| A solid oxide fuel cell (SOFC) is a highly efficient and environmental-friendly energy conversion device. But the progress toward commercialization continues to be a slow struggle mostly due to high operation temperatures (800∼1000°C). Lowering operation temperatures can bring manufacturing costs down and provide high conversion efficiency, and less difficulty in sealing and heat management. However, substantial increase in internal losses, especially activation overpotential, lowers SOFC performance with reduced operation temperatures. Because the activation overpotential is mostly attributed to the oxygen reduction reaction (ORR) on the cathode, tremendous works have been done in order to improve the cathode performance and understand the ORR mechanisms and degradation mechanisms.;A heterogeneous reaction is primarily affected by the interaction between surface and adsorbed species. Therefore, the surface composition and structure are one of the most important factors affecting the cathode performance. Nevertheless, compositional and structural variations of cathode surfaces at high temperatures have not gained much attention. The objective of this study is, firstly, to better understand the surface structure of cathode materials at high temperatures, and, secondly, to improve the SOFC cathode performance based on this knowledge.;La0.6Sr0.4Co0.2Fe0.8O 3 has been widely used for SOFC cathodes. Chemical and structural variations of La0.6Sr0.4Co0.2Fe0.8O 3 (LSCF) surfaces under oxidizing environment were investigated. Cr contamination free LSCF showed the formation of submicron-sized SrOx precipitates on the grain surface. This caused the reduced concentration of transition metals in B sites. The addition of cobalt oxide on the surface of LSCF was found to improve the cathode performance.;In contrast, Cr vapor deposition caused the formation of larger SrCrO 4 particles on the LSCF surface along grain boundaries, and Sr-deficient matrix. The structural analysis identified the phase transition from rhombohedral to cubic perovskite in due to Sr deficiency. A defect chemistry model was presented based on observed phenomena. Electrical conductivity relaxation, AC impedance spectroscopy and temperature programmed isotopic exchange (TPX) were conducted to study the impacts of Cr contamination on the cathode performance.;It was found that Cr contamination was a chemical process for Co-Fe based materials and an electrochemical process for Mn based materials. Unstable Co4+ and Fe4+ lead to the formation of a nucleation agent for the chemical reaction. By contrast, stable Mn4+ does not lead to the formation of a nucleation agent. At last, a new hypothesis for the vaporization of Sr from La0.6Sr0.4Co0.2Fe0.8O 3-delta (LSCF) was proposed based on previous observations, and the evidence for Sr vaporization was provided. |
