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Preparation and characterization of hollow glass microspheres (HGMs) for hydrogen storage using urea as a blowing agent
Microelectronic Engineering 126 (2014) 65–70
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Preparation and characterization of hollow glass microspheres (HGMs) for hydrogen storage using urea as a blowing agent Sridhar Dalai a, S. Vijayalakshmi a,⇑, Pragya Shrivastava a, Santosh Param Sivam b, Pratibha Sharma c a
Centre for Research in Nano-Technology and Science (CRNTS), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India Amity Institute of Nanotechnology, Amity University, Noida, Uttar Pradesh 201303, India c Department of Energy Science and Engineering (DESE), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India b
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 22 May 2014 Accepted 20 June 2014 Available online 30 June 2014 Keywords: Hydrogen storage Hollow glass microspheres (HGMs) Amber glass Blowing agent Flame spheroidisation Urea
a b s t r a c t Hollow glass microspheres (HGMs) are unique class of porous material that can be used for hydrogen storage. They are finely dispersed and free flowing powder having microsphere diameter, 10–200 lm and wall thickness, 0.5–2 lm. In this report, we present the preparation of HGMs from glass frits using air-acetylene flame spheroidisation method. Urea was incorporated in the feed glass as a blowing agent to get porous walled HGMs. Effect of concentration of urea in the feed glass on the development of pores on HGMs wall was investigated. It was observed that, the porosity of the HGMS walls increased with the increase in urea content in the feed glass. Characterization of HGMs prepared was done using FE–SEM, ESEM, FTIR and XRD techniques. The spherical nature and the morphology of the HGMs samples prepared were confirmed using FE–SEM, while the pore diameter and the porosity of the samples was observed in the ESEM images. Hydrogen adsorption experiments were carried out at room temperature (RT) and 200 °C at 10 bar pressure, for 5 h for all the HGMs samples prepared. The HGMs prepared with 2% urea showed large number of micro/nano pores and it showed a hydrogen storage capacity of 2.3 wt.% at 200 °C and 10 bar pressure. The hydrogen adsorption on HGMs at RT and 10 bar pressure was lower than that observed at 200 °C and 10 bar pressure. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Replacement of fossil fuels with hydrogen can lead to a major reduction in the generation of pollutants and provide a path by which current non-renewable fuels can be replaced by a renewable one. The use of hydrogen gas in fuel cell has been considered as a safer and more environment friendly alternative to the use of gasoline and fossil fuels. The use of hydrogen to address the energy economy issue, emphasis has to be given on the production of clean hydrogen, distribution and its storage [1–5]. There are various hydrogen storage methods such as gas storage, liquid hydrogen and solid storage. Compressed hydrogen gas storage requires heavy storage cylinders which has high risk of explosion while transportation. Storage of hydrogen as a liquid at low temperatures requires high costs to maintain the low temperatures. However, the storage of hydrogen in solid materials is feasible except for the high temperature conditions to be applied for the release of stored hydrogen [6–9]. Hollow glass microspheres (HGMs) have ⇑ Corresponding author. Tel.: +91 22 25767671; fax: +91 2225723314. E-mail addresses: [email protected] (S. Dalai), [email protected] (S. Vijayalakshmi), [email protected] (P. Shrivastava), santosh.sivam@ gmail.com (S.P. Sivam), [email protected] (P. Sharma). http://dx.doi.org/10.1016/j.mee.2014.06.017 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
been shown to be a promising hydrogen storage material and it has many advantages over other hydrogen storage techniques. The hydrogen storage capacity of HGMs is 100 MPa of hydrogen. The raw materials for production of HGMs are generally recycled cullets and hence are cost effective and require low energy consumption for production [10–13]. There are several methods used to produce HGMs [14–21], of which the flame sprayed pyrolysis of glass frit will be suitable for preparation of porous walled HGMs for hydrogen storage. The prepared feed glass powder is sprayed into an oxy- acetylene flame, so that the viscosity of glass frits decreases and takes a spherical form due to surface tension. The liquid HGMs cool quickly on leaving the flame, and retain the spherical shape. This method often results good yield of HGMs with uniform diameters and wall thickness [22–24]. The properties of HGMs, such as diffusivity, strength and reversibility, are of important to monitor the hydrogen storage properties of the HGMs. The HGMs has long been proposed for hydrogen storage as a solution to the hydrogen storage for energy applications [25]. Diffusivity of gases depends upon the temperature and size of the gas molecule. As a result, heating HGMs allows the smallest gas atoms and molecules to diffuse through the porous wall of HGMs into hollow cavity [26,27]. To improve the hydrogen storage
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capacity in HGMs, it is necessary to increase the number of micro/ nano pores on the HGMs wall. This can be achieved by adding the blowing agent to the glass feed before flame spheroidisation. The percentage of the blowing agent required is very low; often 0.5– 2% is preferred, depending on its blowing ability [28,29]. In this paper, we report the preparation of HGMs from amber glass frit by flame spraying method. Modification of microsphere wall thickness and porosity were tried by incorporating known amount of blowing agent like urea while preparing the feed glass for HGMs. We have also studied the effect of blowing agent concentration on the hydrogen storage properties of HGMs. 2. Experimental details 2.1. Preparation of HGMs from feed glass with blowing agent using flame spraying method
Transform Infrared spectroscopy and X-ray Diffraction techniques. Field Emission–Scanning Electron Microscope (FE–SEM) JEOL make JSM-7600F was used to study the surface morphology of the feed glass powder and the HGMs. Ultra trace amount of the respective sample dispersed in acetone was spread on the SEM sample holder, dried; sputter coated with a thin layer of platinum to reduce charging and was used for SEM analysis. Environmental scanning electron microscopy (ESEM), FEI QUANTA 200 in back scattered electron (BSE) mode was used to examine the shape and size of the pores on the HGMs walls. The spheres were placed as a thin layer on the conductive tape fixed over the sample holder. The holder was loaded into the sample chamber and evacuated. The sample spot size kept was 4.0– 6.0 nm; pressure 40–70 Pa and accelerating potential of 10–30 kV. The phase structure analysis of feed and products was done using an X-ray diffractometer (XRD), X’Pert-Pro, Pan analytical utilizing Cu Ka X-radiation of wavelength 1.54 Å, and 2h range, 5– 100°. The FTIR spectrum of the feed and product HGMs mixed with KBr were recorded in the range 400–4000 cm 1 at room temperature on a Bruker Fourier Transform Infrared Spectroscopy (Vertex 80).
Broken amber glass bottles from the laboratory were crushed to particle size 1 mm. About 1 kg of the crushed amber glass were further pulverized and sieved to different particle size ranging from 35–120 lm to get the feed glass powder. Based on the results obtained on the effect of feed glass particle size and feed flow rate on the yield of HGMs, it was decided to use the feed particle size, 63–75 lm and feed flow rate 100 mg/min [30] for the HGMs sample preparation reported in this paper. About 2% by weight urea solution in water was prepared and was used to blend urea with the feed glass powder by soak and dry method. Known volume of the urea stock solution was added to a fixed quantity of glass powder to make feed glass powder with 0.3%, 0.5%, 1%, 2%, 3% and 5% of blowing agent. The amber glass powder mixed with the urea solution was stirred at room temperature for 4 h initially and continued stirring at 50 °C till a dried mass of the glass powder was obtained. Further it was dried in an oven at 100 °C, gently ground using an agate mortar- pestle and stored in desiccators. The urea mixed feed glass powder was converted into HGMs using flame spheroidisation method. The schematic diagram of flame spraying setup is shown in Fig. 1. The HGMs prepared were labeled as HAU0.3, HAU0.5, HAU1, HAU2, HAU3 & HAU5 where, HA stands for hollow glass microspheres prepared from amber glass frit, U denotes the urea as blowing agent and the numerical values in the sample code indicate the percentage of urea in the feed glass.
About 100 mg of the HGMs sample was exactly weighed and was placed inside a quartz tube which was then inserted in the sample cell. The entire system was heated to 200 °C and evacuated using a rotary vacuum pump to 10 3mbar for 1 h, prior to the hydrogen adsorption study. This helps to remove any moisture and surface impurities present in the sample. Highly pure (99.99%) hydrogen gas at pressure of 10 bar was introduced into the sample cell for adsorption on the HGMs sample. The decrease in pressure with respect to time at constant temperature was noted using a digital data logger. When hydrogen is send to the sample, both, absorption as well as adsorption of the gas takes place on the sample. The decrease in pressure value was used to calculate the amount of hydrogen uptake by the sample. This data was used to construct the Pressure-Composition (PC) isotherm. Adsorption of hydrogen at room temperature (RT) and 200 °C on all the samples was done at pressure 10 bar for 5 h. A schematic diagram of the Sievert’s type apparatus used for hydrogen storage on HGMs samples is shown in Fig. 2 [31].
2.2. Characterization
3. Results and discussion
The feed glass powder and the HGMs with and without urea were characterized using scanning electron microscopy, Fourier
FE–SEM images of amber feed glass and product HGMs are shown in Fig. 3. The feed particle size in the range 63–75 lm and
2.3. Experiments on hydrogen gas filling
Fig. 1. Schematic diagram of the experimental setup for preparation of HGMs.
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Fig. 2. Block diagram Sievert’s type apparatus.
Fig. 3. FEGSEM images of (a) amber feed glass, (b) product HGMs (HA), (c) corresponding magnified HGMs showing its wall thickness (inset).
feed flow rate 100 mg/min was found to give 95% conversion to uniform sized HGMs. It was found that synthesized HGMs have 10–100 lm diameter and 1to 2 lm wall thickness. 3.1. FTIR and XRD studies on feed and product HGMs FTIR studies on the urea loaded feed glass and product HGMs were conducted in the range 400–4000 cm 1 using Bruker make FTIR spectrometer. The FTIR transmission spectrums of feed and product samples are shown in Fig. 4a. The X-ray Diffractogram data for the feed and HGMs in the 2h range, 5–90° were collected and is shown in Fig. 4b. From Fig. 4a, it was clearly observed that all the feed samples and the respective HGMs samples showed IR transmission bands corresponding to the bending, stretching and the anti-symmetric stretching vibrations of the SiO2 bonds. Apart from that, significant IR signal was observed at wave number, 1423 cm 1, which is corresponding to the asymmetric stretching vibration of N–C–N. The peak at 1627 cm 1 is due to the bending vibration of H–O–H. The peaks at 2925 and 3438 cm 1 are corresponding to the O–H stretching vibration. Thus from the FTIR analysis of the feed glass as well as the HGMs samples, it can be confirmed that the urea
impregnated in the feed glass sample has been decomposed to ammonia and oxides of carbon at high temperature spheroidisation [32–36]. The XRD analysis of the feed and the product HGMs (Fig. 4b) did not show any well-defined peaks, but showed a hump centered at 2h = 22° which shows the amorphous nature with short range crystalline, characteristic of glass substances [37,38]. 3.2. Effect of urea as blowing agent on the morphology of HGMs Inclusion of urea as blowing agent in the feed glass powder showed a very interesting trend in the physical morphology as well as the hydrogen adsorption behavior of the HGMs. The ESEM images of HA, HAU0.3, HAU0.5, HAU1, HAU2, HAU3 and HAU5 are shown in Figs. 5–7. From Fig. 5, it was observed that the formation of pores on the walls of HGMs was enhanced considerably, when urea was incorporated in the feed glass. But the actual nature of the pores, whether it is a through-put pore on the walls or inter-connected pores or only a deep cavity formed on the walls, was not clear from the SEM pictures. From the ESEM images (Fig. 5), it was observed that the use of urea as the blowing agent left the HGMs, highly
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Fig. 4. (a) FTIR spectra, (b) XRD patterns of urea loaded feed samples and the respective HGMs samples with concentration of 0.3, 1, 2 wt.%.
Fig. 5. ESEM images of (a) HA, (b) HAU2, (c) and (d) are magnified images of (a) and (b).
porous compared to the HGMs prepared without urea. This was due to the decomposition of urea to ammonia and oxides of carbon during the flame spheroidisation.
From Fig. 6, it was observed that, there was only a nominal change in pore formation on the surface of HGMs, when the percentage of urea increased from 0.3% to 1%. The ESEM images
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Fig. 6. ESEM images of (a) HAU0.3, (b) HAU0.5, (c) HAU1.
Fig. 7. ESEM images of (a) HAU0.3, (b) HAU2, (c) HAU3, (d) HAU5.
(Fig. 7), showed that, the size and number of pores developed, varied with the percentage of urea used in the feed glass. It was also observed that the number of pores increased when the percentage of urea increased from 0.3% to 2%. When the urea content was increased further to 3 and 5 wt.%, most of the small pores were closed (Fig. 7 c and d) and there was a decrease in pore formation on the surface of HGMs. 3.3. Hydrogen storage in HGMs prepared with urea Hydrogen storage experiments on HAU0.3 samples were conducted at ambient temperature and 200 °C under 10 bar of pressure for 5 h in the Sievert’s type apparatus fabricated in the laboratory. It was observed that, uptake of hydrogen at 200 °C was more than that obtained at room temperature. This is due to
the dependency of diffusivity of gas with temperature and the size of the gas molecule. On heating the HGMs to 200 °C, the diffusivity of hydrogen gas increases and allows hydrogen molecule to diffuse across the microsphere wall into hollow cavity. But at room temperature, the diffusivity of hydrogen gas is very low and no significant uptake of hydrogen was observed. Thus the diffusivity of hydrogen through the microsphere walls increases with temperature. Hence, further experiments on hydrogen adsorption on the HAUx samples were done at 200 °C and 10 bar for 5 h. The hydrogen uptake curves at 200 °C and 10 bar pressure, for samples HAU0.3, 2, 3 & 5 are shown in Fig. 8. The uptake of hydrogen gas on HGMs increased with increase of the urea concentration up to 2 wt.%, but decreased with further increase of the urea concentration from 2 to 5 wt.%. The HAU3 and HAU5 suffered a partial pore closure, as seen from ESEM images (Fig. 7) which hampered
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as blowing agent were capable of storing hydrogen gas when exposed to hydrogen at high temperature and high pressure. The hydrogen adsorption capacity of the urea loaded HGMs increases with the increase in urea loading from 0.3% to 2%. Furthermore an increase of percentage of urea in amber glass feed decreases pore formation on microsphere wall due to the high temperature polymerization of urea happening during flame spheroidisation. The maximum hydrogen storage capacity was found to be 2.3 wt.% for HAU2 at 200 °C for a pressure 10 bar which is better compared with HA. Acknowledgment The authors gratefully acknowledge the support of sophisticated analytical instrument facilities at SAIF, IIT Bombay during this research work. References
Fig. 8. Hydrogen uptake curve for HAU 0.3, 2, 3 & 5 at 200 °C.
Fig. 9. Hydrogen uptake curve for HA and HAU2 at 200 °C.
the diffusivity of hydrogen gas into the microsphere cavity. Pore closure and pore narrowing in the HGMs at higher loading of urea is due to the polymerization of urea during flame spheroidisation leading to the deposition of the polymer molecule at the pore mouth. The hydrogen storage capacity of HA and HAU2 are compared in Fig. 9. It was observed that hydrogen storage capacity of HAU2 is 2.3 wt.% whereas that of HA is about 1 wt.% only. From these results, it is confirmed that more pore formation on surface of HGMs enhances the hydrogen uptake. 4. Conclusion Porous walled HGMs were fabricated from amber glass frit by flame spraying method using urea as blowing agent. Both HGMs with and without urea were found to be amorphous in structure. The HGMs prepared using urea as blowing agent (HAU2) was found to have large number of micro/nanopores on the walls. Thus, it is confirmed that the urea impregnated in the feed glass sample has been decomposed to ammonia and oxides of carbon at high temperature spheroidisation. The HGMs prepared by using urea
[1] Cong Gao, Xiaobo Qi, Zhanwen Zhang, Sufen Chen, Bo Li, Weixing Huang, Int. J. Hydrogen Energy 36 (2011) 9758–9766. [2] S. Dunn, Int. J. Hydrogen Energy 27 (3) (2002) 35–64. [3] M. Momirlan, T.N. Veziroglu, Int. J. Hydrogen Energy 30 (7) (2005) 795–802. [4] D. Mori, K. Hirose, Int. J. Hydrogen Energy 34 (10) (2009) 4569–4574. [5] S.Z. Shi, J.Y. Hwang, Int. J. Hydrogen Energy 32 (2007) 224–228. [6] K.L. Yan, B.G. Sellars, J. Lo, S. Johar, M.K. Murthy, Int. J. Hydrogen Energy 10 (7/ 8) (1985) 517–522. [7] K.L. Lim, H. Kazemian, Z. Yaakob, W.R.W. Daud, Chem. Eng. Technol. 33 (2010) 213–226. [8] R.K. Brow, M.L. Schmitt, J. Eur. Ceram. Soc. 29 (2009) 1193–1201. [9] E. Axinte, Mater. Des. 32 (2011) 1717–1732. [10] Yuqun Xie, Simon D. McAllister, Dean B. Edwards, Francis Cheng, J. Power Sources 196 (2011) 10727–10730. [11] M. Herr, J.A. Lercher. Hydrogen storage in microspheres-final report, ET Energie Technologie GmbH, Sept 2003, Report No.: ET-TN-03-628. [12] D.K. Kohli, R.K. Khardekar, R. Singh, P.K. Gupta, Int. J. Hydrogen Energy 33 (2008) 417–422. [13] B. Duret, A. Saudin, Int. J. Hydrogen Energy 19 (1994) 757–764. [14] M.L. Schmitt, J.E. Shelby, M.M. Hall, J. Non Cryst. Solids 352 (2006) 626–631. [15] G.G. Wicks, L.K. Heung, R.F. Schumacher, Am. Ceram. Soc. Bull. 87 (2008) 23– 28. [16] V.S. Bessmertnyi, V.P. Krokhin, A.A. Lyashko, N.A. Drizhd, Zh.E. Shekhovtsova, Glass Ceram. 58 (2001) 268–269. [17] W.R. Beck, Hollow glass microspheres, United States Patent Application US 20100040881, 2010 Feb 18. [18] F. Veatch, Lyndhurst, H.E. Alford, R.D. Croft, Hollow glass particles and method of producing the same, United States Patent US 3030215, 1962 Apr 17. [19] I. Bica, Mater. Sci. Eng., B 77 (2000) 210–212. [20] C.D. Hendricks, A. Rosencwaig, R.L. Woerner, J.C. Koo, J.L. Dressler, J.W. Sherohman, et al., J. Nucl. Mater. 85 (86) (1979) 107–111. [21] J.Y. Ding, D.E. Day, J. Mater. Res. 6 (1991) 168–174. [22] C.D. Hendricks, Method and apparatus for producing small hollow spheres, United States Patent US 4163637, 1979 Aug 7. [23] V.V. Budov, Glass Ceram. (Engl. Transl.) 47 (3–4) (1990) 77–79. [24] D.B. Rapp, J.E. Shelby, J. Non-Cryst. Solids 349 (2004) 254–259. [25] Xiaobo Qi, Cong Gao, Zhanwen Zhang, Sufen Chen, Bo Li, Sheng Wei, Int. J. Hydrogen Energy 37 (2012) 1518–1530. [26] S. Sachin, M.M. Hall, Int. J. Hydrogen Energy 36 (2011) 9694–9701. [27] R. Kitamura, P. Laurent, Int. J. Hydrogen Energy 34 (2009) 6690–6704. [28] W.R. Beck, Hollow glass microspheres, United States Patent Application 20100040881, 2010, Feb 18. [29] F. Veatch, Lyndhurst, H.E. Alford, R.D. Croft, Hollow glass particles and method of producing the same, United States Patent 3030215, 1962, Apr 17. [30] D. Sridhar, S. Vijayalakshmi, S. Pratibha, Adv. Mater. Res. 678 (2013) 37–41. [31] D. Sridhar, S. Pragya, S. Vijayalakshmi, S. Pratibha, Energy Env. Eng. J. 1 (2) (2012) 64–67. [32] F.A. Sigoli, Y. Kawano, M.R. Davolos Jr., M. Jafelicci, J. Non-Cryst. Solids 284 (2001) 49–54. [33] W. Junpeng, C. Jinshu, L. Ping, J. Wuhan Univ. Technol. Mater. Sci. Ed. 23 (2008) 419–421. [34] M. Handke, W. Mozgawa, Vib. Spectrosc. 5 (1993) 75–84. [35] T. Rajalakshmi, R.S. Quahlid Fareed, R. Dhanasekaran, P. Ramasamy, Jacob Thomas, K. Srinivasan, Mater. Sci. Eng., B 39 (1996) 111–115. [36] E. Roumeli, E. Papadopoulou, E. Pavlidou, G. Vourlias, D. Bikiaris, K.M. Paraskevopoulos, K. Chrissafis, Thermochim. Acta 527 (2012) 33–39. [37] Zhen-guo An, Jing-jie Zhang, Shun-long Pan, Appl. Surf. Sci. 255 (2008) 2219– 2224. [38] M. Kawashita, S. Toda, T. Kokubo, N. Masuda, Key Eng. Mater. 240 (242) (2003) 229–232.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Preparation and characterization of hollow glass microspheres (HGMs) for hydrogen storage using urea as a blowing agent Sridhar Dalai a, S. Vijayalakshmi a,⇑, Pragya Shrivastava a, Santosh Param Sivam b, Pratibha Sharma c a
Centre for Research in Nano-Technology and Science (CRNTS), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India Amity Institute of Nanotechnology, Amity University, Noida, Uttar Pradesh 201303, India c Department of Energy Science and Engineering (DESE), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India b
a r t i c l e
i n f o
Article history: Received 24 October 2013 Received in revised form 22 May 2014 Accepted 20 June 2014 Available online 30 June 2014 Keywords: Hydrogen storage Hollow glass microspheres (HGMs) Amber glass Blowing agent Flame spheroidisation Urea
a b s t r a c t Hollow glass microspheres (HGMs) are unique class of porous material that can be used for hydrogen storage. They are finely dispersed and free flowing powder having microsphere diameter, 10–200 lm and wall thickness, 0.5–2 lm. In this report, we present the preparation of HGMs from glass frits using air-acetylene flame spheroidisation method. Urea was incorporated in the feed glass as a blowing agent to get porous walled HGMs. Effect of concentration of urea in the feed glass on the development of pores on HGMs wall was investigated. It was observed that, the porosity of the HGMS walls increased with the increase in urea content in the feed glass. Characterization of HGMs prepared was done using FE–SEM, ESEM, FTIR and XRD techniques. The spherical nature and the morphology of the HGMs samples prepared were confirmed using FE–SEM, while the pore diameter and the porosity of the samples was observed in the ESEM images. Hydrogen adsorption experiments were carried out at room temperature (RT) and 200 °C at 10 bar pressure, for 5 h for all the HGMs samples prepared. The HGMs prepared with 2% urea showed large number of micro/nano pores and it showed a hydrogen storage capacity of 2.3 wt.% at 200 °C and 10 bar pressure. The hydrogen adsorption on HGMs at RT and 10 bar pressure was lower than that observed at 200 °C and 10 bar pressure. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Replacement of fossil fuels with hydrogen can lead to a major reduction in the generation of pollutants and provide a path by which current non-renewable fuels can be replaced by a renewable one. The use of hydrogen gas in fuel cell has been considered as a safer and more environment friendly alternative to the use of gasoline and fossil fuels. The use of hydrogen to address the energy economy issue, emphasis has to be given on the production of clean hydrogen, distribution and its storage [1–5]. There are various hydrogen storage methods such as gas storage, liquid hydrogen and solid storage. Compressed hydrogen gas storage requires heavy storage cylinders which has high risk of explosion while transportation. Storage of hydrogen as a liquid at low temperatures requires high costs to maintain the low temperatures. However, the storage of hydrogen in solid materials is feasible except for the high temperature conditions to be applied for the release of stored hydrogen [6–9]. Hollow glass microspheres (HGMs) have ⇑ Corresponding author. Tel.: +91 22 25767671; fax: +91 2225723314. E-mail addresses: [email protected] (S. Dalai), [email protected] (S. Vijayalakshmi), [email protected] (P. Shrivastava), santosh.sivam@ gmail.com (S.P. Sivam), [email protected] (P. Sharma). http://dx.doi.org/10.1016/j.mee.2014.06.017 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
been shown to be a promising hydrogen storage material and it has many advantages over other hydrogen storage techniques. The hydrogen storage capacity of HGMs is 100 MPa of hydrogen. The raw materials for production of HGMs are generally recycled cullets and hence are cost effective and require low energy consumption for production [10–13]. There are several methods used to produce HGMs [14–21], of which the flame sprayed pyrolysis of glass frit will be suitable for preparation of porous walled HGMs for hydrogen storage. The prepared feed glass powder is sprayed into an oxy- acetylene flame, so that the viscosity of glass frits decreases and takes a spherical form due to surface tension. The liquid HGMs cool quickly on leaving the flame, and retain the spherical shape. This method often results good yield of HGMs with uniform diameters and wall thickness [22–24]. The properties of HGMs, such as diffusivity, strength and reversibility, are of important to monitor the hydrogen storage properties of the HGMs. The HGMs has long been proposed for hydrogen storage as a solution to the hydrogen storage for energy applications [25]. Diffusivity of gases depends upon the temperature and size of the gas molecule. As a result, heating HGMs allows the smallest gas atoms and molecules to diffuse through the porous wall of HGMs into hollow cavity [26,27]. To improve the hydrogen storage
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capacity in HGMs, it is necessary to increase the number of micro/ nano pores on the HGMs wall. This can be achieved by adding the blowing agent to the glass feed before flame spheroidisation. The percentage of the blowing agent required is very low; often 0.5– 2% is preferred, depending on its blowing ability [28,29]. In this paper, we report the preparation of HGMs from amber glass frit by flame spraying method. Modification of microsphere wall thickness and porosity were tried by incorporating known amount of blowing agent like urea while preparing the feed glass for HGMs. We have also studied the effect of blowing agent concentration on the hydrogen storage properties of HGMs. 2. Experimental details 2.1. Preparation of HGMs from feed glass with blowing agent using flame spraying method
Transform Infrared spectroscopy and X-ray Diffraction techniques. Field Emission–Scanning Electron Microscope (FE–SEM) JEOL make JSM-7600F was used to study the surface morphology of the feed glass powder and the HGMs. Ultra trace amount of the respective sample dispersed in acetone was spread on the SEM sample holder, dried; sputter coated with a thin layer of platinum to reduce charging and was used for SEM analysis. Environmental scanning electron microscopy (ESEM), FEI QUANTA 200 in back scattered electron (BSE) mode was used to examine the shape and size of the pores on the HGMs walls. The spheres were placed as a thin layer on the conductive tape fixed over the sample holder. The holder was loaded into the sample chamber and evacuated. The sample spot size kept was 4.0– 6.0 nm; pressure 40–70 Pa and accelerating potential of 10–30 kV. The phase structure analysis of feed and products was done using an X-ray diffractometer (XRD), X’Pert-Pro, Pan analytical utilizing Cu Ka X-radiation of wavelength 1.54 Å, and 2h range, 5– 100°. The FTIR spectrum of the feed and product HGMs mixed with KBr were recorded in the range 400–4000 cm 1 at room temperature on a Bruker Fourier Transform Infrared Spectroscopy (Vertex 80).
Broken amber glass bottles from the laboratory were crushed to particle size 1 mm. About 1 kg of the crushed amber glass were further pulverized and sieved to different particle size ranging from 35–120 lm to get the feed glass powder. Based on the results obtained on the effect of feed glass particle size and feed flow rate on the yield of HGMs, it was decided to use the feed particle size, 63–75 lm and feed flow rate 100 mg/min [30] for the HGMs sample preparation reported in this paper. About 2% by weight urea solution in water was prepared and was used to blend urea with the feed glass powder by soak and dry method. Known volume of the urea stock solution was added to a fixed quantity of glass powder to make feed glass powder with 0.3%, 0.5%, 1%, 2%, 3% and 5% of blowing agent. The amber glass powder mixed with the urea solution was stirred at room temperature for 4 h initially and continued stirring at 50 °C till a dried mass of the glass powder was obtained. Further it was dried in an oven at 100 °C, gently ground using an agate mortar- pestle and stored in desiccators. The urea mixed feed glass powder was converted into HGMs using flame spheroidisation method. The schematic diagram of flame spraying setup is shown in Fig. 1. The HGMs prepared were labeled as HAU0.3, HAU0.5, HAU1, HAU2, HAU3 & HAU5 where, HA stands for hollow glass microspheres prepared from amber glass frit, U denotes the urea as blowing agent and the numerical values in the sample code indicate the percentage of urea in the feed glass.
About 100 mg of the HGMs sample was exactly weighed and was placed inside a quartz tube which was then inserted in the sample cell. The entire system was heated to 200 °C and evacuated using a rotary vacuum pump to 10 3mbar for 1 h, prior to the hydrogen adsorption study. This helps to remove any moisture and surface impurities present in the sample. Highly pure (99.99%) hydrogen gas at pressure of 10 bar was introduced into the sample cell for adsorption on the HGMs sample. The decrease in pressure with respect to time at constant temperature was noted using a digital data logger. When hydrogen is send to the sample, both, absorption as well as adsorption of the gas takes place on the sample. The decrease in pressure value was used to calculate the amount of hydrogen uptake by the sample. This data was used to construct the Pressure-Composition (PC) isotherm. Adsorption of hydrogen at room temperature (RT) and 200 °C on all the samples was done at pressure 10 bar for 5 h. A schematic diagram of the Sievert’s type apparatus used for hydrogen storage on HGMs samples is shown in Fig. 2 [31].
2.2. Characterization
3. Results and discussion
The feed glass powder and the HGMs with and without urea were characterized using scanning electron microscopy, Fourier
FE–SEM images of amber feed glass and product HGMs are shown in Fig. 3. The feed particle size in the range 63–75 lm and
2.3. Experiments on hydrogen gas filling
Fig. 1. Schematic diagram of the experimental setup for preparation of HGMs.
S. Dalai et al. / Microelectronic Engineering 126 (2014) 65–70
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Fig. 2. Block diagram Sievert’s type apparatus.
Fig. 3. FEGSEM images of (a) amber feed glass, (b) product HGMs (HA), (c) corresponding magnified HGMs showing its wall thickness (inset).
feed flow rate 100 mg/min was found to give 95% conversion to uniform sized HGMs. It was found that synthesized HGMs have 10–100 lm diameter and 1to 2 lm wall thickness. 3.1. FTIR and XRD studies on feed and product HGMs FTIR studies on the urea loaded feed glass and product HGMs were conducted in the range 400–4000 cm 1 using Bruker make FTIR spectrometer. The FTIR transmission spectrums of feed and product samples are shown in Fig. 4a. The X-ray Diffractogram data for the feed and HGMs in the 2h range, 5–90° were collected and is shown in Fig. 4b. From Fig. 4a, it was clearly observed that all the feed samples and the respective HGMs samples showed IR transmission bands corresponding to the bending, stretching and the anti-symmetric stretching vibrations of the SiO2 bonds. Apart from that, significant IR signal was observed at wave number, 1423 cm 1, which is corresponding to the asymmetric stretching vibration of N–C–N. The peak at 1627 cm 1 is due to the bending vibration of H–O–H. The peaks at 2925 and 3438 cm 1 are corresponding to the O–H stretching vibration. Thus from the FTIR analysis of the feed glass as well as the HGMs samples, it can be confirmed that the urea
impregnated in the feed glass sample has been decomposed to ammonia and oxides of carbon at high temperature spheroidisation [32–36]. The XRD analysis of the feed and the product HGMs (Fig. 4b) did not show any well-defined peaks, but showed a hump centered at 2h = 22° which shows the amorphous nature with short range crystalline, characteristic of glass substances [37,38]. 3.2. Effect of urea as blowing agent on the morphology of HGMs Inclusion of urea as blowing agent in the feed glass powder showed a very interesting trend in the physical morphology as well as the hydrogen adsorption behavior of the HGMs. The ESEM images of HA, HAU0.3, HAU0.5, HAU1, HAU2, HAU3 and HAU5 are shown in Figs. 5–7. From Fig. 5, it was observed that the formation of pores on the walls of HGMs was enhanced considerably, when urea was incorporated in the feed glass. But the actual nature of the pores, whether it is a through-put pore on the walls or inter-connected pores or only a deep cavity formed on the walls, was not clear from the SEM pictures. From the ESEM images (Fig. 5), it was observed that the use of urea as the blowing agent left the HGMs, highly
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Fig. 4. (a) FTIR spectra, (b) XRD patterns of urea loaded feed samples and the respective HGMs samples with concentration of 0.3, 1, 2 wt.%.
Fig. 5. ESEM images of (a) HA, (b) HAU2, (c) and (d) are magnified images of (a) and (b).
porous compared to the HGMs prepared without urea. This was due to the decomposition of urea to ammonia and oxides of carbon during the flame spheroidisation.
From Fig. 6, it was observed that, there was only a nominal change in pore formation on the surface of HGMs, when the percentage of urea increased from 0.3% to 1%. The ESEM images
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Fig. 6. ESEM images of (a) HAU0.3, (b) HAU0.5, (c) HAU1.
Fig. 7. ESEM images of (a) HAU0.3, (b) HAU2, (c) HAU3, (d) HAU5.
(Fig. 7), showed that, the size and number of pores developed, varied with the percentage of urea used in the feed glass. It was also observed that the number of pores increased when the percentage of urea increased from 0.3% to 2%. When the urea content was increased further to 3 and 5 wt.%, most of the small pores were closed (Fig. 7 c and d) and there was a decrease in pore formation on the surface of HGMs. 3.3. Hydrogen storage in HGMs prepared with urea Hydrogen storage experiments on HAU0.3 samples were conducted at ambient temperature and 200 °C under 10 bar of pressure for 5 h in the Sievert’s type apparatus fabricated in the laboratory. It was observed that, uptake of hydrogen at 200 °C was more than that obtained at room temperature. This is due to
the dependency of diffusivity of gas with temperature and the size of the gas molecule. On heating the HGMs to 200 °C, the diffusivity of hydrogen gas increases and allows hydrogen molecule to diffuse across the microsphere wall into hollow cavity. But at room temperature, the diffusivity of hydrogen gas is very low and no significant uptake of hydrogen was observed. Thus the diffusivity of hydrogen through the microsphere walls increases with temperature. Hence, further experiments on hydrogen adsorption on the HAUx samples were done at 200 °C and 10 bar for 5 h. The hydrogen uptake curves at 200 °C and 10 bar pressure, for samples HAU0.3, 2, 3 & 5 are shown in Fig. 8. The uptake of hydrogen gas on HGMs increased with increase of the urea concentration up to 2 wt.%, but decreased with further increase of the urea concentration from 2 to 5 wt.%. The HAU3 and HAU5 suffered a partial pore closure, as seen from ESEM images (Fig. 7) which hampered
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as blowing agent were capable of storing hydrogen gas when exposed to hydrogen at high temperature and high pressure. The hydrogen adsorption capacity of the urea loaded HGMs increases with the increase in urea loading from 0.3% to 2%. Furthermore an increase of percentage of urea in amber glass feed decreases pore formation on microsphere wall due to the high temperature polymerization of urea happening during flame spheroidisation. The maximum hydrogen storage capacity was found to be 2.3 wt.% for HAU2 at 200 °C for a pressure 10 bar which is better compared with HA. Acknowledgment The authors gratefully acknowledge the support of sophisticated analytical instrument facilities at SAIF, IIT Bombay during this research work. References
Fig. 8. Hydrogen uptake curve for HAU 0.3, 2, 3 & 5 at 200 °C.
Fig. 9. Hydrogen uptake curve for HA and HAU2 at 200 °C.
the diffusivity of hydrogen gas into the microsphere cavity. Pore closure and pore narrowing in the HGMs at higher loading of urea is due to the polymerization of urea during flame spheroidisation leading to the deposition of the polymer molecule at the pore mouth. The hydrogen storage capacity of HA and HAU2 are compared in Fig. 9. It was observed that hydrogen storage capacity of HAU2 is 2.3 wt.% whereas that of HA is about 1 wt.% only. From these results, it is confirmed that more pore formation on surface of HGMs enhances the hydrogen uptake. 4. Conclusion Porous walled HGMs were fabricated from amber glass frit by flame spraying method using urea as blowing agent. Both HGMs with and without urea were found to be amorphous in structure. The HGMs prepared using urea as blowing agent (HAU2) was found to have large number of micro/nanopores on the walls. Thus, it is confirmed that the urea impregnated in the feed glass sample has been decomposed to ammonia and oxides of carbon at high temperature spheroidisation. The HGMs prepared by using urea
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