04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access The lithium-sulfur battery : design, characterization, and physically-based modeling(2015) Fronczek, David Norman; Bessler, Wolfgang G. (Prof. Dr.)The lithium-sulfur (Li/S) battery is a promising candidate for next-generation electrochemical energy storage. Its unique combination of electrochemical performance, cost effectiveness, and environmental sustainability are unprecedented among battery materials. As of today, however, Li/S batteries are only used for few niche applications; a broader adoption of this technology is impeded by the yet unsatisfactory energy efficiency, self discharge, and limited lifetime. This work contributes to the advancement of Li/S technology in two respects: First, a novel kind of positive electrode, based on coated lithium sulfide, was prepared, tested and optimized. Second, the understanding of the complex chemical and physical processes in the cell was improved by creating and utilizing a computational model of the Li/S battery. For the experimental part of this work, a chemical vapor deposition process was developed to apply a carbon coating to lithium sulfide particles. The focus of this work was on the optimization of the process chain from commercially available chemicals to the final coin cell in general and on the characterization of the materials and electrodes during various processing steps in particular. For the modeling part, an existing multiscale electrochemical modeling framework was extended to enable full-cell simulations of Li/S batteries. The model includes a detailed description of electrochemistry, transport, and the evolution of solid phases in the cell, but also accounts for the electrical double layer and, in a generic fashion, the microstructure of the electrodes. Finally, a phenomenological description of the shuttle effect and associated cell degradation was implemented and analyzed. The parametrization and partial validation of the model makes use of original data collected for this purpose, but also data from literature. Simulation results comprise charge/discharge profiles, cyclic voltammetry, impedance spectra, and the evolution of the chemical composition of both the electrolyte and the electrodes over time. The analysis of these results reveals limiting factors and suggests improved operating conditions. The apt combination of theoretical and empirical methods enabled an improvement of the performance and cycle life of the novel cathode material, but also contributed to a more profound understanding of the Li/S battery.Item Open Access Utilization of various projectiles to mitigate fouling in tubular heat exchangers(2016) Jalalirad, Mohammad Reza; Müller-Steinhagen, Hans (Prof. Dr. Dr.-Ing. habil.)Heat exchangers are the workhorse of most chemical, petrochemical, food processing and power generating processes. Of the many types of heat exchangers, approximately 60% of the market is still dominated by shell and tube heat exchangers. One major problem of heat exchangers and particularly the shell and tube type is directly related to the deposition of unwanted materials on the heat transfer surfaces. Fouling may cause one or more of several major operating problems: i) reduction of heat transfer, ii) under-deposit corrosion, iii) increased pressure loss and iv) flow mal-distribution. There are many different mitigation techniques available in the market to maintain the surface of heat exchangers clean to some extent. Among them are projectiles of various shapes, materials and hardnesses which circulate via a separate loop through the exchanger. The advantages of this method include effective fouling mitigation and stable operating conditions. Having said that, there are nevertheless numerous unanswered questions such as optimum injection interval, minimum required shear force to remove fouling layers, applicability of projectiles at elevated temperatures, minimum required velocity of projectile propulsion, and the criterion for the selection of projectiles for any specific fouling process. The present study, as part of a European Project entitled Clean-Ex, endeavors to address some of these questions. A test rig was designed and constructed to simulate conditions under which fouling occurs in water service processes. The rig includes an online cleaning device which enables introduction of projectiles for various operating scenarios including i) continuous or ii) at different time intervals. A comprehensive set of experimental runs was carried out for crystallization fouling of CaSO4 solutions with and without projectiles. Due to laboratory restrictions, fouling runs were performed at accelerated conditions to rigorously characterize the impact of projectile cleaning in terms of injection intervals and various types of projectiles. The experimental results showed that the projectiles are capable of removing parts of the fouling layer at the early stage of the fouling process. The cleaning efficiency decreases as the fouling layer builds up such that the projectile is not effective when the asymptotic fouling is approached. In addition, shorter injection intervals of the projectiles decrease the asymptotic fouling resistance. Sintering of the fouling layer which hinders the cleaning action of projectiles should be accounted for this phenomenon. Furthermore, all projectiles decreased the induction time of the fouling process. The asymptotic fouling resistance was also approached much quicker compared to the case of no injection. The performance of any projectile lies in a trade-off between its size, texture and stiffness. Stiffness produces a shear force required to dislodge the deposit and size is required to maintain the contact area between projectile and the surface. Accordingly, a criterion was developed to determine the optimum projectile size and stiffness for best cleaning performance. The criterion shows that bigger and softer projectiles cannot last for a long time injection processes. Given the importance of size and stiffness, the projectiles were subsequently divided into two groups of hard and soft due to the required stiffness and velocity to move the projectile within the tube. To discriminate between these two groups, a new term called contact stability factor or Z factor is proposed which is a function of stiffness and size. A mechanistic model has also been developed to predict the asymptotic fouling resistance when projectiles are in operation, based on injection rate, fouling rate and removal rates. The model predicts the asymptotic fouling resistance with an accuracy of 69% based on CaSO4 concentration, saturation concentration, injection interval, shear force and contact stability of the tube with the projectile.Item Open Access Herstellung und Charakterisierung bifunktioneller Sauerstoffelektroden für hochenergetische Lithium-Luft-Batterien(2016) Wittmaier, Dennis; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Der zunehmende Energiebedarf mobiler und portabler Anwendungen sowie das Ziel, den Bedarf an elektrischer Energie des täglichen Lebens aus regenerativen Energien zu decken, führt zu ständig wachsenden Anforderungen an heutige Energiespeicher. Als Energiespeicher mit einer hohen Vielseitigkeit und Energiedichte hat sich die Lithium-Ionen-Batterie (LIB) in vielen Bereichen etabliert. Trotz großer Anstrengungen bei der Erhöhung der Energiedichte stößt die LIB aufgrund der verwendeten Aktivmaterialien an ihre Grenzen. Lithium-Batterien der nächsten Generation versprechen jedoch eine signifikante Steigerung der Energiedichten und besitzen das Potenzial, die Energiespeicher der Zunkunft zu sein. Metall-Luft-Batterien und im Speziellen der Lithium-Luft-Batterie (LAB) wird dabei die größte Steigerung der Energiedichte zugetraut. Bevor diese Batterien jedoch ihren Weg in die Anwendung finden können, müssen viele Fragestellungen geklärt werden. Als eine schwierige Fragestellung gilt die Entwicklung einer bifunktionellen Sauerstoff-/Gasdiffusionselektrode zur Katalyse der Lade- und Entladereaktion der Lithium-Luft-Batterie. Die Herausforderung stellt dabei nicht die Katalyse der Entladereaktion (Sauerstoffreduktion), sondern die bei der Ladung stattfindende Sauerstoffentwicklung dar. In der vorliegenden Arbeit sollte eine bifunktionelle Sauerstoffelektrode entwickelt und charakterisiert werden, welche eine geringe Überspannung bei diesen Reaktionen realisiert. Zur Entwicklung einer solchen Sauerstoffelektrode ist das Auffinden von möglichst effektiven Katalysatoren unverzichtbar. Neben dem umfangreichen Sichten möglicher Katalysatoren mussten auch entsprechende Herstellverfahren zur technischen Umsetzbarkeit untersucht werden. In einem nächsten Schritt konnten dann bifunktionelle Elektroden mit entsprechenden Katalysatoren zum Vergleich untereinander hergestellt werden und strukturelle sowie materialwissenschaftliche Parameter dieser Elektroden weiter untersucht werden. Im Fall der Katalysatoren stellte sich das Co3O4 als vielversprechend heraus, so dass hiermit weiter verfahren wurde. Die Kombinationen aus Co3O4 mit zum einen Ag und zum anderen Ni wurden auf das optimale Verhältnis der beiden Katalysatoren untersucht und mit Maßstab Ag- bzw Ni/IrO2-Elektroden verglichen. Für die sich als sehr vielversprechend herausstellende Kombination aus Ag und Co3O4 wurde anschließend weiter der Einfluss der Elektrodendicke, der Porosität, der Temperatur, der Elektrolytkonzentration und des Gases auf die erreichten Stromdichten untersucht. Als letzte Punkte folgten eine materialwissenschaftliche Untersuchung der verwendeten Materialien über einen Lade- und Entladezyklus und eine Simulation des Einflusses von Strukturparametern auf beispielsweise die Permeation von Gasen durch die Gasdiffusionselektrode (GDE). Es konnte gezeigt werden, dass der Katalysator, die Zusammensetzung, die Elektrodendicke, die Temperatur und die Elektrolytkonzentration den wohl größten Einfluss auf die Stromdichten der Gasdiffusionselektroden haben. Eine Veränderung der Porosität wirkte sich bei den untersuchten GDEs entgegen der Erwartungen stets negativ aus.