Structural and Electrical Characterization of the Novel SrCo_0.9Sb_0.1O_3–δ Perovskite: Evaluation as a Solid Oxide Fuel Cell Cathode Material

Abstract

A novel perovskite oxide with the title composition has been prepared by soft-chemistry procedures followed by thermal treatments at 1000 °C. This polycrystalline sample has been characterized by temperature-dependent neutron powder diffraction (NPD), thermal analysis, electrical conductivity, and thermal expansion measurements, in order to evaluate its potential use as a mixed electronic-ionic conductor in intermediate-temperature solid oxide fuel cells (IT-SOFCs). At room temperature (RT), the sample adopts a tetragonal superstructure of perovskite with a ) a 0 , c ) 2a 0 (a 0 ≈ 3.9 Å) defined in the P4/mmm space group. Co and Sb are distributed at random over the octahedral positions of the perovskite; flattened and elongated (Co,Sb)O 6 octahedra alternate along the c axis, sharing corners in a three-dimensional array (3C-like structure). The refinement of the oxygen occupancy factors yields the crystallographic formula SrCo 0.9 Sb 0.1 O 2.73(4) ; the oxygen vacancies are located at the equatorial O2 and O3 atoms, in alternating layers with different occupancy. O3 atoms exhibit, at RT, large thermal factors of 5.3 Å 2 , suggesting a considerable mobility. This structure is stable up to 500 °C; between 500 and 700 °C, an order-disorder phase transition takes place to give a fully disordered simple-cubic perovskite with a ) a 0 (space group Pm3 j m); this structure is shown to be stable up to 940 °C from NPD data. This is a second-order nonreconstructive transition, which is not observed at the differential thermal analysis curves, although it is probably responsible for a subtle change of slope at 650 °C in the thermal expansion curve. The thermal evolution of the electrical conductivity exhibits a maximum of 300 S • cm -1 at 400 °C; above this electronic transition, the conductivity regularly decreases, but it is still well above the required 100 S • cm -1 in the temperature region 650-850 °C corresponding to the working regime of a IT-SOFC.

Synthesis of silicon doped SrMO3 (M = Mn, Co): stabilization of the cubic perovskite and enhancement in conductivity

“…The conductivity behaviour is similar to previously observed for doping with Nb, Sb (and other cations such as Cr, Ti and Fe) [20][21][22], and can be attributed to the change from a hexagonal perovskite with face sharing of octahedra, to a cubic cell with corner sharing, and mixed…”

“…Doping SrCoO 3-y on the Co site with a range of cations, e.g. Nb, Sb, has been shown to help to stabilise the cubic perovskite polymorph, containing corner linked CoO 6 octahedra, leading to a substantial enhancement in the conductivity [20][21][22]. This stabilisation of the cubic phase on Nb, Sb doping can be attributed to a partial reduction of the Co 4+ towards…”

Synthesis, structure and conductivity of sulfate and phosphate doped SrCoO3

“…In contrast, for P doping, the higher conductivity was observed for x=0.05 compared to x=0.03. The conductivities for all samples show a decrease above ≈350 ○ C, which can be attributed to oxygen loss above this temperature, as observed in related studies on doping SrCoO 3 with similar size cations (Nb, Sb) [28][29][30][31].…”

“…Nb, Sb, Si has been shown to alter the stacking arrangement of the close packed oxygen layers from purely hexagonal to purely cubic (referred to as 2H and 3C respectively). The consequent change from face-shared to corner linked CoO 6 octahedra, leads to a substantial enhancement in the electronic conductivity [28][29][30][31][32]. This stabilisation of the 3C-perovskite framework on Nb, Sb, Si doping can be attributed to a partial reduction of Co 4+ to Co…”

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

“…In a previous work, we have stabilized a 3C perovskite phase by doping the SrCoO 3Àd system with M nþ ¼ Sb 5þ or Mo 6þ in SrCo 1Àx M x O 3Àd (x ¼ 0.05, 0.1, 0.15 and 0.2) [4,5]. In this work we have successfully explored the introduction of M ¼ Nb 5þ .…”