Interaction between microstructure and electrical properties of screen printed cathodes in SOFC single cells

The relationship between microstructural and electrical properties of screen printed cathodes has been investigated by testing single cells with different cathode microstructures under realistic working conditions. To determine the influence of various microstructural properties on the different loss mechanisms occuring during operation, DC and AC electrical measurements have been made. The performance was essentially governed by the structure of the cathode/electrolyte interface, that determined the losses due to activation-polarisation which occurs during electrochemical reaction in the three phase boundary region. It was found, that neither ohmic nor gas diffusion polarisation losses had a significant influence on the cathode performance. For all the single cells investigated, the first period of operation resulted in a drastic alteration of the cathode microstructure especially at the cathode-electrolyte interface, which led to a consequent decrease in cathode resistance. The extent of these changes is influenced by the initial microstructure of the cathode and the operating conditions of the cell. The single cells, differing only in cathode microstructure properties, showed maximum current densities between 500 and 1300 mA/cm(2) at 0.7 V cell-voltage, 950 degrees C, using pure oxygen as the oxidant and hydrogen as the fuel.