CO and H2S in H2: contamination, recovery and mitigation strategies in PEMFCs with ultra-low Pt loaded anode electrodes

Einige der größten Hürden für die Kommerzialisierung von Fahrzeugen mit Brennstoffzellenantrieb (engl. Proton Exchange Membrane Fuel Cell, PEMFC) sind die Kosten und Lebensdauer der Systeme, sowie die fehlende Infrastruktur, die den Wasserstoff (H2) bereitstellt. Zur Verringerung der PEMFC-Kosten ist eine Reduktion des Gehalts an Katalysatoren der Platin (Pt)-Gruppe nötig, was jedoch durch Vergiftungsmechanismen erschwert wird, deren Wirkung bei niedrigeren Pt-Beladungen pro aktiver Fläche ([µgPt/cm²]) zunimmt. Verunreinigungen im H2, die durch kostenintensive Reinigungsverfahren herausgefiltert werden, beeinträchtigen die PEMFC-Leistung und Lebensdauer. Sind Kontaminations- und Erholungsmechanismen bekannt, können Betriebsweisen angepasst und der Katalysatorgehalt einerseits verringert, sowie die Herstellung und Aufreinigung des H2 andererseits kosteneffizienter gestaltet werden. Diese Dissertation untersucht daher die Auswirkung einer Verringerung der Platinbeladung der Anodenelektrode auf die Toleranz gegenüber Kohlenmonoxid (CO) und Schwefelwasserstoff (H2S) im H2 anhand von Einzelzellen unter Verwendung klassischer elektrochemischer Charakterisierungsverfahren. Zunächst ist die Charakterisierung niedrig beladener Elektroden per Zyklovoltammetrie (engl. Cyclic Voltammetry, CV) durch Artefakte erschwert, die normalerweise bei höheren Beladungen (>100 µgPt/cm²) nicht auftreten. Bei niedrigen Beladungen (<50 µgPt/cm²) kann es zu einer spontanen Oxidation von angesammeltem Permeat-Wasserstoff während des kathodischen CV-Vorschubs kommen, was den Pt-Oxid-Reduktionsstrom überlappt und die Bestimmung der aktiven Fläche (engl. Electrochemically Active Surface Area, ECSA) beeinträchtigt. Werden PEMFCs mit gering beladenen Anodenelektroden (<25 µgPt/cm²) und H2 betrieben, der CO in Konzentrationen enthält, die gemäß H2-Qualitätsstandard ISO 14687 zulässig sind, kommt es zu inakzeptablen Spannungsabfällen von bis zu 40%. Dies deutet darauf hin, dass die CO-Toleranz des Katalysators verbessert, oder der ISO-Grenzwert für CO verringert werden sollte. Ist H2S in Konzentrationen gemäß ISO 14687 im H2 enthalten, kommt es während chronoamperometrischer Tests zu Spannungseinbrüchen, die bei niedrig beladenen Elektroden Schwefeldosisabhängig früher auftreten. Andererseits können sich PEMFCs durch Stopp/Start (engl. Shut-Down/Start-Up, SD/SU)-Prozeduren von Schwefelvergiftungen erholen. Gemeinsam mit den erst nach Dutzenden Stunden auftretenden Spannungseinbrüchen wirft diese Erholung durch SD/SUs die Frage auf, ob der ISO-Grenzwert für Schwefelspezies einer Anpassung bedarf. Eine Beschichtung des Pt-Katalysators in der Anodenelektrode mit einer Siliziumoxidschicht (SiO2-Pt/C) verbessert die Leistung und Toleranz der PEMFC beim Betrieb mit reinem und H2S-kontaminiertem H2, verschlechtert jedoch deren CO-Toleranz. Während die Verbesserung der Leistung und H2S-Toleranz auf Einflüsse des SiO2 auf Wassermanagement und Mobilität oxidierter Schwefelspezies zurückzuführen ist, steht die Verschlechterung der CO-Toleranz im Zusammenhang mit erschwerter Bildung und Mobilität der OH-Gruppen, die für die CO-Oxidation notwendig und in erhöhten Potentialen für CO-Oxidation sichtbar sind

Hydrogen Oxidation Artifact During Platinum Oxide Reduction in Cyclic Voltammetry Analysis of Low-Loaded PEMFC Electrodes

An 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

Hydrogen oxidation artifact during platinum oxide reduction in cyclic voltammetry analysis of low-loaded PEMFC electrodes

An 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.Bundesministerium für Wirtschaft und TechnologieProjekt DEA

Tolerance of Silicon Oxide-Coated Pt/C Catalyst Toward CO and H2S Contamination in Hydrogen for Proton Exchange Membrane Fuel Cells

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

Tolerance of silicon oxide‐coated Pt/C catalyst toward CO and H2S contamination in hydrogen for proton exchange membrane fuel cells

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.Bundesministerium für Verkehr und Digitale Infrastruktu

Integration of LCC and LCA results to higher system levels : The German meat and EU tomato cases

This report aims to highlight the potential contribution of food waste reduction to improving the sustainability of agri-food sector, by integrating the Life Cycle Assessment (LCA) and Life Cycle Cost (LCC) results and upscaling them to a higher system level, using Germany meat and EU tomatoes as examples

Interfacial morphology and contact resistance between the catalyst and micro porous layers in proton exchange membrane fuel cells

The interface between the catalyst layer (CL) and the micro porous layer (MPL) in proton exchange membrane fuel cells (PEMFCs) has been studied in ex-situ experiments. The interfacial morphology, specifically the area, origin and dimensions of interfacial gaps in between compressed CLs and MPLs were investigated with high-resolution X-ray micro computed tomography. In a separate experiment, the electric contact resistance (CR) was evaluated using a custom four-point-probe setup for CLs with different compositions as a function of compression pressure and relative humidity (RH). The interfacial gap area (fraction of the interface separated by gaps) was higher for gas diffusion layers (GDL, with MPL) – catalyst coated membrane (CCM) assemblies with large differences in the surface roughness of the CL and MPL. The interfacial gap area decreased with increasing compression and with increased similarity in roughness. Relatively large continuous gaps were found in proximity of specific cracks in the MPL. These are hypothesized to form due to the presence of large pores on the surface of the GDL, in which the MPL sags and cracks. Relatively small gaps form by means of the regular surface roughness features throughout the CL-MPL interface. Smaller pores on the GDL surface serving as substrate for the MPL could reduce the number of MPL crack-induced gaps. Moreover, adjusting the CL and MPL surface roughness parameters to achieve similar orders of roughness can result in fewer enclosed gaps, and therefore, enhance the mating characteristics. The electric CR followed a similar trend for all the CL compositions, featuring a non-linear decrease in resistance with the increase in the compression pressure. Moreover, the CR was also found to increase with the ionomer content in the CL and with the increase in RH. Physical characterization of the CL surfaces revealed that this increase in the ionomer content enhances the surface roughness features and the surface coverage by the ionomer, both of which affecting the electrical CR towards the MPL. With increasing RH, the CR values doubled for all CL compositions as a result of humidity induced ionomer swelling with the uptake of water.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

Tolerance and Recovery of Ultralow-Loaded Platinum Anode Electrodes upon Carbon Monoxide and Hydrogen Sulfide Exposure

The effects of carbon monoxide (CO) and hydrogen sulfide (H2S) in concentrations close to their respective limits in the Hydrogen Quality Standard ISO 14687-2:2012 on the performance of proton exchange membrane fuel cells (PEMFCs) with ultralow-loaded platinum anode catalyst layers (CLs) were investigated. The anodic loadings were 50, 25, and 15 µg/cm², which represent the current state-of-the-art, target, and stretch target, respectively, for future automotive PEMFCs. Additionally, the effect of shut-down and start-up (SD/SU) processes on recovery from sulfur poisoning was investigated. CO at an ISO concentration of 0.2 ppm caused severe voltage losses of ~40–50% for ultralow-loaded anode CLs. When H2S was in the fuel, these anode CLs exhibited both a nonlinear decrease in tolerance toward sulfur and an improved self-recovery during shut-down and start-up (SD/SU) processes. This observation was hypothesized to have resulted from the decrease in the ratio between CL thickness and geometric cell area, as interfacial effects of water in the pores increasingly impacted the performance of ultrathin CLs. The results indicate that during the next discussions on the Hydrogen Quality Standard, a reduction in the CO limit could be a reasonable alternative considering future PEMFC anodic loadings, while the H2S limit might not require modification

Impurities in fuels and air

Despite impressive developments in the technology of proton-exchange membrane fuel cells (PEMFCs), large-scale commercialization still faces issues of low stability and durability that continue to be adversely affected by impurities in the PEMFC system. This article provides a thorough discussion of the impurity and contamination issues that have plagued PEMFC performance. After a brief introduction to the operational principles of the PEMFC, the common sources of impurities in PEMFC systems are discussed, including fuel-side impurities (COx, H2S, and NH3), air-side impurities (NOx, SOx, and COx), and impurities coming from the system components themselves. The effect of contamination and the mechanisms of these impurities on fuel cell operation and performance are presented, followed by a detailed discussion of current strategies for mitigating contamination. Finally, suggestions are provided for future work in PEMFC contamination research