Influence of impurities on th H2/H2O/Ni/YSZ electrode

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    Abstract

    The kinetics of the SOFC anode or more specific the H2/H2O/Ni/SZ electrode (SZ=stabilized zirconia) is widely investigated, but there are large disagreements about the kinetics and mechanisms in the literature. It is reported that impurities from the electrode materials (Ni/SZ) segregate to the surface/interface/TPB (TPB=three phase boundary) and that these impurities have a negative influence on the kinetics. These impurities may be the explanation for the disagreements found in the literature. The purpose of this study is therefore to perform electrochemical measurements in a very clean system to avoid the effects of impurities. This is attempted by using high purity materials, lowering the operation temperature to prevent fast segregation of impurities and by limiting impurities from the environment. A simplified geometry of the real SOFC anode, which is a porous Ni/SZ composite, was studied. The simplified anode was made by pressing a Ni wire against a single crystal of stabilized zirconia. In spite of the efforts of making electrochemical measurements in a very clean system, impurities were still found on the surface of the electrode materials (Ni and SZ) after an electrochemical experiment. The impurities found on the SZ are believed to segregate from the bulk of SZ to the surface. Sulfur was found on the surface of the Ni, but its origin is unclear. A higher impurity level was detected on the surface of the Ni and SZ outside the contact area (between the Ni and YSZ) than inside the contact area. The initial smooth surface of the SZ had developed a hill and valley structure in the contact area after a heat treatment. Also, a ridge around the contact area on the SZ was seen. The polarization resistance at open circuit voltage (500°C, 3% H2O/H2) increased by a factor of 5-19 over 10-20 days before leveling out. The increase in polarization resistance is believed to be
    caused by: 1) Segregated impurities, 2) The built up of a ridge around the contact area and 3) The sulfur adsorption on the Ni wire. An increasing water content in the atmosphere was seen to lower the polarization resistance at OCV and at anodic overpotentials, but it had no effect at cathodic overpotentials. It was hypothesized that water changes the properties of the impurities and hereby promote the electrode reaction at OCV and anodic overpotentials. A strong cathodic polarization <-2000 mV vs. air at 700°C was seen to lower the polarization resistance by a factor of 5-60. It was hypothesized that the strong cathodic polarization reduces or partly reduces the impurities from oxide to metal. Hence, the blocking influence of impurities would be diminished.
    Original languageEnglish
    Place of PublicationRoskilde
    PublisherRisø National Laboratory
    Number of pages126
    ISBN (Electronic)87-550-3502-7
    Publication statusPublished - Sep 2005
    SeriesRisø-PhD
    Number22(EN)

    Keywords

    • Risø-PhD-22(EN)
    • Risø-PhD-22

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