Oxygen reduction kinetics of high performance BaCo0.4Fe0.4M0.1Y0.1O3-δ (M = Mg, Zr) positrode for protonic ceramic fuel cells

Protonic ceramic fuel cells (PCFCs) should exhibit high performance at intermediate temperatures in the range of 400-600 °C. To reduce the operating temperature, more active air electrodes (positrodes) are needed. In the present work, BaCo_0.4Fe_0.4Mg_0.1Y_0.1O_3-δ (BCFMY) is investigated as a positrode material for application in PCFCs as well as solid oxide fuel cells (SOFCs). For SOFCs, the polarization resistance ascribed to the oxygen reduction reaction is proportional to p_O2^-1/4 (p_O2: oxygen partial pressure), suggesting that the rate-determining process is the charge transfer on the mixed ionic-electronic conductors. For PCFCs, this polarization resistance is proportional to p_O2^-^1/2, suggesting that the rate-determining process is the oxygen dissociation. The total polarization resistance for the PCFCs using the BCFMY positrode is 0.066 Ωcm² at 600 °C, lower than that using the BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) positrode. The higher oxygen nonstoichiometry of BCFMY promotes the oxygen dissociation process on the PCFC positrode surface.