Improving electrocatalytic activity through multi-element doping to A-site of Fe-based perovskite cathode for solid oxide fuel cells

Perovskite oxide La_0.5Sr_0.5FeO_3-δ (LSF) is a potential Co-free electrocatalyst for oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFCs). To improve its catalytic activity, this work performs multi-element doping to the A-site elements La and Sr, forming a high-entropy oxide Ba_0.2Sr_0.2La_0.2Pr_0.2Sm_0.2FeO_3-δ (H-LSF) and a medium-entropy material Ba_0.35Sr_0.35La_0.1Pr_0.1Sm_0.1FeO_3-δ (M−LSF). The multi-element doping effects are investigated on various physicochemical properties including crystalline structure, chemical compatibility with electrolyte, valence state of B-site Fe, amount of adsorbed oxygen species, oxygen nonstoichiometric value, sinterability, thermal expansion coefficient, electrical conductivity, chemical oxygen surface exchange coefficient (k_chem), and electrochemical performance. While the multi-element doping does not change the phase structure and compatibility, it decreases the electronic conductivity, increases the oxygen vacancy concentration and k_chem. Consequently, it improves ORR activity such as reduces interfacial polarization resistance and elevates peak power density. The best performance is observed with the medium-entropy perovskite rather than the high-entropy material. M−LSF shows k_chem of 21.5 × 10⁻⁵ cm s⁻¹ at 750 °C, 32 % higher than H-LSF and 4 times as high as LSF. In addition, M−LSF electrode exhibits interfacial polarization resistance of 0.090 ² at 750 °C, about 1/2 of H-LSF and 1/5 of LSF. Furthermore, single cell with M−LSF cathode demonstrates peak power density of 1.72 W cm⁻² at 800 °C, 25 % higher than H-LSF and more than twice as LSF. Therefore, multi-element doping to the A-site of ferrite-based cobalt-free perovskite could be an effective method to improve the cathode performance.