Nanotube-derived carbon foam for hydrogen sorption

Horizontally aligned carbon nanotube arrays: growth mechanism, controlled synthesis, characterization, properties and applications

This review summarizes the growth mechanism, controlled synthesis, characterization, properties and applications of horizontally aligned carbon nanotube arrays.

Carbon Honeycomb High Capacity Storage for Gaseous and Liquid Species

We report an exceptionally stable honeycomb carbon allotrope obtained by deposition of vacuum-sublimated graphite. The allotrope structures are derived from our low temperature electron diffraction and electron microscopy data. These structures can be both periodic and random and are built exclusively from sp^{2}-bonded carbon atoms, and may be considered as three-dimensional graphene. They demonstrate high levels of physical absorption of various gases unattainable in other carbon forms such as fullerites or nanotubes. These honeycomb structures can be used not only for storage of various gases and liquids but also as a matrix for new composites.

Atomistic simulation of the growth of defect-free carbon nanotubes

Atomistic simulation of defect-free single-walled carbon nanotube (SWCNT) growth is essential for the insightful understanding of the SWCNT's growth mechanism. Despite the extensive effort paid in the past two decades, the goal has not been completely achieved, due to the huge timescale discrepancy between atomistic simulation and the experimental synthesis of SWCNTs, as well as the lack of an accurate classical potential energy surface for large scale simulation. Here, we report atomistic simulations of defect-free SWCNT growth by using a new generation of carbon-metal potential and a hybrid method, in which a basin-hopping strategy is applied to facilitate the defect healing during the simulation. The simulations reveal a narrow diameter distribution and an even chiral angle distribution of the growth of SWCNTs from liquid catalyst, which is in agreement with most known experimental observations.

Energy-Driven Kinetic Monte Carlo Method and Its Application in Fullerene Coalescence

Mimicking the conventional barrier-based kinetic Monte Carlo simulation, an energy-driven kinetic Monte Carlo (EDKMC) method was developed to study the structural transformation of carbon nanomaterials. The new method is many orders magnitude faster than standard molecular dynamics or Monte Marlo (MC) simulations and thus allows us to explore rare events within a reasonable computational time. As an example, the temperature dependence of fullerene coalescence was studied. The simulation, for the first time, revealed that short capped single-walled carbon nanotubes (SWNTs) appear as low-energy metastable structures during the structural evolution.

New insights into designing metallacarborane based room temperature hydrogen storage media

Metallacarboranes are promising towards realizing room temperature hydrogen storage media because of the presence of both transition metal and carbon atoms. In metallacarborane clusters, the transition metal adsorbs hydrogen molecules and carbon can link these clusters to form metal organic framework, which can serve as a complete storage medium. Using first principles density functional calculations, we chalk out the underlying principles of designing an efficient metallacarborane based hydrogen storage media. The storage capacity of hydrogen depends upon the number of available transition metal d-orbitals, number of carbons, and dopant atoms in the cluster. These factors control the amount of charge transfer from metal to the cluster, thereby affecting the number of adsorbed hydrogen molecules. This correlation between the charge transfer and storage capacity is general in nature, and can be applied to designing efficient hydrogen storage systems. Following this strategy, a search for the best metallacarborane was carried out in which Sc based monocarborane was found to be the most promising H2 sorbent material with a 9 wt.% of reversible storage at ambient pressure and temperature.

Feasibility of Lithium Storage on Graphene and Its Derivatives

Nanomaterials are anticipated to be promising storage media, owing to their high surface-to-mass ratio. The high hydrogen capacity achieved by using graphene has reinforced this opinion and motivated investigations of the possibility to use it to store another important energy carrier - lithium (Li). While the first-principles computations show that the Li capacity of pristine graphene, limited by Li clustering and phase separation, is lower than that offered by Li intercalation in graphite, we explore the feasibility of modifying graphene for better Li storage. It is found that certain structural defects in graphene can bind Li stably, yet a more efficacious approach is through substitution doping with boron (B). In particular, the layered C3B compound stands out as a promising Li storage medium. The monolayer C3B has a capacity of 714 mAh/g (as Li1.25C3B), and the capacity of stacked C3B is 857 mAh/g (as Li1.5C3B), which is about twice as large as graphite's 372 mAh/g (as LiC6). Our results help clarify the mechanism of Li storage in low-dimensional materials, and shed light on the rational design of nanoarchitectures for energy storage.

Co-adsorption of N2 in the presence of CH4 within carbon nanospaces: evidence from molecular simulations

Molecular simulations were performed to study the separation of CH(4) and N(2) from mixtures of composition x(CH(4))/x(N(2)) = 5/95 and x(CH(4))/x(N(2)) = 10/90 at 50 bar and 298 K on prototype carbon materials with different pore structures. The studied carbon structures include a slit and a tubular pore, that represent the simplest form of activated carbon and carbon nanotubes, respectively, in addition to a realistic porous carbon model with disordered pore structure and a recently introduced carbon foam model, which has a three-dimensional pore structure. The results indicate that, depending on the pressure and composition, the pore structure influences both the CH(4)/N(2) selectivity and the adsorption behaviour of the fluid molecules. The selectivity was decided by the interactions between CH(4) and N(2) molecules within the pore structure, in addition to the solid-fluid interactions. The simulation results indicate that, at least for the case of activated carbons (slit and random pores), it would not be appropriate to predict the binary adsorption behaviour of methane and nitrogen by means of pure component information. Regardless of the pore structure, the simulation results indicate that carbon materials show a CH(4)/N(2) (thermodynamic) selectivity of only 2-3 up to 2 bar at 298 K, and above this pressure, at equilibrium, none of the carbon materials is adequate for the efficient separation of this mixture.

Unfolding the fullerene: nanotubes, graphene and poly-elemental varieties by simulations

Recent research progress in nanostructured carbon has built upon and yet advanced far from the studies of more conventional carbon forms such as diamond, graphite, and perhaps coals. To some extent, the great attention to nano-carbons has been ignited by the discovery of the structurally least obvious, counterintuitive, small strained fullerene cages. Carbon nanotubes, discovered soon thereafter, and recently, the great interest in graphene, ignited by its extraordinary physics, are all interconnected in a blend of cross-fertilizing fields. Here we review the theoretical and computational models development in our group at Rice University, towards understanding the key structures and behaviors in the immense diversity of carbon allotropes. Our particular emphasis is on the role of certain transcending concepts (like elastic instabilities, dislocations, edges, etc.) which serve so well across the scales and for chemically various compositions.

Efficient defect healing in catalytic carbon nanotube growth

The energetics of topological defects (TDs) in carbon nanotubes (CNTs) and their kinetic healing during the catalytic growth are explored theoretically. Our study indicates that, with the assistance of a metal catalyst, TDs formed during the addition of C atoms can be efficiently healed at the CNT-catalyst interface. Theoretically, a TD-free CNT wall with 10(8)-10(11) carbon atoms is achievable, and, as a consequence, the growth of perfect CNTs up to 0.1-100 cm long is possible since the linear density of a CNT is ∼100 carbon atoms per nanometer. In addition, the calculation shows that, among catalysts most often used, Fe has the highest efficiency for defect healing.