Degradation of SOFC Anodes upon Redox Cycling: A Comparison Between Ni/YSZ and Ni/CGO

Abstract

In this experimental study, electrolyte-supported solid oxide fuel cells (SOFCs) with two different anodes were investigated. Specifically, the stability of cells with a nickel/8 mol % yttria-stabilized zirconia (Ni/8YSZ) cermet anodes was compared to those based on nickel/40 mol % gadolinia-doped ceria (Ni/CG40). For this, the cells were characterized by impedance spectroscopy as well as by four-point electrical conductivity measurements. A high frequency process was observed in the Ni/8YSZ anode, which was not detected in the Ni/CG40 anode. After eight redox cycles at 950�C, the cell with the Ni/8YSZ anode showed an increase in the polarization resistance mainly in the high frequency domain. However, the cell with the Ni/CG40 anode showed an increase in both ohmic and polarization resistances, the latter mainly in the low frequency domain. Compared with Ni/CG40, the degradation in Ni/8YSZ upon redox cycling was higher at 850�C but lower at 950�C. For the Ni/8YSZ anode, a significant degradation was seen in the first 3 h after a redox cycle. The increase in the ohmic resistance of the Ni/CG40-based cell is believed to correlate with a decrease in the electrical conductivity of the anode. The latter showed a strong decrease upon a subsequent redox cycling at 950�C. For the Ni/CG40 anode, the degradation in both the conductivity and electrochemical performance significantly improved by decreasing the operation temperature from 950 to 850�C.