04 Fakultät Energie-, Verfahrens- und Biotechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5

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Institute: search.filters.UBSInstitut.Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)Subject: search.filters.subject.660

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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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    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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    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, Marcelo
    Water 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.