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

Abstract

The title compound has been selected from the SrCo1−xSbxO3−δ series for its enhanced electronic conductivity, as high as 500 S cm−1 at 400 °C, and tested in a single cell as a cathode material for solid 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 μm thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) with Sr2MgMoO6 as the anode and SrCo0.95Sb0.05O3-δ as the cathode. The test cells gave a maximum power density of 0.511 and 0.618 W/cm2 for temperatures of 800 and 850 °C, respectively, with pure H2 as fuel and air as oxidant. In the 100−700 °C range, SrCo0.95Sb0.05O3−δ adopts a tetragonal superstructure of perovskite with a = a0, c=2a0 (a0 ≈ 3.9 Å) defined in the P4/mmm space group containing two inequivalent Co positions. Sb atoms are randomly distributed at Co2 positions, whereas Co1 sites do not apparently contain Sb. Flattened and elongated (Co,Sb)O6 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 °C, probably associated with the endothermic peak observed at 816 °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 °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 Å2 in the 700−850 °C range), suggests a significant ionic mobility. Additionally, this cathode material exhibits an extremely low electrode polarization resistance, below 0.1 Ω cm2 in the 750−800 °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 °C.