SrCo_0.95Sb_0.05O_3−δ as Cathode Material for High Power Density Solid Oxide Fuel Cells

Abstract

The title compound has been selected from the SrCo 1− x Sb x O 3−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 La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3−δ (LSGM) with Sr 2 MgMoO 6 as the anode and SrCo 0.95 Sb 0.05 O 3-δ as the cathode. The test cells gave a maximum power density of 0.511 and 0.618 W/cm 2 for temperatures of 800 and 850 °C, respectively, with pure H 2 as fuel and air as oxidant. In the 100−700 °C range, SrCo 0.95 Sb 0.05 O 3−δ adopts a tetragonal superstructure of perovskite with a = a 0, c =2a 0 ( a 0 ≈ 3.9 Å) defined in the P 4/ 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)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 °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 Co 3+ /Co 4+ 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 Å 2 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 cm 2 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.