A new asymmetric SrCo_0.8Fe_0.1Ga_0.1O_3−δ perovskite hollow fiber membrane for stable oxygen permeability under reducing condition

Abstract

A new SrCo_0.8Fe_0.1Ga_0.1O_3−δ (SCFG) perovskite material has been successfully synthesized using an EDTA–CA complexing method. The resultant SCFG powder calcined at 900 °C shows perovskite-type crystal structure upon calcination at 900 °C for 5 h. The oxygen-permeating SCFG hollow fiber membrane prepared from the SCFG powder by the phase inversion–sintering method shows a unique asymmetric structure with porous internal–external surfaces and a dense middle layer. This SCFG hollow fiber membrane is able to achieve a good oxygen permeability of 3.5 mL min⁻¹ cm⁻² at 950 °C and 0.55 mL min⁻¹ cm⁻² at 600 °C when supplied with air using He as a sweep gas. The presence of Ga was found to significantly lower the activation energy of SCFG hollow fiber membrane. The unique asymmetric structure played a critical role in elevating the critical temperature to 900 °C for transition from surface-reaction to bulk-diffusion controlled regime. Furthermore the SCFG hollow fiber membrane is very stable under reducing environment of 20 mL min⁻¹ methane for at least 100 h with an enhanced permeation of 3.4 mL min⁻¹ cm⁻² at 900 °C.

Introduction

Mixed ion-electron conducting oxides with defective perovskite structure (ABO_3−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]. SrCo_0.8Fe_0.2O_3−δ (SCFO) is one of the perovskite-type materials reportedly to have high oxygen permeability [5], [6]. SCFO is derived from La_1−xSr_xCo_1−yFe_yO_3−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 SrCoFeO_x 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 SrCoFeO_x 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], Cr^3+/4+ [12], [13], Ti⁴⁺ [14], Zr⁴⁺ [15], Sc³⁺ [16], [17], Nb⁵⁺ [18], Ta⁵⁺ [19], Ru^4+/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 La_1−xSr_xGa_1−yFe_yO_3−δ perovskite-type materials with gallium as a key element in the B site of ABO_3−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 SrCo_0.8Fe_0.1Ga_0.1O_3−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:

$$t=\frac{R_{\mathrm{A}}+R_{\mathrm{O}}}{\sqrt{R_{\mathrm{B}}+R_{\mathrm{O}}}}$$

where t is tolerance factor and R_A, R_B and R_O 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.