Experimental investigations of the microscopic features and polarization limiting factors of planar SOFCs with LSM and LSCF cathodes

The paper deals with the microscopic and polarization evaluation of planar circular-shaped seal-less SOFC cells from InDEC (R) with an outline of performance limiting factors at reduced temperature. The cells consist of porous NiO-YSZ anode as mechanical support, NiO-YSZ anode active layer, yttria-stabilized zirconia (YSZ) electrolyte, and only differ for the cathode design. A first design (ASCI) with strontium doped lanthanum manganate LSM-YSZ cathode functional layer (CFL) and LSM cathode current collector layer (CCCL); the second design (ASC2) with yttria doped ceria (YDC) barrier layer and lanthanum strontium cobalt ferrite oxide (LSCF) cathode. The microscopic analysis was performed using SEM methods. The electrical characterization was performed by taking current-voltage measurements over a range of temperatures between 650 and 840 degrees C with hydrogen as fuel, and air as oxidant. The analysis of performance showed that at 740 degrees C the voltage of 700 mV is reached at around a double value of current density in the case of ASC2. Further, the dependence of performance on the various polarization contributions was rationalized on the basis of an analytical model. Through a parameter estimation on the experimental data, it was possible to determine some polarization parameters for the two cells such as the effective exchange current densities, ohmic resistance and anodic limiting current density. It is shown that the performance limitation at low temperature is due to activation polarization for ASC1 and ohmic polarization for ASC2. Based on the results of the investigation, it is concluded that LSCF cathodes are really effective for decreasing the cathode activation polarization, allowing the reduction of operating temperature. (c) 2007 Elsevier B.V. All rights reserved.