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), 10mol% Ta 5+ doped in the B site of strontium ferrite perovskite oxide (SrTa 0.1 Fe 0.9 O 3− δ , STF) is investigated and optimized. The effects of Ta 5+ doping on structure, transition metal reduction, oxygen nonstoichiometry, thermal expansion, and electrical performance are evaluated systematically. Via 10mol% Ta 5+ 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 0.2 Ce 0.8 O 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 (O 2 -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·cm 2 and peak power density (PPD) of 749 mW·cm −2 . ORR activity of STF was further improved by introducing 30wt% Sm 0.2 Ce 0.8 O 1.9 (SDC) powder, STF + SDC composite cathode achieving outstanding ASR value of 0.115 Ω·cm 2 at 600°C, even comparable with benchmark cobalt-containing cathode, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 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.