George E. Froudakis

Pillared Graphene: A New 3-D Network Nanostructure for Enhanced Hydrogen Storage

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 H2/L under ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.

Modeling of thermal transport in pillared-graphene architectures.

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.

Improving Hydrogen Storage Capacity of MOF by Functionalization of the Organic Linker with Lithium Atoms

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 %.

Ab initio Study of the Interactions between CO 2 and N-Containing Organic Heterocycles

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.

Curvature dependence of the metal catalyst atom interaction with carbon nanotubes walls

Abstract Interactions of the transition metal atoms with carbon nanotube walls are investigated using a tight-binding molecular dynamics method that allows for spin unrestricted geometry optimization. Comparison with the results for bonding on graphite indicates major differences in bonding sites, magnetic moments and the direction of charge transfer. The significant values of magnetic moments obtained for the metal atoms on nanotube walls is consistent with the recent experimental findings.

Designing 3D COFs with Enhanced Hydrogen Storage Capacity

Hydrogen storage properties have been studied on newly designed three-dimensional covalent-organic framework (3D-COF). The design of these materials was based on the ctn network of the ultralow density COF-102. The structures were optimized by multiscale techniques and the optimized structures were checked for their storage capacities by grand canonical Monte Carlo simulations. Our simulations demonstrate that the gravimetric uptake of one of these new COFs can overpass the value of 25 wt % in 77 K and reach the Department of Energys target of 6 wt % in room temperature, classifying them between the top hydrogen storage materials.

Various bonding configurations of transition-metal atoms on carbon nanotubes: Their effect on contact resistance

Our investigations reveal that the bonding of the transition-metal atoms on a single-wall carbon nanotube (SWCN) depends on the detailed contact conditions. On the basis of our results, we suggest that the early 3-d elements (Sc, Ti, and V) can be expected to be good candidates for making metal–SWCN contacts of low resistance, while contacts employing the late 3-d elements (Fe, Co, and Ni) and Cu are expected to exhibit large contact resistance.

Reticular Synthesis of HKUST-like tbo-MOFs with Enhanced CH4 Storage

Successful implementation of reticular chemistry using a judiciously designed rigid octatopic carboxylate organic linker allowed the construction of expanded HKUST-1-like tbo-MOF series with intrinsic strong CH4 adsorption sites. The Cu-analogue displayed a concomitant enhancement of the gravimetric and volumetric surface area with the highest reported CH4 uptake among the tbo family, comparable to the best performing metal organic frameworks (MOFs) for CH4 storage. The corresponding gravimetric (BET) and volumetric surface areas of 3971 m(2) g(-1) and 2363 m(2) cm(-3) represent an increase of 115% and 47%, respectively, in comparison to the corresponding values for the prototypical HKUST-1 (tbo-MOF-1), and are 42% and 20% higher than that of tbo-MOF-2. High-pressure methane adsorption isotherms revealed a high total gravimetric and volumetric CH4 uptakes, reaching 372 cm(3) (STP) g(-1) and 221 cm(3) (STP) cm(-3), respectively, at 85 bar and 298 K. The corresponding working capacities between 5 and 80 bar were found to be 294 cm(3) (STP) g(-1) and 175 cm(3) (STP) cm(-3) and are placed among the best performing MOFs for CH4 storage particularly at relatively low temperature. To gain insight on the mechanism accounting for the resultant enhanced CH4 storage capacity, molecular simulation study was performed and revealed the presence of very strong CH4 adsorption sites near the organic linker with similar adsorption energetics as the open metal sites. The present findings support the potential of tbo-MOFs based on the supermolecular building layer (SBL) approach as an ideal platform to further enhance the CH4 storage capacity via expansion and functionalization of the quadrangular pillars.

DFT study of hydrogen storage by spillover on graphite with oxygen surface groups.

DFT modeling was used to understand the role of epoxide (C-O-C) and hydroxyl (C-OH) functional groups on the spillover mechanism for hydrogen storage on graphite oxide and oxygen-modified carbons. A primary spillover model was used, consisting of a Pt(4) cluster, a graphite substrate model, and O and OH functional groups adsorbed on graphite. The spillover mechanism was found to proceed via the migration of dissociated hydrogen atoms from the Pt cluster to epoxide groups adjacent to the cluster (to form OH), followed by H migration by hopping on the adsorbed O atoms. The low energy barriers required for the relevant elementary steps indicate that the spillover process is facile when the carbon substrate is decorated with oxygen functionalities, leading to enhanced hydrogen uptake and faster charge/discharge kinetics. However, a reaction path was also identified, in which surface OH groups can react to form water, which can have adverse consequences for hydrogen storage on oxygenated carbons via spillover.

Hydrogen interaction with carbon nanotubes: a review of ab initio studies

In this paper we review the existing theoretical literature on hydrogen storage in single-walled carbon nanotubes. The importance of theoretical simulations for understanding the adsorption procedure and for improving the storage capacity of these nano-materials is underlined. We report two different categories of theoretical approach used for this purpose, i.e. classical modelling and ab initio calculations. For both, advantages and disadvantage are listed. For the ab initio simulations in particular, we present an analytical overview that gives insight into the storage procedures in different cases.