Novel Composite Electrolytes for Solid Oxide Fuel Cell Applications

Solid oxide fuel cells (SOFCs) are devices that allow direct conversion of chemical to electrical energy through an electrochemical reaction in a cleaner and more efficient way than conventional processes (eg. gas turbines). They are characterized by the use of a solid oxide material as the electrolyte. Yttria-stabilised zirconia (YSZ) has traditionally been used in SOFCs electrolytes at temperatures in the range of 850-1000 °C. Recent research is being focused on the development of new materials with increased ionic conductivity at intermediate temperatures (500-800ºC) as alternative materials to YSZ. Rare earth silicates with an apatite-type structure, such as doped lanthanum oxides of general formula La10(MO4)6O2, where M = Ge, Co, Si, Al, or P, are among these materials. The major limitation associated with the manufacture of these materials is their poor sinterability, which requires high sintering temperatures (1600ºC).The present work concerns the production of La9.33Si2Ge4O26 dense materials from La2O3 (99.9%), SiO2 (99.4%) and GeO2 (99.9%) powders by mechanical alloying (MA) followed by conventional and microwave hybrid sintering at 1350ºC. Dry mechanical milling was carried out in protective atmosphere (argon at 200 kPa) by using a rotating speed of 350 rpm for 15h. Prior to the synthesis of the La9.33Si2Ge4O26 mixture, the starting materials were milled separately at 350 rpm in argon at 50 kPa in order to achieve low particle size distributions with a greater ability for sintering. The MA mixture was compacted by uniaxial pressing at 390 MPa followed by sintering for 1 h at 1350 °C. Density of the La9.33Si2Ge4O26 pellets was determined by the boiling test method. SEM/EDS, XRD and atomic force microscopy (AFM) were used for samples characterization.The bulk densities and the open porosities of the pellets (EF and MW for conventional and microwave hybrid sintering, respectively) are presented in Table 1. Dense pellets were obtained in both cases. However, sample MW showed higher density and, consequently, lower porosity than sample EF. Both mechanically alloyed samples are formed by an apatite phase. This phase remains stable during sintering. However, traces of a second phase (La4GeO8) were detected after sintering. This phase was already referred in previous work (Serra et al., 2012). Figure 1 shows the morphology of the two samples obtained by SEM and AFM as well as the corresponding particles size distributions. The sintered samples were as-polished and thermal etched in air at 1300 ºC, for 10 min., for detailed microstructural observation. SEM images at low amplification confirm that sample EF is more porous than sample MW (Figure 1 (a) and (b), respectively). Although the same sintering temperature was used in both sintering processes, sample MW has a coarser grain size (Figure 1 (c) to (f)). This feature was already been reported in previous work (Fang et al., 1994). Mean values of 0.9 and 1.9 m were determined for samples EF and MW, respectively.As conclusion, one may say that microwave hybrid sintering is a novel suitable process for the densification of La9.33Si2Ge4O26 powders for application in SOFCs electrolytes.This research is partially sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by national funds through FCT – Fundação para a Ciência e Tecnologia – under the contract PTDC/EME-PME/102837/2008. The research fellowships granted to Cátia Alves, Márcio Santos and Mafalda Macatrão are also gratefully acknowledged.

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.

Alumina/cerium oxide nano-composite electrolyte for solid oxide fuel cell applications

Lowering the operating temperature of the high-temperature, solid oxide fuel cell (SOFC) improves both the thermodynamic efficiency and the lifetime of this energy efficient technology. Unfortunately the rate of oxygen-ion transport through the solid electrolyte is temperature dependent, and materials previously employed as electrolytes in the high-temperature SOFC perform poorly at intermediate temperatures. Therefore new oxygen-ion conductors with enhanced ionic conductivity at intermediate temperatures are required. The bulk of the existing literature on high-temperature SOFCs has focussed on zirconia-based binary systems as electrolytes, due to their high ionic conductivity and negligible electronic conductivity. Only select compositions within the zirconia-scandia system have demonstrated acceptable ionic conductivity levels at intermediate temperatures; however unstable phase assemblage and the high economic cost of scandia are clear disadvantages. Ceria-based binary systems have demonstrated improved oxygen-ion conductivity at intermediate temperature compared to many zirconia systems, however significant levels of n-type electronic conductivity are observed at low oxygen partial pressures. Consequently it was thought unlikely that significant increases in ionic conductivity would be found in existing zirconia- and ceria-based binary systems, therefore another approach was required in an attempt to improve the performance of these established fluorite systems. The fluorite systems Zr0.75Ce0.08M0.17O1.92 (M = Nd, Sm, Gd, Dy, Ho, Y, Er, Yb, Sc) were prepared and investigated as possible, intermediate-temperature SOFC electrolytes in an attempt to combine the higher conductivity found in the ceria systems with the low electronic conductivity observed in the zirconia systems. Also it was anticipated that systems containing dopants not previously observed to confer high ionic conductivity in either zirconia- and ceria-based binary systems, might exhibit enhanced ionic conductivity with expansion of the zirconia lattice resulting from the addition of ceria. All the as-fired Zr0.75Ce0.08M0.17O1.92 compositions possessed the face-centred cubic structure and lattice parameter measurements revealed the anticipated unit cell enlargement as the size of the dopant cation increased. No unusual microstructural parameters were identified that could be expected to interfere with the ionic transport properties in the as-fired compositions. The electrical conductivity was found to be influenced by the dopant-ion radius, the presence of ceria, low oxygen partial pressures and, in some compositions, the formation of poorly conducting, ordered-pyrochlore microdomains dispersed amongst the cubic defect-fluorite matrix. In a second approach to the formulation of new oxygen-ion conductors suitable for the intermediate-temperature SOFC, compounds possessing structures other than the fluorite structure were considered. An examination of the literature for oxides having the pyrochlore, scheelite and perovskite structures showed that the Sr+2- and Mg+2-doped LaGaO3 perovskites (LSGM) possessed ionic conductivity equal to the highest conducting, zirconia and ceria binary compounds. Therefore the perovskite systems La0.9Sr0.1Ga(0.8-x)InxMg0.2O2.85 (X = 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8) (I-LSGM) were prepared and examined, the objective being to favourably influence structural parameters believed responsible for optimal ionic conductivity, namely the unit cell symmetry and volume. It was found that In+3 systematically substituted for Ga+3 on to the B-site of LSGM at least up to the X = 0.4 composition. While In+3 was found to replace the Ga+3 as expected, Mg+2, which occupies the same crystallographic site, was also replaced by In+3. Up to the X = 0.2 composition, at least two trace level secondary phases were observed to form along with the bulk I-LSGM phase. For I-LSGM compositions with X > 0.2, significantly larger concentrations of the secondary phases were identified. Evidence of a strontium-rich, high-temperature liquid phase was observed also near the grain boundaries on as-sintered and thermally etched surfaces in LSGM and I-LSGM compositions. It is believed that the observed, high sintered density in the complex, doped-LaGaO3 systems is due to the formation of this high-temperature liquid phase. Increasing levels of diffuse scatter and superstructure formation were observed in electron diffraction patterns in the I-LSGM bulk phase (up to X = 0.2), indicating a possible decrease in vacancy concentration and reduced, localised unit cell symmetry. The electrical conductivity in the I-LSGM compositions was believed to be influenced by the distortion of the oxygen-ion conduction path, a reduction in vacancy concentration, formation of stronger dopant-vacancy associates at low temperature and the presence of ordered structures. In addition, phase instability, in the form of subtle ordering in specific crystalline planes, was observed to influence the electrical conductivity as a function of time at intermediate temperatures.

Acceptor-doped La1.9M0.1Ce2O7 (M = Nd, Sm, Dy, Y, In) proton ceramics and in-situ formed electron-blocking layer for solid oxide fuel cells applications

Microstructure formation and oxidation behaviour of the Oxide Dispersion Strengthened (ODS) steels for application as structure material in Nuclear Power Plant was investigated. A mixture composed of Fe and 12 wt. % Cr powder with addition of 0.5 and 1 wt.% ZrO2 particles was milled and isostatic pressed to form a sample coin. The coin was then consolidated in the Arc Plasma Sintering (APS) for 4 minutes. The samples were subjected to the high temperature oxidation test in the Magnetic Suspension Balance (MSB). The oxidation test was carried out at 700°C for 6 hours to evaluate the oxide growth in the early stage of it formation by extraction the mass gain curve. The Scanning Electron Microscope (SEM) imaging and X-ray Diffraction Spectroscopy (EDX) elemental mapping were performed to study the microstructure change and compositional distribution. SEM and EDX observation revealed the time dependent development of the Fe-Cr-phases during consolidation. The oxidation rate behaviour of the samples followed the parabolic rate characteristic for inward oxidation process driven by oxygen inward diffusion through the oxide scale with the maximum weight gain around of 60 g/m2. The oxidation resistance was strongly affected by the formation of the oxide protective layer on the surface. In so far, addition of zirconia particles has played no significant role to the oxidation behaviour.

Advances in the knowledge of the double perovskites derived from the conformation and substitution of the material Sr2MgMoO6-δ as anode with potential application in SOFC cell

Augmentation of grain boundary conductivity in Ca2+ doped ceria-carbonate-composite

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part I: Microstructure optimization

This paper deals with a modeling of the composite timber-concrete ceiling structure. In the analysis of the composite structure, there is the slip using the contact elements and volume finite elements considered. Numerical modeling is complemented by an analytical calculation. This article aims to determine the stiffness of a particular type of glued strip. Totally eight models of the composite timber-concrete structure have been studied.

In this study, we used multilayer graphene oxide (GO) obtained by anodic oxidation of graphite powder in 83% sulfuric acid. The modification of GO was carried out by its interaction with hexamethylenediamine (HMDA) according to the mechanism of nucleophilic substitution between the amino group of HMDA (HMDA) and the epoxy groups of GO, accompanied by partial reduction of multilayer GO and an increase in the deformation of the carbon layers. The structure and properties of modified HMDA-GO were characterized using research methods such as scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectroscopy and Raman spectroscopy. The conducted studies show the effectiveness of using HMDA-OG for modifying epoxy composites. Functionalizing treatment of GO particles helps reduce the free surface energy at the polymer-nanofiller interface and increase adhesion, which leads to the improvement in physical and mechanical characteristics of the composite material. The results demonstrate an increase in the strength and elastic modulus in bending by 48% and 102%, respectively, an increase in the impact strength by 122%, and an increase in the strength and elastic modulus in tension by 82% and 47%, respectively, as compared to the pristine epoxy composite which did not contain GO-HMDA. It has been found that the addition of GO-HMDA into the epoxy composition initiates the polymerization process due to the participation of reactive amino groups in the polymerization reaction, and also provides an increase in the thermal stability of epoxy nanocomposites.

High densification and Li-ion conductivity of Al-free Li7-xLa3Zr2-xTaxO12 garnet solid electrolyte prepared by using ultrafine powders

In this study, an effective photodegrading properties of WO 3 nanoparticles has been enhanced by doping with cerium. To achieve our goal pristine and Ce‐doped WO 3 nanoparticles were prepared by a simple and cost effective co‐precipitation route. Characterization of all the specimens were conducted which showed that there were no noticeable structural changes. WO 3 retained its monoclinic structure even after doping with (3%, 5%, 8%, 10%) cerium which was also confirmed with X‐ray diffraction (XRD) and Raman spectroscopy. In addition, XRD and Fourier transform infra‐red spectroscopy also confirm that no new phases were formed in doped specimens. XRD also depicts that minimum crystallite size is achieved at 3% cerium doping. Scanning electron microscope images show decrease in particle size initially at 3% cerium doping, that is, ~80 ± 2.5 nm. In contrast, further increase in doping percentage increases crystallite and particle sizes. Additionally, defect generation increased at lower doping percentage ~3%. In contrast, further increase in dopant (5%, 8%, 10%) caused defect annihilation process to dominate over defect generation. C/C o ratio and rate constant ( K MB ) values suggest that maximum degradation of methylene blue will occur at 3% cerium doping due to increase in crystalline defects. Ultraviolet visible Spectroscopy further validates C/C o ratio and degradation rate constant values by showing drastic decline in peak intensity at 3% cerium doping. Therefore, 3% cerium‐doped WO 3 is more effective photocatalyst to degrade methylene blue in comparison to most of the previously used photocatalysts. The prepared cerium‐doped WO 3 nanoparticles as a sun light driven photocatalyst, may be utilized for industrial wastewater purification.

CeO{sub 2}-based oxides have recently been shown to have great potential as electrolytes in medium temperature solid oxide fuel cell applications, primarily due to their high ionic conductivity. Steele et al., for example, have examined a cell of the type: O{sub 2}, La{sub 0.6}Sr{sub 0.4}Fe{sub 0.8}Co{sub 0.2}O{sub 3}{vert_bar}Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95}{vert_bar}Ni-ZrO{sub 2}, H{sub 2}/H{sub 2}O at 715{degrees}C. Gd{sub 2}O{sub 3} doped CeO{sub 2} has been reported as having one of the highest oxygen ion conductivities of the ceria-based materials. An ionic conductivity of 8.3 x 10{sup -2} s/cm has been reported for (CeO{sub 2}){sub 0.8}(GdO{sub 1.5}){sub 0.2} at 800{degrees}C, which is approximately four times that of Y{sub 2}O{sub 3}-doped ZrO{sub 2}, at the same temperature. Although the electrical properties of the material have been examined in detail, very little work has considered the microstructural/property relationships, particularly in relation to the mechanical properties. It is well know that CeO{sub 2}-based materials are difficult to density and attempts have been performed to examine this. Preliminary studies have also been undertaken to examine the effect of sintering on the mechanical properties of the material. In this paper we examine the effect of microstructure on the high temperature mechanical properties of (CeO{sub 2}){sub 0.8}(GdO{sub 1.5}){sub 0.2}.

Ba0.5Sr0.5[CoxZn0.2-x]Fe0.8O3–δ, (x = 0, 0.04, 0.08, 0.12) cathode formulations were successfully synthesized by solid state reactions and the effect of cobalt doping at Zn site of Ba0.5Sr0.5Zn0.2Fe0.8O3–δ (BSZF0.2) on the electrical conductivity, the polarization resistance and electrochemical behavior was evaluated. X-ray diffraction patterns indicate that a single cubic perovskite phase of Ba0.5Sr0.4Co0.8Fe0.2O3–δ oxide is successfully obtained. Ba0.5Sr0.5Co0.04Zn0.16Fe0.8O3–δ (BSCZF0.16) exhibited a high electrical conductivity of 10 S/cm at 400 °C in comparison to the BSZF0.2 showing 5.5 S/cm. Further, BSCZF0.16 also possess a low polarization resistance as low as 0.22, 0.38, 0.87, and 1.55 Ω cm2 at 750, 700, 650, and 600 °C in air, respectively. Accordingly, a low activation energy value of 149.8 kJ/mol for BSCZF0.16 in comparison to 159.4 kJ/mol for BSZF0.2 indicates high catalytic efficiency. Enhancement of desirable properties such as electrical conductivity in combination with low-polarization resistance and low-activation energy values can be attributed to the coexistence of Co and Zn in the B-site of BSCZF0.16 leading to the multivalent states which contributes to the enhanced electron transport properties demonstrating BSCZF0.16 as a better cathode for intermediate temperature solid oxide fuel cells applications.