Characterisation of fuel cells and fuel cell systems using 3D x-ray tomography

Three dimensional (3D) computer aided X-ray tomography (CT) has proven to be an extremely useful tool in developing our own as well as in examining commercially available micro-tubular solid oxide fuel cells. The results of 3D-CT measurements became very important for understanding the functionality of our first generation and improving the development of our second fuel cell generation. Also geometrical measurements, especially the roundness and the straightness of the tube, can be evaluated, both critical parameters when the stack is heated and mechanical stress has to be avoided. By using this technique the structure of our first generation cells proved to be of non sufficient quality. Problems like variation of the thickness of the electrolyte tube as well as the homogeneity in thickness of the electrodes deposited can easily be detected by this measuring technique. Microscopic investigation of this problem of course provide equal results, but only after cutting the samples in many slices and many single measurements of different areas of the fuel cell. Using cells with inhomogeneous thickness of course results in drastic variations of the current densities along a single cell. Electrolyte layers that are too thick will result in drastic power loss due to the increased resistance in the ionic conductivity of the electrolyte. If the electrolyte of an electrolyte supported cell is too thin, this can cause mechanical instability. Problems can also occur with the leak tightness of the fuel cell tube. Gas diffusion through the electrode layer can become a problem when the thickness of the electrode layer is too high. On the other hand, if the layers are too thin, the result can be a discontinuous layer, leading to a high electrical series resistance of the electrode. Besides determining the thickness variations also the porosity of the electrolyte needs careful attention. Larger cavities or shrink holes form insulating islands for the ion-stream and are therefore limiting the ionic conductivity. They are also diminishing the mechanical stability and provide problems for depositing a closed electrolyte film in electrode supported cells. Otherwise, too small pores again lead to diffusion problems. Also the brazing connection between cell and stack needs to be analyzed in order to estimate the leak tightness and the mechanical stability of the stack system. In a microtubular SOFC the current collector inside the tube is a very important part of the device which can only be investigated in its fitting to the cell using x-ray techniques, especially the 3D-CT. In our case, already the first measurements have shown that the electrical contact inside the cell is critical. Metallic cages used in commercially available cells provide only very few contact points and thereby their contact resistance contributes substantially to the overall series resistance.