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Takkeun Oh,z Jianlin Li,z Keith L. Duncan,z and Eric D. Wachsmanzw. zDepartment of Materials ... followed by rolling the green tape on a circular rod. SrCe0.9.
J. Am. Ceram. Soc., 92 [8] 1849–1852 (2009) DOI: 10.1111/j.1551-2916.2009.03103.x r 2009 The American Ceramic Society

Journal Fabrication of Thin-Film SrCe0.9Eu0.1O3d Hydrogen Separation Membranes on Ni–SrCeO3 Porous Tubular Supports Heesung Yoon,z Sun-Ju Song,y Takkeun Oh,z Jianlin Li,z Keith L. Duncan,z and Eric D. Wachsmanzw z

Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611

y

Department of Materials Science and Engineering, Chonnam National University, Gwangju, Korea

SrCe0.9Eu0.1O3d thin-film (B30 lm) tubular hydrogen separation membranes were developed in order to obtain high hydrogen fluxes. Fifteen centimeters long, one end closed, NiO–SrCeO3 tubular supports were fabricated by tape casting, followed by rolling the green tape on a circular rod. SrCe0.9 Eu0.1O3d powders were prepared by the citrate process and coated on partially sintered NiO–SrCeO3 tubular supports. Leakage-free hydrogen membrane cells were obtained by adjusting the presintering and final sintering temperatures to reduce the difference of linear shrinkage rates between SrCe0.9Eu0.1O3d thin films and NiO–SrCeO3 supports. A hydrogen flux of 2.2 cm3/min was obtained for the SrCe0.9Eu0.1 O3d on Ni–SrCeO3 tubular hydrogen separation membranes at 9001C using 25% H2 balanced with Ar and 3% H2O as the feed gas and He as the sweep gas. Thus, a 40% single pass yield of pure H2 was achieved with this membrane.

In this study, the fabrication of hydrogen separation membranes was performed by tape casting and rolling end-capped tubular-type supports (Ni–SrCeO3) and by a slurry coating to form dense SrCe0.9Eu0.1O3d (10ESC) hydrogen-permeable membrane thin films on the inner side of tubular supports.

II. Experimental Procedure SrCeO3 powder was used as a support structure without Eu as the aliovalent dopant because electronic–ionic conduction is not functionally necessary for the support. SrCeO3 powder for the support was prepared by the conventional solid-state synthesis method using SrCO3 (99.9%, Alfa Aesar, Wardhill, MA) and CeO2 (99.9%, Alfa Aesar) powders as the starting materials. NiO (99%, Alfa Aesar) powder was used to create porosity by reduction to Ni (catalyst for steam reforming) when the membrane was subsequently exposed to H2. The ratio of the mixture was 30 wt% NiO and 70 wt% SrCeO3. In order to prepare the NiO–SrCeO3 slurry for tape casting, NiO, SrCeO3, and fish oil (as a dispersant) were mixed in an EtOH/toluene solvent by ball milling for 24 h and then, polyvinyl butyral, polyethylene glycol, and di-n-butyl phthalate were added and ball milled for 24 h. The NiO–SrCeO3 slurry was tape cast. NiO–SrCeO3 green tapes were then rolled on a 1/4-in.-diameter steel core polytetrafluoroethylene rod and capped with a circular piece of the tape as shown in Fig. 2. 10ESC powder for the membrane was synthesized by a citrate process using Ce(NO3)3 6H2O (99.5%, Alfa Aesar), Sr(NO3)2 (99.97%, Alfa Aesar), and Eu(NO3)3 6H2O (99.9%, Alfa Aesar) as the starting materials. The molar ratio of the total metal

I. Introduction

T

HE advancement of hydrogen separation technology has become important in energy applications for the production of petrochemicals and pure hydrogen, the latter being of particular interest for use in fuel cells.1,2 Accordingly, high-temperature proton-conducting oxides have received increasing attention for these applications. With proton conductors, the separation process is enabled by ion transport, and results in 100% pure hydrogen in the absence of leaks.3–6 Recently, our group has studied the perovskite-structured, Eu-doped SrCeO3 for use as hydrogen separation membranes due to its mixed protonic–electronic conductivity.7–13 The Wagner equation shows that when transport is bulk diffusion limited, the permeation through a mixed ionic–electronic conducting membrane is inversely proportional to the thickness.14 Therefore, our research has been directed toward the development of thin-film hydrogen membranes deposited on porous tubular supports in order to maximize hydrogen production. Figure 1 illustrates the basic design of our thin-film hydrogen membranes. For a hydrogen production cell, a thin film of Eudoped SrCeO3 is coated on the inner side of a catalytic tubulartype porous support. A methane and steam mixture is made to flow on the outer side of the tube. At operating temperatures, the methane–steam mixture reacts to form CO, CO2, and H2, and due to the hydrogen partial pressure gradient between the outer and the inner side of the membrane, the H2 permeates through the thin film.

S. Zha—contributing editor

Manuscript No. 25875. Received February 13, 2008; approved March 8, 2009. This work was supported by NASA Grants NAG3-2930. w Author to whom correspondence should be addressed. e-mail: [email protected]fl.edu

Fig. 1. Basic design of a tubular-type hydrogen membrane cell for the methane steam-reforming process.

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The 10ESC/NiO–SrCeO3 tubular hydrogen membrane cell was placed in a quartz tube reactor in order to measure the leakage level and hydrogen permeation of the membrane. H2 balanced with Ar was made to flow to the outer (feed) side while He was made to flow to the inner sweep side of the cell. The concentrations of permeated gases on the (sweep) side were measured using a Q100MS Dycor QuadLink mass-spectrometer (Ametek, Paoli, PA).

III. Results and Discussion

Fig. 2. Process sequence for fabricating one end closed green body supports.

nitrates to citric acid and ethylene glycol was 1:2:2 in order to prevent the formation of secondary phases and decrease the calcining temperature.15,16 To preserve membrane integrity, the linear shrinkage rates between the 10ESC film and the support need to be matched during heat treatment so that cracking of the 10ESC film does not occur. In order to measure the shrinkage rate, tape-cast green NiO– SrCeO3 bodies, and uniaxially pressed 10ESC pellets were heat treated at 9501 through 15501C and the shrinkage was measured. After preparing the support, the tubular green body was partially sintered at temperatures between 11001 and 12001C. To prepare the hydrogen permeation cell, a layer of 10ESC was coated on the inner side of the partially sintered tubular support by colloidal coating, and then the 10ESC layer and the NiO– SrCeO3 support were cosintered.

Figure 3 shows the linear shrinkage rates for the NiO–SrCeO3 support and 10 mol% of Eu2O3-doped and undoped SrCeO3 as a function of firing temperature. Doping SrCeO3 with Eu2O3 promotes densification and therefore increases the shrinkage rate. The shrinkage rate of NiO–SrCeO3 was higher than that of SrCeO3. In order to increase the compatibility of the linear shrinkage of 10ESC film and the support, the NiO–SrCeO3 support was partially sintered at 11001–12001C for 4 h. The 10ESC film and the partially sintered NiO–SrCeO3 support were then sintered between 13501 and 14501C. The microstructures of the 10ESC film on NiO–SrCeO3 supports according to the sintering temperature are shown in Fig. 3. Scanning electron microscopy pictures show that the 10ESC film was fully densified above 14001C. Figure 4 shows the crosssectional views of the as-coated 10ESC layer and the fully densified 10ESC, with an apparent thickness of 30 mm, after sintering at 14501C. Subsequent porosity in the Ni–SrCeO3 was achieved by reducing the NiO to Ni in hydrogen at 9001C. Figure 5 shows the end-capped tubular hydrogen membrane cells after each processing step. The NiO–SrCeO3 substrate shrunk around 1%–2% when it was presintered at 11001C for 2 h. After fully sintering it at 14501C for 4 h, the shrinkage was 15%. The end-capped 10ESC/NiO–SrCeO3 tubular cell was sealed with ultra torr fitting and placed into the quartz tube reactor as shown in Fig. 6. The NiO in the support was reduced to Ni,

Fig. 3. Shrinkage rates for 10 mol% Eu-doped and undoped SrCeO3 prepared using a uniaxial press technique and SrCeO3 with a 30 wt% NiO green tape by tape casting, and microstructures for 10 mol% Eu-doped SrCeO3 thin layers that were coated on a NiO–SrCeO3 support partially sintered at 11001C according to the final sintering temperature.

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Fig. 6. Configuration of the hydrogen permeation reactor using a 15-cm-long, one end closed tubular-type hydrogen membrane cell.

Fig. 4. Crosssectional view of (a) a coated 10ESC film on partially sintered NiO–SrCeO3 and (b) a dense 10ESC film on a Ni–SrCeO3 support after sintering and reduction in a hydrogen atmosphere.

creating the desired porosity at the operating temperature. The gas components of the sweep side were measured by the mass spectrometer. As shown in Fig. 7, the Ar tracer concentration remained at the baseline irrespective of the H2 concentration, confirming that the cell is leak free. The permeated hydrogen flux was stable and increased with the hydrogen concentration on the feed side. A hydrogen flux of 2.2 cm3/min was achieved with 25% H2 at Fig. 7. Hydrogen flux and tracer Ar level through the 15-cm-long, one end closed tubular-type hydrogen membrane cell at 9001C according to the hydrogen concentration on the feed side (the feed side: 20 ccm of total flow rate with 3% H2O, the sweep side: 20 ccm of He).

9001C. Further results obtained with this hydrogen membrane are presented elsewhere.17

IV. Conclusions

Fig. 5. Photographs of the 15 cm long, one end closed tubular-type hydrogen membrane cells according to the processing steps.

NiO–SrCeO3 tubular supports for thin-film hydrogen separation membranes were prepared by tape casting and rolling after the optimized slurry for tape casting was achieved. SrCe0.9Eu0.1O3d thin-film hydrogen separation membranes on the inner side of the partially sintered NiO–SrCeO3 tubular supports were obtained by colloidal coating. Leak-free, 15 cm long, and endcapped tubular SrCe0.9Eu0.1O3d/Ni–SrCeO3 hydrogen permeation membranes were prepared by adjusting the presintering and final sintering temperatures. A pure H2 permeation flux of 2.2 cm3/min was achieved at 9001C from a 20 cm3/min feed gas consisting of 25% H2, 3%

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H2O, and balanced Ar. Thus, a 40% single pass yield of pure H2 was achieved with this membrane.

References 1

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S.-J. Song, E. D. Wachsman, S. E. Dorris, and U. Balachandran, ‘‘Electrical Properties of p-Type Electronic Defects in the Protonic Conductor SrCe0.95Eu0.05O3d,’’ J. Electrochem. Soc., 150 [6] A790–5 (2003). 10 S.-J. Song, E. D. Wachsman, J. Rhodes, S. E. Dorris, and U. Balachandran, ‘‘Numerical Modeling of Hydrogen Permeation in Chemical Potential Gradients,’’ Solid State Ionics, 164, 107–16 (2003). 11 S.-J. Song, E. D. Wachsman, J. Rhodes, S. E. Dorris, and U. Balachandran, ‘‘Hydrogen Permeability of SrCe1xMxO3d (x 5 0.05, M 5 Eu, Sm),’’ Solid State Ionics, 167, 99–105 (2004). 12 S.-J. Song, E. D. Wachsman, J. Rhodes, H.-S. Yoon, G. Zhang, S. E. Dorris, and U. Balachandran, ‘‘Hydrogen Permeability and Effect of Microstructure on Mixed Protonic–Electronic Conducting Eu-Doped Strontium Cerate,’’ J. Mater. Sci., 40, 4061–6 (2005). 13 T. Tsuji, H. Kurona, and Y. Yamamura, ‘‘Formation Reaction and Thermodynamic Properties of SrCe1yEuyO3x,’’ Solid State Ionics, 136–137, 313–7 (2000). 14 T. Norby and Y. Larring, ‘‘Mixed Hydrogen Ion–Electronic Conductors for Hydrogen Permeable Membranes,’’ Solid State Ionics, 136–137, 139–48 (2000). 15 I. Yoon, H. Yoon, and B. Kim, ‘‘Characterization of (La, Sr)MnO3/ Gd0.2Ce0.8O1.9 Interface with Citric Acid Contents and Sintering Temperature,’’ J. Korean Inst. Electr. Electron. Mater. Eng., 11 [1] 18–25 (1998). 16 H. Yoon, S. Choi, D. Lee, and B. Kim, ‘‘Synthesis and Characterization of Gd1xSrxMnO3d Cathode for Solid Oxide Cells,’’ J. Power Source, 93, 1–7 (2001). 17 H. Yoon, T. Oh, J. Li, K. Duncan, and E. Wachsman, ‘‘Permeation through SrCe0.9Eu0.1O3d/Ni–SrCeO3 Tubular Hydrogen Separation Membranes,’’ J. Electrochem. Soc., in press. &