04 Fakultät Energie-, Verfahrens- und Biotechnik

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Institute: search.filters.UBSInstitut.Institut für Gebäudeenergetik, Thermotechnik und EnergiespeicherungSubject: search.filters.subject.660

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    Multistep reactions of molten nitrate salts and gas atmospheres
    (2024) Steinbrecher, Julian; Thess, André (Prof. Dr.)
    Dissertation zur Untersuchung der Stabilität von Nitratsalzschmelzen unter verschiedenen atmosphärischen Bedingungen und Temperaturen.
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    Perovskite chromite-based fuel electrode for solid oxide cells (SOCs): towards the understanding of the electrochemical performance
    (2023) Amaya Dueñas, Diana María; Friedrich, K. Andreas (Prof. Dr. rer. nat.)
    The current energy transition is a key driver for the continuous development of fuel cells and electrolyzers due to the rapid growth of the clean energy demand and the need to overcome the intermittency of the power supply of renewable energy sources, such as wind and solar energy. In this regard, solid oxide cells (SOC) are promising systems that allow to overcome such fluctuations: they convert renewable electrical energy into chemical energy in the form of hydrogen and valuable fuels and chemicals, while they can also repower the grid by converting fuels and hydrogen into electrical power. This feature in reversibility has attracted the interest among Power-to-X technologies, which can be exploited by operating SOCs in fuel cell (SOFC), electrolysis (SOEL) and reversible (rSOC) modes. Nevertheless, SOCs are not yet a mature technology due to limitations on the performance of their electrolyte and electrodes. Typical fuel electrodes made of Ni-based cermets are in contact not only with hydrogen, but also with reactants such as natural gas, biogas, steam and carbon dioxide, leading to important operation issues related to high temperatures and poisoning tolerance, which significantly detriment the performance of these systems. Due to the urgent need for the development of sustainable SOC systems in clean energy scenarios, this thesis aims to cover the Ni cermets issues related to SOCs operation, such as nickel agglomeration, nickel migration, structural cell damage and carbon deposition. Therefore, with the motivation to propose alternative fuel electrode materials to the state-of-the-art Ni cermets, formulations of perovskite chromite-based fuel electrodes were investigated in different SOC operating conditions. Firstly, different perovskite compositions were investigated by X-ray diffraction (XRD) to ensure the desired phase. With these crystal structure characterizations, the lanthanum-chromite perovskite with Ni doping (LSCrN) was selected as candidate fuel electrode material with the compositions La0.7Sr0.3Cr0.85Ni0.15O3-δ (L70SCrN) and La0.65Sr0.3Cr0.85Ni0.15O3-δ (L65SCrN). These materials were synthetized by the glycine-nitrate combustion method and ceramic powder morphology was characterized by scanning electron microscopy (SEM). An experimental protocol for the cell manufacturing process was designed and the electrolyte-supported-cells (ESCs) were produced by screen-printing, drying and sintering processes. ESCs were tested in different operating SOC modes: fuel cell (SOFC), steam electrolysis (SOEL), steam and carbon dioxide co-electrolysis (co-SOEL), as well as in reversible mode (rSOC) and even in dry carbon dioxide electrolysis operation. In situ electrochemical characterizations were performed by evaluating the voltage - current response and the electrochemical impedance spectroscopy (EIS). In parallel, the exsolution of nickel particles from the produced LSCrN ceramic powders was investigated by means of temperature programmed reduction (TPR), X-ray spectroscopy (XPS) and XRD techniques. It was shown that the introduction of A-site deficiency promoted the reduction of metallic nickel particles on the perovskite surface. The particle distribution was found to be dependent on the temperature, the atmosphere and the overpotential. In co-SOEL operation, cells with the developed L65SCrN electrode showed a comparable performance to the ones with state-of-the-art Ni cermets, e.g. - 0.8 A·cm-2 at 1.32 V and 860 °C. The long-term stability (~ 1000 hours) suggested that under strongly reducing atmospheres, such as in SOEL at 860 °C, the L65SCrN electrode suffered from accelerated performance degradation due to an alteration of the transport properties. Nonetheless, it was found that a decrease in operating temperature (below 830 °C) could be a suitable strategy to mitigate this durability issue. These findings are related to a gain in performance of the perovskite electrodes against the state-of-the-art Ni electrodes at temperatures between 770 °C and 830 °C, possibly due to lower reaction energy barriers. These outcomes were used as basis for a scale-up analysis from the cell level up to the system level, i.e. up to the MW scale, by analyzing a real case application of SOEL-based systems for hydrogen production. This analysis suggested that the implementation of perovskite electrodes in SOEL systems, together with a decrease of the system operating temperature, would lead to a significant reduction of the number of cells in the stacks and hence of the system components, simplifying the system layout. Additionally, the required amount of Ni raw material would also be significantly decreased, which would mitigate future supply chain issues that the mineral market may experience in the upcoming years. This study paves the way for future alternative electrode development for SOC applications while suggesting potential benefits at the system scale.
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    Temperature reduction as operando performance recovery procedure for polymer electrolyte membrane fuel cells
    (2024) Zhang, Qian; Schulze, Mathias; Gazdzicki, Pawel; Friedrich, Kaspar Andreas
    To efficiently mitigate the reversible performance degradation of polymer electrolyte membrane fuel cells, it is crucial to thoroughly understand recovery effects. In this work, the effect of operando performance recovery by temperature reduction is evaluated. The results reveal that operando reduction in cell temperature from 80 °C to 45 °C yields a performance recovery of 60-70% in the current density range below 1 A cm-2 in a shorter time (1.5 h versus 10.5 h), as opposed to a known and more complex non-operando recovery procedure. Notably, the absolute recovered voltage is directly proportional to the total amount of liquid water produced during the temperature reduction. Thus, the recovery effect is likely attributed to a reorganization/rearrangement of the ionomer due to water condensation. Reduction in the charge transfer and mass transfer resistance is observed after the temperature reduction by electrochemical impedance spectroscopy (EIS) measurement. During non-operando temperature reduction (i.e., open circuit voltage (OCV) hold during recovery instead of load cycling) an even higher recovery efficiency of >80% was achieved.
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    On the mass transport phenomena in proton exchange membrane water electrolyzers
    (2020) García Navarro, Julio César; Friedrich, K. Andreas (Prof. Dr. rer. nat.)
    Proton exchange membrane (PEM) water electrolysis is a technology designed to produce hydrogen using only water and electricity as inputs; it has gained increased attention in industry and academia due to its advantages over incumbent hydrogen generation processes (of which the most widely used are steam reforming and coal gasification) namely, low temperature, carbon-neutral and intermittent operation. PEM electrolysis can be instrumental for creating a hydrogen economy, although still much research needs to be carried out before widespread industrial adoption is achieved. PEM water electrolyzers suffer energy losses associated with the chemical reactions and the transport of charge and mass; of these phenomena, mass transport in PEM electrolyzers is the least understood subject, given the complex nature of the interaction of multiphase flows (mainly consisting of liquid water and evolved gases) through micrometric pores. The subject of multiphase flow in water electrolysis and its relationship with the mass transport phenomena in PEM water electrolysis has been a prevalent subject in the literature. Despite numerous attempts at pinpointing the relationship between mass transport overpotential and the operating parameters, there is no clear consensus about which transport mechanisms dominate, nor about how the component design of PEM electrolyzers affects the mass transport. While the effect of temperature and current density on mass transport losses has been extensively studied and is well understood, there are significantly fewer studies that focus on the effect of water flow and pressure. Both water flow and pressure have a direct effect on mechanisms such as bubble nucleation and two-phase flows that occur in the porous structures within a PEM electrolyzer (electrodes and porous transport layers, PTLs). In this work, I studied the effect of water flow and pressure on the mass transport phenomena in PEM electrolyzers. Chapters 1 and 2 provide an introduction to the topic as well as a description of the materials and experimental setups used. Chapter 3 of this thesis depicts the visualization and modeling of bubble nucleation in an operating PEM electrolyzer. I discovered that bubble detachment radii are largely independent of water flow and I identified two types of bubbles: bubbles that detach after reaching a critical size, and bubbles that fill up the pores of a PTL before detaching. Chapter 3 consists of the measurements I carried out regarding the transport of evolved gas through the water-filled pores of a PTL, where I observed that water flow severely impedes the gas transport through the pores and that such impediment is related to a shear stress exerted by the water flow on the pores. Chapter 5 shows the measuring of mass transport losses using electrochemical impedance spectroscopy (EIS) on an operating PEM electrolyzer; the results indicate that pressure and water flow affect the diffusion of gas in the electrode and that the mass transport overpotential depends on design parameters of the PEM electrolyzer, such as electrode thickness and hydrophobicity. Overall, I derived a theoretical framework based on the assumption that the evolved gas in a PEM electrolyzer permeates through the PTL after diffusing from the active sites to the bubble nucleation sites. Such framework, constructed on the basis of the models regarding gas transport in porous media, can be used to explain the mass transport loses in a PEM electrolyzer that arise from operating with increased water flows and pressures. The model I derived can be used in future work as a guideline to optimize the components of a PEM electrolyzer, in particular regarding the hydrophobicity and pore size distribution of PTLs as well as the composition of the catalyst ink to produce the electrodes. Moreover, this work can also be used to further understand the mass transport losses and optimize the operation of PEM electrolyzers to decrease the energy consumption of hydrogen generation.
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    Experimental analysis of the co-electrolysis operation under pressurized conditions with a 10 layer SOC stack
    (2020) Riedel, Marc; Heddrich, Marc P.; Friedrich, K. Andreas
    This study examines the performance of a solid oxide cell (SOC) stack during co-electrolysis of CO2 and H2O at elevated pressures up to 8 bar. Steady-state and dynamically recorded U(i)-curves were performed in order to evaluate the performance over a wide temperature range and to quantify the area specific resistance (ASR) at different pressure levels. Furthermore, the outlet gas composition at various current densities was analyzed and compared with the thermodynamic equilibrium. The open circuit voltage (OCV) was found to increase with higher pressure due to well known thermodynamic relations. An increase of the limiting current density at elevated pressure was not observed for the investigated stack with electrolyte supported cells. The ASR of the stack was found to decrease slightly with higher pressure. It revealed an increase of the cell resistance with lower H/C ratios in the feed at lower temperatures, whereas the performance of the co-electrolysis was very similar to steam electrolysis for temperatures above 820 °C. Within an impedance study for steam, co- and CO2 electrolysis operation it was shown that pure CO2 electrolysis exhibits a higher pressure sensitivity compared to pure steam or co-electrolysis due to significantly increased activation and diffusion resistances.
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    A sustainable CVD approach for ZrN as a potential catalyst for nitrogen reduction reaction
    (2024) Glauber, Jean-Pierre; Lorenz, Julian; Liu, Ji; Müller, Björn; Bragulla, Sebastian; Kostka, Aleksander; Rogalla, Detlef; Wark, Michael; Nolan, Michael; Harms, Corinna; Devi, Anjana
    In pursuit of developing alternatives for the highly polluting Haber-Bosch process for ammonia synthesis, the electrocatalytic nitrogen reduction reaction (NRR) using transition metal nitrides such as zirconium mononitride (ZrN) has been identified as a potential pathway for ammonia synthesis. In particular, specific facets of ZrN have been theoretically described as potentially active and selective for NRR. Major obstacles that need to be addressed include the synthesis of tailored catalyst materials that can activate the inert dinitrogen bond while suppressing hydrogen evolution reaction (HER) and not degrading during electrocatalysis. To tackle these challenges, a comprehensive understanding of the influence of the catalyst's structure, composition, and morphology on the NRR activity is required. This motivates the use of metal–organic chemical vapor deposition (MOCVD) as the material synthesis route as it enables catalyst nanoengineering by tailoring the process parameters. Herein, we report the fabrication of oriented and facetted crystalline ZrN thin films employing a single source precursor (SSP) MOCVD approach on silicon and glassy carbon (GC) substrates. First principles density functional theory (DFT) simulations elucidated the preferred decomposition pathway of SSP, whereas ab initio molecular dynamics simulations show that ZrN at room temperature undergoes surface oxidation with ambient O2, yielding a Zr-O-N film, which is consistent with compositional analysis using Rutherford backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS) depth profiling. Proof-of-principle electrochemical experiments demonstrated the applicability of the developed ZrN films on GC for NRR and qualitatively hint towards a possible activity for the electrochemical NRR in the sulfuric acid electrolyte.
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    Degradation study on solid oxide steam electrolysis
    (2020) Hörlein, Michael Philipp; Friedrich, K. Andreas (Prof. Dr.)
    Untersuchung der Degradation von Festoxidzellen im Elektrolysebetrieb von Wasserdampf anhand von Variationen der Betriebsbedingungen.
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    Solar-driven water electrolysis : new multijunction solar cells and electrolysis materials
    (2023) Branco, Bruno; Friedrich, K. Andreas (Prof. Dr. rer. nat.)
    The unpredictability and intermittency associated with solar energy render it unsuccessful to fully replace fossil fuels. Expansion of the deployment of renewable energy will also depend on appropriate energy storage methods and water electrolysis arises as one of the most promising of such technologies. However, solar-driven water electrolysis currently relies on noble metal and metal oxide electrocatalysts and bipolar plates, perfluorinated polyelectrolytes, and III-V multijunction solar cells that have high economic and environmental burdens. Developing and implementing zero-carbon and cost-effective solutions is thus fundamental to reduce the global dependence on fossil fuels and achieve carbon-neutrality. This work focuses on testing proton-exchange membranes based on fluorinated polymers, preparing membrane-electrode assemblies by printing methodologies, and combining a water electrolysis cell with perovskite-based multijunction solar cells to enable high solar-to-hydrogen conversion. The research is part of the eSCALED project that aims at creating new materials and devices to eventually combine into an artificial leaf, mimicking natural photosynthesis. Firstly, in chapter 2, to ensure reproducible and comparable results with literature work when using state-of-the-art materials, a water electrolysis setup and cell were built and optimized. The optimization process involved the study of the cell clamping torque, cell temperature, flow field’s material, porous transport layers (PTLs) materials and sample storage environment. After the improvement of the resistive losses, reaction kinetics, and overall stability, the proton exchange membrane water electrolysis reached current densities as high as 1 A cm−2 below 1.70 V. These results established the benchmark for the new electrolysis materials and preparation methodology investigated in later chapters. In chapter 3, a monolithic two-terminal multijunction solar cell that combined a wide-bandgap perovskite (PVK) semiconductor with a narrow-bandgap crystalline silicon (c-Si) was connected to the water electrolysis cell for solar-driven water electrolysis operation under 1-Sun equivalent light intensity. Two-terminal multijunction solar cells provide an open-circuit voltage (Voc) beyond the standard cell potential for water electrolysis while also increasing the power conversion efficiency (PCE) above the limit of series-connected single-junction cells. The PVK-Si tandem solar cell attains a Voc above 1.75 V, enough to conduct water electrolysis, while reaching high current densities that enable a solar-to-hydrogen efficiency (STH) of 21.5% when using state-of-the-art catalysts and membranes. This STH value is presently the highest reported value for a system operating without sunlight concentration. The system also represents the first example of a two-terminal PVK-Si multijunction solar cell coupled to a flow electrochemical cell operating in normal sunlight. In chapter 4, the PVK-Si tandem solar cell is replaced by an all-perovskite tandem solar cell that provides a higher Voc of almost 2 V, widening the operating voltage range at the expense of some current density. The slightly increased Voc is beneficial in case less efficient, but cost-effective electrocatalysts are used as the overpotential for water electrolysis rises. Solar-driven water electrolysis conducted with the all-perovskite tandem solar cell reached a STH close to 19% while using a comparatively inexpensive semiconductor and state-of-the-art catalysts and membranes. Additionally, this chapter describes the optimization of a narrow-bandgap perovskite solar cell to potentially increase the PCE of the all-perovskite tandem solar cell and STH of the coupled system. The use of bulk additives and top surface treatments of the perovskite layer enabled a maximum PCE of 18.6%, a 3.1% increase over the previous procedure. Such optimization combined with further improvements on the wide-bandgap sub-cell might elevate the PCE of the all-perovskite tandems above 26% and STH to almost 20%. In the next chapter, the water electrolysis performance and hydrogen permeability of different proton exchange membranes (PEMs) made of sulfonated derivatives of polypentafluorostyrene and poly(arylene thioether)s are compared to Nafion as standard membrane. These membranes were developed by other researchers in eSCALED. The ionic transport properties of the new PEMs are mostly better than Nafion, however, Nafion still outperformed them in terms of energy and faradaic efficiency in water electrolysis. This was mainly attributed to the larger water uptake and swelling ratios of the new membranes that increased mass transfer losses at the electrodes and allowed more hydrogen crossover. Hence, the design of new ionomers for PEMs should combine high ion transport properties and low water uptakes to avoid excessive gas permeation and energy losses. Titanium is widely used to manufacture the porous transport layers and bipolar plates, however, it results in high capital cost, which is higher than the one associated with the noble metal catalysts, decreasing the economic viability of water electrolysis. The growing field of printed electronics may provide a suitable answer to attain high throughput and low-cost manufacturing of electrodes. Graphite-based electrodes printed directly on Nafion with diverse patterns are studied in chapter 6. The patterns allowed proton transport across the membrane that resulted in successful water electrolysis, but also revealed several shortcomings in terms of resistive losses, graphite oxidation, reproducibility and overall performance. Additional improvements on the ink formulation and testing other patterns may enhance the efficiency of these electrodes, whilst using minimal amount of materials. In the last chapter, a life cycle assessment (LCA) of a solar-driven water electrolysis device that integrates the new materials (PEM and molecular catalysts developed in eSCALED project) is conducted. In this LCA, the eSCALED device is further compared with a device employing state-of-the-art materials. The study considers all the environmental impacts from raw material extraction (cradle) to manufacture (gate) of the devices to identify the most environmentally critical processes and materials. Overall, the environmental impact and energy demand of the eSCALED device were larger than of the state-of-the-art device. The low efficiency of the molecular catalysts in particular, prevents operation of the electrochemical cell at high current densities, resulting in larger material and energy consumption in the coupled system (photovoltaics and electrolysis components). Finally, the identification of the most environmentally impactful processes led to a better understanding of the environmental burden of the devices and where to improve them in the future.