Degradation of (La,Sr)(Co,Fe)O3-Δ SOFC Cathodes at the Nanometre Scale and below

Abstract

For developing solid oxide fuel cells operating at intermediate temperatures (ITSOFCs), 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)O3-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 (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ (LSCF6428) 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 advanced ion and electron microscopy techniques including focus ion beam (FIB) tomography, high resolution (scanning) transmission electron microscopy ((s)TEM) and analytical STEM. The systematic combination of bulk and high-resolution analysis on the same cells allows, for the first time, to directly correlate Cr induced performance degradation with subtle and localized structural/chemical changes of the cathode down to the atomic scale. Up to two orders of magnitude reduction in conductivity, oxygen surface exchange rate and diffusivity were observed in Cr poisoned LSCF6428 samples. These effects are associated with the formation of nanometer size SrCrO4; grain boundary segregation of Cr; enhanced B-site element exsolution (both Fe and Co); and reduction in the Fe valence, the latter two being related to Cr substitution in the LSCF phase. The finding that significant degradation of the cathode happens before obvious microscale change points to new critical SOFC degradation mechanisms effective at the nanometer scale and below, which 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.