Spray pyrolysis of electrolyte interlayers for vacuum plasma-sprayed SOFC

The effects of gadolinia-doped ceria (CGO, Ce_0.8Gd_0.2O_1.9−x) and yttria-doped zirconia (8YSZ, Zr_0.92Y_0.08O_2−x) interlayers prepared by spray pyrolysis between vacuum plasma-sprayed 8YSZ electrolytes (8YSZ–VPS) and screen-printed (La_0.8Sr_0.2)_0.98MnO₃ cathodes (LSM) on the power output of solid oxide fuel cells (SOFC) are investigated. Amorphous thin films are deposited and then converted to nanocrystalline electrolyte–cathode interlayers during the first heat-up cycle of a SOFC to the operating temperature. CGO thin films between the YSZ plasma-sprayed electrolyte and the LSM cathode increased the power output by more than 20% compared to cells without interlayers, whereas YSZ films degraded the power output of cells. It is assumed that CGO improves the charge transfer at the electrolyte–cathode interface and that the CGO layer prevents the formation of undesirable insulation of La-zirconate at the interface 8YSZ/LSM.     

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.     

Redox behaviour,chemical compatibility and electrochemical performance of Sr2MgMoO6 − δ as SOFC anode

The double perovskite Sr₂MgMoO_6 − δ (SMM) has been proposed as a potential anode material for direct hydrocarbon oxidation in solid oxide fuel cells (SOFCs). The oxygen nonstoichiometry and electrical conductivity dependence of Sr₂MgMoO_6 − δ have been determined as a function of the oxygen partial pressure by coulometric titration and impedance spectroscopy techniques. The chemical compatibility of Sr₂MgMoO_6 − δ with most of the typical electrolytes commonly used in SOFCs i.e. La_0.8Sr_0.2Ga_0.8Mg_0.2O_3 − δ (LSGM), Ce_0.8Gd_0.2O_2 − δ (CGO) and Zr_0.84Y_0.16O_2 − δ (YSZ), was investigated. Reactivity between SMM and all these electrolytes has been found above 1000 °C, although the reaction is most severe with ZrO₂-based electrolytes. Area-specific polarisation resistance of the SMM/LSGM/SMM symmetrical cells indicates that the polarisation resistance increases with the firing temperature of the electrodes due to chemical interaction between LSGM and SMM layers. A CGO buffer layer between the anode and electrolyte was also used to prevent an excessive interdiffusion of ionic species between these components, resulting in better performance. Power densities of 330 and 270 mW cm^− 2 were reached at 800 °C for SMM/CGO/LSGM/LSCF and SMM/LSGM/LSCF electrolyte-supported cells, respectively; with 600-μm-thick LSGM electrolyte, using humidified H₂ as fuel and air as oxidant. XPS and XRPD studies on SMM powders annealed in air and diluted CH₄ atmospheres showed that the surface of SMM powders is mainly formed by SrMoO₄ and metal carbonates.     

A-deficit LSCF for intermediate temperature solid oxide fuel cells

A-deficit La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) was synthesized by a citrate complexation (Pechini) route. Using La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ as cathode material, a superior cell performance with the maximum power density of 309, 470 and 855 mW cm^? 2 at 600, 650 and 700 °C was achieved, in contrast with the maximum power density of 266, 354 and 589 mW cm^? 2 using conventional La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ as cathode material at the same temperatures. The reason of this improvement was analyzed on the basis of defect chemistry. Thermal shrinkage experiment testified that the oxygen vacancies in La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ are more mobile than in La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ. Furthermore, theoretical calculation in terms of their composition and the shift of peak position in XRD pattern showed that the concentration of oxygen vacancies of La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ is higher than that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ. Therefore, the oxygen ion conductivity via vacancies transfer mechanism is enhanced, which induces the polarization resistance of La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ being decreased with a result of cell performance improved.     

Evaluation of Sr- and Mn-substituted LaAlO3 as potential SOFC anode materials

Lanthanum–aluminate-based oxides, (La_0.8Sr_0.2)_1−yAl_1−xMn_xO_3−δ (x = 0, 0.3, 0.5; y = 0 or 0.06) (LSAM), were synthesized and evaluated in detail as potential anode materials for solid oxide fuel cells (SOFCs). The electrical conductivity of LSAM (Mn ≥ 30 mol%) is dominated by p-type electronic conduction and can be treated as a diluted system of lanthanum manganites, (La,Sr)MnO₃. At 810 °C, the electrical conductivity of (La_0.8Sr_0.2)_0.94Al_0.5Mn_0.5O_3−δ (LSAM8255b) reaches 12 S/cm in air and 2.7 S/cm in humidified Ar/4% H₂ (p(O₂) ≈ 10^− 18 bar). The thermal expansion coefficients of LSAM8255a and LSAM8255b match YSZ very well and no chemical reaction was observed between these two perovskite materials and YSZ up to at least 1400 °C. Fairly good electrochemical performance was observed for an LSAM8255b–YSZ composite anode. At 850 °C, the polarization resistances are only 0.34 and 0.50 Ω cm² in wet (∼3% H₂O) Ar/20% H₂ and wet Ar/20% CH₄, respectively. In addition, an exposure to Ar/20% CH₄/3% H₂O for 35 h did not cause any apparent carbon deposition on the electrode. However, the chemical stability of LSAM8255a and LSAM8255b in a typical anode environment under open circuit conditions does not seem sufficient, leading to performance degradation with time in wet Ar/20% H₂ or wet Ar/20% CH₄. Furthermore, relatively large chemical expansion (0.3–0.5%) was observed when the atmosphere was switched from air to wet Ar/4% H₂, which might cause intolerable stress on the thin film electrolyte layer for a large-area anode-supported planar SOFC, but which might be tolerable for small geometries or electrolyte-supported SOFCs.     

La0.75Sr0.25Cr0.5Mn0.5O3−δ + Cu composite anode running on H2 and CH4 fuels

La_1−xSr_xCr_1−xM_xO_3−δ (M = Cr, Fe, V) system has been studied as anode materials for solid oxide fuel cells (SOFCs). The perovskite La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) is stable in both H₂ and CH₄ atmospheres at temperatures up to 1000°C. However, in the reducing atmospheres of H₂ and CH₄, its electronic conductivity is greatly reduced from its value in air. We have characterized LSCM as the anode of a SOFC having 250 μm-thick La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) as the electrolyte and SrCo_0.8Fe_0.2O_3−δ (SCF) as the cathode. We report a comparison of the overpotentials at the following anodes: (1) La_0.4Ce_0.6O_1.8 (LDC) + NiO composite in H₂, (2) porous LSCM in H₂ and CH₄, (3) porous LSCM impregnated with CuO in H₂ and CH₄ and (4) porous LSCM impregnated with CuO and sputtered with Pt in H₂ and CH₄. An LSCM + CuO + Pt anode gave a maximum power output at 850 °C of 850 mW/cm² and 520 mW/cm², respectively, with H₂ and CH₄ as fuel whereas anode (1) gave 1.4 W/cm² at 800 °C in H₂. There was no noticeable coke formation in CH₄ with anodes (2), (3) and (4), which demonstrates that the perovskite oxide is a plausible option for the anode of a SOFC operating with hydrocarbon fuels. We also report the moisture effect in the H₂ and CH₄ fuel-oxidation process.     

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.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Defect interactions in La0.3Sr0.7Fe(M′)O3−δ (M′ = Al,Ga) perovskites: Atomistic simulations and analysis of p(O2)-T-δ diagrams

Atomistic modelling showed that a key factor affecting the p(O₂) dependencies of point defect chemical potentials in perovskite-type La_0.3Sr_0.7Fe_1−xM′_xO_3−δ (M′ = Ga, Al; x = 0–0.4) under oxidizing conditions, relates to the coulombic repulsion between oxygen vacancies and/or electron holes. The configurations of A- and B-site cations with stable oxidation states have no essential influence on energetics of the mobile charge carriers, whereas the electrons formed due to iron disproportionation are expected to form defect pair clusters with oxygen vacancies. These results were used to develop thermodynamic models, adequately describing the p(O₂)-T-δ diagrams of La_0.3Sr_0.7Fe(M′)O_3−δ determined by the coulometric titration technique at 923–1223 K in the oxygen partial pressure range from 1 × 10^− 5 to 0.5 atm. The thermodynamic functions governing the oxygen intercalation process were found independent of the defect concentration. Doping with aluminum and gallium leads to increasing oxygen deficiency and induces substantial changes in the behavior of iron cations, increasing the tendencies to disproportionation and hole localization. Despite similar oxygen nonstoichiometry in the Al- and Ga-substituted ferrites at a given dopant content, the latter tendency is more pronounced in the case of aluminum-containing perovskites.     

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.     

Performance of perovskite-related oxide cathodes in contact with lanthanum silicate electrolyte

The cathodic performance of selected mixed-conducting electrodes, including perovskite-type SrMn_0.6Nb_0.4O_3 ? δ, Sr_0.7Ce_0.3Mn_0.9Cr_0.1O_3 ? δ and Gd_0.6Ca_0.4Mn_0.9Ni_0.1O_3 ? δ, and Ruddlesden–Popper La₂Ni_0.5Cu_0.5O_4 + δ, LaSr₂Mn_1.6Ni_0.4O_7 ? δ, La₄Ni_3 ? xCu_xO_10 ? δ (x = 0–0.1) and La_3.95Sr_0.05Ni₂CoO_10 ? δ, was evaluated in contact with apatite-type La₁₀Si₅AlO_26.5 solid electrolyte at 873–1073 K and atmospheric oxygen pressure. The electrochemical activity of porous nickelate-based layers was found to correlate with the concentration of mobile ionic charge carriers and bulk oxygen transport, thus lowering in the series La₄Ni_2.9Cu_0.1O_10 ? δ > La₄Ni₃O_10 ? δ > La_3.95Sr_0.05Ni₂CoO_10 ? δ and decreasing on copper doping in K₂NiF₄-type La₂Ni_1 ? xCu_xO_4 ? δ. The relatively high overpotentials of nickelate-based cathodes, varying in the range ? 240 to ? 370 mV at 1073 K and current density of ? 200 mA/cm², are primarily associated with surface diffusion of silica from La₁₀Si₅AlO_26.5, which partially blocks the electrochemical reaction zone. As compared to the intergrowth nickelate materials, the manganite-based electrodes exhibit substantially worse electrochemical properties, in correlation with the level of oxygen-ionic and electronic conduction in Mn-containing phases. The effects of cation interdiffusion between the cell components as a performance-deteriorating factor are briefly discussed.     

A ceria layer as diffusion barrier between LAMOX and lanthanum strontium cobalt ferrite along with the impedance analysis

A thin interlayer of samarium doped ceria (SDC) is applied as diffusion barrier between La_1 ? xSr_xCo_yFe_1 ? yO₃ x = 0.1–0.4, y = 0.2–0.8 (LSCF) cathode and La_1.8Dy_0.2Mo_1.6W_0.4O₉ (LDMW82) electrolyte to obstruct Mo–Sr diffusion and solid state reaction in the intermediate temperature range of SOFC. We demonstrate the effectiveness of the diffusion barrier through contrasting the clearly defined interfaces of LSCF/SDC/LDMW82 against a rugged growing product layer of LSCF/LDMW82 in 800 °C thermal annealing, and analyze the product composition and the probable new phase. In addition, the measured polarization resistance is considerably lower for the half-cell with a diffusion barrier. Therefore, the electrochemical performance of the LSCF cathode is investigated on the SDC-protected LDMW82. The cell with LSCF (x = 0.4) persistently outperforms the one with x = 0.2 in polarization resistance because of its small low-frequency contribution. The activation energy of polarization resistance is also lower for La_0.6Sr_0.4Co_yFe_1 ? yO₃ (112–135 kJ/mol), than that for La_0.8Sr_0.2Co_yFe_1 ? yO₃ (156–164 kJ/mol). La_0.6Sr_0.4Co_yFe_1 ? yO₃ y = 0.4–0.8 is the proper composition for the cathode interfaced to SDC/LDMW82.     

Structure and oxygen stoichiometry of SrCo0.8Fe0.2O3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ

High temperature X-ray diffraction (HT-XRD), temperature programmed desorption (TPD), thermogravimetric analysis–differential thermal analysis (TGA/DTA) and neutron diffraction were combined to determine the structure and oxygen stoichiometry of SrCo_0.8Fe_0.2O_3−δ (SCF) and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) up to 1273 K in the pO₂ range of 1 to 10^− 5 atm. Formation of the vacancy-ordered brownmillerite phase, SrCo_0.8Fe_0.2O_2.5, was observed as a region of zero oxygen release in the TPD measurements and confirmed by HT-XRD and TGA/DTA. No ordering was observed in the BSCF system by any of the techniques utilized in this work. The oxygen vacancy concentration of BSCF was found to be considerably higher than that of SCF and always higher than that of the ordered brownmillerite phase of SCF, δ = 0.5. The combination of a high vacancy concentration and absence of ordering leads to higher oxygen permeation fluxes through BSCF membranes in comparison to SCF.     

LFN and LSCFN perovskites — structure and transport properties

Structural, electronic and transport properties of LFN (LaFe_1−zNi_zO₃) and LSCFN (La_1−xSr_xCo_1−y −zFe_yNi_zO₃) perovskites synthesized by a modified citric acid method were studied. Structure of the samples was characterized by X-ray studies with Rietveld method analysis. Magnetic properties and valence states of iron ions were characterized by ⁵⁷Fe Moessbauer spectroscopy performed at RT, which were found to be greatly dependent on the chemical composition of the samples. Electrical conductivity was measured in the 20–800 °C temperature range and for some compositions relatively high values (exceeding 100 S cm^− 1) were observed in the 600–800 °C range. Chemical stability studies in relation to Ce_0.8Gd_0.2O_1.9 electrolyte, performed for selected perovskite samples, revealed decreasing stability with increasing Ni concentration and formation of solid solutions in CGO/perovskite composites. The coefficient of thermal expansion (CTE) of LFN perovskites was found to match that of CGO electrolyte (CTE in the 10–13 · 10^− 6 K^− 1 range).     

Preparation and characteristics of Pr1.6Sr0.4NiO4+YSZ as composite cathode of solid oxide fuel cells

Composite cathodes of (1?x wt%)Pr_1.6Sr_0.4NiO₄+(x wt%)Y₂O₃-stabilized ZrO₂ (YSZ; x=0, 10, 20, 30, 40) abbreviated as Pr_1.6Sr_0.4NiO₄+xYSZ, were prepared. The composite cathodes with x>0 matched with electrolyte YSZ in thermal expansion coefficient (TEC) better than the cathode Pr_1.6Sr_0.4NiO₄ did. Pr_1.6Sr_0.4NiO₄+20YSZ exhibited the best performance on cathode overpotential and impedance. When the cathode overpotential was 0.1 V, the polarization current density of Pr_1.6Sr_0.4NiO₄+20YSZ was 0.28 A cm^?2, which is about 5.6 times higher than that of Pr_1.6Sr_0.4NiO₄, 0.05 A cm^?2. The area-specific resistance (ASR) for Pr_1.6Sr_0.4NiO₄+20YSZ was about 17.7% of that for Pr_1.6Sr_0.4NiO₄ at 750 °C.     

Oxygen non-stoichiometry and electrical conductivity of Pr2−xSrxNiO4±δ with x = 0–0.5

Oxygen non-stoichiometry and electrical conductivity of the Pr_2−xSr_xNiO_4±δ series with x = 0.0–0.5 were investigated in Ar/O₂ (pO₂ = 2.5 to 21 000 Pa) within a temperature range of 20–1000 °C. The equilibrium values of oxygen non-stoichiometry and electrical conductivity of these nickelates were determined as functions of temperature and oxygen partial pressure (pO₂). The nickelates with x = 0–0.5 appear to be p-type semiconductors in the investigated temperature and pO₂ ranges. The nickelates with x = 0.3–0.5 show very feebly marked pO₂ dependencies of the conductivity. Pr_1.7Sr_0.3NiO_4±δ shows the anomalies of the conductivity versus oxygen partial pressure which can be related to the orthorhombic–tetragonal crystal structure transformations. The conductivity of the Pr_2−xSr_xNiO_4±δ samples correlates with the average oxidation state of the nickel cations. The samples with x = 0.5 have the highest nickel oxidation state (≈ 2.5+), the highest [Ni³⁺]/[Ni²⁺] ratio close to 1 and show the highest conductivity (≈ 120 S/cm) in the whole pO₂ and temperature ranges investigated.     

Extraordinary fast oxide ion conductivity in La1.61GeO5−δ thin film consisting of nano-size grain

Thin films of La_1.61GeO_5−δ, a new oxide ionic conductor, were fabricated on dense polycrystalline Al₂O₃ substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La_1.61GeO_5−δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La_1.61GeO_5−δ thin films. It was also found that the annealed La_1.61GeO_5−δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La_1.61GeO_5−δ thin films increased with the decreasing thickness as compared to that in bulk La_1.61GeO_5−δ. In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La_1.61GeO_5−δ film with a thickness of 373 nm is as high as 0.05 S cm^− 1 (log(σ/S cm^− 1) = − 1.3) at 573 K.     

Oxygen nonstoichiometry and defect equilibrium in La2 − xSrxNiO4 + δ

Nonstoichiometric variation of oxygen content in La_2 ? xSr_xNiO_4 + δ (x = 0, 0.1, 0.2, 0.3, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range between 873 and 1173 K and the P(O₂) range between 10^? 20 and 1 bar. La_2 ? xSr_xNiO_4 + δ showed the oxygen excess and the oxygen deficient compositions depending on P(O₂), temperature, and the Sr content. The value of partial molar enthalpy of oxygen approaches zero as δ increases in the oxygen excess region, which indicate that the interstitial oxygen formation reaction is suppressed as δ increase. The relationship between δ and logP(O₂) were analyzed by two types of defect equilibrium models. One is a localized electron model, and the other is a delocalized electron model. Both models can well explain the oxygen nonstoichiometry of La_2 ? xSr_xNiO_4 + δ with a regular solution approximation.     

Kinetic demixing and decomposition of oxygen permeable membranes

Oxygen flux through La_0.5Sr_0.5Fe_1−xCo_xO_3−δ (x = 0, 0.5 and 1) membranes has been determined as a function of oxygen partial pressure, temperature and time. The flux was diffusion controlled for low pO₂ gradients while larger pO₂ gradients caused a surface exchange controlled flux. The activation energy of the oxygen flux varied in the range 67–105 kJ/mol. After about 1 month at 1150 °C in an O₂/N₂ gradient the membranes were examined for kinetic demixing and decomposition. On the reducing side only the original perovskite phase was observed at the surface, while on the oxidizing side various secondary phases were observed dependent on the composition at the Fe/Co-site and the Sr + La/Fe + Co ratio of the materials. Moreover, kinetic demixing of the main perovskite phase was also observed, particularly near the surfaces. Grain growth and pore coalescence resulting in membrane expansion were also observed in some cases. The present findings are discussed with regard to the long term chemical stability of the membranes.