- Email: [email protected]
Bismuth doping effects on the structure, electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3O3−δ ceramic membranes
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 6 9 4 e1 2 6 9 9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Bismuth doping effects on the structure, electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3O3Ld ceramic membranes Jianying Yang a, Hailei Zhao a,b,*, Xiaotong Liu a, Yongna Shen a, Lihua Xu a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab. of New Energy Materials and Technology, Beijing 100083, China
article info
abstract
Article history:
A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB, x ¼ 0e0.2) ceramic membranes were
Received 11 February 2012
prepared by solid state reaction method. The doping effects on the phase structure,
Received in revised form
structural stability, electrical conductivity and oxygen permeability were investigated.
2 June 2012
Little amount of Bi (x ¼ 0e0.08) can maintain the cubic perovskite structure of BSCFB
Accepted 5 June 2012
materials while more Bi (x > 0.08) will result in the generation of other impurities. Even in
Available online 27 June 2012
the Bi solid solution range, Bi doping is unfavorable for the enhancement of structural stability of BSCFB membranes. The electrical conductivity decreases with Bi doping level,
Keywords:
while the oxygen permeability of BSCF membrane can be increased remarkably with little
Bismuth
amount of Bi doping (x ¼ 0.05). More Bi leads to the structure deterioration of membrane
Doping
surface under oxygen permeation condition, resulting in a severe decrease in oxygen
BSCF
permeability. Considering the overall performance, a low Bi doping amount such as
Oxygen permeation membrane
x ¼ 0.05 is favored for the membrane applications. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The fast development of modern society brings great convenience and prosperity to human being but at the expense of the huge consumption rate of energy. The severe emission of greenhouse gases and the worn out of fossil fuel make it urgent to explore new clean energy sources. Meanwhile, hydrogen is expected to play a significant role in the future energy system [1] because it can provide viable, sustainable options for meeting the world’s energy requirements [2]. Among the diversified methods of producing hydrogen, the partial oxidation of methane (POM) in a reaction device equipped with mixed ionic and electronic conducting (MIEC)
membrane is regarded an efficient and low cost technology [3] and thus has attracted much attention. The MIEC membranes can combine the separation of oxygen from air and the catalytic oxidation in a single process serving not only as sustainable oxygen sources but also barriers to N2 to avoid the generation of NOx pollutants [4]. Furthermore, the intermediate temperature needed for oxygen permeation of membranes can be attained by the heat generated by the POM reaction itself [5] thus being energy-saving. MIEC membrane is one of the most crucial parts of this POM technology. From the perspective of application, the membrane must possess sufficient oxygen permeability and sustainable structural stability to withstand harsh conditions [6].
* Corresponding author. University of Science and Technology Beijing, School of Materials Science and Engineering, 30 Xueyuan Rd., Haidian District, Beijing 100083, China. Tel./fax: þ86 10 82376837. E-mail address: [email protected] (H. Zhao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.013
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Since Teraoka [7] first reported the high oxygen permeation flux of SrCo0.8Fe0.2O3d (SCFO) membrane, many researches have been made based on this kind of materials. Shao [6,8] first reported that the doping of barium into SCFO, namely BaxSr1xCoyFe1yO3d (BSCF) could suppress the oxidation of Co3þ and Fe3þ to higher valence states of Co4þ and Fe4þ in the lattice, and stabilize the perovskite structure under lower oxygen partial pressures. Higher oxygen permeation rates were also achieved from this doping strategy. Then, the BSCF material system was widely explored and regarded as promising MIEC materials for both oxygen permeation devices and IT-SOFCs [9e13]. Bismuth is an interesting element because bismuth oxides show high oxygen ionic conductivity [14] and many works have been reported that Bi doping could improve the oxygen permeability of oxide membranes. Li [15] investigated the effect of Bi doping at the A-site of Sr10n/2BinFe20Om and Sr1xBixFeO3, and found that the oxygen flux increased with the increasing bismuth content in both oxides. Shao reported that the increasing bismuth content in BaBixCo0.2Fe0.8xO3d resulted in an increasing oxygen flux, where Shao also indicated that a low bismuth content was favored due to the large expansion coefficient of the material accompanied by the valence change from Bi5þ to Bi3þ at high temperature [16]. Sunarso investigated the structure and oxygen permeability of barium bismuth oxides [17]; barium bismuth iron oxides [18] and Bi doped BaSc0.1Co0.9O3d [19,20]. The proper introduction of bismuth could result in the significant improvement of the oxygen permeability while maintaining the stability of the cubic perovskite membranes. Our previous work revealed that Ba0.6Sr0.4Co0.7Fe0.2 Nb0.1O3d is a potential oxygen permeation membrane with good structural stability against reducing atmosphere and acceptable oxygen permeability [5]. In this study, Bi was employed to dope Ba0.6Sr0.4Co0.7Fe0.3O3d with the aim of enhancing the oxygen permeability. A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB) membranes were synthesized and their lattice structure, electrical conductivity, oxygen permeability and structural stability were investigated.
2.
Experimental
2.1.
Synthesis and preparation
Bi doped Ba0.6Sr0.4Co0.7Fe0.3O3d powders were prepared by the conventional solid state reaction method. BaCO3, SrCO3, Co(CH3COO)2$4H2O, Fe2O3 and Bi2O3 (all in A.R. grade) were weighed according to the stoichiometric formula Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB, x ¼ 0, 0.05, 0.08, 0.12, 0.2) and mixed by planet ball milling in the ethyl alcohol medium for 4 h with ZrO2 balls. After drying, the powders were screened through a 140-mesh screen and then calcined at 900 C for 10 h with both heating and cooling rates of 3 C/ min. The calcined powders were finely grounded with an agate mortar and screened through a 140-mesh screen again, followed by pressing under a uniaxial pressure with stainless steel molds to prepare green disks and bars. The green disks and bars were sintered in air at different temperatures for 10 h with both heating and cooling rates
of 3 C/min to characterizations.
2.2.
achieve
dense
samples
for
various
Characterization
The lattice structure of the densified membranes was evaluated by X-ray Diffraction (XRD, RIGAKU D/MAX-A Diffractometer) using Cu Ka radiation at room temperature. The scanning electronic microscope (SEM, LEO-1450) was employed to observe the morphology of microstructure of the membranes. The structural stability of BSCFB membranes was examined in flowing Ar for 10 h and 5% H2/Ar for 0.5 h at 900 C with a flowing rate of 60 ml/min. The electrical conductivity was measured by four-terminal DC method from 200 to 900 C in different atmospheres at a heating rate of 5 C/min. The data were recorded in an interval of 50 C after equilibrium at each temperature to reach the stable state.
2.3.
Oxygen permeation measurement
Disk-shaped membranes were subjected to an oxygen permeation measuring system. The tested membranes were finely polished to 1.2 mm thick and sealed between a quartz tube and an alumina tube with silver rings between. No nitrogen leakage was guaranteed for oxygen permeation measurements. Air was fed from a gas cylinder to the oxygen rich side at a flow rate of 90 ml/min [STP], while helium was applied as sweeping gas at a flow rate of 60 ml/min. The gas flow rates were controlled by the mass flow controller. Gas chromatography was used to analyze the oxygen content in the sweeping gas and the oxygen permeation flux was calculated by Eq. (1): 1
JO2 ml cm2 min
¼
CO2 F S 1 CO2
(1)
where CO2 is the measured concentration of oxygen in the sweeping gas; F is the fixed flow rate of helium and S is the effective membrane surface area (cm2) for permeation. Tests were performed upon cooling from 900 to 820 C at a cooling rate of 1 C/min and a 10 min was held to reach a stable permeation flux for each test point.
3.
Results and discussion
3.1.
Lattice structure and structural stability
The structure of the sintered membranes was evaluated by XRD and the results are shown in Fig. 1. For samples with low Bi doping level (x ¼ 0, 0.05, 0.08), pure cubic perovskite structure was formed and the XRD peaks shifted to smaller angle with increasing Bi doping amount, indicating the gradual lattice expansion. This is attributed to the bigger radius of Bi ˚ , CN ¼ 6, ion than that of Co and Fe ions at B-site (Co3þ: 0.61 A ˚ , CN ¼ 6, HS) [21]. According to Bhalla [22], the HS; Fe3þ: 0.645 A perovskite structure with a general formula of ABO3 correlates ˚ ) and medium sized B to large sized A cation (1.10e1.80 A ˚ ). Bi has ionic radius of 1.03 and 0.76 A ˚ for cation (0.62e1.00 A Bi3þ and Bi5þ in a six-coordination, respectively [21], so it is reasonable to say that Bi was doped into B-site of BSCF with
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valence state 5þ. Nevertheless, the XRD peaks shifted back to bigger angle when the Bi doping level increasing from x ¼ 0.08 to x ¼ 0.12 (as shown in Fig. 1(b)). Meanwhile impurity peaks identified as barium bismuth oxide were detected. Taking into account of the fact that either Bi3þ or Bi5þ has a smaller ion ˚ , respecradius than Ba2þ and Sr2þ at A-site (1.61 and 1.44 A tively, in a 12-coordination) [21], the contracted lattice parameter and the emergence of barium bismuth oxide imply that some Bi ions may have occupied the A-site when Bi doping amount excesses 0.08. For BSCFB samples with x ¼ 0.12 and 0.2, they show not only low melting points and narrow sintering temperature range, but also weak mechanical strength. Accordingly, the following investigations were only focused on the samples with x ¼ 0, 0.05 and 0.08. Oxygen permeation membranes for POM applications are constantly operated in complicated environment. The feeding side provides an oxidation atmosphere while the sweeping side a reduction one. Therefore, the structural stability of the membrane is an important factor for practical use. The structural stability of the BSCFB membranes was examined in inert Ar and 5% H2/Ar reducing atmospheres, respectively. Fig. 2 shows the XRD patterns of the samples treated in flowing Ar at 900 C for 10 h as well as the original BSCF without heat treatment for comparison. The initial cubic phase was preserved and no impurity was detected for both Bi-doped samples, indicating the good stability of all the BSCFB membranes in inert atmospheres. The reduction tolerance of the membranes was tested in an even more severe atmosphere of 5% H2/Ar at 900 C for 0.5 h. As illustrated in Fig. 3, the XRD peaks of the treated samples become much weaker and the amount of impurities increases with increasing Bi doping content, demonstrating that Bi doping is unfavorable for the structural stability of Ba0.6Sr0.4Co0.7Fe0.3O3d material. This differs from the Nbdoping in Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3d membrane, which displays a good structural stability even after heating for 1 h in 5% H2/Ar [5]. It is worth to mention that the peaks of the remained perovskite structural for all samples shift to small angles further especially those after treatment in 5% H2/Ar, indicating the lattice expansion associated with the lattice oxygen release in low oxygen and reducing atmospheres. This
Fig. 1 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0, 0.05, 0.08, 0.12, 0.2) membranes in 2 Theta range of (a) 20e80 ; (b) 30e33 .
Fig. 2 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after treated in flowing Ar for 10 h.
could be attributed to the repulsion force arising between the mutually exposed cations after oxygen ions were released and the increased cation sizes due to the reduction of B-site ions [10].
3.2.
Electrical conductivity
MIEC materials have electronic and ionic conductivity simultaneously due to the co-existence of free electrons or electron holes and oxygen vacancies. Nevertheless, the electronic conductivity of perovskite-type oxide is typically 100e1000 times higher than the ionic conductivity [23]. Therefore, it is rational to refer the measured total conductivity mainly to the electronic conductivity. The electrical conductivity of BSCFB with x ¼ 0.05 under different oxygen partial pressures is displayed in Fig. 4. The electrical conductivity decreases with decreasing oxygen partial pressure, suggesting the p-type semi-conductor feature of BSCFB membranes with electronic holes as charge
Fig. 3 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after treated in 5% H2/Ar for 0.5 h.
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Fig. 4 e Electrical conductivity of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0.05) samples in different atmospheres.
carriers. The conductivity of the samples decreased in particular temperature range, 500e600 C in Ar and 550e650 C in air atmosphere, respectively. This was related to the lattice oxygen loss associated with the elimination of electronic holes, which could be described as Eq. (2). In Ar atmosphere, the sample underwent a more severe oxygen loss started at a lower temperature because of the low oxygen partial pressure. 1 O o þ 2h /Vo þ O2 ðgÞ 2
(2)
Fig. 5 depicts the temperature dependence of electrical conductivities of the BSCFB samples (a) with the corresponding Arrhenius plots of Ln (sT ) vs. 1/T (b). The electrical conductivity decreases with increasing Bi doping level. Discussion in Section 3.1 suggests that Bi taking 5þ valence state at B-site and similar reports can be found in literature [16,24]. For the substitution of Bi for Fe in BSCFB system, the charge balance can be achieved through the decrease in oxygen vacancy concentration and/or the decrease in valence state of Co and Fe ions. The decreased electrical conductivity of BSCFB with increasing Bi doping level suggests the decreased charge carrier concentration (electron holes), which is caused by the reduction of Co and Fe ion [25]. Besides, from 550 to 650 C, there is a temporary decrease in electrical conductivities with temperature, which is attributed to the lattice oxygen release of the samples, accompanying the reduction of some of the Co and Fe ions from Co4þ/Fe4þ to Co3þ/Fe3þ states [8]. It is worth to note that the degree of conductivity decrease in this temperature range is alleviated with Bi substitution. This should be ascribed to the decreased Co4þ/Fe4þ concentration in BSCFB with increasing Bi doping level, as reflected in the conductivity change behavior. The linear relationship between Ln (sT ) and reciprocal temperature suggests the conduction behavior of BSCFB being thermally activated hopping process of small polarons. The tuning of the line corresponds to the lattice oxygen release process. The activation energy (Ea) for the electron conduction can be
Fig. 5 e Electrical conductivity of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld samples (a) temperature dependence; (b) Arrhenius plots.
Fig. 6 e Oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes at different temperatures.
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Fig. 7 e Cross-section near the feeding side of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after oxygen permeation measurement, (a) x [ 0; (b) x [ 0.05; (c) x [ 0.08.
derived from the slope of the fitted line according to Eq. (3). In higher temperature range, the lower slope, corresponding to lower Ea value, could be ascribed to the weakened defect association resulted from the decreased electronic holes [26]. s¼
A Ea exp T kT
3.3.
(3)
Oxygen permeability
The oxygen permeability of BSCFB membranes with 1.2 mm thickness was examined in the temperature range of 820e900 C. The results are shown in Fig. 6. It is obvious that the oxygen permeation flux increases with temperature for all samples. The higher oxygen diffusivity and surface exchange activity as well as the increase of oxygen vacancy concentration enhance the oxygen flux together at higher temperatures
[17,27]. A highest flux of 1.049 ml cm2 min1 is obtained at 900 C with sample x ¼ 0.05, which is similar with the value achieved by Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3d membrane under the same thickness (ca. 1.2 mm) [5]. The oxygen permeation flux increases considerably from x ¼ 0 to x ¼ 0.05. This improvement can be attributed to the lowered average metaleoxygen bonding energy from Bi doping strategy [16]. However, more Bi doping, e.g. sample with x ¼ 0.08, results in lower oxygen permeability. After oxygen permeation measurement, the membranes were subjected to SEM observation. Fig. 7 shows the crosssection microstructure of the used membrane near the feeding side. Samples with x ¼ 0 and 0.05 exhibit much clearer surface structure. However, sample with x ¼ 0.08 underwent more severe phase deterioration during the measurement. As shown in Fig. 7(c), a large amount of impurities were produced, which is identified as BaO and Bi2O4 by XRD (Fig. 8). Although the low BieO bonding energy is favorable for oxygen transport, it makes Bi easy to be reduced and precipitated. This is the main reason which should be accounted for the low oxygen permeability of sample with x ¼ 0.08. All the results demonstrate that Bi doping can enhance the oxygen permeability while lower the structure stability. Considering the overall aspects, a low Bi doping amount such as x ¼ 0.05 is favored for the membrane applications.
4.
Fig. 8 e XRD pattern of feeding side surface of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0.08) membrane after oxygen permeation measurement.
Conclusions
A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (x ¼ 0e0.2) oxygen permeation membranes were synthesized by the conventional solid reaction method. Low Bi doping level can maintain the cubic structure with Bi occupying B-site with 5þ valence state while higher doping level results in the generation of impurities. Even in the B-site doping range, Bi doping cannot enhance the structural stability of BSCFB materials. Bi ions most possibly take the 5þ valence state at B-site, which leads to the decreased lattice oxygen loss of BSCFB membranes in
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 6 9 4 e1 2 6 9 9
the temperature range of 550e650 C, corresponding to the reduction of Co and Fe ions from Co4þ/Fe4þ to Co3þ/Fe3þ states and reflected by the alleviated temporary decrease in electrical conductivity. The electrical conductivity decreases with Bi doping amount and increases with oxygen partial pressure. Little amount of Bi (x ¼ 0.05) can increase the oxygen permeability remarkably while more Bi doping (e.g. x ¼ 0.08) easily leads to structure deterioration under permeation condition and thus exhibiting a low oxygen permeability. Considering overall aspects, a low Bi doping amount is favored.
[11]
[12]
[13]
Acknowledgment
[14]
This work was kindly supported by 863 Program of National High Technology Research Development Project of China (No. 2006AA11A189), Beijing Natural Science Foundation (No. 2102031) and Open Project of Shanghai Key Laboratory of Modern Metallurgy and Materials Processing (SELF-2010-02).
[15]
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[18] [1] Mueller-Langer F, Tzimas E, Kaltschmitt M, Peteves S. Techno-economic assessment of hydrogen production processes for the hydrogen economy for the short and medium term. Int J Hydrogen Energy 2007;32:3797e810. [2] Elam CC, Gregoire Padro´ CE, Sandrock G, Luzzi A, Lindblad P, Hagen EF. Realizing the hydrogen future: the International Energy Agency’s efforts to advance hydrogen energy technologies. Int J Hydrogen Energy 2003;28:601e7. [3] Balachandran U, Lee TH, Wang S, Dorris SE. Use of mixed conducting membranes to produce hydrogen by water dissociation. Int J Hydrogen Energy 2004;29:291e6. [4] Balachandran U, Dusek JT, Sweeney SM, Poeppel RB, Mieville RL, Maiya PS, et al. Methane to syngas via ceramic membranes. Am Ceram Soc Bull 1995;74:71e5. [5] Li Y, Zhao H, Xu N, Shen Y, Lu X, Ding W, et al. Systematic investigation on structure stability and oxygen permeability of Sr-doped BaCo0.7Fe0.2Nb0.1O3d ceramic membranes. J Membr Sci 2010;362:460e70. [6] Shao Z, Yang W, Cong Y, Dong H, Tong J, Xiong G. Investigation of the permeation behavior of a Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen membrane. J Membr Sci 2000;172:177e88. [7] Teraoka Y, Zhang HM, Furukawa S, Yamazoe N. Oxygen permeation through perovskite-type oxides. Chem Lett 1985; 14:1743e6. [8] Shao Z, Xiong G, Tong J, Dong H, Yang W. Ba effect in doped Sr(Co0.8Fe0.2)O3d on the phase structure and oxygen permeation properties of the dense ceramic membranes. Sep Purif Technol 2001;25:419e29. [9] Shao Z, Xiong G, Dong H, Yang W, Lin L. Synthesis, oxygen permeation study and membrane performance of a Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen-permeable dense ceramic reactor for partial oxidation of methane to syngas. Sep Purif Technol 2001;25:97e116. [10] Chen Z, Ran R, Zhou W, Shao Z, Liu S. Assessment of Ba0.5Sr0.5Co1yFeyO3d ( y ¼ 0.0e1.0) for prospective
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application as cathode for IT-SOFCs or oxygen permeating membrane. Electrochim Acta 2007;52:7343e51. Zeng P, Chen Z, Zhou W, Gu H, Shao Z, Liu S. Re-evaluation of Ba0.5Sr0.5Co0.8Fe0.2O3d perovskite as oxygen semi-permeable membrane. J Membr Sci 2007;291:148e56. Zhao H, Shen W, Zhu Z, Li X, Wang Z. Preparation and properties of BaxSr1xCoyFe1yO3d cathode material for intermediate temperature solid oxide fuel cells. J Power Source 2008;182:503e9. Gao D, Zhao J, Zhou W, Ran R, Shao Z. Influence of highenergy ball milling of the starting powder on the sintering; microstructure and oxygen permeability of Ba0.5Sr0.5Co0.5Fe0.5O3d membranes. J Membr Sci 2011;366: 203e11. Sammes NM, Tompsett GA, Na¨fe H, Aldinger F. Bismuth based oxide electrolytesdstructure and ionic conductivity. J Eur Ceram Soc 1999;19:1801e26. Li S, Cong Y, Fang L, Yang W, Lin L, Meng J, et al. Oxygen permeating properties of the mixed conducting membranes without cobalt. Mater Res Bull 1998;33:183e8. Shao Z, Xiong G, Cong Y, Yang W. Synthesis and oxygen permeation study of novel perovskite-type BaBixCo0.2Fe0.8xO3d ceramic membranes. J Membr Sci 2000; 164:167e76. Sunarso J, Liu S, Lin YS, Diniz da Costa JC. Oxygen permeation performance of BaBiO3d ceramic membranes. J Membr Sci 2009;344:281e7. Sunarso J, Liu S, Lin YS, Diniz da Costa JC. Structure effect on the oxygen permeation properties of barium bismuth iron oxide membranes. J Membr Sci 2010;351:44e9. Sunarso J, Motuzas J, Liu S, Diniz da Costa JC. Bi-doping effects on the structure and oxygen permeation properties of BaSc0.1Co0.9O3d perovskite membranes. J Membr Sci 2010; 361:120e5. Sunarso J, Liu S, Lin YS, Diniz da Costa JC. High performance BsBiScCo hollow fibre membranes for oxygen transport. Energy Environ Sci 2011;4:2516e9. Shannon R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A 1976;32. Bhalla AS, Guo R, Roy R. The perovskite structure e a review of its role in ceramic science and technology. Mater Res Innov 2000;4:3e26. Zhang K, Ran R, Ge L, Shao Z, Jin W, Xu N. Systematic investigation on new SrCo1yNbyO3d ceramic membranes with high oxygen semi-permeability. J Membr Sci 2008;321: 436e43. Zeng P, Shao Z, Liu S, Xu Z. Influence of M cations on structural, thermal and electrical properties of new oxygen selective membranes based on SrCo0.95M0.05O3d perovskite. Sep Purif Technol 2009;67:304e11. Zhao H, Xu N, Chen Y, Wei W, Chen N, Ding W, et al. Investigation of mixed conductor BaCo0.7Fe0.3xYxO3d with high oxygen permeability. J Phys Chem C 2010;114: 17975e81. Zhang J, Zhao H, Li Y, Xu N, Ding W, Lu X, et al. Effects of iron content on structural evolution, electrical properties and thermochemical stability of BaCo0.9xFexNb0.1O3d ceramic membrane. Int J Hydrogen Energy 2010;35:814e20. Tong J, Yang W, Zhu B, Cai R. Investigation of ideal zirconium-doped perovskite-type ceramic membrane materials for oxygen separation. J Membr Sci 2002;203: 175e89.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Bismuth doping effects on the structure, electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3O3Ld ceramic membranes Jianying Yang a, Hailei Zhao a,b,*, Xiaotong Liu a, Yongna Shen a, Lihua Xu a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab. of New Energy Materials and Technology, Beijing 100083, China
article info
abstract
Article history:
A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB, x ¼ 0e0.2) ceramic membranes were
Received 11 February 2012
prepared by solid state reaction method. The doping effects on the phase structure,
Received in revised form
structural stability, electrical conductivity and oxygen permeability were investigated.
2 June 2012
Little amount of Bi (x ¼ 0e0.08) can maintain the cubic perovskite structure of BSCFB
Accepted 5 June 2012
materials while more Bi (x > 0.08) will result in the generation of other impurities. Even in
Available online 27 June 2012
the Bi solid solution range, Bi doping is unfavorable for the enhancement of structural stability of BSCFB membranes. The electrical conductivity decreases with Bi doping level,
Keywords:
while the oxygen permeability of BSCF membrane can be increased remarkably with little
Bismuth
amount of Bi doping (x ¼ 0.05). More Bi leads to the structure deterioration of membrane
Doping
surface under oxygen permeation condition, resulting in a severe decrease in oxygen
BSCF
permeability. Considering the overall performance, a low Bi doping amount such as
Oxygen permeation membrane
x ¼ 0.05 is favored for the membrane applications. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The fast development of modern society brings great convenience and prosperity to human being but at the expense of the huge consumption rate of energy. The severe emission of greenhouse gases and the worn out of fossil fuel make it urgent to explore new clean energy sources. Meanwhile, hydrogen is expected to play a significant role in the future energy system [1] because it can provide viable, sustainable options for meeting the world’s energy requirements [2]. Among the diversified methods of producing hydrogen, the partial oxidation of methane (POM) in a reaction device equipped with mixed ionic and electronic conducting (MIEC)
membrane is regarded an efficient and low cost technology [3] and thus has attracted much attention. The MIEC membranes can combine the separation of oxygen from air and the catalytic oxidation in a single process serving not only as sustainable oxygen sources but also barriers to N2 to avoid the generation of NOx pollutants [4]. Furthermore, the intermediate temperature needed for oxygen permeation of membranes can be attained by the heat generated by the POM reaction itself [5] thus being energy-saving. MIEC membrane is one of the most crucial parts of this POM technology. From the perspective of application, the membrane must possess sufficient oxygen permeability and sustainable structural stability to withstand harsh conditions [6].
* Corresponding author. University of Science and Technology Beijing, School of Materials Science and Engineering, 30 Xueyuan Rd., Haidian District, Beijing 100083, China. Tel./fax: þ86 10 82376837. E-mail address: [email protected] (H. Zhao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.013
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Since Teraoka [7] first reported the high oxygen permeation flux of SrCo0.8Fe0.2O3d (SCFO) membrane, many researches have been made based on this kind of materials. Shao [6,8] first reported that the doping of barium into SCFO, namely BaxSr1xCoyFe1yO3d (BSCF) could suppress the oxidation of Co3þ and Fe3þ to higher valence states of Co4þ and Fe4þ in the lattice, and stabilize the perovskite structure under lower oxygen partial pressures. Higher oxygen permeation rates were also achieved from this doping strategy. Then, the BSCF material system was widely explored and regarded as promising MIEC materials for both oxygen permeation devices and IT-SOFCs [9e13]. Bismuth is an interesting element because bismuth oxides show high oxygen ionic conductivity [14] and many works have been reported that Bi doping could improve the oxygen permeability of oxide membranes. Li [15] investigated the effect of Bi doping at the A-site of Sr10n/2BinFe20Om and Sr1xBixFeO3, and found that the oxygen flux increased with the increasing bismuth content in both oxides. Shao reported that the increasing bismuth content in BaBixCo0.2Fe0.8xO3d resulted in an increasing oxygen flux, where Shao also indicated that a low bismuth content was favored due to the large expansion coefficient of the material accompanied by the valence change from Bi5þ to Bi3þ at high temperature [16]. Sunarso investigated the structure and oxygen permeability of barium bismuth oxides [17]; barium bismuth iron oxides [18] and Bi doped BaSc0.1Co0.9O3d [19,20]. The proper introduction of bismuth could result in the significant improvement of the oxygen permeability while maintaining the stability of the cubic perovskite membranes. Our previous work revealed that Ba0.6Sr0.4Co0.7Fe0.2 Nb0.1O3d is a potential oxygen permeation membrane with good structural stability against reducing atmosphere and acceptable oxygen permeability [5]. In this study, Bi was employed to dope Ba0.6Sr0.4Co0.7Fe0.3O3d with the aim of enhancing the oxygen permeability. A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB) membranes were synthesized and their lattice structure, electrical conductivity, oxygen permeability and structural stability were investigated.
2.
Experimental
2.1.
Synthesis and preparation
Bi doped Ba0.6Sr0.4Co0.7Fe0.3O3d powders were prepared by the conventional solid state reaction method. BaCO3, SrCO3, Co(CH3COO)2$4H2O, Fe2O3 and Bi2O3 (all in A.R. grade) were weighed according to the stoichiometric formula Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (BSCFB, x ¼ 0, 0.05, 0.08, 0.12, 0.2) and mixed by planet ball milling in the ethyl alcohol medium for 4 h with ZrO2 balls. After drying, the powders were screened through a 140-mesh screen and then calcined at 900 C for 10 h with both heating and cooling rates of 3 C/ min. The calcined powders were finely grounded with an agate mortar and screened through a 140-mesh screen again, followed by pressing under a uniaxial pressure with stainless steel molds to prepare green disks and bars. The green disks and bars were sintered in air at different temperatures for 10 h with both heating and cooling rates
of 3 C/min to characterizations.
2.2.
achieve
dense
samples
for
various
Characterization
The lattice structure of the densified membranes was evaluated by X-ray Diffraction (XRD, RIGAKU D/MAX-A Diffractometer) using Cu Ka radiation at room temperature. The scanning electronic microscope (SEM, LEO-1450) was employed to observe the morphology of microstructure of the membranes. The structural stability of BSCFB membranes was examined in flowing Ar for 10 h and 5% H2/Ar for 0.5 h at 900 C with a flowing rate of 60 ml/min. The electrical conductivity was measured by four-terminal DC method from 200 to 900 C in different atmospheres at a heating rate of 5 C/min. The data were recorded in an interval of 50 C after equilibrium at each temperature to reach the stable state.
2.3.
Oxygen permeation measurement
Disk-shaped membranes were subjected to an oxygen permeation measuring system. The tested membranes were finely polished to 1.2 mm thick and sealed between a quartz tube and an alumina tube with silver rings between. No nitrogen leakage was guaranteed for oxygen permeation measurements. Air was fed from a gas cylinder to the oxygen rich side at a flow rate of 90 ml/min [STP], while helium was applied as sweeping gas at a flow rate of 60 ml/min. The gas flow rates were controlled by the mass flow controller. Gas chromatography was used to analyze the oxygen content in the sweeping gas and the oxygen permeation flux was calculated by Eq. (1): 1
JO2 ml cm2 min
¼
CO2 F S 1 CO2
(1)
where CO2 is the measured concentration of oxygen in the sweeping gas; F is the fixed flow rate of helium and S is the effective membrane surface area (cm2) for permeation. Tests were performed upon cooling from 900 to 820 C at a cooling rate of 1 C/min and a 10 min was held to reach a stable permeation flux for each test point.
3.
Results and discussion
3.1.
Lattice structure and structural stability
The structure of the sintered membranes was evaluated by XRD and the results are shown in Fig. 1. For samples with low Bi doping level (x ¼ 0, 0.05, 0.08), pure cubic perovskite structure was formed and the XRD peaks shifted to smaller angle with increasing Bi doping amount, indicating the gradual lattice expansion. This is attributed to the bigger radius of Bi ˚ , CN ¼ 6, ion than that of Co and Fe ions at B-site (Co3þ: 0.61 A ˚ , CN ¼ 6, HS) [21]. According to Bhalla [22], the HS; Fe3þ: 0.645 A perovskite structure with a general formula of ABO3 correlates ˚ ) and medium sized B to large sized A cation (1.10e1.80 A ˚ ). Bi has ionic radius of 1.03 and 0.76 A ˚ for cation (0.62e1.00 A Bi3þ and Bi5þ in a six-coordination, respectively [21], so it is reasonable to say that Bi was doped into B-site of BSCF with
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valence state 5þ. Nevertheless, the XRD peaks shifted back to bigger angle when the Bi doping level increasing from x ¼ 0.08 to x ¼ 0.12 (as shown in Fig. 1(b)). Meanwhile impurity peaks identified as barium bismuth oxide were detected. Taking into account of the fact that either Bi3þ or Bi5þ has a smaller ion ˚ , respecradius than Ba2þ and Sr2þ at A-site (1.61 and 1.44 A tively, in a 12-coordination) [21], the contracted lattice parameter and the emergence of barium bismuth oxide imply that some Bi ions may have occupied the A-site when Bi doping amount excesses 0.08. For BSCFB samples with x ¼ 0.12 and 0.2, they show not only low melting points and narrow sintering temperature range, but also weak mechanical strength. Accordingly, the following investigations were only focused on the samples with x ¼ 0, 0.05 and 0.08. Oxygen permeation membranes for POM applications are constantly operated in complicated environment. The feeding side provides an oxidation atmosphere while the sweeping side a reduction one. Therefore, the structural stability of the membrane is an important factor for practical use. The structural stability of the BSCFB membranes was examined in inert Ar and 5% H2/Ar reducing atmospheres, respectively. Fig. 2 shows the XRD patterns of the samples treated in flowing Ar at 900 C for 10 h as well as the original BSCF without heat treatment for comparison. The initial cubic phase was preserved and no impurity was detected for both Bi-doped samples, indicating the good stability of all the BSCFB membranes in inert atmospheres. The reduction tolerance of the membranes was tested in an even more severe atmosphere of 5% H2/Ar at 900 C for 0.5 h. As illustrated in Fig. 3, the XRD peaks of the treated samples become much weaker and the amount of impurities increases with increasing Bi doping content, demonstrating that Bi doping is unfavorable for the structural stability of Ba0.6Sr0.4Co0.7Fe0.3O3d material. This differs from the Nbdoping in Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3d membrane, which displays a good structural stability even after heating for 1 h in 5% H2/Ar [5]. It is worth to mention that the peaks of the remained perovskite structural for all samples shift to small angles further especially those after treatment in 5% H2/Ar, indicating the lattice expansion associated with the lattice oxygen release in low oxygen and reducing atmospheres. This
Fig. 1 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0, 0.05, 0.08, 0.12, 0.2) membranes in 2 Theta range of (a) 20e80 ; (b) 30e33 .
Fig. 2 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after treated in flowing Ar for 10 h.
could be attributed to the repulsion force arising between the mutually exposed cations after oxygen ions were released and the increased cation sizes due to the reduction of B-site ions [10].
3.2.
Electrical conductivity
MIEC materials have electronic and ionic conductivity simultaneously due to the co-existence of free electrons or electron holes and oxygen vacancies. Nevertheless, the electronic conductivity of perovskite-type oxide is typically 100e1000 times higher than the ionic conductivity [23]. Therefore, it is rational to refer the measured total conductivity mainly to the electronic conductivity. The electrical conductivity of BSCFB with x ¼ 0.05 under different oxygen partial pressures is displayed in Fig. 4. The electrical conductivity decreases with decreasing oxygen partial pressure, suggesting the p-type semi-conductor feature of BSCFB membranes with electronic holes as charge
Fig. 3 e XRD patterns of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after treated in 5% H2/Ar for 0.5 h.
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Fig. 4 e Electrical conductivity of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0.05) samples in different atmospheres.
carriers. The conductivity of the samples decreased in particular temperature range, 500e600 C in Ar and 550e650 C in air atmosphere, respectively. This was related to the lattice oxygen loss associated with the elimination of electronic holes, which could be described as Eq. (2). In Ar atmosphere, the sample underwent a more severe oxygen loss started at a lower temperature because of the low oxygen partial pressure. 1 O o þ 2h /Vo þ O2 ðgÞ 2
(2)
Fig. 5 depicts the temperature dependence of electrical conductivities of the BSCFB samples (a) with the corresponding Arrhenius plots of Ln (sT ) vs. 1/T (b). The electrical conductivity decreases with increasing Bi doping level. Discussion in Section 3.1 suggests that Bi taking 5þ valence state at B-site and similar reports can be found in literature [16,24]. For the substitution of Bi for Fe in BSCFB system, the charge balance can be achieved through the decrease in oxygen vacancy concentration and/or the decrease in valence state of Co and Fe ions. The decreased electrical conductivity of BSCFB with increasing Bi doping level suggests the decreased charge carrier concentration (electron holes), which is caused by the reduction of Co and Fe ion [25]. Besides, from 550 to 650 C, there is a temporary decrease in electrical conductivities with temperature, which is attributed to the lattice oxygen release of the samples, accompanying the reduction of some of the Co and Fe ions from Co4þ/Fe4þ to Co3þ/Fe3þ states [8]. It is worth to note that the degree of conductivity decrease in this temperature range is alleviated with Bi substitution. This should be ascribed to the decreased Co4þ/Fe4þ concentration in BSCFB with increasing Bi doping level, as reflected in the conductivity change behavior. The linear relationship between Ln (sT ) and reciprocal temperature suggests the conduction behavior of BSCFB being thermally activated hopping process of small polarons. The tuning of the line corresponds to the lattice oxygen release process. The activation energy (Ea) for the electron conduction can be
Fig. 5 e Electrical conductivity of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld samples (a) temperature dependence; (b) Arrhenius plots.
Fig. 6 e Oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes at different temperatures.
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Fig. 7 e Cross-section near the feeding side of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld membranes after oxygen permeation measurement, (a) x [ 0; (b) x [ 0.05; (c) x [ 0.08.
derived from the slope of the fitted line according to Eq. (3). In higher temperature range, the lower slope, corresponding to lower Ea value, could be ascribed to the weakened defect association resulted from the decreased electronic holes [26]. s¼
A Ea exp T kT
3.3.
(3)
Oxygen permeability
The oxygen permeability of BSCFB membranes with 1.2 mm thickness was examined in the temperature range of 820e900 C. The results are shown in Fig. 6. It is obvious that the oxygen permeation flux increases with temperature for all samples. The higher oxygen diffusivity and surface exchange activity as well as the increase of oxygen vacancy concentration enhance the oxygen flux together at higher temperatures
[17,27]. A highest flux of 1.049 ml cm2 min1 is obtained at 900 C with sample x ¼ 0.05, which is similar with the value achieved by Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3d membrane under the same thickness (ca. 1.2 mm) [5]. The oxygen permeation flux increases considerably from x ¼ 0 to x ¼ 0.05. This improvement can be attributed to the lowered average metaleoxygen bonding energy from Bi doping strategy [16]. However, more Bi doping, e.g. sample with x ¼ 0.08, results in lower oxygen permeability. After oxygen permeation measurement, the membranes were subjected to SEM observation. Fig. 7 shows the crosssection microstructure of the used membrane near the feeding side. Samples with x ¼ 0 and 0.05 exhibit much clearer surface structure. However, sample with x ¼ 0.08 underwent more severe phase deterioration during the measurement. As shown in Fig. 7(c), a large amount of impurities were produced, which is identified as BaO and Bi2O4 by XRD (Fig. 8). Although the low BieO bonding energy is favorable for oxygen transport, it makes Bi easy to be reduced and precipitated. This is the main reason which should be accounted for the low oxygen permeability of sample with x ¼ 0.08. All the results demonstrate that Bi doping can enhance the oxygen permeability while lower the structure stability. Considering the overall aspects, a low Bi doping amount such as x ¼ 0.05 is favored for the membrane applications.
4.
Fig. 8 e XRD pattern of feeding side surface of Ba0.6Sr0.4Co0.7Fe0.3LxBixO3Ld (x [ 0.08) membrane after oxygen permeation measurement.
Conclusions
A series of Ba0.6Sr0.4Co0.7Fe0.3xBixO3d (x ¼ 0e0.2) oxygen permeation membranes were synthesized by the conventional solid reaction method. Low Bi doping level can maintain the cubic structure with Bi occupying B-site with 5þ valence state while higher doping level results in the generation of impurities. Even in the B-site doping range, Bi doping cannot enhance the structural stability of BSCFB materials. Bi ions most possibly take the 5þ valence state at B-site, which leads to the decreased lattice oxygen loss of BSCFB membranes in
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the temperature range of 550e650 C, corresponding to the reduction of Co and Fe ions from Co4þ/Fe4þ to Co3þ/Fe3þ states and reflected by the alleviated temporary decrease in electrical conductivity. The electrical conductivity decreases with Bi doping amount and increases with oxygen partial pressure. Little amount of Bi (x ¼ 0.05) can increase the oxygen permeability remarkably while more Bi doping (e.g. x ¼ 0.08) easily leads to structure deterioration under permeation condition and thus exhibiting a low oxygen permeability. Considering overall aspects, a low Bi doping amount is favored.
[11]
[12]
[13]
Acknowledgment
[14]
This work was kindly supported by 863 Program of National High Technology Research Development Project of China (No. 2006AA11A189), Beijing Natural Science Foundation (No. 2102031) and Open Project of Shanghai Key Laboratory of Modern Metallurgy and Materials Processing (SELF-2010-02).
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