A multiscale theoretical approach was used to investigate hydrogen storage in a novel three-dimensional carbon nanostructure. This novel nanoporous material has by design tunable pore sizes and surface areas. Its interaction with hydrogen was studied thoroughly via ab initio and grand canonical Monte Carlo calculations. Our results show that, if this material is doped with lithium cations, it can store up to 41 g H 2 /L under ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.Hydrogen is considered to be one of the most promising energy fuels for automobiles, and its use can be further extended to smaller portable devices, like mobile phones and laptops. It can be stored in either liquid or gas phase, provided that an efficient storage device exists. United States' Department of Energy (D.O.E.) has established requirements that have to be met by 2010, regarding the reversible storage of hydrogen according to which the required gravimetric density should be 6 wt % and the volumetric capacity should be 45 g of H 2 /L.1 By moving in that direction, the appropriate material for hosting hydrogen has to be developed.Initially, metal alloys, such as LaNi 5 , TiFe, and MgNi, were proposed as storage tanks since by chemical hydrogenation they form metal hydrides. Hydrogen can then be released by dehydrogenation of the hydride. Regarding vehicle applications, metal hydrides can be distinguished into high or low temperature materials. This depends on the temperature at which hydrogen absorption or desorption is taking place and if this is above or below 150°C, respectively. La-based and Ti-based alloys are examples of some low temperature materials with their main drawback as the very low gravimetric capacity (<2 wt %) they provide. 2,3 Controversially, high temperature materials like Mg-based alloys can reach a theoretical maximum capacity of 7.6 wt %, suffering though from poor hydrogenation/ dehydrogenation kinetics and thermodynamics. [3][4][5] In every case, metal alloys, besides the cost, are heavy for commercial production focused on mobile applications.On the other hand, light nanoporous materials can store hydrogen by physisorption that allows fast loading and unloading. However, as the interaction between H 2 and host material is dominated by weak van der Waals forces, only a small amount of H 2 can be stored at room temperature. In this case, high surface area and appropriate pore size are key parameters for achieving high hydrogen storage. Nanoporous carbon structures fulfill this criterion, placing them since the beginning among of the best candidates for hydrogen storage media. [6][7][8][9] After the synthesis of carbon nanotubes (CNTs), 10 the scientific research focused on their storage capability. Initial studies showed indeed that CNTs can be considered as a good material for reversible hydrogen storage, 11-13 but it was revealed later that under ambient conditions, pristine CNTs are not that promising.14-16 Further studies showed that doping CNTs with lithi...

No abstract

No abstract

Carbon nanotubes (CNT) and graphene are considered as potential future candidates for many nano/microscale integrated devices due to their superior thermal properties. Both systems, however, exhibit significant anisotropy in their thermal conduction, limiting their performance as three-dimensional thermal transport materials. From thermal management perspective, one way to tailor this anisotropy is to consider designing alternative carbon-based architectures. This paper investigates the thermal transport in one such novel architecture-a pillared-graphene (PG) network nanostructure which combines graphene sheets and carbon nanotubes to create a three-dimensional network. Nonequilibrium molecular dynamics simulations have been carried out using the AIREBO potential to calculate the thermal conductivity of pillared-graphene structures along parallel (in-plane) as well as perpendicular (out-of-plane) directions with respect to the graphene plane. The resulting thermal conductivity values for PG systems are discussed and compared with simulated values for pure CNT and graphite. Our results show that in these PG structures, the thermal transport is governed by the minimum interpillar distance and the CNT-pillar length. This is primarily attributed to scattering of phonons occurring at the CNT-graphene junctions in these nanostructures. We foresee that such architecture could potentially be used as a template for designing future structurally stable microscale systems with tailorable in-plane and out-of-plane thermal transport.

A multiscale theoretical approach was used for the investigation of hydrogen storage in the recently synthesized carbon nanoscrolls. First, ab initio calculations at the density functional level of theory (DFT) were performed in order to (a) calculate the binding energy of H2 molecules at the walls of nanoscrolls and (b) fit the parameters of the interatomic potential used in Monte Carlo simulations. Second, classical Monte Carlo simulations were performed for estimating the H2 storage capacity of "experimental size" nanoscrolls containing thousands of atoms. Our results show that pure carbon nanoscrolls cannot accumulate hydrogen because the interlayer distance is too small. However, an opening of the spiral structure to approximately 7 A followed by alkali doping can make them very promising materials for hydrogen storage application, reaching 3 wt % at ambient temperature and pressure.

In the garden of dispersion: High-accuracy ab initio calculations are performed to determine the nature of the interactions and the most favorable geometries between CO(2) and heteroaromatic molecules containing nitrogen (see figure). Dispersion forces play a key role in the stabilization of the dimer, because correlation effects represent about 50 % of the total interaction energy. The interactions between carbon dioxide and organic heterocyclic molecules containing nitrogen are studied by using high-accuracy ab initio methods. Various adsorption positions are examined for pyridine. The preferred configuration is an in-plane configuration. An electron donor-electron acceptor (EDA) mechanism between the carbon of CO(2) and the nitrogen of the heterocycle and weak hydrogen bonds stabilize the complex, with important contributions from dispersion and induction forces. Quantitative results of the binding energy of CO(2) to pyridine (C(5)H(5)N), pyrimidine, pyridazine, and pyrazine (C(4)H(4)N(2)), triazine (C(3)H(3)N(3)), imidazole (C(3)H(4)N(2)), tetrazole (CH(2)N(4)), purine (C(5)H(4)N(4)), imidazopyridine (C(6)H(5)N(3)), adenine (C(5)H(5)N(5)), and imidazopyridamine (C(6)H(6)N(4)) for the in-plane configuration are presented. For purine, three different binding sites are examined. An approximate coupled-cluster model including single and double excitations with a perturbative estimation of triple excitations (CCSD(T)) is used for benchmark calculations. The CCSD(T) basis-set limit is approximated from explicitly correlated second-order Møller-Plesset (MP2-F12) calculations in the aug-cc-pVTZ basis in conjunction with contributions from single, double, and triple excitations calculated at the CCSD(T)/6-311++G** level of theory. Extrapolations to the MP2 basis-set limit coincide with the MP2-F12 calculations. The results are interpreted in terms of electrostatic potential maps and electron density redistribution plots. The effectiveness of density functional theory with the empirical dispersion correction of Grimme (DFT-D) is also examined.

A combination of quantum and classical calculations have been performed in order to investigate hydrogen storage in metal-organic frameworks (MOFs) modified by lithium alkoxide groups. Ab initio calculations showed that the interaction energies between the hydrogen molecules and this functional group are up to three times larger compared with unmodified MOF. This trend was verified by grand canonical Monte Carlo (GCMC) simulations in various thermodynamic conditions. The gravimetric capacity of the Li-modified MOFs reached the value of 10 wt % at 77 K and 100 bar, while our results are very promising at room temperature, too, with 4.5 wt %.

We propose structural and electronic properties of recently synthesized SiC nanotubes. The nanotubes with a Si to C ratio of 1:1 exhibit rich morphologies and are shown to belong to two distinct categories that are close in energies but show significant differences in electronic and transport properties. Similarities and differences are invoked with the case of BN nanotubes to explain the observed surface reconstruction.