04 Fakultät Energie-, Verfahrens- und Biotechnik

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Institute: search.filters.UBSInstitut.Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)Subject: search.filters.subject.620

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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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    Wetting behavior of aprotic Li-air battery electrolytes
    (2021) Kube, Alexander; Bienen, Fabian; Wagner, Norbert; Friedrich, Kaspar Andreas
    The open architecture of cathodes in Li–air batteries implies the need for open porosity with adequate pore size distribution and surface energy optimization with regard to the electrolyte. The interaction of liquid and cathode material, especially the wetting properties, which depend on cathode material, roughness and porosity, and electrolyte properties, needs to be understood properly to avoid flooding and assure high active areas. In this work, contact angle goniometry, capillary rise method, and pressure saturation curves are used to investigate the wetting properties of dimethyl sulfoxide (DMSO), tetraethylene glycol dimethyl ether (Tetraglyme), a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) and water on a gas diffusion layer (GDL) Sigracet 39BC, and a pure flat polytetrafluorethylene (PTFE) foil. Contact angle measurement shows that all three organic solvents wet the GDL hydrophobic agent PTFE. Capillary rise measurements show that all sample liquids slowly imbibe into the porous network. While for Tetraglyme an efficient penetration is limited by the high viscosity, water flow rate is slowed down by the hydrophobic pore network of the GDL. Pressure saturation curves for DMSO, Tetraglyme, and EC:DMC can be obtained for the first time and are compared with the water pressure saturation curve.
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    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, Nada
    Platinum 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.