New Nb-doped SrCo1−xNbxO3−δ perovskites performing as cathodes in solid-oxide fuel cells

The high-temperature cubic phase of SrCoO 3− δ is a promising cathode material for solid oxide fuel cells (SOFC) due to its high electrical conductivity and oxygen permeation flux. However, this phase is not stable below 900 °C where a 3C-cubic to 2H-hexagonal phase transition takes place when the sample is slowly cooled down. In this work we have stabilized a 3C-tetragonal P4/mmm structure for SrCo 1− x Nb x O 3− δ with x = 0.05. We have followed the strategy consisting of introducing a highly-charged cation at the Co sublattice, in order to avoid the stabilization of the unwanted 2H structure containing face-sharing octahedra. The characterization of this oxide included X-ray (XRD) and neutron powder diffraction (NPD) experiments. SrCo 0.95 Nb 0.05 O 3− δ adopts a tetragonal superstructure of perovskite with a = a 0 , c = 2 a 0 ( a 0 ≈ 3.9 A) defined in the P4/mmm space group containing two inequivalent Co positions. Flattened and elongated (Co,Nb)O 6 octahedra alternate along the c axis sharing corners in a three-dimensional array (3C-like structure). 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). The test cells gave a maximum power density of 0.4 and 0.6 W/cm 2 for temperatures of 800 and 850 °C, respectively, with pure H 2 as fuel and air as oxidant. 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 suggests a significant ionic mobility.