Impedance Measurements for Development and Quality Control of Tubular Solid Oxide Fuel Cells

摘要

Siemens Westinghouse Power Corp. manufactures and supplies SOFC -facilities. Installations with a power of 100 kW are currently in use, a pilot plant with a power of 1 MW will be delivered in 2002. The electrochemical cell is of a tubular geometry and consists of a cylindrical, porous tube with integrated cathode, an electrolyte layer and the anode at the outside. The length of a standard design cell is about 1,8 meters and the cell has a diameter of 22 millimeters. Due to the process of fabrication as well as to the very thin electrolyte layer (about 30 μm) the implementation of reference electrodes to separate single electrode potentials is not possible. But, the determination of the polarization resistances as well as the determination of the additional contributions is an unavoidable task for the quality control and the improvement of the fuel cell. The performance of impedance measurements of high precision in the frequency range from 1 mHz to 1 MHz was made possible in 1999. For this purpose short tube segments, cut from the original tubes, were mounted in houses of Al_2O_3 and sealed appropriately at the end faces. The relaxation parameters obtained from impedance measurements of a working cell at 950°C can be assigned to different processes. First of all, the conductive pathways, which are of extraordinary form in the cylindrical cell design, are taken into account. Furthermore, these ohmic contributions can be separated from the activation polarization of the electrodes and from contributions due to gas diffusion.