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Composite mixed ionic-electronic conducting ceramic for intermediate temperature oxygen transport membrane
Ceramics International xxx (xxxx) xxx–xxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Composite mixed ionic-electronic conducting ceramic for intermediate temperature oxygen transport membrane ⁎
Ming-Wei Liao, Tai-Nan Lin , Wei-Xin Kao, Chun-Yen Yeh, Yu-Ming Chen, Hong-Yi Kuo Nuclear Fuels and Materials Division, Institute of Nuclear Energy Research, No.1000 Wunhua Rd., Jiaan Village, Longtan District, Taoyuan City 32546, Taiwan (R.O.C)
A R T I C L E I N F O
A BS T RAC T
Keywords: Membranes Composites Laser surface melting
The dense ceramic substrate formed by a mixed ionic-electronic conducting (MIEC) material can be used as an oxygen transport membrane (OTM), enabling the transport of high flux oxygen with certain selectivity and gas separation at high temperatures (800 ~ 900 °C). In recent years, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has been reported to be a promising MIEC material for oxygen permeation due to its relatively high oxygen ion conductivity at high temperatures. However, the catalytic efficiency of BSCF is relatively low among the MIEC materials, resulting in the dramatic decrease of oxygen permeation at temperatures below 800 °C. In the present study, a composite MIEC ceramic consisting of a BSCF substrate and the catalytic La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) layer has been proposed. A simple method of laser surface melting is executed to fabricate the composite oxygen transport membrane. The scanning electron microscope (SEM) investigations show that LSCF powders can be well-adherent to the BSCF surface after laser scanning melting process. The oxygen permeation flux reaches 0.5 ml min−1 cm−2 for pure BSCF membrane with thickness of 420 µm, while the BSCF membrane substrate with laser scanning LSCF exhibits substantial improvement on oxygen permeation up to 60% at 700 °C. The result suggests that the composite MIEC ceramic has significant potential for intermediate temperature oxygen transport membrane.
1. Introduction The greenhouse effect on the global warming becomes a critical issue nowadays and the urgent demand has arisen for reduction of CO2 emission. Oxy-fuel combustion by burning fossil fuels in pure oxygen offers easily capture of CO2 due to the simple flue gas composition without nitrogen [1]. However, the additional cost of supplying pure oxygen is unfavorable to the development of Oxy-fuel combustion. Oxygen transport membrane (OTM) has received increasing attention in recent years as a potential technology of oxygen separation from air [2-6]. Comparing to other traditional oxygen separation techniques such as pressure swing adsorption or cryogenic distillation, OTM can offer high purity oxygen, less power consumption, and capability of system integration. Typically, OTM consists of a dense mixed ionicelectronic conducting (MIEC) ceramic membrane with a perovskite structure which can provide excellent ionic and electronic conductivity, leading to attractive oxygen flux [7]. Among the MIEC materials, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has been reported to be a promising material for oxygen permeation due to its relatively high oxygen ion conductivity at high temperatures [8,9]. In general, the oxygen
⁎
permeation flux through an OTM can be improved by the reduction of membrane thickness. The oxygen permeation flux becomes surfaceexchange-speed-limited as the thickness is thin enough. However, the decrease of membrane thickness results in the lower mechanical strength and leads to an unexpected failure risk during operation. A few researches devoted to investigate the asymmetric OTM containing a thick (several hundred micrometers) porous supported substrate and a thin dense transport layer [10–12]. The thick porous supported substrate exhibits solid mechanical strength and provides more surface catalytic areas for oxygen reduction which is beneficial to surface exchange process of oxygen ion. On the other hand, it has been reported that the BSCF with relatively low catalytic efficiency among the MIEC materials is ideally utilized for single-chamber fuel-cell [13]. Thus, the operation temperature range is limited due to the dramatic decrease of oxygen permeation below 800 °C. Coating some metal catalysts on a BSCF membrane is a feasible solution for further improvement of oxygen permeation flux. For example, Ag nanoparticles which were considered as excellent catalysts have been applied on surface modification of BSCF [14]. A significant improvement of oxygen permeation flux by the Ag surface modification can be obtained,
Corresponding author. E-mail address: [email protected] (T.-N. Lin).
http://dx.doi.org/10.1016/j.ceramint.2017.05.222
0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liao, M.-W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.222
Ceramics International xxx (xxxx) xxx–xxx
M.-W. Liao et al.
especially at relatively low temperatures (700 ~ 800 °C). Although the positive effects of metal catalysts on oxygen flux enhancement of BSCF membranes have been confirmed, there are still some issues in real application including particle growth, diffusion, or oxidation at the selected operation temperature range. In a previous study, MIEC of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) was a potential cathode material for high performance solid oxide fuel cells (SOFCs) due to its good electronic and catalytic property [15]. As a candidate of OTM material, LSCF showed excellent chemical stability against carbonation in CO2 containing atmosphere [16–18] and even much higher O2 conversion efficiency than BSCF [13] despite of the lower ionic conductivity. The differences of surface exchange efficiency between BSCF and LSCF have been investigated [19–21]. In general, BSCF possessed more oxygen vacancy with lower surface exchange coefficient k, while LSCF showed a comparable surface exchange rate and even slightly higher k especially in the case of low partial pressure [20] or limited thickness [21]. To enhance the oxygen permeation flux at lower temperature, some groups discussed on the application of BSCF catalytic layer on the membrane [10,22]. However, very few studies have been focused on the effect of LSCF catalytic layer. It is thus of interest in our work to investigate the performance improvement which can be realized by combining the BSCF membrane support and a LSCF catalytic layer. Accordingly, in the present study we prepared a composite MIEC ceramic device consisting of a dense BSCF substrate and the catalytic LSCF layer. A simple method of laser surface melting is executed to bind the LSCF powders onto the BSCF surface. The thickness of OTM substrates was fixed to investigate the difference of oxygen permeation flux between pure BSCF and composite. An asymmetric OTM with similar thickness was compared to investigate the oxygen permeation flux in different operation temperatures.
Fig. 1. Schematic diagram of (a) Laser melting method for fabricating composite BSCF membranes with LSCF powder on the surface. (b) A setup of the measurement apparatus.
2. Material and methods of OTM is 2 cm2 which determined by the inner diameter of the tube. The Ag wire was used as gasket material. Following the sealing process at 960 °C, pure nitrogen and Ar sweep gas were fed in the system for leakage test. A gas chromatography (Micro GC 3000, INFICON) was used to analysis gas composition of the output gas. If the output gas containing the leakage of nitrogen which is less than 0.3%, the input gas was switched from nitrogen to air for oxygen separation measurement. The oxygen permeation flux can be determined by the flux and oxygen ratio of the Ar sweep gas.
The BSCF powders were synthesized using the glycine nitrate combustion (GNC) process. Stoichiometric amounts of Ba(NO3)2 (SHOWA, 99%), Sr(NO3)2 (Merck, 99%), Co(NO3)2·6H2O (SHOWA, 98%), and Fe(NO3)3·9H2O (Merck, 99%) were used as the starting raw materials. Metal nitrates were dissolved in distilled water and then the glycine was added to the solution. The mixture was heated on a hot plate, evaporated to a viscous gel, and ignited with a flame, resulting in a BSCF ash of a dark-gray color. The phase identification of BSCF was performed by X-ray diffraction (XRD, Bruker, D8 advance). The casting slurries were prepared by mixing BSCF powders, the dispersant, binder, plasticizer as well. An asymmetric BSCF membrane was fabricated by the slurry containing 6 w.t.% graphite powder (Alfa Aesar) as the pore former for the porous supported layer and the slurry for the dense membrane without any pore former. The porosity of porous substrates was determined by Archimedes method. Both the dense BSCF membrane and asymmetric BSCF membrane were prepared by tape casting process (ECS, Model CS-8). The green membrane was then subjected to a hot-press process via a laminator for several times. The laminated tapes were further sintered at 1100 °C for 4 h to form the BSCF membrane. The deposition of LSCF was performed by means of the CO2 Laser melting method (Universal, PLS6MW). First, the BSCF membranes were covered with commercial LSCF powders (KTX material). Then the optimized Laser power of 4 W with scanning speed of 200 mm/s were used to perform the binding of LSCF powders onto the BSCF membrane. The schematic diagram of the Laser melting method is shown in Fig. 1(a). The surface morphologies and microstructures of all BSCF membranes were examined by field emission scanning electron microscopy (FE-SEM, JEOL, 7010). In order to clarify the BSCF and LSCF in details, the composite OTM was further analyzed by energy dispersive spectroscopy (EDS, Oxford) for elements distribution. For measurement of oxygen permeation flux, the BSCF membranes were setup on an alumina tube of the testing stage (ProboStat, NorECs), as shown in Fig. 1(b). Therefore, the active area
3. Results and discussion 3.1. Microstructure and morphology of BSCF membrane The BSCF membranes were prepared by tape casting process, followed by sintering at 1100 °C for 4 h. Fig. 2 presents the SEM micrographs of BSCF membranes before and after laser scanning the LSCF powder on the membrane surface, respectively. In Fig. 2(a), it can be observed that the pure BSCF membrane without pore former shows a dense structure with the grain size of several micrometers, indicating that the gas-tightness of the membrane has been achieved. From Fig. 2(b), the melting result of LSCF on the BSCF surface can be achieved after the laser scanning process. A significant difference in surface morphology can be observed between the original BSCF and laser scanned region in Fig. 2(b). It shall be noted that the noncontinuous CO2 laser output is 500 point per inch (DPI) and the heateffect zone for each pulse is about the circle with a diameter of 100 µm. Therefore, the local heat effect region on the BSCF surface was melted and then solidified repeatedly during the laser scanning process. On the other hand, the LSCF powders were not to be influenced by laser due to its loose structure results in less accumulation and absorption of energy [23]. That is, Laser energy in LSCF powder was scattered and diffused to the air. Consequently, it is clearly seen that the LSCF powders were 2
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Fig. 2. Top-view SEM micrographs of sintered BSCF membranes (a) before Laser scanning (b) after Laser scanning process for combining BSCF and LSCF on the surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. EDS mapping results on the melted BSCF and LSCF region of membrane following the Laser scanning process.
bonded LSCF remains powder configuration without densification after the Laser melting process. These LSCF powders with particle sizes ranging from 0.6 to 1.5 µm provide extremely large specific surface area of ~ 12 m2/g, which is favorable to the oxygen permeation behavior. It can enhance the overall surface exchange efficiency. Actually for comparison, we executed the experiments of Laser melting BSCF powder as well. However, the applied Laser energy which was high enough to laser-melt the BSCF membrane could also cause the densification of the BSCF powder due to the lower sintering temperature of BSCF than LSCF. This is one reason that we choose the LSCF powder as the catalytic layer.
enveloped in re-solidified BSCF matrix regions after the laser scanning melting process to bind the LSCF powder on the BSCF surface. In order to confirm whether the covered powder are LSCF or not, the EDS mapping analysis has been performed in the laser scanning region. As shown in Fig. 3, the green color points representing lanthanum element can be determined as LSCF while the dark orange points representing barium element can be determined as BSCF. According to the SEM image and EDS mapping results, it can be seen the co-existence of BSCF matrix with the laser-scanning-melted LSCF. It is evident that a composite OTM of dense BSCF membrane substrate with the catalytic LSCF has been fabricated successfully. It is also noteworthy that the
3
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Fig. 4. Oxygen permeation flux measurement results of the dense BSCF membranes with and without Pt deposition on the surface.
Fig. 5. Oxygen permeation flux measurement results of the all BSCF membranes with thickness of 420 µm, thickness of 480 µm, asymmetric structure, and composite material.
3.2. Oxygen permeation flux measurement results
same membrane thickness of 420 µm, the composite BSCF membrane with laser-scanning-melted LSCF particles on the surface (as shown in the SEM image of Fig. 2) exhibits a slightly increasing oxygen permeation flux at 900 °C from 1.14 to 1.23 ml min−1 cm−2 and a significant increase at 700 °C from 0.51 to 0.80 ml min−1 cm−2. It is also noted that the correlation between oxygen permeation flux and temperature is transformed from linear-like to second order due to a coupled transport process. The oxygen atoms on the surface of composite membrane were reduced to oxygen ions by BSCF and LSCF, and then transferred through a dense BSCF membrane. As the temperature decreases, catalytic activity decreases rapidly much than the ionic transportation and LSCF catalytic layer provides large specific area to sustaining enough surface exchange efficiency [7]. Comparing to the metal catalyst, the LSCF catalytic layer which considered as a MIEC material can transport the oxygen ion directly thus has larger effective catalytic area. The significant increase in oxygen permeation flux at 700 °C can be thus attributed to the improvement of specific catalytic area caused by the LSCF powder, which is a consistent result as the reported literature for surface modification of activation layer [10,18,22]. In addition, a 480 µm asymmetric BSCF membrane which consists of 120 µm dense structure and 360 µm porous substrate with the porosity of 20.4% and the pore size of 5–10 µm was measured for comparison. It is seen that while the outstanding raise of oxygen permeation flux at 900 °C due to the significant decrease in thickness of dense layer, the performance at 700 °C is not improved. In fact, the porous substrates have to trade pore structure off against mechanical strength. For supporting purpose, the porosity and pore size are limited in the asymmetric membrane, implying the lower specific surface area. Therefore, a thick supporting porous layer with a limited specific surface area is not benefit to improve the oxygen permeation especially at low temperature.
For investigating the difference between the metal catalysts and MIEC catalysts in our composite OTM, the Pt deposition has been executed on a dense BSCF membrane for comparison. Fig. 4 shows the oxygen permeation flux measurement results of the dense BSCF membranes with and without Pt deposition on the surface. It is observed that the oxygen permeation flux of dense BSCF membrane without Pt deposition decreases with operating temperature from 900 to 700 °C. By contrast, the BSCF membrane with Pt deposition shows the slightly lower oxygen permeation flux at 900 °C but higher value at 700 °C despite of similar whole trend in the measuring temperature region. According to the Wagner equation [24]:
JO2 = −
RT 4F L 2 2
lnP"O2
∫lnP
σelσion d lnPO2 ′O2 σel + σion
The oxygen permeation flux is proportional to the operating temperature T and oxygen partial pressure PO2, but inversely proportional to the membrane thickness L. However, with the same thickness, temperature, and partial pressure between two BSCF membranes investigated in Fig. 4, the results imply that the variation of oxygen permeation flux by Pt deposition is mainly attributed to the surface exchange reaction. At the high temperature of 900 °C, the BSCF exhibits the excellent oxygen catalytic activity and the whole oxygen permeation flux is not limited by oxygen exchange reaction. Therefore, the Pt deposition blocks some oxygen conductive route and the oxygen permeation flux value decreases accordingly. As the temperature decreases, the oxygen catalytic activity of BSCF gradually decreases, resulting in a transformation from mass transfer control to surface exchange speed control. That is, the oxygen exchange reaction becomes a major factor to dominate the oxygen permeation flux at lower temperature. As a result, the BSCF membrane with Pt deposition exhibits an improvement in oxygen permeation flux at low temperature comparing to that without Pt deposition. However, degradation in oxygen permeation flux from 0.61 ml min−1 cm−2 to 0.52 ml min−1 cm−2 was observed in BSCF membrane with Pt deposition after operation for 1 h, suggesting the Pt layer was deterioration or vaporization. Fig. 5 presents the oxygen permeation flux measurement results of the BSCF membranes with different preparation conditions. For the dense BSCF membrane, it is shown that membrane of 420 µm thick exhibits a higher oxygen permeation flux than that of 480 µm at high temperature of 900 °C. The fact that the decrease of thickness results in an increase of oxygen permeation flux is consistent with Wagner equation. However the difference of oxygen permeation flux between these two samples (membrane of 420 µm and 480 µm) at 700 °C is nearly negligible, indicating that the oxygen exchange reaction dominates the permeation flux at lower temperature as discussed. With the
4. Conclusions Composite MIEC ceramic membrane consisting of a dense BSCF substrate and LSCF powder on the surface has been fabricated by tape casting and laser scanning melting method. From the surface morphology observation and EDS mapping analysis, the BSCF substrate was melted then re-solidified to bind the LSCF powder during the laser scanning process. The oxygen permeation flux measurement results show that the composite BSCF membrane exhibits a slightly increase at 900 °C from 1.14 to 1.23 ml min−1 cm−2 and a significant increase at 700 °C from 0.51 to 0.80 ml min−1 cm−2, compared to a dense BSCF membrane with equal thickness. The oxygen exchange reaction becomes a major factor to dominate the oxygen permeation flux at relatively low temperature of 700 °C. Thus, the laser-scanning-melted 4
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LSCF powder with high catalytic activity provides an alternative improvement to compensate the low oxygen exchange reaction of BSCF at 700 °C. The present works provide a feasible method for fabricating intermediate temperature oxygen transport membrane with a LSCF catalytic layer without additional sintering process.
[9]
[10]
Conflict of interest
[11]
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
[12]
Acknowledgements
[14]
This work is sponsored by Ministry of Science & Technology under Contract No. MOST 104-3111-Y042A-062.
[15]
[13]
[16]
References
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improvement of Ba0.5Sr0.5Co0.8Fe0.2Ox perovskite ceramic membranes, J. Membr. Sci. 369 (2011) 174–181. K. Efimov, Q. Xu, A. Feldhoff, Transmission electron microscopy study of Ba0.5Sr0.5Co0.8Fe0.2O3-delta perovskite decomposition at intermediate temperatures, Chem. Mater. 22 (2010) 5866–5875. S. Baumann, J.M. Serra, M.P. Lobera, S. Escolástico, F. Schulze-Küppers, W.A. Meulenberg, Ultrahigh oxygen permeation flux through supported Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes, J. Membr. Sci. 377 (2011) 198–205. Z.H. Chen, R. Ran, Z.P. Shao, H. Yu, J.C.D. da Costa, S.M. Liu, Further performance improvement of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite membranes for air separation, Ceram. Int. 35 (2009) 2455–2461. P. Lemes-Rachadel, G.S. Garcia, R.A. Francisco Machado, D. Hotza, J.C. Diniz da Costa, Current developments of mixed conducting membranes on porous substrates, Mater. Res.-Ibero-Am. J. Mater. 17 (2014) 242–249. Z. Shao, S.M. Haile, A high-performance cathode for the next generation of solidoxide fuel cells, Nature 431 (2004) 170–173. A. Leo, S. Liu, J.C.D. da Costa, The enhancement of oxygen flux on Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) hollow fibers using silver surface modification, J. Membr. Sci. 340 (2009) 148–153. A.P. Jamale, C.H. Bhosale, L.D. Jadhav, Electrochemical behavior of LSCF/GDC interface in symmetric cell: an application in solid oxide fuel cells, J. Alloy. Compd. 623 (2015) 136–139. J.H. Park, S. Do Park, Oxygen permeability and structural stability of La0.6Sr0.4Co0.2Fe0.8O3-δ membrane, Korean J. Chem. Eng. 24 (2007) 897–905. S. Gupta, M.K. Mahapatra, P. Singh, Lanthanum chromite based perovskites for oxygen transport membrane, Mat. Sci. Eng.: R: Rep. 90 (2015) 1–36. J.M. Serra, J. Garcia-Fayos, S. Baumann, F. Schulze-Küppers, W. Meulenberg, Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3−δ membranes, J. Membr. Sci. 447 (2013) 297–305. F.S. Baumann, J. Maier, J. Fleig, The polarization resistance of mixed conducting SOFC cathodes: a comparative study using thin film model electrodes, Solid State Ion. 179 (2008) 1198–1204. C. Niedrig, S.F. Wagner, W. Menesklou, S. Baumann, E. Ivers-Tiffée, Oxygen equilibration kinetics of mixed-conducting perovskites BSCF, LSCF, and PSCF at 900 °C determined by electrical conductivity relaxation, Solid State Ion. 283 (2015) 30–37. L. Almar, J. Szász, A. Weber, E. Ivers-Tiffée, Oxygen transport kinetics of mixed ionic-electronic conductors by coupling focused ion beam tomography and electrochemical impedance spectroscopy, J. Electrochem. Soc. 164 (2017) F289–F297. Y. Hayamizu, M. Kato, H. Takamura, Effects of surface modification on the oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane, J. Membr. Sci. 462 (2014) 147–152. A. Streek, P. Regenfuss, H. Exner, Fundamentals of energy conversion and dissipation in powder layers during laser micro sintering, Phys. Procedia 41 (2013) 858–869. W.K. Hong, G.M. Choi, Oxygen permeation of BSCF membrane with varying thickness and surface coating, J. Membr. Sci. 346 (2010) 353–360.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Composite mixed ionic-electronic conducting ceramic for intermediate temperature oxygen transport membrane ⁎
Ming-Wei Liao, Tai-Nan Lin , Wei-Xin Kao, Chun-Yen Yeh, Yu-Ming Chen, Hong-Yi Kuo Nuclear Fuels and Materials Division, Institute of Nuclear Energy Research, No.1000 Wunhua Rd., Jiaan Village, Longtan District, Taoyuan City 32546, Taiwan (R.O.C)
A R T I C L E I N F O
A BS T RAC T
Keywords: Membranes Composites Laser surface melting
The dense ceramic substrate formed by a mixed ionic-electronic conducting (MIEC) material can be used as an oxygen transport membrane (OTM), enabling the transport of high flux oxygen with certain selectivity and gas separation at high temperatures (800 ~ 900 °C). In recent years, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has been reported to be a promising MIEC material for oxygen permeation due to its relatively high oxygen ion conductivity at high temperatures. However, the catalytic efficiency of BSCF is relatively low among the MIEC materials, resulting in the dramatic decrease of oxygen permeation at temperatures below 800 °C. In the present study, a composite MIEC ceramic consisting of a BSCF substrate and the catalytic La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) layer has been proposed. A simple method of laser surface melting is executed to fabricate the composite oxygen transport membrane. The scanning electron microscope (SEM) investigations show that LSCF powders can be well-adherent to the BSCF surface after laser scanning melting process. The oxygen permeation flux reaches 0.5 ml min−1 cm−2 for pure BSCF membrane with thickness of 420 µm, while the BSCF membrane substrate with laser scanning LSCF exhibits substantial improvement on oxygen permeation up to 60% at 700 °C. The result suggests that the composite MIEC ceramic has significant potential for intermediate temperature oxygen transport membrane.
1. Introduction The greenhouse effect on the global warming becomes a critical issue nowadays and the urgent demand has arisen for reduction of CO2 emission. Oxy-fuel combustion by burning fossil fuels in pure oxygen offers easily capture of CO2 due to the simple flue gas composition without nitrogen [1]. However, the additional cost of supplying pure oxygen is unfavorable to the development of Oxy-fuel combustion. Oxygen transport membrane (OTM) has received increasing attention in recent years as a potential technology of oxygen separation from air [2-6]. Comparing to other traditional oxygen separation techniques such as pressure swing adsorption or cryogenic distillation, OTM can offer high purity oxygen, less power consumption, and capability of system integration. Typically, OTM consists of a dense mixed ionicelectronic conducting (MIEC) ceramic membrane with a perovskite structure which can provide excellent ionic and electronic conductivity, leading to attractive oxygen flux [7]. Among the MIEC materials, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has been reported to be a promising material for oxygen permeation due to its relatively high oxygen ion conductivity at high temperatures [8,9]. In general, the oxygen
⁎
permeation flux through an OTM can be improved by the reduction of membrane thickness. The oxygen permeation flux becomes surfaceexchange-speed-limited as the thickness is thin enough. However, the decrease of membrane thickness results in the lower mechanical strength and leads to an unexpected failure risk during operation. A few researches devoted to investigate the asymmetric OTM containing a thick (several hundred micrometers) porous supported substrate and a thin dense transport layer [10–12]. The thick porous supported substrate exhibits solid mechanical strength and provides more surface catalytic areas for oxygen reduction which is beneficial to surface exchange process of oxygen ion. On the other hand, it has been reported that the BSCF with relatively low catalytic efficiency among the MIEC materials is ideally utilized for single-chamber fuel-cell [13]. Thus, the operation temperature range is limited due to the dramatic decrease of oxygen permeation below 800 °C. Coating some metal catalysts on a BSCF membrane is a feasible solution for further improvement of oxygen permeation flux. For example, Ag nanoparticles which were considered as excellent catalysts have been applied on surface modification of BSCF [14]. A significant improvement of oxygen permeation flux by the Ag surface modification can be obtained,
Corresponding author. E-mail address: [email protected] (T.-N. Lin).
http://dx.doi.org/10.1016/j.ceramint.2017.05.222
0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liao, M.-W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.222
Ceramics International xxx (xxxx) xxx–xxx
M.-W. Liao et al.
especially at relatively low temperatures (700 ~ 800 °C). Although the positive effects of metal catalysts on oxygen flux enhancement of BSCF membranes have been confirmed, there are still some issues in real application including particle growth, diffusion, or oxidation at the selected operation temperature range. In a previous study, MIEC of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) was a potential cathode material for high performance solid oxide fuel cells (SOFCs) due to its good electronic and catalytic property [15]. As a candidate of OTM material, LSCF showed excellent chemical stability against carbonation in CO2 containing atmosphere [16–18] and even much higher O2 conversion efficiency than BSCF [13] despite of the lower ionic conductivity. The differences of surface exchange efficiency between BSCF and LSCF have been investigated [19–21]. In general, BSCF possessed more oxygen vacancy with lower surface exchange coefficient k, while LSCF showed a comparable surface exchange rate and even slightly higher k especially in the case of low partial pressure [20] or limited thickness [21]. To enhance the oxygen permeation flux at lower temperature, some groups discussed on the application of BSCF catalytic layer on the membrane [10,22]. However, very few studies have been focused on the effect of LSCF catalytic layer. It is thus of interest in our work to investigate the performance improvement which can be realized by combining the BSCF membrane support and a LSCF catalytic layer. Accordingly, in the present study we prepared a composite MIEC ceramic device consisting of a dense BSCF substrate and the catalytic LSCF layer. A simple method of laser surface melting is executed to bind the LSCF powders onto the BSCF surface. The thickness of OTM substrates was fixed to investigate the difference of oxygen permeation flux between pure BSCF and composite. An asymmetric OTM with similar thickness was compared to investigate the oxygen permeation flux in different operation temperatures.
Fig. 1. Schematic diagram of (a) Laser melting method for fabricating composite BSCF membranes with LSCF powder on the surface. (b) A setup of the measurement apparatus.
2. Material and methods of OTM is 2 cm2 which determined by the inner diameter of the tube. The Ag wire was used as gasket material. Following the sealing process at 960 °C, pure nitrogen and Ar sweep gas were fed in the system for leakage test. A gas chromatography (Micro GC 3000, INFICON) was used to analysis gas composition of the output gas. If the output gas containing the leakage of nitrogen which is less than 0.3%, the input gas was switched from nitrogen to air for oxygen separation measurement. The oxygen permeation flux can be determined by the flux and oxygen ratio of the Ar sweep gas.
The BSCF powders were synthesized using the glycine nitrate combustion (GNC) process. Stoichiometric amounts of Ba(NO3)2 (SHOWA, 99%), Sr(NO3)2 (Merck, 99%), Co(NO3)2·6H2O (SHOWA, 98%), and Fe(NO3)3·9H2O (Merck, 99%) were used as the starting raw materials. Metal nitrates were dissolved in distilled water and then the glycine was added to the solution. The mixture was heated on a hot plate, evaporated to a viscous gel, and ignited with a flame, resulting in a BSCF ash of a dark-gray color. The phase identification of BSCF was performed by X-ray diffraction (XRD, Bruker, D8 advance). The casting slurries were prepared by mixing BSCF powders, the dispersant, binder, plasticizer as well. An asymmetric BSCF membrane was fabricated by the slurry containing 6 w.t.% graphite powder (Alfa Aesar) as the pore former for the porous supported layer and the slurry for the dense membrane without any pore former. The porosity of porous substrates was determined by Archimedes method. Both the dense BSCF membrane and asymmetric BSCF membrane were prepared by tape casting process (ECS, Model CS-8). The green membrane was then subjected to a hot-press process via a laminator for several times. The laminated tapes were further sintered at 1100 °C for 4 h to form the BSCF membrane. The deposition of LSCF was performed by means of the CO2 Laser melting method (Universal, PLS6MW). First, the BSCF membranes were covered with commercial LSCF powders (KTX material). Then the optimized Laser power of 4 W with scanning speed of 200 mm/s were used to perform the binding of LSCF powders onto the BSCF membrane. The schematic diagram of the Laser melting method is shown in Fig. 1(a). The surface morphologies and microstructures of all BSCF membranes were examined by field emission scanning electron microscopy (FE-SEM, JEOL, 7010). In order to clarify the BSCF and LSCF in details, the composite OTM was further analyzed by energy dispersive spectroscopy (EDS, Oxford) for elements distribution. For measurement of oxygen permeation flux, the BSCF membranes were setup on an alumina tube of the testing stage (ProboStat, NorECs), as shown in Fig. 1(b). Therefore, the active area
3. Results and discussion 3.1. Microstructure and morphology of BSCF membrane The BSCF membranes were prepared by tape casting process, followed by sintering at 1100 °C for 4 h. Fig. 2 presents the SEM micrographs of BSCF membranes before and after laser scanning the LSCF powder on the membrane surface, respectively. In Fig. 2(a), it can be observed that the pure BSCF membrane without pore former shows a dense structure with the grain size of several micrometers, indicating that the gas-tightness of the membrane has been achieved. From Fig. 2(b), the melting result of LSCF on the BSCF surface can be achieved after the laser scanning process. A significant difference in surface morphology can be observed between the original BSCF and laser scanned region in Fig. 2(b). It shall be noted that the noncontinuous CO2 laser output is 500 point per inch (DPI) and the heateffect zone for each pulse is about the circle with a diameter of 100 µm. Therefore, the local heat effect region on the BSCF surface was melted and then solidified repeatedly during the laser scanning process. On the other hand, the LSCF powders were not to be influenced by laser due to its loose structure results in less accumulation and absorption of energy [23]. That is, Laser energy in LSCF powder was scattered and diffused to the air. Consequently, it is clearly seen that the LSCF powders were 2
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Fig. 2. Top-view SEM micrographs of sintered BSCF membranes (a) before Laser scanning (b) after Laser scanning process for combining BSCF and LSCF on the surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. EDS mapping results on the melted BSCF and LSCF region of membrane following the Laser scanning process.
bonded LSCF remains powder configuration without densification after the Laser melting process. These LSCF powders with particle sizes ranging from 0.6 to 1.5 µm provide extremely large specific surface area of ~ 12 m2/g, which is favorable to the oxygen permeation behavior. It can enhance the overall surface exchange efficiency. Actually for comparison, we executed the experiments of Laser melting BSCF powder as well. However, the applied Laser energy which was high enough to laser-melt the BSCF membrane could also cause the densification of the BSCF powder due to the lower sintering temperature of BSCF than LSCF. This is one reason that we choose the LSCF powder as the catalytic layer.
enveloped in re-solidified BSCF matrix regions after the laser scanning melting process to bind the LSCF powder on the BSCF surface. In order to confirm whether the covered powder are LSCF or not, the EDS mapping analysis has been performed in the laser scanning region. As shown in Fig. 3, the green color points representing lanthanum element can be determined as LSCF while the dark orange points representing barium element can be determined as BSCF. According to the SEM image and EDS mapping results, it can be seen the co-existence of BSCF matrix with the laser-scanning-melted LSCF. It is evident that a composite OTM of dense BSCF membrane substrate with the catalytic LSCF has been fabricated successfully. It is also noteworthy that the
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Fig. 4. Oxygen permeation flux measurement results of the dense BSCF membranes with and without Pt deposition on the surface.
Fig. 5. Oxygen permeation flux measurement results of the all BSCF membranes with thickness of 420 µm, thickness of 480 µm, asymmetric structure, and composite material.
3.2. Oxygen permeation flux measurement results
same membrane thickness of 420 µm, the composite BSCF membrane with laser-scanning-melted LSCF particles on the surface (as shown in the SEM image of Fig. 2) exhibits a slightly increasing oxygen permeation flux at 900 °C from 1.14 to 1.23 ml min−1 cm−2 and a significant increase at 700 °C from 0.51 to 0.80 ml min−1 cm−2. It is also noted that the correlation between oxygen permeation flux and temperature is transformed from linear-like to second order due to a coupled transport process. The oxygen atoms on the surface of composite membrane were reduced to oxygen ions by BSCF and LSCF, and then transferred through a dense BSCF membrane. As the temperature decreases, catalytic activity decreases rapidly much than the ionic transportation and LSCF catalytic layer provides large specific area to sustaining enough surface exchange efficiency [7]. Comparing to the metal catalyst, the LSCF catalytic layer which considered as a MIEC material can transport the oxygen ion directly thus has larger effective catalytic area. The significant increase in oxygen permeation flux at 700 °C can be thus attributed to the improvement of specific catalytic area caused by the LSCF powder, which is a consistent result as the reported literature for surface modification of activation layer [10,18,22]. In addition, a 480 µm asymmetric BSCF membrane which consists of 120 µm dense structure and 360 µm porous substrate with the porosity of 20.4% and the pore size of 5–10 µm was measured for comparison. It is seen that while the outstanding raise of oxygen permeation flux at 900 °C due to the significant decrease in thickness of dense layer, the performance at 700 °C is not improved. In fact, the porous substrates have to trade pore structure off against mechanical strength. For supporting purpose, the porosity and pore size are limited in the asymmetric membrane, implying the lower specific surface area. Therefore, a thick supporting porous layer with a limited specific surface area is not benefit to improve the oxygen permeation especially at low temperature.
For investigating the difference between the metal catalysts and MIEC catalysts in our composite OTM, the Pt deposition has been executed on a dense BSCF membrane for comparison. Fig. 4 shows the oxygen permeation flux measurement results of the dense BSCF membranes with and without Pt deposition on the surface. It is observed that the oxygen permeation flux of dense BSCF membrane without Pt deposition decreases with operating temperature from 900 to 700 °C. By contrast, the BSCF membrane with Pt deposition shows the slightly lower oxygen permeation flux at 900 °C but higher value at 700 °C despite of similar whole trend in the measuring temperature region. According to the Wagner equation [24]:
JO2 = −
RT 4F L 2 2
lnP"O2
∫lnP
σelσion d lnPO2 ′O2 σel + σion
The oxygen permeation flux is proportional to the operating temperature T and oxygen partial pressure PO2, but inversely proportional to the membrane thickness L. However, with the same thickness, temperature, and partial pressure between two BSCF membranes investigated in Fig. 4, the results imply that the variation of oxygen permeation flux by Pt deposition is mainly attributed to the surface exchange reaction. At the high temperature of 900 °C, the BSCF exhibits the excellent oxygen catalytic activity and the whole oxygen permeation flux is not limited by oxygen exchange reaction. Therefore, the Pt deposition blocks some oxygen conductive route and the oxygen permeation flux value decreases accordingly. As the temperature decreases, the oxygen catalytic activity of BSCF gradually decreases, resulting in a transformation from mass transfer control to surface exchange speed control. That is, the oxygen exchange reaction becomes a major factor to dominate the oxygen permeation flux at lower temperature. As a result, the BSCF membrane with Pt deposition exhibits an improvement in oxygen permeation flux at low temperature comparing to that without Pt deposition. However, degradation in oxygen permeation flux from 0.61 ml min−1 cm−2 to 0.52 ml min−1 cm−2 was observed in BSCF membrane with Pt deposition after operation for 1 h, suggesting the Pt layer was deterioration or vaporization. Fig. 5 presents the oxygen permeation flux measurement results of the BSCF membranes with different preparation conditions. For the dense BSCF membrane, it is shown that membrane of 420 µm thick exhibits a higher oxygen permeation flux than that of 480 µm at high temperature of 900 °C. The fact that the decrease of thickness results in an increase of oxygen permeation flux is consistent with Wagner equation. However the difference of oxygen permeation flux between these two samples (membrane of 420 µm and 480 µm) at 700 °C is nearly negligible, indicating that the oxygen exchange reaction dominates the permeation flux at lower temperature as discussed. With the
4. Conclusions Composite MIEC ceramic membrane consisting of a dense BSCF substrate and LSCF powder on the surface has been fabricated by tape casting and laser scanning melting method. From the surface morphology observation and EDS mapping analysis, the BSCF substrate was melted then re-solidified to bind the LSCF powder during the laser scanning process. The oxygen permeation flux measurement results show that the composite BSCF membrane exhibits a slightly increase at 900 °C from 1.14 to 1.23 ml min−1 cm−2 and a significant increase at 700 °C from 0.51 to 0.80 ml min−1 cm−2, compared to a dense BSCF membrane with equal thickness. The oxygen exchange reaction becomes a major factor to dominate the oxygen permeation flux at relatively low temperature of 700 °C. Thus, the laser-scanning-melted 4
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LSCF powder with high catalytic activity provides an alternative improvement to compensate the low oxygen exchange reaction of BSCF at 700 °C. The present works provide a feasible method for fabricating intermediate temperature oxygen transport membrane with a LSCF catalytic layer without additional sintering process.
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Conflict of interest
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We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Acknowledgements
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This work is sponsored by Ministry of Science & Technology under Contract No. MOST 104-3111-Y042A-062.
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