Abstract
Gas and ion diffusivity within the membrane electrode assembly (MEA) are of central importance for the functioning of a proton exchange membrane (PEM) fuel cell. These properties are, among other things, directly impacted by the microstructure of the layering and the morphology of the individual layers. Different types of preparation were used to prepare the inner layer composite of the MEA/PEM of a fuel cell. The preparation quality provided for high resolution electron microscopy and the microstructure quantification were compared. The conventional mechanical preparation is compared to planar ion polishing, and cryo ion polishing at -80 °C. Advantages and disadvantages of the methods are discussed by way of examples related to the microstructure such as porosity, elemental distribution, and the measurement of the layer thickness. It is shown that cryo ion polishing facilitates a largely artifact-free preparation of the highly thermo-sensitive, partially highly porous layered PTFE composites, and the porous catalyst layers. Thus, not only easily ascertainable layer thicknesses but also qualitative features and important quantitative microstructural parameters, such as nanometer-sized pore structures and the porosity of the various composite layers can be revealed.
Kurzfassung
Für die Funktion einer Protonenaustauschmembran-Brennstoffzelle (engl. Proton Exchange Membrane Fuel Cell, PEM) sind die Gas- und Ionendiffusionsfähigkeit innerhalb der Membran-Elektroden-Einheit (engl. membrane electrode assemebly, MEA) von zentraler Bedeutung. Diese Eigenschaften werden direkt durch die Mikrostruktur des Schichtaufbaus und die Morphologie der einzelnen Schichten mitbestimmt. Verschiedene Präparationsvarianten wurden am inneren Schichtverbund einer Brennstoffzelle der MEA / PEM angewandt. Die Präparationsqualität für hochaufgelöste Elektronenmikroskopie und für die Gefügequantifizierung wurde verglichen. Die konventionelle mechanische Präparation wird mit einer flächigen Ionenpolitur und einer Kryo-Ionenpolitur bei -80 °C verglichen. Die Vor- und Nachteile der Methoden werden anhand von Gefügebeispielen, wie Porosität, Elementverteilung und Schichtdickenmessung, diskutiert. Es wird gezeigt, dass mittels der Kryo-Ionenpolitur die thermisch sehr empfindlichen, teilweise hochporösen PTFEPolymerschichtverbunde und die porösen Katalysatorschichten weitestgehend artefaktfrei präpariert werden können. Damit erschließen sich neben den leicht zugänglichen Schichtdicken auch qualitative Merkmale und wichtige quantitative Gefügekenngrößen, wie nanometerfeine Porenstrukturen und Porosität der verschiedenen Verbundschichten.
About the authors
Andreas Kopp is currently pursuing his PhD in the area of microstructural investigations and quality assurance of Li-ion batteries. He gained his bachelor’s degree in mechanical engineering and his research master’s degree in Advanced Materials & Manufacturing at Aalen University in the . He is research associate at the Materials Research Institute Aalen focusing on XRD, SEM and FIB analysis.
Jan Niedermeier has been working as a research associate at the material research institute at Aalen University since 2019, developing methods for the quality evaluation of li-ion battery electrodes. He received a bachelor's degree in mechanical engineering focused on new materials and a master's degree in the research master's program Advanced Materials and Manufacturing at Aalen University. His research activities include materials analysis using computed tomography and microscopy.
References / Literatur
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