Strontium-doped Lanthanum Cobaltite Research Articles

Phase equilibria and defect chemistry of the La-Sr-Co-O system

Strontium doped lanthanum cobaltite (LSC, La1−xSrxCoO3−δ) is a promising oxygen electrode material for intermediate temperature solid oxide cells (SOCs) and oxygen membranes. Its application in service is however impeded by its thermodynamic instability. The present work is aiming to identify the phase stability of LSC by thermodynamic modeling of the La-Sr-Co-O system using the CALculation of PHAse Diagrams (CALPHAD) method coupled with experiments. The calculated results based on the developed database are validated using experimental data both from literature and the present work. General agreement is achieved between model predictions and experimental results. Other properties of the LSC perovskite, such as oxygen non-stoichiometry and cation distribution, are also calculated and predicted based on the developed La-Sr-Co-O database. These calculations can provide important guidance in designing LSC-based materials for various applications and the established thermodynamic functions can serve as input for further thermodynamic calculations on altered and even more complex perovskites.

Room temperature magnetic shape-memory effect in strontium-doped lanthanum cobaltite single crystals

In this study, we investigated the temperature dependence of magnetostriction in the twinned rhombohedral perovskite structure-based lanthanum cobaltite La0.8Sr0.2CoO3. The magnetostriction measured in a magnetic field applied along the pseudocubic [111]c axis showed large hysteresis and residual strains. The residual strains are caused by the twin deformation, which we observed using x-ray diffraction. We found that the critical magnetic field for the twin deformation of La0.8Sr0.2CoO3 increases with temperature. The twin deformation occurs in a sufficiently strong magnetic field (∼6 T), even in the paramagnetic region at room temperature. We also confirmed that the temperature dependence of the critical magnetic field for twin deformation is related to the temperature dependence of the magnetization.

Alternative pathways to α,β-unsaturated ketones via direct oxidative coupling transformation using Sr-doped LaCoO3 perovskite catalyst.

A strontium-doped lanthanum cobaltite perovskite material was prepared, and used as a recyclable and effective heterogeneous catalyst for the direct oxidative coupling of alkenes with aromatic aldehydes to produce α,β-unsaturated ketones. The reaction afforded high yields in the presence of di-tert-butylperoxide as oxidant. Single oxides or salts of strontium, lanthanum and cobalt, and the undoped perovskite offered a lower catalytic activity than the strontium-doped perovskite. Benzaldehyde could be replaced by benzyl alcohol, dibenzyl ether, 2-oxo-2-phenylacetaldehyde, 2-bromoacetophenone or (dimethoxymethyl) benzene in the oxidative coupling reaction with alkenes. To our best knowledge, reactions between these starting materials with alkenes are new and unknown in the literature.

Synthesis of triphenylpyridines via an oxidative cyclization reaction using Sr-doped LaCoO3 perovskite as a recyclable heterogeneous catalyst.

An La0.6Sr0.4CoO3 strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reaction between ketoximes and phenylacetic acids. The transformation proceeded via the oxidative functionalization of the sp3 C–H bond in phenylacetic acid. The La0.6Sr0.4CoO3 catalyst demonstrated a superior performance to that of the pristine LaCoCO3 as well as a series of homogeneous and heterogeneous catalysts. Furthermore, the La0.6Sr0.4CoO3 catalyst was facilely recovered and reused without considerable decline in its catalytic efficiency. To the best of our knowledge, the combination of ketoximes with easily available phenylacetic acids is novel.

Production of syngas by solid oxide electrolysis: A case study

An investigation of syngas production using solid oxide electrolysis was carried out. A conventional solid oxide cell based on Ni supporting cathode, thin yttria-stabilized zirconia electrolyte, yttria-doped ceria interlayer, and strontium-doped lanthanum cobaltite and ferrite-based perovskite anode (Ni-YSZ/YSZ/YDC/LSFC) was used for the reduction of CO2 and water to syngas. This process was assisted by H2 added to the reactant in various amounts to maintain the Ni sites in a metallic state. This was necessary to favour CO2 reaction and to avoid ohmic constraints that may derive from the occurrence of Ni re-oxidation as consequence of the presence of oxidising species like CO2 and water. The outlet gas was analysed by gas chromatography. The presence of CO and CH4 beside CO2 and H2 was detected in the outlet stream. Analysis of outlet gas composition revealed that CO was produced by both electrochemical and catalytic mechanisms. Suitable CO2 conversion was achieved using dry gas.

Performance stability of strontium-doped lanthanum cobaltite ceramic cathode synthesized by a wet chemical method

Porous strontium-doped lanthanum cobaltite (La0.5Sr0.5CoO3−δ, LSC) thin film electrodes were deposited by a metal-organic chemical solution deposition method. These films were used as cathodic electrodes replacing precious metal electrodes for low-temperature solid oxide fuel cell (LT-SOFC) applications. They were characterized electrochemically and compared with devices that had platinum (Pt) cathodes. Pt was prepared by a DC magnetron sputtering method and LSC-YSZ-Pt and Pt-YSZ-Pt (cathode-electrolyte-anode) SOFC unit cells were fabricated. The performance of each cell was continuously evaluated under operating conditions and the performance of fuel cells with a Pt cathode was superior to those using a LSC cathode at the initial stages of operation; however, the output power density started to decrease as a function of time due to the degradation that comes from the heat vulnerability of Pt. As a consequence, the performance of Pt-YSZ-Pt was significantly lower than LSC-YSZ-Pt, and the electrode resistance grew to be about four times larger compared to before operation. In contrast, neither the output power density nor the electrode resistance of LSC-YSZ-Pt changed during operation. These results imply that LSC has better long-term performance stability compared to Pt and feasibility of using ceramic electrode for low-temperature solid oxide fuel cells.

Synthesis and characterization of nanoporous strontium-doped lanthanum cobaltite thin film using metal organic chemical solution deposition

By employing strontium as a dopant of lanthanum cobaltite (LaCoO3), strontium-doped lanthanum cobaltite (La1−xSrxCoO3−δ, LSC) thin film was fabricated using a metal organic chemical solution deposition (MOCSD) method. Lanthanum nitrate hexahydrate [La(NO3)36H2O], strontium acetate [Sr(CH3COO)2], and cobalt acetate tetrahydrate [Co(CH3COO)24H2O] were used as precursors. The coating process was performed through a spin coating method on a substrate, which were then heat treated under various temperature conditions. Electrical properties, microstructures, and crystalline structures with respect to sintering temperature were analyzed. According to these analyses, the change in surface morphology, phase shift, and conductive properties were closely related, which could explain their respective behaviors. Furthermore, sintered strontium-doped lanthanum perovskite oxides showed various conductivities according to the amount of dopant. With the molar ratio of strontium that is stoichiometrically equivalent to lanthanum (La0.5Sr0.5CoO3−δ) thin film showed the best conductivity in the sintering temperature range of 650–700°C, with perovskite phases formed at this temperature condition. As the electrically conductive properties of the thin film are a function of thickness, the films were coated several times to a thickness of approximately 300nm, with the lowest resistivity (approximately 9.06×10−4Ωcm) observed at the optimized sintering temperature and solution composition.

Characterization of LSCO|BCZY|LSCO for Potential Application in IT-SOFC

A high purity of strontium-doped lanthanum cobaltite with formula of La0.6Sr0.4CoO3-δ (LSCO) was synthesized via a combined citrate-EDTA route. LSCO slurry was prepared by mixing LSCO and polyvinyl pyrrolidone (PVP) in ethanol solution. This slurry was manually painted onto both surfaces of yttrium-doped cerate-zirconate, BaCe0.54Zr0.36Y0.1O2.95 (BCZY) electrolyte to fabricate a symmetrical cell of LSCO|BCZY|LSCO. The scanning electron microscopy (SEM) analysis result revealed that the LSCO was well adhered onto the BCZY electrolyte with no formation of crack or air gap/hole at the LSCO|BCZY interface. Elemental composition of LSCO cathode and BCZY electrolyte elements such as lanthanum (La), barium (Ba) and cerium (Ce) at the interface region was confirmed by electron dispersive spectroscopy (EDS) analysis. The electrochemical performance of the fired cell was analyzed in air by an electrochemical impedance spectroscopy (EIS) as a function of temperature ranging from 500 – 800°C. It is found that the fabricated cell exhibits low polarization resistance (Rp) at the operating temperatures and the values are comparable with those reported in literature. This significant result indicates that LSCO is a promising candidate to be used as a cathode material for BCZY electrolyte at intermediate temperatures.

Durability of high performance Ni–yttria stabilized zirconia supported solid oxide electrolysis cells at high current density

We report the durability of a solid oxide electrolysis cell (SOEC) with a record low initial area specific resistance (ASR) and a record low degradation rate. The cell consists of a Ni–yttria stabilized zirconia (YSZ) cermet as support and active fuel electrode, a YSZ electrolyte, a gadolinia doped ceria (CGO) inter-diffusion barrier, and a strontium doped lanthanum cobaltite (LSC)–CGO composite oxygen electrode. The cell was tested at 800 °C and −1 A cm−2 converting 31% of a 0.1:0.45:0.45 mixture of H2:H2O:CO2 for approximately 2700 h, demonstrating an initial ASR of 200 mΩ cm2 and a steady degradation rate of ≤12 mV (or 0.9%) per 1000 h. Electrochemical impedance spectroscopy (EIS) was used to study in situ changes in the electrochemical response of the cell and the retrieved data was treated to deconvolute resistive contributions from the physiochemical processes occurring within the cell. The results showed rapid initial fuel electrode degradation during the first 350 h followed by partial reactivation. The serial resistance was found to increase with time but in an exponentially decaying behavior. A discussion is made based on the detailed electrochemical results together with post-mortem microstructural analysis.

INVITED TALK - In Situ Electronic Structure Measurements and Correlations to Reactivity on Hetero-Structures for Solid Oxide Fuel Cells

Strontium-doped lanthanum cobaltite is one of the most promising cathode candidates for intermediate temperature solid oxide fuel cells. Recently, the hetero-interfaces between the perovskite (La,Sr)CoO3 (LSC113) and the Ruddlesden-Popper (La,Sr)2CoO4 (LSC214) phases have shown highly enhanced oxygen exchange kinetics. This observation offers the potential for a new class of cathode structure design, either via multilayering or by forming vertical heteroepitaxial nanocomposites, with high densities of these special interfaces. In this work, nanoscale multilayers and vertically aligned nanocomposites of LSC113/ 214 have been prepared by pulsed laser deposition. Their surface chemistry was characterized by Auger Electron Spectroscopy with high spatial resolution, and their surface electronic structure was probed by scanning tunneling microscopy at elevated temperatures and in oxygen gas. The intimate contact of LSC214 to the LSC113 phase, rendered the charge transfer on the LSC214 surface to be more facile, contributing to enhanced oxygen reduction reactivity near the LSC113/ 214 interfacial regions intersecting the surface. The enhancement of the ORR kinetics is also confirmed by EIS measurement at relatively low temperatures below 400 oC. The surface chemistry at higher temperatures ages by time and hides the improvements that arise from the coupling of these two phases through their interface. These observations lead to a novel means for SOFC cathode fabrication with enhanced performance and provide guidelines for identifying other promising cathode candidates.

Synthesis and Characterization of Strontium-Doped Lanthanum Cobaltite Cathode Material Prepared by a Modified Combined Complexing Method

A potential candidate for intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material which is strontium-doped lanthanum cobaltite, La0.6Sr0.4CoO3-α (LSCO) has been synthesized by a complexing method. Citric acid (CA) and ethylenediaminetetraacetic acid (EDTA) were used as a combined chelating agent and ethylene glycol (EG) was employed as surfactant. The obtained powder was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). TGA results showed the thermal decomposition of the precursor gel was completed at 700 °C. A single perovskite phase of LSCO with cubic structure was obtained at calcination temperature of 1000 °C with heating/cooling rate of 10 °C min-1 as confirmed by XRD analysis. SEM result revealed that the morphology of the powder was spherical in shape with diameter ranging from 250 to 650 nm. Apparently, the bulk sample consists of almost homogeneous and identical particles.

ChemInform Abstract: Elementary Kinetic Modeling of Solid Oxide Fuel Cell Electrode Reactions

AbstractReview: 96 refs.

Impedance Modeling of Solid Oxide Fuel Cell Cathodes

A 1-dimensional impedance model for a solid oxide fuel cell cathode is formulated and applied to a cathode consisting of 50/50 wt% strontium doped lanthanum cobaltite and gadolinia doped ceria. A total of 42 impedance spectra were recorded in the temperature range: 555-852°C and in the oxygen partial pressure range 0.028-1.00 atm. The recorded impedance spectra were successfully analyzed using the developed impedance model in the investigated temperature and oxygen partial pressure range. It is also demonstrated that the model can be used to predict how impedance spectra evolve with different physical parameters such as the cathode thickness.

A comparative study on polarization behavior of (La,Sr)MnO 3 and (La,Sr)CoO 3 cathodes for solid oxide fuel cells

The cathodic polarization behavior of strontium-doped lanthanum manganite (LSM) fired on stabilized zirconia electrolyte was investigated. The performance of LSM was enhanced significantly during discharge operation. The subsequent relaxation behavior under open-circuit state proceeded with two steps. A comparative study was also performed on strontium-doped lanthanum cobaltite (LSC) electrodes. The cathode potential of LSC with stabilized zirconia electrolyte decreased significantly at first and then increased gradually under current-static cathodic polarization. The subsequent relaxation in the open-circuit state also proceeded by two steps. When the interlayer of Sm-doped ceria (SDC) was introduced between electrolyte and cathode, the performance enhancement under cathodic polarization became much smaller regardless of the cathode materials, LSM and LSC. The mechanisms for the polarization behavior of LSM and LSC were discussed based on the obtained results.

Electrochemical and gas phase parameters of cathodes for intermediate temperature solid oxide fuel cells

A series of cyclic voltammetry, chronoamperometry and electrochemical impedance experiments have been carried out in order to investigate the effect of cathode composition and porosity on the electrochemical characteristics of strontium-doped lanthanum, praseodymium and gadolinium cobaltite cathodes. The impedance responses at different electrode potentials of the half cell and symmetric single cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cell cathode and anode reactions contributions based on the results of simultaneous analysis of half cells and symmetric single cells has been made by differential impedance real part vs. ac frequency plot analysis method. Noticeable influence of cathode chemical composition, meso-porosity and macro-porosity on the electrochemical activity of the oxygen electroreduction has been demonstrated. Seeming activation energy values have been calculated and discussed.

Sulfur as Pollutant Species on the Cathode Side of a SOFC System

In order to link electrochemical performance degradation of the cathode in a large solid oxide fuel cell (SOFC) to the presence of pollutant species, a spatially-resolved study of contaminants was performed. Distribution maps of pollutants over the cell allowed to identify their sources. Besides chromium and silicon, sulfur was found as a major pollutant species. Its preferential reaction with strontium doped lanthanum cobaltite (LSC), forming strontium sulfate SrSO4, compared to strontium doped lanthanum manganite (LSM) is revealed here. When sulfur poisoning arises in combination with chromium, a sulfur-containing strontium chromate compound is formed.

Effect of Cell Geometry on the Electrochemical Parameters of Solid Oxide Fuel Cell Cathodes

A series of electrochemical impedance experiments has been carried out in order to investigate the effect of cell composition and geometry on the determination of electrochemical characteristics of strontium-doped lanthanum cobaltite cathodes. The impedance responses at different electrode potentials of the half-cell and symmetric single-cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cells has been achieved by a differential impedance real part vs ac frequency plot analysis method. The results indicate that the three-electrode half-cell configuration is more suitable for fundamental research of material parameters at different electrode potentials (measured vs reference electrode), whereas the simpler two-electrode single-cell setup allows the estimation of cathode performance in approximate working conditions.

Strontium-Doped Lanthanum Cobaltite and Manganite: Highly Active Catalysts for Toluene Complete Oxidation

La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) catalysts have been fabricated via a strategy of citric acid complexing and hydrothermal treatment. The oxidation of toluene was used as a probe reaction for ...

Functionally graded composite cathodes for solid oxide fuel cells

Functionally graded solid oxide fuel cell (SOFC) cathodes have been prepared from mixtures of strontium-doped lanthanum manganite (LSM) and gadolinia-doped ceria (CGO) using slurry spraying techniques. Similar samples were also prepared from mixtures of LSM and ytrria-stabilised zirconia (YSZ). A current collector comprising a mixture of LSM and strontium-doped lanthanum cobaltite (LSCO) was then applied to both cathode types. Samples were characterised using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Characterisation using EIS techniques showed that cathodes incorporating CGO into the structure gave improved performance over those fabricated using YSZ. These performance gains were most noticeable as the temperature was decreased towards 700 °C, and were maintained during the testing of three cell membrane electrode assemblies fabricated to the Rolls-Royce design.

Oxygen reaction on strontium-doped lanthanum cobaltite dense electrodes at intermediate temperatures

La 0.7Sr 0.3CoO 3− δ (LSC) powder was characterized by thermogravimetry. Thin, dense layers of LSC were deposited by RF magnetron sputtering on YSZ pellets. The electrochemical behavior was studied as functions of temperature (300–530°C) and oxygen partial pressure (0.21–2×10 −5 bar) by impedance spectroscopy. Impedance diagrams were decomposed into two elements, characteristic of the electrode polarization. The low frequency contribution can be described either as a limiting Warburg diffusion loop or as a semi-circle. These results can be interpreted in terms of a progressive evolution of the limiting process as functions of oxygen pressure and temperature, i.e. a finite diffusion process in series with dissociative sorption process. Under low oxygen pressure, at temperatures higher than 475°C, gas-phase diffusion polarization becomes significant.