Investigation and optimization of high-valent Ta-doped SrFeO3-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), 10 mol% Ta5+ doped in the B site of strontium ferrite perovskite oxide (SrTa0.1Fe0.9O3-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 10 mol% Ta5+ doping, the thermal expansion coefficient (TEC) decreased from 34.1.6×10-6 K-1 (SrFeO3-δ) to 14.6×10-6 K-1 (STF), which is near the TEC of electrolyte (13.3×10-6 K-1 for Sm0.2Ce0.8O1.9, SDC), indicates excellent thermomechanical compatibility. At 550-750ºC, 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 ºC, which matched well with the inflection points on the thermos-gravimetric analysis and electrical conductivity curves. At 600 ºC, the STF electrode shows area-specific resistance (ASR) of 0.152 Ωcm2 and peak power density (PPD) of 749 mW cm-2. ORR activity of STF was further improved by introducing 30wt% SDC powder, STF+SDC composite cathode achieving outstanding ASR value of 0.115 Ωcm2 at 600ºC, even comparable with benchmark cobalt-containing cathode, Ba0.5Sr0.5Co0.8Fe0.2O3-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.