Markötter, Henning: Entwicklung und Optimierung von radiographischen und tomographischen Verfahren zur Charakterisierung von Wassertransportprozessen in PEM - Brennstoffzellenmaterialien. , Technische Universität Berlin, 2013
http://nbn-resolving.de/urn:nbn:de:kobv:83-opus-38956
Open Access Version
(available 01.01.3000)Abstract:
Water transport in polymer electrolyte membrane fuel cells (PEMFC) was non-destructively studied during operation with synchrotron X-ray radiography and tomography. The focus was set on the influence of the three-dimensional morphology of the cell materials on the water distribution and transport. Water management is still one of the mayor issues in PEMFC research. If the fuel cell is too dry, the proton conductivity (of the membrane) decreases leading to a performance loss and, in the worst case, to an irreversible damage of the membrane. On the other hand, the presence of water hinders the gas supply and causes a decrease in the cell performance. For this reason, effective water transport is a prerequisite for successful fuel cell operation. In this work the three-dimensional water transport through the gas diffusion layer (GDL) and its correlated with the 3D morphology of the cell materials has been revealed for the first time. It was shown that water is transported preferably through only a few larger pores which form transport paths of low resistance. This effect is pronounced because of the hydrophobic properties of the employed materials. In addition, water transport was found to be bidirectional, i. e. at appropriate locations a back and forth transport between GDL and flow field channels was observed. Furthermore, liquid water in the GDL was found to agglomerate preferably at the ribs of the flow field. This can be explained by condensation due to a temperature gradient in the cell and by the position, which is sheltered from the gas flow. Larger water accumulations in the gas supply channels were mainly attached to the channel wall opposing the GDL. The gas flow can bypass these agglomerations allowing a continuous gas supply. Moreover, it was shown that randomly distributed cracks in the micro porous layers (MPL) play an important role for the agglomeration of liquid water as they form preferred low resistance transport paths. In this work also perforated MPL/GDL-materials were investigated. It had been shown in complementary measurements that depending on process parameters perforated MPL/GDL materials can have either a positive or in other cases a negative impact on the cell performance (gains of up to 20 % vs. losses of same magnitude). The water transport was found to be responsible for the different behavior. At its best, the perforations have a drainage effect which facilitates effective water removal. In other cases a flooding of the whole local pore area around the perforation was observed. This area was obviously heat affected by laser perforation procedure and showed a hydrophilic behavior. The transport through the perforations was also found to be bidirectional. In this work, specially adapted measuring techniques were applied to analyze various aspects of water management. For example the combination of dynamic radiographic and three-dimensional tomographic measurements has been proven as valuable method to interpret transport phenomena in terms of the underlying cell structure. On top of that a method is applied, which allows for an increased spatial resolution in tomography and the easy switch between radiographic and tomographic measure mode. By comparing the tomographic data of the cell measured subsequent to operation with the dry reference state it was possible to extract the three-dimensional quasi in situ water distribution. This allows for more detailed analyses, for example, statistical water cluster size distributions. The extracted water distribution was also used by a group at the ZSW Ulm for the model validation of a grand canonical Monte Carlo simulation.