Universität Stuttgart
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Item Open Access Membrane electrode assembly for water electrolysis(2023) Nguyen, Thi Hai Van; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Maintaining a sufficient energy supply while minimizing the impact on the environment and climate is one of the greatest social and scientific challenges of our times. There are various fields of research and technological developments in the context of global warming and limitless growing energy demand, and the focus of this PhD programme is on artificial photosynthesis, more specifically on the assembly of Membrane electrode assembly for water electrolyzer part. Mimicking photosynthesis in a scheme to trap solar energy in chemical bonds (fuels) is a scientific and technological challenge. Having a cost-effective and reliable process stays one of the main limitations in order to achieving the long-term goal of this approach. In this work, within the European eSCALED project, the elaboration of Membrane Electrode Assembly (MEA) for water electrolysis by introducing new materials and low-cost fabrication methods was investigated.Item Open Access Development of hydrodynamic density functional theory for mixtures and application to droplet coalescence(Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Stierle, Rolf; Groß, Joachim (Prof. Dr.-Ing.)Predicting accurately coalescence phenomena is critical to the accurate description of the hydrodynamics of fluids and their mixtures. A promising framework for the development of models for such phenomena is dynamic density functional theory. Dynamic density functional theory enables the analysis of dynamical processes in inhomogeneous systems of pure fluids and fluid mixtures at the molecular level. In this work, a hydrodynamic density functional theory model for mixtures in conjunction with Helmholtz energy functionals based on the PC-SAFT equation of state is proposed, that obeys the first and second law of thermodynamics and simplifies to the isothermal Navier-Stokes equation for homogeneous systems. The hydrodynamic density functional theory model is derived from a variational principle and accounts for both viscous forces and diffusive molecular transport. A Maxwell-Stefan model is applied for molecular transport. This work identifies a suitable expression for the driving force for molecular diffusion of inhomogeneous systems that captures the effect of interfacial tension. The proposed hydrodynamic density functional theory is a non-local theory that requires the computation of weighted (spatial averaged) densities around each considered spatial coordinate by convolution, which is computationally expensive. This work uses Fourier-type transforms to determine the weighted densities. A pedagogical derivation is presented for the efficient computation of the convolution integrals occurring in the Helmholtz energy functionals in Cartesian, cylindrical, and spherical coordinates on equidistant grids using fast Fourier and similar transforms. The applied off-the-shelf algorithms allow to reduce dimensionality and complexity of many practical problems. Furthermore, an algorithm for a fast first-order Hankel transform is proposed, allowing fast and easy density functional theory calculations in rotationally symmetric systems. Application of the hydrodynamic density functional theory model using a well-balanced finite-volume scheme to one-dimensional droplet and bubble coalescence of pure fluids and binary mixtures is presented. The required transport coefficients, shear viscosity and Maxwell-Stefan diffusion coefficients, are obtained by applying entropy scaling to inhomogeneous fluids. The considered systems show a qualitative difference in the coalescence characteristics of droplets compared to bubbles. This constitutes a first step towards predicting the phase rupture leading to coalescence using dynamic density functional theory.Item Open Access Integrated process design and optimisation for the production of cellobiose lipids from Ustilago sp.(2024) Oraby, Amira; Rupp, Steffen (Prof. Dr.)Item Open Access Multistep reactions of molten nitrate salts and gas atmospheres(2024) Steinbrecher, Julian; Thess, André (Prof. Dr.)Dissertation zur Untersuchung der Stabilität von Nitratsalzschmelzen unter verschiedenen atmosphärischen Bedingungen und Temperaturen.Item Open Access Perovskite chromite-based fuel electrode for solid oxide cells (SOCs): towards the understanding of the electrochemical performance(2023) Amaya Dueñas, Diana María; Friedrich, K. Andreas (Prof. Dr. rer. nat.)The current energy transition is a key driver for the continuous development of fuel cells and electrolyzers due to the rapid growth of the clean energy demand and the need to overcome the intermittency of the power supply of renewable energy sources, such as wind and solar energy. In this regard, solid oxide cells (SOC) are promising systems that allow to overcome such fluctuations: they convert renewable electrical energy into chemical energy in the form of hydrogen and valuable fuels and chemicals, while they can also repower the grid by converting fuels and hydrogen into electrical power. This feature in reversibility has attracted the interest among Power-to-X technologies, which can be exploited by operating SOCs in fuel cell (SOFC), electrolysis (SOEL) and reversible (rSOC) modes. Nevertheless, SOCs are not yet a mature technology due to limitations on the performance of their electrolyte and electrodes. Typical fuel electrodes made of Ni-based cermets are in contact not only with hydrogen, but also with reactants such as natural gas, biogas, steam and carbon dioxide, leading to important operation issues related to high temperatures and poisoning tolerance, which significantly detriment the performance of these systems. Due to the urgent need for the development of sustainable SOC systems in clean energy scenarios, this thesis aims to cover the Ni cermets issues related to SOCs operation, such as nickel agglomeration, nickel migration, structural cell damage and carbon deposition. Therefore, with the motivation to propose alternative fuel electrode materials to the state-of-the-art Ni cermets, formulations of perovskite chromite-based fuel electrodes were investigated in different SOC operating conditions. Firstly, different perovskite compositions were investigated by X-ray diffraction (XRD) to ensure the desired phase. With these crystal structure characterizations, the lanthanum-chromite perovskite with Ni doping (LSCrN) was selected as candidate fuel electrode material with the compositions La0.7Sr0.3Cr0.85Ni0.15O3-δ (L70SCrN) and La0.65Sr0.3Cr0.85Ni0.15O3-δ (L65SCrN). These materials were synthetized by the glycine-nitrate combustion method and ceramic powder morphology was characterized by scanning electron microscopy (SEM). An experimental protocol for the cell manufacturing process was designed and the electrolyte-supported-cells (ESCs) were produced by screen-printing, drying and sintering processes. ESCs were tested in different operating SOC modes: fuel cell (SOFC), steam electrolysis (SOEL), steam and carbon dioxide co-electrolysis (co-SOEL), as well as in reversible mode (rSOC) and even in dry carbon dioxide electrolysis operation. In situ electrochemical characterizations were performed by evaluating the voltage - current response and the electrochemical impedance spectroscopy (EIS). In parallel, the exsolution of nickel particles from the produced LSCrN ceramic powders was investigated by means of temperature programmed reduction (TPR), X-ray spectroscopy (XPS) and XRD techniques. It was shown that the introduction of A-site deficiency promoted the reduction of metallic nickel particles on the perovskite surface. The particle distribution was found to be dependent on the temperature, the atmosphere and the overpotential. In co-SOEL operation, cells with the developed L65SCrN electrode showed a comparable performance to the ones with state-of-the-art Ni cermets, e.g. - 0.8 A·cm-2 at 1.32 V and 860 °C. The long-term stability (~ 1000 hours) suggested that under strongly reducing atmospheres, such as in SOEL at 860 °C, the L65SCrN electrode suffered from accelerated performance degradation due to an alteration of the transport properties. Nonetheless, it was found that a decrease in operating temperature (below 830 °C) could be a suitable strategy to mitigate this durability issue. These findings are related to a gain in performance of the perovskite electrodes against the state-of-the-art Ni electrodes at temperatures between 770 °C and 830 °C, possibly due to lower reaction energy barriers. These outcomes were used as basis for a scale-up analysis from the cell level up to the system level, i.e. up to the MW scale, by analyzing a real case application of SOEL-based systems for hydrogen production. This analysis suggested that the implementation of perovskite electrodes in SOEL systems, together with a decrease of the system operating temperature, would lead to a significant reduction of the number of cells in the stacks and hence of the system components, simplifying the system layout. Additionally, the required amount of Ni raw material would also be significantly decreased, which would mitigate future supply chain issues that the mineral market may experience in the upcoming years. This study paves the way for future alternative electrode development for SOC applications while suggesting potential benefits at the system scale.Item Open Access Einfluss der Reaktionskinetik und Mischung auf die Selektivität in reaktiven Blasenströmungen(2022) Gast, Sebastian; Nieken, Ulrich (Prof. Dr.-Ing.)In dieser Arbeit wird das bisher noch unzureichend erforschte Wechselspiel zwischen Fluiddynamik, Stoffübergang und chemischer Reaktion in Blasenströmungen untersucht. Um die gegenseitigen Abhängigkeiten dieser Prozesse zu verstehen, müssen diese zuerst getrennt voneinander ohne die Beeinflussung der anderen Prozesse betrachtet werden. Um die Reaktionskinetik ohne Einfluss des Stofftransportes zu bestimmen, wurde ein neuer Kinetikreaktor entwickelt. Hierbei wird der Stoffübergang von der Gas- in die Flüssigphase räumlich von der Reaktion getrennt. Diese räumliche Entkopplung erlaubt die Untersuchung der Reaktionskinetik in homogener flüssiger Phase ohne jegliche Stofftransportlimitierung. Als Modellsystem wurde die Kinetik der unkatalysierten Toluoloxidation ermittelt und parametriert. Das selektive Reaktionsnetzwerk der Toluoloxidation, bestehend aus konkurrierenden Folge- und Parallelreaktionen bietet die notwendigen Voraussetzungen für die Studie der zuvor genannten Abhängigkeiten der Fluiddynamik, Stoffübergang und chemischer Reaktion in Blasenströmungen. Die ermittelte Reaktionskinetik erwies sich in numerischen Simulationen als zu langsam für die Interaktion mit der Blasenumströmung. Dies konnte experimentell in einer transparenten Hochdruckblasensäule technischer Größe bei industriellen Bedingungen von 30 bar und 190°C bestätigt werden. In weiterführenden Simulationen wurde die um einen Faktor KF beschleunigte Reaktionskinetik verwendet, um den Einfluss der nicht idealen Vermischung im Nachlauf einer Blase auf die Reaktion und das erzeugte Produktspektrum zu untersuchen. Es konnte gezeigt werden, dass nur Reaktionen durch die Blasenströmung beeinflusst werden, welche in einem Zeitbereich von 0.1 < Da_1 < 1000, dem sogenannten mischungsmaskierten Bereich, ablaufen. Langsamere oder schnellere Reaktionen laufen in der Bulkphase beziehungsweise ausschließlich an der Blasenoberfläche ab und werden nicht durch die unvollständige Vermischung im Nachlauf der Blase beeinflusst. Der größte Einfluss auf den Verlauf der Reaktion wird dabei von einer durch den stationären Blasenwirbel erzeugte Transportbarriere verursacht. Diese verhindert den Abtransport der erzeugten Produkte. Bei einem gleichzeitig konstanten Zustrom an Edukt werden Folgereaktionen gefördert. Dies führt zu einer starken Veränderung des Produktspektrums gegenüber des Reaktionsablaufes bei ideal vermischten Bedingungen. Darüber hinaus wurde ein Compartment Modell aufgestellt, um den Einfluss der nicht ideal vermischten Bedingungen einer Blasenumströmung auf die ablaufende Reaktion zu beschreiben. Das Compartment Modell basiert auf einem modifizierten Oberflächenerneuerungsmodell zur Darstellung der Abläufe an der Blasenoberfläche und einem Verweilzeitmodell zur Abbildung der unvollständigen Vermischung im Nachlauf der Blase. Es ist in der Lage, die identifizierte Abhängigkeit der Reaktion von Fluiddynamik und Stoffübergangs und -transport bei deutlich reduziertem Rechenaufwand zu reproduzieren und ist damit für den Einsatz in großskaligen Simulationen wie Euler-Euler und Euler-Lagrange geeignet.Item Open Access Reaction mechanism development and numerical modeling of biomass gasification process(2021) Fernando, Niranjan; Riedel, Uwe (Prof. Dr. rer. nat.)Item Open Access On the mass transport phenomena in proton exchange membrane water electrolyzers(2020) García Navarro, Julio César; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Proton exchange membrane (PEM) water electrolysis is a technology designed to produce hydrogen using only water and electricity as inputs; it has gained increased attention in industry and academia due to its advantages over incumbent hydrogen generation processes (of which the most widely used are steam reforming and coal gasification) namely, low temperature, carbon-neutral and intermittent operation. PEM electrolysis can be instrumental for creating a hydrogen economy, although still much research needs to be carried out before widespread industrial adoption is achieved. PEM water electrolyzers suffer energy losses associated with the chemical reactions and the transport of charge and mass; of these phenomena, mass transport in PEM electrolyzers is the least understood subject, given the complex nature of the interaction of multiphase flows (mainly consisting of liquid water and evolved gases) through micrometric pores. The subject of multiphase flow in water electrolysis and its relationship with the mass transport phenomena in PEM water electrolysis has been a prevalent subject in the literature. Despite numerous attempts at pinpointing the relationship between mass transport overpotential and the operating parameters, there is no clear consensus about which transport mechanisms dominate, nor about how the component design of PEM electrolyzers affects the mass transport. While the effect of temperature and current density on mass transport losses has been extensively studied and is well understood, there are significantly fewer studies that focus on the effect of water flow and pressure. Both water flow and pressure have a direct effect on mechanisms such as bubble nucleation and two-phase flows that occur in the porous structures within a PEM electrolyzer (electrodes and porous transport layers, PTLs). In this work, I studied the effect of water flow and pressure on the mass transport phenomena in PEM electrolyzers. Chapters 1 and 2 provide an introduction to the topic as well as a description of the materials and experimental setups used. Chapter 3 of this thesis depicts the visualization and modeling of bubble nucleation in an operating PEM electrolyzer. I discovered that bubble detachment radii are largely independent of water flow and I identified two types of bubbles: bubbles that detach after reaching a critical size, and bubbles that fill up the pores of a PTL before detaching. Chapter 3 consists of the measurements I carried out regarding the transport of evolved gas through the water-filled pores of a PTL, where I observed that water flow severely impedes the gas transport through the pores and that such impediment is related to a shear stress exerted by the water flow on the pores. Chapter 5 shows the measuring of mass transport losses using electrochemical impedance spectroscopy (EIS) on an operating PEM electrolyzer; the results indicate that pressure and water flow affect the diffusion of gas in the electrode and that the mass transport overpotential depends on design parameters of the PEM electrolyzer, such as electrode thickness and hydrophobicity. Overall, I derived a theoretical framework based on the assumption that the evolved gas in a PEM electrolyzer permeates through the PTL after diffusing from the active sites to the bubble nucleation sites. Such framework, constructed on the basis of the models regarding gas transport in porous media, can be used to explain the mass transport loses in a PEM electrolyzer that arise from operating with increased water flows and pressures. The model I derived can be used in future work as a guideline to optimize the components of a PEM electrolyzer, in particular regarding the hydrophobicity and pore size distribution of PTLs as well as the composition of the catalyst ink to produce the electrodes. Moreover, this work can also be used to further understand the mass transport losses and optimize the operation of PEM electrolyzers to decrease the energy consumption of hydrogen generation.Item Open Access Herstellung, Charakterisierung und Testung von Zeolithkatalysatoren für die Alkoholumsetzung(2024) Dittmann, Daniel; Dyballa, Michael (PD Dr.)Item Open Access Agglomeratstabilität von Nanopartikeln in Flammen zur Untersuchung der Freisetzung von Nanopartikeln bei der Abfallverbrennung(2023) Lang, Inge-Maria; Seifert, Helmut (Prof. Dr.-Ing.)Diese Arbeit untersucht die Stabilität von agglomerierten Ceroxid-Nanopartikeln in Flammen. Hierzu wurde Ceroxid-Aerosol in Laborflammen, in eine Drehrohr-Pilotanlage und in eine industrielle Sonderabfallverbrennungsanlage, eingebracht. Die Partikelgrößenverteilungen sowie CeO2-Konzentrationen im Abgas, Abwasser und Reststoffen der Abgasreinigung wurden gemessen. Es konnte gezeigt werden, dass sich CeO2-Agglomerate bereits weit unter dem Schmelzpunkt des Bulkmaterials zersetzen und im Abgas hohe Konzentrationen von Nanopartikeln bilden. Trotzdem tritt bei der thermischen Abfallbehandlung keine Freisetzung von CeO2 -Nanopartikeln in die Umgebung auf, da diese im Abgas mit dem Flugstaub agglomerieren und in der Abgasreinigung zurückgehalten werden. In Laboruntersuchungen mit einer Propan-Vormischflamme zersetzen sich CeO2-Agglomerate im Bereich von 1.400 bis 1.750°C und bilden hohe Konzentrationen von Nanopartikeln im Bereich von 7-15 nm. Die fahlgelbe Flammenfärbung weist auf die Bildung gasförmiger Cer-Spezies hin, die im kühleren Abgas Partikel bilden, deren mit HR-TEM bestimmte Gitterkonstante mit CeO2 übereinstimmt. Durch parallele Untersuchungen in einem Rohrofen im Temperaturbereich bis 1.600°C wurde ein reiner Temperatureffekt ausgeschlossen. Bei der Zersetzung dürften somit reduzierende Flammenbestandteile eine wesentliche Rolle spielen. Bei den Tracer-Versuchen an der Pilotanlage am Campus Nord des KIT und an der Rückstandsverbrennungsanlage in Dormagen wurde gleichermaßen vorgegangen, indem eine Ceroxid-Suspension mit einem Partikeldurchmesser von 40 nm in den Brennraum eingedüst und im Rauchgas die Konzentration und die Größenverteilung von Ceroxid bestimmt wurde. Im Rauchgas beider Anlagen wurden Partikel mit einem Durchmesser von weniger als 20 nm gemessen. Somit finden hier die gleichen Prozesse statt, welche, wie in den Laborversuchen, zur Bildung einer neuen Partikelfraktion führen. Die elementspezifische Massenverteilung des Cers durch die ICP-MS Analyse der einzelnen Impaktorstufen zeigt die Agglomeration der Ceroxidpartikel mit dem Flugstaub. Die Wiederfindungsrate im Rauchgas lag bei 30% des eindosierten Ceroxid-Tracers. In der nassen Rauchgasreinigung (RGR) der Rückstandsverbrennungsanlage (RVA) wurden 99,99%, bezogen auf die insgesamt zudosierte Tracer-Menge, abgeschieden. Die Bilanzierung der wässrigen Stoffströme der RGR zeigt, dass 69% der Tracermenge in Quensche und saurem Wäscher abgeschieden werden. Im Filtrat der Waschwasserbehandlung, in der alle Stoffströme der RGR gereinigt werden, lag die Konzentration an Cer unterhalb der Nachweisgrenze. D. h., dass die gesamte Menge an abgeschiedenen Partikeln aus der RVA im Filterkuchen abgeschieden wird.