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Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage
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Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage Sridhar Dalai a, S. Vijayalakshmi a,*, Pratibha Sharma b, Ko Yeon Choo c a
Centre for Research in Nano-Technology and Science (CRNTS), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India b Department of Energy Science and Engineering (DESE), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India c Korea Institute of Energy Research (KIER), Daejon, South Korea
article info
abstract
Article history:
The development of a safe and efficient method for hydrogen storage is essential for the
Received 6 November 2013
use of hydrogen with fuel cells for vehicular applications. Hollow glass microspheres
Received in revised form
(HGMs) have characteristics suitable for hydrogen storage and are expected to be a po-
25 February 2014 Accepted 11 March 2014
tential hydrogen carrier to be used for energy release applications. The HGMs with 10 e100 mm diameters, 100e1000 A pore width and 3e8 mm wall thicknesses are expected to be
Available online xxx
useful for hydrogen storage. In our research we have prepared HGMs from amber glass powder of particle size 63e75 mm using flame spheroidisation method. The HGMs samples
Keywords:
with magnesium and iron loading were also prepared to improve the heat transfer property
Metal loaded hollow glass micro-
and thereby increase the hydrogen storage capacity of the product. The feed glass powder
spheres (HGMs)
was impregnated with calculated amount of magnesium nitrate hexahydrate salt solution
Hydrogen storage
to get 0.2e3.0 wt% Mg loading on HGMs. Required amount of ferrous chloride tetrahydrate
Amber glass
solution was mixed thoroughly with the glass feed powder to prepare 0.2e2 wt% Fe loaded
Flame spheroidisation
HGMs. Characterizations of all the HGMs samples were done using FEG-SEM, ESEM and
Magnesium oxide
FTIR techniques. Adsorption of hydrogen on all the Fe and Mg loaded HGMs at 10 bar
Iron oxide
pressure was conducted at room temperature and at 200 C, for 5 h. The hydrogen adsorption capacity of Fe loaded sample was about 0.56 and 0.21 weight percent for Fe loading 0.5 and 2.0 weight percentage respectively. The magnesium loaded samples showed an increase of hydrogen adsorption from 1.23 to 2.0 weight percentage when the magnesium loading percentage was increased from 0 to 2.0. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ91 22 25767671; fax: þ91 22 25764890. E-mail addresses: [email protected] (S. Dalai), [email protected], [email protected] (S. Vijayalakshmi), pratibha_sharma@ iitb.ac.in (P. Sharma), [email protected] (K.Y. Choo). http://dx.doi.org/10.1016/j.ijhydene.2014.03.062 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Introduction
Materials and methods
Presently, for the energy security and global warming, hydrogen has been globally recognized as an important fuel of the future. But the greatest challenge to introduce hydrogen to the global energy economy system lies in the availability of a suitable material for hydrogen storage [1e4]. Although the gravimetric energy density of hydrogen is about three times higher than that of gasoline, the energy content per volume of the hydrogen stored material is less by four times. The weight, volume, energy efficiency, refueling times, cost and also safety aspects cut the use of hydrogen as an energy carrier for on board vehicles [5,6]. For hydrogen to replace fossil fuels, safe and efficient storage techniques with high gravimetric and volumetric efficiencies are required. The various hydrogen storage methods available are, physisorption, metal hydrides, complex hydrides, chemical hydrides, micro porous metal organic frameworks, carbon nanotubes, cryogenic storage, and high performance composite cylinder. But each and every approach has advantages and disadvantages related to cost, volumetric and gravimetric efficiencies, reversibility of uptake and release of hydrogen, storage capacity, delivery rate, lifetime, and safety [7e9]. Hollow glass microspheres (HGMs) have been shown to be a potential material for hydrogen storage and it has many advantages over other hydrogen storage techniques. The main attraction of HGMs for hydrogen storage lies in the light weight, low density, nontoxic nature, good mechanical strength, zero environmental pollution, low cost for production [10e14]. There are several methods available to fabricate HGMs [15e23], the suitable method being flame sprayed pyrolysis of glass frit [24,25]. For hydrogen filling in HGMs, the HGMs is placed at high pressure environment and heated to high temperature to accelerate the gas diffusion inside the microspheres. The filled HGMs are then cooled to ambient temperature so that gas contained in the hollow cavity of HGMs will be retained due to the low diffusivity of hydrogen at room temperature. The poor thermal conductivity of the glasses lead to partial filling of the hydrogen gas during adsorption and also partial release of stored gas during desorption [26,27]. In order to increase the hydrogen storage capacity of HGMs, it was necessary to increase the heat transfer property of the HGMs. This can be achieved by adding the metals like iron and magnesium and other transition elements to the glass feed [28,33].
Preparation of HGMs using flame spraying method Broken amber glasswares from the laboratory waste were crushed to particle size w1 mm. About 1 kg of the crushed amber glass were further pulverized and sieved to different particle size ranging from 35 to 120 mm to get the feed glass powder. The different parameters like particle size, feed flow rate were optimized to get best possible yields of uniform sized HGMs. From the data obtained, it was decided to use feed particle size of 63e75 mm and feed flow rate of 100e200 mg/min [34]. The metal dopants used were magnesium and iron for which magnesium nitrate hexahydrate and ferrous chloride tetrahydrate were used in the feed glass preparation. A 2% stock solution of magnesium nitrate hexahydrate and ferrous chloride tetrahydrate was prepared by dissolving weighed quantity of the respective salt in distilled water. Calculated volumes of these solutions were used to soak the amber feed glass powder to get 0.2, 0.5, 1, 2 weight percent iron loading and 0.2, 0.5, 1, 2, 3 weight percent magnesium loading on the feed glass powder. The amber glass powder mixed with the metal salt solution was stirred at room temperature for 4 h initially and then 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 [27]. The metal loaded feed glass powder was converted into HGMs using a flame spheroidisation method. The schematic diagram of flame spraying setup is shown in Fig. 1. The HGMs prepared were labeled as HAMg0.2, HAMg0.5, HAMg1, HAMg2 and HAMg3 where, HA stands for hollow glass microspheres prepared from amber glass frit, Mg denotes the magnesium loading and the numerical values in the sample code indicate the percentage of magnesium in the feed glass. Similarly, HGMs with iron were labeled as HAFe0.2, HAFe0.5, HAFe1 and HAFe2.
Characterization The feed glass powder and the HGMs were characterized using scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR) techniques. Field Emission Gun-
Fig. 1 e Schematic diagram of the experimental setup for preparation of HGMs. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Scanning Electron Microscope (FEG-SEM) JEOL make JSM7600F 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 was kept 4.0e6.0 nm; pressure 40e70 Pa and accelerating potential of 10e30 kV. The FTIR spectrum of the feed and product HGMs mixed with KBr were recorded in the range 400e4000 cm1 at room temperature on a Bruker Fourier Transform Infrared Spectroscopy (Vertex 80).
Experiments on hydrogen gas filling About 100 mg of the HGMs was accurately 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 103mbar, 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 known pressure 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 was send to the sample, both, absorption as well as adsorption of the gas takes place on the sample. The pressure reduction was used to calculate the amount of hydrogen uptake by the sample. This data was used to construct hydrogen uptake curve on each of the HGMs sample prepared. Adsorption of hydrogen at room temperature (RT) and 200 C for 5 h on all the samples was done at pressure 10 bar. A schematic diagram of the Sievert’s type apparatus used for hydrogen storage on HGMs samples is shown in Fig. 2 [35].
Results and discussion The FEG-SEM images of amber feed glass and product HGMs are shown in Fig. 3. The feed particle size in the range 63e75 mm and feed flow rate 100e200 mg/min was found to give w95% conversion to uniform sized HGMs. It was also found that these HGMs had 10e100 mm diameter and 1e2 mm wall thickness.
FTIR studies on feed and product HGMs The FTIR analysis of the magnesium and iron loaded feed glass and product HGMs were conducted in the range 400 cm1 to 4000 cm1 using Bruker make FTIR spectrometer. The FTIR transmission spectrums of feed and HAMgx samples are shown in Fig. 4(a) and (b). The FTIR peak assignments and its wave numbers are presented in Table 1.
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Fig. 2 e Block diagram Sievert’s type apparatus. From Fig. 4(a) and (b), 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. The very weak IR signal observed at 662e670 cm1 in almost all the samples is due to the CoeO stretching vibration. Another significant IR signal observed was at wave numbers, 1374 and 1386 cm1, which are corresponding to the symmetric stretching of NeO bond. It was seen that the intensity of the IR signal at 1374 and 1386 cm1 increases with the increase in magnesium loading in the feed glass powder. But it was absent in the HGMs samples which confirms the decomposition of magnesium nitrate hexahydrate to MgO during the high temperature spheroidisation. In the feed and product samples, peak at 3438 cm1 and at 1644 cm1 are corresponding to the OeH stretching and HeOeH in-plane bending vibration respectively. Thus from the FTIR analysis of the feed glass and the HGMs samples, it can be confirmed that the magnesium nitrate impregnated in the feed glass sample has been decomposed to magnesium oxide at high temperature spheroidisation [36e45].
Effect of metal loading on the morphology of HGMs Incorporating metal salt 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 HAMg0.2, HAMg0.5, HAMg1 and HAMg2 are shown in Fig. 5. The FEG-SEM and EDAX images of HGMs prepared with and without magnesium loading in amber glass powder are shown in Fig. 6. From Fig. 5, it was observed that the number of pores increased, when the percentage of magnesium increased from 0.2% to 2%. When the magnesium content was increased further to 3 wt%, most of the small pores were closed and a deposition of a thin layer was found on the microspheres (Fig. 6(b)). Using the EDAX facility in SEM, this thin layer was confirmed to be magnesium rich material. Complete blending of the amber glass powder with magnesium nitrate hexahydrate was possible only up to 2% magnesium, beyond which the agglomeration of Mg/MgO takes place leading to the closure of pores due to deposition of metal rich patches on the surface. It indicates that the percentage of magnesium loading
Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Fig. 3 e FEG-SEM images of (a) amber feed glass, (b) product HGMs (HA).
has a significant effect on the preparation of HGMs. Attempts to make HAMg5 and HAMg10 failed due to the high hygroscopicity of the magnesium salt used for impregnation of the feed glass powder. From Fig. 6, it is found that the surface of HGMs without magnesium is smooth, while the surface of HAMg3 is not. There is a MgO deposition on the surface of HGMs. Chemical analysis of this layer was done by EDX. The EDX data (Fig. 6(d)) taken at the thick patches seen on the Mg loaded HGMs showed the presence of magnesium. The intensity of the magnesium in the HGMs increased with the increase in magnesium in the feed glass powder (Fig. 6(c)). It is concluded that magnesium loading on the feed glass via. magnesium nitrate hexahydrate beyond 2% of magnesium leads to deposition of magnesium oxide on the surface of the HGMs walls leading to the non availability of nanopores for H2 diffusion. Effect of different concentration level of Iron loading on HGMs is shown in Fig. 7. Ferrous chloride tetrahydrate solution was used for loading iron in the feed glass powder. Pore formation was confirmed in the HAFe0.2 & HAFe0.5 samples. But in HAFe1 and HAFe2, absolutely no pores were visible in ESEM images.
Hydrogen storage in metal loaded HGMs Adsorption of hydrogen on HA was conducted at ambient 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 observed at room temperature. The diffusivity of hydrogen increases at higher temperature which helps to increase the diffusion of the hydrogen gas through the pores in the microsphere wall. Further experiments on hydrogen adsorption on the HAMgx and HAFex samples were done at 200 C and 10 bars for 5 h. Hydrogen uptake on magnesium loaded HGMs are shown in Fig. 8. The uptake of hydrogen gas on HGMs increased with increase of the magnesium concentration up to 2 wt%, but decreases with further increase in the magnesium concentration to 3 wt%. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity. From ESEM image (Fig. 7), it is observed that the pore formation on HGMs wall decreases as the concentration of iron
Fig. 4 e FTIR spectra of magnesium loaded feed samples and the respective HGMs samples with concentration of (a) 0.2, 0.5 wt% (b) 1, 2 wt%. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Table 1 e FTIR peak assignments of magnesium loaded feed glass and product HGMs. S. no.
1 2 3 4 5 6 7
Peak assignment
Bending vibration of SieOeSi Stretching vibration of MgeO Stretching vibration of OeSieO Anti-symmetric stretching vibration of SieOeSi Symmetric stretching vibration of NeO In-plane bending vibration of HeOH Stretching vibration of OeH
Wavenumber (cm1) Fig. 4(a)
Fig. 4(b)
483 658 766 1035 1374 1644 3438
478 654 761 1031 1386 1641 3437
increases from 0.2 to 2 wt%. So in case of iron loaded HGMs, the hydrogen uptake decreased with increase in the weight percentage of Fe (Fig. 9). The hydrogen adsorption capacity of Fe loaded sample was 0.56 and 0.21 wt% for Fe loading 0.5 and 2.0 wt% respectively. The ferrous chloride tetrahydrate salt was not able to form porous microsphere walls during spheroidisation. Also at higher loading of Fe, the Fe/FeO was blocking the very few pores available, thereby reducing the hydrogen adsorption capacity of the sample from 0.56 to 0.21 wt%. The metal salt used for iron loading on the feed glass powder was ferrous chloride tetrahydrate which may not be a suitable precursor that can function as a blowing agent as well as metal dopant during flame spheroidisation. The hydrogen uptake on HGMs samples with and without magnesium loading at 10 bar pressure and 200 C are shown in Fig. 10. It was observed that hydrogen storage capacity of
Literature range for wavenumber (cm1)
400e600 [36] 440e700 [39e42] 700e800 [36e38] 1000e1200 [37] 1340e1410 [43e45] 1550e1650 [36,44] 3400e3600 [36,44,45]
HAMg2 sample is 2 wt% whereas on HA is 1.23 wt%. From these results, it is confirmed that nanopore formation on surface of HGMs enhances the hydrogen uptake.
Conclusion Magnesium or Iron loaded HGMs were successfully prepared by flame spheroidisation method using Mg/Fe metal salt impregnated amber glass powder. A suitable metal loading in required proportion on the HGMs helps in improving the hydrogen storage capacity. All metal loaded HGMs were capable of storing hydrogen gas at 200 C and 10 bar pressure. The hydrogen adsorption capacity of the magnesium loaded HGMs increased from 1.23 wt% to 2.0 wt% with the increase in magnesium loading from 0 to 2%. But iron loading on the feed
Fig. 5 e ESEM images of (a) HAMg0.2, (b) HAMg0.5, (c) HAMg1, (d) HAMg2. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Fig. 6 e FEG-SEM images of (a) HA, (b) HAMg3, (c) & (d) are EDX images of a and b.
glass powder had an adverse effect on the hydrogen storage capacity of the HAFex samples. Ferrous chloride tetrahydrate which was used for iron loading on the glass powder is not a good candidate to be used as a blowing agent. Hence the pore formation on the microsphere walls was meager and
whatever little pores developed was getting blocked by the Fe/ FeO formed during flame spheroidisation. It is also concluded that the hydrogen storage on the HGMs occurs due to diffusion of hydrogen molecule through the nanopores on the wall. Hence it is concluded that high temperature and high pressure
Fig. 7 e ESEM images of (a) HAFe0.2, (b) HAFe0.5, (c) HAFe1, (d) HAFe2. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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are required to for hydrogen storage. It is also concluded that, the more the pore formation on the HGMs walls, the better the hydrogen uptake. Magnesium loaded HGMs is an interesting material for hydrogen storage.
Acknowledgment The authors gratefully acknowledge the support of sophisticated analytical instrument facilities at SAIF, IIT Bombay during this research work.
references
Fig. 8 e Hydrogen uptake curve for HAMg0.2, 0.5, 1, & 2 at 200 C.
Fig. 9 e Hydrogen uptake for HAFe0.5 & 2 at 200 C.
Fig. 10 e Hydrogen uptake curve for HA and HAMg2 at 200 C.
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Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage Sridhar Dalai a, S. Vijayalakshmi a,*, Pratibha Sharma b, Ko Yeon Choo c a
Centre for Research in Nano-Technology and Science (CRNTS), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India b Department of Energy Science and Engineering (DESE), Indian Institute of Technology Bombay (IIT B), Powai, Mumbai, India c Korea Institute of Energy Research (KIER), Daejon, South Korea
article info
abstract
Article history:
The development of a safe and efficient method for hydrogen storage is essential for the
Received 6 November 2013
use of hydrogen with fuel cells for vehicular applications. Hollow glass microspheres
Received in revised form
(HGMs) have characteristics suitable for hydrogen storage and are expected to be a po-
25 February 2014 Accepted 11 March 2014
tential hydrogen carrier to be used for energy release applications. The HGMs with 10 e100 mm diameters, 100e1000 A pore width and 3e8 mm wall thicknesses are expected to be
Available online xxx
useful for hydrogen storage. In our research we have prepared HGMs from amber glass powder of particle size 63e75 mm using flame spheroidisation method. The HGMs samples
Keywords:
with magnesium and iron loading were also prepared to improve the heat transfer property
Metal loaded hollow glass micro-
and thereby increase the hydrogen storage capacity of the product. The feed glass powder
spheres (HGMs)
was impregnated with calculated amount of magnesium nitrate hexahydrate salt solution
Hydrogen storage
to get 0.2e3.0 wt% Mg loading on HGMs. Required amount of ferrous chloride tetrahydrate
Amber glass
solution was mixed thoroughly with the glass feed powder to prepare 0.2e2 wt% Fe loaded
Flame spheroidisation
HGMs. Characterizations of all the HGMs samples were done using FEG-SEM, ESEM and
Magnesium oxide
FTIR techniques. Adsorption of hydrogen on all the Fe and Mg loaded HGMs at 10 bar
Iron oxide
pressure was conducted at room temperature and at 200 C, for 5 h. The hydrogen adsorption capacity of Fe loaded sample was about 0.56 and 0.21 weight percent for Fe loading 0.5 and 2.0 weight percentage respectively. The magnesium loaded samples showed an increase of hydrogen adsorption from 1.23 to 2.0 weight percentage when the magnesium loading percentage was increased from 0 to 2.0. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ91 22 25767671; fax: þ91 22 25764890. E-mail addresses: [email protected] (S. Dalai), [email protected], [email protected] (S. Vijayalakshmi), pratibha_sharma@ iitb.ac.in (P. Sharma), [email protected] (K.Y. Choo). http://dx.doi.org/10.1016/j.ijhydene.2014.03.062 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Introduction
Materials and methods
Presently, for the energy security and global warming, hydrogen has been globally recognized as an important fuel of the future. But the greatest challenge to introduce hydrogen to the global energy economy system lies in the availability of a suitable material for hydrogen storage [1e4]. Although the gravimetric energy density of hydrogen is about three times higher than that of gasoline, the energy content per volume of the hydrogen stored material is less by four times. The weight, volume, energy efficiency, refueling times, cost and also safety aspects cut the use of hydrogen as an energy carrier for on board vehicles [5,6]. For hydrogen to replace fossil fuels, safe and efficient storage techniques with high gravimetric and volumetric efficiencies are required. The various hydrogen storage methods available are, physisorption, metal hydrides, complex hydrides, chemical hydrides, micro porous metal organic frameworks, carbon nanotubes, cryogenic storage, and high performance composite cylinder. But each and every approach has advantages and disadvantages related to cost, volumetric and gravimetric efficiencies, reversibility of uptake and release of hydrogen, storage capacity, delivery rate, lifetime, and safety [7e9]. Hollow glass microspheres (HGMs) have been shown to be a potential material for hydrogen storage and it has many advantages over other hydrogen storage techniques. The main attraction of HGMs for hydrogen storage lies in the light weight, low density, nontoxic nature, good mechanical strength, zero environmental pollution, low cost for production [10e14]. There are several methods available to fabricate HGMs [15e23], the suitable method being flame sprayed pyrolysis of glass frit [24,25]. For hydrogen filling in HGMs, the HGMs is placed at high pressure environment and heated to high temperature to accelerate the gas diffusion inside the microspheres. The filled HGMs are then cooled to ambient temperature so that gas contained in the hollow cavity of HGMs will be retained due to the low diffusivity of hydrogen at room temperature. The poor thermal conductivity of the glasses lead to partial filling of the hydrogen gas during adsorption and also partial release of stored gas during desorption [26,27]. In order to increase the hydrogen storage capacity of HGMs, it was necessary to increase the heat transfer property of the HGMs. This can be achieved by adding the metals like iron and magnesium and other transition elements to the glass feed [28,33].
Preparation of HGMs using flame spraying method Broken amber glasswares from the laboratory waste were crushed to particle size w1 mm. About 1 kg of the crushed amber glass were further pulverized and sieved to different particle size ranging from 35 to 120 mm to get the feed glass powder. The different parameters like particle size, feed flow rate were optimized to get best possible yields of uniform sized HGMs. From the data obtained, it was decided to use feed particle size of 63e75 mm and feed flow rate of 100e200 mg/min [34]. The metal dopants used were magnesium and iron for which magnesium nitrate hexahydrate and ferrous chloride tetrahydrate were used in the feed glass preparation. A 2% stock solution of magnesium nitrate hexahydrate and ferrous chloride tetrahydrate was prepared by dissolving weighed quantity of the respective salt in distilled water. Calculated volumes of these solutions were used to soak the amber feed glass powder to get 0.2, 0.5, 1, 2 weight percent iron loading and 0.2, 0.5, 1, 2, 3 weight percent magnesium loading on the feed glass powder. The amber glass powder mixed with the metal salt solution was stirred at room temperature for 4 h initially and then 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 [27]. The metal loaded feed glass powder was converted into HGMs using a flame spheroidisation method. The schematic diagram of flame spraying setup is shown in Fig. 1. The HGMs prepared were labeled as HAMg0.2, HAMg0.5, HAMg1, HAMg2 and HAMg3 where, HA stands for hollow glass microspheres prepared from amber glass frit, Mg denotes the magnesium loading and the numerical values in the sample code indicate the percentage of magnesium in the feed glass. Similarly, HGMs with iron were labeled as HAFe0.2, HAFe0.5, HAFe1 and HAFe2.
Characterization The feed glass powder and the HGMs were characterized using scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR) techniques. Field Emission Gun-
Fig. 1 e Schematic diagram of the experimental setup for preparation of HGMs. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Scanning Electron Microscope (FEG-SEM) JEOL make JSM7600F 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 was kept 4.0e6.0 nm; pressure 40e70 Pa and accelerating potential of 10e30 kV. The FTIR spectrum of the feed and product HGMs mixed with KBr were recorded in the range 400e4000 cm1 at room temperature on a Bruker Fourier Transform Infrared Spectroscopy (Vertex 80).
Experiments on hydrogen gas filling About 100 mg of the HGMs was accurately 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 103mbar, 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 known pressure 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 was send to the sample, both, absorption as well as adsorption of the gas takes place on the sample. The pressure reduction was used to calculate the amount of hydrogen uptake by the sample. This data was used to construct hydrogen uptake curve on each of the HGMs sample prepared. Adsorption of hydrogen at room temperature (RT) and 200 C for 5 h on all the samples was done at pressure 10 bar. A schematic diagram of the Sievert’s type apparatus used for hydrogen storage on HGMs samples is shown in Fig. 2 [35].
Results and discussion The FEG-SEM images of amber feed glass and product HGMs are shown in Fig. 3. The feed particle size in the range 63e75 mm and feed flow rate 100e200 mg/min was found to give w95% conversion to uniform sized HGMs. It was also found that these HGMs had 10e100 mm diameter and 1e2 mm wall thickness.
FTIR studies on feed and product HGMs The FTIR analysis of the magnesium and iron loaded feed glass and product HGMs were conducted in the range 400 cm1 to 4000 cm1 using Bruker make FTIR spectrometer. The FTIR transmission spectrums of feed and HAMgx samples are shown in Fig. 4(a) and (b). The FTIR peak assignments and its wave numbers are presented in Table 1.
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Fig. 2 e Block diagram Sievert’s type apparatus. From Fig. 4(a) and (b), 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. The very weak IR signal observed at 662e670 cm1 in almost all the samples is due to the CoeO stretching vibration. Another significant IR signal observed was at wave numbers, 1374 and 1386 cm1, which are corresponding to the symmetric stretching of NeO bond. It was seen that the intensity of the IR signal at 1374 and 1386 cm1 increases with the increase in magnesium loading in the feed glass powder. But it was absent in the HGMs samples which confirms the decomposition of magnesium nitrate hexahydrate to MgO during the high temperature spheroidisation. In the feed and product samples, peak at 3438 cm1 and at 1644 cm1 are corresponding to the OeH stretching and HeOeH in-plane bending vibration respectively. Thus from the FTIR analysis of the feed glass and the HGMs samples, it can be confirmed that the magnesium nitrate impregnated in the feed glass sample has been decomposed to magnesium oxide at high temperature spheroidisation [36e45].
Effect of metal loading on the morphology of HGMs Incorporating metal salt 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 HAMg0.2, HAMg0.5, HAMg1 and HAMg2 are shown in Fig. 5. The FEG-SEM and EDAX images of HGMs prepared with and without magnesium loading in amber glass powder are shown in Fig. 6. From Fig. 5, it was observed that the number of pores increased, when the percentage of magnesium increased from 0.2% to 2%. When the magnesium content was increased further to 3 wt%, most of the small pores were closed and a deposition of a thin layer was found on the microspheres (Fig. 6(b)). Using the EDAX facility in SEM, this thin layer was confirmed to be magnesium rich material. Complete blending of the amber glass powder with magnesium nitrate hexahydrate was possible only up to 2% magnesium, beyond which the agglomeration of Mg/MgO takes place leading to the closure of pores due to deposition of metal rich patches on the surface. It indicates that the percentage of magnesium loading
Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Fig. 3 e FEG-SEM images of (a) amber feed glass, (b) product HGMs (HA).
has a significant effect on the preparation of HGMs. Attempts to make HAMg5 and HAMg10 failed due to the high hygroscopicity of the magnesium salt used for impregnation of the feed glass powder. From Fig. 6, it is found that the surface of HGMs without magnesium is smooth, while the surface of HAMg3 is not. There is a MgO deposition on the surface of HGMs. Chemical analysis of this layer was done by EDX. The EDX data (Fig. 6(d)) taken at the thick patches seen on the Mg loaded HGMs showed the presence of magnesium. The intensity of the magnesium in the HGMs increased with the increase in magnesium in the feed glass powder (Fig. 6(c)). It is concluded that magnesium loading on the feed glass via. magnesium nitrate hexahydrate beyond 2% of magnesium leads to deposition of magnesium oxide on the surface of the HGMs walls leading to the non availability of nanopores for H2 diffusion. Effect of different concentration level of Iron loading on HGMs is shown in Fig. 7. Ferrous chloride tetrahydrate solution was used for loading iron in the feed glass powder. Pore formation was confirmed in the HAFe0.2 & HAFe0.5 samples. But in HAFe1 and HAFe2, absolutely no pores were visible in ESEM images.
Hydrogen storage in metal loaded HGMs Adsorption of hydrogen on HA was conducted at ambient 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 observed at room temperature. The diffusivity of hydrogen increases at higher temperature which helps to increase the diffusion of the hydrogen gas through the pores in the microsphere wall. Further experiments on hydrogen adsorption on the HAMgx and HAFex samples were done at 200 C and 10 bars for 5 h. Hydrogen uptake on magnesium loaded HGMs are shown in Fig. 8. The uptake of hydrogen gas on HGMs increased with increase of the magnesium concentration up to 2 wt%, but decreases with further increase in the magnesium concentration to 3 wt%. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity. From ESEM image (Fig. 7), it is observed that the pore formation on HGMs wall decreases as the concentration of iron
Fig. 4 e FTIR spectra of magnesium loaded feed samples and the respective HGMs samples with concentration of (a) 0.2, 0.5 wt% (b) 1, 2 wt%. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Table 1 e FTIR peak assignments of magnesium loaded feed glass and product HGMs. S. no.
1 2 3 4 5 6 7
Peak assignment
Bending vibration of SieOeSi Stretching vibration of MgeO Stretching vibration of OeSieO Anti-symmetric stretching vibration of SieOeSi Symmetric stretching vibration of NeO In-plane bending vibration of HeOH Stretching vibration of OeH
Wavenumber (cm1) Fig. 4(a)
Fig. 4(b)
483 658 766 1035 1374 1644 3438
478 654 761 1031 1386 1641 3437
increases from 0.2 to 2 wt%. So in case of iron loaded HGMs, the hydrogen uptake decreased with increase in the weight percentage of Fe (Fig. 9). The hydrogen adsorption capacity of Fe loaded sample was 0.56 and 0.21 wt% for Fe loading 0.5 and 2.0 wt% respectively. The ferrous chloride tetrahydrate salt was not able to form porous microsphere walls during spheroidisation. Also at higher loading of Fe, the Fe/FeO was blocking the very few pores available, thereby reducing the hydrogen adsorption capacity of the sample from 0.56 to 0.21 wt%. The metal salt used for iron loading on the feed glass powder was ferrous chloride tetrahydrate which may not be a suitable precursor that can function as a blowing agent as well as metal dopant during flame spheroidisation. The hydrogen uptake on HGMs samples with and without magnesium loading at 10 bar pressure and 200 C are shown in Fig. 10. It was observed that hydrogen storage capacity of
Literature range for wavenumber (cm1)
400e600 [36] 440e700 [39e42] 700e800 [36e38] 1000e1200 [37] 1340e1410 [43e45] 1550e1650 [36,44] 3400e3600 [36,44,45]
HAMg2 sample is 2 wt% whereas on HA is 1.23 wt%. From these results, it is confirmed that nanopore formation on surface of HGMs enhances the hydrogen uptake.
Conclusion Magnesium or Iron loaded HGMs were successfully prepared by flame spheroidisation method using Mg/Fe metal salt impregnated amber glass powder. A suitable metal loading in required proportion on the HGMs helps in improving the hydrogen storage capacity. All metal loaded HGMs were capable of storing hydrogen gas at 200 C and 10 bar pressure. The hydrogen adsorption capacity of the magnesium loaded HGMs increased from 1.23 wt% to 2.0 wt% with the increase in magnesium loading from 0 to 2%. But iron loading on the feed
Fig. 5 e ESEM images of (a) HAMg0.2, (b) HAMg0.5, (c) HAMg1, (d) HAMg2. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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Fig. 6 e FEG-SEM images of (a) HA, (b) HAMg3, (c) & (d) are EDX images of a and b.
glass powder had an adverse effect on the hydrogen storage capacity of the HAFex samples. Ferrous chloride tetrahydrate which was used for iron loading on the glass powder is not a good candidate to be used as a blowing agent. Hence the pore formation on the microsphere walls was meager and
whatever little pores developed was getting blocked by the Fe/ FeO formed during flame spheroidisation. It is also concluded that the hydrogen storage on the HGMs occurs due to diffusion of hydrogen molecule through the nanopores on the wall. Hence it is concluded that high temperature and high pressure
Fig. 7 e ESEM images of (a) HAFe0.2, (b) HAFe0.5, (c) HAFe1, (d) HAFe2. Please cite this article in press as: Dalai S, et al., Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.062
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are required to for hydrogen storage. It is also concluded that, the more the pore formation on the HGMs walls, the better the hydrogen uptake. Magnesium loaded HGMs is an interesting material for hydrogen storage.
Acknowledgment The authors gratefully acknowledge the support of sophisticated analytical instrument facilities at SAIF, IIT Bombay during this research work.
references
Fig. 8 e Hydrogen uptake curve for HAMg0.2, 0.5, 1, & 2 at 200 C.
Fig. 9 e Hydrogen uptake for HAFe0.5 & 2 at 200 C.
Fig. 10 e Hydrogen uptake curve for HA and HAMg2 at 200 C.
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