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High-capacity hydrogen storage in Li-decorated (AlN)n (n = 12, 24, 36) nanocages
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High-capacity hydrogen storage in Li-decorated (AlN)n (n [ 12, 24, 36) nanocages Guangzhao Wang, Hongkuan Yuan*, Anlong Kuang, Wenfeng Hu, Guolin Zhang, Hong Chen* School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
article info
abstract
Article history:
The capability of Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages for hydrogen storage has
Received 14 August 2013
been studied by using density functional theory (DFT) with the generalized gradient
Received in revised form
approximation (GGA). It is found that each Al atom is capable of binding one H2 molecule
15 December 2013
up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy
Accepted 22 December 2013
of 0.189, 0.154, and 0.144 eV/H2 in the pristine (AlN)n (n ¼ 12, 24, 36) nanocages, respectively.
Available online 20 January 2014
Further, we find that Li atoms can be preferentially decorated on the top of N atoms in
Keywords:
each Li atom with an average binding energy of 0.145, 0.154, 0.102 eV/H2 in the Lin(AlN)n
(AlN)n nanocages
(n ¼ 12, 24, 36) nanocages, respectively. Both the polarization of the H2 molecules and the
Li decoration
hybridization of the Li-2p orbitals with the H-s orbitals contribute to the H2 adsorption on
(AlN)n (n ¼ 12, 24, 36) nanocages without clustering, and up to two H2 molecules can bind to
Hydrogen storage
the Li atoms. Thus, the Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages can store hydrogen up
Density functional theory
to 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of H2 molecules lying in the range of 0.1e0.2 eV/H2 are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. It is also pointed out that when allowed to interact with each other, the agglomeration of Lidecorated (AlN)n nanocages would lower the hydrogen storage capacity. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is being considered as an ideal energy carrier due to its efficiency, abundance, and environmental friendliness. To achieve economical feasibility, hydrogen storage materials with high gravimetric and volumetric densities, as specified by the Department of Energy (DOE) goal of 9.0 wt% mass ratio and 81 g/L volumetric capacity by 2015 for mobile application [1], must be developed, and hydrogen recycling should be performed reversibly under ambient conditions [2e4]. Therefore, hydrogen storage materials should be composed of
elements lighter than aluminum, and the binding energy per hydrogen molecule should be within the range of 0.1e0.3 eV [5,6]. For more than a decade a large number of studies have been devoted to finding and designing such storage materials. Carbon-based nanostructures have been widely studied as potential hydrogen storage media, such as carbon nanotubes [7e12], carbon fullerenes [13e17], graphenes [18e22]. However, the hydrogen storage capacity of these nanostructures greatly diminishes at room temperature and ambient pressure due to the weak physical absorption of H2 molecules [2] as like metaleorganic frameworks [23e26]. One possible approach to
* Corresponding authors. Tel.: þ86 23 68367040. E-mail addresses: [email protected] (H. Yuan), [email protected] (H. Chen). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.138
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increase their chemical activity is to decorate the nanostructures with metal atoms to improve storage performance. Decorating carbon nanostructures with early transition metals [27e32] has been presented to enhance the hydrogen adsorption energies up to 0.2e0.6 eV/H2 via Kubas interaction (hybridization of the d states with the H2 states) [33,34]. Unfortunately, theoretical studies show that transition-metals decorated on carbon surfaces undergo clustering during reversible hydrogenation [27,35]. The alkali metals and alkaliearth metals can produce a uniform decoration due to their lightweight and lower cohesive energies than those of transition metals. Although the Li-decorated C60 nanocage was demonstrated by Sun et al. [36] to bind up to 40H2 molecules with an approximate gravimetric density of hydrogen storage of 9 wt%, the average binding energy of H2 molecules is 0.075 eV/H2, which is so small that the storage system has to be operated at lower than ambient temperature. Theoretical studies have found that the hydrogen storage capacity of Cadecorated C60 nanocage [37], graphene [38], nanotube [39] is more than 5 wt%. However, a recent study claimed that the density functional theory calculations overestimated significantly the binding energy of H2 molecules onto the Ca1þ cation centers [40]. Therefore, the challenge has been to find novel nanostructures where clustering of metal atoms can be prevented without compromising the binding energy of hydrogen. Recent efforts have been paid to BN and AlN nanostructures. A recent report claimed that the BN nanotubes could bind hydrogen more strongly than carbon nanotubes [41], which is attributed to the heteropolar bonding in boron nitride. The Li doped BN fullerene was found to hold 18H2 molecules up to 3.7 wt% with an average binding energy of 0.146 eV/H2 [42], and the enhancement of the binding energy of H2 molecules is attributed to the polarization of the H2 molecules induced by positively charged Li atoms. With the synthesis of AlN nanostructures in experiment [43e47], much attention has been devoted to the capacity of AlN nanostructures for hydrogen storage. The hydrogen chemisorption of single-walled AlN nanotubes has been investigated by density functional theory calculation [48], indicating that a single H atom chemisorption introduces spin-polarized magnetic moment to hydrogenated AlN nanotubes. The adsorption of atomic and molecular hydrogen on AlN nanowires has been investigated by Li et al. [49] by using the first-principles calculations, and it was found that the hydrogen atoms prefer to adsorb on top of neighboring threefold-coordinated N and Al atoms in pairs, while the hydrogen molecule prefers to adsorb on top of threefold-coordinated Al atoms in the nanowire. The AlN nanostructures (nanocages, nanocones, nanotubes, and nanowires) can bind H2 molecules primarily due to polarization mechanism, leading to a hydrogen storage capacity of 4.7 wt% [50]. Especially, the Ni-decorated (AlN)12 nanocage was studied [51] as a hydrogen storage media from density functional theory calculations, indicating that high weight percentage hydrogen storage cannot be achieved in NiAl12N12 nanocage due to the thermodynamical unstability. In present work, we have used the first-principles calculations to systematically investigate the possibility of Li decorated on (AlN)n (n ¼ 12, 24, 36) nanocages and the hydrogen adsorption capacity of Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages. To our knowledge, the hydrogen storage properties of alkali metal
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decorated AlN nanostructures have not been examined so far. We showed that Li can be decorated on the surfaces of (AlN)n strongly by the hybridization between Li-p orbitals and AlN nanocages without clustering, and the Li-decorated (AlN)n nanocages can absorb H2 molecules up to higher gravimetric density of hydrogen storage with the desirable binding energy.
2.
Calculational method
All our calculations were carried out by using a first-principles method based on density functional theory (DFT) [52e54] as implemented in the Vienna ab initio simulation package (VASP) [55e57]. Generalized gradient approximation (GGA) [58,59] with PW91 [60] was used to treat the exchange and correlation effects of electrons. The projector-augmented wave (PAW) method [61] was adopted to describe the core electrons. To obtain self solutions to the KohneSham equation, the Brillouin zone was sampled by (222) special mesh k-points. The cutoff energy for the plane wave-basis was set as 450 eV, and the convergence threshold was chosen to be 105 eV in energy. The binding energy of (AlN)n nanocage with hydrogen adsorption is defined as Eb1 ¼ EðAlNÞn þ iEH2 EiH2 þðAlNÞn i;
(1)
where EðAlNÞn is the energy of the (AlN)n nanocage, EH2 is the energy of a free H2 molecule, EiH2 þðAlNÞn is the total energy of the (AlN)n nanocage with i adsorbed H2 molecules. The binding energy of Li atoms decorated on (AlN)n nanocage is defined as Eb2 ¼ EðAlNÞn þ mELi EmLiðAlNÞn m;
(2)
where EðAlNÞn , ELi, and EmLiðAlNÞn are the energy of the pristine (AlN)n nanocage, the energy of an isolated Li atom, and the total energy of the system with m Li atoms decorated on the (AlN)n nanocage, respectively. The binding energy of H2 molecules adsorbed on the Li-decorated (AlN)n nanocage is defined as Eb3 ¼ EmLiðAlNÞn þ iEH2 EiH2 þmLiðAlNÞn i;
(3)
where EiH2 þmLiðAlNÞn is the total energy of mLi-decorated (AlN)n nanocage with i adsorbed H2 molecules, EmLi(AlN)n is the total energy of mLi-decorated (AlN)n nanocage, and EH2 is the energy of an isolated H2 molecule. The binding energy of the two 12Lidecorated (AlN)12 nanocage is defined as Eb4 ¼ 2E12LiðAlNÞ12 E12LiðAlNÞ12 112LiðAlNÞ12
(4)
where E12LiðAlNÞ12 is the energy of an isolated 12Li-decorated (AlN)12 nanocage, and E12LiðAlNÞ12 112LiðAlNÞ12 is the total energy of the system of two 12Li-decorated (AlN)12 nanocages interacting with each other.
3.
Results and discussions
3.1. Hydrogen adsorption on (AlN)n (n ¼ 12, 24, 36) nanocages The geometry of (AlN)12 nanocage is optimized and its eventual configuration (see Fig. 1(a)) has Th symmetry composed of
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Fig. 1 e (a) The optimized configuration of (AlN)12 nanocage. (b) The charge density difference of (AlN)12 nanocage. (c) 12H2 molecules absorbed on the (AlN)12 nanocage.
six four-numbered rings and eight six-numbered rings. There are two different types of AleN bond: the AleN bond length in four-numbered ring is 1.855 A, while the AleN bond length between four-numbered rings is 1.791 A. These calculation results are in accordance with previous studies [50,62e65]. The charge density difference with respective to free atoms (see Fig. 1(b)) indicates that the charge transfer from the Al atoms to N atoms makes the Al atoms positively charged. A charge analysis also indicates that the Al atom carries a charge of þ0.47e. As in previous studies [66], the positively charged Al ions on the cage surface become the available sites for adsorption of H2 molecules. The (AlN)12 nanocage can bind up to 12H2 molecules with an average binding energy of 0.189 eV/H2 (see Fig. 1(c)) and with a slightly stretched HeH bond length of 0.761 A. The average distance between the H2 molecule and neighboring Al atom is 2.245 A. The lengths of two different types of AleN bond become 1.857 and 1.794 A, which approach the corresponding values of the pristine (AlN)12 nanocage, indicating that the geometry of (AlN)12 nanocage is hardly changed upon adsorption of H2 molecules. The charge analysis reveals that each H2 molecule carries a charge of þ0.07e, implying that the H2 molecules transfer charge to Al atoms. Furthermore, the partial density of states (PDOS) of (AlN)12e12H2 system is shown in Fig. 2. The main peaks of Al 3s orbitals located at 7.35 and 4.45 eV hybridize with H s orbitals. Therefore, both the polarization of the H2 molecules and the orbital hybridization between H s orbitals and Al 3s orbitals are responsible for the H2 adsorption on the (AlN)12 nanocage. To know whether and how the hydrogen storage capacity of AlN nanocages depends on the cluster sizes, we have also studied the hydrogen adsorption on (AlN)24 and (AlN)36 nanocages. Previous studies [64,65] confirmed that the geometries of (AlN)24 and (AlN)36 nanocages have S4 and Th symmetries, respectively. Compared with the (AlN)12 nanocage, there are more different types of AleN bond in (AlN)24 and (AlN)36 nanocages. The (AlN)24 nanocage is composed of six four-numbered rings and twenty six-numbered rings, the AleN bond lengths vary from 1.790 to 1.858 A (see Fig. 3(a)).
While the (AlN)36 nanocage is composed of six four-numbered rings and thirty-two six-numbered rings, the AleN bond lengths vary from 1.790 to 1.837 A (see Fig. 3(d)). The charge density differences of the (AlN)24 and (AlN)36 nanocages (see Fig. 3(b) and (e)) indicates that the charge transfer from Al atoms to N atoms makes the Al atoms positively charged. Similar to (AlN)12 nanocage, each Al atom can be decorated with one H2 molecule, thus the (AlN)24 and (AlN)36 nanocages are capable of binding up to 24 and 36H2 molecules, respectively. The optimized configuration of (AlN)24 nanocage decorated with 24H2 molecules is shown in Fig. 3(c). Here the average binding energy of H2 molecules is 0.154 eV/H2, the HeH bond lengths vary from 0.752 to 0.761 A which yields an average bond length of 0.757 A, and the average distance beA. The optitween the Al atoms and H2 molecules is 2.476 mized geometry configuration of (AlN)36 nanocage decorated with 36H2 molecules is shown in Fig. 3(f). Here the average binding energy of H2 molecules is 0.144 eV/H2, the average HeH bond length is 0.756 A, and the average distance between A. It is evident that the the Al atoms and H2 molecules is 2.625 average binding energy of H2 molecules decreases slowly as the size of AlN nanocages increases, but the gravimetric
Fig. 2 e The partial density of states (PDOS) of Al and H atoms in (AlN)12e12H2 system. Fermi level is set to 0.
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Fig. 3 e (a) The optimized configuration of (AlN)24 nanocage. (b) The charge density difference of (AlN)24 nanocage. (c) 24H2 molecules adsorbed on the (AlN)24 nanocage. (d) The optimized configuration of (AlN)36 nanocage. (e) The charge density difference of (AlN)36 nanocage. (f) 36H2 molecules adsorbed on the (AlN)36 nanocage.
density of hydrogen storage of 4.7 wt% keep unchange with AlN nanocage sizes.
3.2. Hydrogen adsorption on Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages Firstly, the favorite position of a single Li atom decorated on the (AlN)12 nanocage is searched. In our calculations we consider all possible different decorated sites, as shown in Fig. 4, which are the top of the Al and N atoms (A1,A2), the hollow of the four-numbered rings (H1) and the hollow of the six-numbered rings (H2), the bridge site over the AleN bond in four-numbered rings (B1) and between four-numbered rings (B2). The most favorable decorated site is the A2 site, in which the Li atom has a larger binding energy compared with the hollow of the six-numbered rings (H2) (The binding energies of Li atom on A2 and H2 sites are 1.507 and 1.265 eV, respectively). Besides, the top of Al atom (A1), the hollow of the four-numbered ring (H1), and the two different bridge sites over AleN bond (B1, B2) turn out to be unstable. When a Li atom is decorated on these sites, it moves to the top of the N atom after structural relaxation. The fully optimized structure of the stable configuration is shown in Fig. 5(a). With the adsorption of Li atom on the A2 site, the distances between the Li atom and three neighboring Al atoms are
3.232, 3.232, and 3.076 A, respectively, while the distance between the Li atom and neighboring N atom is 1.840 A. The charge density difference of a single Li decorated (AlN)12 nanocage is shown in Fig. 5(b). The charge transfer from Li atom to (AlN)12 nanocage makes the Li atom positively charged. This can also be confirmed by the charge analysis which indicates that the Li atom carries a charge of þ0.36e. To
Fig. 4 e The different position of Li atom decorated on the (AlN)12 nanocage.
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Fig. 5 e (a) The optimized configuration of single Li atom decorated (AlN)12 nanocage. (b) The charge density difference of single Li atom decorated (AlN)12 nanocage. (c) One H2 molecule adsorbed on single Li atom decorated (AlN)12 nanocage. (d) Two H2 molecules adsorbed on single Li atom decorated (AlN)12 nanocage. (e) Three H2 molecules adsorbed on single Li atom decorated (AlN)12 nanocage.
further understand the binding mechanism of the Li atom decorated on the (AlN)12 nanocage, the partial density of states (PDOS) of the N and Li atoms in Li-(AlN)12 system are shown in Fig. 6(a). It can be seen that the hybridization of the N 2p with Li 2p orbitals from 7 to 0 eV is responsible for the formation of LieN bond, resulting in Li atoms bind to the surface of the (AlN)12 nanocage. The positively charged Li atom in Li(AlN)12 system is an available site for adsorption of H2 molecules. Upon decorating the first hydrogen molecule near the Li atom, the configuration after relaxation is shown in Fig. 5(c). It indicates that the H2 molecule can be adsorbed on the Li atom with a binding energy of 0.178 eV/H2 and with a slightly stretched HeH bond length of 0.756 A. The LieN distance is 1.868 A and the disA. tance between the H2 molecule and Li atom is about 2.172 As two H2 molecules are adsorbed to the Li atom, the optimized configuration is shown in Fig. 5(d). The average binding A, energy is 0.172 eV/H2, the average HeH bond length is 0.756 the LieN distance becomes 1.879 A, and the average distance A. To between the two H2 molecules and the Li atom is 2.182 know the maximum number of hydrogen molecules a Li atom can absorb, we doped more H2 molecules near the Li atom and optimized the geometry. Three H2 molecules can be adsorbed to a single Li atom and the fourth H2 molecule is moved to a distance of 3.946 A from the Li atom. Thus, as shown in Fig. 5(e) a single Li atom was found to adsorb up to three H2 molecules
with the average binding energy of 0.180 eV/H2 and with the average HeH bond length of 0.761 A. The LieN distance is 1.888 A and the average distance between the H2 molecules and Li atom is about 2.094 A. The analysis of the charge transfer indicates that the Li atom and the H2 molecule carry a charge of þ0.32 and þ0.07e, respectively. This indicates that the positively charged Li atom adsorbs H2 molecules primarily due to the polarization of the H2 molecule induced by the charged Li atom. While a small positively charge of H2 molecule indicates that the charge transfer from the H2 molecule to Li atom, so there exits a hybridization between the H2 molecule and Li atom. Fig. 6(b) shows that the PDOS of the Li and H atoms, from which we can see more clearly that the peak center around 9.28 eV corresponds to the hybridization of the Li 2p with the H s orbitals. Therefore, the H2 adsorption on the Li-decorated (AlN)12 nanocage is attributed to both the polarization of the H2 molecules induced by the charges of Li atom and the hybridization of the Li 2p states with H s orbitals. Now we consider the capability of 12 Li decorated (AlN)12 nanocage to store hydrogen. The fully optimized configuration is shown in Fig. 7(a). The average distances between the Li atom and three neighboring Al atoms, between the Li atom and neighboring N atom, and between two adjacent Li atoms are 2.579, 1.891, and 3.392 A, respectively. The average binding energy is 1.788 eV/Li, which is larger than in the Li2 dimer (0.95 eV/Li) and in the Li bulk cohesive energy (1.63 eV/Li). Thus
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Fig. 6 e (a) The partial density of states (PDOS) of N and Li atoms in Li-(AlN)12 system. (b) The partial density of states (PDOS) of Li and H atoms in Li-(AlN)12eH2 system. Fermi level is set to 0.
the clustering of Li atoms would be suppressed and 12 Li atoms are isolated decorated on the top of N atoms in the (AlN)12 nanocage. For the case of 12Li-decorated (AlN)12 nanocage, the situations of one, two, three H2 molecules adsorbed on each Li atom are investigated. When each Li atom adsorbs one H2 molecule (see Fig. 7(b)), the binding energy of H2 molecule is 0.183 eV, the average HeH bond length is 0.761 A, and the average distance between the H2 molecule and Li atom is 2.245 A. When each Li atom adsorbs two H2 molecules (see A Fig. 7(c)), the average bond length of the H2 molecules is 0.757 and the average binding energy of the H2 molecules is 0.145 eV/ H2. When three H2 molecules are decorated on each Li atom, only two H2 molecules are adsorbed on the top of Li atoms after relaxation and the third H2 molecule cannot be adsorbed. We draw a conclusion that up to two H2 molecules can be bind to each Li atom with the average binding energy of 0.145 eV/H2 and with the gravimetric density of 7.7 wt% in the 12Li-decorated (AlN)12 nanocage. A similar procedure has been followed for the Li decorated (AlN)24 and (AlN)36 nanocages. The optimized configurations of 24Li-decorated (AlN)24 and 36Li-decorated (AlN)36
nanocages are shown in Fig. 8(a) and (c), respectively. The (AlN)24 and (AlN)36 nanocages would be decorated with 24 Li and 36 Li atoms with the average binding energies of 1.664 and 1.632 eV/Li, respectively. As shown in Fig. 8(b) and (d), the 24Lidecorated (AlN)24 nanocage and 36Li-decorated (AlN)36 nanocage can bind up to 48 and 72H2 molecules with an average binding energy of 0.154 and 0.102 eV/H2, respectively. These data indicate that all the Li-decorated (AlN)n nanocages have the same hydrogen storage capacity of 7.7 wt%. While the above results are promising for isolated Lidecorated (AlN)n nanocages, one may wonder whether it is possible to synthesize a cluster assemble material composed of Li-decorated (AlN)n nanocages. If the Li-decorated (AlN)n nanocages could synthesize clusters, do the clusters retain their original capacity to store hydrogen? The eventual configuration of the cluster material composed of Lidecorated (AlN)n nanocages is difficult to determine, but the coalescence of Li-decorated (AlN)n nanocages may be prevented due to the repulsion between the positively Li atoms decorated on the (AlN)n nanocages. Hence, it is likely that the Li-decorated (AlN)n interacts with each other by the Li atoms.
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Fig. 7 e (a) The optimized configuration 12Li-decorated (AlN)12 nanocage. (b) 12H2 molecules adsorbed on 12Li-decorated (AlN)12 nanocage. (c) 24H2 molecules adsorbed on 12Li-decorated (AlN)12 nanocage.
Fig. 8 e (a) The optimized configuration 24Li-decorated (AlN)24 nanocage. (b) 48H2 molecules adsorbed on 24Li-decorated (AlN)24 nanocage. (c) The optimized configuration 36Li-decorated (AlN)36 nanocage. (d) 72H2 molecules adsorbed on 36Lidecorated (AlN)36 nanocage.
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To solve these problems, we study the interaction between two 12Li-decorated (AlN)12 nanocages. At first, we start with an initial configuration where the two adjacent Li atoms of one 12Li-(AlN)12 nanocage interact with the triangle face of the other 12Li-decorated (AlN)12 nanocage as shown in Fig. 9(a). The optimized geometry configuration of this system is given in Fig. 9(b), it is obvious that the geometry configuration of the interaction part of this system changes while the geometry configuration of the other part almost no changes. This structure has a binding energy of 3.519 eV, which suggests that this system has lower energy as compared to the two isolated 12Li-decorated (AlN)12 nanocages. It means that when the isolated 12Li-decorated (AlN)12 nanocages interact with each other, they may form clusters. The Li atoms of the interaction part would not adsorb H2 molecules as many as the original condition because of the limitation of space. It is difficult to detail calculate the gravimetric density of hydrogen of this system, but it is obvious that the hydrogen storage capacity is lower than 7.7 wt%. It should also be emphasized that the generalized gradient approximation (GGA) method underestimates the binding energy. Hence in our systems the binding energy may be higher than the calculated results [42].
4.
Conclusion
In conclusion, the capability of pristine and Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages to store hydrogen have been
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investigated by using density functional theory with general gradient approximation (GGA). It have been demonstrated that the positively charged Al atoms in (AlN)n (n ¼ 12, 24, 36) nanocages can hold n H2 molecules up to the gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, 0.144 eV/H2, respectively. Due to the hybridization between N 2p orbitals and Li 2p orbitals, the Li atoms can be strongly decorated on the top of N atoms in (AlN)n nanocages without clustering, and the average binding energies of Li atoms are 1.788, 1.664, and 1.632 eV/Li for the (AlN)n (n ¼ 12, 24, 36), respectively. The full nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages can bind up to 2n H2 molecules with an average binding energy of 0.145, 0.154, and 0.102 eV/H2, respectively. Thus, the hydrogen storage capacity of nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages is 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of hydrogen molecules lying in the range of 0.1e0.2 eV/H2 are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. Both the polarization mechanism and the hybridization of the Li 2p states with the H s orbitals contribute to the adsorption of H2 molecules on the nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages. In addition, it should also be emphasized that when the Lidecorated (AlN)n nanocages interact with each other, it is possible to synthesize clusters composed of Li-decorated (AlN)n nanocages. This would lower the hydrogen storage capacity. We hope that the present study may lead to design and synthesis of ideal hydrogen storage materials.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant Nos. 11175146 and 10904125, the Natural Science Foundation of Chongqing under Grant Nos. CSTC-2011BA6004 and CSTC-2008BB4253 and the Fundamental Research Funds for the Central Universities under Grant No. XDJK2012C038.
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Fig. 9 e (a) The initial configuration of the system of two 12Li-decorated (AlN)12 nanocages. (b) The optimized configuration of the system of two 12Li-decorated (AlN)12 nanocages.
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High-capacity hydrogen storage in Li-decorated (AlN)n (n [ 12, 24, 36) nanocages Guangzhao Wang, Hongkuan Yuan*, Anlong Kuang, Wenfeng Hu, Guolin Zhang, Hong Chen* School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
article info
abstract
Article history:
The capability of Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages for hydrogen storage has
Received 14 August 2013
been studied by using density functional theory (DFT) with the generalized gradient
Received in revised form
approximation (GGA). It is found that each Al atom is capable of binding one H2 molecule
15 December 2013
up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy
Accepted 22 December 2013
of 0.189, 0.154, and 0.144 eV/H2 in the pristine (AlN)n (n ¼ 12, 24, 36) nanocages, respectively.
Available online 20 January 2014
Further, we find that Li atoms can be preferentially decorated on the top of N atoms in
Keywords:
each Li atom with an average binding energy of 0.145, 0.154, 0.102 eV/H2 in the Lin(AlN)n
(AlN)n nanocages
(n ¼ 12, 24, 36) nanocages, respectively. Both the polarization of the H2 molecules and the
Li decoration
hybridization of the Li-2p orbitals with the H-s orbitals contribute to the H2 adsorption on
(AlN)n (n ¼ 12, 24, 36) nanocages without clustering, and up to two H2 molecules can bind to
Hydrogen storage
the Li atoms. Thus, the Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages can store hydrogen up
Density functional theory
to 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of H2 molecules lying in the range of 0.1e0.2 eV/H2 are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. It is also pointed out that when allowed to interact with each other, the agglomeration of Lidecorated (AlN)n nanocages would lower the hydrogen storage capacity. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is being considered as an ideal energy carrier due to its efficiency, abundance, and environmental friendliness. To achieve economical feasibility, hydrogen storage materials with high gravimetric and volumetric densities, as specified by the Department of Energy (DOE) goal of 9.0 wt% mass ratio and 81 g/L volumetric capacity by 2015 for mobile application [1], must be developed, and hydrogen recycling should be performed reversibly under ambient conditions [2e4]. Therefore, hydrogen storage materials should be composed of
elements lighter than aluminum, and the binding energy per hydrogen molecule should be within the range of 0.1e0.3 eV [5,6]. For more than a decade a large number of studies have been devoted to finding and designing such storage materials. Carbon-based nanostructures have been widely studied as potential hydrogen storage media, such as carbon nanotubes [7e12], carbon fullerenes [13e17], graphenes [18e22]. However, the hydrogen storage capacity of these nanostructures greatly diminishes at room temperature and ambient pressure due to the weak physical absorption of H2 molecules [2] as like metaleorganic frameworks [23e26]. One possible approach to
* Corresponding authors. Tel.: þ86 23 68367040. E-mail addresses: [email protected] (H. Yuan), [email protected] (H. Chen). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.138
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increase their chemical activity is to decorate the nanostructures with metal atoms to improve storage performance. Decorating carbon nanostructures with early transition metals [27e32] has been presented to enhance the hydrogen adsorption energies up to 0.2e0.6 eV/H2 via Kubas interaction (hybridization of the d states with the H2 states) [33,34]. Unfortunately, theoretical studies show that transition-metals decorated on carbon surfaces undergo clustering during reversible hydrogenation [27,35]. The alkali metals and alkaliearth metals can produce a uniform decoration due to their lightweight and lower cohesive energies than those of transition metals. Although the Li-decorated C60 nanocage was demonstrated by Sun et al. [36] to bind up to 40H2 molecules with an approximate gravimetric density of hydrogen storage of 9 wt%, the average binding energy of H2 molecules is 0.075 eV/H2, which is so small that the storage system has to be operated at lower than ambient temperature. Theoretical studies have found that the hydrogen storage capacity of Cadecorated C60 nanocage [37], graphene [38], nanotube [39] is more than 5 wt%. However, a recent study claimed that the density functional theory calculations overestimated significantly the binding energy of H2 molecules onto the Ca1þ cation centers [40]. Therefore, the challenge has been to find novel nanostructures where clustering of metal atoms can be prevented without compromising the binding energy of hydrogen. Recent efforts have been paid to BN and AlN nanostructures. A recent report claimed that the BN nanotubes could bind hydrogen more strongly than carbon nanotubes [41], which is attributed to the heteropolar bonding in boron nitride. The Li doped BN fullerene was found to hold 18H2 molecules up to 3.7 wt% with an average binding energy of 0.146 eV/H2 [42], and the enhancement of the binding energy of H2 molecules is attributed to the polarization of the H2 molecules induced by positively charged Li atoms. With the synthesis of AlN nanostructures in experiment [43e47], much attention has been devoted to the capacity of AlN nanostructures for hydrogen storage. The hydrogen chemisorption of single-walled AlN nanotubes has been investigated by density functional theory calculation [48], indicating that a single H atom chemisorption introduces spin-polarized magnetic moment to hydrogenated AlN nanotubes. The adsorption of atomic and molecular hydrogen on AlN nanowires has been investigated by Li et al. [49] by using the first-principles calculations, and it was found that the hydrogen atoms prefer to adsorb on top of neighboring threefold-coordinated N and Al atoms in pairs, while the hydrogen molecule prefers to adsorb on top of threefold-coordinated Al atoms in the nanowire. The AlN nanostructures (nanocages, nanocones, nanotubes, and nanowires) can bind H2 molecules primarily due to polarization mechanism, leading to a hydrogen storage capacity of 4.7 wt% [50]. Especially, the Ni-decorated (AlN)12 nanocage was studied [51] as a hydrogen storage media from density functional theory calculations, indicating that high weight percentage hydrogen storage cannot be achieved in NiAl12N12 nanocage due to the thermodynamical unstability. In present work, we have used the first-principles calculations to systematically investigate the possibility of Li decorated on (AlN)n (n ¼ 12, 24, 36) nanocages and the hydrogen adsorption capacity of Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages. To our knowledge, the hydrogen storage properties of alkali metal
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decorated AlN nanostructures have not been examined so far. We showed that Li can be decorated on the surfaces of (AlN)n strongly by the hybridization between Li-p orbitals and AlN nanocages without clustering, and the Li-decorated (AlN)n nanocages can absorb H2 molecules up to higher gravimetric density of hydrogen storage with the desirable binding energy.
2.
Calculational method
All our calculations were carried out by using a first-principles method based on density functional theory (DFT) [52e54] as implemented in the Vienna ab initio simulation package (VASP) [55e57]. Generalized gradient approximation (GGA) [58,59] with PW91 [60] was used to treat the exchange and correlation effects of electrons. The projector-augmented wave (PAW) method [61] was adopted to describe the core electrons. To obtain self solutions to the KohneSham equation, the Brillouin zone was sampled by (222) special mesh k-points. The cutoff energy for the plane wave-basis was set as 450 eV, and the convergence threshold was chosen to be 105 eV in energy. The binding energy of (AlN)n nanocage with hydrogen adsorption is defined as Eb1 ¼ EðAlNÞn þ iEH2 EiH2 þðAlNÞn i;
(1)
where EðAlNÞn is the energy of the (AlN)n nanocage, EH2 is the energy of a free H2 molecule, EiH2 þðAlNÞn is the total energy of the (AlN)n nanocage with i adsorbed H2 molecules. The binding energy of Li atoms decorated on (AlN)n nanocage is defined as Eb2 ¼ EðAlNÞn þ mELi EmLiðAlNÞn m;
(2)
where EðAlNÞn , ELi, and EmLiðAlNÞn are the energy of the pristine (AlN)n nanocage, the energy of an isolated Li atom, and the total energy of the system with m Li atoms decorated on the (AlN)n nanocage, respectively. The binding energy of H2 molecules adsorbed on the Li-decorated (AlN)n nanocage is defined as Eb3 ¼ EmLiðAlNÞn þ iEH2 EiH2 þmLiðAlNÞn i;
(3)
where EiH2 þmLiðAlNÞn is the total energy of mLi-decorated (AlN)n nanocage with i adsorbed H2 molecules, EmLi(AlN)n is the total energy of mLi-decorated (AlN)n nanocage, and EH2 is the energy of an isolated H2 molecule. The binding energy of the two 12Lidecorated (AlN)12 nanocage is defined as Eb4 ¼ 2E12LiðAlNÞ12 E12LiðAlNÞ12 112LiðAlNÞ12
(4)
where E12LiðAlNÞ12 is the energy of an isolated 12Li-decorated (AlN)12 nanocage, and E12LiðAlNÞ12 112LiðAlNÞ12 is the total energy of the system of two 12Li-decorated (AlN)12 nanocages interacting with each other.
3.
Results and discussions
3.1. Hydrogen adsorption on (AlN)n (n ¼ 12, 24, 36) nanocages The geometry of (AlN)12 nanocage is optimized and its eventual configuration (see Fig. 1(a)) has Th symmetry composed of
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Fig. 1 e (a) The optimized configuration of (AlN)12 nanocage. (b) The charge density difference of (AlN)12 nanocage. (c) 12H2 molecules absorbed on the (AlN)12 nanocage.
six four-numbered rings and eight six-numbered rings. There are two different types of AleN bond: the AleN bond length in four-numbered ring is 1.855 A, while the AleN bond length between four-numbered rings is 1.791 A. These calculation results are in accordance with previous studies [50,62e65]. The charge density difference with respective to free atoms (see Fig. 1(b)) indicates that the charge transfer from the Al atoms to N atoms makes the Al atoms positively charged. A charge analysis also indicates that the Al atom carries a charge of þ0.47e. As in previous studies [66], the positively charged Al ions on the cage surface become the available sites for adsorption of H2 molecules. The (AlN)12 nanocage can bind up to 12H2 molecules with an average binding energy of 0.189 eV/H2 (see Fig. 1(c)) and with a slightly stretched HeH bond length of 0.761 A. The average distance between the H2 molecule and neighboring Al atom is 2.245 A. The lengths of two different types of AleN bond become 1.857 and 1.794 A, which approach the corresponding values of the pristine (AlN)12 nanocage, indicating that the geometry of (AlN)12 nanocage is hardly changed upon adsorption of H2 molecules. The charge analysis reveals that each H2 molecule carries a charge of þ0.07e, implying that the H2 molecules transfer charge to Al atoms. Furthermore, the partial density of states (PDOS) of (AlN)12e12H2 system is shown in Fig. 2. The main peaks of Al 3s orbitals located at 7.35 and 4.45 eV hybridize with H s orbitals. Therefore, both the polarization of the H2 molecules and the orbital hybridization between H s orbitals and Al 3s orbitals are responsible for the H2 adsorption on the (AlN)12 nanocage. To know whether and how the hydrogen storage capacity of AlN nanocages depends on the cluster sizes, we have also studied the hydrogen adsorption on (AlN)24 and (AlN)36 nanocages. Previous studies [64,65] confirmed that the geometries of (AlN)24 and (AlN)36 nanocages have S4 and Th symmetries, respectively. Compared with the (AlN)12 nanocage, there are more different types of AleN bond in (AlN)24 and (AlN)36 nanocages. The (AlN)24 nanocage is composed of six four-numbered rings and twenty six-numbered rings, the AleN bond lengths vary from 1.790 to 1.858 A (see Fig. 3(a)).
While the (AlN)36 nanocage is composed of six four-numbered rings and thirty-two six-numbered rings, the AleN bond lengths vary from 1.790 to 1.837 A (see Fig. 3(d)). The charge density differences of the (AlN)24 and (AlN)36 nanocages (see Fig. 3(b) and (e)) indicates that the charge transfer from Al atoms to N atoms makes the Al atoms positively charged. Similar to (AlN)12 nanocage, each Al atom can be decorated with one H2 molecule, thus the (AlN)24 and (AlN)36 nanocages are capable of binding up to 24 and 36H2 molecules, respectively. The optimized configuration of (AlN)24 nanocage decorated with 24H2 molecules is shown in Fig. 3(c). Here the average binding energy of H2 molecules is 0.154 eV/H2, the HeH bond lengths vary from 0.752 to 0.761 A which yields an average bond length of 0.757 A, and the average distance beA. The optitween the Al atoms and H2 molecules is 2.476 mized geometry configuration of (AlN)36 nanocage decorated with 36H2 molecules is shown in Fig. 3(f). Here the average binding energy of H2 molecules is 0.144 eV/H2, the average HeH bond length is 0.756 A, and the average distance between A. It is evident that the the Al atoms and H2 molecules is 2.625 average binding energy of H2 molecules decreases slowly as the size of AlN nanocages increases, but the gravimetric
Fig. 2 e The partial density of states (PDOS) of Al and H atoms in (AlN)12e12H2 system. Fermi level is set to 0.
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Fig. 3 e (a) The optimized configuration of (AlN)24 nanocage. (b) The charge density difference of (AlN)24 nanocage. (c) 24H2 molecules adsorbed on the (AlN)24 nanocage. (d) The optimized configuration of (AlN)36 nanocage. (e) The charge density difference of (AlN)36 nanocage. (f) 36H2 molecules adsorbed on the (AlN)36 nanocage.
density of hydrogen storage of 4.7 wt% keep unchange with AlN nanocage sizes.
3.2. Hydrogen adsorption on Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages Firstly, the favorite position of a single Li atom decorated on the (AlN)12 nanocage is searched. In our calculations we consider all possible different decorated sites, as shown in Fig. 4, which are the top of the Al and N atoms (A1,A2), the hollow of the four-numbered rings (H1) and the hollow of the six-numbered rings (H2), the bridge site over the AleN bond in four-numbered rings (B1) and between four-numbered rings (B2). The most favorable decorated site is the A2 site, in which the Li atom has a larger binding energy compared with the hollow of the six-numbered rings (H2) (The binding energies of Li atom on A2 and H2 sites are 1.507 and 1.265 eV, respectively). Besides, the top of Al atom (A1), the hollow of the four-numbered ring (H1), and the two different bridge sites over AleN bond (B1, B2) turn out to be unstable. When a Li atom is decorated on these sites, it moves to the top of the N atom after structural relaxation. The fully optimized structure of the stable configuration is shown in Fig. 5(a). With the adsorption of Li atom on the A2 site, the distances between the Li atom and three neighboring Al atoms are
3.232, 3.232, and 3.076 A, respectively, while the distance between the Li atom and neighboring N atom is 1.840 A. The charge density difference of a single Li decorated (AlN)12 nanocage is shown in Fig. 5(b). The charge transfer from Li atom to (AlN)12 nanocage makes the Li atom positively charged. This can also be confirmed by the charge analysis which indicates that the Li atom carries a charge of þ0.36e. To
Fig. 4 e The different position of Li atom decorated on the (AlN)12 nanocage.
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Fig. 5 e (a) The optimized configuration of single Li atom decorated (AlN)12 nanocage. (b) The charge density difference of single Li atom decorated (AlN)12 nanocage. (c) One H2 molecule adsorbed on single Li atom decorated (AlN)12 nanocage. (d) Two H2 molecules adsorbed on single Li atom decorated (AlN)12 nanocage. (e) Three H2 molecules adsorbed on single Li atom decorated (AlN)12 nanocage.
further understand the binding mechanism of the Li atom decorated on the (AlN)12 nanocage, the partial density of states (PDOS) of the N and Li atoms in Li-(AlN)12 system are shown in Fig. 6(a). It can be seen that the hybridization of the N 2p with Li 2p orbitals from 7 to 0 eV is responsible for the formation of LieN bond, resulting in Li atoms bind to the surface of the (AlN)12 nanocage. The positively charged Li atom in Li(AlN)12 system is an available site for adsorption of H2 molecules. Upon decorating the first hydrogen molecule near the Li atom, the configuration after relaxation is shown in Fig. 5(c). It indicates that the H2 molecule can be adsorbed on the Li atom with a binding energy of 0.178 eV/H2 and with a slightly stretched HeH bond length of 0.756 A. The LieN distance is 1.868 A and the disA. tance between the H2 molecule and Li atom is about 2.172 As two H2 molecules are adsorbed to the Li atom, the optimized configuration is shown in Fig. 5(d). The average binding A, energy is 0.172 eV/H2, the average HeH bond length is 0.756 the LieN distance becomes 1.879 A, and the average distance A. To between the two H2 molecules and the Li atom is 2.182 know the maximum number of hydrogen molecules a Li atom can absorb, we doped more H2 molecules near the Li atom and optimized the geometry. Three H2 molecules can be adsorbed to a single Li atom and the fourth H2 molecule is moved to a distance of 3.946 A from the Li atom. Thus, as shown in Fig. 5(e) a single Li atom was found to adsorb up to three H2 molecules
with the average binding energy of 0.180 eV/H2 and with the average HeH bond length of 0.761 A. The LieN distance is 1.888 A and the average distance between the H2 molecules and Li atom is about 2.094 A. The analysis of the charge transfer indicates that the Li atom and the H2 molecule carry a charge of þ0.32 and þ0.07e, respectively. This indicates that the positively charged Li atom adsorbs H2 molecules primarily due to the polarization of the H2 molecule induced by the charged Li atom. While a small positively charge of H2 molecule indicates that the charge transfer from the H2 molecule to Li atom, so there exits a hybridization between the H2 molecule and Li atom. Fig. 6(b) shows that the PDOS of the Li and H atoms, from which we can see more clearly that the peak center around 9.28 eV corresponds to the hybridization of the Li 2p with the H s orbitals. Therefore, the H2 adsorption on the Li-decorated (AlN)12 nanocage is attributed to both the polarization of the H2 molecules induced by the charges of Li atom and the hybridization of the Li 2p states with H s orbitals. Now we consider the capability of 12 Li decorated (AlN)12 nanocage to store hydrogen. The fully optimized configuration is shown in Fig. 7(a). The average distances between the Li atom and three neighboring Al atoms, between the Li atom and neighboring N atom, and between two adjacent Li atoms are 2.579, 1.891, and 3.392 A, respectively. The average binding energy is 1.788 eV/Li, which is larger than in the Li2 dimer (0.95 eV/Li) and in the Li bulk cohesive energy (1.63 eV/Li). Thus
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Fig. 6 e (a) The partial density of states (PDOS) of N and Li atoms in Li-(AlN)12 system. (b) The partial density of states (PDOS) of Li and H atoms in Li-(AlN)12eH2 system. Fermi level is set to 0.
the clustering of Li atoms would be suppressed and 12 Li atoms are isolated decorated on the top of N atoms in the (AlN)12 nanocage. For the case of 12Li-decorated (AlN)12 nanocage, the situations of one, two, three H2 molecules adsorbed on each Li atom are investigated. When each Li atom adsorbs one H2 molecule (see Fig. 7(b)), the binding energy of H2 molecule is 0.183 eV, the average HeH bond length is 0.761 A, and the average distance between the H2 molecule and Li atom is 2.245 A. When each Li atom adsorbs two H2 molecules (see A Fig. 7(c)), the average bond length of the H2 molecules is 0.757 and the average binding energy of the H2 molecules is 0.145 eV/ H2. When three H2 molecules are decorated on each Li atom, only two H2 molecules are adsorbed on the top of Li atoms after relaxation and the third H2 molecule cannot be adsorbed. We draw a conclusion that up to two H2 molecules can be bind to each Li atom with the average binding energy of 0.145 eV/H2 and with the gravimetric density of 7.7 wt% in the 12Li-decorated (AlN)12 nanocage. A similar procedure has been followed for the Li decorated (AlN)24 and (AlN)36 nanocages. The optimized configurations of 24Li-decorated (AlN)24 and 36Li-decorated (AlN)36
nanocages are shown in Fig. 8(a) and (c), respectively. The (AlN)24 and (AlN)36 nanocages would be decorated with 24 Li and 36 Li atoms with the average binding energies of 1.664 and 1.632 eV/Li, respectively. As shown in Fig. 8(b) and (d), the 24Lidecorated (AlN)24 nanocage and 36Li-decorated (AlN)36 nanocage can bind up to 48 and 72H2 molecules with an average binding energy of 0.154 and 0.102 eV/H2, respectively. These data indicate that all the Li-decorated (AlN)n nanocages have the same hydrogen storage capacity of 7.7 wt%. While the above results are promising for isolated Lidecorated (AlN)n nanocages, one may wonder whether it is possible to synthesize a cluster assemble material composed of Li-decorated (AlN)n nanocages. If the Li-decorated (AlN)n nanocages could synthesize clusters, do the clusters retain their original capacity to store hydrogen? The eventual configuration of the cluster material composed of Lidecorated (AlN)n nanocages is difficult to determine, but the coalescence of Li-decorated (AlN)n nanocages may be prevented due to the repulsion between the positively Li atoms decorated on the (AlN)n nanocages. Hence, it is likely that the Li-decorated (AlN)n interacts with each other by the Li atoms.
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Fig. 7 e (a) The optimized configuration 12Li-decorated (AlN)12 nanocage. (b) 12H2 molecules adsorbed on 12Li-decorated (AlN)12 nanocage. (c) 24H2 molecules adsorbed on 12Li-decorated (AlN)12 nanocage.
Fig. 8 e (a) The optimized configuration 24Li-decorated (AlN)24 nanocage. (b) 48H2 molecules adsorbed on 24Li-decorated (AlN)24 nanocage. (c) The optimized configuration 36Li-decorated (AlN)36 nanocage. (d) 72H2 molecules adsorbed on 36Lidecorated (AlN)36 nanocage.
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To solve these problems, we study the interaction between two 12Li-decorated (AlN)12 nanocages. At first, we start with an initial configuration where the two adjacent Li atoms of one 12Li-(AlN)12 nanocage interact with the triangle face of the other 12Li-decorated (AlN)12 nanocage as shown in Fig. 9(a). The optimized geometry configuration of this system is given in Fig. 9(b), it is obvious that the geometry configuration of the interaction part of this system changes while the geometry configuration of the other part almost no changes. This structure has a binding energy of 3.519 eV, which suggests that this system has lower energy as compared to the two isolated 12Li-decorated (AlN)12 nanocages. It means that when the isolated 12Li-decorated (AlN)12 nanocages interact with each other, they may form clusters. The Li atoms of the interaction part would not adsorb H2 molecules as many as the original condition because of the limitation of space. It is difficult to detail calculate the gravimetric density of hydrogen of this system, but it is obvious that the hydrogen storage capacity is lower than 7.7 wt%. It should also be emphasized that the generalized gradient approximation (GGA) method underestimates the binding energy. Hence in our systems the binding energy may be higher than the calculated results [42].
4.
Conclusion
In conclusion, the capability of pristine and Li-decorated (AlN)n (n ¼ 12, 24, 36) nanocages to store hydrogen have been
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investigated by using density functional theory with general gradient approximation (GGA). It have been demonstrated that the positively charged Al atoms in (AlN)n (n ¼ 12, 24, 36) nanocages can hold n H2 molecules up to the gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, 0.144 eV/H2, respectively. Due to the hybridization between N 2p orbitals and Li 2p orbitals, the Li atoms can be strongly decorated on the top of N atoms in (AlN)n nanocages without clustering, and the average binding energies of Li atoms are 1.788, 1.664, and 1.632 eV/Li for the (AlN)n (n ¼ 12, 24, 36), respectively. The full nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages can bind up to 2n H2 molecules with an average binding energy of 0.145, 0.154, and 0.102 eV/H2, respectively. Thus, the hydrogen storage capacity of nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages is 7.7 wt%, approaching the U.S. Department of Energy (DOE) target of 9 wt% by the year 2015, and the average binding energies of hydrogen molecules lying in the range of 0.1e0.2 eV/H2 are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. Both the polarization mechanism and the hybridization of the Li 2p states with the H s orbitals contribute to the adsorption of H2 molecules on the nLi-decorated (AlN)n (n ¼ 12, 24, 36) nanocages. In addition, it should also be emphasized that when the Lidecorated (AlN)n nanocages interact with each other, it is possible to synthesize clusters composed of Li-decorated (AlN)n nanocages. This would lower the hydrogen storage capacity. We hope that the present study may lead to design and synthesis of ideal hydrogen storage materials.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant Nos. 11175146 and 10904125, the Natural Science Foundation of Chongqing under Grant Nos. CSTC-2011BA6004 and CSTC-2008BB4253 and the Fundamental Research Funds for the Central Universities under Grant No. XDJK2012C038.
references
Fig. 9 e (a) The initial configuration of the system of two 12Li-decorated (AlN)12 nanocages. (b) The optimized configuration of the system of two 12Li-decorated (AlN)12 nanocages.
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