Production of Microtubular SOFC Fuel Cells and Their Use in Stacks With Ultra Short Warm-Up Time

Microtubular Solid oxide fuel cells have some very desirable advantages over their planar counter-parts, especially a very high mechanical and thermal stability. The investigations presented concern the fabrication of electrolyte supported tubular cells with a diameter of 3 to 5 mm and a length of 50 mm. Such electrolyte tubes were commercially available fabricated using extrusion and firing methods. The electrolyte material used was 8YSZ and the wall thickness of the electrolyte was about 200μm. The inside of the electrolyte tubes were coated with anode materials (Ni-YSZ) from different sources using a special dip coating method followed by drying and sintering under various conditions. The outside of the tubes were coated using commercially available cathode materials (LSM) from different sources by painting, spraying and dip coating. Again various sintering parameters were examined. Ni-wires were used to provide electrical contact to the anode. We used Ag wire, Ag and Pt paste on the cathode side to provide electrical contact. In this configuration single cell tests were performed in an oven at 850°C and pure Hydrogen was used as a fuel. The next step was to arrange several cells in a stack. The stack concept is modular, so scaling up using our base module (the so called microreader) is easy to do. Micro reactors with various numbers of cells were tested in an oven at our operating temperatures of between 800 and 900°C, and as with the single cells, pure Hydrogen was used as a fuel. Also, for a better fuel utilisation always two single cells were connected in series in the microreactor unit so that the exhaust gas of the firsl cell was used in the second cell. Here the cells were electrically connected in parallel. In order to prevent burning of the fuel off gas at the end of a tube and to make fuel flux from cell to cell possible, while still the cells were mounted only on one side, all tubular cells were blocked at one end. At the other end. the cells were fixed to a manifold providing the hydrogen supply. The gas was supplied through an inner tube bringing the fuel up to the blocked end of the cell. Here the fuel supply is similar to the air supply in the Siemens-Westinghouse concept, only with the off gas of the first cell passing through a second cell. This type of stack is extremely insensitive against thermal shocks because of the small dimensions of the single cells and because, in the stack concept, the cells are fixed on one side only. It is possible to warm up the stack from room temperature to operating temperature in less than 10 minutes.