Effects of dopants and trace elements at the Ni / ScYSZ interface

Michael Stenbæk Schmidt

    Research output: Book/ReportPh.D. thesisResearch

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    Abstract

    The interfaces between the various materials and phases in solid oxide fuel cells (SOFCs) play a fundamental role, when optimizing SOFC performance. The industrial grade materials commonly used in SOFCs contain numerous impurities. At the catalyst / electrolyte interfaces in SOFC cermet anodes, these impurities are believed to be responsible for losses in electrochemical performance. Impurities such as silica have been shown to segregate to the three phase boundary (TPB) in SOFC cermet anodes.

    A three-electrode configuration, with a high purity (99.999 %) nickel pointelectrode acting as the working electrode, was used as a simplified model of the SOFC cermet anode. Electrochemical impedance spectroscopy was used to characterise the electrode polarisation resistances on selected electrolyte materials at 850 °C in humidified hydrogen atmospheres. Electrode reactions were characterized on two types of scandia and yttria co-doped zirconia electrolytes (ScYSZ) with different purity levels, a ScYSZ electrolyte doped with alumina and an yttria stabilized zirconia (YSZ) electrolyte.

    A point electrode setup provided a well defined electrode / electrolyte interface (EEI) and TPB during electrochemical characterisation, which, after lifting the electrode off the electrolyte, could be studied by surface analysis techniques. Surface analysis was performed using low acceleration voltage scanning electron microscopy (SEM), atomic force microscopy (AFM), time of flight secondary ion mass spectrometry (ToF-SIMS) and xray photoelectron spectroscopy (XPS).

    For all electrolyte materials the electrode polarization resistances dropped rapidly during the first 50 hours of measurements at 850 °C before stabilising. From ToFSIMS and SEM analysis, rim zones with relatively low impurity concentrations were identified at the outer perimeter of the EEI on the electrolyte surfaces. It was suggested that the rim zones were formed, as the nickel electrode expanded, due to metallic creep. The subsequent drops in polarization resistance are believed to be a direct consequence of fewer impurities in the reaction path at the anode. Chemical analysis of the various EEIs showed that larger impurity concentrations measured in the EEIs resulted in larger electrode polarization resistances.

    It was found that prolonged anodic polarisations activated the electrodes. This effect was suggested to be caused by the removal of impurities by water, which formed at the TPB. The electrodes slowly deactivated when the anodic polarization was lifted which was believed to be a result of renewed impurity segregation and concentrations at the TPB.

    Dynamic processes occurred on the electrodes when they were subjected to anodic polarization. Chronoamperometry revealed a sawtooth oscillation pattern in the current signal which was hypothesised to be caused by formation and subsequent reduction of Ni(OH)2 on the electrode surface. Increasing the water content in the atmosphere promoted the dynamic process. Nickel was transported away from the electrode which could be a possible cause for the degradation observed in SOFC anodes
    Original languageEnglish
    Place of PublicationRoskilde
    PublisherRisø National Laboratory
    Number of pages151
    ISBN (Print)978-87-550-3708-3
    Publication statusPublished - 2008
    SeriesRisø-PhD
    Number44(EN)

    Keywords

    • Risø-PhD-44(EN)
    • Risø-PhD-44
    • Risø-PhD-0044

    Cite this

    Schmidt, M. S. (2008). Effects of dopants and trace elements at the Ni / ScYSZ interface. Risø National Laboratory. Risø-PhD, No. 44(EN)