SrCo_(0.95)Sb_(0.05)O_(3-δ) as Cathode Material for High Power Density Solid Oxide Fuel Cells

摘要

The title compound has been selected from the SrCo_(1-x)Sb_xO_(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 Sr2MgMoO6 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 H2 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 perovskile with a = a0, c = 2a0 (ao ≈ 3.9 A) defined in the P4/mmm space group containing two inequivalent Co positions. Sb atoms are randomly distributed at Co2 positions, whereas Co 1 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 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 A~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.