Investigation into the effect of Si doping on the cell symmetry and performance of Sr_1−yCa_yFeO_3−δ SOFC cathode materials

In this paper we report the successful incorporation of silicon into Sr_1−yCa_yFeO_3−δ perovskite materials for potential applications as electrode materials for Solid Oxide Fuel Cells. It is observed that Si doping leads to a change from a tetragonal or orthorhombic structure (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). The structures of the phases, SrFe_0.85Si_0.15O_3−δ, Sr_0.75Ca_0.25Fe_0.85Si_0.15O_3−δ and Sr_0.5Ca_0.5Fe_0.85Si_0.15O_3−δ, were analysed using neutron powder diffraction. The data confirmed the cubic unit cell, with no long range oxygen vacancy ordering. Conductivity measurements showed an improvement in the conductivity on Si doping, especially for samples with high Ca content. Composite electrodes comprising 50% Ce_0.9Gd_0.1O_1.95 and 50% Sr_1−yCa_y(Fe/Si)O_3−δ on dense Ce_0.9Gd_0.1O_1.95 pellets were therefore examined in air. An improvement in the area specific resistances (ASR) values is observed for the Si-doped samples with respect to the undoped samples. Thus the results show that silicon can be incorporated into Sr_1−yCa_yFeO_3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode material in Solid Oxide Fuel Cells (SOFC).