OPUS Online Publikationen der Universität Stuttgart
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NOx production via an atmospheric microwave air plasma torch
No full textAbstractIn order to achieve climate neutrality, preferably accompanied by cost reduction, plasmochemical processes open up attractive alternatives for the currently very energy‐intensive production of fertilizers. They can be cheaper, more environmentally friendly and agiler than the current state‐of‐the‐art for fixation of nitrogen building blocks. NOx can be obtained from atmospheric nitrogen in an air‐powered plasma torch operating in‐loco and on‐demand, currently yielding a maximum NOx concentration of 2.8 %. The torch can be further improved in efficiency and is designed for an easy upscaling.Projekt DEA
Co‐doping approach for enhanced electron extraction to TiO2 for stable inorganic perovskite solar cells
No full textInorganic perovskite CsPbI3 solar cells hold great potential for improving the operational stability of perovskite photovoltaics. However, electron extraction is limited by the low conductivity of TiO2, representing a bottleneck for achieving stable performance. In this study, a co‐doping strategy for TiO2 using Nb(V) and Sn(IV), which reduces the material's work function by 80 meV compared to Nb(V) mono‐doped TiO2, is introduced. To gain fundamental understanding of the processes at the interfaces between the perovskite and charge‐selective layer, transient surface photovoltage measurements are applied, revealing the beneficial effect of the energetic and structural modification on electron extraction across the CsPbI3/TiO2 interface. Using 2D drift‐diffusion simulations, it is found that co‐doping reduces the interface hole recombination velocity by two orders of magnitude, increasing the concentration of extracted electrons by 20%. When integrated into n-i-p solar cells, co‐doped TiO2 enhances the projected TS80 lifetimes under continuous AM1.5G illumination by a factor of 25 compared to mono‐doped TiO2. This study provides fundamental insights into interfacial charge extraction and its correlation with operational stability of perovskite solar cells, offering potential applications for other charge‐selective contacts.Deutsche ForschungsgemeinschaftProgramme d’Investissement d’AvenirBundesministerium für Bildung und ForschungBundesministerium für Wirtschaft und Klimaschut
Modelling, simulation and experimental characterisation of hydrogen-bromine flow batteries
No full textLarge-scale energy storage systems are considered one of the key components of power grids based on renewable energy resources. The electricity supply and demand are normally in balance owing to the grid inertia of e.g. rotational energy of steam turbines. With increasing shares of renewables in the energy mix which provide electricity intermittently, the supply and demand fall out of equilibrium and, to prevent blackouts and brownouts or frequency variations, the deployment of large-scale energy storage systems is necessary.
Batteries represent just one category within the realm of energy storage systems, with lithium-ion technology reigning as the most prevalent choice. However, due to lithium-ion batteries’ finite cycle lifespan, safety concerns, reliance on limited natural resources, and constraints on energy density, there are many incentives to seek for alternative approaches.
Flow batteries address the lithium-ion technology’s issues as they store the energy in the form of liquid electrolyte in separate tanks. One of the promising flow battery systems is the hydrogen-bromine flow battery (HBFB) system which combines high power densities with low cost due to inexpensive constituents of the electrolyte: hydrogen gas, bromine and hydrobromic acid.
To better understand the working principles, determine the desired operating conditions and to optimise the HBFB, modelling and simulation tools are utilised. However, a proper parametrisation of such models remains a key challenge on the way to accurate performance prediction. In this doctoral thesis, three particular topics pertaining to the HBFB system modelling and characterisation are addressed in detail: the thermodynamics of the bromine-based electrolytes, mass transport properties of electroactive species in concentrated, aqueous solutions and modelling and simulation of porous electrodes.
In the first part, a considerable effort is devoted to the modelling and experimental validation of the open-circuit potential (OCP) of the bromine electrode and the open-circuit voltage (OCV) of the HBFB cell. It is shown how solution thermodynamic activity and formation of polybromides affect the predicted OCP values and how different they are from the ones modelled with a basic Nernst equation. An improved, physics-based OCV model is described and validated experimentally in a specially designed setup, offering a maximum prediction error reduction from 20% to less than 1% compared to the basic Nernst equation.
The second part deals with mass transport by diffusion and migration of electroactive species in the HBFB posolyte: dissolved bromine and bromide ions. The impact of elevated electrolyte concentrations of more than 8 moles per litre in HBr on the effective diffusion rates is determined empirically with the use of ultramicroelectrodes in a dedicated experimental setup, and theoretical equations to model the limiting current densities of bromide and bromine are proposed which showed satisfactory agreement.
Lastly, the problem of upscaling of theoretical equations used to model the performance of porous electrodes (PE) is described. Pore-scale models are able to capture local variations of the variables of interest, however they suffer from high computational cost. Here, a mathematical upscaling method (the volume-averaging method, VAM) is used to computationally analyse several different PE geometries and to elaborate effective transport parameters. As shown in the thesis, they can be then used in macroscopic flow battery models to save on computational effort while still capturing the impact of pore-scale effects such as pore geometry or the impact of electrochemical reaction on the effective diffusion coefficient. Here, the novelty is the upscaling of coupled diffusion-advection equation with a full Butler-Volmer-type heterogeneous reaction at the solid-liquid interface.Große Energiespeichersysteme gelten als eine der Schlüsselkomponenten von Stromnetzen, die auf erneuerbaren Energiequellen basieren. Angebot und Nachfrage elektrischer Energie müssen normalerweise ausgeglichen sein, was bisher durch die Trägheit rotierender Massen der Dampfturbinen ermöglicht wird. Mit steigenden Anteilen erneuerbarer Energien im Energiemix, die intermittierend Strom liefern, geraten Angebot und Nachfrage aus dem Gleichgewicht. Um Stromausfälle oder Frequenzschwankungen zu verhindern, ist die Integration von Energiespeichersystemen mit hohem Energieinhalt in die Stromnetze notwendig.
Batterien stellen nur eine Sorte von Energiespeichersystemen dar, wobei am häufigsten die Lithium-Ionen-Technologie zum Einsatz kommt. Aufgrund der Abhängigkeit von begrenzten natürlichen Ressourcen und Einschränkungen der Energiedichte von Lithium-Ionen-Batterien gibt es viele Anreize, nach alternativen Batterietechnologien zu suchen.
Flow-Batterien zur stationären Energiespeicherung können potentiell die Einschränkungen der Lithium-Ionen-Technologie überwinden. Dabei wird die Energie in Form von flüssigem Elektrolyten in separaten Tanks gespeichert.
Eine vielversprenchende Flow-Batterie ist die Wasserstoff-Brom-Flussbatterie (Engl. hydrogen-bromine flow battery, HBFB). Sie besteht aus preiswerten Komponenten, d.h. dem Elektrolyten - Wasserstoffgas, Brom und hydrobromischer Säure - und ermöglicht eine hohe Leistungsdichte bei niedrigen Systemkosten.
Um die Funktionsprinzipien besser zu verstehen, die gewünschten Betriebsbedingungen zu bestimmen und die HBFB zu optimieren, werden mathematische Modellierung und numerische Simulation eingesetzt. Die angemessene Parametrisierung der Modelle bleibt jedoch eine wichtige Herausforderung auf dem Weg zu einer genauen Leistungsvorhersage. In dieser Doktorarbeit werden drei spezielle Themen zur Modellierung und Charakterisierung des HBFB-Systems behandelt:
die Thermodynamik der brombasierten Elektrolyte, die Massentransporteigenschaften elektroaktiver Spezies in konzentrierten, wässrigen Lösungen und die Modellierung und Simulation poröser Elektroden.
Im ersten Teil wird auf die Modellierung und experimentelle Validierung des Leerlaufpotentials (Engl.: open-circuit potential, OCP) der Brom-Elektrode und der Leerlaufspannung (Engl.: open-circuit voltage, OCV) der HBFB-Zelle einge- gangen. Es wird gezeigt, wie die thermodynamische Aktivität der Lösung und die Bildung von Polybromiden die vorhergesagten OCP-Werte beeinflussen und zu deutlich anderen Werten führt als mittels einfacher Nernstgleichung ermittelt. Ein verbessertes, physikalisches OCV-Modell wird vorgestellt und experimentell in einem speziell konzipierten Messaufbau validiert, wodurch der Vorhersagefehler von 20% auf weniger als 1% im Vergleich zur einfachen Nernst-Gleichung reduziert wird.
Der zweite Teil behandelt den Massentransport durch Diffusion und Migration elektroaktiver Spezies im HBFB-Posolyten: gelöstes Brom und Bromidionen. Der Einfluss erhöhter Elektrolytkonzentrationen von mehr als 8 Mol pro Liter in HBr auf die effektiven Diffusionsraten wird empirisch mit Hilfe von Ultramikroelektroden in einem speziellen experimentellen Aufbau bestimmt, und mit theoretischen Gleichungen der Grenzstromdichten von Bromid und Brom modelliert, was zu einer zufriedenstellenden Übereinstimmung führt.
Schließlich wird das Problem der Hochskalierung theoretischer Gleichungen zur Modellierung der Leistung poröser Elektroden (PE) beschrieben. Mit Hilfe von porenskaligen Modelle, lassen sich lokale Variationen der interessierenden Variablen (z.B. Konzentration von Spezies) berechnen, dabei ist jedoch ein hoher Rechenaufwand nötig. In dieser Arbeit wird eine mathematische Methode zur Hochskalierung (die Volumenmittelungsmethode, Englisch: volume averaging Metod, VAM) verwendet, um mehrere verschiedene poröse Elektroden-geometrien rechnerisch zu analysieren und effektive Transportparameter zu bestimmen. Wie in der Dissertation gezeigt wird, können sie dann in makroskopischen Flussbatteriemodellen verwendet werden, um Rechenaufwand zu sparen, während immer noch Effekte auf der Porenskala wie Porengeometrie oder die Auswirkungen von elektrochemischer Reaktion auf den effektiven Diffusionskoeffizienten erfasst werden können. Die Neuheit besteht hier in der Hochskalierung der gekoppelten Diffusions-Advektions-Gleichung mit einer vollständigen Butler-Volmer-typischen heterogenen Reaktion an der Fest-Flüssig-Grenzfläche
What makes a branched aromatic compound a crystallization chaperone? Insights from a comparison of three organic scaffolds
No full textSome tetraaryladamantane (TAA) octa‐ and tetraethers have the ability to crystallize into well‐ordered lattices without full desolvation. In many cases, the solvates then yield high‐resolution X‐ray crystal structures of the encapsulated liquids. To shed light on this unusual effect of TAAs as crystallization chaperones, we have synthesized a series of spirobiflourene and porphyrin derivatives with four phenyl arms also found in TAA chaperones. Despite the structural similarity, neither of the non‐TAA compounds showed promising crystallization properties. Six new X‐ray crystal structures were obtained, but neither gave a high‐resolution structure of an encapsulated guest. Quantum chemical computations suggest that conformational changes have low activation barriers for the TAAs, which may help to adapt to the structures of guest molecules in tightly packed arrangements. These findings on supramolecular chemistry in the crystalline state may help to design new chaperones with improved properties.Deutsche Forschungsgemeinschaft (DFG
Human-nature interaction in urban socio-ecological systems
No full textHuman-nature interactions are an ubiquitous part of life and are integral for forming strong human-nature relationships that benefit people, while also potentially supporting non-human nature. Indeed, diverse human-nature interactions have been associated with many positive outcomes related to mental and physical well-being, learning, inspiration, development, and the cultivation of a strong sense of nature-connectedness. These positive outcomes for humans can additionally have cascading benefits for non-human nature with, for example, people who feel more connected to nature being more likely to support pro-environmental and pro-biodiversity actions. Despite these positive outcomes, people are interacting less with their natural environment, especially in urban areas, which reduces the opportunity for many urban residents to experience such nature-derived benefits. In this context, it is increasingly important to understand human-nature interactions in urban areas in particular, so that such interactions can be supported year-round to enhance the lives of both diverse urban residents and urban nature. This support for urban people and nature is especially critical as Earth and its inhabitants face the triple planetary crisis of climate change, pollution, and biodiversity loss.
In recent years, the research community has made substantial strides in studying urban human-nature interactions and relationships. Yet, there are several aspects of these relationships that have received less research attention, but are nevertheless essential to better understand human-nature interactions, their perception, and subsequent impact on urban life. Therefore, the overarching objective of this dissertation is to examine a subset of these understudied, yet critical, aspects of human-nature interactions within urban socio-ecological systems. The research will contribute to a fuller understanding of urban human-nature interactions that can ultimately help support both people and nature, especially in times of urban migration and densification.
To accomplish this objective, a framework for urban human-nature interactions was developed to guide and contextualize this research. This framework describes urban human-nature interactions according to their occurrences, perceptions, outcomes, and relationships between these factors. The framework additionally emphasizes the potential impact of attributes of urban socio-ecological systems in which human-nature interactions are imbedded like Covid-19 pandemic conditions, geographic location, and seasonality. Using this framework in the examination of the current research discourse, supported the identification of understudied aspects of urban human-nature interactions related to their occurrence, perception, outcome, and socio-ecological context that led to the four main research questions examined in this dissertation, namely:
How did the Covid-19 pandemic affect human-nature interactions in European cities (RQ1)? How is urban human-nature interaction and sociocultural background of Stuttgart residents associated with a specified human health outcome (i.e. depression risk; RQ2)? What characterizes human-nature interaction of Stuttgart’s urban park visitors specifically in winter (RQ3)? What is the current global research discourse on human-nature interactions in winter and what are common themes and relevant gaps for future research (RQ4)?
A research methodology that included diverse study designs and analyses across geographic scales, seasons, and Covid-19 pandemic conditions was developed to examine these research questions. The results of the four published articles comprising this dissertation suggest first, that the Covid-19 pandemic affected human-nature interaction patterns in European cities and that existing geographic trends of human-nature interaction were enhanced during the pandemic. Second, high-engagement human-nature interaction in the form of urban gardening was associated with a lower odds of depression risk in a Stuttgart case study, with important nuances for residents with a migration history. Third, certain park visitation behaviors and landscape features were associated with high-engagement nature interaction in Stuttgart’s urban parks, though proportionally few visitors demonstrated high-engagement nature interaction. Fourth, a scoping review of the literature revealed the body of research examining winter human-nature interactions focused on outcomes related to health, recreation and tourism, and culture is relatively small and could benefit from more explicit research attention. Finally, results of all four studies together highlight that urban human-nature interactions are diverse and context specific, affected by urban residents’ unique socio-cultural backgrounds (e.g., migration history and age) and aspects of the socio-ecological systems in which they are imbedded (e.g., Covid-19 pandemic conditions, geographic location, and seasonality).
Therefore, adaptive strategies that aim to legitimately include the many perspectives, needs, and preferences of urban residents should be prioritized to help support urban human-nature interactions that are positive for both people and non-human nature alike. From an urban planning standpoint, the holistic integration of measures that support high-quality urban nature and high engagement human-nature interaction can contribute to meeting goals outlined in overarching policy frameworks including the 2030 Agenda for Sustainable Development, the EU Biodiversity Strategy for 2030, and the European Green Deal more broadly
Raman characterization of dioxygen species as defects in single-crystal ZnO including their pressure dependence
No full textThe defects in zinc oxide crystals are of crucial importance for their usability in many applications and are not yet fully understood. Here, we demonstrate that dioxygen species are present as defects in the grown ZnO, resulting in a bending of the atom layers that lie perpendicular to the c-axis. In the Raman spectra, these defects cause the appearance of bands different from the known bands of perfect ZnO crystals allowed by symmetry. These additional Raman bands, which have been frequently reported for ZnO in the past, can thus be fully explained by the presence of dioxygen species, and the widespread assumption of second-order modes for the assignments of these bands is not necessary. Furthermore, the Raman spectrum belonging to perfect zinc oxide in the ideal wurtzite structure is presented, obtained from small domains in ZnO(0001) crystals exposed to pressures up to 2 GPa. The dependence of the O-O stretching modes on the applied pressure proves the presence of dioxygen species in ZnO, which is also confirmed by phonon calculations of structure models with embedded dioxygen species. The surface quality of the ZnO crystals studied is also reflected in the Raman spectra and is included in the analysis
Bacterial minicell‐based biohybrid sub‐micron swimmers for targeted cargo delivery
No full textBacterial biohybrid microrobots possess significant potential for targeted cargo delivery and minimally invasive therapy. However, many challenges, such as biocompatibility, stability, and effective cargo loading, remain. Bacterial membrane vesicles, also referred to as minicells, offer a promising alternative for creating sub‐micron scale biohybrid swimmers (minicell biohybrids) due to their active metabolism, non‐dividing nature, robust structure, and high cargo‐carrying capacity. Here, a biohybrid system is reported that utilizes motile minicells, ≈400 nm in diameter, generated by aberrant cell division of engineered Escherichia coli ( E. coli ), for the first time. Achieving over 99% purification from their parental bacterial cells, minicells are functionalized with magnetic nanoparticles (MNPs) to enable external magnetic control. Minicell biohybrids are capable of swimming at an average speed of up to 13.3 µm s -1 and being steered under a uniform magnetic field of 26 mT. Furthermore, they exhibit a significantly high drug loading capacity (2.8 µg mL -1 ) while maintaining their motility and show pH‐sensitive release of anticancer drug doxorubicin hydrochloride (DOX) under acidic conditions. Additionally, drug‐loaded minicell biohybrids notably reduce the viability of SK‐BR‐3 breast cancer cells in vitro. This study introduces minicell biohybrids and establishes their potential as magnetically guided, drug‐loaded biohybrid systems for targeted therapies in future medical applications.Max Planck SocietyDeutscher Akademischer Austauschdienst (DAAD
Large‐scale interlaboratory study along the entire process chain of laser powder bed fusion : bridging variability, standards, and optimization across metals and polymers
No full textLaser powder bed fusion is a cornerstone technology for additive manufacturing (AM) of metals and polymers, yet challenges in achieving consistent reproducibility and process optimization persist. Addressing these requires a systematic understanding of the interactions between feedstock, process parameters, and final part characteristics throughout the entire production chain. This study presents results from a comprehensive interlaboratory investigation conducted by 32 research institutions, evaluating six feedstock, including nanoparticle‐modified aluminum alloy and polyamide powders, under standardized protocols. Data analysis encompasses 69 powder properties, 15 process parameters per print, and 78 part features, culminating in a dataset of over 1.2 million correlations. Advanced statistical methods and machine learning are employed to identify critical variability drivers, such as the impact of nanoparticle modifications on powder flowability and thermal conductivity, as well as the influence of process parameters on reproducibility. Newly introduced dimensionless figures of merit provide universal metrics to describe and predict thermal and mechanical interactions, simplifying process optimization and material characterization. The findings, supported by an open‐access dataset adhering to findable, accessible, interoperable, and reusable principles, advance understanding of material–process–structure-property relationships. They establish a benchmark for future research and lay the foundation for improving the reliability, quality, and sustainability of AM processes.Deutsche Forschungsgemeinschaf
Novel microwave plasma source for plasma chemistry in gases
No full textIn order to reduce the use and combustion of fossil raw materials, increasing reliance must be placed on renewable energies. Many strategies are currently being researched to enable the use of electrical energy from renewable resources for chemical synthesis [1, 2]. This contribution shows the possibility of obtaining basic chemicals by conversion using microwave plasma technology. A challenge for renewable energies, in addition to the storage problem, is the dependence on weather conditions. The plasma technology approach provides excellent flexibility in controlling of the process and thus in utilizing the fluctuating availability of cost‐effective renewable energies. This enables efficient and on‐demand operation.Projekt DEA
Quantitative determination of the electric field strength in a plasmon focus from ponderomotive energy shifts
No full textSpectroscopic photoemission microscopy is used to detect and quantify a ponderomotive shift in the energy of electrons that are emitted from a surface plasmon polariton focus. The focus is formed on an atomically flat Au(111) surface by an Archimedean spiral and is spatiotemporally separated from the circularly polarized light pulse used to excite the spiral. A spectroscopic analysis of electrons emitted from the focus exhibits a peaked above-threshold electron emission spectrum. From the shift of the peaks as function of laser power the field strength of the surface plasmon polariton was quantitatively determined without free parameters. Estimations of the Keldysh parameter γ = 4.4 and the adiabaticity parameter δ = 4700 indicate that electron emission occurs in a regime of multiplasmon absorption and nonlocalized surface plasmon fields.Carl-Zeiss-StiftungERCBW StiftungIQSTDeutsche ForschungsgemeinschaftUniversity of Duisburg-Esse