The dispersion of transition and alkaline-earth metals on defective graphenes is studied using first-principles calculations. The effect of vacancy defects on binding properties of metal atoms to the graphene and with hydrogen molecules is particularly investigated. It is shown that vacancy defects enhance efficiently the metal binding energy and thus its dispersion, particularly for alkaline-earth metals. Mg on vacancy defects shows a substantial increase in its binding energy and hydrogen uptake capacity. Among metals considered, Ca-vacancy complexes are found to exhibit the most favorable hydrogen adsorption characteristics in terms of the binding energy and the capacity.

The hydrogen adsorption on alkaline-earth metal dispersed in doped graphenes was studied through ab initio calculations. Substitutional doping in graphenes is explored to control the ionic state of the metal atoms that plays a crucial role for dispersion and hydrogen adsorption. It was found that the adsorption behavior, particularly in Ca-dispersed graphene complexes, exhibits a crossover between the multipole Coulomb and Kubastype ͑or orbital͒ interactions as the ionic state of Ca and the number of adsorbed hydrogen molecules change. The level exchange in s and d orbitals of Ca is responsible for the crossover. This finding enables the optimization of hydrogen adsorption and metal dispersion in graphitic materials, which is useful for developing solid hydrogen storage and efficient catalysts. DOI: 10.1103/PhysRevB.79.155437 PACS number͑s͒: 68.43.Bc, 68.55.Ln, 73.20.Hb The interaction of hydrogen molecules with metal atoms is a key physicochemical process involved in many energy related technology such as fuel cell catalysts and hydrogen storage. For example, developing storage systems of hydrogen in nondissociative forms has been a great challenge in hydrogen-based fuel cell research.1-4 Transition metal ͑TM͒ complexes have been studied extensively in this respect as they were shown to be very promising in terms of hydrogen binding strength and storage capacity at ambient conditions. 5-10 Quantum mechanical simulations showed that the hydrogen binding energy is about 0.3-0.7 eV depending on TM and the storage capacity can reach as high as 6 wt %.5-10 The Kubas-type ͑or orbital͒ interaction between H 2 and TM is known to be responsible for such a large number of H 2 adsorbed on TM with significant binding energies.11 While many theoretical studies proved the strong potential of TM dispersion, experimental evidence has not been positive. The metal clustering was pointed out as a probable source for degradation of TM complexes.12,13 Adsorption or catalytic functionality of metal atoms with hydrogen so far has been directly associated with metal d orbitals. Nontransition metals such as Li, Na, Mg, Al, and Ca usually form hydrides, where H 2 dissociate to form chemical bonding with metal atoms.1,2 Lochan and Head-Gordon 14 reported interesting theoretical results, which show that ionic alkalimetal atoms such as Li + , Na + , Mg 2+ , and Al 3+ can hold up to six hydrogen molecules. This indicates that, once alkalimetal or alkaline-earth metal ͑AEM͒ exists as bare ion, it can be a binding center for multiple H 2 without dissociation. As the binding energy of metal atoms to substrate depends on their ionic state, the metal dispersion can also be affected by their charge state. While many studies have dealt with metaldispersed medium for hydrogen storage, 5-10 the detailed mechanism and its practical implication are not comprehensively addressed yet. Here we study the dispersion of non-d metal atoms, such as Be, Mg, Al, and Ca, in doped graphenes and the hydrogen adsorption on these metals with the use of ab initio...

We investigate the mechanism of dihydrogen adsorption onto Ca cation centers, which has been the significant focus of recent research for hydrogen storage. We particularly concentrate on reliability of commonly used density-functional theories, in comparison with correlated wave function theories. It is shown that, irrespective of the chosen exchange-correlation potentials, density-functional theories result in unphysical binding of H 2 molecules onto Ca 1þ system. This suggests that several previous publications could contain a serious overestimation of storage capacity at least in part of their results. DOI: 10.1103/PhysRevLett.103.216102 PACS numbers: 68.43.Bc, 84.60.Ve One of the greatest challenges of scientific communities worldwide is a pollution-free and renewable energy source. Hydrogen storage with high enough volumetric and gravimetric density is particularly important as an energy carrier for a mobile system [1,2]. It has been discussed that, in order for the storage and discharge to be cycled near room temperature, hydrogen adsorbents need to have a binding affinity with the hydrogen molecule of a few tens of kJ/mol [3,4]. Through a very particular chemistry between the open d shell of metal atoms and H 2 molecular orbitals, an optimal strength of hydrogen adsorption can be achieved [5,6]. In order to realize such a chemistry in the form of a practical hydrogen storage system, numerous previous theory articles investigated nanostructures with dispersed transition metal (TM) atoms [3,[7][8][9]. However, experimental trials to synthesize an open-TM-based hydrogen storage system have been unsuccessful for various reasons. The most prominent barrier is the strong tendency of aggregation of TM atoms which renders the suggested models of dispersed TMs rather hyphothetical [10]. As an alternative, numerous research groups are now focused toward the alkaline-earth metals (AEMs) which have less tendency of aggregation and are believed to have similar binding affinity with dihydrogen adsorbates as TMs [11][12][13][14][15]. In particular, the systems of dispersed Ca atoms have been suggested as possessing the most salient properties.In the series of computational searches for hydrogen storage materials, the density-functional theory (DFT) has been used most widely, mainly because of its practicality [16]. However, common implementations of DFT involve approximations in the exchange-correlation potential, and thus the accuracy of DFTs could not be perfectly trusted. In this regard, it is very pertinent to investigate the capability of common forms of DFT for hydrogen adsorption onto AEMs. We show that DFTs deviate significantly from the correlated wave function theories in the description of the dihydrogen adsorption onto the Ca 1þ system. We discuss that the valence configuration of the Ca cation can be sharply switched between 4s and 3d upon hydrogen adsorption [17]. This sort of interaction is not widely noticed, and the reliability of DFTs for that is intriguing in the context of hydrogen storage as wel...

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We investigated noncovalent molecular adsorptions onto carbon nanotubes (CNTs), using density-functional theory methods including recently developed hybrid functionals. Planar aromatic molecules have greater binding strength than fully saturated nonaromatic molecules, and adsorption geometries are consistent with the features of intermolecular π−π stacking. We show that modifications with alcohol or a thiol terminal group increase the adsorption strength consistently throughout the considered molecules. This result may be used in experiments involving noncovalent sidewall functionalizations of CNTs.

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