Effects of trivalent dopants on phase stability and catalytic activity of YBaCo4O7-based cathodes in solid oxide fuel cells

Abstract

In order to understand the doping and co-doping effects of trivalent cations (Al³⁺, Ga³⁺, and Fe³⁺) in the swedenborgite oxide YBaCo₄O₇ as a cathode in intermediate-temperature solid oxide fuel cells (IT-SOFCs), four series of YBaCo₄O₇-based materials, including YBaCo_4−xAl_xO_7+δ, YBaCo_4−x−yGa_xAl_yO_7+δ, YBaCo_3.2Ga_0.8−xFe_xO_7+δ, and YBaCo_3.5−xAl_0.5Fe_xO_7+δ, have been synthesized and investigated. The long-term phase stability has been evaluated by X-ray diffraction after the samples were annealed at 600–800 °C under ambient air for 120 h. Although Al has been suggested to have better phase stabilization capability than Ga in the literature, severe decomposition above 700 °C occurs in all the YBaCo_4−xAl_xO_7+δ samples. Fe has much weaker phase stabilization capability than Al and Ga, but excellent phase stability is still maintained with a small substitution of Fe for Ga. All the materials with a stable phase at high temperatures exhibit well-matched thermal expansion coefficients (8.0–9.5 × 10⁻⁶ K⁻¹) with common electrolyte materials, which alleviates the thermal stress during the SOFC operation. In comparison to YBaCo_3.2Ga_0.8O_7+δ, a partial substitution of Ga by Fe enhances the catalytic activity for the oxygen reduction reaction. In addition, the performance of the anode-supported single cell with the YBaCo_3.2Ga_0.7Fe_0.1O₇–Ce_0.8Gd_0.2O_1.9 composite cathode reached 1.0 W cm⁻² with H₂ fuel at 700 °C. Thus, this work provides a guideline on trivalent dopants for developing swedenborgite-based cathode materials with a high phase stability and catalytic activity in IT-SOFCs.