Investigation into the effect of Si doping on the performance of Sr_1−yCa_yMnO_3−δ SOFC cathode materials

In this paper we report the successful incorporation of silicon into Sr_1−yCa_yMnO_3−δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a ²⁹Si enriched Sr_1−yCa_yMn_1−xSi_xO_3−δ perovskite system is confirmed by ²⁹Si MAS NMR measurements at low B₀ field. The very large paramagnetic shift (∼3000–3500 ppm) and anisotropy (span ∼4000 ppm) suggests that the Si⁴⁺ 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 Mn⁴⁺ 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 Sr_1−yCa_yMn_1−xSi_xO_3−δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr_0.5Ca_0.5Mn_1−xSi_xO_3−δ series; x = 0.075 for Sr_0.7Ca_0.3Mn_1−xSi_xO_3−δ; x = 0.10 for Sr_0.8Ca_0.2Mn_1−xSi_xO_3−δ; and x = 0.15 for SrMn_1−xSi_xO_3−δ. Composites with 50% Ce_0.9Gd_0.1O_1.95 were examined on dense Ce_0.9Gd_0.1O_1.95 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.