Lanthanum-doped Ceria Research Articles

Boosting the Stability and Performance of SOFCs: A New Approach to Oxygen Storage Capacity Using Lanthanum-Doped Ceria Interlayer Technology

Being carbon neutral is one of the world's most important objectives. More than 65 percent of the countries that emit harmful greenhouse gases have committed to achieving net-zero emissions by the middle of the century.[1] There are numerous essential ways to gather energy for worldwide missions, including fuel cells, solar power, wind power, etc. The growing marketplace of fuel cells is following the trend of green-energy demand. Solid oxide fuel cells (SOFCs) are gaining popularity in the next-generation energy industry due to their high energy conversion efficiency, fuel flexibility, and low emissions.[2]The main challenges for the commercialization of SOCs that need to address are high performance, long-term durability, and effective cost. Stabilized zirconia such as Yttria-stabilized zirconia (YSZ) and Scandia-stabilized zirconia (ScSZ) is commonly used as a self-standing electrolyte support. The inability of zirconia-based electrolytes to react with Sr-rich cathode materials like LSC, LSCF, BSCF, etc. Therefore, the electrodes and the zirconia-based electrolyte must have the best-interlayered buffer structure. Optimizing interlayer microstructures to make them more effective at high-performance ESCs is of great interest. Several Rare-earth doped ceria (RDC) materials, including (Gd, Ce)O2, (Sm, Ce)O2, and (La, Ce)O2, have been used as efficient interlayers to lessen reactivity and preserve ionic conductivity in the interface. However, the difference in the thermal expansion coefficient (TEC) between the GDC and the YSZ poses a significant problem for starting and halting SOCs. In the present work, we focus on the commercial lanthanum-doped ceria (Ce0.6La0.4O1.8 - LDC) as a buffer layer between the YSZ electrolyte and the LSCF cathode because of its higher oxygen storage capacitance. The reactivity of the YSZ/GDC composite was higher than that of the YSZ/LDC composite. And the total conductivity of the LDC/YSZ composite is higher than GDC/YSZ composite even though LDC has lower conductivity than GDC. Moreover, the mismatching of TEC was reduced by replacing the GDC with the LDC bilayer.Consequently, the maximum power density for the electrolyte-supported cell SOFC with LDC interlayer was ~0.55 W.cm−2 at 1073 K. A variety of additives (Co, Mn, and Cu) was added to the LDC buffer layer to examine the sintering properties. The addition of copper not only obtained the highest microstructural density but also improve mechanical stability under cost-effective processes.

Using Ex Situ and In Situ HERFD-XANES to Reveal the Superior Oxidation and Reduction Cycling of Ceria Nanocubes Dispersed in Silica Aerogel.

The oxygen storage capacity of ceria-based catalytic materials is influenced by their size, morphology, and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces; however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes, and ceria nanocubes embedded inside a highly porous silica aerogel by high-energy resolution fluorescence detection-X-ray absorption near edge spectroscopy at the Ce L3 edge. In situ measurements showed an increased reversibility of redox cycles in ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement in the redox capacity and increased thermal stability of this catalytic material compared to the surfactant-capped nanocubes. Ex situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce4+ compared to that of the undoped ceria nanocubes.

NO Oxidation on Lanthanum-Doped Ceria Nanoparticles with Controlled Morphology

The present work aims to assess the impact of morphology and reducibility on lanthanum-doped ceria nanocatalysts with controlled morphology on the NO oxidation reaction. Specifically, samples were prepared using a hydrothermal method incorporating lanthanum at varying molar concentrations (0, 5, 10, and 15 mol.%) into ceria with a controlled morphology (nanocubes and nanorods). The structural, compositional, and redox characterization of these catalysts has been performed via scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (X-EDS), inductively coupled plasma (ICP), hydrogen temperature-programmed reduction (H2-TPR), and oxygen storage capacity (OSC). NO oxidation catalytic tests were conducted, and the results were compared with estimated curves (obtained by considering the proportions of the corresponding components), which revealed the presence of a synergistic effect between lanthanum and ceria. The degree of enhancement was found to depend on both the morphology and the amount of lanthanum incorporated into CeO2. These findings may facilitate the optimization of features concerning ceria-based nanocatalysts for the removal of NOx emissions from exhaust gases.

(Invited) Chemical Stability and Performance of Rare Earth Nickelate Oxygen Electrodes for Reversible Solid Oxide Cells

Composite rare-earth nickelate-rare-earth doped ceria oxygen electrodes have inherent chemical instabilities. Over the last year, we have made progress on achieving chemical stability and high performance using a strategy of heavily doping the ceria phase with rare-earth cations. A higher dopant concentration of a rare-earth cation in the ceria phase ensures thermodynamic stability of the nickelate phase in contact with the ceria phase. In particular, composite oxygen electrodes comprising lanthanum nickelate La2NiO4+δ (LNO) - lanthanum doped ceria (LDC) and neodymium nickelate Nd2NiO4+δ (NNO) - neodymium doped ceria (NDC) have been electrochemically tested in both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. The performance of these cells have been compared to a composite strontium doped lanthanum manganite (LSM)-8 mol% yttria stabilized zirconia (YSZ) electrode. Both nickelate based electrodes achieve far superior performance compared to the LSM electrodes. We also report impedance measurements from symmetrical cells featuring LNO and NNO electrodes, analyzed using a distribution of relaxation times (DRT) approach. The DRT analysis reveals significant differences in the rate controlling steps in oxygen reduction and incorporation steps between the two materials. Lastly, oxygen surface exchange results for LNO and NNO are also reported which show different oxygen exchange kinetics when measured on dense versus porous materials which have implications for deploying these materials in reversible solid oxide cells.

Tuning of Shape, Defects, and Disorder in Lanthanum-Doped Ceria Nanoparticles: Implications for High-Temperature Catalysis

Tuning of Shape, Defects, and Disorder in Lanthanum-Doped Ceria Nanoparticles: Implications for High-Temperature Catalysis

Understanding the conduction mechanism of acceptor-doped ceria oxygen ion conductors by photoluminescence analysis

Doping or co-doping is a universal and extremely effective strategy for enhancing ion conductivity by modulating the coordination surroundings of oxygen vacancies in oxygen ion conductors. Herein, the structural origin and conduction mechanism of bulk conduction upon cation doping, exemplified in Ce0.85Sm0.15-xLaxO2-δ (x = 0, 0.03, 0.06, 0.09, 0.12, and 0.15) polycrystalline oxygen ion conductors, were investigated by employing the structure-sensitive photoluminescence properties of Sm3+ ions. Photoluminescence results show that the oxygen vacancies, introduced by La3+ substitutions, moved from near the coordination positions of La3+ ions to those of Sm3+ and Ce4+ ions. This occurred when the Sm3+ ions were substituted by La3+ ions. Compared with Samarium doped Ceria (SDC), the oxygen vacancies in Lanthanum-doped ceria (LDC) are more likely to be located around the Ce4+ ions rather than around the trivalent doping cations. For acceptor-doped CeO2-based oxygen ion conductors, the key factor in the activation energy of bulk conduction, Eαbulk, is the cation electronegativity on the oxygen vacancy migration path rather than the defect association and local lattice distortion on the oxygen vacancy migration path in the lattice. The significance of cation electronegativity to ion conduction proposed in this study would be beneficial for understanding the conduction mechanism and for optimising the electrical properties of various ion conductors. In addition, the strategy developed in this study to shed light on the relationship between cation doping and local structures by photoluminescence could be extended to other ion conductors in solid oxide fuel cells, oxygen sensors, oxygen concentration cells, and memristors.

Synergistic Improvement in Coating with UV Aging Resistance and Anti-Corrosion via La-Doped CeO2 Powders

Benefitting from a suitable band gap, ceria is an excellent material for UV shielding. By solid solution doping and specific micromorphology, its band gap can be effectively controlled. In this paper, ceria doped with lanthanum via oxalate precipitation is combined with a high-temperature roasting process. The properties of the prepared samples are characterized by UV–Vis diffuse reflectance spectroscopy (DRS), Raman, XRD, FESEM and XPS. The absorption threshold of materials is clearly red-shifted in the ultraviolet band, which originates from the electron-phonon generation. To further reveal the mechanism, the density function theory calculation (DFT) is implemented to study the influence of lanthanum concentrations on ceria’s band gap. It is demonstrated that the band gap can even be narrowed to 2.97 eV by optimizing the sintering temperature and lanthanum-doped concentration. To investigate its improved anti-aging properties under ultraviolet rays, different amounts of 5% lanthanum-doped ceria is mixed with an Al-based coating and then coated on the Q235 steel. Combined with an ultraviolet light irradiation experiment and electrochemical test technology, the corrosion resistance of the modified coatings is evaluated. The coating with 20% La-doped ceria provides the best corrosion resistance performance.

Lanthanum-doped ceria interlayer between electrolyte and cathode for solid oxide fuel cells

ABSTRACT Gadolinia-doped ceria (GDC) is widely used as an interlayer for preventing the formation of SrZrO3 between yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobalt iron oxides (LSCF) cathode for solid oxide fuel cells (SOFCs). However, the diffusion of Gd ions to YSZ electrolyte results in the decrease in ionic conductivity due to the formation of Gd2(Zr, Ce)2O7. In the present work, we focus on lanthanum-doped ceria (LDC) as an interlayer between YSZ electrolyte and LSCF cathode. The ionic conductivity of YSZ/LDC composite was higher than that of YSZ/GDC composite in spite of the lower conductivity of LDC than GDC. The maximum power density for the microtubular SOFC with LDC interlayer was better than that with GDC interlayer. While the ohmic resistance decreased by the change from GDC to LDC interlayers, the polarization resistance for oxygen reduction reaction slightly increased as a result of distribution of relaxation times (DRT) analysis. The chemical compositions of Zr, Ce and La was gradually varied at the interface between YSZ electrolyte and LDC interlayer, which affected the both of ohmic and polarization resistances.

Lanthanum-Doped Ceria Nanocomposite: A Highly Stable Monolithic Catalyst for Direct Synthesis of Dimethyl Carbonate

That dimethyl carbonate is directly synthesized from methanol and carbon dioxide is an effective and environmental approach to solve the greenhouse effect. For the sake of solving the problems of low DMC productivity and poor catalysts stability in presence of the formed water. Here we design and prepare a serial of spherical La-doped ceria nanoparticles (Ce1-xLaxOδ nanocomposites, x=0.00, 0.05, 0.10, 0.15, and 0.20) via a co-precipitation method. These Ce1-xLaxOδ composites are ground into slurry by ball milled and then coated on cordierite honeycomb ceramics to obtain Ce1-xLaxOδ monolithic catalysts. These Ce1-xLaxOδ composites are characterized extensively by TEM, XRD, Raman spectroscopy, N2 adsorption-desorption isotherms, H2-TPR and XPS. The characterization results show that the Ce1-xLaxOδ composites nanoparticles possesses richer surface oxygen vacancies, higher BET surface area and smaller particle size than that of pure CeO2 nanoparticle. Besides, catalytic activity test shows these Ce1-xLaxOδ monolithic catalysts exhibit better catalytic performance than that of pure CeO2 nanoparticles. Among them, Ce0.95La0.05Oδ monolithic catalyst exhibits the highest CH3OH conversion and DMC yield, which is in good line with the oxygen vacancy content measured by XPS. Finally, the Ce0.95La0.05Oδ monolithic catalyst also shows an excellent durability of more than 100 hours, which is mainly due to the doping effect of lanthanum into the ceria oxides tailoring the structure and surface properties of the catalyst.

Study of phase stability of SrTi0.3Fe0.7O3−δ perovskite in reducing atmosphere: Effect of microstructure

Increasing SOFC electrode's surface area by modification of its microstructure is a well-known technique to reduce electrode polarization resistance. This is because reduced grain size and increased porosity promote diffusion and surface reactions, thus improving the electrode performance. However, a modified microstructure also causes differences in phase stability and in chemical compatibility with other SOFC materials.In this work, we study the effect of particle size in both the electrode performance and the phase stability under different fuel conditions and temperatures. SrTi0.3Fe0.7O3−δ (STF) is both prepared via solid state reaction (STF-SSR) and also by an alternative sol-gel route (STF-SG). The sintering temperature is reduced dramatically with the sol-gel method, hence inducing a higher porosity and a much smaller grain size. As particle size is reduced the stability under fuel conditions is also diminished, so decomposition induced by segregation of metallic Fe and SrO occurs at lower temperatures for the STF-SG sample. The stability under reducing conditions is studied by combined techniques such as TGA, TPR, XRD, SEM and TEM. Performance as anode and cathode is evaluated by Electrochemical Impedance Spectroscopy (EIS) by using electrolyte supported symmetrical cells. Prior to electrochemical experiments, the reactivity between La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) electrolyte and STF was studied, and also between STF and a Lanthanum Doped Ceria (LDC) buffer layer. It is seen that microstructure also plays a key role in the chemical stability of the STF. The impact of particle size reduction is higher for the anodic polarization resistance, which is reduced twice from STF-SSR to STF-SG.

Measurement of ion mobility based on a reversible migration process in solids

Ion mobility, which facilitates ion migration in lattices, not only is important for selection of solid electrolytes from a large number of candidate materials but also plays an essential role in the study of the fundamentals of ion transport in solids. In the present work, a simple approach is developed for measuring the mobility of oxygen ions in a typical solid electrolyte, lanthanum-doped ceria. Changes in the concentration of oxygen vacancies, which are equivalent to variations in oxygen ions, are recorded using integrated peak intensity ratios of Raman spectra under a reversible electric field. A model of the migration process of oxygen ions in a reversing electric field is proposed based on the experimental results. The mobility of oxygen ions is determined by analyzing the time interval between two steady states based on a simplified model. By altering the electric field, this approach is applicable to the measurement of the mobilities of different ion species.

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.

Characteristics of La0.8Sr0.2Ga0.8Mg0.2O3-δ-supported micro-tubular solid oxide fuel cells with LaCo0.4Ni0.6-xCuxO3-δ cathodes

In this study, micro-tubular solid oxide fuel cells (T-SOFCs) with extruded La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte as the mechanical support and LaCo0.4Ni0.6O3-δ (LCNO) or LaCo0.4Ni0.4Cu0.2O3-δ (LCNCO) as cathodes were prepared and characterized. Partial substitution of Cu for the Ni-ion positions in the LCNO lattices was found to significantly enhance the densification and accelerate the grain growth. The porosity-corrected electrical conductivity was significantly increased from 1275 S/cm for LCNO ceramic to 1537 S/cm for LCNCO ceramic, because the acceptor doping was compensated by the formation of hole carriers that produced additional polarons and significantly augmented the electrical conductivity. SOFCs with three configurations were built in this study, including Cell A that had a lanthanum-doped ceria (LDC) buffer layer inserted between the LSGM electrolyte and the LCNCO cathode, Cell B that used an LCNO-LSGM composite cathode, and Cell C that featured an LCNCO-LSGM composite cathode. Among the three cells, Cell C with 263 μm of LSGM electrolyte possessed the lowest Ohmic resistance of 0.89 Ω cm2, a polarization resistance of 0.69 Ω cm2, and the highest maximum power density of 178 mW cm−2 at 750 °C.

Sub-nanometer surface chemistry and orbital hybridization in lanthanum-doped ceria nano-catalysts revealed by 3D electron microscopy

Surface chemical composition, electronic structure, and bonding characteristics determine catalytic activity but are not resolved for individual catalyst particles by conventional spectroscopy. In particular, the nano-scale three-dimensional distribution of aliovalent lanthanide dopants in ceria catalysts and their effect on the surface electronic structure remains unclear. Here, we reveal the surface segregation of dopant cations and oxygen vacancies and observe bonding changes in lanthanum-doped ceria catalyst particle aggregates with sub-nanometer precision using a new model-based spectroscopic tomography approach. These findings refine our understanding of the spatially varying electronic structure and bonding in ceria-based nanoparticle aggregates with aliovalent cation concentrations and identify new strategies for advancing high efficiency doped ceria nano-catalysts.

Preventing of LaNiO3 Formation at the LSGM-NiO Interface via LDC Protective Layer and Proper Processing Route for Solid Oxide Fuel Cells

LSGM based SOFC cells are prone to suffer from performance degradation during operation, when they are used with Ni-containing anodes. In this study, one of the causes for performance degradation in an LSGM-based SOFC system was investigated. XRD and SEM studies coupled with EDX measurements revealed that formation of an insulating LaNiO3 layer is possible in such cells depending on the processing conditions. This ionic insulator phase forms a layer on the anode side of the LSGM and may block oxygen ion diffusion through electrolyte. A possible solution to hinder the formation of LaNiO3 phase is the use of doped ceria (XDC; where X = La or Sm) as a protective layer with proper processing route. Cross-sectional EDX analyses coupled with SEM showed that lanthanum doped ceria (LDC) was a very effective protection layer for LSGM. In contrast to LDC, samarium doped ceria (SDC) protective layer was ineffective. An inexpensive and feasible fabrication procedure for SOFC assembly was demonstrated with a thin (1-5 µm) and dense protective layer-electrolyte couple coated onto the anode.

Asymmetric Lanthanum Doped Ceria Membrane with Proton Conductive and Hydrogen Separation Capability for Solid Oxide Fuel Cell

An asymmetric lanthanum doped ceria (LDC) membrane was prepared by sintering process. The membrane consisted of two layers, dense LDC and porous Ni-LDC layers with a total thickness of approximately 850 μm. According to the XRD pattern, sintering process did not cause any chemical changes to the membrane. The membrane had a crystalline dense LDC layer with a highest hydrogen permeation flux of 1.3 × 10 -3 mol∙m -2 ∙s -1 , observed at a hydrogen partial pressure of 65.9 kPa and operating temperature of 800 o C. The hydrogen permeation increased as the partial pressure of the hydrogen gas increased. The proton conducting permeation became more dominant as the hydrogen partial pressure decreased. The highest value was observed at 20.3 kPa of hydrogen partial pressure, where 50.6% of the total permeation came from proton conducting ability of the membrane. While, the decreased in the operating temperature decreased the proton conductive permeation flux. With the decrease in the sintering temperature, the amount of pores in the dense LDC layer increased. The LDC membrane with both proton conductivity and hydrogen separation capability shows a promising potential as a hydrogen separation membrane and as a solid electrolyte for the solid oxide fuel cell.

Surface reaction controlling hydrogen permeation through lanthanum-doped ceria membrane with asymmetric structure

Surface reaction controlling hydrogen permeation through lanthanum-doped ceria membrane with asymmetric structure

Ionic conductivity investigation in lanthanum (La) and strontium (Sr) co-doped ceria system

Effect of strontium doping on ionic conductivity of lanthanum-doped ceria has been investigated. Distribution and average radius of oxygen vacancies formed from divalent and trivalent dopant cations in co-doped system has been calculated using hard-sphere model. Phase identification, microstructure and ionic conductivities of samples have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and AC impedance spectroscopy, respectively. X-ray diffraction reveals that all the samples are single phase with cubic fluorite-type structure. Dense ceramics with density ∼95% of the theoretical density have been obtained without using any sintering aid. The results show that the samples co-doped with La and Sr exhibit higher ionic conductivity and lower activation energy than those of ceria doped with La only in the intermediate temperature range. The highest ionic conductivity has been observed for Ce0.89La0.07Sr0.04O1.925 in the samples investigated (4.01 × 10−3 S cm−1) at 500 °C, which is 20 times as high as that of Ce0.85La0.15O1.925. Concept of average oxygen vacancy radius has been found to be a useful in calculating the lattice parameter in this system.

Nanosized Ce1−xLaxO2−δ/Al2O3 solid solutions for CO oxidation: Combined study of structural characteristics and catalytic evaluation

Nanosized lanthanum-doped ceria (Ce1−xLaxO2−δ) and the same dispersed on alumina support (Ce1−xLaxO2−δ/Al2O3) were synthesized following a simple and high yielding procedure. Formation of solid solutions and interaction with the support were monitored by X-ray diffraction (XRD), Raman spectroscopy (RS; UV-RS and Vis-RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and diffuse reflectance spectroscopy (UV-DRS) techniques. XRD results confirmed the formation of ceria–lanthana solid solutions. HREM studies revealed well distributed and small sized Ce–La–oxide particles of ∼7nm size over the surface of alumina support. Formation of defect sites or oxygen vacancies was investigated by RS using 325 and 632.81nm excitation laser lines which confirmed their presence in the investigated samples. RS studies further deduced that evolution of oxygen vacancies (∼570cm−1) was due to different ionic valence state of dopant (La3+) from that of Ce4+, while the development of MO8-type complex (∼600cm−1) was due to different ionic radius of dopant (La3+, 0.110nm) from that of Ce4+ (0.097nm). XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminates formation. TPR data revealed that surface modification with support enhances the reducibility of the doped ceria. Catalytic evaluation showed a 50% CO conversion temperature of 634K per 66mg of CL for the CL/A catalyst, which is lower than the CL sample (636K per 100mg of CL), possibly on account of an increased dispersion, more oxygen vacancies and easy reducibility.

Microstructural Aspects of Cation Interdiffusion Across the LSCF/GDC Interface

To evaluate the long-term durability of a reaction barrier made of gadolinium doped ceria (GDC), we have measured cation diffusion in the LSCF/GDC system. Interdiffusion and microstructural evolution were investigated for samples annealed at temperatures of 1000 to 1200 oC and times from 30 to 672 h. Significant diffusion of lanthanum into GDC and diffusion of ceria and gadolinium into LSCF were observed. X-ray diffraction (XRD) revealed the formation of lanthanum doped ceria (LDC) in the vicinity of the interface. Furthermore, EDS analysis showed the uneven distribution of lanthanum into the polycrystalline ceria. Those compositional patched patterns are thought to be the result of the interplay of diffusion along grain boundaries and bulk diffusion, without excluding the possible effect of crystallographic relationship at the interface.