Enhanced Oxygen Permeation Behavior of Ba0.5Sr0.5Co0.8Fe0.2O3

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Enhanced oxygen permeation behavior of BSCF membranes in CO2–containing atmosphere with SDC functional shell Kun Zhang, Chi Zhang, Ling Zhao, Bo Meng, Jian Liu, and Shaomin Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02218 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Enhanced oxygen permeation behavior of BSCF membranes in CO2–containing atmosphere with SDC functional shell Kun Zhang,† Chi Zhang,† Ling Zhao,‡ Bo Meng,§ Jian Liu†, Shaomin Liu*,† †

Department of Chemical Engineering, Curtin University, Perth WA 6845, Australia

‡

Department of Material Science and Chemistry, CUG-AU Institute of Function Materials,

China University of Geosciences, Wuhan 430074, China §

School of Chemical Engineering, Shandong University of Technology, Zibo255049, China

ABSTRACT: The deployment of clean energy technologies has faced an uphill battle to reduce the cost. Ion conducting membranes for cost-effective oxygen production help to overcome this bottleneck. The existing high performance perovskite membrane such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is featured with short running life due to the low material stability; surface decoration with robust ion conducting layer is one of the important strategies for improvement. To this purpose, in this work, an ultrathin dense Sm0.2Ce0.8O1.9 (SDC) with approximate 100 nm thickness has been successfully coated on BSCF perovskite membrane surface for highly efficient oxygen production. Compared with the pristine BSCF membrane, this new modified structure offers the enhanced performance of oxygen flux due to the better surface exchange kinetics. Most importantly, the long term oxygen permeation test under CO2 atmosphere shows that the SDC-shell protected BSCF membrane has improved stability comparable with pure BSCF membrane. 1 ACS Paragon Plus Environment

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1. INTRODUCTION In the 21st century, the energy consumption is getting increasing on an unprecedented rate to satisfy the growth of world economy and population. Our contemporary society is facing serious energy and environment problems. Currently, the main energy production method is based on the conventional fossil fuel combustion power system. However, these fossil fuel sources like oil, coal or others will not last forever but are gradually running out. Moreover, the over reliance of fossil fuel is polluting our planet with its side product-greenhouse gases (i.e. CO2) that are capturing heat from the sun in a dangerous level and leading to severe climate change. As an alternative, much faith has been placed onto renewable sources of energy. There has been the widespread public opinion that green energy can replace fossil energy to absolutely solve the energy and environmental issues. Contrary to public perceptions, renewable energy is not the silver bullet that will soon solve all our problems. Despite of the great progress in wind or solar power, the world has to continue the usage of fossil fuels with higher CO2 emission to meet the immediate requirement in energy demand growth for the next 50 years, especially from developing countries. It is forecasted that even with major technological breakthrough, renewable energy could only make up for 30% energy supply by the middle of this century.1,2 Therefore, it is urgent to improve energy efficiency and lower the waste streams, especially for energy-intensive chemical industry. As it seems, there is no easy way to solve this problem, which must be challenged with a clever combination of all possible solutions. Membrane reactors combining the reaction and separation in one unit hold many advantages for chemical reaction processes.3-10 The production process can be intensified by the in-situ separation effect from the membrane, which can cause higher conversion of reactants and higher selectivity of products. With the help of the membrane technology, the energy efficiency can be dramatically improved with low emission due to the increased selectivity to desired products. 2 ACS Paragon Plus Environment

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Among the various membranes, dense ceramic mixed conducting oxygen permeable membrane,11-15 which can be used in coal gasification or oxy-fuel combustion power station system with CO2 sequestration, have received much interest due to their great potential to cut down the oxygen production cost as most of clean energy technologies need oxygen as the feed gas. By integrating these ceramic membranes which exhibit both ionic and electronic conducting ability to obtain pure oxygen from air into coal power station can reduce energy cost by 35% or more compared to these conventional oxygen processes.16-18 In these membrane processes, ceramic membranes usually have to be exposed into certain extremely hostile operating conditions, such as erosive environment and high temperature and pressure. Therefore, the development of highly efficient and stable oxygen permeation membrane materials is the prerequisite for the practical application. Pioneering work on La1-xSrxCo1yFeyO3-δ

by Teraoka et al. provided the first example of high oxygen permeation fluxes from

mixed conducting perovskite membranes.19 Subsequently, much research effort from the scientific community has been placed in this area and many other membrane materials with different oxygen transport properties have been reported. Among these reported membranes, an exciting membrane of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) has been developed,20,21 which shows surprisingly high oxygen flux rates at intermediate-high temperatures. This material has been extensively studied as the oxygen separation membranes under different operating conditions and scaling up considerations.22-25 However, it is hard for perovskite containing alkaline-earth metals to survive from the attack by H2O, H2, CO2 or SO2 erosive gas due to the reactions between the perovskite and these gases .26-30 Review of all the perovskite membranes for O2 separation implies that a good ceramic membrane addressing all the application criteria is very rare; there is always a balance between the chemical stability and oxygen flux with improvement in one property but to the detriment of the other property.11 The current work is not the case, which provides a good strategy to use the robust oxygen ion

3 ACS Paragon Plus Environment

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conducting ceramic layer as the shell to protect the relatively weak perovskite membrane surface for performance improvement not only in the flux rate but also in the stability. This surface modification method has long been applied in previous research to increase the surface exchange rate of oxygen permeable membranes.31-33 In our previous research, noble metal such as silver or platinum was decorated on the SDC membrane surface to form short circuit providing the electron transport necessary for oxygen permeation via mixed conduction concept.34 Inspired by this research, other works on surface modification topic using different short-circuited membrane designs have been carried out. For example, mixed ionic-electronic conducting perovskite with excellent oxygen-exchange kinetics is used to replace expensive noble metal on the SDC or GDC membranes.35-36 Recently, modified surface decorations such as tunable segmented structures and dual-phase surface coating have also been developed to further improve the oxygen permeation flux rates and the stability of membranes.37-38 In particular, Caro’s group published an interesting paper using the Nafion® membrane for hydrogen separation via a similar short-circuit design without applying an electrical voltage for hydrogen separation at room temperature.39 In this paper, we propose to use a Sm0.2Ce0.8O1.9 (SDC) functional layer coated on BSCF membrane to improve its stability performance. The SDC functional shell can be easily made on the BSCF membrane surface using the precursor solution followed by the thermal treatment, which can greatly strengthen BSCF membrane stability against CO2 acid gas, while with improved oxygen permeation flux. This technique is versatile and can be expanded to other perovskite membranes for performance enhancement. 2. EXPERIMENTAL SECTION Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) composites oxides were synthesized by a combined EDTAcitrate complexing sol-gel process. Ba(NO3)2, Sr(NO3)2, Co(NO3)2·6H2O and Fe(NO3)3·9H2O (all in A.R. grade) were applied as the raw materials for the metal-ion sources. For the precise 4 ACS Paragon Plus Environment

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controlling of the doping level, the metal nitrates were first made into aqueous solutions (~1 M) separately with their precise concentrations determined by standard EDTA titration technique. The required amount of metal nitrates in the state of aqueous solution were then prepared into a mixed solution according to the aimed composition, followed by the introduction of EDTA and citric acid as the complexing agents at the mole ratio of total metal ions: EDTA: citric acid=1 : 1 : 2. The pH value of the system was controlled at ~ 7 with the help of NH3·H2O. Continuous heating at 90 oC resulted in evaporating water from the solution until a transparent purple gel was obtained, which was pre-fired at 250 oC and followed by calcination at 950 oC in air for 5 h to get the products with the final composition and phase structure. For the fabrication of dense membranes, the as-synthesized BSCF oxide powders were pressed into disk-shape membranes in a stainless steel mold (15.0 mm in diameter) under a hydraulic pressure of approximately 1.5×106 Pa. These green membranes were further sintered in an electrical box furnace at 1100 oC in air for 5 h at a heating /cooling rate of 1-2 o

C min-1. Sm0.2Ce0.8O1.9 (SDC) was introduced onto the surface of BSCF membrane with a

dip-coating process using nitrate solutions, which were prepared from individual nitrates. The concentration was 0.3 M of the total metal ions. The coating was carried out by dipping the BSCF membrane into the solution, drawing slowing up and sintering at 800 oC for 2 h. Scanning Electron Microscopy (SEM) images were obtained using a Zesis EVO 40XVP at an accelerating voltage of 15 kV. The electrical conductivity relaxation (ECR) test was conducted by measuring the conductivity in a process of an abrupt step change in oxygen partial pressure around the samples, which can be created by introducing mixtures of N2 and O2 of known oxygen content. Permeation properties of the membranes were investigated by the gas chromatography (GC) method using a high-temperature oxygen permeation apparatus. A silver paste was used


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as the sealant to fix the membrane disk onto a dense quartz tube and exposed an effective area of ~ 0.45 cm2 at the sweep side for permeation study. Helium was applied as the sweep gas to create an oxygen partial pressure gradient across the membrane, which also acted as the carrier gas to bring the permeated oxygen to a Shimadzu 2014A equipped with a 5Å capillary molecule column and thermal conductivity detector for quantitative oxygen concentration analysis. The oxygen permeation flux was calculated by: 1

−2

−1

Jo 2 (ml cm min ) = [C O 2

0.21  28  2 F − C N2 × ×   ]× 0.79  32  S

(1)

Where CO2 and CN2 are the measured concentrations of oxygen and nitrogen in the gas on the sweep side, respectively (mol ml-1), F is the flow rate of the exit gas on the sweep side (ml min-1), and S is the membrane geometric surface area of the sweep side (cm2). 3. RESULTS AND DISCUSSION A typical X-ray diffraction (XRD) pattern of SDC coated BSCF membrane is depicted in Figure 1. The peaks can be clearly indexed into two sets of characteristic peaks: BSCF phase and SDC phase.20,34 The XRD pattern indicates that SDC coating layer can be formed with the simple dip-coating process. No additional peaks could be observed, suggesting the absence of reaction between the samarium and cerium nitrate with BSCF membrane during the heating process.


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Figure 1. XRD pattern of BSCF membrane dip-coated with SDC shell (*: BSCF, #: SDC)

Figure 2 is the SEM and TEM images for BSCF membrane with SDC protective layer. It can be seen from Figure 2A that the dense pure BSCF membrane is fabricated after sintering. The grain sizes vary from 10 ~ 25 µm. Meanwhile, the morphology of BSCF membrane surface loaded with SDC thin layer is obtained. As shown in Figure 2B, an obvious SDC shell covering on the surface of BSCF membrane can be observed. Looking through the SDC layer, the grain boundary of BSCF membrane can still be clearly identified, which implies that the SDC shell has very thin thickness. It should be noted that there are some cracks (marked by red arrows in Figure 2B) appearing on the SDC shell after the thermal treatment, suggesting that the SDC layer cannot totally cover the BSCF membrane surface. This phenomenon probably results from the mismatch of sintering properties between SDC and BSCF, i.e. different thermal expansion coefficients.


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10 µm

10 µm

200 nm

2 nm

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Figure 2. SEM and TEM images for BSCF membrane with SDC protective layer: (A) SEM image of pure BSCF membrane surface. (B) SEM image of BSCF membrane surface with SDC layer. (C, D) TEM images of BSCF membrane cross section with SDC layer.

oxide. However, it is interesting to find that the every single BSCF grain on the sintered membrane surface is fully occupied by the SDC shell, leaving cracks along the grain boundary area on the BSCF membrane surface. According to TEM image (Figure 2C), the SDC shell can be confined down to nano-size with the thickness of around 100 nm. In addition, SDC layer adheres well to the BSCF disk membrane, which can be further evidenced by Figure 2D. Figure 3 demonstrates the oxygen permeation fluxes through 1 mm-thick BSCF membranes with SDC functional shell. Oxygen partial pressure on the oxygen rich side atmosphere was maintained at 0.21 atm by applying ambient air as the feed gas, while a constant helium sweep rate of 100 ml min-1 was applied to the other side of the membrane


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surface to create a medium oxygen pressure gradient across the membrane. After applying the SDC thin layer on the membrane surface, the oxygen flux through BSCF membrane experienced an obvious increase from ~3.2 to 3.7 ml cm-2 min-1 at 900 oC. Over the entire test temperature range between 600 and 900 oC, there is a steady increase of oxygen flux through BSCF membrane with SDC shell as compared to pure BSCF membrane. This improvement shows that coating the SDC thin layer on the BSCF membrane surface may have a positive effect on its oxygen permeation ability. As well known, SDC is a typical oxygen ion conducting oxide which has been widely used as the electrolyte for solid oxide fuel cell due to its excellent oxygen ion transport ability at intermediate-low temperatures.40 For oxygen permeable membranes, it is necessary to have sufficient electronic conducting ability for oxygen reduction or oxidation to take place on the membrane surface. The reactions occurring on the surface involves the following processes:

1 O + V ⋅⋅ + 2 e⋅ → O× (feed side) O 2 2 O Oo× → 1 O2 + VO⋅⋅ + 2e⋅ (permeate side) 2

(2) (3)

These surface reactions indicate that electron conduction is required to guarantee the oxygen exchange reactions on the surface to transfer the molecular oxygen to lattice oxygen (feed side) or oxygen ion reduction to release the molecular oxygen (permeate side). Therefore, it sounds like a mission impossible for pure oxygen ion conductors to play as oxygen permeation membranes without any external circuit for electron transport. However, certain electronic conductivity can be observed on SDC under the operation temperatures above 600 o

C due to the thermal reduction from Ce4+ to Ce3+.41 This thermal reduction causes the

reduction of the open-circuit voltage and an energy loss, which has to be avoided in the application of SDC electrolyte in SOFCs. This thermal-sensitive electronic conductivity of SDC provides the possibility that it can be used as surface modification material for the mixed conducting BSCF oxygen permeation membrane at high temperatures (> 600 oC).42,43 9 ACS Paragon Plus Environment

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On the other hand, the thickness of SDC shell applied on the BSCF membrane is very small, which can be as thin as less than one hundred nano-meter. It was found that the electronic conductivity could be enhanced with reducing the grain size of SDC oxide down to the nanolevel.44 Thus, it is reasonable to believe that this thin SDC shell can offer sufficient electrons for the oxygen reduction on the BSCF membrane at high temperatures. Meanwhile, it has been reported that SDC shows fast oxygen transport ability at relative lower temperatures, which is qualified for the electrolyte membrane with high ionic conductivity operated at the temperatures down to 500 oC. Therefore, the introduction of SDC thin shell is favorable for the oxygen permeation process through BSCF dense membrane.

Figure 3. Oxygen flux through BSCF membrane with/without SDC layer

It is well known that oxygen permeation ability of a dense ceramic membrane is closely related to its oxygen bulk transport rate and surface exchange kinetics. In our case, the improvement of oxygen flux through BSCF membrane with SDC thin layer may be resulted from enhanced oxygen exchange rate on the membrane surface. To further investigate the oxygen surface reaction shell, ECR technique was applied to identify the surface exchange 10 ACS Paragon Plus Environment

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coefficients of BSCF membrane with SDC shell. Figure 4 exhibits the typical transient conductivity response between 600 and 800 oC upon abruptly changing the oxygen partial pressure from 0.05 to 0.25 atm and the temperature dependence of the fitted kex of BSCF membrane with SDC shell. As can be clearly seen in Figure 4, the BSCF membrane with SDC shell has much faster oxygen exchange reaction process as compared to pure BSCF membrane. The thickness of the sample is around 0.6 mm. From the temperature dependence of surface exchange coefficients, a steady rise of kex of BSCF membrane with SDC shell was observed within the test temperature range. For example, at 750 oC, the value of kex increased from 0.007 cm s-1 to 0.015 cm s-1. This result explains that the fast oxygen surface exchange process accounts for the improved oxygen flux through the BSCF membrane surface with SDC shell.

Figure 4. (A, B) ECR response curves of BSCF membrane with/without SDC layer at various temperatures after sudden change of oxygen partial pressure from 0.05 to 0.25 atm. (C) The temperature dependence of the fitted kex.


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Because of the contained alkaline earth elements, perovskite type mixed conducting membranes are very sensitive to react with acidic gases such as CO2. Many perovskite membranes achieved with high oxygen permeation flux, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and Ba(Co, Fe, Nb)O3-δ tends to suffer from decomposition of the structure and hence partial or even completely loss of the oxygen permeation ability in the CO2 containing atmosphere.20,45,46 To test the permeation stability of the SDC shell protecting BSCF membrane in the presence of CO2, the oxygen permeation experiment was conducted using the gas mixture with CO2 and He as the sweep gas. As seen in Figure 5a, during the limited test time of around 5 h, both BSCF membrane with SDC shell and pure BSCF membrane have relative stable oxygen flux under 100% pure He as the sweep gas. After switching the sweep gas from the pure He to the 10% CO2 containing mixture gas, the flux through pure BSCF membrane continuously drops down, leveling off at only 1.2 ml cm-2 min-1 after 8 h. After changing back to pure He sweep gas, the flux cannot be recovered to its original value, which is consistent with the result reported in previous works.34,38 Interestingly, it can be observed that after experiencing a slight oxygen flux decline, the BSCF membrane with SDC shell can be fully restored to its origin oxygen permeation ability with the sweep gas changed back to pure He. This similar temporary oxygen decrease on perovskite membranes in CO2 containing atmosphere has been observed by other researchers as well.47,48 It was believed that it may be caused by the adsorption effect of CO2 on the membrane surface, not by the poison effect from the reaction between CO2 and membrane material to form carbonate. Therefore, this CO2 stability test result shows that the BSCF membrane with SDC shell has a stronger tolerance against acidic CO2 gas than pure BSCF. As well known, SDC is a very tough material to resist most of acidic gases, which can even survive from concentrated nitric acid.49,50 Therefore, the introduction of SDC shell on the BSCF membrane is favorable for its stability against acidic CO2. However, this SDC shell is not a completed thin film, which


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Figure 5. Long term oxygen permeation test under CO2 at 850oC: (A) pure BSCF membrane. (B) BSCF membrane with SDC layer.

cannot totally cover the entire surface of BSCF membrane. As seen from SEM image (Figure 2B), there are some cracks along the BSCF grain boundary, which may give some space for CO2 to attack the BSCF membrane. However, from our test result, the SDC shell even with these cracks can still protect the BSCF membrane from erosive CO2. As discussed above, SDC shell can greatly enhance the oxygen surface exchange rate on the BSCF membrane. Therefore, the oxygen surface reaction on the crack section or porous SDC area is faster than the area covering by the dense SDC shell; which results in the oxygen gathering together at


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these cracks to form the redundant oxygen-rich zone covering the BSCF membrane surface. This can further explain the SDC shell with cracks can still help to prevent the BSCF membrane from acidic CO2 erosion. Of course, to verify this hypothesis, much more experimental work is still required.

4. CONCLUSION This work provides a good example to use the robust ion conductors to decorate the relatively weak perovskite oxide membranes for oxygen separation performance improvement. To illustrate the working principle, Sm0.2Ce0.8O2 (SDC) has been chosen as the robust ion conducting layers to be coated on Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes. The fluorite SDC protective layer has been successfully integrated on the perovskite BSCF membrane by using precursor solution with thermal treatment. The oxygen flux through the BSCF membrane was improved due to the faster oxygen surface exchange rate from the SDC layer. Most importantly, the SDC protective layer offers the BSCF membrane a better stability evidenced by the stable oxygen permeation performance under CO2 atmosphere over 70 hours long term operation; at similar case, the pure BSCF membrane was failed due to the poor material stability. This method is versatile. The stability improvement of other perovskite membranes using SDC or other ion conductors like gadolinium doped ceria (GDC) is also feasible.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +61-8-92669056. Fax: 61-8-92662681

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT Authors gratefully acknowledge the financial support provided by the Australian Research Council through the Future Fellow Program (FT12100178) and the research funding from the Natural Science Foundation of China (21176146 & 21476131).

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