Mg-doped Lanthanum Gallate Research Articles

Ionic and Electronic Transport in Nanocrystalline La0.9Sr0.1Ga0.9Mg0.1O3-Δ

A major limitation for the development of reversible solid oxide cells (R-SOCs) is the delamination between the oxygen electrode and electrolyte in electrolysis mode. Previously, Virkar reported that higher electronic conductivity within the predominantly ionically conducting electrolyte led to a lower tendency for the formation of high internal pressure [1]. Therefore an electrolyte having a small amount of electronic conduction may be beneficial for relieving the delamination between the oxygen electrode and electrolyte in R-SOCs. Strontium- and magnesium-doped lanthanum gallate (LSGM) has been widely investigated as an electrolyte for medium-low temperature (400-650 ºC) R-SOCs due to its high ionic conductivity and compatibility with some perovskite electrodes. Compared to well-established yttria stabilized zirconia electrolytes, which have ionic transference numbers close to 1 across a wide range of oxygen partial pressures, LSGM can exhibit non-negligible p-type electronic conductivity under oxidizing conditions [2, 3]. Furthermore, limited studies have demonstrated enhanced electronic transference numbers at grain boundaries compared to the bulk, attributed to space charge effects [3, 4]. Therefore, it is of interest to understand the impact of nanostructuring and grain boundaries on the ionic and electronic transport properties of LSGM for relieving the delamination issue in R-SOCs. In this work, two methods - field assisted sintering technique/spark plasma sintering (FAST/SPS) and traditional pressureless sintering were employed to prepare dense La0.9Sr0.1Ga0.9Mg0.1O3-δ (LSGM9191) pellets from nanopowders synthesized by the Pechini method. Post-sintering anneals were applied to the FAST/SPS pellets to oxidize them and control grain size. Microstructure was evaluated by scanning and transmission electron microscopy with EELS/EDS line scans at grain boundaries, and transport properties were measured by 2-point ac-impedance spectroscopy over the temperature range using porous Pt/Ag electrodes. Equivalent circuit fitting with application of microstructural models was applied to evaluate local conductivities. The impact of grain size on grain core (gc), grain boundary (gb) and total conductivity were studied. Grain size did not affect the gc conductivity, and the gb and total conductivity increased with grain growth. Specific gb conductivity decreased with increasing grain size. Additionally, the dependence of the conductivities on oxygen partial pressure (pO2) and applied dc bias was measured to assess any electronic contribution to the conductivity. We found that the gb conductivities of LSGM9191 pellets prepared by FAST/SPS with different grain sizes (ranging from ~100nm to 800 nm) did not exhibit pO2 dependence, showing different behavior than some previous research [5]. The dependence of gb conductivity on dc bias (both in N2 or 21% O2) was also smaller than previous reports [6, 7]. Therefore, the electronic conductivity and space charge effect was limited, which might be caused by the high dopant content in the present work compared to previous studies and/or by impurities/ amorphous material/ organic residue trapped at the GBs as a result of the FAST/SPS process. However, the gb conductivity of a LSGM9191 pellet prepared by pressureless sintering did show a small pO2 dependence, indicating some p-type conductivity, albeit lower than in previous reports. The results suggest that factors such as grain size, doping level, grain boundary chemistry, and processing routes could be modified to tailor mixed conduction in LSGM for R-SOC applications. Acknowledgements Support from WPI-I2CNER to NHP and a JSPS Fellowship (201702103) to TC are gratefully acknowledged. Reference [1] Anil V. Virkar, Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells, International of Hydrogen Energy, 35 (2010) 9527-9543 [2] V.V. Kharton et al., Electron-hole transport in (La0.9Sr0.1)0.98Ga0.8Mg0.2O3- δ electrolyte: effects of ceramic microstructure, Electrochimica Acta 48 (2003) 1817-1828 [3] H.J. Park et al., Space Charge Effects on the Interfacial Conduction in Sr-Doped Lanthanum Gallates: A Quantitative Analysis. Phys. Chem. C 2007, 111, 14903-14910 [4] N.H. Perry, Local electrical and dielectric properties of nanocrystalline solid oxide fuel cell electrolytes, Northwestern University, Ph.D. thesis, 2009 [5] H.J. Park et al., Mixed conduction behavior in nanostructured lanthanum gallate, Electrochemistry Communications 11(2009) 962-964 [6] C.T. Chen et al., Current–voltage characteristics of grain boundaries in polycrystalline Sr-doped LaGaO3 Phys. Chem. Chem. Phys., 2012, 14, 9047–9049 [7] Raghvendra • R. K. Singh • P. Singh, Influence of small DC bias field on the electrical behaviour of Sr- and Mg-doped lanthanum gallate, Appl. Phys. A (2014) 116:1793–1800

Microstructure and electrical conductivity of fast fired Sr- and Mg-doped lanthanum gallate

La0.9Sr0.1Ga0.8Mg0.2O3−δ solid electrolytes were consolidated by fast firing aiming to investigate the effects of the sintering method on densification, microstructure and ionic conductivity. Powder mixtures were prepared by solid state reaction at 1250 and 1350°C for 12h, and fast fired at 1450 and 1500°C temperatures for 5 and 10min. The content of impurity phases was found to be quite low with this sintering method. Relatively high density (>90% of the theoretical value) and low porosity (<1.5%) were readily obtained for powder mixtures calcined at 1250°C. The activation energy for conduction was approximately 1eV. Specimens fast fired at 1450°C for 10min with a mean grain size of 2.26µm reached the highest value of total ionic conductivity, 22mScm−1, at 600°C.

Phase composition, densification and electrical conductivity of La0.9Sr0.1Ga0.8Mg0.2O3−δ consolidated by the two-stage sintering method

The influence of the method of sintering on densification, phase assemblage and electrical conductivity of Sr- and Mg-doped lanthanum gallate was investigated aiming to optimize the sintering process of this solid electrolyte. Powder mixtures were prepared by solid state reactions, and the sintering of green compacts was carried out by the two-stage method varying the temperature of the first stage (T1) and the isothermal temperature (T2) and time (t2) profile. The type and content of predominant impurity phases depend on specific parameters of the sintering process. In the first stage of sintering, the content of impurity phases, such as LaSrGaO4 and La4Ga2O9, decreases with increasing temperature. The relative density of sintered specimens reaches 99% whenever T1 is 1500°C. The mean grain size varies from 1.45 to 4.9µm. At 600°C the electrical conductivity attains 12mScm−1 for T1 1400°C and T2 1350°C for 5h, with activation energy of approximately 1eV.

Influence of small DC bias field on the electrical behaviour of Sr- and Mg-doped lanthanum gallate

One of the promising electrolyte materials for solid oxide fuel cells application, Sr- and Mg-doped lanthanum gallate La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), is synthesized by conventional solid state ceramic route. X-ray Rietveld analysis confirms the formation of main orthorhombic phase at room temperature along with a few minor secondary phases. SEM micrograph reveals the grain and grainboundary morphology of the system. Electrical conductivity of the LSGM sample is measured in the temperature range 573–873 K and in the frequency range 20 Hz–1 MHz at a few small DC bias fields (at 0.0, 0.5, 1.0, 1.5 and 2.0 V). The conductivity spectra show power-law behaviour. Electrical conductivity of the sample is found to be weakly dependent on DC bias field. This is attributed to field-dependent bulk and grainboundary conduction processes. In the present system, under investigated bias field range, the possibility of formation of Schottky barrier is ruled out. The concept of grainboundary channel (pathway) modulation on the application of bias field is proposed.

(La,Ba)CoO3 and Pr1.9(Ni,Cu,Ga)O4 Composite Oxide as Active Cathode for Intermediate Temperature Solid Oxide Fuel Cells Using Doped LaGaO3 Electrolyte Films

Ba0.5La0.5CoO3 (BLC) and Pr1.9(Ni,Cu,Ga)O4 (PNCG) composite cathode were investigated to enhance electrochemical performance in solid oxide fuel cells using Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte films at intermediate temperatures (773–973 K). Effect of the calcination temperature of the composite cathode was studied on chemical compatibility, area specific resistance (ASR), and power generation property. ASR of the composite cathode was smaller than that of SSC but increased with increasing calcination temperature by chemical reaction. Improved maximum power density of 1.270 and 0.146 Wcm-2 at 973 K and 773 K was achieved on a single cell using BLC-PNCG composite as cathode on LSGM film.

Synthesis of La0.85Sr0.15Ga0.8Mg0.2O3 − δ powder by carbonate co-precipitation combining with azeotropic-distillation process

La0.85Sr0.15Ga0.8Mg0.2O3−δ (LSGM1520) powder is synthesized by a carbonate co-precipitation coupling with azeotropic-distillation process. The composition of the as-prepared precipitate is determined by X-ray diffraction (XRD), elemental analysis (EA), thermogravimetric and differential scanning calorimeter (TG-DSC) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The post-distilled precursor is submitted to various thermal treatments and characterized by XRD and transmission electron microscopy (TEM). The results show that the components of the powder calcined at 1000°C contain a main perovskite phase and small amounts of secondary phases (namely LaSrGaO4 and LaSrGa3O7). TEM observation reveals that the powder has a narrow grain size distribution, which turns out to be suitable for sintering (>97% relative density for a pellet sintered at 1400°C during 4h). The conductivity of sintered pellets is measured and the highest conductivity at 800°C of LSGM1520 is 0.186Scm−1, which is nearly three times as high as that of La0.8Sr0.2Ga0.8Mg0.2O3−δ (0.0644Scm−1) prepared by the similar carbonate method. It suggests that a new route of synthesizing Sr- and Mg-doped lanthanum gallate (LSGM) with good sinterability has been developed.

Improved sintering and electrical properties of La-doped CeO2 buffer layer for intermediate temperature solid oxide fuel cells using doped LaGaO3 film prepared by screen printing process

Effects of a sintering agent for La-doped ceria (LDC) as a buffer layer to prevent a chemical reaction between Ni in anode and Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte during sintering were studied for improving sintering and electrical properties. Electrochemical performance of anode-supported solid oxide fuel cells (SOFCs) using LDC and LSGM films prepared by screen printing and co-sintering (1,350 °C) was also investigated. The prepared cell with dense LDC (ca. 17 μm) and LSGM electrolyte (ca. 60 μm) films showed an open circuit voltage close to the theoretical value of 1.10 V and a high maximum power density (0.831 W cm–2) at 700 °C. The addition of 1 wt.% LSGM to porous LDC buffer layer was effective for improving the sintering density and electrical conductivity, resulting in the high power density due to the decreased internal resistance loss.

Improved power generation performance of solid oxide fuel cells using doped LaGaO3 electrolyte films prepared by screen printing method II. Optimization of Ni–Ce0.8Sm0.2O1.9 cermet anode support

A Ni and Sm-doped ceria (Ce0.8Sm0.2O1.9, SDC) cermet anode as a porous support of doped LaGaO3 film prepared by a wet coating and co-firing process was investigated. Different preparation methods and compositions were used to improve the power density of intermediate temperature solid oxide fuel cells. NiO–SDC precursor powder with fine particles and a porous microstructure with high surface area was synthesized by a modified impregnation method and compared with that synthesized by a ball milling method. In addition, an open circuit voltage, which is almost equal to the theoretical value of 1.1 V, and maximum power densities of 835, 277, and 67 mW cm−2 at 700, 600, and 500 °C, respectively, were achieved on a single cell supported by a 75 wt% Ni–SDC cermet anode when a 60 μm thick Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte was used. The improved power density was explained by the enlarged reaction area for the anode as a result of the low polarization resistance of the anode by high porosity and uniform distribution of Ni and SDC particles. Although a small amount of Ni diffused to the interface between the La-doped ceria (LDC) buffer layer and the LSGM electrolyte film, an adverse reaction that deteriorates cell performance seemed to be suppressed, and thus, reasonably high power density was achieved on the cell using the LSGM film prepared by the screen printing method with optimization of the anode substrate structure and composition.

SiO[sub 2]-Tolerant Grain-Boundary Conduction in Sr- and Mg-Doped Lanthanum Gallate

The effect of SiO 2 impurity on the grain-boundary conduction in Sr- and Mg-doped lanthanum gallate (La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-δ , LSGM) was studied. The grain-boundary conduction of LSGM was slightly affected, even by the addition of 500-2000 ppm SiO 2 , while the apparent grain-boundary resistivity was increased up to ∼780 times in gadolinia-doped ceria by the doping of 500-2000 ppm SiO 2 . The high tolerance of the grain-boundary conduction against siliceous impurity in the LSGM specimen was explained by the gathering of the acidic siliceous phase near the basic SrO component.

Mixed conduction behavior in nanostructured lanthanum gallate

Electrical conduction in nanostructured 10 mol% Sr- and 15 mol% Mg-doped lanthanum gallate was investigated. The grain boundary conductivity shows mixed ionic and electronic conduction, while the grain conductivity shows ionic conduction. This is attributed to the increase of the contribution of the electron-hole in the grain boundary by the space charge effect. The overall electrical conductivity is governed by the grain boundary conductivity such that nanostructured lanthanum gallate becomes a mixed conductor.

Fabrication of GDC/LSGM/GDC tri-layers on polypyrrole-coated NiO-YSZ by electrophoretic deposition for anode-supported SOFC

The formation of thin tri-layers of Sr- and Mg-doped lanthanum gallate (LSGM) and Gd-doped ceria (GDC) solid electrolytes was performed by electrophoretic deposition (EPD) on a Ni-YSZ anode for use in solid oxide fuel cell (SOFC) applications. The surface of the NiO-YSZ substrate was coated with a conductive polypyrrole to provide electric conduction, which is essential for the EPD processing. The GDC/LSGM8282/GDC laminar coating was performed by sequential deposition on the same substrate. The electrolyte layers were easily sintered together on the anode without cracking and peering-off at the interfaces.

Synthesis and characterization of Sr- and Mg-doped Lanthanum gallate electrolyte materials prepared via the Pechini method

The powders of Sr- and Mg-doped lanthanum gallate (La 0.85Sr 0.15Ga 0.80Mg 0.2O 2.825; LSGM) were synthesized by the Pechini method. The XRD pattern indicates that the main phase (LaGaO 3) exists in the uncalcined powders. The LSGM materials are composed of the main phase without secondary phases when calcined at 1400 °C. The LSGM materials contain fewer amounts of secondary phases than those prepared by the sol–gel method and solid-state reaction method at the same calcination temperature. TEM image of the powders indicate that the average grain size is about 80 nm. The conductivity increases with the testing temperature increasing. The curve of ln( σT) vs 1/ T exists two straight lines intersecting at T * ( T * is about 602 °C). It indicates that activation energy of oxygen-vacancy motion at lower temperatures is greater than that at higher temperatures.

Synthesis of strontium- and magnesium-doped lanthanum gallate by glycine-nitrate combustion method

Sr- and Mg-doped lanthanum gallate powders with the composition of La 0.85Sr 0.15Ga 0.85Mg 0.15O 2.85 were synthesized by a glycine-nitrate combustion method. Powders prepared under different fuel combustion conditions were investigated by XRD and TEM. The results show that, under slightly rich fuel condition, the product powders contain less impurity phases, and powders prepared by the glycine-nitrate combustion contain far less impurity phases and have smaller particle sizes than those prepared by solid-state reaction method or acrylamide polymerization technique.

Thermal shock and thermal fatigue study of Sr- and Mg-doped lanthanum gallate

Thermal shock and thermal fatigue resistance of La 0.85Sr 0.15Ga 0.85Mg 0.15O 2.85 (LSGM) were investigated using a water-quench technique. Thermal shock resistance test shows that the residual strength decreases gradually as the thermal shock temperature increases, and the critical temperature difference, Δ T c, is about 75 °C. The SEM observations show that the primary fracture mode does not change and intergranular fracture is the primary fracture mode before and after thermal shock test. Thermal fatigue measurements of LSGM and 2 wt% Al 2O 3 strengthened LSGM (named as LSGMA) show that the addition of Al 2O 3 did not enhance the crack extension resistance.

Development of intermediate-temperature solid oxide fuel cells for direct utilization of hydrocarbon fuels

In this study, we compare the performance of solid oxide fuel cells (SOFC) having composite Cu-based anodes but made with the following low-temperature electrolytes: samaria-doped ceria (SDC), Sr- and Mg-doped lanthanum gallate (LSGM), and scandia-stabilized zirconia (ScSZ). Performance (V-I) curves and impedance spectra were measured using H 2 and n-butane fuels at 973 K. The results suggest that the use of electrolyte materials with higher ionic conductivity can lead to improved anodes for direct-utilization SOFC, although the performance of each of the cells in n-butane appears to be at least partially limited by the electrochemical oxidation reaction.

Elastic properties of Sr- and Mg-doped lanthanum gallate at elevated temperature

Abstract The elastic moduli, i.e., Young’s modulus, shear modulus and Poisson’s ratio, of a sintered La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3− δ bulk have been experimentally determined in the temperature range from room temperature to 1373 K using a resonance technique. Anomalous elastic properties were observed over a wide temperature range from 473 to 1173 K. In the results for internal friction and in X-ray diffraction measurements at elevated temperature, two varieties of structural changes were seen in La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3− δ in the examined temperature range. The results agreed with the findings of a previous crystallographic study of the same composition system by Slater et al. In addition, the temperature range in which a successive structural change occurred in La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3− δ was the same as that exhibiting the anomalous elastic properties. Taking all the results together, it can be inferred that the successive structural change in the significant temperature range is responsible for the elastic property anomaly of La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3− δ .

Preparation of La1-xSrxGa0.8Mg0.2O3-.DELTA. Electrolyte for Solid Oxide Fuel Cell by Citrate Method Using Industrial Raw Materials.

Sr- and Mg-doped lanthanum gallate which exhibits high ionic conductivity was synthesized by a citrate method using industrial raw materials. X-ray diffraction analysis revealed that the powder by a citrate method was single-phase. Electron probe micro analysis (EPMA) showed that the sintered body which was formed by synthesized powder by a citrate method and fired at 1723 K for 6 h had inhomogeneous texture caused by segregation of Sr and Ga elements. Scanning electron microscope (SEM) images and densimetry revealed that the sintered body synthesized by a citrate method had high density, the sintered bodies synthesized by a citrate method had much higher oxide ion conductivity than those by a solid state reaction method.

Preparation and electrochemical characterization of Sr- and Mn-doped LaGaO3 as anode materials for LSGM-based SOFCs

Sr- and Mn-doped lanthanum gallate powders (LSGMn, La0.9Sr0.1Ga1 − xMnxO3 − δ, x = 0.20, 0.35, 0.43) were prepared by glycine-nitrate combustion synthesis. X-ray diffraction patterns indicate the perovskite structure was formed without any second phase after calcining the powders at 1000°C for 4 h. Compacts of powders synthesized under stoichiometric combustion were sintered to densities over 95% of theoretical values. The electrical conductivity of this material in both air and H2 were characterized using AC impedance spectroscopy. It showed that the conductivity in H2 atmosphere is lower than that in air due to p-type electrical conduction in this material, and the electrical conductivity increases remarkably with increasing manganese content. Good chemical stability of La0.9Sr0.1Ga1 − xMnxO3 − δ in H2 atmosphere as well as the relatively high conductivity makes it an appropriate anode material for Sr- and Mg-doped lanthanum gallate (LSGM)-based IT-SOFCs. Preliminary fuel cell performance measurements were performed, showing promising electrochemical properties of such anode materials.

Thermodynamic investigations of Sr- and Mg-doped lanthanum gallate by Knudsen effusion mass spectrometry and defect chemical analysis

The vaporization of the La1−xSrxGa1−yMgyO3−(x+y)/2 perovskite phases was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1608–1901 K. The partial pressures of the gaseous species O2, Mg, Sr, SrO, Ga, GaO, Ga2O and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of thermodynamic activities of Ga2O3 (1700 K), MgO (1800 K) and SrO (1800 K) in the perovskite phase at the temperatures given in parentheses. The results are compared with the thermodynamic data of LaGaO3(s) without additives. The influence of the Mg concentration on the thermodynamic activity of Ga2O3 was carefully studied and modeled by a defect chemical analysis.

Vaporization of Sr- and Mg-Doped Lanthanum Gallate and Implications for Solid Oxide Fuel Cells

Vaporization of the and perovskite phases was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1618-1886 K. The partial pressures of the gaseous species Mg, Sr, SrO, Ga, GaO, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of thermodynamic activities of the components at 1800 K. The results are compared with thermodynamic data of without additives. Implications of the present data for the potential use of the material in solid oxide fuel cell technology are discussed as well. © 2001 The Electrochemical Society. All rights reserved.