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 Application of ion chromatography for the reliable quantification of ammonium in electrochemical ammonia synthesis experiments : a practical guide(2023) Bragulla, Sebastian C. H.; Lorenz, Julian; Harms, Corinna; Wark, Michael; Friedrich, K. AndreasAssessing novel electrocatalysts for the electrochemical ammonia synthesis (EAS) requires reliable quantitative trace analysis of electrochemically produced ammonia to infer activity and selectivity. This study concerns the development of an ion chromatography (IC) method for quantitative trace analysis of ammonium in 0.1 M sulfuric acid electrolyte, which is applied to EAS gas-diffusion electrode (GDE) experiments with commercial chromium nitride as electrocatalyst. The developed IC method is highly sensitive, versatile, and reliable, achieving a limit of quantification (LOQ) of 6 μg l-1 (6 ppbmol) ammonium. The impacts of the sample matrix, dilution, and neutralization, as well as contamination, on the quantitative analysis by IC are analyzed. Experimental constraints result in an effective LOQ including dilution of 60 μg l-1 for the determination of ammonium in 0.1 M sulfuric acid electrolyte, owing to necessary sample dilution. The practical guide presented herein is intended to be very relevant for the field of EAS as a guideline and applicable to a broad range of catalyst systems and ion chromatography devices.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 Advancement of segmented cell technology in low temperature hydrogen technologies(2020) Biswas, Indro; Sánchez, Daniel G.; Schulze, Mathias; Mitzel, Jens; Kimmel, Benjamin; Gago, Aldo Saul; Gazdzicki, Pawel; Friedrich, K. AndreasThe durability and performance of electrochemical energy converters, such as fuel cells and electrolysers, are not only dependent on the properties and the quality of the used materials. They strongly depend on the operational conditions. Variations in external parameters, such as flow, pressure, temperature and, obviously, load, can lead to significant local changes in current density, even local transients. Segmented cell technology was developed with the purpose to gain insight into the local operational conditions in electrochemical cells during operation. The operando measurement of the local current density and temperature distribution allows effective improvement of operation conditions, mitigation of potentially critical events and assessment of the performance of new materials. The segmented cell, which can replace a regular bipolar plate in the current state of the technology, can be used as a monitoring tool and for targeted developments. This article gives an overview of the development and applications of this technology, such as for water management or fault recognition. Recent advancements towards locally resolved monitoring of humidity and to current distributions in electrolysers are outlined.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 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 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 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.Item Open Access Hydrogen oxidation artifact during platinum oxide reduction in cyclic voltammetry analysis of low-loaded PEMFC electrodes(2020) Prass, Sebastian; St-Pierre, J.; Klingele, Matthias; Friedrich, K. Andreas; Zamel, NadaAn artifact appearing during the cathodic transient of cyclic voltammograms (CVs) of low-loaded platinum on carbon (Pt/C) electrodes in proton exchange membrane fuel cells (PEMFCs) was examined. The artifact appears as an oxidation peak overlapping the reduction peak associated to the reduction of platinum oxide (PtOx). By varying the nitrogen (N2) purge in the working electrode (WE), gas pressures in working and counter electrode, upper potential limits and scan rates of the CVs, the artifact magnitude and potential window could be manipulated. From the results, the artifact is assigned to crossover hydrogen (H2X) accumulating in the WE, once the electrode is passivated towards hydrogen oxidation reaction (HOR) due to PtOx coverage. During the cathodic CV transient, PtOx is reduced and HOR spontaneously occurs with the accumulated H2X, resulting in the overlap of the PtOx reduction with the oxidation peak. This feature is expected to occur predominantly in CV analysis of low-loaded electrodes made of catalyst material, whose oxide is inactive towards HOR. Further, it is only measurable while the N2 purge of the WE is switched off during the CV measurement. For higher loaded electrodes, the artifact is not observed as the electrocatalysts are not fully inactivated towards HOR due to incomplete oxide coverage, and/or the currents associated with the oxide reduction are much larger than the spontaneous HOR of accumulated H2X. However, owing to the forecasted reduction in noble metal loadings of catalyst in PEMFCs, this artifact is expected to be observed more often in the future.Item Open Access Exploring the interface of skin‐layered titanium fibers for electrochemical water splitting(2021) Liu, Chang; Shviro, Meital; Gago, Aldo S.; Zaccarine, Sarah F.; Bender, Guido; Gazdzicki, Pawel; Morawietz, Tobias; Biswas, Indro; Rasinski, Marcin; Everwand, Andreas; Schierholz, Roland; Pfeilsticker, Jason; Müller, Martin; Lopes, Pietro P.; Eichel, Rüdiger‐A.; Pivovar, Bryan; Pylypenko, Svitlana; Friedrich, K. Andreas; Lehnert, Werner; Carmo, MarceloWater electrolysis is the key to a decarbonized energy system, as it enables the conversion and storage of renewably generated intermittent electricity in the form of hydrogen. However, reliability challenges arising from titanium‐based porous transport layers (PTLs) have hitherto restricted the deployment of next‐generation water‐splitting devices. Here, it is shown for the first time how PTLs can be adapted so that their interface remains well protected and resistant to corrosion across ≈4000 h under real electrolysis conditions. It is also demonstrated that the malfunctioning of unprotected PTLs is a result triggered by additional fatal degradation mechanisms over the anodic catalyst layer beyond the impacts expected from iridium oxide stability. Now, superior durability and efficiency in water electrolyzers can be achieved over extended periods of operation with less‐expensive PTLs with proper protection, which can be explained by the detailed reconstruction of the interface between the different elements, materials, layers, and components presented in this work.