Improved Phase Stability and CO₂ Poisoning Robustness of Y-Doped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ SOFC Cathodes at Intermediate Temperatures

The outstanding oxygen permeability of the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and its applicability as a cathode in solid oxide fuel cells are remarkable, yet have been hindered by the formation of secondary phases at T < 840 °C and the subsequent degradation. The other main drawback to BSCF is related to the formation of carbonates in the presence of CO2. These degrade its excellent oxygen surface-exchange kinetics. In this work, 10% Y-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF10Y) is electrochemically, microstructurally, and chemically characterized in O2- and CO2-containing atmospheres as porous cathodes in symmetrical cells with Gd-doped ceria as electrolyte. Experiments in oxygen/nitrogen gas mixtures (pO2 = 0.02–0.40 atm) at T = 600–900 °C showed high performance with a cathode specific resistance of 49.9 mΩ cm2 at 600 °C in air (pO2 = 0.21 atm), which is very comparable to 47.8 mΩ cm2 for undoped BSCF, but which deteriorates constantly under the same conditions. Moreover, adding significant amounts of CO2 (from 1% to 3% vol) to air at 700 °C shows that BSCF10Y has a remarkably enhanced tolerance toward the adsorption of CO2 molecules (compared to the undoped BSCF cathodes). The electrode microstructure was analyzed by focused ion beam (FIB) combined with scanning electron microscopy (SEM). The chemical composition was analyzed by scanning transmission electron spectroscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDXS). The presence of secondary phases in the BSCF10Y cathodes was strongly reduced or even suppressed compared to undoped BSCF, which explains the superior oxygen reduction reaction kinetics over time. Also, the segregated volume fraction of carbonates is greatly reduced after 50 h in 1% CO2. The improved stability of the cubic phase in BSCF10Y compared to BSCF is linked to the monovalent agent and higher radius of the Y-dopant, while the improved stability in CO2-containing atmospheres is attributed to the higher acidity of Y3+ compared to the A-site cations and the high concentration of the Y-dopant, which is located on both the A- and B-sites of the perovskite.