Degradation of (La,Sr)(Co,Fe)O3‐δ SOFC Cathodes at the Nanometre Scale and Below

Abstract

For developing solid oxide fuel cells (SOFCs) operating at intermediate temperatures, metallic materials have become a preferential choice for the interconnect due to their low cost and excellent physical and chemical properties. However the presence of chromium in all commonly used metallic alloys has been found to cause poisoning of the cathode leading to rapid electrochemical performance degradation of the cathodes including one of the most promising (La,Sr)(Co,Fe)O 3‐δ (LSCF) perovskite oxides [1‐3]. Despite the extensive research on the chromium deposition and poisoning processes, careful microstructural studies, especially at the nanoscale, are rare, which can provide valuable information for the fundamental understanding of the Cr poisoning mechanisms required for developing Cr tolerant cathode materials. In this paper, we examine the Cr poisoning mechanisms in LSCF materials by correlating the bulk electrochemical properties of the cell with their structural and chemical change at multi‐scales down to the nanometer level. Cells with LSCF cathodes were prepared, and the effect of Cr poisoning on the electrochemical behavior of the cell was assessed by impedance spectroscopy. The change in nano/microstructure and chemistry due to poisoning were studied in parallel by a combination of several advanced electron microscopy techniques including focus ion beam (FIB) tomography, high resolution (scanning) transmission electron microscopy ((s)TEM) and analytical STEM. Our results show that Cr poisoned samples exhibit multiscale changes especially at the nanoscle including formation of nanometer size Cr rich phases (Figure 1), Cr segregation at LSCF grain boundaries (Figure 2), alternation of local LSCF stoichiometry and structure (Figure 3), and change of valence state of the B site elements. These observed nanoscale changes are consistent with the impedance data measured from the same samples that shows the reduction of both oxygen surface reaction rate and oxygen diffusion by 1‐2 orders of magnitude after Cr poisoning. The work revealed critical degradation mechanisms effective at the nano to atomic scale and provide new insight for the development of future poisoning resistant electrode materials not only for SOFCs but also for other devices such as solid oxide electrolysis cells.