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 (Rs) during testing. Oscillations of the Rs, 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.

► We identify that all oscillations in SC-SOFCs are attributed to the cyclic oxidation and reduction processes occurring on the Ni metal surface. ► The in situ measurements of the local anode resistances (Rs) associated with the voltages (V) and the actual temperatures of the cell (Tc) provide a new insight into identifying the regime of the Ni/NiO redox. ► Oscillation patterns in SC-SOFCs 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 in the redox process. ► 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.