Brennstoffzellen-Allianz Baden-Württemberg: Forschungsaversum 2009 - Entwicklung einer Kompositschicht mit hoher Ionenleitfähigkeit für Metallsubstrat getragene Festelektrolytbrennstoffzellen (SOFCs) im Temperaturbereich von 600-800 °C

Ziel ist die Entwicklung eines alternativen Elektrolytmaterials (YSZ/ScSZ-Komposits) für SOFCs im abgesenkten Temperaturbereich unter Berücksichti-gung der Perkolationstheorie. Der neue Komposit-Elektrolyt soll den Vorteil des reinen ScSZ-Materials, eine hohe ionische Leitfähigkeit im Temperaturbereich zwischen 600-800 °C, gepaart mit den Kostenvorteilen des Standardmaterials YSZ besitzen

The Solution Precursor Plasma Spraying Process for Making Zirconia Based Electrolytes

Ceramic layers, such as yttria-stabilized zirconia or scandia-stabilized zirconia, used for functional layers of solid oxide fuel cells, i.e. the gas tight oxygen ion conductive electrolyte or as ceramic component in the porous cermet anode, were obtained by the Solution Precursor Plasma Spray (SPPS) process. The influence of different solvent types on microstructure was analyzed by comparison of coatings sprayed with water-based solution to ethanol-based one. Use of solvent with low surface tension and low boiling point enhances splat formation, coating micro-structure and crystalline structure. Parameter adjustment to receive coatings from nitrate solutions with ethanol as solvent was carried out. Results of Raman spectroscopy indicate that an intermediate of both nitrates (zirconyl and scandium nitrate hydrate) was deposited

Sprayed and Constrained-Sintered Zirconia Based Electrolytes

The current development in solid oxide fuel cells (SOFCs) is focused on reducing the operating temperature below 800 °C. Though, the reduced operating temperature promotes durability of cells and decreases stringent demands on peripheral components, the ionic conductivity of electrolytes decreases following Arrhenius law. To solve this problem two different ways are possible: a) reducing the thickness of the conventionally used yttria-stabilized zirconia (YSZ) electrolyte by using nanostructured particles as feedstock or b) by using an electrolyte with improved ionic conductivity for intermediate temperature (IT)-SOFCs. Conventional sintering of electrolytes is performed over several hours at temperatures above 1400 °C. In this paper, plasma sprayed YSZ electrolyte layers were sintered under constraint and non-constraint conditions in the temperature range of 800 to 1520 °C. Thereby, the influence of particle size on sintering kinetics and microstructure development was analysed. By comparison of nanostructured YSZ and conventional YSZ layers, differences in the sintering rate were determined. Lower dL/L0 of nanostructured compared to conventional plasma sprayed YSZ during heating above 900 °C was measured, indicating initiation of densification at lower temperature compared to conventional YSZ. A higher shrinkage rate of the nanostructured YSZ layer strongly suggests that observed differences in sintering rates are due to different particle sizes of the powder feedstock. Experimental shrinkage rates were assigned to different mass transfer effects according to the sintering model of Coble. It was observed that the sintering of free-standing coatings differ from that of coatings on substrates which was explained by theory of constrained sintering. Changes of the microstructural characteristics were identified using scanning electron microscopy. According to the constraint from the substrate, the YSZnano sample constrained sintered at 1000 °C has achieved a lower thickness compared to the free-standing sample sintered at 1520 °C. Comparing the values of both YSZ electrolytes sintered at 1325 °C to the ones of the as-sprayed layers, a faster sintering process of the nanostructured YSZ can be supposed. The lamellar microstructure in the as-sprayed samples was found to significantly reduce the electrical conductivity of the YSZ electrolytes, measured by 4-point dc method. Sintering of as-sprayed electrolyte layers at sufficient temperature increased the electrical conductivity, due to changes of the microstructure densification

Investigation of Plasma-Sprayed Zirconia-Based Electrolytes

Solid oxide fuel cells (SOFC) operating in the temperature range between 800–1000 °C are devices converting directly chemical energy into electrical energy. The SOFC electrolyte layer typically consisting of yttria-stabilized zirconia (YSZ) was prepared using atmospheric plasma spraying technology. Plasma spraying is a cost-effective technique for the production of functional layers in SOFCs. Properties, such as microstructure, conductivity of YSZ electrolyte layers were investigated by Scanning electron microscopy (SEM), X-ray diffraction (XRD), 4-point dc method, and mercury intrusion porosimetry and Raman spectroscopy. Raman spectroscopy is a powerful tool for the investigation of structural features, for example, crystallinity, molecular orientation, and phase composition, especially of inorganic thin films. In this study we show that depending on the preparation conditions the crystal growth and the density of the plasma-sprayed thin films can be influenced significantly. Therefore Raman spectra as well as XRD and SEM pictures show subtle differences concerning the crystallinity of various sample

Entwicklung von Funktionsschichten für die Festoxidbrennstoffzelle

Brennstoffzellen sind elektrochemische Energiewandler, welche die chemische Energie eines Brennstoffes direkt in elektrische Energie umwandeln. Die verschiedenen Brennstoffzellen-Typen können nach dem eingesetztem Elektrolyten und der Arbeitstemperatur (Niedertemperatur- bzw. Hochtemperatur-Brennstoffzelle) klassifiziert werden. Zu den Hochtemperatur-Brennstoffzellen zählt die Festoxidbrennstoffzelle (SOFC: Solid Oxide Fuel Cell). Die gegenwärtige Entwicklung in Festoxidbrennstoffzellen zielt auf die Verringerung der Betriebstemperatur auf deutlich unter 800 °C ab. Beim bisher in den meisten Anwendungen als Standardmaterial für Elektrolytschichten eingesetzten Yttriumoxid stabilisierten Zirkonoxids (YSZ) sinkt jedoch durch das Herabsetzen der Betriebstemperatur die Leitfähigkeit gemäß dem Arrhenius-Gesetz. Um dies zu umgehen, kann entweder die Dicke der Elektrolytschicht durch Verwendung von Nanomaterialien heruntergesetzt oder ein anderes Material als Elektrolyt verwendet werden. Vorgestellt werden hier Untersuchungen zum Sinterverhalten von konventionellem und nanostrukturiertem YSZ in einem Temperaturbereich von 800 °C bis 1520 °C. Dabei ist zu berücksichtigen, dass das vorhandene Substrat das Sinterverhalten beeinflussen kann. Folglich wurden Plasma gesprühte Elektrolytschichten ohne Substrat (frei) und mit Substrat (constrained) gesintert. Die Bestimmung der thermischen Längenänderung von freien Elektrolytschichten erfolgte in einem Dilatometer. Charakterisiert wurden die gesprühten und gesinterten Schichten mittels REM, Hg-Porosimetrie und elektrischen Leitfähigkeitsmessungen

Stand der Entwicklung der Festoxidbrennstoffzelle am DLR

Brennstoffzellen sind elektrochemische Energiewandler, welche die chemische Energie eines Brennstoffes direkt in elektrische Energie umwandeln. Die verschiedenen Brennstoffzellen-Typen können nach dem eingesetztem Elektrolyten und der Arbeitstemperatur (Niedertempe-ratur- bzw. Hochtemperatur-Brennstoffzelle) klassifiziert werden. Zu den Hochtemperatur-Brennstoffzellen zählt die Festoxidbrennstoffzelle (SOFC: Solid Oxide Fuel Cell). Voraussetzung für die Markteinführung der SOFC ist, wie bei anderen Brennstoffzellentypen, eine wesentlich kostengünstigere Herstellung und eine hinreichende Lebensdauer. Das Institut für Technische Thermodynamik am DLR (Deutschen Zentrum für Luft- und Raumfahrt) stützt sich bei der Verfolgung dieser Ziele auf plasmaspritztechnische Methoden und Verfah-ren, insbesondere auf das Vakuumplasmaspritz-Verfahren (VPS). Im Gegensatz zu gesinter-ten Zellen, bei der eine der drei Funktionsschichten der Zelle auch die mechanische Stabilität der Zelle garantiert, wird dieses beim DLR-Verfahren von einem porösen metallischen Trä-gersubstrat übernommen, auf dem die Zelle in einem Folgebeschichtungsprozess aufgetragen wird. Dadurch ist es möglich, alle drei Schichten (Anode, Elektrolyt und Kathode) sehr dünn auszuführen mit Schichtdicken von jeweils 30-50 µm. Die Entwicklung der einzelnen Kom-ponenten der SOFC, die in den letzten Jahren zu einer erheblichen Verbesserung der Leistungsfähigkeit geführt hat, wird vorgestellt

Brennstoffzellen-Allianz Baden Württemberg: Forschungsaversum 2009 - Entwicklung einer Komposit- Elektrolytschicht mit hoher Ionenleitfähigkeit für Metallsubstrat getragene Festelektrolytbrennstoffzellen (SOFCs) im Temperaturbereich von 600-800 °C

Ermittlung des idealen Mischungsverhältnisses eines Komposit-Elektrolyts o Ermittlung einer Sinterroute für Komposit-Elektrolyte aus Scandiumoxid stabilisiertem Zirkonoxid (ScSZ) / Yttriumoxid stabilisiertem Zirkonoxid (YSZ) o Ermittlung des Beitrags der Bulk (Volumen)- / und Korngrößenleitfähigkeit mittels elektrochemischer Impedanzspektroskopie (EIS) o Analyse mittels der Methoden Raster-Elektronenmikroskopie (REM), Energiedispersive Röntgenspektroskopie (EDX), Röntgen-Diffraktometrie (XRD) - Ermittlung des Sinterverhaltens Plasma gespritzter YSZ-Schichten mittels 4-Punkt-Messung der Leitfähigket o Analyse mittels Elektronen-Rückstreu-Diffraktometrie (EBSD) o Analyse mittels der Methoden REM, EDX, XRD - Prozessanpassung und Parameterentwicklung für das Solution Precursor Plasma Spraying (SPPS) o Analyse mittels der Methoden REM, EDX, XRD, Ramanspektroskopie

Constrained and non-constrained sintering of plasma-sprayed zirconia based electrolytes for SOFCs

The current development in SOFCs is focused on reducing the operating temperature below 800°C. The reduced operating temperature promotes durability of cells and decreases stringent demands on peripheral components. However, with lower temperature the ionic conductivity of electrolytes decreases. Thickness reduction of the conventionally used yttria-stabilized zirconia electrolyte by using nanostructured particles as feedstock is a possibility to avoid this problem. Another possibility is the use of an electrolyte with improved ionic conductivity for intermediate temperature SOFCs, e.g. scandia-stabilized zirconia. Within this work zirconia based electrolyte layers were deposited on metal substrates using plasma spraying. As all thermal sprayed coatings contain some porosity, which influences the cell performance, the sprayed electrolyte layers were sintered in a second step. Plasma-sprayed coatings were under compressive stresses. In order to achieve a better understanding of differences in the sintering behaviour, plasma sprayed layers were sintered under constrained and non-constrained conditions in the temperature range of 800 to 1520 °C and characterised. Sintering properties, microstructure, and conductivity of sprayed and sintered electrolyte layers were investigated

XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells

In electrodes of low temperature fuel cells like polymer electrolyte membrane fuel cells (PEFC) or alkaline fuel cells (AFC) the reactants and the water must be transported. For this purpose the pore system in the electrodes needs a hydrophilic character for the transport of the water and a hydrophobic character for the transport of the gases. The degree of the hydrophobicity determines whether the pore system will be flooded by the reaction water. In the case of PEFC, this is also determined by the degree of the required humidification of the reaction gases. In AFC hydrophobicity determines the extension of the three-phase reaction zone. Caused by the strong influence of hydrophobicity on the transport processes, the electrochemical performance and the optimized operation conditions are also affected by hydrophobicity. Typically polytetrafluoro-ethylene (PTFE) is used to make the electrodes hydrophobic, because PTFE has a high chemical stability. Hydrophobicity depends on the concentration of PTFE on the electrode surface. The PTFE concentration, which is related to the hydrophobic character, can be determined by XPS. The changes in the PTFE content and structure of the electrode of a PEFC was investigated by cyclic voltammetry and XPS and correlated with the performance of the cell in long-term operation. With both methods an initial significant increase in free and electrochemically active surface platinum area is observed. This activation is associated with a degradation of the PTFE in the electrode which is responsible for the hydrophobic properties of the electrode. With further operation the performance of the cell decreases because the water management becomes more critical. Generally, it is shown that XPS can be used for the investigation of the hydrophobicity of electrodes prepared by various manufacturing techniques as well as of changes in their hydrophobicity induced by the electrochemical operation