High-Entropy Perovskite as a High-Performing Chromium-Tolerant Cathode for Solid Oxide Fuel Cells.

To achieve chromium tolerance and high performance, a new series of high-entropy perovskites (HEPs) are investigated as cathode materials for solid oxide fuel cells (SOFCs). Multiple rare-earth, alkaline-earth, and high-order transition metal elements are used for the A-site of this ABO 3 structure. A pure phase is achieved through the designed combination of different elements in seven out of eight candidates. Due to the retaining of alkaline-earth elements Sr and/or Ba, the electrical conductivities of these HEPs are in the order of 100 S/cm at 550-700 °C, a value that can practically eliminate the electronic resistance of the porous cathode. Three out of eight candidates show similar or better performance than the (La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3-δ (LSCF) benchmark. It is found that A-site elements can cast a substantial influence on the overall performance even with a change as small as 10% of the total cations. It seems that each element has its individual "phenomenal activity" that can be transferred from one candidate to the other in the general setting of the perovskite structure, leading to the best candidate by using the three most active elements simultaneously at the A-site. Excellent Cr tolerance has been observed on the (La 0.2 Sr 0.2 Pr 0.2 Y 0.2 Ba 0.2 )Co 0.2 Fe 0.8 O 3-δ sample, showing degradation of only 0.25%/kh during a 41 day operation in the presence of Cr, while LSCF increases by 100% within the first day in the same condition. X-ray photoelectron spectroscopy discovers no Sr segregation as LSCF is found in this HEP; rather, the active element Y takes more A-sites on the outermost layer after long-term operation.