Chlorine-Anion Doping Induced Lattice Distortion to Enhance the ORR and OER Activities of a La0.5Sr0.5FeO3–δ Oxygen Electrode in Reversible Solid Oxide Cells

Fe-based perovskite La0.5Sr0.5FeO3-δ (LSF) is a potential oxygen electrode material for reversible solid oxide cells (R-SOCs) due to low cost and good thermal matching with common electrolytes. However, its catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are insufficient compared with traditional Co-based perovskite. Herein, we propose incorporating chlorine-anions (Cl-) into LSF as the oxygen electrode for R-SOCs, synergistic optimization of lattice defects, electronic structure, and surface chemical properties of Fe-based perovskite, thereby significantly enhancing their ORR and OER catalytic activity. Experimental results combined with density functional theory (DFT) calculations showed that appropriate Cl- doping can increase the LSF unit cell volume and induce local lattice distortion and then significantly reduce oxygen vacancy formation energy (EVo··) and increase oxygen vacancy (Vo··) concentration. DFT calculations confirmed that Cl- doping can reduce the hybridization between Fe 3d and O 2p and weaken the covalency of metal-oxygen bonds (Fe-O), thereby facilitating the formation and redistribution of Vo··. The oxygen surface exchange coefficient (kchem) and bulk diffusion coefficient (Dchem) of La0.5Sr0.5FeO2.95-δCl0.05 (LSFCl0.05) at 750 °C were increased by 3 times and 4 times, respectively. Distribution of relaxation time (DRT) showed that Cl- doping significantly enhances the adsorption and dissociation steps in the ORR process of the LSF oxygen electrode, thereby reducing the polarization impedance. A single cell with a LSFCl0.05 oxygen electrode achieved a maximum power density of 1.493 W·cm-2 in solid oxide fuel cell (SOFC) mode and a high electrolysis current density of 2.306 A·cm-2 in solid oxide electrolysis cell (SOEC) mode at 750 °C, and the performances were improved by 50% and 53%, respectively. Additionally, it demonstrated excellent long-term stability in both two modes and cycle stability between SOFC and SOEC modes. This work significantly enhances the catalytic activities of Fe-based perovskites for ORR/OER, providing a practically feasible avenue for developing high performance oxygen electrode materials for R-SOCs.