Deciphering the Interaction of Single-Phase La 0.3 Sr 0.7 Fe 0.7 Cr 0.3 O 3-δ with CO 2 /CO Environments for Application in Reversible Solid Oxide Cells.

A detailed study aimed at understanding and confirming the reported highly promising performance of a La 0.3 Sr 0.7 Fe 0.7 Cr 0.3 O 3-δ (LSFCr) perovskite catalyst in CO 2 /CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO 2 atmospheres (pure CO 2 , CO alone (balance N 2 ), and a 90-70% CO 2 /10-30% CO containing mixture), related to the different conditions that could be encountered during CO 2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO 2 (balance N 2 , pO 2 = 10 -11 -10 -12 atm) without any decomposition. However, in 30% CO (balance N 2 , pO 2 ∼ 10 -26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO 2 /10-30% CO (pO 2 = 10 -17 -10 -18 atm) mixtures at 800 °C, in 70 CO 2 /30 CO, minor impurities of SrCO 3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO 2 /CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO 2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.