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Various influence of surface modification on permeability and phase stability through an oxygen permeable membrane
Author’s Accepted Manuscript Various influence of surface modification on permeability and phase stability through an oxygen permeable membrane Jian Xue, Guowei Weng, Li Chen, Yanpeng Suo, Yanying Wei, Armin Feldhoff, Haihui Wang www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(18)32640-1 https://doi.org/10.1016/j.memsci.2018.12.040 MEMSCI16723
To appear in: Journal of Membrane Science Received date: 23 September 2018 Revised date: 13 December 2018 Accepted date: 15 December 2018 Cite this article as: Jian Xue, Guowei Weng, Li Chen, Yanpeng Suo, Yanying Wei, Armin Feldhoff and Haihui Wang, Various influence of surface modification on permeability and phase stability through an oxygen permeable m e m b r a n e , Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.12.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Various influence of surface modification on permeability and phase stability through an oxygen permeable membrane Jian Xue,1,2,* Guowei Weng,1 Li Chen,1 Yanpeng Suo,2 Yanying Wei,1 Armin Feldhoff2 and Haihui Wang1,* 1
School of Chemistry & Chemical Engineering, South China University of Technology, No.
381 Wushan Road, Guangzhou 510640, China. 2
Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover,
Callinstrasse 3A, D-30167 Hannover, Germany.
ABSTRACT: Good oxygen permeability and stability of oxygen transport membrane are highly necessary for practical applications. Herein, through different theories, the oxygen transport limitation step through the Ruddlesden-Popper (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG) membrane was demonstrated firstly, which suggest surface modification can be an effective approach to improve the oxygen separation performance. After coating, various influences of different side surface modification on permeability and phase stability were observed. The sweep-side coated membrane exhibits largely enhanced permeation fluxes than feed-side coated membrane, and the feed-side coated membrane shows better phase stability under the same conditions (these results reveal that the sweep side is the permeability limitation side and the feed is the phase stability limitation side). The both side coated membrane combines abovementioned advantages which shows 57 % enhanced and stable oxygen permeation flux at 800 oC. The observed various modification effects on the 1
permeation performance are discussed based on the surface exchange properties and the mechanism of the oxygen transporting membrane in detail.
Graphic Abstract
Surface modification on different side possesses various influence on the performance of oxygen permeation membranes. The sweep-side coated membrane exhibits largely enhanced permeation fluxes compared to that through feed-side coated membrane, while the feed-side coated membrane shows better phase stability under the same conditions. The both side modified membrane combines abovementioned advantages, and it shows 57 % enhanced and stable oxygen permeation flux during the long-term test at 800 oC. The observed catalytic effects are discussed based on the oxygen surface exchange properties of the material and the mechanism of the oxygen transporting membrane in detail.
KEYWORDS: Oxygen permeation membrane; Gas separation; Mixed conductor; Ruddlesden-Popper oxide; Phase stability 2
INTRODUCTION
Oxygen is an important raw material for human and possesses widespread application in modern society. Annually, a hundred million tonnes of oxygen are separated from air for practical using [1]. Normally, oxygen are produced through cryogenic distillation, electrolysis of water or pressure swing adsorption. While, the first two methods are energy-intensive while the latter cannot produce high purity oxygen [2-7].
Recently, new advanced technology through dense ceramic membrane with mixed conductivity (both oxygen ions and electrons, abbreviated as MIEC) has attracted lots attentions for oxygen purification with absolute selectivity from an oxygen-containing atmosphere at high temperature under a driving force (oxygen concentration or electrical potential gradient) [8-15]. Such membranes systems features high energy-efficiency, clean, simplified operation and can be integrated with some industrial reactions such as methane coupling to ethane or aromatics and partial oxidation of methane into synthesis gas [16-23].
In a dense MIEC membrane, as shown in Figure 1, oxygen could be separated through three progressive steps: (i) feed side: oxygen insertion surface-exchange process (oxygen adsorption and dissociation); (ii) bulk membrane: diffusion of charged oxygen carriers (oxygen vacancies or interstitial oxygen) and electrons or electron holes simultaneously; (iii) sweep side: oxygen release surface-exchange process, (oxygen association and desorption) [24-31].
3
Figure 1. Schematic diagram of the different sections involved in the oxygen transport during oxygen permeation.
For a relatively thick membrane, the oxygen diffusion process (step ii) acts a dominant role in separation and the permeation flux can be expressed by the Wagner equation [9, 32, 33]:
∫
(1)
Where jO2 is the oxygen permeation flux, R is the gas constant, F is the Faraday constant, T is temperature, L is the thickness, σel is electronic conductivity, σion is ionic conductivity, and
are oxygen partial pressure at feed side or sweep side, respectively. As described in
Eq. 1, the oxygen flux has a linear relationship with the reciprocal of the membrane thickness. Therefore, reducing the membrane thickness could increase the permeation flux, while the limiting impact of surface exchange becomes larger. When membrane thickness is less than a marginal value LC (the characteristic thickness, where the separation process is equally controlled by surface-exchange and bulk-diffusion), the surface exchange step turns into the rate-controlling one [9, 34]. Therefore, further enhancement of oxygen flux can be obtained through porous catalytic layer surface coating [35-43]. 4
In our previous work, the K2NiF4-type membrane exhibited twice higher oxygen permeation flux when A-site deficiency was introduced in (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG) [44]. However, this material was not stable in oxidizing atmosphere at intermediate temperature, so that the feed side (high oxygen concentration) membrane surface experienced decomposing in practical oxygen separation processes [45]. Therefore, in this study, to overcome the phase stability limitations and furtherly improve the oxygen permeation flux, surface modification was applied. The typical perovskite La0.6Sr0.4CoO3 (LSC) catalyst was applied here [46, 47], the influence of surface coating on permeability and stability of thin (PL)1.9NCG membrane in oxygen-containing atmosphere were investigated in detail (see Materials and Methods in the Supporting Information) [48-52].
RESULTS AND DISCUSSION
Characterization of Materials. In general, oxygen permeation process is governed by surface exchange and/or bulk diffusion. If bulk diffusion is the controlling step, the oxygen flux can be described by Wagner equation (Eq. 1) and the oxygen flux has a linear relationship with the reciprocal of the membrane thickness [9, 32, 33]. However, as shown in Figure 2a, the relationship is not linear anymore for (PL)1.9NCG membrane when the thickness is less than 1.6 mm, indicating the surface exchange comes into play [44].
5
Figure 2. Oxygen permeability of (PL)1.9NCG: (a) Dependence of the normalized oxygen flux on the inverse membrane thickness [31]; (b) the characteristic membrane thickness (LC) and values of the constant (C), which were fitted through the experimental results shown in Figure 2a.
If the oxygen permeability is controlled by both bulk diffusion and surface exchange under small oxygen gradient, the Wagner equation is modified to [53, 54],
(
∫
)
(2)
Where LC (LC = D/k, D and k are the coefficients of bulk diffusion and surface exchange respectively) is the characteristic thickness. As shown in Eq. 2, two parameters (LC and L) determine the oxygen permeation flux under the fixed oxygen gradient. After simple modification of Eq. 2 by combining the thickness-independent variables into a constant C, the specific oxygen flux can be described as [40, 55]:
(
)
(3)
Both the C and LC values could be estimated by using the least-squares fitting method and are varied independently [40, 55]: 6
(
)
(4)
The fitting results at different temperatures are shown in Figure 2b. With increasing temperature from 800 to 975 oC, the LC values decrease from 0.43 to 0.22 mm and C values increase from 0.17 to 0.39 cm3.min-1.cm-1. This implies that the surface exchange become more dominate for the oxygen permeation through the (PL)1.9NCG membrane with decreasing temperature and membrane thickness. Under the premise of membrane mechanical strength, the (PL)1.9NCG membrane thickness was reduced to 0.6 mm in which the surface exchange plays a relative significant role in the oxygen separation process.
Figure 3. The Effects of the oxygen partial pressure in the feed side on the oxygen fluxes through the (PL) 1.9NCG membrane.
To further demonstrate the limitation step of the 0.6 mm (PL)1.9NCG membrane, another theory proposed by Lin et al. was applied [56]. For thin membrane (electron holes contribute to the electronic conductivity), the permeability was controlled by surface reaction, which can be described as [56]: 7
(
)
(5)
Where is the surface permeation constant. When the permeation process is limited by surface exchange, the oxygen flux would be linearly proportional to
. Therefore,
the oxygen fluxes were tested by changing the oxygen partial pressure in feed side (Figure 3). The oxygen permeability enhanced with the increasing oxygen partial pressure due to the increased oxygen gradient. For instance, 0.6 mL/min cm2 oxygen flux can be increased to 1.2 mL/min cm2 at 975 oC when the feeding oxygen concentrate increased from 0.1 to 0.6. Then these results were fitted by Eq. 5 as shown in Figure 4a. The fitted lines were linear at low temperatures and low oxygen partial pressure, while become curve with increasing temperature and oxygen gradient. Moreover, Jacobson et al. proposed that the oxygen fluxes should be linearly proportional with oxygen gradient (the concentration of oxygen carriers does not vary significantly) if the separation process was limited by bulk diffusion [57, 58]:
(6)
Where is related to the concentration and diffusivity of oxygen ions, thickness of membrane. Based on Eq. 6, the results in Figure 3 were also simulated as shown in Figure 4b. The oxygen fluxes showed no any linear relationship with the
, which illustrate the
transport step was not controlled by bulk diffusion. Combining abovementioned results (Figure 2 and 4), it can be concluded that the oxygen separation process through 0.6 mm (PL)1.9NCG membrane was largely limited by surface exchange, especially at low temperatures. Therefore, enhanced oxygen flux can be achieved through surface modification. 8
Figure 4. Relationship between oxygen fluxes and (a)
; (b)
through the 0.6 mm (PL)1.9NCG membrane.
Herein, a typical perovskite LSC was applied as the surface exchange catalyst [46, 47]. Figure 5a depicts the room temperature PXRD patterns of the LSC and (PL)1.9NCG samples after treated at 1000 oC for 10 h. The PXRD patterns of LSC are identified as single perovskite phase and that of (PL)1.9NCG is confirmed as K2NiF4 structure phase, no peaks other than abovementioned two phase have been observed on the PXRD patterns [44, 45]. The SEM images of the modified membrane are shown in Figure 5b. It could be distinguished that the coated LSC layer is porous with around 200 m thickness. The porosity of the LSC layer was come from the organics combustion, which can supply lots adsorption or desorption sites and increase surface area of the (PL)1.9NCG membrane [36, 59, 60].
9
Figure 5. (a) Powder XRD patterns of LSC and (PL)1.9NCG oxides after treated at 1000 oC for 10 h. The crystal structures of LSC and (PL)1.9NCG are Hexagonal and Tetragonal respectively; (b) SEM images of the cross-section of the LSC-coated (PL)1.9NCG membrane.
Oxygen Permeability of the Membranes. Figure 6a compares the oxygen fluxes of (PL)1.9NCG membrane with or without LSC catalyst coated. The sweep-side and both sweepand feed-sides coated (PL)1.9NCG membranes show significant enhancement in oxygen permeation fluxes while the feed-side coated one shows modest improvement. Figure 6b shows the dependence of the oxygen flux improvement through the LSC modified (PL)1.9NCG membranes on temperatures. The improvement percentage of both side coated membrane are higher than other two type membranes and decreases from 57% to 36% with increasing temperature as the relative controlling influence of bulk diffusion becomes more significant, similar results are also observed by other researchers [61]. The enhancement of sweep-side coated membrane are 51% at 800 oC and 25% at 975 oC, that are much higher than that of feed-side coated membrane which are around 5%. Similar results were reported in GdBaCo2O5+δ modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane, in which the oxygen permeation flux through air-side coating rise by 16%, while increase 23% with sweep-side modification [60]. Joo et al. also found a modest increase in the oxygen flux with a LSC feed-side coated dual-phase membrane, while an order of magnitude improvement was observed through the sweep-side coated membrane [40].
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Figure 6. (a) Influence of temperature on the oxygen permeation fluxes; (b) Percentage improvement of oxygen fluxes of
LSC-coated membranes compared to the uncoated one at different temperatures.
This clearly indicates that the surface exchange at feed-side (oxygen-rich side) is not the limiting process through the 0.6 mm (PL)1.9NCG membrane and the oxygen flux is indeed controlled by oxygen surface exchange at sweep-side (oxygen-lean side). With this in mind, surface exchange properties of (PL)1.9NCG are estimated by thermogravimetric (TG) methods. When the atmosphere is changed between air and nitrogen, the material undergo a weight loss in oxygen release process and a weight gain in oxygen insertion process. As shown in Figure 7a, the weight of the (PL)1.9NCG sample decrease when atmosphere is switched from air to N2 (oxygen release) and increase when atmosphere is switched back from N2 to air (oxygen insertion) at different temperature. At all temperatures, the (PL)1.9NCG sample lose weight slowly in oxygen release process (oxygen association and desorption) and gain weight rapidly in oxygen insertion period (oxygen adsorption and disassociation), similar results are also found in other materials [38, 62, 63].
11
Figure 7. (a) Weight changes of the (PL)1.9NCG during air → N2 → air cycle at different temperatures; (b) The calculated surface-exchange coefficient ki and kr of (PL)1.9NCG at different temperatures.
From this TG curves, the oxygen insertion coefficient ki and the oxygen release coefficient kr can be calculated by the theoretical model proposed by Lin et al., relative equation was simplified as [62-64]: ( )
( )
( )
( )
(
)
(7)
Where the w(0) is the weight of material at the time of nitrogen switching to air (scale as t = 0) and w(e) is the steady-state weight of the material after atmosphere switching. The surface exchange rate coefficients ki and kr calculated by Eq. 7 are described in Figure 7b. The ki increases from 1.2 to 2.0 *10-6 cm.s-1 with increasing temperature from 800 oC to 950 oC, while the kr is around 0.15 *10-6 cm.s-1 in the tested temperature range. Most important, the values of ki are much larger than those of kr at different temperatures, which clearly indicated that the oxygen release is the limit of the (PL)1.9NCG of investigated thickness (0.6 mm), similar phenomenon are also found in other materials [38, 40, 64]. Therefore, it could be expected that the oxygen release on the sweep side is the rate limiting step in (PL)1.9NCG membrane and the modification on the sweep surface (oxygen release side) will result in 12
larger oxygen flux improvement than that on the feed surface (oxygen insertion side). This is consistent with the oxygen separation results shown in Figure 6.
Figure 8. (a) Long-term stability through the unmodified and LSC modified (PL) 1.9NCG membrane at different temperatures.32 (b) Post-XRD tests of the spent (PL)1.9NCG ceramic surfaces (strip the catalytic layer).
Figure 8a shows the long-term separation performance through the uncoated and LSC coated (PL)1.9NCG membranes at 900, 850 and 800 oC. In our previous report, the uncoated (PL)1.9NCG membrane (the model is described in Figure 9a) underwent a phase decomposition into Pr4Ni3O10-δ′ (a higher member of the Ruddlesden-Popper series) and praseodymium oxide phases under oxygen-containing atmosphere at intermediate temperature, so that its oxygen permeation fluxes decreased with time as also shown in Figure 8a [45]. For instance, the oxygen fluxes through the uncoated (PL)1.9NCG membrane decreased 1% at 900 oC, ca. 13% at 850 oC and ca. 43% at 800 oC over a period of 100 h due to the phase decomposition on feed side as shown in Figure 8b. The oxygen permeation fluxes at low temperatures decreased more than that at high temperature due to the lowering temperature result in more oxidative conditions (higher oxygen chemical potential) [45].
13
Figure 9. Models of the LSC coated (PL)1.9NCG membrane: a) uncoated; b) sweep-side coated; c) feed-side coated; d) both sides coated.
The oxygen fluxes declines through the sweep-side coated (PL)1.9NCG membrane (the model is described in Figure 9b) decrease ca. 1% at 850 oC and 6% at 800 oC of the initial permeation fluxes, which are slower than that of the uncoated one. The enhanced phase stability by the modification on the sweep surface might result from its higher oxygen permeability. During the oxygen permeation process, oxygen is partly permeated from feed side (decomposition side with high O2 chemical potential) to sweep side (low O2 chemical potential). Therefore, higher oxygen permeation fluxes lead to lower O2 chemical potential near the feed-side surface under the same operation condition as show in Figure 9b [1]. The feed side of the spent sweep-side coated membrane was also examined by XRD as shown in Figure 8b, which partially decomposed but not as serious as the uncoated one. Therefore, the 14
sweep-side coated (PL)1.9NCG membrane exhibits enhanced phase stability in oxygen separation process.
Moreover, the feed-side coated membrane shows more stable permeation fluxes even at 800 oC. When the feed side is modified, the catalytic layer reduces the oxygen chemical potential on the membrane surface as shown in Figure 9c. Therefore, the catalytic layer will be a protective layer to prevent the membrane being in direct contact with the high oxygen chemical potential atmosphere, so that the feed-side coated membrane did not decompose at low temperature as also demonstrated by XRD shown in Figure 8b. Therefore, the feed-side coated membrane shows better phase stability than the uncoated one. Similar approach was applied to improve the CO2 tolerance of 80 vol.% Ce0.9Gd0.1O2-δ-20 vol.% La0.7Sr0.3MnO3±δ dual-phase membrane, in which the membrane possessed enhanced stability after composite catalyst coating under pure CO2 [40]. When both sides of the membrane are modified with catalytic LSC layer (the model is described in Figure 9d), the advantages of the feed- and sweep-side coated membrane are combined that it possessed both higher oxygen permeation and better phase stability at intermediate temperature range under oxidative atmosphere, as shown in Figure 6 and Figure 8. Therefore, surface modification on different side possesses various influence on the oxygen permeability and stability. The enhancement of oxygen permeability and stability through the (PL)1.9NCG membrane reveal the surface-modification method was an effective strategy for turning the membrane separation performance, which could accelerate their practical applications in oxygen separation, membrane reactors and solid state fuel cell.
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4. CONCLUSIONS
The dependence of the oxygen permeability of (PL)1.9NCG on the membrane thickness indicates the surface exchange limited permeation fluxes with thinner membrane and suggests a possibility to improve the separation performance through coating. The enhancement of oxygen permeation fluxes through sweep-side coated membrane by a porous layer of LSC are 51% at 800 oC and 25% at 975 oC, which are higher than that through feed-side coated one (around 5%). Concluded from the TG results, the desorption rate of (PL)1.9NCG is much lower than the adsorption rate, which suggests that the surface exchange on the sweep side is the transport limiting process so that the coating on the sweep-side surface shows obvious enhancement on oxygen fluxes than the feed-side coated one. Moreover, the sweep-side coated membrane also shows slightly improved phase stability compared with the uncoated membrane due to its improved oxygen permeation flux. The feed-side coated membrane shows better phase stability not only than the uncoated membrane but also than the sweep-side coated membrane under the same conditions, which are resulted from the reducing oxygen chemical potential on the membrane surface by coating layer. Abovementioned results suggests that the sweep side is the permeability limitation side and the feed is the phase stability limitation side, so that the both side modified membrane possesses all the advantages of one side coating which shows 57 % enhanced and stable oxygen flux during the long-term test at 800 oC.
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AUTHOR INFORMATION
Corresponding Author
* E-mail: [email protected]; [email protected].
Notes The authors declare no competing financial interest
ACKNOWLEDGMENT The Authors thank the financial support from National Natural Science Foundation of China (No. 21706076, 21536005, 51621001), Natural Science Foundation of Guangdong (2017A030310431,
2014A030312007),
Guangzhou
Technology
Project
(No.
201804010210), China Postdoctoral Science Foundation (No. 2018T110870) and the Deutsche Forschungsgemeinschaft (DFG, FE928/7-1).
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[44] J. Xue, Q. Liao, W. Chen, H.J.M. Bouwmeester, H. Wang, A. Feldhoff, A new CO2-resistant Ruddlesden–Popper oxide with superior oxygen transport: A-site deficient (Pr0.9 La0.1)1.9 (Ni0.74 Cu0.21 Ga0.05)O4+δ, J. Mater. Chem. A 3 (2015) 19107-19114. [45] X. Jian, A. Schulz, H. Wang, A. Feldhoff, The phase stability of the Ruddlesden-Popper type oxide (Pr0.9La0.1)2.0Ni0.74Cu0.21Ga0.05O4+δ in an oxidizing environment, J. Membr. Sci. 497 (2016) 357-364. [46] R. Ganeshananthan, A. Virkar, Measurement of surface exchange coefficient on porous La0.6Sr0.4CoO3−δ samples by conductivity relaxation, 152 (2005) A1620-A1628. [47] H. Luo, K. Efimov, H. Jiang, A. Feldhoff, H. Wang, J. Caro, CO2-stable and cobalt-free dual-phase membrane for oxygen separation, Angew. Chem. Int. Edit. 50 (2011) 759-763. [48] A. Feldhoff, M. Arnold, J. Martynczuk, T.M. Gesing, H. Wang, The sol–gel synthesis of perovskites by an EDTA/citrate complexing method involves nanoscale solid state reactions, Solid State Sci. 10 (2008) 689-701. [49] H. Luo, B. Tian, Y. Wei, H. Wang, H. Jiang, J. Caro, Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ, AIChE J. 56 (2009) 604-610. [50] H. Wang, R. Wang, D.T. Liang, W. Yang, Experimental and modeling studies on Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) tubular membranes for air separation, J. Membr. Sci. 243 (2004) 405-415. [51] J. Xue, Q. Liao, Y. Wei, Z. Li, H. Wang, A CO2-tolerance oxygen permeable 60Ce0.9Gd0.1O2−δ–40Ba0.5Sr0.5Co0.8Fe0.2O3−δ dual phase membrane, J. Membr. Sci. 443 (2013) 124-130. 23
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Highlights
Various influence of different side surface modification were observed;
Oxygen permeation fluxes was largely improved by surface coating on the sweep side;
Feed-side coated membranes possess better phase stability under the same conditions;
The improvements are explained in detail based on the oxygen transport mechanism.
25
PII: DOI: Reference:
S0376-7388(18)32640-1 https://doi.org/10.1016/j.memsci.2018.12.040 MEMSCI16723
To appear in: Journal of Membrane Science Received date: 23 September 2018 Revised date: 13 December 2018 Accepted date: 15 December 2018 Cite this article as: Jian Xue, Guowei Weng, Li Chen, Yanpeng Suo, Yanying Wei, Armin Feldhoff and Haihui Wang, Various influence of surface modification on permeability and phase stability through an oxygen permeable m e m b r a n e , Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.12.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Various influence of surface modification on permeability and phase stability through an oxygen permeable membrane Jian Xue,1,2,* Guowei Weng,1 Li Chen,1 Yanpeng Suo,2 Yanying Wei,1 Armin Feldhoff2 and Haihui Wang1,* 1
School of Chemistry & Chemical Engineering, South China University of Technology, No.
381 Wushan Road, Guangzhou 510640, China. 2
Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover,
Callinstrasse 3A, D-30167 Hannover, Germany.
ABSTRACT: Good oxygen permeability and stability of oxygen transport membrane are highly necessary for practical applications. Herein, through different theories, the oxygen transport limitation step through the Ruddlesden-Popper (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG) membrane was demonstrated firstly, which suggest surface modification can be an effective approach to improve the oxygen separation performance. After coating, various influences of different side surface modification on permeability and phase stability were observed. The sweep-side coated membrane exhibits largely enhanced permeation fluxes than feed-side coated membrane, and the feed-side coated membrane shows better phase stability under the same conditions (these results reveal that the sweep side is the permeability limitation side and the feed is the phase stability limitation side). The both side coated membrane combines abovementioned advantages which shows 57 % enhanced and stable oxygen permeation flux at 800 oC. The observed various modification effects on the 1
permeation performance are discussed based on the surface exchange properties and the mechanism of the oxygen transporting membrane in detail.
Graphic Abstract
Surface modification on different side possesses various influence on the performance of oxygen permeation membranes. The sweep-side coated membrane exhibits largely enhanced permeation fluxes compared to that through feed-side coated membrane, while the feed-side coated membrane shows better phase stability under the same conditions. The both side modified membrane combines abovementioned advantages, and it shows 57 % enhanced and stable oxygen permeation flux during the long-term test at 800 oC. The observed catalytic effects are discussed based on the oxygen surface exchange properties of the material and the mechanism of the oxygen transporting membrane in detail.
KEYWORDS: Oxygen permeation membrane; Gas separation; Mixed conductor; Ruddlesden-Popper oxide; Phase stability 2
INTRODUCTION
Oxygen is an important raw material for human and possesses widespread application in modern society. Annually, a hundred million tonnes of oxygen are separated from air for practical using [1]. Normally, oxygen are produced through cryogenic distillation, electrolysis of water or pressure swing adsorption. While, the first two methods are energy-intensive while the latter cannot produce high purity oxygen [2-7].
Recently, new advanced technology through dense ceramic membrane with mixed conductivity (both oxygen ions and electrons, abbreviated as MIEC) has attracted lots attentions for oxygen purification with absolute selectivity from an oxygen-containing atmosphere at high temperature under a driving force (oxygen concentration or electrical potential gradient) [8-15]. Such membranes systems features high energy-efficiency, clean, simplified operation and can be integrated with some industrial reactions such as methane coupling to ethane or aromatics and partial oxidation of methane into synthesis gas [16-23].
In a dense MIEC membrane, as shown in Figure 1, oxygen could be separated through three progressive steps: (i) feed side: oxygen insertion surface-exchange process (oxygen adsorption and dissociation); (ii) bulk membrane: diffusion of charged oxygen carriers (oxygen vacancies or interstitial oxygen) and electrons or electron holes simultaneously; (iii) sweep side: oxygen release surface-exchange process, (oxygen association and desorption) [24-31].
3
Figure 1. Schematic diagram of the different sections involved in the oxygen transport during oxygen permeation.
For a relatively thick membrane, the oxygen diffusion process (step ii) acts a dominant role in separation and the permeation flux can be expressed by the Wagner equation [9, 32, 33]:
∫
(1)
Where jO2 is the oxygen permeation flux, R is the gas constant, F is the Faraday constant, T is temperature, L is the thickness, σel is electronic conductivity, σion is ionic conductivity, and
are oxygen partial pressure at feed side or sweep side, respectively. As described in
Eq. 1, the oxygen flux has a linear relationship with the reciprocal of the membrane thickness. Therefore, reducing the membrane thickness could increase the permeation flux, while the limiting impact of surface exchange becomes larger. When membrane thickness is less than a marginal value LC (the characteristic thickness, where the separation process is equally controlled by surface-exchange and bulk-diffusion), the surface exchange step turns into the rate-controlling one [9, 34]. Therefore, further enhancement of oxygen flux can be obtained through porous catalytic layer surface coating [35-43]. 4
In our previous work, the K2NiF4-type membrane exhibited twice higher oxygen permeation flux when A-site deficiency was introduced in (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG) [44]. However, this material was not stable in oxidizing atmosphere at intermediate temperature, so that the feed side (high oxygen concentration) membrane surface experienced decomposing in practical oxygen separation processes [45]. Therefore, in this study, to overcome the phase stability limitations and furtherly improve the oxygen permeation flux, surface modification was applied. The typical perovskite La0.6Sr0.4CoO3 (LSC) catalyst was applied here [46, 47], the influence of surface coating on permeability and stability of thin (PL)1.9NCG membrane in oxygen-containing atmosphere were investigated in detail (see Materials and Methods in the Supporting Information) [48-52].
RESULTS AND DISCUSSION
Characterization of Materials. In general, oxygen permeation process is governed by surface exchange and/or bulk diffusion. If bulk diffusion is the controlling step, the oxygen flux can be described by Wagner equation (Eq. 1) and the oxygen flux has a linear relationship with the reciprocal of the membrane thickness [9, 32, 33]. However, as shown in Figure 2a, the relationship is not linear anymore for (PL)1.9NCG membrane when the thickness is less than 1.6 mm, indicating the surface exchange comes into play [44].
5
Figure 2. Oxygen permeability of (PL)1.9NCG: (a) Dependence of the normalized oxygen flux on the inverse membrane thickness [31]; (b) the characteristic membrane thickness (LC) and values of the constant (C), which were fitted through the experimental results shown in Figure 2a.
If the oxygen permeability is controlled by both bulk diffusion and surface exchange under small oxygen gradient, the Wagner equation is modified to [53, 54],
(
∫
)
(2)
Where LC (LC = D/k, D and k are the coefficients of bulk diffusion and surface exchange respectively) is the characteristic thickness. As shown in Eq. 2, two parameters (LC and L) determine the oxygen permeation flux under the fixed oxygen gradient. After simple modification of Eq. 2 by combining the thickness-independent variables into a constant C, the specific oxygen flux can be described as [40, 55]:
(
)
(3)
Both the C and LC values could be estimated by using the least-squares fitting method and are varied independently [40, 55]: 6
(
)
(4)
The fitting results at different temperatures are shown in Figure 2b. With increasing temperature from 800 to 975 oC, the LC values decrease from 0.43 to 0.22 mm and C values increase from 0.17 to 0.39 cm3.min-1.cm-1. This implies that the surface exchange become more dominate for the oxygen permeation through the (PL)1.9NCG membrane with decreasing temperature and membrane thickness. Under the premise of membrane mechanical strength, the (PL)1.9NCG membrane thickness was reduced to 0.6 mm in which the surface exchange plays a relative significant role in the oxygen separation process.
Figure 3. The Effects of the oxygen partial pressure in the feed side on the oxygen fluxes through the (PL) 1.9NCG membrane.
To further demonstrate the limitation step of the 0.6 mm (PL)1.9NCG membrane, another theory proposed by Lin et al. was applied [56]. For thin membrane (electron holes contribute to the electronic conductivity), the permeability was controlled by surface reaction, which can be described as [56]: 7
(
)
(5)
Where is the surface permeation constant. When the permeation process is limited by surface exchange, the oxygen flux would be linearly proportional to
. Therefore,
the oxygen fluxes were tested by changing the oxygen partial pressure in feed side (Figure 3). The oxygen permeability enhanced with the increasing oxygen partial pressure due to the increased oxygen gradient. For instance, 0.6 mL/min cm2 oxygen flux can be increased to 1.2 mL/min cm2 at 975 oC when the feeding oxygen concentrate increased from 0.1 to 0.6. Then these results were fitted by Eq. 5 as shown in Figure 4a. The fitted lines were linear at low temperatures and low oxygen partial pressure, while become curve with increasing temperature and oxygen gradient. Moreover, Jacobson et al. proposed that the oxygen fluxes should be linearly proportional with oxygen gradient (the concentration of oxygen carriers does not vary significantly) if the separation process was limited by bulk diffusion [57, 58]:
(6)
Where is related to the concentration and diffusivity of oxygen ions, thickness of membrane. Based on Eq. 6, the results in Figure 3 were also simulated as shown in Figure 4b. The oxygen fluxes showed no any linear relationship with the
, which illustrate the
transport step was not controlled by bulk diffusion. Combining abovementioned results (Figure 2 and 4), it can be concluded that the oxygen separation process through 0.6 mm (PL)1.9NCG membrane was largely limited by surface exchange, especially at low temperatures. Therefore, enhanced oxygen flux can be achieved through surface modification. 8
Figure 4. Relationship between oxygen fluxes and (a)
; (b)
through the 0.6 mm (PL)1.9NCG membrane.
Herein, a typical perovskite LSC was applied as the surface exchange catalyst [46, 47]. Figure 5a depicts the room temperature PXRD patterns of the LSC and (PL)1.9NCG samples after treated at 1000 oC for 10 h. The PXRD patterns of LSC are identified as single perovskite phase and that of (PL)1.9NCG is confirmed as K2NiF4 structure phase, no peaks other than abovementioned two phase have been observed on the PXRD patterns [44, 45]. The SEM images of the modified membrane are shown in Figure 5b. It could be distinguished that the coated LSC layer is porous with around 200 m thickness. The porosity of the LSC layer was come from the organics combustion, which can supply lots adsorption or desorption sites and increase surface area of the (PL)1.9NCG membrane [36, 59, 60].
9
Figure 5. (a) Powder XRD patterns of LSC and (PL)1.9NCG oxides after treated at 1000 oC for 10 h. The crystal structures of LSC and (PL)1.9NCG are Hexagonal and Tetragonal respectively; (b) SEM images of the cross-section of the LSC-coated (PL)1.9NCG membrane.
Oxygen Permeability of the Membranes. Figure 6a compares the oxygen fluxes of (PL)1.9NCG membrane with or without LSC catalyst coated. The sweep-side and both sweepand feed-sides coated (PL)1.9NCG membranes show significant enhancement in oxygen permeation fluxes while the feed-side coated one shows modest improvement. Figure 6b shows the dependence of the oxygen flux improvement through the LSC modified (PL)1.9NCG membranes on temperatures. The improvement percentage of both side coated membrane are higher than other two type membranes and decreases from 57% to 36% with increasing temperature as the relative controlling influence of bulk diffusion becomes more significant, similar results are also observed by other researchers [61]. The enhancement of sweep-side coated membrane are 51% at 800 oC and 25% at 975 oC, that are much higher than that of feed-side coated membrane which are around 5%. Similar results were reported in GdBaCo2O5+δ modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane, in which the oxygen permeation flux through air-side coating rise by 16%, while increase 23% with sweep-side modification [60]. Joo et al. also found a modest increase in the oxygen flux with a LSC feed-side coated dual-phase membrane, while an order of magnitude improvement was observed through the sweep-side coated membrane [40].
10
Figure 6. (a) Influence of temperature on the oxygen permeation fluxes; (b) Percentage improvement of oxygen fluxes of
LSC-coated membranes compared to the uncoated one at different temperatures.
This clearly indicates that the surface exchange at feed-side (oxygen-rich side) is not the limiting process through the 0.6 mm (PL)1.9NCG membrane and the oxygen flux is indeed controlled by oxygen surface exchange at sweep-side (oxygen-lean side). With this in mind, surface exchange properties of (PL)1.9NCG are estimated by thermogravimetric (TG) methods. When the atmosphere is changed between air and nitrogen, the material undergo a weight loss in oxygen release process and a weight gain in oxygen insertion process. As shown in Figure 7a, the weight of the (PL)1.9NCG sample decrease when atmosphere is switched from air to N2 (oxygen release) and increase when atmosphere is switched back from N2 to air (oxygen insertion) at different temperature. At all temperatures, the (PL)1.9NCG sample lose weight slowly in oxygen release process (oxygen association and desorption) and gain weight rapidly in oxygen insertion period (oxygen adsorption and disassociation), similar results are also found in other materials [38, 62, 63].
11
Figure 7. (a) Weight changes of the (PL)1.9NCG during air → N2 → air cycle at different temperatures; (b) The calculated surface-exchange coefficient ki and kr of (PL)1.9NCG at different temperatures.
From this TG curves, the oxygen insertion coefficient ki and the oxygen release coefficient kr can be calculated by the theoretical model proposed by Lin et al., relative equation was simplified as [62-64]: ( )
( )
( )
( )
(
)
(7)
Where the w(0) is the weight of material at the time of nitrogen switching to air (scale as t = 0) and w(e) is the steady-state weight of the material after atmosphere switching. The surface exchange rate coefficients ki and kr calculated by Eq. 7 are described in Figure 7b. The ki increases from 1.2 to 2.0 *10-6 cm.s-1 with increasing temperature from 800 oC to 950 oC, while the kr is around 0.15 *10-6 cm.s-1 in the tested temperature range. Most important, the values of ki are much larger than those of kr at different temperatures, which clearly indicated that the oxygen release is the limit of the (PL)1.9NCG of investigated thickness (0.6 mm), similar phenomenon are also found in other materials [38, 40, 64]. Therefore, it could be expected that the oxygen release on the sweep side is the rate limiting step in (PL)1.9NCG membrane and the modification on the sweep surface (oxygen release side) will result in 12
larger oxygen flux improvement than that on the feed surface (oxygen insertion side). This is consistent with the oxygen separation results shown in Figure 6.
Figure 8. (a) Long-term stability through the unmodified and LSC modified (PL) 1.9NCG membrane at different temperatures.32 (b) Post-XRD tests of the spent (PL)1.9NCG ceramic surfaces (strip the catalytic layer).
Figure 8a shows the long-term separation performance through the uncoated and LSC coated (PL)1.9NCG membranes at 900, 850 and 800 oC. In our previous report, the uncoated (PL)1.9NCG membrane (the model is described in Figure 9a) underwent a phase decomposition into Pr4Ni3O10-δ′ (a higher member of the Ruddlesden-Popper series) and praseodymium oxide phases under oxygen-containing atmosphere at intermediate temperature, so that its oxygen permeation fluxes decreased with time as also shown in Figure 8a [45]. For instance, the oxygen fluxes through the uncoated (PL)1.9NCG membrane decreased 1% at 900 oC, ca. 13% at 850 oC and ca. 43% at 800 oC over a period of 100 h due to the phase decomposition on feed side as shown in Figure 8b. The oxygen permeation fluxes at low temperatures decreased more than that at high temperature due to the lowering temperature result in more oxidative conditions (higher oxygen chemical potential) [45].
13
Figure 9. Models of the LSC coated (PL)1.9NCG membrane: a) uncoated; b) sweep-side coated; c) feed-side coated; d) both sides coated.
The oxygen fluxes declines through the sweep-side coated (PL)1.9NCG membrane (the model is described in Figure 9b) decrease ca. 1% at 850 oC and 6% at 800 oC of the initial permeation fluxes, which are slower than that of the uncoated one. The enhanced phase stability by the modification on the sweep surface might result from its higher oxygen permeability. During the oxygen permeation process, oxygen is partly permeated from feed side (decomposition side with high O2 chemical potential) to sweep side (low O2 chemical potential). Therefore, higher oxygen permeation fluxes lead to lower O2 chemical potential near the feed-side surface under the same operation condition as show in Figure 9b [1]. The feed side of the spent sweep-side coated membrane was also examined by XRD as shown in Figure 8b, which partially decomposed but not as serious as the uncoated one. Therefore, the 14
sweep-side coated (PL)1.9NCG membrane exhibits enhanced phase stability in oxygen separation process.
Moreover, the feed-side coated membrane shows more stable permeation fluxes even at 800 oC. When the feed side is modified, the catalytic layer reduces the oxygen chemical potential on the membrane surface as shown in Figure 9c. Therefore, the catalytic layer will be a protective layer to prevent the membrane being in direct contact with the high oxygen chemical potential atmosphere, so that the feed-side coated membrane did not decompose at low temperature as also demonstrated by XRD shown in Figure 8b. Therefore, the feed-side coated membrane shows better phase stability than the uncoated one. Similar approach was applied to improve the CO2 tolerance of 80 vol.% Ce0.9Gd0.1O2-δ-20 vol.% La0.7Sr0.3MnO3±δ dual-phase membrane, in which the membrane possessed enhanced stability after composite catalyst coating under pure CO2 [40]. When both sides of the membrane are modified with catalytic LSC layer (the model is described in Figure 9d), the advantages of the feed- and sweep-side coated membrane are combined that it possessed both higher oxygen permeation and better phase stability at intermediate temperature range under oxidative atmosphere, as shown in Figure 6 and Figure 8. Therefore, surface modification on different side possesses various influence on the oxygen permeability and stability. The enhancement of oxygen permeability and stability through the (PL)1.9NCG membrane reveal the surface-modification method was an effective strategy for turning the membrane separation performance, which could accelerate their practical applications in oxygen separation, membrane reactors and solid state fuel cell.
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4. CONCLUSIONS
The dependence of the oxygen permeability of (PL)1.9NCG on the membrane thickness indicates the surface exchange limited permeation fluxes with thinner membrane and suggests a possibility to improve the separation performance through coating. The enhancement of oxygen permeation fluxes through sweep-side coated membrane by a porous layer of LSC are 51% at 800 oC and 25% at 975 oC, which are higher than that through feed-side coated one (around 5%). Concluded from the TG results, the desorption rate of (PL)1.9NCG is much lower than the adsorption rate, which suggests that the surface exchange on the sweep side is the transport limiting process so that the coating on the sweep-side surface shows obvious enhancement on oxygen fluxes than the feed-side coated one. Moreover, the sweep-side coated membrane also shows slightly improved phase stability compared with the uncoated membrane due to its improved oxygen permeation flux. The feed-side coated membrane shows better phase stability not only than the uncoated membrane but also than the sweep-side coated membrane under the same conditions, which are resulted from the reducing oxygen chemical potential on the membrane surface by coating layer. Abovementioned results suggests that the sweep side is the permeability limitation side and the feed is the phase stability limitation side, so that the both side modified membrane possesses all the advantages of one side coating which shows 57 % enhanced and stable oxygen flux during the long-term test at 800 oC.
16
AUTHOR INFORMATION
Corresponding Author
* E-mail: [email protected]; [email protected].
Notes The authors declare no competing financial interest
ACKNOWLEDGMENT The Authors thank the financial support from National Natural Science Foundation of China (No. 21706076, 21536005, 51621001), Natural Science Foundation of Guangdong (2017A030310431,
2014A030312007),
Guangzhou
Technology
Project
(No.
201804010210), China Postdoctoral Science Foundation (No. 2018T110870) and the Deutsche Forschungsgemeinschaft (DFG, FE928/7-1).
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Highlights
Various influence of different side surface modification were observed;
Oxygen permeation fluxes was largely improved by surface coating on the sweep side;
Feed-side coated membranes possess better phase stability under the same conditions;
The improvements are explained in detail based on the oxygen transport mechanism.
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