Sm0.5Sr0.5CoO3 cathode material from glycine-nitrate process: Formation, characterization, and application in LaGaO3-based solid oxide fuel cells

Cathode material Sm_0.5Sr_0.5CoO₃ (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm_0.5Sr_0.5CoO₃ were investigated. The single cell performance was also tested using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm₂O₄ phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10⁻⁶ K⁻¹ from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm² at 800 and 850 °C, respectively.     

Synthesis and characterization of lanthanum strontium manganite

La_0.85Sr_0.15MnO₃ (LSM) powder has been prepared by co-precipitation. Phase evolution studies by DTA, TGA and XRD indicated initiation of LSM formation above 750 °C. Densification behaviour of LSM powder with and without pore former was different. Sintered LSM was characterized for electrical conductivity, thermal expansion and microstructural development.     

Direct synthesis of La9.33Si6O26 ultrafine powder via sol–gel self-combustion method

Single phase La_9.33Si₆O₂₆ ultrafine powder, as a kind of highly activated precursor to prepare medium-to-low temperature electrolyte for solid oxide fuel cells (SOFCs), has been successfully synthesized via a non-aqueous sol–gel and self-combustion approach from the starting materials: lanthanum nitrate (La(NO₃)₃·6H₂O), citric acid, ethylene glycol (EG), tetraethyl orthosilicate (TEOS) and ammonium nitrate. The details of gel's self-combustion were investigated by DTA–TG and the structural characterization of as-synthesized powder from self-combustion was performed by XRD and SEM. The results show that La_9.33Si₆O₂₆ single phase of apatite-type crystal structure can be directly synthesized by sol–gel self-combustion method without further calcinations on the condition that the molar ratio (R) of NO₃⁻ to citric acid and ethylene glycol being 6:1. Such powders composed of well-dispersed particles with an average size of 200 nm and a specific surface area of 5.54 m²/g. It can be sintered to 90% of its theoretical density at 1500 °C for 10 h, about 200 °C lower than the sintering temperature for the powder derived from traditional solid reactions. The sintered material has a thermal expansion coefficient of 9.2 × 10⁻⁶ K⁻¹ between room temperature and 800 °C.     

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.8Ti0.2O3−δ solid solutions

Series of perovskite-type compounds La_1−aCa_aCr_0.8Ti_0.2O_3−δ (a=0–1.0) were synthesized by the ceramic technique in air (final heating 1350 °C). The crystal structure of the compounds after cooling in air to room temperature was characterized as orthorhombic in space group Pbnm. Analysis of the lattice constants shows a noticeable decrease with increasing Ca content. All compounds prepared were stable in air and in a stream of Ar/1 Pa O₂ at 20–1400 °C, as also in Ar/5% H₂ (pH₂O/pH₂=0.01) at 850–1000 °C. Oxygen stoichiometry and electrical conductivity of the solid solutions with a=0.0–1.0 are investigated. Increasing Ca contents decrease the stability of the oxides in respect to the thermal dissociation of oxygen. All compounds are p-type semiconductors in the temperature range 20–1000 °C at oxygen partial pressures of 10⁻¹⁵ to 0.21×10⁵ Pa. A maximum conductivity of about 30 S/cm in air at 1000 °C is observed for the composition with a=0.6 corresponding to a ratio of Cr³⁺/Cr⁴⁺=1 at an oxygen stoichiometry near 3.0, and oxidation states of La, Ca, Ti, and O ions of 3+, 2+, 4+, and 2−, respectively.     

Photoelectrochemical study of lanthanide titanium oxides, Ln2Ti2O7 (Ln = La, Sm, and Gd)

Photocatalytic activities for hydrogen evolution of lanthanide titanium oxides, Ln₂Ti₂O₇ (Ln = La, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb) prepared by a solid-state reaction were studied. Hydrogen gas was clearly evolved in distilled water suspension of La₂Ti₂O₇ and Sm₂Ti₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.04, −0.02, +0.27 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.29, 2.79, 2.82 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively. The photocatalytic activities of Ln₂Ti₂O₇ (La, Sm, and Gd) were discussed along with detailed band structures estimated in this study. From the band structures, Sm₂Ti₂O₇ is a possible candidate of photocatalyst responding to visible light.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Cu元素在二元等原子比合金中的扩散现象

研究了Cu₅₀Ni₅₀、Cu₅₀Co₅₀、Fe₅₀Ni₅₀、Fe₅₀Cr₅₀ 4种二元等原子比合金在镀铜并经850 ℃×10 h扩散处理后扩散层的显微组织及成分分布。结果表明,经过850 ℃×10 h扩散处理后,在Cu₅₀Ni₅₀等原子比合金中,由于适配空位的存在,Ni元素与Cu元素之间发生了明显的互扩散,并且Cu元素往往在晶界处发生互扩散;在Cu-Fe₅₀Ni₅₀扩散偶中,Cu元素和Fe元素之间发生了互扩散;但是在Cu-Fe₅₀Cr₅₀和Cu-Cu₅₀Co₅₀扩散偶中,各个元素均没有发生扩散。     

The characteristics of Li(CoxNi1 − x)O2 cathode powders formed from the fine-sized Co3O4/NiO precursor powders

Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 1) cathode powders were prepared by solid state reaction method using Co₃O₄/NiO precursor powders obtained by spray pyrolysis. The effect of the ratios of cobalt and nickel components on the characteristics of Co₃O₄/NiO precursor and Li(Co_xNi_1 − x)O₂ cathode powders were investigated. The Co₃O₄/NiO precursor powders with the ratios of cobalt and nickel components as 1/0, 0.75/0.25 and 0.5/0.5 had submicron size and regular morphologies. On the other hand, the Co₃O₄/NiO powders with the high contents of nickel component had aggregated morphologies of submicron size primary powders. The fine-sized precursor powders formed the fine-sized LiCoO₂ and Li(Co_0.75Ni_0.25)O₂ cathode powders by solid state reaction with LiOH powders. However, the high contents of the nickel component of the Co₃O₄/NiO precursor powders formed the Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 0.5) cathode powders with aggregated morphologies and large sizes. The discharge capacities of the powders increased with increasing the nickel content into the Li(Co_xNi_1 − x)O₂ cathode powders up to 188 mAh/g.     

三层共烧制备LaCrO3基连接体/复合阳极/YsZ电解质的研究

将La_0.7Ca_0.3Cr_0.97O_3-δ（LCC）连接材料引入到NiO/YSZ阳极中,制备NiO/YSZ/LCC三相复合阳极,并进行烧结特性、微观结构、电导率、热嘭胀系数等性能对比测试.结果表明,NiO/YSZ/LCC新型复合阳极具有优良的综合性能.采用浆料浸渍法在NiO/YSZ/LCC阳极戈撑体两个表面上分别制备LCC和YSZ湿膜,1400℃空气条件下三层共烧4 h后,获得致密LCC连接体和YSZ电解质薄膜.     

Hydriding kinetics of the La1.5Ni0.5Mg17–H system prepared by hydriding combustion synthesis

An experimental investigation of the hydrogen absorption rate in the two-phase (–β) region of La_1.5Ni_0.5Mg₁₇ powder under the condition of various pressures and temperatures is presented. The results are well interpreted using the Jander diffusion model, [1−(1−ξ)^1/3]²=k(T,P)t, which suggests that the rate-controlling step of hydrogen absorption in La_1.5Ni_0.5Mg₁₇ is three-dimensional diffusion. An apparent activation energy for such diffusion process of 90±1 kJ/mol H₂ has been obtained from the absorption data.     

La–Mg–Ni ternary hydrogen storage alloys with Ce2Ni7-type and Gd2Co7-type structure as negative electrodes for Ni/Mh batteries

Hydrogen strorage alloys with formula La_1.5Mg_0.5Ni₇ were prepared by induction melting followed by different annealing treatments (1073, 1123 and 1173 K) for 24 h. The alloy composition, alloy microstructure and electrochemical properties were investigated, respectively. The results showed that the multi-phase structure of as-cast alloy was converted into a double-phase structure (Gd₂Co₇-type phase and Ce₂Ni₇-type phase) through annealing treatments. Mg atoms were mainly located in Laves unit of Gd₂Co₇-type unit cell and Ce₂Ni₇-type unit cell. The electrochemical capacity of alloy electrodes after annealing treatment could be up to 390 mAh/g. The cyclic stability of alloy electrodes was significantly improved by annealing treatments; After 150 charge/discharge cycles, the capacity retention ratio of alloy annealed at 1173 K was the highest (81.9%). The high rate dischargeability of alloy electrodes was also improved due to annealing treatment.     

Facile autocombustion synthesis of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite via a modified complexing sol–gel process with NH4NO3 as combustion aid

Ammonia nitrate was applied as an oxidizer and combustion trigger to modify the normal combined EDTA-citrate complexing method into a process with autocombustion and low ignition temperature properties. Therefore, the synthesis procedure was greatly simplified. The effect of NH₄NO₃/metal ions to organic mole ratios and the heating temperature on the autocombustion behavior and the properties of the powders derived were investigated in detail. The critical amount of NH₄NO₃ for the autocombustion to occur was identified at the NO₃⁻ to citric acid to EDTA mole ratio of around 10:2:1. After the experimental optimization, well-crystallized nanostructured La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) powder with a specific surface area as high as 21 m²/g was obtained; it is comparable with that obtained from a normal complexing process. By adjusting the combustion parameters, the properties of the powders then derived can be tailored for different applications, such as nanograined dense membrane for oxygen separation membrane, and porous cathode for fuel cells and sensors.     

Magnetic entropy change in polycrystalline La1−xKxMnO3 perovskites

Polycrystalline samples of potassium doped lanthanum manganites having nanometric crystallite size have been synthesized by pyrophoric method. The Curie temperature (T_C) of the prepared samples is found to be strongly dependent on K content and spans between 260 and 309 K. Close to T_C, large change in magnetic entropy has been observed in all the samples. The maximum magnetic entropy change observed for samples with different concentration of K, exhibits a linear dependence with the applied magnetic field. Adiabatic temperature change at T_C at 1 T also increases with K doping and attains a maximum of 2.1 K for La_0.85K_0.15MnO₃. Estimated relative cooling power of La_1−xK_xMnO₃ compounds is nearly one-third of pure Gd. In addition to the tuneability of T_C between 260 and 310 K, higher chemical stability, lower eddy current heating and inexpensive preparation technique; the magnetic entropy change in La_0.85K_0.15MnO₃ compound at 1 T magnetic field is found to be 3.00 J/kg K and is 89% to that known for the prototype magnetic refrigerant (pure Gd). Our result on magnetocaloric properties suggests that La_1−xK_xMnO₃ compounds are attractive as a possible refrigerant for near room temperature magnetic refrigeration.     

La0.9Sr0.1Ga0.8Mg0.2O3−δ sintered by spark plasma sintering (SPS) for intermediate temperature SOFC electrolyte

The La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) powders for intermediate temperature SOFC electrolyte have been synthesized by glycine-nitrate combustion process. The as-synthesized powders show almost pure perovskite phase. And then, the as-synthesized powders were sintered by SPS at 1300 °C to prepare electrolyte. The SEM, XRD and AC impedance were employed to characterize the microstructure, phase and electrical conductivities. Results show that the grain size is very fine, less than 1 μm, and the relative density of the pellet after sintering by SPS is about 94.7%. There is very little amount of secondary phases after SPS and the grain boundary and secondary phase resistance is very small. The electrolyte sintered by SPS shows higher conductivities than that sintered by conventional method at the same temperature. The activation energy at lower temperatures (400–700 °C) and higher temperatures (700–800 °C) is about 0.94 and 0.49 eV, respectively. Spark plasma sintering is a promising and effective method to sinter the LSGM electrolyte.     

A comparative study on the hydriding kinetics of Zr-based AB2 hydrogen storage alloys

Two kinetic models (Jander model and Chou model) are used to investigate the hydrogen absorption kinetic mechanism of Zr-based AB₂ type Laves phase alloys (Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55). The analysis shows that the rate-controlling step is the diffusion process at high temperatures in the range from 673 K to 923 K with a low hydrogen concentration (solid solution phase). Both models can well describe the experimental data but Chou model is preferred. Chou model is simpler and easier to use for analyzing the experimental results. The activation energies calculated using Chou model with the least square method are 29.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, 43.8 kJ/mol H₂ for Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and 48.5 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55, which are close to the values reported in the literature (28.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5 and 40.3 ± 1.5 kJ/mol H₂ for both Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55).     

Nuclear structure and magnetic properties of perovskite compounds La1−xNdxFe0.5Cr0.5O3 (x = 0.1, 0.15 and 0.2)

Perovskite oxides with the composition La_1−xNd_xFe_0.5Cr_0.5O₃ (x = 0.10, 0.15 and 0.20) have been studied. The samples have been prepared using the standard solid-state reaction method in air. The X-ray and neutron diffractograms indicates that the samples crystallize in the orthorhombic perovskite structure with space group Pnma (No. 62). The Nd-substitution causes minor changes in the cell parameters and bond lengths but the nuclear structure remains orthorhombic in the whole range of substitutions and in the temperature range of 10–700 K. The magnetic structure of the samples at room temperature and below is G-type antiferromagnetic with an average magnetic moment of the Fe/Cr ions of 3.29(3) μ_B/atom at 10 K independent of the Nd content. At room temperature the average magnetic moment of the Fe/Cr ions reduces to 1.23 μ_B/atom. At temperatures below 250 K a weak but increasing uncompensated spontaneous magnetic moment develops that reaches a magnitude of about 0.5 emu/g (or 0.02 μ_B per Fe/Cr site) at 10 K. This moment saturates rather rapidly in the magnetization versus magnetic field curve and is at higher field superposed on the response of the antiferromagnetic spin system.     

一步法合成La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ-Ce0.8Gd0.2O2-δ对称电极应用于SOFC性能研究

一步法合成La_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3-δ-Ce_0.8Gd_0.2O_2-δ(LSCFN-CGO)混合电导对称电极,并采用La_0.8Sr_0.2Ga_0.83Mg_0.17O_3-δ(LSGM)作为电解质制备了结构为LSCFN-CGO‖_LSGM‖LSCFN-CGO的对称电池。分别使用X射线衍射(XRD)仪和扫描电子显微镜(SEM)对LSCFN-CGO粉体物相及电极微观结构进行分析。一步法制得的LSCFN-CGO电极粉体为纯相,LSCFN钙钛矿相与CGO萤石相具备极好的化学相容性,且烧结得到了良好微观结构的对称电极。采用H₂(3%H₂O)、C₃H₈(3%H₂O)为燃料气测试电池性能,850℃电池最大功率密度可分别达980和869mW/cm²。稳定性测试在C₃H₈(3%H₂O)气氛中0.3 A/cm²的恒流放电条件下进行,全程共420 h,衰减较小,期间进行8次电极氧化还原循环,对称电极具有理想的碳基燃料下氧化还原再生能力。结果表明,采用一步法合成混合电导电极是一种简便、优化的电极制备方法,具有广阔的应用前景。     

Influence of the rare earth composition on the properties of Ni–MH electrodes

Lattice parameters, hydrogen absorption properties and electrochemical cycling properties up to 240 cycles have been measured as a function of the Ce content for alloys of composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 (0≤x≤0.82). The results show the strong increase of the plateau pressure correlated to the cell volume decrease as a function of x. On the other hand, the hydrogen capacity measured in solid–gas reaction as well as the electrochemical capacity decreases slightly. The results show that both La and Ce have to be present to achieve a good cycle life, the cycling degradation being almost independent of their relative quantities in a broad range of concentrations.     

Self-propagating high-temperature synthesis of La(Sr)Ga(Mg)O3−δ for electrolyte of solid oxide fuel cells

This paper describes self-propagating high-temperature synthesis (SHS) of an electrolyte for solid oxide fuel (SOFC), in comparison to a conventional solid-state reaction method (SRM). Doped-lanthanum gallate: La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM9182) and LSGM9173 as the SOFC electrolyte, was prepared by the SHS and sintered at different temperatures, for measuring the electrical conductivity of the sintered LSGM and the power generating performance at 1073 K, in comparison to the SRM. In the SHS, the LSGM powders with smaller size were obtained and easily sintered at the 100 K-lower temperature, 1673 K, than in the SRM. Most significantly, the electrical conductivity of the sintered LSGM9182 was as high as 0.11 S cm⁻¹ and its maximum power density was a value of 245 mW cm⁻² in the cell configuration of Ni/LSGM9182 (0.501 mm in thickness)/Sm_0.5Sr_0.5CoO₃. The conclusion was that the proposed SHS-sintering method with many benefits of minimizing the energy requirement and the processing time in the production, easing temperature restriction for the sintering, and improving the electrolyte performance up to a conventional level is practicable for producing the LSGM-electrolyte of SOFC at an intermediate-temperature application.     

Photoelectrochemical study of lanthanide zirconium oxides, Ln2Zr2O7 (Ln = La, Ce, Nd and Sm)

Photocatalytic activities for hydrogen evolution of lanthanide zirconium oxides, Ln₂Zr₂O₇ (Ln = La, Ce, Nd and Sm) prepared by a solution reaction method were investigated. Under the illumination of 500 W Xenon lamps, hydrogen gas was clearly evolved in a distilled water suspension of La₂Zr₂O₇, Sm₂Zr₂O₇ and Nd₂Zr₂O₇. Under the visible-light illumination, hydrogen gas was evolved in a distilled water suspension of Nd₂Zr₂O₇ and Sm₂Zr₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.64, −0.52, −0.31 and +0.04 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.52, 2.86, 2.67 and 2.53 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively. Due to the effect of 4f orbital electrons, the band gap energy of these compounds becomes narrower than in ZrO₂ and as a consequence, Sm₂Zr₂O₇ and Nd₂Zr₂O₇ show the photocatalytic activity under the visible-light.