A first-principles study of lithium-decorated hybrid boron nitride and graphene domains for hydrogen storage Zi-Yu Hu, Xiaohong Shao, Da Wang, Li-Min Liu, and J. Karl Johnson Citation: The Journal of Chemical Physics 141, 084711 (2014); doi: 10.1063/1.4893177 View online: http://dx.doi.org/10.1063/1.4893177 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Lithium-decorated oxidized graphyne for hydrogen storage by first principles study J. Appl. Phys. 116, 174304 (2014); 10.1063/1.4900435 Adsorption of hydrogen atoms on graphene with TiO2 decoration J. Appl. Phys. 113, 153708 (2013); 10.1063/1.4802445 Theoretical prediction of hydrogen storage on Li-decorated boron nitride atomic chains J. Appl. Phys. 113, 064309 (2013); 10.1063/1.4790868 Lithium-decorated oxidized porous graphene for hydrogen storage by first principles study J. Appl. Phys. 112, 124312 (2012); 10.1063/1.4770482 First-principles study on the enhancement of lithium storage capacity in boron doped graphene Appl. Phys. Lett. 95, 183103 (2009); 10.1063/1.3259650

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THE JOURNAL OF CHEMICAL PHYSICS 141, 084711 (2014)

A first-principles study of lithium-decorated hybrid boron nitride and graphene domains for hydrogen storage Zi-Yu Hu,1,2 Xiaohong Shao,1,a) Da Wang,2 Li-Min Liu,2,a) and J. Karl Johnson3 1

College of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China Beijing Computational Science Research Center, Beijing 100084, People’s Republic of China 3 Departments of Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA 2

(Received 9 May 2014; accepted 29 July 2014; published online 28 August 2014) First-principles calculations are performed to investigate the adsorption of hydrogen onto Lidecorated hybrid boron nitride and graphene domains of (BN)x C1−x complexes with x = 1, 0.25, 0.5, 0.75, 0, and B0.125 C0.875 . The most stable adsorption sites for the nth hydrogen molecule in the lithium-decorated (BN)x C1−x complexes are systematically discussed. The most stable adsorption sites were affected by the charge localization, and the hydrogen molecules were favorably located above the C-C bonds beside the Li atom. The results show that the nitrogen atoms in the substrate planes could increase the hybridization between the 2p orbitals of Li and the orbitals of H2 . The results revealed that the (BN)x C1−x complexes not only have good thermal stability but they also exhibit a high hydrogen storage of 8.7% because of their dehydrogenation ability. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4893177] I. INTRODUCTION

Hydrogen storage is an important issue for hydrogen applications.1, 2 For commercial applications, it is necessary to develop materials that can reversibly store molecular hydrogen under ambient conditions with a gravimetric efficiency of 6 wt.%, that also possess acceptable desorption kinetics. To achieve this challenging amount of storage, many solid adsorbents have been developed, such as pure and metal-doped carbonaceous materials,3, 4 metal alloys and metal oxides,5, 6 metal and complex hydrides,7, 8 metalorganic frameworks,9 covalent organic frameworks,10 zeolitic imidazolate frameworks,11 and boron based materials.12 Among the adsorbents studied, BN-based materials have been considered to be promising for hydrogen storage owing to the heteropolar binding nature of the B and N atoms.13 BNnanotubes have good thermal and chemical stability and can store hydrogen up to 2.6 wt.%.14 BN-cages can adsorb hydrogen up to 4 wt.%, but the structure can break, even at room temperature.15 In addition, much attention has been paid to a plane structural carbon sheet (graphene), owing to its spectacular electronic properties.16 However, pristine graphene is unsuitable for hydrogen storage.17 By considering two disparate materials with similar lattice parameters and crystal structures, Ci et al.18 synthesized new materials made of hybrid boron nitride and graphene domains complementary to those of hexagonal BN (h-BN) and graphene. Hexagonal hybrid boron nitride and graphene (h-BNC) is a promising system for fundamental physical investigations, such as charge localization and possible metal-insulator transitions. Recently, Wu et al.19 studied carbon doped boron nitride cages, which are favorable for hydrogenation/dehydrogeneration. h-BNC can have different properties, including different electronic struca) Electronic addresses: [email protected] and [email protected]

0021-9606/2014/141(8)/084711/10/$30.00

tures and hydrogen storage ability for different BN and C concentrations. Designing and synthesizing new materials is a promising solution for enhancing the hydrogen storage capacity. Another potential solution is to decorate existing promising materials. Li is considered as a suitable decorating candidate because Li atoms are lightweight and can polarize the hydrogen molecules.20 In Li-decorated systems, the hydrogen storage capacity is influenced by the orbital hybridization intensity of both the H-s and Li-p (or/and s) orbitals and the polarization mechanism.21 The hydrogen molecules interact with the Li decorated substrates through Dewar22 and Kubas23 interactions. With a combination of these interactions, the hybridization of the p orbitals of the substrates with the partial occupancy of the Li-p (or/and s) orbitals enhances the binding energies of the hydrogen molecules. When the Li atoms react with the substrates, the adsorbed Li atoms donate s electrons in the substrates. This leads to partially filled p orbitals in the substrates, which can re-donate electrons to the low-lying Lip orbitals, resulting in strong s-p or p-p hybridizations between the Li atoms and the substrate. The introduced hydrogen molecules and the polarization effect cause the charges on both sides of the hydrogen molecules deplete and accumulate, because of the hybridization of the p orbitals in the substrate with the σ orbitals in the hydrogen molecules. A previous study24 showed that Li-decorated B-doped graphene sheets had strong p-p hybridization, which resulted in a high hydrogen storage capacity. h-BNC has more types of orbitals (such as, B-p, N-p, and C-p), than pure BN and pure graphene. This makes it easier to form s-p or p-p hybridizations between the Li-p (or/and s) orbitals and the substrates. The hybrid boron nitride and graphene domains have different electronic structures, corresponding to different hybridizations and different abilities to host hydrogen molecules. Therefore, for hydrogen storage it is important to discuss the Li-decorated

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hybridizations of h-BNC complexes. During this procedure, the adsorption sites can greatly affect the adsorption, including the binding and adsorption energies, and can even affect the capacity of hydrogen molecules that it can host. For pure h-BN and graphene, the hollow sites are considered as the most stable sites for Li atoms on the substrate.20, 25 Previous studies have shown that the top-Li site is the most stable site for Li-decorated systems with one hydrogen molecule introduced.20 However, Liu et al.26 discussed the hydrogen storage of Li-doped graphene and the results showed that the bridge vertical site is the most stable adsorption site for hydrogen molecules. The most stable adsorption sites are mainly affected by the charge localization. The charge localizations are different for h-BNC with different concentrations of BN and C, therefore, the adsorption sites may be different. Consequently, it is still a challenge to investigate the adsorption sites, which are closely related to the binding and adsorption energies. Motivated by these questions, a systematic study of hydrogen adsorption into the Li-decorated complexes, (BN)x C1−x where x = 1, 0.25, 0.5, 0.75, 0, and B0.125 C0.875 are investigated in this work. The properties of the electronic structure of the pure substrate planes and the Li-decorated complexes are discussed first. The results showed that the boron and nitrogen atoms in the substrate planes resulted in strong s-p and p-p hybridizations between the Li atoms and the (BN)x C1−x complexes. Second, the initial adsorption sites for H2 on the Li-decorated complexes are discussed, showing that it is favorable for the first introduced hydrogen molecule to be located above the C-C bonds, beside a Li atom and parallel to a Li atom. Then, the bonding characteristics and energetics for the absorption of hydrogen onto the Li-decorated complexes are compared and analyzed. The results show that Li-decorated (BN)x C1−x achieved a high hydrogen storage of 8.7%, indicating that h-BNC is a promising adsorbent for hydrogen storage.

II. COMPUTATIONAL METHOD

Density functional theory (DFT) and moleculardynamics (MD) calculations were performed using the Vienna ab initio simulation package.27 Previous studies have shown that the local density approximation (LDA) is a better choice for alkali doped carbon and boron nitride materials.28, 29 Therefore, LDA30 was used to calculate the exchange correlation potential, along with the projector augmented wave method.31 The plane-wave energy cutoff was set to 400 eV. The adsorption of Li atoms and hydrogen molecules was calculated in a super cell with the lattice parameters, a = b = 4.94 Å with a 2 × 2 cell for the (BN)x C1−x plane, where x = 1, 0.25, 0.5, 0.75, 0, and B0.125 C0.875 . According to the Monkhorst-Pack scheme,32 the Brillouin zone was sampled by 19 × 19 × 1 special mesh points in K-space for a 2 × 2 cell of (BN)x C1−x . A 20 Å vacuum region was employed for all of the systems to avoid interactions between the periodic images. The force convergence criterion was set to 0.01 eV/Å for optimization.

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III. RESULTS AND DISCUSSION A. Electronic structure of the pure h-BNC complexes

The (BN)x C1−x structures are based on the BN monolayer, and the (BN)x C1−x plane (x = 0.25, 0.5, 0.75) are constructed through replacing the original B-N of the BN monolayer with C-C gradually. After the construction, the lattice constant the atomic structure were fully optimized. Here x means the ration between the B-N and C-C. The electronic structures of the (BN)x C1−x plane (x = 1, 0.25, 0.5, 0.75, 0) were calculated first. Fig. 1 shows the molecular orbitals and band structures for the five BN complexes. Figs. 1(a)–1(e) and 1(a )–1(e ) show the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs), respectively. Figs. 1(a) and 1(a ) show the HOMO and LUMO for the pure BN complex. The HOMO was mainly localized on the B sites, while the LUMO was localized on the N sites, as a result from the electro-negativity of the nitrogen atoms and the electro-positivity of the boron atoms. This demonstrates that the hybridization is sp2 .33 When the graphene domains increased, the wavefunctions showing the localization of the HOMOs and LUMOs changed, corresponding to the change in the hybridization of the different complexes. For (BN)x C1−x , the hybridization changed to sp3 , which is different to the other complexes. When the complex was turned into pure graphene, the HOMO was parallel and the LUMO was vertical, which is a typical graphitic sp2 characteristic.34 The conclusion drawn above could also be investigated from the band structures. In the pure BN sheet, the wavefunctions of the π and π * bands were predominantly composed of atomic pz orbitals (orientated perpendicular to the plane). Because of the high electro-negativity of nitrogen, the π band wavefunction was located on the nitrogen atoms, while the π * band was located on the boron atoms.35 The strong difference in the electro-negativity between B and N led to a large band gap of 5.01 eV at the K-point.18 As the graphene domains increased, the band gap gradually became smaller. In addition, the anti-bonding π * bands quickly moved with as the graphene domains increased, while the π bands changed slightly. This corresponded to larger wavefunction changes in the localization of the LUMOs than that of the HOMOs. When the complex was pure graphene, the π and π * bands degenerated at the K-point, leading to linear crossing of the two bands. We plot all the HOMOs and LUMOs for the different compositions of (BN)x C1−x . When the x is zero, the HOMOs and LUMOs represent the pure BN; When x becomes 1, the HOMOs and LUMOs represent the pure graphene. The HOMOs of the graphene became two ribbons parallel localized on the C-C bonds, while the LUMOs separated vertical localized on the C-C bonds.

B. Thermal stability

Previous studies have shown that BNC materials have good thermal stability. It is of interest to know whether the stability is maintained when the alkali metal atoms Li are introduced into the system and whether they are absorbed by

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FIG. 1. The HOMOs (a–e) and the LUMOs (a –e ) at an isovalue of 0.05 e/Å3 in 2 × 2 cells of bare (BN)x C1−x (x = 1, 0.25, 0.5, 0.75, 0) sheets and the energy band structures (lower panels) for the five different sheets. The two colors, red and yellow, denote the positive and negative signs of the wavefunction, respectively. The zero band energy was set to the Fermi energy, EF .

the hydrogen molecules. To answer these questions, finitetemperature ab initio MD simulations were carried out with Li atoms and hydrogen molecules in a 2 × 2 (BN)x C1−x (where x = 1, 0.25, 0.5, 0.75 0) and B0.125 C0.875 cells. The time step was 1 fs and the simulation had 3000 time steps. The atomic temperature was controlled by a Nosé-Hoover thermostat with converging temperatures of T = 300, 400, 500, and 600 K. A minimum vacuum distance of 20 Å in the z-direction was employed in the “super box cell” to minimize spurious interactions between the periodic images. For a single Li atom on the BNC complexes (except for the pure BN complex), the bonds between the adsorbed Li atoms and the complexes were sustained. Even at temperatures up to 600 K within 3000 time steps, no structural deformation was observed. This meant that the structures were stable when the alkali metal atoms Li were introduced into the complexes.

C. Interactions between the Li atoms and h-BNC complexes

Li atoms are considered as a good candidate for hydrogen storage. To enhance the hydrogen uptake, Li atoms were adsorbed into the complexes. The binding energies of the five complexes were obtained using the equation: Eb = −(EBNC + Emetal − EBNC+metal ), where Eb is the binding energy of a Li atom, EBMC+metal , EBNC , and Emetal are the total energies of the adsorbed BNC, pure BNC and metallic atoms in the same cell, respectively. Table I shows the calculated binding energies between the Li atoms and either pure BN or graphene, which are 0.13 and 0.86 eV, respectively, which are in agreement with previous results.20, 36, 37 For the two phases, the binding energies between the Li atoms and the complexes increased with an increasing concentration of carbon atoms in the h-BNC sheets.

TABLE I. Binding energies between the Li atoms and the (BN)x C1-x complexes. Complex −Eb (eV)

This work Refs.

(BN)1 C0

(BN)0.75 C0.25

(BN)0.5 C0.5

(BN)0.25 C0.75

(BN)0 C1

0.13 0.131

0.24 ...

0.62 ...

0.81 ...

0.86 0.861, 2

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FIG. 2. The most stable configurations in the basal plane and the cross section of Li in different complexes. The atoms are colored as follows: C is white, Li is green, B is light red, and N is navy blue.

The configurations of the five Li-decorated BN, (BN)x C1−x , (BN)0.5 C0.5 , (BN)0.25 C0.75 and graphene complexes are presented in Fig. 2. For the pure BN sheet and graphene, the most stable adsorption sites were the hollow sites because of their high symmetry. For (BN)0.75 C0.25 , the nitrogen atom (on the left) was surrounded by three boron atoms, forming a local electron-deficient system. Therefore, it was favorable for the Li atom to reside on the N-top site. For the same reason, the adsorbed Li atom slightly deviated above the (BN)0.5 C0.5 and (BN)0.25 C0.75 sheets, compared with the BN and graphene sheets. The right panel in Fig. 2 shows that the adsorption height decreased as the concentration of the carbon atoms increased. This phenomenon is consistent with the binding energies in Table I. The large binding energy meant that there were strong adsorption interactions between the Li atoms and the substrate, which corresponded to the low adsorption height. The interactions between the Li atoms and the h-BNC complexes, as well as the electronic structures of the Lidecorated h-BNC complexes are shown in Fig. 3. The partial density of states (PDOS) for each element in the

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five different h-BNC complexes are plotted on the left in Figs. 3(a)–3(e), while the corresponding HOMOs and LUMOs are shown on the right. For the elements B, N, and C, the hybridizations near the Fermi level were mainly dominated by p-orbitals. Only the p-orbitals of B, N, and C are shown. Fig. 3(a) shows the PDOS of the Li-decorated pure BN complex. The s-orbitals of the adsorbed Li atoms mainly stepped into the Fermi level, indicating that the Li-decorated BN sheet remained metallic, the interactions between the Li atoms and the substrate were weak and the charge transfer was small. Compared with the electronic localization of the HOMO, it is apparent that the hybridization changed from sp2 to sp3 after Li atoms were absorb into the substrate. The electronic localization of the LUMO was around the Li atom, which is consistent with the results where the charge transfer was not strong between the Li atoms and the substrate complex. With an increase in the graphene domains, the PDOS of each element (Fig. 3(b)) crossed the Fermi-level, but for the pure BN complex does not show such feature. The decrease in the peak for Li-s near the Fermi-level indicated that the metallic state slightly decreased. The p-orbitals for each element crossed into the Fermi-level, causing a strong hybridization between the Li-s and Li-p orbitals and the substrates. Meanwhile, from the LUMO (on the right), it is suggested that the charge of the Li-s orbitals can transfer into the substrate, leading to the wavefunctions becoming localized above the atoms in the substrate. In contrast, the back-donated electrons in the Li-p orbitals caused the spectra for the p-orbitals in the substrate to shift to the Fermi-level. The hybridized peaks at the Fermi-level are not obvious, indicating that the charge transfer between them was not strong. The same phenomenon was observed in the Li-decorated (BN)0.5 C0.5 complex. The hybridization between the p-orbitals in the substrate and the Li-s, and p-orbitals increased, owing to an increase in the charge transfer rate between the adsorbed Li atoms and the (BN)0.5 C0.5 complex. The PDOS of the Li-decorated (BN)0.25 C0.75 complex is shown in Fig. 3(d), where the adsorbed Li atom donated s electrons to the substrate, leading to partially filled B-p, Np, and C-p orbitals. Meanwhile, the empty Li-p orbitals split under the strong ligand field generated by the (BN)0.25 C0.75 complex. The substrate back-donated electrons to the lowlying Li-p orbitals, resulting in strong p-p and s-p hybridizations between the Li atoms and (BN)0.25 C0.75 complex, which became equal. The PDOS (shown in Fig. 3(d)) shows an increase in the intensity of the N-p orbitals. Only the N-p orbitals gradually increased with a decrease in the BN concentration, which is different to the cases in Figs. 3(a)–3(c). It can be concluded that N plays an important role as the states of the Li-p orbitals decrease. To further verify this, the PDOS of the Li-decorated N-doped graphene sheet were analyzed and the same result was obtained. A previous study24 showed that the interactions between the Li atoms and low concentration B-doped graphene are responsible for the p-p hybridization. The number of H2 molecules adsorbed was influenced by the hybridization intensity of the H-s and Li-p (s) orbitals. Therefore, it is possible that smaller BN domains led to an increase in the hybridization of the Li-p (s) orbitals, making the Li-decorated (BN)0.25 C0.75 complex feasible for

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FIG. 3. The left panels show the corresponding PDOS for a Li atom bonded to (BN)x C1−x for (a) x = 1, (b) x = 0.25, (c) x = 0.5, (d) x = 0.75, and (e) x = 0, with units of states/eV. The dashed line indicates the position of the Fermi-level. The corresponding HOMOs and LUMOs for the (BN)x C1−x complexes are shown in the middle and right panels, respectively. The isovalue of 0.05 e/Å3 was used. The red and yellow colors represent the positive and negative signs of the wavefunction, respectively.

hydrogen storage. The hybridization remained as sp2 , shown by the HOMO of the (BN)0.25 C0.75 complex. Contamination of the LUMO caused the p-orbitals of each element to shift across the Fermi-level. For pure graphene, the C-p hybridization was smaller than that for the h-BNC complex. Interestingly, unlike (BN)0.5 C0.5 and (BN)0.25 C0.75 complex, the hybridization changed from sp2 to sp3 (see the HOMOs).38 Similar to the Li-decorated pure BN sheet and (BN)0.75 C0.25 , there has a large positive charge area above the Li atom in the LUMOs shown in Fig. 3(e). But the hybridized peaks at the Fermi-level are obvious, especially, the strong hybridization between C-p orbitals and Li-p orbitals makes the graphene having larger binding energy with the Li atom. In addition, the possibility of Li clustering is also considered in our work. As shown in the supplementary material,39 three typical complexes of (BN)0.25 C0.75 , B0.125 C0.875 along with pure graphene are employed. The results show that the isolated configuration is more stable by 0.34 eV and 0.23 eV. It can be easily concluded that the Li atoms prefer to isolate

adsorb on to the (BN)0.25 C0.75 not cluster into the dimerized configuration. D. The most stable adsorption sites for hydrogen in the Li-decorated h-BNC complexes

To further understand the adsorption behavior of the hydrogen molecules in these complexes, the adsorption behavior of each H2 molecule is discussed and compared. The different high-symmetry (HS) adsorption sites could affect both the stability and the adsorption energy of the hydrogen molecules. Most studies have suggested that the top-site is the most stable site for the introduced hydrogen molecules in the Li-decorated systems. This work focusses on the HS adsorption sites for the hydrogen molecules in the Li-decorated h-BNC complexes. To find the most stable adsorption configuration, all of the HS adsorption sites on the pure sheets (BN and C) and the doping sheets ((BN)0.75 C0.25 , (BN)0.5 C0.5 , (BN)0.25 C0.75 , and B0.125 C0.875 ) were calculated. Several

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TABLE II. The initial configurations for 1 to 3 hydrogen molecules at different sites on a graphene sheet. The atoms are colored as follows: C is white, Li is purple, and H is green.

studies showed that boron can enhance the hydrogen storage ability.24, 37 The B0.125 C0.875 plane complex structures is based on the graphene monolayer, and the B0.125 C0.875 structure is constructed through replacing the original C atom of the graphene monolayer with B gradually. After the construction, the lattice constant of the atomic structure were fully optimized. B0.125 C0.875 is considered in the following study. First, the adsorption of H2 onto graphene is discussed. Five initial hydrogen adsorption sites around the Li atom are considered: (1) the top-Li sites (T-Li: hydrogen is above a Li atom); (2) the top carbon sites (TC1: hydrogen is located above the carbon atom and the molecular direction is vertical to the Li atom; TC2: hydrogen is located above the carbon atom and the molecular direction is parallel to the Li atom); and (3) the bridge carbon sites (BC1: hydrogen is located above the C-C bonds, beside the Li atom and the molecular orientation is vertical to a plane crossing the Li atom and parallels to the substrate; BC2: hydrogen is located above the C-C bonds, beside the Li atom and the molecular orientation is parallel to the Li atom). The adsorption energies of the T-Li, TC1, TC2, BC1, and BC2 were 0.09, 0.02, 0.17, 0.12, and 0.18 eV, respectively. Therefore, the BC2 site was the most stable site, which is consistent with the literature. These results show that the adsorption sites greatly affect the adsorption energy and behavior. Similar configurations could be achieved by introducing two or three hydrogen molecules. Four possible initial sites for 2H2 are listed in Table II. The two hydrogen molecules located at the BC1 sites were the most stable sites. Compared with 1H2 adsorption sites, there is no Top-site for 2H2 adsorbed configuration, and the hexatomic adsorbed 2H2 configurations at the substrate have the symmetrical, so some of the others of H2 configurations are equal to the cases of possible configurations listed in Table II. The same case happened in the 3H2 adsorption. For 3H2 , the three hydrogen molecules were all placed at the BC2 sites, which corresponded to the most stable sites. The simulation results indicated that the effect of the different adsorption sites

became weak when the number of adsorbed H2 molecules was increased to four or five. The most stable adsorption sites for the other Lidecorated h-BNC complexes ((BN)0.75 C0.25 , (BN)0.5 C0.5 , (BN)0.25 C0.75 , and B0.125 C0.875 ) are shown in Table III. There are many possible sites where the hydrogen molecules can adsorb onto for these complexes, such as: the T-Li, TC1, TC2, BC1, BC2 or the TB1, TB2, BB1, BB2 or the TN1, TN2, BN1, BN2 sites (the definition for the TB1, TB2, BB1, BB2 and TN1, TN2, BN1, BN2 sites are similar to that for TC1, TC2, BC1, BC2). After a large number of calculations on all of the possible initial adsorbed sites mentioned above, the most stable adsorption configurations for different numbers of adsorbed hydrogen molecules were obtained (listed in Table III). The initial adsorbed structures (the geometric configuration before geometric relaxations) for the most stable configurations presented certain regular rules that the initial adsorbed structures of the stable configurations in each row are the same. For 1H2 , the initial adsorption sites were all located at the BC2 sites for all of the complexes, except for the (BN)0.75 C0.25 sheet. For 2H2 and 3H2 , similar configurations could be achieved, which is different to (BN)0 C1 and pure graphene. For the (BN)0.75 C0.25 sheet, the most stable site for the Li atoms was located above the nitrogen atoms. Therefore, the initial configuration was different from other sheets, corresponding to Fig. 2. The sites for 4H2 and 5H2 were also considered in this work. The adsorption energies for the different sites nearly had the same value for all five different sheets. E. Hydrogen storage

The most stable final adsorption configuration for different numbers of H2 molecules onto the Li-decorated complexes, and the energies and geometries that are related to the adsorption of molecular H2 are summarized in Fig. 4. The binding energies for these fragments

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TABLE III. All possible configurations for the hydrogen molecules at different sites on the (BN)1 C0 , (BN)0.75 C0.25 , (BN)0.5 C0.5 , (BN)0.25 C0.75 , (BN)0 C1 , and B0.125 C0.875 complexes. The colors of the atoms are: C is white, Li is purple, H is green, B is light red, and N is navy blue.

were obtained using: En = E(n−1)H +adsorbed _Li+BNC + E1H 2 2 −EnH +adsorbed _Li+BNC , where n is the number of hy2 drogen molecules, En is the binding energy of the nth hydrogen molecule, E1H is the energy for one isolated 2 hydrogen molecule, and EnH +adsorbed _Li+BNC is the en2 ergy for the optimized structures that include n hydrogen molecules adsorbed onto the Li-decorated (BN)x C1−x planes. The average binding energies for these fragments were calculated using: Enav = (Eadsorbed _Li+BNC + EnH 2 −EnH +adsorbed _Li+BNC )/n, where dL is the distance between 2 a Li atom and the surface and dnav is the average distance between a hydrogen molecule and a Li atom. The H2 bond lengths in these fragments are listed in the corresponding panels (in red) in Fig. 4. Now, the adsorption of one H2 molecule is discussed. There were differences between the two H atomic charges before and after one H2 molecule was adsorbed onto the Lidecorated (BN)x C1−x (x = 0.25, 0.5, 0.75, 0) and B0.125 C0.875 were obtained: (BN)1 C0 (0.03 e, 0.01 e), (BN)0.75 C0.25 (0.06 e, 0.03 e), (BN)0.5 C0.5 (0.07 e, 0.04 e), (BN)0.25 C0.75 (0.08 e, 0.05 e), (BN)0 C1 (0.08 e, 0.06 e), and B0.125 C0.875 (0.09 e, 0.05 e). This led to the hydrogen molecules titling toward the Li atoms, which is in agreement with previous studies.20, 24, 26, 36, 37 In addition, the H–H bonds extended from 0.75 to ∼0.79 Å because of the polarization interactions between Li+ and H2 . The binding energies for one H2 molecule adsorbed onto the Li-decorated (BN)1 C0 , (BN)0.75 C0.25 , (BN)0.5 C0.5 , (BN)0.25 C0.75 , and (BN)0 C1 substrates were 0.18, 0.22, 0.23, 0.25, and 0.28 eV, respectively, showing that the ionic adsorption mechanism caused the molecules to strongly interact with the Li-decorated (BN)x C1−x complexes. When the second H2 was adsorbed around the Li atom, the binding energy becomes 0.13 eV, which is larger than that of a single hydrogen molecule (0.03 eV). This can be explained by the PDOS in Fig. 5. The upper panels in Fig. 5 show that the molecular level of H2 at ∼−8 eV was broadened and the existence of the H2 –H2 orbital interactions led to an increase in the binding energy. The Li-decorated pure BN system could not host three or more H2 molecules.

In this case, the amount of adsorption that could be obtained was about 3.6 wt.%. While, the third and fourth H2 molecules were adsorbed onto the Li-decorated (BN)x C1−x (x = 0.25, 0.5, 0.75, 0) and B0.125 C0.875 systems, the hydrogen molecules were bonded around the Li atoms at a certain angle. The stronger polarization interactions also made the bond lengths of the H2 molecules extend from 0.75 to 0.80 Å. In addition, there was a sudden drop in the binding energy (on the right side of the adsorbed configuration) for the third and fourth H2 molecules, owing to the strong steric interactions between the adsorbed H2 molecules. When a fifth H2 molecule was added above the Li atom, the binding energies for the (BN)0.75 C0.25 , (BN)0.5 C0.5 and (BN)0.25 C0.75 complexes were 0.05, 0.06, and 0.08 eV, respectively, which achieved a high hydrogen storage amount of 8.7%. The (BN)0 C1 and B0.125 C0.875 systems could not host five H2 molecules. To make sure that the fifth H2 could be adsorbed onto the Li-decorated (BN)0.75 C0.25 , (BN)0.5 C0.5 and (BN)0.25 C0.75 complexes, the PBE functional with the generalized gradient approximation (GGA)40 was used. The calculated binding energies for (BN)0.75 C0.25 , (BN)0.5 C0.5 , and (BN)0.25 C0.75 for the fifth H2 molecule were 0.015, 0.014, and 0.012 eV, respectively, which are smaller than those with the LDA.41 While these two functionals gave the same conclusions, (BN)0.75 C0.25 , (BN)0.5 C0.5 , and (BN)0.25 C0.75 could host five H2 molecules. In general, the calculated results with the GGA were smaller than with the LDA. Thus, the (BN)0.75 C0.25 , (BN)0.5 C0.5 and (BN)0.25 C0.75 systems should be able to host five hydrogen molecules, yielding a H2 storage capacity of ∼8.7 wt.%. As mentioned above, the hybridization of the (BN)0.25 C0.75 complex was much stronger than that of the other complexes. This makes (BN)0.25 C0.75 a potential candidate for hydrogen storage. Here, the average binding energy per H2 molecule in the Li-decorated (BN)x C1−x complex was within the ideal energy window of 0.2–0.4 eV/H2 , indicating that it has an acceptable release rate at ambient temperature.42 Furthermore, we also use the vdW corrections to calculate the hydrogen adsorption energy. The details can be found in the supplementary material.39 The results present the

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FIG. 4. The most stable adsorption configurations and the maximum hydrogen contents (wt.%) for the corresponding Li atoms adsorbed onto (BN)x C1−x (x = 1, 0.25, 0.5, 0.75, 0) and B0.125 C0.875 are shown for the (a) side and (b) top view. dnav (n = 1–5) is the average distance between the adsorbed H2 molecules and the surface and dL is the distance between an adsorbed Li atom and the surface. E1 is the binding energy of the first H2 molecule absorbed by a Li atom; En (n = 2–5) is the binding energy of the last (nth) H2 molecule adsorbed by a Li atom and Enav is the average binding energy of n H2 molecules adsorbed by a Li atom. The distances between the adsorbed H2 molecules are listed in red.

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FIG. 5. PDOS (upper panels) for the H-s (red/blue curve) and Li-2p (gray curve) orbitals for 1–5 H2 molecules adsorbed onto a Li atom bonded to the (BN)0.25 C0.75 (2 × 2) configuration. Contour plots of the difference charge densities are presented in the lower panels. They contain the titled adsorbed H2 molecules and the Li atoms adsorbed onto a (BN)0.25 C0.75 sheet. The differential charge densities were calculated using: ρ = ρnH +Li/(BN ) C − ρnH 2 o.25 0.75 2 − ρLi/(BN) C . The red and blue colors indicate the charge depletion and accumulation, respectively. o.25 0.75

similar rules according to the above LDA results. Such as the pure graphene complex, the adsorption energies of 1H2 and 4H2 are obtained by vdW are 0.15, 0.21 eV, respectively, and the BN0.75 C0.25 complex, the adsorption energies of 1H2 and 5H2 using vdW corrections are 0.26, 0.10 eV, while, the LDA results are 0.18, 0.22 eV for pure graphene and 0.25, 0.08 eV for BN0.75 C0.25 complex, respectively. Thus the results with LDA are reliable compared with those with the corresponding functional with vdW corrections. To further understand the hybridization and polarization mechanisms, the calculated PDOS for the H-s and Li-2p orbitals and the differences in the two dimensional charge densities for the adsorbed H2 molecules onto the Li-decorated (BN)0.25 C0.75 complex were plotted in Fig. 5. The PDOS of the H-s and Li-2p orbitals indicated that the H-s orbitals below the Fermi level with ∼−8 eV, broadened with an increasing number of H2 molecules, indicating that there were interactions between the H2 molecules. In addition, the orbital hybridizations between the H-s and Li-p (s) orbitals become weaker as the number of adsorbed H2 molecules increased. Conversely, the amount of electron transferred from the Li atoms to H2 molecules still increased through the orbital hybridizations, which reduced the net charge of the Li atoms, and thus, it decreased the capacity that the Li atoms could polarize the H2 molecules. This explains the drop in the binding energies for the addition of the third, fourth, and fifth H2 molecules. The number of peaks corresponded with the adsorbed number of H2 molecules, which is different to the Ca-H2 system.37, 43 The absence of d-orbitals on the Li atoms did not cause unphysical over binding, compared with the Ca-H2 system that involved hybridization of the d-orbitals.

The charge accumulation and depletion at both sides of the adsorbed hydrogen molecules are plotted in Fig. 5 and clearly show that the H2 molecules were strongly polarized by the Li+ ions. The depletion of the Li ions made it nearly become a positively charged ion, resulting in polarization. The effect of polarization was responsible for the adsorption of H2 . The fifth H2 molecule was adsorbed above the Li atom because of the polarization effect. The charge density above the Li atom was positive, i.e., the charge transfer between the Li ions and the substrate could increase the binding energy of the Li-decorated complexes. The charged states between the Li ions and the substrate were much lower than those between Li and H2 . The charges were transferred from the Li atoms to the adsorbed H2 molecules and filled in the H2 σ *, leading to the bonds in the hydrogen molecules stretching. This indicated that the adsorption of hydrogen molecules was a typical “Kubas” type.23 IV. CONCLUSIONS

In summary, the characteristics of hydrogen adsorption on a Li-decorated carbon doped boron nitride (BN)x C1−x (x = 1, 0.25, 0.5, 0.75, 0) and B0.125 C0.875 planes were investigated. The calculated results showed that the Li-decorated (BN)0.25 C0.75 compound can spontaneously store hydrogen with a storage capacity up to 8.7%. The adsorption mechanisms were discussed and the results showed that the nitrogen atoms that were introduced into the substrate planes could increase the hybridization of the 2p-orbitals for Li, along with the orbitals for H2 . The existence of boron atoms in the substrate planes can avoid clustering of the Li atoms. The successive energies of the adsorbed hydrogen molecules were in the

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rage of ∼0.2–0.4 eV/H2 , which is feasible for hydrogen desorption. Therefore, Li-decorated (BN)0.25 C0.75 is a potential candidate for a hydrogen storage material. ACKNOWLEDGMENTS

This work was supported by the NSFC (Grant Nos. 51102009 and 51222212), the Fundamental Research Funds for the Central Universities (Grant No. JD1109), the CAEP foundation (Grant No. 2012B0302052), and the MOST of China (973 Project, Grant No. 2011CB922200). The authors are grateful to Professor M. Pu from the State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology for the useful discussions. 1 L.

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A first-principles study of lithium-decorated hybrid boron nitride and graphene domains for hydrogen storage.

First-principles calculations are performed to investigate the adsorption of hydrogen onto Li-decorated hybrid boron nitride and graphene domains of (...
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