Anionic and cationic co-doping to enhance the oxygen transport property of Bi_0.5Sr_0.5FeO_3-δ as a high-performance air electrode for reversible solid oxide cells

Reversible solid oxide cells (RSOCs) have attracted considerable interest due to their efficient energy conversion. However, the low catalytic activity of air electrodes limits the application of RSOCs. Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ developed in our previous work exhibits a comparatively desirable performance and is expected to be further optimized. Herein, a novel strategy of anionic F and cationic Ta co-doped Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3-δ-xF_x (x = 0, 0.05, 0.10, 0.15, denoted as, BSFTF0, BSFTF5, BSFTF10, BSFTF15) materials are obtained to enhance the oxygen transport property of Bi_0.5Sr_0.5FeO_3-δ for RSOCs. The polarization resistance (Rp) of BSFTF10 (0.017 Ω cm²) decreases by 66 % compared with that of BSFTF0 (0.051 Ω cm²) at 800 °C. In fuel cell mode, the single cell with BSTF10 reaches the maximum power density of 908 mW cm⁻², which is approximately 72 % higher than the only Ta-doped BSFTF0 (527 mW cm⁻²) at 800 °C. In electrolysis mode, the single cell with BSFTF10 exhibits a high electrolysis current density of 1679 mA cm⁻², indicating an approximately 90 % increase compared to that of BSFTF0 at 800 °C and 1.5 V with 70 % CO₂-30 % CO feed gas. The bulk chemical diffusion (D_chem) of BSFTF10 is up to 5.12 × 10⁻⁴ cm² s⁻¹, which is 4.3 times higher than that of BSFTF0. The superior oxygen transport property of BSFTF10 is attributed to the high electronegativity of F⁻ (4.0) and low electronegativity of Ta⁵⁺(1.8), which may weaken the chemical bonding between B-site ions and O²⁻, generating more oxygen vacancies and accelerating the rate of oxygen transfer. The results indicate that F and Ta co-doping is an effective strategy to develop a highly catalytically active air electrode for RSOCs featuring oxygen transport properties.