Development of Solid Oxide Fuel Cells and Short Stacks for Mobile Application

At the German Aerospace Center (DLR) in Stuttgart, a lightweight stack design for mobile applications was developed in cooperation with the automotive industry (BMW, Munich; Elring-Klinger, Dettingen; Rhodius, Weissenburg). This concept is based on the application of stamped metal sheet bipolar plates into which porous metallic substrate-supported cells (MSCs) are integrated. The paper concentrates on the one hand on the investigation of plasma sprayed button cells with a diameter of 48 mm on porous metallic substrates during reduction/oxidation and thermal cycling. On the other hand, another focus lies in the electrochemical testing of short stacks in the cassette arrangement. The microstructure of the cells was characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and energy dispersive microanalysis (EDX) before and after operation. The cells and short stacks were electrochemically characterized mainly by long-term measurements (life cycle), by current-voltage measurements, and by impedance spectroscopy. In order to understand the nature of degradation mechanisms, the open-circuit voltages (OCV), the ohmic resistances, and the polarization resistances, during dynamic operation are compared and discussed. In order to distinguish between degradation effects due to the dynamic operation and usual stationary effects, these values are compared to values of noncycled cells. All of the cells investigated were able to withstand ten redox and ten thermal cycles without severe failure. Their redox- and thermal-cycling behavior are strongly dependent on their OCVs, which decrease during cycling. This proves that thermomechanical stresses in the electrolyte layer play a major role for the electrochemical performance of the cells during cycling. The improvement of the electrodes during the first 200 h of operation and the ohmic resistance of the cells are not significantly influenced by the cycling. The first four-cell short stack with the cassette arrangement shows promising results with an OCV of ∼4 V and an overall power of 92 W at 800°C. The performances of the single cells are in the range of 180-220 mW/cm 2 . The differences in cell performance can be attributed to different polarization resistances of the cells in the cassettes, which might be caused by a nonuniform gas supply in the short stack.