Understanding the Electrochemical Performance of SOFC Stacks

In the past much effort was spent for the detailed understanding of the electrochemical behavior of SOFC single cells. However, these results can only hardly be transferred to the SOFC stack technology because of huge differences in fuel utilization, in temperature distribution, in gas flow design and in electrical current load. Therefore, proper and detailed understanding of the electrochemical behavior of the SOFC stacks is obligatory for the successful introduction and acceptance of the SOFC technology into the global energy market. The paper presents both experimental and modeling results of the electrochemical behavior of SOFC short stacks with anode supported cells (ASC) in the cassette design at different operating conditions. The focus lies on the investigation of the different resistances and the voltage losses in the stack layers, which can be classified in ohmic, electrode polarization and gas concentration ones. The influence of the different operation conditions, e.g. temperature, current density and fuel utilization is outlined. The stacks were investigated by current-voltage curves, electrochemical impedance spectroscopy (EIS), gas analysis and long term measurements. Results of both integral and locally resolved measurements along the fuel gas flow path are presented. The modeling results were obtained with a two dimensional elementary kinetic model which represents a single cell along the gas channels and through the thickness of the membrane-electrode assembly.