Redox of Ni/YSZ anodes and oscillatory behavior in single-chamber SOFC under methane oxidation conditions

A single-chamber solid oxide fuel cell made of Ni/YSZ anodes, YSZ electrolytes and SDC-impregnated LSM cathodes was tested in methane-oxygen mixture at furnace temperature equal to 700 °C. Two Ag wires were arranged on the anode surface to in situ measure the changes of the local anode resistance (R_s) during testing. Oscillations of the R_s, the cell voltage and the actual temperature of the cell were observed and attributed to Ni/NiO redox cycles. Steady-state tests of the cell showed the corresponding oscillation patterns mainly depended on methane-to-oxygen ratio (M = 1, 2). Higher current density (J) to a certain extent could suppress NiO reduction and promote Ni oxidation. Scanning electron micrographs confirmed that Ni/NiO redox cycles had occurred mainly near the anode surface. The obtained results imply that gradual reoxidation of the Ni-anodes accompanying the various oscillation behaviors plays an important role in the degradation of the SC-SOFCs, especially under oxidation conditions.