Ohmic resistance of nickel infiltrated chromium oxide scales in solid oxide fuel cell metallic interconnects

• Interaction of Ni with Cr2O3 decreases ohmic losses of degraded metallic interconnects.
• NiCr2O4 spinel is formed and decomposed during redox cycling at the Ni(O)/Cr2O3 interface.
• Increased el. conductivity by Ni particle rearrangement after spinel decomposition
• Superposition of different phase change mechanisms incl. Ostwald ripening
• Extensive characterization incl. BIB-SEM, AFM, XRD and el. conductivity measurements

Oxide scale formation on metallic interconnects contributes to the overall degradation of solid oxide fuel cell (SOFC) stacks. On the anode side, thermally grown oxide scale might contain additional nickel, nickel oxide, or nickel chromium spinel phases — depending on the applied operation conditions. Ni originates from Ni-meshes, often applied as current collector, from Ni-containing anodes or from Ni-containing coatings. Ni particles released during thermo redox cycles from adjacent Ni-containing components might be interspersed into the oxide scale. This study aims at investigating the influence of Ni on the electrical conductivity of oxide scales. For this purpose pellets of Cr2O3 were mixed with different amounts of Ni and then investigated in-situ under both reducing and oxidizing gas atmospheres at 850 °C. The formed crystals were analyzed using X-ray diffraction, whereas the resulting microstructures were quantified using scanning electron microscopy. During oxidation Ni is converted into NiO, and the latter interacts with Cr2O3 to form a NiCr2O4 spinel phase. Subsequent exposure to reducing conditions leads to an almost instantaneous decomposition of NiCr2O4 spinel, resulting in finely dispersed elementary Ni. This rearrangement of Ni by spinel decomposition leads to a significant improvement of the electrical conductivity of the Cr2O3 pellets compared to their initial state.

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