Dynamic Octahedral Breathing in Oxygen-Deficient Ba0.9Co0.7Fe0.2Nb0.1O3‑δ Perovskite Performing as a Cathode in Intermediate-Temperature SOFC

• Published in: Inorganic chemistry Vol. 55; no. 6; pp. 3091 - 3097
• Main Authors: Gong, Yudong, Sun, Chunwen, Huang, Qiu-an, Alonso, Jose Antonio, Fernández-Díaz, Maria Teresa, Chen, Liquan
• Format: Journal Article
• Language: English
• Published: American Chemical Society 21.03.2016
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b03002

Ba0.9Co0.7Fe0.2Nb0.1O3‑δ outperforms as a cathode in solid-oxide fuel cells (SOFC), at temperatures as low as 700–750 °C. The microscopical reason for this performance was investigated by temperature-dependent neutron powder diffraction (NPD) experiments. In the temperature range of 25–800 °C, Ba0.9Co0.7Fe0.2Nb0.1O3‑δ shows a perfectly cubic structure (a = a 0), with a significant oxygen deficiency in a single oxygen site, that substantially increases at the working temperatures of a SOFC. The anisotropic thermal motion of oxygen atoms considerably rises with T, reaching B eq ≈ 5 Å2 at 800 °C, with prolate cigar-shaped, anisotropic vibration ellipsoids that suggest a dynamic breathing of the octahedra as oxygen ions diffuse across the structure by a vacancies mechanism, thus implying a significant ionic mobility that could be described as a molten oxygen sublattice. The test cell with a La0.8Sr0.2Ga0.83Mg0.17O3‑δ electrolyte (∼300 μm in thickness)-supported configuration yields a peak power density of 0.20 and 0.40 W cm–2 at temperatures of 700 and 750 °C, respectively, with pure H2 as fuel and ambient air as oxidant. The electrochemical impedance spectra (EIS) evolution with time of the symmetric cathode fuel cell measured at 750 °C shows that the Ba0.9Co0.7Fe0.2Nb0.1O3‑δ cathode possesses a superior ORR catalytic activity and long-term stability. The mixed electronic–ionic conduction properties of Ba0.9Co0.7Fe0.2Nb0.1O3‑δ account for its good performance as an oxygen-reduction catalyst.