The performance of solid oxide fuel cells is affected by various polarization losses, usually grouped in ohmic, activation and concentration polarization. Under typical operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometry: as an example, the performance of a tubular cell is strongly limited by the ohmic polarization due to the long current path of electrons, while in a planar cell each of these losses has a comparable effect. It is therefore of interest, in case of planar geometry, to investigate the performance limiting factors. In this paper, a performance evaluation of planar circular-shaped seal-less SOFC cells from InDEC® was performed, with an outline of the limiting factors at reduced temperature. Two different designs of planar cells are considered: both have porous NiO-YSZ anode as mechanical support, NiO-YSZ anode active layer, yttria-stabilized zirconia (YSZ) electrolyte, and only differ for the cathode design: (1) strontium doped lanthanum manganate (LSM)-YSZ cathode functional layer (CFL) and LSM cathode current collector layer (CCCL); (2) yttria doped ceria (YDC) blocking layer and lanthanum strontium cobalt ferrite oxide (LSCF) functional layer. The characterization was performed by taking V-I measurements over a range of temperatures between 650°C and 840°C with hydrogen as fuel, and air as oxidant. The dependence of the cell performance on the various polarization contributions was rationalized on the basis of a analytical model, through a parameter estimation on the experimental data, devoted to the determination of the temperature dependence of the area specific resistance (ASR) and of the cathode exchange current density: in particular, 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. Finally, a microstructural analysis with SEM and optical microscopy has been performed on the ASC2 cell after the polarization testing. The aim of this investigation was in particular to evaluate the degradation phenomena occurring in the anodic structure and over the interfaces between the various active layers. The ASC2 elastic modulus has also been estimated before and after polarization testing in order to evaluate the decreasing of the mechanical strength of the cell after a complete thermal cycle. The results describe a mechanical degradation of the structure and of the distribution of the phases.
Noise Improvement of a-Si Microbolometers by the Post-Metal Annealing Process
