Investigation and optimization of high-valent Ta-doped SrFeO_3−δ as air electrode for intermediate-temperature solid oxide fuel cells

To explore highly active and thermomechanical stable air electrodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs), 10mol% Ta⁵⁺ doped in the B site of strontium ferrite perovskite oxide (SrTa_0.1Fe_0.9O_3−δ, STF) is investigated and optimized. The effects of Ta⁵⁺ doping on structure, transition metal reduction, oxygen nonstoichiometry, thermal expansion, and electrical performance are evaluated systematically. Via 10mol% Ta⁵⁺ doping, the thermal expansion coefficient (TEC) decreased from 34.1 × 10⁻⁶ (SrFeO_3−δ) to 14.6 × 10⁻⁶ K⁻¹ (STF), which is near the TEC of electrolyte (13.3 × 10⁻⁶ K⁻¹ for Sm_0.2Ce_0.8O_1.9, SDC), indicates excellent thermomechanical compatibility. At 550–750°C, STF shows superior oxygen vacancy concentrations (0.262 to 0.331), which is critical in the oxygen-reduction reaction (ORR). Oxygen temperature-programmed desorption (O₂-TPD) indicated the thermal reduction onset temperature of iron ion is around 420°C, which matched well with the inflection points on the thermos-gravimetric analysis and electrical conductivity curves. At 600°C, the STF electrode shows area-specific resistance (ASR) of 0.152 Ω·cm² and peak power density (PPD) of 749 mW·cm⁻². ORR activity of STF was further improved by introducing 30wt% Sm_0.2Ce_0.8O_1.9 (SDC) powder, STF + SDC composite cathode achieving outstanding ASR value of 0.115 Ω·cm² at 600°C, even comparable with benchmark cobalt-containing cathode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF). Distribution of relaxation time (DRT) analysis revealed that the oxygen surface exchange and bulk diffusion were improved by forming a composite cathode. At 650°C, STF + SDC composite cathode achieving an outstanding PPD of 1117 mW·cm⁻². The excellent results suggest that STF and STF + SDC are promising air electrodes for IT-SOFCs.