Optimization of BaxSr1−xCo0.9Nb0.1O3−δ perovskite as oxygen semi-permeable membranes by compositional tailoring
Introduction
Oxygen is the second-largest volume industrial gas and the third largest chemical commodity in the world. It is used as an important raw material in diverse applications covering many industries including chemical processing, glass manufacturing, health service, refining, pulp and paper, steel manufacturing, and water/wastewater treatment. In the near future, the oxygen market may be massively expanded as all large scale clean energy technologies, which require O2 as feed. This is reflected in many international policies to sustain security of energy supply and reduce greenhouse gases.
Traditionally, the industrial scale production of oxygen is mainly by cryogenic air separation, a process that compresses and cools atmospheric air, then separates the resulting liquid into its components in a distillation column. Recently, vacuum pressure swing adsorption (VPSA), a non-cryogenic technology that produces oxygen from air by using an adsorbent in a pressure swing process to remove nitrogen, has also been applied. Both processes, however, have low energy efficiencies and high costs. The quest for a lower cost option for oxygen separation is promoting increasing interest in ceramic membranes based on mixed ionic and electronic conducting (MIEC) oxides [1], [2], [3]. When there is a difference in oxygen partial pressure across the MIEC membrane, oxygen at the oxygen rich side will adsorb onto the membrane surface and dissociate into oxygen ions and electron holes via a series of complicated intermediate steps. These charged species then move towards the oxygen lean side of the membrane surface where they re-combine to oxygen. The molecular oxygen finally desorbs from the membrane surface and releases into the oxygen lean side of the membrane [4]. Macroscopically, it appears as if molecular oxygen can permeate through the MIEC membrane. Theoretically, the membrane has 100% O2 permselectivity because of its structure and the ionic diffusion nature of oxygen. Furthermore, selected membranes have permeability even higher than that of microporous polymer membranes. It is believed that the capital cost for both plant construction and operation could be significantly reduced when the ceramic MIEC membrane technology is successfully adopted for oxygen generation [5]. In addition, these MIEC membranes are also of interest as membrane reactors for partial oxidation of hydrocarbons to value-added products at high temperatures, to achieve higher product yield/selectivity than that of the reaction equilibrium allows [4], [6], [7].
Compared with microporous polymer membranes, the oxygen permeation through MIEC membranes is related to membrane composition rather than morphology. Typical MIEC membrane materials are composed of a perovskite-type composite oxide [8], [9], [10], [11]. The simplest perovskite has the nominal composition of ABO3, where A can be Ln3+ and Ba2+/Sr2+ in twelve coordination with oxygen anion, while B is typically a transition metal element in six coordination with oxygen anion. By appropriate doping in both A- and B-sites, these materials can have a considerable amount of free oxygen vacancies [12], [13], [14], which are the carriers for oxygen anions, while electron hopping between the multivalent B-site cations leads to electronic conduction of the oxide [15].
Teraoka et al. developed membranes with high oxygen permeability on the basis of SrCo0.8Fe0.2O3−δ perovskite [16], [17]. The high flux is attributed to the high concentration of oxygen vacancies in the lattice created from the total substitution of La3+ by Sr2+ in the A-site of perovskite, but the material has low chemical and structural stability in practical operation conditions [18]. Later, many researchers explored new perovskite membranes with improved phase stability and oxygen permeability by optimizing the composition of the ABO3 perovskite structure via A-site or B-site doping strategies [6], [19], [20], [21], [22], [23].
Very recently, Nagai et al. reported that Nb is the optimal B-site dopant for improving the phase stability and oxygen permeability of SrCoO3−δ-based materials compared with other metal elements including Cr, Fe, Al, Ga, Ti, Zr, Sn, and V [24], [25]. We have also conducted a systematic investigation of the SrCo1−yNbyO3−δ membranes and found that SrCo0.9Nb0.1O3−δ gives the best performance in terms of phase stability, electrical conductivity and oxygen permeability [26]. However, the studies also indicated that the introduction of Nb lowered the oxygen vacancy concentration in the perovskite lattice because of the high oxidation state of Nb(5+), and increased the average oxidation state of cobalt ion in SrCo1−yNbyO3−δ. Thereby, a decline of oxygen permeation flux with further increase in Nb doping concentration was observed [26]. In our previous studies, we have demonstrated that the partial substitution of Sr2+ in SrCo0.8Fe0.2O3−δ by a larger cation Ba2+ promoted the reduction of cobalt ion to a lower average oxidation state, creating more oxygen vacancies in the lattice structure [27]. The partial substitution of Sr2+ by Ba2+ in SrCo0.9Nb0.1O3−δ may also be a potential way to increase the oxygen vacancy concentration and the oxygen permeation fluxes.
In this study BaxSr1−xCo0.9Nb0.1O3−δ membranes were synthesized, and the effect of barium doping levels on the properties of the membranes, such as phase structure, carbonate formation rate under carbon dioxide atmosphere, electrical conductivity and oxygen permeation fluxes, were systemically investigated. The composition of the membrane was optimized.
Section snippets
Powder synthesis and membrane fabrication
BaxSr1−xCo0.9Nb0.1O3−δ oxides, abbreviated as BSCNx (x = 0.0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0) were synthesized by a mechano-chemical activation-promoted solid-state reaction with BaCO3, SrCO3, Nb2O5 and Co2O3 (all in analytical grades) applied as the raw materials for the metal sources. These chemicals, according to the cation stoichiometry of the aimed composition of BSCNx were weighted and well mixed using a FRITSCH Pulverisette 6 high-energy ball miller under acetone liquid media at
Phase structure and stability
To ensure the as-synthesized BSCNx oxides formed their most stable phase structure, the solid precursors were calcined at 1100 °C for 5 h under an air atmosphere. In some cases repeated calcinations were conducted until no difference in diffraction patterns was observed between the products from the last two calcinations. The room temperature X-ray diffraction patterns of BSCNx are shown in Fig. 1a. All BSCNx samples displayed a single perovskite-type phase with cubic lattice symmetry at the Ba
Conclusions
The Ba doping concentration had a significant effect on the lattice constant, carbonate formation rate under carbon dioxide atmosphere, and electrical conductivity of BaxSr1−xCo0.9Nb0.1O3−δ oxides. Pure phase cubic perovskite can be obtained at x ≤ 0.8 in air, while the further increase of Ba doping concentration resulted in the formation of BaNb3O6 and BaCoO3 impurity phases. Ba doping resulted in an increase of the lattice constant of the cubic perovskite structure. Low level substitution of Sr
Acknowledgements
This work was supported by the National Science Research Foundation of China contract Nos. 20701020 and 20703024, and Fok Ying Tung Education Foundation under contract No. 111073. The authors acknowledge the financial support from the Australia Research Council for the project DP 0985578.
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