Structure-designed gadolinia doped ceria interlayer for solid oxide fuel cell

A bi-structured gadolinia doped ceria (GDC) interlayer, which has a dense layer combining tightly with yttria-stabilized zirconia (YSZ) film and a porous layer with rough surface for contacting closely with the cathode, is prepared from a two-step sputtering process. Compared to the cell with a dense GDC interlayer, the single cell with the bi-structured GDC interlayer depicts greatly reduced ohmic and polarization resistances and increased electrochemical performance.     

碱土金属双掺杂的Ce_(0.9)Ca_(0.1-x)Sr_xO_(1.9)中温固体电解质的性能与应用

以固相反应方法合成了碱土 (Ca ,Sr)双掺杂的氧化铈基固溶体材料Ce0 .9Ca0 .1 -xSrxO1 .9(x =0 ,0 0 4,0 0 5 ,0 0 6 ,0 1)。结构研究表明 :碱土双掺杂的CeO2 呈立方萤石结构。利用阻抗谱研究了材料的离子导电性 ,发现碱土双掺杂有利于提高材料离子导电率 ,掺杂两种碱土金属离子的等效半径接近临界离子半径时导电率最高。将此系列材料作为电解质进行了燃料电池试验 ,发现电池的输出功率高于YSZ电解质及碱土单掺杂氧化铈 ,且电池输出开路电压亦高于单掺杂的情况。     

Ca,Sr共掺杂铈氧化物及其在甲烷氧化偶联反应中的应用

采用柠檬酸法制备了Ca,Sr共掺杂的CeO_2催化剂,发现共掺杂催化剂较单掺杂或未掺杂催化剂呈现出更好的催化甲烷氧化偶联(OCM)反应性能.通过表征可知,Ca,Sr共掺杂催化剂的物相为CeO_2和SrCO_3,Ca高度分散或掺杂于CeO_2之中;CeO_2的粒径明显变小;表面呈中等程度碱性;Ce基催化剂上的亲电氧物种数量随着碱土金属的掺杂而增大,亲电氧物种与晶格氧摩尔比(O_2-2+O-2)/O-2的大小顺序(CeSrCaCe SrCeO_2)与C2选择性一致;且SrCO_3相的存在有助于Ce基氧化物催化甲烷氧化偶联反应(OCM)性能的改善.     

Design of nanostructured ceria-based solid electrolytes for development of IT-SOFC

A considerable interest has been shown in the application of doped ceria (CeO₂) compounds for “intermediate” (300–500 °C) temperature operation of solid oxide fuel cells. The microdomains with ordered structure of oxygen vacancy were observed in the microstructure of the M-doped CeO₂-sintered bodies (where M: Gd, Y, and Dy). We have previously shown that the conductivity of doped CeO₂-sintered bodies was lower when the sintered body contained large microdomains within grains. As a consequence of this observation, we have examined the grain size dependence and dopant content on conductivity in specimens where we adjust the microdomain size and a degree of oxygen vacancy ordering in the microdomains by controlling the microstructure. The microdomain size control in Dy-doped CeO₂ specimens was obtained by combining pulsed electric current sintering and conventional sintering. Using these techniques, we were able to improve the conductivity in Dy-doped CeO₂ specimens to a point where it became comparable to that of the more conventional Gd-doped CeO₂ specimens. It is concluded that by combining ultimate high-resolution analysis of these nanostructures with the adjusting processing route design, it is possible to further develop these materials in CeO₂-doped fuel cell application.     

89Y magic-angle spinning NMR of Y2Ti2-xSnxO7 pyrochlores

The yttrium local environment in the series of pyrochlores Y2Ti2-xSnxO7 was studied using 89Y NMR. Oxides with the pyrochlore structure exhibit a range of interesting physical and chemical properties, resulting in many technological applications, including the encapsulation of lanthanide- and actinide-bearing radioactive waste. The use of the nonradioactive Y3+ cation provides a sensitive probe for any changes in the local structure and ordering with solid solution composition, through 89Y (I = 1/2) NMR. We confirm that a single pyrochlore phase is formed over the entire compositional range, with Y found only on the eight-coordinated A site. A significant (approximately 15 ppm) chemical shift is observed for each Sn substituted into the Y second neighbor coordination environment. The spectral signal intensities of the possible combinations of Sn/Ti neighbors match those predicted statistically assuming a random distribution of Sn4+/Ti4+ on the six-coordinated pyrochlore B site.     

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.     

Unravelling the enhanced reactivity of bulk CeO2 doped with gallium: A periodic DFT study

Doping CeO₂ with gallium leads to promising materials with application in hydrogen purification processes for fuel cells. The bulk ceria–gallia is investigated by ab initio calculations. The outstanding reactivity is explained by important relaxations induced by gallium in the ceria host, having a strong impact in the electronic structure. As a result, the mixed oxide is found to be more reducible than the pure oxides in agreement with experimental data. It is thus possible to correlate structure and reactivity of the doped system on the molecular level, representing a step forward to the rational design of materials with selected properties.     

Catalytically Active and Spectator Ce3+ in Ceria‐Supported Metal Catalysts

Identification of active species and the rate‐determining reaction steps are crucial for optimizing the performance of oxygen‐storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce³⁺ species in a ceria‐supported platinum catalyst during CO oxidation are short‐lived and therefore cannot be observed under steady‐state conditions. Using time‐resolved resonant X‐ray emission spectroscopy, we quantitatively correlated the initial rate of Ce³⁺ formation under transient conditions to the overall rate of CO oxidation under steady‐state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce³⁺ was present as spectators. This approach can be applied to various catalytic processes involving oxygen‐storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.     

Nanostructured, Gd-doped ceria promoted by Pt or Pd: investigation of the electronic and surface structures and relations to chemical properties

Nanostructured ceria doped with other rare earth elements is a good oxygen ion conductor, which gives rise to various catalytic applications such as the construction of membranes for syngas production by partial oxidation of methane. This article focuses on the Gd-doped cerium dioxides, which can be modified with Pt or Pd to enhance the reactivity of the lattice oxygen in interaction with methane. The aim of the work is the elucidation of correlations between the structural, electronic, and chemical properties of these nanomaterials. Detailed studies were performed for a series of samples with and without surface modification by noble metals using a complex combination of physicochemical methods: XRD, TEM, CH(4) TPR, XPS, SIMS, and FTIR spectroscopy of adsorbed CO. XPS and TPR data revealed that surface modification with noble metals enhances the reducibility of the doped ceria support, where the effect is more pronounced for Pd than for Pt. The formation of highly cationic Pd species due to strong metal support interactions provides a possible explanation for this behavior. Furthermore, the results obtained in the present work for the Gd-doped ceria system are compared to those obtained previously for the Pr-doped ceria system.     

Fine tuning of gadolinium doped ceria electrolyte nanoparticles via reverse microemulsion process

Gadolinium doped ceria (Gd–CeO₂) nanoparticles have been synthesized by an reverse microemulsion system using cyclohexane as the oil phase, a non-ionic surfactant Igepal CO 520 and their mixed aqueous solutions of gadolinium III nitrate hexahydrate and cerium III nitrate hexahydrate as the water phase. The control of particle size was achieved by varying the water to surfactant molar ratio. The synthesized and calcined powders were characterized by thermogravimetry-differential thermal analysis (TGA-DTA), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The XRD results show that all the samples calcined at 700 °C were single phase cubic fluorite structure. The average size of the particle was found to increase with increase in water to surfactant molar ratio (R). The mean diameter of the particle for various value of R varies between 8–15 nm (SEM) and 7.5–11 nm (TEM), respectively. EDS confirm the presence of gadolinia and ceria phase in the nanopowder calcined at 700 °C. FTIR analysis was carried to monitor the elimination of residual oil and surfactant phases from the microemulsion-derived precursor and calcined powder. Raman spectroscopy and DTA evidenced the formation of a solid solution of gadolinium doped ceria at room temperature.     

一种通过纳米粒子的组装制备基于二氧化铈中孔材料的简易方法

用HF或者HCl作联合剂,三嵌段共聚物表面活性剂作模板剂,通过二氧化铈纳米粒子(或者过渡金属掺杂的二氧化铈纳米粒子)组装形成具有热稳定和晶化孔壁的基于二氧化铈的中孔材料。焙烧该合成的超分子模板中孔结构的材料可以形成具有高比表面的基于二氧化铈的中孔材料,这些中孔材料用不同的光谱技术表征。通过D₂-OH交换测得的二氧化铈表面的羟基在组装过程和中孔材料的稳定性方面至关重要。联结剂中的卤素离子(F和Cl离子)可以替代中孔材料的表面羟基,从而影响这些中孔材料的结构稳定性和光学活性,而用具有3 d的过渡金属在组装前掺杂二氧化铈纳米粒子可以显著地提高中孔材料的光学活性,这种提高主要归结为通过掺杂可以促使能量转移的提高。     

生物质气为燃料的燃料电池理论电动势计算

The electromotive force (e.m.f.) of solid oxide fuel cells using biomass produced gas (BPG) as the fuels is calculated at 700-1,200 K using an in-house computer program, based on thermodynamic equilibrium analysis. Tour program also predicts the concentration of oxygen in the fuel chamber as well as the concentration of equilibrium species such as H2, CO, CO2 and CH4. Compared with using hydrogen as a fuel, the e.m.f.for cells using BPG as the fuels is relative low and strongly influenced by carbon deposition. To remove carbon deposition, the optimum amount of H2O to add is determined at various operating temperatures.Further the e.m.f, for cells based on yttria stabilized zirconia and doped ceria as electrolytes are compared.The study reveals that when using BPG as fuel, the depression of e.m.f, for a SOFC using doped ceria as electrolyte is relatively small when compared with that using Yttria stabilized zirconia.     

A theoretic insight into the catalytic activity promotion of CeO2 surfaces by Mn doping

In this paper, we investigated the primary reduction and oxygen replenishing processes over Mn substitutionally doped CeO(2)(111) surfaces by density functional theory with the on-site Coulomb correction (DFT + U). The results indicated that Mn doping could make the surface much more reducible and the adsorbed O(2) could be effectively activated to form superoxo (O(2)(-)) and/or peroxo species (O(2)(2-)). The Mn doping induced the Mn 3d-O 2p gap state instead of Ce 4f acting as an electrons acceptor and donor during the first oxygen vacancy formation and O(2) replenishing, which helped to lower the formation energy of the first and second oxygen vacancies to -0.46 eV and 1.40 eV, respectively. In contrast, the formation energy of a single oxygen vacancy in the pure ceria surface was 2.08 eV and only peroxo species were identified as the O(2) molecule adsorbed. Our work provides a theoretical and electronic insight into the catalytic redox processes of Mn doped ceria surfaces, which may help to understand the enhanced catalytic performances of MnO(x)-CeO(2) oxides, as reported in previous experimental works.     

Solid-state NMR study of the anionic conductor Ca-doped Y(2)Ti(2)O(7)

Doping the pyrochlore (Y(2)Ti(2)O(7)) with the Ca(2+) has been reported to improve the oxygen ionic conductivity. (17)O and (89)Y solid-state NMR has been used in work reported here to probe the local environment of the oxygen and yttrium sites in these materials. Although the conductivity of these materials is only moderate, variable-temperature, one-dimensional (17)O NMR experiments and two-dimensional (17)O magnetization exchange spectroscopy show that oxygen-ion jumps occur between the oxygen sites nearby titanium. Motion at ambient temperatures involves the O1 and local environments created by the Ca(2+) doping; motion does not involve the O2 oxygen site coordinated to four yttrium ions.     

Unravelling the Mystery of Solid Solutions: A Case Study of 89Y Solid-State NMR Spectroscopy

Incorporating heteroatoms in functional materials is an invaluable approach to modulate their properties, assuming a solid solution is formed. However, thorough understanding of key structural information on the resulting solid solution, such as the local environment of cations and vacancies, remains a challenge. Solid-state NMR (SSNMR) spectroscopy is a powerful structural characterization tool, very sensitive to the local environment. Due to the difficulty in signal acquisition and spectral interpretation, SSNMR spectroscopy is relatively less known to chemists and materials scientists. Herein, we present an introductory review to demonstrate how to use ⁸⁹Y SS NMR spectroscopy to unravel the mystery of solid solutions. In general, ⁸⁹Y chemical shift varies with different cation/vacancy arrangements in Y coordination spheres, providing ultrafine structural information in the atomic scale. As a case study and an extreme condition, the approach demonstrated in this review can be extended to other systems.     

Defect clustering and local ordering in rare earth co-doped ceria

Defect clustering and local ordering in rare earth co-doped ceria were studied by computer simulation and electron diffraction, respectively. The simulation of electrically neutral defect clusters containing up to four oxygen vacancies revealed that the permutation of different dopant cations in a co-doped cluster could have a significant influence on the binding energy of the cluster. Moreover, the growth of larger clusters (number of oxygen vacancies ≥ 3) could be restrained by a co-doping effect. The selected area electron diffraction study indicated that the restrained growth of larger clusters will further lead to a suppression of the local ordering of oxygen vacancies in co-doped ceria. The correlation between defect clustering, local ordering of oxygen vacancies and ionic conduction in co-doped ceria was discussed.     

Catalytic properties of Ni/ceria-yttria electrode materials for partial oxidation of methane

The catalytic properties of electrode materials Ni/Ce1-xYxO2-δ (x = 0.05, 0.10, 0.15 and 0.20) were investigated for partial oxidation of methane (POM). The CeO2-Y2O3 solid solutions were prepared by co-precipitaion method. The Ni-based catalysts supported on the solid solutions were obtained using the impregnation method. Structural, surface and redox characteristics of the prepared catalysts were systematically examined by means of X-ray diffraction (XRD), N2 adsorption-desorption (Brunauer-Emmet-Teller BET method), H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) methods. The results indicated that yttria doped in the ceria system, forming a good solid solution, readily induced more defects and oxygen vacancies that favored the improvement of catalytic activity and coking resistance. In the temperature range of 600-850 ℃, Ni/Ce0.90Y0.10O1.95 catalyst exhibited the best catalytic activity among the four tested catalysts, with the CH4 conversion, CO selectivity and H2 selectivity of 78.8%, 90.6% and 89.8%, respectively, at 850 ℃. And the H2/CO molar ratio in products of Ni/Ce0.90Y0.10O1.95 catalyst was closer to the theoretical value of 2.0. The excellent coking resistant behaviors for all catalysts were clearly manifested by Thermal Analysis.     

Oxygen Storage Materials for Automotive Catalysts: Ceria-Zirconia Solid Solutions

This paper reviews progress in the development of oxygen storage materials for automotive exhaust catalysts. The research was mainly conducted as a study and development exercise in the author's laboratory in Japan.Ceria-lanthana solid solutions (CL) and the first generation of ceriazirconia solid solutions (CZ) were developed as excellent oxygen storage materials for automotive catalysts in the 1980s. These materials consist of ceria doped with less than 20 mol% of La⁴⁺ or Zr⁴⁺. An increase in oxygen defects in CL and CZ under reductive conditions is responsible for an enhanced oxygen storage capability on the cerium atoms. An accurate measure of the oxygen storage capacity (OSC) per cerium is very important for theoretical and practical treatments of the catalyst. The term partial OSC was introduced to describe this capacity and to differentiate it from the usual definition of the OSC, known also as the total OSC. After the development of CL and CZ, a new technology was developed to dissolve more than 20 mol% of zirconia in the ceria, allowing second generation CZ and third generation CZ (known as ACZ, which is doped with alumina) to be successfully developed in the 1990s. The partial OSC of these materials increases with increasing amounts of zirconia dissolved in the ceria, and also with decreasing material particle size after an engine durability test. In the case of ACZ, alumina was added to CZ based on the diffusion barrier concept, in which a diffusion barrier layer inhibits the coagulation of CZ and A when the material is required for duty at high temperature in air.Furthermore, the relationship between the total or partial OSC and the structure of the ceriazirconia solid solutions is explained in this paper.For ceriazirconia solid solutions composed of equimolar CeO₂ and ZrO₂(Ce/Zr=1), the total or partial OSC of the -phase CeZrO₄, in which the cerium and zirconium ions are regularly distributed, was about twice as large as that of a ceriazirconia solid solution with a relatively irregular distribution of cerium and zirconium ions, and about five times larger than that of a mixture of ceria powder and zirconia containing only a small amount of ceriazirconia solid solution. It corresponds to about 89% of the theoretical maximum value.For a ceriazirconia solid solution composed of non-equimolar CeO₂ and ZrO₂(Ce/Zr 1), the partial OSC of a ceria--phase solid solution with a zirconia content of between 30 and 50mol% is much higher than that of a ceriazirconia solid solution of the same zirconia content. The partial OSC of a -phase and zirconia mixed oxide, which is formed by reducing the material at 1200 °C, reaches a value above 0.20 mol-O₂/mol-Ce (about 80% of the theoretical maximum value of the partial OSC), when the zirconia content is between 50 and 80 mol%.The Toyota Motor Corp. has put automotive three-way catalysts containing the first, second and third generations of CZ into practical use on a global basis.     

Spectroscopic characterization of microscopic hydrogen-bonding disparities in supercritical water

The local hydrogen-bonding environment in supercritical water (380 degrees C, 300 bars, density 0.54 gcm3) was studied by x-ray Raman scattering at the oxygen K edge. The spectra are compared to those of the gas phase, liquid surface, bulk liquid, and bulk ice, as well as to calculated spectra. The experimental model systems are used to assign spectral features and to quantify specific local hydrogen-bonding situations in supercritical water. The first coordination shell of the molecules is characterized in more detail with the aid of the calculations. Our analysis suggests that approximately 65% of the molecules in supercritical water are hydrogen bonded in configurations that are distinctly different from those in liquid water and ice. In contrast to liquid water the bonded molecules in supercritical water have four intact hydrogen bonds and in contrast to ice large variations of bond angles and distances are observed. The remaining approximately 35% of the molecules exhibit two free O-H bonds and are thus either not involved in hydrogen bonding at all or have one or two hydrogen bonds on the oxygen side. We determine an average O-O distance of 3.1+/-0.1 A in supercritical water for the H bonded molecules at the conditions studied here. This and the corresponding hydrogen bond lengths are shown to agree with neutron- and x-ray-diffraction data at similar conditions. Our results on the local hydrogen-bonding environment with mainly two disparate hydrogen-bonding configurations are consistent with an extended structural model of supercritical water as a heterogeneous system with small patches of bonded molecules in various tetrahedral configurations and surrounding nonbonded gas-phase-like molecules.