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Item Open Access Entwicklung laserspektroskopischer Methoden zur Analyse der Verdunstungseigenschaften von Brennstofftropfen(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2021) Werner, Stefanie; Riedel, Uwe (Prof. Dr. rer. nat.)Die steigenden Emissionen des klimaschädlichen Treibhausgases CO2 durch die Verbrennung von fossilen, endlichen Energieträgern müssen möglichst schnell und nachhaltig reduziert werden. Ein vielversprechender Lösungsansatz zur Reduzierung der Schadstoffemissionen bei der Verbrennung liegt in dem Einsatz von alternativen und erneuerbaren Brennstoffen. Als Energieträger bieten sich auf Grund ihrer hohen Energiedichte vor allem flüssige Brennstoffe an. Diese werden typischerweise durch Druckzerstäubung in die Brennkammer eingebracht, verdunstet und dann mit dem Oxidationsmittel vermischt und verbrannt. Die Verdunstung der kleinen Brennstofftropfen des sogenannten Sprays ist von entscheidender Bedeutung für den Gesamtverbrennungsprozess in Verbrennungsmotoren und Gasturbinen. Im Allgemeinen bestimmt die Verdunstungsrate die Verbrennungsrate. Daher sind Modelle notwendig, die eine genaue Vorhersage der Brennstoffverdunstung ermöglichen. Zur Validierung dieser Modelle werden quantitative Messungen unter genau definierten Randbedingungen benötigt. Da die Prozesse in technischen Brennkammern sehr komplex sind, werden Experimente zur Tropfenverdunstung häufig mit linearen, monodispersen Tropfenketten durchgeführt, um die Kopplung zwischen den verschiedenen Effekten zu minimieren. Durch die geringe Größe der Tropfen (typischerweise wenige hundert Mikrometer oder weniger), erfordert die experimentelle Untersuchung eine hohe räumliche Auflösung. In dieser Arbeit wurden quantitative, laseroptische Messtechniken mit hoher räumlicher Auflösung zur experimentellen Untersuchung der Tropfenverdunstung an monodispersen Tropfenketten entwickelt. Mit den Messtechniken wurden Validierungsdaten für die Verdunstungseigenschaften von verschiedenen Brennstoffen bestimmt. Konzentrationsmessungen von verdunsteten Kohlenwasserstoffen wurden unter Verwendung von Infrarot-Laserabsorptionsspektroskopie und laserinduzierter Fluoreszenzspektroskopie (LIF) durchgeführt. Tropfenketten wurden mit einem Tropfenkettengenerator erzeugt, welcher vertikal in einem Strömungskanal installiert wurde. Die untersuchten Brennstoffe waren Cyclohexan, iso-Octan, n-Heptan, n-Pentan, 1-Butanol und Anisol. Der Strömungskanal wurde mit einer laminaren Luftströmung bei verschiedenen Temperaturen (313 K - 430 K) durchströmt. Da die untersuchten Tropfen einen Durchmesser in der Größenordnung von 120 bis 160 µm hatten und die Konzentrationsgradienten nahe der Tropfenoberfläche groß waren, war eine hohe räumliche Auflösung der Messtechniken erforderlich. Die Absorptionsmessungen wurden mit der Infrarotstrahlung eines HeNe-Lasers bei λ = 3,39 µm durchgeführt, um die CH-Streckschwingung der Kohlenwasserstoffe anzuregen. Die für die Quantifizierung der Brennstoffkonzentrationen benötigten Absorptionsquerschnitte wurden in einer beheizten Gaszelle für Temperaturen von 300 K - 773 K bestimmt. Die räumliche Auflösung im Strömungskanal betrug < 50 µm über eine Länge von 2 mm (Halbwertsbreite). Durch die Zylindersymmetrie und gute Stabilität der Tropfenketten konnten zeitliche Mittelungs- und Tomografieverfahren angewandt werden. Hierdurch konnten radiale Konzentrationsprofile an mehreren Positionen im Strömungskanal erhalten werden. Aus dem Anstieg der Dampfkonzentration an verschiedenen Messpositionen konnte die Verdunstungsrate bestimmt werden. Die Verdunstungsraten wurden in Abhängigkeit von der Mantelstromtemperatur (313 K - 430 K), der Tropfengeschwindigkeit (8 m/s - 23 m/s), der Tropfenerzeugungsfrequenz (12 kHz - 75 kHz) und dem Tropfenabstand (300 µm - 685 µm) gemessen. Im untersuchten Temperaturbereich steigt die Verdunstungsrate des Brennstoffs linear mit der Temperatur an. Die Reihenfolge der Brennstoffe in Bezug auf die Verdunstungsrate entspricht den Siedepunkten der einzelnen Brennstoffe. Da technische Brennstoffe häufig eine Mischung mehrerer Komponenten sind, ist die Untersuchung von Brennstoffgemischen von großem Interesse. Daher wurde ein Messverfahren entwickelt, um binäre Gemische zu untersuchen. Das Verfahren wurde verwendet, um eine Mischung aus Cyclohexan und Anisol zu untersuchen. Zwei Messtechniken - laserinduzierte Fluoreszenz (LIF) und Infrarot Absorptionsspektroskopie - wurden verwendet, um beide Spezies zu messen. Um λ = 3,39 µm ist der Absorptionsquerschnitt von Cyclohexan um etwa den Faktor 8 größer als von Anisol. Im untersuchten Fall war die Konzentration aufgrund des höheren Dampfdrucks ebenfalls deutlich größer. Daher konnte das Infrarot-Absorptionssignal praktisch ausschließlich Cyclohexan zugeordnet werden. Anisol hat bei Anregung bei λ = 266 nm eine sehr gute Fluoreszenzquantenausbeute, während Cyclohexan keine Fluoreszenz zeigt. LIF ermöglicht daher die Quantifizierung von Anisol (oder anderen Aromaten) ohne Interferenz durch Kohlenwasserstoffe. Es wurde ein Messverfahren entwickelt, welches Halationseffekte vermeidet, die typischerweise in planaren LIF-Experimenten an Tropfenketten auftreten. Kalibrationsmessungen, die im gleichen Strömungskanal durchgeführt wurden, ermöglichten die Quantifizierung der verdunsteten Anisolkonzentrationen. Die räumliche Auflösung betrug 80 µm. Ähnlich wie bei den Einzelkomponentenmessungen wurden Verdunstungsraten bestimmt. Wie aufgrund des niedrigeren Dampfdrucks zu erwarten, ist die Verdunstungsrate von Anisol niedriger als die von Cyclohexan. Die Verdunstungsrate von Cyclohexan in der binären Mischung stimmt gut mit den Einzelkomponentenmessungen überein. Das entwickelte Messverfahren ist sehr vielversprechend für weitere Untersuchungen an Mehrkomponentenmischungen. In dieser Arbeit konnte damit erstmals mit hoher räumlicher Auflösung die Verdunstung von Brennstoffkomponenten mittels Absorptionsspektroskopie in der Nähe von Brennstofftropfen untersucht werden. Zusätzlich wurden in Kombination mit laserinduzierter Fluoreszenzspektroskopie Messungen an binären Mischungen durchgeführt. Damit steht ein wertvoller Datensatz zur Validierung von numerischen Simulationen zur Verfügung.Item Open Access Application of ion chromatography for the reliable quantification of ammonium in electrochemical ammonia synthesis experiments : a practical guide(2023) Bragulla, Sebastian C. H.; Lorenz, Julian; Harms, Corinna; Wark, Michael; Friedrich, K. AndreasAssessing novel electrocatalysts for the electrochemical ammonia synthesis (EAS) requires reliable quantitative trace analysis of electrochemically produced ammonia to infer activity and selectivity. This study concerns the development of an ion chromatography (IC) method for quantitative trace analysis of ammonium in 0.1 M sulfuric acid electrolyte, which is applied to EAS gas-diffusion electrode (GDE) experiments with commercial chromium nitride as electrocatalyst. The developed IC method is highly sensitive, versatile, and reliable, achieving a limit of quantification (LOQ) of 6 μg l-1 (6 ppbmol) ammonium. The impacts of the sample matrix, dilution, and neutralization, as well as contamination, on the quantitative analysis by IC are analyzed. Experimental constraints result in an effective LOQ including dilution of 60 μg l-1 for the determination of ammonium in 0.1 M sulfuric acid electrolyte, owing to necessary sample dilution. The practical guide presented herein is intended to be very relevant for the field of EAS as a guideline and applicable to a broad range of catalyst systems and ion chromatography devices.Item Open Access PEM single cells under differential conditions : full factorial parameterization of the ORR and HOR kinetics and loss analysis(2022) Gerling, Christophe; Hanauer, Matthias; Berner, Ulrich; Friedrich, K. AndreasThe anode and cathode kinetics are parameterized based on differential cell measurements. Systematic parameter variations are evaluated to disentangle the dependencies of the electrochemical impedance spectroscopy (EIS) signatures in H2/H2 mode. We introduce a new CO recovery protocol for both electrodes that enables to accurately characterize the hydrogen oxidation reaction (HOR) kinetics. Then, we demonstrate that a compact Tafel kinetics law captures the oxygen reduction reaction (ORR) kinetics for a full factorial grid of conditions, covering a wide range of relative humidities (rH), temperatures, oxygen partial pressures and current densities. This yields the characteristic activation energy and effective reaction order, and we reconcile models that make different assumptions regarding the rH dependency. Moreover, we analyze O2 transport contributions by steady-state and transient limiting current techniques and heliox measurements. Although the rising uncertainty of loss corrections at high current densities makes it impossible to unambiguously identify an intrinsic potential-dependent change of the Tafel slope, our data support that such effect needs not be considered for steady-state cathodic half-cell potentials above 0.8 V.Item Open Access A new approach to modeling solid oxide cell reactors with multiple stacks for process system simulation(2022) Tomberg, M.; Heddrich, M. P.; Sedeqi, F.; Ullmer, D.; Ansar, S. A.; Friedrich, K. AndreasReactors with solid oxide cells (SOC) are highly efficient electrochemical energy converters, which can be used for electricity generation and production of chemical feedstocks. The technology is in an upscaling phase. Thereby demanding development of strategies for robust and efficient operation or large SOC reactors and plants. The present state of technology requires reactors with multiple stacks to achieve the appropriate power. This study aims to establish and apply a simulation framework to investigate process systems containing SOC reactors with multiple stacks. Focusing especially on the operating behavior of SOC reactors under transient conditions, by observing the performance of all cells in the reactor. For this purpose, a simulation model of the entire SOC reactor consisting of multiple stacks, pipes, manifolds, and thermal insulation was developed. After validation on stack and reactor level, the model was used to investigate the fundamental behavior of the SOC reactors and the individual stacks in various operation modes. Additionally, the influences of local degradation and reactor scaling on the performance were examined. The results show that detailed investigation of the reactors is necessary to ensure operability and to increase efficiency and robustness. Furthermore, the computing performance is sufficient to develop and validate system controls.Item Open Access Novel pyrrolidinium-functionalized styrene-b-ethylene-b-butylene-b-styrene copolymer based anion exchange membrane with flexible spacers for water electrolysis(2023) Xu, Ziqi; Delgado, Sofia; Atanasov, Vladimir; Morawietz, Tobias; Gago, Aldo Saul; Friedrich, K. AndreasAnion exchange membranes (AEM) are core components for alkaline electrochemical energy technologies, such as water electrolysis and fuel cells. They are regarded as promising alternatives for proton exchange membranes (PEM) due to the possibility of using platinum group metal (PGM)-free electrocatalysts. However, their chemical stability and conductivity are still of great concern, which is appearing to be a major challenge for developing AEM-based energy systems. Herein, we highlight an AEM with styrene-b-ethylene-b-butylene-b-styrene copolymer (SEBS) as a backbone and pyrrolidinium or piperidinium functional groups tethered on flexible ethylene oxide spacer side-chains (SEBS-Py2O6). This membrane reached 27.8 mS cm-1 hydroxide ion conductivity at room temperature, which is higher compared to previously obtained piperidinium-functionalized SEBS reaching up to 10.09 mS cm-1. The SEBS-Py206 combined with PGM-free electrodes in an AWE water electrolysis (AEMWE) cell achieves 520 mA cm-2 at 2 V in 0.1 M KOH and 171 mA cm-2 in ultra-pure water (UPW). This high performance indicates that SEBS-Py2O6 membranes are suitable for application in water electrolysis.Item Open Access Crystal structure and XPS study of titanium-substituted M-type hexaferrite BaFe12−xTixO19(2023) Mehnert, Kim-Isabelle; Häßner, Manuel; Dreer, Yanina Mariella; Biswas, Indro; Niewa, RainerThe M-type barium hexaferrite substituted with titanium, BaFe12−xTixO19, was synthesized from sodium carbonate flux and the obtained single crystals with a maximum degree of substitution of up to about x = 0.9 were characterized. XPS measurements were carried out for the identification of side products and in particular in order to assign the valence states of the transition-metal constituents. Due to the aliovalent exchange of iron(III) with titanium(IV), an additional charge balance needs to occur. No titanium(III) was detected, while the amount of iron(II) increased in the same order of magnitude as the amount of titanium(IV); thus, the major charge balancing is attributed to the reduction of iron(III) to iron(II). According to the XPS data, the amount of titanium(IV) typically is slightly higher than that of iron(II). This is in line with a tendency to a minor formation of vacancies on the transition-metal sites becoming more important at higher substitution levels according to PXRD and WDS measurements, completing the picture of the charge-balance mechanism. XRD taken on single crystals indicates the distribution of titanium and vacancies over three of the five transition-metal sites.Item Open Access Identification of the underlying processes in impedance response of sulfur/carbon composite cathodes at different SOC(2022) Gerle, Martina; Wagner, Norbert; Häcker, Joachim; Nojabaee, Maryam; Friedrich, K. AndreasFor lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported.Item Open Access Hydrogen oxidation artifact during platinum oxide reduction in cyclic voltammetry analysis of low-loaded PEMFC electrodes(2020) Prass, Sebastian; St-Pierre, J.; Klingele, Matthias; Friedrich, K. Andreas; Zamel, NadaAn artifact appearing during the cathodic transient of cyclic voltammograms (CVs) of low-loaded platinum on carbon (Pt/C) electrodes in proton exchange membrane fuel cells (PEMFCs) was examined. The artifact appears as an oxidation peak overlapping the reduction peak associated to the reduction of platinum oxide (PtOx). By varying the nitrogen (N2) purge in the working electrode (WE), gas pressures in working and counter electrode, upper potential limits and scan rates of the CVs, the artifact magnitude and potential window could be manipulated. From the results, the artifact is assigned to crossover hydrogen (H2X) accumulating in the WE, once the electrode is passivated towards hydrogen oxidation reaction (HOR) due to PtOx coverage. During the cathodic CV transient, PtOx is reduced and HOR spontaneously occurs with the accumulated H2X, resulting in the overlap of the PtOx reduction with the oxidation peak. This feature is expected to occur predominantly in CV analysis of low-loaded electrodes made of catalyst material, whose oxide is inactive towards HOR. Further, it is only measurable while the N2 purge of the WE is switched off during the CV measurement. For higher loaded electrodes, the artifact is not observed as the electrocatalysts are not fully inactivated towards HOR due to incomplete oxide coverage, and/or the currents associated with the oxide reduction are much larger than the spontaneous HOR of accumulated H2X. However, owing to the forecasted reduction in noble metal loadings of catalyst in PEMFCs, this artifact is expected to be observed more often in the future.Item Open Access A review of the MSCA ITN ECOSTORE : novel complex metal hydrides for efficient and compact storage of renewable energy as hydrogen and electricity(2020) Hadjixenophontos, Efi; Dematteis, Erika Michela; Berti, Nicola; Wołczyk, Anna Roza; Huen, Priscilla; Brighi, Matteo; Le, Thi Thu; Santoru, Antonio; Payandeh, Seyed Hosein; Peru, Filippo; Dao, Anh Ha; Liu, Yinzhe; Heere, MichaelHydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.Item Open Access In situ laboratory for plastic degradation in the Red Sea(2022) Brümmer, Franz; Schnepf, Uwe; Resch, Julia; Jemmali, Raouf; Abdi, Rahma; Kamel, Hesham Mostafa; Bonten, Christian; Müller, Ralph-WalterDegradation and fragmentation of plastics in the environment are still poorly understood. This is partly caused by the lack of long-term studies and methods that determine weathering duration. We here present a novel study object that preserves information on plastic age: microplastic (MP) resin pellets from the wreck of the SS Hamada, a ship that foundered twenty-nine years ago at the coast of Wadi el Gemal national park, Egypt. Its sinking date enabled us to precisely determine how long MP rested in the wreck and a nearby beach, on which part of the load was washed off. Pellets from both sampling sites were analyzed by microscopy, X-ray tomography, spectroscopy, calorimetry, gel permeation chromatography, and rheology. Most pellets were made of low-density polyethylene, but a minor proportion also consisted of high-density polyethylene. MP from inside the wreck showed no signs of degradation compared to pristine reference samples. Contrary, beached plastics exhibited changes on all structural levels, which sometimes caused fragmentation. These findings provide further evidence that plastic degradation under saltwater conditions is comparatively slow, whereas UV radiation and high temperatures on beaches are major drivers of that process. Future long-term studies should focus on underlying mechanisms and timescales of plastic degradation.