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Hydrogen storage of nanostructured TiO2-impregnated carbon nanotubes
international journal of hydrogen energy 34 (2009) 961–966
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen storage of nanostructured TiO2-impregnated carbon nanotubes Sami-ullah Rather, Naik Mehraj-ud-din, Renju Zacharia, Sang Woon Hwang, Ae Rahn Kim1, Kee Suk Nahm* Nanomaterial Research Center and School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, Republic of Korea
article info
abstract
Article history:
Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO2-nanorods and
Received 28 November 2007
nanotubes has been performed at room temperature and moderate hydrogen pressures of
Received in revised form
8–18 atm. Under hydrothermal synthesis conditions, nanorods (NRs) and nanoparticles
26 June 2008
(NPs) are found to form either of the two polymorphic phases, i.e., nanorods are formed of
Accepted 28 September 2008
predominantly anatase phase while nanoparticles are formed of rutile phase. NRs and NPs
Available online 9 December 2008
are introduced into the CNT matrix via the wetness-impregnation method. These composites store up to 0.40 wt.% of hydrogen at 298 K and 18 atm, which is nearly five
Keywords:
times higher the hydrogen uptake of pristine CNTs. The excess amount of hydrogen stored
Spillover
in TiO2-impregnated CNTs is determined from the amount of TiO2 in the sample and the
Nanoparticles
measured hydrogen uptake of TiO2 nanoparticles. Higher hydrogen uptake of NP-impreg-
Nanotube
nated CNTs when compared pristine CNTs is accounted for by considering initial binding
Rutile
of hydrogen on TiO2 and subsequent spillover in CNT–TiO2-NPs.
Adsorption
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Sievert’s volumetric apparatus
reserved.
Anatase
1.
Introduction
Tremendous R & D activities are presently pursued world-wide to fulfill the requirements of hydrogen storage for transportation applications and make the hydrogen competitive with the fossil fuels. For transportation applications, the US Department of energy’s gravimetric and volumetric hydrogen storage targets for the year 2010 are 6.5 wt.% and 62 kg H2 m3, respectively. Among the most vehemently investigated hydrogen storage options, only the chemisorption and physisorption-mediated hydrogen storage satisfy these mid- and long-term storage capacity targets [1]. Physisorption of
hydrogen in nanostructured porous materials, such as carbon nanotubes (CNTs), clathrate hydrates, zeolites, metal organic frameworks, etc., is a promising method of storing hydrogen and has been intensely investigated in the past [2–5]. Nanostructured composite materials formed by carbon nanotubes and metal- or metal oxide-nanoparticles are particularly interesting hydrogen storage materials. It is acknowledged that these novel hybrid materials exhibit non-classical s-p-d hybridization and spillover phenomena that lead their enhanced hydrogen storage capacity [6–9]. Nanostructured TiO2 is a novel and an intensively studied class of structurally organized, nano-sized material. It
* Corresponding author. Tel.: þ82 63 270 2311; fax: þ82 63 270 2306. E-mail address: [email protected] (K.S. Nahm). 1 Specialized Graduate School of Hydrogen and Fuel Cell Engineering and Technology, Chonbuk National University, Chonju 561-756, Republic of Korea. 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.09.089
962
international journal of hydrogen energy 34 (2009) 961–966
exhibits superior photochemical and catalytic efficiency as compared to the bulk titanium dioxide, which is attributed to its high specific surface area and porosity [10,11]. Previously, the interaction of TiO2 with hydrogen was largely studied from the perspectives of surface passivation and relaxation of TiO2 [12,13]. Early reports by Henrich and Kurtz and also by Pan et al. show evidence for only weak interaction of molecular hydrogen with the rutile polymorph [14,15]. Nevertheless, the latter study suggested that atomic hydrogen binds to the rutile surface, even at room temperature [15]. Interestingly, hydrogen is able to diffuse in and out of the bulk TiO2, effectively making it a possible hydrogen storage media [16]. Heterolytic cleavage of molecular hydrogen and successive adsorption of proton (Hþ) on un-coordinated oxygen atom, and the hydride type ion (H) on Ti atoms was perceived from the ab initio calculations performed by Leconte et al. [17]. Bavykin et al. only recently reported that up to 3.8 wt.% of hydrogen can be adsorbed on TiO2-nanotubes over a wide range of temperature (196 to þ100 C) [18,19]. This high storage capacity is accounted for by considering the intercalation of molecular hydrogen in between walls of the titania nanotubes. Furthermore, nanostructured TiO2 is well known for catalytically improving the hydrogenation of Mg, as shown by Jung et al. and Oelerich et al. [20,21]. Here, we report the hydrogen uptake of nanostructured TiO2 and TiO2-impregnated CNTs, measured at room temperature and moderate pressures. The main objective is to examine the enhancement of the hydrogen storage capacity of purified CNT by impregnating with nanostructured TiO2. The effect of TiO2 in tailoring the hydrogen storage capacities of the carbon phase in the impregnated sample is analyzed by considering the hydrogen storage capacities of nanostructured TiO2 and the carbon phase.
2.
Experimental
TiO2-NRs and NPs were prepared via the hydrothermal reaction of commercial anatase and TiCl3 precursors. For the synthesis of titania nanorods, around 2 g of commercial anatase (99%, Sigma Aldrich) was refluxed with 100 ml of 10 M NaOH at 120 C for nearly 48 h. After the completion of the reaction, the reaction medium was neutralized by the addition of 0.1 M HCl and was washed with de-ionized water and filtered. For the preparation of TiO2 nanoparticles, 6 ml of TiCl3 (99%, Sigma Aldrich) suspended in aqueous medium was hydrolyzed with 54 ml of 0.5 M HCl at 70 C for 3 h. The morphological characterization of nanostructured titania was performed using S-4700 Hitachi field emission scanning electron microscope (FESEM) at an accelerating voltage of 10 keV and energy dispersive X-ray (EDX) techniques. For the preparation of TiO2-impregnated CNT, 0.05 g of the TiO2-NP or TiO2NR was ultrasonically mixed with purified CNT (98%, CNT Inc., 4 ¼ 30–100 nm) in aqueous medium. The resulting materials are referred to as CNT–TiO2-NP and CNT–TiO2-NR, respectively. They were characterized by powder X-ray diffraction (XRD, D/MAX 2500 Rigaku, Cu Ka radiation) and thermogravimetric analysis (10 C/min, TGA, TGAQ50). Hydrogen storage studies of the samples were carried out at 25 C using a Sievert’s volumetric equipment. Nearly 0.05 g of sample was used
in each storage study. Prior to each adsorption experiment, the samples were out-gassed for nearly 6 h by heating to 200 C under continuous evacuation up to 104 Torr. The degassing was typically performed until a constant and reproducible background pressure was obtained. For the adsorption experiments, ultra high pure hydrogen gas (99.9999%) was admitted into the reservoir and maintained until the equilibrium was established. The initial pressure of hydrogen in the reservoir was maintained constant between 15 and 30 atm. The final equilibrium pressure in a typical adsorption experiment was found to be between 8 and 18 atm. The gravimetric storage capacities were determined from the pressure drop of hydrogen gas using the ideal gas equation. Further details of the experimental setup and the hydrogen storage experiment procedures are reported elsewhere [22].
3.
Results and discussions
The powder XRD spectra of CNT–TiO2-NR and CNT–TiO2-NP are compared in Fig. 1a, b. The XRD spectra are characterized by several intense peaks which can be ascribed to both nanotubes and TiO2. The peaks at 2q ¼ 26.1 and 44 correspond to reflection from (002) and (100) facets of CNTs. The peaks at 2q ¼ 25.3 , 27.3 , 36.1 , 37.8 , 48 , 41 , 54.3 , 56.5 , 63 , 69 , 69.9 , and 74.8 are indexed on the basis of polymorphic phases of crystalline TiO2 (JCPDS PD File No. 34-0180, 21-1276, and 21-1272). Interestingly, from the X-ray diffraction data, we find that the nanostructured TiO2 is composed of either anatase structure (for NRs) or rutile structure (for NPs). The highest intensity peaks that appear at 2q ¼ 25.3 and 27.30 in the XRD patterns ‘a’ and ‘b’ correspond to (101) and (110) planes of anatase and rutile phases. These planes are the most energetically favorable surfaces for anatase and rutile phases and form up to 94% and 56% of the available surface area in Wulff construction [23]. The TiO2-NPs have a diameter distribution that ranges from 27 to 70 nm. The average diameter and length of
Fig. 1 – The XRD spectra of TiO2-impregnated CNTs. (a) The diffraction pattern of the nanorod-based CNTs predominantly contains the anatase polymorphs while (b) the nanoparticlebased samples contain the rutile polymorphs.
international journal of hydrogen energy 34 (2009) 961–966
963
Fig. 2 – Representative field emission scanning electron microscope images of (a) pristine CNT, (b) TiO2-NP, (c) TiO2-NR, (d) CNT–TiO2-NP, (e) CNT–TiO2-NR, and panel (f) gives the energy dispersive X-ray response of CNT–TiO2-NP. The scale bar corresponds to (a) 500, (b) 300, (c) 500, (d) 500, and (e) 500 nm, respectively.
nanorods are w185 nm and w1.8 mm. In Fig. 2, we present the morphological characterization of the pristine CNT, TiO2-NP, TiO2-NR, and TiO2-impregnated CNTs. Fig. 2 (panel f) shows the energy dispersive X-ray analysis data of CNT–TiO2-NP. The existence of nanorods with anatase structure may appear as an anomaly, since it is known experimentally that above a critical size, TiO2 assume the rutile phase [24]. Nevertheless, the thermodynamical ordering of anatase and rutile phase is not the function of the strictly size alone, and depends also on the other experimental conditions, for instance, the reaction temperature [25,26]. In Fig. 3, thermogravimetric and differential thermal plots of the impregnated materials are
Fig. 3 – The thermogravimetric and differential thermal analysis of (a, c) CNT–TiO2-NR and (b, d) CNT–TiO2-NP samples performed in the oxygen atmosphere. The ramp rate is 10 8C/min. The left vertical axis corresponds to the TGA, while the right one corresponds to the differential weight loss.
depicted. CNT–TiO2-NR shows a single monotonous weight fall in the temperature range of 580–690 C. This feature stems from the gasification of CNT. The onset of the decomposition is at 582 C as obtained from the tangential drawn to TGA curves. The maximum weight loss corresponding to the gasification of CNTs reached at w700 C is 68%, indicating 32% of TiO2 in the TiO2-CNT sample. The Tmax, i.e., the temperature at which the rate of gasification becomes maximum, is 612 C. The thermal decomposition profile of CNT–TiO2-NPs, on the other hand, shows a two-stepped decomposition profile with Tmax ¼ 512 and 626 C. The total weight loss recorded for the nanoparticle impregnated sample is 88.62%. The minor peak that appeared at 512 C possibly arises from the oxidation of amorphous carbon. Slight up-shift of the Tmax of CNT–TiO2-NP when compared with that of CNT–TiO2-NR, implies better thermal stability of the NP-impregnated TiO2. In Fig. 4, we present the temporal evolution of hydrogen uptake of TiO2-impregnated CNTs, at hydrogen equilibrium pressure of 18 atm and 298 K. We have collected the transient hydrogen uptake of the CNT–TiO2-NP (A) for equilibrium pressures (a) 8, (b) 11.5, (c) 14, and (d) & (e) 18 atm and CNT– TiO2-NR (B) for equilibrium pressures of (a) 8.5, (b) 15, and (c) & (d) 18 atm. The maximum hydrogen storage capacities of these samples are 0.40 and 0.35 wt.%. Even if the amount of hydrogen stored in impregnated CNTs is not significant for practical hydrogen storage, it is noteworthy to consider that the impregnated materials exhibit nearly 5 times the storage capacity of pristine CNTs; the latter being w0.075 wt.% [inset of Fig. 4]. Summary of hydrogen adsorption of purified CNT, CNT–TiO2-NP, and CNT–TiO2-NR at RT and hydrogen pressure of 18 atm are provided in Table 1. Now, in order to resolve the contribution of TiO2, we consider the individual contributions of TiO2 and the carbon phases in the composites. In a separate uptake experiment,
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international journal of hydrogen energy 34 (2009) 961–966
Fig. 4 – Transient hydrogen storage capacity of CNT–TiO2 sample recorded at various equilibrium pressures and 298 K. We have data recorded at (a) 8, (b) 11.5, (c) 14, and (d) & (e) 18 atm for CNT–TiO2-NP (A) and (a) 8.5, (b) 15, and (c) & (d) 18 atm for CNT–TiO2-NR (B). Inset shows the transient hydrogen storage capacity of pristine CNT measured under similar experimental conditions.
we measured the hydrogen storage capacity of TiO2-NPs and NRs. The resulting hydrogen uptake is found to be 0.63 and w1.00 wt.%, respectively. Now, by assuming that the storage capacity of nanostructured TiO2 does not change appreciably in the impregnated sample, we employ them to estimate the hydrogen adsorbed in the carbon phase. For this, we use the following relation:
and NPs are predominantly made of anatase and rutile, respectively. Therefore, the difference in their hydrogen uptake can be qualitatively addressed if the hydrogen binding on the (101) and (110) surfaces of anatase and rutile are considered. The energetically favorable adatom binding sites of these surfaces are previously reported by Barnard and Zapol [12]. On a stoichiometric anatase (101) surface, this is a hollow
Wt: of H2 in CNT þ Wt: of H2 in TiO2 100 Wt: of H2 in CNT þ Wt: of H2 in TiO2 þ Wt: of CNT þ Wt: of TiO2
HSCNT–TiO2 ¼
where HSCNT–TiO2 is the experimentally measured hydrogen storage capacity of the nanotube-TiO2 composite material. The amounts of TiO2 and CNTs in the respective samples are obtained from the TGA analysis. The amount of hydrogen uptake by the carbon nanotubes in the NP-impregnated sample is 0.37 wt.%, nearly 5 times higher when compared with the pure CNTs. Similar calculations using the TiO2-NRs indicate that the hydrogen uptake by the carbon nanotubes has diminished by w43% as compared with that of pristine CNTs. Initially, we address the higher hydrogen storage capacity of TiO2-NRs vs TiO2-NPs by considering the available binding sites on the surfaces of NRs and NPs. As pointed earlier, NRs
Table 1 – Summary of hydrogen storage capacity studies of pure CNT, CNT–TiO2-NP, and CNT–TiO2-NR. Samples Pristine CNT CNT–TiO2-NP CNT–TiO2-NR
Temperature Eq. pressure Hydrogen content (K) (atm) (wt.%) 298 – –
18 – –
0.075 0.40 0.35
site which is surrounded by two 2-fold coordinated oxygen, two 5-fold coordinated Ti atoms, and two 6-fold coordinated Ti atoms (see the illustration in Fig. 5) [27]. In comparison, the most energetically favorable site on a stoichiometric rutile (110) surface is the hollow site, which is surrounded by one bridging oxygen (BrO) and one 5-fold Ti atoms. These sites are depicted as gray spheres in the schematic of stoichiometric anatase (101) and rutile (110) phases in Fig. 5. Owing to the greater coordination of adatoms on anatase compared to that on the rutile, the hydrogen adsorbed on an anatase surface is thermodynamically more stable. Since in our experiments, hydrogen is stored isothermally with sufficiently long time to reach the thermodynamic equilibrium, it can be understood that the high hydrogen uptake by the TiO2-NR stems from the stronger interaction of molecular hydrogen with the binding sites on anatase (101) plane. On a reduced TiO2 surface, the hydrogen may have similar coordination effects, if we draw comparisons from Iddir et al.’s calculations for Pt on TiO2 [27]. Even then, asymmetrically position Ti atoms on an anatase surface render stronger ineractions with hydrogen molecules, which is eventually leads to higher hydrogen uptake. The hydrogen uptake of the titania-impregnated carbon nanotubes can be also readily understood by considering the interaction of hydrogen with the respective polymorphs.
international journal of hydrogen energy 34 (2009) 961–966
965
Fig. 5 – Schematic rendition of the lowest energy sites (gray) on stoichiometric (a) rutile (110) and (b) anatase (101) surfaces. On the rutile the lowest energy site is coordinated to one bridging oxygen (BrO) and a 5-fold coordinated Ti (Ti5c), while on the anatase the lowest binding site is coordinated to two 2-fold coordinated oxygens, two 5-fold coordinated Ti, and three 6fold coordinated (Ti6c). Red and blue spheres represent oxygen and titanium atoms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Weak interaction of hydrogen with the atoms on the rutile (110) plane of NPs results in its diffusive spillage to CNT binding sites. Atoms of the anatase (101) plane of NRs, on the other hand, strongly bind hydrogen and therefore, exhibit negligible spillover. This leads to a lowered hydrogen uptake in the NP–CNT composite. An interesting fact is that the gravimetric storage density of hydrogen in nanostructured TiO2 is itself more than that in CNTs (w1 vs 0.08 wt.%) and one might consider using them for hydrogen uptake instead of CNTs. This has been previously reported by Bavykin et al. [18]. Nevertheless, nanostructured TiO2 possess significantly lower specific surface area (w of the order of 200 m2 g1) in comparison to the higher specific surface area MWCNTs [18]. Additionally, the kinetics of hydrogen adsorption in TiO2 is activation limited and proceeds extremely slow at ambient temperature [18]. We presume that the lower hydrogen densities of TiO2-impregnated CNTs as compared to that of CNTs possibly arise due to un-hydrogenated TiO2 particles that exist within the composite matrix. Even if the hydrogen storage capacities of the impregnated CNTs are not sufficient for the practical applications, it is important to consider that phase-selected TiO2 nanoparticles are efficient in storing and spilling hydrogen onto the CNTs. Further theoretical as well as experimental investigations are, indeed, necessary in order to quantitatively understand the microscopic origin of the hydrogen adsorption and spillover in TiO2-based nanomaterials.
4.
Conclusion
In summary, the hydrogen uptake of nanostructured TiO2carbon nanotube composite materials, performed at room temperature and moderate hydrogen pressures of 8–18 atm is presented. TiO2 nanoparticles and nanorods, prepared using hydrothermal reaction of commercial anatase and TiCl3, respectively, are subsequently introduced into the carbon nanotube matrix via the wetness-impregnation method. Nanostructured TiO2 prepared from different precursors resulted in phase-selective formation of anatase-dominant NRs and rutile-dominant NPs. The maximum reversible hydrogen storage capacity of CNT–TiO2 nanoparticles composites at room temperature and an equilibrium pressure of 18 atm is 0.40 wt.%, nearly five times larger uptake of pristine
CNTs. The actual amounts of hydrogen stored in the CNTphase of the composites are obtained from the composition of composite and the hydrogen uptake of the TiO2-phase. The recalculation indicated that TiO2-NPs enhanced the hydrogen uptake of CNTs up to 5 times, though NRs showed higher storage capacity in the pure form. High hydrogen storage capacity NRs as compared with the NPs (w1.00 vs 0.63 wt.%) and inefficient hydrogen spillover in CNT–TiO2-NR composites are attributed to relatively stronger binding of hydrogen on anatase (101) surface of NRs than on rutile (110) surface of NPs.
Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) KRF2005-210-D00037.
references
[1] Satyapal S, Petrovic J, Read C, Thomas G, Ordaz G. The U.S. department of energy’s national hydrogen storage project: progress towards meeting hydrogen-powered vehicle requirements. Catal Today 2006;120:246–56. [2] Benard P, Chahine R. Storage of hydrogen by physisorption on carbon and nanostructured materials. Scr Mater 2007;56: 803–8. [3] Lee H, Lee JW, Kim DY, Park J, Seo YT, Zeng H, et al. Tuning clathrate hydrates for hydrogen storage. Nature 2005;434: 743–6. [4] Zuttel AM. Materials for hydrogen storage. Mater Today 2003; 6:24–33. [5] Rosi NL, Eckert J, Eddaoudi M, Vodak DT, O’Keeffe M, Yaghi OM. Hydrogen storage in microporous metal-organic frameworks. Science 2003;300:1127. [6] Lueking A, Yang RT. Hydrogen spillover from a metal oxide catalyst onto carbon nanotubesdimplications for hydrogen storage. J Catal 2002;206:165–8. [7] Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501–4. [8] Zacharia R, Rather S-U, Hwang SW, Nahm KS. Spillover of physisorbed hydrogen from sputter-deposited arrays of platinum nanoparticles to multi-walled carbon nanotubes. Chem Phys Lett 2007;434:286–91.
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[9] Lueking A, Yang RT. Hydrogen storage in carbon nanotubes: residual metal content and pretreatment temperature. AIChE J 2003;49:1556–68. [10] Maira AJ, Yeung KL, Lee CY, Yue PL, Chan CK. Size effects in gas-phase photo-oxidation of trichloroethylene using nanometer-sized TiO2 catalysts. J Catal 2000;192:185. [11] Ferroni M, Guidi V, Martinelli G, Faglia G, Nelli P, Sberveglieri. Characterization of TiO2 gas sensor. Nanostruct Mater 1996; 7:709–18. [12] Barnard AS, Zapol P. Effects of particle morphology and surface hydrogenation on the phase stability of TiO2. Phys Rev B 2004;70:235403–13. [13] Yang Y, Sushchikh, Mills G, Metiu H, Mcfarland E. Reactivity of TiO2 with hydrogen and deuterium. Appl Surf Sci 2004;229:346–51. [14] Henrich VE, Kurtz R. Surface electronic structure of TiO2: atomic geometry, ligand coordination, and the effect of adsorbed hydrogen. Phys Rev B 1981;23. 6280–87. [15] Pan JM, Maschhoff BL, Diebold U, Madey TE. Interaction water, oxygen and hydrogen with TiO2 (110) surfaces having different defect densities. J Vac Sci Technol A 1992;10:2470. [16] Johnson OW, Paek S-H, DeFord JW. Diffusion of H and D in TiO2: suppression of internal fields by isotope exchange. J Appl Phys 1975;46:1026–33. [17] Leconte J, Markoits A, Skalli MK, Minot C, Belmajdoub A. Periodic ab initio study of the hydrogenated rutile TiO2 (110) surface. Surf Sci 2002;497:194–204. [18] Bavykin DV, Lapkin AA, Plucinski PK, Friedrich JM, Walsh FC. Reversible storage of molecular hydrogen by sorption into multilayered TiO2 nanotubes. J Phys Chem B 2005;41:19422–7.
[19] Bavykin DV, Friedrich JM, Walsh FC. Protonated titanates and TiO2 nanostructured materials: synthesis, properties, and applications. Adv Mater 2006;18:2807–24. [20] Jung KS, Kim DH, Lee EY, Lee KS. Hydrogen sorption of magnesium hydride doped with nano-sized TiO2. Catal Today 2007;120:270–5. [21] Oelerich W, Klassen T, Bormann T. Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials. J Alloys Compd 2001;315:237. [22] Zacharia R, Kim KY, Kibria AKMF, Nahm KS. Enhancement of hydrogen storage capacity of carbon nanotubes via spill-over from vanadium and palladium nanoparticles. Chem Phys Lett 2005;412:369. [23] Madhavan R, David V, Selloni A, King-Smith RD. First principles calculation of stoichiometric TiO2 surfaces. Phys Rev B 1994;49:16721. [24] Zhang HZ, Banfield JF. Thermodynamic analysis of phase stability of nanocrystalline titania. J Mater Chem 1998;8: 2073–6. [25] Cristallo, Roncari E, Rinaldo A, Trifiro F. Study of anatase– rutile transition phase in monolithic catalyst V2O5/TiO2 and V2O5–WO3/TiO2. Appl Catal A Gen 2001;209:249. [26] Djaoued Y, Badilescu S, Ashrit PV, Bersani D, Lottici PP, Robichaud J. Study of anatase to rutile phase transition in nanocrystalline titania films. J Sol-Gel Sci Technol 2002;24: 255. [27] Iddir H, Skavysh V, Ogut S, Browning ND, Disko MM. Preferential growth of Pt on rutile TiO2. Phys Rev B 2006;73: 041403(R).
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen storage of nanostructured TiO2-impregnated carbon nanotubes Sami-ullah Rather, Naik Mehraj-ud-din, Renju Zacharia, Sang Woon Hwang, Ae Rahn Kim1, Kee Suk Nahm* Nanomaterial Research Center and School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, Republic of Korea
article info
abstract
Article history:
Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO2-nanorods and
Received 28 November 2007
nanotubes has been performed at room temperature and moderate hydrogen pressures of
Received in revised form
8–18 atm. Under hydrothermal synthesis conditions, nanorods (NRs) and nanoparticles
26 June 2008
(NPs) are found to form either of the two polymorphic phases, i.e., nanorods are formed of
Accepted 28 September 2008
predominantly anatase phase while nanoparticles are formed of rutile phase. NRs and NPs
Available online 9 December 2008
are introduced into the CNT matrix via the wetness-impregnation method. These composites store up to 0.40 wt.% of hydrogen at 298 K and 18 atm, which is nearly five
Keywords:
times higher the hydrogen uptake of pristine CNTs. The excess amount of hydrogen stored
Spillover
in TiO2-impregnated CNTs is determined from the amount of TiO2 in the sample and the
Nanoparticles
measured hydrogen uptake of TiO2 nanoparticles. Higher hydrogen uptake of NP-impreg-
Nanotube
nated CNTs when compared pristine CNTs is accounted for by considering initial binding
Rutile
of hydrogen on TiO2 and subsequent spillover in CNT–TiO2-NPs.
Adsorption
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Sievert’s volumetric apparatus
reserved.
Anatase
1.
Introduction
Tremendous R & D activities are presently pursued world-wide to fulfill the requirements of hydrogen storage for transportation applications and make the hydrogen competitive with the fossil fuels. For transportation applications, the US Department of energy’s gravimetric and volumetric hydrogen storage targets for the year 2010 are 6.5 wt.% and 62 kg H2 m3, respectively. Among the most vehemently investigated hydrogen storage options, only the chemisorption and physisorption-mediated hydrogen storage satisfy these mid- and long-term storage capacity targets [1]. Physisorption of
hydrogen in nanostructured porous materials, such as carbon nanotubes (CNTs), clathrate hydrates, zeolites, metal organic frameworks, etc., is a promising method of storing hydrogen and has been intensely investigated in the past [2–5]. Nanostructured composite materials formed by carbon nanotubes and metal- or metal oxide-nanoparticles are particularly interesting hydrogen storage materials. It is acknowledged that these novel hybrid materials exhibit non-classical s-p-d hybridization and spillover phenomena that lead their enhanced hydrogen storage capacity [6–9]. Nanostructured TiO2 is a novel and an intensively studied class of structurally organized, nano-sized material. It
* Corresponding author. Tel.: þ82 63 270 2311; fax: þ82 63 270 2306. E-mail address: [email protected] (K.S. Nahm). 1 Specialized Graduate School of Hydrogen and Fuel Cell Engineering and Technology, Chonbuk National University, Chonju 561-756, Republic of Korea. 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.09.089
962
international journal of hydrogen energy 34 (2009) 961–966
exhibits superior photochemical and catalytic efficiency as compared to the bulk titanium dioxide, which is attributed to its high specific surface area and porosity [10,11]. Previously, the interaction of TiO2 with hydrogen was largely studied from the perspectives of surface passivation and relaxation of TiO2 [12,13]. Early reports by Henrich and Kurtz and also by Pan et al. show evidence for only weak interaction of molecular hydrogen with the rutile polymorph [14,15]. Nevertheless, the latter study suggested that atomic hydrogen binds to the rutile surface, even at room temperature [15]. Interestingly, hydrogen is able to diffuse in and out of the bulk TiO2, effectively making it a possible hydrogen storage media [16]. Heterolytic cleavage of molecular hydrogen and successive adsorption of proton (Hþ) on un-coordinated oxygen atom, and the hydride type ion (H) on Ti atoms was perceived from the ab initio calculations performed by Leconte et al. [17]. Bavykin et al. only recently reported that up to 3.8 wt.% of hydrogen can be adsorbed on TiO2-nanotubes over a wide range of temperature (196 to þ100 C) [18,19]. This high storage capacity is accounted for by considering the intercalation of molecular hydrogen in between walls of the titania nanotubes. Furthermore, nanostructured TiO2 is well known for catalytically improving the hydrogenation of Mg, as shown by Jung et al. and Oelerich et al. [20,21]. Here, we report the hydrogen uptake of nanostructured TiO2 and TiO2-impregnated CNTs, measured at room temperature and moderate pressures. The main objective is to examine the enhancement of the hydrogen storage capacity of purified CNT by impregnating with nanostructured TiO2. The effect of TiO2 in tailoring the hydrogen storage capacities of the carbon phase in the impregnated sample is analyzed by considering the hydrogen storage capacities of nanostructured TiO2 and the carbon phase.
2.
Experimental
TiO2-NRs and NPs were prepared via the hydrothermal reaction of commercial anatase and TiCl3 precursors. For the synthesis of titania nanorods, around 2 g of commercial anatase (99%, Sigma Aldrich) was refluxed with 100 ml of 10 M NaOH at 120 C for nearly 48 h. After the completion of the reaction, the reaction medium was neutralized by the addition of 0.1 M HCl and was washed with de-ionized water and filtered. For the preparation of TiO2 nanoparticles, 6 ml of TiCl3 (99%, Sigma Aldrich) suspended in aqueous medium was hydrolyzed with 54 ml of 0.5 M HCl at 70 C for 3 h. The morphological characterization of nanostructured titania was performed using S-4700 Hitachi field emission scanning electron microscope (FESEM) at an accelerating voltage of 10 keV and energy dispersive X-ray (EDX) techniques. For the preparation of TiO2-impregnated CNT, 0.05 g of the TiO2-NP or TiO2NR was ultrasonically mixed with purified CNT (98%, CNT Inc., 4 ¼ 30–100 nm) in aqueous medium. The resulting materials are referred to as CNT–TiO2-NP and CNT–TiO2-NR, respectively. They were characterized by powder X-ray diffraction (XRD, D/MAX 2500 Rigaku, Cu Ka radiation) and thermogravimetric analysis (10 C/min, TGA, TGAQ50). Hydrogen storage studies of the samples were carried out at 25 C using a Sievert’s volumetric equipment. Nearly 0.05 g of sample was used
in each storage study. Prior to each adsorption experiment, the samples were out-gassed for nearly 6 h by heating to 200 C under continuous evacuation up to 104 Torr. The degassing was typically performed until a constant and reproducible background pressure was obtained. For the adsorption experiments, ultra high pure hydrogen gas (99.9999%) was admitted into the reservoir and maintained until the equilibrium was established. The initial pressure of hydrogen in the reservoir was maintained constant between 15 and 30 atm. The final equilibrium pressure in a typical adsorption experiment was found to be between 8 and 18 atm. The gravimetric storage capacities were determined from the pressure drop of hydrogen gas using the ideal gas equation. Further details of the experimental setup and the hydrogen storage experiment procedures are reported elsewhere [22].
3.
Results and discussions
The powder XRD spectra of CNT–TiO2-NR and CNT–TiO2-NP are compared in Fig. 1a, b. The XRD spectra are characterized by several intense peaks which can be ascribed to both nanotubes and TiO2. The peaks at 2q ¼ 26.1 and 44 correspond to reflection from (002) and (100) facets of CNTs. The peaks at 2q ¼ 25.3 , 27.3 , 36.1 , 37.8 , 48 , 41 , 54.3 , 56.5 , 63 , 69 , 69.9 , and 74.8 are indexed on the basis of polymorphic phases of crystalline TiO2 (JCPDS PD File No. 34-0180, 21-1276, and 21-1272). Interestingly, from the X-ray diffraction data, we find that the nanostructured TiO2 is composed of either anatase structure (for NRs) or rutile structure (for NPs). The highest intensity peaks that appear at 2q ¼ 25.3 and 27.30 in the XRD patterns ‘a’ and ‘b’ correspond to (101) and (110) planes of anatase and rutile phases. These planes are the most energetically favorable surfaces for anatase and rutile phases and form up to 94% and 56% of the available surface area in Wulff construction [23]. The TiO2-NPs have a diameter distribution that ranges from 27 to 70 nm. The average diameter and length of
Fig. 1 – The XRD spectra of TiO2-impregnated CNTs. (a) The diffraction pattern of the nanorod-based CNTs predominantly contains the anatase polymorphs while (b) the nanoparticlebased samples contain the rutile polymorphs.
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Fig. 2 – Representative field emission scanning electron microscope images of (a) pristine CNT, (b) TiO2-NP, (c) TiO2-NR, (d) CNT–TiO2-NP, (e) CNT–TiO2-NR, and panel (f) gives the energy dispersive X-ray response of CNT–TiO2-NP. The scale bar corresponds to (a) 500, (b) 300, (c) 500, (d) 500, and (e) 500 nm, respectively.
nanorods are w185 nm and w1.8 mm. In Fig. 2, we present the morphological characterization of the pristine CNT, TiO2-NP, TiO2-NR, and TiO2-impregnated CNTs. Fig. 2 (panel f) shows the energy dispersive X-ray analysis data of CNT–TiO2-NP. The existence of nanorods with anatase structure may appear as an anomaly, since it is known experimentally that above a critical size, TiO2 assume the rutile phase [24]. Nevertheless, the thermodynamical ordering of anatase and rutile phase is not the function of the strictly size alone, and depends also on the other experimental conditions, for instance, the reaction temperature [25,26]. In Fig. 3, thermogravimetric and differential thermal plots of the impregnated materials are
Fig. 3 – The thermogravimetric and differential thermal analysis of (a, c) CNT–TiO2-NR and (b, d) CNT–TiO2-NP samples performed in the oxygen atmosphere. The ramp rate is 10 8C/min. The left vertical axis corresponds to the TGA, while the right one corresponds to the differential weight loss.
depicted. CNT–TiO2-NR shows a single monotonous weight fall in the temperature range of 580–690 C. This feature stems from the gasification of CNT. The onset of the decomposition is at 582 C as obtained from the tangential drawn to TGA curves. The maximum weight loss corresponding to the gasification of CNTs reached at w700 C is 68%, indicating 32% of TiO2 in the TiO2-CNT sample. The Tmax, i.e., the temperature at which the rate of gasification becomes maximum, is 612 C. The thermal decomposition profile of CNT–TiO2-NPs, on the other hand, shows a two-stepped decomposition profile with Tmax ¼ 512 and 626 C. The total weight loss recorded for the nanoparticle impregnated sample is 88.62%. The minor peak that appeared at 512 C possibly arises from the oxidation of amorphous carbon. Slight up-shift of the Tmax of CNT–TiO2-NP when compared with that of CNT–TiO2-NR, implies better thermal stability of the NP-impregnated TiO2. In Fig. 4, we present the temporal evolution of hydrogen uptake of TiO2-impregnated CNTs, at hydrogen equilibrium pressure of 18 atm and 298 K. We have collected the transient hydrogen uptake of the CNT–TiO2-NP (A) for equilibrium pressures (a) 8, (b) 11.5, (c) 14, and (d) & (e) 18 atm and CNT– TiO2-NR (B) for equilibrium pressures of (a) 8.5, (b) 15, and (c) & (d) 18 atm. The maximum hydrogen storage capacities of these samples are 0.40 and 0.35 wt.%. Even if the amount of hydrogen stored in impregnated CNTs is not significant for practical hydrogen storage, it is noteworthy to consider that the impregnated materials exhibit nearly 5 times the storage capacity of pristine CNTs; the latter being w0.075 wt.% [inset of Fig. 4]. Summary of hydrogen adsorption of purified CNT, CNT–TiO2-NP, and CNT–TiO2-NR at RT and hydrogen pressure of 18 atm are provided in Table 1. Now, in order to resolve the contribution of TiO2, we consider the individual contributions of TiO2 and the carbon phases in the composites. In a separate uptake experiment,
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Fig. 4 – Transient hydrogen storage capacity of CNT–TiO2 sample recorded at various equilibrium pressures and 298 K. We have data recorded at (a) 8, (b) 11.5, (c) 14, and (d) & (e) 18 atm for CNT–TiO2-NP (A) and (a) 8.5, (b) 15, and (c) & (d) 18 atm for CNT–TiO2-NR (B). Inset shows the transient hydrogen storage capacity of pristine CNT measured under similar experimental conditions.
we measured the hydrogen storage capacity of TiO2-NPs and NRs. The resulting hydrogen uptake is found to be 0.63 and w1.00 wt.%, respectively. Now, by assuming that the storage capacity of nanostructured TiO2 does not change appreciably in the impregnated sample, we employ them to estimate the hydrogen adsorbed in the carbon phase. For this, we use the following relation:
and NPs are predominantly made of anatase and rutile, respectively. Therefore, the difference in their hydrogen uptake can be qualitatively addressed if the hydrogen binding on the (101) and (110) surfaces of anatase and rutile are considered. The energetically favorable adatom binding sites of these surfaces are previously reported by Barnard and Zapol [12]. On a stoichiometric anatase (101) surface, this is a hollow
Wt: of H2 in CNT þ Wt: of H2 in TiO2 100 Wt: of H2 in CNT þ Wt: of H2 in TiO2 þ Wt: of CNT þ Wt: of TiO2
HSCNT–TiO2 ¼
where HSCNT–TiO2 is the experimentally measured hydrogen storage capacity of the nanotube-TiO2 composite material. The amounts of TiO2 and CNTs in the respective samples are obtained from the TGA analysis. The amount of hydrogen uptake by the carbon nanotubes in the NP-impregnated sample is 0.37 wt.%, nearly 5 times higher when compared with the pure CNTs. Similar calculations using the TiO2-NRs indicate that the hydrogen uptake by the carbon nanotubes has diminished by w43% as compared with that of pristine CNTs. Initially, we address the higher hydrogen storage capacity of TiO2-NRs vs TiO2-NPs by considering the available binding sites on the surfaces of NRs and NPs. As pointed earlier, NRs
Table 1 – Summary of hydrogen storage capacity studies of pure CNT, CNT–TiO2-NP, and CNT–TiO2-NR. Samples Pristine CNT CNT–TiO2-NP CNT–TiO2-NR
Temperature Eq. pressure Hydrogen content (K) (atm) (wt.%) 298 – –
18 – –
0.075 0.40 0.35
site which is surrounded by two 2-fold coordinated oxygen, two 5-fold coordinated Ti atoms, and two 6-fold coordinated Ti atoms (see the illustration in Fig. 5) [27]. In comparison, the most energetically favorable site on a stoichiometric rutile (110) surface is the hollow site, which is surrounded by one bridging oxygen (BrO) and one 5-fold Ti atoms. These sites are depicted as gray spheres in the schematic of stoichiometric anatase (101) and rutile (110) phases in Fig. 5. Owing to the greater coordination of adatoms on anatase compared to that on the rutile, the hydrogen adsorbed on an anatase surface is thermodynamically more stable. Since in our experiments, hydrogen is stored isothermally with sufficiently long time to reach the thermodynamic equilibrium, it can be understood that the high hydrogen uptake by the TiO2-NR stems from the stronger interaction of molecular hydrogen with the binding sites on anatase (101) plane. On a reduced TiO2 surface, the hydrogen may have similar coordination effects, if we draw comparisons from Iddir et al.’s calculations for Pt on TiO2 [27]. Even then, asymmetrically position Ti atoms on an anatase surface render stronger ineractions with hydrogen molecules, which is eventually leads to higher hydrogen uptake. The hydrogen uptake of the titania-impregnated carbon nanotubes can be also readily understood by considering the interaction of hydrogen with the respective polymorphs.
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Fig. 5 – Schematic rendition of the lowest energy sites (gray) on stoichiometric (a) rutile (110) and (b) anatase (101) surfaces. On the rutile the lowest energy site is coordinated to one bridging oxygen (BrO) and a 5-fold coordinated Ti (Ti5c), while on the anatase the lowest binding site is coordinated to two 2-fold coordinated oxygens, two 5-fold coordinated Ti, and three 6fold coordinated (Ti6c). Red and blue spheres represent oxygen and titanium atoms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Weak interaction of hydrogen with the atoms on the rutile (110) plane of NPs results in its diffusive spillage to CNT binding sites. Atoms of the anatase (101) plane of NRs, on the other hand, strongly bind hydrogen and therefore, exhibit negligible spillover. This leads to a lowered hydrogen uptake in the NP–CNT composite. An interesting fact is that the gravimetric storage density of hydrogen in nanostructured TiO2 is itself more than that in CNTs (w1 vs 0.08 wt.%) and one might consider using them for hydrogen uptake instead of CNTs. This has been previously reported by Bavykin et al. [18]. Nevertheless, nanostructured TiO2 possess significantly lower specific surface area (w of the order of 200 m2 g1) in comparison to the higher specific surface area MWCNTs [18]. Additionally, the kinetics of hydrogen adsorption in TiO2 is activation limited and proceeds extremely slow at ambient temperature [18]. We presume that the lower hydrogen densities of TiO2-impregnated CNTs as compared to that of CNTs possibly arise due to un-hydrogenated TiO2 particles that exist within the composite matrix. Even if the hydrogen storage capacities of the impregnated CNTs are not sufficient for the practical applications, it is important to consider that phase-selected TiO2 nanoparticles are efficient in storing and spilling hydrogen onto the CNTs. Further theoretical as well as experimental investigations are, indeed, necessary in order to quantitatively understand the microscopic origin of the hydrogen adsorption and spillover in TiO2-based nanomaterials.
4.
Conclusion
In summary, the hydrogen uptake of nanostructured TiO2carbon nanotube composite materials, performed at room temperature and moderate hydrogen pressures of 8–18 atm is presented. TiO2 nanoparticles and nanorods, prepared using hydrothermal reaction of commercial anatase and TiCl3, respectively, are subsequently introduced into the carbon nanotube matrix via the wetness-impregnation method. Nanostructured TiO2 prepared from different precursors resulted in phase-selective formation of anatase-dominant NRs and rutile-dominant NPs. The maximum reversible hydrogen storage capacity of CNT–TiO2 nanoparticles composites at room temperature and an equilibrium pressure of 18 atm is 0.40 wt.%, nearly five times larger uptake of pristine
CNTs. The actual amounts of hydrogen stored in the CNTphase of the composites are obtained from the composition of composite and the hydrogen uptake of the TiO2-phase. The recalculation indicated that TiO2-NPs enhanced the hydrogen uptake of CNTs up to 5 times, though NRs showed higher storage capacity in the pure form. High hydrogen storage capacity NRs as compared with the NPs (w1.00 vs 0.63 wt.%) and inefficient hydrogen spillover in CNT–TiO2-NR composites are attributed to relatively stronger binding of hydrogen on anatase (101) surface of NRs than on rutile (110) surface of NPs.
Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) KRF2005-210-D00037.
references
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