Lanthanum Cobalt Research Articles

Effect of thermoelectric material of Ca or Fe-doped LaCoO3

As the Perovskite-type Lanthanum Cobalt Oxide of LaCoO3 is nontoxic and thermally stable even at high temperature, this material is expected as a candidate for thermoelectric applications. The thermoelectric performance of a material is often evaluated by the dimensionless figure-of-merit, ZT (=S[Formula: see text]T/[Formula: see text]), or S[Formula: see text] in the ZT equation. S[Formula: see text] shows the electrical characteristic as a Power factor (PF). It has been reported Seebeck coefficient of LaCoO3 is higher than other oxide materials at room temperature even though electrical conductivity and ZT are lower values. In this study, calcium-doped lanthanum cobaltite La[Formula: see text]Ca[Formula: see text]CoO3 (x = 0.00, 0.05, 0.10 and 0.15) and iron-doped lanthanum cobaltite LaCo[Formula: see text]Fe[Formula: see text]O3 (y = 0.05, 0.10 and 0.15) have been prepared by solid-phase process. The X-ray diffraction patterns of the calcium-doped samples and iron-doped samples show cubic perovskite structure. Electric conductivities were improved by Ca or Fe substitution and showed a tendency to increase with increasing the temperature. The sample substituted with Fe 5 mol.% showed the maximum PF, 0.510 ([Formula: see text] W/K2m) at 548 K, and the sample substituted with Ca 15 mol.% showed the maximum PF, 0.152 ([Formula: see text] W/K2m) at 498 K.

Sol-gel synthesized and microwave heated Pb0.8-yLayCo0.2TiO3 (y = 0.2–0.8) nanoparticles: Structural, morphological and dielectric properties

In this work, the Pb0.8-yLayCo0.2TiO3 (y = 0.2–0.8) (PLCT) nanoparticles were prepared by sol-gel method and further subjected to microwave heating. The formed tetragonal reflection planes of PLCT samples were on par with PbTiO3 (PT) and lanthanum cobalt titanate (LCT) tetragonal phases. Especially, for y = 0.2–0.4, PLCT attained the PT structure while for y = 0.6–0.8, PLCT acquired the LCT structure. That is, the structural transformation occurred from tetragonal PT to tetragonal LCT phases at higher La-contents. The high resolution transmission electron microscope (HRTEM) and field emission scanning electron microscopes (FESEM) showed complete formation of nanofibers at y = 0.8 which may reveal drug delivery system applications. The Fourier transform infrared spectra (FTIR) were recorded in order to find the presence of metal oxide bonds. The dielectric properties evidenced that the transition temperatures (Tc) were found to be increasing from 713 to 783 K with increase of y-value. Furthermore, the temperature and frequency dependence of dielectric constant (ε′), dielectric loss (ε′′), ac-electrical conductivity (σac), real (M′) and imaginary (M′′) parts of complex dielectric modulus (M*) was described. The Power law was used to fit the logσac versus logω plots in order to determine the dc-conductivity (σdc) and exponent (n) values of the samples at room temperature (RT). Later on, the Arrhenius plots (lnσac versus T−1 plots) were drawn to find the activation energies. The results expressed the existence of two activation energies at low and high temperature regions due to slope change before and after Tc.

Oxygen Reduction Reaction of Perovskite Electrocatalysts in Alkaline Media at a Rotating Disk Electrode: Mechanism Analysis

Owing to their high theoretical specific energy density offered by the metal-oxygen chemistry, rechargeable metal-air batteries have garnered much attention in recent years. The major technical challenge of the batteries lies in developing novel low-cost and durable bifunctional oxygen electrocatalysts. In search of such materials, Linear sweep voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS) are conventional techniques used to characterize their catalytic activity. However, due to the complicated multi-step and parallel reactions occurring simultaneously at the oxygen electrode, the classical methods of data analysis are not applicable, such as Levich Equation, Equivalent Circuit method. In this paper, we proposed a physics-based model to include multiple oxygen transfer pathways and further estimate parameters from experimental data of Lanthanum Cobalt Oxide (LCO) catalyst at a rotating disk electrode (RDE). The model is capable to predict reaction kinetics of both sequential two-step oxygen reduction mechanism and direct one-step four-electron transfer mechanism, as well as peroxide disproportion reaction. Nonlinear parameter estimation technique is applied to obtain the relevant kinetic and transport parameters. The modeling results will give experimentalists a guideline in understanding the ORR mechanism for LCO catalyst.

Role of Strain in Surface Segregation of La 1-X SrxCo0.2Fe0.8O3

Strontium doped lanthanum cobalt ferrite (LSCF), a widely used cathode material in solid oxide fuel cells (SOFC), can have long-term stability issues that can adversely affect electrochemical performance. Heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3 with varying Sr content (x = 0.4, 0.3, 0.2) were deposited on single crystal NdGaO3, SrTiO3 and GdScO3 substrates by pulsed laser deposition. The lattice mismatch between the films and the substrate led to different strains in the films. The extent of Sr-rich precipitate formation on the film surface was quantified using total reflection X-ray fluorescence (TXRF), and atomic force microscopy (AFM). The microstructure and the nature of the bonding of the surface Sr-rich phases were investigated by scanning/transmission electron microscopy (S/TEM) and hard x-ray photoelectron spectroscopy (HAXPES), respectively. The strain in the thin films was measured by high-resolution x-ray diffraction (HRXRD). The combined effects of the strontium content and strain on the extent of surface phase formation will be discussed.

Magnesium oxide as synergistic catalyst for oxygen reduction reaction on strontium doped lanthanum cobalt ferrite

MgO has been demonstrated to have positive effects on the oxygen reduction reaction (ORR) at the strontium doped lanthanum cobalt ferrite (LSCF) cathode for solid oxide fuel cells. MgO particles, which were prepared by the infiltration technique using magnesium nitrate as the precursor, are compatible with LSCF under the experimental conditions for electrode fabrication. Electrical conductivity relaxation investigation demonstrated that the chemical oxygen surface exchange coefficient at 750 °C can be improved by a factor up to 2.4 through coating LSCF surface with MgO particles. Electrochemical impedance analysis showed that the area specific interfacial polarization resistance can be obviously reduced, such as from 0.49 Ω cm2 to 0.31 Ω cm2 at 650 °C when a porous LSCF electrode supported on samaria-doped ceria electrolyte was infiltrated with 1.01 wt% MgO particles. Distribution of relaxation time (DRT) analysis suggested that the resistance reduction is associated with the charge transfer process. Thus, MgO can enhance ORR on LSCF electrocatalyst, like the previously reported alkaline earth metal compounds of BaCO3, SrCO3 and CaO.

Improvement of thermoelectric properties of lanthanum cobaltate by Sr and Mn co-substitution

We report thermoelectric (TE) properties of Sr and Mn co-substituted LaCoO3 system from room temperature to 700 K. Sr-substitutions at La and Mn at Co site in LaCoO3 improves the electrical conductivity (σ). Thermal conductivity (κ) of all the samples increases with the increase in temperature but decreases with the substitution in LaCoO3. An estimation of the electronic thermal conductivity (κe) suggests a dominant phonon contribution to thermal conductivity in this system. A maximum value of the figure of merit is 0.14 at 480 K for La0.95Sr0.05Co0.95Mn0.05O3.

Synthesis, structure and thermoelectric properties of $$\mathrm{La}_{1-x}\mathrm{Na}_{x}\mathrm{CoO}_{3 }$$ La 1 - x Na x CoO 3 perovskite oxides

Monovalent ion doped lanthanum cobaltate $$\hbox {La}_{1-x}\hbox {Na}_{x}\hbox {CoO}_{3 }$$ ( $$0 \le x \le 0.25$$ ) compositions were synthesized by the nitrate–citrate gel combustion method. All the heat treatments were limited to below 1123 K, in order to retain the Na stoichiometry. Structural parameters for all the compounds were confirmed by the Rietveld refinement method using powder X-ray diffraction (XRD) data and exhibit the rhombhohedral crystal structure with space group R-3c (No. 167). The scanning electron microscopy study reveals that the particles are spherical in shape and sizes, in the range of 0.2–0.5 $$\upmu $$ m. High temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements were performed on the high density hot pressed pellets in the temperature range of 300–800 K, which exhibit p-type conductivity of pristine and doped compositions. The X-ray photoelectron spectroscopy (XPS) studies confirm the monotonous increase in $$\hbox {Co}^{4+}$$ with doping concentration up to $$x = 0.15$$ , which is correlated with the electrical resistivity and Seebeck coefficient values of the samples. The highest power factor of $$10~\upmu \hbox {W~mK}^{-2 }$$ is achieved for 10 at% Na content at 600 K. Thermoelectric figure of merit is estimated to be $${\sim }1 \times 10^{-2}$$ at 780 K for 15 at% Na-doped samples.

Synthesis of rose like structured LaCoO3 assisted functionalized carbon nanofiber nanocomposite for efficient electrochemical detection of anti-inflammatory drug 4-aminoantipyrine

For the first time, lanthanum cobalt oxide was prepared in a rose like structure and assisted with functionalized carbon nanofiber (f-CNF/LaCoO3) using the hydrothermal technique. Further, the prepared f-CNF/LaCoO3 was used as an electrode material for sensitive electrochemical detection of non-steroid anti-inflammatory drug 4-Aminoantipyrine (4-AAP). The formation of f-CNF/LaCoO3 nanocomposite was characterized by using various analytical techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and amperometric i-t technique were performed to scrutinize the electrochemical activity of f-CNF/LaCoO3/GCE toward the electrochemical oxidation of 4-AAP. The proposed sensor exhibits excellent electrochemical performance with wide linear range (0.001–1374.87 μM) and ultra low detection limit of as low as concentration (1.0 nM) of 4-AAP. Moreover, the excellent selectivity of the sensor was recorded and the practicability of the sensor also demonstrated for detection of 4-AAP in the human urine sample.

Electrochemical Performance and Stability of (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ Cathode Powders Modified by Atomic Layer Deposition

For solid oxide fuel cell (SOFC) systems to become economically competitive, continuous development of new materials and processes is required not only to enhance SOFCs durability but also to reduce their cost. The cathode electrode is known to contribute the most to SOFC performance degradation and efficiency loss. In the case of strontium doped lanthanum cobalt iron oxide, it has been frequently suggested that strontium migration to the cathode surface under operating conditions plays an important role in the degradation. Recently, there have been extensive efforts to modify the surface of the cathode in order to improve the reliability. Most of these attempts have been focused on surface modification subsequent to the cell fabrication (e.g. infiltration). An attractive alternative is to modify the cathode powder surface prior to incorporating the powder into the SOFC. This approach would simplify the manufacturing flow, allowing a “drop-in replacement” of existing cathode powders, while potentially providing similar or even better performance enhancements. In this work, commercial (La0.6Sr0.4)0.95Co0.2Fe0.8O3- d (LSCF) powders have been modified by atomic layer deposition (ALD) using zirconia-, ceria- and hafnia-based oxides. The effect of coating thickness (number of ALD cycles) and dopant addition have been investigated. Morphology and porosity of sintered samples based on ALD coated powders and the pristine powder were found to be rather similar. On the other hand, the electrochemical performance of cathodes based on the ALD treated powders was significantly different from that of the unmodified cathode. This paper examines the details of electrochemical behavior of ALD modified and unmodified powders. Electrochemical impedance spectroscopy (EIS) analysis of symmetrical cells under air at 600-800 °C temperature range showed that high and medium frequency arcs were highly affected in ALD treated samples whereas the low frequency region stayed unchanged. The effect of surface modification of LSCF powders on durability of the LSCF-based cathode is under comprehensive assessment.

Doped LaCoO3 perovskite with Fe: A catalyst with potential antibacterial activity

The present work is aimed at investigating the antimicrobial activity of Lanthanum cobalt ferrate (LaCo1-XFeXO3) perovskite nanoparticles against Gram-negative Pseudomonas aeruginosa MTCC 1688 and Gram-positive Staphylococcus aureus MTCC 96. A series of LaCo1-XFeXO3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) perovskite were synthesized by citrate precursor sol-gel method. Samples have been characterised by X-ray diffraction (XRD), Scanning Electron microscope (SEM), Branauer-Emmett-Teller (BET) nitrogen adsorption technique. The average size of the nanoparticles varied from 10 to 100 nm and XRD revealed the formation of a single phase perovskite structure after annealing the precursor at 800 °C for 8 h. Surface area was found to be highest for LaCo0.2Fe0.8O3 (17.48 m2/g). Antibacterial activity was assessed according to the Radial Diffusion Assay (RDA). The results indicates promising antibacterial activity of LaCo1-XFeXO3 on both Gram-positive and Gram-negative bacteria. MIC values for S.aureus and P.aeruginosa, were in the range of 50–200 μg/mL, the enhanced antibacterial activity has attributed to increased doping of Fe stoichiometric ratio of the perovskite nanoparticles. MBC studies showed that the growth of S.aureus and P.aeruginosa could be ceased above 100 μg/mL of LaCo1-XFeXO3. Thus the data supports the use of such nanoparticles as a potential antimicrobial agents specially with regards to water purification technology.

Thermodynamic Stability of La0.6Sr0.4Co0.2Fe0.8O3- δ in Carbon Dioxide Impurity: A Comprehensive Experimental and Computational Assessment

The thermodynamic stability of strontium doped lanthanum cobalt iron oxide has been examined using computational approach under solid oxide fuel cell device operating conditions. Reaction phase equilibria have been computed for ambient conditions and for enhanced carbon dioxide in the cathode gas. Such a computational approach which takes into account materials stability a priori in addition to performance metrics such as oxygen reduction kinetics can be very useful in designing materials with long term performance stability for high temperature solid state ionic devices such as fuel cells and electrolyzers.

In-situ formed Ce0.8Gd0.2O1.9 barrier layers on yttria stabilized zirconia backbones by infiltration - A promising path to high performing oxygen electrodes of solid oxide cells

Oxygen electrodes for solid oxide cells were prepared by a consecutive infiltration of a gadolinium doped ceria (Ce0.8Gd0.2O1.9, CGO) barrier layer and a lanthanum cobalt nickelate (La0.95Co0.4Ni0.6O3, LCN) electro catalyst layer into a porous yttrium doped zirconia (YSZ) backbone. The influences of the following parameters on the microstructure of the formed CGO barrier layer and on the electrochemical performance of the cells were studied: i) surfactants and wetting agents, ii) ceria/gadolinia coverage, iii) calcination profiles and iv) exposure temperature during testing. The infiltration process of the CGO barrier layer was optimized and a slow, pH controlled decomposition of urea containing precursor solutions was found to be most effective. For these cells a decrease of 84% in the polarization resistance (Rp) was achieved compared to cells without a barrier layer. Furthermore, a better initial performance and only a small increase of the cell-resistance with increasing exposure temperatures during testing were obtained. A complete and homogenous covering of the YSZ backbone with Ce0.8Gd0.2O1.9 was found to be necessary to maintain high performance also at higher exposure temperatures (>800°C).

Anomalous behavior of displacement correlation function and strain in lanthanum cobalt oxide analyzed both from X-ray powder diffraction and EXAFS data

A combined X-ray powder diffraction (XPD) and high-resolution extended X-ray absorption fine structure (EXAFS) at the Co and Ga K-edges study has been performed for LaCoO3 and LaGaO3 ceramics, the latter sample was used as a reference without spin transitions. Based on the X-ray diffraction data, we have found that isotropic atomic displacement parameters (ADP) or mean-squared displacement of the Co–O bond exhibit gradual growth below ~50 K, wherein the strain dependencies testify rapid increase below 150 K for the LaCoO3 having rhombohedral structure. No similar features could be observed for LaGaO3 sample. Above ~100 K the isotropic ADP of the Co–O bond indicate a gradual growth, whereas strain curves show distinct bend near the spin-state transition temperature at about 150 K. According to the EXAFS data, the correlated parallel mean squared relative displacement (MSRD||) of Co–O and Ga–O bonds exhibit a gradual growth above 150 K; however, in the LaCoO3 this parameter is notably bigger. It is supposed that at low temperature the cobalt ions are dominantly in low-spin (LS) state, while certain amount of Co3+ ions located within the surface layer of the crystallines have high-spin state (HS). Temperature growth leads to a gradual transformation of the HS state of the cobalt ions into the highly-hybridized intermediate-spin (IS) state, while the cobalt ions located in the inner part of the crystallines remain LS configuration up to 150 K. Further temperature increase leads to a spin transition of the Co3+ ions located within the crystallines from the LS state into the IS one.

Experimental and thermodynamic evaluation of La1−xSrxMnO3±δ and La1−xSrxCo1−yFeyO3−δ cathodes in Cr-containing humidified air

Chemical and structural stability of strontium doped lanthanum manganite (LSM) and lanthanum cobalt ferrite (LSCF) cathodes in Cr-containing humidified air has been studied by a combination of experimental and thermodynamic approaches. During 100 h tests performed in flowing air (3% H2O) at 1023 K, the electrochemical performance of LSM/yttria doped zirconia (YSZ)/Pt half-cells exhibited a relatively faster degradation in current (I–t) at 0.5 V applied bias than the LSCF/gadolinium-doped ceria (GDC)/Pt half-cells. Cr species from the gas phase deposited predominantly at LSM/YSZ interface while LSCF showed mainly surface deposition throughout the electrode. Raman spectra indicate SrCrO4 formation on the post tested LSCF cathode but not on the post tested LSM cathode. The polarization resistance of the LSM cathode also increased significantly compared to that of the LSCF cathode. A linear programming approach coupled with first-principles thermodynamics suggests that the stoichiometric LSM remains stable and unreacted for the whole range of experimental PCrO3 and temperature conditions whereas the formation of SrCrO4 on LSC cathode is energetically favored at 1023 K supporting the experimental findings.

On the Properties and Long-Term Stability of Infiltrated Lanthanum Cobalt Nickelates (LCN) in Solid Oxide Fuel Cell Cathodes

Infiltration as a fabrication method for solid oxide fuel cells (SOFC) electrodes is offering significant improvements in cell performance at reduced materials and fabrication costs, especially when combined with co-sintering. However, important questions regarding the long-term performance and microstructural stability remain unanswered. Here, we present the results of a three-year project, where large footprint anode-supported SOFCs with a co-sintered cathode backbone and infiltrated La0.95Co0.4Ni0.6O3 (LCN) cathodes were developed and thoroughly characterized. The initial long-term performance and stability of this new cell type was investigated for 1500+ hours, coupled with STEM-EDS investigation of the microstructural changes in the infiltrated electrodes. Additionally, electrodes were further aged at elevated temperatures (750 - 900°C) for periods reaching up to 5000 hours, while following changes in the electrode properties using SEM, BET area, and in-plane conductivity measurements. Finally, the mechanical properties of co-sintered cathode backbone cells were determined in four-point bending tests carried out both at room temperature and at 800°C in air. Based on these results, degradation mechanisms were identified and recommendation for safe operation conditions in real life application could be formulated.

Synthesis of La:Co:TiO<sub>2</sub> Nanocomposite and Photocatalytic Degradation of Tartaric Acid in Water at Various Parameters

In this study, nanocomposites of La:Co:TiO2 was prepared by the co-precipitation method. The alcoholic route of synthesis was adopted here which was gives greenish homogeneous powder of La:Co:TiO2. The material was found in the nanodiamension by the SEM analysis. The rutile and anatase both phases were present in XRD analysis of the synthesized materials. The particle size was found 24 and 82 nm in case of La:Co:TiO2 and pure Titania respectively. The surface area of Titania and La:Co:TiO2 nanocomposites were found 6.4 and 43.2 m2/g. The band gap energy of Titania and La:Co:TiO2 nanocomposites were found 3.2 eV and 3.0 eV respectively. The photodegradation of Tartaric Acid was investigate at different parameters such as temperature, concentration, pH of reaction mixture, dose of photocatalyst and time of illumination of UV-Visible light. The photodegradation of Tartaric Acid occurs 60-80% in presence of Cobalt lanthanum modified Titania and in presence of neat Titania only 20-40% degradation was observed. It is found that photodegradation of Tartaric Acid follow the first order mechanism and its rate constant is become doubled when temperature is rise by 10°C.

Chemical characterization of surface precipitates in La0.7Sr0.3Co0.2Fe0.8O3-δ as cathode material for solid oxide fuel cells

Abstract In this study, a strontium doped lanthanum cobalt ferrite thin film with 30% Sr on A-site, denoted as La 0.7 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ or LSCF-7328, is investigated before and after annealing at 800 °C under CO 2 containing atmosphere for 9 h. The formation of secondary phases on surface of post-annealed LSCF-7328 is observed using atomic force microscope (AFM) and scanning electron microscope (SEM). The extent of Sr segregation at the film surface is monitored using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The bonding environment of the secondary phases formed on the surface is investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Scanning transmission electron microscopy (STEM) and related spectroscopy techniques are used for microstructural and quantitative elemental analyses of the secondary phases on surface. These studies reveal that the secondary phases on surface consist of SrO covered with a capping layer of SrCO 3 . The formation of Co-rich phases is observed on the surface of post-annealed LSCF-7328.

Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δ as Cathode Material for Solid Oxide Fuel Cells.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3-δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Understanding Hydrogen Peroxide Decomposition Activity of Transition Metal Oxide Catalysts in Oxygen Reduction Reaction

Energy storage is essential for large-scale integration of renewable energy such as solar and wind into the electricity grid. Fuel cells and metal-air batteries are promising technologies for grid-scale energy storage owing to their high energy densities and environmental friendliness. However, it is well known that the slow kinetics of the oxygen reduction reaction (ORR) at the cathode limits the efficiency of these fuel cells and metal-air batteries. Among transition metal oxides, calcium-doped lanthanum cobalt oxide is one of the promising catalysts for ORR because of the ability of these materials to support both oxygen evolution and oxygen reduction reactions [1]. In previous work, we demonstrated that in transition metal oxide–carbon composites catalysts, carbon is the primary electrocatalyst for the two electron electro-reduction of oxygen to hydrogen peroxide and the transition metal oxide decomposes the hydroperoxide to generate additional oxygen [2]. Therefore, we have focused on understanding the role of mixed metal oxides on decomposition of hydrogen peroxide for designing more active carbon-based catalyst systems. Specifically, we have studied the effect of manganese substitution of cobalt in calcium-doped lanthanum cobalt oxide on the hydrogen decomposition rate of the catalysts. Manganese-substituted compounds of the formula La0.6Ca0.4Co1-xMnxO3 compound with x value = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1 were synthesized by the sol-gel method using the Pechini process. As previously described [3], a two-step heat treatment was employed for the synthesis of these compounds. In the first step the material subjected to controlled combustion at 120-170 oC, then the material was collected, ground and heat-treated in a second step sintering process at 700oC for 2 hours. The decomposition characteristics of hydrogen peroxide on various transition metal oxides was measured by oxygen reduction studies on rotating-ring-disk (coated glassy carbon) electrode, and by steady state polarization studies of the oxidation and reduction of hydrogen peroxide. We have demonstrated that the mixed potential of the oxide electrode under open circuit conditions could be predicted from the kinetics of the conjugate reactions of peroxide oxidation and reduction. We also find that the higher rate of oxidation of hydroperoxide correlated directly with the improved oxygen reduction activity in the composite catalyst. The presentation will discuss these findings. Figure 1

Surface Segregation in Strontium Doped Lanthanum Cobalt Ferrite: Effect of Composition and Strain

The oxygen reduction reaction (ORR) at the cathode is often the rate-controlling step in the electrochemical reactions occurring in the solid oxide fuel cells (SOFCs). Strontium doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during SOFC operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon, and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x Sr x Co0.2Fe0.8O3-δ with various Sr contents (x = 0.4, 0.3, 0.2) were deposited by Pulsed Laser Deposition (PLD) on single crystal NdGaO3, SrTiO3 and GdScO3 substrates, leading to different strains in the films. The LSCF thin films were annealed at 800 °C for 10 hours in air. The extent of Sr segregation at the post-annealed film surface was quantified using synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The formation of secondary phases were found and their surface area coverage was quantified by Atomic Force Microscope (AFM). Transmission electron microscope (TEM) and related spectroscopy techniques were used for microstructural and quantitative elemental analyses of the secondary phases on surface which were found to be Sr-rich phases. The bonding environment of the Sr-rich phases formed on the surface were investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strains of LSCF thin films on various substrates were measured using high-resolution X-ray diffraction (HRXRD) and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.