Crystal structure and electrical properties of LnCoO₃ (Ln=La, Pr, Tb) perovskite

The rare-earth cobalt-based perovskite oxides LnCoO₃ are promising electronic functional materials. The different synthesis conditions and microstructures led to obviously different results in previous investigations. In this study, LnCoO₃ (Ln=La, Pr, Tb) with different Ln f-electron configurations [La³⁺ (4f⁰), Pr³⁺ (4f³), and Tb³⁺ (4f⁹)] were investigated through careful synthesis controlling. The bond valence analysis, Mulliken population charge analysis and XPS analysis confirm that the Ln and Co ions in LnCoO₃ are all in + 3 oxide state (i.e. La³⁺, Pr³⁺, Tb³⁺, and Co³⁺). The different A-site cations have no significant effect on the [CoO₆] octahedra size in LnCoO₃ and average Co–O bond lengths are all ~ 1.93 Å. The band structure of LnCoO₃ shows similar coupling distribution between Co-3d and O-2p bands, which originates from the similarity of the [CoO₆] octahedra. This band structure leads to similar OER and ORR catalytic activities of LnCoO₃. The OER overpotential of LnCoO₃ is 463–506 mV, which is ~ 100 mV lower than that of commercial Pt/C catalysts, and the ORR half-wave potential is 0.63–0.67 V. The conductivity (σ) of LnCoO₃ is 0.11 S cm⁻¹ for LaCoO₃, 0.04 S cm⁻¹ for PrCoO₃ and 3.91 × 10^–4 S cm⁻¹ for TbCoO₃ at room temperature. This study reveals the [CoO₆] octahedra in LnCoO₃ perovskite is the key factor to their band structure and electrocatalytic behavior, providing an important knowledge for the research and development of these materials.