Mixed ionic-electronic conducting (MIEC) materials are promising candidates of oxygen transport membranes (OTMs). They have attracted great interests in scientific research field over the last two decades due to the high purity and energy efficiency of oxygen separation from air for different applications in catalytic membrane reactors such as partial oxidation of methane (POM). For application, the structural stability of MIEC membranes under operating conditions is a very important factor.
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 materials SrTi_0.65-xFe_0.35NixO_3-δ (x=0, 0.05, 0.075, 0.1) (STFN) are sintered at 1350-1400 ? for 5 h. Functionality, i.e. mixed ionic and electronic conductivity, is introduced by substitution of Ti by Fe. Ni is a very promising element for catalysis for instance achieving >95% CH4 conversion and > 96% syngas selectivity. Therefore, Ni is introduced into STFN for improving the catalytic performance.
The XRD patterns showed that the materials are single phase after sintering. The permeation rate of STFN slightly increased with Ni content and the permeation rate of STFN01 is close to the benchmark LSCF6428 around 850 ?. Thermochemical stability tests were performed by annealing samples for 72 h in a gas stream of 25 Vol-% CO + 50 Vol-% H2+ 25 Vol-% H2O forming different syngas compositions at 600 ?, 700 ?, 800 ? and 900 ?. XRD analyses revealed that the thermochemical stability of STFN decreased with increasing Ni content. Therefore, Ni content should be limited to 5 mol-% in the B-site of STFN. Furthermore, electrochemical impedance spectroscopy showed STFN exhibits good conductivity whereas the conductivity decreases with temperature and increases with Ni content. Ni exsolution phenomenon of STFN was found after annealing in syngas or pure H₂, which is beneficial for catalysis in a membrane reactor. The results indicated that STFN (the substitution of Ni should be limited to 5 mol-%) can be a promising material for POM in membrane reactors due to its high oxygen permeation rate and good stability under syngas.