Characterisation of Ce0.8Gd0.2O1.9/3Y-TZP composite electrolytes—effects of weight % 3Y-TZP particles

Solid oxide fuel cell electrolytes are currently manufactured from zirconia. This is due to its high ionic conductivity and mechanical strength. However, a disadvantage of zirconia is the high temperatures required for efficient operation, greater than 900°C for cells not utilising thin supported electrolytes. A study to characterise the properties of an alternative composite solid electrolyte, based on a gadolinia doped ceria matrix surrounding yttria stabilised tetragonal zirconia particles is presented. Composite samples produced using 1, 2, 5, 10 and 20 wt% zirconia particles were characterised using a range of different experimental techniques including scanning electron microscopy, transmission electron microscopy, X-ray diffraction, mechanical testing and impedance spectroscopy. Results showed a lowering in flexure strength of the experimental composite samples with the addition of particles. It is considered that the particles acted as regions where cracks could initiate.     

Characterisation of Ce0.8Gd0.2O1.9/3Y-TZP composite electrolytes—Effects of 3Y-TZP particle treatment

Solid oxide fuel cell (SOFC) electrolytes are currently manufactured from yttria stabilised zirconia. A limitation of this material is the high operating temperatures needed for efficient operation, approximately 1000°C, as this can lead to sealing problems and require expensive nickel/chromium alloy auxiliary components. Alternative electrolytes, which allow operation at reduced temperatures, such as gadolinium doped cerium oxide have been suggested. However these ceramic materials often exhibit inferior mechanical properties. In this paper the manufacture and characterisation of composite electrolytes consisting of 3% yttria stabilised zirconia particles in a 20% gadolinia doped ceria matrix is described. Results suggest improvements in strength can be achieved by the composite approach.     

Computer Modeling of Ionic Conductivity in Low Temperature Doped Ceria Solid Electrolytes

Solid oxides, such as ceria (CeO₂) doped with cations of lower valance, are potential electrolytes for future solid oxide fuel cells. This is due to the theoretically high ionic conductivity at low operation temperature. This paper investigates the feasibility of two potential electrolytes which are samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) to replace the traditional yttria-stablized zirconia (YSZ). Molecular simulation techniques were employed to study the influence of different dopant concentrations at different operation temperatures on the ionic conductivity from the atomistic perspective. Simulation results show that the optimized ionic conductivity occurs at 11.11mol% concentration using both dopants of Gd₂O₃ and Sm₂O₃. The temperature effect was also examined under a fixed concentration simulation to check how low temperature they still function. The predicted ionic conductivities have been verified with published experimental results and show reasonable agreements. This simulation technique reveals a clear picture with qualitative and quantitative connection between the choice of the dopant and the improvement of the ionic conductivity of fuel cell electrolytes.     

A sol-powder coating technique for fabrication of yttria stabilised zirconia

Yttria stabilised zirconia has been prepared using a simple sol-powder coating technique. The polymeric yttria sol, which was prepared using 1,3 propanediol as a network modifier, was homogeneously mixed with nanocrystalline zirconia powder and it showed a dual function: as a binder which promoted densification and a phase modifier which stabilised zirconia in the tetragonal and cubic phases. Thermal analysis and X-ray diffraction revealed that the polymeric yttria sol which decomposed at low temperature into yttrium oxide could change the m → t phase transformation behaviour of the zirconia, possibly due to the small particle size and very high surface area of both yttria and zirconia particles allowing rapid alloying. The sintered samples exhibited three crystalline phases: monoclinic, tetragonal and cubic, in which cubic and tetragonal are the major phases. The weight fractions of the individual phases present in the selected specimens were determined using quantitative Rietveld analysis.     

Effect of submicron grains on ionic conductivity of nanocrystalline doped ceria

Doped ceria has been considered for high oxygen ion conductivity for solid oxide fuel cells. In the present study, 20 mole% samarium doped nano ceria powder was prepared by wet chemical synthesis and sintered at different temperatures to retain submicron grains (> 92-96% density). ionic conductivity of the sintered pellets was measured using impedance spectroscopy as a function of temperature (200-800 degrees C). The total maximum conductivity was 1.0 x 10(-2)S.cm(-1) (at 600 degrees C) for samples sintered at 1200 degrees C. The activation energy at higher test temperature decreases with the decrease in the sintering temperature (by 25%). The grain boundary, grain interior conductivity and activation energy of the electrolyte were correlated to the resulting microstructure. It has been demonstrated that use of doped nano ceria powder as precursor not only reduced the sintering temperature but also provided segregation free grain boundary for engineering higher conductivity dense electrolytes.     

Microstructure behaviour and influence on thermally grown oxide formation of double‐ceramic‐layer EB‐PVD thermal barrier coatings annealed at 1,300 °C under ambient isothermal conditions

To resist high thermal loads in turbines effectively, turbine blades are protected by thermal barrier coatings in combination with additional air cooling. State‐of‐the‐art yttria stabilised zirconia top coats do not operate at temperatures higher than 1,200 °C. Promising candidates for alternative top coats are pyrochlores, lanthanum zirconate and gadolinium zirconate. But lifetime of pyrochlores is short because of spallation. However, combinations of yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate as multilayer systems are promising top layers operating at higher temperatures than yttria stabilised zirconia. Such thermal barrier coatings top coats as double‐ceramic‐layer systems consisting of 7 wt.% yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate were deposited by Electron Beam‐Physical Vapour Deposition. The focus of the work was set on the influence of the coating design and the microstructure variation generated at different rotating speeds on the adhesion and thermally grown oxide behaviour after isothermal oxidation at 1,300 °C. Phase formation of the thermal barrier coatings top coats was obtained using X‐ray diffraction. After isothermal oxidation tests for 50 h at 1,300 °C, both, microstructure change and the formation of the thermally grown oxide were investigated. While the pyrochlore single‐ceramic‐layer are completely spalled off, microstructure of the double‐ceramic‐layer reveals only crack initiation. The thermally grown oxide thickness was determined by means of scanning electron microscopy. A high aluminum and oxygen content in the thermally grown oxide is found using X‐ray spectroscopy. Existence of α‐phase in Al₂O₃ was proved by X‐ray diffraction. After isothermal testing, no phase transformation can be detected regarding the double‐ceramic‐layer coatings.     

RETRACTED ARTICLE: Determination of dopant of ceria system by density functional theory

Oxides with the cubic fluorite structure, e.g., ceria (CeO₂), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy–dopant interactions, represented by association (binding) energies of vacancy–dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately. An erratum to this article can be found at     

Fabrication–microstructure–performance relationships of reversible solid oxide fuel cell electrodes–review

Abstract

A reversible solid oxide fuel cell system can act as an energy storage device by storing energy in the form of hydrogen and heat, buffering intermittent supplies of renewable electricity such as tidal and wave generation. The most widely used electrodes for the cell are lanthanum strontium manganate–yttria stabilised zirconia and Ni–yttria stabilised zirconia. Their microstructure depends on the fabrication techniques, and determines their performance. The concept and efficiency of reversible solid oxide fuel cells are explained, along with cell geometry and microstructure. Electrode fabrication techniques such as screen printing, dip coating and extrusion are compared according to their advantages and disadvantages, and fuel cell system commercialisation is discussed. Modern techniques used to evaluate microstructure such as three-dimensional computer reconstruction from dual beam focused ion beam–scanning electron microscopy or X-ray computed tomography, and computer modelling are compared. Reversible cell electrode performance is measured using alternating current impedance on symmetrical and three electrode cells, and current/voltage curves on whole cells. Fuel cells and electrolysis cells have been studied extensively, but more work needs to be done to achieve a high performance, durable reversible cell and commercialise a system.     

Thermal shock fracture behaviour of ZrO2 based ceramics

Thermal shock fracture behaviour of alumina, mullite, silicon carbide, silicon nitride and various kinds of zirconia based ceramics, such as magnesia partially stabilized zirconia (Mg-PSZ), yttria and ceria doped tetragonal zirconia polycrystals (Y-TZP and Ce-TZP), Y-TZP/Al₂O₃ composites and yttria doped cubic stabilized zirconia (Y-CSZ), was evaluated by the quenching method using water, methyl alcohol and glycerin as quenching media. Thermal shock fracture of all materials seemed to proceed by the thermal stress due to convective heat transfer accompanied by boiling of the solvents under the present experimental conditions. Thermal shock resistance of zirconia based ceramics increased with increasing the fracture strength, but that of Y-TZP and Y-TZP/Al₂O₃ composites was anormalously lower than the predicted value.     

Effect of sintering temperature on structural and electrical properties of gadolinium doped ceria (Ce<Subscript>0·9</Subscript>Gd<Subscript>0·1</Subscript>O<Subscript>1·95</Subscript>)

Gadolinium doped ceria oxide is one of the promising materials as an electrolyte for IT-SOFCs. Ce_0·9Gd_0·1O_1·95 (GDC10) powder was prepared by solid state reaction and sintered at 1473 K, 1573 K, 1673 K and 1773 K. All samples were studied using X-ray diffraction, scanning electron micrograph and d.c. conductivity measurement. The crystallinity and surface morphology of the samples improved with sintering temperature. Further, the electrical conductivity measurement indicated that the conduction mechanism is mainly ionic. The conductivity of samples sintered at 1673 K and 1773 K at 800°C are of the order of 0·1 S-cm⁻¹. The activation energies decreased from 1·25–0·82 eV with increase in sintering temperature.     

Stability of zirconia-ceria-yttria ceramics in hostile environments

The phase chemistry, conductivity in air and forming gas, and stability in warm humid environments of zirconia-ceria-yttria ceramics are described. The compositions studied lie between 12 mol % ceria/88 mol % zirconia and 3 mol % yttria/97 mol % zirconia and fall within the tetragonal phase field of this ternary system. At temperatures below 700° C, reducing atmospheres were found to affect only the conductivity of the end-member ceria-zirconia ceramic, having no effect on the ternary compositions investigated. Compositions containing more than 10 mol % ceria were found to be more stable to stabilizer loss in water vapour at 132° C than those containing less than 10 mol % ceria.     

Effect of ZnO nanoparticles on the structure and ionic relaxation of poly(ethylene oxide)-LiI polymer electrolyte nanocomposites

The effect of ZnO nanoparticles on the structure and ionic relaxation of LiI salt doped poly(ethylene oxide) (PEO) polymer electrolytes has been investigated. X-ray diffraction, high resolution transmission electron microscopy and field emission scanning electron microscopy show that ZnO nanoparticles dispersed in the PEO-LiI polymer electrolyte reduce the crystallinity of PEO and increase relative smoothness of the surface morphology of the nanocomposite electrolyte. The electrical conductivity of the nanocomposites is found to increase due to incorporation of ZnO nanoparticles. We have shown that the structural modification due to insertion of ZnO nanoparticles results in the enhancement of the mobility i.e., the hopping rate of mobile Li+ ions and hence the ionic conductivity of PEO-LiI-ZnO nanocomposite electrolyte.     

Electrical properties and microstructure in the system ceria-alkaline earth oxide

The ionic conduction of oxygen in the ceria-alkaline earth oxide system was investigated as a function of temperature, partial pressure of oxygen and oxide composition, together with its crystal structure, density, and microstructure. Undoped ceria and its solid solution with alkaline earth oxides have a cubic fluorite structure. The ionic conductivity of ceria is greatly enhanced by additions of calcia and strontia, even when they are added in excess of the solubility limit. The conductivities of ceria-calcia and ceria-strontia were much higher than those of calcia-and yttria-stabilized zirconia. Up to the limit of calcia and strontia, the ionic transference number was nearly unity in the temperature range between 600 and 900°C. With an increase in calcia and strontia content, the ionic conductivity was little affected by the presence of a second phase of CaO and SrCeO₃.     

Sm_2O_3掺杂CeO_2纳米材料的电子顺磁共振研究

利用溶胶-凝胶法制备Sm2O3掺杂CeO2的电解质材料。通过粉末X射线衍射谱对所制备的电解质材料进行平均晶粒尺寸和晶格常数分析,并利用电子顺磁共振仪进行测试,分析样品的电子顺磁共振特性。结果表明:所制备的粉末样品晶粒尺寸较小、具有立方萤石型结构,而且Sm2O3掺杂CeO2的固体电解质材料有缺陷结构,随焙烧温度越高,缺陷结构越多;其次不同比例的Sm3+掺入CeO2后,材料中缺陷结构也有很明显的变化。     

Bismuth oxide-based solid electrolytes for fuel cells

During the last three decades, a large number of investigations has been reported pertaining to the science and technology of solid oxide fuel cells (SOFCs) based mainly on the yttria-stabilized zirconia (YSZ) electrolyte. Because of the problems associated with the high temperature of operation (~ 1000°C) of the YSZ-based cells, there has been a substantial effort to develop alternative electrolytes with ionic conductivity comparable to that of YSZ at relatively lower temperatures. This review presents a systematic evolution in the area of the development of new electrolytes based on bismuth sesquioxide for fuel cell applications at moderate temperatures.     

Electric conductivity and oxygen permeability of modified cerium oxides

Electrical conductivities of samarium (Sm), terbium (Tb), praseodymium (Pr) and zirconium (Zr) doped ceria membranes were measured in T = 600–900°C and pO₂ = 10^–22–0.21 atm. Doping Sm and Pr in CeO₂ respectively enhances the ionic conductivity and electron-hole conductivity of ceria. Sm-Pr doped ceria exhibits both n-type (at lower pO₂) and p-type (at high pO₂) electronic and ionic conductivity in the temperature range studied. Adding Tb in the Sm doped ceria causes a reduction in ionic conductivity. Zr-Pr doped ceria is an n-type electronic conductor at low and p-type electronic conductor at high pO₂. The ionic conductivity of Zr-Pr doped ceria is lower than Sm doped ceria but higher than pure ceria. Oxygen permeation flux through the Zr-Pr doped ceria membrane, dominated by the slow oxygen ionic conduction, is similar to the ytttria doped bismuth oxide membrane, and smaller, but close to that of perovskite-type lanthanum cobaltite membrane.     

Preparation and thermal evolution of sol-gel derived transparent ZrO2 and MgO-ZrO2 gel monolith

Transparent gel monoliths of pure and MgO-doped zirconia having dopant concentrations in the range 0 to 15 mol % were prepared by chemical polymerization of zirconium n-propoxide and magnesium acetate tetrahydrate using 2-methoxy ethanol as solvent. The thermal evolution of amorphous gels was studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. The crystallization of pure and doped zirconia gels occurred in the temperature range 360 to 450° C. The first crystalline phase to appear is tetragonal for pure and 2 mol % doped zirconia, and cubic for 3 to 15 mol % doped samples. Both crystallization and decomposition temperatures are found to increase with increasing dopant concentration, approaching a saturation value for 10 mol % doped samples. It has been established that the transformation of the cubic to the monoclinic phase takes place via a metastable tetragonal phase. A linear relationship between the lattice parameter of cubic zirconia and MgO concentration has been established. X-ray diffraction studies have also revealed that the entire amount of MgO used in preparing doped zirconia gels remains in a single MgO-ZrO₂ crystalline phase formed initially by thermal treatment.[/p]     

Electrochemical properties of composite electrolytes based on poly(ethylene oxide)/poly(ethylene imine) containing the inorganic silica fillers

In this study, poly(ethylene oxide) (PEO) and poly(ethylene imine) (PEI) polymer blends containing inorganic silica fillers were studied in order to enhance the ion conductivity and interfacial properties. Lithium perchlorate (LiCIO4) as a salt, and silica (SiO2) as the inorganic filler were introduced in the polymer electrolyte composites and were examined to evaluate their use to improve the ionic conductivity. The addition of inorganic fillers in polymer electrolytes has resulted in high ionic conductivity at a room temperature. The structure and morphology of the solid polymer electrolytes were evaluated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The ionic conductivity was measured by an AC impedance method. The enhanced conductivity was dependent on the decreased crystallinity and more heterogeneous morphologies.     

Oxygen ion mobility and conductivity prediction in cubic yttria-stabilized zirconia single crystals

The CALPHAD (calculation of phase diagrams) approach is applied to predict the oxygen vacancy concentration at different temperatures and yttria concentrations of cubic yttria-stabilized zirconia (c-YSZ) single crystals. The quantitative mobility diagrams of oxygen ions are developed in a wide range of temperature and yttria concentration, using the experimental data from the literature. Therefore, the ionic conductivity of c-YSZ single crystals is predicted, using the mobility and oxygen vacancy concentration. Particularly, the conductivity of low-yttria c-YSZ is predicted by applying the CALPHAD approach for the first time. The conductivity prediction of low-yttria c-YSZ can be crucial, since new applications may be designed based on this new information. The activation energy and pre-exponential factor diagrams versus yttria concentration are also plotted.     

Effect of boron oxide on the cubic-to-monoclinic phase transition in yttria-stabilized zirconia

Specimens of yttria fully stabilized zirconia with different amounts of boron oxide have been studied by X-ray diffraction at room temperature and at higher temperatures up to 1250 °C. A boron oxide-assisted cubic-to-monoclinic phase transformation was determined in the temperature range 800-1250 °C. In situ high temperature X-ray diffraction experiments gave evidences of the dependence of the phase transformation on the heating rate. The possibility of tuning the cubic-monoclinic phase ratio by suitable addition of boron oxide before pressing and sintering is proposed.