Synchrotron X-ray absorption edge imaging was used to investigate the ruthenium distribution in both fresh and aged Pt/Ru-based membrane electrode assemblies (MEA) of direct methanol fuel cells. MEAs aged in different ways were analyzed: artificially aged by MeOH depletion and aged for 1700 h in an operating fuel cell stack. An element sensitive tomographic technique – differential X-ray absorption edge tomography – was applied allowing for a 3D-visualization of the ruthenium distribution within the MEA. We found a markedly changed Ru distribution after aging which is correlated to the GDL structure, the flow field geometry, and CO2 transport in the methanol solution.
The three-dimensional water distribution and water transport paths in the gas diffusion layer (GDL) and the adjacent micro-porous layer (MPL) of a polymer electrolyte membrane fuel cell (PEMFC) were analyzed during cell operation. The technique of quasi in-situ X-ray tomography was used for a 3D visualization of the water distribution and the structure of the GDL at different operating conditions. Based on findings from in-situ radiographic measurements water transport paths were detected and subsequently examined by tomography. The combination of these 2D and 3D techniques allows for a fully three-dimensionally resolved visualization of transport paths through the GDL.
The 3-dimensional spatial distribution of liquid water in different gas diffusion layer (GDL) materials was analyzed using synchrotron X-ray tomography. The capability of the method was demonstrated by virtually separating the GDL components in order to facilitate individual analysis of fiber material, liquid water and gas filled pore spaces. The influence of hydrophobic surface treatment on the water distribution in the GDL was illustrated by analyzing three GDL materials with different degrees of hydrophobicity. In the least hydrophobic sample, liquid water tends to form larger clusters which stretch out about several hundred µm inside the porous GDL. In contrast, only small water clusters were found in the strongly hydrophobic material with high Polytetrafluoroethylene (PTFE)-content as the liquid is partially pressed out of the GDL. Additionally, the influence of fiber orientation on the water distribution in the felt material was demonstrated.
We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium particles to their support and their freely accessible surface area. The shape analysis shows the ruthenium particles deviate significantly from spherical symmetry which increases their surface to volume ratio. The morphological studies help to understand the structure formation mechanisms during the fabrication as well as the high effectiveness of these catalysts in the oxygen reduction reaction at the cathode side of fuel cells.
Auf Grund des hohen Wirkungsgrades und der vielfältigen Einsatzmöglichkeiten stellen Brennstoffzellen sowie Brennstoffzellenverbünde eine vielversprechende Energiequelle dar. Daher ist es wichtig, die Prozesse während des Betriebs einer Brennstoffzelle exakt charakterisieren zu können, um diese im nächsten Schritt zu optimieren.
So ist ein ausbalanciertes Wassermanagement in einer mit Wasserstoff betriebenen Zelle ausschlaggebend für ihre Leistungsfähigkeit. Für eine dauerhafte Leistungsfähigkeit muss das entstehende Wasser die Membran feucht halten, um so die Protonenleitfähigkeit aufrecht zu erhalten. Zu viel Wasser kann jedoch die Gaszufuhr durch die porösen Materialien in Richtung Membran verhindern.
Alterungsphänomene der einzelnen Komponenten können dieses sensible Wassermanagement stören und so die Leistungsfähigkeit einer Brennstoffzelle
herabsetzen. Zum tieferen Verständnis der Wasserverteilung werden zerstörungsfreie Methoden, wie zum Beispiel die ex situ Neutronentomografie oder die in situ Synchrotronradiografie eingesetzt. Letztere Methode kann während des Brennstoffzellenbetriebs in Kombination mit weiteren Messmethoden, wie der ortsaufgelösten Stromdichtemessung, eingesetzt werden. So können einzelne Brennstoffzellenkomponenten, wie die Gasdiffusionsschichten, charakterisiert werden.