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Protonic conductivity nanostructured ceramic film with improved resistance to carbon dioxide at elevated temperatures
Surface & Coatings Technology 200 (2005) 1252 – 1258 www.elsevier.com/locate/surfcoat
Protonic conductivity nanostructured ceramic film with improved resistance to carbon dioxide at elevated temperatures Xinqing Ma *, Jinxiang Dai, Heng Zhang, David E. Reisner US Nanocorp, Inc., 74 Batterson Park Road, Farmington, CT 06032, USA Available online 12 September 2005
Abstract As a high value product, hydrogen is a clean fuel with zero emission, and thereby its applications alleviate the threat of ‘‘Greenhouse effect.’’ From the ‘‘hydrogen economy’’ point of view, the predominant way of producing hydrogen is reforming from fossil fuels. A dense ceramic protonic conductive film can be used to separate hydrogen from other reformed products or syngas in gasification process at high temperatures. However, currently existing ceramic films are proven to severely degrade in CO2-containing environments. In this work, a codoped BaCeO3 material was proposed for better CO2 resistance and higher protonic conductivity. Nanostructured BaCeO3-based film was fabricated from nano-grain feedstock using air plasma spray. The hydrogen permeable film has been demonstrated superior to currently available ceramic protonic films for hydrogen separation in terms of chemical stability, protonic and electronic conductivity, and thermomechanical properties in the temperature range of 600 – 800 -C. This work has demonstrated a methodology to improve chemical stability, solid ionic conductivity, as well as good mechanical integrity for a protonic membrane system, using a doping composition technique and incorporating a nanostructured membrane manufacturing process. D 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Protonic conductivity; Ceramic film; CO2-resistance; High temperature
1. Introduction Hydrogen is a clean fuel in the sense that its applications will not emit toxic gas and green house effect CO2 gas [1]. Hydrogen is a highly valuable product, but not an available primary fuel. The annual hydrogen consume and production is expected to increase as the world is moving toward the application of hydrogen energy carrier because of environmental concern. Today’s water electrolysis technology is neither cost effective nor practical for producing hydrogen because the energy efficiency is low and the process consumes electric energy, a clean energy. Economically, the predominant way is to produce hydrogen from fossil fuel, such as coal and natural gas [2]. The produced hydrogen is usually mixed with other products. Therefore, it is important to develop a hydrogen separation technology to divide hydrogen from by-products. Using a separation * Corresponding author. E-mail address: [email protected] (X. Ma). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.07.099
process, high purity hydrogen can be produced from the reformed fuel gas. Now, worldwide development effort is focusing on the hydrogen separation membrane technology. Hydrogen permeable membranes include metallic membranes, porous inorganic membranes and dense ceramic membranes [3 –6]. In particular, ceramic membranes are of importance and interest compared to metallic hydrogenpermeable membranes such as palladium or its alloys. The advantages of ceramic membranes include high temperature application, up to 600 –900 -C, high resistance to aggressive corrosion degradation including oxidation and sulfidation, no hydrogen brittleness problem, no sulfur poisoning in H2S and SO2 atmospheres, low-cost and robust performance. Perovskite-based oxide ceramic BaCeO3 has been wellidentified in high proton conductivity. However, its decomposition in the presence of acidic gas CO2 or SO2 and moisture has been reported in extensive studies [7 –10]. The material degradation is caused by high basicity of BaCeO3, which causes it to react with CO2/SO2 and form BaCO3, CeO2, BaSO4, or Ba(OH)2 depending on environment and
X. Ma et al. / Surface & Coatings Technology 200 (2005) 1252 – 1258
temperature. Therefore, BaCeO3 is not a viable and practical membrane material used for hydrogen separation from synthetic fuel gas at high temperature range of 600 –900 -C. Rare earth element (R = Y, La, Gd or Nd, etc.) doped BaCeO3 (BaCe1 X RX O3) may increase chemical stability, but a high dopant content (X > 0.6) is required [11 – 13]. Inversely, high dopant content will change lattice parameters or even crystalline structure and hence degrade protonic conductivity of barium cerates. Therefore, the challenging is to develop a new BaCeO3-based ceramic that will compromise the chemical stability and proton conductivity. This work has investigated the synthesis routes for nanostructured Zr-substituted barium cerate-based materials and their chemical stability in CO2-containing environment at 600 –800 -C. Nanostructured dense membranes were fabricated from the synthesized barium cerate-based materials by atmospheric plasma spray techniques. The plasma sprayed dense membranes were evaluated in microstructure, phase composition, protonic/electronic conductivity and mechanical integrity.
2. Experimental procedures 2.1. Synthesis of nanocrystalline membrane materials Zr-substituted BCY (BaCe0.9 X ZrX Y0.1O3 a ) was synthesized by solid-state reaction routes. The starting materials included barium carbonate as the Ba source, and nano-sized CeO2, Y2O3 and ZrO2 powders as Ce, Y and Zr sources. The nano-oxides (< 20 nm) were synthesized with US Nanocorp’s (USN) patented synthesis technique, a wet chemical reaction in an aqueous solution [14]. Three routes were developed for the synthesis of BCZYs with the Zr dopant concentration X, varying from 0.0 to 0.4. The synthesis routes were: (i) Route 1: high temperature reaction at 1550 -C for 5 h followed by powder reconstitution; (ii) Route 2: high temperature reaction starting nano-powders reconstitution followed by a solid state reaction at 1650 -C for 4 h; and (iii) Route 3: low temperature reaction at 1450 -C followed by powder reconstitution and plasma-assisted reaction and densification. 2.2. Dense membrane deposition by plasma spraying Air plasma spray was employed to produce the BCZY membrane. The optimization of plasma spray conditions was centered on attaining high density (> 95%) and a nanostructure. A Metco 9 MB-gun plasma spray system was used for producing the dense membrane on a porous support. The major plasma spray parameters included working gas, current/voltage, powder feed rate and specific setups for powder feeding. The influence of different particle sized feedstock with varying morphology and density, on the resultant membrane density, was investigated. The membranes were deposited on a porous
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Ni + BaCeO3 cermet substrate that was designed and fabricated by USN. 2.3. Microstructure and phase evaluation of the sprayed membranes The sprayed membranes were examined on cross-section microstructures using an optical microscope and scanning electron microscopy (SEM). The defects in the membranes were examined on pores, cracks, unmelted inclusions, and interface delamination. Phase compositions in as-sprayed membranes and the tested samples were determined by Xray diffraction (XRD). The grain sizes of the membranes were estimated indirectly from the broadening peak widths in XRD spectra, and measured directly from TEM images. 2.4. Nanostructured membrane performance evaluation The chemical stability of the synthesized BCZY powders (X = 0, 0.2, 0.3, 0.4) and sprayed membranes (X = 0.4) were tested in flowing pure CO2 (100 ml/min) at 600– 800 -C. The test samples were put in alumina boats separately, and then positioned in a tubular furnace. CO2 was kept flowing through the quartz tube during the whole test period at a rate of 100 ml/min. The furnace was heated at 7 -C/min and constantly held at 600, 700 and 800 -C for 2 h. Finally, it was cooled to ambient temperature at 7 -C/min. The chemical stability of the tested materials at each temperature was determined by XRD phase analysis. The electrical conductivity of the dense membranes was estimated by electrochemical impedance spectroscopy method. The test cell consisted of a porous Pt (anode)|a BaCe0.5Zr0.4Y0.1O3 a membrane/cermet support|a porous Pt (cathode). The electrical conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 a membrane was measured at 400 – 900 -C separately in dry H2 or dry H2 + CO2 (Flow rate H2/CO2: 100/70 ml/min) under ambient pressure. A 4-probe AC electrochemical impedance spectroscopy (EIS) measurement was conducted using a Solartron 1250 impedance analyzer, with an EG and G Model 273 Potentiostat/ Galvonostat, under control of a computer operated by Zplot software. An EIS analysis method was used to derive protonic and electronic conductivity from the total electrical conductivity, based on an equivalent circuit for the testing system. 2.5. Evaluation of mechanical integrity As reported in literatures, BaCeO3-based ceramics exhibit a thermal expansion anomaly at higher temperatures (> 600 -C) that may degrade membrane/support integrity. Therefore, a rapid thermal shock test comprising a cycle with 2-min heat-up to 600 -C, 3-min holding, and 4-min cooling to room temperature, was conducted for the evaluation of mechanical integrity of the membrane-support system. The tested membrane was examined especially at
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the membrane/support interface using metallographic analysis to identify any damages (cracks, delamination and spallation).
3. Results and analyses 3.1. Synthesis of nanocrystalline membrane materials The study has focused on developing optimal synthesis routes that will achieve a complete reaction and single phase structure while maximize retention of nano-sized grains and maintain stoichiometry. Powder synthesis via different routes. First, it was found that the minimum temperature for solid-state reaction increased from 1000 -C to 1400 -C with increasing Zr dopant content from X = 0.0 –0.4 in BaCe0.9 X ZrX Y0.1O3 a . In route 1, the powder was synthesized at a higher temperature 1550 -C for 5 h, and then was reconstituted into flowable feedstock. The higher temperature was selected based primarily on promoting a complete reaction and enhanced synthesis efficiency for scaled-up quantities. The powder was completely reacted and remained single phase, but the feedstock had unfavorable characteristics, viz. low density, and hollow spherical particles. In route 2, the reaction was conducted after powder agglomeration, allowing densification of the feedstock and removal of the binder prior to plasma spray. The major concerns with high temperature routes 1 and 2 were the stoichiometric deviation in BCZYs (literatures reported that barium may evaporate and become CeO2 rich above 1550 -C), as well as grain growth. Some studies have revealed that the solid state reaction can occur at 1350 -C, but post sintering at 1500 -C was required to obtain a single perovskite phase [9,15]. Therefore, based on route 2, the third route was developed, in which the solid-state reaction was conducted at a relatively low temperature 1400 -C followed by plasma-
Fig. 1. XRD patterns for the synthesized BaCe0.9
assisted rapid sintering, to form a single-phase high-density feedstock. X-ray diffraction analysis verified the complete reaction and presence of a single perovskite phase in the BCZY (X = 0.4) synthesized in this route. Nanostructure and lattice parameters. The XRD patterns of the synthesized BCZYs with Zr dopant contents varying from 0.0 to 0.4 are exhibited in Fig. 1, where a-Al2O3 powder was added to the synthesized powder to calibrate the peak position for exact determination of the lattice parameters. The XRD data identified a single phase and a nanostructure from the broadening of the peaks under the synthesis condition at 1450 -C for 12 h. The grain sizes ranged from 20 to 35 nm estimated from the broadening peak (002) according to Scherrer equation. The smaller grain size correlated with more Zr dopant. TEM images analyzed the particle sizes as 30 –60 nm in Fig. 2. XRD broadening is attributed mostly to the fraction of small particles, and the grain size may be underestimated with respect to the actual size observed in TEMs. The lattice parameters of the BCZYs were estimated from the XRD data and are listed in Table 1. The BCZYs have an orthorhombic lattice structure, and the lattice parameters are consistent with published data [7,15,16]. 3.2. Dense membranes produced by plasma spraying The primary plasma spray parameters were investigated to obtain a dense, thin, and adherent nanostructured membrane. Based on the microstructures of membranes made in the different spray trials, feedstock size and density, velocity and trajectory of droplets in plasma plume were identified as major factors determining the membrane density in addition to plasma power and standoff distance. In comparison with a conventional powder feed setup via an external powder port, a modified anode for the 9 MB plasma gun facilitated internal feeding into the axis of the plasma flume. This setup provided the benefit of even heating and
X ZrX Y0.1O3 a .
materials with varying Zr content X.
X. Ma et al. / Surface & Coatings Technology 200 (2005) 1252 – 1258
Fig. 2. TEM image of the synthesized BaCeZrYO3 powder having a nanometer grain size of 30 – 60 nm.
better melting of the feedstock powders by containment in the central ‘‘hot’’ zone. Besides the improvement in melting, high momentum of the molten droplet is more critical upon impacting on the substrate. Thus, a high-velocity plasma nozzle was used in the setup. In plasma spray practice, we determined that the unmelted particles contributed mostly to porosity formation, and these originated from injected powders transiting in the less ‘‘hot’’ boundary zone. USN has developed a method to shield unmelted particles with a plate located in front of the substrate. With the current air plasma spray system, the thickness limitation is about 50 Am considering the major factors about feedstock powder size and droplet velocity. Based on our experience with thin films and membranes [17], a thin layer <20 Am will require a sufficient droplet velocity to completely flatten into pan-cake like splats in addition to the use of fine size particles and their total melting during the plasma spray process.
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BCZY membrane with a high density (> 97%) has indicated that it had a uniform thickness (the hole on the support surface was completely filled as the arrow indicates), was crack-free, and well bonded to the porous cermet support. High resolution SEM revealed a case of complete melting of the droplets. This membrane was fabricated using a fine feedstock and the shielding setup. It also was noticed that cracks in the support did not extend into the membrane, indicating some toughness of the membrane. A 150-Amthick membrane made in the setup using a high-speed nozzle. Somewhat greater porosity was identified in the membrane, mostly from the shorter residence time for the powder traveling in the plasma. The production of nanostructured membranes was verified by XRD analysis and TEM images as well. In Fig. 3, the TEM image shows the sprayed BCZY membrane has equiaxial grains with a grain size distribution of 20 – 100 nm. The diffraction pattern indicates a fully crystallized phase structure. The formation of the nanostructure results
3.3. Microstructure and phase evaluation of the sprayed membrane Dense membranes were applied onto the substrates, having a thickness 60 –150 Am and a density more than 95%. The cross-section of the membrane of a 60-Am-thick Table 1 Lattice parameters for the synthesized BaCe Materials BaCe0.9 X ZrX Y0.1O3 X = 0.0 X = 0.0 [17] X = 0.1 X = 0.1 [17] X = 0.2 X = 0.3 X = 0.4
0.8
Zr0.1Y0.1O3
˚ Lattice Parameters, A d
a
b
c
Unit cell ˚ 3) volume (A
8.806 8.776 8.776 8.734 8.716 8.688 8.678
6.222 6.221 6.221 6.170 6.184 6.149 6.134
6.247 6.224 6.224 6.216 6.177 6.152 6.137
342.2 339.8 339.8 335.0 332.9 328.7 326.7
Determined by X-ray diffraction analysis.
Fig. 3. TEM images of the nanostructured membrane (a) and the electron diffraction pattern (b).
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form two aspects of (i) retained nanograins from the nanostructured feedstock and (ii) re-generated nanostructure in the melted splats during rapid solidification. 3.4. Chemical stability of the nanostructured membrane in CO2 atmosphere Fig. 4a presents the XRDs for the BCZY powders with different Zr dopant contents (X = 0.0 – 0.4) after being treated at 800 -C for 2 h in flowing CO2. The formation of reaction by-product BaCO3 was verified in the material that had a Zr content X = 0.0, 0.1, 0.2 or 0.3; however, there was little BaCO3 in the material with X = 0.4. The diminishing diffraction intensity of BaCO3 with increasing Zr content in the membranes also indicates better CO2 resistance. Moreover, the effect of exposure temperature on CO2resistance of these materials is significant. The chemical stability temperature was evaluated: BCZY (X = 0.0, 0.1) less than 600 -C, BCZY (X = 0.2, 0.3) up to 700 -C and BCZY (X = 0.4) up to 800 -C. The chemical stability of a plasma sprayed BCZY (X = 0.4) membrane was tested under
Fig. 5. Protonic and electronic conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is membrane vs. temperature tested in dry H2 atmosphere.
the same conditions. Fig. 4b indicates that there was no major phase difference in the membranes CO2-treated at 800 -C. Longer term and higher temperature chemical stability for the membrane in fuel/CO2 mixtures will be a major issue and need to be investigated in further tests. 3.5. Electrical conductivity of the membranes
Fig. 4. XRD patterns in (a) the synthesized powders with different Zr contents and (b) the plasma sprayed membrane after treated in CO2 at 800 -C for 2 h.
Membrane conductivity in dry H2. The protonic and electronic conductivity of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is plotted via temperature in Fig. 5. From the initial results, three important aspects of the nano-membrane were identified under the test conditions: (i) the membrane is a mixed conductor of protonic conductivity (r P) and electronic conductivity (r E); (ii) the membrane has a high electronic conductivity r E. With increasing temperature, the ratio of r P to r E becomes 1.0 at about 800 -C; and (iii) the membrane has relatively higher proton conductivity than similar ceramic membranes. For example, the r P of the nano-membrane is 12 10 3 S/cm at 700 -C in dry H2, in comparison with that of BaCeZrNd0.1O3 5 10 3 S/cm at 725 -C [15], and BaCeZrYO3 1 10 3 S/cm at 700 -C [7]. The conductivity r P of BaCeYO3 usually reduces with Zr doping, but the Zr-substituted membrane has the same high r P as BaCe0.9Y0.1O3 (10 10 3 S/cm) at 700 -C. Therefore, it is assumed that the increase in conductivity of the membrane is associated with its nanostructure, and thereby a higher conductivity could be achieved by the reduction in nanograin size. Membrane conductivity in dry H2 + CO2. The plots of conductivity r P and r E of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) membrane is via temperature in mixed H2 + CO2 are given in Fig. 6. In general, the membrane still behaved as a mixed conductor and has a unity ratio of r P/r E at 700 -C.
X. Ma et al. / Surface & Coatings Technology 200 (2005) 1252 – 1258
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rating a nanostructured membrane concept into the manufacturing process. Main results are summarized below:
Fig. 6. Protonic and electronic conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is membrane vs. temperature tested in mixed H2 + CO2 atmosphere.
However, the decrease in r P and r E is readily observed. From the results of CO2 stability tests at 600 – 800 -C, it is clear that the reduced conductivity is not related to the membrane’s stability. Further, it is noticed that the low r P and r E values occur almost over the entire temperature range from 450– 700 -C and maintain the similar linear relation with temperature. Thus, the membrane conductivity depends on the membrane’s surrounding atmosphere as mentioned in several literature articles [9,11,18], and future study will address to the membrane’s characterization in synthetic gas environment. 3.6. Thermo-mechanical strength of the membrane/support system A thermal cycle test was performed between room temperature and 600 -C. With a total 30 cycles, no membrane failed by pulling away from the support. When the membrane was repeatedly tested between room temperature to 900 -C 2– 3 times, there is no observable membrane failure. It also was observed that the membrane on a severely deformed support (caused by the oxidation of Ni in the cermet), still adhered to the support with a few macrocracks. Therefore, it is believed that the cermet support has well matched physical properties with those of the membrane, and will be applicable to the fabrication of H2 separators in planar or tubular form.
4. Conclusions This work has developed a methodology for improving chemical stability, solid ionic conductivity, together with acceptable mechanical properties for a protonic membrane system, by using a doping material technique and incorpo-
1. The Zr ratio, X, in BaCe0.9 X ZrX Y0.1O3 (‘‘BCZY’’) was optimized based on chemical stability under CO2 atmosphere at temperatures from 600 to 800 -C. BaCeZrYO3 (X = 0) reacts with CO2 at 600 -C while BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is stable at 800 -C. The ceramic’s CO2-resistance increases dramatically with increasing Zr content. 2. Low-cost solid state reaction routes were developed for the materials synthesis. Low temperature reaction and plasma-assisted rapid sintering process ensured a complete synthesis reaction, retention of nanograins and minimal stoichiometric deviation. 3. Innovative plasma spray methods were developed for producing high-density BaCe0.5Zr0.4Y0.1O3 (X = 0.4) membranes. The optimized techniques facilitated membrane production with a density > 95% and thickness of only 60 –150 Am. A fully crystalline nanostructured (50 – 120 nm) membrane was verified by TEM analysis. The membrane is single phased with a perovskite (orthorhombic) structure. 4. The BaCeZrYO3 membrane (X = 0.4) exhibited much improved chemical stability at 600 –800 -C in flowing CO2 compared to BaCeO3. 5. The membrane of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) was verified to be mixed conductor. It was demonstrated a high electronic conductivity with a protonic to electronic ratio of unity at 700– 800 -C, which allows non-galvanic mode hydrogen separation at the temperature most suitable for IGCC process. The nanostructured membrane also demonstrated an enhanced protonic conductivity compared to the similar protonic materials in literatures. 6. Thermo-mechanical properties of the membrane/support system are satisfactory. The thermal shock tests verified no presence of cracks or delamination at the membrane/ support interface.
Acknowledgements The authors gratefully acknowledge financial support by the U.S. Department of Energy under Contract No. DEFG01-04ER04-19. The authors thank Dr. X. Huang, Global Fuel Cell Center, the University of Connecticut, for the his experimental assistance in partially conducting the EIS measurement of the membranes; and Dr. D. Xiao, Inframat Corporation, for beneficial discussion.
References [1] B. Gaudernack, S. Lynum, Int. J. Hydrogen Energy 23 (1998) 1087. [2] J. Bockris, Int. J. Hydrogen Energy 24 (1999) 1. [3] S. Uemiya, W. Kato, A. Uyama, M. Kajiwara, T. Kojima, E. Kikuchi, Sep. Purif. Technol. 22 – 23 (2001) 309.
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[4] H. Ohya, S. Sato, A. Ishii, Y. Negishi, K. Matsumoto, Int. J. Hydrogen Energy 18 (1993) 475. [5] R.M. de Vos, W.F. Maier, H. Verweij, J. Membr. Sci. 158 (1999) 277. [6] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3 – 4 (1981) 359. [7] K. Katahira, Y. Kohchi, T. Shimura, H. Iwahara, Solid State Ionics 138 (2000) 91. [8] M.J. Scholten, J. Schonman, J.C. Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [9] K.H. Ryu, S.M. Haile, Solid State Ionics 125 (1999) 355. [10] S. Gopalan, A.V. Virkar, J. Electrochem. Soc. 140 (1993) 1060.
[11] S. Wienstroer, H.D. Wiemhofer, Solid State Ionics 101 – 103 (1997) 1113. [12] R.V. Kumar, J. Alloys Compd. 250 (1997) 501. [13] K. Hyun, S.M. Haile, Solid State Ionics 125 (1999) 355. [14] P.R. Strutt, B.H. Kear, R. Boland, U.S Pat. No.6,025,034, (2000). [15] S. Wienstroer, H.D. Wiemhofer, Solid State Ionics 101 – 103 (1997) 1113. [16] G. Ma, T. Shimura, H. Iwahra, Solid State Ionics 110 (1998) 103. [17] X.Q. Ma, F. Borit, V. Guipont, M. Jeandin, J. Adv. Mater. 34 (2002) 52. [18] T. Norby, Y. Larring, Solid State Ionics 136 – 137 (2000) 139.
Protonic conductivity nanostructured ceramic film with improved resistance to carbon dioxide at elevated temperatures Xinqing Ma *, Jinxiang Dai, Heng Zhang, David E. Reisner US Nanocorp, Inc., 74 Batterson Park Road, Farmington, CT 06032, USA Available online 12 September 2005
Abstract As a high value product, hydrogen is a clean fuel with zero emission, and thereby its applications alleviate the threat of ‘‘Greenhouse effect.’’ From the ‘‘hydrogen economy’’ point of view, the predominant way of producing hydrogen is reforming from fossil fuels. A dense ceramic protonic conductive film can be used to separate hydrogen from other reformed products or syngas in gasification process at high temperatures. However, currently existing ceramic films are proven to severely degrade in CO2-containing environments. In this work, a codoped BaCeO3 material was proposed for better CO2 resistance and higher protonic conductivity. Nanostructured BaCeO3-based film was fabricated from nano-grain feedstock using air plasma spray. The hydrogen permeable film has been demonstrated superior to currently available ceramic protonic films for hydrogen separation in terms of chemical stability, protonic and electronic conductivity, and thermomechanical properties in the temperature range of 600 – 800 -C. This work has demonstrated a methodology to improve chemical stability, solid ionic conductivity, as well as good mechanical integrity for a protonic membrane system, using a doping composition technique and incorporating a nanostructured membrane manufacturing process. D 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Protonic conductivity; Ceramic film; CO2-resistance; High temperature
1. Introduction Hydrogen is a clean fuel in the sense that its applications will not emit toxic gas and green house effect CO2 gas [1]. Hydrogen is a highly valuable product, but not an available primary fuel. The annual hydrogen consume and production is expected to increase as the world is moving toward the application of hydrogen energy carrier because of environmental concern. Today’s water electrolysis technology is neither cost effective nor practical for producing hydrogen because the energy efficiency is low and the process consumes electric energy, a clean energy. Economically, the predominant way is to produce hydrogen from fossil fuel, such as coal and natural gas [2]. The produced hydrogen is usually mixed with other products. Therefore, it is important to develop a hydrogen separation technology to divide hydrogen from by-products. Using a separation * Corresponding author. E-mail address: [email protected] (X. Ma). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.07.099
process, high purity hydrogen can be produced from the reformed fuel gas. Now, worldwide development effort is focusing on the hydrogen separation membrane technology. Hydrogen permeable membranes include metallic membranes, porous inorganic membranes and dense ceramic membranes [3 –6]. In particular, ceramic membranes are of importance and interest compared to metallic hydrogenpermeable membranes such as palladium or its alloys. The advantages of ceramic membranes include high temperature application, up to 600 –900 -C, high resistance to aggressive corrosion degradation including oxidation and sulfidation, no hydrogen brittleness problem, no sulfur poisoning in H2S and SO2 atmospheres, low-cost and robust performance. Perovskite-based oxide ceramic BaCeO3 has been wellidentified in high proton conductivity. However, its decomposition in the presence of acidic gas CO2 or SO2 and moisture has been reported in extensive studies [7 –10]. The material degradation is caused by high basicity of BaCeO3, which causes it to react with CO2/SO2 and form BaCO3, CeO2, BaSO4, or Ba(OH)2 depending on environment and
X. Ma et al. / Surface & Coatings Technology 200 (2005) 1252 – 1258
temperature. Therefore, BaCeO3 is not a viable and practical membrane material used for hydrogen separation from synthetic fuel gas at high temperature range of 600 –900 -C. Rare earth element (R = Y, La, Gd or Nd, etc.) doped BaCeO3 (BaCe1 X RX O3) may increase chemical stability, but a high dopant content (X > 0.6) is required [11 – 13]. Inversely, high dopant content will change lattice parameters or even crystalline structure and hence degrade protonic conductivity of barium cerates. Therefore, the challenging is to develop a new BaCeO3-based ceramic that will compromise the chemical stability and proton conductivity. This work has investigated the synthesis routes for nanostructured Zr-substituted barium cerate-based materials and their chemical stability in CO2-containing environment at 600 –800 -C. Nanostructured dense membranes were fabricated from the synthesized barium cerate-based materials by atmospheric plasma spray techniques. The plasma sprayed dense membranes were evaluated in microstructure, phase composition, protonic/electronic conductivity and mechanical integrity.
2. Experimental procedures 2.1. Synthesis of nanocrystalline membrane materials Zr-substituted BCY (BaCe0.9 X ZrX Y0.1O3 a ) was synthesized by solid-state reaction routes. The starting materials included barium carbonate as the Ba source, and nano-sized CeO2, Y2O3 and ZrO2 powders as Ce, Y and Zr sources. The nano-oxides (< 20 nm) were synthesized with US Nanocorp’s (USN) patented synthesis technique, a wet chemical reaction in an aqueous solution [14]. Three routes were developed for the synthesis of BCZYs with the Zr dopant concentration X, varying from 0.0 to 0.4. The synthesis routes were: (i) Route 1: high temperature reaction at 1550 -C for 5 h followed by powder reconstitution; (ii) Route 2: high temperature reaction starting nano-powders reconstitution followed by a solid state reaction at 1650 -C for 4 h; and (iii) Route 3: low temperature reaction at 1450 -C followed by powder reconstitution and plasma-assisted reaction and densification. 2.2. Dense membrane deposition by plasma spraying Air plasma spray was employed to produce the BCZY membrane. The optimization of plasma spray conditions was centered on attaining high density (> 95%) and a nanostructure. A Metco 9 MB-gun plasma spray system was used for producing the dense membrane on a porous support. The major plasma spray parameters included working gas, current/voltage, powder feed rate and specific setups for powder feeding. The influence of different particle sized feedstock with varying morphology and density, on the resultant membrane density, was investigated. The membranes were deposited on a porous
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Ni + BaCeO3 cermet substrate that was designed and fabricated by USN. 2.3. Microstructure and phase evaluation of the sprayed membranes The sprayed membranes were examined on cross-section microstructures using an optical microscope and scanning electron microscopy (SEM). The defects in the membranes were examined on pores, cracks, unmelted inclusions, and interface delamination. Phase compositions in as-sprayed membranes and the tested samples were determined by Xray diffraction (XRD). The grain sizes of the membranes were estimated indirectly from the broadening peak widths in XRD spectra, and measured directly from TEM images. 2.4. Nanostructured membrane performance evaluation The chemical stability of the synthesized BCZY powders (X = 0, 0.2, 0.3, 0.4) and sprayed membranes (X = 0.4) were tested in flowing pure CO2 (100 ml/min) at 600– 800 -C. The test samples were put in alumina boats separately, and then positioned in a tubular furnace. CO2 was kept flowing through the quartz tube during the whole test period at a rate of 100 ml/min. The furnace was heated at 7 -C/min and constantly held at 600, 700 and 800 -C for 2 h. Finally, it was cooled to ambient temperature at 7 -C/min. The chemical stability of the tested materials at each temperature was determined by XRD phase analysis. The electrical conductivity of the dense membranes was estimated by electrochemical impedance spectroscopy method. The test cell consisted of a porous Pt (anode)|a BaCe0.5Zr0.4Y0.1O3 a membrane/cermet support|a porous Pt (cathode). The electrical conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 a membrane was measured at 400 – 900 -C separately in dry H2 or dry H2 + CO2 (Flow rate H2/CO2: 100/70 ml/min) under ambient pressure. A 4-probe AC electrochemical impedance spectroscopy (EIS) measurement was conducted using a Solartron 1250 impedance analyzer, with an EG and G Model 273 Potentiostat/ Galvonostat, under control of a computer operated by Zplot software. An EIS analysis method was used to derive protonic and electronic conductivity from the total electrical conductivity, based on an equivalent circuit for the testing system. 2.5. Evaluation of mechanical integrity As reported in literatures, BaCeO3-based ceramics exhibit a thermal expansion anomaly at higher temperatures (> 600 -C) that may degrade membrane/support integrity. Therefore, a rapid thermal shock test comprising a cycle with 2-min heat-up to 600 -C, 3-min holding, and 4-min cooling to room temperature, was conducted for the evaluation of mechanical integrity of the membrane-support system. The tested membrane was examined especially at
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the membrane/support interface using metallographic analysis to identify any damages (cracks, delamination and spallation).
3. Results and analyses 3.1. Synthesis of nanocrystalline membrane materials The study has focused on developing optimal synthesis routes that will achieve a complete reaction and single phase structure while maximize retention of nano-sized grains and maintain stoichiometry. Powder synthesis via different routes. First, it was found that the minimum temperature for solid-state reaction increased from 1000 -C to 1400 -C with increasing Zr dopant content from X = 0.0 –0.4 in BaCe0.9 X ZrX Y0.1O3 a . In route 1, the powder was synthesized at a higher temperature 1550 -C for 5 h, and then was reconstituted into flowable feedstock. The higher temperature was selected based primarily on promoting a complete reaction and enhanced synthesis efficiency for scaled-up quantities. The powder was completely reacted and remained single phase, but the feedstock had unfavorable characteristics, viz. low density, and hollow spherical particles. In route 2, the reaction was conducted after powder agglomeration, allowing densification of the feedstock and removal of the binder prior to plasma spray. The major concerns with high temperature routes 1 and 2 were the stoichiometric deviation in BCZYs (literatures reported that barium may evaporate and become CeO2 rich above 1550 -C), as well as grain growth. Some studies have revealed that the solid state reaction can occur at 1350 -C, but post sintering at 1500 -C was required to obtain a single perovskite phase [9,15]. Therefore, based on route 2, the third route was developed, in which the solid-state reaction was conducted at a relatively low temperature 1400 -C followed by plasma-
Fig. 1. XRD patterns for the synthesized BaCe0.9
assisted rapid sintering, to form a single-phase high-density feedstock. X-ray diffraction analysis verified the complete reaction and presence of a single perovskite phase in the BCZY (X = 0.4) synthesized in this route. Nanostructure and lattice parameters. The XRD patterns of the synthesized BCZYs with Zr dopant contents varying from 0.0 to 0.4 are exhibited in Fig. 1, where a-Al2O3 powder was added to the synthesized powder to calibrate the peak position for exact determination of the lattice parameters. The XRD data identified a single phase and a nanostructure from the broadening of the peaks under the synthesis condition at 1450 -C for 12 h. The grain sizes ranged from 20 to 35 nm estimated from the broadening peak (002) according to Scherrer equation. The smaller grain size correlated with more Zr dopant. TEM images analyzed the particle sizes as 30 –60 nm in Fig. 2. XRD broadening is attributed mostly to the fraction of small particles, and the grain size may be underestimated with respect to the actual size observed in TEMs. The lattice parameters of the BCZYs were estimated from the XRD data and are listed in Table 1. The BCZYs have an orthorhombic lattice structure, and the lattice parameters are consistent with published data [7,15,16]. 3.2. Dense membranes produced by plasma spraying The primary plasma spray parameters were investigated to obtain a dense, thin, and adherent nanostructured membrane. Based on the microstructures of membranes made in the different spray trials, feedstock size and density, velocity and trajectory of droplets in plasma plume were identified as major factors determining the membrane density in addition to plasma power and standoff distance. In comparison with a conventional powder feed setup via an external powder port, a modified anode for the 9 MB plasma gun facilitated internal feeding into the axis of the plasma flume. This setup provided the benefit of even heating and
X ZrX Y0.1O3 a .
materials with varying Zr content X.
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Fig. 2. TEM image of the synthesized BaCeZrYO3 powder having a nanometer grain size of 30 – 60 nm.
better melting of the feedstock powders by containment in the central ‘‘hot’’ zone. Besides the improvement in melting, high momentum of the molten droplet is more critical upon impacting on the substrate. Thus, a high-velocity plasma nozzle was used in the setup. In plasma spray practice, we determined that the unmelted particles contributed mostly to porosity formation, and these originated from injected powders transiting in the less ‘‘hot’’ boundary zone. USN has developed a method to shield unmelted particles with a plate located in front of the substrate. With the current air plasma spray system, the thickness limitation is about 50 Am considering the major factors about feedstock powder size and droplet velocity. Based on our experience with thin films and membranes [17], a thin layer <20 Am will require a sufficient droplet velocity to completely flatten into pan-cake like splats in addition to the use of fine size particles and their total melting during the plasma spray process.
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BCZY membrane with a high density (> 97%) has indicated that it had a uniform thickness (the hole on the support surface was completely filled as the arrow indicates), was crack-free, and well bonded to the porous cermet support. High resolution SEM revealed a case of complete melting of the droplets. This membrane was fabricated using a fine feedstock and the shielding setup. It also was noticed that cracks in the support did not extend into the membrane, indicating some toughness of the membrane. A 150-Amthick membrane made in the setup using a high-speed nozzle. Somewhat greater porosity was identified in the membrane, mostly from the shorter residence time for the powder traveling in the plasma. The production of nanostructured membranes was verified by XRD analysis and TEM images as well. In Fig. 3, the TEM image shows the sprayed BCZY membrane has equiaxial grains with a grain size distribution of 20 – 100 nm. The diffraction pattern indicates a fully crystallized phase structure. The formation of the nanostructure results
3.3. Microstructure and phase evaluation of the sprayed membrane Dense membranes were applied onto the substrates, having a thickness 60 –150 Am and a density more than 95%. The cross-section of the membrane of a 60-Am-thick Table 1 Lattice parameters for the synthesized BaCe Materials BaCe0.9 X ZrX Y0.1O3 X = 0.0 X = 0.0 [17] X = 0.1 X = 0.1 [17] X = 0.2 X = 0.3 X = 0.4
0.8
Zr0.1Y0.1O3
˚ Lattice Parameters, A d
a
b
c
Unit cell ˚ 3) volume (A
8.806 8.776 8.776 8.734 8.716 8.688 8.678
6.222 6.221 6.221 6.170 6.184 6.149 6.134
6.247 6.224 6.224 6.216 6.177 6.152 6.137
342.2 339.8 339.8 335.0 332.9 328.7 326.7
Determined by X-ray diffraction analysis.
Fig. 3. TEM images of the nanostructured membrane (a) and the electron diffraction pattern (b).
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form two aspects of (i) retained nanograins from the nanostructured feedstock and (ii) re-generated nanostructure in the melted splats during rapid solidification. 3.4. Chemical stability of the nanostructured membrane in CO2 atmosphere Fig. 4a presents the XRDs for the BCZY powders with different Zr dopant contents (X = 0.0 – 0.4) after being treated at 800 -C for 2 h in flowing CO2. The formation of reaction by-product BaCO3 was verified in the material that had a Zr content X = 0.0, 0.1, 0.2 or 0.3; however, there was little BaCO3 in the material with X = 0.4. The diminishing diffraction intensity of BaCO3 with increasing Zr content in the membranes also indicates better CO2 resistance. Moreover, the effect of exposure temperature on CO2resistance of these materials is significant. The chemical stability temperature was evaluated: BCZY (X = 0.0, 0.1) less than 600 -C, BCZY (X = 0.2, 0.3) up to 700 -C and BCZY (X = 0.4) up to 800 -C. The chemical stability of a plasma sprayed BCZY (X = 0.4) membrane was tested under
Fig. 5. Protonic and electronic conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is membrane vs. temperature tested in dry H2 atmosphere.
the same conditions. Fig. 4b indicates that there was no major phase difference in the membranes CO2-treated at 800 -C. Longer term and higher temperature chemical stability for the membrane in fuel/CO2 mixtures will be a major issue and need to be investigated in further tests. 3.5. Electrical conductivity of the membranes
Fig. 4. XRD patterns in (a) the synthesized powders with different Zr contents and (b) the plasma sprayed membrane after treated in CO2 at 800 -C for 2 h.
Membrane conductivity in dry H2. The protonic and electronic conductivity of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is plotted via temperature in Fig. 5. From the initial results, three important aspects of the nano-membrane were identified under the test conditions: (i) the membrane is a mixed conductor of protonic conductivity (r P) and electronic conductivity (r E); (ii) the membrane has a high electronic conductivity r E. With increasing temperature, the ratio of r P to r E becomes 1.0 at about 800 -C; and (iii) the membrane has relatively higher proton conductivity than similar ceramic membranes. For example, the r P of the nano-membrane is 12 10 3 S/cm at 700 -C in dry H2, in comparison with that of BaCeZrNd0.1O3 5 10 3 S/cm at 725 -C [15], and BaCeZrYO3 1 10 3 S/cm at 700 -C [7]. The conductivity r P of BaCeYO3 usually reduces with Zr doping, but the Zr-substituted membrane has the same high r P as BaCe0.9Y0.1O3 (10 10 3 S/cm) at 700 -C. Therefore, it is assumed that the increase in conductivity of the membrane is associated with its nanostructure, and thereby a higher conductivity could be achieved by the reduction in nanograin size. Membrane conductivity in dry H2 + CO2. The plots of conductivity r P and r E of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) membrane is via temperature in mixed H2 + CO2 are given in Fig. 6. In general, the membrane still behaved as a mixed conductor and has a unity ratio of r P/r E at 700 -C.
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rating a nanostructured membrane concept into the manufacturing process. Main results are summarized below:
Fig. 6. Protonic and electronic conductivity of the nanostructured BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is membrane vs. temperature tested in mixed H2 + CO2 atmosphere.
However, the decrease in r P and r E is readily observed. From the results of CO2 stability tests at 600 – 800 -C, it is clear that the reduced conductivity is not related to the membrane’s stability. Further, it is noticed that the low r P and r E values occur almost over the entire temperature range from 450– 700 -C and maintain the similar linear relation with temperature. Thus, the membrane conductivity depends on the membrane’s surrounding atmosphere as mentioned in several literature articles [9,11,18], and future study will address to the membrane’s characterization in synthetic gas environment. 3.6. Thermo-mechanical strength of the membrane/support system A thermal cycle test was performed between room temperature and 600 -C. With a total 30 cycles, no membrane failed by pulling away from the support. When the membrane was repeatedly tested between room temperature to 900 -C 2– 3 times, there is no observable membrane failure. It also was observed that the membrane on a severely deformed support (caused by the oxidation of Ni in the cermet), still adhered to the support with a few macrocracks. Therefore, it is believed that the cermet support has well matched physical properties with those of the membrane, and will be applicable to the fabrication of H2 separators in planar or tubular form.
4. Conclusions This work has developed a methodology for improving chemical stability, solid ionic conductivity, together with acceptable mechanical properties for a protonic membrane system, by using a doping material technique and incorpo-
1. The Zr ratio, X, in BaCe0.9 X ZrX Y0.1O3 (‘‘BCZY’’) was optimized based on chemical stability under CO2 atmosphere at temperatures from 600 to 800 -C. BaCeZrYO3 (X = 0) reacts with CO2 at 600 -C while BaCe0.5Zr0.4Y0.1O3 (X = 0.4) is stable at 800 -C. The ceramic’s CO2-resistance increases dramatically with increasing Zr content. 2. Low-cost solid state reaction routes were developed for the materials synthesis. Low temperature reaction and plasma-assisted rapid sintering process ensured a complete synthesis reaction, retention of nanograins and minimal stoichiometric deviation. 3. Innovative plasma spray methods were developed for producing high-density BaCe0.5Zr0.4Y0.1O3 (X = 0.4) membranes. The optimized techniques facilitated membrane production with a density > 95% and thickness of only 60 –150 Am. A fully crystalline nanostructured (50 – 120 nm) membrane was verified by TEM analysis. The membrane is single phased with a perovskite (orthorhombic) structure. 4. The BaCeZrYO3 membrane (X = 0.4) exhibited much improved chemical stability at 600 –800 -C in flowing CO2 compared to BaCeO3. 5. The membrane of BaCe0.5Zr0.4Y0.1O3 (X = 0.4) was verified to be mixed conductor. It was demonstrated a high electronic conductivity with a protonic to electronic ratio of unity at 700– 800 -C, which allows non-galvanic mode hydrogen separation at the temperature most suitable for IGCC process. The nanostructured membrane also demonstrated an enhanced protonic conductivity compared to the similar protonic materials in literatures. 6. Thermo-mechanical properties of the membrane/support system are satisfactory. The thermal shock tests verified no presence of cracks or delamination at the membrane/ support interface.
Acknowledgements The authors gratefully acknowledge financial support by the U.S. Department of Energy under Contract No. DEFG01-04ER04-19. The authors thank Dr. X. Huang, Global Fuel Cell Center, the University of Connecticut, for the his experimental assistance in partially conducting the EIS measurement of the membranes; and Dr. D. Xiao, Inframat Corporation, for beneficial discussion.
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