SrCo0.95Sb0.05O3-δ as Cathode Material for High Power Density Solid Oxide Fuel Cells

The title compound has been selected from the SrCo1-xSbxO3-delta for solid series for its enhanced electronic conductivity, as high as 500 S cm(-1) at 400 degrees C, and tested in a single cell as a cathode material oxide fuel cells (SOFC). The characterization of this oxide included X-ray (XRD) and "in situ" temperature-dependent neutron powder diffraction (NPD) experiments, thermal analysis, and impedance spectroscopy. In the test cell, the electrodes were supported on a 300 mu m thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-delta (LSGM) with Sr2MgMoO6 as the anode and SrCo0.95-Sb0.05O3-delta as the cathode. The test cells gave I maximum power density of 0.511 and 0.618 W/cm(2) for temperatures of 800 and 850 degrees C, respectively, with pure H-2 as fuel and air as oxidant. In the 100-700 degrees C range SrCO0.95Sb0.05O3-delta adopts a tetragonal superstructure of perovskite with a = a(0), c = 2a(0) (a(0) approximate to 3.9 angstrom) defined in the p4/mmm space group containing two inequivalent Co positions. Sb atoms are randomly distributed at Co2 positions, whereas Co I sites do not apparently contain Sb. Flattened and elongated (Co,Sb)O-6 octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). This material experiences a phase transition from the tetragonal superstructure to a simple cubic perovskite between 700 and 850 degrees C, probably associated with the endothermic peak observed at 816 degrees C in the DTA curve. This phase transition is related to the disordering of oxygen vacancies from the three available positions in the tetragonal structure to a single oxygen site in the cubic unit cell with an average thermal factor and occupancy. This structure is stable up to 930 degrees C; at this temperature the oxygen stoichiometry is 2.46(4). The good performance of this material as a cathode is related to its mixed electronic-Ionic conduction (MIEC) properties, which can be correlated to the investigated structural features: the Co3+/Co4+ redox energy at the top of the O 2p bands accounts for the excellent electronic conductivity, which is favored by the corner-linked perovskite network. The considerable number of oxygen vacancies, with the oxygen atoms showing high displacement factors (4-6 angstrom(2) in the 700-850 degrees C range), suggests a significant ionic mobility. Additionally, this cathode material exhibits an extremely low electrode polarization resistance, below 0.1 Omega cm(2) in the 750-800 degrees C range. The thermal expansion is compatible with the electrolyte and the nonreconstructive tetragonal-to-cubic transition does not involve an abrupt change in unit-cell volume, which increases smoothly over the entire temperature interval up to 930 degrees C