Defect equilibria and partial molar properties of (La,Sr)(Co,Fe)O3−δ

The oxygen nonstoichiometry δ of La_1−xSr_xCo_1−yFe_yO_3−δ (x = 0.6 and y = 0.2, 0.4) was investigated by thermogravimetry in the range 703 ≤ T/°C ≤ 903 and 1E−5 < pO₂/atm < 1. The oxygen deficit increases with increasing T and decreasing pO₂. Electronic conductivities σ were measured as a function of pO₂ in the range 1E−5 < pO₂/atm < 1 at 700 ≤ T/°C ≤ 900. At constant T, a p-type pO₂-dependence of σ is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation ΔH_ox^θ and ΔS_ox^θ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO₂^θ = 1 atm), (h_O − H_O^θ) and (s_O − S_O^θ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (h_O − H_O^θ) decreases with increasing δ, while (s_O − S_O^θ) is constant within the limits of error. Defect chemical modelling was performed by an ideal solution model under consideration of three different valence states for B-site ions (Co or Fe). The dependence of σ on δ is modelled, using calculated defect concentrations as functions of δ. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies.     

Characterization of Sr2.7Ln0.3Fe1.4Co0.6O7 (Ln = La,Nd, Sm,Gd) intergrowth oxides as cathodes for solid oxide fuel cells

Perovskite-related intergrowth oxides Sr_2.7Ln_0.3Fe_1.4Co_0.6O_7 ? δ (Ln = La, Nd, Sm, and Gd) have been investigated as cathode materials for solid oxide fuel cells (SOFC). With decreasing size of the Ln³⁺ ions, the unit cell volume, oxygen content, thermal expansion coefficient (TEC), and total electrical conductivity decrease from Ln = La to Gd. The decreasing unit cell volume and oxygen content is attributed to the decreasing size of Ln³⁺ ions from Ln = La to Gd and a consequent preference for lower coordination numbers. While the decrease in the ionicity of the Ln–O bonds from Ln = La to Gd causes a decrease in the TEC, the increasing amount of oxygen vacancies leads to a decrease in electrical conductivity arising from a thermally activated semiconducting behavior. The cathode polarization conductance (R_p^? 1) measured using the ac-impedance spectroscopy and the catalytic activity for the oxygen reduction reaction in SOFC decrease from Ln = La to Gd partly due to the decrease in electrical conductivity.     

Oxygen nonstoichiometry,mixed valency and mixed conduction in (La,Sr)(Co,Fe)O3−δ

Defect chemistry for a mixed conductor, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was studied. Samples were treated under controlled oxygen partial pressure, P(O₂), conditions at 1273 K [10^− 11.1 ≤ P(O₂)/atm ≤ 1], and cooled to room temperature. Oxygen nonstoichiometry and valences of transition metal ions for the treated samples were evaluated by iodometry and X-ray absorption spectroscopy, respectively. With decreasing P(O₂), preferential reduction of Co³⁺ to Co²⁺ was observed, while iron preserved its higher valence above 3 under conditions studied. A dependency of its electrical conductivity on P(O₂) was discussed along with a change in concentration of oxygen vacancies and mixed valences.     

Oxygen reduction at sol–gel derived La0.8Sr0.2Co0.8Fe0.2O3 cathodes

The perovskite material, La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF), substituted by Sr and Fe at the A and B sites, was prepared using the sol–gel (SG) method, followed by heating at 900 °C for 4 h. The X-ray powder diffraction pattern for the SG derived LSCF material showed good agreement with the literature data. Scanning electron microscopy showed that the LSCF structure is highly porous, facilitating gas transfer and maximizing the number of active sites for the oxygen reduction reaction (ORR) at the cathode of a solid oxide fuel cell. Transmission electron microscopy (TEM) was employed to determine the SG–LSCF particle size and distribution. The kinetics of the ORR were investigated at SG–LSCF, deposited by screen-printing on a samarium-doped ceria (SDC) electrolyte, using electrochemical impedance spectroscopy and cyclic voltammetry at temperatures ranging from 400 to 700 °C. The results showed that the SG–LSCF cathode is stable and exhibits a high exchange current density (and low charge transfer resistance), yielding an apparent activation energy for the ORR of ca. 120 kJ/mol. It was also found that the SG–LSCF on SDC cathode was approximately one order of magnitude more active than standard manganite-based composite cathodes, deposited on yttria stabilized zirconia, studied under otherwise identical operating conditions.     

(La0.75Sr0.25)(Cr0.5Mn0.5)O3/YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells

The synthesis and performance of (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃/Y₂O₃–ZrO₂ (LSCM/YSZ) composites are investigated as alternative anodes for the direct utilization of methane (i.e., natural gas) in solid oxide fuel cells. Addition of YSZ phase greatly improves the adhesion and reduces the electrode polarization resistance of the LSCM/YSZ composite anodes. LSCM/YSZ composite anodes show reasonably good performance for the methane oxidation reaction in wet CH₄ and the best electrode performance was achieved for the composite with LSCM contents of 50–60 wt.% with polarization resistances of 2–3 Ω cm² in 97% CH₄/3% H₂O at 850 °C. The electrode impedance for the methane oxidation in wet CH₄ on the LSCM/YSZ composite anodes was characterized by three separable arcs and the electrode behavior could be explained based on the ALS model for the reaction on the MIEC electrode. The results indicate that electrocatalytic activity of the LSCM/YSZ composite anodes for the methane oxidation is likely limited by the oxygen vacancy diffusion in the substituted lanthanum chromite-based materials.     

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mössbauer measurements on nanocrystalline perovskite structured manganite La_0.8Sr_0.2Mn_0.8Fe_0.19 ⁵⁷Fe_0.01O₃ as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 Å. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe³⁺ as well as Fe⁴⁺ ions are distributed in two different environments – Fe³⁺ in low symmetric site surrounded by Mn³⁺ ions only while Fe⁴⁺ in high symmetric site with at least one Mn³⁺ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe⁴⁺ to Fe³⁺. Another transition at 3.7 GPa, represents the presence of Fe³⁺ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.     

Synthesis and characterization of La0.9Sr0.1Ga0.8Mg0.2O3-δ electrolyte for intermediate temperature solid oxide fuel cells (ITSOFC)

Experimental investigations on new materials for application as electrolyte in electrolyte supported planar Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC) operating below 800 °C is in progress at our laboratory. Sr and Mg doped Lanthanum gallate (LSGM) powder was prepared by glycine — nitrate combustion method. The prepared LSGM powder is relatively finer than that prepared through other techniques such as solid-state reaction. The measurements comprising XRD, particle size, density, TGA/DTA were made. Thin sections of circular pellets were fabricated and annealed at different temperatures ranging between 1000 and 1300 °C. The sintering behaviour of LSGM was investigated to obtain information on the densification factor, relative percentage shrinkage/expansion in volume, while annealing and the resulting apparent porosity values. Bismuth oxide is found to be an effective sintering aid in general. So the effect of bismuth oxide addition on LSGM was investigated through sintering studies, XRD, TGA/DTA, SEM and conductivity measurements. The results obtained on LSGM with and without bismuth oxide addition are discussed with respect to the requirement of an electrolyte for ITSOFC applications. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Development and characterization of a quasi-ternary diagram based on La0.8Sr0.2(Co,Cu,Fe)O3 oxides in view of application as a cathode contact material for solid oxide fuel cells

Oxides resulting from discrete changes in composition within the quasi-ternary system La_0.8Sr_0.2CuO_2.4 + δ–La_0.8Sr_0.2CoO_3 ? δ–La_0.8Sr_0.2FeO_3 ? δ were investigated under similar experimental conditions with the objective of obtaining an overview of the variation of the relevant properties for possible applications as cathode contact layer in SOFCs. Twenty-two oxide compositions within this system were systematically selected and synthesized under identical conditions by the Pechini method. The distribution of the different crystallographic phases at 1050 °C within this quasi-ternary phase diagram, the DC electrical conductivity at 800 °C and the thermal expansion coefficients are presented. Perovskites of different compositions issued from this ternary diagram were tested as cathode contact material between an La_0.8Sr_0.2FeO₃ cathode and a Crofer22APU interconnect by resistance measurements at 800 °C. The application of a MnCo_1.9Fe_0.1O₄ spinel protection reduced the interfacial reaction between the Crofer22APU and the cathode contact material. Electrical resistance measurements at 800 °C in air up to 1000 h and the analysis by scanning electron microscopy/energy-dispersive X-ray spectroscopy of the sample cross-sections were carried out to verify the surface stability and the electrical performance.     

Simultaneous thermogravimetry - mass spectrometry for a solid oxide fuel cell cathode: (La0.7Sr0.3)0.9MnO3

A SOFC cathode related perovskite material, (La_0.7Sr_0.3)_0.9MnO₃, has been investigated by simultaneous thermogravimetry - mass spectrometry from room temperature to 1770 K. Water, carbon dioxide and oxygen were detected by mass spectrometry. Water and carbon dioxide evolution can be interpreted by assuming that prior to the thermogravimetry-mass spectrometry measurement about 0.5 % of the lanthanum component had reacted with carbon dioxide and water to form La₂(CO₃)₃*8H₂O, which dehydrated and decomposed via La₂O₂CO₃ into La₂O₃ and evolving H₂O and CO₂ during the present experiment. The observation that the lanthanum strontium manganite emitted oxygen in two stages can be ascribed to the two different oxygen sites in the perovskite lattice, that is, the oxygen excess and deficient regions.     

A mechanistic study on the activation process of (La,Sr)MnO3 electrodes of solid oxide fuel cells

The mechanism of the activation process for the O₂ reduction on (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes is investigated by examining the electrochemical behavior of LSM under cathodic and anodic polarization conditions and the relaxation behavior of LSM under open circuit. A comparative study is also performed on a LSM electrode after dilute acid etching treatment. It has been shown that the segregated SrO has a significant inhibiting effect on the surface exchange process such as dissociative adsorption, incorporation and diffusion of oxygen species on the LSM surface, resulting in the initially very high impedance for the O₂ reduction on LSM electrodes. A mechanism involving the incorporation of SrO into LSM lattice with the concomitant removal of cation vacancies is proposed for the activation effect of cathodic current passage/polarization in solid oxide fuel cells.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

On the sol-gel synthesis and characterization of (BaSr)(CeZr)O3-based fuel cell electrolytes

Ionics - Proton-conducting ceramics have been very useful due to their potential applications in fuel cells, electrolysers, and hydrogen membranes. The proton-conducting fuel cell electrolytes...     

高温固体氧化物燃料电池实验演示

介绍一种高效低污染的新型能源－高温固体氧化物燃料电池的工作原理及其演示实验装置。     

Characterisation of the reduction properties of La0.5Sr0.5Fe0.5Co0.5O3

We report here on the characterisation by temperature programmed reduction, ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy of the phases resulting from treatment of the perovskite-related material La_0.5Sr_0.5Fe_0.5Co_0.5O₃ in a flowing 90% hydrogen/10% nitrogen atmosphere. The results show that treatment of La_0.5Sr_0.5Fe_0.5Co_0.5O₃ (which contains approximately 50% Fe⁴⁺ and 50% Fe³⁺) in the flowing 90% hydrogen/10% nitrogen atmosphere at 600°C does not result in the reduction of any of the constituent elements of the material and that the perovskite structure is still retained. The Mössbauer spectrum recorded following heating in the gaseous reducing environment at 1,000°C shows the presence of metallic iron, an Fe³⁺-containing phase with parameters compatible with the presence of SrLaFeO₄ which has a K₂NiF₄-type structure, and a paramagnetic Fe³⁺ phase. The X-ray absorption spectroscopy results show the presence of metallic cobalt. The Mössbauer spectrum recorded following heating at 1,200°C continues to show the Fe³⁺-containing components plus a larger contribution from metallic iron. The X-ray absorption spectroscopy results show the presence of metallic cobalt, SrLaFeO₄, La₂O₃ and SrO.     

Transport properties and electrochemical activity of YBa(Co,Fe)4O7 cathodes

The layered phases derived from YBaCo₄O₇ exhibit a relatively low thermal expansion, significant mixed conductivity and attractive electrochemical activity in contact with doped LaGaO₃ solid electrolyte. These properties may be of interest for intermediate-temperature SOFC cathodes. Moderate doping of yttrium-barium cobaltite with iron decreases slightly the total conductivity, predominantly p-type electronic, and have no essential effect on the oxygen ion transport and cathodic polarization at 1073 K. However, hexagonal YBaCo₄O₇, where the average cobalt oxidation state is + 2.25, and its derivatives appear metastable at temperatures below 1050–1100 K. Oxygen uptake at intermediate temperatures in air leads to the phase decomposition, accompanied by increasing conductivity and dramatic volume contraction.     

A stable BaCeO3-based proton conductor for solid oxide fuel cells

In order to improve the chemical stability of BaCeO₃, Ti⁴⁺ was introduced into B site of BaCeO₃ to modify the chemical stability. XRD test demonstrates that \textBaC\texte_0.6\textT\texti_0.2\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.6}}{\text{T}}{{\text{i}}_{0.2}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCTY) keeps its original pervoskite-type structure at a high doping level of 20%. After exposure in 94% N₂ + 3% CO₂ + 3% H₂O at 700 °C for 10 h, BCTY exhibited adequate chemical stability while decomposition was found in \textBaC\texte_0.8\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.8}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCY). Accordingly, the conductivity of BCTY reaches 0.0072 S/cm at 700 °C in humidified hydrogen which is a little lower than BCY (0.0085). Besides, BCTY displayed better sintering characteristics than BCY at high temperatures and the relative density reaches 96.4% and 94.8%, respectively. The two samples also exhibited similar thermal expansion behavior from 30 to 1,000 °C. A fuel cell with BCTY as electrolyte exhibited 244 mW/cm² at 700 °C and the stable short-term performance further proved the stability of BCTY.     

Structure and electrochemical properties of Sm0.5Sr0.5Co1 − xFexO3 − δ cathodes for solid oxide fuel cells

Crystal structure, thermal expansion coefficient, electrical conductivity and cathodic polarization of compositions in the system Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ with 0 ≤ x ≤ 0.9 were studied as function of Co / Fe ratio and temperature, in air. Two phases, including an Orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9, were observed in samples of Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ at room temperature. The adjustment of thermal expansion coefficient (TEC) to electrolyte, which is one of the main problems of SSC, could be achieved to lower TEC values with more Fe substitution. High electrical conductivity above 100 S/cm at 800 °C was obtained for all specimens, so they could be good conductors as cathodes of IT-SOFC. The polarization behavior of SSCF as a function of Fe content was evaluated by means of AC impedance using LSGM electrolyte. It was discovered that the Area Specific Resistance (ASR) of SSCF increased as the amount of substitution of Fe for Co increased. When the amount of Fe reached to 0.4, the highest ASR was obtained and then the resistance started decreasing above that. The electrode with a composition of Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3 − δ showed high catalytic activity for oxygen reduction operating at temperature ranging from 700 to 800 °C.     

High temperature properties of the n = 2 Ruddlesden–Popper phases (La,Sr)3(Fe,Ni)2O7−δ

The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden–Popper (R–P) phases Sr_3−xLa_xFe_2−yNi_yO_7−δ with 0 ≤ x ≤ 0.3 and 0 ≤ y ≤ 1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO₂ (10^− 5 ≤ pO₂ ≤ 1 atm) in the temperature range 400 ≤ T ≤ 1000 °C indicate that the Sr₃FeNiO_7−δ phase is able to accommodate a large oxygen non-stoichiometry (δ ∼ 1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550 ≤ T ≤ 1000 °C. The electrical transport of the Sr₃FeNiO_7−δ phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation.     

An intermediate temperature solid oxide fuel cell utilizing superior oxide ion conducting electrolyte, doubly doped LaGaO3 perovskite

LaGaO₃-based perovskite oxide doped with Sr and Mg exhibits high ionic conduction over a wide oxygen partial pressure. In this study, the stability of the LaGaO₃ based oxide was investigated. It became clear that LaGaO₃ based oxide is very stable for reduction and oxidation. SOFCs utilizing LaGaO₃-based perovskite type oxide for electrolyte were further studied for the decreased temperature solid oxide fuel cells. The power generation characteristics of cells were strongly affected by the electrode, both anode and cathode. It became clear that Ni and LnCoO₃ (Ln: rare earth) are suitable for anode and cathode, respectively. Rare earth cations in the Ln-site of Co-based perovskite cathode also have a great effect on the power generation characteristics. In particular, high power density could be attained in the temperature range from 973 to 1273 K by using doped SmCoO₃ for the cathode. The electrical conductivity of SmCoO₃ increases with increasing Sr amount doped for the Sm site and attained the maximum at Sm_0.5Sr_0.5CoO₃. The cathodic overpotential and the internal cell resistance exhibit almost opposite dependence on the amount of doped Sr. Consequently, the power density of the cell reaches a maximum when Sm_0.5Sr_0.5CoO₃ is used for cathode. On this cell, the maximum power density is as high as 0.58 W/cm² at 1073 K, although a 0.5 mm thick electrolyte is used. Therefore, this study reveals that the LaGaO₃ based oxide for electrolyte and the SmCoO₃ based oxide for cathode are promising for solid oxide fuel cells at intermediate temperature. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June, 14–17, 1998.