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j.matlet.2018.07.104

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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
[1]. 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: gregzhangsg@gmail.com (S. Zhang), ning.han@curtin.edu.au
(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 [2]. Via this short-circuit concept, higher oxygenpermeability and enhanced CO2 resistance can be simultaneously
achieved in Ce0.8Gd0.2O2-CoFe2O4 [8] and Pr0.1Gd0.1Ce0.8O2CoFe2O4 [9] 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) [12]. 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.
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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 [14]. 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 [14]. 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 [15].
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 [16].
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.
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