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 PEM single cells under differential conditions : full factorial parameterization of the ORR and HOR kinetics and loss analysis(2022) Gerling, Christophe; Hanauer, Matthias; Berner, Ulrich; Friedrich, K. AndreasThe anode and cathode kinetics are parameterized based on differential cell measurements. Systematic parameter variations are evaluated to disentangle the dependencies of the electrochemical impedance spectroscopy (EIS) signatures in H2/H2 mode. We introduce a new CO recovery protocol for both electrodes that enables to accurately characterize the hydrogen oxidation reaction (HOR) kinetics. Then, we demonstrate that a compact Tafel kinetics law captures the oxygen reduction reaction (ORR) kinetics for a full factorial grid of conditions, covering a wide range of relative humidities (rH), temperatures, oxygen partial pressures and current densities. This yields the characteristic activation energy and effective reaction order, and we reconcile models that make different assumptions regarding the rH dependency. Moreover, we analyze O2 transport contributions by steady-state and transient limiting current techniques and heliox measurements. Although the rising uncertainty of loss corrections at high current densities makes it impossible to unambiguously identify an intrinsic potential-dependent change of the Tafel slope, our data support that such effect needs not be considered for steady-state cathodic half-cell potentials above 0.8 V.Item Open Access High-resolution analysis of ionomer loss in catalytic layers after operation(2018) Morawietz, T.; Handl, M.; Oldani, C.; Gazdzicki, P.; Hunger, Jürgen; Wilhelm, Florian; Blake, John; Friedrich, K. Andreas; Hiesgen, R.The function of catalytic layers in fuel cells and electrolyzers depends on the properties of the ionically conductive phase, which are most commonly perfluorinated ionomers based on Nafion and Aquivion. An analysis by atomic force microscopy reveals that the ultrathin ionomer films around Pt/C agglomerates have a thickness distribution ranging from 3.5 nm to 20 nm. Their conductivity and gas permeation properties determine the fuel cell performance to a large extend. For electrodes in Aquivion-based membrane-electrode-assemblies operation-induced structure changes were investigated by means of material- and conductivity-sensitive atomic force microscopy, infrared spectroscopy and electron-dispersive X-ray analysis. The observed thinning of the ultrathin ionomer films was mainly caused by polymer degradation deduced from reduced swelling after long-time operation and a significant loss of ionomer with operation time detected by infrared spectroscopy. From the linear thickness increase of the ultrathin films with rising humidity, a mainly layered structure of the ionomer was deduced. An influence of thickness of such ultrathin ionomer films on fuel cell lifetime was found by analysis of differently prepared membrane-electrode-assemblies, where a linear increase of irreversible degradation rate with ionomer film thickness in the electrodes of unused membrane-electrode-assemblies was found.Item Open Access Influence of cycling profile, depth of discharge and temperature on commercial LFP/C cell ageing : post-mortem material analysis of structure, morphology and chemical composition(2020) Simolka, Matthias; Heger, Jan-Frederik; Kaess, Hanno; Biswas, Indro; Friedrich, K. AndreasThe paper presents post-mortem analysis of commercial LiFePO4 battery cells, which are aged at 55 °C and - 20 °C using dynamic current profiles and different depth of discharges (DOD). Post-mortem analysis focuses on the structure of the electrodes using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the chemical composition changes using energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). The results show that ageing at lower DOD results in higher capacity fading compared to higher DOD cycling. The anode surface aged at 55 °C forms a dense cover on the graphite flakes, while at the anode surface aged at - 20 °C lithium plating and LiF crystals are observed. As expected, Fe dissolution from the cathode and deposition on the anode are observed for the ageing performed at 55 °C, while Fe dissolution and deposition are not observed at - 20 °C. Using atomic force microscopy (AFM), the surface conductivity is examined, which shows only minor degradation for the cathodes aged at - 20 °C. The cathodes aged at 55 °C exhibit micrometer size agglomerates of nanometer particles on the cathode surface. The results indicate that cycling at higher SOC ranges is more detrimental and low temperature cycling mainly affects the anode by the formation of plated Li.Item Open Access A new approach to modeling solid oxide cell reactors with multiple stacks for process system simulation(2022) Tomberg, M.; Heddrich, M. P.; Sedeqi, F.; Ullmer, D.; Ansar, S. A.; Friedrich, K. AndreasReactors with solid oxide cells (SOC) are highly efficient electrochemical energy converters, which can be used for electricity generation and production of chemical feedstocks. The technology is in an upscaling phase. Thereby demanding development of strategies for robust and efficient operation or large SOC reactors and plants. The present state of technology requires reactors with multiple stacks to achieve the appropriate power. This study aims to establish and apply a simulation framework to investigate process systems containing SOC reactors with multiple stacks. Focusing especially on the operating behavior of SOC reactors under transient conditions, by observing the performance of all cells in the reactor. For this purpose, a simulation model of the entire SOC reactor consisting of multiple stacks, pipes, manifolds, and thermal insulation was developed. After validation on stack and reactor level, the model was used to investigate the fundamental behavior of the SOC reactors and the individual stacks in various operation modes. Additionally, the influences of local degradation and reactor scaling on the performance were examined. The results show that detailed investigation of the reactors is necessary to ensure operability and to increase efficiency and robustness. Furthermore, the computing performance is sufficient to develop and validate system controls.Item Open Access Tolerance of silicon oxide‐coated Pt/C catalyst toward CO and H2S contamination in hydrogen for proton exchange membrane fuel cells(2023) Prass, Sebastian; Nerlich, Leon; Singh, Rajveer; Godoy, Andres O.; Jankovic, Jasna; Friedrich, K. Andreas; Zamel, NadaPlatinum on graphitized low surface area carbon (Pt/C) is coated with a silicon oxide thin film and is employed as anode catalyst to manipulate the tolerance of proton exchange membrane fuel cells toward carbon monoxide and hydrogen sulfide contamination. The SiO2 coating, prepared by successive hydrolysis of 3-aminopropyl-triethoxisilane and tetraethoxysilane, forms clusters in proximity to Pt in sizes comparable to the catalyst particles, leaving most of the carbon surfaces free. The performance with and without CO is investigated in situ at relative humidities (RH) of 100%, 70%, and 40%. When operated with neat hydrogen, SiO2-Pt/C shows marginally better performance owing to an improved protonic conduction due to the water retaining hydrophilic SiO2. Upon operation with CO-contaminated fuel, the SiO2-Pt/C performs worse than that of Pt/C particularly at high RH. CO stripping measurements reveal an increase in CO oxidation potential for the SiO2-Pt/C, suggesting an increased CO coverage and consequently higher anode overpotentials during operation with CO-contaminated fuel. Upon operation with H2S in the fuel, the SiO2 coating extends the lifetime until the cell voltage broke down, which is attributed to the enhanced water retention due to SiO2 and the solubility of sulfuric species.Item Open Access Advanced impedance analysis for performance degradation during low-temperature CO2 electroreduction(2024) Chen, Qinhao; Kube, Alexander; Kopljar, Dennis; Friedrich, Kaspar AndreasElectrochemical impedance spectroscopy (EIS) is a powerful tool commonly used to study electrochemical systems. Nevertheless, its application in CO2 electroreduction has been so far limited due to its complex reaction mechanism and environment. Although initial findings have demonstrated the viability of applying EIS analysis in CO2 electrolyzers, the assignment of individual processes in the impedance spectra remains ambiguous. Therefore, a more detailed investigation, especially focused on its application in evaluating degradation mechanisms, is essential. In this study, a stable gas diffusion electrode (GDE) system was developed for a comprehensive EIS and distribution of relaxation time (DRT) evaluation to assess key degradation mechanisms under accelerated stress conditions such as high current density and low operating temperature. Validated by post-mortem analysis and complementary methods, we demonstrate the viability of this approach for operando monitoring of CO2 electroreduction by assigning individual mechanistic processes in the GDE and linking them to performance degradation over time.Item Open Access Elucidating the performance limitations of alkaline electrolyte membrane electrolysis : dominance of anion concentration in membrane electrode assembly(2020) Razmjooei, Fatemeh; Farooqui, Azharuddin; Reissner, Regine; Gago, Aldo Saul; Ansar, Syed Asif; Friedrich, Kaspar AndreasAnion exchange membrane water electrolyzers (AEMWEs) offer a cost‐effective technology for producing green hydrogen. Here, an AEMWE with atmospheric plasma spray non‐precious metal electrodes was tested in 0.1 to 1.0 M KOH solution, correlating performance with KOH concentration systematically. The highest cell performance was achieved at 1.0 M KOH (ca. 0.4 A cm-2 at 1.80 V), which was close to a traditional alkaline electrolysis cell with ≈6.0 M KOH. The cell exhibited 0.13 V improvement in the performance in 0.30 M KOH compared with 0.10 M KOH at 0.5 A cm−2. However, this improvement becomes more limited when further increasing the KOH concentration. Electrochemical impedance and numerical simulation results show that the ohmic resistance from the membrane was the most notable limiting factor to operate in low KOH concentration and the most sensitive to the changes in KOH concentration at 0.5 A cm-2. It is suggested that the effect of activation loss is more dominant at lower current densities; however, the ohmic loss is the most limiting factor at higher current densities, which is a current range of interest for industrial applications.Item Open Access Visualization of local ionic concentration and diffusion constants using a tailored electrochemical strain microscopy method(2019) Simolka, M.; Heim, C.; Friedrich, K. Andreas; Hiesgen, R.A tailored electrochemical strain microscopy technique is presented and used to analyze the ionic mobility and diffusion coefficients in composite Si/C anodes. The resulting surface displacement after a voltage pulse is proportional to the ionic concentration change and is measured by the deflection of an atomic force microscopy tip. The results show a higher ionic mobility at the steps of silicon composite anode microcrystals compared to the crystal centers. Diffusion coefficients are extracted from the time dependence of the surface displacement. Mappings with nanoscale resolution of local diffusion coefficients are displayed. The results demonstrate higher diffusion coefficients at the steps.Item Open Access A segmented cell measuring technique for current distribution measurements in batteries, exemplified by the operando investigation of a Zn-air battery(2021) Kube, Alexander; Meyer, Jens; Kopljar, Dennis; Wagner, Norbert; Friedrich, Kaspar AndreasA transimpedance amplifier circuit as well as an instrumental amplifier circuit were used to measure current densities of a zinc-air battery with an integrated segmented current collector foil. Error calculation showed that the transimpedance amplifier is superior to the used instrumental amplifier, but both methods provide valuable and consistent results. They both showed comparable results with operando insight into the current distribution of the battery. The knowledge about those distributions is essential to avoid fast degradation of battery materials and irreversible capacity loss due to heterogeneous dissolution of the anode during discharge. In this work we showed that oxygen starvation as well as gas flow rate leads to large current gradients. It was also demonstrated that heterogeneous current distributions on cathode side induces also a heterogenous dissolution behavior on the anode, resulting in irreversible capacity loss.Item Open Access Novel pyrrolidinium-functionalized styrene-b-ethylene-b-butylene-b-styrene copolymer based anion exchange membrane with flexible spacers for water electrolysis(2023) Xu, Ziqi; Delgado, Sofia; Atanasov, Vladimir; Morawietz, Tobias; Gago, Aldo Saul; Friedrich, K. AndreasAnion exchange membranes (AEM) are core components for alkaline electrochemical energy technologies, such as water electrolysis and fuel cells. They are regarded as promising alternatives for proton exchange membranes (PEM) due to the possibility of using platinum group metal (PGM)-free electrocatalysts. However, their chemical stability and conductivity are still of great concern, which is appearing to be a major challenge for developing AEM-based energy systems. Herein, we highlight an AEM with styrene-b-ethylene-b-butylene-b-styrene copolymer (SEBS) as a backbone and pyrrolidinium or piperidinium functional groups tethered on flexible ethylene oxide spacer side-chains (SEBS-Py2O6). This membrane reached 27.8 mS cm-1 hydroxide ion conductivity at room temperature, which is higher compared to previously obtained piperidinium-functionalized SEBS reaching up to 10.09 mS cm-1. The SEBS-Py206 combined with PGM-free electrodes in an AWE water electrolysis (AEMWE) cell achieves 520 mA cm-2 at 2 V in 0.1 M KOH and 171 mA cm-2 in ultra-pure water (UPW). This high performance indicates that SEBS-Py2O6 membranes are suitable for application in water electrolysis.