LaCoO3 Research Articles

Magnetism and phase transitions in LaCoO3

Neutron scattering and magnetometry measurements have been used to study phase transitions in LaCoO3 (LCO). For H ≤ 100 Oe, evidence for a ferromagnetic (FM) transition is observed at Tc ≈ 87 K. For 1 kOe ≤ H ≤ 60 kOe, no transition is apparent. For all H, Curie–Weiss analysis shows predominantly antiferromagnetic (AFM) interactions for T > Tc, but the lack of long-range AFM order indicates magnetic frustration. We argue that the weak ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and nanoparticles. The lattice strain is present at the bulk surfaces and at the interfaces between the LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly affected by the Co–O–Co angle (γ), in agreement with recent band calculations which predict that ferromagnetic long-range order can only take place above a critical value, γC. Consistent with recent thin film estimations, we find γC = 162.8°. For γ > γC, we observe power-law behavior in the structural parameters. γ decreases with T until the critical temperature, To ≈ 37 K; below To the rate of change becomes very small. For T < To, FM order appears to be confined to regions close to the surfaces, likely due to the lattice strain keeping the local Co–O–Co angle above γC.

Synergy effects of Pr1.91Ni0.71Cu0.24Ga0.05O4 and Ba0.5La0.5CoO3 composite on cathodic activity for intermediate temperature solid oxide fuel cells

Synergy effects of mixing Pr2NiO4 and La(Ba)CoO3 on cathodic performance were investigated using a LaGaO3-based oxide electrolyte. It was observed that cathodic overpotential could be suppressed significantly by mixing Pr2NiO4 with La(Ba)CoO3 base oxide, in spite of a decrease in the electrical conductivity. Because LaCoO3-doped with Ba shows high oxygen dissociation activity and Pr2NiO4 shows high oxide ion and hole conductivity, the active sites for oxygen dissociation could be extended in three dimensions. In response to the improved number of active sites, diffusion overpotential was decreased significantly by mixing Pr2NiO4-based oxide with LaCoO3-based oxide. The maximum power density of 117 mW cm−2 was achieved at temperatures as low as 673 K using a LaGaO3 thin film electrolyte (5-μm thick) and a composite oxide of Pr2NiO4–La(Ba)CoO3 cathode.

LaCoO3: Promising cathode material for protonic ceramic fuel cells based on a BaCe0.2Zr0.7Y0.1O3−δ electrolyte

Symmetric cells (cathode/electrolyte/cathode) were prepared using BaCe0.2Zr0.7Y0.1O3−δ (BCZY27) as proton conducting electrolyte and LaCoO3 (LC) infiltrated into a porous BCZY27 backbone as cathode. Single phased LC was formed after annealing in air at 600 °C for 2 h. Scanning electron micrographs showed the presence of the infiltrated LC in the full cathode depth. Transmission electron micrographs revealed LC grains (60–80 nm) covering partly the BCZY27 grains (200 nm–1 μm). Impedance spectra were recorded at 500 °C and 600 °C, varying the oxygen partial pressure and the water vapour pressure. Two arcs correspond to the cathode contribution: a middle range frequency one (charge transfer) and a low frequency one (oxygen dissociation/adsorption). The area specific resistances (ASRs) of both contributions increase when decreasing the oxygen partial pressure. The low frequency arc is independent on the water vapour pressure while the charge transfer ASR values increase with higher pH2O. The cathode ASRs of 0.39 and 0.11 Ω cm2 at 500 and 600 °C respectively, in air (pH2O = 0.01 atm) are the lowest reported to the authors’ knowledge for PCFC cathodes. Furthermore, this work shows that the presence of oxide ion conduction in the cathode material is not necessary for good performance.

Synergy Effects of Pr2NiO4 and Ba(La)CoO3 on Cathodic Activity for Intermediate Temperature Solid Oxide Fuel Cells

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Novel sol–gel methodology to produce LaCoO3 by acrylamide polymerization assisted by γ-irradiation

In this paper we report the synthesis of LaCoO3 (LCO) nano-particles with two methodologies: the conventional sol–gel reaction of acrylamide (AA) polymerization using a cross-linking agent (methylenebisacrylamide or MBA) with the activation of the polymerization reaction by thermo-chemical initiator (azobisisobutyrnitrile or AIBN). The second was a novel sol–gel methodology in which the polymerization of AA monomers was done without MBA and the initiation was achieved by gamma radiation. With thermochemical initiator a xerogel with a foam and porous structure was obtained, while the gamma-irradiation of the mixture leads to the formation of a compact resin with entrapped cations. X-ray diffraction (XRD) shows that formation of the product begins around 500°C and according to analysis of microscopy images of powders calcined in 700°C the average sizes of particles are 20nm and 42nm for samples obtained using γ-irradiation and AIBN as initiators, respectively. TEM images also show differences in particle morphology. Those synthesized using AIBN as initiator are dispersed, while those with γ-irradiation are in aggregates.

Enhanced one dimensional mobility of oxygen on strained LaCoO3(001) surface

Mechanisms by which lattice strain alters the oxygen reduction reaction (ORR) kinetics are important to understand in order to increase the ORR activity of solid oxide fuel cell cathodes. Here we assess the mechanistic and quantitative effects of strain on oxygen diffusion on the LaCoO3(LCO)(001) surface using density functional theory calculations. Planar tensile strain is found to reduce the migration barrier of oxygen vacancy anisotropically on the LCO(001) surface, inducing an enhanced mobility along the [10] direction and a suppressed mobility along the [110] direction. The increase of space around Co that the oxygen (vacancy) traverses with a curved path is the cause of the enhanced mobility along the [10]. The increasing octahedral distortions with planar tensile strain inhibit the migration of oxygen vacancy along the [110] direction. Furthermore, the mobility of the adsorbed oxygen atom is suppressed with increasing strain due to its stronger adsorption on the surface. On the basis of rate theory estimates, the significantly lower energy barrier for oxygen vacancy diffusion is expected to dominate the other degrading factors and actually accelerate the ORR kinetics on LCO(001) up to 3% strain. The insights obtained here are useful for designing strategies to control the desired anisotropic and uni-directional oxygen transport along strained hetero-interfaces.

High Performance Electrode for Medium-Temperature Solid Oxide Fuel Cells: Control of Microstructure of La ( Sr ) CoO3 Cathodes with Highly Dispersed Pt Electrocatalysts

In order to enhance gas diffusion rates in a porous (LSC) cathode, additional μm-sized pores were introduced by sintering a LSC paste containing fine polymer beads coated on yttria-stabilized zirconia (YSZ) electrolyte. A thin and dense samaria-doped ceria interlayer was used to avoid unfavorable solid-state reactions between LSC and YSZ. Control of the microstructure of LSC cathode appreciably lowered the IR-free overpotential without an increase of the ohmic resistance. The performance of the LSC cathode with optimized microstructure was enhanced further with highly dispersed Pt catalysts at 3 wt % loading, especially at low operating temperature. The current density on the Pt-LSC cathode at an overpotential of −50 mV was 1 A/cm2 at 800°C in air. © 2001 The Electrochemical Society. All rights reserved.

Ba(La)CoO3 Cathode for Intermediate Temperature Operating Solid Oxide Fuel Cell Using LaGaO3 Based Electrolyte

Ba(La)CoO3 Cathode for Intermediate Temperature Operating Solid Oxide Fuel Cell Using LaGaO3 Based Electrolyte

Intermediate Temperature Solid Oxide Fuel Cells with Doped Lanthanum Gallate Electrolyte, La(Sr)CoO3 Cathode, and Ni-SDC Cermet Anode

Intermediate Temperature Solid Oxide Fuel Cells with Doped Lanthanum Gallate Electrolyte, La(Sr)CoO3 Cathode, and Ni-SDC Cermet Anode

Powder prepared by spray pyrolysis as an electrode material for solid oxide fuel cells

A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. Examples that are discussed include the following powders: (La0.8Sr0.2)0.9MnO3-YSZ (La(Sr)MnO3-YSZ), La0.6Sr0.4CoO3 (La(Sr)CoO3), (CeO2)0.8(SmO1.5)0.2-NiO (SDC-NiO) and La0.9Sr0.1Ga0.8Mg0.2O3-NiO (LSGM-NiO). For all these powders, spherical, non-agglomerated, submicrometer particles were obtained from aqueous solution of metal salts into a furnace using an ultrasonic atomizer at 1.7 MHz. After SP some of the particles exhibit a hollow-shell morphology. Subsequent calcination at 1000°C yielded crystalline particles. The electrical performance of Ni-SDC/LSGM/La(Sr)CoO3 fuel cells operating at 800°C, prepared from the powders obtained by SP, is reported.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

The LaGa1−x−yCoxMgyO3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa1−x−yCoxMgyO3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10−6 K−1 at 300–1100 K. Doping La(Ga,Co)O3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa1−x−yCoxMgyO3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.

High-Performance Electrode for Medium-Temperature Solid Oxide Fuel Cells Control of Microstructure of La(Sr)CoO3 Cathode with Highly Dispersed Pt Electrocatalysts

High-Performance Electrode for Medium-Temperature Solid Oxide Fuel Cells Control of Microstructure of La(Sr)CoO3 Cathode with Highly Dispersed Pt Electrocatalysts

Built-in voltages and asymmetric polarization switching in Pb(Zr,Ti)O3 thin film capacitors

Asymmetric polarization switching of Pb(Zr,Ti)O3 (PZT) thin films with different electrode configuration has been studied in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 (LSCO) heterostructures in which the conducting oxide (La,Sr)CoO3 and/or LaCoO3 (LCO) have been used as an electrode. Polarization-voltage (P-V) hysteresis loop of LSCO/PZT/LSCO was symmetric. However, LCO/PZT/LSCO showed a largely asymmetric P-V hysteresis loop and large relaxation of the remanent polarization at the negatively poled state, eventually leading to an imprint failure. On the other hand, LSCO/PZT/LCO exhibited large relaxation of the positively poled state. The asymmetric behavior of the polarized states implies the presence of an internal electric field inside the PZT layer. It is suggested that the internal electric field is caused by built-in voltages at LCO/PZT and LSCO/PZT interfaces. The built-in voltages at LCO/PZT and LSCO/PZT interfaces were 0.6 V and 0.12 V, respectively.

Ferroelectric field effect in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 heterostructures

Epitaxial LaCoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 (LSCO) heterostructures have been grown on LaAlO3 by pulsed laser deposition for investigating ferroelectric field effect. In the heterostructure, semiconducting LaCoO3 was used as a conducting channel layer, instead Si. The resistivity of the LaCoO3 (LCO) channel layer was found to be dependent on an oxygen ambient, primarily the ambient oxygen pressure during deposition. The resistivity of the LCO layer varied in the range of 0.1 – 100 Ω cm. Ferroelectric field effect induced in LaCoO3 layer was observed by measuring the resistance modulation of the LCO layer with respect to the polarized state of the PZT layer. The resistance modulation of 9% was obtained in the 680 Å thick LCO layer. Further the resistance modulation was improved up to 45% after applying dc bias. It is suggested that the LCO/PZT/LSCO heterostructure can be used as a ferroelectric field effect transistor.

Asymmetric switching and imprint in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 heterostructures

(La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 (LSCO) heterostructures have been grown on LaAlO3 substrates by pulsed laser deposition (PLD) to investigate asymmetric polarization switching of Pb(Zr,Ti)O3 (PZT) thin films with different electrode configuration. P-V hysteresis loop of LSCO/PZT/LSCO was symmetric. However, LaCoO3 (LCO)/PZT/LSCO showed a largely asymmetric P-V hysteresis loop and large relaxation of the remanent polarization at the negatively poled state, which means that the negatively poled state was unstable. On the other hand, LSCO/PZT/LCO exhibited large relaxation of the positively poled state. The asymmetric behavior of the polarized states implies the presence of an internal electric field inside the PZT layer. It is suggested that the internal electric field caused by built-in voltages at LCO/PZT and LSCO/PZT interfaces. The built-in voltages at LCO/PZT and LSCO/PZT interfaces were 0.6 eV and 2.1 eV, respectively.

Electrochemical filters for oxygen sensors with improved surface properties

Oxygen sensors based on YSZ (yttria stabilized zirconia) are best known for high temperature measurements. The protection of such oxygen sensors with dense electrochemical filters is an effective way to increase their life time in aggressive industrial atmospheres. This work reports on the performance of sensors protected by composite or single phase mixed conducting filters. The first group includes materials based on GCO (Gd2O3 doped CeO2)+LC (LaCoO3), and on YSZ+LCM (La(Mn,Co)O3). Sensors protected with GCO+LC composites showed the best performance amongst the composite filters already evaluated. Electroded single phase GCO filters were also tested to check on the role of surface reaction kinetics on the overall oxygen transport process across the filter. The fastest sensor responses to a sudden decrease in oxygen partial pressure were obtained with deposited electrodes either on both surfaces of the filter or on the filter cathode side. These preliminary results suggest that the cathodic process is more relevant than the anodic one in determining the filter permeability (and protected sensor performance), which fulfils the logical requirement of electrode free surfaces in contact with the furnace atmosphere.

Electrical properties of the system lanthanum lead cobalt titanium oxide

Measurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La1−x Pb x−δ Co1−x Ti x O3−δ for 0·2≤x≤0·9 have been made in the temperature range 300–800 K. AC conductivity,σ a.c, of all the samples were measured as a function of temperature (300–573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5–6 orders of magnitude, depends on both the compositionx and the structure of the samples. All the samples exhibitp-type electronic conduction. The value of Seebeck coefficient,α, for samples withx≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour ofα for samples withx≤0·5 and for samples withx>0·5 over the entire temperature range is similar to that of La CoO3. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures.