Ta/Nb doping motivating cubic phase stability and CO₂ tolerance of Sr₂Co_1.6Fe_0.4O_6- double perovskite for high-performing SOFC cathode

Developing cathodes that are both phase-stable and tolerant to carbon dioxide (CO2) is a crucial and challenging task for solid-oxide fuel cells (SOFCs) operating at reduced temperatures. The activity of oxygen reduction reaction (ORR) in SOFC cathodes is typically influenced by factors such as geometric features and oxygen vacancies. In this study, new and robust cathodes for SOFCs, Sr2Co1.5Fe0.4Ta0.1O5+delta (SCFT) and Sr2Co1.5Fe0.4Nb0.1O5+delta (SCFN) are developed by doping a small amount of niobium (Nb) and tantalum (Ta) into Sr2Co1.6Fe0.4O5+delta (SCF) using a simple solid-state reaction technique. The effects of Nb/Ta doping on the crystal structure, oxygen vacancies, electrical conductivity, and electrochemical properties of SCFT and SCFN are investigated. The results reveal that these materials possess a fully cubic structure in the Fm-3m space group, with improved oxygen vacancy and electrical conductivity. They also exhibit good chemical compatibility with electrolytes at 1100 degrees C for 8 h and remain thermally stable when exposed to air and CO2 at 700 degrees C, without any additional phase formation. The area-specific resistance (ASR) of the SCFT cathode is found to be only 0.06 Omega cm(2), whereas the SCFN cathode has an ASR of 0.10 Omega cm(2). The superior performance of SCFT can be attributed to the higher oxygen vacancy, which enhances oxygen surface exchange kinetics and diffusivity compared to SCFN. When tested in a single cell using humidified H-2 as the fuel and ambient air as the oxidant, the SCFT cathode achieves a power density of 656 mW cm(-2) at 700 degrees C, while the SCFN cathode reaches 559 mW cm(-2), indicating the potential of SCFT as a promising cathode material for SOFCs. The excellent performance of these materials can be attributed to their mixed electronic-ionic conduction properties, unique structural features, and high observed oxygen vacancy, which contribute to their remarkable electronic conductivity and significant ionic mobility.