Magnesium-Doped Sr2(Fe,Mo)O6−δ Double Perovskites with Excellent Redox Stability as Stable Electrode Materials for Symmetrical Solid Oxide Fuel Cells

Abstract

In this work, magnesium-doped $Sr_{2}Fe_{1.2}Mg_{0.2}Mo_{0.6}O_{6−δ}$ and $Sr_{2}Fe_{0.9}Mg_{0.4}Mo_{0.7}O_{6−δ}$ double perovskites with excellent redox stability have been successfully obtained. The physicochemical properties including: crystal structure properties, redox stability, thermal expansion properties in oxidizing and reducing conditions, oxygen content as a function of temperature and transport properties, as well as the chemical compatibility with typical electrolytes have been systematically investigated. The in situ oxidation of reduced samples using high-temperature XRD studies shows the crystal structure of materials stable at up to a high-temperature range. The in situ reduction and oxidation of sinters with dilatometer measurements prove the excellent redox stability of materials, with the thermal expansion coefficients measured comparable with electrolytes. The oxygen nonstoichiometry δ of compounds was determined and recorded in air and argon up to 900 °C. $Sr_{2}Fe_{1.2}Mg_{0.2}Mo_{0.6}O_{6−δ}$ oxide presents satisfactory values of electrical conductivity in air ($56.2 \ S·cm^{−1}$ at 600 °C) and reducing conditions ($10.3 \ S·cm^{−1}$ at 800 °C), relatively high coefficients D and k, and good ionic conductivity (cal. $0.005 \ S·cm^{−1}$ at 800 °C). The stability studies show that both compounds are compatible with $Ce_{0.8}Gd_{0.2}O_{1.9}$ but react with the $La_{0.8}Sr_{0.2}Ga_{0.8}Mg_{0.2}O_{3−d}$ electrolyte. Therefore, the magnesium-doped double perovskites with excellent redox stability can be potentially qualified as electrode materials for symmetrical SOFCs and are of great interest for further investigations.