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 [1-4]. Despite the extensive research on the chromium deposition and poisoning processes, careful microstructural studies at multi-scale lengths 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 electrochemical 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 and analytical (scanning) transmission electron microscopy ((s)TEM). Our results show that apart from change at the micrometer level, Cr poisoned samples with an increase in the total polarization resistance by one order of magnitude exhibit subtle nanoscale changes such as formation of nanometer size Cr rich phases, Cr segregation at LSCF grain boundaries and alternation of LSCF bulk stoichiometry. This nanoscale evolution correlates well with the impedance results obtained from the same samples, which shows that the area specific resistance of poisoned sample increased due predominantly to the increase of the oxygen reduction component related with a decrease both in the oxygen exchange rate at the LSCF internal surface and the oxygen diffusion in the electrode.

References

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