22 research outputs found
The design and experimental investigation of an alumina reticulate porous ceramic heat exchanger for high temperatures
No full textUniversity of Minnesota Master of Science thesis. June 2014. Major: Mechanical Engineering. Advisor: Dr. Jane H. Davidson. 1 computer file (PDF); xvi, 85 pages, appendices A-D.The present study focuses on the design, modeling and testing of an alumina heat exchanger filled with reticulate porous ceramic (RPC). The heat exchanger has been designed to operate reliably at temperatures up to 1773 K, integrate seamlessly with the reactor designed for isothermal CO2 and H2O splitting using ceria and obtain an effectiveness of >0.85 for the range of flow rates anticipated during operation of the isothermal reactor. The RPC morphology, namely porosity and pore density and the geometry of the heat exchanger are selected based on the results of a fluid flow and heat transfer model of the heat exchanger. A prototype was also tested at temperatures up to 1240 K. The permeability, inertial coefficient, overall heat transfer coefficient, effectiveness and pressure drop were measured.Banerjee, Aayan. (2014). The design and experimental investigation of an alumina reticulate porous ceramic heat exchanger for high temperatures. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/172118
Multi-scale modelling of a Li-ion battery electrode with aligned pores
No full textThere is a growing interest in increasing the accessible capacity of Li-ion battery electrodes without compromising the power. This is a challenging task since as the electrode thickness increases, the ionic transport in the electrolyte becomes limiting. In order to facilitate ionic transport, we investigate the effect of the introduction of micro-channels aligned along the electrode thickness by using a multi-scale modelling approach. A volume-averaged electrochemical model is developed to take into account charge transport in concentrated solution, charge transfer at the solid/electrolyte interface, and electron migration and lithium diffusion in the electrode material [1]. Carbon nanoparticles are embedded within the electrode material to increase the effective electronic conductivity, which is evaluated through TauFactor [2] by simulating random dispersions of nanoparticles. A multi-scale optimisation strategy is adopted: at the micro-scale, the volume fraction of carbon nanoparticles is optimised in order to enhance the effective electronic conductivity
without compromising the electrode capacity; at the electrode level, the diameter, pitch and length of micro-channels are varied to maximise the power density. Results show that the electronic conductivity is significantly increased as soon as percolation of the carbon nanoparticles is achieved. At the electrode level, as long as the volume fraction of microchannels is constant, smaller pores lead to higher current densities. Finally, a positive electrode fabricated via freeze-casting according to optimised geometric parameters identified by the model is produced, tested and reconstructed with 3D X-ray
tomography. The difference between model predictions and real electrochemical performance may in part be attributed to the pore morphology of the actual sample, which is more complex than the prismatic forms captured in this model
Computational study of Ni-CGO as a solid oxide fuel cell anode
No full textIn this thesis, two computational works are presented to address the identified gap in two key operational reactions in SOFCs: partial methane oxidation (POM) and electro-oxidation of hydrogen (HOR). The model for both works are constructed based on intrinsic kinetics and provide a detailed description of physical and electrochemical phenomena occurring in the composite. In the POM study, a quasi 2D reactive-diffusion model is employed to reveal the main mechanism causing the unique profile of CO observed in transient experiments. Through mechanism analysis, a dynamic equilibrium of oxygen concentration established by oxygen bulk diffusion and spill-over effect was found to be the primary factor. A sensitivity study of the processes involved also suggests the presence of co-limiting mechanisms of these diffusive fluxes in the release of CO. Furthermore, the model has shown its capability to estimate the catalyst intrinsic properties by fitting parameters.
On the other hand, the HOR study was conducted to identify the role of Nickel in HOR catalysis on the Ni-CGO surface. The constructed model provides a comprehensive description of the processes occurreing in the electrode including charge transport, electrochemical reaction and mass transfer in the gas phase. The resultant model is capable of reproducing EIS data of two different composites: pure CGO and an impregnated Ni-CGO electrode. Followed by a process analysis, a hypothesis regarding to Nickel’s role was identified. Nickel could not only act as electron conductor, but also as an active surface facilitating reactant adsorption. Further study to validate this hypothesis is proposed in the conclusion sectionOpen Acces
Integrated Multiscale Modeling of Solid Oxide Electrodes, Cells, Stacks, and Systems
No full textAn integrated multiscale multiphysics framework to model solid oxide electrochemical cell systems is discussed. The framework directly couples electrode, single cell, stack, and system models using a hierarchical approach with minimal loss of information across the scales. The success of the model framework in better understanding solid oxide cells at the microscale and the industrial scale showcases it as a powerful tool for the rational design and upscaling of novel lab-scale materials and devices for next-generation chemical and electrochemical technologies.</p
Hierarchical Modeling of Solid Oxide Cells: From micro-kinetics to 3-D stacks
No full textIn dieser Arbeit wurde ein physikalisches Mehrskalen-Kontinuum-Modell zur Untersuchung von Feststoffoxidzellen (Solide Oxide Cells, SOCs), welche chemische in elektrische Energie oder umgekehrt umwandeln können, entwickelt und detailliert untersucht. Zur Beschreibung von Feststoffoxidzellen sind eine Vielzahl unterschiedlicher chemischer und physikalischer Prozesse in verschiedenen Größenordnungen zu berücksichtigen: von einzelnen Partikeln bis zur Kombination von mehreren Zellen zu sog. Stacks („Stapel“). Um die Leistungs- und Lebensdauerbeschränkungen solcher Zellen aufzudecken, wird eine hierarchische Multiskalen-Herangehensweise benötigt, die die chemischen Reaktionen im Nanometer-Bereich, die Transportphänomene im MikrometerBereich für eine einzelne Zelle bzw. im Millimeter-bis-Meter-Bereich für einen Stack koppelt. Das angewandte Modell bildet einen konsistenten mathematischen Rahmen, um die physikalischen Prozesse in all diesen Größenordnungen zu beschreiben. Dabei werden die fundamentalen Gleichungen für den Massen-, Impuls-, Ladungs- und Energieerhalt gelöst, wobei die heterogenen Reaktionen auf der Elektrodenoberfläche und die elektrochemischen Reaktionen in den elektronisch-ionischen Grenzflächen als volumetrische Quellenterme behandelt werden. Die Eigenschaften des Modells werden an zwei Beispielen demonstriert. Zuerst wird die Produktion von Wassergas (H2+ CO) via CoElektrolyse von H2O und CO2 in einer Feststoffoxid-Knopfzelle, einer einzelnen Repeating Unit („Wiederholungseinheit“) und einem Stack untersucht, um die Auswirkungen der Hochskalierung auf Leistung, Kapitalkosten und Lebensdauer aufzudecken. Um realistische kinetische Parameter zu erhalten, wurde das Modell an Messdaten für die Polarisierung, die Temperatur und die Gaszusammensetzung am Ausgang angepasst, welche von Fu et al. in einer Einzel- Ni-GDC|YSZ|LSM-YSZ-Zelle gemessen wurden [ECS Transactions 35, 2949-2956 (2011)]. Außerdem wurde das so kalibrierte Modell verwendet, um 3D-Kontour-Plots zu erstellen, welche die Leistung einer einzelnen Zelle eines F-Design-Stacks des Forschungszentrums Jülich [Fuel Cells 7, 204-210 (2007)] abbilden. Dabei wurden am Ausgang zwei verschiedene H2/CO-Verhältnisse gewählt, welche kommerziell für die Hydroformylierung und die Fischer-Tropsch-Synthese zum Einsatz kommen. Eine parametrische Analyse zeigte die optimalen Betriebsbedingungen für eine Repeating Unit auf, wodurch die Bedeutung von Aufenthaltsdauer, Spannung und Temperatur für die Effizienz und die Wassergas-Ausbeute unterstreichen wird. Im Falle einer Hochskalierung von einzelnen Zellen zu kommerziell eingesetzten Stacks führte der Betrieb bei höchster Effizienz zu den geringsten Temperaturgradienten innerhalb des Gerätes. Jedoch die geringen Stromdichten führten zu höheren Kapitalkosten und größeren Elektrolysator-Flächen für eine industrielle Wassergas-Produktion. Die Simulation eines Stacks für verschiedene Betriebspunkte zeigte auf, dass der Stack im Bereich des schnellen Massen- und Ladungstransports vom Wärmetransport limitiert wird. Nachdem die analytischen und die Optimierungsmöglichkeiten eruiert wurden, dient das zweite Beispiel dazu, die Universalität des Modells zu verbessern. Dafür wurden kinetische Parameter bestimmt, welche intrinsisch für ein Material und damit unabhängig von der Mikrostruktur der Elektrode sind. Zuletzt wurde das Modell für eine Simulation einer FeststoffoxidHalbzelle genutzt, welche aus einer porösen LSM-YSZ-(Sauerstoff)-Elektrode, die auf einen dichten YSZ-Elektrolyten gesintert wurde, besteht. Dabei wurde die Kinetik der Sauerstoffreduktionsreaktion untersucht, wobei eine intrinsische Kinetik für die YSMLSZ-Elektrode erhalten wurde. Für die Modellierung der elektrochemischen Reduktion von O2 wurden drei verschiedene Mechanismen verwendet. Jeder Mechanismus beinhaltete hierbei den parallelen Transport von Sauerstoff-Spezies über die Partikeloberfläche und durch das Teilchen hindurch, wobei drei verschiedene elektrische Potentiale die Reduktion antrieben. Die Mechanismen wurden mit drei Sets von elektrochemischen Impedanz-Spektren und Polarisationskurven verglichen, welche von Barbucci et al. [J. Appl. Electrochem, 39, 513–521 (2009)], Cronin et al. [J. Electrochem. Soc., 159(4), B385–B393 (2012)] und Nielsen und Hjelm [Electrochimica Acta, 115, 31– 45 (2014)] über einen weiten Bereich von Betriebstemperaturen (873 K bis 1173 K), Einlass-O2-Konzentrationen ( 5% bis 100%) und Überpotentialen (-1 V bis +1 V) aufgenommen wurden. Zwei der drei Mechanismen konnten quantitativ diese drei experimentellen Sets ohne neue Parametrisierung der Kinetik wiedergeben. Durch Analyse der thermodynamischen Parameter und der Kinetik konnte jener Mechanismus, der die Chemisorption eines gasförmigen O2-Moleküls unter Bildung einer Super-Oxo-O2 -Spezies auf der LSM-Oberfläche postuliert, als der am besten der Realität entsprechende identifiziert werden. Diese zwei Beispiele heben somit hervor, dass diese Modellierung für SOC- bzw. Stack-Design, -Überwachung und -Steuerung ein geeignetes Werkzeug darstellt
Benchmarking solid oxide electrolysis cell-stacks for industrial Power-to-Methane systems via hierarchical multi-scale modelling
No full textPower-to-Gas (PtG) is prognosticated to realize large capacity increases and create substantial revenues within the next decade. Due to their inherently high efficiencies, solid oxide electrolysis cells (SOECs) have the potential to become one of the core technologies in PtG applications. While thermal integration of the high-temperature SOEC module with downstream exothermic methanation is a very potent concept, the performance of SOECs needs to be boosted to amplify the technologies impact for future large-scale plants. Here, we use a combined experimental and modelling approach to benchmark commercial electrolyte- (ESC) and cathode-supported cell (CSC) designs on industrial-scale planar SOEC stack performance. In a first step, comprehensive electrochemical and microstructural analyses are carried out to parametrize, calibrate and validate a detailed multi-physics 2D cell model, which is then used to study the cells’ behaviour in detail. The analysis reveals that there exists a cell-specific threshold steam conversion of ∼80% for the ESC and ∼75% for the CSC design, which represents a maximum of the total (heat plus electrical) electrolysis efficiency. Moreover, while the ESC-design suffers from performance reductions under pressurized conditions, considerable performance increases of ∼9% at 20 atm (700 °C, 1.35 V) compared to atmospheric pressure are predicted for the CSC design, showcasing a unique advantage of the CSC cell for process integration with the catalytic methanation. Subsequently, based on a 3D stack model, a scale-up to the industrial stack size is conducted. To comparatively assess stack performances under application-oriented conditions, optimization studies are carried out for 150-cell stack units based on the two cell designs individually. When optimally selecting the stack operation points, the model predicts the CSC-based stack to reach a high capacity up to 36.6 kW (∼10.6 Nm H h) at 1.35 V and 700 °C, whilst ensuring reasonably low temperature gradients (<10 K cm) and sweep gas cooling requirements (<30 sccm cm). Thus, CSC-design stacks incorporating such a highly active cell design can be expected to further boost the competitiveness of high-temperature electrolysis in PtG plant concepts
Simulation of bi-layer cathode materials with experimentally validated parameters to improve ion diffusion and discharge capacity
No full textThe prospect of thick graded electrodes for both higher energy and higher-power densities in lithium-ion batteries is investigated. The simulation results discussed in previous reports on next-generation graded electrodes do not recognize the effect of material processing conditions on microstructural, transport and kinetic parameters. Hence, in this work, we focus on the effect of material processing conditions on particle morphology and its subsequent influence on microstructure (porosity and tortuosity), along with the resultant transport (solid-phase diffusivity) and kinetic (reaction rate constant) properties of synthesized single-layer cathodes. These experimental insights are employed to simulate the benefits of 400 μm thick bi-layer graded cathodes with two different particle sizes and porosities in each layer. The microstructural, transport, and kinetic information are obtained through 3D imaging and electrochemical impedance spectroscopy (EIS) techniques. These parameters are used to develop bi-layer numerical models to understand transport phenomena and to predict cell performance with such graded structures. Simulation results highlight that bi-layer cathodes display higher electrode utilization (solid phase lithiation) next to the current-collector compared to conventional monolayer cathodes with an increase of 39.2% in first discharge capacity at 2C. Additionally, the simulations indicate that an improvement of 47.7% in energy density, alongside a marginal increase of 0.6% in power density, can be achieved at 4C by structuring the porosity in the layer next to the separator to be higher than the porosity in the layer next to the current-collector
Study of Bi-layer electrodes for lithium-ion batteries through simulation and experiment
No full textThe need to develop secondary lithium-ion batteries (LIBs) with high-energy and high-power density is imperative for the advancement of portable devices, electric vehicles (EV), and integrated renewable energy systems in order to meet future energy demands while reducing fossil fuel dependency and mitigating global environmental issues. LIB designed for high energy density suffer from low power density. Therefore, increasing energy density without compromising power density represents a great challenge for battery researchers and manufacturers. One of the promising avenues for improving energy and power density is electrode engineering through grading a thick (≥200 µm) electrode by spatially varying microstructures (i.e., porosities and/or particle sizes). To date, most of the reports on graded electrodes have focused on the modelling effort to predict cell performance. While previous modelling studies have predicted both considerable and marginal improvement in cell performance, very few experimental studies have been conducted to validate the performance of such electrode designs. In addition, the simulation results discussed in previous reports on next-generation graded electrodes do not recognize the effect of ball milling conditions on microstructural, transport and kinetic parameters.
At first, this thesis focusses on investigating the effect of material processing conditions on NMC particle morphology and the resulting microstructural (i.e., porosity, tortuosity), transport (solid phase diffusivity) and kinetic (reaction rate constant) properties of synthesized single-layer cathodes. The cathode microstructures composed of small particles are found to be less porous and more tortuous than the cathode microstructures composed of big particles. An increase in solid-phase diffusivity and reaction rate constants due to decrease in crystallinity and increase in interfacial surface area respectively are observed for cathodes with small particles. Next, these experimentally obtained parameters are used to develop a coupled electrochemical-thermal model and simulate the performance of a 400 µm thick bi- layer cathode with two different particle sizes and porosities in each layer. The simulation results predict that higher electrode utilization and discharge capacity at higher C-rates (1-4C) can be achieved by positioning large particles with higher porosity in a layer next to the separator and small particles with lower porosity in a layer next to the current-collector. The bi-layer cathodes also show promising performance for both energy and power applications. Motivated by the simulation results, a 200 µm thick bi-layer cathode was synthesized in order to compare cell performance against monolayer cathodes. The bi-layer cathode exhibited high discharge capacity with increasing C-rates (1-2C) compared to monolayer cathodes during initial cycling performance. However, long term cycling (100 cycles) analysis reveals that bi-layer cathode retains no advantage over a conventional electrode (i.e., a monolayer cathode composed of big particles) in a half-cell configuration. Finally, the benefits of grading such microstructures are contrasted against the constraints associated with their synthesis through traditional slurry-casting methods.Open Acces
Revisiting the promise of bi-layer graded cathodes for improved Li-ion battery performance
No full textImproving power and energy density by grading electrode microstructures is a promising topic in the field of battery electrode engineering. While previous modelling studies have predicted both considerable and marginal improvements in cell performance, very few experimental studies have been conducted to validate the performance of graded electrodes. In this article, we report on the fabrication of a bi-layer graded lithium-ion battery cathode by varying both the particle size and the porosity in each layer. Structural analyses were carried out via 2D (scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX)) and 3D (X-ray computed tomography (XCT) and focused-ion beam tomography (FIB)) imaging techniques. The bi-layer cathode (BLC) exhibits an increase of 62.8% in discharge capacity at 2C compared to a conventional single layer electrode. The polarization and electrochemical impedance spectroscopy data indicate that the improved capacity performance of the BLC can be attributed to reduced charge transfer resistance and increased solid phase diffusivity. However, capacity retention performance reveals that the BLC retained no advantage over a conventional electrode in a half-cell configuration after 100 cycles. At 1C, the BLC displayed only minimal improvement in power (4.6%) and energy (7.6%) density based on first discharge capacity. As such, noting the extra challenges involved in manufacturing such graded electrode structures, it is recommended that their use is best focused on higher C rate applications and that more work is needed to demonstrate the retention of the higher C rate performance gain over multiple cycles
Optimizing solid oxide fuel cell performance to re-evaluate its role in the mobility sector
No full textA sustainable, interconnected, and smart energy network in which hydrogen plays a major role cannot be dismissed as a utopia anymore. There are vast international and industrial ambitions to reach the envisioned system transformation, and the decarbonization of the mobility sector is a central pillar comprising a huge economic share. Solid oxide fuel cells (SOFCs) are one of the most promising technologies in the brigade of clean energy devices and have potentially wide applicability for transportation, due to their high efficiencies and impurity tolerance. To uncover future pathways to boost the cell’s performance, we propose a detailed multiscale modeling methodology to evaluate the direct impact of cell materials and morphologies on commercial-scale system performance. After acquiring intrinsic electrokinetics decoupled from mass and charge transport of different anode and cathode materials via a half-cell model, a full cell model is employed to identify the most promising electrode combination. Subsequently, a scale-up to the system level is performed by coupling a 3-D kW-stack model to the balance of plant components while focusing on morphological optimization of the membrane electrode assembly (MEA). On optimally tailoring the MEA, model results demonstrate that an advanced cell design comprising a Ni fiber-CGO matrix structured anode and a LSCF-infiltrated CGO cathode could reach a stack power density of 1.85 kW L–1 and a net system efficiency of 52.2% for operation at <700 °C, with manageable stack temperature gradients of <14 K cm–1. The model-optimized power density is substantially higher than those of commercial stacks and surpasses industrial targets for SOFC-based range extenders. Thus, with further cell and stack development targeting the performance limiting processes elucidated in the paper, commercial SOFCs could, alongside range extenders, also act as prime movers in larger scale transport applications such as trucks, trains, and ships
