Introduction

Due to its merits of cleanness, renewability, abundance in nature, and high energy density per unit mass, hydrogen has been recognized as an appealing energy carrier for the future world. It has the potential to reduce our dependence on fossil fuels, which are limited in resource and harmful to the environment1,2,3,4. One bottleneck of using hydrogen for vehicular applications is the lack of safe and efficient hydrogen storage materials5,6,7 that store molecular H2 reversibly with high gravimetric density and fast kinetics for adsorption, as well as desorption, under the conditions of moderate temperature and pressure8,9. An ideal H2 storage system would be one that binds hydrogen in molecular form and with an adsorption energy (AE) in the regime of 0.1–0.5 eV per H2, that is, intermediate between the physisorbed and chemisorbed states10,11. Although advances have been made towards meeting the U.S. DOE’s targets for hydrogen storage, an ideal system is yet to be designed and synthesized. Therefore, seeking novel hydrogen storage materials has remained an important issue.

Previous experiments and theoretical calculations have shown that metal-decorated carbon fullerenes and nanotubes12,13,14,15,16,17,18,19, as well as their boron-, nitrogen- and beryllium-substituted nanostructures20,21,22, might be good candidates for the storage of H2 molecules. For instance, Zhang and co-workers showed that the reversible hydrogen storage of transition-metal-coated C60 and C48B12 may be as high as 9 wt%21. Yildirim et al. revealed that Ti-coated single-walled carbon nanotubes can store 8 wt% of H223. To avoid the clustering problem of transition metal atoms on the surface of carbon nanostructures, Yoon and co-workers18 found that Ca can achieve homogeneous monolayer coating, which is superior to other metal elements. They concluded that up to 8.4 wt% of hydrogen can be stored in Ca32C60 with an AE of 0.2–0.4 eV/H2. Through first-principles computations, Sun et al.13 predicted that Li-decorated fullerene C60 (Li12C60) can store up to 9 wt% of H2, albeit with a rather weak AE of 0.075 eV/H2. Furthermore, Yoshida et al.17 measured the hydrogen absorption of Li9C60 based on experiments and confirmed that up to ~2.6 wt % H2 can be stored at 250 °C and 30 bar H2. For lithium-doped fullerenes (Lix-C60-Hy) with a Li:C60 mole ratio of 6:1, a reversible uptake of 5 wt% H2 at 350 °C and 105 bar H2 and desorption onset temperature of ~270 °C was observed15. Subsequently, another experimental results16 showed that up to 9.5 wt % deuterium (D2) are absorbed in Li12C60 under a pressure of 190 bar and a temperature below 100 °C.

Boron is the lighter neighbor of carbon in the periodic table, which possesses the similar merit as carbon in terms of light weight and potential applications for hydrogen storage. For this purpose, its chemical hydrides24,25,26 were studied, as were relevant model nanostructures, such as boron monolayer sheets, fullerenes, and nanotubes27,28,29. In particular, following the proposal of the celebrated Ih B80 buckyball30, which is built upon the C60 motif by capping all 20 surface hexagons, a number of papers were devoted to hydrogen storage using B80 coated with metals (M = Li, Na, K, Be, Mg, Ca, Sc, Ti, and V)27,31,32,33. However, B80 was subsequently found to favor core-shell type structures at various theoretical levels34,35. It is thus not feasible to pursue any realistic technological applications of B80 buckyball as hydrogen storage materials.

Very recently, the first all-boron fullerenes or borospherenes, D2d B40 and D2d B40−, were observed in a combined experimental and theoretical study36, marking the onset of the borospherene chemistry, whose future development may be envisioned to parallel that of the fullerenes. Endohedral M@B40 (M = Ca, Sr) and exohedral M&B40 (M = Be, Mg) metalloborospherenes were also predicted, which further support the structural, electronic, and chemical robustness of the B40 borospherene37. Closely following B40, chiral B39−, B41+, and B422+ borospherenes were also studied38,39, which expand the borospherene family and may eventually lead to new boron-based nanomaterials.

Borospherene B40 possesses a cube-like cage structure, whose six hexagonal and heptagonal holes each occupy a face of the cube. It also has eight triangular, close-packing B6 structural blocks, each on an apex of the cube. All B atoms are on the surface of the cage, which is an ideal, well-defined system for chemistry. B40 differs from carbon fullerenes in terms of structure and bonding, and the pursuit of borospherene-based nanomaterials for hydrogen storage is thus intriguing from a fundamental point-of-view. Furthermore, borospherenes are lighter than carbon fullerenes, which make the former systems better candidates to reach a higher gravimetric capacity for hydrogen storage. Relevant to this topic, Dong et al.40 predicted on the basis of density-functional theory (DFT) calculations that Ti-decorated B40 fullerene (Ti6B40) is capable of storing up to 34 H2 molecules with a maximum gravimetric density of 8.7 wt% and a reversible storage capacity of 6.1 wt%. To our knowledge, the U.S. DOE has set a target of 7.5 wt% for hydrogen storage capacity for the year of 201541,42.

In this work, we choose to study lithium-decorated borospherene B40 as a potential candidate for hydrogen storage via extensive DFT calculations. Since boron-based nanomaterials are also candidates for lithium storage, the current ternary B-Li-H system is quite unique28,29,43,44. Compared to transition metal, Li as the lightest metal definitely will facilitate the improvement of hydrogen storage capacity for the metal-decorated B40 system. The Lim-B40-nH2 system differs markedly from Lim-C60-nH2 or Ti6-B40-nH2, which have an AE value that is either rather small (0.075 eV)13 or too large (up to 0.82 eV)40. Even the recently proposed Li8-B6-nH2 system44 only has an AE of less than 0.1 eV. Our computational data show that Li-decorated B40 appears to be a promising medium for hydrogen storage. The Li atoms readily attach to the top of hexagonal and heptagonal holes on B40, forming a series of charge-transfer complexes from Cs Li&B40(1), C2v Li&B40(2), up to D2d Li6&B40(3). The Lim&B40 complexes can adsorb three H2 molecules per Li site with a moderate AE of 0.11–0.25 eV/H2. The Li6&B40(3) complex stores up to 34 H2 with an average AE of 0.10 eV/H2. The first 18 H2 of these possess ideal AEs, which suggest a gravimetric density of 7.1 wt%. Furthermore, the eight close-packing, triangular B6 corner sites of B40 are also suitable for Li-decoration and H2 adsorption. In a desirable Li14&B40(7) complex, up to 42 H2 molecules can be stored with AEs of 0.12–0.32 eV/H2, which corresponds to a gravimetric density of 13.8 wt%.

Results and Discussion

Isolated B40 Borospherene for H2 Adsorption

The first all-boron fullerene called as borospherene36, D2d B40 (1A1), possesses a very large HOMO–LUMO gap of 3.13 eV at the PBE0 level that indicates its overwhelming stability, which is comparable to that of Ih C60 (1Ag) (3.02 eV) calculated at the same level. All the valence electrons in B40 are either delocalized σ or π bonds and there is no localized 2c–2e bond, unlike the C60 fullerene. In fact, the surface of B40 is not perfectly smooth and exhibits unusual heptagonal faces which may play a role that release the surface strains, in contrast to C60 fullerene whose surface makes up of pentagons and hexagons and presents the least strain. And the diameter of B40 is 6.2 Å, slightly smaller than the value of C60 (7.1 Å), which makes B40 more comfortable to accommodate a range of small molecules inside the cage.

We initially studied H2 adsorption on the isolated B40 borospherene. The optimized structures of B40H2, H2@B40 and 2H2@B40 are shown in Fig. 1. In the C2 B40H2 dihydride, the H2 molecule tends to form two B–H covalent bonds with the tetracoordinate-B sites, which dissociate H2. The dissociative AE of a single H2 is up to 1.30 eV. For H2 storage inside the cage, only one H2 molecule can be encapsulated into B40, which is marginally exothermal with an AE of 0.24 eV. Interestingly, once such an encapsulation is completed, the H2 molecule cannot escape due to substantial energy barriers (>3 eV). The AE of a second H2 inside the cage is found to be endothermic by 1.32 eV, which is thus not feasible experimentally. In short, the above results show that an isolated B40 borospherene is not a good candidate for hydrogen storage directly. The B40-H2 interactions appear to be different from the case of C60. The latter is known to interact with H2 via weak van der Waals forces45. As a comparison, our calculation results show that the dissociative AE of a single H2 for C60H2 is only ~0.18 eV at the same level. However, similar to B40, only one hydrogen molecule can reside inside the C60 cage with a negative AE value of ~0.22 eV.

Figure 1
figure 1

Top and side views of optimized configurations of B40H2, H2@B40 and 2H2@B40.

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Configurations of Li-Decorated B40

As shown in Fig. 2, we start with a single Li atom interacting with B40. Relative stability of Li atom bound on heptagonal and hexagonal holes were considered. The exohedral Cs Li&B40(1), in which Li caps a heptagon, turns out to be more stable by 0.20 eV with respect to C2v Li&B40(2). In the latter species, Li caps a hexagon. The BE for Li is 3.08 and 2.88 eV in 1 and 2, respectively. Thus, Li prefers to bind on top of the heptagonal hole of B40. The Li–B distance in 1 is 2.34 Å, compared to 2.33 Å in 2 (Table 1). Clearly, the BE of Li on the center of heptagon or hexagon in B40 is substantially higher than those on the pentagon of C60 (1.80 eV), in Li2 dimer (0.95 eV), and in the Li bulk (1.63 eV)13. This should help suppress the potency of Li aggregation to form clusters on B40 surface, suggesting that Li is a suitable adsorbate to decorate B40. As shown in Table 1, electron transfer occurs from Li to borospherene B40 cage in 1 and 2, resulting in a positive charge of 0.87–0.88 |e| on Li as revealed in the Bader charge analysis. The ionized Li atom hints a possibility for H2 adsorption via the polarization mechanism18.

Table 1 Calculated Binding Energy (BE) of Li to B40, Li–B Bond Distance, and Atomic Charge on Li Atom for Li&B40(1), Li&B40(2), and Li6&B40(3).
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Figure 2
figure 2

Top and side views of the optimized configurations of Cs Li&B40(1), C2v Li&B40(2), and D2d Li6&B40(3), in which one Li atom is coated on a heptagonal hole of B40, one Li atom is on a hexagonal hole, and six Li atoms are on the heptagonal/hexagonal holes, respectively.

The B atom is in orange and Li is in purple.

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To increase the coverage of Li on B40, we place one Li atom on top of every hexagon and heptagon hole and reach exohedral D2d Li6&B40(3) (Fig. 2). In complex 3, six Li atoms remain isolated on the surface holes, resulting in a highly symmetric D2d geometry. The average BE of Li in 3 is 3.07 eV/Li, which is comparable to that in Li&B40(1) (3.08 eV) and is slightly larger than that in Li&B40(2) (2.88 eV). There appears to be a collective effect for Li adsorption because six Li atoms in 3 have a higher total BE (18.48 eV) than six individual Li atoms in 1 and 2 combined (18.08 eV). This fact suggests that Li6&B40(3) is a favorable configuration for Li-decoration. Remarkably but not surprisingly, our computational data indicate that 3 is at least 6.29 eV more stable than B40 attached by a compact Li6 cluster (Fig. S1). Therefore, surface aggregation of Li for island clusters is unlikely in the Li6&B40 system. The Li–B distance in 3 is 2.33 Å, nearly identical to those in 1 and 2. Bader charge analysis shows that the atomic charge on each Li atom in 3 is +0.87 |e|.

We further analyzed the isosurfaces of charge density differences in complexes Cs Li&B40(1), C2v Li&B40(2), and D2d Li6&B40(3), as depicted in Fig. 3. Here the yellow and blue colors represent electron accumulation and depletion regions, respectively. From the charge density variations of 1–3, which are induced by the adsorption of Li atoms onto B40, it is obvious that charge transfer from Li atom to B40 indeed takes place upon Li decoration.

Figure 3
figure 3

Top views of the isosurface of charge density differences of Cs Li&B40(1), C2v Li&B40(2), and D2d Li6&B40(3).

Yellow color represents the electron accumulation region, and blue represents the electron depletion region.

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Figure 4 shows the frontier canonical molecular orbitals (CMOs) of 1–3, which are compared with those of D2d B40. Upon attachment of the first Li atom on B40, the lowest unoccupied molecular orbital (LUMO) of B40 (Fig. 4d) becomes half-filled due to charge transfer, which are the singly occupied molecular orbitals (SOMOs) in 1 and 2 (Fig. 4a,b). Note these three CMOs are virtually identical. Likewise, in line with their lower symmetry, LUMO (a′) of 1 and LUMO (b2) of 2 correspond to the degenerate LUMO + 1 of D2d B40. In D2d Li6&B40(3) (Fig. 4c), six electrons are transferred from Li to the B40 cage, which successively occupy the LUMO and degenerate LUMO + 1 of D2d B40. The latter LUMO + 1 become the highest occupied molecular orbital (HOMO) in 3, which are also doubly degenerated due to the same high symmetry of D2d. As a consequence, the LUMO + 2 (b1) of D2d B40 becomes the LUMO in 3. The spatial distributions of the frontier CMOs in these four species are remarkably similar, demonstrating the electronic robustness of the B40 cage motif along this series. The calculated Wiberg bond indices associated with Li are all less than 0.30 in 1–3, which further indicate that Li does not participate in the covalent bonding of B40.

Figure 4
figure 4

Molecular orbital pictures of the LUMOs and HOMOs (or SOMOs) of Cs Li&B40(1), C2v Li&B40(2), and D2d Li6&B40(3), as compared with those of borospherene D2d B40.

The orbitals are aligned according to their shapes.

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The calculated HOMO-LUMO energy gaps of 1, 2, and 3 are 1.39, 1.41, and 1.42 eV, respectively, which differ from that of D2d B40 (3.13 eV at the same level), suggesting the possibility to tune the electronic properties of borospherenes via metal-decoration, similar to the case of C60 buckyball.

H2 Adsorption on Li-Decorated B40

The Cs Li&B40(1), C2v Li&B40(2), and D2d Li6&B40(3) clusters are well-defined molecular systems for the adsorption of H2 molecules. H2 can be added successively to the systems, from one H2 molecule up to six per Li site. The largest complex corresponds to the adsorption of 34 H2 molecules to 3, that is, Li6&B40(3)-34 H2. Tables S1 and 2 summarize the AEs and the equilibrium Li–B, Li–H, and H–H distances for the Lim-B40-nH2 (m = 1, 6; n = 1–34) complexes.

When one H2 molecule is introduced to 1, due to the polarization interaction between the charged Li atom and the H2 molecule, the Li–B bond distance is slightly enlarged (by 0.01 Å) to 2.35 Å. The H–H distance is found to be 0.76 Å. The equilibrium Li–H distance is 1.97 Å, which is comparable to the value of 2.04 Å in the case of a free Li+ ion interacting with H246. The AE of the first H2 to 1 is 0.25 eV, which is in quantitative agreement with that in Li+H2 (0.25 eV)46.

With more H2 molecules being attached to 1, the average AE of H2, consecutive AE of H2, the Li–B distance, and the distances between H2 and Li change accordingly. As shown in Table S1 and Fig. 5a, a single Li atom in 1, coated on a heptagonal hole, can adsorb up to six H2 molecules with an average AE of 0.11 eV/H2. From one to six H2, the average AE decreases from 0.25 to 0.11 eV/H2, whereas the consecutive AE decrease from 0.25 to 0.05 eV/H2. This effect may be partially due to the steric repulsion47 when the number of H2 molecules increases. In line with this trend, the Li–B distances are elongated gradually from 2.35 to 2.43 Å. However, the H–H bond distance is nearly constant in the range of 0.75–0.76 Å, which is the value of free H2 molecule, consistent with the nature of molecular adsorption. The Li–H distances span a rather wide range from 1.97 to 2.91 Å. Notably, there is an abrupt increase in the Li–H distances from 1–3 H2 to 1–4 H2, so that the first three H2 are closer to the Li site than the next three. In other words, the adsorption of the first three H2 molecules forms an inner core with Li, upon which the additional H2 molecules adsorb loosely. The data of consecutive AE confirm this to be indeed the case: The first three H2 possess an AE of 0.25–0.11 eV, in contrast to 0.04–0.05 eV for the next three (Table S1). In fact, the structure of 1–4 H2 can be constructed on the basis of 1–3 H2 by adding one H2 on the top of Li. However, when the fifth and sixth H2 are put on successively in 1–5 H2 and 1–6 H2, they flee away after structural optimization as shown in Fig. 5a. Therefore, the Li site in 1 may adsorb three H2 molecules comfortably, whereas additional H2 are only physisorbed.

Figure 5
figure 5

Top and side views of the optimized configurations for successive addition of H2 molecules on (a) Li&B40(1) with a Li-coated heptagonal hole and (b) Li&B40(2) with a Li-coated hexagonal hole.

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Basically, the adsorption of H2 on 2 is rather similar to that on 1. Up to five H2 molecules may be adsorbed on the Li site in 2 (Fig. 5b). Again, the first three H2 are located closely to Li, with the fourth H2 being situated symmetrically on top of Li at a substantially larger distance. For the 2–5 H2 case, there is a structural rearrangement for the H2 molecules, suggesting that this Li site can potentially adsorb up to four H2 molecules at a reasonable strength. Nonetheless, the fifth H2 only interact with Li loosely. On the basis of the consecutive AEs (0.22–0.17 eV for the first three H2; Table S1), we conclude that the Li site in 2 is capable of adsorbing at least three H2 molecules with further possibility for a fourth, whereas additional H2 molecules should be considered physisorbed.

Based on the above results of H2 adsorption on single Li-decorated B40, we constructed and optimized the H2 adsorption configurations on the Li6&B40(3) complex, which aims at exploring the hydrogen storage capacity. The starting configurations were constructed by attaching the corresponding H2 molecules around Li atoms above the 4 heptagonal and 2 hexagonal holes on the B40 cage. Successively, 6 H2, 12 H2, 18 H2, 24 H2, and up to 34 H2 are adsorbed on Li6&B40(3), whose optimized structures are shown in Fig. S2 and 6. The former four cases correspond to the adsorption of one to four H2 on each Li site. For the 34 H2 case, that is, Li6&B40-34 H2(4), 6 H2 are adsorbed on each heptagonal Li site and 5 H2 are on each hexagonal Li site, as depicted in Fig. 6. The total interaction energy of 34 H2 in 4 is 3.43 eV, yielding an average AE of 0.10 eV/H2. The calculated consecutive AEs are collected in Table 2, which reflects the adsorption nature more faithfully. Similar to 1 and 2, the first three H2 for each Li in 4 are located close to the adsorption site, resulting in the 3–6 H2, 3–12 H2, and 3–18 H2 complexes with reasonable AEs of 0.18–0.11 eV/H2. Additional H2 molecules in 3–24 H2 and 4 complexes are farther away for the Li sites with relatively weak AEs of 0.03–0.05 eV/H2, hinting physisorption in nature. In summary, up to 34 H2 molecules may be adsorbed in 4, among which the first 18 interact in moderate strength with the Li site, corresponding to a gravimetric density of 7.1 wt%.

Table 2 Calculated Average and Consecutive Adsorption Energy (AE) of H2, Bond Distances of Li–B, Li–H, and H–H in the Li6&B40-nH2 (n = 6, 12, 18, 24, 34) Complexes.
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Figure 6
figure 6

Top and side views of the optimized configuration of Li6&B40-34 H2(4).

The B atom is in orange, Li in purple, and H in white.

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H2 Adsorption on Li-Decorated B40H16

Borospherene B40 as an electron-deficient system36 is generally considered to be more reactive than C60, we can thus engineer and passivate B40 at least partially with B–H bonds, which may benefit the adsorption and release of H2 molecules. B40 has 16 tetracoordinate and 24 pentacoordinate sites, where the former are anticipated to be more reactive. A model B40H16 cage cluster is readily constructed via 16 B–H bonds for the tetracoordinate B sites, which can also be decorated with six Li atoms, resulting in a D2d Li6&B40H16(5) complex as depicted in Fig. 7. The Li–B distance in 5 remains to be 2.33 Å, which is very close to that in 3. In complex 5, each Li atom carries a charge of 0.88 |e|. Interestingly, the B–H bonds markedly alter the Li-decoration properties in 5 and the average BE of Li atom now increases to 4.17 eV per Li, compared to 3.07 eV in Li6&B40(3).

Figure 7
figure 7

Top and side views of the optimized configurations of Li6&B40H16(5) and Li6&B40H16-34 H2(6).

The B atom is in orange, Li in purple, and H in white.

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Li6&B40H16(5) can also adsorb from 6 H2, 12 H2, 18 H2, 24 H2, and up to 34 H2 molecules, resulting in a series of 5-nH2 complexes (Fig. S3). The optimized structure for Li6&B40H16-34 H2(6) is shown in Fig. 7. Note that hydrogen remains in the molecular state with a uniform H–H distance of 0.75 Å in all 5-nH2 species. For the first 6 H2 molecules in 5–6 H2, the average AE amounts to 0.22 eV/H2. The average Li–B and Li–H distances, 2.33 and 1.97 Å, respectively, are almost the same as those in Li6&B40-6 H2 (that is, 3–6 H2). With further H2 adsorption, the average AEs for the first 18 H2 in 5-nH2 decrease slightly down to 0.17 eV/H2, which are in the ideal thermodynamic range for reversible hydrogen storage10,11. The Li6&B40H16(5) complex thus behaves rather similar to Li6&B40(3) in terms of hydrogen storage properties, except for the B–H passivation in 5. The 18 “core” H2 in Li6&B40H16-34 H2(6) represents a gravimetric density of 6.5 wt%, where an additional 8.6 wt% of dissociated H atoms and loosely physisorbed 16 H2 are not counted.

On the Possibility of Doubling the H2 Adsorption Sites: Li-Decorated Triangular B6 Corners

To further improve the hydrogen storage capacity of Li-decorated B40, we also attempted to place Li atoms on top of the close-packing, triangular B6 corner sites of the cube-like B40 cage. As a test case, adsorption of a single H2 molecule on a corner Li site is optimized (Fig. S4). The BE of Li is 1.87 eV, which is lower than those in Li&B40(1) and Li&B40(2), but the value still represents a reasonable strength. In fact, it is comparable to the corresponding value for C60 (1.80 eV)13. Moreover, the AE for the first H2 amounts to 0.28 eV, which is comparable to and even slightly greater than that in Li&B40(1)-H2 (0.25 eV) or Li&B40(2)-H2 (0.22 eV; Table S1). The above data hint that a triangular B6 corner site on B40 is as promising as, if not better than, a hexagonal/heptagonal site for hydrogen storage. The calculated consecutive AEs of Li&B40-nH2 (n = 1–3) with H2 molecules adsorbed on a corner Li site are at the range of 0.22–0.28 eV/H2. For Li8&B40-nH2 (n = 8, 16, 24) with H2 molecules adsorbed on eight corner Li site, the consecutive AEs change from 0.17 to 0.36 eV/H2, which are ideal for reversible hydrogen storage and release

In this way, one can more than double the number of sites for Li-decoration from six in Li6&B40(3) and Li6&B40H16(5) to fourteen in Li14&B40(7), owing to the eight triangular B6 corners (versus six hexagonal/heptagonal holes). The optimized structure of Li14&B40(7) is shown in Fig. 8. Here, upon Li decoration, the boron structure distorts considerably from the free-standing B40 borospherene, but the cage motif maintains. The average BE is 2.57 eV/Li. Following the strategy for Li6&B40(3) and Li6&B40H16(5), we build a series of model complexes: 7–14 H2, 7–28 H2, and Li14&B40-42 H2(8), whose optimized structures are depicted in Fig. 8. The calculated average AE for the first 14 H2 in 7–14 H2 is 0.32 eV/H2, for the second 14 H2 in 7–28 H2 is 0.22 eV/H2, and for the third 14 H2 in Li14&B40-42 H2(8) is 0.12 eV/H2, suggesting that all these H2 molecules are thermodynamically favorable for a hydrogen storage material10,11. For the extreme case of 8, a maximum gravimetric density of 13.8 wt% is obtained. We do not exclude the possibility of further H2 adsorption onto the 8 complex, albeit those additional H2 are anticipated to interact rather loosely with the Li sites.

Figure 8
figure 8

Optimized configurations of Li14&B40(7) and its H2 adsorption complexes: Li14&B40-14 H2, Li14&B40-28 H2, and Li14&B40-42 H2(8).

The B atom is in orange, Li in purple, and H in white.

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Concluding Remarks

In conclusion, we have carried out a comprehensive density-functional study on the lithium-decoration of B40 borospherene and the potential utilization of Li-B40 complexes as a novel nanomaterial for hydrogen storage. We showed that all six heptagonal and hexagonal holes on B40 surface can be decorated with Li atoms and each Li site is capable of adsorbing up to six or five H2 molecules. This results in an ultimate Li6&B40-34 H2 complex, in which 18 H2 are bound to Li sites with ideal adsorption energies of 0.11–0.18 eV per H2, corresponding to a gravimetric density of 7.1 wt%. The additional 16 H2 are physisorbed in nature. We further showed that the eight close-packing, triangular B6 corner sites on the B40 cage are also readily decorated with Li, which more than double the number of sites for hydrogen storage. The corresponding Li14&B40-42 H2 complex can store all 42 H2 molecules at adsorption energies of 0.12–0.32 eV per H2, suggesting a maximum gravimetric density of 13.8 wt%. The Li-B40-H2 complexes as a hydrogen storage material differ markedly from the prior Li-C60-H2 and Ti-B40-H2 systems. The Li-C60-H2 complex13 adsorbs H2 rather loosely and is thus not efficient for hydrogen storage, whereas the Ti-B40-H2 complex40 bounds H2 too strongly, for which a substantial portion of H2 stored are not reversible for release. In fact, preliminary attempts also suggest that the structural integrity of B40 unit is maintained when they are allowed to interact with each other. Considering the presence of chemical bondings between them, we forecast it is possible to construct boron-based nanomaterials for hydrogen storage using lithium-decorated B40 unit as a building block or connecting the exohedral metalloborospherene with organic linkers. And the hydrogen storage capacity of the boron-based nanomaterials could be better than previously reported carbon-based counterparts.

Methods

All calculations were based on DFT, using a plane-wave basis set with the Projector Augmented Wave (PAW)48,49 pseudopotential method as implemented in the Vienna ab initio Simulation Package (VASP)50,51. Generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)52 functional was adopted to treat the electron exchange correlation. The GGA-PBE method has been previously utilized to treat Li-decorated fullerenes and heterofullerenes for hydrogen storage19,53 which is thus a suitable choice for our current system. The dispersion corrected DFT (DFT-D) scheme54,55,56 was used to describe the van der Waals (vdW) interaction. The supercell approach was used, where the B40-based systems were placed at the center of a 25 × 25 × 25 Å3 vacuum space. And only the Γ point was used to sample the Brillouin zone. The energy cutoff for the plane-wave basis set was set to 500 eV. All structures were fully relaxed until the force acting on each atom was less than 10−2 eV/Å and a tolerance in total energy was at least 10−4 eV.

The binding energies (BEs) for the Li-decorated B40 are defined as Eb = −(ELi-B40 −EB40 − mELi)/m, where ELi-B40 is the total energy of Li-decorated B40, EB40 and ELi are the total energies of an isolated B40 and a Li atom, respectively, and m is the number of Li atoms. Similarly, the average AE for H2 is defined as Ea = −(ELi-B40-nH2 − ELi-B40 − nEH2)/n and the consecutive AE is defined as ΔE = −(ELi-B40-nH2 − ELi-B40-(n-1)H2 − EH2), where ELi-B40-nH2 and ELi-B40-(n-1)H2 are the total energies of n and (n–1) H2 adsorbed on the Li-decorated B40, respectively. ELi-B40 also represents the total energy of Li-decorated B40, EH2 is the total energy of isolated H2 molecule, and n stands for the number of adsorbed H2 molecules.

We note that for comparison with D2d B40 in our previous work (ref. 36), the HOMO-LUMO energy gaps of 1, 2, and 3 were calculated using the Gaussian 09 package57, which is usually used for calculations on the isolated molecules. And the corresponding structures were optimized at the PBE0 levels with the 6–311 + G* basis set58,59, which has been benchmarked in prior works as a reliable method for boron clusters.

Additional Information

How to cite this article: Bai, H. et al. Lithium-Decorated Borospherene B40: A Promising Hydrogen Storage Medium. Sci. Rep. 6, 35518; doi: 10.1038/srep35518 (2016).