Origin of Microstructural Degradation in Nickel Oxide–Scandia–Ceria-Stabilized Zirconia during Reduction

The electrochemical performance of ceramic–metal composite (cermet) electrodes in solid oxide cells (SOCs) is correlated to the ionic conductivity of the ion-conducting phase. This study explores the effect of replacing the 8 mol % yttria-stabilized zirconia (YSZ) in the fuel electrode of state-of-the-art SOCs with 10 mol % scandia and 1 mol % ceria-stabilized zirconia (10SSZ). The electrochemical behavior of electrodes with a 56:44 wt % (NiO:YSZ/10SSZ) ratio, symmetrically screen-printed on YSZ pellets and sintered at 1400 °C for 5 h, is investigated. The area-specific resistance (ASR) in Ar/3% H2 between 800 and 700 °C for Ni–10SSZ is found to be much higher compared to the conventional Ni–YSZ under these conditions, even though the ionic conductivity of scandia-stabilized zirconia (SSZ) is greater than that of YSZ. Increasing the initial NiO content in NiO–10SSZ from 56 to 65 wt % also did not improve the electrochemical performance relative to Ni–YSZ. Based on scanning electron microscopy (SEM) analysis, we attribute the poor performance of Ni–10SSZ to the detachment and contact loss between Ni and 10SSZ particles. The transmission electron microscopy (TEM) analysis reveals scandium segregation at the NiO grain boundaries after sintering NiO–10SSZ at 1400 °C in air, which may contribute toward such detachment.