Heat Loss Reduction in Combustion of Cu(Ni)–Al Powder Systems Due to Their Microstructural Transformation

The paper reviews published data on structural–phase changes in Cu–Al and Ni–Al metal powders during in situ combustion. The heatconservation properties of various structures located around reactive cells in the combustion wave zone are revealed. The mechanism providing for nearly adiabatic combustion conditions is discussed and a microstructural model for this mechanism is justified.     

Effect of end-group modification on the adsorption of poly(ethylene oxide)-b-poly(butylene oxide) diblock copolymers at the solid–liquid interface

The adsorption of poly(ethylene oxide)-b-poly(butylene oxide) diblock copolymers at the solid–liquid interface was studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The effect of modifying the end group of the hydrophilic block was investigated by comparing the behaviour of trimethylammonium- and dimethylamino-tipped copolymers, designated as TE _m B _n and DE _m B _n , respectively. For adsorption from aqueous solution onto a gold surface, results for DE₄₉B₂₂ were similar to those of the T-analogue, but for DE₈₀B₃₄ adsorbed amounts were substantially higher, and for DE₂₇B₂₅ enormously higher, than for the T-analogue. It is suggested that very high levels of adsorption are associated with the formation of a multilayer structure.     

Lowering the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular solid oxide fuel cells

To prevent undesirable reactions between the cathode and electrolyte materials in cathode-supported solid oxide fuel cells (SOFCs), the co-sintering temperature of these two layers must be lowered. In the present work, we employed different strategies to lower the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular SOFCs by increasing the cathode sintering shrinkage and adding sintering aids to the electrolyte. Strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ) were used as the cathode and electrolyte materials, respectively. To facilitate densification of the electrolyte layer by controlling the shrinkage of the cathode support, the particle size of the LSM powder was reduced by high-energy ball milling and different amounts of micro-crystalline cellulose pore former were used. Sintering aids, namely NiO and Fe₂O₃, were also added to the YSZ electrolyte to further improve its low-temperature sintering. Our results indicate that with the improvement in the cathode support shrinkage and use of the small amounts of sintering aids, the cathode–electrolyte co-sintering temperature can be reduced to 1250–1300 °C. It was also observed that the presence of the sintering aids helps to reduce the reactivity between the LSM cathode and YSZ electrolyte.     

Liquid-like H2O adsorption layers to catalyze the Ca(OH)2/CO2 solid–gas reaction and to form a non-protective solid product layer at 20°C

Porous calcium hydroxide particles have been equilibrated with water vapor in the relative pressure range 0.4–0.85 at the temperature of 20°C. Then the particles were used in the reaction with gaseous carbon dioxide at the same temperature and at pressure of 0.65 kPa. The system is converted up to 85% into a non-protective layer of calcium carbonate which is all distributed inside the initial porous particles. The same reaction with dry-calcium hydroxide powders converts up to 10% at a temperature of 100°C. The observed catalytic effect is dependent upon the initial amount of water adsorbed. A minimum number of four layers of water adsorbed onto the calcium hydroxide surfaces is required to promote the catalytic effect.     

Electrochemical performance of La2NiO4+δ cathode for intermediate-temperature solid oxide fuel cells

The application of the La₂NiO_4+δ (LNO), one of the Ruddlesden–Popper series materials, as a cathode material for intermediate temperature solid oxide fuel cells is investigated in detail. LNO is synthesized via a complex method using ethylenediaminetetraacetic acid (EDTA) and citric acid. The effect of the calcination temperature of the LNO powder and the sintering temperature of the LNO cathode layer on the anode-supported cell, Ni–YSZ/YSZ/GDC/LNO, is characterized in view of the charge transfer resistance and the mass transfer resistance. Charge transfer resistance was not significantly affected by calcination and sintering temperature when the sintering temperature was not lower than the calcination temperature. Mass transfer resistance was primarily governed by the sintering temperature. The unit cell with the LNO cathode sintered at 1100 °C with 900 °C-calcined powder presented the lowest polarization resistance for all the measured temperatures and exhibited the highest fuel cell performances, with values of 1.25, 0.815, 0.485, and 0.263 W cm⁻² for temperatures of 800, 750, 700, and 650 °C, respectively.     

Characteristics of Cu and Mo-doped Ca3Co4O9−δ cathode materials for use in solid oxide fuel cells

In this study, Cu and Mo ions were doped in Ca₃Co₄O_9−δ to improve the electrical conductivity and electrochemical behavior of Ca₃Co₄O_9−δ ceramic and the performance of a solid oxide fuel cell (SOFC) single cell based on NiO-SDC/SDC/doped Ca₃Co₄O_9−δ-SDC were examined. Cu substitution in the monoclinic Ca₃Co₄O_9−δ ceramic effectively enhanced the densification, slightly increased the grain size, and triggered the formation of some Ca₃Co₂O₆; however, no second phase was found in porous Mo-doped Ca₃Co₄O_9−δ ceramics even when the sintering temperature reached 1050 °C. Substitution of Cu ions caused slight increase in the Co³⁺ and Co⁴⁺ contents and decrease in the Co²⁺ content; however, doping with Mo ions showed the opposite trend. Doping the Ca₃Co₄O_9−δ ceramic with a small amount of Cu or Mo increased its electrical conductivity. The maximum electrical conductivity measured was 218.8 S cm⁻¹ for the Ca₃Co_3.9Cu_0.1O_9−δ ceramic at 800 °C. The Ca₃Co_3.9Cu_0.1O_9−δ ceramic with a coefficient of thermal expansion coefficient of 12.1×10⁻⁶ K⁻¹ was chosen as the cathode to build SOFC single cells consisting of a 20 μm SDC electrolyte layer. Without optimizing the microstructure of the cathode or hermetically sealing the cell against the gas, a power density of 0.367 Wcm⁻² at 750 °C was achieved, demonstrating that Cu-doped Ca₃Co₄O_9−δ can be used as a potential cathode material for IT-SOFCs.     

Effect of GDC addition method on the properties of LSM–YSZ composite cathode support for solid oxide fuel cells

Equal amounts of Gd_0.1Ce_0.9O_2−δ (GDC) were added to La_0.65Sr_0.3MnO_3−δ/(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM/YSZ) powder either by physical mixing or by sol–gel process, to produce a porous cathode support for solid oxide fuel cells (SOFCs). The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm⁻² at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by sol–gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs.     

Interfacial characterization of YSZ-to-steel joints with Ag–Cu–Pd interlayers for solid oxide fuel cell applications

Yttria-stabilized zirconia (YSZ)/stainless steel joints made using two commercial silver-based interlayers containing palladium (58Ag–32Cu–10Pd and 65Ag–20Cu–15Pd), were systematically analyzed for the microstructures of the interlayer matrices, interlayer–steel interface, and interlayer–YSZ interface using scanning electron microscopy (SEM) and transmission electron microscopy coupled with energy dispersive spectroscopy (TEM/EDS). In the interlayer matrix, a face-centered cubic (FCC) Cu-rich phase formed in the vicinity of the YSZ and dissolved a significant amount of Zr from the ZrO₂ and minor amounts of Fe and Cr from the steel. The Cu-rich phase in the interlayer matrix in the vicinity of the steel substrate was the ordered phase Cu₃Pd with antiphase boundaries (APBs) and the L1₂ crystal structure. Silver particles precipitated within the Cu₃Pd phase in 58Ag–32Cu–10Pd; a Fe(Cr) needle-like phase, instead of Ag particles, precipitated within the Cu₃Pd phase in 65Ag–20Cu–15Pd. Although no reaction products at the interlayer–steel interface were found, dislocations appeared within the Ag- and Cu-rich phases. At the interlayer–YSZ interface, two reaction products, SiO₂ (impurity in YSZ) and Ti₃O₅ (from the reaction of YSZ with Ti impurities in the steel) were observed. Diffusion and chemical reactions led to the compositional changes and interface reconstruction, thereby yielding metallurgically sound YSZ/steel joints.     

Selective hydrogenation of styrene oxide to 2-phenyl ethanol over polyurea supported Pd–Cu catalyst in supercritical carbon dioxide

2-Phenyl ethanol (2-PEA) is an important chemical which finds several applications in perfumes, deodorants, soaps and detergents. It is prepared by different polluting and dangerous routes. The current work is concerned with production of 2-PEA by hydrogenation of styrene oxide using polyurea encapsulated catalysts (EnCat) in methanol and supercritical carbon dioxide (scCO₂). Bimetallic Pd–Cu catalyst encapsulated with polyurea, Pd–Cu EnCat, is the best catalyst. The epoxide ring in styrene oxide is selectively hydrogenated to give 2-PEA in scCO₂ without formation of any isomerization or deoxygenated products, which are formed in methanol. A complete conversion of styrene oxide with 100% selectivity to 2-PEA was obtained without addition of any promoter. Effects of various parameters were studied and a bifunctional site Langmuir–Hinshelwood–Hougen–Watson kinetic model was found to be in good agreement with the experimental data. The process is clean and green.     

The influence of oxide–oxide interaction on the catalytic properties of Co/Al2O3 in CO hydrogenation

The influence of oxide–oxide interaction on the catalytic properties of cobalt in CO hydrogenation is investigated on the example of a Co/Al₂O₃ catalyst. Oxide–oxide interaction was prevented by modification of the alumina surface with magnesia. It has been shown that oxide–oxide interaction affects catalytic activity and the amount of carbon deposited on the catalyst surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Glycothermal synthesis of zirconia–rare earth oxide solid solutions

The reactions of mixtures of zirconium alkoxide and cerium or yttrium acetate in 1,4-butanediol at 300°C (glycothermal reaction) yielded microcrystalline tetragonal zirconia–rare earth oxide solid solutions. The products maintained large surface areas even after calcination at high temperatures. The surface area of the products with low rare earth contents strongly depended on the drying conditions because of compaction at the drying stage, and the product dried by microwave irradiation had the highest surface area.     

Screen printing of sol–gel-derived electrolytes for solid oxide fuel cell (SOFC) application

The idea of using the sol–gel technique for producing low-cost components for solid oxide fuel cell (SOFC) application has attracted great interest. Besides its economic advantages, the sol–gel technique additionally offers the chance to reduce either the thickness of the electrolyte and therefore to reduce ohmic resistances or to lower the sintering temperature of single components like the electrolyte layer, due to the clearly reduced particle sizes of colloidal distributed particles in the sol.The work presented here deals with the development of sols and their application in combination with yttria fully stabilized zirconia mixed-oxide powders for the preparation of screen-printing pastes. Besides physical, chemical and thermal characterization of the sols, variations of the composition of the sol as well as of the pastes composed of sol and mixed-oxide powder were evaluated for preparing dense, gas-tight layers sintered at various temperatures, resulting in sufficient gas-tightness to ensure high power density SOFCs. Additionally, technological screen-printing parameters were studied.Single cell tests (50 mm × 50 mm) revealed current densities of approx. 1 A/cm². These values are comparable to current densities obtained by cells based on normal electrolyte layers, which were prepared in parallel.     

Thermodynamic investigation of vanadium oxide in CaO–SiO2–VOx system at 1873 K

Owing to the complexity of the multivalence states of vanadium oxides in slag systems and experimental difficulties, thermodynamic properties of vanadium oxides have not been established yet. In the present study, the mixed-valence states and activities of the vanadium oxides in CaO–SiO₂–VO_x slag were investigated experimentally at 1873 K and oxygen partial pressures of 3.2 × 10^–9 and 3.1 × 10^–7 atm. After the CaO–SiO₂–VO_x slag had equilibrated with a platinum strip, the mixed-valence states of the vanadium oxides in the slag were estimated by performing X-ray photoelectron spectroscopy, and the activities of the vanadium oxides in the slag were calculated using the activity of vanadium in the platinum strip at equilibrium using thermodynamic equations. At an oxygen partial pressure of 3.2 × 10^–9 atm, V³⁺ was the dominant ion and V⁴⁺ was the second most abundant ion. With increasing VO_x content or basicity (CaO/SiO₂ ratio), the fraction of V³⁺ decreased, whereas that of V⁴⁺ increased. The activity of VO_1.5 was greater than those of the other vanadium oxides. On the other hand, when the oxygen partial pressure increased to 3.1 × 10^–7 atm, V⁴⁺ became the dominant ion. As the slag basicity increased, the fraction of V⁴⁺ increased further, whereas that of V³⁺ decreased to less than that of V⁵⁺. The activity of VO_1.5 was greater than those of the other vanadium oxides, limiting the effect of the slag basicity. Consequently, the valence state of vanadium oxide was affected by the slag basicity at a low oxygen partial pressure by acting as a network modifier. In contrast, at a higher oxygen partial pressure, the activity of vanadium oxide increased further but was not affected by the slag basicity because of its contribution to the network structure formation. The present findings can be applied to optimize the slag composition in steel refining or vanadium pentoxide production processes to increase the yield rate of vanadium.     

Characterization of Ba0.5Sr0.5M1−xFexO3−δ (M = Co and Cu) perovskite oxide cathode materials for intermediate temperature solid oxide fuel cells

Ba_0.5Sr_0.5Co_1?xFe_xO_3?δ (x = 0.2, 0.6, and 0.8) and Ba_0.5Sr_0.5Cu_1?xFe_xO_3?δ (x = 0.6 and 0.8) perovskite oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells. All the samples synthesized by a citrate–EDTA complexing method were single-phase cubic perovskite solid solutions. Then, the thermal expansion coefficient, electrical conductivities, the oxygen vacancy concentrations, the polarization resistances (R_p), and the power densities were measured. An increase in the Co content resulted in a decrease in the polarization resistance, the electrical conductivities at low temperatures, and the inflection point of the thermal expansion coefficient, but it led to an increase in the electrical conductivities at high temperatures, the oxygen vacancy concentrations, and the maximum power densities. The Cu-based system has similar behavior to the Co-based system; yet, in terms of the electrical conductivities, high Cu content gave a better result than low content for the entire range of temperatures.     

Kinetics of sulfur transfer from H2S to digenite (Cu2−xS) at 500°C

The kinetics of sulfur transfer from H₂S to cuprous sulfide (digenite) at 500°C has been established by the resistance relaxation technique. The resistance measurements have been carried out by the van der Pauw method, which uses a four probe configuration. The rate of the forward reaction decreases with the increase in the activity of sulfur in the sulfide (rate a _s ^–0.55 ) while the rate of the backward reaction is found to be nearly independent of the sulfur activity. Based on these results, the rate limiting step for sulfur transfer reaction to digenite is shown to be: H₂S (g) + 2e^– = S^2–(ad) + H₂(g).     

Hydrothermal synthesis of ZrO2–Y2O3 solid solutions at low temperature

Ultrafine powders of ZrO₂–Y₂O₃ solid solutions have been synthesized by hydrothermal treatment at 110°C. Zirconia gel, crystalline Y₂O₃ and various mineralizing solutions have been utilized as precursors for the hydrothermal synthesis. Yttria-stabilized zirconia (YSZ) with different Y₂O₃ content and characterized by different crystallite sizes have been produced by changing the hydrothermal treatment temperature, and the nature and concentration of the mineralizer solution. The role of mineralizer solutions on the crystallization-stabilization of zirconia gel at low temperature of hydrothermal treatment is discussed.     

Gas–liquid–solid reaction system for CO2 electroreduction to formate without using supporting electrolyte

The use of bismuth-based catalysts is promising for formate production by the electroreduction of CO₂ captured from waste streams. However, compared to the extensive research on catalysts, only a few studies have focused on electrochemical reactor performance. Hence, this work studied a continuous-mode gas–liquid–solid reaction system for investigating CO₂ electroreduction to formate using Bi-catalyst-coated membrane electrodes as cathodes. The experimental setup was designed to analyze products obtained in both liquid and gas phases. The influence of relevant variables (e.g., temperature and input water flow) was analyzed, with the thickness of the liquid film formed over the cathode surface being a key parameter affecting system performance. Promising results, including a high formate concentration of 34 g/L with faradaic efficiency for formate of 72%, were achieved.     

Statistical design of experiments for evaluation of Y–Zr–Ti oxides as anode materials in solid oxide fuel cells

Abstract

Mixed conducting anode materials for solid oxide fuel cells are desirable in order to extend the electron transfer reaction zone for fuel gas conversion and to minimise the nickel content for achieving a redox stable anode. Partial substitution by titania in yttria stabilised zirconia (YSZ) is known to increase the electronic conductivity in reducing atmospheres. Nine different compositions were selected from the quasi ternary phase diagram according to principles used in statistical design of experiments covering the whole stoichiometric regime relevant for ionic applications. The dc electrical conductivity values increase strongly with high Ti contents under reducing (Ar–4%H₂) conditions, whereas they decrease continuously with increasing Ti content under oxidising conditions (air). The results clearly show that the chosen screening process for materials selection can considerably reduce the number of samples. For solid oxide fuel cell anodes, the compositions in the YO_1.5–ZrO₂–TiO₂ system should be restricted to low Ti contents.