A-site composition engineering in high-entropy AFeO3 perovskite SOFC cathodes and unraveling oxygen reduction mechanisms

Abstract

High-entropy perovskites (HEPs) have emerged as promising cathode materials for solid oxide fuel cells (SOFCs) due to their tunable properties and enhanced electrochemical performance. In this study, a series of iron-based HEP cathodes, AFeO₃ (A = Sr, Ba, Bi, La, Pr, Nd, Sm), were synthesized via a high-entropy strategy to investigate the influence of A-site composition on structural and electrochemical properties. The introduction of low-valence A-site elements (Sr and Ba) and Bi³⁺ with highly polarized 6s² lone pair electrons was found to facilitate the formation of oxygen vacancies and modulate the valence state of iron ions, leading to lattice distortion and a phase transformation from cubic to orthorhombic structures. Electrochemical impedance spectroscopy (EIS) combined with distribution of relaxation time (DRT) analysis revealed distinct oxygen ion transport mechanisms under varying conditions. The incorporation of divalent and bismuth elements reduced the formation energy of oxygen vacancies and promoted the formation of high-valence iron ions, significantly enhancing oxygen adsorption, transport, and reduction capabilities. The optimized cathode, (Sr_0.2Ba_0.2Bi_0.2La_0.2Pr_0.2)FeO₃, exhibited an area-specific resistance (ASR) of 0.03 Ω cm² at 800 °C, outperforming conventional Fe-based perovskite cathodes. This work provides valuable insights into the design of high-performance HEP cathodes for SOFCs, highlighting the critical role of A-site composition in optimizing oxygen reduction reaction (ORR) kinetics and thermal expansion behavior.