Rare Earth Doped Ceria Research Articles

Boosting Electrochemical Performance via Extra-Role of La-Doped CeO2-δ Interlayer for "Oxygen Provider" at High-Current SOFC Operation.

Utilizing rare earth doped ceria in solid oxide cells (SOCs) engineering is indeed a strategy aimed at enhancing the electrochemical devices' durability and activity. Particularly, Gd-doped ceria (GDC) is actively used for barrier layer and catalytic additives in solid oxide fuel cells (SOFCs). In this study, experiments are conducted with La-doped CeO2 (LDC), in which the Ce sites are predominantly occupied by La, to prevent the formation of the Ce-Zr solid solution. This LDC is comparably used as a functional interlayer between the electrolyte and cathode if sintered at lower temperatures to avoid La2Zr2O7 impurity. In addition, the high substitution of La3+ into the ceria lattice improves the oxygen non-stoichiometry of LDC, leading to accelerated electrochemical high performance by the additional role of LDC for oxygen supplier capacitance at high current operation. Thus, it is confirmed that the improved SOFC high performance is achieved at the maximum power density (MPD) of ≈2.15W cm-2 at 800°C when the optimized LDC buffer layer is hired at the anode-supported typed-Samsung's SOFC by lowering the sintering temperature to prevent LDC's impurity reaction.

Unveiling the local structure of doubly doped CeO2: a synchrotron X-ray pair distribution function study

For the first time the structural properties at the local scale of doubly-doped ceria systems are discussed.

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.

Development of Durable Materials for Cost Effective Water Splitting

Saint-Gobain, a global leader in ceramic and polymer based materials and components has been developing solutions for fuel and electrolysis cells for over 20 years. These solutions include; electrode and electrolyte materials, innovative cell and stack designs, ceramic interconnects, seals (both glass-ceramic and polymer based) and hot box designs. Although high temperature electrolysis promises to be an efficient solution to the production of hydrogen, durability and performance improvements are still required to produce devices that are economically feasible. A promising material family for the air electrode are rare-earth nickelate oxides with the Ruddlesden-Popper structure. These oxides are known to have a point defect structure in which oxygen interstitials play a key role in the oxygen surface exchange and transport through the lattice. Although the performance of the rare-earth nickelates can be high, they unfortunately react and decompose when in contact with rare-earth doped ceria. Ceria is typically added as a second phase from the standpoint of buffering the coefficient of thermal expansion, providing additional oxygen ionic conducting paths, and for forming the buffer layer preventing adverse reactions between the oxygen electrode and the electrolyte which are typically yttrium or scandium doped zirconia.This work focuses on stabilizing the rare-earth nickelate electrodes through compositional modifications while developing cell-design agnostic material solutions using these materials. The driving force for nickelate decomposition was determined to be the high limits for cation solubility within typical ceria-based barrier layers. A solution to this problem is presented. In addition, towards improving the performance of cells the oxygen diffusion mechanism was modeled allowing the calculation of ionic conductivity for a variety of cation substitutions within the nickelate structure. Fundamental material properties such as exchange coefficient, oxygen incorporation into the lattice as a function of partial pressure, and electrode resistance over time were measured. Finally, an innovative processing technique for the simultaneous multilayer coating of highly loaded slurries was developed. This process provides a reduction in the number of processing steps required to produce cells while also allowing for good layer adhesion as well as thickness control at the 2.5 micron range.

(Invited) Boosting Solid Oxide Fuel Cell Performance through the Effect of of Interstitial Oxide Ion Supplying of Oxygen Storage Capacitance Materials at Cathodic Functional Layer

Chemical compatibility between the hierachical interface components in solid oxide fuel cells (SOFCs) is an emerging critical issues that significantly impact the durability of SOFCs. However, there are several challenging issues to be solved: cost-effective manufacturing configuration, lowering the operating temperature, and durability. The creation of novel materials or long-lasting electrode/electrolyte interfaces that permit high activity (oxygen reduction or fuel oxidation) while maintaining long-term stability is one of the major barriers to the commercialization of the SOFC Rare-earth doped ceria (REDC) is commonly used to enhance durability and activity in solid oxide cells (SOCs) engineering, with Gd-doped ceria (GDC) being a popular choice for interlayer and additives in solid oxide fuel cells (SOFCs). However, the Ce diffusion into the lattice of ZrO2of YSZ-based electrolyte as a solid solution at high-temperature manufacturing still is remained to be solved. In this study, we conducted La-doped CeO2 (LDC), whose Ce-sites are highly occupied with La to avoid a solid solution of Ce-Zr, as a functionalized interlayer between YSZ-based electrolyte and cathode. When LDC (La-doped CeO2, LDC, Ce0.6La0.4O2−δ) or Bi doped Cerium Oxide buffer layer was optimized at lowering sintering temperature to prevent a La2Zr2O7 impurity reaction, the improved electrochemical performance was achieved at the maximum power density (MPD) of ~1.43 W cm-2 at 800 oC. The high substitution of La3+ into the ceria lattice improves the oxygen non-stoichiometry of LDC, leading to improve the oxide ion supplying of the cathode side during the high current operation. Consequently, we firstly propose that the larger oxygen storage or supplying capacitance (OSC) effect of the interdiffusion layer composition (La, Zr, Ce, Y)O2 in La-doped CeO2 (LDC) is responsible for the positive enhancement of the oxygen reduction reaction (ORR) at high current ranges through interstitial oxygen supply at the cathode/electrolyte interface rather than (Gd, Zr, Ce, Y)O2 in GDC. The OSC of the LDC is higher than that of the GDC, and it effectively enhances the electrochemical performance of the cathode interface in the SOFCs because it may be affected by the high oxygen carrier concentration of the LDC, the higher oxygen releasing, and storage capacitance. This revealed that the effect of oxygen releasing, and storage capacity of the buffer layer might be the cause of improved performance. Consequently, the cell containing the high OSC buffer layer fabricated at 1250 oC to avoid interfacial insulating layers, such as La2Zr2O7 impurities, successfully displayed a dramatically high-power density value. Figure 1

(Invited) Ruddlesden-Popper Oxides As Bi-Functional Oxygen Electrodes

Incorporating more renewables into our energy mix is a major challenge that confronts our transition to a carbon free economy. Solid oxide electrochemical cells when operated alternately in fuel cell and electrolysis modes, enable the incorporation and integration of far greater amounts of solar and wind energy into our power grid. As islanded systems, they can also function as distributed generation systems thereby decreasing our reliance on the power grid. In this talk, we present a recent survey of results on rSOCs based on rare earth nickelate – rare-earth doped ceria composite oxygen electrodes that exhibit excellent cycling behavior when operated alternately in solid oxide fuel cell (SOFC) and solid oxide electrochemical cell (SOEC) modes of operation. The degradation in such cells is significantly mitigated by mode-switching operation when compared to operating in single mode operation in electrolysis mode. By comparing distributed relaxation times (DRT) analysis of impedance spectra obtained from single-mode (electrolysis) operated cells, and cells operated in switched-modes, we obtain clues into cell degradation mechanisms.

Lanthanide element variation in rare earth doped ceria – FeCo2O4 dual phase oxygen transport membranes

The increased interest in dual-phase membrane materials for oxygen separation leads to the continuous optimization of their composition. Rare-earth doped ceria is a promising candidate as the ion-conducting phase in the membrane. Spinel-structured FeCo2O4 was investigated as an electronic conducting phase forming an additional electronic conducting perovskite-structured phase during sintering when combined with Ce1−xLnxO2−δ. The influence of rare-earth lanthanide elements, i.e., Gd and Sm, as well as their concentration, i.e. x = 0.1 and 0.2, on the final phase composition and microstructure as well as its related functional properties in particular oxygen permeation is analyzed. 20 mol.% doping of either Gd or Sm reveals a multi-phase microstructure after sintering. Moreover, segregation of Gd/Sm, iron, and cobalt is found at the ceria-ceria grain boundaries in Ce0.8Sm0.2O2−δ- and Ce0.9Gd0.1O2−δ-based composites. In contrast, 10 mol.% Gd–doping leads to a dual-phase membrane material without the formation of any other phase. In all cases, the percolation threshold is reached at approx. 20 vol% of the electron-conducting phase in the system leading to similar maximum permeation rates determined by the ionic conductivity of the ceria phase.

(Invited) Reversible Solid Oxide Electrochemical Cells for Grid Scale Storage of Renewable Energy

Incorporating more renewables into our energy mix is the central challenge that confronts our transition to a carbon free economy. Reversible solid oxide electrochemical cells (rSOCs) when operated alternately as fuel cell power systems and as electrolysis systems, enable the incorporation and integration of far greater amounts of solar and wind energy into our power grid. As standalone systems, they can also function as distributed generation systems thereby decreasing our reliance on the power grid. In this talk, we present recent results on rSOCs based on rare earth nickelate – rare-earth doped ceria composite oxygen electrodes that exhibit excellent cycling behavior when operated reversibly in solid oxide fuel cell (SOFC) and solid oxide electrochemical cell (SOEC) modes of operation. The degradation in such cells is significantly mitigated by mode-switching operation when compared to operating in single mode operation in electrolysis mode. By comparing distributed relaxation times (DRT) analysis of impedance spectra obtained from single-mode (electrolysis) operated cells, and cells operated in switched-modes, we obtain clues into cell degradation mechanisms.

Solid Oxide Cell Electrode Nanocomposites Fabricated by Inkjet Printing Infiltration of Ceria Scaffolds.

The enhancement of solid oxide cell (SOC) oxygen electrode performance through the generation of nanocomposite electrodes via infiltration using wet-chemistry processes has been widely studied in recent years. An efficient oxygen electrode consists of a porous backbone and an active catalyst, which should provide ionic conductivity, high catalytic activity and electronic conductivity. Inkjet printing is a versatile additive manufacturing technique, which can be used for reliable and homogeneous functionalization of SOC electrodes via infiltration for either small- or large-area devices. In this study, we implemented the utilization of an inkjet printer for the automatic functionalization of different gadolinium-doped ceria scaffolds, via infiltration with ethanol:water-based La1−xSrxCo1−yFeyO3−δ (LSCF) ink. Scaffolds based on commercial and mesoporous Gd-doped ceria (CGO) powders were used to demonstrate the versatility of inkjet printing as an infiltration technique. Using yttrium-stabilized zirconia (YSZ) commercial electrolytes, symmetrical LSCF/LSCF–CGO/YSZ/LSCF–CGO/LSCF cells were fabricated via infiltration and characterized by SEM-EDX, XRD and EIS. Microstructural analysis demonstrated the feasibility and reproducibility of the process. Electrochemical characterization lead to an ASR value of ≈1.2 Ω cm2 at 750 °C, in the case of nanosized rare earth-doped ceria scaffolds, with the electrode contributing ≈0.18 Ω cm2. These results demonstrate the feasibility of inkjet printing as an infiltration technique for SOC fabrication.

Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria

Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries.

High Pressure X-ray Diffraction as a Tool for Designing Doped Ceria Thin Films Electrolytes

Rare earth-doped ceria thin films are currently thoroughly studied to be used in miniaturized solid oxide cells, memristive devices and gas sensors. The employment in such different application fields derives from the most remarkable property of this material, namely ionic conductivity, occurring through the mobility of oxygen ions above a certain threshold temperature. This feature is in turn limited by the association of defects, which hinders the movement of ions through the lattice. In addition to these issues, ionic conductivity in thin films is dominated by the presence of the film/substrate interface, where a strain can arise as a consequence of lattice mismatch. A tensile strain, in particular, when not released through the occurrence of dislocations, enhances ionic conduction through the reduction of activation energy. Within this complex framework, high pressure X-ray diffraction investigations performed on the bulk material are of great help in estimating the bulk modulus of the material, and hence its compressibility, namely its tolerance toward the application of a compressive/tensile stress. In this review, an overview is given about the correlation between structure and transport properties in rare earth-doped ceria films, and the role of high pressure X-ray diffraction studies in the selection of the most proper compositions for the design of thin films.

(Invited) Compositionally Engineered Ceria-Nickelate Composite Electrodes as Oxygen Electrodes in SOFCs and SOECs

Composite electrodes comprising a rare-earth nickelate phase and a doped rare-earth ceria phase with a high level of rare-earth doping have been studied as oxygen electrodes. While rare-earth nickelates have shown great promise as oxygen electrodes in both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOECs), they become unstable in contact with rare-earth doped ceria. In this work we describe a strategy to eliminate this instability and enable the use of these electrodes as stable high performance oxygen electrodes in both SOFC and SOEC applications. The performance of such composite electrodes is explored using a symmetrical cell approach, and through the electrical conductivity relaxation (ECR) method and compare it to state-of-the-art oxygen electrodes comprising strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ) composite electrodes. The symmetrical cell tests which reveal the stabilized nickelate-ceria electrodes to be superior, are also well-correlated with the results of testing complete cells in both SOFC and SOEC modes. Based on these results we make the case for deploying these stabilized nickelate-ceria composite electrodes as oxygen electrodes in reversible SOFC-SOEC systems.

Effects of Inadequate Current Collection on Impedance of Solid State Electrodes

Impedance measurements of SOEC, SOFC, and other high temperature solid state electrodes often employ a current collector to insure a laterally homogeneous current density. During recent measurements of H2O and CO2 reduction on rare-earth doped ceria electrodes, we found that inadequate current collection leads to distortion of the impedance response. This distortion not only involves quantitative inaccuracies due to reduced active area, but also changes in the shape and frequency response of the electrodes. In order to better understand these results, we have developed a general model, based on frequency-resolved finite element analysis, for the impacts of finite current collection on impedance. This model predicts a number of surprising results, including severe distortions particularly in the high frequency portion of the spectrum, appearance of apparent Warburg features when none is present in the actual electrode response, shifts in characteristic resistance and frequency of the electrodes, and misattribution of electrolyte resistance to electrode response. This model may help explain a number of seemingly anomalous impedance features often reported by authors.

Electrical properties of rare earth doped ceria electrolyte for solid oxide fuel cell applications

This paper attempt to investigate the structural and electrical properties of the boron doped Samarium doped Ceria (SDC) electrolyte prepared by simple solid state reaction method. The prepared sample was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and impedance Spectroscopy for their composition, size, and electrical conductivity. In this present work, we are evaluating the electrical conductivity of the prepared electrolyte. The sintered pellet exhibits good surface morphology. The frequency dependence of dielectric properties is obtained from impedance spectroscopy studies. Results of analysis reveal that nanocrystalline codoped sample was successfully prepared at low processing temperature with good ionic conductivity suitable for solid oxide fuel cell applications

Effect of rare earth doping on the enhancement of photocatalytic performance of ceria nanocrystals under natural sunlight

The photocatalytic properties of CeO2, Gd:CeO2 and Sm:CeO2 nanocrystals have been investigated under natural sunlight. The structural and morphological analyses of the synthesized samples were carried out by X-ray powder diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. The UV–Vis absorption of pure and doped ceria confirmed the reduction in the band gap values. Fluorescence spectra of CeO2 show strong emission peaks variation at 424 and 466 nm and for rare earth doped ceria at 466 nm. XPS analysis revealed in the composition, Ce4+ to Ce3+ ratio and energy states of the prepared samples. The photocatalytic performance of Sm:CeO2 was relatively high compared to pure and Gd:CeO2 in the degradation of methylene blue dye. The variation in the photocatalytic performance was discussed based on the oxygen vacancy and Ce3+ concentrations. The present study confirmed that the incorporation of rare earth ions increased the oxygen vacancies, which caused changes in Ce4+/Ce3+ ratio thereby enhancing the visible photocatalytic activities.

Structural and ionic transport mechanism of rare earth doped cerium oxide nanomaterials: Effect of ionic radius of dopant cations

This work focuses on the effect of ionic radius of dopant cations on the structural and electrical properties of different rare earth doped ceria based nanoparticles. The lattice parameter of the doped samples increases linearly with dopant ionic radius. The Gd3+ doped ceria shows minimum r.m.s. strain among the others. The conductivity and activation energy of the samples are significantly affected by the dopant ionic radius. The Gd3+ doped ceria shows highest conductivity and lowest activation energies among the other rare earth doped ceria. The conductivity decreases and activation energy increases for the dopants having ionic radii higher and lower with respect to Gd3+. The ion movements at grain and grain boundary are also found to depend on the ionic radius of the dopants. Scaling of frequency dependent modulus spectra reveals the temperature and ionic radius independent relaxation mechanism. The dielectric constant shows that the stability of defect associates is higher for the samples having ionic radius lower and higher than the ionic radius of Gd3+ ions. The conductivity values are well supported by the VRH model.

Imaging Space Charge Regions in Sm-Doped Ceria Using Strain-Based Scanning Probe Techniques

Scanning probe methods have long been employed to conduct local electrochemistry. However, connecting these measurements to macroscopic electrochemical performance of an electrode can be challenging. Workers have recently begun exploring strain-based methods sensitive to local defect concentrations in materials. These include electrochemical strain microscopy (ESM) and more recently scanning thermo-ionic microscopy (STIM). The basis for these techniques will be discussed, with examples involving Li battery materials and the study of grain boundaries in rare-earth doped ceria. Also discussed will be the possible application of these techniques as local witnesses of defect concentrations in complex electrode structures during impedance or other transient measurements.

Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ → Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tail...

Synergistic effects of ultrasonication and ethanol washing in controlling the stoichiometry, phase-purity and morphology of rare-earth doped ceria nanoparticles

Over a period of last thirty years, use of ethanol has been historically reported for obtaining nanopowders with low agglomeration for various oxide systems. In addition to these benefits, we show for the first time that treatments in ethanol medium coupled with an ultrasonication step can impart crucial additional advantages in controlling the phase purity and stoichiometry/composition for such systems. This is an important issue especially for any complex multicationic oxide nanoparticles system and hence we selected one of the most popular catalyst systems of doped-ceria (CeO2) nanoparticles with very high (50%) level of rare-earth (lanthanum) doping for this case study. The effect of an ultrasonication combined ethanol treatment was compared with the other solvent media (pure water and ethanol) without ultrasonication. The underlying mechanism for this process involves lowering the deprotonation rate in ethanol medium which eventually reduces the condensation of the individual metal oxides while the ultrasonication ensures the reproducibility of the synthesis by providing a homogeneous colloidal solution for each washing stages. This novel modification in synthesis of nanoparticles aims to provide meaningful solutions in optimising the phase, composition and morphology of multicationic complex system of nanocrystals.

Structural/surface characterization and catalytic evaluation of rare-earth (Y, Sm and La) doped ceria composite oxides for CH3SH catalytic decomposition

A series of rare earth (Y, Sm and La) doped ceria composite oxides and pure CeO2 were synthesized and evaluated by conducting CH3SH catalytic decomposition test. Several characterization studies, including XRD, BET, Raman, H2-TPR, XPS, FT-IR, CO2-TPD and CH3SH-TPD, were undertaken to correlate structural and surface properties of the obtained ceria-based catalysts with their catalytic performance for CH3SH decomposition. More oxygen vacancies and increased basic sites exhibited in the rare earth doped ceria catalysts. Y doped ceria sample (Ce0.75Y0.25O2-δ), with a moderate increase in basic sites, contained more oxygen vacancies. More structural defects and active sites could be provided, and a relatively small amount of sulfur would accumulate, which resulted in better catalytic performance. The developed catalyst presented good catalytic behavior with stability very similar to that of typical zeolite-based catalysts reported previously. However, La doped ceria catalyst (Ce0.75La0.25O2-δ) with the highest alkalinity was not the most active one. More sulfur species would be adsorbed and a large amount of cerium sulfide species (Ce2S3) would accumulate, which caused deactivation of the catalysts. The combined effect of increased oxygen vacancies and alkalinity led to the catalytic stability of Ce0.75Sm0.25O2-δ sample was comparable to that of pure CeO2 catalyst.