Distribution of relaxation times analysis and interfacial effects of LSCF fired at different temperatures

Abstract

This paper describes the effects of (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCF) firing temperature on the electrochemical performance and the chemical interactions at the interface of Gd-doped ceria (GDC) barrier layer and yttria-stabilized zirconia (YSZ) electrolyte for commercial cells. Initial scanning electron microscopy (SEM) analysis indicate that the optimal cathode porosity is obtained at a firing temperature of 1100 °C; however, the peak power is not observed at the same firing temperature. At 750 °C and using humidified hydrogen, the peak power densities are found to be 0.607, 0.959, 1.41, and 0.842 W/cm2 for a cathode firing temperature of 900, 1000, 1050, and 1100 °C, respectively. The power density data indicate an increasing trend in performance as the firing temperature increases; however, the electrochemical performance drops drastically at a firing temperature of 1100 °C. Using impedance spectroscopy (IS) and distribution of relaxation times (DRT) analysis, it is determined that the decrease in cell performance is mostly due to a slower charge transfer process. This is further confirmed using high resolution SEM/EDS and STEM/EDS analysis which show that Sr is present at the GDC/YSZ interface with the highest concentration at 1100 °C. This indicates that an insulating layer of strontium zirconate (possibly SrZrO3) has formed and is mostly responsible for low cell performance. In addition, STEM/EDS analysis shows that Zr and Ce have interdiffused with each other as well. It is further shown that more Zr (in addition to interdiffusion) has moved into the barrier layer in order to react with Sr to form the strontium zirconate.