CO2-tolerant (Y,Tb)Ba(Co,Ga)4O7 cathodes with low thermal expansion for solid oxide fuel cells

Abstract

Oxide catalysts with alkaline earth metal dopants, such as Sr²⁺ and Ba²⁺, are widely applied as cathode materials in intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, one of the irreversible performance degradation mechanisms is the formation of BaCO₃ and SrCO₃ on the cathode surface when CO₂ in air reacts with the cathode. This study presents a series of CO₂-tolerant swedenborgite oxides (Y,Tb)Ba(Co,Ga)₄O_7+δ with high catalytic activity and relatively low thermal expansion coefficients (TECs, 9.2–10.2 × 10⁻⁶ K⁻¹). The doping effect of Tb and Ga on the phase stability in air and CO₂-containing air is comprehensively investigated. Unlike the >500% increase in area-specific resistance (ASR) of Co-containing perovskite oxides exposed to 5% CO₂ in air in the literature, the ASRs of (Y,Tb)Ba(Co,Ga)₄O_7+δ–Gd-doped ceria (GDC) composite cathodes only increase by ∼120% at 600 °C. In addition, the ASRs recover to their initial values after switching the gas back to pure air within 1 h, indicating good CO₂ tolerance and recovery capability. The low number of oxygen vacancies in (Y,Tb)Ba(Co,Ga)₄O_7+δ might explain the high CO₂ tolerance compared to Co-containing perovskite materials. Furthermore, a high electrochemical performance of 1.17 W cm⁻² at 700 °C is demonstrated with an anode-supported single cell and Y_0.5Tb_0.5BaCo_3.2Ga_0.8O_7+δ–GDC composite cathode.