Rare Earth Doped Ceria Research Articles

Interface Stability of Perovskite Cathodes and Rare-Earth Doped Ceria Interlayer in SOFCs

The reaction and interdiffusion characteristics at the interface of rare-earth doped ceria (, M = Gd, La) and perovskite-type cathode materials (, and ) were investigated and their effect on the long-term reliability of solid oxide fuel cell (SOFC) performance are discussed. Secondary phase formation was observed at the ceria side of the interface by the reaction of the rare-earth component in ceria and transition metal components in perovskite. A significant diffusion of transition metals (cobalt or iron) and strontium from perovskite to ceria was observed. The iron content in perovskite and the type of rare earth in ceria strongly affect the stability of secondary phase and extent of transition metal diffusion in ceria. The bulk diffusivity was determined as a function of temperature. The iron component exhibited a fast and inhomogeneous migration in ceria, forming the iron-rich inclusions. The strontium component exhibited the tendency of the fast grain boundary diffusion in ceria. The effects of mass transport on the SOFC materials durability are discussed.

Degradation Behavior at Interface of LSCF Cathodes and Rare Earth Doped Ceria

The reaction and interdiffusion characteristics at the interface of rare earth doped ceria (M0.2Ce0.8O1.9, M = Gd, La) and perovskite- type cathode materials (La0.8Sr0.2CoO3, La0.8Sr0.2FeO3, and La0.8Sr0.2(Co, Fe)O3) were investigated, and their effect on the long-term reliability of SOFC performance was discussed. A secondary phase formed at the interface by the reaction of rare earth component in ceria and transition metal components in perovskite. A significant diffusion of transition metals (cobalt or iron) and strontium from perovskite to ceria was observed, whereas little diffusion of cerium was observed in perovskites. The iron content in perovskite and the type of rare earth in ceria strongly affect the stability of secondary phase and extent of impurity diffusion in ceria. Strontium component exhibited the tendency of the fast grain boundary diffusion in ceria.

Thermochemical compatibility between selected (La,Sr)(Co,Fe,Ni)O 3 cathodes and rare earth doped ceria electrolytes

In this paper the results of thermal expansion coefficient measurements of different singly and doubly doped ceria electrolytes, together with results for selected cathode materials from (La,Sr)(Co,Fe,Ni)O 3 system are given. A high temperature chemical stability of the cathode–electrolyte interface was measured on 1:1 wt. mixtures of previously characterized ceria and perovskite powders. The samples were heated at 800, 1000 or at 1200 °C in air for 6 or 100 h. Chemical reactivity investigations were conducted using XRD with Rietveld analysis. For all heated samples the crystal structure of both components were preserved. However, their lattice parameters evolved to a different extent, suggesting the existence of cation exchange. A formation of the solid state solution between Ce 1− x RE x O 2− x/2 and La(Co,Fe,Ni)O 3− δ was found, with mobile La cation. On the basis of the obtained results a qualitative mechanism of the observed reaction was proposed.

Hydrothermal synthesis and characterization of rare earth doped ceria nanoparticles

Ceria (CeO 2) and rare earth doped ceria (RE = Pr, Gd and Sm) of different doping concentration (10, 20 and 30 mol%) were synthesized by hydrothermal synthesis method, achieving nano-particles with weak agglomeration. The nano-particles were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), BET specific surface area measurement and thermal analysis. The size of all doped nano-particles was in the range of 13–25 nm, while the un-doped ceria nano-particles were in the range of 12–16 nm. The effect of precursor concentration on microstructure and morphology of the final powder is reported in this paper. The increase of precursor concentration led to decreases in crystallite size, increase in lattice parameter and increase in the BET specific surface area with Ce 3+ from 0.005 to 0.015 M. The highest rare earth doping ratio is found to be 20, 10 and 20 mol% for Pr, Gd and Sm, respectively.

Thermodynamic and kinetic considerations on degradations in solid oxide fuel cell cathodes

Analyses have been made to understand the chemical behaviors of the perovskite cathode/rare earth doped ceria interlayer/YSZ (yttria stabilized zirconia) electrolyte multilayer structure and their relation to the electrochemical performance. Particular focus was given to the thermodynamic activities of SrO in the cathodes and their relation to the reaction with rare earth doped ceria or YSZ. The possibility of Sr diffusion from the cathode through the rare earth doped ceria to the YSZ electrolyte to form SrZrO 3 was analyzed in terms of the SrO activity. It has been found that the SrO activity at the cathode/ceria interfaces is important for the steepening of the chemical potential gradient of SrO inside the doped ceria and for the enhancement of the solubility of SrO in doped ceria. Two possible effects influencing the SrO activities have been examined; these are chemical reactions between perovskite cathodes and doped ceria and kinetic decomposition under the oxygen potential gradient. Effects of cathodic polarization have been also examined in terms of changes in chemical equilibria and change in oxygen potential gradients.

Reaction of Ce<sub>1-x</sub>Re<sub>x</sub>O<sub>2-δ</sub> Nanopowders Synthesis

One of the methods for powder synthesis that is both cost and time effective is the selfpropagating room temperature synthesis. We applied this method to synthesize rare earth doped ceria nanopowders. Since they exhibit very high ionic conductivity at intermediate temperatures these compositions are attractive for a new generation of nanostructured ceramics applicable in solid oxide fuel cells as electrolytes. In this paper we paid our attention to the reaction based on methathetical pathway, whereby solid solution nanopowders of rare earth elements with ceria were obtained at room temperature. Compositions of Ce1-xRexO2-δ (Re = Y , Nd) were synthesized with x ranging from 0 to 0.20. The reaction course is discussed and the properties of the obtained powders are presented.

Ceria: Relation among thermodynamic, electronic hole and proton properties

The proton solubility and the hole conductivity of the rare earth doped ceria have been examined in their relations to the thermodynamic properties of doped ceria under the assumption that the hypothetical species, LnOOH and LnOO (Ln = Rare earth), can be regarded as constituents for representing protons and holes in the fluorite lattice. Focus is made on the dopant dependence, the host dependence and the temperature dependence in the rare earth doped zirconia(or ceria) fluorite lattice. The chemical potentials of the rare earth dopant are less stabilized in the ceria-based oxides than in the zirconia-based ones. The proton solubility in the ceria-based, zirconia-based, and ceria–zirconia solid solutions has been well interpreted in terms mainly of the hydroxidation energy and the stabilization energy of LnO 1.5 in the fluorite lattice. Since the dopant dependence of the stabilization energy of LnO 1.5 is stronger than the hydroxidation energy, the proton solubility becomes high in the smaller dopants. To account for less dopant-dependent behavior in the hole conduction, the peroxidation energy is assumed to have about the same dopant dependence as the stabilization energy. The calculated temperature dependences of proton solubility and hole concentration were compared with available experimental data; it has been suggested that holes and protons in ceria reach to saturation levels with lowering temperature. Some discussions are made on the possible explanation on recently observed anomalous hole conductivity in nano-size Ce 0.8Gd 0.2O 1.9 in terms of plausible effects of miscibility gap, associated Gd enrichment, and simultaneous formation of Ce 3+ and holes.

Electrolytes for Solid-Oxide Fuel Cells

AbstractThree solid-oxide fuel cell (SOFC) electrolytes, yttria-stabilized zirconia (YSZ), rare-earth–doped ceria (REDC), and lanthanum strontium gallium magnesium oxide (LSGM), are reviewed on their electrical properties, materials compatibility, and mass transport properties in relation to their use in SOFCs. For the fluorite-type oxides (zirconia and ceria), electrical properties and thermodynamic stability are discussed in relation to their valence stability and the size of the host and dopant ions. Materials compatibility with electrodes is examined in terms of physicochemical features and their relationship to the electrochemical reactions. The application of secondary ion mass spectrometry (SIMS) to detect interface reactivity is demonstrated. The usefulness of doped ceria is discussed as an interlayer to prevent chemical reactions at the electrode–electrolyte interfaces and also as an oxide component in Ni–cermet anodes to avoid carbon deposition on nickel surfaces. Finally, the importance of cation diffusivity in LSGM is discussed, with an emphasis on the grain-boundary effects.

Thermal expansion and Debye temperature of rare earth-doped ceria

Thermal expansions of un-doped ceria and rare earth-doped ceria (Ce 1− x M x O 2− x/2 , M = La, Sm, Dy, Yb) were measured in the temperature range from 100 to 800 K by TMA. Debye temperatures of these compounds were measured at room temperature by an ultrasonic pulse method. The thermal expansion coefficients of rare earth-doped CeO 2 increased with increasing ionic radius of doped ions and its M content expect Dy ion. The Debye temperature of rare earth-doped ceria estimated from the longitudinal and transverse sonic wave velocities decreased with increasing M content, probably due to generation of oxygen vacancy by doping the rare earth elements. The relation between thermal expansion coefficient and Debye temperature was discussed.

Transport properties of ceria–zirconia–yttria solid solutions {(CeO 2) x(ZrO 2) 1− x} 1− y(YO 1.5) y ( x = 0–1, y = 0.2, 0.35)

The electron and hole conductivity, oxygen transport property and thermal expansion behavior of {(CeO 2) 0.5(ZrO 2) 0.5} 0.65(YO 1.5) 0.35 solid solution were investigated and the effect of yttrium content was examined by comparing the data with those of {(CeO 2) 1− x (ZrO 2) x } 0.8(YO 1.5) 0.2. The electron conductivity of {(CeO 2) 0.5(ZrO 2) 0.5} 0.65(YO 1.5) 0.35 measured by ion blocking method was one order lower than that of {(CeO 2) 0.5(ZrO 2) 0.5} 0.8(YO 1.5) 0.2. The increase of yttrium content suppressed the phase transition from cubic to pyrochlore structure in reducing atmosphere. The oxygen isotope diffusivity and surface exchange rate constant of {(CeO 2) 0.5(ZrO 2) 0.5} 0.65(YO 1.5) 0.35 are considered to be comparable to those of {(CeO 2) 0.5(ZrO 2) 0.5} 0.8(YO 1.5) 0.2. The chemical expansion due to the reduction of cerium ion was observed as well as for rare earth doped ceria. The compatibility of the material as an anode substrate in solid oxide fuel cells is discussed.

Self-propagating room temperature synthesis of nanopowders for solid oxide fuel cells (SOFC)

Ceria based solid solutions are promising ceramic electrolytes for SOFC which will be able to function at intermediate temperatures. In accordance with common trends to decrease the production and operating costs of new generation of SOFC materials the reaction based on metathetical pathway is described, whereby solid solution powders of rare earth doped ceria in the nanoscale range were obtained at room temperature. By simple hand mixing of reactants the reaction was enhanced, propagating afterwards by itself. Nanometric ceria powders doped with Y or Nd, as well as co doped with both cations were synthesized. Composition of Ce 1 − x Me x O 2 − y ranged from x = 0–0.25. The reaction course is described in detail and the properties of the obtained powders are presented.

Significant effect of water on surface reaction and related electrochemical properties of mixed conducting oxides

The interaction between water vapor and fluorite-type oxide ion conductors, and its effects on surface and electrochemical properties are investigated. High deuterium ion intensity was observed for rare-earth doped ceria annealed in D 2O-containing atmospheres. It indicates that a pretty amount of deuterium is absorbed in the ceria polycrystalline. The oxygen isotope exchange rate on YSZ surface increased with water vapor pressure, indicating that the interaction between water and oxygen vacancy in YSZ is significant. The interfacial conductivity at platinum/YSZ interface in air at T=968 K was drastically improved by addition of 1 kPa of water vapor pressure. This improvement was clearly observed for YSZ electrolyte, however, only a small effect was observed for Ce 0.8Gd 0.2O 1.9.

Oxygen Partial Pressure Dependence of Creep for Yttria-Doped Ceria Ceramics

Rare earth-doped ceria ceramics are potential candidates for an electrolyte in medium-temperature solid oxide fuel cells (SOFC). However, the mechanical properties under the practical working environments of SOFCs have not been not fully investigated for CeO2 based electrolytes. In this study, creep behaviors of CeO2-YO1.5 were investigated at stresses of 20–39 MPa and temperatures of 1045 to 1125°C under controlled oxygen partial pressure (PO2). The stress exponent n≈0.96 and the activation energy Q≈264 kJ/mol of creep rate were obtained experimentally in air. Because this value is much higher than that for anion diffusion in this material, it was concluded that creep is controlled by a cation diffusion mechanism. It was found that the oxygen partial pressure dependence of the steady-state strain rate has a peak at PO2=10−14 atm and the profile is similar to electrical conductivity.

Determination of hydrogen solubility in oxide ceramics by using SIMS analyses

Hydrogen solubility in oxide ceramics was evaluated by annealing in D2O-containing atmosphere and subsequent secondary ion mass spectrometry (SIMS). A good proportional relation was obtained between the relative intensity of deuterium ion in SIMS and literature data of hydrogen solubility for YSZ and Sr(Yb)CeO3−δ. The hydrogen solubility in CeO2 and rare-earth doped ceria (Ce0.8M0.2O1.9, M=Y, La, Nd, Sm, Gd, Yb) was evaluated. The hydrogen solubility in ceria dramatically increased by rare earth substitution, and it depended on the lattice parameter of Ce0.8M0.2O1.9.

Thermal and mechanical properties of rare earth-doped ceria ceramics

Oxalate coprecipitation method was applied to prepare rare earth-doped ceria powders with a composition of Ce 0.8R 0.2O 1.9 (R=Sm, La). The precipitate produced was decomposed at 600°C in air to the oxide solid solution. After milling with α-alumina ball, the powders were densified to 98–99% calculated density at 1600°C. No influence of doped rare earth element on the thermal expansion coefficient of CeO 2 was measured (11.4×10 −6–11.8×10 −6 K −1). The Ce 0.8Sm 0.2O 1.9 formed by colloidal processing and with 86–98% relative density offered 53–81 MPa of four-point flexural strength and 9.1 of Weibull modulus. Large flaws of 200–300 μm sizes produced near surface were fracture origins. The Young's modulus and Vicker's hardness were also measured for Ce 0.8Sm 0.2O 1.9. These measured thermal and mechanical data were compared with those of component materials of solid oxide fuel cell.

Characterization of Rare Earth-Doped Ceria Powders Dervied from Oxalate Coprecipitate

Characterization of Rare Earth-Doped Ceria Powders Dervied from Oxalate Coprecipitate

Surface Properties and Aqueous Processing of Rare Earth-Doped Ceria Powders by Coprecipitation Method

Surface Properties and Aqueous Processing of Rare Earth-Doped Ceria Powders by Coprecipitation Method

A Systematic investigation of the dc electrical conductivity of rare-earth doped ceria

The dc electrical conductivity of rare-earth doped ceria has been measured as a function of temperature (300–600 K) and composition (0.05–15 mol% M2O3) on using the complex impedance technique. Five dopants have been selected, yttrium and the lanthanides Yb, Gd, Nd, and La. For all of them, the variations of the activation energy versus dopant concentration are similar and characterized by the existence of a minimum. This peculiar property can be understood if attractive interactions between immobile dopant ions and mobile oxygen vacancies are taken into account. From an analysis of the experimental results, it is concluded that this interaction extends at least to third or fourth nearest neighbors depending on the size and the electronic configuration of the dopant ion.