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 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 the first generation cells proved to be of insufficient quality. Problems like the variation in thickness of the electrolyte tube as well as the homogeneity in thickness of the electrodes deposited can easily be detected by this nondestructive technique. Microscopic investigations 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 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 electrode film in electrode supported cells.
Solid oxide fuel cells are known to be able to handle a large variety of different fuels. Because of the greenhouse effect the use of carbon dioxide neutral gases or liquids are of special interest. In this context wood-gas has a big potential to be an alternative fuel for solid oxide fuel cells (SOFCs). The gas is generated by a fluidized bed steam gasifier and consists of various components such as 25 Vol % carbon monoxide, 20 Vol % carbon dioxide, 10 Vol % methane, 2.5 Vol % ethylene, 0.5 Vol % propylene, 2 Vol % nitrogen, and the rest hydrogen (values in dry state). The water concentration of the original pyrolysis gas is about 35 Vol %. Besides these main ingredients there are of course many impurities like dust, tars, ammonia, hydrogen sulphide, and hydrogen chloride present in the product gas. Especially the last two ones may lead to degeneration of the fuel cell anode and must therefore be almost totally removed before feeding the gas into the cell. In order to reduce energy losses, hot gas cleaning systems are favored. This, however, limits the possibility to reduce the impurity concentrations to very low levels. Therefore the aim of this work is to define the maximum acceptable output concentrations for the hydrogen chloride adsorber also in combination with hydrogen sulphide, since for a micro-tubular SOFC there are as yet hardly any data available. In order to determine the influence of the hydrogen chloride on the performance of the fuel cell, different concentrations of this impurity were fed to the cell. Here, also the flow rate was changed while the electrochemical output was determined. In addition it was analyzed if there were any effects when changing from pure hydrogen to the HCl containing fuel. This was investigated at 1123 K and 1173 K, which are the preferred working temperatures for our cells. Cooling down as well as heating up procedures were tested with cells between 1173 K and 573 K. In a second series of experiments, combinations of hydrogen chloride and hydrogen sulphide of variable concentrations were tested. As before, changing between pure hydrogen and the acid containing fuel at above given temperatures was analyzed by determining the cell performance. In parallel to the above experiments, synthetic wood gas was used for operating the microtubular fuel cell while monitoring the electrochemical output with time.
In this study different concentrations of methane in hydrogen have been fed into a microtubular SOFC while determining the cell performance over time. Also the effect of adding water to the gas as a reforming agent has been analyzed with respect to power output and cell stability. In addition to the characterization by measuring the power with time gas chromatography and impedance spectroscopy were used in order to obtain further information about the cell behavior. Post mortem electron microscopy and atomic force microscopy analysis has shown the influences of the blends on the anode microstructure after different test series. Also the influence of the working temperature and of the rate of fuel utilization on the cell performance was investigated. Depending on the working conditions stable cell performance could be achieved but also three different degradation mechanisms could be found.
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