Effect of Antisite Defects on the Formation of Oxygen Vacancies in Sr2FeMoO6: Implications for Ion and Electron Transport

To face worldwide energy-related environmental concerns, solid oxide fuel cell (SOFC) technology emerges as a promising route for clean and efficient production of electricity. Within this context, great efforts have been devoted to the development of SOFC devices able to run at intermediate temperatures and to retain electrochemical performance as good as in the high temperature regime. To this end, materials that have characteristics of mixed ionic and electronic conductors (MIECs) have been proposed as electrodes for SOFC applications. Among many proposed systems, MIEC electrodes based on strontium iron molybdenum oxide (Sr2Fe2-xMoxO6-delta) have been proven to be extremely efficient for intermediate temperature SOFC. However, to advance SFMO-based electrodes further, a detailed understanding of the physical and chemical processes involved and of the corresponding electronic and structural features is needed. As a first step in this direction, we investigate via quantum mechanics the Sr2FeMoO6 (SFMO) material, with a particular emphasis on characterizing the formation of bulk oxygen vacancies, which is a key component of the oxide ion diffusion process in SOFC electrodes. To explore the feasibility of vacancy formation in different local environments, we studied ordered SFMO as well as SFMO with Fe-Mo-Mo-Fe antisite defects. The formation energy for oxygen vacancies along M-O-M' bonds is predicted to follow the trend Fe-O-Fe < Fe-O-Mo < Mo-O-Mo. Therefore, oxygen diffusion should be enhanced for local higher concentrations of iron. Moreover, the reduced material may have enhanced electronic conductivity, as judged by its altered electronic structure. Our results and analysis of the reasons behind this trend highlight the importance of further experimental and theoretical investigations on Fe-rich SFMO-based materials.