GdBaM2-xFexO6-δ (M = Co, Mn) Perovskite-type Oxides as SOFCs Cathodes: Influence of Ordering Effects on the Crystal Structure and Properties

Perovskites of general formula A_1-xA^’_xM_1-yM’_yO_3-δ (A = lanthanide atom, A’ = alkali earth atom and M, M’ = transition metal atom) are promising candidates as electrodes for solid oxide fuel cells (SOFCs) [1, 2]. Among them, A-site ordered perovskites as for instance GdBaCo₂O_5+δ, also called layered-type perovskites, show better electrochemical behaviour than materials with random distribution of the cations. The high oxygen mobility in these layered-type perovskites seems to be related to the A-cation ordering [3, 4].

We are studying the influence of the crystal structure, in particular of the ion ordering effects, on the electrochemical properties of doubled perovskites with composition GdBaM_2-xFe_xO_6-δ (M = Co, Mn). The synthesis conditions affect both the oxygen stoichiometry and cation and/or anion vacancies ordering in the crystal structure of the materials.  We study the crystal structure of the compounds by Selected Area Electron Diffraction (SAED), High Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) (High Angle Annular Dark Field HAADF-STEM and Annular Bright Field ABF-STEM) and Electron Energy-Loss Spectroscopy (EELS). The use of aberration-corrected transmission electron microscopy techniques provides atomic resolution information. We have found cation ordering (mainly associated to the A cations, although B-cation ordering has also been observed) and anion vacancies ordering in some of these perovskites. HAADF-STEM images (also called Z-contrast images) confirm the layered-type ordering of the A cations. Direct imaging of the oxygen sublattice is obtained by ABF-STEM images and the phase image of the restored exit wave, which allows location of the oxygen vacancies in the structure.

We study the electrochemical behavior of the materials on symmetrical two-electrode cells using Ce_0.9Gd_0.1O_2-δ as electrolyte. Area specific resistance (ASR) associated to the electrode polarisation process at different temperatures is determined by AC impedance spectroscopy. We found that ordering effects of the crystal structure of the materials have a great impact on ASR values. In general, layered-type ordering of Gd and Ba in the structure decreases ASR values but different types of ordering of anion vacancies also play an important role in the physical properties of these materials.   

References:

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