Materials Letters 230 (2018) 161–165 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue Enhancing O2-permeability and CO2-tolerance of La2NiO4+d membrane via internal ionic-path Qing Wei a, Shuguang Zhang a,⇑, Bo Meng a, Ning Han b,⇑, Zhonghua Zhu c, Shaomin Liu b a School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia c School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia b a r t i c l e i n f o Article history: Received 12 April 2018 Received in revised form 20 June 2018 Accepted 24 July 2018 Available online 25 July 2018 Keywords: Ceramic composites Fiber technology a b s t r a c t Novel La2NiO4+d-Sm0.2Ce0.8O1.9 dual-phase hollow fiber membrane was developed via a combined phase inversion-sintering process. The enhanced O2-permeability is due to the existence of LNO and SDC interface crossing the composite membrane not only from the surface, but also from the bulkiness, which greatly promotes the oxygen ionic transport rates. Such dual-phase membrane shows great CO2resistance without sacrificing the oxygen permeation flux value when swept by pure CO2 compared with helium. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction Current tonnage O2 production still mainly relies on cryogenic distillation technology, one 100-years-old, high capital and energy intensive technique. From an economic and clean energy or environmental perspective, it is urgent to develop a new low-cost and energy-efficient technique that can satisfy the increasing demand of oxygen for industries. Recently, ceramic-based ionic transport membranes for oxygen separation have been developed given its potential to replace the conventional cryogenic method . Mixed ionic-electronic conducting (MIEC) ceramics exhibiting simultaneously high ionic and electronic conductivities have been proposed as a promising selection for oxygen separation membrane to improve the viability of zero emission technology [2–4]. Many application circumstances involve the CO2 presence, thus requiring the membrane to be CO2-resistant. However, a tradeoff phenomenon generally exists in MIEC membranes between CO2-resistance and O2-permeability in particular for the state of art single phase perovskite or Ruddlesden-Popper membranes [5–7]. CO2-resistance is an enabling property for implementation of oxygen-selective membranes in clean energy technologies like oxyfuel combustion for CO2 capture, the oxidative coupling of methane, partial oxidation of methane to synthesis gas and aromatization of formaldehyde. Developing dual-phase membrane is a ⇑ Corresponding authors. E-mail addresses: firstname.lastname@example.org (S. Zhang), email@example.com (N. Han). https://doi.org/10.1016/j.matlet.2018.07.104 0167-577X/Ó 2018 Elsevier B.V. All rights reserved. promising strategy that can not only conquer the CO2 resistance but also raise the O2 flux value. There are some robust ceramic membranes but limited by either a low ionic or electronic conductivity [2,6]. Recently, an internal electronic short-circuit method has been reported to improve these fluorite-type ionic membranes via the addition of electronic conducting material as the second phase . Via this short-circuit concept, higher oxygenpermeability and enhanced CO2 resistance can be simultaneously achieved in Ce0.8Gd0.2O2-CoFe2O4  and Pr0.1Gd0.1Ce0.8O2CoFe2O4  dual-phase membranes where the addition of spinel phase facilitates the electronic conduction. Compared to those perovskite MIEC membranes, La0.6Sr0.4Co0.2Fe0.8O3 d and Ba0.5Sr0.5Co0.8Fe0.2O3 d, with higher oxygen fluxes and lower stability [10,11], La2NiO4+d (Cobalt-free) is more robust but limited by the oxygen flux due to the low ionic conductivity (0.04 S cm 1 at 800 °C) . Similarly, La2NiO4+d membrane can be improved by the addition of the second phase (i.e. fluorite-type oxide) to increase its ionic conductivity [6,13]. In this work, La2NiO4+d-Sm0.2Ce0.8O2-d (LNO-SDC) dual-phase hollow fiber was developed as the hollow fiber geometry can provide the largest membrane area per unit volume. In this dual-phase system, because of its excellent CO2-resistant characteristic, good phase stability and high ionic conductivity, fluorite-type SDC is applied to build an extra oxygen ionic conducting passage to compensate the disadvantage of Ruddlesden-Popper La2NiO4 material, i.e. inherent low oxygen ionic conductivity. 162 Q. Wei et al. / Materials Letters 230 (2018) 161–165 2. Experimental section La2NiO4+d was synthesized via sol-gel route and calcined at 1000 °C . SDC powder was purchased from NingboSOFCMAN/Corporation. The dual-phase membrane has the composition of 60 wt%LNO + 40 wt%SDC (LNO-SDC). Hollow fiber precursor was prepared via the combined phase inversion-sintering process . Both precursors were sintered at 1400 °C for 4 h to get the desired ceramic hollow fiber membranes. The morphologies and crystal structures of the hollow fibers and powders were characterized by scanning electron microscope (SEM/BSEM, FEI/Sirion-200) and powder X-ray diffraction (XRD, Bruker/D8/Advance). The thermal expansion coefficients (TECs) were probed using dilatometry (DIL-402C/Netzsch/Germany). Composition of permeate gas was analyzed through gaschromatograph (Agilent/6890N) and the calculation of oxygen fluxes through the membrane can be referred elsewhere . 3. Results and discussion Characteristic peaks of (1 1 1), (2 0 0), (2 2 0) planes of SDC and (1 1 3), (2 0 0), (0 2 4) planes of LNO calcined at 1000 °C could be clearly observed agreeing well with the cubic SDC and orthorhombic LNO and with space-group of Fm3m and Fmmm (Fig. 1a), respectively. The stick patterns of orthorhombic LNO (JCPDS/ PDF#01-079-0951) and cubic SDC (JCPDS/PDF#01-075-0158) are also included as baselines for comparison purpose. Fig. 1b/c illustrates the crystal structure of LNO/SDC composite. The characteristic peaks of LNO-SDC composite (sintered at 1400 °C) match well with the individual LNO and SDC, which indicates the good compatibility between these two phases. TECs of LNO, SDC and LNO-SDC bar-samples sintered at 1400 °C are 13.96 10 6, 11.98 10 6 and 11.42 10 6 K 1 , respectively, which are calculated from Fig. 1d. These similar TEC values indicate their excellent thermo-mechanical compatibility, which ensures the physical integrity of LNO and SDC interface during thermal cycling up to 1000 °C. Fig. 2a/b exhibit the oxygen fluxes of LNO and LNO-SDC swept by helium from 750 to 1000 °C. For both membranes, the increase in temperature leads to an enhanced oxygen flux. For example, at 100 mL min 1, the oxygen fluxes of LNO and LNO-SDC increased from 0.13 to 2.0 and from 0.45 to 2.9 mL min 1 cm 2, respectively, from 750 to 1000 °C. This is due to the enhancement of oxygen ion bulk-diffusion rate and oxygen surface-exchange rate with operating temperature rise. Likewise, oxygen fluxes also display positive correlation with helium flow rate. For instance, at 1000 °C, an increase in helium from 20 to 120 mL min 1 translates to an increase in oxygen flux from 1.26 to 2.0 and from 1.45 to 3.05 mL min 1 cm 2 for LNO and LNO-SDC, respectively. Higher helium flow rate manifests into higher oxygen partial pressure difference between the feed-side and the permeate-side. A careful inspection of the fluxes from the two membranes indicates that at similar operating conditions, LNO-SDC composite membrane gave a better performance than LNO at all temperatures examined, highlighting the fact that the addition of SDC can promote the bulk-ionic transfer process of the resultant composite membrane with mechanism showing in Fig. 2h. SDC is a well-known oxygen ionic conductor, the continuous SDC path by 40 wt% addition inside the dual-phase membrane will promote the bulk-oxygenionic conducting process [6,8]. However, the enhancement factor of LNO-SDC relative to LNO decreased with the rise of operating temperature. To give one example, at a constant air feed and helium sweep rates of 100 and 120 mL min 1, the enhancement factors at 800 and 950 °C is dwindled from 1.6 to 0.60 as shown in Fig. 2c. The decreased enhancement factor with the Fig. 1. (a) Powder XRD patterns of SDC, LNO, ball-milled, and 1400 °C sintered LNO-SDC; (b/c) Crystal structure of LNO/SDC; (d) TECs of SDC, LNO, and LNO-SDC. Q. Wei et al. / Materials Letters 230 (2018) 161–165 163 Fig. 2. Oxygen permeation fluxes of (a) LNO; (b) LNO-SDC; (c) Temperature-dependent enhancement factors; SEM images, (d) Cross-sections; (e) BSEM; (f/g) External/ internal-surfaces; (h) Schematics of LNO-SDC composite membranes. temperature improvement reflects the alternation of the relative rate-determining step in the overall O2 transport process. Such controlling step is shifted from bulk diffusion at lower operating temperatures to surface reactions at higher temperatures of these LNO-based membranes (Fig. S1). A similar observation has been made on other ceramic membranes . SEM images of tested LNO-SDC hollow fiber are shown in Fig. 2d–g. The sandwich cross-section structure could still be maintained and clearly observed in Fig. 2d. For example, two finger-like porous inter-layers were integrated well with the closing dense central-layer, which is evolved from the precursor morphology achieved from the different precipitation rates at different locations during the phase inversion process. Fig. 2e displays BSEM image of inter-surface. The contrasting appearance of the different grains indicates the presence of two different phases. The dark grey and light grey areas in the figure represent LNO and SDC phases, respectively. SEM images of external/internal-surfaces are shown in Fig. 2f/g. There is no obvious difference of surfaces swept by helium or air. The influence from CO2 presence in different concentration in the permeate side on the oxygen permeability of the resultant LNO-SDC membrane is presented in Fig. 3. The oxygen fluxes gradually decreased with the increase of CO2 concentration (Fig. 3a) with a maximum cutting down by 12% using 100% pure CO2 as the sweep gas when compared with the inert gas without CO2. Noteworthy that the LNO-SDC membrane performed stably in CO2 containing atmosphere even swept by pure CO2. Fig. 3b displays the oxygen flux variation with different CO2 sweep gas flowrates and operation temperatures. The variation trend is similar with that on helium flow rate. The recovery test under He/pureCO2/He for about 900 min is given in Fig. 3c. Full-recovery of permeability under helium proves the strong CO2 tolerance of LNOSDC membrane. The stability tests of LNO-SDC powder (I) and membrane (II) were also conducted in pure CO2 (Fig. 3d). As can be observed, two XRD patterns are very similar and no impurity phase can be observed in the sample treated in CO2 atmosphere. Long-time permeation test could further verify the excellent CO2 resistance. SEM images of LNO-SDC after CO2 test are displayed in Fig. 3e–j. The sandwich cross-section structure is clearly displayed in Fig. 3e–g. External/internal-surfaces are shown in Fig. 3h/i. Fig. 3j displays BSEM image of the inter-surface (treated with CO2). Few physical structure damage have been observed on all these images after CO2 treatment. 4. Conclusions By introducing Sm0.2Ce0.8O1.9 (SDC) with higher ionic conductivity into La2NiO4+d (LNO) system, a novel LNO-SDC dual-phase membrane was successfully developed. LNO-SDC membrane not only displays enhanced oxygen permeability by a factor up to 2.3 compared to pure LNO membrane, but also presents high CO2 resistance. Such robust dual-phase membrane has the potential to overcome the low stability problem of single phase perovskite oxide membranes, thus opening new opportunities for many advanced applications in clean energy area or membrane reactors for chemical production. 164 Q. Wei et al. / Materials Letters 230 (2018) 161–165 Fig. 3. Oxygen permeation fluxes at various CO2 concentrations (a); Changing CO2 sweep rates (b); Stability test under He/pure-CO2/He (c); Stability test (d); SEM images, (e–g) Cross-sections; (h/i) External/internal-surface; (j) BSEM. Acknowledgement Appendix A. Supplementary data This work was supported by National Natural Science Foundation of China (21476131, 21376143), Australian Research Council (DP160104937), Shandong Provincial Natural Science Foundation (ZR2012BQ010), Scientific Research Foundation for the Returned Overseas Chinese Scholars, Joint Research and Development Program of Zibo City SDUT and Key Research and Development Program of Shandong Province. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2018.07.104. References  Y. Teraoka, T. Nobunaga, K. Okamoto, N. Miura, N. Yamazoe, Solid State Ionics 48 (1991) 207–212. Q. 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