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 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 Fouling during solution polymerization in continuously operated reactors(2021) Zander, Christian; Nieken, Ulrich (Prof. Dr.-Ing.)Specialty polymers are mostly produced in discontinuous processes in tank reactors due to the need for flexibility in the production of this product class. Milli-structured, continuously operated reactors are promising alternatives for process intensification to increase energy efficiency and space-time-yield, reduce time-to-market for new products and maintain flexibility. A major obstacle for the transfer of batch processes to such reactor systems is the formation of fouling deposits, which grow and block the reactor. To overcome this obstacle, knowledge of the mechanisms of the formation of fouling deposits is essential. In this thesis, fouling during the polymerization of N-Vinylpyrrolidone (NVP) in aqueous solution is studied both experimentally and in simulations to gain insight into the underlying mechanism, find a model-based description of this mechanism and make suggestions how to prevent or at least decrease the formation of fouling deposits. First, results from experiments in different kinds of tank and tubular reactors are presented. In all these reactor systems, fouling deposits are formed by an insoluble polymer gel, which adheres strongly to metal surfaces. Initially, the polymer gel is formed in regions with increased local residence time, e.g. in dead-water zones of static mixer elements, at baffles of tank reactors or at walls of tubular reactors without mixer elements. Once fouling deposits have been formed, they grow by reaction and lead to clogging of tubular reactors systems. Since a polymer gel is formed, side reactions that lead to high-molecular and branched polymer chains must play an important role for the formation of deposits. Kinetic models that are based on a recently suggested reaction mechanism and predict microstructural property distribution are presented and validated using continuously stirred tank reactor (CSTR) experiments. The results confirm the suggested reaction mechanism in which creation and propagation of terminal double bonds lead to branched or crosslinked polymer chains. Although gelation of the bulk phase does not occur, fouling deposits are formed at the baffles of the tank reactor and in other poorly mixed regions of the reactor. This observation emphasizes the importance of the flow pattern and diffusive mass transport for the formation of fouling deposits. To demonstrate the interplay of the flow pattern, the reaction and diffusive mass transport, simulations using a transient CFD solver including a reduced version of the reaction kinetics model together with a model for diffusive mass transport are presented. The mass transport model is able to describe diffusive transport of statistical moments and is, therefore, consistent with the reaction kinetics model. Simulations in different two-dimensional geometries confirm that regions with increased local residence time lead to the formation of polymer gels. These regions, e.g. regions close to reactor walls or dead-water zones, cause concentration gradients, which induce mass transport between such regions and the bulk phase. Due to their lower diffusion coefficients in comparison to low molecular species, polymer molecules accumulate in these regions, which increases the viscosity locally. Because of the viscosity gradients, the flow pattern is distorted and the size of regions with increased residence time expands. The combination of an increased residence time, high polymer and low monomer contents promotes the formation of polymer gels by side reactions. Together with the adhesion of macromolecules on metal surfaces, this seems to be the relevant mechanism for the formation of fouling deposits. Therefore, strategies to decrease fouling should focus on surface modifications, which reduce adhesion of macromolecules, as well as the elimination of dead-water zones and viscosity gradients.Item Open Access Proton-conducting membranes for the artificial leaf(2023) Bosson, Karell; Tovar, Günter E. M. (Prof.)With the aim of producing proton conducting membranes with improved proton conductivity and mechanical properties, the poly(pentafluorostyrene)-b-(butyl acrylate) (PPFS-b-PBuA) system was investigated. The study mainly focuses on the influence of the forming polymer nanostructures on the conductivity properties of the membranes. A series of well-defined PPFS-b-PBuA block copolymers (BCPs) were synthesized via nitroxide-mediated controlled radical polymerization (NMP). Spontaneous self-assembly of the BCP element was induced via a targeted change in polymer composition. Moreover, by adjusting the molar composition via enrichment of one of the blocks after synthesis, controlled self-assembly of the BCPs was realized. This was done by combining the corresponding homopolymer with the block copolymer to form a polymer blend - one of the blocks mixed to the BCP. Forming such polymer blends expanded the range of available techniques for tailoring the morphology for desired applications. Sulfonation of BCPs for the preparation of proton-conducting membranes was carried out by a para-fluoro thiol "click" reaction using sodium 3-mercapto-1-propanesulfonate (SMPS). The accessibility of fluorine in the para position of the phenylene group of PPFS provides countless opportunities for polymer functionalization by nucleophilic substitution. After modification of BCP, the self-assembly ability was retained, and higher conductivities were obtained compared to random copolymers. In addition, complementary studies were conducted on the use of printing techniques for membrane upscaling and evaluation of their life cycle.Item Open Access Synthesis and characterization of anion exchange blend membranes for vanadium redox flow battery applications(2021) Cho, Hyeongrae; Friedrich, K. Andreas (Prof. Dr. rer. nat.)In this dissertation, Anion Exchange Blend Membranes (AEBMs) were synthesized and applied in Vanadium Redox Flow Batteries (VRFBs). In the first paper, AEBMs were systematically optimized for VRFBs by varying the composition of polymers components. A bromomethylated poly (2, 6-dimethyl-1,4-phenylene oxide) (Br-PPO) was used as an anionic exchange precursor which was quaternized with 1,2,4,5-tetramethylmidazole (TMIm). A Polybenzimidazole-OO (PBI-OO, produced by Fuma-Tech) was used as a matrix polymer to provide mechanical strength. A minor amount of sulfonated polymer was used as an ionical cross-linker. Those AEBMs showed comparable Energy Efficiency (EE) with Nafion 212 membranes and one of the synthesized AEBMs (BM-TMIm 4) showed a superior Coulombic Efficiency (CE) of almost no decreasing after 300 charging-discharging cycles with a significant capacity retention of 77% of the initial value for after 300 charging-discharging cycles at a current density of 40 mA/cm2. Therefore, AEBMs are promising candidates for long-term operation in VRFBs if the proportion and type of the different components in the blend system is carefully adjusted. In the first paper, the composition of AEBMs were optimized for use in VRFBs. In the second paper, AEBMs were prepared with different polymer combinations. These AEBMs consisted of 3 polymer components. 1) F6-PBI (fluorinated PBI) or PBI-OO (non-fluorinated PBI): PBI was used as a matrix polymer, 2) Br-PPO: Br-PPO was used as an anion exchange polymer precursor by quaternizing with TMIm to provide anion exchange sites, 3) a partially fluorinated polyether or a non-fluorinated poly (ether sulfone): sulfonated polymer was used as an ionical cross-linker. The same weight ratios of three components were used in blend membranes, while different combinations of polymers were used. Similar properties of blend membranes such as ion exchange capacity, conductivity and swelling behavior showed since same amount of anion exchange polymer in each blend membrane was used. In VRFB test, all blend membranes showed better performances than the commercial membranes (Nafion: a cation exchange membrane and FAP 450: an anion exchange membrane) in terms of coulombic-, voltage- and energy efficiencies. One of the blend membranes (BM-TMIm4 FF), which is composed fluorinated polymers, exhibited excellent capacity retention showing no capacity decay over 550 charging-discharging cycles run at a current density of 40 mA/cm2. The outstanding performance of fluorinated polymers-based blend membranes probably is due to the highly stability of F6-PBI in an acidic condition. A pure F6-PBI membrane showed no structural changes in 30 % sulfuric acid solution for 9 days confirmed by Fourier-Transfrom Infrared Spectroscopy (FT-IR spectra), while a PBI-OO membrane was sulfonated after few days. Thus, it can be concluded that if proper matrix polymer chosen for blend membrane, the AEBMs in VRFBs are expected to exhibit superior performance. In the paper 3, AEBMs were synthesized by 3 steps based on Poly(pentafluorostyrene) (PPFSt) for use in VRFBs. Firstly, 1-(2-dimethylaminoethyl)-5-mercaptotetrazole was grafted onto PPFSt by nucleophilic substitution on the para-position. Secondly, the tertiary amino groups were quaternized with iodomethane to provide anion exchange sites. Thirdly, AEBMs were fabricated by blending of synthesized anion exchange polymer with F6-PBI. The blend membrane containing of 30% F6PBI showed better VRFBs performance that Nafion membrane in terms of energy efficiency, Open Circuit Voltage (OCV) and charging-discharging cycling. While the blend membrane containing of 40% F6-PBI displayed much longer OCV time and capacity retention by a charging-discharging test than that of Nafion membrane. It can be concluded that AEBMs are strong candidate for VRFB applications. The AEBMs tested in VRFBs have shown better performances than that of commercial reference membranes of a cation exchange membrane (Nafion) and an anion exchange membrane (FAP 450). By considering the battery test results of AEBMs in this study, therefore, it can be concluded that the AEBMs are very promising candidates as separators in VRFBs if the matrix polymer is chosen properly. One further potential application of those blend membranes can be used in phosphoric acid doped high temperature proton exchange membrane fuel cells (PA-doped HT-PEMFCs), since the AEBMs in this dissertation showed high thermal stability. In recent paper (Nat Energy 1, 16120 (2016)), anion exchange membrane showed very promising results in PA-doped PEMFCs displaying much better performances than that of a polybenzimidazole membrane (a standard membrane for PA-doped HT-PEMFCs) in the FC test. The results in our patent have shown also that those anion exchange membranes exhibited promising performances in the fuel cell test. Therefore, anion exchange blend membranes synthesized in this study are expected to excellent performances for phosphoric acid doped HT-PEMFCs.Item Open Access Die Zusammensetzungsanalyse und die katalytische Aktivitätsuntersuchung von bimetallischen Nanopartikeln(2022) Muntean, Alex; Seipenbusch, Martin (PD Dr.-Ing. habil.)Item Open Access Structural analysis of the multi-layer bellows in the ball valves(2023) Oleynikov, Andrey; Merten, Clemens (Prof. Dr.-Ing. habil.)Item Open Access Combined material and process design for the adsorption refrigeration process(2022) Scherle, Marc; Nieken, Ulrich (Prof. Dr.-Ing.)Item Open Access Polymer fouling in tubular reactors for radical polymerizations(2024) Welzel, Stefan; Nieken, Ulrich (Prof. Dr.-Ing.)Item Open Access Proton-conducting (blend) membranes based on sulfonated/phosphonated and basic polymers(2024) De Azpiazu Nadal, Ignasi; Tovar, Günter (Prof.)Aiming at new proton-conducting membranes, this thesis deals with the syntheses and characterizations of highly sulfonated poly(arylene sulfides) and other polysulfides for application as polymer electrolytes. The study focuses mainly on the analysis of the polymer structures that would improve the conductivity of current proton conducting membranes while maintaining their mechanical stability. In a first step, several polymers are obtained from which poly(arylene sulfide)s polymers look more promising for further functionalization. They are obtained by using mild reaction conditions of a polycondensation reaction between 4,4 ́-thiobisbenzenethiol (TBBT) and decafluorobiphenyl. Optimization of this reaction allows for the obtainment of higher molecular weights than the ones reported in the literature. In a second step, poly(arylene sulfides) were phosphonated and sulfonated by a nucleophilic aromatic substitution (SNAr) displacement reaction of the fluorine atoms of the fluorinated polymer sub-units using different agents. Highly sulfonated polymers were obtained when using sodium 3-mercapto-1-propanesulfonate and resulted in water soluble ionomers. Kinetic studies of this reaction were performed and several new sulfonated poly(arylene sulfides) were obtained. Finally, stable polymer electrolyte membrane (PEM) with enhanced mechanical and chemical stability were obtained by blending these obtained ionomers with polybenzimidazole (PBIOO). These membranes were further characterized and in the best case a PEM with new sulfonated ionomer showed a conductivity 40 % higher than Nafion 212, used as a golden reference material. The best performing PEM’s obtained were further used in an electrolytic cell being part of eSCALED, a H2020 (MSC-ITN-2017. GA# 765376) European project which aim is to obtain a device that does the artificial photosynthesis in a more efficient way than the current devices.