Showing papers by "Vladislav V. Kharton published in 2015"

Development of bilayer glass-ceramic SOFC sealants via optimizing the chemical composition of glasses—a review

Abstract: Glasses and glass-ceramics (GCs), in particular alkaline-earth alumino silicate-based compositions, are becoming the most common sealing materials for gas-tight sealing applications in solid oxide fuel cells (SOFCs). The present review aims at reporting the systematic procedures put forward developing a novel concept of diopside-based bilayer GC seal, which contains a rigid GC and a self-healing (SH) GC. The concept behind the bilayer GCs is (i) a small gradient in the coefficient of thermal expansion (CTE) will lead to lower thermal expansion mismatch between the sealing layers and other SOFC components, thus providing enhanced mechanical reliability for the stack; and (ii) cracks produced due to minor thermal stresses in the rigid GC layer can be healed by the SH GC layer due to sufficient amorphous content. In general, at high temperature, highly crystallized glass behaves as a rigid glass. On the other hand, due to low viscosity behavior, partially crystallized glass provides a SH behavior. Various glasses in the field of diopside crystalline materials have been systematically designed by varying the chemical composition of glass to achieve desired combination of functional properties for the rigid and SH GC layers. The glass Sr-0.3 where Sr replaced 30 % of Ca was revealed as a highly reliable rigid GC seal for high-temperature electrochemical applications. On the other hand, SH features have been achieved in 30 % Sr-containing diopside-based glass with Gd2O3 for MgO + SiO2 substitution (denoted as Gd-0.3). These GCs exhibit similar thermal properties and excellent thermal stability along a period of 1000 h, while differing in their amorphous fractions, and revealed excellent thermal stability along a period of 1000 h. The bilayered GC synthesized from Sr-0.3 and Gd-0.3 showed good wetting and bonding ability to the SOFC metallic Crofer22APU components. The results revealed superior performance for the newly proposed bilayer GCs in comparison to single-layer sealants.

Performance Optimization of Cermet SOFC Anodes: An Evaluation of Nanostructured NiO

Abstract: The electrochemical performance of the Ni -containing cermet anodes , widely used for all types of SOFCs, i essentially governed by the triple phase boundary (TPB) formed by metal, solid electrolyte and gaseous phase. Although the TPB length and electrode surface area may be drastically increased using nanostructured components, information on the resultant effects in terms of the cermet properties and SOFC production technology is still scarce. The present work is centered on the appraisal of NiO morphologies and optimization of pretreatment conditions for the powders used for anode screenprinting, with commercial nanocrystalline nickel (II) oxide as a model starting material. A series of NiO powders were prepared via anneali g at various temperatures (300 – 1100 C) followed by chemical, structural and morphological characterization. The results of X -ray diffraction (XRD), scanning and transmission electron microscopy (SEM/ TEM), thermogravimetry and Raman spectroscopy reveal coreshell structure of nanosized NiO grains, formed due to surface hydration and oxidation under ambient conditions. Thermally induced desorption, and likely oxidation of organic components of the screen -printing pastes by the hyperstoichiometric oxygen, lead to poor quality of the electrode layers, thus making it necessary to introduce an additional powder preannealing step . The optimum pre-treatment temperature nabling to remove the absorbates, simultaneously preserving submicron grain size in the ele ctrodes, corresponds to approximately 700 C.

Analysis of Interfacial Processes at the SOFC Electrodes by In-Situ Raman Spectroscopy

Abstract: The present work is centered on the developments of a new technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions, combined with electrochemical measurements. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the triple-phase boundary zone by passing the beam through transparent single-crystal solid electrolyte onto the interface, varying current density, temperature and atmosphere over the working electrode. The results of case studies, focused on reduction of Ni-containing cermet anodes, are presented.

Ion transport in dual-phase SrFe1−xТаxO3−δ (x = 0.03 − 0.10): effects of redox cycling

Abstract: The incorporation of tantalum cations in mixed-conducting SrFe1-xTaxO3−δ (x = 0.03 − 0.10) results in the formation of single cubic perovskite-like phases in oxidizing atmospheres while under reducing conditions phase separation is observed, accompanied with an appearance of brownmillerite-type nanodomains on the background of the perovskite-like matrix. For SrFe0.97Ta0.03O3−δ after reduction, the x-ray and electron diffraction studies combined with transmission electron microscopy evidence the formation of approximately 30 vol.% brownmillerite phase with an average domain size of 20–40 nm. The oxygen partial pressure dependencies of the total conductivity in the \( {p}_{{\mathrm{O}}_2} \) range from 10−20 to 0.5 atm at 700–950 °C show that the electron transport parameters remain virtually independent on the dopant content and domain structure. Contrary to the materials with higher dopant content, however, the ion conduction in SrFe0.97Ta0.03O3−δ tends to substantially increase on redox cycling. This behavior was attributed to the brownmillerite domain disintegration and rearrangement, induced by cyclic formation and disappearance of oxygen vacancies.

Interdiffusion and Charge Transport Across Surface-Modified Current Collectors in Planar SOFCs

Abstract: Current collectors are a key component of planar SOFCs, separating air and fuel supplied onto the electrodes and connecting the cells in series. One of main challenges in the SOFC technology development is to suppress degradation processes, often associat ed with the interconnect materials, and to provide low contact resistivity in oxidizing atmospheres. The present work i s focused on the studies of near-surface interdiffusion phenomena in Crofer 22 APU ferritic steel interconnects with Nibased protective layers. Particular emphasis was centered on the area -sp cific resistance (ASR) between the current collectors and La0.8Sr0.2MnO3 (LSM) cathodes , which exhibit time dependencies governed by the protective inter layer composition and interface microstructure alterations. The ASR changes, tested during over 30,000 hours at atmospheric oxygen pressure, can be described in terms of a model assuming that the current across the interconnector | LSM interface is essentially controlled by electron transfer via the interfacial Schottky barrier. The experimental observations validate th is approach, explaining the junction resistivity and Schottky barrier height variations as a result of metal interdiffusion between the current collector and Ni -based protective coatin g.