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Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 5 7 4 e3 5 7 9
Available at www.sciencedirect.com
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Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane D. Silambarasan, V.J. Surya, V. Vasu, K. Iyakutti* School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu 625 021, India
article info
abstract
Article history:
Single walled carbon nanotubes (SWCNTs) dispersed in 2-propanol are deposited on the
Received 21 October 2010
alumina substrate using drop cast method. The deposited SWCNTs are characterized using
Received in revised form
the techniques SEM, EDS and FTIR. Then the SWCNTs are functionalized with BH3 using
2 December 2010
LiBH4 as the precursor. FTIR, XPS and CHNS techniques are used to confirm the func-
Accepted 5 December 2010
tionalization. The functional groups are identified from FTIR studies. The various elements
Available online 7 January 2011
present in the functionalized SWCNTs are identified from XPS and CHNS studies. The functionalized samples are hydrogenated and the hydrogen storage capacity of these
Keywords:
samples is estimated using CHNS studies.
Carbon nanotubes
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Functionalization
reserved.
Borane Hydrogen storage capacity
1.
Introduction
Nanotechnology is the fastest growing area in materials science and engineering. Since the nanomaterials have interesting physical properties, they have attracted great interest in recent years. The high surface/volume ratio of these materials has significant implications with respect to energy storage. The shrinking availability of fossil fuels and global warming leads to the consideration of hydrogen as the potential alternative for energy services. Hydrogen is one of the most abundant elements on earth. Moreover, it is the lightest element and has the highest energy-to-mass ratio. There is no pollutant emission during energy production. Nanostructured materials have potential promise in hydrogen storage because of their unique features such as adsorption on the surface, inter- and intragrain boundaries, and bulk absorption [1,2]. Out of the several carbon nanostructures, the nanotubes discovered by Iijima [3] are now the major breakthrough in the technological development. Since hydrogen
exists in the form of gas at ambient pressure and temperature, the most appropriate way to store hydrogen is in an adsorbed form on a suitable media. But the problem while using hydrogen as the fuel is the lack of hydrogen storage media, hence the development of advanced hydrogen storage media is needed. Carbon nanotubes are one of the most promising materials, can adsorb and release a large quantity of hydrogen easily, reliably, safely and efficiently [4]. Dillon et al. [5] discovered that SWCNTs have a high reversible hydrogen storage capacity. Thereafter many studies have been done widely [6e11]. The hydrogen storage capacity of boron substituted carbon nanotubes was investigated by Sankaran et al. [12] and a maximum of 2 wt% hydrogen storage capacity was observed by them. Hydrogen storage properties of asgrown, purified SWCNT and nanocrystalline platinum dispersed SWCNT have been investigated in the pressure range of 1e100 bar, at 298 and 125 K, the platinum dispersed SWCNT showed a hydrogen uptake up to 3.03 wt% at 125 K and at 78 bar [13]. Lipson et al. [14] reported that, the
* Corresponding author. Tel.: þ91452 2458471x390. E-mail address: [email protected] (K. Iyakutti). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.028
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 5 7 4 e3 5 7 9
electrochemical loading of SWCNTs encapsulated by thin Pd coating resulted in a storage capacity of 8e12 wt% H2. This hydrogen absorption occured mainly within the interior of the SWCNT and depended on the SWCNT type. Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO2 nanorods and nanotubes has been performed at room temperature and moderate hydrogen pressures by Rather et al. [15] and these composites stored up to 0.40 wt% of hydrogen at 298 K and 18 atm, which is nearly five times higher the hydrogen uptake of pristine CNTs. Recently [16] the multi-walled carbon nanotubes (MWCNTs) treated by microwave are used for hydrogen storage and the hydrogen storage capacity of the MWCNTs increased to nearly 0.35 wt% over 0.1 wt% for the pristine sample and increased further to 0.4 wt %, with improved stability after subsequent heat-treatment. Lee et al. [17] activated the MWCNTs using CO2 in the temperature range 500e1100 C and found that the hydrogen storage capacity has enhanced to 0.78 wt% when the activation temperature is increased to 900 C [17]. Rashidi et al. [18] investigated the effect of purity of CNTs on hydrogen storage. They observed a maximum hydrogen adsorption capacity of 0.8 wt% in the case of CNTs with 95% purity and it may be raised up to 1 wt% with some purification. In addition, they compared the hydrogen storage capacity of CNTs with that of some activated carbon and reported that, there were no considerable H2-storage in pure carbon nanotubes and activated carbon at room temperature. On the theoretical side Yildirim et al. [19] reported that, the transition metal atoms adsorbed on SWCNT can store hydrogen molecules up to 8 wt %. After that, Iyakutti et al. [20,21] suggested that, the CNTs functionalized with metal hydrides, can be used as a hydrogen
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storage medium. Among the hydrides tried BH3 [22] is found to be more suitable than the other metal hydrides. The present experimental investigation deals with the hydrogenation of SWCNTs functionalized with borane which is a follow up of the above theoretical prediction.
2.
Theoretical predictions
As mentioned earlier, BH3 is found to be more suitable than the other metal hydrides. Functionalization with BH3 enhances the binding energy of hydrogen molecules as well as increases the storage capacity [22]. BH3 can be deposited on the SWCNTs using LiBH4 as the precursor. Lithium borohydride LiBH4, yields diborane on heating [23]. Generally, boranes are the compounds comprising boron and hydrogen. The parent member BH3 dimerizes to form diborane, B2H6. Diborane is a colorless gas at room temperature. It has been reported that at room temperature there is no isolated monoborane, BH3. It is in equilibrium mixture with B2H6. B2H6 4 2BH3 But at very low pressure and at a temperature of about 200 C, a small fraction of B2H6 is cleaved into BH3. So there is a definite possibility of producing borane. In our case we deal with a hydrogen storage medium which can operate at or above room temperature. Though borane is unstable at room temperature, our theoretical calculations reveal that (CNT þ BH3) complex is stable [22]. Since BH3 remains in
Fig. 1 e a. SEM image of purchased SWCNTs. b. SEM image of SWCNTs after sonication.
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Fig. 2 e EDS analysis.
equilibrium mixture with B2H6, we can use B2H6 4 2BH3 to functionalize the SWCNTs. Depending upon their interaction with SWCNTs they will remain as such or split up into BH3 molecules. On higher coverage of BH3 molecules on SWCNTs we can observe dimerization of BH3 molecules, which leads to the formation of B2H6 or it can remain as BH3. Moreover, diborane is metastable at normal pressures up to 50 C, it will decompose into 2BH3 molecules. The existence of B2H6 as such or as 2BH3 depends upon temperature and pressure in which our system (SWCNTs) is maintained.
3.
Experimental
3.1.
CNT deposition
The purchased SWCNTs with 99% purity are used for experiments without any further purification. Alumina substrates of dimension 19 mm 19 mm 0.65 mm (alumina substrate was taken as it will not react during the heating process) are cleaned with ethanol, acetone by means of sonication for 15 min. SWCNTs were taken with 2-propanol in the ratio of 5 mg/ml. This was then kept in an ultrasonic bath at room temperature for 1 h to allow the CNTs to be dispersed uniformly. The dispersed solution is then deposited drop wise using a micro pipette on the substrates that are maintained in air w60 C. The highly volatile 2-proponal gets evaporated leaving SWCNTs on the substrates. After deposition the substrates are heated to 300 C to remove the impurities if any. The morphology of the dispersed SWCNTs is analyzed using the SEM pictures and the functional groups are identified through FTIR. The weight percentage of the carbon present in our sample is analyzed by energy-dispersive x-ray analysis (EDS). The SEM pictures, ED spectrum and FTIR spectrum are shown in Figs. 1a, 1b, 2 and 3a respectively.
3.2.
Functionalization
LiBH4 mixed with di ethyl ether in the ratio of 25 mg/ml is taken and is drop casted over the surface of SWCNTs. Then the substrates are heated to 275 C (decomposition temperature of LiBH4) for 1 h which yields lithium hydride, boranes
Fig. 3 e a. FTIR image of SWCNTs. b. FTIR image of SWCNTs functionalized with LiBH4.
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and hydrogen. The presence of BH3 in the functionalized samples is confirmed by the FTIR study and the corresponding FTIR spectrum is shown in Fig. 3b. We have used X-ray photoelectron spectroscopy to find the elements present in our sample. The weight percentage of the hydrogen present in our functionalized sample is estimated using CHNS elemental analysis.
3.3.
Hydrogenation
The functionalized sample is loaded in the Seivert like hydrogenation setup and hydrogenated as detailed below. The sample is maintained at 50 C for 100 min and the hydrogen is allowed to flow for 20 min at a flow rate of 3 L/min, and then the sample is left in the chamber to attain the room temperature. After hydrogenation, the hydrogen content present in our sample is again estimated using CHNS elemental analysis. The experiments are repeated for different hydrogenation conditions. The results are given in Table 1.
4.
Fig. 4 e XPS spectrum.
X-ray photoelectron spectroscopy is used to find the elements present in our sample. The full survey spectrum for our functionalized sample is shown in Fig. 4. The lower binding energy component is assigned to boron atoms bonded only to the other boron atoms [27], whereas the higher energy component represents the oxidized boron [28]. The C 1s component at 286.3 eV is assigned to CeO or CeOH bond [29], a small peak at 290.9 eV is due to Li2CO3. The main O 1s component is assigned to boron oxide [30]. The de-convolved XPS spectra (Fig. 5) show the individual components. The hydrogen storage capacity of the hydrogenated samples is estimated using CHNS elemental analysis in the machine e Elementar Vario EL III e Germany. If oxygen were to absorb H2 then OH groups will appear in the FTIR spectrum. Obviously the peak corresponding to OH group is not appearing in the FTIR spectra (Fig. 3b). Therefore, the hydrogen absorption is mainly due to carbon. The portion of boron absorption attributed to the presence of oxygen is determined from the XPS of the functionalized sample and the wt % of boron is 3.7. The hydrogenation results (Table 1), we have got are encouraging. There is a lot of scope for improvement since there are four parameters involved and are to be optimized for the maximum hydrogen storage capacity. Also from the EDS studies, it is observed that the wt% of carbon present in our sample is 66.29. The remaining 33.71 wt% of oxygen present in the sample may be due to CNT deposition in open atmosphere. This can be avoided by annealing the samples in inert atmospheric conditions. This will also improve the hydrogen storage capacity.
Results and discussion
Fig. 1a and b show the scanning electron microscopy (SEM) images of purchased SWCNTs and the dispersed SWCNTs (sonicated for 1 h in 2-propanol) respectively. The average diameter of SWCNTs is estimated to be 22 nm. Fig. 2 shows the ED spectrum of CNT after deposition. From the EDS studies, it is observed that the wt% of carbon present in our sample is 66.29. The remaining 33.71 wt% of oxygen present in the sample may be due to CNT deposition in open atmosphere. The functional groups are determined from the bands appear in the FTIR spectrum. Fig. 3a shows the FTIR spectrum of SWCNTs coated on alumina substrates. A broad absorption peak at 3436 cm1 is attributed due to the hydroxyl group. This might have resulted from water during the purification. The peak at 1631 cm1 is due to C]C stretching modes of vibration. The absorption peak at 1240 cm1 is assigned to CeC stretching vibration [24]. The FTIR spectrum of SWCNTs functionalized with LiBH4 is shown in Fig. 3b. The higher the BH3 dispersion, the higher is its reactivity with SWCNT. If the stretching bonds corresponding to BH3/CNT are more then naturally the reactivity of BH3 and CNT is more. In addition to the above peaks the new absorption peak at 2355 cm1 occurs due to asymmetric BeH stretching [25] in borane complex and a peak at 1182 cm1 has been assigned to the bending mode of BeH bond in the BH3 group [26]. It is to be noted that the peak corresponding to OH group is not observed in the FTIR spectrum.
Table 1 e Hydrogenation results. Time of heating (min) 40 100 100
Temperature of the substrate ( C)
Flow rate (liter/min)
Flow duration (Min)
Storage capacity (wt%)
100 100 50
1 1 3
4 20 20
0.68 0.87 1.5
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5.
Conclusion
The SWCNTs have been successfully functionalized with BH3 as predicted theoretically using LiBH4 as the precursor. From XPS studies, the presence of Li, B, and O are observed in our samples. The presence of BH3 is confirmed using FTIR studies. The hydrogen storage capacity of the functionalized SWCNTs is found to be 1.5 wt% at 50 C. The hydrogen absorption is mainly due to carbon. Hydrogenation depends upon various parameters which are to be optimized to achieve maximum hydrogen storage capacity. Surely, we will be able to achieve the 2015 DOE target.
Acknowledgments Author DS thanks Madurai Kamaraj University (MKU) for USRF. The authors VV and KI thank MKU and UGC-UPE. The authors KI and VJS thank Council of Scientific and Industrial Research (CSIR) for financial assistance under Emeritus Scientist Scheme. The financial assistance by Asian Office of Aerospace Research and Development (No. 09-4157) and the help by Dr. R. Ponnappan, the project manager are acknowledged with thanks.
references
Fig. 5 e XPS of Li, B, C, O.
[1] Baburaj EG, Froes FH, Shutthanandan V, Thevuthasan S. Low cost synthesis of nanocrystalline titanium aluminides. In: Interfacial chemistry and engineering annual report, vol. 4; 2000 (4 pp). [2] Schulz R, Boily S, Zaluski L, Zaluka A, Tessier P, Stro¨mOlsen JO. Nanocrystalline materials for hydrogen storage. Innovation in Metallic Mater; 1995:529e35. [3] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56e8. [4] Nikitin A, Ogasawara H, Mann D, Denecke R, Zhang Z, Dai H, et al. Hydrogenation of single-walled carbon nanotubes. Phys Rev Lett 2005;95:225507 (4 pp). [5] Dillon AC, Jones KM, Bekkendahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single- walled carbon nanotubes. Nature 1997;386:377e9. [6] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127e9. [7] Cheng HM, Liu C, Fan YY, Li F, Su G, Cong HT, et al. Synthesis and hydrogen storage of carbon nanofibers and single-walled carbon nanotubes. Z Metallkd 2000;91:306e11. [8] Gundiah G, Govindaraj A, Rajalakshmi N, Dhathatheyan KS, Rao CNR. Hydrogen storage in carbon nanotubes and related materials. J Mater Chem 2003;13:209e13. [9] Smith MR, Britner EW, Shi W, Jhonson JK, Bockrath BC. Chemical activation of single-walled carbon nanotubes for hydrogen adsorption. J Phys Chem 2003;107:3752e60. [10] Anson A, Benham M, Jagiello J, Callejas MA, Benito AM, Maser WK, et al. Hydrogen adsorption on a single-walled carbon nanotube material: a comparative study of three different adsorption techniques. Nanotechnology 2004;15: 1503e8. [11] Han KS, Kim HS, Song MS, Park MS, Han SS, Lee JY, et al. Atmospheric-pressure plasma treatment to modify
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 5 7 4 e3 5 7 9
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
hydrogen storage properties of multiwalled carbon nanotubes. Appl Phys Lett 2005;86:263105 (3 pp). Sankaran M, Viswanathan B. Hydrogen storage in boron substituted carbon nanotubes. Carbon 2007;45:1628e35. Reddy ALM, Ramaprabhu S. Hydrogen adsorption properties of single-walledcarbon nanotube-Nanocrystalline platinum composites. Int J Hydrogen Energy 2008;33:1028e34. Lipson AG, Lyakhov BF, Saunin EI, Tsivadze AY. Evidence for large hydrogen storage capacity in single-walled carbon nanotubes encapsulated by electroplating Pd onto a Pd substrate. Phys Rev B 2008;77:081405 (4 pp). Rather S, Mehraj-ud-din N, Zacharia R, Hwang SW, Kim AR, Nahm KS. Hydrogen storage of nanostructured TiO2impregnated carbon nanotubes. Int J Hydrogen Energy 2009; 34:961e6. Geng HZ, Kim TH, Lim SC, Jeong HK, Jin MH, Jo YW, et al. Hydrogen storage in microwave-treated multi-walled carbon nanotubes. Int J Hydrogen Energy 2010;35:2073e82. Lee SY, Park SJ. Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors. Int J Hydrogen Energy 2010;35:6757e62. Rashidi AM, Nouralishahi A, Khodadadi AA, Mortazavi Y, Karimi A, Kashefi K. Modification of single wall carbon nanotubes (SWNT) for hydrogen storage. Int J Hydrogen Energy 2010;35:9489e95. Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501 (4 pp). Iyakutti K, Kawazoe Y, Rajarajeswari M, Surya VJ. Aluminum hydride coated single-walled carbon nanotube as a hydrogen storage medium. Int J Hydrogen Energy 2009;34:370e5.
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[21] Surya VJ, Iyakutti K, Venkataramanan NS, Mizuseki H, Kawazoe Y. The role of Li and Ni metals in the adsorbate complex and their effect on the hydrogen storage capacity of single walled carbon nanotubes coated with metal hydrides, LiH and NiH2. Int J Hydrogen Energy 2010;35:2368e76. [22] Surya VJ, Iyakutti K, Rajarajeswari M, Kawazoe Y. Functionalization of single-walled nanotube with borane for hydrogen storage. Physica E 2009;41:1340e6. [23] Kostka J, Lohstroh W, Fitchner M, Hahn H. Diborane release from LiBH4/silica-gel mixtures and the effect of additives. J Phys Chem C 2007;111:14026e9. [24] Socrates G. Infrared characteristic group frequencies. Chichester: Wiley-Interscience; 1980. [25] Nakamoto K. Infrared and raman spectra of inorganic and coordination compounds. Newyork: John Wiley and Sons; 1978. [26] Kaldor A, Porter RF. Infrared spectra of the pyrolysis products of borane carbonyl in an argon matrix. J Amer Chem Soc 1971;93:2140e5. [27] Guo L, Singh RN, Gleebe HJ. Growth of boron-rich nanowires by chemical vapor deposition (CVD). J Nanomater; 2006:58237 (6 pp). [28] Ong CW, Huang H, Zheng B, Kwok RWM, Hui YY, Lau WM. Xray photoemission spectroscopy of nonmetallic materials: Electronic structures of boron and BxOy. J App Phys 2004;95: 3527e34. [29] Vickerman JC. Surface analysis e the principle techniques. Chichester: John Wiley and Sons; 1997. [30] Xu TT, Zheng JG, Wu NQ, Nicholls AW, Roth JR, Dikin DA, et al. Crystalline boron nanoribbons: synthesis and characterization. Nano Lett 2004;4:963e8.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane D. Silambarasan, V.J. Surya, V. Vasu, K. Iyakutti* School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu 625 021, India
article info
abstract
Article history:
Single walled carbon nanotubes (SWCNTs) dispersed in 2-propanol are deposited on the
Received 21 October 2010
alumina substrate using drop cast method. The deposited SWCNTs are characterized using
Received in revised form
the techniques SEM, EDS and FTIR. Then the SWCNTs are functionalized with BH3 using
2 December 2010
LiBH4 as the precursor. FTIR, XPS and CHNS techniques are used to confirm the func-
Accepted 5 December 2010
tionalization. The functional groups are identified from FTIR studies. The various elements
Available online 7 January 2011
present in the functionalized SWCNTs are identified from XPS and CHNS studies. The functionalized samples are hydrogenated and the hydrogen storage capacity of these
Keywords:
samples is estimated using CHNS studies.
Carbon nanotubes
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Functionalization
reserved.
Borane Hydrogen storage capacity
1.
Introduction
Nanotechnology is the fastest growing area in materials science and engineering. Since the nanomaterials have interesting physical properties, they have attracted great interest in recent years. The high surface/volume ratio of these materials has significant implications with respect to energy storage. The shrinking availability of fossil fuels and global warming leads to the consideration of hydrogen as the potential alternative for energy services. Hydrogen is one of the most abundant elements on earth. Moreover, it is the lightest element and has the highest energy-to-mass ratio. There is no pollutant emission during energy production. Nanostructured materials have potential promise in hydrogen storage because of their unique features such as adsorption on the surface, inter- and intragrain boundaries, and bulk absorption [1,2]. Out of the several carbon nanostructures, the nanotubes discovered by Iijima [3] are now the major breakthrough in the technological development. Since hydrogen
exists in the form of gas at ambient pressure and temperature, the most appropriate way to store hydrogen is in an adsorbed form on a suitable media. But the problem while using hydrogen as the fuel is the lack of hydrogen storage media, hence the development of advanced hydrogen storage media is needed. Carbon nanotubes are one of the most promising materials, can adsorb and release a large quantity of hydrogen easily, reliably, safely and efficiently [4]. Dillon et al. [5] discovered that SWCNTs have a high reversible hydrogen storage capacity. Thereafter many studies have been done widely [6e11]. The hydrogen storage capacity of boron substituted carbon nanotubes was investigated by Sankaran et al. [12] and a maximum of 2 wt% hydrogen storage capacity was observed by them. Hydrogen storage properties of asgrown, purified SWCNT and nanocrystalline platinum dispersed SWCNT have been investigated in the pressure range of 1e100 bar, at 298 and 125 K, the platinum dispersed SWCNT showed a hydrogen uptake up to 3.03 wt% at 125 K and at 78 bar [13]. Lipson et al. [14] reported that, the
* Corresponding author. Tel.: þ91452 2458471x390. E-mail address: [email protected] (K. Iyakutti). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.028
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 5 7 4 e3 5 7 9
electrochemical loading of SWCNTs encapsulated by thin Pd coating resulted in a storage capacity of 8e12 wt% H2. This hydrogen absorption occured mainly within the interior of the SWCNT and depended on the SWCNT type. Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO2 nanorods and nanotubes has been performed at room temperature and moderate hydrogen pressures by Rather et al. [15] and these composites stored up to 0.40 wt% of hydrogen at 298 K and 18 atm, which is nearly five times higher the hydrogen uptake of pristine CNTs. Recently [16] the multi-walled carbon nanotubes (MWCNTs) treated by microwave are used for hydrogen storage and the hydrogen storage capacity of the MWCNTs increased to nearly 0.35 wt% over 0.1 wt% for the pristine sample and increased further to 0.4 wt %, with improved stability after subsequent heat-treatment. Lee et al. [17] activated the MWCNTs using CO2 in the temperature range 500e1100 C and found that the hydrogen storage capacity has enhanced to 0.78 wt% when the activation temperature is increased to 900 C [17]. Rashidi et al. [18] investigated the effect of purity of CNTs on hydrogen storage. They observed a maximum hydrogen adsorption capacity of 0.8 wt% in the case of CNTs with 95% purity and it may be raised up to 1 wt% with some purification. In addition, they compared the hydrogen storage capacity of CNTs with that of some activated carbon and reported that, there were no considerable H2-storage in pure carbon nanotubes and activated carbon at room temperature. On the theoretical side Yildirim et al. [19] reported that, the transition metal atoms adsorbed on SWCNT can store hydrogen molecules up to 8 wt %. After that, Iyakutti et al. [20,21] suggested that, the CNTs functionalized with metal hydrides, can be used as a hydrogen
3575
storage medium. Among the hydrides tried BH3 [22] is found to be more suitable than the other metal hydrides. The present experimental investigation deals with the hydrogenation of SWCNTs functionalized with borane which is a follow up of the above theoretical prediction.
2.
Theoretical predictions
As mentioned earlier, BH3 is found to be more suitable than the other metal hydrides. Functionalization with BH3 enhances the binding energy of hydrogen molecules as well as increases the storage capacity [22]. BH3 can be deposited on the SWCNTs using LiBH4 as the precursor. Lithium borohydride LiBH4, yields diborane on heating [23]. Generally, boranes are the compounds comprising boron and hydrogen. The parent member BH3 dimerizes to form diborane, B2H6. Diborane is a colorless gas at room temperature. It has been reported that at room temperature there is no isolated monoborane, BH3. It is in equilibrium mixture with B2H6. B2H6 4 2BH3 But at very low pressure and at a temperature of about 200 C, a small fraction of B2H6 is cleaved into BH3. So there is a definite possibility of producing borane. In our case we deal with a hydrogen storage medium which can operate at or above room temperature. Though borane is unstable at room temperature, our theoretical calculations reveal that (CNT þ BH3) complex is stable [22]. Since BH3 remains in
Fig. 1 e a. SEM image of purchased SWCNTs. b. SEM image of SWCNTs after sonication.
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Fig. 2 e EDS analysis.
equilibrium mixture with B2H6, we can use B2H6 4 2BH3 to functionalize the SWCNTs. Depending upon their interaction with SWCNTs they will remain as such or split up into BH3 molecules. On higher coverage of BH3 molecules on SWCNTs we can observe dimerization of BH3 molecules, which leads to the formation of B2H6 or it can remain as BH3. Moreover, diborane is metastable at normal pressures up to 50 C, it will decompose into 2BH3 molecules. The existence of B2H6 as such or as 2BH3 depends upon temperature and pressure in which our system (SWCNTs) is maintained.
3.
Experimental
3.1.
CNT deposition
The purchased SWCNTs with 99% purity are used for experiments without any further purification. Alumina substrates of dimension 19 mm 19 mm 0.65 mm (alumina substrate was taken as it will not react during the heating process) are cleaned with ethanol, acetone by means of sonication for 15 min. SWCNTs were taken with 2-propanol in the ratio of 5 mg/ml. This was then kept in an ultrasonic bath at room temperature for 1 h to allow the CNTs to be dispersed uniformly. The dispersed solution is then deposited drop wise using a micro pipette on the substrates that are maintained in air w60 C. The highly volatile 2-proponal gets evaporated leaving SWCNTs on the substrates. After deposition the substrates are heated to 300 C to remove the impurities if any. The morphology of the dispersed SWCNTs is analyzed using the SEM pictures and the functional groups are identified through FTIR. The weight percentage of the carbon present in our sample is analyzed by energy-dispersive x-ray analysis (EDS). The SEM pictures, ED spectrum and FTIR spectrum are shown in Figs. 1a, 1b, 2 and 3a respectively.
3.2.
Functionalization
LiBH4 mixed with di ethyl ether in the ratio of 25 mg/ml is taken and is drop casted over the surface of SWCNTs. Then the substrates are heated to 275 C (decomposition temperature of LiBH4) for 1 h which yields lithium hydride, boranes
Fig. 3 e a. FTIR image of SWCNTs. b. FTIR image of SWCNTs functionalized with LiBH4.
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and hydrogen. The presence of BH3 in the functionalized samples is confirmed by the FTIR study and the corresponding FTIR spectrum is shown in Fig. 3b. We have used X-ray photoelectron spectroscopy to find the elements present in our sample. The weight percentage of the hydrogen present in our functionalized sample is estimated using CHNS elemental analysis.
3.3.
Hydrogenation
The functionalized sample is loaded in the Seivert like hydrogenation setup and hydrogenated as detailed below. The sample is maintained at 50 C for 100 min and the hydrogen is allowed to flow for 20 min at a flow rate of 3 L/min, and then the sample is left in the chamber to attain the room temperature. After hydrogenation, the hydrogen content present in our sample is again estimated using CHNS elemental analysis. The experiments are repeated for different hydrogenation conditions. The results are given in Table 1.
4.
Fig. 4 e XPS spectrum.
X-ray photoelectron spectroscopy is used to find the elements present in our sample. The full survey spectrum for our functionalized sample is shown in Fig. 4. The lower binding energy component is assigned to boron atoms bonded only to the other boron atoms [27], whereas the higher energy component represents the oxidized boron [28]. The C 1s component at 286.3 eV is assigned to CeO or CeOH bond [29], a small peak at 290.9 eV is due to Li2CO3. The main O 1s component is assigned to boron oxide [30]. The de-convolved XPS spectra (Fig. 5) show the individual components. The hydrogen storage capacity of the hydrogenated samples is estimated using CHNS elemental analysis in the machine e Elementar Vario EL III e Germany. If oxygen were to absorb H2 then OH groups will appear in the FTIR spectrum. Obviously the peak corresponding to OH group is not appearing in the FTIR spectra (Fig. 3b). Therefore, the hydrogen absorption is mainly due to carbon. The portion of boron absorption attributed to the presence of oxygen is determined from the XPS of the functionalized sample and the wt % of boron is 3.7. The hydrogenation results (Table 1), we have got are encouraging. There is a lot of scope for improvement since there are four parameters involved and are to be optimized for the maximum hydrogen storage capacity. Also from the EDS studies, it is observed that the wt% of carbon present in our sample is 66.29. The remaining 33.71 wt% of oxygen present in the sample may be due to CNT deposition in open atmosphere. This can be avoided by annealing the samples in inert atmospheric conditions. This will also improve the hydrogen storage capacity.
Results and discussion
Fig. 1a and b show the scanning electron microscopy (SEM) images of purchased SWCNTs and the dispersed SWCNTs (sonicated for 1 h in 2-propanol) respectively. The average diameter of SWCNTs is estimated to be 22 nm. Fig. 2 shows the ED spectrum of CNT after deposition. From the EDS studies, it is observed that the wt% of carbon present in our sample is 66.29. The remaining 33.71 wt% of oxygen present in the sample may be due to CNT deposition in open atmosphere. The functional groups are determined from the bands appear in the FTIR spectrum. Fig. 3a shows the FTIR spectrum of SWCNTs coated on alumina substrates. A broad absorption peak at 3436 cm1 is attributed due to the hydroxyl group. This might have resulted from water during the purification. The peak at 1631 cm1 is due to C]C stretching modes of vibration. The absorption peak at 1240 cm1 is assigned to CeC stretching vibration [24]. The FTIR spectrum of SWCNTs functionalized with LiBH4 is shown in Fig. 3b. The higher the BH3 dispersion, the higher is its reactivity with SWCNT. If the stretching bonds corresponding to BH3/CNT are more then naturally the reactivity of BH3 and CNT is more. In addition to the above peaks the new absorption peak at 2355 cm1 occurs due to asymmetric BeH stretching [25] in borane complex and a peak at 1182 cm1 has been assigned to the bending mode of BeH bond in the BH3 group [26]. It is to be noted that the peak corresponding to OH group is not observed in the FTIR spectrum.
Table 1 e Hydrogenation results. Time of heating (min) 40 100 100
Temperature of the substrate ( C)
Flow rate (liter/min)
Flow duration (Min)
Storage capacity (wt%)
100 100 50
1 1 3
4 20 20
0.68 0.87 1.5
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5.
Conclusion
The SWCNTs have been successfully functionalized with BH3 as predicted theoretically using LiBH4 as the precursor. From XPS studies, the presence of Li, B, and O are observed in our samples. The presence of BH3 is confirmed using FTIR studies. The hydrogen storage capacity of the functionalized SWCNTs is found to be 1.5 wt% at 50 C. The hydrogen absorption is mainly due to carbon. Hydrogenation depends upon various parameters which are to be optimized to achieve maximum hydrogen storage capacity. Surely, we will be able to achieve the 2015 DOE target.
Acknowledgments Author DS thanks Madurai Kamaraj University (MKU) for USRF. The authors VV and KI thank MKU and UGC-UPE. The authors KI and VJS thank Council of Scientific and Industrial Research (CSIR) for financial assistance under Emeritus Scientist Scheme. The financial assistance by Asian Office of Aerospace Research and Development (No. 09-4157) and the help by Dr. R. Ponnappan, the project manager are acknowledged with thanks.
references
Fig. 5 e XPS of Li, B, C, O.
[1] Baburaj EG, Froes FH, Shutthanandan V, Thevuthasan S. Low cost synthesis of nanocrystalline titanium aluminides. In: Interfacial chemistry and engineering annual report, vol. 4; 2000 (4 pp). [2] Schulz R, Boily S, Zaluski L, Zaluka A, Tessier P, Stro¨mOlsen JO. Nanocrystalline materials for hydrogen storage. Innovation in Metallic Mater; 1995:529e35. [3] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56e8. [4] Nikitin A, Ogasawara H, Mann D, Denecke R, Zhang Z, Dai H, et al. Hydrogenation of single-walled carbon nanotubes. Phys Rev Lett 2005;95:225507 (4 pp). [5] Dillon AC, Jones KM, Bekkendahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single- walled carbon nanotubes. Nature 1997;386:377e9. [6] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127e9. [7] Cheng HM, Liu C, Fan YY, Li F, Su G, Cong HT, et al. Synthesis and hydrogen storage of carbon nanofibers and single-walled carbon nanotubes. Z Metallkd 2000;91:306e11. [8] Gundiah G, Govindaraj A, Rajalakshmi N, Dhathatheyan KS, Rao CNR. Hydrogen storage in carbon nanotubes and related materials. J Mater Chem 2003;13:209e13. [9] Smith MR, Britner EW, Shi W, Jhonson JK, Bockrath BC. Chemical activation of single-walled carbon nanotubes for hydrogen adsorption. J Phys Chem 2003;107:3752e60. [10] Anson A, Benham M, Jagiello J, Callejas MA, Benito AM, Maser WK, et al. Hydrogen adsorption on a single-walled carbon nanotube material: a comparative study of three different adsorption techniques. Nanotechnology 2004;15: 1503e8. [11] Han KS, Kim HS, Song MS, Park MS, Han SS, Lee JY, et al. Atmospheric-pressure plasma treatment to modify
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 5 7 4 e3 5 7 9
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
hydrogen storage properties of multiwalled carbon nanotubes. Appl Phys Lett 2005;86:263105 (3 pp). Sankaran M, Viswanathan B. Hydrogen storage in boron substituted carbon nanotubes. Carbon 2007;45:1628e35. Reddy ALM, Ramaprabhu S. Hydrogen adsorption properties of single-walledcarbon nanotube-Nanocrystalline platinum composites. Int J Hydrogen Energy 2008;33:1028e34. Lipson AG, Lyakhov BF, Saunin EI, Tsivadze AY. Evidence for large hydrogen storage capacity in single-walled carbon nanotubes encapsulated by electroplating Pd onto a Pd substrate. Phys Rev B 2008;77:081405 (4 pp). Rather S, Mehraj-ud-din N, Zacharia R, Hwang SW, Kim AR, Nahm KS. Hydrogen storage of nanostructured TiO2impregnated carbon nanotubes. Int J Hydrogen Energy 2009; 34:961e6. Geng HZ, Kim TH, Lim SC, Jeong HK, Jin MH, Jo YW, et al. Hydrogen storage in microwave-treated multi-walled carbon nanotubes. Int J Hydrogen Energy 2010;35:2073e82. Lee SY, Park SJ. Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors. Int J Hydrogen Energy 2010;35:6757e62. Rashidi AM, Nouralishahi A, Khodadadi AA, Mortazavi Y, Karimi A, Kashefi K. Modification of single wall carbon nanotubes (SWNT) for hydrogen storage. Int J Hydrogen Energy 2010;35:9489e95. Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501 (4 pp). Iyakutti K, Kawazoe Y, Rajarajeswari M, Surya VJ. Aluminum hydride coated single-walled carbon nanotube as a hydrogen storage medium. Int J Hydrogen Energy 2009;34:370e5.
3579
[21] Surya VJ, Iyakutti K, Venkataramanan NS, Mizuseki H, Kawazoe Y. The role of Li and Ni metals in the adsorbate complex and their effect on the hydrogen storage capacity of single walled carbon nanotubes coated with metal hydrides, LiH and NiH2. Int J Hydrogen Energy 2010;35:2368e76. [22] Surya VJ, Iyakutti K, Rajarajeswari M, Kawazoe Y. Functionalization of single-walled nanotube with borane for hydrogen storage. Physica E 2009;41:1340e6. [23] Kostka J, Lohstroh W, Fitchner M, Hahn H. Diborane release from LiBH4/silica-gel mixtures and the effect of additives. J Phys Chem C 2007;111:14026e9. [24] Socrates G. Infrared characteristic group frequencies. Chichester: Wiley-Interscience; 1980. [25] Nakamoto K. Infrared and raman spectra of inorganic and coordination compounds. Newyork: John Wiley and Sons; 1978. [26] Kaldor A, Porter RF. Infrared spectra of the pyrolysis products of borane carbonyl in an argon matrix. J Amer Chem Soc 1971;93:2140e5. [27] Guo L, Singh RN, Gleebe HJ. Growth of boron-rich nanowires by chemical vapor deposition (CVD). J Nanomater; 2006:58237 (6 pp). [28] Ong CW, Huang H, Zheng B, Kwok RWM, Hui YY, Lau WM. Xray photoemission spectroscopy of nonmetallic materials: Electronic structures of boron and BxOy. J App Phys 2004;95: 3527e34. [29] Vickerman JC. Surface analysis e the principle techniques. Chichester: John Wiley and Sons; 1997. [30] Xu TT, Zheng JG, Wu NQ, Nicholls AW, Roth JR, Dikin DA, et al. Crystalline boron nanoribbons: synthesis and characterization. Nano Lett 2004;4:963e8.