Investigation into the effect of Si doping on the performance of SrFeO3−δ SOFC electrode materials

Abstract

In this paper we report the successful incorporation of silicon into SrFeO_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 cell (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). Annealing experiments in 5% H₂/95% N₂ (up to 800 °C) also showed the stabilization of the cubic form for the Si-doped samples under reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast to the cubic cell of the reduced Si doped system, reduction of undoped SrFeO_3−δ leads to the formation of a brownmillerite structure with ordered oxide ion vacancies. SrFe_0.90Si_0.10O_3−δ and SrFe_0.85Si_0.15O_3−δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mössbauer spectroscopy data were also recorded for SrFe_0.90Si_0.10O_3−δ, and indicated the presence of only Fe³⁺ and Fe⁵⁺ (i.e. disproportionation of Fe⁴⁺ to Fe³⁺ and Fe⁵⁺) for such doped samples. Conductivity measurements showed an improvement in the conductivity on Si doping. Composite electrodes with 50% Ce_0.9Gd_0.1O_1.95 were therefore examined on dense Ce_0.9Gd_0.1O_1.95 pellets in two different atmospheres: air and 5% H₂/95% N₂. In both atmospheres an improvement in the area specific resistance (ASR) values is observed for the Si-doped samples. Thus the results show that silicon can be incorporated into SrFeO_3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFCs.