Doped Lanthanum Gallate Research Articles

Approaches for the preparation of dense ceramics and sintering aids for Sr/Mg doped lanthanum gallate: focus review

This review focuses on Sr/Mg doped lanthanum gallate and its use as a dense electrolyte for medium-temperature solid oxide electrochemical devices. Methods for synthesizing the material are discussed, including techniques that lower the temperature required to attain a single-phase crystalline structure. The effects of secondary phases and grain size on conductivity of ceramics are also investigated. Moreover, the techniques used for compacting electrolyte powder and reducing energy expenses, as well as the impacts of sintering aids on ceramic properties, are analyzed. The potential advantages and disadvantages of the proposed alterations to generate an electrolyte from doped lanthanum gallate are evaluated.

Recent Progress in the Design, Characterisation and Application of LaAlO3- and LaGaO3-Based Solid Oxide Fuel Cell Electrolytes.

Solid oxide fuel cells (SOFCs) are efficient electrochemical devices that allow for the direct conversion of fuels (their chemical energy) into electricity. Although conventional SOFCs based on YSZ electrolytes are widely used from laboratory to commercial scales, the development of alternative ion-conducting electrolytes is of great importance for improving SOFC performance at reduced operation temperatures. The review summarizes the basic information on two representative families of oxygen-conducting electrolytes: doped lanthanum aluminates (LaAlO3) and lanthanum gallates (LaGaO3). Their preparation features, chemical stability, thermal behaviour and transport properties are thoroughly analyzed in terms of their connection with the target functional parameters of related SOFCs. The data presented here will serve as a starting point for further studies of La-based perovskites, including in the fields of solid state ionics, electrochemistry and applied energy.

Analysis of thermotransport and thermal and ionic conductivity in doped lanthanum gallate (LSGM) using molecular dynamics

The lattice thermal conductivity, ionic conductivity and thermotransport of doped lanthanum gallate (LSGM) were investigated using molecular dynamics (MD) using the Green-Kubo formalism across a wide temperature range. The lattice thermal conductivity was estimated by the heat current autocorrelation function directly from the simulation. Data from the autocorrelation function and the tracer diffusion coefficient of the mean squared displacement were used to calculate the ionic conductivity. The Green-Kubo formalism was employed to calculate the Onsager cross-coefficients (LAq and LqA). The results showed that the concentration of the dopant had a significant effect on both the thermotransport and the ionic conductivity. The results of this study demonstrated good agreement with the available experimental and theoretical data.

Electrical Conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-D/La1.55Sr0.45Ga3O7+D Composite Electrolyte

Doped lanthanum gallate constitutes a family of solid electrolytes with high ionic conductivity, wide electrolytic domain and good chemical stability. The combination of their properties turns some compounds of this family, promising candidates for application in solid oxide fuel cells operating at intermediate temperatures. In this work, La0.9Sr0.1Ga0.8Mg0.2O3-δ - x La1.55Sr0.45Ga3O7+δ, with x = 0, 1 and 5 wt.% were investigated aiming to obtain a composite solid electrolyte with optimized properties compared to the parent material. Composite electrolytes were prepared by solid state reaction method and evaluated for densification, phase content and microstructure. Electrical conductivity measurements were carried out as a function of temperature and oxygen partial pressure.Increase on densification and microstructural homogeneity were obtained with increasing fraction of the melillite phase. The electrical conductivity determined by impedance spectroscopy revealed a decrease of the grain resistivity for increasing amounts of La1.55Sr0.45Ga3O7+δ.

Electrical Conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-D/La1.55Sr0.45Ga3O7+D Composite Electrolyte

Doped lanthanum gallate constitutes a family of solid electrolytes with high ionic conductivity, wide electrolytic domain, and good chemical stability. The combination of its properties turns some compounds of this family promising candidates for application in solid oxide cells operating at intermediate temperatures. In this work, La0.9Sr0.1Ga0.8Mg0.2O3-y - xLa1.55Sr0.45Ga3O7+δ with x = 0, 1, and 5 wt.% were investigated aiming to obtain a composite solid electrolyte with optimized properties compared to the parent material. Composite electrolytes were prepared by solid state reaction method and evaluated for densification, phase content, and microstructure. Electrical conductivity measurements were carried out as a function of temperature and oxygen partial pressure. Increase on densification and structural homogeneity were obtained with increasing fraction of the melilite phase. The electrical conductivity determined by impedance spectroscopy revealed a decrease of the bulk resistivity for increasing amounts of La1.55Sr0.45Ga3O7+δ.

Thermal and spectroscopic characterization of sol–gel-synthesized doped lanthanum gallate

In this study, a complex oxide solid solution consisting of lanthanum gallate with partial substitutions for strontium and magnesium (La0.9Sr0.1Ga0.8Mg0.2O2.85) was synthesized by the sol–gel route, aiming to obtain a sinter active powder and a final material that could be stoichiometrically controlled. The thermal behavior of the synthesized powder involves several steps of decomposition. The linear shrinkage of green compacts up to 1500 °C was 31%. The bulk conductivity of sintered specimens increases with sintering temperature up to 1400 °C. The stoichiometry was maintained for sintering temperatures up to 1450 °C. Elemental mapping obtained by energy-dispersive spectroscopy evidenced magnesium segregation at the grain boundaries. The overall results evidence the suitability of the synthesis method for preparing doped lanthanum gallate.

Parameterization of dilute Ising model for iron-containing lanthanum gallate and aluminate solid solutions based on first-principles calculations

Calculations of the cubic models of two solid solutions LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 have been performed within the hybrid density functional theory. Multiple configurations have been considered for the solid solutions resulting from a different distribution of Fe atoms over the p-metal (Ga or Al) positions accounting for different spin orientation for Fe atoms themselves. It was demonstrated that 27 structures for both LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 should be treated to account for all the possible configurations in case of cubic 2 × 2 × 2 supercell. Optimized geometry, energy, and electron properties were calculated for all the obtained configurations. Statistical weights and probabilities were estimated for each symmetry non-equivalent configuration of the solid solutions within the canonical ensemble. A new parameterization for the dilute Ising model has been proposed. In this model, we account for non-magnetic contributions, which are absent in the simple Ising model, using the lattice approach based on the concept of interchange energy. Two model parameters (the magnetic coupling constant and interchange energy) were fitted to the calculated total energies of all considered configurations of both solid solutions. The dilute Ising model confirmed the benefit of Fe-clustering in doped lanthanum gallate against aluminate. Different signs of the estimated interchange energy enable us to explain the reasons for such differences.

Fluorescence characterization of heavily Eu3+-doped lanthanum gallate glass spheres with high quenching concentration.

Highly Eu2O3-doped (up to 30mol%) La2O3-Ga2O3 glasses were synthesized by an aerodynamic levitation technique. The red emission associated with D05 level of Eu3+was most intense at 20mol% Eu2O3, indicating a small effect of concentration quenching even at high Eu concentration. The fluorescence decay curves indicated that the dominant transition mechanism at high Eu concentration was fast energy migration among the Eu3+ ions, which averaged the environment of the Eu, yielding a nearly single-exponential decay of D05→F27 emission. Lifetime of the D05 level gradually decreased with Eu2O3 over the 5-25mol% and rapidly decreased at 30mol%, very consistent with the observed emission spectra.

Influence of Yttria-Stabilized Zirconia on Microstructure and Electrical Properties of Doped Lanthanum Gallate

Lanthanum gallate with partial substitutions for strontium and magnesium shows higher ionic conductivity than traditional ionic conductors such as yttria-stabilized zirconia (8YSZ), and has been considered for application as solid electrolyte and electrode in Solid Oxide Fuel Cells operating at intermediate temperatures (500-700oC). Parallel to the development of new solid electrolytes, improvement of the already known electrolytes has been one goal. In this work, the effects of adding 8YSZ to La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), on the microstructure and electrical performance were investigated. Compound electrolytes consisting of up to 20 wt.% 8YSZ were prepared by solid state reactions. Sintered pellets with up to 10 wt.% 8YSZ achieved near full density after sintering at 1450oC and reduction in the fraction of impurity phases. Addition of 8YSZ promoted grain growth to LSGM. The higher ionic conductivity was obtained for 1 wt.% 8YSZ addition with slight decrease of the apparent activation energy.

Sintering evaluation of doped lanthanum gallate based on thermodilatometry

The sintering behavior of La0.9Sr0.1Ga0.8Mg0.2O3-δ oxide-ion conductor was systematically investigated by thermodilatometry. The shrinkage data obtained with heating rates of 4, 7, 10 and 12 °C min−1 were analyzed by the constant rate of heating model and by construction of the master sintering curve. Validation of the master sintering curve was carried out by measurements of density in conventionally sintered specimens. Slight anisotropy of shrinkage data was found and changes to the basic equation of density was proposed to account for this effect. Plotting the data determined by the constant rate of heating model versus density allowed an easy identification of the density range of constant activation energy. The activation energy (865 kJ mol−1) obtained from the master sintering curve correlates quite well with that (874 kJ mol−1) obtained by the constant rate of heating model.

Effect of grain size on the electrical properties of strontium and magnesium doped lanthanum gallate electrolytes

The aim of this research is to investigate the effect of average grain size on electrical and electrochemical performances of La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM). For this purpose, the glycine-nitrate method is used to synthesize the LSGM electrolytes and subsequently sintered at 1350, 1400, 1450 and 1500 °C in a muffle furnace with temperature holding for 10 h. XRD study reveals that all of the sintered LSGM ceramic electrolytes crystallize to a single phase of orthorhombic perovskite structure without any impurity phases. Results from scanning electron microscopy (SEM) study demonstrate that increasing sintering temperature from 1350 to 1500 °C leads to increase in average grain size from 1.02 to 4.87 μm. Electrochemical AC impedance tests show that the bulk resistance is nearly independent on the average grain size, and the grain boundary resistance changes because of the difference in the average grain size. The LSGM sample sintered at 1500 °C achieves the maximum electrical conductivity 9.19 × 10−2 S cm−1 at 800 °C. The maximum power density of assembled cell with LSGM sintered at 1500 °C as an electrolyte is 0.76 W cm−2 at 800 °C in air atmosphere.

Development of tubular anode-supported solid oxide fuel cell cell and 4-cell-stack based on lanthanum gallate electrolyte membrane for mobile application

Anode-supported tubular solid oxide fuel cells and 4-cell-stack with lanthanum-doped ceria barrier layer and strontium, magnesium doped lanthanum gallate tri-layer electrolytes membrane are investigated. The single cell exhibits peak power densities of 1061 mW cm−2 and 917 mW cm−2 at 800 °C fueled with bubbled hydrogen and methane (3 vol % water), respectively. Energy dispersive X-ray detector result demonstrates that no measurable carbon is detected in the anode directly operated in methane during the whole testing process.The 4-cell-stack assembled with single cells provides maximum output power of 1.66 W and open circuit voltage of 3.13 V at 800 °C with hydrogen as fuel. While the 4-cell-stack presents maximum output power of 1.38 W and open circuit voltage of 3.88 V at 800 °C fueled with methane. A stability test of each cell in 4-cell-stack is developed at constant current densities (J = 0 A cm−2, J = 0.1 A cm−2, J = 0.4 A cm−2) directly fueled with 75 ml min−1 methane at 800 °C. The results show that the voltages of each cell in 4-cell-stack stabilize during the whole durability test. The results demonstrate that the stability and dependability of 4-cell-stack is good, and it is promising for mobile application of intermediate temperature solid oxide fuel cells.

Synthesis and photoluminescence properties of a novel red phosphor SrLaGaO4:Mn4+

AbstractA novel Mn4+‐doped strontium lanthanum gallate red phosphor SrLaGaO4:Mn4+has been successfully prepared via the conventional solid‐state reaction method. Phase purity, photoluminescence excitation/emission spectra, concentration quenching, decay curves, and temperature‐dependent photoluminescence have been investigated systematically. SrLaGaO4:Mn4+phosphor exhibits broad excitation band from 250 to 600 nm and emits intense red light centered at 716 nm arising from spin‐forbidden transition,2E →4A2of Mn4+. The optimal dopant concentration of Mn4+is determined to be 0.2 mol%. Dipole‐dipole interaction is supposed to be the mechanism of concentration quenching. The crystal‐field strength Dq, the Racah parameters B and C, and the nephelauxetic ratioβ1of SrLaGaO4:Mn4+have been calculated according to its luminescent spectra. Our systematic investigation on this new phosphor can provide a reference for the development of red‐emitting phosphor.

Solid oxide fuel cells incorporating doped lanthanum gallate films deposited by radio-frequency magnetron sputtering at various Ar/O2 ratios and annealing conditions

In this study, we prepared solid oxide fuel cells (SOFCs) incorporating a dense La0.87Sr0.13Ga0.88Mg0.12O3−δ (LSGM) film deposited by radio-frequency (RF) magnetron sputtering on an NiO-Sm0.2Ce0.8O2−δ (NiO-SDC) anode substrate. The influences of the Ar-O2 deposition atmosphere and the subsequent annealing conditions on the characteristics of the LSGM film and on the performance of the resulting SOFCs was investigated. LSGM targets with La0.76Sr0.24Ga0.63Mg0.37O3−δ composition and a deposition atmosphere with Ar/O2 ratios ranging from 10/0 to 5/5 were used throughout the study. Owing to the amorphous nature of the as-deposited LSGM films, the film deposited in an Ar/O2 = 10/0 atm required post-annealing at 950 °C for 4 h for crystallization and removal of the SrLaGa2O7 content, while the films deposited at lower Ar/O2 ratios required higher crystallization temperatures: for instance, the films deposited at Ar/O2 = 6/4 had to be annealed at 1100 °C for 2 h. The anode-supported substrates were then screen-printed with a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)-LSGM cathode layer and fired to form SOFC cells. The single cell incorporating an LSGM film with La0.87Sr0.13Ga0.88Mg0.12O3−δ composition deposited in a pure Ar atmosphere exhibited the lowest cell resistance and thus the highest maximum power density (MPD) at all operating temperatures. The total resistance of the single cell incorporating a 4.3 μm-thick LSGM film decreased from 0.386 to 0.121 Ω cm2 as the temperature increased from 650 to 850 °C, and the open circuit voltages ranged from 0.941 to 0.861 V. The maximum power density of the single cell was 0.422, 0.736, and 1.105 W cm−2 at 650, 750, and 850 °C, respectively.

Synchronous enhancement of H2O/CO2 co-electrolysis and methanation for efficient one-step power-to-methane

Solid oxide electrolysis cells (SOECs) enable the integration of H2O/CO2 co-electrolysis and methanation reactions in a single reactor. The product gas may therefore potentially be sent directly to the existing natural gas pipelines. However, current power-to-methane (P2M) routes are not able to achieve acceptable CO2-to-CH4 conversion in a single SOEC reactor for direct integration into natural gas networks. In order to obtain higher CO2-to-CH4 conversions, a two-dimensional multiscale electro-thermo tubular SOEC model was developed to describe in detail the P2M processes along the whole cell and optimize the materials, flow modes, and operating conditions for CH4 production. The numerical simulations revealed that the stabilized Zr-based SOEC is unable to offer a sufficiently high electrochemical performance at methanation-favoring temperatures (250–650 °C). Such a temperature mismatch limits the peak CH4 production ratio in the tubular-stabilized Zr-based SOEC (with an inlet CO2 content of 20%) to only 13% at 1.5 V. In contrast, a tubular strontium and magnesium doped lanthanum gallate (LSGM)-based SOEC showed remarkable performance at or below 650 °C, improving the CH4 production ratio to over 50% at 550 °C and 1.3 V for the same inlet gas composition. However, the operating temperature was still too high to obtain an acceptable CO2-to-CH4 conversion for direct supply to natural gas pipelines. The model revealed that the H2O/CO2 co-electrolysis and methanation reactions can be synchronously enhanced by feeding a cold gas in counter-flow mode under pressurized conditions. At 29 bar, the tubular LSGM-based SOEC could achieve a CO2-to-CH4 conversion ratio of 98.7%, with an electricity-to-gas efficiency of 94.5% in the thermal neutral mode. The produced gas can be directly sent to the existing natural gas pipelines so that the curtailed renewable power can be stored more efficiently and utilized over a wider area.

Ionic Conductivity of Doped Lanthanum Gallate and Strontium Gallate Composites

Polycrystalline ceramic materials based on strontium- and magnesium-doped lanthanum gallate exhibit high ionic conductivity compared to the conventional yttria-stabilized zirconia, and find potential application as solid electrolyte for solid oxide fuel cells operating at intermediate temperatures (500-700°C). Sintering of this solid electrolyte is usually carried out at high temperatures being responsible for loss of Ga and consequent formation of impurity phases. In this work, the composition La0.9Sr0.1Ga0.8Mg0.2O3- δ with small additions of strontium gallate was prepared by solid state reaction and the influence of the additive on electrical conductivity and phase composition of the solid electrolyte was investigated. The sintered density of composites is higher than 95% of the theoretical value and the average grain size amounts 2.40 µm after sintering at 1350°C. The contents of impurity phases decreased with increasing strontium gallate additions. The overall ionic conductivity is higher for composites than that of the base material.

Ionic Conductivity of Doped Lanthanum Gallate and Strontium Gallate Composites

Polycrystalline ceramic solid electrolytes based on strontium- and magnesium-doped lanthanum gallate exhibit high ionic conductivity and find potential application in solid oxide fuel cells operating at intermediate temperatures. Sintering of this solid electrolyte is usually carried out at high temperatures, being responsible for loss of Ga and consequent formation of impurity phases. In this work, composites consisting of La0.9Sr0.1Ga0.8Mg0.2O3-δ with additions of strontium gallate were prepared by solid state reaction, and the influence of the additive on electrical conductivity and phase composition of the composite electrolyte was investigated. The sintered density of composites is higher than 95% of the theoretical value after sintering at 1350°C. The contents of impurity phases decreased with increasing additions of strontium gallate. The overall ionic conductivity of the composites is higher than that of LSGM. This effect is attributed to the decrease in the fraction of free MgO at grain boundaries in the composites.

Symmetrical solid oxide fuel cell with strontium ferrite-molybdenum electrodes

The work contains the results of studies of a promising composite material of Sr2Fe1.5Mo0.5O6 + Ce0.8Sm0.2O1.9 for electrodes of symmetrical solid oxide fuel cells. It is shown that conductivity of the composite at 800°C is about 10 and 15 S/cm, for air and humid hydrogen, respectively, and polarization resistance of the electrodes in contact with the electrolyte based on doped lanthanum gallate under the same conditions is about 0.26 and 0.12 Ohm cm2. Tests of a symmetrical fuel cell with a planar design and the supporting gallate electrolyte with the thickness of 300 μm show that the cell can develop the power of about 0.5 W/cm2 at 800°C when air is supplied to the cathode and humid hydrogen is supplied to the anode. Analysis of polarization losses shows that the polarization of the oxygen electrode considerably exceeds the polarization of the anode.

Microstructure and Electrochemical Properties of La0.8Sr0.2Ga0.8Mg0.2O3 Thin Film Deposited by Vacuum Cold Spray for Solid Oxide Fuel Cells

Doped lanthanum gallate (La0.8Sr0.2Ga0.8Mg0.2O3, LSGM) is considered as a promising electrolyte material for the intermediate temperature-solid oxide fuel cell (IT-SOFC), due to its high ionic conductivity at 600~800 °C. However, the common methods to fabricate the LSGM electrolyte (e.g., solid phase sintering) often lose their advantages owing to the depletion of Ga and the interfacial reaction between the LSGM and electrodes. Vacuum cold spray (VCS) is an emerging method to fabricate thin (﹤20 μm) and dense ceramic electrolyte membrane. In the present study, the LSGM electrolyte layer with a thickness of ~10 μm is prepared by VCS system assisted with in-situ particle heating approach (25~400 °C). The coating density and the ionic conductivities were increased with heating temperature. The results showed that VCS is a promising method to prepare the electrolyte film applied in the IT-SOFC.

Iron–Air Battery Operating at High Temperature

AbstractA simple configuration for an Fe–air rechargeable battery operating at high temperature was investigated. Two different ceramic electrolytes, that is, gadolinia‐doped ceria (CGO) and strontium/magnesium‐doped lanthanum gallate (LSGM), were studied. Strontium‐doped lanthanum ferrite‐cobaltite (LSFCO) perovskite was used as the air electrode, whereas an Fe0/Fe2O3–CGO composite was used as the Fe electrode. The synthesized CGO and LSGM supporting electrolytes were characterized by proper ionic conductivity for this application (7.4×10−2 S cm−1 at 800 °C). The LSFCO perovskite electrode showed excellent reversibility in symmetrical cells. Low overpotentials were recorded for the evolution of oxygen (0.1 V at 1.7 A cm−2) and the oxygen reduction process (0.1 V at 2.4 A cm−2) at 800 °C. The high‐temperature battery based on the CGO electrolyte showed a pronounced propensity to spontaneously discharge. This was caused by redox behavior involving the interconversion between the Ce4+ and Ce3+ ions in the crystallographic structure, which caused a parasitic electron drag through the electrolyte. On the other hand, the LSGM‐based battery showed promising cyclability characteristics at 800 °C with an open‐circuit voltage higher than 1 V, an electric capacity of 0.3 Ah g−1, and an energy density of 0.22 Wh g−1. However, significant improvements in the coulombic efficiency (≈42 %) for this battery system are necessary for practical applications.