Introduction

Hydrogen has long been considered as a clean, abundant and efficient energy carrier1,2. Developing appropriate storage media is of the importance for practical application of hydrogen energy. As an earth-abundant element, boron is widely applied for hydrogen storage with its chemical hydrides and nanostructural forms3. Boron-based chemical hydrogen storage materials such as borohydrides (e.g., LiBH4 and NaBH4) are promising compounds because of their high hydrogen capacities4,5,6. However, due to kinetic and/or thermodynamic limitations, the chemical hydrides suffer from poor reversibility7, there are still difficulties in practical application of borohydrides8. An efficient solution is to find suitable all-boron nanostructures as replacement.

Since the bulk boron cannot be found in nature, the design and synthesis of bulk boron allotropes still keeps challenging to theoretical and experimental chemists. It attracts more interest on all-boron fullerenes after the theoretical prediction of B80 fullerene9, which is a hollow cage-like cluster resembling the C60. It is revealed that all of the boron allotropes are based on different arrangements of B12 icosahedrons9,10. After that, various types of boron fullerene nanostructures were proposed and simulated by theoretical calculations, such as B32 + 8n (from B32 to B80)11, B32 + 6n (from B32 to B56)12, 80 n2 boron fullerenes series (from B80 to B2000)13, B10014, etc.

Boron fullerenes are seen as efficient hydrogen storage media since they are light-weight and have the capability to bind with metal adatoms. Combined with the fact that isolated transition metal (TM) has the ability to bind a certain number of hydrogens in molecular form, theoretical simulations on hydrogen adsorption by metal-adsorbed boron fullerenes were reported15,16,17. By density functional theory (DFT) calculations, Li et al.16 declaimed that Ca-coated B80 fullerene can store up to 8.2 wt% H2 with an adsorption energy of 0.12-0.40 eV/H2. Before that, Zhou et al.17 reported the hydrogen adsorption on alkli-metal (Na, K) doped B80. They found that B80Na12 and B80K12 show fairly low adsorption energies (0.07 eV/H2 and 0.09 eV/H2), indicating that alkli-metal is unsuitable for hydrogen storage. So far, all the theoretical investigations are based on the “proposed” boron fullerenes. Their applications in hydrogen storage may be unfeasible due to the uncertainty of the adsorbents.

Recently, an all-boron fullerene-like cage cluster B40− was produced and observed18. Its neutral counterpart B40 exhibits the fullerene-like cage (D2d symmetry) and is calculated to be the most stable structure among the B40 allotropes. The relevant theoretical simulation indicates that B40 fullerene is thermally stable at temperature as high as 1000 K18. This is the first experimental evidence for the existence of all-boron fullerene.

For the hydrogen storage materials, transition metal (TM) atoms are important components due to their strong attraction to hydrogen molecules19,20,21,22. Among the TMs, titanium (Ti) is regarded as an ideal binding metal in nanomaterials since it takes great advantages in hydrogen storage, which has been concluded16. Because of the outstanding performance in hydrogen storage, Ti-decorated nanostructures have been widely reported19,23,24,25,26,27,28,29,30,31,32. However, previous computational researches on hydrogen storage of B8016,17 indicated that Ca is the appropriate adsorbate for boron fullerene due to the stable adsorption and high storage capacity. So which kind of metal atom would be the best adsorbate for B40 as hydrogen storage material? Here we perform DFT calculations on the binding capability of different metal atoms (Ca and TM: Sc, Ti, V, Fe, Co, Ni, Cu) decorated B40 fullerene. The simulations on hydrogen storage by metal-decorated B40 fullerene are also carried out.

Results and Discussion

The surface of B40 fullerene contains 48 boron triangles, embedded by 4 heptagonal rings and 2 hexagonal rings. The hexagons are planar while the heptagons are non-planar. We placed metal atoms on different sites of surface of B40 and calculated the binding energy (Ebind) following

where n is the number of metal adatom coated on B40. EM, EB40 and EnM@B40 stand for the total energies of metal adatom, B40 and the metal-coated B40 complex, respectively. We first calculated the binding energies of single metal atom on different binding sites of B40, including the centers of hexagon and heptagon, as well as the B-B bridges around hexagon (B1) and heptagon (B2). We take 8 different metal adatoms (Sc, Ti, V, Fe, Co, Ni, Cu and Ca) for comparison. As shown in Fig. 1, the centers of hexagon and heptagon are confirmed as the energy-favorable sites due to the significantly higher Ebind than sites B1 and B2. Ca atoms even cannot stably bind to the B-B bridges. To avoid the metal adatoms forming cluster on surface of B4023,33, the metal species should meet the requirement that the binding energies are higher than their corresponding crystalline cohesive energies (Ecoh)19,34.

Figure 1
figure 1

The binding energy (Ebind) of single metal adatom on different binding sites of B40, 8 different metal adatoms are used as comparison. B1 and B2 represent the B-B bridge sites around hexagon and heptagon, respectively. The “hex” and “hept” are marked to denote the location of hexagons and heptagons. Pink ball: boron atom, grey ball: metal atom.

Full size image

Figure 1 indicates that Sc, Ti and Ni show higher binding energies with B40 than their cohesive energies, both on the centers of hexagon and heptagon. Thus Sc, Ti and Ni could be used as good adsorbates to decorate B40. The average binding energies of 1-6 metal adatoms (Sc, Ti and Ni) on different facets of B40 are listed in Table 1.When there are more than 4 Sc atoms, the Sc-coated B40 complexes will distort and the cause instability of the fullerene-like substrate. Oppositely, the introduction of more Ti and Ni atoms will not affect the geometric structure of B40 significantly. When all of the hexagonal and heptagonal facets are coated by Ti or Ni atoms, the Ti6B40 or Ni6B40 complexes keep stable and provide high Ebind.

Table 1 The average binding energies (Ebind) of 1-6 metal adatoms (Sc, Ti and Ni) on different facets of B40, the cohesive energies (Ecoh) of the metal are shown as comparison47
Full size table

It is worth noting that due to the differences in valence electron configuration, Ni and Ti show significantly different bonding structures and binding energies with the different facets of B40. By comparing the binding energies with equal number of metal atom in Table 1, it can be concluded that Ti is more energy-favorable on hexagon, while Ni is more energy-favorable on heptagon. To reveal the bonding rules, we performed Mayor bond order35 analysis on single Ti- and Ni-decorated B40 fullerene, as shown in Fig. 2. Different binding conformations on hexagonal ring and heptagonal ring are named M@hexagon and M@heptagon, respectively. The bonding structures reveal that Ni covalently bonds with all the surrounding boron atoms, but Ti only forms 4 and 3 stable covalent bonds (with bond order value larger than 0.5) when binds to hexagon and heptagon respectively. Considering their valence electron configurations (Ti: 3d24s2, Ni: 3d84s2), the rich valence electrons determine that Ni can form as much as 7 weak Ni-B covalent bonds, while Ti only forms up to 4 Ti-B covalent bonds due to its 4 valence electrons. Ti-B average bond length (~2.17 Å) is longer than Ni-B average bond length (~2.0 Å), which explains why the Ti-coated hexagon expands in Fig. 2(a) compared with Ni@hexagon in Fig. 2(c). However, the Ti-coated heptagon changes slightly, mostly due to its non-planar arrangement of boron atoms. Ti@hexagon shows higher stability than Ti@heptagon since there are more Ti-B covalent bonds. Similarly, Ni@heptagon is more stable than Ni@hexagon because of the 7 covalent bonds. This is the reason why Ti is more energy-favorable on hexagon while Ni is more energy-favorable on heptagon.

Figure 2
figure 2

Bonding structures of single Ti- or Ni-decorated B40: (a) Ti@hexagon, (b) Ti@heptagon, (c) Ni@hexagon and (d) Ni@heptagon. Covalent M-B bonds are shown with the bond order values (digits in blue color).

Full size image

Another important finding is that the average binding energy is related with the number of metal adatoms on different facets. That is, for Ti-decorated B40 fullerene, the average binding energy increases as the number of Ti atoms on heptagon increases and decreases as the number of Ti atoms on hexagon decreases. Differently, for Ni-decorated B40 fullerene, the average binding energy decreases as the number of Ni atoms increases for both binding sites. It can be inferred that there exists attractive interaction between the decorated Ti atoms, while the interaction between the coated Ni atoms is repulsive. In summary, when all the hexagonal and heptagonal rings are embedded by metal atoms, the binding of Ti will be stronger than Ni and also the strongest among the chosen metal species. The stable binding of Ti on B40 leads to promising applications of the Ti-decorated B40 fullerene. Here we consider it as a suitable candidate for hydrogen storage.

According to the well-known 18-electron rule19,36, the maximum number of adsorbed hydrogen molecules (Nmax) is limited by the valence electrons that participating in covalent bonds. For metal-decorated B40 fullerenes we design here, the 18-electron rule can be specified as

where nv(M) represents the valence electron number of the metal element, nv(B40) represents the electrons contributed by B40, which is 4 for Ti@hexagon and 3 for Ti@heptagon. The Nmax is calculated to be 5 and 5.5 for Ti@hexagon and Ti@heptagon, which demonstrates that the single Ti-decorated B40 can store up to 5 and 6 H2 molecules when Ti atom binds to hexagon and heptagon, respectively. However, the Nmax is calculated to be 1 for Ni-decorated B40 fullerene. Obviously the Ni-derad B40 fullerene is inefficient as hydrogen storage medium.

We use average adsorption energy (Eads) to evaluate the adsorption capability of the Ti-decorated B40 fullerene. We also define consecutive adsorption energy (ΔE) as the energy gained by successive additions of H2 molecules to evaluate the reversibility for storage of H2 molecules. They are calculated based on the following formulas

and

where n stands for the number of adsorbed H2 molecules. ETi@B40 and EH2 are the total energies of Ti-decorated B40 and H2 molecule. ETi@B40 + nH2 and ETi@B40 + (n-1)H2 are the total energies of Ti-decorated B40 with n and (n-1) H2 molecules, respectively. For efficient hydrogen storage at ambient conditions, the ideal adsorption energy should be in the range of 0.16-0.42 eV/H237,38 to realize reversible adsorption and desorption. This energy range leads to intermediate between physisorption and chemisorptions16.

The calculated Eads and ΔE are summarized in Table 2. With all of the ΔE larger than 0.2 eV/H2, our simulations confirm that the maximum adsorption numbers of H2 molecules can reach 5 for Ti@hexagon and 6 for Ti@heptagon, respectively. For H2 adsorption on Ti@hexagon, the first H2 molecule exhibits significantly larger adsorption energy than the following H2 molecules. Addition of the second to fifth H2 molecule gains energies within 0.2-0.3 eV per H2 and they are adsorbed around the first H2, as shown in Fig. 3(a). Our analysis on the Ti-H2 distance reveals that for the Ti@hexagon, the first added H2 molecule keeps a close distance to the Ti atom (1.950 ~ 1.975 Å in Table 2). Particularly, affected by the 4 surrounding H2 molecules, the 1st H2 molecule of 5 H2 molecules adsorbed Ti@hexagon will be closer to the Ti atom. Moreover, as shown in Fig. 3(b), the first H2 molecule always shares the highest occupied molecular orbital (HOMO) with the adsorbent, indicating the strong chemical adsorption between the first H2 molecule and Ti@hexagon.

Table 2 Calculated average adsorption energies (Eads) and consecutive adsorption energies (ΔE) by the successive addition of H2 molecules to Ti@hexagon and Ti@heptagon, as well as the distance between Ti atom and the first (Ti-1st H2) or second (Ti-2nd H2) added H2 molecule.
Full size table
Figure 3
figure 3

(a) & (c) Top view of successive addition of H2 molecules on Ti@hexagon & Ti@heptagon. (b) & (d) HOMO distributions on Ti@hexagon & Ti@heptagon with H2 molecules adsorbed, the HOMO isovalue is set as 0.03 e/Å3. Pink ball: B atom, grey ball: Ti atom, white ball: H atom.

Full size image

The case of adsorption on Ti@heptagon is different. As we can see in Table 2, the first and second H2 molecules both show strong binding to the Ti atom. This can be attributed to the extra 3d electron of Ti, which doesn’t participate in forming covalent Ti-B bond. Figure 3(d) indicates that the Ti 3d orbital overlaps with the H 1s orbital when there is one or two H2 molecules adsorbed. With the addition of third H2 molecule, the overlapping between Ti and H2 is interrupted. From the addition of 3rd to 6th H2 molecule, the HOMOs only distribute on surface of Ti@heptagon, indicating the weakening of the H2-Ti interaction. On the other hand, distances between Ti and the first two H2 molecules become significantly larger with the addition of 3rd to 6th H2 molecules, which is consistent with the HOMO analysis. Addition of the third to sixth H2 molecules gains consecutive adsorption energies within 0.3–0.4 eV per H2, which also meets the requirement for reversible uptake and release of H2 molecules.

As displayed in Fig. 3, it should be pointed out that either the geometric structures or the distribution of HOMOs of the adsorption substrate (Ti-decorated B40 fullerene) keep stable and are little changed with the increasing of adsorbed H2 molecules, revealing the high stability of Ti-decorated B40 fullerene. The geometric and electronic structure of the substrate is little affected by the addition of H2 molecules, which is important for the realization of reversible hydrogen storage.

To check if the first adsorbed H2 molecule will dissociate into two hydrogen ions on centered Ti atom and form dihydride complex, as mentioned in similar work19,22,24,25,27, we also modeled the dihydride contained complexes (B40TiH2) as initial configurations and performed full geometry optimization. Our simulation results (as displayed in Figure S1) show that the dihydride complex is less stable than our determined local minimum (about 1.10 eV higher in total energy). Meanwhile, singlet state should be considered as the ground state for dihydrogen adsorbed Ti@B40 complexes due to the higher stability. The dihydrogen molecule with a slight elongation of H-H is determined as the local minimum for adsorption of the first H2 molecule on Ti-decorated B40.

To look insight of the influence of B40 in adsorbing hydrogen, we checked all the possible adsorption sites of undecorated B40 for H2 adsorption. Calculation results show that the B40 fullerene itself is unsuitable for H2 adsorption with Eads ranges from 0.15 eV to 0.20 eV (as listed in Table S1). All of the distances from the adsorbed H2 to B40 surface are larger than 2.8 Å, indicating the nature of weak physisorption. To see whether the H2 molecule will transfer from Ti to B40 when adsorbs to Ti-decorated B40, the possibility of H2 adsorption onto B40 in Ti-decorated B40 (Ti6B40) is also checked. Our simulations elucidate that comparing with the H2 adsorption on Ti atoms, the H2 adsorption on B40 is rather weaker with Eads around 0.2 eV (Table S1). Adsorption energies of H2 on B40 in Ti6@B40 complex enhance slightly compared with the undecorated one (for the same adsorption site), indicating that the decoration of Ti atoms won’t improve the adsorption performance of B40 for H2 much. For our modeled Ti6B40 complexes, the Ti atoms exhibit high attraction for hydrogen molecules due to the high localization of FMO on them, as shown in Figure S2. This localization won’t be significantly affected by the increasing H2 molecules, making the transfer of H2 molecule to B40 difficult to happen.

Based on the calculation results of hydrogen adsorption on single Ti-decorated B40, we constructed and optimized the adsorption configuration of H2 molecules on Ti6B40 complex. As shown in Fig. 4 (the atomic coordinates of the optimized Ti6B40 and Ti6B40 with 34 H2 molecules adsorbed are listed in Table S2 and S3), up to 34 H2 molecules are adsorbed around the Ti adatoms [named Ti6B40(H2)34]. Our calculated gravimetric density of hydrogen stored by Ti6B40 can reach 8.7 wt%, with an average adsorption energy of 0.37 eV/H2. As we have mentioned above, the first H2 molecule on Ti@hexagon and the first two H2 molecules on Ti@heptagon have stronger binding with the Ti atoms than the following H2 molecules. We measured the average distance between the H2 molecules and the corresponding Ti atoms for Ti6B40(H2)34. For H2 adsorption on hexagon-embedded Ti atoms, the average distance of the 1st H2 molecules to Ti atom is 1.952 Å, almost the same distance with the occasion of 5 H2 molecules adsorbed Ti@hexagon. However, for H2 adsorption on heptagon-embedded Ti atoms, the average distances of the 1st and 2nd H2 molecules to Ti atom are 2.052 Å and 2.358 Å, respectively. The values are significantly larger than the occasion of 6 H2 molecules adsorbed Ti@heptagon, indicating the repulsive interaction from H2 molecules on other facets. Analysis on H2-Ti distance demonstrates that the increase of H2 molecule mainly affects the hydrogen adsorption on heptagon-embedded Ti atoms, which is the origin of reduction of the average H2 adsorption energy.

Figure 4
figure 4

The optimized structure of Ti6B40 complex with 34 H2 molecules adsorbed. Pink ball: B atom, grey ball: Ti atom, white ball: H atom.

Full size image

Evaluating from our calculation results on successive addition of H2 molecules, among the 34 adsorbed H2 molecules on Ti6B40, 24 of them have moderate adsorption energies within the range of 0.2-0.4 eV/H2, corresponding to a reversible storage capacity of 6.1 wt%. It is notable that the bonding type and geometric structure of the B40Ti6 complex is also little affected by the adsorption of H2 molecules. The favorable consecutive adsorption energy assures the reversible storage of hydrogen molecules under ambient conditions.

B40 is a newly discovered boron nanostructure and also the first experimentally observed all-boron fullerene. Here we performed computational investigations on hydrogen storage capacity of Ti-decorated B40 fullerene. Comparative calculations reveal that, among the chosen metal species, Ti exhibits the strongest binding on surface of B40. Ti-decorated B40 fullerene exhibits strong adsorption and high capacity for H2 molecules. Single Ti decorated B40 fullerene can store up to 5 and 6 H2 molecules for Ti@hexagon and Ti@heptagon, respectively. All of the adsorption happens on Ti atom and B40 shows weak capability in adsorbing H2 molecules. This leads to a maximum storage capacity of 34 H2 molecules for Ti6B40 complex with an average adsorption energy of 0.37 eV/H2, corresponding to a gravimetric density of 8.7 wt%. The consecutive adsorption energy of H2 molecules within the range of 0.2–0.4 eV/H2 assures the reversible storage of 6.1 wt% under ambient conditions. Our computational investigations confirm that the Ti-decorated B40 fullerene is favorable for hydrogen adsorption, which makes it promising as a new hydrogen storage material.

Methods

Density functional theory (DFT) calculations are carried out by DMol3 program39,40. The generalized gradient approximation (GGA) functional by Perdew and Wang (PW91)41, along with a double numerical basis set including p-polarization function (DNP), is applied for the geometry optimization and property calculations. Dispersion-corrected DFT (DFT-D)42,43,44 scheme put forward by Ortmann, Bechstedt and Schmidt (OBS)45 is used to describe the van der Waals (vdW) interaction. DFT semi-core pseudo-potentials (DSPPs)46 are employed to efficiently treat with the core electron of TM element after Ca. Self-consistent-field (SCF) convergence tolerance is set to 1 × 10−6 Ha. The convergence threshold values are specified as 1 × 10−5 Ha for energies, 2 × 10−3 Ha/Å for gradient and 5 × 10−3 Å for displacement, respectively.

Reliability of PW91/DNP level in treating metal-boron system has been proven by Zhou et al.17 in calculating the binding of alkli-metal (AM) on B80 fullerene as well as the hydrogen storage capacity of B80-AM complexes. The incorporation of DFT-D scheme further improves the accuracy in evaluating weak interactions.

Additional Information

How to cite this article: Dong, H. et al. New Ti-decorated B40 fullerene as a promising hydrogen storage material. Sci. Rep. 5, 09952; doi: 10.1038/srep09952 (2015).