TU Darmstadt / ULB / TUbiblio

About the Proton Conductivity of BaFeO2.5+δ Epitaxial Thin Films in the Intermediate Temperature Range

Benes, Alexander (2019):
About the Proton Conductivity of BaFeO2.5+δ Epitaxial Thin Films in the Intermediate Temperature Range.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8688],
[Ph.D. Thesis]

Abstract

Reduction of the operating temperature to an intermediate temperature range between 350 °C and 650 °C is a necessity to further increase the competitiveness of Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs) with existing energy conversion technologies. By lowering the operating temperature several high-temperature-related issues can be eliminated leading to lower costs. Motivated by the goal of lower operating temperatures the application of proton-conducting oxides has become an active and broad area of research. The incorporation of these proton-conducting materials entails problems such as ohmic resistances in the electrolyte as well as polarization resistances at the air electrode. To lower the air electrode polarizations, materials, which can be used as effective electrode catalysts on the air electrode, are required to conduct protons and electrons at the same time. Therefore, this thesis focuses on a thorough investigation of the proton conduction in the promising material system BaFeO2.5+δ (BFO). The experiments are conducted on expitaxially grown BaFeO2.5+δ thin films, deposited by pulsed laser deposition on (001)- and (111)-oriented Nb:SrTiO3 substrates. To monitor changes occurring to the thin films, they are examined before and after the electrochemical characterization: This investigation includes the analysis of structural and microstructural information by X-ray diffraction and scanning electron microscopy. Additionally, Mößbauer spectroscopy is used to determine the local coordination and oxidation state of Fe throughout the complete film. For the purpose of accounting for changes to the surface composition the films are furthermore examined using X-ray photoelectron spectroscopy. For the characterization of the conductive properties Electrochemical Impedance Spectroscopy (EIS) is used, yielding a measurable protonic contribution. This protonic contribution can successfully be separated from the total conductivity by comparing measurements in wet and dry atmospheres (Ar or air, respectively). Thereby, the bulk proton conductivity of BFO can be estimated for the first time between 200 °C and 300 °C (3.6 × 10−6 S cm−1 at 300 °C). At temperatures above 300 °C the influence of oxidizing measurement atmosphere and water loss reveals a strong dependence on the conductivity. For the goal of reducing the operating temperature to the intermediate temperature range it is not only important to find and employ well-suited electrode catalysts but furthermore essential to reduce ohmic resistances in the electrolyte. For this reason the search for possible deposition techniques, which enable the deposition of the respective proton conductors according to the required attributes, is subject of research efforts. Based on these grounds two deposition methods, which to date have not been used to synthesize Y-doped BaZrO3 (one of the most promising proton conductors) thin films, were investigated in order to assess their potential of depositing high-quality films. In detail Laser-Assisted Chemical Vapor Deposition (LA-CVD) and Aerosol-Assisted Chemical Vapor Deposition (AA-CVD) are used for this purpose. The deposition parameters of the films are varied aiming to obtain stoichiometric smooth films with perovskite type structure. Both deposition methods present problems with relation to the required film attributes. The identified obstacles, for which solutions/workarounds are suggested, are based on thermodynamical as well as experimental facts.

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Benes, Alexander
Title: About the Proton Conductivity of BaFeO2.5+δ Epitaxial Thin Films in the Intermediate Temperature Range
Language: English
Abstract:

Reduction of the operating temperature to an intermediate temperature range between 350 °C and 650 °C is a necessity to further increase the competitiveness of Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs) with existing energy conversion technologies. By lowering the operating temperature several high-temperature-related issues can be eliminated leading to lower costs. Motivated by the goal of lower operating temperatures the application of proton-conducting oxides has become an active and broad area of research. The incorporation of these proton-conducting materials entails problems such as ohmic resistances in the electrolyte as well as polarization resistances at the air electrode. To lower the air electrode polarizations, materials, which can be used as effective electrode catalysts on the air electrode, are required to conduct protons and electrons at the same time. Therefore, this thesis focuses on a thorough investigation of the proton conduction in the promising material system BaFeO2.5+δ (BFO). The experiments are conducted on expitaxially grown BaFeO2.5+δ thin films, deposited by pulsed laser deposition on (001)- and (111)-oriented Nb:SrTiO3 substrates. To monitor changes occurring to the thin films, they are examined before and after the electrochemical characterization: This investigation includes the analysis of structural and microstructural information by X-ray diffraction and scanning electron microscopy. Additionally, Mößbauer spectroscopy is used to determine the local coordination and oxidation state of Fe throughout the complete film. For the purpose of accounting for changes to the surface composition the films are furthermore examined using X-ray photoelectron spectroscopy. For the characterization of the conductive properties Electrochemical Impedance Spectroscopy (EIS) is used, yielding a measurable protonic contribution. This protonic contribution can successfully be separated from the total conductivity by comparing measurements in wet and dry atmospheres (Ar or air, respectively). Thereby, the bulk proton conductivity of BFO can be estimated for the first time between 200 °C and 300 °C (3.6 × 10−6 S cm−1 at 300 °C). At temperatures above 300 °C the influence of oxidizing measurement atmosphere and water loss reveals a strong dependence on the conductivity. For the goal of reducing the operating temperature to the intermediate temperature range it is not only important to find and employ well-suited electrode catalysts but furthermore essential to reduce ohmic resistances in the electrolyte. For this reason the search for possible deposition techniques, which enable the deposition of the respective proton conductors according to the required attributes, is subject of research efforts. Based on these grounds two deposition methods, which to date have not been used to synthesize Y-doped BaZrO3 (one of the most promising proton conductors) thin films, were investigated in order to assess their potential of depositing high-quality films. In detail Laser-Assisted Chemical Vapor Deposition (LA-CVD) and Aerosol-Assisted Chemical Vapor Deposition (AA-CVD) are used for this purpose. The deposition parameters of the films are varied aiming to obtain stoichiometric smooth films with perovskite type structure. Both deposition methods present problems with relation to the required film attributes. The identified obstacles, for which solutions/workarounds are suggested, are based on thermodynamical as well as experimental facts.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Fachgebiet Materialdesign durch Synthese
11 Department of Materials and Earth Sciences > Material Science > Joint Research Laboratory Nanomaterials
Date Deposited: 23 Jun 2019 19:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8688
URN: urn:nbn:de:tuda-tuprints-86889
Referees: Clemens, Prof. Dr. Oliver and Albert, Prof. Dr. Barbara
Refereed / Verteidigung / mdl. Prüfung: 2 May 2019
Alternative Abstract:
Alternative abstract Language
Die Reduzierung der Betriebstemperatur in einen intermediären Temperaturbereich zwischen 350 °C und 650 °C ist Voraussetzung, um die Wettbewerbsfähigkeit von Festoxidbrennstoff- und Festoxidelektrolysezellen gegenüber anderen Energiewandlungstechniken zu erhöhen. Durch das Senken der Temperatur können Probleme, die durch hohe Temperaturen verursacht werden, vermieden werden, was zu einer Verringerung der Kosten führt. Der durch das Ziel sinkender Betriebstemperaturen motivierte Einsatz von protonenleitenden Oxiden ist somit zu einem aktiven und breiten Forschungsfeld herangewachsen. Beim Einsatz dieser protonenleitenden Materialien ist neben im Elektrolytmaterial auftretenden ohmschen Widerständen vor allem der an der Luftelektrode auftretende Polarisationswiderstand problematisch. Um diese Polarisation zu verringern müssen Materialien, die als effektive Elektrodenkatalysatoren eingesetzt werden können, sowohl elektronische Leitfähigkeit als auch eine ausreichende Protonenleitfähigkeit aufweisen. Aus diesem Grund widmet sich diese Arbeit einer umfassenden Untersuchung der Protonenleitfähigkeit am vielversprechenden Materialsystem BaFeO2.5+δ (BFO). Die Experimente werden an epitaktisch gewachsenen BaFeO2.5+δ Dünnschichten, welche per Laserstrahlverdampfen (Pulsed Laser Deposition) auf (001) und (111)-orientierte Nb:SrTiO3 Substrate abgeschieden sind, durchgeführt. Um Veränderungen an den Dünnschichten zu beobachten, werden die Filme sowohl vor als auch nach der elektrochemischen Charakterisierung eingehend untersucht: Dazu gehört die Auswertung struktureller und mikrostruktureller Informationen mittels Röntgenbeugung und Rasterelektronenmikroskopie. Zusätzlich wird mittels Mößbauer Spektroskopie die lokale Umgebung und der Oxidationszustand von Fe über das komplette Volumen des Films untersucht. Um auch Änderungen der Zusammensetzung der Oberfläche zu verfolgen wird weiterhin Röntgenphotoelektronenspektroskopie eingesetzt. Für die Charakterisierung der Leitfähigkeit wird die elektrochemische Impedanzspektroskopie verwendet, wobei ein rein protonischer Anteil messbar ist. Dieser protonische Anteil kann erfolgreich von der Gesamtleitfähigkeit separiert werden, indem die Messungen in feuchten und trockenen Atmosphären (Ar oder Luft) verglichen werden. Dadurch ist es zum ersten Mal möglich, eine Volumenprotonenleitfähigkeit von BFO zwischen 200 °C und 300 °C (3.6 x 10-6 Scm-1) abzuschätzen. Bei Temperaturen über 300 °C zeigt die Leitfähigkeit einen starken Einfluss von oxidierender Atmosphäre sowie Wasserverlust. Für das Ziel der Reduzierung der Betriebstemperatur in einen intermediären Temperaturbereich ist es nicht nur wichtig, gut geeignete Elektrodenkatalysatoren einzusetzen, sondern weiterhin auch die ohmschen Widerstände im Elektrolytmaterial gering zu halten. Aus diesem Grund ist die Suche nach möglichen Abscheidetechniken, die es ermöglichen, dünne Schichten der entsprechenden Protonenleiter mit dem benötigten Anforderungsprofil abzuscheiden, Gegenstand der aktuellen Forschung. Darauf aufbauend wurden in dieser Arbeit zusätzlich zwei Dünnschichtabscheidemethoden, die bisher nicht für Y-dotiertes BaZrO3 (einer der vielversprechendsten Protonenleiter) verwendet wurden, auf ihr Potential hin untersucht, hochwertige Filme abzuscheiden. Hierzu werden Laser-gestützte Chemische Gasphasenabscheidung (Laser-Assisted Chemical Vapor Deposition) und Aerosol-gestützte Chemische Gasphasenabscheidung eingesetzt. Die Abscheideparameter werden variiert um stöchiometrische und glatte Filme in der Perovskitstruktur zu erhalten. Beide Abscheidetechniken weisen Probleme bei der Erfüllung der geforderten Filmcharakteristika auf. Die identifizierten Problematiken der Herstellungsrouten, für welche abschließend Abhilfen bzw. Lösungsansätze vorgeschlagen werden, beruhen sowohl auf thermo-dynamischen als auch auf experimentellen Gegebenheiten.German
Export:

Optionen (nur für Redakteure)

View Item View Item