr_0.4FeO_3-δ (LSFSc) has been synthesized for utilization as an integrated ceramic interconnector of tubular-solid oxide cells (SOCs). Redox stability and electric conductivity of LSFSc were improved by optimizing the scandium (Sc) doping concentration, the pH of the synthetic solutions and the calcination temperature of the organic precursors. The crystalline phases of LSFSc were stable when the pH of the synthetic solution was below 2 and the calcination temperature was over 1200 °C. As the Sc concentration increased, redox stability was improved while the electrical conductivity decreased. To consider the trade-off relationship between electrical conductivity and phase stability, La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ can be considered as one of the stable compositions for an integrated ceramic interconnector of tubular-SOCs.     

制备方法对Pr0.6Sr0.4FeO3-δ结构与性能的影响

采用甘氨酸-硝酸盐、Pechini、柠檬酸-硝酸盐以及尿素-硝酸盐等4种不同的湿化学方法,制备了Pr0.6Sr0.4FeO3-δ复合氧化物粉体.用X射线衍射分析了材料中钙钛矿物相的形成过程及其与中温电解质的化学相容性.用扫描电镜研究了样品的微结构.结果表明:不同方法得到的素坯经1 000℃煅烧2 h即形成钙钛矿结构的固溶体.Pechini法制备的非晶产物煅烧后钙钛矿物相的纯度最高.素坯经1200℃煅烧2 h,所得陶瓷体的总气孔率均为43%～49%;体积密度以柠檬酸-硝酸盐法粉体的样品最高,甘氨酸-硝酸盐法最低.在室温到800℃的温度范围内,Pechini法制备的陶瓷体的热膨胀系数为12.15×10-6/K,与电解质Sm0.2Ce0.8O1.9(SDC)及La0.8Sr0.2Ga0.8Mg0.2O3-δ(LSGM)的数值一致.X射线衍射揭示产物与中温电解质SDC及LSGM具有良好的化学相容性.     

Effects of Al content on the oxygen permeability through dual-phase membrane 60Ce0.9Pr0.1O2-δ-40Pr0.6Sr0.4Fe1-xAlxO3-δ

Ceramic dual-phase oxygen transport membranes with the composition of 60 wt% Ce_0.9Pr_0.1O_2-δ-40 wt%Pr_0.6Sr_0.4Fe_1-xAl_xO_3-δ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0) (60CPO-40PSF_1-xA_xO) based on 60Ce_0.9Pr_0.1O_2-δ-40Pr_0.6Sr_0.4FeO_3-δ doped Al was successfully synthesized through a modified Pechini method. Crystal structure, surface microtopography and oxygen permeability are investigated systematically. The cell parameters of perovskite phase first increased and then decreased with the increase of Al content, which is related to the radius of the Al³⁺ and the formation of impurity phase. As x ranges from 0.1 to 0.8, the oxygen permeability of the materials first increases and then decreases, and the maximum value of oxygen permeation rate for 60CPO-40PSF_1-xA_xO membranes with 0.4 mm thickness at 1000 °C is 1.12 mL min⁻¹ cm⁻² when x = 0.4. XRD measurements revealed high temperature stability and CO₂-tolerant property of the dual-phase composites. The partial replacement of Fe³⁺/Fe⁴⁺ by Al³⁺ causes the material not only to exhibit good stability, but also to increase the oxygen permeability of the membranes.     

阴极材料用Pr0.6-xNdxCa0.4FeO3-δ陶瓷的特性

以Pechini法合成了ABO3型钙钛矿结构的Pr0.6-xNdxCa0.4FeO3-δ(x=01,0.3,0.5)系列稀土复合氧化物粉体.用Fourier变换红外光谱和激光共焦Raman光谱对粉体烧结后样品的化学键及物相进行了表征.用热膨胀仪测定烧结样品的热膨胀系数.通过扫描电镜观察样品用作阴极材料时的微结构及阴极/电解质[钐掺杂氧化铈(samarium-doped ceria,SDC)]复合层的断口形貌.结果表明:1 200℃煅烧2h的样品的主晶相为正交钙钛矿,x=0.3的样品是正交与立方相的混合晶.在室温～1 000 ℃范围内,烧结样品的平均热膨胀系数为12.76x10-6/K,与SDC及La0.8Sr0.2Ga0.85Mg0.15O3-δ(LSGM)电解质的热膨胀系数一致.烧结样品内部孔隙分布均匀,孔隙率约35%,阴极/电解质复合层界面清晰.将复合氧化物粉体和SDC在1 200℃煅烧10h没有检测出第三相.     

High temperature microwave absorbing properties of plasma sprayed La0.6Sr0.4FeO3-δ/MgAl2O4 composite ceramic coatings

Microwave absorbing materials (MAM) which can be used in high temperature and oxidation environments are strongly demanded in application, for conventional MAM fail due to various factors at high temperature. In this work, ceramic coatings composed of La_0.6Sr_0.4FeO_3-δ and MgAl₂O₄ have been successfully prepared via atmospheric plasma spraying and the microwave absorbing properties at high temperature are first reported. When the coatings of LSF/MAS matched with the metal substrate on thermal expansion, they can be treated repeatedly in the temperature range of 300 K–1173 K without peeling off. Meantime, the ceramic coatings with thickness of 1.5 mm showed considerable microwave absorption in the range of 8 GHz–18 GHz at high temperatures between 673 K–1173 K. The absorption bandwidth of LSF30 was 3.5 GHz for RL < −10 dB and 8 GHz for RL < −5 dB in the temperature range of 673 K–1173 K. LSF/MAS possesses the advantages of thin thickness and broad bandwidth at high temperatures, suggesting great potential as MAM in ultra-temperature environment.     

Synthesis,CO2-tolerance and rate-determining step of Nb-doped Ce0.8Gd0.2O2−δ–Pr0.6Sr0.4Co0.5Fe0.5O3−δ ceramic membranes

In this work, CO₂-tolerant Ce_0.8Gd_0.2O_2δ–Pr_0.6Sr_0.4Co_0.5Fe_0.5−xNb_xO_3−δ (CG–PSCF_0.5−xN_x; x=0–0.125) dual-phase dense oxygen permeation membranes were successfully developed. The crystal structure, microstructure, oxygen permeability, rate-determining step and CO₂ tolerance were systematically investigated. The experimental results showed that the increase in CG content improved oxygen permeability and CO₂ tolerance. Thermogravimetry–differential-scanning-calorimetry analysis, X-ray photoelectron spectra and oxygen permeation tests indicated that the increase in Nb content caused a slight decrease in oxygen permeability, while the long-term CO₂ resistance can be improved significantly. According to the adopted permeation model, the weight ratio and thickness affect the oxygen permeability and permeation resistance distribution. By examining the distribution of three permeation resistances, we identified the rate-determining step and then optimized the weight ratio of the two phases, as well exploring the effects of thickness on oxygen permeability. All these experiments confirm that CG–PSCF_0.5−xN_x dual-phase membranes have great CO₂ tolerance and potential application in oxy-fuel combustion.     

Creep behavior of porous La0.6Sr0.4Co0.2Fe0.8O3-δ substrate material for oxygen separation application

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000?°C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.     

Structural and mechanical properties of La0.6Sr0.4M0.1Fe0.9O3-δ (M: Co,Ni and Cu) perovskites

La_0.6Sr_0.4M_0.1Fe_0.9O_3-δ (M: Co, Ni and Cu) perovskite nanostructures were synthesized using low frequency ultrasound assisted synthesis technique and effect of substitution of Fe by Co, Ni and Cu on crystal structure and mechanical properties in La_0.6Sr_0.4FeO_3-δ perovskite was studied. The HRTEM and Rietveld refinement analyses revealed the uniform equi-axial shape of the obtained nanostructures with the existence of La_0.6Sr_0.4M_0.1Fe_0.9O_3−δ with mixed rhombohedral and orthorhombic structures. Substitution by Cu decreases the melting point of La_0.6Sr_0.4FeO_3-δ. The results of mechanical characterizations show that La_0.6Sr_0.4Co_0.1Fe_0.9O_3−δ and La_0.6Sr_0.4Ni_0.1Fe_0.9O_3−δ have ferroelastic behavior and comparable elastic moduli, however, substitution by Ni shows higher hardness and lower fracture toughness than Co in B-site doping.     

Surface Exchange of Oxygen in La1−x Sr x FeO3−δ (x = 0, 0.1)

The electronic charge carrier concentration in La_1?x Sr _x FeO_3?δ was shown to depend on the partial pressure of O₂ (pO ₂). Chemical diffusion coefficient and surface exchange coefficient, k _chem, were determined by conductivity relaxation in O₂/N₂ and CO/CO₂ mixtures. k _chem was proportional to pO ₂ ^1.06 in O₂/N₂, while in CO/CO₂ k _chem was controlled by a reaction mechanism involving both CO and CO₂.     

Tuning Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode to high stability and activity via Ce-doping for ceramic fuel cells

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) fuel-cell cathode stands out because of its ultrahigh ionic conductivity and excellent electrocatalytic activity, but it is still very subject to instability. Here, a new strategy of Ce doping is proposed to boost the stability and activity of the BSCF cathode. A one-pot combustion method is employed to synthesize (Ba_0.5Sr_0.5)_1–xCe_xCo_0.8Fe_0.2O_3-δ (x=0–0.2) cathodes. Both BSCF and (Ba_0.5Sr_0.5)_0.9Ce_0.1Co_0.8Fe_0.2O_3-δ have a cubic perovskite structure. (Ba_0.5Sr_0.5)_0.8Ce_0.2Co_0.8Fe_0.2O_3-δ shows two phases of cubic perovskite and fluorite ceria. Proper Ce doping can boost the electrical conductivity of BSCF, and can dramatically reduce the polarization resistance of BSCF cathode. Ce doping significantly improved BSCF cathode long-term stability by 160 h. Moreover, ten-percent Ce doping in BSCF highly improves single-cell output performance from 516.33 mW cm^?2 to 629.75 mW cm^?2 at 750 °C. The results reveal that Ce doping as a potential strategy for enhancing the stability and activity of BSCF cathode is promising.     

Chemical compatibility of RE0.6M0.4Mn0.8Co0.2O3-δ (RE=La,Pr, Nd,Sm and Gd; M=Sr and Ca) with yttria stabilized zirconia

The chemical compatibility of one new series, RE_0.6M_0.4Mn_0.8Co_0.2O_3-δ (RE=La, Pr, Nd, Sm and Gd; M=Sr and Ca), with yttria stabilized zirconia (YSZ) has been examined. Powder mixtures of perovskites and YSZ were annealed at 1200° for 24h in air. RE_0.6M_0.4Mn_0.8Co_0.2O_3-δ showed different compatibility towards YSZ for M=Sr and Ca, respectively. The formation of SrZrO₃ was identified by X-ray diffraction analysis as a reaction product for RE_0.6Sr_0.4Mn_0.8Co_0.2O_3-δ. In contrary, there is no reaction product for RE_0.6Ca_0.4Mn_0.8Co_0.2O_3-δ. EDX analysis revealed that there is a larger difference between the solubility of Sr and Ca in YSZ. The bond-valence model was used to discuss the different compatibility of Sr and Ca series perovskites with YSZ. The present studies also showed that the formation of SrZrO₃ was favored for larger disorder effect due to size difference between A-site RE³⁺ and Sr²⁺ in RE_0.6Sr_0.4Mn_0.8Co_0.2O_3-δ.     

Effects of Sr doping on high temperature electric properties of CaMnO3+δ - based compounds

Ca_1－xSr_xMnO₃（x=0,0.05,0.1）ceramic samples were prepared by using the sol-gel method and sintering in air with nitrales as starting materials. The phase composition,microstructure and electric properties of Sr doped samples were investigated. The results showed that the Ca_1－xSr_xMnO₃ ceramic samples was single phase and had condensed microstructure. Sr doped samples had better electric properties. The substitution of Sr for Ca had no significant effect on Seabeek coefficient but decreased its electrical resistivity,the highest power factor was 24.03×10^-5 W/（m·K²）at 600℃ for the sample of Ca_0.9Sr_0.1MnO₃.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Semiconductor ionic based Sm0.2Ce0.8O2−δ-La0.6Sr0.4Fe0.8Cu0.2O3-δ heterostructure for low temperature ceramic fuel cells

Structural design/doping strategy is an efficient method to prepare electrolytes with high oxygen ionic conductivity, but there is still hindrance for solid oxide fuel cell (SOFC) commercialization. Recent advances in semiconductor ionic materials have developed a novel strategy in designing low-temperature electrolyte materials. Here, a heterostructure composite of LSFC (La_0.6Sr_0.4Fe_0.8Cu_0.2O_3-δ) and SDC (Sm_0.2Ce_0.8O_2?δ) is developed. The LSFC-SDC composite exhibits a high ionic conductivity, >0.1S/cm at 550 °C. With symmetrical NCAL (Ni_0.8Co_0.15Al_0.05LiO_2-δ)-coated electrode, cells with SDC-LSFC electrolyte exhibit high open-circuit voltage (OCV), and achieve a significant power improvement (>1000 mW/cm²) compared with pure SDC electrolyte at 550 °C. The short-term stability result has proven the operating ability of SDC-LSFC electrolyte under fuel cell environment (H₂/air). This work demonstrates a new developing route of low-temperature solid oxide fuel cell (LTSOFC), which is different from the conventional SOFC.     

Improved electrochemical performance of Bi doped La0.8Sr0.2FeO3-δ nanofiber cathode for IT-SOFCs via electrospinning

In this study, perovskite La_0.8-xBi_xSr_0.2FeO_3-δ (LBSF, x = 0.0–0.5) nanofibers with great crystallinity were prepared by electrospinning method and used as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). The symmetric cells of nanofiber-based LBSF electrode on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte show excellent electrochemical performance. The La_0.4Bi_0.4Sr_0.2FeO_3-δ (LBSF4) cathode has the best performance with a polarization resistance (R_P) of 0.126 Ω cm² at 650 °C. The anode-supported single cell with LBSF4 as the cathode film and Ni-SDC as the anode has a maximum power density of 448 mW cm^-2 at 650 °C using wet H₂ as the fuel. In addition, the LBSF4 cathode with fibrous structure exhibits outstanding electrochemical behavior. The catalytic activity of the cathode was improved due to the incorporation of the Bi element, indicating that LBSF4 is promising as a cathode material in the field of IT-SOFCs.     

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.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part I: Influence on oxygen permeation,electrical properties,reductive stability,and lattice parameters

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840?°C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y³⁺ was doped in small concentrations (1–10?mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20–30%), a doping concentration of 10?mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800?°C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.