Towards quantitative Low Energy Ion Scattering on CaSiO3_3 from Comparison to Multiple-Scattering-Resolved Dynamical Binary Collision Approximation Simulations

We perform Low Energy Ion Scattering with 1\,keV He ions on CaSiO$_3$ using a commercial electrostatic detector system and determine the charge fraction of scattered ions from comparison with Binary Collision Approximation simulations. The simulations take dynamical surface changes due to surface cleaning Ar sputtering into account and scattered He particles are separated into single, dual, and multiple scattering trajectories. We find that the charge fraction of single and dual scattered He is about 10 times higher than the one for multiple collisions. Our results show that quantitative concentration profiles can be inferred from this method, if the charge fraction components are determined first

Understanding oxygen anionic-electronic defects under high electric fields: Resistive switches devices

Nanoscale resistive switches (ReRAMs) were recently proposed as new class of non-volatile memories by switching non-linearly between low- and high-resistance values through application of voltage pulses in the ns-range. Through this paper we firstly introduce the topic of resistive switching oxides under high electric fields, their charge transport mechanism and often named memristive characteristics; and critically address open questions. In the second part we turn, to innovative new approaches in making of doped oxides and interface designs to novel device structures for oxide-based switches based on own results: Here, we will firstly discuss a mixed anionic electronic conductor model experiment, being a Gd-doped ceria series with tuned doping concentration to affect the defect association and mobility of the oxide switching bits in a systematic manner. We find a clear correlation between concentration and mobility of oxygen ionic carriers and resistive switching response, and discuss those down to the changes in the near order structures connected therein. Secondly, we exemplify the switching characteristics based on either compressively or tensely strained Gd0.1Ce0.9O2-x heterostructures modulated by Er2O3 or Sm2O3 layers, respectively, and discuss directly the device implication. Thereby, we present a new type of a model material device concept entitled a strained ReRAM . Here, new material engineering of oxides beyond doping is discussed to control resistive switching device properties like retention, Roff/Ron ratios and power consumption by interfacial strain engineering of mixed conducting oxide . Thirdly, we grow nanoscopically-flat LaFeO3 switching bits and demonstrate in a model experiment for amorphous and epitaxially grown films the implication of grain-boundary free but varying defect levels of the structures on resistive switching. Fourthly, we turn to the role of electric field and frequency dependencies of SrTiO3-based ReRAMs. Here, electrochemical impedance spectroscopy, cyclic voltammetry and chronoamperometry are used to investigate optimum operation concerning fast switching and stable retention with high resistance modulation. We show that two different switching mechanisms can be individually addressed depending on electric field strength and switching times. The Memristor-based Cottrell analysis is used to successfully determine diffusion constant characteristics of the materials and separating capacitive and memristive contributions. Finally, we conclude on the role of oxygen anionic-electronic carriers and transfer for oxide-based switches, and discuss the applicability for bits and circuits of potential memory and logic applications. References S. Schweiger, M. Kubicek, F. Messerschmitt, C. Murer, J.L.M. Rupp, ACS Nano, 8, 5, 5032, 2014. F. Messerschmitt, M. Kubicek, S. Schweiger, J.L.M. Rupp, Adv. Funct. Mater. 24, 47, 7448, 2014. F. Messerschmitt, M. Kubicek, J.L.M. Rupp, Adv. Funct. Mater. 25, 32, 5117, 2015. M. Kubicek, R. Schmitt, F. Messerschmitt, J.L.M. Rupp ACS Nano, 9, 11, 10737, 201

The Chemical Evolution of the La0.6Sr0.4CoO3−δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

© The Author(s) 2018Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)212921411

Mechanisms of electrochemical oxygen exchange on mixed conducting model electrodes in H2-H2O atmosphere

Zusammenfassung in deutscher SpracheSolid oxide cells are on the verge of commercialization, but still much research is attributed to optimizing long term stability, for example by decreasing the operating temperature significantly below 800°C. For this goal, new electrode materials with high activity for oxygen exchange at intermediate temperatures are searched for. Mixed ionic and electronic conductors (MIECs) are a promising class of electrode materials, because all species (oxygen ions, electrons and atmospheric oxygen) necessary for electrochemical oxygen exchange are accessible on the entire surface area. While already much research effort was dedicated to the investigation of mixed conducting oxygen electrodes, fundamental investigations of the fuel side electrodes are rather scarce and only ceria-based materials have been studied on a detailed mechanistic level. In this thesis, mixed conducting perovskite-type electrode materials are in the focus of research. The main goal was the development of methods and models to characterize mixed conducting acceptor doped perovskite-type materials in H2+H2O atmospheres. Electrochemical impedance spectroscopy (EIS) was used to quantify the area-specific resistance of the surface reaction, as well as electronic and ionic conductivity of mixed conductors. Additional analytic methods, such as ambient pressure X ray photoelectron spectroscopy (AP-XPS) and 18O isotope exchange with subsequent depth profiling were employed to gain a better insight into the mechanisms of electrochemical oxygen exchange at the surface. For the impedance measurements, thin film model electrodes were deposited on YSZ substrates. As an experimental challenge, the electronic conductivity of acceptor-doped oxides in H2+H2O atmosphere is typically rather low, resulting in a complex interplay of electrochemical reactions and in-plane charge transport, which complicates the interpretation of impedance spectra. Therefore, a novel electrode design was developed, consisting of a mixed conducting thin film and metallic current collectors. Two interdigitating metallic current collectors are placed in a microelectrode, which allows in-plane measurements between the current collectors as well as electrochemical measurements versus a large counter electrode. Equivalent circuit models for quantifying the spectra of both measurement modes were developed and applied to simultaneously fit both spectra, using the same parameter set. In this manner, electronic conductivity as well as the area-specific resistance of the surface reaction and the chemical capacitance can be determined on a single microelectrode. This method together with in-plane measurements on microelectrodes on MgO substrates was employed to investigate the effect of the Fe content in SrTix1-FexO¬3 - on the electronic and ionic conductivity as well as the area specific resistance of the surface oxygen exchange reaction. The derived equivalent circuit models predict that the electrochemically active zone in weakly electron conducting materials the surface chemistry in operating conditions was gained by ambient pressure XPS investigation of model cells with a thin film working electrodes of La0.6Sr0.4CoO3-- (LSC), La0.6Sr0.4FeO3-- (LSF) or SrTi0.7Fe0.3O3-- (STF). This enabled very surface-sensitive chemical ais near to technological operating conditions. The cells were investigated during well-defined electrochemical polarization in oxidizing (O2) and in reducing (H2+H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization caused reversible shifts in the measured binding energy. These shifts can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2+H2O. Cathodic polarization in reducing atmosphere leads to the evolution of Fe0 particles that can be reversibly oxidized and reduced by very small applied potentials. The current-voltage characteristics are very steep when metallic Fe is present and rather shallow without the presence of metallic Fe, which indicates a high catalytic activity of the formed Fe particles. The evolution of metallic Fe could be reproduced in a lab-based UHV XPS analyzer, which proves that the electrode bulk oxygen partial pressure is more decisive than the atmospheric conditions.Festoxidbrennstoffzellen sind eine vielversprechende Technologie der Stromerzeugung und auf dem Weg zur kommerziellen Produktion. Trotzdem ist noch viel Forschungsarbeit zur Verbesserung der Langzeitstabilität, zum Beispiel durch Reduktion der Betriebstemperatur weit unter 800°C, notwendig. Für dieses Ziel werden neue Elektrodenmaterialien mit hoher Sauerstoffaustauschrate gesucht. Gemischte Elektronen- und Ionen leiter sind eine vielversprechende Klasse von Materialien für diesen Zweck, da die Sauerstoffaustauschreaktion auf der gesamten Elektrodenoberfläche stattfinden kann. Gemischte Leiter werden derzeit vorrangig als Sauerstoffelektroden untersucht, während mechanistische Studien gemischter Leiter für brennstoffseitige Elektroden derzeit fast ausschließlich für Ceroxid-basierte Materialien verfügbar sind. Die elektrochemischen Eigenschaften von gemischt leitenden Materialien mit Perowskit-Struktur sind der Fokus dieser Dissertation. Ein Hauptziel ist die Entwicklung von Methoden und Modellen, um gemischt leitende Materialien in H2+H2O Atmosphäre zu untersuchen. Mit Elektrochemischer Impedanzspektroskopie wurde die elektronische und ionische Leitfähigkeit, sowie der flächenbezogene Widerstand der elektrochemischen Sauerstoffaustauschreaktion gemischter Leiter bestimmt. Mit in-operando Photoelektronen Spektroskopie und 18O Isotopenaustausch Experimente wurde die Sauerstoffaustauschreaktion weiter untersucht. Die Impedanzmessungen wurden an Dünnfilmelektroden auf Yttrium-stabilisierten Zirkonoxidsubstraten durchgeführt. Die vergleichsweise geringe elektronische Leitfähigkeit gemischter Leiter wie SrTi1-xFexO3-- und La0.6Sr0.4FeO3-- in H2+H2O Atmosphäre stellte eine experimentelle Herausforderung dar, welche durch das Hinzufügen metallischer Stromsammler gelöst werden konnte. Ein neues Design von Mikroelektroden wurde entwickelt, bei welchem jede Elektrode zwei ineinandergreifende Stromsammler enthält. Diese Geometrie ermöglicht -in-plane- Kontaktierung, bei welcher die Impedanz zwischen den Stromsammlern innerhalb einer Elektrode gemessen wird, und -elektrochemische- Kontaktierung, bei welcher die Mikroelektrode gegen eine makroskopische Gegenelektrode kontaktiert wird. Für beide Kontaktmodi wurden Ersatzschaltbilder entwickelt, welche gleichzeitig mit nur einem Parameterset gefittet werden. Durch diese Methode können elektronische und ionische Leitfähigkeit, sowie der flächenbezogene Widerstand der Sauerstoffaustauschreaktion an nur einer Mikroelektrode bestimmt werden. Diese Methode wurde zusammen mit -in-plane- Messungen an Mikroelektroden auf isolierenden MgO Substraten zur Untersuchung des Einflusses der Eisendotierung in SrTixFe1 xO3-- Dünnfilmen auf die elektronische und ionische Leitfähigkeit, sowie den Widerstand der Sauerstoffaustauschreaktion eingesetzt. Laut dem Ersatzschaltbild für gemischte Leiter mit geringer über die Oberflächenchemie konnte durch in-operando Photoelektronenspektroskopie an elektrochemischen Modellzellen mit La0.6Sr0.4CoO¬3 - (LSC), La0.6Sr0.4FeO¬3 - (LSF) oder SrTi0.7Fe0.3O¬3 - (STF) Dünnfilmelektroden gewonnen werden. Die Oberflächenchemie wurde während definierter elektrochemischer Polarisation der Elektroden in oxidierender (O2) und reduzierender (H2+H2O) Atmosphäre untersucht. Ändern des Sauerstoffpartialdruckes in den Elektroden durch Wechsel der Atmosphäre sowie durch elektrochemische Polarisation verursachten reversible Änderungen der Bindungsenergie aller beobachteten Peaks. Diese Änderungen konnten auf eine Verschiebung der Fermienergie in Abhängigkeit vom Sauerstoffpartialdruck zurückgeführt werden. Der Oxidationszustand von oberflächennahem Eisen hängt vom Sauerstoffpartialdruck in der Elektrode ab. In oxidierender Atmosphäre ist Eisen hauptsächlich als Fe3+ vorhanden, während in reduzierender Atmosphäre Fe2+ und Fe3+ koexistieren. Kathodische Polarisation in reduzierender Atmosphäre führt sogar zur reversiblen Bildung von metallischem Eisen. Dieses bildet Nanopartikel an der Oberfläche, welche die Aktivität für H2O Elektrolyse stark verbessern. Die Bildung von Fe0 Nanopartikeln konnte auch in einem labor-basierten UHV Spektrometer nachgewiesen werden, was nahelegt, dass der Sauerstoffpartialdruck in der Elektrode entscheidender ist als die Atmosphäre.15

Electrochemical reactions and transport paths of SrTi0.7Fe0.3O3 thin film model electrodes in H2-H2O atmosphere

Zsfassung in dt. SpracheSrTi1-xFexO3 (STFO) ist ein gemischter Leiter in Perovskitstruktur mit ge-ringer elektronischer Leitf ahigkeit, verglichen mit anderen häufig untersuchten gemischten Leitern (z.B.Sr-dotiertes LaMnO3 (LSM) oder LaCoO3 (LSC)). In Sauerstoffatmosphäre ist die Elektrodenkinetik jedoch für ei-ne potentielle Anwendbarkeit als SOFC (Solid Oxide Fuel Cell) Kathode ausreichend. Da STFO auch in reduzierender Atmosphäre hohe chem-sche Stabilität aufweist, ist es prinzipiell auch als Anodenmaterial verwend-bar. Für eine mögliche Anwendung sind hohe elektronische und ionische Leitfähigkeit, sowie eine katalytisch aktive Oberfläche und geringe Degradation entscheidende Parameter. Um diese Parameter zu bestimmen, wurden Dünnschicht-Modellelektroden mit definierter Geometrie und Oberfläche un-tersucht. Dünnfilme aus SrTi0.7Fe0.3O3 wurden mit Pulsed Laser Deposition auf einkristallinen Yttrium-stabilisierten Zirkoniumoxid Substraten abgeschieden, und anschließend photolithographisch mikrostrukturiert. Ein Dünnfilm Platin-Gitter wurde ober- oder unterhalb der Elektroden aufgetragen, um eine ausreichende Verteilung der Elektronen, trotz geringer elektro-nischer Leitfähigkeit, zu gewährleisten. In H2-H2O - Atmosphäre wurden die elektrochemischen Eigenschaften und Defektkonzentrationen dieser Mikro-elektroden mit Impedanzspektroskopie und Gleichstrommessungen untersucht. Im thermodynamischen Gleichgewicht ist die elektrochemische Wasserspaltungsreaktion ratenbestimmend. Eine für die Untersuchung von Mi-kroelektroden optimiertes Design von Metallgitter und Mikroelektroden mit zwei getrennten Metallstrukturen pro Elektrode wurde verwendet. Mit dieser Elektrodengeometrie können elektronische und ionische Leitfähigkeit, sowie die Reaktionsrate der Wasserspaltungsreaktion, und die chemische Kapazität an nur einer Mikroelektrode bestimmt werden.SrTi1-xFexO3 is a perovskite type mixed conductor with relatively low elec-tronic conductivity, compared to other mixed conducting electrode materi-als such as Sr-doped LaMnO3 (LSM) or LaCoO3 (LSC). Nevertheless, it shows a highly catalytic surface for the Solid Oxide Fuel Cell (SOFC) cathode reaction. Its high stability under reducing conditions makes it also principally applicable as a SOFC anode or SOEC cathode. For potential practical application, high electrochemical surface ctivity, as well as elec-tronic and ionic conductivity, chemical stability and low degradation are crucial. In order to investigate these properties, geometrically well defined thin lm microelectrodes of SrTi0.7Fe0.3O3 have been deposited on single crystalline yttria-stabilized zirconia substrates via pulsed laser depositionand photolithographic techniques. To achieve sufficient lateral electronic transport within the film, a thin film platinum grid was deposited on top or beneath the electrode. The electro- and defect chemical properties of these microelectrodes were investigated by means of impedance and DC-bias measurements as function of temperature in H2-H2O atmosphere. Under equilibrium conditions, the electrode resistance is dominated by the electrochemical surface reaction. By applying a specific electrode and metal grid design with two separated metal structures on one microelectrode, the surface reaction rate, electronic and ionic conductivity as well as chemical capacitance can be identified on a single microelectrode. The results are discussed in terms of electronic and ionic defect models in solids10

Electrochemical Stability Window and Electrolyte Breakdown Mechanisms of Lithium Lanthanum Titanate

Perovskite-type La$_{0.57}$Li$_{0.29}$TiO$_3$ (LLTO) is a promising solid electrolyte material with high Li-ion conductivity. However, its experimental electrochemical stability window is not precisely known, and thus the compatibility with potential electrode materials is partly unclear. In this contribution, we present results from electrochemical and analytical experiments to elucidate the stability of LLTO when being polarized with Li-ion-blocking Pt electrodes. Above 2.5 V, a darkened color front starts moving from the cathode to the anode, leading to electrolyte degradation. While first-principles calculations predict the appearance of new phases as decomposition products, we find zones with modified defect chemical properties originating from the anode and cathode. The darkened zone forming at the cathode contains Ti$^{3+}$ polarons with high mobility, which leads to a mixed ion-electron conductivity, already for a very small Li excess concentration. Next to the anode a spatially very confined, weakly conductive Li depletion zone forms. The spatially confined but substantial Li depletion near the anode could be quantified by analytical laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In contrast to first-principles calculations, no new phases were found near the anode, according to synchrotron-based grazing incidence XRD measurements

The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of >2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells