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Open Access
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.
Open Access
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.
Open Access
Techno-economic analysis of integration possibilities for fluctuating renewable energy sources in the Power and Biomass to Liquid process
(2024) Habermeyer, Felix; Thess, André D. (Prof. Dr.)
Sustainable aviation fuels provide the opportunity to reduce the aviation industry’s climate impact while avoiding a complete replacement of the current aircraft fleet. The European Union directs its member states to a gradual uptake of sustainable aviation fuel from 2 %vol. in 2025 to 63 %vol. by 2050 with the ReFuelEU directive. Yet, biomass-based fuel production in Europe is limited by the availability of sustainable biomass. This limitation can be mitigated by the Power Biomass to Liquid process, which attains near full biogenic carbon conversion to fuel by the addition of electrolytic hydrogen. This work evaluates the economic feasibility and global warming impact of sustainable aviation fuel production via the Power Biomass to Liquid process. Different options to enhance process performance and reduce its footprint are analyzed. This includes a discussion of process configurations and integration options for fluctuating energy sources. Based on flowsheet simulations in Aspen Plus, production costs, emissions and fuel production volume are estimated under different economic and regional boundary conditions. The production cost for the Power Biomass to Liquid process is highly sensitive to the electricity price. In fact, the electricity cost is the largest cost contributor followed by the cost for the biomass and the electrolyzer investment. The electricity’s carbon footprint is also shown to be the determining factor for the fuel’s global warming potential. Therefore, regions with inexpensive and green electricity, either from their national grid mix or their renewable energy potential, are the ideal sites for the Power Biomass to Liquid process. In a region-specific analysis, Norway and Sweden present good production sites due to their suitable grid conditions. Ireland is a promising production site based on its onshore wind potential. High electricity prices and emissions can also be avoided by operating the Power Biomass to Liquid process dynamically. Yet, dynamically operated electrolysis units add substantial cost when over-dimensioned. Therefore, an optimum between reduced electricity costs and increased capital expenses has to be found. A cost reduction can also be achieved by identifying process configurations suited for the regionspecific boundary conditions. A higher CO2 recycle ratio, for example, leads to an enhanced product yield at the cost of a larger hydrogen demand. Due to the increased electricity demand for hydrogen production, this is only cost-effective at low electricity prices. When considering forest residues with an availability of 33 % as the feedstock for the Power Biomass to Liquid process, around 25 Mt/a sustainable aviation fuel can be produced within Europe. This output could be even higher when agricultural residues can also be utilized. Yet, this production volume of sustainable aviation fuel depends upon low-carbon electricity. When considering grid connected operation in Europe today, only around 5 Mt/a can be produced when adhering to the European sustainable aviation fuel definition of 70 % emission reduction compared to fossil fuel.
Open Access
Effect of polytetrafluorethylene content in Fe‐N‐C‐based catalyst layers of gas diffusion electrodes for HT‐PEM fuel cell applications
(2024) Zierdt, Tanja; Müller‐Hülstede, Julia; Schmies, Henrike; Schonvogel, Dana; Wagner, Peter; Friedrich, K. Andreas
Fe-N-C catalysts are a promising alternative to replace cost-intensive Pt-based catalysts in high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) electrodes. However, the electrode fabrication needs to be adapted for this new class of catalysts. In this study, gas diffusion electrodes (GDEs) are fabricated using a commercial Fe-N-C catalyst and different polytetrafluorethylene (PTFE) binder ratios, varying from 10 to 50 wt % in the catalyst layer (CL). The oxygen reduction reaction performance is investigated under HT-PEMFC conditions (160 °C, conc. H3PO4 electrolyte) in a half-cell setup. The acidophilic character of the Fe-N-C catalyst leads to intrusion of phosphoric acid electrolyte into the CL. The strength of the acid penetration depends on the PTFE content, which is visible via the contact angles. The 10 wt % PTFE GDE is less capable to withdraw product water and electrolyte and results into the lowest half-cell performance. Higher PTFE contents counterbalance the acid drag into the CL and impede flooding. The power density at around 130 mA mgCatalyst−2 increases by 34 % from 10 to 50 wt % PTFE.
Open Access
Development of a high temperature and high power PCM storage for standby operation
(2024) Johnson, Maike; Thess, André (Prof. Dr.)
Thermal energy storages integrated in industrial processes allow for higher degrees of energy flexibility and reduction of fossil fuel use. The combination of latent heat thermal energy storage systems with water/steam processes can lead to optimized thermal gradients and therefore good system efficiencies. Storage units and systems have been proven at pilot scale. The integration in industrial processes remains a challenge, due to the size of the systems as well as the hurdles in design, permitting and build. This thesis encompasses the parametrization, design, build, integration and initial operation of a megawatt-scale latent heat thermal energy storage unit, producing superheated steam for an operating cogeneration plant and industrial process customers. The storage unit can produce superheated steam at more than 300 °C and 25 bar at a mass flow rate of 8 t/h for at least 15 minutes. The data of the dispatchable production of superheated steam for more than 20 minutes show the thermal power and capacity at 5.5 MW and 1.9 MWh. In order to develop this storage system and show the feasibility of the novel aspects of producing superheated steam in once-through operation, providing megawatt-scale thermal power and capacity, and integrating it into an operating system, various steps are involved. These steps are a combination of upscaling the thermal power and capacity, designing for a feasible build and for given system requirements, and system integration development. The upscaling in thermal power results in the development of a very dense fin structure and tight tube-spacing. The upscaling in capacity requires the development of a design model with capabilities for analysis of thermal losses and possible non-ideal flow through the headers, as well as more banal aspects such as transportability and weight considerations as well as physical filling capability with the pelleted salt during commissioning, and accessibility for permitting bodies. System integration in an operating system considers charging and discharging with the available components and maximization of benefits to the plant. This integration, requiring not just a design and optimization of the storage technology itself but of the whole system, is a novel point of view for the development of latent heat storage systems. It is not critical that the storage itself provide all of the parameters, but that the system integration makes this feasible.
Open Access
Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells
(2024) Sarkezi-Selsky, Patrick; Friedrich, K. Andreas (Prof. Dr.)
Polymer electrolyte membrane fuel cells (PEMFCs) offer a compelling powertrain solution for the e-mobility sector and in particular for heavy-duty applications. Generating electrical energy by electrochemical reaction of hydrogen and oxygen to water, these fuel cells present (when using green hydrogen) a climate-friendly technology in support of reducing overall greenhouse gas emissions. During cell operation, the reactand gases are consumed at the electrodes and water is produced in the cathode catalyst layer (CCL). In dependence of the operating conditions, this product water can condensate and block as a liquid phase available transport paths for the reactand gases. In order to prevent advancing liquid water accumulation and therewith flooding of the cell, the product water has to be therefore removed efficiently. Thus, stable fuel cell (FC) operation is only possible with an appropriate water management. In this context, the porous gas diffusion layer (GDL) plays a pivotal role by ensuring both homogeneous reactand gas distribution to the electrodes and efficient water removal from the cathode catalyst layer. Consequently, optimal water management is only achievable with an appropriate design of the GDL material properties, which requires a profound understanding of the capillary transport phenomena on the pore scale. In long-term operation, the GDL is furthermore in general subject to different aging processes, due to the harsh conditions typically present in automotive applications. With proceeding material degradation, the water removal capabilities of the GDL can decrease over time, leading to a progressively deteriorated gas transport and increasing cell performance losses upon aging. In order to achieve the component durability needed to meet the lifetime requirements for long-term fuel cell operation, degradation phenomena and their repercussions on the water management therefore have to be fully understood. Owing to the complexity of multiphase transport mechanisms in porous media, however, conventional experimental testing can be expensive, time-consuming and with limited depth of detail. In recent years, pore-scale modeling (PSM) has therefore gained increasing popularity as a comparatively fast and inexpensive technique for investigation of porous media transport processes directly at the pore scale. In this work, liquid water transport through a carbon felt GDL was thoroughly investigated using multiphase PSM simulations and real microstructural data. At first, a 3D Color-Gradient Lattice-Boltzmann model was developed and validated against analytical references. GDL microstructures of a plain and an impregnated carbon fiber substrate of a Freudenberg H14 GDL were then reconstructed via segmentation of high-resolution X-ray micro-computed tomography (µCT) images. For the microstructure reconstruction of the impregnated GDL, an in-house algorithm was furthermore developed to distinguish a hydrophobic additive component (polytetrafluoroethylene (PTFE)) from the support material (carbon fibers). Subsequently, a first computational domain for the simulation of GDL liquid water transport was generated according to the experimental boundary conditions of a test bench for measuring capillary pressure-saturation (pc - S) relations. Here, special attention was paid to the GDL surface regions by explicit consideration of two semipermeable membranes according to the experimental setup. Starting from an initially dry GDL, liquid water intrusion was then simulated by gradually decreasing the gas pressure until full saturation was reached. Whereas the obtained pc - S relation is not unique but dependent on the wetting history of the GDL, drainage of liquid water was simulated as well, thereby recovering the complete capillary hysteresis. Parametric studies then showed that the obtained pc - S characteristics are significantly affected by boundary effects owing to the highly porous GDL surface regions. In addition, microstructure-resolved simulations require a high resolution of the porous medium in order to predict capillary behavior reasonably. Assuming uniform wettability with a carbon fiber contact angle of θ_CF = 65° for the plain fiber substrate, the simulated pc - S curves were furthermore in good agreement with the experimental reference. Considering mixed wettability for the impregnated fiber substrate, on the other hand, the simulations showed an expected shift of the pc - S characteristics towards higher capillary pressures owing to the hydrophobicity of the additive (PTFE) but could otherwise not reproduce the experimentally observed significant enlargement of the capillary hysteresis. This discrepancy was primarily related to a measurement artifact. After validation of the simulated capillary hystereses, novel pc - S relations were furthermore derived for both the intrusion and drainage of liquid water in the plain and impregnated H14 fiber substrate. On the other hand, the widely-used Leverett relation was found to be incapable to describe the capillary characteristics of the plain and impregnated carbon felt GDL appropriately. In order to investigate the impact of aging on the GDL water management, a second computational domain was generated, mimicking the operando conditions during fuel cell operation. For this purpose, the GDL microstructure reconstruction was virtually compressed corresponding to a typical cell assembly clamping pressure. In addition, a microporous layer (MPL) was considered as well by reconstruction of its macropores via segmentation of µCT images of an impregnated and MPL-coated Freudenberg H14 GDL. Generating a computational setup, the GDL/MPL assembly was then sandwiched between a liquid and a gas buffer zone representing the catalyst layer (CL) and the gas channel (GC) according to operando conditions. In order to investigate aging effects, the microstructure reconstructions were furthermore degraded virtually assuming PTFE loss from the GDL and increase of the MPL macroporosity as the main aging scenarios. Corresponding to stationary cell operation, liquid water transport through the partially aged GDL and MPL was then simulated with a constant inlet velocity. Once the liquid phase percolated through to the gas channel (i.e., at breakthrough), the simulations were halted and the invasion patterns were then characterized by means of local and overall saturation. In order to investigate mass transport limitations due to liquid water accumulation in the pore space, effective gas transport properties were furthermore determined for the partially saturated and aged GDLs and MPLs. Subsequent simulations then showed that with the MPL in the pristine state loss of PTFE had no measureable effect on the liquid water transport through the degraded GDL. This observation was related to the dominant impact of the MPL on the capillary transport by strongly restricting the liquid water invasion sites into the GDL. Upon aging of the MPL, on the other hand, significant degradation effects were observed, as indicated by rising breakthrough saturations and an increasingly disturbed gas transport. Once the MPL was already degraded, aging of the GDL was then found to impact the liquid water transport and consequently the effective gas transport as well.
Open Access
Experimental operation, modelling and simulation of solid oxide cell reactors with multiple stacks for process systems
(2024) Tomberg, Marius; Friedrich, K. Andreas (Prof. Dr. rer. nat.)
Solid Oxide Cell (SOC) reactors are highly efficient electrochemical energy converters. They can be operated in electrolysis mode (SOEC) to produce chemical feedstocks and in fuel cell mode (SOFC) to convert chemical energy into electricity. These characteristics enable them to meet the challenges of the energy transition and the increasing penetration of renewables, such as intermittency and integration into specific industrial sectors and processes. SOC reactors can therefore play a central role in the energy system of the future. Today’s large-scale SOC reactors are composed of multiple stacks/subreactors, resulting in a modular design. However, such an arrangement leads to several operational challenges for further scaling and operation. The objective of this dissertation is to establish a general understanding of the operational behavior of large SOC reactors and to contribute to the deployment of SOC reactors in the future energy system by developing generic scaling, operation and control strategies. In this work, above objective is addressed by experimental and numerical studies of SOC reactors with multiple stacks. The approach is based on the construction and operation of test rigs to study large reactors, the development and application of a simulation framework and a strong interaction between these two. The experiments demonstrated the general operational behavior and provided parameterization as well as validation data for the simulations. These, in turn, supported the experimental investigations, for example, by providing estimates of operating parameters for specific operating points. Finally, the simulations were used to develop operation and control strategies that were iteratively improved by using feedback from the experiments. A unique test rig for the investigation of SOC reactors with multiple stacks was built, which contains a blower for off-gas recirculation at SOC reaction temperature and a pressure vessel for operation under pressure. This pressurized reactor test rig was used to study a modular 30 kW SOFC reactor with multiple stacks. In addition, a simulation framework for the study of process systems with SOC reactors was created. The simulation framework has two unique features. First, it allows modelling of complete SOC reactors consisting of multiple stacks, pipes, manifolds, thermal insulation, and thermal interaction between all these components. Second, the framework provides the capability for transient simulation of not only the SOC reactors, but also all the required BoP components. Both the test rig and the simulation framework were used to develop generic strategies for reliable operation. In a measurement campaign with the pressurized SOC reactor test rig, fuel gas, reactant conversion, and pressure were varied in stationary and transient experiments. The experimental results showed that the operating conditions of the individual stacks of large SOFC reactors vary largely due to flow distribution and heat losses. Methods for the investigation of these critical characteristic parameters were derived from the experimental results. Furthermore, the impact of pressurization and fuel gas recirculation on the SOFC reactor was analyzed. These experimental investigations showed the need to understand the behavior of large SOFC reactors with multiple stacks to increase the performance and robustness of complete process systems. Therefore, the simulation framework was applied to an entire SOC reactor consisting of multiple stacks, pipes, manifolds, and thermal insulation. After experimental validation on stack and reactor level, the model was used to investigate the fundamental behavior of the SOC reactor and its individual stacks in fuel cell and electrolysis mode. Subsequently, the simulation framework was applied to develop operational and control strategies. An example that also provides generic conclusions, is the model of a megawatt scale flexible electrolysis system consisting of twelve reactors with a nominal load of 80 kW. The model was used to define crucial and efficient operation points and to establish transitions between these by comparing different strategies and control approaches. The simulation results showed that systems with SOCs can be operated more transiently than usually assumed. For instance, the start-up time from a hot standby point was reduced by 80 %, while at the same time the temperature gradients were significantly reduced. Furthermore, by taking advantage of the modular nature of state-of-the-art reactors, fast power modulation was achieved. In addition to the study of electrolysis systems, operating strategies for fuel cell operation were developed with a focus on the challenges that arose from the experiments with the pressurized SOC reactor test rig. A control strategy was developed for sub-reactors with separate electrical channels but shared reactant processing units. It uses the operating parameters of each stack in each sub-reactor to improve power and efficiency. This was successfully tested on the pressurized SOC reactor test rig. In addition, a feed forward temperature control was developed and experimentally validated, resulting in a significantly improved controller and the possibility of faster power ramps. A unique test rig was operated for the first scientific investigations focusing on SOC reactors with multiple stacks, and a simulation framework was developed to study large SOC reactors in process systems. Both activities contributed to a better understanding of large reactors and to the integration and operation of SOC reactors in the future energy system. In the process, unique experimental results were obtained and operating as well as control strategies were developed.
Open Access
Experimental characterization and numerical simulation of differential PEM fuel cells
(2024) Gerling, Christophe; Friedrich, Kaspar Andreas (Prof. Dr. rer. nat.)
To accomplish a global market entry with polymer electrolyte membrane fuel cells, several challenges have to be tackled. One of them is finding the right compromise between functionality, lifetime and costs. For this purpose, understanding the influence of design, materials and operating conditions on the fuel cell behavior is essential. In this work, the focus is set on the operating conditions. The first aim is to address the lack of consistent datasets for electrochemical parameters of state-of-the-art materials in the literature by using in situ characterization techniques. The second aim is to propose a new, simple but complete parameterized performance model. When a fuel cell is operated, several loss mechanisms can be observed. Close attention is paid to the oxygen reduction reaction (ORR) since it represents the highest contribution, even though other mechanisms are also characterized in order to deconvolute the performance signatures. In order to ensure high data quality and minimize unwanted in-plane effects, single cells under differential conditions are used. The base of this work consists of a comprehensive dataset containing polarization, electrochemical impedance spectroscopy (EIS), limiting current and voltammetry data obtained from systematic parameter variations in H2/O2, H2/N2 and H2/H2 configurations. In the first publication, the hydrogen crossover and the protonic resistances of both the cathode catalyst layer (CCL) and the membrane are characterized. First, the equivalency of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) to determine the hydrogen crossover is demonstrated and validated with an online gas analysis. Then, an H2/N2 measurement campaign with a full factorial variation of the relative humidity RH and the temperature T is carried out. Based on the CV data, a model for the hydrogen permeation coefficient is parameterized and with the EIS spectra the parameters of a specific transmission line model (TLM) for blocking electrodes are fitted. Especially the membrane and cathodic ionomer resistances R_PEM and R_p are thereby fitted as functions of RH and T and then used to parameterize models of the ionomer conductivities. Although the values that are measured are comparable to values reported in the literature, distinct deviations and trends can be observed which highlights the need for specific characterization of new materials. Finally, the change of the ionomer conductivity caused by modified water management under load (H2/O2) is investigated and an approach to estimate the local CCL relative humidity is discussed. The deviations of the ionomer resistances under load from those in the H2/N2 state reveal that a correction of polarization data for ohmic contributions based on H2/N2 data is not recommended. In the second publication, proton pump (H2/H2) measurements provide a quantification of the anode loss contributions which are strongly dependent on RH and the hydrogen partial pressure p_H2. Within these O2-free experiments, carbon monoxide (CO) poisoning on the catalysts from trace impurities in the gas feed is observed. Thus, a new recovery procedure to counter CO poisoning is presented which allows for a reliable parameterization of the hydrogen oxidation reaction (HOR) based on linearized Butler-Volmer (BV) kinetics. Then, a dataset containing polarization curves for a full factorial variation of T, RH and the oxygen partial pressure p_O2 is created and corrected by the ionomer resistances and hydrogen crossover. This dataset is used to parameterize the ORR reaction based on a Tafel law and allows to prove that RH has no significant influence on the ORR performance. Subsequently, O2 mass transport contributions are discussed based on limiting current techniques and heliox measurements and a global loss analysis is performed. The latter shows that up to 0.1 A/cm2, anode and oxygen transport contributions do not interfere with the parameterization of the ORR. Moreover, the analysis reveals that the simple Tafel law with one intrinsic slope of -70 mV/dec is sufficient to capture the ORR kinetics at half-cell potentials above 0.8 V. Below 0.8 V, the data deviates from the Tafel line. Furthermore, the EIS spectra under load show trends that are not covered by the Tafel law even at very small current densities. Deviations between Tafel slopes obtained from polarization data and EIS charge transfer resistances hint at more complex kinetics, which is investigated further in the third publication. In the third publication, the low-frequency inductive loop in EIS and its responsibility for the discrepancies in the Tafel slopes between polarization curves (-70 mV/dec) and the charge transfer resistance (<= -100 mV/dec) is demonstrated. Based on the H2/O2 test run, the low-frequency contributions are deciphered by subtracting the local slopes of the polarization curves (calculated by numerical derivation) from the low-frequency real-axis-intercepts of the capacitive EIS that occur at approximately 1 Hz. Based on this knowledge and on the ionomer resistances under load, the slow humidification effects leading to inductivities can be separated from the platinum oxidation contributions. Therewith, a new simple representation of the cathode kinetics containing the ORR and platinum poisoning effects that result in a low-frequency inductive loop is parameterized. This reaction kinetics is integrated into a transient 1D through-plane membrane electrode assembly (MEA) model which is used for an extensive parameter study. This model yields consistent results and both polarization curves and EIS match the experiments well at high humidity conditions, thus reconciling steady-state and dynamic performance signatures of PEMFCs. In Chapter 5, the link between the publications and their significance in the literature context are discussed. Additionally, the MEA model is extended by ionomer hydration dynamics. The parameterizations of R_PEM and R_p depending on RH and the current density j is obtained by fitting polynomials to selected data of the H2/O2 test run. This final MEA model is now able to simulate all humidity conditions meaningfully and even slightly enhances the predictions at high RH compared to the model from the third publication. Finally, an extensive loss analysis containing both overpotentials and differential resistances is carried out and depicts the most important loss mechanisms in state-of-the-art PEMFCs depending on the operating parameters.
Open Access
Aggregation of distributed energy resources in energy communities : a bottom-up analysis
(2024) Sarfarazi, Seyedfarzad; Bertsch, Valentin (Prof. Dr.)
This dissertation is motivated by the ongoing decentralization of the German energy system and the challenges faced by policymakers, regulators, and emerging market actors in ensuring the efficient integration of decentralized energy systems (DESs). Currently, a significant research gap exists between studies focused on the techno-economic analysis of energy communities (ECs) and those examining the overall system integration of DESs. The former often overlooks the feedback effects of ECs on the broader energy system, while the latter frequently lacks the detailed examination necessary to fully capture the diverse and complex nature of EC business models. In response, this thesis seeks to address this gap by proposing novel methodological developments that bridge these two bodies of literature. Specifically, it introduces a methodology for modeling the operation of DESs within ECs as a Stackelberg energy trading game and develops innovative techniques to find the Stackelberg equilibrium and derive an optimal real-time pricing scheme for ECs. Additionally, the thesis evaluates the systemic impacts of DESs using the agent-based electricity market model AMIRIS, among other methods. The methods developed in this thesis enable a holistic analysis of EC integration and can support critical political decision-making processes.
Open Access
Investigation of effects of performance recovery procedures for polymer electrolyte membrane fuel cells (PEMFCs)
(2024) Zhang, Qian; Friedrich, K. Andreas (Prof. Dr. rer. nat.)

Universität Stuttgart

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