Synthesis and hydrogen storage properties of carbon nanotubes
Introduction
Ever since their discovery by Iijima [1], carbon nanotubes (CNTs) have attracted considerable research interest owing to their unique physical and chemical properties [2]. The applications of CNTs are various, such as field emission, electrical conductivity, electrochemical systems (e.g. lithium ion batteries), hydrogen storage devices and molecular sieves [3]. Hydrogen storage devices are of significant importance in the present scenario of depleting conventional energy sources. It is found that pores of molecular dimension can adsorb large quantities of gases, enhanced density of the adsorbed materials inside the pores being a consequence of the attractive potential of the porous walls [4]. CNTs, with their impressive chemical stability, large surface area, hollowness and light mass, can be considered as ideal candidates for hydrogen adsorption [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Recently, promising results on the hydrogen storage properties of CNTs have been reported [5], [6].
Dillon et al. [8] first measured hydrogen adsorption capacity of as-prepared carbon soot containing single-walled carbon nanotubes (SWNT) at 133 K, from which they extrapolated a hydrogen adsorption capacity of 5–10 wt% for pure SWNT. Ye et al. [9] reported a hydrogen adsorption capacity of above 8 wt% for high-purity crystalline ropes of SWNT at a cryogenic temperature of 80 K and pressure . Liu et al. [10] analyzed the effects of pretreatments of SWNT on their hydrogen adsorption capacity. It has been reported that SWNT pickled in an aqueous solution of HCl and heat treated under vacuum condition can adsorb up to 4.2 wt% [11].
Comparatively, multi-walled carbon nanotubes (MWNT) also seem to have an attractive future for storage. Chen et al. [12] carried out experiments on hydrogen storage capacity of alkali-doped MWNT and reported that the hydrogen intake can achieve 20 and 14 wt% for Li-doped (653 K) and K-doped (room temperature) nanotubes. Gundiah et al. [13] have examined the hydrogen adsorption properties of well-characterized samples of CNTs and reported a maximum storage capacity of 3.7 wt%.
Since the properties of CNTs are expected to be dependent on their diameters and helicity [14], considerable efforts have been made to study the synthesis and growth mechanism of CNTs [15], [16]. CNTs are usually prepared by arc evaporation [17], laser ablation [18] or metal catalyzed chemical vapor deposition (CVD) [19]. Catalysts with large surface area having active catalytic centers are vital for the large scale production of CNTs using CVD [20].
In the present study, we report the synthesis of MWNT by the catalytic decomposition of acetylene using novel rare-earth (RE) based C15 type alloy hydride catalysts (, and ). The as-grown and purified CNTs were characterized by powder X-ray diffraction (XRD), TGA, SEM, TEM and Raman Spectroscopy. The hydrogen adsorption properties of these samples were investigated and the results have been discussed.
Section snippets
Experimental details
RE based hydrogen storage alloys (, and were prepared by arc melting the constituent pure elements in stoichiometric proportions in an arc furnace under argon atmosphere. Each alloy was hydrogenated to a maximum storage capacity using a high pressure Sieverts apparatus. Fine powders of alloys were obtained after several cycles of hydrogenation/dehydrogenation.
MWNT were synthesized by the decomposition of acetylene over RE based alloy hydride powders using a fixed-bed
Results and discussion
XRD patterns of RE based alloys show the formation of single phase with a C 15 type cubic structure (Fig. 1). These catalyst precursors, after several hydrogenation and dehydrogenation cycles, were found to be finely powdered to about . The presence of transition metals Ni, Co and Fe and the hydrogen decrepitation method of powdering the above mentioned catalysts into fine particles in the presence of hydrogen provide fresh surfaces with large surface area which act as effective
Conclusions
Due to the large decrepitation upon hydrogenation/dehydrogenation process, RE based alloy hydrides are suitable catalysts for the production of MWNT with a purity of 95% by the pyrolysis of acetylene over these catalysts by CVD technique. The hydrogen adsorption studies of as-grown and purified MWNT show that the hydrogen adsorption capacity increases with purification and with decrease of temperature. Maximum hydrogen storage capacity of 3.0 wt% at about 95 bar has been obtained at 298 K for
Acknowledgments
The authors are grateful to DRDO and IITM for financial support. One of the authors, R.B. Rakhi is grateful to Council of Scientific and Industrial Research, India, for the financial support provided in the form of Senior Research Fellowship.
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