In situ decorated SrCO3 nanorods on a porous electrode: detrimental or beneficial toward oxygen reduction for solid oxide fuel cells?

Abstract

The majority of highly active cathodes for solid oxide fuel cells (SOFCs) contain alkaline-earth metals to create oxygen vacancies and improve electronic conductivity. However, many of these cathodes unavoidably face segregation, and more importantly, the segregation and the subsequent impurity-induced species (e.g., SrCO₃) are generally regarded to be detrimental to the oxygen reduction reaction (ORR). Herein, we used a classic cathode, La_0.2Sr_0.8CoO_3−δ (LSC), to understand the effects of SrCO₃ on the ORR in SOFCs. Using in situ CO₂-induced transformation, SrCO₃ nanorods are successfully decorated on LSC. We observe that, compared to the cell with bare LSC, the cells with SrCO₃-modified LSC exhibit improved ORR activity, as represented by a decreased area-specific polarization resistance of 0.1340 Ω cm² (650 °C) using a symmetric cell and a maximum power density of 1.211 W cm⁻² (650 °C) using an anode-supported fuel cell. Electrochemical impedance spectra deconvoluted using the equivalent circuit model and the distribution of relaxation times indicate that the enhanced ORR activity is closely associated with accelerated O₂ dissociation. Additionally, density functional theory calculations reveal that the SrCO₃-modified LSC lowers the dissociation energy of O₂ molecules from 1.86 eV to 0.68 eV, and the calculated Bader charge suggests that the generated SrCO₃ facilitates electron supply to LSC, both of which speed up the ORR kinetics. The activity enhancement is also valid for other representative cathode materials (e.g., BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ and Sm_0.5Sr_0.5CoO_3−δ). Therefore, our work not only demonstrates that the in situ-formed SrCO₃ nanorods are beneficial for O₂ dissociation but also presents a general strategy to induce activity enhancement.