Magnesium-doped Lanthanum Gallate Research Articles

Preparation of bulk-like La0.8Sr0.2Ga0.8Mg0.2O3-δ coatings for porous metal-supported solid oxide fuel cells via plasma spraying at increased particle temperatures

While porous metal-supported solid fuel oxide cells (PMS-SOFCs) have the potential to decrease the cost and increase the start-up speed of power units, the available fabrication processes remain too cumbersome for industrial production. In this study, we prepared bulk-like strontium and magnesium-doped lanthanum gallate (LSGM) coatings using atmospheric plasma spraying (APS) at an increased particle temperature. The large equiaxed grains inside the coatings indicated the epitaxial growth of the splat interfaces and improvement in the coating quality. With increased particle temperature, the conductivity of dense LSGM coatings directly deposited by APS was comparable to that of the bulk material, and cell performance was also significantly enhanced. The maximum power density of the PMS-SOFC at 700 °C was 831 mW cm−2 and 596 mW cm−2 when high and low particle temperatures were used, respectively. These results indicated that the quality of the coating was improved by increasing the in-flight particle temperature.

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.

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

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.

Improvement in the sintering and electrical properties of strontium- and magnesium-doped lanthanum gallate by MoO3 dopant

Abstract The LSGM powders by doping 0–2.5 at.% of MoO3 were synthesized and investigated. The microstructure and electrical properties of the samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and electrochemical impedance spectroscopy (EIS) respectively. The results showed that the phase purity, densification and conductivity of the LSGM samples could be improved by a proper addition of MoO3. The LSGM specimen by doping 2 at.% MoO3 exhibited the best phase purity, the best density, the largest grain size and the lowest activation energy. Moreover, it was found that the conductivity of the LSGM by doping 2 at.% MoO3 was 3.91 × 10−4 S cm−1 at 450 °C, which was 2.82 times as high as LSGM without MoO3 doping. Moreover, the thermal expansion coefficient of the LSGM-2Mo sample changed little by MoO3 doping. Therefore, the MoO3 doped LSGM may be a potential electrolyte material for anode-supported solid oxide fuel cells (SOFCs) without requiring buffer layer between anode and electrolyte.

Influence of Substrate on Conductivity of Sr and Mg Doped Lanthanum Gallate Thin Film Electrolyte for Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) consisting of ion conductive ceramics has various advantages such as the high generating efficiency, the utilization of hydrocarbon-containing fuel and the non-necessity of expensive rare metal catalyst. Yttria-stabilized Zirconia (YSZ) or Gadolinium-doped Ceria (GDC) are mainly used as the electrolyte for SOFC. Strontium- and Magnesium-doped Lanthanum Gallate (La0.9Sr0.1Ga0.8Mg0.2O3-δ, LSGM) has higher conductivity than YSZ, negligible electronic conduction and high chemical stability in a wide oxygen partial pressure range compared to GDC.Recently, it was reported that YSZ and GDC thin films have different values of ion conductivity from those of the bulk. LSGM should have the similar phenomenon. The reason for this change of the conductivity is assumed to be the lattice strain in the thin film, the difference between the bulk resistivity and the grain boundary resistivity or space charge layers at homo- or hetero-junctions. However, the mechanism of the conductivity change of LSGM thin film from the bulk is not revealed.In this study, the radio frequency magnetron sputtering, which is a reliable method for preparing thin film, was used for obtaining various thickness films on sapphire and quartz substrate. However, the deposited film was amorphous. To obtain the crystallized LSGM film, we investigated the influence of the annealing temperature. By proper post-annealing, crystallized LSGM film was obtained. We evaluated the crystallinity by X-ray diffraction, observed the microstructure with a scanning electron microscope and measured the conductivity by electrochemical impedance spectroscopy. In this paper, we report the effect on the electrical conductivity of the difference of thermal expansion co efficiency between LSGM thin film and the substrate.

Ionic Conductivity of Chemically Synthesized La<sub>0.9</sub>Sr<sub>0.1</sub>Ga<sub>0.8</sub>Mg<sub>0.2</sub>O<sub>3-δ </sub>Solid Electrolyte

Strontium and magnesium-doped lanthanum gallate is a solid electrolyte with higher ionic conductivity than yttria-stabilized zirconia. This perovskite-type ionic conductor has been investigated for application as solid electrolyte in intermediate-temperature solid oxide fuel cells. In this work the La0.9Sr0.1Ga0.8Mg0.2O3-δ composition was synthesized by a soft chemistry route seeking for a more homogeneous microstructure. Densification was carried out by the conventional method and by fast firing. The ionic conductivity of sintered specimens were evaluated by impedance spectroscopy. The apparent density was about 90% and 94% for fast fired and conventionally sintered specimens, respectively. No significant difference was found for the grain conductivity of specimens fast fired at 1500 oC for 5 min and sintered by the conventional method at 1450 oC for 4 h.

Electrical conductivity of LSGM–YSZ composite materials synthesized via coprecipitation route

Composites of Strontium- and Magnesium-doped lanthanum gallate, La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM), and yttria-stabilized zirconia, Zr0.9Y0.1O1.95 (YSZ), with weight ratios of 9.5:0.5, 9:1 and 8.5:1.5 were first prepared by co-precipitation route. The component compounds, LSGM and YSZ were also prepared by the same route for comparative study. X-ray Rietveld analyses revealed that the sintered LSGM–YSZ composites contain mainly perovskite orthorhombic LSGM phase along with fluorite YSZ phase similar to that of the cubic zirconia. Scanning Electron Microscopic image of the composite depicted the spherical- and oval-shaped grains. XPS spectra of the composite exhibited the LSGM along with a trace amount of the YSZ constituents. Electrical conductivity of the three systems was measured in the frequency range 20 Hz–1 MHz and in the temperature range of 400–700 °C. LSGM–YSZ composite electrolyte with 9:1 ratio was found to exhibit enhanced electrical conductivity in comparison to LSGM and YSZ systems. Moreover, it exhibited lower activation energy than that of their individual components.

Investigations on structural and electrical properties of calcium substituted LSGM electrolyte materials for IT-SOFC

Strontium and magnesium doped lanthanum gallate, La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) and a few compositions of calcia (CaO) incorporated LSGM, La(0.9−x)CaxSr0.1Ga0.8Mg0.2O2.85; x≤0.10, were prepared by conventional solid state ceramic route. The X-ray diffraction patterns showed orthorhombic LSGM phase along with a few impure secondary phases. SEM micrograph depicted fairly dense grain morphology of the system. The textural properties were also found to be quite suitable for solid oxide fuel cell electrolyte applications. The electrical conductivity of the systems was measured in the frequency range 20Hz to 1MHz and temperature range 300–800°C. The total conductivity of the system was found to first increase up to x=0.05 and thereafter it decreased for higher concentrations of the dopant. A small amount of calcia (upto x=0.05) scavenges the intergranular secondary impure phases resulting in a significant increase in grainboundary conduction. The presence of extra secondary phases in the higher compositions has been attributed to decrease in electrical conductivity.

Electrical conductivity of barium substituted LSGM electrolyte materials for IT-SOFC

Strontium and magnesium doped lanthanum gallate, La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM), and Ba-incorporated LSGM with compositions La0.9−xBaxSr0.1Ga0.8Mg0.2O2.85;x≤0.15 (LBSGM) were prepared by conventional solid state route. The X-ray diffraction patterns of the LBSGM system showed the main orthorhombic LSGM phases along with a few impure secondary phases. SEM micrographs exhibited fairly densed grain morphology of the system. The XPS spectra of the system indicated the presence of the constituent elements. It was found that sintered samples are stable in air and the constituent elements were found to exist in their characteristic valence states. The electrical conductivity of the LBSGM system was measured in the frequency range 20Hz to 1MHz and a temperature range of 300–800°C. The electrical conductivity of the system was found to increase slightly first up to x=0.07 and thereafter it decreased for higher dopant concentrations. The formation of additional Ba-derived secondary phases in the compositions with x>0.07 may lead to a decrease in electrical conductivity.

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte via ethylene glycol route and its characterizations for IT-SOFC

Strontium and magnesium doped lanthanum gallate (La0.9Sr0.1Ga0.8Mg0.2O3−δ), known as LSGM, was first prepared via ethylene glycol method. This route of preparation showed improved electrical conductivity, better surface area and high density. X-ray diffraction patterns of LSGM sintered at different temperatures indicated that pure LSGM phase was formed after sintering at 1400°C. X-ray Rietveld refinement confirmed the formation of pure perovskite orthorhombic phase of the LSGM. The sintered sample showed 99% relative density. Scanning electron microscopic study of LSGM also depicted fairly densed grain morphology. X-ray photoelectron spectroscopy measurement confirmed the stability of the sintered sample in air and the existence of constituent elements in their characteristic valence states. A surface without porosity was observed in BET measurement. Average thermal expansion coefficient was found to be 9.78×10−6/°C in the measured temperature range (RT–1000°C). The frequency dependent electrical conductivity of the sample was measured in the temperature range 400–800°C. Total electrical conductivity of the LSGM pellet was found to be 0.056Scm−1 at 800°C.

Effect of Attrition Milling and Calcination Temperature on Phase Composition of Strontium-and Magnesium-Doped Lanthanum Gallate

Oxygen-ion conductors based on strontium-and magnesium-doped lanthanum gallate have been proposed to be used as solid electrolyte in solid oxide fuel cells operating at intermediate temperatures (500-700 °C), due to their high ionic conductivity and stability over a wide range of oxygen partial pressures. In this work, the effect of attrition milling on phase composition of powder and consolidated specimens prepared by solid state synthesis has been investigated. The results show that both the attrition milling and the calcination temperature play a major role in the phase composition. Powders with negligible amount of secondary phases were obtained after two steps of calcination at high temperature followed by attrition milling.

A solid oxide cell yielding high power density below 600 °C

Here we report solid oxide cells with thin strontium- and magnesium-doped lanthanum gallate electrolytes that yield power densities of 1.06 W cm−2 at 550 °C and 0.81 W cm−2 at 500 °C when operated on humidified hydrogen and ambient air. Cost-effective ceramic processing and chemical solution impregnation methods were utilized, yielding a dual micron- and nano-scale architecture that is essential for achieving good low-temperature performance.

A reduced temperature solid oxide fuel cell with nanostructured anodes

Reduced-temperature solid oxide fuel cells (SOFCs), featuring thin strontium- and magnesium-doped lanthanum gallate (LSGM) electrolytes and nano-scale Ni anodes, showed high power densities of 1.20 W cm−2 at 650 °C and 0.39 W cm−2 at 550 °C when operated on humidified hydrogen fuel and air oxidant.

Plasma-Sprayed Strontium- and Magnesium-Doped Lanthanum Gallate Ceramics

La1−xSrxGa1−yMgyO3−d (LSGM) powder of different composition and a grain size of about 5–10 μm were plasma sprayed on Ti substrates in order to study the effect of plasma spraying of LSGM on the microstructure and chemical composition of the resulting ceramic LSGM layers. As-sprayed LSGM layers contain a considerable amount of amorphous phase besides LSGM grains, whereas postannealed layers mainly consist of LSGM besides very small amounts of second phases. Evaporation of mainly Ga was observed, which considerably altered the chemical composition of the layers.

Low-temperature sintering and electrical properties of strontium- and magnesium-doped lanthanum gallate with V2O5 additive

The effects of a V2O5 additive on the low-temperature sintering and ionic conductivity of strontium- and magnesium-doped lanthanum gallate (LSGM: La0.8Sr0.2Ga0.8Mg0.2O2.8) are studied. The LSGM powders prepared by the glycine nitrate method are mixed with 0.5–2at.% of VO5/2 and then sintered at 1100–1400°C in air for 4h. The apparent density and phase purity of the LSGM specimens are increased with increasing sintering temperature and VO5/2 concentration due to the enhanced sintering and mass transfer via the intergranular liquid phase. The 1at.% VO5/2-doped LSGM specimen sintered at 1300°C exhibits a high oxide ion conductivity of ∼0.027Scm−1 at 700°C over a wide range of oxygen partial pressure (PO2=10−27−1 atm), thereby demonstrating its potential as a useful electrolyte for anode-supported solid oxide fuel cells (SOFCs) without the requirement for any buffer layer between the electrolyte and anode.

A novel design of anode-supported solid oxide fuel cells with Y 2O 3-doped Bi 2O 3, LaGaO 3 and La-doped CeO 2 trilayer electrolyte

Anode-supported solid oxide fuel cells (SOFCs) with a trilayered yttria-doped bismuth oxide (YDB), strontium- and magnesium-doped lanthanum gallate (LSGM) and lanthanum-doped ceria (LDC) composite electrolyte film are developed. The cell with a YDB (18 μm)/LSGM (19 μm)/LDC (13 μm) composite electrolyte film (designated as cell-A) shows the open-circuit voltages (OCVs) slightly higher than that of a cell with an LSGM (31 μm)/LDC (17 μm) electrolyte film (designated as cell-B) in the operating temperature range of 500–700 °C. The cell-A using Ag-YDB composition as cathode exhibits lower polarization resistance and ohmic resistance than those of a cell-B at 700 °C. The results show that the introduction of YDB to an anode-supported SOFC with a LSGM/LDC composite electrolyte film can effectively block electronic transport through the cell and thus increased the OCVs, and can help the cell to achieve higher power output.