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Ball-milled carbon and hydrogen storage
International Journal of Hydrogen Energy 27 (2002) 425–432
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Ball-milled carbon and hydrogen storage Kalpana Awasthia , R. Kamalakaranb , A.K. Singha , O.N. Srivastavaa;∗ a Department
of Physics, Banaras Hindu University, Varanasi 221005, India India Technology Centre, Bangalore 560060, India
b GE
Abstract We report the formation of carbon in di1erent nanoparticle forms obtained by ball-milling of graphitic carbon. Ball-milling of graphite was carried out in Szegvari attritor at room temperature for varied times i.e. 24, 48 and 100 h in hexane medium. The characterization of ball-milled graphitic carbon (BMC) samples was done by X-ray di1ractometry, scanning electron microscopy and transmission electron microscopy. The self-coagulated carbon agglomerates were obtained for the case of 24 and 100 h BMC samples. The formation of coiled nanotubes and nano:bres was observed in the BMC sample. The BMC samples with and without nickel (Ni) catalyst were subjected to hydrogenation cycling in a Sievert’s type apparatus fabricated in our laboratory. It has been found that BMC sample can adsorb hydrogen. The hydrogen adsorption capacity has been found to be ∼ 0:6 wt%. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ball-milled carbon; Ball-milling; XRD; SEM; TEM; Hydrogen adsorption
1. Introduction Tailoring and controlling the material properties have attracted scienti:c attention and has led to several discoveries. One of them, is to use the often observed exotic properties of materials in the nanoparticle form. It has been reported in several cases that structure and properties can be monitored simply by controlling the size of the particle without changing any physical or chemical parameters [1]. For example, graphitic nano:bres can adsorb, desorb hydrogen di1erently from conventional carbon, the carbon in the cluster-assembled form-C60 fullerene is a good solid lubricant and carbon nanotubes can be either insulator, semiconductor or metal [2]; photoluminescence of silicon is known to depend on the size of the silicon particles which comfortably covers the whole visible spectrum [3]. Indeed, today an observable trend in the scienti:c development is to try novel forms of materials to meet the speci:c requirement for particular application. One of them is to look for solid materials which can absorb hydrogen reversibly, in copious amounts. The fast depletion of fossil fuels and its use leading to serious pollution e1ects, have alarmingly alerted the ∗
Corresponding author. Tel.=fax: +91-542-368-468. E-mail address: [email protected] (O.N. Srivastava).
scientists to look for alternative fuels. Hydrogen is possibly the most suitable future fuel. Hydrogen with bond energy of 104 kcal=mol and favourable combustion properties, is an established, excellent fuel. But, its storage and transportation demands further development. Hydrogen is generally stored in highly pressurized cylinders which demands necessary safety measures against leakage, explosion, etc., when it is used in several applications including its use as fuel in road transportation. This mode of storing hydrogen may be more acceptable for stationary uses, for example, in gensets, etc., but carrying the highly pressurized gas cylinders for vehicular use is not a preferable option. An alternative to this is to store hydrogen in the liquid form but this requires the costly and cumbersome cryogenics as it demands temperature as low as 20 K. Yet another possibility is to store hydrogen in the metal hydride form [4]. This mode has been investigated quite extensively and a variety of intermetallics are now known which can store hydrogen reversibly with a wt% of about 1.5. A good material of choice is one which, can store hydrogen reversibly and in large quantities, is easily available, can be handled easily, does not demand high purity of hydrogen, is economically bene:cial and interfaces with nature. Carbon, being an abundant and cheap material and also being a cycleable material in natural processes, makes this discovery
0360-3199/02/$ 20.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 1 3 4 - 3
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K. Awasthi et al. / International Journal of Hydrogen Energy 27 (2002) 425–432
notable. The typical highest estimate of hydrogen adsorption in carbon materials has been 5:3 wt%, at a temperature of 77 K [5,6]. A stunning discovery in this regard was reported by Chambers et al. [7] that graphitic nano:bres can store hydrogen at about 67 wt%, which is much more than the storage capacities in intermetallics. It has been reported that single-walled carbon nanotubes can store 5 –10 wt% of hydrogen [8,9]. The e1orts on exploring hydrogen storage in carbon nanoparticles have been rather sparse. Chen et al. (1999) have reported the occurrence of disordered and nanoporous carbon powder when the graphitic powder is ball-milled at room temperature [10]. The purpose of the present communication is to report the formation of graphitic nanoparticles by ball-milling, its structural characterization by X-ray di1ractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the hydrogen storage behaviour of ball-milled graphitic carbon (BMC). 2. Experimental techniques 2.1. Ball-milled carbon Vacuum annealed graphite rods were used as the starting material for obtaining the ball-milled carbon. Annealing of the graphite rods ensured the removal of impurities such as sulphur, often present in the as-obtained graphite rods. Ball-milling of graphite was carried out in Szegvari attritor at room temperature for varied times i.e. 24, 48 and 100 h, the grinding speed of which was maintained at 400 rpm. Two grams of roughly crushed graphite rods were loaded into the milling container (of volume 1400 cm3 ) with 2:5 kg of steel balls and 500 ml of hexane was added as the wetting medium. The resulting material after 24 (BMC-1), 48 (BMC-2) and 100 h (BMC-3) was taken out from the milling container and placed into glass beakers. 2.2. Characterization of BMC sample The resulting BMC sample was removed from beaker and was characterized by employing XRD (Philips PW-1710), SEM (Philips XL-20) and TEM (Philips CM 12) techniques. 2.3. Hydrogenation behaviour To determine the hydrogen adsorption behaviour of BMC samples, Sievert’s apparatus [11] was used. The hydrogen pressure reading was recorded by digital pressure indicator (model PRM-300 MC2 with a least count of 0:1 atm). The apparatus=reactor was thoroughly checked for a possible leak at 75 atm hydrogen pressure prior to use. To avoid instrumental error the apparatus was standardized by making hydrogenation runs without sample. The hydrogenation behaviour was determined by monitoring the variation of
the hydrogen pressure on cooling the reactor with and without the sample under the same experimental conditions. The pressure reduced more in the presence of the sample. Thus, while without the sample the pressure drop, apparently only due to cooling e1ect, was from 50.3 to 16:0 atm, the same with the sample was from 50.3 to 15:6 atm. This clearly indicated the adsorption=absorption of hydrogen by the BMC sample. The BMC sample (2 g) was loaded into the reactor and pressurized upto 50:3 atm of hydrogen at room temperature. The reactor with sample was cooled to 77 K by immersing the reactor in liquid nitrogen. 3. Results and discussion The resulting material after 24 (BMC-1), 48 (BMC-2) and 100 h (BMC-3) of milling at room temperature, appeared as a dispersion of :ne graphite particles in hexane. Interestingly, the particles settling was very slow and eventually dried in 2 days to coagulate into a Mat disc (diameter = 3:5 cm; thickness = 0:7 cm, and density = 0:47 g=cm3 ). The diameter of the disc resulting from coagulation of graphite particles of ball-milled graphite was smaller than that of the beaker. This implies the tendency of particles to coagulate. The fact that particles settling was so slow also signi:es that the particles are very small. This can be understood in terms of Stokes’ law according to which the size of the particles sedimenting in a Muid, usually a liquid, is given by d = (18h=Ogt)1=2 , where d; ; h; O; g; t are equivalent Stokes’ diameter, absolute viscosity of the Muid, distance through which particle has fallen in time t, di1erence in density between the particle and the Muid and acceleration due to gravity . This is valid when there is no particle-to-particle interaction, and no wall e1ect and particles are sedimenting under laminar Mow in the Muid. 3.1. Structural and microstructural characteristics The XRD pattern of BMC-1, BMC-2 and BMC-3 samples is shown in Fig. 1(a) – (c). The analysis of this pattern revealed that the peaks were explicable in terms of the known hexagonal structure of graphite. The particle sizes as calculated from the XRD pattern using Scherrer’s formula are P for BMC-1, BMC-2 and BMC-3 sam145, 115 and 50 A ples, respectively. The relevant intensity of 00.2 and 10.1 peaks in Fig. 1(a) and (b) are attributed to the texturing that can be noticed, the peaks are sharper in Fig. 1(a) BMC-1 sample than those in Fig. 1(b) BMC-2 sample. This indicates that the particles are smaller in BMC-2 sample than in ◦ BMC-1 sample. The peak at 16 is present due to the tape ◦ used to stick the sample (Fig. 1(b)). The peak at ∼ 39 is unidenti:ed, it is presumably from the impurity of the as-obtained graphite. It is noticed that, this impurity peak has disappeared in Fig. 1(c), which suggests that long time ball-milling destroyed the crystallinity of the impurity. The
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Fig. 1. (a) – (c) X-ray di1ractograms (XRD) of the ball-milled (24 h) carbon BMC-1, ball-milled (48 h) carbon BMC-2 and ball-milled (100 h) carbon BMC-3 samples. (d) The XRD taken from the pelletized BMC-1 showing a strong 00.2 peak. This indicates the texturing in the pellet (e).
XRD (Fig. 1(d)) taken from the pelletized sample revealed a strong 00.2 peak signifying the texturing in the pellet. When compared with XRD of the as-obtained graphitic rod we found the following di1erences: XRD from the pellet with X-ray facing the Mat and curved (Fig. 1(e)) surfaces were quite di1erent. From the Mat surface XRD exhibited a
much stronger 00.2 peak signifying the said texturing. The sharpness in peak 10.1 was found in Fig. 1(e) which was attributed to the texturing and pelletizing process. Fig. 2(a) is the SEM micrograph of BMC-1 sample which shows the Maky particles of graphite. The SEM pictures, in Fig. 2(b) and (c) show the agglormeration of the carbon particles.
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Fig. 2. (a) – (c) Scanning electron micrographs of the BMC-1, BMC-2 and BMC-3 samples.
Microstructural characterization of BMC samples was done by transmission electron microscopic investigations. The nanotubes are not a dominant feature in BMC-1 sample. The tube-like structure is not seen clear in Fig. 3. Fig. 4 is the microstructure of BMC-2 sample which shows the fragmentation of graphite. For the sample ball-milled for 100 h (BMC-3), dense carbon agglomerate structure was observed as typi:ed by Fig. 5. Detailed electron microscopic studies of BMC-2 sample revealed the formation of nanotubes and nano:bres of graphite. They were randomly present in this sample. In order to isolate the nanotubes and nano:bres from the BMC-2 sample, annealing experiments were ◦ carried out. BMC-2 (2 g) was annealed at 700 C in air for 30 min. Electron beam annealing of the sample in the TEM resulted in the formation of some novel structures i.e., torus, coiled nanotubes and nano:bres of carbon. Fig. 6(a) – (c) correspond to the torus, coiled nanotube and nano:bres observed in the sample. The formation of nanotubes in the BMC sample is understandable keeping in mind the fact that
Fig. 3. Transmission electron micrograph (TEM) exhibiting a tubule-like structure in BMC-1 sample.
K. Awasthi et al. / International Journal of Hydrogen Energy 27 (2002) 425–432
Fig. 4. The TEM micrograph of BMC-2 sample depicting the fragmentation of graphite.
Fig. 5. The TEM micrograph of BMC-3 sample representing the dense carbon agglomerate.
small thin graphitic sheets (Makes) can be formed during the suitable milling time, very small thin layers will then curl to form nanotubes. Ball-milling for a short period may not be enough for the formation of thin graphitic sheets. The structure can be destroyed if the ball-milling is done for a very long time. The details of the formation of nanotubes and nano:bres in the BMC sample is being investigated presently and results will be forthcoming. 3.2. Hydrogenation characteristics The hydrogenation properties of various known forms of carbon i.e., graphitic nano:bres, nanotubes, fullerenes, etc., seem to be strongly correlated to their structural and
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microstructural properties. In the present case, we conjecture that the ball-milled carbon, consisting of small Maky microcrystals, would be ideally suited for hydrogen adsorption. In order to verify this, hydrogenation studies on the as-processed and characterized BMC samples were carried out, both with and without nickel as a catalyst. The :rst set of experiments was performed on a BMC-1 sample which was powdered and loaded into the reactor. At room temperature (300 K), the reactor with the sample was pressurized at ∼ 50:3 atm of hydrogen. There was no drop in the initial pressure, when the reactor with the sample was kept at room temperature for 24 h. This proves that the BMC sample does not adsorb hydrogen at room temperature. However, when the reactor with the sample was cooled to 77 K by immersing the reactor in liquid nitrogen, the pressure in the reactor dropped. During the second cycle of hydrogenation, the pressure in the reactor dropped from 50:3 atm at 300 K to 15:9 atm at 77 K in 15 min and did not change further, which is slightly less in comparison to that without sample. Care was taken to maintain the same level of the liquid nitrogen in the reactor throughout the experiment. Subsequent cycles (of heating and cooling) showed no further reduction in the :nal pressure reading. For the second set of experiments, the agglomerates of BMC-1 were added with Ni (1 atomic (at%)) and thoroughly mixed with the help of an electric mixer. The dry grinding renders the separation of agglomerated carbon particles. The microporous nature of the sample can be seen which is potentially suited for hydrogen adsorption. This sample (BMC-1 + Ni (1 at%)) was loaded in the reactor to determine its hydrogenation characteristics. The reactor with the sample was pressurized up to 50:3 atm of hydrogen at room temperature and was then dipped in liquid nitrogen and cooled for 30 min. This was then allowed to heat up to the room temperature. The heating and cooling cycles were repeated six times. The pressure at the lowest temperature was found to decrease with the number of cycles. This signi:es the activation of the sample when it is made to undergo the above-mentioned heating–cooling cycle. The :nal pressure noted after six cycles of cooling and heating (from 300 to 77 K) was 15:6 atm which is lower in comparison to that for without sample which was 16:0 atm. This signi:es that some hydrogen has been adsorbed in the BMC sample. The reactor with the sample was heated from 77 K to room temperature (300 K), the pressure in the reactor was found to recover to the initial value (∼ 50:3 atm). This signi:es that BMC sample desorbs the adsorbed hydrogen when the reactor with the sample was heated from 77 to 300 K. Similar experiments were performed on a BMC-3 sample with Ni (1 at%) and Ni (3 at%). Fig. 7(a) and (b) shows the change of pressure with respect to time, when the reactor was cooled from 300 K to 77 K. Based on these observations the amount of hydrogen adsorbed in BMC-1 sample was calculated to be 0:1 wt% and for BMC-1 with Ni (1 at%) it was found to be 0:42 wt%. It can be concluded that this enhancement in the hydrogen adsorption is due to the presence of Ni as a catalyst. In
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Fig. 6. The TEM images of torus (a),coiled nanotube (b) and nano:bres (c),obtained from annealed BMC-2 sample.
sample BMC-3 + Ni(1 at%), the amount of hydrogen adsorbed was calculated to be 0:63 wt%. We have also found that increasing the Ni content to 3 at% of Ni in BMC-3 sample adsorbed hydrogen corresponding to 0:58 wt%. This apparent decrease in the wt% of hydrogen is understandable in the following way: Ni is presumably an activator as it helps in splitting hydrogen molecule in to hydrogen atoms and hydrogen atoms are adsorbed more than the hydrogen molecule, but Ni does not adsorb= absorb hydrogen under the conditions of the experiment; it is the ball-milled carbon which is the main material responsible for adsorbing hydrogen. When we increase the Ni content, the active part for adsorption namely carbon is reduced. Therefore, wt% of adsorbed hydrogen is less. Indeed, if we calculate the hydrogen adsorbed per 100 g of carbon the numbers are approximately the same for both the samples namely 1 at% Ni in BMC-3 and 3 at% Ni in BMC-3. These values for hydrogen adsorption are 0.63 and 0:58 wt% for BMC-3 with
1 and 3 at% Ni, respectively. Table 1 summarizes the results of these investigations on the hydrogen adsorption in BMC samples. E1orts to increase the hydrogen storage capacity from BMC are being done presently and results will be forthcoming.
4. Conclusions Based on the above results, the following conclusions can be drawn : 1. Ball-milling of graphitic carbon leads to the carbon nanoparticles which coagulate on their own. 2. Self-coagulated carbon agglomerates were formed by 24 and 100 h of ball-milling. Further dry grinding of these BMC agglomerates resulted in the formation of microporous carbon.
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Fig. 7. (a) and (b) show the pressure versus time plot for the reactor without and with ball-milled carbon (BMC-1 and BMC-3) samples.
3. The graphitic nanotubes and nano:bres were found to be present even though sparsely in 48 h ball-milled sample. 4. Ball-milled carbon does not adsorb=absorb hydrogen at room temperature. However, it adsorbs
hydrogen at 77 K, the maximum storage capacity being ∼ 0:6 wt%. 5. Ball-milled carbon adsorbs hydrogen more eQciently when mixed with Ni (≈ 1 at%) and cooled to liquid nitrogen.
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Table 1 Summary of results on hydrogen adsorption studies Sl. no.
1. 2. 3. 4. 5. 6.
Sample status of reactor
Empty reactor Reactor with 2 g Reactor with 2 g Reactor with 2 g Reactor with 2 g Reactor with 2 g
BMC-1 BMC-1 BMC-1 BMC-3 BMC-3
agglomerates agglomerates powder + Ni-1 at% powder + Ni-1 at% powder + Ni-3 at%
Temperature (K)
Pressure (atm)
Initial
Final
Initial
Final
300 300 300 300 300 300
77 300 77 77 77 77
50.3 50.3 50.3 50.3 50.3 50.3
16.0 50.3 15.9 15.6 15.4 15.5
Acknowledgements The authors are thankful to Professor A.R. Verma, Professor C.N.R. Rao, Professor Y.C. Simhadri and Professor T. N. Veziroglu, President, IAHE, USA for their encouragement. Helpful discussions with Dr. R.S.Tiwari, technical assistance from Sri Vijay Kumar (Scienti:c OQcer) are gratefully acknowledged. Financial assistance from AICTE and UGC is gratefully acknowledged. References [1] Gleiter H. Nanocrystalline materials. Progress in Material Science 1989;33:223–315. [2] Mintmire JW, Dunlap BJ, White CT. Are fullerene tubules metallic. Phy Rev Lett 1992;68:631–4. [3] Cullis AG, Canham LT, Calcott PDG. The structural and luminescence properties of porous silicon. J Appl Phys 1997;82(3):909–65.
Adsorbed hydrogen (wt%) — — 0.1 0.42 0.63 0.58
[4] Andresen AF, Maeland AJ. Hydride for energy storage. Pergamon: Oxford, 1978. [5] Noh JS, Agrawal RK, Schwarz A. Hydrogen storage system using activated carbon. Int J Hydrogen Energy 1987;12(10):693–700. [6] Hynek S, Fuller W, Bentley J. Hydrogen storage by carbon sorption. Int J Hydrogen Energy 1997;22(6):601–10. [7] Chambers A, Park C, Baker RTK, Rodrigues NM. Hydrogen storage in graphite nano:bers. J Phys Chem 1998;B102(22):4253–6. [8] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386(27):377–9. [9] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127–9. [10] Chen Y, Gerald JF, Chadderton LT, Cha1ron L. Nanoporous carbon produced by ball milling. Appl Phys Lett 1999;74(19):2782–4. [11] Ramakrishna K, Singh SK, Singh AK. Srivastava ON. Solid state materials for hydrogen storage. Progress in Hydrogen Energy 1987;81–110.
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Ball-milled carbon and hydrogen storage Kalpana Awasthia , R. Kamalakaranb , A.K. Singha , O.N. Srivastavaa;∗ a Department
of Physics, Banaras Hindu University, Varanasi 221005, India India Technology Centre, Bangalore 560060, India
b GE
Abstract We report the formation of carbon in di1erent nanoparticle forms obtained by ball-milling of graphitic carbon. Ball-milling of graphite was carried out in Szegvari attritor at room temperature for varied times i.e. 24, 48 and 100 h in hexane medium. The characterization of ball-milled graphitic carbon (BMC) samples was done by X-ray di1ractometry, scanning electron microscopy and transmission electron microscopy. The self-coagulated carbon agglomerates were obtained for the case of 24 and 100 h BMC samples. The formation of coiled nanotubes and nano:bres was observed in the BMC sample. The BMC samples with and without nickel (Ni) catalyst were subjected to hydrogenation cycling in a Sievert’s type apparatus fabricated in our laboratory. It has been found that BMC sample can adsorb hydrogen. The hydrogen adsorption capacity has been found to be ∼ 0:6 wt%. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ball-milled carbon; Ball-milling; XRD; SEM; TEM; Hydrogen adsorption
1. Introduction Tailoring and controlling the material properties have attracted scienti:c attention and has led to several discoveries. One of them, is to use the often observed exotic properties of materials in the nanoparticle form. It has been reported in several cases that structure and properties can be monitored simply by controlling the size of the particle without changing any physical or chemical parameters [1]. For example, graphitic nano:bres can adsorb, desorb hydrogen di1erently from conventional carbon, the carbon in the cluster-assembled form-C60 fullerene is a good solid lubricant and carbon nanotubes can be either insulator, semiconductor or metal [2]; photoluminescence of silicon is known to depend on the size of the silicon particles which comfortably covers the whole visible spectrum [3]. Indeed, today an observable trend in the scienti:c development is to try novel forms of materials to meet the speci:c requirement for particular application. One of them is to look for solid materials which can absorb hydrogen reversibly, in copious amounts. The fast depletion of fossil fuels and its use leading to serious pollution e1ects, have alarmingly alerted the ∗
Corresponding author. Tel.=fax: +91-542-368-468. E-mail address: [email protected] (O.N. Srivastava).
scientists to look for alternative fuels. Hydrogen is possibly the most suitable future fuel. Hydrogen with bond energy of 104 kcal=mol and favourable combustion properties, is an established, excellent fuel. But, its storage and transportation demands further development. Hydrogen is generally stored in highly pressurized cylinders which demands necessary safety measures against leakage, explosion, etc., when it is used in several applications including its use as fuel in road transportation. This mode of storing hydrogen may be more acceptable for stationary uses, for example, in gensets, etc., but carrying the highly pressurized gas cylinders for vehicular use is not a preferable option. An alternative to this is to store hydrogen in the liquid form but this requires the costly and cumbersome cryogenics as it demands temperature as low as 20 K. Yet another possibility is to store hydrogen in the metal hydride form [4]. This mode has been investigated quite extensively and a variety of intermetallics are now known which can store hydrogen reversibly with a wt% of about 1.5. A good material of choice is one which, can store hydrogen reversibly and in large quantities, is easily available, can be handled easily, does not demand high purity of hydrogen, is economically bene:cial and interfaces with nature. Carbon, being an abundant and cheap material and also being a cycleable material in natural processes, makes this discovery
0360-3199/02/$ 20.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 1 3 4 - 3
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notable. The typical highest estimate of hydrogen adsorption in carbon materials has been 5:3 wt%, at a temperature of 77 K [5,6]. A stunning discovery in this regard was reported by Chambers et al. [7] that graphitic nano:bres can store hydrogen at about 67 wt%, which is much more than the storage capacities in intermetallics. It has been reported that single-walled carbon nanotubes can store 5 –10 wt% of hydrogen [8,9]. The e1orts on exploring hydrogen storage in carbon nanoparticles have been rather sparse. Chen et al. (1999) have reported the occurrence of disordered and nanoporous carbon powder when the graphitic powder is ball-milled at room temperature [10]. The purpose of the present communication is to report the formation of graphitic nanoparticles by ball-milling, its structural characterization by X-ray di1ractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the hydrogen storage behaviour of ball-milled graphitic carbon (BMC). 2. Experimental techniques 2.1. Ball-milled carbon Vacuum annealed graphite rods were used as the starting material for obtaining the ball-milled carbon. Annealing of the graphite rods ensured the removal of impurities such as sulphur, often present in the as-obtained graphite rods. Ball-milling of graphite was carried out in Szegvari attritor at room temperature for varied times i.e. 24, 48 and 100 h, the grinding speed of which was maintained at 400 rpm. Two grams of roughly crushed graphite rods were loaded into the milling container (of volume 1400 cm3 ) with 2:5 kg of steel balls and 500 ml of hexane was added as the wetting medium. The resulting material after 24 (BMC-1), 48 (BMC-2) and 100 h (BMC-3) was taken out from the milling container and placed into glass beakers. 2.2. Characterization of BMC sample The resulting BMC sample was removed from beaker and was characterized by employing XRD (Philips PW-1710), SEM (Philips XL-20) and TEM (Philips CM 12) techniques. 2.3. Hydrogenation behaviour To determine the hydrogen adsorption behaviour of BMC samples, Sievert’s apparatus [11] was used. The hydrogen pressure reading was recorded by digital pressure indicator (model PRM-300 MC2 with a least count of 0:1 atm). The apparatus=reactor was thoroughly checked for a possible leak at 75 atm hydrogen pressure prior to use. To avoid instrumental error the apparatus was standardized by making hydrogenation runs without sample. The hydrogenation behaviour was determined by monitoring the variation of
the hydrogen pressure on cooling the reactor with and without the sample under the same experimental conditions. The pressure reduced more in the presence of the sample. Thus, while without the sample the pressure drop, apparently only due to cooling e1ect, was from 50.3 to 16:0 atm, the same with the sample was from 50.3 to 15:6 atm. This clearly indicated the adsorption=absorption of hydrogen by the BMC sample. The BMC sample (2 g) was loaded into the reactor and pressurized upto 50:3 atm of hydrogen at room temperature. The reactor with sample was cooled to 77 K by immersing the reactor in liquid nitrogen. 3. Results and discussion The resulting material after 24 (BMC-1), 48 (BMC-2) and 100 h (BMC-3) of milling at room temperature, appeared as a dispersion of :ne graphite particles in hexane. Interestingly, the particles settling was very slow and eventually dried in 2 days to coagulate into a Mat disc (diameter = 3:5 cm; thickness = 0:7 cm, and density = 0:47 g=cm3 ). The diameter of the disc resulting from coagulation of graphite particles of ball-milled graphite was smaller than that of the beaker. This implies the tendency of particles to coagulate. The fact that particles settling was so slow also signi:es that the particles are very small. This can be understood in terms of Stokes’ law according to which the size of the particles sedimenting in a Muid, usually a liquid, is given by d = (18h=Ogt)1=2 , where d; ; h; O; g; t are equivalent Stokes’ diameter, absolute viscosity of the Muid, distance through which particle has fallen in time t, di1erence in density between the particle and the Muid and acceleration due to gravity . This is valid when there is no particle-to-particle interaction, and no wall e1ect and particles are sedimenting under laminar Mow in the Muid. 3.1. Structural and microstructural characteristics The XRD pattern of BMC-1, BMC-2 and BMC-3 samples is shown in Fig. 1(a) – (c). The analysis of this pattern revealed that the peaks were explicable in terms of the known hexagonal structure of graphite. The particle sizes as calculated from the XRD pattern using Scherrer’s formula are P for BMC-1, BMC-2 and BMC-3 sam145, 115 and 50 A ples, respectively. The relevant intensity of 00.2 and 10.1 peaks in Fig. 1(a) and (b) are attributed to the texturing that can be noticed, the peaks are sharper in Fig. 1(a) BMC-1 sample than those in Fig. 1(b) BMC-2 sample. This indicates that the particles are smaller in BMC-2 sample than in ◦ BMC-1 sample. The peak at 16 is present due to the tape ◦ used to stick the sample (Fig. 1(b)). The peak at ∼ 39 is unidenti:ed, it is presumably from the impurity of the as-obtained graphite. It is noticed that, this impurity peak has disappeared in Fig. 1(c), which suggests that long time ball-milling destroyed the crystallinity of the impurity. The
K. Awasthi et al. / International Journal of Hydrogen Energy 27 (2002) 425–432
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Fig. 1. (a) – (c) X-ray di1ractograms (XRD) of the ball-milled (24 h) carbon BMC-1, ball-milled (48 h) carbon BMC-2 and ball-milled (100 h) carbon BMC-3 samples. (d) The XRD taken from the pelletized BMC-1 showing a strong 00.2 peak. This indicates the texturing in the pellet (e).
XRD (Fig. 1(d)) taken from the pelletized sample revealed a strong 00.2 peak signifying the texturing in the pellet. When compared with XRD of the as-obtained graphitic rod we found the following di1erences: XRD from the pellet with X-ray facing the Mat and curved (Fig. 1(e)) surfaces were quite di1erent. From the Mat surface XRD exhibited a
much stronger 00.2 peak signifying the said texturing. The sharpness in peak 10.1 was found in Fig. 1(e) which was attributed to the texturing and pelletizing process. Fig. 2(a) is the SEM micrograph of BMC-1 sample which shows the Maky particles of graphite. The SEM pictures, in Fig. 2(b) and (c) show the agglormeration of the carbon particles.
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Fig. 2. (a) – (c) Scanning electron micrographs of the BMC-1, BMC-2 and BMC-3 samples.
Microstructural characterization of BMC samples was done by transmission electron microscopic investigations. The nanotubes are not a dominant feature in BMC-1 sample. The tube-like structure is not seen clear in Fig. 3. Fig. 4 is the microstructure of BMC-2 sample which shows the fragmentation of graphite. For the sample ball-milled for 100 h (BMC-3), dense carbon agglomerate structure was observed as typi:ed by Fig. 5. Detailed electron microscopic studies of BMC-2 sample revealed the formation of nanotubes and nano:bres of graphite. They were randomly present in this sample. In order to isolate the nanotubes and nano:bres from the BMC-2 sample, annealing experiments were ◦ carried out. BMC-2 (2 g) was annealed at 700 C in air for 30 min. Electron beam annealing of the sample in the TEM resulted in the formation of some novel structures i.e., torus, coiled nanotubes and nano:bres of carbon. Fig. 6(a) – (c) correspond to the torus, coiled nanotube and nano:bres observed in the sample. The formation of nanotubes in the BMC sample is understandable keeping in mind the fact that
Fig. 3. Transmission electron micrograph (TEM) exhibiting a tubule-like structure in BMC-1 sample.
K. Awasthi et al. / International Journal of Hydrogen Energy 27 (2002) 425–432
Fig. 4. The TEM micrograph of BMC-2 sample depicting the fragmentation of graphite.
Fig. 5. The TEM micrograph of BMC-3 sample representing the dense carbon agglomerate.
small thin graphitic sheets (Makes) can be formed during the suitable milling time, very small thin layers will then curl to form nanotubes. Ball-milling for a short period may not be enough for the formation of thin graphitic sheets. The structure can be destroyed if the ball-milling is done for a very long time. The details of the formation of nanotubes and nano:bres in the BMC sample is being investigated presently and results will be forthcoming. 3.2. Hydrogenation characteristics The hydrogenation properties of various known forms of carbon i.e., graphitic nano:bres, nanotubes, fullerenes, etc., seem to be strongly correlated to their structural and
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microstructural properties. In the present case, we conjecture that the ball-milled carbon, consisting of small Maky microcrystals, would be ideally suited for hydrogen adsorption. In order to verify this, hydrogenation studies on the as-processed and characterized BMC samples were carried out, both with and without nickel as a catalyst. The :rst set of experiments was performed on a BMC-1 sample which was powdered and loaded into the reactor. At room temperature (300 K), the reactor with the sample was pressurized at ∼ 50:3 atm of hydrogen. There was no drop in the initial pressure, when the reactor with the sample was kept at room temperature for 24 h. This proves that the BMC sample does not adsorb hydrogen at room temperature. However, when the reactor with the sample was cooled to 77 K by immersing the reactor in liquid nitrogen, the pressure in the reactor dropped. During the second cycle of hydrogenation, the pressure in the reactor dropped from 50:3 atm at 300 K to 15:9 atm at 77 K in 15 min and did not change further, which is slightly less in comparison to that without sample. Care was taken to maintain the same level of the liquid nitrogen in the reactor throughout the experiment. Subsequent cycles (of heating and cooling) showed no further reduction in the :nal pressure reading. For the second set of experiments, the agglomerates of BMC-1 were added with Ni (1 atomic (at%)) and thoroughly mixed with the help of an electric mixer. The dry grinding renders the separation of agglomerated carbon particles. The microporous nature of the sample can be seen which is potentially suited for hydrogen adsorption. This sample (BMC-1 + Ni (1 at%)) was loaded in the reactor to determine its hydrogenation characteristics. The reactor with the sample was pressurized up to 50:3 atm of hydrogen at room temperature and was then dipped in liquid nitrogen and cooled for 30 min. This was then allowed to heat up to the room temperature. The heating and cooling cycles were repeated six times. The pressure at the lowest temperature was found to decrease with the number of cycles. This signi:es the activation of the sample when it is made to undergo the above-mentioned heating–cooling cycle. The :nal pressure noted after six cycles of cooling and heating (from 300 to 77 K) was 15:6 atm which is lower in comparison to that for without sample which was 16:0 atm. This signi:es that some hydrogen has been adsorbed in the BMC sample. The reactor with the sample was heated from 77 K to room temperature (300 K), the pressure in the reactor was found to recover to the initial value (∼ 50:3 atm). This signi:es that BMC sample desorbs the adsorbed hydrogen when the reactor with the sample was heated from 77 to 300 K. Similar experiments were performed on a BMC-3 sample with Ni (1 at%) and Ni (3 at%). Fig. 7(a) and (b) shows the change of pressure with respect to time, when the reactor was cooled from 300 K to 77 K. Based on these observations the amount of hydrogen adsorbed in BMC-1 sample was calculated to be 0:1 wt% and for BMC-1 with Ni (1 at%) it was found to be 0:42 wt%. It can be concluded that this enhancement in the hydrogen adsorption is due to the presence of Ni as a catalyst. In
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Fig. 6. The TEM images of torus (a),coiled nanotube (b) and nano:bres (c),obtained from annealed BMC-2 sample.
sample BMC-3 + Ni(1 at%), the amount of hydrogen adsorbed was calculated to be 0:63 wt%. We have also found that increasing the Ni content to 3 at% of Ni in BMC-3 sample adsorbed hydrogen corresponding to 0:58 wt%. This apparent decrease in the wt% of hydrogen is understandable in the following way: Ni is presumably an activator as it helps in splitting hydrogen molecule in to hydrogen atoms and hydrogen atoms are adsorbed more than the hydrogen molecule, but Ni does not adsorb= absorb hydrogen under the conditions of the experiment; it is the ball-milled carbon which is the main material responsible for adsorbing hydrogen. When we increase the Ni content, the active part for adsorption namely carbon is reduced. Therefore, wt% of adsorbed hydrogen is less. Indeed, if we calculate the hydrogen adsorbed per 100 g of carbon the numbers are approximately the same for both the samples namely 1 at% Ni in BMC-3 and 3 at% Ni in BMC-3. These values for hydrogen adsorption are 0.63 and 0:58 wt% for BMC-3 with
1 and 3 at% Ni, respectively. Table 1 summarizes the results of these investigations on the hydrogen adsorption in BMC samples. E1orts to increase the hydrogen storage capacity from BMC are being done presently and results will be forthcoming.
4. Conclusions Based on the above results, the following conclusions can be drawn : 1. Ball-milling of graphitic carbon leads to the carbon nanoparticles which coagulate on their own. 2. Self-coagulated carbon agglomerates were formed by 24 and 100 h of ball-milling. Further dry grinding of these BMC agglomerates resulted in the formation of microporous carbon.
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Fig. 7. (a) and (b) show the pressure versus time plot for the reactor without and with ball-milled carbon (BMC-1 and BMC-3) samples.
3. The graphitic nanotubes and nano:bres were found to be present even though sparsely in 48 h ball-milled sample. 4. Ball-milled carbon does not adsorb=absorb hydrogen at room temperature. However, it adsorbs
hydrogen at 77 K, the maximum storage capacity being ∼ 0:6 wt%. 5. Ball-milled carbon adsorbs hydrogen more eQciently when mixed with Ni (≈ 1 at%) and cooled to liquid nitrogen.
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Table 1 Summary of results on hydrogen adsorption studies Sl. no.
1. 2. 3. 4. 5. 6.
Sample status of reactor
Empty reactor Reactor with 2 g Reactor with 2 g Reactor with 2 g Reactor with 2 g Reactor with 2 g
BMC-1 BMC-1 BMC-1 BMC-3 BMC-3
agglomerates agglomerates powder + Ni-1 at% powder + Ni-1 at% powder + Ni-3 at%
Temperature (K)
Pressure (atm)
Initial
Final
Initial
Final
300 300 300 300 300 300
77 300 77 77 77 77
50.3 50.3 50.3 50.3 50.3 50.3
16.0 50.3 15.9 15.6 15.4 15.5
Acknowledgements The authors are thankful to Professor A.R. Verma, Professor C.N.R. Rao, Professor Y.C. Simhadri and Professor T. N. Veziroglu, President, IAHE, USA for their encouragement. Helpful discussions with Dr. R.S.Tiwari, technical assistance from Sri Vijay Kumar (Scienti:c OQcer) are gratefully acknowledged. Financial assistance from AICTE and UGC is gratefully acknowledged. References [1] Gleiter H. Nanocrystalline materials. Progress in Material Science 1989;33:223–315. [2] Mintmire JW, Dunlap BJ, White CT. Are fullerene tubules metallic. Phy Rev Lett 1992;68:631–4. [3] Cullis AG, Canham LT, Calcott PDG. The structural and luminescence properties of porous silicon. J Appl Phys 1997;82(3):909–65.
Adsorbed hydrogen (wt%) — — 0.1 0.42 0.63 0.58
[4] Andresen AF, Maeland AJ. Hydride for energy storage. Pergamon: Oxford, 1978. [5] Noh JS, Agrawal RK, Schwarz A. Hydrogen storage system using activated carbon. Int J Hydrogen Energy 1987;12(10):693–700. [6] Hynek S, Fuller W, Bentley J. Hydrogen storage by carbon sorption. Int J Hydrogen Energy 1997;22(6):601–10. [7] Chambers A, Park C, Baker RTK, Rodrigues NM. Hydrogen storage in graphite nano:bers. J Phys Chem 1998;B102(22):4253–6. [8] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386(27):377–9. [9] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127–9. [10] Chen Y, Gerald JF, Chadderton LT, Cha1ron L. Nanoporous carbon produced by ball milling. Appl Phys Lett 1999;74(19):2782–4. [11] Ramakrishna K, Singh SK, Singh AK. Srivastava ON. Solid state materials for hydrogen storage. Progress in Hydrogen Energy 1987;81–110.