Silver-doped strontium niobium cobaltite as a new perovskite-type ceramic membrane for oxygen separation
Introduction
Oxygen is one of the most significant chemicals in modern industries. Therefore, the mass production of oxygen in an efficient way has been intensively pursued. In particular, a recent emphasis on the oxy-fuel process during the utilization of fossil fuels will expect a fast-expanded demand on oxygen in the near future [1], [2]. An oxygen separation process driven by oxygen partial pressure gradient via a single-phase/dual-phase mixed oxygen-ion and electronic conducting membrane has received particular attention. This mixed conducting ceramic membrane can be potentially coupled in industrial applications such as the high-temperature catalytic membrane reactors and aforementioned oxy-fuel combustion [3], [4], [5], [6], [7], [8], [9], [10].
Extensive research activities have been directed towards the development of practical single-phase mixed conducting ceramic membranes with both high oxygen-ion conductivity and overwhelming electronic conductivity at elevated temperatures [11], [12], [13], [14]. Several reviews on single-phase mixed conducting ceramic membranes for efficient oxygen separation are available which provide not only material compositions, but also preparation methods and applications [15], [16], [17], [18], [19]. Typical single-phase mixed conducting perovskite oxides developed from 2010 to now which have exhibited a relatively high oxygen permeability include SrCo0.9Nb0.1O3-δ (SNC) [20], Pr0.6Sr0.4Co0.8Fe0.2O3-δ (PSCF) [21], BaBi0.05Sc0.1Co0.85O3-δ (BBSC) [22], Sr(Co0.8Fe0.2)0.9Ta0.1O3-δ [23], BaCo0.7Fe0.22Y0.08O3-δ (BCFY) [24], BaCo0.7Fe0.2Sn0.1O3-δ [25], BaBi0.05Co0.8Nb0.15O3-δ (BBCN) [26], and SrCo0.75Fe0.2Mo0.05O3-δ (SCFM) [27]. However, continuous research efforts have to be made since none of the available materials can satisfy practical applications.
The oxygen permeation occurs mainly via an oxygen-ion diffusion mechanism in the mixed conducting membranes, thus it is important to maximize the oxygen-ion conductivity in order to promote high oxygen permeability [20], [28], [29]. For perovskite-type membranes, one general strategy to maximize oxygen-ion conductivity is to increase the disordered oxygen vacancy concentration inside the lattice because oxygen vacancies typically work as the charge carrier for oxygen ions [30]. Increasing the oxygen mobility in the perovskite oxide is another important way to improve the oxygen-ion conductivity. It was reported that oxygen mobility in the perovskite oxide is closely associated with the oxygen-metal bond energy [20], [31]. Lowering the bond energy between cations and oxygen ions in perovskite oxides may facilitate the oxygen diffusion. Since the oxygen diffusion through ceramic membrane is accompanied by the concurrent transportation of electrons in a reverse direction, a high electronic conductivity is also required. In addition, membranes made of perovskite-type oxides are usually required to be sintered at elevated temperatures to form a dense structure with intimate grain boundaries [32]. Therefore from an energy-saving perspective, a relatively low sintering temperature for the membrane densification is preferred.
It is known that silver-oxygen bond energy is quite low, which may be beneficial to improving the oxygen mobility in perovskite oxides [20], [31], [33]. In addition, silver is a good sintering aid thanks to its low melting point of ~ 960 °C [34]. However, simple silver oxide is thermodynamically unstable at elevated temperatures and easily decomposes to metallic silver at ~ 300 °C [35]. One advantageous feature of the perovskite structure is that it can stabilize some unusual oxidation states of cations that are unlikely to be observed in simple oxides [36], [37], [38]. Very recently, through advanced synthesis, we have successfully prepared a silver-doped perovskite oxide at elevated temperatures, which were then applied as host to develop silver nanoparticles modified perovskite composites as superior electrode for oxygen reduction reaction in solid oxide fuel cells [39]. However, up to now, there is no report available about the application of silver-doped perovskite oxides as ceramic membrane for oxygen separation at elevated temperatures.
In this study, we demonstrate for the first time that silver-doped strontium niobium cobaltite with a nominal composition of Sr0.95Ag0.05Co0.9Nb0.1O3-δ (SANC) can be applied as a superior ceramic membrane for oxygen separation. A comparison with silver-free SrCo0.9Nb0.1O3-δ (SNC) membrane was made from various important features such as structure, sintering capability, mechanical strength, electrical conductivity, bulk chemical diffusion and surface exchange coefficients, and oxygen permeation fluxes. Insights towards the beneficial effect of silver doping on key properties and oxygen permeation performance of the perovskite oxide were also obtained. The current study provides a new doping strategy for the development of high-performance ceramic membranes for oxygen separation.
Section snippets
Preparation of perovskite oxides and membranes
SANC was synthesized through a conventional solid-state reaction method using SrCO3, Ag2O, Co3O4 and Nb2O5 in analytical grade (Sinopharm Chemical Reagent Co., Ltd) as raw materials. These starting materials were well mixed by high-energy ball milling (Fritsch, Pulverisette 6) under acetone medium at a rotation speed of 400 rpm for 60 min. After drying the slurry, the mixed powders were sintered at 1175 °C in air for 10 h to obtain the perovskite oxide. The sintered powders were ground by ball
Results and discussion
Shown in Fig. 1 are the room-temperature XRD patterns with Rietveld refinement of the as-synthesized SANC and SNC, respectively. Diffraction patterns of both SANC and SNC showed a typical perovskite structure in a cubic symmetry with no split diffraction peak at the 2-theta of around 33°. In addition, no characteristic diffraction peak of silver at the 2-theta of 38° was observed in the XRD pattern of SANC. It suggests that silver was likely to be doped into the perovskite lattice as expected
Conclusions
A single-phase mixed conducting silver-doped SANC perovskite oxide has been successfully synthesized using a conventional solid-state reaction method. The doping of silver into the perovskite lattice effectively promoted the sintering and improved mechanical strength of the SANC membrane. In addition, both the bulk diffusion and surface exchange properties of SANC were enhanced after the silver doping. As a result, the silver-doped SANC membrane exhibited higher oxygen fluxes with respect to
Acknowledgements
The authors thank the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Changjiang Scholars Program (T2011170), the National Natural Science Foundation of China (No. 21576135 and 51702125), the Pearl River S&T Nova Program of Guangzhou (No. 201806010054), the China Postdoctoral Science Foundation (2017M620401), and the Youth Fund in Jiangsu Province under Contract No. BK20150945.
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