Ultrahigh-Capacity Molecular Hydrogen Storage of a Lithium

Article pubs.acs.org/JPCC

Ultrahigh-Capacity Molecular Hydrogen Storage of a LithiumDecorated Boron Monolayer Jiling Li,† Hongyu Zhang,‡ and Guowei Yang*,† †

J. Phys. Chem. C 2015.119:19681-19688. Downloaded from pubs.acs.org by UNIV OF NEBRASKA-LINCOLN on 09/02/15. For personal use only.

State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China ‡ Department of Physics, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Recently, a novel boron monolayer with the “hexagon holes” density of η = 1/8 was repeatedly predicted to be the most stable boron sheet in different literatures. Its fascinating porous characteristic structure and sufficient surface space seem attractive and motivate researchers to perform further investigation about it. Herein, we demonstrated that the Li-decorated 1/8-boron monolayer is a kind of ultrahigh capacity hydrogen storage medium. We also established that Li atoms can be attached above the centers of the hexagonal holes in the novel 1/8-boron monolayer due to the charge transfer from Li atoms to boron atoms, and the electric field induced by the positive charged Li atoms attracts and polarizes the H2 molecules and makes the binding strong enough for potential applications to store H2 molecules but not dissociate them. Detailed calculations showed that the two-sided Li-decorated 1/8boron monolayer has an ultrahigh hydrogen storage capacity averagely to bind up to four H2 molecules for each Li atom with an ideal binding energy of 0.23 eV/H2, which is just in the ideal binding energy scope (0.2−0.4 eV/H2) for reversible hydrogen storage and corresponding to a hydrogen uptake of 15.26 wt %. These findings suggested a possible method of engineering a new structure for ultrahigh-capacity hydrogen storage materials with the reversible adsorption and desorption of hydrogen molecules, and they were expected to motivate an active line of experimental efforts.

I. INTRODUCTION Previous advances in materials have recognized hydrogen as an ideal alternative energy source for its high efficiency and environmental friendliness.1 However, the implementation of hydrogen in practical applications is currently limited by the lack of an economic method for hydrogen storage. Accordingly, a great deal of effort has been devoted to searching for suitable materials that can store hydrogen with high gravimetric/ volumetric density and good reversibility.1−4 Generally, there are two routes for the interaction between hydrogen and the substrate materials: either physisorbed in molecular form or chemisorbed in atomic form, respectively, corresponding to low hydrogen storage with easy hydrogen desorbtion and high hydrogen storage but difficult hydrogen desorbtion. To achieve the reversible hydrogen uptake and release at ambient conditions, an ideal storage system would be one where hydrogen binds molecularly, with the binding energy between 0.2 and 0.4 eV/H2,5 which is intermediate between the physisorbed and chemisorbed states. In pursuit of this goal, in the past years, various substrate materials, especially lightelement nanostructures composed of carbon,6−10 boron, or nitrogen,11−13have been extensively investigated to store hydrogen due to their lightweight and large surface area. Unfortunately, previous reports indicated that pristine lightelement nanostructures are chemically too inert to be useful for practical hydrogen storage.14As to this point, to improve the © 2015 American Chemical Society

hydrogen storage capacity of the pristine nanomaterials, one approach is extensively adopted by coating their surfaces using metal atoms, such as transition or alkali metals.15−17 Compared with the transition metals, the alkali atoms are lightweight. Consequently, coating the pristine nanostructures with alkali atoms will not considerably increase the self-weight of the substrate materials and sequentially be favorable to obtain higher gravimetric density of hydrogen. Additionally, the alkali atoms are predicted to be adsorbed on the surfaces of pure nanostructures uniformly.18 On the other hand, a review of the previous literature shows that carbon nanostructures have been extensively studied19and emerged as attractive candidate materials for hydrogen storage.7−10 Boron is the nearest neighbor of carbon in the Periodic Table. Accordingly, the corresponding boron nanostructures may be promising hydrogen storage media due to their low weight and large specific surface area.11−13 Just recently, a new class of two-dimensional (2D) material, the boron monolayer composed of triangular and hexagonal motifs, was identified to be energetically more stable than those of only triangular lattices or hexagonal lattices.20−23Among them, the so-called “α-sheet” is predicted to be most stable. Note that the Received: June 27, 2015 Revised: August 11, 2015 Published: August 14, 2015 19681

DOI: 10.1021/acs.jpcc.5b06164 J. Phys. Chem. C 2015, 119, 19681−19688

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The Journal of Physical Chemistry C “hexagon hole density”, defined as η = (No. of hexagon holes)/ (No. of atoms in the original triangular sheet), is proposed to describe the concentration of the hexagon in a flat triangular sheet. The calculated results indicated that between η = 0 and η = 1/3, corresponding, respectively, to the flat triangular and hexagonal boron sheets, the maximum binding energy (Eb) of the most stable structure occurs for the α-sheet (η = 1/9).20 Subsequently, two other boron monolayers also composed of triangular and hexagonal motifs are reported to have lower total energy than that of the “α-sheet”, with the value η of 1/8 and 2/15, respectively. Noticeably, in both of the literatures, the η = 1/8 boron monolayer is predicted to be the most stable boron monolayer.21 During the systemic research by the particle swarm optimization (PSO) algorithm, a variety of boron monolayers were proposed, and for the second time, the results indicated that most of the stable boron sheets have η values between 1/9 and 2/15.22 Indeed, the η = 1/8 boron (1/8boron) monolayer is then again confirmed to be the most stable boron monolayer in the recent literature.23 This special 1/8-boron monolayer seems fascinating and motivates us to perform further investigations about it. The equilibrium configuration of an attractive 1/8-boron monolayer is constructed and shown in Figure 1. Remarkably, it has the

II. METHODOLOGICAL METHODS All the computations about the structures and energies are based on the first-principles density-functional theory by using the SIESTA computation code,24−26 which performs fully selfconsistent calculations solving the standard Kohn−Sham equations. A flexible linear combination of numerical atomicorbital basis sets is used for the description of valence electrons, and norm-conserving nonlocal pseudopotentials are adopted for the atomic cores. The pseudopotentials are constructed using the Trouiller−Martins scheme27 to describe the interaction of valence electrons with the atomic cores. The nonlocal components of pseudopotential are expressed in the fully separable form of Kleiman and Bylander.28,29 The Perdew−Burke−Ernzerhof (PBE) form generalized gradient approximation (GGA) corrections are adopted for the exchange-correction potential.30 The atomic orbital set employed throughout is a double-ζ plus polarization (DZP) function. For all atoms, the basis functions are strictly localized within radii that correspond to the confinement energy of 0.02 Ry. The numerical integrals are performed and projected on a real space grid with an equivalent cutoff of 120 Ry to calculate the self-consistent Hamiltonian matrix. To test the convergence accuracy, we have adopted another real space grid with the equivalent cutoff of 180 Ry for one of the considered complexes. The difference in total energies between the results is less than 0.07 meV/atom. To determine the equilibrium configurations of all the considered nanostructures, we relax all the atomic coordinates involved by using a conjugate gradient (CG) algorithm, until the maximum atomic forces are less than 0.02 eV/Å. To explore the redistribution of electron properties after the Li-decorated and H2-adsorbed, the electron density calculations were specially performed using the DMOL3 package.31,32 The geometrical structures were first reoptimized at the same theoretical levels with that adopted in the Siesta for both the one-sided Li-decorated boron monolayer and the 12 H2 molecules adsorbed one-sided Li-decorated boron monolayer. By the analysis of the structural parameters and bond lengths after optimization, the equilibrium configurations from DMOL3 were found to be the same as those from Siesta. Therefore, the total and deformation electron densities were then, respectively, calculated for the single-point configurations of the one-sided Li-decorated boron monolayer and 12 H2 molecules adsorbed one-sided Li-decorated boron monolayer to explain the enhancement effect of the Li-decorated boron monolayer for hydrogen storage.

Figure 1. Optimized configuration of the pristine 1/8-boron monolayer: (a) side view and (b) top view.

III. RESULTS AND DISCUSSION As mentioned above, the η = 1/8 boron monolayer (1/8-boron monolayer) is repeatedly confirmed to be the most stable boron monolayer theoretically in different literatures.21−23 Accordingly, we first construct the geometrical configuration of the pristine 1/8-boron monolayer, and the optimized configurations are shown in Figure 1(a) and (b), corresponding to the side and top views, respectively. It is clear that the equilibrium configuration obtained currently keeps perfect monolayer structural characteristics and the same with that previously proposed. Boron atoms are aggregated and wellregulated to be triangular and hexagonal motifs by forming B− B bonds, with the bond lengths distributed from 1.66 to 1.76 Å and the average value of 1.71 Å. The triangular and hexagonal motifs and the resulting porous structural characteristic of this

typically porous characteristic structure due to the “hexagonal hole” involved, which can provide sufficient surface space to accommodate more H2 molecules and greatly enhance the capacity of hydrogen storage as a subtract nanomaterial. Therefore, the further investigations about this η = 1/8 boron monolayer for hydrogen storage are thus desirable. Considering the above, in this contribution, we perform a systemic theoretical exploration for the feasibility of the functionalized η = 1/8 boron monolayer proposed as hydrogen storage nanomaterials and focus on Li as the introduced metal atoms to enhance the hydrogen storage capacity. 19682


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“hexagonal hole” monolayer could provide sufficient space and are thus favorable for hydrogen storage. By this motivation, we then consider the Li atoms decorated on the surface of the 1/8-boron monolayer and perform the study on the hydrogen storage capacity of the Li-decorated complex. By adopting a 2 × 1 supercell, we consider the adsorption of a single Li atom and vary the decorated position of the Li atom above the surface of the boron monolayer. After the structure optimization, we find out that the site above the center of the hexagonal hole obviously becomes the preferred place for the Li atom to adsorb. Besides, to estimate the stability for the Li decorated on the 1/8-boron monolayer, binding energies for the decorated Li atoms are an important quantity and defined as E b = (mE Li + E Bsheet − E Li − Bsheet)/m

(1)

Herein, ELi−Bsheet is the total energy of the Li-decorated boron monolayer; EBsheet and ELi are the energies of the pristine boron sheet and a single Li atom, respectively; and m is the number of Li atoms involved in the Li-decorated boron sheet. Due to the special “hexagonal hole” structure and large surface area, the 1/8-boron monolayer as the substrate nanomaterial is expected to provide enough sites to adsorb Li atoms. Hence, we further investigate the deposition of multi-Li atoms decorated on the 1/8-boron monolayer. In addition, to get enough hydrogen gravimetric density, the decorated Li atoms should be separated far enough from each other so that each Li atom has enough space to bind more H2 molecules. Since the centers of the hexagons are the preferred sites, in the following study, we directly discuss the structures and properties of the Li-decorated boron monolayer with the Li atoms just above the centers of each hexagon hole. At the first step, the Li atoms are assigned to be decorated on only one side of the 1/8-boron monolayer (one-sided Lidecorated boron sheet). The optimized equilibrium configuration is obtained, as shown in Figure 2(a) and (b), respectively, corresponding to the side and top views. It is obvious that the Li atoms remain dispersed above the centers of the hexagonal holes, resulting in a high regular and symmetric geometrical complex. The average distance of the Li atoms to the 1/8-boron monolayer is 1.64 Å. Simultaneously, accompanied with the adsorption of Li atoms, the B−B bond lengths in the 1/8-boron monolayer are slightly enlarged, with the average value changed from 1.71 to 1.73 Å. This structural distortion and the interaction between Li and B atoms will exert an influence on the electronic properties of the decorated Li atom and the one-sided Li-decorated 1/8-boron monolayer. Indeed, through the Mulliken population analysis, we find out that there are charges transferring from the Li atom to the boron atoms in the boron monolayer, with the average quantity of 0.083 eV for each Li atom. Further, to explore the redistribution of electron density resulting from the Li adsorption, we plot the isosurface of the electron density difference for the Li-decorated 1/8-boron sheet. The electron density differences are defined as Δρ = ρLi‑decorated‑Bsheet − (ρBsheet + ρLi), where ρLi‑decorated‑Bsheet is the total electron density of the optimized Li-decorated boron sheet. ρBsheet and ρLi are, respectively, the electron density of the corresponding boron sheet and the dispersed Li atoms extracted from the optimized one-sided Li-decorated boron sheet, without the relaxation. These results are shown in Figure 3. Herein, the red and green,

Figure 2. Optimized configurations of the one-sided Li-decorated boron monolayer: (a) side view and (b) top view.

respectively, denote the electron accumulation and depletion. Clearly, we can see that the electron depletion corresponding to the loss of electron mainly occurs around all the Li atoms and the minority of the boron atoms at the hexagonal vertexes in the 1/8-boron sheet. However, the electron is accumulated exactly between the Li atoms and boron monolayer, including the space around the majority of boron atoms. The electron accumulation and depletion at difference places induce a local electric field and thus promote the Li-decorated 1/8-boron monolayer to store hydrogen. Moreover, the charge transfer makes the positive charged Li atom stably distributed and having the interaction with the negative charged boron monolayer with the binding energy of 2.23 eV/Li, as listed in Table 1. This binding energy is higher than the cohesive energy 1.63 eV of bulk Li.33 This result indicates that during the adsorption Li atoms can be distributed dispersedly without being clustered. Indeed, there is no Li aggregation occurring during our calculations. Then, in order to investigate the interaction of hydrogen on this one-sided Li-decorated boron sheet, H2 molecules are introduced into this complex. As the Mulliken population analysis above and electron density differences show in Figure 3, the charges transferring from the Li atoms to the boron atoms make the Li atoms positively charged and the center of positive charges absorb the H2 molecules. So, at the first step of simulation about the adsorption of the H2 molecules on the Lidecorated boron sheet, one H2 molecule was introduced exactly around each adsorbed Li atom. Herein, the considered onesided Li-decorated boron sheet is composed of one 2 × 1 supercell with six Li atoms involved in and thus correspondingly six H2 molecules in all to be adsorbed. The optimized equilibrium configuration is shown in Figure 4(a), corresponding to the side and top views, respectively. Due to the interaction between the electric field and the adsorbed H2 molecule, the H−H bond lengths expand lightly from 0.775 Å of an isolated H2 molecule to 0.783 Å. In order to evaluate the adsorbed form of H2 molecules on the one-sided Li-decorated 19683


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hole in the pristine boron sheet adsorbs one H2 molecule, that is, six H2 molecules adsorbed in all. The calculated adsorption energy is 0.19 eV/H2, which is much smaller than that of the H2 molecules on the Li-decorated boron sheet (0.27 eV/H2) and unfavorable for being out of the ideal scope (0.2−0.4 eV/H2) for reversible hydrogen storage. So, in the following, we keep our attention on the hydrogen storage of the Li-decorated 1/8boron monolayer. Due to the special “hexagonal hole” structural characteristic and large surface area of the considered boron monolayer, we further perform the calculations to study more H2 molecules approaching the one-sided Li-decorated boron sheet. At first, we studied the structure and stability of two H2 molecules around each Li atom, that is, 12 H2 molecules in all adsorbed on the one-sided Li-decorated boron sheet. The optimized structure is given in Figure 4(b). It is obvious that hydrogen atoms still remain in the molecular state with the uniform bonding length of 0.784 Å. The average adsorption energy of two H2 molecules around each Li to the one-sided Li-decorated boron sheet is 0.31 eV/H2. The corresponding results are listed in Table 2. Then step by step, we increase the number of H2 molecules by adding additional hydrogen molecules around each Li atom in the one-sided Li-decorated boron sheet. The fully optimized configurations of the multi-H2 adsorbed onesided Li-decorated boron sheet are shown in Figure 4(b)−(d), following the order of increasing one H2 molecule around each Li atom attach step. As more H2 molecules approach the onesided Li-decorated boron sheet, the corresponding average H2 adsorption energies are 0.31 eV/H2, 0.38 eV/H2, and 0.29 eV/ H2 correspondingly for 2H2, 3H2, and 4H2 around each Li atom, respectively, as listed in Table 2. All these average adsorption energies are in the scope of the ideal binding energy intermediate between the physisorbed and chemisorbed states. Moreover, for the optimized configuration, all the hydrogen atoms retain their molecular identity during the process of increasing the number of H2 molecules, and the H−H bond lengths still remain unchanged with the uniform value of 0.78 Å. The significant charge transfer and the resulting electric field induce strong enough interaction to take up so many H2 molecules and not dissociate them. However, as more H2 molecules approach the one-sided Lidecorated boron sheet, there will be too overfull H2 molecules to be bonded, and thus they move away from the space near the surface of the one-side Li-decorated boron sheet, which could be seen from the optimized configurations of the five H2 molecules around each Li atom adsorbed system shown in Figure 4(e). Consequently, the binding energy is greatly decreased to 0.21 eV/H2 due to the weak interaction of the overflowing H2 with the boron sheet and the steric repulsion accompanying the increasing number of H2 molecules. Thus, we ignore more H2 molecules than four H2molecules around each Li atom stored in the one-sided Li-decorated boron sheet and conclude that a Li atom decorated on the boron sheet has the capacity of accommodating averagely four H2 molecules, corresponding to the gravimetric density of 8.90 wt %. To lend further insight into the adsorption mechanism of H2 molecules on the surface of the one-side Li-decorated boron sheet, we plotted the isosurface of electron density difference for the one-sided Li-decorated boron sheet in the presence of two H2 molecules around each Li as an example. The electron density differences were defined completely identically with that of the Li-decorated boron sheet discussed just above. We distinguish that, herein, Δρ = ρH2+Li‑decorated‑Bsheet − (ρBLi‑decorated‑Bsheet

Figure 3. Electron density differences with an isovalue of 0.015 e/Å3 for the one-sided Li-decorated 1/8-boron monolayer: (a) side view and (b) top view. The green and red indicate electron depletion and accumulation, respectively.

Table 1. Calculated Binding Energies of Li Atoms on the 1/ 8-Boron Monolayer (Eb), the Average Distances of the Li Atom to the Boron Sheet (DLi‑sheet), and the Average Charge Transfer of Each Li Atom to the Boron Monolayer complex one-sided Li-decorated boron sheet two-sided Li-decorated boron sheet

Eb (eV/Li)

DLi‑Bsheet (Å)

average charge transfer on each Li atom (e)

−2.23

1.64

0.083

−2.18

1.57

0.056

boron sheet, we calculate the hydrogen adsorption energy according to the definition Ea = (ELi‑decorated‑Bsheet+ nEH2 − EH2+Li‑decorated‑Bsheet)/n, where EH2+Li‑decorated‑Bsheet, ELi‑decorated‑Bsheet, and EH2 are the total energies of the H2-adsorbed Li-decorated boron sheet, the separated Li-decorated boron sheet, and the isolated H2 molecule, respectively. The calculated adsorption energy of six H2 molecules adsorbed on the one-sided Lidecorated boron sheet is 0.27 eV/H2, which is in the ideal binding energy scope (0.2−0.4 eV/H2) for reversible hydrogen storage. To compare with the situation that H2 molecules directly adsorbed on the pristine boron sheet, we also considered the situation that each center of the hexagonal 19684


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Figure 4. Optimized configurations of the multi-H2 adsorbed on the one-sided Li-decorated 1/8-boron monolayer with the number of H2 molecules increasing, following the order of increasing one H2 molecule around each Li atom at each step.

Table 2. Calculated Average Distance of the Li Atom to the Decorated Boron Monolayer (DLi‑sheet), the H−H Bond Lengths in the Adsorbed H2 Molecule (dH−H), the Gravimetric Density of the H2 Molecules in the H−Li−B Complex, and the Average Adsorption Energies for Each H2 Adsorbed in the Li-Decorated Boron Sheet (Ea) complex one-sided Lidecorated boron sheet

two-sided Lidecorated boron sheet

1H2 2H2 3H2 4H2 1H2 2H2 3H2 4H2

DLi‑Bsheet (Å)

dH−H (Å)

gravimetric density

Ea (eV/H2)

1.58 1.63 1.66 1.66 1.66 1.66 1.68 1.67

0.783 0.784 0.781 0.782 0.795 0.793 0.788 0.785

2.38 4.65 6.82 8.90 4.31 8.26 11.90 15.26

0.27(0.43) 0.31 0.28 0.29 0.27(0.42) 0.28 0.26 0.23(0.34)

wt wt wt wt wt wt wt wt

% % % % % % % %

+ ρH2), where ρH2+Li‑decorated‑Bsheet is the total electron density of the optimized H2-adsorbed Li-decorated boron sheet and ρB Li‑decorated‑Bsheet and ρ H2 are the electron density of the corresponding one-sided Li-decorated boron sheet and the adsorbed H2 molecules directly extracted from the optimized H2-adsorbed one-sided Li-decorated boron sheet, respectively, without the rerelaxation. These results are shown in Figure 5. As shown, when the two H2 molecules around each Li atom are adsorbed, the electron accumulation and depletion occur, respectively, at the two ends of each H2 molecule, suggesting the polarization of the H2 molecule under the electric field induced in the Li-decorated boron sheet. Indeed, the Mulliken population analysis indicates that H2 molecules have local dipole moments ranging from 0.06 to 0.16 debye in these two H2-adsorbed averagely each Li complex. This polarization interaction dominates the adsorption of the H2 molecules on the Li-decorated boron sheet and gives the explanation of how the introduced Li atoms enhance the capacity of the pristine boron sheet for hydrogen storage. Furthermore, we perform the exploration of the Li atoms decorated on both sides of the 1/8-boron monolayer. The optimized equilibrium configuration of this two-sided Lidecorated boron sheet is shown in Figure 6(a) and (b), corresponding to the side and top view, respectively. From this figure, it is clear that all the Li atoms still remain dispersed above the centers of the hexagonal holes. The average distance of the Li atoms to the boron sheet is 1.57 Å, slightly smaller

Figure 5. Electron density differences with an isovalue of 0.007 e/Å3 for the 12 H2 molecules absorbed on the one-sided Li-decorated 1/8boron monolayer: (a) side view and (b) top view. The green and red indicate electron depletion and accumulation, respectively.

than that of the one-sided Li-decorated 1/8-boron monolayer (1.64 Å). The average binding energy of each Li atom decorated on this two-sided Li-decorated boron sheet is 2.18 eV/Li, slightly smaller than that of each Li on the one-sided Lidecorated Bsheet (2.23 eV/Li) and still much larger than the cohesive energy of bulk Li (1.63 eV/Li).33The Mulliken population analysis indicates that the average quantity of the transferred charges from the Li atom to the boron monolayer in the two-sided Li-decorated boron sheet is 0.056 e/Li, slightly 19685


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two-sided Li-decorated boron sheet is 0.27 eV. The corresponding gravimetric density of one H2 molecule around each Li-adsorbed two-sided Li-decorated boron sheet is 4.31 wt %. Then we gradually increase the number of H2 molecules introduced into the complex. The increased number is in order of the average increase of one H2 molecule for one Li atom in each step. The fully optimized configurations of the multi-H2 adsorbed on the two-sided Li-decorated boron sheet are shown in Figure 7(b)−(d), increasing one H2 molecule for each Li each step. The corresponding average H2 adsorption energy is 0.28 eV/H2, 0.26 eV/H2, and 0.23 eV/H2 respectively, as listed in Table 2. As seen in these figures, accompanied by the increase of the H2 molecules, all the adsorbed H2 molecules are divided into two cases: the inner and the outer relative to the Li−B sheet. For the first case, the inner H2 molecules, the distance of H2 molecules is about the average value of 1.8 Å to the covered Li layer. For the second case, H2 molecules being outer of the Li−B sheet, the average distance to the Li layer is less than 4.8 Å. Besides, the charge transfer from Li atoms to boron atoms and the electric field induced by the positively charged Li atoms attract and polarize all the H2 molecules and make the binding strong enough to absorb the outer H2 molecules. In fact, we, respectively, recalculated the binding energy of each individual H2 molecule in the outer by performing the single-point energy calculation through subtracting the corresponding H2 molecule from the H2adsorbed two-sided Li-decorated complex. The binding energy of each individual H2 molecule in the outer is in the range of 0.12−0.24 eV, comparable with the average value of 0.23 eV. Herein, the adopted GGA normally underestimates this effect and gives lower energies. Therefore, all these binding energies are basically in the scope of the ideal values between the physisorbed and chemisorbed states. Besides, the Mulliken population analysis and electron density differences have shown that the electric field induced by the positively charged Li atoms and boron would attract and polarize these outer H2 molecules

Figure 6. Optimized configurations of the two-sided Li-decorated boron monolayer: (a) side view and (b) top view.

smaller than that in the one-sided Li-decorated boron sheet (0.083 e/Li), as listed in Table 1. Finally, the conditions that more H2 molecules adsorbed on the two-sided Li-decorated boron sheet are further considered. The step begins from the status that one H2 molecule around each Li atom, that is, 12 H2 molecules in all, are first introduced into the two-sided Li-decorated boron sheet. The obtained equilibrium configuration is shown in Figure 7(a), the up and down respectively, corresponding to the side and top views. The average adsorption energy of each H2 molecule on the

Figure 7. Optimized configurations of the multi-H2 adsorbed on the two-sided Li-decorated 1/8-boron monolayer with the number of H2 molecules increasing, following the order of increasing one H2 molecule around each Li atom at each step. 19686




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and make the binding strong enough. Upon all these, we think that all the H2 molecules considered in the system, averagely 4 H2 molecules around each Li atom, could be counted for in the calculation of capacity to store hydrogen. The two-sided Lidecorated boron sheet to adsorb averagely 4H2 molecules around each Li atom corresponds to the gravimetric density of 15.26 wt %. This gravimetric density, with the binding energy of 0.23 eV/H2, is much higher than that of 8.2 wt % in the 12Ca-decorated-B80 and 7.6 wt % in the (9,0) boron nanotube about the hydrogen storage of the boron substrate nanomaterials.13 Additionally, it is widely regarded that GGA normally underestimates the binding interaction, and LDA overestimates the dispersion interaction and gives lower binding energies. The real adsorption energy may lie between the GGA and LDA results. Aiming at this point, we then readopt the local density approximation (LDA) with the Perdew−Wang-92 (PW92) functional when evaluating the binding energy of the H2 molecules when they adsorb on the one-sided Li-decorated complex and the two-sided version, for the case of one H2 molecule around each Li atom. The calculated binding energy is 0.43 and 0.42 eV/H2, respectively, as listed in the brackets in Table 2. This means the real adsorption energies lie between 0.27 eV/H2 from the GGA and ∼0.42 eV/H2 from LDA, being in the ideal binding energy scope (0.2−0.4 eV/H2) for reversible hydrogen storage. Finally, one thing that should be mentioned is that the adopted GGA methods have the limitation of partially including the long-range type of electrostatic interaction. Consequently, for comparison, we have recalculated the four H2 molecules around each Li atom two-sided Li-decorated boron sheet by including the molecular mechanics potentials, with the dispersion potential of the Grimme type. During the calculation, C6 is set to 0.14, and the sum of van der Waals raddi for the interacting H atom is 1.001 Å, according to ref 34. The corresponding results are listed in the brackets in Table 2. The adsportion energy of 0.34 eV/H2 is still in the ideal adsorption energy scope of 0.2−0.4 eV/H2. Therefore, for the H−Li−B compounds considered, the van der Waals interactions will not distinctly modify the results obtained in the present work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Basic Research Program of China (2014CB931700) and State Key Laboratory of Optoelectronic Materials and Technologies supported this work and partially by Shanghai Supercomputer Center.

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REFERENCES

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IV. CONCLUSION In summary, we have shown the Li-decorated 1/8-boron monolayer rise of ultrahigh capacity hydrogen storage medium. Li atoms can be attached above the centers of the hexagonal holes in the novel 1/8-boron monolayer due to the charge transfer from Li atoms to boron atoms. It was found that the electric field induced by the positively charged Li atoms attracts and polarizes the H2 molecules and makes the binding strong enough for potential applications to store H2 molecules but not dissociate them. The further results indicated that the two-side Li-decorated boron monolayer has an ultrahigh hydrogen storage capacity to averagely store up to four H2 molecules with an ideal binding energy of 0.23 eV/H2, corresponding to a hydrogen uptake of 15.26 wt %. The steric repulsion accompanying the increase of H2 molecules may be a source of additional storage restriction. These theoretical predictions suggested a possible method of engineering new structures for ultrahigh-capacity hydrogen storage materials, and these findings are expected to motivate an active line of experimental efforts. 19687


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Ultrahigh-Capacity Molecular Hydrogen Storage of a Lithium

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