Characterization and hydrogen storage in multi-walled carbon nanotubes grown by aerosol-assisted CVD method

doi:10.1016/j.diamond.2014.01.016

Diamond and Related Materials

Volume 43, March 2014, Pages 66-71

https://doi.org/10.1016/j.diamond.2014.01.016 Get rights and content

Highlights

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MWCNTs prepared by aerosol-assisted CVD possess defects.
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The catalyst support has a significant effect on CNT production.
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Raman spectroscopy is in good agreement with our TEM and TGA observation.
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The H₂ adsorption behaviors depend of their structural and morphological characteristics.

Abstract

The characterization and hydrogen storage capacity of multi-walled carbon nanotubes (MWCNTs) have been studied in the present work. MWCNTs with high purity and bulk yield were achieved from a mixture of camphor/alcohol on a Ni/zeolite support by aerosol-assisted chemical vapor deposition (AACVD). The morphology, surface quality and structure of MWCNTs were characterized by transmission electron microscopy (TEM). Crystallinity and defects of the MWCNTs were studied by Raman spectroscopy and thermo gravimetric analysis (TGA). Hydrogen storage properties of MWCNTs were investigated using a quartz crystal microbalance (QCM). Values between 1.2 and 2.0 wt.% of adsorbed H₂ were reached depending on the exposure pressure. The results also showed that the remaining zeolite present in the as-prepared MWCNT powder adsorbs hydrogen, allowing better adsorption performance of the CNT12 and CNT13 samples. The hydrogen adsorption behavior of CNTs is significantly affected by their structural and morphological characteristics.

Introduction

Carbon nanotubes (CNTs) are among the most interesting materials in current nanotechnological research, due to their low chemical activity, high aspect to ratio, high mechanical strength, and conductivity [1]. There have been extensive reports in recent years to demonstrate the growth of CNTs under different conditions [2], [3], [4]. Among CNTs synthesis methods, chemical vapor deposition (CVD) is the most suitable and economic method at low temperatures and ambient pressure when compared with laser, or arc process [5]. However, these methods are associated with a low yield and the formation of many undesired products [5]. On the other hand, aerosol assisted chemical vapor deposition (AACVD) offers more opportunities to obtain larger amounts of CNTs. Using AACVD, various inorganic porous materials were also investigated as support materials by researchers in producing single- and multi-walled CNTs [2], [3]. These studies showed that the resulting structure, the morphology and applications are dependent on the experimental conditions [2], [3]. The applications for CNTs have enormous potentials such as novel nanoscale electronic devices, electron field emitters, energy storage and energy conversion devices, sensors, and lithium ion batteries [4], [5], [6], [7]. Recently, attention has focused on carbon-based materials due to the usage of CNTs as a safe hydrogen storage medium [5], [8], [9], [10], [11]. To date, many studies have revealed that hydrogen storage capacity is enhanced by added metals to carbon structures, such as Fe, Ni, Co, Cu, Au, Ag, Pt, Ca, Li, K, Al and Pd [3], [5], [12], [13], [14] or by using CNT-based composites [15], [16], [17]; however, the properties of hydrogen storage in these materials is still being researched at a basic level. Actually, the reproducibility of the reported hydrogen storage capacity of CNTs is poor, and the mechanism of how hydrogen is stored in CNTs remains unclear [8]. Liu et al. pointed out that certain amount of hydrogen (less than 1.7 wt.% under a pressure of ~ 12 MPa and at room temperature) can be stored in CNTs [8], which indicate that CNTs cannot fulfill the benchmark of 6.5 wt.% set by the U.S. Department of Energy (DOE) for hydrogen storage systems [8], [13], [18]. Recently, very low values of hydrogen storage capacity of CNTs started to emerge, in particular, those experimentally obtained at room temperature. The reproducibility of the reported high hydrogen capacity of CNTs is poor, and the mechanism of how the hydrogen is stored in CNTs remains unclear [8]. However, experimental studies on hydrogen storage of CNTs and CNT-based hybrid structures, as well as, the hydrogen adsorption mechanism and hydrogen adsorption sites are studied [5], [13].

In the present work, synthesis of MWCNTs has been obtained in an AACVD process under nitrogen and argon flow, and their hydrogen adsorption capacity at room temperature under different loading pressures were studied. Zeolite is investigated as a suitable support for growing CNTs and storing hydrogen, allowing better adsorption performance in between carbon nanotubes. Transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and Raman spectroscopy were used to evaluate the structure, grade of graphitization and quality characteristics of MWCNTs.

Section snippets

Preparation of MWCNT

MWCNTs were synthesized using an aerosol-assisted CVD method. Approximately 2.0 g of camphor, mixed with 10 mL of isopropyl alcohol were placed in an ultrasonic nebulizer. Pure nitrogen (N₂) and argon (Ar) gas were used to transport the precursor mist generated in the atomization chamber to a horizontal quartz tube (length: 50 cm, diameter: 3.0 cm) inserted in a furnace. For CNT12 and CNT13 samples, nickel particles were used as catalyst impregnated in zeolite (molecular sieve: alkalimetal aluminum

Results and discussion

Apart from the first studies on the synthesis of CNTs, there are plenty of reports using thermal decomposition of many hydrocarbons on zeolites (such as Y-type high-silica zeolite (HSZ-390HUA), or aluminophosphate crystals (AFI))) as mesoporous substrates or support [2]. However, it is difficult to compare the present results with those reported, because the CVD method presents more sensitive parameters and conditions (temperature, flow, gas carrier) on CNTs production. However, from the

Conclusion

In this study, MWCNTs were produced by AACVD using a camphor/alcohol mixture and Ni catalyst and zeolite as precursors. The identified characteristics of as-synthesized MWCNTs as well as hydrogen storage capacity were studied. TEM micrographs revealed that the formed MWCNTs have an average diameter in between 20 nm and 50 nm. Also, some defects and a low degree of crystallinity are observed on the graphitic layers. From Raman spectroscopy we conclude that the results are in good agreement with

Primary novelty statement

Hydrogen adsorption capacities of multi-wall carbon nanotubes (MWCNTs) at room temperature under loading pressures were studied. Defects and a low degree of crystallinity are observed on the graphitic layers by TEM, TGA and RS. The defects sites, the tube width and the low crystallinity would promote an enhancement in the H2 storage properties of the MWCNTs.

Acknowledgments

This work was partially supported by the Chilean Government Research Agencies FONDECYT (Grant no. 11110001) and CONICYT (Grant no. ACT1117). The authors thank Professor Alejandro Cabrera from Physics Department of the Pontifícia Universidad Católica de Chile for the provision of laboratory equipments for this research.

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Characterization and hydrogen storage in multi-walled carbon nanotubes grown by aerosol-assisted CVD method

Highlights

Abstract

Introduction

Section snippets

Preparation of MWCNT

Results and discussion

Conclusion

Primary novelty statement

Acknowledgments

Mater. Lett.

Carbon

Appl. Surf. Sci.

J. Nanomater.

Acta

Carbon

Comput. Mater. Sci.

Carbon

Int. J. Hydrogen Energy

J. Electroanal. Chem.

Sensors Actuators B

Thin Solid Films

Carbon

Int. J. Hydrogen Energy

Carbon