Investigation of the degradation of LSM-YSZ SOFC cathode by electrochemical impedance spectroscopy

摘要

The aim of this PhD study was to investigate degradation of the LSM-YSZ cathode of anode supported Ni-YSZ/YSZ/LSM-YSZ solid oxide fuel cells. The chosen cathode materials LSM25 and 8YSZ were investigated for their compatibility and stability, to confirm that expansion/contraction or decreasing conductivity would not be a problem during degradation experiments of the cells. The experiments carried out for this purpose include x-ray diffraction, conductivity and dilatometry. LSM-YSZ/YSZ/LSM-YSZ symmetrical cells were prepared and investigated by means ofelectrochemical impedance spectroscopy, at different operating conditions. An equivalent circuit was developed for the symmetrical cell, describing the processes taking place at the LSM-YSZ cathode. This equivalent circuit was applied in degradation studies, where the processes affected by degradation over time could be pinpointed. Furthermore, it was discovered that impurities in air cause significant degradation of the cathode. Humidity was found to increase the degradation rate, but other impurities might also be present and increasing degradation. Then the anode supported Ni-YSZ/YSZ/LSM-YSZ single cells were prepared and tested. It was found that at the applied operating conditions the impedance data could not be deconvoluted as anode and cathode processes were overlapping. Nonetheless it appeared that at OCV the degradation of the cathode is similar for symmetrical and single cells. Under current degradation was significantly lower, so real performance and degradation data can only be obtained on single cells as symmetrical cells can only be tested at OCV. For single cells degradation caused by impurities from air was also observed.