Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials

Abstract

Brownmillerite-type oxides Ca2Fe2-xCoxO5-δ (x = 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) have been explored as possible cathodes for solid oxide fuel cells (SOFC). The samples have been prepared, characterized and tested as cathode materials in single solid-oxide fuel cells. As shown in a neutron powder diffraction (NPD) study at RT, for x = 0 and 0.2 the compounds crystallized in a single phase with brownmillerite-type structure (s.g. Pcmn), whereas for x = 0.4, 0.6, 0.8, 1, 1.2, and 1.4 the samples crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite (s.g Pcmb). This superstructure consists of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with (Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. In an “in situ” NPD experiment of Ca2Fe0.8Co1.2O5-δ at the working temperature of the SOFC, this compound shows the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that suggest a large lability and mobility. In single test cells these cathode materials generated a maximum power of 412 mW/cm2 at 850 °C with pure H2 as a fuel. The electrodes were supported on a 300-mm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM). The measured thermal expansion coefficients between 300 and 850 °C exhibit an excellent chemical compatibility with the electrolyte. The obtained compounds display a semiconductor-like behavior with conductivity values at the SOFCs working temperatures (650–850 °C) that are sufficient to yield a good performance in IT-SOFC.