溶胶-自蔓延法制备La0.8Sr0.2Mn0.8Ni0.2O3催化剂的优化条件

采用溶胶-凝胶-自蔓延法合成较大比表面积的钙钛矿型复合氧化物La_0.8Sr_0.2Mn_0.8Ni_0.2O₃催化剂,研究了pH、苹果酸用量和焙烧温度对催化剂的影响,通过XRD、BET和SEM等分析手段对催化剂进行了表征。结果表明,pH=3、苹果酸用量为金属离子总物质的量2倍和焙烧温度600 ℃为最佳制备工艺条件。催化剂在低温对CO催化燃烧显示出良好的催化活性。     

Phase formation between La(Sr)Ga(Mg)O3 and Ce(La)O2 for solid oxide fuel cell applications

In strontium- and magnesium-doped LaGaO₃ (LSGM) electrolyte-based solid oxide fuel cells (SOFC), lanthanum-doped CeO₂ (LDC) is usually used as buffer layer material to prevent reactions between LSGM electrolyte and NiO-based anode. In literature, based on results for one particular LSGM composition, a fixed buffer layer composition of 40% La-doped ceria (LDC40) has been used even with electrolytes of different LSGM compositions. In this study, we report the results of a comprehensive study of phase formations between various LSGM and LDC compositions. Our results show that only one LSGM/LDC combination results in no additional phases. For the other combinations, at least one and often two additional phases, LaSrGaO₄ and LaSrGa₃O₇, result. Because LaSrGa₃O₇ has much lower conductivity, it is necessary to select combinations that avoid this phase. We propose that the combination that results in no additional phase should be considered favorably for SOFCs. For other LSGM compositions, LDC50 should be used as a buffer layer instead of LDC40 as is presently done in SOFC studies. Alternately, if LDC40 is preferred for buffer layer, then lower Sr content LSGM compositions should be used as electrolytes. These combinations are likely to lead to better long-term SOFC performance.     

Microstructure and electrical conductivity of La0.9Sr0.1 Ga0.8Mg0.2O2.85-Ce0.8Gd0.2O1.9 composite electrolytes for SOFCs

(100-x) wt.% La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 - x wt.% Ce_0.8Gd_0.2O_1.9 (x = 0, 5, 10, 20) electrolytes were prepared by solid-state reaction. The composition, microstructure, and electrical conductivity of the samples were investigated. At 300 ~ 600°C, the pure La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 electrolyte has a higher conductivity compared to the composite electrolytes, but at 650 ~ 800°C the 95 wt.% La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 - 5 wt.% Ce_0.8Gd_0.2O_1.9 composite electrolyte presents the highest conductivity, reaching 0.035 S cm⁻¹ at 800°C. The cell performances based on La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85-Ce_0.8Gd_0.2O_1.9 electrolytes were measured using Sr₂CoMoO₆-La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 as anode and Sr₂Co_0.9Mn_0.1NbO₆ -La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 as cathode, respectively. At 800°C, the measured open-circuit voltages are higher than 1.08 V, and the maximum power density and current density of the fuel cell prepared with 95 wt.% La_0.9Sr_0.1 Ga_0.8Mg_0.2O_2.85 - 5 wt.% Ce_0.8Gd_0.2O_1.9 electrolyte reach 192 mW cm⁻² and 720 mA cm⁻², respectively.     

Fabrication of Solid Oxide Fuel Cells with a Thin (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O3–δ Electrolyte on a Sr0.8La0.2TiO3 Support

This paper describes Sr_0.8La_0.2TiO₃ (SLT)‐supported solid oxide fuel cells with a thin (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3–δ (LSGM) electrolyte and porous LSGM anode functional layer (AFL). Optimized processing for the SLT support bisque firing, LSGM electrolyte layer co‐firing, and LSGM AFL colloidal composition is presented. Cells without a functional layer yielded a power density of 228 mW cm^–2 at 650 °C, while cells with a porous LSGM functional layer yielded a power density of 434 mW cm^–2 at 650 °C. Cells with an AFL yielded a higher open circuit voltage, possibly due to reduced Ti diffusion into the electrolyte. Infiltration produced Ni nanoparticles within the support and AFL, which proved crucial for the electrochemical activity of the anode. Power densities increased with increasing Ni loadings, reaching 514 mW cm^–2 at 650 °C for 5.1 vol.% Ni loading. Electrochemical impedance spectroscopy analysis indicated that the cell resistance was dominated by the cathode and electrolyte resistance with the anode resistance being relatively small.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 powder by gel combustion route with two-step doping strategy

A two-step doping strategy was applied to the synthesis of La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM1020) powder by a gel combustion method. The Mg-doped LaGaO₃ powder was prepared in the first step, and Sr incorporation in the Mg-doped LaGaO₃ powder was done in the second step to obtain the final LSGM1020 powder. The two-step procedure is effective in preparing higher purity powders than the traditional one-step procedure. Rietveld refinement of X-ray powder diffraction (XRD) patterns shows that incorporation of Mg in LaGaO₃ in the first step enlarges the LaGaO₃ lattice: this facilitates the incorporation of Sr in the second doping step and thus high purity powder is obtained. Relatively phase pure LSGM1020 powder with only 3.1% of LaSrGaO₄ was obtained after calcination at 1300 °C for 5 h. Therefore, the two-step doping strategy is an effective procedure for the preparation of LSGM powders with high Sr- and Mg-doping levels.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 by successive freeze-drying and self-ignition of a hydroxypropylmethyl cellulose solution

The present paper reports the synthesis of La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 perovskite powders by a method combining freeze-drying and self-ignition of an aqueous solution of metallic nitrates containing hydroxypropylmethyl cellulose. The precursor powder obtained after self-ignition was submitted to various thermal treatments and the resulting powders were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption–desorption isotherm analysis, mercury porosimetry and laser granulometry. It turns out that this synthesis method yields single-phase powders with good homogeneity and sinterability properties. The precursor powder treated at 1200 °C presents a coral-like structure which collapses under application of low uniaxial pressure, resulting in a narrow grain size distribution suitable for sintering (98.8% relative density for a pellet sintered at 1400 °C during 1 h). The fact that no milling step is necessary is an additional advantage of this method, which shows promising prospects for the synthesis of other multicationic oxides.     

La0.6Sr0.4Co0.2Fe0.8O3 as the anode and cathode for intermediate temperature solid oxide fuel cells

The perovskite material La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF-6428) has been considered as both the anode and cathode in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (550–700°C). Solid electrolyte coulometry (SEC) has been used to measure the oxygen non-stoichiometry as a function of temperature and ambient oxygen partial pressure, thus enabling kinetic data relating to oxygen transport in cathodes to be correlated with the material oxygen vacancy concentration. The catalytic activity towards methane oxidation, and susceptibility to deactivation through carbon deposition have both been investigated by temperature programmed methods, and compared with data for the conventional Ni/YSZ anode material.     

Influence of Microwave‐Assisted Pechini Method on La0.80Sr0.20Ga0.83Mg0.17O3–δ Ionic Conductivity

With the aim of investigating the microwave influence on the electrolyte material properties, La_0.80Sr_0.20Ga_0.83Mg_0.17O_2.815 was prepared by both a conventional and a microwave‐assisted sol–gel Pechini method. With respect to the conventional Pechini method (hereafter SGP), the microwave assisted process (hereafter MWA‐SGP) guaranteed a faster procedure, reducing the time needed to remove the excess solvents to complete the polyesterification reaction from some days to a few hours. In fact, when a MWA‐SGP method was used, powders having higher phase purity were obtained. The sintering process at 1,450 °C of the powders prepared by both methods yielded pellets with similar density values (≥92% of theoretical). Nevertheless, only by microwave‐assisted process single‐phase products were obtained and no secondary phases such as tetragonal LaSrGaO₄ and LaSrGa₃O₇ were detected. These by‐products have been demonstrated to be detrimental for conductivity. Indeed, pellets obtained by MWA‐SGP method showed oxygen ionic conductivity values higher (about 30–40%) than those checked for SGP samples, thus demonstrating the important role of the microwave process on reducing time and costs and on improving the electrolyte properties.     

Perovskite-type oxide ACo0.8Bi0.2O2.87 (A=La0.8Ba0.2): a catalyst for low-temperature CO oxidation

Perovskite-type oxide ACo_0.8Bi_0.2O_2.87 (A=La_0.8Ba_0.2) has been investigated as a catalyst for the oxidation of carbon monoxide. X-ray diffraction results revealed that the catalyst is single-phase and cubic in structure. The results of chemical analysis indicated that in ACo_0.8Bi_0.2O_2.87, bismuth is pentavalent whereas cobalt is trivalent as well as bivalent; in La_0.8Ba_0.2CoO_2.94, cobalt ions exist as Co³⁺ and Co⁴⁺. The substitution of Bi for Co enhanced the catalytic activity of the perovskite-type oxide significantly. Over the Bi-incorporated catalyst, at equal space velocities and with the rise in CO/O₂ molar ratio, the temperature for 100% CO conversion shifted to a higher range; at a typical space velocity of 30000 h^–1 and a CO/O₂ molar ratio of 0.67/1.00, 100% CO conversion was observed at 250°C. Over ACo_0.8Bi_0.2O_2.87, at equal CO/O₂ molar ratio, the temperature for 100% CO conversion decreased with a drop in space velocity; the lowest being 190°C at a space velocity of 5000 h^–1. The result of O₂-TPD study illustrated that the presence of Bi ions caused the lattice oxygen of La_0.8Ba_0.2CoO_3– to desorb at a lower temperature. The results of TPR, ¹⁸O/¹⁶O isotopic exchange, and CO-pulsing investigations demonstrated that the lattice oxygen of the Bi-doped catalyst is highly mobile.     

The Effect of Plasma Spraying Power on La0.58Sr0.4Co0.2Fe0.8O3–δ–La0.8Sr0.2Ga0.8Mg0.2O3–δ Composite Cathode Interlayer Microstructure and Cell Performance

The metal‐supported intermediate temperature solid oxide fuel cells with a porous nickel substrate, a nano‐structured LDC (Ce_0.55La_0.45O_2–δ)–Ni composite anode, an LDC diffusion barrier layer, an LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ) electrolyte, an LSCF (La_0.58Sr_0.4Co_0.2Fe_0.8O_3–δ)–LSGM composite cathode interlayer and an LSCF cathode current collector are fabricated by atmospheric plasma spraying. Four different plasma spraying powers of 26, 28, 30, and 34 kW are used to fabricate the LSCF–LSGM composite cathode interlayers. Each cell with a prepared LSCF–LSGM composite cathode interlayer has been post‐heat treated at 960 °C for 2 h in air with an applied pressure of 450 g cm^–2. The current‐voltage‐power and AC impedance measurements indicate that the LSCF–LSGM composite cathode interlayer formed at 28 kW plasma spraying power has the best power performance and the smallest polarization resistance at temperatures from 600 to 800 °C. The microstructure of the LSCF–LSGM composite cathode interlayer shows to be less dense and composed of smaller dense regions as the plasma spraying power decreases to 28 kW. The durability test of the cell with an optimized LSCF–LSGM composite cathode interlayer gives a degradation rate of 1.1% kh^–1 at the 0.3 A cm^–2 constant current density and 750 °C test temperature.     

Mechanism and kinetic modeling for steam reforming of toluene on La0.8Sr0.2Ni0.8Fe0.2O3 catalyst

Reaction mechanism for steam reforming of toluene is proposed for La_0.8Sr_0.2Ni_0.8Fe_0.2O₃ perovskite catalyst. The proposed mechanism was derived from various characterization results such as temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) water, TPSR toluene, TPD O₂ and in situ DRIFT of toluene decomposition, and steam reforming of toluene. Five kinetic models were developed based on the proposed dual‐site reaction mechanism using Langmuir–Hinshelwood approach. Subsequently, the parameters of the kinetic models were estimated by nonlinear least square regression. A good agreement was obtained between experimental and model predicted results for the rate determining step based on reaction between adsorbed aldehyde and adsorbed oxygen. The adsorbed aldehyde species is produced from the reaction between adsorbed C₂H₂ or CH₂ and adsorbed oxygen while the adsorbed oxygen species can come from the oxygen from water activation, lattice oxygen species, and/or the redox property of some metals such as Fe. This shows that the adsorbed oxygen species plays important role in this reaction. © 2014 American Institute of Chemical Engineers AIChE J 60: 4190–4198, 2014     

用柠檬酸-硝酸盐合成La0.8Sr0.2Ga0.85Mg0.15O3-δ及其性能

用柠檬酸-硝酸盐水系溶液为前驱体合成了具有钙钛矿结构的中温电解质La_(0.8)Sr_(0.2)Ga_(0.85)Mg_(0.15)O_(3-s-δ)(LSGM)。用DTA-TGA和X射线衍射仪分析了LSGM材料中钙钛矿相的形成过程,用热膨胀仪和交流复阻抗谱研究了样品的烧结、热膨胀和电学性能。研究结果表明:用柠檬酸-硝酸盐溶液制备LSGM所得到的非晶产物在800℃时开始形成钙钛矿相,1400℃烧结6 h已经完全转变成稳定钙钛矿相,LSGM样品在1450℃烧结6 h,相对密度已经达到98％。1450℃烧结6 h的LSGM样品阻抗谱研究表明:与固相法制备的LSGM相比,用柠檬酸-硝酸盐溶液合成的LSGM晶界电阻和杂相电阻都很小,不影响样品的电导。表明用湿化学法合成LSGM有利于提高纯度,改善导电性能。850℃时样品的电导率为6.0×10~(-2)S/cm,900℃时单电池的最大输出功率密度为12.2 mW/cm~2,短路电流密度达刭55.2 mA/cm~2。     

Synthesis and characterization of La0.85Sr0.15Ga0.85Mg0.15O3−δ electrolyte by steric entrapment synthesis method

Strontium and magnesium doped lanthanum gallate La_0.85Sr_0.15Ga_0.85Mg_0.15O_3−δ (LSGM) oxygen ionic conducting ceramics were prepared by a steric entrapment synthesis (SES) method, which is a polymeric precursor synthesis method by using polyvinyl alcohol in aqueous solution. The perovskite LSGM phase formed essentially at a calcination temperature of 900 °C. Pure and single perovskite LSGM phase with high relative density of 97.1% was obtained after sintering at 1450 °C, while the relative density of the LSGM sample sintered at the same temperature by solid state reaction (SSR) method was 80.6% in present experiment. Comparing with SSR synthesis method, the sintering temperature by SES can be reduced at least 100 °C. Impedance spectra revealed that the grain-boundary resistivity of LSGM synthesized by SES was smaller than that by SSR method, and the conductivities of the samples by SES were higher than those by SSR method at all measuring temperatures.     

A highly conductive Ce0.8Nd0.2O1.9 electrolyte with double sintering aids for intermediate-temperature solid oxide fuel cells

In this paper, the influence of Bi/Zn mass ratio on the phase composition, microstructure, sintering properties, and electrical properties of Bi/Zn co-added Nd_0.2Ce_0.8O_1.9 (NDC) used for intermediate-temperature solid oxide fuel cells (SOFCs) was investigated. At 700 °C, the total conductivity of the NDC-based electrolyte (3Bi/1Zn-NDC) with the mass ratio 3:1 for Bi₂O₃ and ZnO was as high as 5.89 × 10^?2 S cm^?1, 4.60 and 4.51 times higher than the single addition of 4 wt% Bi₂O₃ and 4 wt% ZnO, respectively. In addition, the 3Bi/1Zn-NDC electrolyte exhibited a good physical and chemical compatibility with the electrode materials. The open circuit voltage (OCV) of the cell supported by the 3Bi/1Zn-NDC electrolyte was 0.67 V, and the output power density could reach 402.25 mW cm^?2 at 700 °C. It showed stable power output and OCV in the long-term stability test within 50 h. Overall, the combination of 3 wt% Bi₂O₃ and 1 wt% ZnO was a very effective dual sintering aid for NDC electrolyte.     

Densification of the entropy stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O

The first entropy-stabilized oxide, (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O, was reported in 2015. Initial studies synthesized this material using solid state processing and were limited to densities < 80%. Here, we report a straightforward solid state route to sinter samples to densities up to 98% of the theoretical by identifying the role of oxygen and promoting the resulting mechanisms in densification. Previous works have studied effects of cation stoichiometry on the entropy-driven reaction to form a single phase, but few have explored the associated effects of anion stoichiometry and/or redox chemistry on both phase stability and densification. We demonstrate here that tuning heating rate and pO₂ during heating of initially-homogeneous calcined powders can enhance densifying diffusion processes and enable reliable sintering of dense Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O samples.     

La1.7Ca0.3Ni0.75Cu0.25O4‐δ‐Layered Perovskite as Cathode on La0.9Sr0.1Ga0.8Mg0.2O3 or Ce0.8Gd0.2O2 Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

La₂NiO_4+δ‐based oxides, mixed ionic–electronic conductors with K₂NiF₄‐type structure, have been considerably investigated in recent decades as electrode materials for advanced solid oxide fuel cells (SOFCs) due to their high electrical conductivity and oxidation reduction reaction (ORR) activity. In this study, La_1.7Ca_0.3Ni_0.75Cu_0.25O_4+δ was investigated as a potential cathode on La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ electrolyte support. Furthermore, La_1.7Ca_0.3Ni_0.75Cu_0.25O_4+δ was examined on thin Ce_0.8Gd_0.2O₂ (GDC) electrolyte with Ni‐GDC anode support for intermediate temperature SOFCs (IT‐SOFCs). La_1.7Ca_0.3Ni_0.75Cu_0.25O_4‐δ cathode with gadolinium doped ceria (GDC) electrolyte and NiO‐GDC anode support showed a maximum power density of 0.75 W/cm² in H₂ and lower polarization resistance, Rp (<0.1 Ω cm²), in impedance spectroscopy at 700°C.     

溶胶凝胶法合成La_(0.7)Sr_(0.3)Fe_(0.8)Co_(0.2)O_3影响因素的研究

以合成中温固体氧化物燃料电池阴极La0.7Sr0.3Fe0.8Co0.2O3(LSFC)粉体为研究对象,探讨了PVA改进的溶胶凝胶法合成粉体的影响因素,获得最优实验条件。当pH值=7,柠檬酸与金属离子摩尔比为1.6∶1;PVA与硝酸盐质量比为1∶4时,能够形成形态良好且透明澄清的溶胶,再经12 h的陈化形成凝胶,然后在155℃烘箱中使凝胶膨化制得LSFC前驱体。LSFC前躯体在900℃煅烧2 h后形成晶体结构稳定、粒径分布均匀、具有单一钙钛矿结构的粉体。     

复合氧化物La0.8Sr0.2CoO3的合成及其析氧电催化

通过有机酸辅助法在较低温度下合成了La0.8Sr0.2CoO3复合氧化物,XRD结果表明,用该方法合成的La0.8Sr0.2CoO3具有单相钙钛矿结构,从稳态物化曲线得出,在碱性溶液中,在La0.8Sr0.2CoO3表面的析氧Tafel斜率为65mV/dec,OH-的反应级数为1,在分析了反应机理后,得出了析氧反应的动力学方程,恒电流测试结果表明,用该方法制备的La0.8Sr0.2CoO3/Ni电极,在碱性溶液中具有良好的析氧催化活性.     

Low‐Temperature Sintering and Microwave Dielectric Properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–(Ca0.8Sr0.2)TiO3 Composite Ceramics

0.9(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄–0.1(Ca_0.8Sr_0.2)TiO₃ (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited ε_r = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τ_f = 15 ppm/°C. The effects of LiF–Fe₂O₃–V₂O₅ (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF₂/0.28MgF₂/0.6LiF and MgV₂O₆ were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca₅Mg₄(VO₄)₆ as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: ε_r = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τ_f = ?10 ppm/°C.