Investigation into the effect of Si doping on the performance of Sr1−yCayMnO3−δ SOFC cathode materials

Abstract

In this paper we report the successful incorporation of silicon into Sr1-yCayMnO3-δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a (29)Si enriched Sr1-yCayMn1-xSixO3-δ perovskite system is confirmed by (29)Si MAS NMR measurements at low B0 field. The very large paramagnetic shift (~3000-3500 ppm) and anisotropy (span ~4000 ppm) suggests that the Si(4+) species experiences both Fermi contact and electron-nuclear dipolar contributions to the paramagnetic interaction with the Mn(3+/4+) centres. An improvement in the conductivity is observed for low level Si doping, which can be attributed to two factors. The first of these is attributed to the tetrahedral coordination preference of Si leading to the introduction of oxide ion vacancies, and hence a partial reduction of Mn(4+) to give mixed valence Mn. Secondly, for samples with high Sr levels, the undoped systems adopt a hexagonal perovskite structure containing face sharing of MnO6 octahedra, while Si doping is shown to help to stabilise the more highly conducting cubic perovskite containing corner linked octahedra. The level of Si, x, required to stabilise the cubic Sr1-yCayMn1-xSixO3-δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr0.5Ca0.5Mn1-xSixO3-δ series; x = 0.075 for Sr0.7Ca0.3Mn1-xSixO3-δ; x = 0.10 for Sr0.8Ca0.2Mn1-xSixO3-δ; and x = 0.15 for SrMn1-xSixO3-δ. Composites with 50% Ce0.9Gd0.1O1.95 were examined on dense Ce0.9Gd0.1O1.95 pellets. For all series an improvement in the area specific resistances (ASR) values is observed for the Si-doped samples. Thus these preliminary results show that silicon can be incorporated into perovskite cathode materials and can have a beneficial effect on the performance.

Oxyanions in perovskites: from superconductors to solid oxide fuel cells

“…Successful Si doping has also been reported into SrFeO 3−x , with this work showing that up to 15% Si incorporation can be achieved leading to the cubic perovskites SrFe 1−x Si x O 3−y (x ≤ 0.15). 76,80 An interesting feature of this system was that whereas undoped SrFeO 3−x was observed to convert to brownmillerite type Sr 2 Fe 2 O 5 on heat treatment under reducing conditions, the Si doped samples preserved their cubic symmetry under such conditions (Fig. 8).…”

“…In this case, only silicate doping has been proven so far to stabilize the highly conducting cubic perovskite. 74,76 Relatively high levels of Si (15%) were required to achieve this stabilisation, although this level could be lowered by partial substitution of Sr by Ca. 29 Si NMR studies provided direct confirmation for the incorporation of Si into the perovskite structure, and these Si doped Sr 1−y Ca y MnO 3−z were shown to thermally stable against transformation back to the hexagonal perovskite.…”

Calcium manganite as oxygen electrode materials for reversible solid oxide fuel cell

“…Both BaMnO 3-δ and SrMnO 3-δ were reported to have hexagonal structure 10 and show very low electric conductivity 11 , but CaMnO 3−δ (CM) has an orthorhombic GdFeO 3 structure and shows a conductivity of higher than 1 S cm -1 at 700 o C 12 . The addition of oxyanion dopants (such as Si, B and P) in to a Ca 1-y Sr y MnO 3-δ system was reported to increase the conductivity by increasing the ratio of Mn III and inhibit the formation of hexagonal SrMnO 3-δ 11, 13, 14 .…”

Synthesis and Characterization of Phosphate Doped BaPr1-y(Y/Yb/Tm)yO3-δ

“…In addition to this oxyanion doping has been shown to enhance the CO2 stability of the doped Ba2(In/Sc)2O5 systems, which is attributed to the introduction of these acidic dopants reducing the basicity of the system [11][12][13][14]. These oxyanion dopants have also been shown to be able to be accommodated into a range of other perovskites, both electrolyte and electrode systems, showing that the perovskite structure is amenable to their incorporation [15][16][17][18][19][20][21][22][23].…”