Recent advances in the preparation and utilization of carbon nanotubes for hydrogen storage

Recent progress in the production, purification, and experimental and theoretical investigations of carbon nanotubes for hydrogen storage are reviewed. From the industrial point of view, the chemical vapor deposition process has shown advantages over laser ablation and electric-arc-discharge methods. The ultimate goal in nanotube synthesis should be to gain control over geometrical aspects of nanotubes, such as location and orientation, and the atomic structure of nanotubes, including helicity and diameter. There is currently no effective and simple purification procedure that fulfills all requirements for processing carbon nanotubes. Purification is still the bottleneck for technical applications, especially where large amounts of material are required. Although the alkalimetal-doped carbon nanotubes showed high H2 weight uptake, further investigations indicated that some of this uptake was due to water rather than hydrogen. This discovery indicates a potential source of error in evaluation of the storage capacity of doped carbon nanotubes. Nevertheless, currently available single-wall nanotubes yield a hydrogen uptake value near 4 wt% under moderate pressure and room temperature. A further 50% increase is needed to meet U.S. Department of Energy targets for commercial exploitation. Meeting this target will require combining experimental and theoretical efforts to achieve a full understanding of the adsorption process, so that the uptake can be rationally optimized to commercially attractive levels. Large-scale production and purification of carbon nanotubes and remarkable improvement of H2 storage capacity in carbon nanotubes represent significant technological and theoretical challenges in the years to come.     

Synthesis of carbon nanotube-TiO(2) nanotubular material for reversible hydrogen storage

A material consisting of multi-walled carbon nanotubes (MWCNTs) and larger titania (TiO(2)) nanotube arrays has been produced and found to be efficient for reversible hydrogen (H(2)) storage. The TiO(2) nanotube arrays (diameter ～60?nm and length ～2-3?μm) are grown on a Ti substrate, and?MWCNTs a few μm in length and ～30-60?nm in diameter are grown inside these TiO(2) nanotubes using chemical vapor deposition with cobalt as a catalyst. The resulting material has been used in H(2) storage experiments based on a volumetric method using the pressure, composition, and temperature relationship of the storage media. This material can store up to 2.5?wt% of H(2) at 77?K under 25?bar with more than 90% reversibility.     

纳米碳管电化学储氢的研究进展

纳米碳管的储氢是近年来纳米碳管领域研究的一个热点。纳米碳管储氢研究有两种方法，一种是气相法，另一种是电化学法。本文对纳米碳管电化学储氢的基本原理、纳米碳管电化学储氢的理论计算以及氢与纳米碳管的相互作用机制，特别是目前单壁和多壁纳米碳管电化学储氢的实验研究进展进行了综述，展望了利用其电化学储氢特性作为高性能电池的可能性。     

载气种类对单壁碳纳米管管径的影响研究

单壁碳纳米管的管径对其性能、特别是储氢性能有极其重要的影响,但至今未见制备过程中系统控制单壁碳纳米管管径的报道.本文分别以氦气、氮气和氩气为载气,采用催化裂解法制备了不同直径范围的单壁碳纳米管.HRTEM和Raman光谱分析表明,以氦气、氩气为载气制得的碳管直径分布范围相对较窄,平均直径分别约为1.6和5.0nm.以氮气为载气时碳管直径分布相对较宽,约为2.0～4.5nm.氮气与碳反应生成氮化碳可能是导致单壁碳纳米管直径分布相对较宽的主要原因.分别以氦气、氮气和氩气为载气制得的单壁碳纳米管,在273K,15MPa时质量储氢分数依次为4.21%、6.30%和8.05%.     

碳纳米管储氢

近年来,碳纳米管由于其独特的力学、电学等性能以及在众多方面的潜在应用,越来越受到世界各国科学家的关注.最近,碳纳米管由于其大表面积和中空的结构,被应用于氢气储存.本文介绍了该领域最新的一些研究结果     

Selection of carbon nanotubes with specific chiralities using helical assemblies of flavin mononucleotide

The chirality of single-walled carbon nanotubes affects many of their physical and electronic properties. Current production methods result in nanotubes of mixed chiralities, so facile extraction of specific chiralities of single-walled carbon nanotubes is an important step in their effective utilization. Here we show that the flavin mononucleotide, a common redox cofactor, wraps around single-walled carbon nanotubes in a helical pattern that imparts efficient individualization and chirality selection. The cooperative hydrogen bonding between adjacent flavin moieties results in the formation of a helical ribbon, which organizes around single-walled carbon nanotubes through concentric pi-pi interactions between the flavin mononucleotide and the underlying graphene wall. The strength of the helical flavin mononucleotide assembly is strongly dependent on nanotube chirality. In the presence of a surfactant, the flavin mononucleotide assembly is disrupted and replaced without precipitation by a surfactant micelle. The significantly higher affinity of the flavin mononucleotide assembly for (8,6)-single-walled carbon nanotubes results in an 85% chirality enrichment from a nanotube sample with broad diameter distribution.     

Deposition of platinum nanoparticles on carbon nanotubes by supercritical fluid method

Carbon nanotube-supported platinum nanoparticles with a 5-15 nm diameter size range can be synthesized by hydrogen reduction of platinum(ll) acetylacetonate in methanol modified supercritical carbon dioxide. X-ray photoelectron spectroscopy and X-ray diffraction spectra indicate that the carbon nanotubes contain zero-valent platinum metal and high-resolution transmission electron microscopy images show that the visible lattice fringes of platinum nanoparticles are crystallites. Carbon nanotubes synthesized with 25% by weight of platinum nanoparticles exhibit a higher activity for hydrogenation of benzene compared with a commercial carbon black platinum catalyst. The carbon nanotube-supported platinum nanocatalyst can be reused at least six times for the hydrogenation reaction without losing activity. The carbon nanotube-supported platinum nanoparticles are also highly active for electrochemical oxidation of methanol and for reduction of oxygen suggesting their potential use as a new electrocatalyst for proton exchange membrane fuel cell applications.     

Molecular dynamic simulations on tensile mechanical properties of single-walled carbon nanotubes with and without hydrogen storage

Using a bond order potential, molecular dynamics (MD) simulations have been performed to study the mechanical properties of single-walled carbon nanotubes (SWNTs) under tensile loading with and without hydrogen storage. (10,10) armchair and (17,0) zigzag carbon nanotubes have been studied. Up to the necking point of the armchair carbon nanotube, two deformation stages were identified. In the first stage, the elongation of the nanotube was primarily due to the altering of angles between two neighbor carbon bonds. Young's Modulus observed in this stage was comparable with experiments. In the second stage, the lengths of carbon bonds are extended up to the point of fracture. The tensile strength in this stage was higher than that observed in the first stage. Similar results were also found for the zigzag carbon nanotube with a lower tensile strength. Hydrogen molecules stored in the nanotubes reduced the maximum tensile strength of the carbon nanotubes, especially for the armchair type. The effect may be attributed to the competitive formation between the hydrogen–carbon and the carbon–carbon bonds.     

Hydrogen Storage of Magnesium-Based Nanocomposites System

Hydrogen storage in traditional metallic hydrides can deliver about 1.5 to 2.0 wt pct hydrogen but magnesium hydrides can achieve more than 7 wt pct. However, these systems suffer from high temperature release drawback and chemical instability problems. Recently, big improvements of reducing temperature and increasing kinetics of hydrogenation have been made in nanostructured Mg-based composites. This paper aims to provide an overview of the science and engineering of Mg materials and their nanosized composites with nanostructured carbon for hydrogen storage. The needs in research including preparation of the materials, processing and characterisation and basic mechanisms will be explored. The preliminary experimental results indicated a promising future for chemically stable hydrogen storage using carbon nanotubes modified metal hydrides under lower temperatures.     

SiC nanotubes: A novel material for hydrogen storage

A multiscale theoretical approach is used for the investigation of hydrogen storage in silicon-carbon nanotubes (SiCNTs). First, ab initio calculations at the density functional level of theory (DFT) showed an increase of 20% in the binding energy of H2 in SiCNTs compared with pure carbon nanotubes (CNTs). This is explained by the alternative charges that exist in the SiCNT walls. Second, classical Monte Carlo simulation of nanotube bundles showed an even larger increase of the storage capacity in SiCNTs, especially in low temperature and high-pressure conditions. Our results verify in both theoretical levels that SiCNTs seem to be more suitable materials for hydrogen storage than pure CNTs.     

Chemisorption of hydrogen molecules on carbon nanotubes: charging effect from first-principles calculations

We report a systematic investigation of the charging effect on hydrogen molecule chemisorption on (3, 3), (5, 5), (5, 0), and (8, 0) carbon nanotubes by first-principles calculations. The influence of injected charge on the chemisorption energy barriers is found to be sensitive to the nanotube diameter and chirality. The calculated results also indicate that electron injection is more effective in lowering the energy barrier for armchair carbon nanotubes while hole injection is more effective for zigzag nanotubes. The origin of these interesting trends and systematics can be understood by a close examination of the underlying electronic structure and the electron transfer between the hydrogen molecules and the nanotubes.     

流动催化法连续制备碳纳米管及其形态和结构的研究

以二茂铁作为催化剂来源、以苯作为碳源、氢气和氩气分别作为载气和稀释气体,在１１００℃连续地合成了碳纳米管．碳纳米管的生成分二个过程：催化生长和表面无定形碳的生成．所得到的碳纳米管的内径为３～６ｎｍ,而外径约为２０～７０ｎｍ碳纳米管的外径随气体流速的增加而变细,在较细的碳纳米管中观察到了由催化生长而成的具有光滑薄壁的原始碳纳米管．生成的碳纳米管的长度达数十微米、直径较均匀,其端部大多为圆形,但也观察到其他的形状的端部．     

Nickel-cobalt nanoparticles supported on single-walled carbon nanotubes and their catalytic hydrogenation activity

Single-walled carbon nanotubes were synthesized from graphite using the arc discharge technique. A nickel/yttrium/graphite mixture was used as the catalyst. After purification by sonication in a Triton X-100 solution, nickel-cobalt metal nanoparticles were deposited on the surface of the single-walled carbon nanotubes. The resulting material and/or the nanotubes themselves were characterized by physisorption, Raman spectroscopy, high-resolution transition electron microscopy and X-ray diffraction. Raman spectroscopy indicates that the nanotubes, prepared by the arc discharge technique, are semi-conducting with a diameter centering at 1.4 nm. The average nickel-cobalt particle size is estimated to be in the region of 8 nm. The catalytic activity of the material was examined for the hydrogenation of unsaturated fatty acid methyl esters obtained from avocado oil. The carbon nanotube supported nickel-cobalt particles effectively hydrogenate polyunsaturated methyl linoleate to monounsaturated methyl oleate. In contrast to a conventional nickel on kieselghur catalyst, further hydrogenation of methyl oleate to undesired methyl stearate was not observed.     

The effect of carbon nanotube dimensions and dispersion on the fatigue behavior of epoxy nanocomposites

Fatigue is one of the primary reasons for failure in structural materials. It has been demonstrated that carbon nanotubes can suppress fatigue in polymer composites via crack-bridging and a frictional pull-out mechanism. However, a detailed study of the effects of nanotube dimensions and dispersion on the fatigue behavior of nanocomposites has not been performed. In this work, we show the strong effect of carbon nanotube dimensions (i.e.?length, diameter) and dispersion quality on fatigue crack growth suppression in epoxy nanocomposites. We observe that the fatigue crack growth rates can be significantly reduced by (1) reducing the nanotube diameter, (2) increasing the nanotube length and (3) improving the nanotube dispersion. We qualitatively explain these observations by using a fracture mechanics model based on crack-bridging and pull-out of the nanotubes. By optimizing the above parameters (tube length, diameter and dispersion) we demonstrate an over 20-fold reduction in the fatigue crack propagation rate for the nanocomposite epoxy compared to the baseline (unfilled) epoxy.     

One-step fabrication of carbon nanotubes decorated with graphitic nanoflakes for energy storage systems

We have used a bias-assisted microwave plasma chemical vapor deposition system to synthesize carbon nanotubes presenting graphitic nanoflakes, named coral-like carbon nanotubes, and well-aligned carbon nanotubes on carbon cloth substrates. Applying an external bias of -100 V led to the growth of well-aligned carbon nanotubes. In the absence of an external bias, the coral-like nanotubes presenting graphite nanoflakes were formed. The specific surface areas of the well-aligned and coral-like carbon nanotubes electrodes were 90.31 and 143.69 m2/g, respectively. In terms of energy storage, we estimated the capacitance of the coral-like carbon nanotube electrode to be ca. 194 F/g in an electrolyte of 1 M H2SO4. This value is almost double that of the well-aligned carbon nanotubes electrode (104 F/g), presumably because the presence of the carbon nanoflakes had a positive influence on the migration and adsorption of ions within the electrode. The fitting results indicated that the coral-like carbon nanotubes electrode behaved as a traditional electrochemical capacitor. Durability tests revealed that the coral-like carbon nanotube electrode was reliable, with a decay of 9% in capacitance over 1000 cycles.     

Effect of elliptical deformation on molecular polarizabilities of model carbon nanotubes from atomic increments

The interacting induced dipole polarization model implemented in our program POLAR is used for the calculation of the dipole-dipole polarizability alpha. The method is tested with single-wall carbon nanotube models as a function of nanotube radius and elliptical deformation. The results for polarizability follow the same trend as reference calculations performed with our version of the program PAPID. For the zigzag tubes, the polarizability is found to follow a remarkably simple law, that is, it varies as the inverse of the radius. A dramatic effect is also found with elliptical deformation. It is found that the polarizability and related properties can be modified continuously and reversibly by the external radial deformation. These results suggest an interesting technology in which mechanical deformation can control chemical properties of the carbon nanotubes. POLAR calculations differentiate more effectively than PAPID computations among single-wall nanotube models with increasing radial deformation. Different effective polarizabilities are calculated for the atoms at the highest and lowest curvature sites. POLAR calculations discriminate more efficiently than PAPID computations between the effective polarizabilities of the highest and lowest curvature sites. This remarkable and significant tunable polarizability can have important implications for metal coverage of metals on nanotubes and selective adsorption and desorption of foreign atoms and molecules on nanotubes and can lead to a wide variety of technological applications, such as catalysts, hydrogen storage, magnetic tubes, etc.     

General theories for the electrical transport properties of carbon nanotubes

We have shown that the general theories of metals and semiconductors can be employed to understand the diameter and voltage dependency of current through metallic and semiconducting carbon nanotubes, respectively. The current through a semiconducting multiwalled carbon nanotube (MWCNT) is associated with the energy gap that is different for different shells. The contribution of the outermost shell is larger as compared to the inner shells. The general theories can also explain the diameter dependency of maximum current through nanotubes. We have also compared the current carrying ability of a MWCNT and an array of the same diameter of single wall carbon nanotubes (SWCNTs) and found that MWCNTs are better suited and deserve further investigation for possible applications as interconnects.     

Plumbing carbon nanotubes

Since their discovery, the possibility of connecting carbon nanotubes together like water pipes has been an intriguing prospect for these hollow nanostructures. The serial joining of carbon nanotubes in a controlled manner offers a promising approach for the bottom-up engineering of nanotube structures--from simply increasing their aspect ratio to making integrated carbon nanotube devices. To date, however, there have been few reports of the joining of two different carbon nanotubes. Here we demonstrate that a Joule heating process, and associated electro-migration effects, can be used to connect two carbon nanotubes that have the same (or similar) diameters. More generally, with the assistance of a tungsten metal particle, this technique can be used to seamlessly join any two carbon nanotubes--regardless of their diameters--to form new nanotube structures.     

Effects of carbon nanotubes and metal catalysts on hydrogen storage in magnesium nanocomposites

This paper reports a study on nanostructured magnesium composites with carbon nanotubes (CNTs) and catalytic transition metals with high H2 adsorption capacity and fast adsorption kinetics at reduced hydrogenation temperatures. Nanostructures in such a composite are shown to be responsible for improvements in both adsorption capacity and kinetics. It is found that the carbon nanotubes significantly increase the hydrogen storage capacity, and the catalytic transition metals (Fe and Ti) greatly improve the kinetics. This could be understood from the enhancement of diffusion by CNTs and decrease in energy barrier of hydrogen dissociation at the magnesium surface.     

Electrochemical hydrogen storage in single-walled carbon nanotube paper

Single-walled carbon nanotube (SWNT) papers were successfully prepared by dispersing SWNTs in Triton X-100 solution, then filtered by PVDF membrane (0.22 microm pore size). The electrochemical behavior and the reversible hydrogen storage capacity of single-walled carbon nanotube (SWNT) papers have been investigated in alkaline electrolytic solutions (6 N KOH) by cyclic voltammetry, linear micropolarization, and constant current charge/discharge measurements. The effect of thickness and the addition of carbon black on hydrogen adsorption/desorption were also investigated. It was found that the electrochemical charge-discharge mechanism occurring in SWNT paper electrodes is somewhere between that of carbon nanotubes (physical process) and that of metal hydride electrodes (chemical process), and consists of a charge-transfer reaction (Reduction/Oxidation) and a diffusion step (Diffusion).