Ni-doping influence on functional properties of SrTi_0.65Fe_0.35O_3-δ for use as oxygen transport membranes

Pure strontium titanate, SrTiO₃, exhibits excellent thermodynamic stability but negligible electronic and ionic conductivity in a wide range of temperatures and oxygen partial pressures. In order to improve the conductivity, B-site doping strategy is used in this work. The SrTi_0.65-xFe_0.35Ni_xO_3-δ (x=0, 0.05, 0.075, 0.1) (STFN_x) powders were synthesized by a solid-state reaction method at 1200 °C and then sintered into membranes at 1350/1400 °C for 5 h. Functionality, i.e. mixed ionic and electronic conductivity, is introduced by substitution of Ti by Fe and Ni. In addition, Ni is proven to improve the catalytic performance by exsolution phenomenon. The XRD patterns show that the materials are single phase after sintering in air. The oxygen permeance and the ionic conductivity of STFN_x increase with an increasing Ni content and are close to benchmark La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ at around 850 °C. Thermochemical stability tests were performed by annealing samples in syngas with/without H₂S and clean H₂. XRD analysis and thermogravimetry analysis (TGA) reveal that STFN005 exhibits good thermochemical stability in reducing atmospheres and the stability of STFN_x decreases with increasing Ni content. Well distributed Fe/Ni exsolution particles can be found even with the lowest Ni amount doped material SrTi_0.6Fe_0.35Ni_0.05O_3-δ after annealing in reducing atmosphere, which will be beneficial to catalytic performance in a membrane reactor. Therefore, 5 % Ni doped STFN005 can be a promising material in catalytic membrane reactors, e.g. for partial oxidation of methane (POM).