Undoped Ceria Research Articles

Control of oxygen vacancies and Ce+3 concentrations in doped ceria nanoparticles via the selection of lanthanide element

The effect of lanthanides that have positive association energies with oxygen vacancies, such as samarium and neodymium, and the elements with negative association energies, such as holmium and erbium, on ionization state of cerium and, consequentially, the oxygen vacancy concentration in doped ceria nanoparticles are investigated in this article. Structural and optical characterizations of the doped and undoped ceria nanoparticles, synthesized using chemical precipitation, are carried out using transmission electron microscopy, X-ray diffractometry, optical absorption spectroscopy, and fluorescence spectroscopy. It is deduced that the negative association energy dopants decrease the conversion of Ce+4 into Ce+3 and, hence, scavenge the oxygen vacancies, evidenced by the observed increase in the allowed direct bandgap, decrease in the integrated fluorescence intensity, and increased the size of doped nanoparticles. The opposite trends are obtained when the positive association dopants are used. It is concluded that the determining factor as to whether a lanthanide dopant in ceria acts as a generator or scavenger of oxygen vacancies in ceria nanoparticles is the sign of the association energy between the element and the oxygen vacancies. The ability to tailor the ionization state of cerium and the oxygen vacancy concentration in ceria has applications in a broad range of fields, which include catalysis, biomedicine, electronics, and environmental sensing.

Effect of co-doping of Mg and La on conductivity of ceria

The aim of present investigation is to study the effect of co-doping of lanthanum and magnesium on the ionic conductivity of CeO2 for its use as a solid electrolyte in the intermediate temperature solid oxide fuel cells (IT SOFC). Attempts have been made to synthesize compositions with x=0.075, 0.07, 0.06, 0.05 and 0.00 and y=0.00, 0.01, 0.03, 0.05 and 0.15 in the system Ce1−x−yMgxLayO2−x−y/2 (CML) by auto-combustion method. X-ray diffraction patterns show that all the samples have fluorite structure similar to undoped ceria. Microstructures of thermally etched samples have been studied by scanning electron microscopy (SEM). In order to evaluate the contributions of grains σg, grainboundaries σgb and electrode–specimen interface to the total conductivity σt, use is made of AC impedance analysis. Impedance measurements are made in the frequency range 1Hz to 1MHz and temperature range 250–500°C. The results show that the conductivity of the system increases with increasing lanthanum content. There is no enhancement in conductivity due to co-doping with La and Mg.

A concerted migration mechanism of mixed oxide ion and electron conduction in reduced ceria studied by first-principles density functional theory

Ceria based oxides are regarded as key oxide materials for energy and environmental applications, such as solid oxide fuel cells, oxygen permeation membranes, fuel cell electrodes, oxygen storage, or heterogeneous catalysis. This great versatility in applications is rendered possible by the fact that rare earth-doped ceria is a pure oxygen ion conductor while undoped ceria, CeO(2-δ), is a mixed oxygen ion-electron conductor. To get deeper insight into the mixed conduction mechanism of oxygen ions and electrons from atomistic and electronic level viewpoints we have applied first-principles density functional theory (DFT + U method). The calculation results show that oxygen vacancies strongly attract localized electrons, forming associates between them. The migration energy of an oxygen vacancy in such an associate is substantially lowered compared to the unassociated case due to the simultaneous positional rearrangement of localized electrons during the ionic jump process. Accordingly, we propose a concerted migration mechanism of oxygen vacancies and localized electrons in reduced ceria; this mechanism results in an increased diffusivity of oxygen vacancies supported by localized electrons compared with that in pure oxide ion conductors.

Effects of Grain Boundary Decoration on the Electrical Conduction of Nanocrystalline CeO2

In this study, we investigate the effect of decorating the grain boundaries of nanocrystalline undoped ceria on the electrical transport properties. For the decoration, different acceptors (Yb, Y, Bi) were chosen. On decoration, the conduction switches from electronic to ionic. Upon sintering the grains are characterized by a core-shell configuration, in which the core remains undoped while the shell is heavily doped as a consequence of the diffusion of the acceptors toward the grain interior. The shell dominates the overall transport properties of the nanocrystalline ceria and is found to be in the mesoscopic regime.

Preparation and characterization of co-doped (Ce0.80La0.15Al0.05O1.90) and multiple-doped (Ce0.80Sm0.10Gd0.05Al0.05O1.90 and Ce0.80Gd0.10Sm0.05Al0.05O1.90) ceria

Attempts have been made to synthesize a few compositions Ce0.80La0.15Al0.05O1.90, Ce0.80Sm0.10Gd0.05Al0.05O1.90, and Ce0.80Gd0.10Sm0.05Al0.05O1.90 by citrate–nitrate auto-combustion method. The aim of the present investigation was to study the effect of co-doping and multiple doping on the ionic conductivity of CeO2 for its use as solid electrolyte in intermediate temperature solid oxide fuel cells. XRD patterns showed that all the samples have fluorite-type crystal structure similar to undoped ceria. Microstructures of thermally etched samples have been studied by scanning electron microscopy. Contributions of grains σg and grain boundaries σgb to the total conductivity σT, have been determined using impedance analysis. Impedance measurements were made in the frequency range 1 Hz–1 MHz and temperature range 250–500 °C. Our experimental results show that multiple doping is more effective than co-doping for improving the oxide ion conductivity of ceria in the materials investigated in the present study.

Low-Temperature Deposition of Undoped Ceria Thin Films in scCO2 As Improved Interlayers for IT-SOFC

Cerium oxide films were deposited onto various yttria-stabilized zirconia (YSZ) electrolytes using supercritical fluid deposition (SFD) in a cold wall reactor. Deposition of ceria thin films (20–240 nm) was carried out via hydrolysis of a Ce precursor at 250–300 °C (typically 30 min) in supercritical CO2 (scCO2). The as-deposited films were continuous, dense and crystalline. They exhibited some carbon contamination which was eliminated in all cases by postannealing the films at 400 °C. Film microstructure depends on the hydrolysis conditions (water to cerium molar ratio), on the substrate nature (Si wafers or YSZ pellets) and on the YSZ pellet thickness (100 μm or 1 mm). Ceria films were then used as interlayers for intermediate temperature solid oxide fuel cells (IT-SOFC) between the YSZ electrolyte and the Nd2NiO4+δ cathode layer subsequently deposited. The electrochemical properties of the SOFC symmetrical half cells were investigated by electrochemical impedance spectroscopy (EIS). The presence of the undoped ceria layer deposited in scCO2 lowers the resistance of the cell, compared both to cells with an optimized doped ceria layer deposited by screen printing and to cells without any interlayer.

Low-Temperature Water–Gas Shift: Doping Ceria Improves Reducibility and Mobility of O-Bound Species and Catalyst Activity

A series of platinum loaded catalysts supported on cation (Me)-doped cerium dioxide (Me = Ba, La, Y, Hf and Zn) was prepared by co-precipitation of the Me-nitrates and impregnation of a Pt precursor. Low temperature water–gas shift activity depends on the nature of dopant employed, varying in the order of Ba > Y > Hf > La > undoped ceria > Zn. TPR-XANES measurements with flowing hydrogen reveal that adding dopants to ceria facilitate ceria reduction and increases the extents of both surface shell and bulk reduction of ceria. Experimental results confirm past theoretical models that dopants enhance both O-mobility and reducibility of ceria. DRIFTS measurements of the transient decomposition of formates in steam suggest that formate half-life follows the trend Zn > undoped ceria > La > Hf > Y > Ba, indicating that the formate decomposition rate is enhanced by the addition of most of the dopants tested. Taken together, the results suggest that dopant addition improves the WGS rate by increasing the O-mobility of O-bound associated intermediates. Therefore, less Pt and Ce, which are expensive, is required to achieve comparable levels of activity.

CO2 and H2O Splitting for Thermochemical Production of Solar Fuels Using Nonstoichiometric Ceria and Ceria/Zirconia Solid Solutions

The solar thermochemical splitting of CO2 and H2O with ceria and Zr-doped ceria for CO and H2 production is considered. The two-step process is composed of the thermal reduction of the ceria-based compound followed by the oxidation of the nonstoichiometric ceria with CO2/H2O to generate CO/H2, respectively. As a reference, the reactivity of pure undoped ceria was first characterized during successive thermochemical cycles using a thermobalance. Then, Zr0.25Ce0.75O2 was synthesized using different soft chemical synthesis routes to evaluate the influence of the powder morphology on the reactivity during the reduction and the oxidation steps. The reduction yield of ceria was significantly improved by doping with Zr as well as the CO/H2 production yields, but the kinetic rates of the oxidation step for doped ceria were lower than for pure ceria. CO and H2 production of 241 and 432 μmol/g, respectively, have been measured. A kinetic analysis of the CO2-splitting step allowed one to estimate the activation ener...

Influences of the processes on the microstructures and properties of the plasma sprayed IT-SOFC anode

By using the atmospheric plasma spraying (APS) technique for SOFC fabrication, we can avoid the thermal failure between the components of SOFC which made from the traditional sintering method at high temperature. The advantage of porous metal supported SOFC produced by the APS process is that the process is simple and low cost. The purpose of this study was to manufacture a porous Ni/CeO 2 anode of intermediate temperature SOFC. By introducing the pore former into the feedstocks, the porous structure of the Ni/CeO 2 anode was obtained by plasma spraying method. In this study, the feedstock of the NiO/CeO 2/Na 2CO 3 powder made by the spray granulation is spherical; the powder structure is compact and a hollow hole inside the powder, hence more porosity (29.1%) of the as-sprayed coating was found and the pore distribution is more uniform. The remainder oxygen vacancies are existed in the reduced anode coating. Use of undoped ceria as an anode material appears to be feasible. The NiO/CeO 2/Na 2CO 3 powder formed the higher bonding strength coating with 33.6 MPa. The electron conductivity was obtained with around 200 S/cm in 700 °C. The other properties of the anode were discussed in the paper.

Doped / Undoped Ceria Buffer Layers for Improved LT SOFC Performances with Pr2NiO4+δ Cathode

The praseodymium nickelate oxide Pr2NiO4+δ, a mixed conducting oxide, was evaluated as cathode for Low Temperature Solid Oxide Fuel Cell at T = 873 K, using zirconia-type electrolytes. The electrochemical performance of the porous cathode has been improved by optimisation of its microstructure combined with the use of a doped ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr2NiO4+δ powders with a median particle size of 0.4 µm. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions and tested. The best results were obtained when using a dense Ce0.8Gd0.2O1.9 (20CGO) barrier layer. Moreover, we developed a new route for depositing ceria thin films: the supercritical fluid deposition (SFD) technique. Interestingly, we could evidence that the deposit of a dense interlayer of a few tens of nanometer contributes to the lowering of the polarization resistance, but also that this result can be achieved with undoped ceria, although this material is never used for SOFC because of its low ionic conductivity level.

Experimentally Validated Simulations of Undoped Ceria Electrodes for H2 Oxidation and H2O Electrolysis in Solid Oxide Electrochemical Cells

We present here the formulation and validation of a one-dimensional model of undoped CeO2-x electrodes for fuel oxidation and H2O splitting in high-temperature solid oxide cells. The model describes dense thin-film electrodes with a porous Au current collector, dense YSZ supporting electrolyte, and a porous LSM/YSZ composite counter electrode. The model is formulated as a set of differential algebraic equations which are integrated over time to steady-state, and is compared to V-i curves from single cell tests to yield estimates for important transport, thermodynamic, and kinetic parameters governing electrochemical performance of CeO2-x electrodes in solid oxide cells. These results represent a quantitative framework for the analysis of electrochemical results on ceria electrodes.

Experimentally Validated Simulations of Undoped Ceria Electrodes for H2 Oxidation and H2O Electrolysis in Solid Oxide Electrochemical Cell Electrodes

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Doped / Undoped Ceria Buffer Layers for Improved LT SOFC Performances with Pr2NiO4+δ Cathode

not Available.

Evaluating H2O Electrolysis on Ceria with Thin-Film Electrodes

Undoped ceria with its facile redox at high-temperatures has potential as an effective material for electrolysis. This study explores the electrochemical performance of thin-film ceria electrodes for H2O electrolysis at 700 °C. The electrochemical characterization is complemented by in situ XPS measurements, which show that the highly reduced ceria surface increases in degree of reduction with electrolysis voltage. Results also show that H2O electrolysis is enhanced with increased H2 partial pressure. Analysis of electrochemical impedance spectra and voltammetric measurements show that the electrolysis reactions depend strongly on H2 pressure. These trends, which are not readily explained by simple surface kinetic models, do indicate that increased ceria surface reduction by the presence of H2 product enhances activity for H2O electrolysis. The results were used to create a microkinetic model to assess surface and subsurface processes in ceria for H2O electrolysis.

Time–Temperature–Transformation (TTT) Diagrams for Crystallization of Metal Oxide Thin Films

AbstractTime–temperature–transformation (TTT) diagrams are proposed for the crystallization of amorphous metal oxide thin films and their specific characteristics are discussed in comparison to glass‐based materials, such as glass‐ceramics and metallic glasses. The films crystallize from amorphous to full crystallinity in the solid state. As an example the crystallization kinetics for a single‐phase metal oxide, ceria, and its gadolinia solid solutions are reported made by the precipitation thin‐film method spray pyrolysis. The crystallization of an amorphous metal oxide thin film generally follows the Lijschitz–Sletow–Wagner (LSW) Ostwald ripening theory: Below the percolation threshold of 20 vol% single grains crystallize in the amorphous phase and low crystallization rates are measured. In this state no impact of solute on crystallization is measurable. Once the grains form primary clusters above the threshold the solute slows down crystallization (and grain growth) thus shifting the TTT curves of the doped ceria films to longer times and higher temperatures in comparison to undoped ceria. Current views on crystallization of metal oxide thin films, the impact of solute dragging, and primary TTT diagrams are discussed. Finally, examples on how to use these TTT diagrams for better thermokinetic engineering of metal oxide thin films for MEMS are given, for example, for micro‐Solid Oxide Fuel Cells and resistive sensors. In these examples the electrical properties depend on the degree of crystallinity and, thereby, on the TTT conditions.

Oxygen vacancy migration in ceria and Pr-doped ceria: A DFT+U study

Oxygen vacancy formation and migration in ceria (CeO(2)) is central to its performance as an ionic conductor. It has been observed that ceria doped with suitable aliovalent cationic dopants improves its ionic conductivity. To investigate this phenomenon, we present total energy calculations within the framework of density functional theory to study oxygen vacancy migration in ceria and Pr-doped ceria (PDC). We report activation energies for oxygen vacancy formation and migration in undoped ceria and for different migration pathways in PDC. The activation energy value for oxygen vacancy migration in undoped ceria was found to be in reasonable agreement with the available experimental and theoretical results. Conductivity values for reduced undoped ceria calculated using theoretical activation energy and attempt frequency were found in reasonably good agreement with the experimental data. For PDC, oxygen vacancy formation and migration were investigated at first, second, and third nearest neighbor positions to a Pr ion. The second nearest neighbor site is found to be the most favorable vacancy formation site. Vacancy migration between first, second, and third nearest neighbors was calculated (nine possible jumps), with activation energies ranging from 0.41 to 0.78 eV for first-nearest-neighbor jumps. Overall, the presence of Pr significantly affects vacancy formation and migration, in a complex manner requiring the investigation of many different migration events. We propose a relationship illuminating the role of additional dopants toward lowering the activation energy for vacancy migration in PDC.

Free Volumes Associated with Sintering in Gadolinium Doped Ceria Solid Solutions

Gadolinium‐doped ceria (GDC) solid solution prepared by the oxalate coprecipitation method is investigated by X‐ray diffraction (XRD), complex impedance analysis, and positron lifetime spectroscopy. XRD reveals an expansion of GDC lattice constant by doping gadolinium into a ceria host crystal, in agreement with an oxygen vacancy model. The ionic conductivity of GDC measured at 773 K in air is two orders of magnitude higher than that of undoped ceria. Positron lifetime spectroscopy reveals the presence of vacancy‐sized free volumes and nanovoids in interfaces among crystallites. It is found that the vacancy‐sized free volumes shrink with increasing sintering temperatures. In the present paper, recent advances in the studies of GDC by XRD, complex impedance measurement, and positron lifetime spectroscopy are reviewed to gain an insight into the sintering mechanism.

MORPHOLOGY CONTROL OF CERIUM PHOSPHATES VIA SOLUTION PROCESSES FOR NEW SUNSCREEN APPLICATION

Plate-like hydrated cerium phosphate, Ce2(PO4)2HPO4 · H2O , ca. 3 μm in diameter, was obtained by the hydrothermal reaction of Ce(SO4)2 in H3PO4 aqueous solution at 150–200°C. Ce2(PO4)2HPO4 · H2O transformed to CeP2O7–CePO4 mixture around 700°C while maintaining the plate-like morphology. Plate-like CePO4 and CeP2 O 7 were also synthesized as single phase by heating plate-like Ce2(PO4)2HPO4 · H2O at 700°C with cerium hydroxide and NH4H2PO4 , respectively. Plate-like cerium phosphates showed excellent comfort, absorption ability of UV-ray and lower oxidation catalytic activity than undoped ceria. The UV-shielding ability was in the order Ce2(PO4)2HPO4 · H2O > CeP2O7 > CeP2O7–CePO4 mixture > CePO4 > K0.8Li0.266Ti1.734O4.

Identifying Defects in Ceria-Based Nanocrystals by UV Resonance Raman Spectroscopy

Local structures in ceria-based materials were investigated by UV resonance Raman spectroscopy using a 363.8 nm laser line for the first time. In the spectra of a highly crystalline and undoped ceria, overtone LO series are clearly detected due to the multiphonon relaxation by the UV resonance Raman effect. Gd3+ doping in ceria additionally activates the disorder bands that are attributable to lattice spaces with or without an oxygen vacancy. The crystal size dependency of the UV Raman spectra suggests that Ce3+ ions preferably form the MO8-type (M = Ce3+) complex in the undoped ceria nanoparticles and are eliminated by oxidation while heating in air. Gd3+ ions also tend to form a MO8-type complex in the nanocrystalline ceria matrix. However, they diffuse to form the more thermodynamically stable defect clusters that accompany the grain growth upon heating.

SYNTHESIS OF PLATE-LIKE CERIUM PHOSPHATES AND THEIR PHOTOCHEMICAL PROPERTIES VIA SOFT CHEMICAL REACTIONS

Plate-like cerium phosphates have attracted attention because of their unique UV-shielding ability with low photocatalytic activity and excellent comfort when applied on skin. Plate-like hydrated cerium phosphate, Ce2(PO4)2HPO4·H2O, ca 3 μm in diameter, was obtained by the hydrothermal reaction of Ce(SO4)2 in H3PO4 aqueous solution at 200 °C for 3 h. Ce2(PO4)2HPO4·H2O transformed to CeP2O7-CePO4, mixture around 700 °C with the transcription of plate-like morphology. Plate-like CePO4 and CeP2O7 were also synthesized as single phase by heating plate-like Ce2(PO4)2HPO4·H2O with cerium hydroxide and NH4H2PO4, respectively. Plate-like cerium phosphates showed excellent comfort, absorption ability of UV-light and lower oxidation catalytic activity than undoped ceria. The UV-shielding ability of them was in the order Ce2(PO4)2HPO4·H2O > CeP2O7 > CeP2O7-CePO4 mixture > CePO4 > K0.8Li0.266Ti1.734O4.