In recent years microstructural measurements techniques such as Focused-Ion-Beam Scanning-Electron-Microscopy or X-ray Computed Tomography, together with numerical simulation programs offered the possibility to get a deep insight and understanding of the microstructural geometry of Solid Oxide Fuel Cells electrodes. In this work the versatile applicability of the microstructural analysis, and the various possible results are summarised including porosity distribution, the equivalent pore diameter, number of pores and the effective diffusion coefficient. The latter is described as scaling factor for the mass-diffusion within the microstructure. The determination of the porosity distribution revealed a rather inhomogeneous metallic support microstructure, with a significant denser layer at the gas-channel metallic support interface, which inhibits the mass-transport, and may stem from the sintering process. The knowledge of the pore diameters on the other hand, allows estimating whether or not Knudsen diffusion has to be taken into account in the simulations. The effective diffusion coefficient is obtained by commonly applied homogenisation techniques. However, a significant improvement over the state-of-the-art approach was achieved by a novel treatment of the boundary conditions. It was shown that the new approach for defining periodic boundary conditions for the lateral boundaries yields more reliable and representative results on smaller geometries. This allows for a considerable reduction of the required computational resources and computation times.
Georg Reiss1, Henrik Lund Frandsen2, Wilhelm Brandstätter3, André Weber4. Journal of Power Sources, Volume 273, 2015, Pages 1006-1015.Show Affiliations
- ICE Strömungsforschung GmbH, Hauptplatz 13, 8700 Leoben, Austria
- Technical University of Denmark, Department of Energy Storage and Conversion, Frederiksborgvej 399, 4000 Roskilde, Denmark.
- Montanuniversität Leoben, Lehrstuhl für Petroleum and Geothermal Energy Recovery, Franz-Josef-Straße 18, 8700 Leoben, Austria.
- Karlsruher Institut für Technologie (KIT), Institut für Werkstoffe der Elektrotechnik (IWE), Adenauerring 20b, D-76131 Karlsruhe, Germany.
Metallic supported Solid Oxide Fuel Cells (SOFCs) are considered as a durable and cost effective alternative to the state-of-the-art ceramic supported cell designs. In order to understand the mass and charge transport in the metal-support of this new type of cell a novel technique involving X-ray tomography and micro-structural modelling is presented in this work. The simulation technique comprises a novel treatment of the boundary conditions, which leads to more accurate effective transport parameters compared to those, which can be achieved with the conventional homogenisation procedures. Furthermore, the porosity distribution in the metal-support was determined, which provided information about the inhomogeneous nature of the material. In addition to that, transport parameters for two identified, different dense layers of the metal-support are evaluated separately.
The results of the evaluation show three significant findings. Firstly, that the effective transport parameters are up to a factor of 20 lower compared to other Solid Oxide Fuel Cells anodes. Secondly, that the micro-structure can have regions, which are much denser, especially the first 100 μm of the interface between gas-channel and support-structure. Thirdly, that the calculation of the transport parameters depends on the correct application of boundary conditions.