The interaction of H2 with multidecker C6−nBnH6Sc (n = 0–6) complexes

https://doi.org/10.1016/j.commatsci.2015.11.005Get rights and content

Highlights

  • The structures and hydrogen storage properties of C6−nBnH6Sc (n = 0–6) units were theoretically investigated.

  • The double hybrid method of MPW2PLYPD was found more suitable for Kubas interaction.

  • Introduction of the B atom enhances the interaction between Sc atom and the C6−nBnH6.

  • C5B6H6Sc, C4B2H6Sc, C2B4H6Sc, and B6H6Sc can effectively adsorb four, four, four and three H2 molecules, respectively.

Abstract

The structures and hydrogen adsorption properties of C6−nBnH6Sc (n = 0–6) were investigated with density functional methods and MP2 method. The double hybrid method of MPW2PLYPD was found more suitable for this system. Our calculations showed that the local geometries of C6−nBn change from planar ring to three-dimensional structure with the increasing n. Introduction of the B atoms enhances the interaction between Sc and the C6−nBnH6 substrate. The number of B atoms also affects the H2 adsorption properties of C6−nBnH6Sc. Our calculations show that C5B6H6Sc, C4B2H6Sc, C2B4H6Sc, and B6H6Sc can adsorb four, four, four and three H2 molecules, respectively. The H2 adsorption energies of C3B3H6Sc and CB5H6Sc are remarkably smaller than these of others. The natural bond orbital analysis indicated that there is no positive correlation between the adsorption energy and the charge on Sc atom. Electronic structure analysis was performed to explain the poor H2 adsorption energies of C3B3H6Sc and CB5H6Sc.

Introduction

In recent years, the energy crisis and environmental pollution originated by the use of fossil fuels have spurred an initiative to find alternative fuels. Hydrogen has been recognized as an ideal energy carrier and has the potential to reduce our dependence on fossil fuels [1], [2]. However, one of the bottlenecks of using hydrogen for vehicular application is the lack of proper onboard hydrogen storage materials. Extensive researches for efficient hydrogen storage materials led to the finding of several promising candidates including metal hydrides [3], [4], [5], metal nitrides, carbides [6], [7], clathrate hydrates [8], carbon-based nanostructures [9], [10], [11] and metal organic frameworks (MOF) [12], [13], zeolites [14], etc. Unfortunately, none of these materials achieves the target set by the U.S. Department of Energy (DOE) [15].

In the last decade, metal decorated carbon fullerenes and nanotubes have gained intensive attention for hydrogen storage. The doped metal can enhance the binding between substrate and hydrogen. The metal decorated materials can be classified to two groups based on the different interaction mechanism with hydrogen. The first group of materials are doped with alkali metal or alkaline-earth metal atoms, which adsorb the hydrogen molecules via static electronic interaction. The second group of materials are doped with transition metal (TM) atoms, which adsorb the hydrogen molecules via Kubas interaction [16]. In most of the cases, Alkali or alkaline-earth metal doped material can achieve high hydrogen storage capacity because of its light weight. Fox example, Han and his coworkers [17] have reported that the Li+ doped conjugated microporous polymers can achieve the hydrogen storage capacity of 6.1 wt% at 1 bar and 77 K. The small binding energies between Li+ ion and hydrogen molecules make them not suitable for hydrogen storage at ambient condition. Sun and his coworker [18] reported that Li doped fullerene, Li12C60, can store up to 60 hydrogen molecules with a binding energy of 7.2 kJ/mol per H2 (0.075 eV/H2). More recently, Xu et al. [19] reported that Li-decorated graphene can reach the hydrogen storage capacity of 13 wt% with the adsorption energy of 0.19 eV/H2 based on the first-principle plane wave calculations. In their calculations, van der Waals interaction was considered by DFT-D2 method of Grimme [20], while the zero-point energy (ZPE) was not included. It was obvious that Xu’s result is essentially different from that of Sun’s.

Replacing the alkali metal atom with TM atom can enhance the interaction with hydrogen molecules [21], [22]. TM atom adsorbs hydrogen molecules by Kubas interaction. For example, based on theoretical calculations, Zhao and his coworkers found that Sc decorated C60 and C48B12 are capable of storing hydrogen with hydrogen gravimetric densities of about 7 wt% and 8.77 wt%, respectively, and the adsorption energies can achieve about 0.3 eV/H2 [23]. Yildirim found that Ti decorated carbon nanotubes can achieve the hydrogen storage capacity of about 8 wt% with the adsorption energies beyond 0.3 eV per H2 [24]. Unfortunately, Sun and his coworkers [25] found that TM atoms on C60 will be clustered rather than scattered, which will significantly reduce the capacity of hydrogen storage. To overcome the clustering of the TM atoms, the first step is to enhance the interaction between the TM atom and the supporting material [26]. And the second is to construct appropriate framework to suppress the clustering of the hydrogen storage unit [27], [28].

Park and his coworkers [29] found that Ti atoms are well dispersed on B-substituted graphene due to the strong interaction between Ti atom and B-substituted graphene. Dixit and his coworkers [28] suggested that in scandium-decorated MOF-5, the substitution of boron for the carbon atom in an aromatic carbon six-membered ring can enhance the interaction between the ring and scandium atom. In fact, in the MOF and COF materials, some TM atoms may be embedded, and then hydrogen molecules can be added to these TM atoms. Zou et al. [27] have developed a two-step doping strategy to modify the COF as hydrogen storage material. The organic building units in the COF were doped with boron atoms first, and then metal atoms were introduced in the B-doped COF to trap the H2 molecules. The introduction of B atom can suppress the clustering of Sc, Ti atoms. To design the TM decorated COF or MOF hydrogen storage materials, it is very important to tune the binding energy between the TM atom and the organic substrate and the adsorption energy to hydrogen molecule.

In this work, the interactions between Sc atom and C6−nBnH6 units were explored systemically to show the effect of B-substitution on the binding energies of Sc to the C6−nBnH6 units. We also want to know how B-substitution changes the adsorption energies of hydrogen molecules.

Section snippets

Computational method

Although the density functional theory (DFT) method of both local density approximation (LDA) and general gradient approximation (GGA) were used in many theoretical works to explore the Kubas interaction in the hydrogen storage materials, it was believed that pure LDA and GGA methods are not suitable for the calculation of weak interaction. Many investigations demonstrated that MP2 method should be a more reliable choice for calculating the weak interaction than DFT method [30], [31]. In order

The Geometries of C6−nBnH6Sc (n = 0–6)

In this section we discuss the geometries of C6−nBnH6Sc (n = 0–6). The optimized structures of C6−nBnH6Sc were depicted in Fig. 1. It is very obvious that the local structures of the C6−nBnH6 substrates evolve from planar six-membered ring to three dimension structure with the increasing n.

The most stable C6H6Sc is a doublet C2v isomer, with the Sc atom sitting above the center of the deformed benzene ring. Pandey and his coworkers had reported a quartet C6v isomer as the ground state of C6H6Sc

Conclusions

In summary, the structures and the hydrogen storage properties of C6−nBnH6Sc were explored with the double hybrid method of MPW2PLYPD. A series of benchmark calculations were performed on the ScH2+(H2)m (m = 1–3) systems first. The MPW2PLYPD method was found more suitable to calculate the interaction energies between Sc and H2 molecules. When the C atoms in C6H6 were substituted by B atoms gradually, the local geometries of the C6−nBnH6 change from the planar ring to three-dimensional structure.

Acknowledgment

This work is financially supported by the National Natural Science Foundation of China (21373131), the Program for New Century Excellent Talents in University (NCET-12-1035) and Shanxi Scholarship Council of China.

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