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Preparation and hydrogen permeation properties of BaCe0.95Nd0.05O3−δ membranes
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Chinese Chemical Letters 19 (2008) 1256–1259 www.elsevier.com/locate/cclet
Preparation and hydrogen permeation properties of BaCe0.95Nd0.05O3d membranes Ming Ya Cai a, Hui Xia Luo a, Zhong Li a, Armin Feldhoff b, Ju¨rgen Caro b, Hai Hui Wang a,* a
The Key Lab of Enhanced Heat Transfer and Energy Conservation of Chinese Ministry of Education, School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou 510640, China b Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstr. 3-3A, D-30179 Hannover, Germany Received 31 March 2008
Abstract Dense mixed proton and electron conducting membrane made of BaCe0.95Nd0.05O3d (BCNd5) was prepared by pressing followed by sintering. X-ray diffraction (XRD) was used to characterize the phase structure of both the powder and the sintered membranes. The microstructure of the sintered membranes was studied by scanning electron microscopy (SEM). Hydrogen permeation through the BCNd5 membrane was studied using a high temperature permeator. The hydrogen permeation fluxes under wet conditions are higher than those under dry conditions, which is due to H+ hopping via surface OH groups. At 925 8C, a hydrogen permeation flux of 0.02 mL/min cm2 was obtained under wet condition, which recommends BCNd5 as a potential material for hydrogen-selective membranes. # 2008 Hai Hui Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Hydrogen; Mixed conductor; Perovskite; Membrane
Hydrogen is gaining more and more attention mainly because it is generally regarded as an important future fuel. The most important source for hydrogen is steam reforming of e.g. natural gas followed by water gas shift and cleaning steps. There is a high demand for new technologies for the cheap and effective separation of hydrogen. High temperature perovskite mixed proton and electron conducting membranes can provide a simple and efficient means of separating hydrogen from gas streams thus offering an alternative to existing methods of hydrogen recovery. In the early 1980s, Iwahara et al. first reported proton conductivity in SrCeO3 materials [1]. Later, BaCeO3, SrZrO3, CaZrO3 and SrTiO3 were also found to show proton conductivity [2]. The two most extensively studied high temperature proton-conducting ceramics are Yb-doped SrCeO3 (SCYb) and Nd-doped BaCeO3 (BCNd). Compared to SCYb, BCNd shows higher proton conductivity. For example, under a pure H2 atmosphere the proton conductivities of SCYb and BCNd are about 0.7 102 and 2.2 102 S/cm at 900 8C, respectively [3]. Despite some information on proton conductivity of BCNd is already available, information on the hydrogen permeation properties of BCNd is still lacking. In this paper, we studied the preparation of BaCe0.95Nd0.05O3d (BCNd5) as well as its hydrogen permeability.
* Corresponding author. E-mail address: [email protected] (H.H. Wang). 1001-8417/$ – see front matter # 2008 Hai Hui Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.06.054
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Fig. 1. XRD patterns of BCNd5 powder and the sintered BCNd5 membrane.
BCNd5 powders were synthesized via a combined citrate and EDTA complexing method according to Shao et al. [4] Proper amounts of Ba(NO3)2, Ce(NO3)3 and Nd(NO3)3 were dissolved in water and followed by the addition of citric acid, EDTA, and NH3. The reaction mixture was then heated under constant stirring to obtain a gel. Afterwards the gel was pre-calcined for 10 h at 350 8C. The pre-calcined powders were grounded and finally fired for 10 h at 950 8C to get a pure perovskite phase. The powders were then coldly pressed under 14 MPa to prepare ‘‘green’’ membranes which were sintered at 1350 8C with a dwelling time of 10 h. The densities of the sintered membranes were determined by the Archimedes method using ethanol. Only those membranes that had relative densities higher than 95% were used for hydrogen permeation studies. The hydrogen permeation experiments were carried out on the self-made high temperature permeation cell (as shown in supplement materials). A ceramic glass powder (KeramikGlasur, UHLIG, Germany) was used as the ceramic binding agent to seal the disc into a dense quartz tube at the solidified temperature of 1040 8C. During the hydrogen permeation experiment, a H2/He mixture was introduced to the feed side, while argon was used as the sweeping gas on the permeate side. The downstream chamber effluent was
Fig. 2. SEM pictures of the sintered BCNd5 membrane: (A) and (B) surface view; (C) and (D) cross-section of the broken membrane.
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analyzed by on-line gas chromatography (Agilent Technologies, 6890, equipped with a Carboxen 1000 column). The hydrogen permeation flux was calculated as following: pffiffiffi 2F H2 C He F 2 (1) J H2 ðmL=min cm Þ ¼ CH2 S F He Where CH2 , CHe are the hydrogen and helium concentrations calculated from GC calibration. F H2 and F He are the flow rates of H2 and He in the feed side, respectively. F is the total flow rate of the effluent gases in the permeate side which could be calculated from the change in the Ne concentrations before and after the permeator. S is the membrane area. The leakage was evaluated by measuring the amount of He in the permeate stream. In our experiments, the leakage is lower than 2%, which was subtracted when the hydrogen permeation flux was calculated.X-ray diffraction (XRD) patterns of BCNd5 powder calcined at 950 8C for 10 h and of the membrane sintered at 1350 8C for 10 h shows that both the powder and the membrane exhibit perovskite structure, as shown in Fig. 1. Fig. 2 shows scanning electron microscope (SEM) views of BCNd5 membranes sintered at 1350 8C for 10 h. A dense membrane with clear grain boundaries could be prepared. The grain size is around 1 mm, as shown in Fig. 2A and B. Fig. 2C and D shows the SEM micrograph of the fractured cross-section, no hole was observed in the bulk phase of the membrane indicating that the membrane was compact.The hydrogen permeation fluxes through the BCNd5 membrane with a thickness of 1.0 mm as a function of temperature were measured under both dry and wet conditions, as shown in Fig. 3. It can be seen that the hydrogen permeation fluxes increase with temperature in the temperature range of 825–925 8C. At 925 8C, the hydrogen permeation flux reaches 0.017 mL/min cm2 under dry condition. Song et al. [5] investigated the hydrogen permeability of SrCe1xMxO3d (x = 0.05, M = Eu, Sm) (SCM). It was found that the hydrogen permeation flux is about 0.0035 mL/min cm2 at 850 8C under dry condition. Li et al. [6] obtained a hydrogen permeation flux of 0.008 mL/min cm2 through BaCe0.9Mn0.1O3d (BCM) at 900 8C. Obviously, the hydrogen permeation flux through our membrane is higher than those on SCM and BCM membranes, respectively. As shown in Fig. 3, the hydrogen permeation fluxes under wet conditions are much higher than those under dry conditions, which is due to H+ hopping via surface OH groups. In the presence of steam, OH groups are formed on the membrane surface and the bulk according to: H2 O þ VO þ OO $ 2OHO with VO, OO and OHO indicating an oxygen vacancy, a lattice oxygen and a hydroxyl ion, which represents an interstitial proton associating strongly with a neighboring oxygen ion. The mm-thick BCNd5 membrane shows appreciable hydrogen permeability (0.017 mL/min cm2 under wet conditions). However, the absolute value of hydrogen permeation can be still improved by a more effective permeator design, e.g. decreasing the membrane thickness to the micrometer range or improving the surface exchange kinetics by coating the membrane surface with a porous layer. The most suitable configuration of the membrane may be the hollow fiber geometry with an asymmetric structure, which has a dense layer around 100 mm and a porous layer [7].
Fig. 3. Hydrogen permeation flux as a function of temperature under dry and wet condition. Membrane thickness: 1.0 mm, feed side: dry condition = 80 mL/min H2 + 20 mL/min He, wet condition = 80 mL/min H2 + 15 mL/min He + 5 mL/min steam, sweep side: 29.5 mL/min Ar + 0.5 mL/min Ne.
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Acknowledgment The authors greatly acknowledge the financial support by Natural Science Foundation of Guangdong Province (No. 7300769), by Program for New Century Excellent Talents in University (No. NECT-07-0307) and by Fok Ying Tung Education Foundation (No. 114019). References [1] [2] [3] [4] [5] [6] [7]
H. Iwahara, T. Esaka, H. Uchida, et al. Solid State Ionics 3-4 (1981) 359. C. Zuo, S.E. Dorris, U. Balachandran, et al. Chem. Mater. 18 (2006) 4648. S. Cheng, V.K. Gupta, J.Y.S. Lin, Solid State Ionics 176 (2005) 2653. Z.P. Shao, G.X. Xiong, Y. Cong, et al. J. Membr. Sci. 172 (2000) 177. S.J. Song, E.D. Wachsman, J. Rhodes, et al. Solid State Ionics 167 (2004) 99. G.T. Li, G.X. Xiong, S.S. Sheng, et al. Chin. Chem. Lett. 12 (10) (2001) 937. H.H. Wang, S. Werth, T. Schiestel, et al. Angew. Chem. Int. Ed. 44 (2005) 6906.
Chinese Chemical Letters 19 (2008) 1256–1259 www.elsevier.com/locate/cclet
Preparation and hydrogen permeation properties of BaCe0.95Nd0.05O3d membranes Ming Ya Cai a, Hui Xia Luo a, Zhong Li a, Armin Feldhoff b, Ju¨rgen Caro b, Hai Hui Wang a,* a
The Key Lab of Enhanced Heat Transfer and Energy Conservation of Chinese Ministry of Education, School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou 510640, China b Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstr. 3-3A, D-30179 Hannover, Germany Received 31 March 2008
Abstract Dense mixed proton and electron conducting membrane made of BaCe0.95Nd0.05O3d (BCNd5) was prepared by pressing followed by sintering. X-ray diffraction (XRD) was used to characterize the phase structure of both the powder and the sintered membranes. The microstructure of the sintered membranes was studied by scanning electron microscopy (SEM). Hydrogen permeation through the BCNd5 membrane was studied using a high temperature permeator. The hydrogen permeation fluxes under wet conditions are higher than those under dry conditions, which is due to H+ hopping via surface OH groups. At 925 8C, a hydrogen permeation flux of 0.02 mL/min cm2 was obtained under wet condition, which recommends BCNd5 as a potential material for hydrogen-selective membranes. # 2008 Hai Hui Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Hydrogen; Mixed conductor; Perovskite; Membrane
Hydrogen is gaining more and more attention mainly because it is generally regarded as an important future fuel. The most important source for hydrogen is steam reforming of e.g. natural gas followed by water gas shift and cleaning steps. There is a high demand for new technologies for the cheap and effective separation of hydrogen. High temperature perovskite mixed proton and electron conducting membranes can provide a simple and efficient means of separating hydrogen from gas streams thus offering an alternative to existing methods of hydrogen recovery. In the early 1980s, Iwahara et al. first reported proton conductivity in SrCeO3 materials [1]. Later, BaCeO3, SrZrO3, CaZrO3 and SrTiO3 were also found to show proton conductivity [2]. The two most extensively studied high temperature proton-conducting ceramics are Yb-doped SrCeO3 (SCYb) and Nd-doped BaCeO3 (BCNd). Compared to SCYb, BCNd shows higher proton conductivity. For example, under a pure H2 atmosphere the proton conductivities of SCYb and BCNd are about 0.7 102 and 2.2 102 S/cm at 900 8C, respectively [3]. Despite some information on proton conductivity of BCNd is already available, information on the hydrogen permeation properties of BCNd is still lacking. In this paper, we studied the preparation of BaCe0.95Nd0.05O3d (BCNd5) as well as its hydrogen permeability.
* Corresponding author. E-mail address: [email protected] (H.H. Wang). 1001-8417/$ – see front matter # 2008 Hai Hui Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.06.054
M.Y. Cai et al. / Chinese Chemical Letters 19 (2008) 1256–1259
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Fig. 1. XRD patterns of BCNd5 powder and the sintered BCNd5 membrane.
BCNd5 powders were synthesized via a combined citrate and EDTA complexing method according to Shao et al. [4] Proper amounts of Ba(NO3)2, Ce(NO3)3 and Nd(NO3)3 were dissolved in water and followed by the addition of citric acid, EDTA, and NH3. The reaction mixture was then heated under constant stirring to obtain a gel. Afterwards the gel was pre-calcined for 10 h at 350 8C. The pre-calcined powders were grounded and finally fired for 10 h at 950 8C to get a pure perovskite phase. The powders were then coldly pressed under 14 MPa to prepare ‘‘green’’ membranes which were sintered at 1350 8C with a dwelling time of 10 h. The densities of the sintered membranes were determined by the Archimedes method using ethanol. Only those membranes that had relative densities higher than 95% were used for hydrogen permeation studies. The hydrogen permeation experiments were carried out on the self-made high temperature permeation cell (as shown in supplement materials). A ceramic glass powder (KeramikGlasur, UHLIG, Germany) was used as the ceramic binding agent to seal the disc into a dense quartz tube at the solidified temperature of 1040 8C. During the hydrogen permeation experiment, a H2/He mixture was introduced to the feed side, while argon was used as the sweeping gas on the permeate side. The downstream chamber effluent was
Fig. 2. SEM pictures of the sintered BCNd5 membrane: (A) and (B) surface view; (C) and (D) cross-section of the broken membrane.
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M.Y. Cai et al. / Chinese Chemical Letters 19 (2008) 1256–1259
analyzed by on-line gas chromatography (Agilent Technologies, 6890, equipped with a Carboxen 1000 column). The hydrogen permeation flux was calculated as following: pffiffiffi 2F H2 C He F 2 (1) J H2 ðmL=min cm Þ ¼ CH2 S F He Where CH2 , CHe are the hydrogen and helium concentrations calculated from GC calibration. F H2 and F He are the flow rates of H2 and He in the feed side, respectively. F is the total flow rate of the effluent gases in the permeate side which could be calculated from the change in the Ne concentrations before and after the permeator. S is the membrane area. The leakage was evaluated by measuring the amount of He in the permeate stream. In our experiments, the leakage is lower than 2%, which was subtracted when the hydrogen permeation flux was calculated.X-ray diffraction (XRD) patterns of BCNd5 powder calcined at 950 8C for 10 h and of the membrane sintered at 1350 8C for 10 h shows that both the powder and the membrane exhibit perovskite structure, as shown in Fig. 1. Fig. 2 shows scanning electron microscope (SEM) views of BCNd5 membranes sintered at 1350 8C for 10 h. A dense membrane with clear grain boundaries could be prepared. The grain size is around 1 mm, as shown in Fig. 2A and B. Fig. 2C and D shows the SEM micrograph of the fractured cross-section, no hole was observed in the bulk phase of the membrane indicating that the membrane was compact.The hydrogen permeation fluxes through the BCNd5 membrane with a thickness of 1.0 mm as a function of temperature were measured under both dry and wet conditions, as shown in Fig. 3. It can be seen that the hydrogen permeation fluxes increase with temperature in the temperature range of 825–925 8C. At 925 8C, the hydrogen permeation flux reaches 0.017 mL/min cm2 under dry condition. Song et al. [5] investigated the hydrogen permeability of SrCe1xMxO3d (x = 0.05, M = Eu, Sm) (SCM). It was found that the hydrogen permeation flux is about 0.0035 mL/min cm2 at 850 8C under dry condition. Li et al. [6] obtained a hydrogen permeation flux of 0.008 mL/min cm2 through BaCe0.9Mn0.1O3d (BCM) at 900 8C. Obviously, the hydrogen permeation flux through our membrane is higher than those on SCM and BCM membranes, respectively. As shown in Fig. 3, the hydrogen permeation fluxes under wet conditions are much higher than those under dry conditions, which is due to H+ hopping via surface OH groups. In the presence of steam, OH groups are formed on the membrane surface and the bulk according to: H2 O þ VO þ OO $ 2OHO with VO, OO and OHO indicating an oxygen vacancy, a lattice oxygen and a hydroxyl ion, which represents an interstitial proton associating strongly with a neighboring oxygen ion. The mm-thick BCNd5 membrane shows appreciable hydrogen permeability (0.017 mL/min cm2 under wet conditions). However, the absolute value of hydrogen permeation can be still improved by a more effective permeator design, e.g. decreasing the membrane thickness to the micrometer range or improving the surface exchange kinetics by coating the membrane surface with a porous layer. The most suitable configuration of the membrane may be the hollow fiber geometry with an asymmetric structure, which has a dense layer around 100 mm and a porous layer [7].
Fig. 3. Hydrogen permeation flux as a function of temperature under dry and wet condition. Membrane thickness: 1.0 mm, feed side: dry condition = 80 mL/min H2 + 20 mL/min He, wet condition = 80 mL/min H2 + 15 mL/min He + 5 mL/min steam, sweep side: 29.5 mL/min Ar + 0.5 mL/min Ne.
M.Y. Cai et al. / Chinese Chemical Letters 19 (2008) 1256–1259
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Acknowledgment The authors greatly acknowledge the financial support by Natural Science Foundation of Guangdong Province (No. 7300769), by Program for New Century Excellent Talents in University (No. NECT-07-0307) and by Fok Ying Tung Education Foundation (No. 114019). References [1] [2] [3] [4] [5] [6] [7]
H. Iwahara, T. Esaka, H. Uchida, et al. Solid State Ionics 3-4 (1981) 359. C. Zuo, S.E. Dorris, U. Balachandran, et al. Chem. Mater. 18 (2006) 4648. S. Cheng, V.K. Gupta, J.Y.S. Lin, Solid State Ionics 176 (2005) 2653. Z.P. Shao, G.X. Xiong, Y. Cong, et al. J. Membr. Sci. 172 (2000) 177. S.J. Song, E.D. Wachsman, J. Rhodes, et al. Solid State Ionics 167 (2004) 99. G.T. Li, G.X. Xiong, S.S. Sheng, et al. Chin. Chem. Lett. 12 (10) (2001) 937. H.H. Wang, S. Werth, T. Schiestel, et al. Angew. Chem. Int. Ed. 44 (2005) 6906.