Abstract: Proton-conducting solid oxide fuel cells (H-SOFCs) are expected to be an ideal energy conversion device operating below 600 °C. However, the lack of robust electrodes with high catalytic activity remains a major obstacle to commercialization. Herein, B-site high-entropy modification is first performed on K₂NiF₄-type oxide to obtain a novel cathode material, La_1.2Sr_0.8Mn_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2O_4+δ (LSMFCNC). By harnessing the unique properties of multiple elements, the B-site transition metals in LSMFCNC evolve into various compound states, resulting in crystal structure expansion and a decrease in the bond strength between cations and oxygen ions, thereby promoting the formation of oxygen vacancies and achieving high proton/oxygen diffusion rates with excellent oxygen reduction reaction (ORR) activity. Ultimately, H-SOFCs using the high-entropy LSMFCNC cathode demonstrate an excellent cell performance with power outputs of 1759 mW/cm² at 700 °C and 1126 mW/cm² at 600 °C. This performance is superior to La_1.2Sr_0.8NiO₄ (LSNO) and other K₂NiF₄-type cathodes reported in the literature. The outstanding electrochemical performance and fine operational stability suggest that LSMFCNC could be a potential cathode alternative for low-temperature H-SOFC operation. This study provides a new approach to developing highly active and durable cathodes for H-SOFCs.

Key words: H-SOFC, K₂NiF₄, High-entropy, Lattice expansion, O^2-/H⁺ diffusion