A new asymmetric SrCo0.8Fe0.1Ga0.1O3−δ perovskite hollow fiber membrane for stable oxygen permeability under reducing condition
Graphical Abstract
Highlights
► SrCo0.8Fe0.1Ga0.1O3−δ (SCFG) hollow fiber membrane was successfully developed for oxygen permeation. ► Achieve permeability of 0.55–3.5 mL min−1 cm−2 from 600 to 950 °C with He sweep gas. ► Ga introduction enhances phase stability and decreases membrane activation energy. ► Asymmetric configuration increase transition temperature to bulk-diffusion control. ► Ability to withstand reducing environment under methane for duration of 100 h.
Introduction
Mixed ion-electron conducting oxides with defective perovskite structure (ABO3−δ) have been receiving considerable attention as oxygen permeation membranes. They have great potential for applications in low cost high purity oxygen generator [1], fuel cell electrodes [2], gas sensors [3] and integrated separation–oxidation membrane reactors [4]. SrCo0.8Fe0.2O3−δ (SCFO) is one of the perovskite-type materials reportedly to have high oxygen permeability [5], [6]. SCFO is derived from La1−xSrxCo1−yFeyO3−δ (LSCF) series of perovskite-type materials, in which the highest oxygen vacancy concentration is obtained when x=1, whereby the A site, La3+ is completely substituted by Sr2+. Therefore, SCFO has the highest oxygen permeability in the LSCF series of perovskite-type materials.
Yin et al. [7] has recently reported that some types of SrCoFeOx were stable in reducing environment during oxygen sorption/desorption test. However, when SCFO exists in the perovskite ABO3 form, it has good oxygen permeability but limited chemical and mechanical stability [8], [9]. This is because cobalt and iron oxides are easily reduced at high temperature causing the perovskite structure to be easily decomposed. The observation of Yin et al. [7] on the high stability of SrCoFeOx based materials in reducing environments motivated us to further explore the development of this perovskite structure for oxygen permeable hollow fiber membrane. One of the routes to modify the SCFO perovskite structure is via doping with higher valence cations such as lanthanum or other rare earth elements with a 3+ valence in the A site. However the concentration of oxygen vacancy decreases greatly with the substitution of 3+ valence dopants in the A site, causing the oxygen permeability to be much lower than that of SCFO.
Another route to modify SCFO structure is to substitute part of B site elements with transition metal cations, while the A site element maintains a 2+ valence. Following with this idea, Al3+ [10], [11], Cr3+/4+ [12], [13], Ti4+ [14], Zr4+ [15], Sc3+ [16], [17], Nb5+ [18], Ta5+ [19], Ru4+/5+ [20], etc. have been used as cationic dopants in the B site of SCFO to substitute part of Fe3+ or Co3+. Among all the cationic dopants, those with a valence of 4+ or higher should decrease the oxygen vacancy concentration by forming non-defective A2+B4+O3 electronically neutral material. So the element with a stable 3+ valence should be better than the higher valence cation in order to obtain higher oxygen flux.
It is believed that gallium oxide easily forms a solid solution with SCFO perovskite material because Ga3+ has a similar radius with Fe3+ and Co3+. A series of La1−xSrxGa1−yFeyO3−δ perovskite-type materials with gallium as a key element in the B site of ABO3−δ have been reported [21], [22]. Since Ga has a fixed 3+ valence, the substitution of Ga in the SCFO perovskite should enhance the stability of the membrane in the presence of reducing gases such as CH4. Therefore, based on the advantage of gallium, there is great potential for utilization of this gallium-doped SCFO (i.e. SCFG) perovskite membrane for reactions such as partial oxidation of methane, water splitting, reforming of hydrocarbons and other reactions that require oxygen in the presence of reducing gases. It has been reported that gallium doping could block part of Co–O–Co bond and decrease the electronic conductivity of the perovskite material [23]. However, when the doping ratio of gallium in SCFO material is low (i.e. Ga doping of 0.1), the decrease in electronic conductivity should be within tolerable limits.
Therefore, using a Ga doping of 0.1 in this study, we have synthesized a SrCo0.8Fe0.1Ga0.1O3−δ (SCFG) perovskite crystal by substituting part of iron with gallium in the SCFO material. Based on the following ionic radii values (Sr2+:121 pm, Co2+(ls-low spin): 65 pm, Co2+(hs-high spin): 74 pm, Co3+(ls): 52.5 pm, Co3+(hs): 61 pm, Co4+: 53 pm, Fe3+(hs): 64.5 pm, Fe3+(ls): 55 pm, Ga3+: 62 pm and O2−: 140 pm) [24], the Goldschmidt tolerance factor was calculated using the following equation in order to validate the intactness of the SCFG perovskite structure:where t is tolerance factor and RA, RB and RO are the respective ionic radii of the lattice constituents. The tolerance factor for the SCFG perovskite material is 0.951–1.035, which is within the optimal values of 0.95<t<1.04 for a stable cubic perovskite structure [25], [26]. Using this new material, the SCFG hollow fiber membrane with a unique asymmetric structure has been successfully fabricated with good oxygen permeability and stability under reducing environment.
Section snippets
Preparation of SCFG material
The SrCo0.8Fe0.1Ga0.1O3−δ (SCFG) material was prepared via an EDTA–CA complexing method as reported by Meng et al. [27]. In this work, ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) of 99.5+% purities were used as chelating agents. Analytical purity of Sr(NO3)2, Co(NO3)2·6H2O, Fe(NO3)3·9H2O and high purity (99.99+%) of Ga(NO3)3 were used as the precursors. pH value was adjusted to 6 by nitric acid and ammonia solution. All the above materials were sourced from Sigma-Aldrich Reagent
Material properties
The SCFG powder prepared by the EDTA–CA complexing and calcination method has very fine and well-structured particles, as shown in Fig. 2. The particle size is as small as 20–30 nm, therefore making it easier to form a dense hollow fiber membrane. From Fig. 3, the XRD graph of the powders calcined at 900 °C shows perovskite-type structure. However, the peaks are not sharp and the baseline has a high background noise, showing that the powder even after calcination at 900 °C still has very small
Conclusions
SrCo0.8Fe0.1Ga0.1O3−δ (SCFG) perovskite-type material has been successfully synthesized and used for the fabrication of an oxygen-permeating SCFG hollow fiber membrane using a phase inversion method. The O2-TPD of the SCFG membrane shows that the incorporation of Ga in the SCFO structure did not affect the oxygen bonding in the perovskite crystalline lattice. The SCFG hollow fiber shows a unique asymmetric structure with porous internal and external surfaces but a dense middle layer. The oxygen
Acknowledgments
The authors gratefully thank the National University of Singapore, A⁎STAR, NEA and National Science Foundation of China for generously supporting this work (A⁎STAR SERC Grant no. 092-138-0022 and RP no. 279-000-292–305; NEA-ETRP Grant no.1002114 and RP no. 279-000-333–490; National Science Foundation of China 210761118).
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