Mapping phase instability to electrochemical degradation in SrTi1−xFexO3−δ under solid oxide cell fuel-electrode conditions

Abstract

SrTi_1−xFe_xO_3−δ (STF) is a promising fuel electrode into which reducible cations can be substituted to yield exsolution of catalytically active nanoparticles, improving performance. Since prior studies have focused on STF with Fe content x = 0.7, questions remain as to whether other compositions provide better stability and performance over a range of fuel compositions. The present study examines a wider composition range from x = 0.5 to 0.8 at 850 °C. Furthermore, phase evolution during exposure to varying H₂/H₂O is observed in situ using X-ray diffraction and correlated directly with impedance spectroscopy observations of electrochemical characteristics and stability. All STF compositions exhibited good phase and polarization resistance stability in the perovskite structure for a p_H2/p_H2O ratio of 1. However, decomposition occurred in more reducing fuels (p_H2/p_H2O ≥ 10), into Ruddlesden–Popper (R–P) perovskite, strontium oxide, and metallic Fe, after an incubation time of ∼2–4 h, the same time frame over which polarization resistance increased rapidly. That is, the results show a clear correlation between phase decomposition and rapid electrochemical degradation. The results suggest that the R–P transformed oxide was less electrochemically active despite the presence of exsolved Fe. Decreasing the Fe content x resulted in slower and reduced decomposition, but slightly higher initial polarization resistance values.