La-doped SrTiO3 Research Articles

Preparation and Characterization of Alternative Ni-Based Anode Materials for SOFCs

Introduction Ni is widely used as the anode material of SOFCs as a cermet containing ionic conductor and electronic conductor. In the SOFC power generation, the oxygen partial pressure in the downstream region of the SOFC systems increases, the metallic Ni can be oxidized to NiO, and thus volume expansion occurs1. Aggregation of adjacent Ni particles can degrade the electrode structure, which makes it difficult to fully utilize the fuel supplied. We have succeeded in fabricating an SOFC anode which is resistant to redox cycling using oxide conductors, La-doped SrTiO3 (LST) and Gd-doped CeO2 (GDC). In this study, we aim to develop (i) a novel electrode in which a metallic Ni catalyst is dissolved from the electrode material and precipitated on the surface of the anode in a reducing atmosphere (Ni-LST-GDC anode), and (ii) a novel electrode using a Ni-based alloy in the electrode framework (Ni-alloy-GDC anode)2,3. Experimental Alternative anode materials were prepared via wet process2,3. The cells were prepared by screen-printing the anode and the cathode followed by each sintering process in air. A platinum mesh was used for the current collector, and a Pt-based reference electrode was deposited on the cathode side for overvoltage separation between the anode and the cathode. The electrochemical performance of the fuel cells was evaluated from the I-V characteristics at the operating temperature of 800°C, using 3%-humidified hydrogen fuel. In order to evaluate the electrode activity, the electrode overvoltage was measured. The current interruption method was applied for evaluating the electrode overvoltages. Results and discussion Figure 1 shows the I-V characteristics of the cells with the newly prepared Ni-LST-GDC anode, the Ni-impregnated anode, and the LST-GDC anode without adding Ni, at 800°C, in 3%-humidified hydrogen fuel. Regarding the anode voltage and the anodic overvoltage, the Ni-LST-GDC anode exhibited better performance than the anode composed only of LST and GDC. In comparison with the Ni-impregnated anode, however, the cell performance was still lower both in anode voltage, overvoltage, and IR loss. Figure 2 shows STEM micrographs indicating that the Ni catalyst particle precipitated could locate on the GDC. Anode materials design principles of alternative Ni-based anodes will be discussed. But most of Ni particles located on the electron-conductive LST. In contrast, the cell with Ni-alloy anode exhibited comparable performance compared to that with the Ni-SSZ cermet.

Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO3/LaAlO3(001) superlattices

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO$_3$)$_n$/(LaAlO$_3$)$_n$(001) superlattices ($n=1,3$) and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with $n=3$ shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the LaNiO$_3$ region and the LaAlO$_3$ spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO$_3$ we predict in- and cross-plane Seebeck coefficients of $\pm 600$ $\mu$V/K and an in-plane power factor of $11$ $\mu$W/K$^2$cm for an estimated relaxation time of $\tau = 4$ fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped SrTiO$_3$ or layered cobaltates and are associated with the opening of a small gap ($0.29$ eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials.

Application of La-Doped SrTiO3 in Advanced Metal-Supported Solid Oxide Fuel Cells

Composite materials frequently allow the drawbacks of single components to be overcome thanks to a synergistic combination of material- and structure-specific features, leading to enhanced and also new properties. This is the case of a metallic-ceramic composite, a nickel-chromium-aluminum (NiCrAl) foam impregnated with La-doped Strontium Titanate (LST). This particular cermet has very interesting properties that can be used in different fields of application, namely: mechanical robustness provided by the metal foam; and chemical stability in harsh conditions of temperature and atmosphere by promotion of a thin protective layer of alumina (Al2O3); high electronic conductivity given by a percolating ceramic conducting phase, i.e., La-doped Strontium Titanate. In this paper, its application as a current collector in a metal-supported Solid Oxide Fuel Cells (SOFC) was studied. Firstly, the electronic properties of different compositions, stoichiometric and under stoichiometric, of LST were analyzed to choose the best one in terms of conductivity and phase purity. Then, LST chemical stability was studied in the presence of Al2O3 at different temperatures, gas compositions and aging times. Finally, stability and conductivity of LST-impregnated NiCrAl foam composite materials were measured, and LST was found to be fully compatible with the NiCrAl foam, as no reactions were detected in oxidizing and reducing atmosphere after up to 300 h operation at 750 °C and 900 °C between the Al2O3 layer and LST. Results showed that the composite is suitable as a current collector in innovative designs of metal-supported SOFC, like the Evolve cell, in which the metallic part is supposed not only to provide the structural stability to the cell, but also to play the role of current collector due to the impregnation of ceramic material.

Direct preparation of La-doped SrTiO3 thermoelectric materials by mechanical alloying with carbon burial sintering

Thermoelectric Sr1-xLaxTiO3 (x=0, 0.02, 0.05, 0.08) nanoparticles were directly prepared by mechanical alloying, followed by carbon burial sintering to produce the bulk counterparts, for the purpose of obtaining pure phase and fine microstructures in a facile process. Electrical and thermal transport properties have been measured over the temperature range from 300K to 1100K. La was successfully doped into the SrTiO3 during the milling process and acted as an n-type dopant. Core structure and superstructure in bulk samples, leads to a relatively high absolute Seebeck coefficient. The dimensionless figure-of-merit ZT changes with the increasing of La content and temperature, and the maximum ZT value can reach 0.06 at 300K for x=0.02 and 0.20 at 1000K for x=0.08. This strategy for bulk thermoelectric materials is simple, cost-effective and has great potential in large-scale industrial applications.

Electronic structural, optical and phonon lattice dynamical properties of pure- and La-doped SrTiO3: An ab initio thermodynamics study

To better understand the electronic, optical, and thermodynamic behaviors of lanthanum doped strontium titanate (La-STO) with different doping levels at high temperature, ab initio thermodynamics have been employed to investigate their electronic structures and thermodynamic evolutions versus temperatures. The results show that with increasing La-doping level, the corresponding structures of La-STO become less stable with an increasing number of phonon soft modes. The calculated thermodynamic properties show that with increasing La-doping levels, the free energy and enthalpy changes of La-STOs are decreased at given temperature. For all La-STOs with different La-doping levels investigated, we found that in three cases doping with 25%, 37.5% and 75% La substituted for Sr, the diagonal elements of their dielectric constant matrix are not all equal, which means these crystal structures are uniaxial and anisotropic. Our results indicate that doping La into STO could alter the optical properties of pure STO to be birefringent.

Electrochemcial Performances of La-Doped SrTiO3 Anode Materials for Intermediate Temperature-Solid Oxide Fuel Cells

Anode plays a very important role in the performance of solid oxide fuel cells (SOFCs) [1, 2]. In order to improve the cell performance proper anode materials should be selectd with suitable cell fabrication techniques. La-doped SrTiO3 (La0.4Sr0.6TiO3, LST) has received many attentions as an alternative anode material to state-of-art Ni-cermets anode for solid oxide fuel cells (SOFCs), because of its high redox stability, good electrical conductivity, and similar thermal expansion properties of yttria-stablized zirconia (YSZ) elecrolytes [3, 4]. Nowdays, decreasing the operating temperature of SOFCs down to intermediate temperature (IT) range (less than 800) is required to improve long-term durability and operational stability with economical advantages [5]. Hence, in this work, LST anode materials are fabricated with the highly conductive Nd0.1Ce0.9O2 (NDC) electrolytes and characterized to investigate the feasibility of SOFCs at the IT. In addition, LST is composited with the NDC to improve the ionic conductivity and decrease the thermal expansion mismatch of anode materails to the electrolyte. The Electrochmical impedance spectra (EIS) analysis shows that the polarization resistance of LST-NDC composite anode decreases with increasing of NDC concentrations, and 60:40 (LST:NDC in weight fraction) exhibits the lowest polarization resistance value at 650. The maximum power density of LST-NDC composite anode-cell is 143 mW cm-2 at 650, and thus we confirm the possbilty of LST as an anode material for the application of IT-SOFCs.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10−4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.

Electrochemcial Performances of La-Doped SrTiO3 Anode Materials for Intermediate Temperature-Solid Oxide Fuel Cells

The La0.4Sr0.6TiO3 (LST) is considered as a candidate material for anodes to solve the drawbacks of Ni-based cermets in solid oxide fuel cells (SOFCs). In this work, LST materials are fabricated with highly conductive Nd0.1Ce0.9O2 (NDC) electrolytes and characterized to investigate the feasibility of SOFCs at the intermediate temperature (IT). In addition, LST is composited with the NDC to improve the ionic conductivity and decrease the thermal expansion mismatch of anode materials to the electrolyte. The electrochemical impedance spectra analysis shows that the area specific resistance of the LST-NDC composite anode decreases with increasing NDC concentrations and a ratio of 7:3 (LST:NDC) in weight fraction exhibits the lowest polarization resistance value at 650. The maximum power density of the LST-NDC composite anode-cell is 143 mW cm-2 at 650. We thus confirm that LST could be used as an anode material for the application of IT-SOFCs.

Electrochemical Characteristics of Nickel and Ceria Infiltrated Nano-Structured La0.2Sr0.8TiO3 Anodes for Solid Oxide Fuel Cells By Infiltration

A La-doped SrTiO3 (LST) is an alternative anode material of solid oxide fuel cells (SOFC). The LST has a high electronic conductivity and chemically stable under a reduced atmosphere. In addition, it has excellent carbon deposition resistance and high tolerance of sulfur in hydrocarbon fuels. However, the LST suffers from a low catalytic activity toward the oxygen reduction reaction. Thus, it needs to improve the catalyst activity of the LST anode. In this study, nickel and gadolia-doped ceria (GDC) infiltrated nano-structured LST anodes were proposed in order to improve the catalytic activity of the LST-based oxide anode. Nano-sized Ni and GDC were homogeneously dispersed in a LST backbone by infiltration. Ni/GDC infiltrated LST showed higher catalytic activity than Ni and GDC infiltrated ones, which means that the synergistic effect can be expected in the Ni/GDC infiltrated LST. It seems that the polarization resistance (Rp) depended on the infiltrated Ni and GDC content and their dispersion state or microstructure. We analyzed the correlation between the microstructure of the infiltrated anode and electrochemical characteristics.

Robust SOFC anode materials with La-doped SrTiO3 backbone structure

Redox-stable Sr0.9La0.1TiO3 (SLT) with Ce0.9Gd0.1O2 (GDC) and (Sc2O3)0.1(CeO2)0.01(ZrO2)0.89 (SSZ) was used to fabricate robust anodes for zirconia-based electrolyte-supported SOFCs. The anode consisted of three layers: a layer of 60wt% GDC-40wt% SLT or 40wt% SSZ-60wt% SLT as a functional layer and two layers of SLT as current conducting layers. An impregnation method has been employed to decorate nano-sized Ni and CeO2 into the ceramic anode to improve electrochemical performance and redox stability. It was found that the catalyst impregnated ceramic anode has improved redox stability compared with the conventional Ni-SSZ cermet. The incorporation of the oxide catalyst, CeO2, during co-impregnation process was found to even further improve redox stability of the anode, since CeO2 particles have a lower tendency to aggregate during redox cycling.

Fabrication and ionic conductivity of oriented lanthanum silicate films with apatite-type structure

We have successfully fabricated c-axis oriented films of apatite-type lanthanum silicate (LSO) using sputtering technique. Two kinds of thin films, La2SiO5 and La2Si2O7, were successively deposited on yttria-stabilized zirconia (YSZ) or La-doped SrTiO3 (STO) single crystal substrates by RF magnetron sputtering in pure Ar atmosphere at room temperature. Annealing the films in air at 1100°C promoted the crystal formation. A surface layer of the obtained specimens were c-axis oriented apatite-type LSO, which was confirmed by in-plane and out-of-plane X-ray diffraction patterns. An activation energy of the ionic conductivity of the LSO sample using STO substrate was 0.74eV, which agrees well with that of polycrystalline La9.33Si6O26 though the conductivity is as low as 2×10−5Scm−1 at 600°C. Using the laminated thin film method, we have accomplished to fabricate c-axis oriented apatite-type LSO with shorter time and lower temperature than the bulk diffusion synthesis between La2SiO5 and La2Si2O7.

Thermoelectric properties of n-type SrTiO3

We present an investigation of the thermoelectric properties of cubic perovskite SrTiO3. The results are derived from a combination of calculated transport functions obtained from Boltzmann transport theory in the constant scattering time approximation based on the electronic structure and existing experimental data for La-doped SrTiO3. The figure of merit ZT is modeled with respect to carrier concentration and temperature. The model predicts a relatively high ZT at optimized doping and suggests that the ZT value can reach 0.7 at T = 1400 K. Thus ZT can be improved from the current experimental values by carrier concentration optimization.

Highly redox-resistant solid oxide fuel cell anode materials based on La-doped SrTiO3 by catalyst impregnation strategy

An anode backbone using 40 wt% (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (SSZ)-Sr0.9La0.1TiO3 (SLT) cermet was prepared for SSZ electrolyte-supported SOFC single cells. 15 mgcm−2 Ce0.9Gd0.1O2 (GDC) was impregnated to totally cover the SSZ-SLT anode backbone surface acting as a catalyst, and the cell voltage achieved 0.865 V at 200 mAcm−2 using (La0.75Sr0.25)0.98MnO3 (LSM)-SSZ cathode in 3%-humidified hydrogen fuel at 800 °C. Cell performance was substantially improved from 0.865 V to >0.97 V when 0.03 mgcm−2 Pd or Ni was further incorporated as a secondary catalyst into the anode layer. 50 redox cycles were performed to investigate redox stability of this high performance anode. It was found that even after the 50 redox cycle long-term degradation test, cell voltage at 200 mAcm−2 was retained around 0.94 V, higher than the cell performance using the conventional Ni-SSZ cermet anode. The catalytically-active reaction sites at ceria-Pd or ceria-Ni may account for the excellent performance, and the extremely low metal catalyst concentration prevent serious metal aggregation in achieving excellent redox stability.

Thermoelectric properties of the ceramic oxide Sr1−x La x TiO3

The effect of lanthanum on the electric and the thermoelectric properties of the ceramic oxide Sr1−xLaxTiO3 (where x = 0.0, 0.04, 0.06, 0.08 and 0.12 mole) have been studied. La-doped SrTiO3 was prepared by using the conventional mixed-oxide reaction method. XRD patterns indicated that almost all the La atoms incorporated into the SrTiO3 crystal provided charge carriers. The lattice parameter increases with increasing La doping content. The relative densities of all the samples varied from 89.6% to 94.8%. The electrical conductivity increased with La doping up to 0.08 moles and then decreased as the content of La was increased above 0.08 moles. The thermal conductivity decreased with increasing La content. The largest absolute value of the Seebeck coefficient, 394 μVK−1 at 973 K, was observed at x = 0.04. The Sr0.92La0.08TiO3 sample showed its maximum electrical conductivity at 773 K and its largest ZT value of 0.20 at 973 K.

The Effect of Mesoporous Structure on the Thermoelectric Properties of Nonstoichiometric La-Doped SrTiO3

We investigated the effect of mesoporous structure on the thermoelectric properties of La-doped SrTiO3 films through the analyses of their structural, electrical, and thermoelectric properties. The mesoporous structure induces a decrease in thermal conductivity and an increase in the Seebeck coefficient because of increased phonon and carrier scattering. The increase of electrical resistivity was smaller than the changes in the Seebeck coefficient and thermal conductivity. As a result, the incorporation of Brij-S10 surfactant into La-doped SrTiO3 films induced an increase of the figure of merit. The maximum value of the figure of merit—approximately 0.025 at 200°C—was obtained with a Brij-S10 molar ratio of 0.1; this figure of merit was approximately 35% higher than that of a La-doped SrTiO3 film without the Brij-S10 surfactant. The results of our study suggest that the mesoporous structure could play a role in enhancing the thermoelectric properties.

High temperature fiber-optic evanescent wave hydrogen sensors using La-doped SrTiO3 for SOFC applications

Advanced sensors are needed for development of next-generation fossil fuel power generation technologies and for enhancing efficiencies of existing power generation systems. Optical waveguide-based sensing technologies have become increasingly important for harsh environment energy applications. In this manuscript, we present sensing results for fiber-optic evanescent wave hydrogen sensors employing La-doped SrTiO3 layers as the active sensing element. These sensors show a rapid, reproducible sensing response to hydrogen fuel gas streams at elevated temperatures (600–800°C). The presence of hydrogen results in a reversible and reproducible decrease in near-infrared transmission through the sensor. Sensors were also tested directly in the anode assembly of an operating solid oxide fuel cell (SOFC) with the sensor response correlating with both H2 concentration and SOFC cell potential.

Exploring the Role of La Codoping beyond Charge Compensation for Enhanced Hydrogen Evolution by Rh-SrTiO3.

In this theoretical study, we investigate recent observation of enhancement of hydrogen evolution efficiency of Rh-doped SrTiO3 due to codoping with La at the Sr lattice site. Using hybrid density functional theory, we have systematically studied the electronic structure of (Rh, La)-codoped SrTiO3 and compared with that of Rh-doped SrTiO3, La-doped SrTiO3, and undoped SrTiO3. The aim of the present study has been to explore the role of different factors toward the observed enhanced photoactivity of (Rh, La)-codoped SrTiO3. Doping with only Rh significantly reduces the photoabsorption energy by introducing localized acceptor states between the valence band and conduction band. Unfortunately, these states act as efficient sources for charge carrier trapping. Besides, the oxygen vacancy found to be present in the Rh-doped SrTiO3 as a charge compensating defect also accelerates the electron-hole recombination rate. We have shown that codoping with La and Rh leads to the formation of clean band structure without encountering any midgap states. Introduction of La into the Rh-doped SrTiO3 not only reduces the quantity of Rh(4+) species but also suppresses the oxygen vacancy due to formation of a charge-compensated system. The presence of La favors Rh doping into the crystal structure of SrTiO3 by reducing the formation energy. Moreover, the conduction band minima are found to be shifted in the upward direction significantly due to codoping with Rh and La, thereby increasing the reducing behavior at the conduction band. This leads to enhancement of hydrogen evolution activity of SrTiO3 during photocatalytic water splitting under visible light.

Rapid Synthesis and Formation Mechanism of Core-Shell-Structured La-Doped SrTiO3 with a Nb-Doped Shell

To provide a convenient and practical synthesis process for metal ion doping on the surface of nanoparticles in an assembled nanostructure, core-shell-structured La-doped SrTiO3 nanocubes with a Nb-doped surface layer were synthesized via a rapid synthesis combining a rapid sol-precipitation and hydrothermal process. The La-doped SrTiO3 nanocubes were formed at room temperature by a rapid dissolution of NaOH pellets during the rapid sol-precipitation process, and the Nb-doped surface (shell) along with Nb-rich edges formed on the core nanocubes via the hydrothermal process. The formation mechanism of the core-shell-structured nanocubes and their shape evolution as a function of the Nb doping level were investigated. The synthesized core-shell-structured nanocubes could be arranged face-to-face on a SiO2/Si substrate by a slow evaporation process, and this nanostructured 10 μm thick thin film showed a smooth surface.

Optical and Chemi-Resistive Sensing in Extreme Environments: La-Doped SrTiO3 Films for Hydrogen Sensing at High Temperatures

For efficient operation of next-generation fossil fuel technologies, development of sensors capable of withstanding harsh environments is required. Optical waveguide based sensing platforms have become increasingly important, but a need exists for materials that exhibit useful changes in optical properties in response to changing gas atmospheres at high temperatures. In this manuscript, the onset of a near-IR absorption associated with an increase in free carrier density in doped metal oxide films to form so-called conducting metal oxides is discussed in the context of results obtained for undoped and La-doped SrTiO3 films. Film characterization results are presented along with measured changes in optical absorption resulting from various high temperature treatments in a range of gas atmospheres. Optical property changes are also discussed in the context of a simple model for optical absorption in conducting metal oxide thin films. The combination of experimental results and theoretical modeling presented here suggests that such materials have potential for high temperature optical gas sensing applications. Simulated sensing experiments were performed at 600–800 °C, and a useful, rapid, and reproducible near-IR optical sensing response to H2 confirms that this class of materials shows great promise for optical gas sensing.

Local Structure and Energetics of Pr- and La-Doped SrTiO3 Grain Boundaries and the Influence on Core–Shell Structure Formation

Using atomistic simulation techniques, we investigate the structural and energetic effects of Pr- and La-doping on the grain boundaries of SrTiO3. Recent experimental studies have shown that Pr-doping of SrTiO3 produces a large thermoelectric power factor and that this is a result of Pr-rich grain boundaries formed in a core–shell structure. This behavior was not observed for La-doping and dopant-rich grain boundaries were not formed. In this work, for both the infinitely dilute limit and solid solutions with dopant concentrations of up to ∼15 mol %, Pr-doping at the grain boundaries is found to be far more favorable than doping in the bulk. La-doping at the grain boundaries is also found to be favored over bulk doping, however, to a far lesser extent than that calculated for Pr. The binding energy between Pr ions and charge compensating Sr vacancies is also stronger than for La ion defect clusters. By analyzing the local structures around these defect clusters, we provide possible explanations as to why there is a significant energetic benefit for grain-boundary doping.