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

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 YBaCo4-xAlxO7+, YBaCo4-x-yGaxAlyO7+, YBaCo3.2Ga0.8-xFexO7+, and YBaCo3.5-xAl0.5FexO7+, 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 degrees 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 degrees C occurs in all the YBaCo4-xAlxO7+ 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 x 10(-6) K-1) with common electrolyte materials, which alleviates the thermal stress during the SOFC operation. In comparison to YBaCo3.2Ga0.8O7+, 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 YBaCo3.2Ga0.7Fe0.1O7-Ce0.8Gd0.2O1.9 composite cathode reached 1.0 W cm(-2) with H-2 fuel at 700 degrees 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.