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

Abstract

The outstanding oxygen permeability of the perovskite Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−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 CO 2 . These degrade its excellent oxygen surface-exchange kinetics. In this work, 10% Y-doped Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF10Y) is electrochemically, microstructurally, and chemically characterized in O 2 -and CO 2 -containing atmospheres as porous cathodes in symmetrical cells with Gd-doped ceria as electrolyte. Experiments in oxygen/nitrogen gas mixtures (pO 2 = 0.02−0.40 atm) at T = 600−900 °C showed high performance with a cathode specific resistance of 49.9 mΩ cm 2 at 600 °C in air (pO 2 = 0.21 atm), which is very comparable to 47.8 mΩ cm 2 for undoped BSCF, but which deteriorates constantly under the same conditions. Moreover, adding significant amounts of CO 2 (from 1% to 3% vol) to air at 700 °C shows that BSCF10Y has a remarkably enhanced tolerance toward the adsorption of CO 2 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% CO 2 . 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 CO 2 -containing atmospheres is attributed to the higher acidity of Y 3+ 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.

High-Performance and CO2-Durable Composite Cathodes toward Electrocatalytic Oxygen Reduction: Ce0.8Sm0.2O1.9 Nanoparticle-Decorated Double Perovskite EuBa0.5Sr0.5Co2O5+δ

“…The lower ABE value means a stronger CO 2 durability of chemical bonds. Our results imply that BaCO 3 and SrCO 3 phases prefer to be generated after exposure to the CO 2 atmosphere, which is in accordance with other reports on the perovskite cathodes, such as BSCF, Nd-Ta codoped SrCoO 3−δ , and Sc-Nb codoped SrCoO 3−δ . The ABE values of Ce–O and Sm–O bonds are −200.8 and −37.3 kJ mol –1 in SDC, respectively.…”

“…In contrast to classic perovskite cathodes, the electrode performance of EBSC is still inadequate, especially operating at reduced temperature. In addition, it is found that the carbonates are readily generated in the perovskite cathodes due to high reaction activity of Ba/Sr elements exposed to CO 2 , giving rise to a performance degradation of the fuel cell. ,, Therefore, the EBSC cathode seems to be not suitably applied for SOFCs in the presence of CO 2 .…”

A Zn-Doped Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite Cathode with Enhanced ORR Catalytic Activity for SOFCs

“…Nevertheless, to assure the practical applications of BSCF cathode, the chemical and structural instabilities at intermediate temperatures is a crucial issue that needs to be addressed [4,23]. Several groups has attempted to obtain the stable cubic crystal structure of BSCF by partially substituting the reducible B-site Fe and Co ions with less reducible or constant valence metal ions (e.g., Mo 6+ , Nb 3+ , Zr 4+ , Y 3+ ) [24][25][26][27][28][29][30]. The B-site Zn-doping was also found to stabilize the cubic perovskite oxides, e.g., Ba 0.5 Sr 0.5 Zn 0.2 Fe 0.8 O 3−δ and BaCo 0.4 Fe 0.4 Zn 0.2 O 3-δ , thus conferring stable long-term oxygen permeability and CO 2 -resistance for the derived ceramic membranes [28,31].…”

Progress in Ce0.8Gd0.2O2−δ protective layers for improving the CO2 stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ O2-transport membranes

“…[5][6][7][8][9] BSCF supported membranes have achieved up to 12.2 ml•min -1 •cm -2 at 1000 °C under Air/Argon gradient, 9 but the main problem of these materials is their low stability in operation conditions on CO2 containing atmospheres, which will be present to some extent in the CPO reaction. Several strategies to improve the stability against CO2 include the doping of the perovskite structure with other transition metals like Zr or Y, 10 the combination of BSCF with a CO2 stable material in a dual-phase composite 11,12 (e.g. gadolinium doped ceria) or, as the goal of this work, the development of CO2 protective layers of ionic or MIEC materials.…”