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Metal decorated monolayer BC2N for hydrogen storage
Computational Materials Science 60 (2012) 181–185
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Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Metal decorated monolayer BC2N for hydrogen storage Yu Sheng Wang a,b, Peng Fei Yuan a, Meng Li a, Wei Fen Jiang b, Qiang Sun a, Yu Jia a,⇑ a b
Center of Clean Energy and Quantum Structures, and School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450052, China College of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou, Henan 450011, China
a r t i c l e
i n f o
Article history: Received 4 November 2011 Received in revised form 11 March 2012 Accepted 14 March 2012 Available online 19 April 2012 Keywords: Hydrogen storage Nanostructure Binding energy
a b s t r a c t Using first-principles density functional calculations, we show that metal decorated monolayer BC2N can serve as a high-capacity hydrogen storage medium with the gravimetric density of 7.69–10.0 wt.%. In particular, Li and Ca decorated BC2N are systematically investigated and found to be feasible for hydrogen storage without metal clustering owing to the Coulomb repulsion between the metal adatoms. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of H2 molecules and the average adsorption energies are 0.24 and 0.26 eV/H2, respectively. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Because of its abundant, renewable, pollution-free, nontoxic properties, and packing more energy per unit mass than any other fuel, hydrogen has been actively considered as the best substitute for fossil fuels in the future [1–4]. However, realizing hydrogen economy has substantial difficulties to overcome. Among these, the most difficult challenge is to find materials that can store hydrogen with large gravimetric and volumetric density and operate under ambient thermodynamic conditions. The US Department of Energy (DOE) has set the gravimetric density of 9 wt.% in 2015 for usable specific energy from H2. To achieve this goal, the storage materials should consist of only light elements. Another important criterion is the binding energy of the adsorbed H2 molecules should ideally be about 0.2 eV/H2 [5]. If the binding energy is too high, release of the hydrogen will be difficult at moderate operating temperatures, while if the binding energy is too weak, storage of the hydrogen will be ineffective. Current hydrogen storage technologies, such as cryogenic liquid, high-pressure gas cells, lowtemperature adsorbates, metal hydrides and chemical storage, cannot meet all the industry requirements. In recent years, metal-decorated nanostructures have widely attracted attention as potential hydrogen storage medias due to their light weights and large surface areas, which have achieved better results than the corresponding pristine substrate materials. For example, coating carbon nanostructures (i.e. graphenes, carbon fullerenes, carbon nanotubes) with alkaline metals (AMs) [6–9] and
⇑ Corresponding author. Tel./fax: +86 3716776118. E-mail address: [email protected] (Y. Jia). 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.03.028
alkaline-earth metals (AEMs) [10–13] can adsorb H2 with high gravimetric densities through static Coulomb interactions. On the other hand, transition metal (TM) decorated materials have H2 adsorption energies in the range of 0.3–0.8 eV/H2 via the Kubas mechanism [14–17]. However, TM tends to cluster due to its high cohesive energy, and the storage capability falls short of theoretical predictions. Moreover, the dissociation of the first H2 molecule added to the TM atoms will also lower the gravimetric density [15,16]. Hence the challenge to find appropriate H2 sorbents is far from being solved. Since carbon nanostructures have attracted rapidly growing research interest because of its various intriguing properties and potential applications in future electronics [18], the synthesis of new carbon-based compounds has also stimulated considerable interest for some years. A possibility is the formation of the BxCyNz compounds where carbon atoms of graphite are partially substituted by boron and nitrogen atoms and there has been some success in preparing BxCyNz compositions [19,20]. These compounds have been the focus of intensive studies recently because of the relevance of such materials to graphene, which is a two-dimensional network sp2-hybridized carbon atom. In addition, the BC2N thin films have been synthesized using chemical vapor deposition method with BCl3 and CH3CN as the starting materials. The atomic arrangement and electronic structure of the monolayer structures have been studied extensively [20,21]. Previous studies prove that metal decorated graphene can be used as good hydrogen storage material. The BC2N sheet has graphene-like structure which inspires us to consider whether metal decorated monolayer BC2N is also efficient hydrogen storage medium. So far as I know, no relevant research has been concentrated on the hydrogen storage of BC2N.
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B 4.98 Å
C N
T3
T2 T1
B2
B1 B3
B4
Fig. 1. The unit cell and optimized geometry for BC2N layer.
In this article, we conduct theoretical studies of high-capacity hydrogen storage media consisting of monolayer BC2N and metal atoms. As a prototype structure of BC2N sheets we consider the geometry which has inversion symmetry, each C atom has a C, B, and N atom nearest neighbor, while B (N) atom has two C atoms and one N (B) atom as nearest neighbors (see Fig. 1). Metals considered here include AMs (Li, Na, and K), AEMs (Be, Mg, and Ca), TMs (Ti, V, Cr, and Fe) and Al. It is found that the adsorption of metal atoms (except K) above and below the hollow site of boron carbon hexagon in BC2N resulting in stable configurations. AMs, Ca and Al as adatoms should be feasible for hydrogen storage. Each metal atom can bind up to 3–4 H2 molecules, respectively, corresponding to storage capacities of 7.69–10.0 wt.%. The average adsorption energies of H2 molecules on Li, Na, K, Ca and Al decorated BC2N are 0.24, 0.14, 0.17, 0.26 and 0.18 eV/H2, respectively, which are in the required energy window allowing hydrogen adsorption and desorption feasible at ambient temperatures and pressures. 2. Computational method All our calculations are carried out using first-principles method based on density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [22,23]. Prior works have proven that the physisorption results obtained from local density approximation (LDA) are in substantial agreement with experiments [24,25]. Therefore, LDA with projector-augmented wave pseudopotentials is used. The supercell contains four C atoms, two B atoms and two N atoms in our calculations. The energy cutoff for the plane-wave basis set is 450 eV with a vacuum layer thickness larger than 15 Å. The Brillouin zone of the supercell is sampled by 4 4 1 k-points within the Monkhorst–Pack scheme. The number of k-points in Brillouin zone is carefully tested by using the larger k points, 6 6 1, thus to ensure that the changes of total energy are less than 103 eV in our calculations. All the obtained structures are optimized without any symmetry constraints until the force on each atom is less than 102 eV/ Å and the total energy changes are less than 104 eV. 3. Results and discussion Our discussion begins with the binding of single metal atoms on BC2N sheet. There are seven different adsorption sites as shown in Fig. 1, which are the hollow center of the B–C hexagon (T1), the hollow center of the B–C–N hexagon (T2), the hollow center of the C–N hexagon (T3), the bridge of C–C bond (B1), the bridge of B–C (B2), the bridge of C–N (B3), and the bridge of B–N (B4). After full relaxation, it is found that the bridge sites are energetically unfavorable compared to T sites for all metals. The T1 site is energetically favorable for most bond metal atoms except K which prefers T3 site. The calculated binding energies (BEs) and the corresponding cohesive energies of the respective metal atoms are shown in Fig. 2. It is clear from Fig. 2a that the BEs of AMs and Ca are higher than the corresponding cohesive energies of bulk metals, which imply a rel-
Fig. 2. (a) Calculated metal binding energies on a BC2N layer and the corresponding experimental cohesive energies of the respective metal atoms including AMs, AEMs and Al (solid bars) (Ref. [26]). The distances between metal atoms and BC2N layer are denoted by solid squares (b) TMs.
ative stability of atomic metal atoms against clustering. While the BEs of Be, Mg and Al are lower than the corresponding cohesive energies. The BEs are correlated with the metal–BC2N distance (dMB) as shown in Fig. 2a. It turns out that, for the atoms in the same group, the smaller dMB is, the larger BE is. Fig. 2b displays the BEs and the corresponding cohesive energies of TMs. The BEs are little than the corresponding cohesive energies for all the TMs. That is to say, TMs tend to cluster. Hence, we next study the hydrogen storage of metal decorated BC2N only including Li, Na, K, Ca and Al (the BE of Al is comparable to the corresponding cohesive energy). Moreover, the BEs of the same metal atoms placed on the T1 site of BC2N sheet are consistently higher than that on top of the hexagon of a pure graphene sheet [6,10,27]. Meanwhile, in the case of metal decorated BC2N sheet, the metal–metal interaction is indeed negligible owing to the large distance of about 4.98 Å (see Fig. 1). These results indicate that the metal–BC2N interaction is strong and then stable. It is well known that the diffusion of metal atoms on the substrate might lead to clustering [28], which will limit the hydrogen uptake. To further confirm the stability of metal atoms on BC2N sheet, as a prototype system we test the total energy as the Li atom moves on the substrate. As shown in Fig. 3, there are four Li atoms on a (2 2) cell of BC2N. When the bottom-left Li atom moves through the positions from M0 to M6, its z coordinate is optimized whereas the remaining three Li atoms are fully relaxed. Beyond the position M4 of this Li atom, the Coulomb repulsion will push the bottomright Li atom to maintain a distance with it. The variation of the energy with the positions of Li atoms is indicated in Fig. 3b. It is found that the calculated diffusion barrier from M0 to M4 is 0.491 eV. For the position M5, z coordinate of the bottom right Li atom is optimized while two other coordinates keep unchangeably. After full relaxation, the vertical distance between the bottom-left Li and the BC2N sheet dLi-sheet increases from 1.676 Å to 3.723 Å while the distance between two bottom Li atoms dLi–Li is 2.784 Å. Similarly, in the case of position M6, dLi-sheet is 4.270 Å and dLi–Li is 2.852 Å. Thus, the original structure is energetically favorable owing to the repulsion between metal Li atoms, which can effectively hinder the mobility of metal on the substrate. Therefore, the long
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(b)
(a) Li
M 0 M1 M2 M3 M4 M 5 M6 Fig. 3. (a) Top view of the (2 2) cell of Li/BC2N complex. (b) The variation in total energy as the bottom-left Li atom moves through the positions from M0 to M6 indicated in (a).
distance of metal–metal, the relative strong bonding between the metal atoms and the BC2N layer, and the Coulomb repulsion between the metal atoms lead to the prevention of metal clustering on the surface. We consider next the double-sided adsorption of metal atoms in order to increase the available surface area for hydrogen storage. As shown in Fig. 1, there are three possible sites, i.e. T1, T2, and T3, for the second metal atom to be positioned on the other side of the BC2N layer. In fact, according to the calculated results, the two metal atoms prefer to occupy the T1 sites but on opposite sides of the BC2N layer except K atoms which prefer to locate on opposite T3 sites. The corresponding BEs of the respective metal atoms are also shown in Fig. 2. Compared to the systems with the single metal adatoms, the BEs are almost no changes. Therefore, each metal atom, such as AMs, Ca and Al, can stay on its site stably as well as the strong binding itself. We now enter the next phase of our calculation, namely, to study H2 adsorption on metal-decorated BC2N. The average adsorption energy Ead per H2 to the system is defined as the following term: Ead ¼ ½Etotal EmþBC2 N nEH2 =n, where n indicates the number of H2 molecules. Etotal, EmþBC2 N and EH2 are the energies of the considered system, the metal-decorated BC2N and hydrogen molecules, respectively. By this definition, the positive value of Ead corresponds that the adsorption is exothermic and hence stable. As prototype systems, we study in detail about the hydrogen storage properties of Li/BC2N and Ca/BC2N. Fig. 4 shows the average H2 adsorption energy on the Li(Ca) decorated BC2N. Each Li adatom can bind four H2 molecules while each Ca adatom binds five H2 molecules. There is a significant difference in adsorption energy of H2 between Li/BC2N and Ca/BC2N. The Ead in Li decorated BC2N is found to lie in the range of 0.19–0.26 eV/H2 while Ead in Ca decorated BC2N is in the range of 0.06–0.27 eV/H2. Table 1 lists the average bond length of H–H (dH–H) (Å), the average distance between Li(Ca) atom and H2 molecule (dLi–H/dCa–H) of these two systems with different numbers of H2. It is evident from Table 1 that
all the bond lengths of H2 are longer than that in the isolated molecular state (0.750 Å). In the case of hydrogen molecules adsorption on the Li decorated BC2N, the average bond length of dH–H increases from 0.787 Å (for the one H2 adsorption case) to 0.788 Å (for the two H2 adsorption case), and then decreases to 0.778 Å (for the four H2 adsorption case). This is in accord with the fact that the average adsorption energy of H2 to the Li/BC2N system first increase from 0.24 to 0.26 eV/H2 and then decrease to 0.19 eV/H2, as shown in Fig. 4. As the number of adsorbed H2 molecules increases from two to four, the average distance between Li atom and H2 is slightly increased from 1.937 Å to 2.458 Å. This finding also corroborates the decrease of the average adsorption energy of H2. For the system of nH2/Ca/BC2N, the average dH–H increases from 0.782 Å (for the one H2 adsorption case) to 0.824 Å (for the four H2 adsorption case), and then decreases to 0.813 Å (for the five H2 adsorption case). While the average dCa–H decreases from 2.501 Å to 2.291 Å then increases to 2.413 Å. Correspondingly, the average adsorption energy of H2 first increase from 0.06 to 0.28 eV/H2 and then decrease to 0.26 eV/H2. In order to investigate the magnitude of charges transferred from the adatoms to BC2N, we performed a Bader charge analysis on these systems [29]. It is found that each Li adatom donates 0.95 valence electrons to BC2N, therefore, Li atom becomes ionized and then binds H2 through static Coulomb interaction [7,30]. To further understand the adsorption mechanism, the PDOS of the nH2/Li/BC2N (n = 1, 2) systems and the three-dimensional electron charge density difference for H2/Li/BC2N are plotted in Fig. 5. As shown in Fig. 5a, the peak of Li 2s (2p) orbitals around 9.28 eV hybridizes with the H s orbitals, resulting in a electron transfer between H s orbitals and Li 2s (2p) orbitals in the H2/Li/BC2N system. From the different charge density inserted in Fig. 5a, we can clearly find that there is an induced dipole for the adsorbed hydrogen molecule, with electron density accumulation on the side close to the Li/BC2N and depletion on the side away from the Li atom. Previous studies have shown that the band broadening of the molecular level of H2 below the Fermi energy indicates a significant H2–H2 interaction that in turn increases its adsorption energy to the substrate [10]. The same mechanism is found in the 2H2/Li/BC2N system where the band broadening of about 8 eV in Fig. 5b. As a
Table 1 Average bond length of H–H (dH–H) (Å), bond length between metal atom (M = Li and Ca) and H2 molecule (dM–H) (Å). The number of H2
Fig. 4. Calculated H2 adsorption energy on the Li(Ca) decorated BC2N. The optimized configurations of hydrogen molecules adsorbed on Ca decorated BC2N are shown as inserts.
1 2 3 4 5
Li
Ca
dH–H (Å)
dLi–H (Å)
dH–H (Å)
dCa–H (Å)
0.787 0.788 0.781 0.778
1.918 1.937 2.237 2.458
0.782 0.804 0.827 0.824 0.813
2.501 2.370 2.282 2.291 2.413
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Y.S. Wang et al. / Computational Materials Science 60 (2012) 181–185 Table 2 The maximum number of adsorbed H2, the average H2 binding energy (BE) and the corresponding hydrogen gravimetric density of H2 adsorbed on the double-sided decorated BC2N with different metal atoms (Li, Na, K, Ca, Al).
Fig. 5. The PDOS for (a) all H atoms and Li atom in H2/Li/BC2N, (b) all H atoms and Li atom in 2H2/Li/BC2N. Fermi level is set to zero. The isosurface of charge density difference with an isovalue of 0.002 e/Å3 for the system of H2/Li/BC2N is shown as an insert. Red and blue colors of the isosurface represent electron accumulation and deletion regions after H2 adsorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
result, the nature of interaction between H2 molecules and Li-decorated BC2N relates not only to the polarization mechanism but also to the orbital hybridization. This phenomenon is similar to the cases of Li-doped B2C graphene [30]. It is important to note that the adsorption energies of the first two H2 molecules in Li/BC2N are larger than those in Ca/BC2N. In the case of Ca-decorated BC2N, Ca adatom is also ionized. Again, the Ca ion adsorbs H2 molecules through static Coulomb interaction. The Bader charge analysis shows that 0.71 electrons are transferred from Ca adatom to the BC2N sheet which is little than that of Li atom in Li/BC2N system. This is the reason why the Li ion catches first two H2 molecules more strongly than the Ca ion. In addition, the first and the second H2 molecules prefer to be parallel to the BC2N layer (see Fig. 5), hence the average binding energy is generally small [10]. In contrast, the average binding energy for three or more H2 molecules which are titled around Ca adatom is larger. Fig. 6 gives the PDOS of the nH2/Ca/BC2N (n = 1–4) systems. Similarly, the orbital hybridization of H s orbitals and Ca s (d) orbitals is also found in H2/Ca/BC2N system (see Fig. 6a). In the case of 2H2/Ca/BC2N, the H s orbitals located far below the Fermi level are broadened and split to two peaks indicating the H2–H2 interaction is significant. Consequently, both the polarization mechanism and the orbital hybridization contribute to the adsorption of the H2 molecules to Ca/ BC2N. As more H2 are adsorbed on Ca, the band broadening of the H s orbitals becomes more extraordinary. On the one hand, these is orbital hybridization between the H s orbitals and Ca d (p) orbitals far below the Fermi level. On the other hand, H s orbi-
Fig. 6. The PDOS for (a) all H atoms and Ca atom in H2/Ca/BC2N, (b) all H atoms and Ca atom in 2H2/Ca/BC2N, (c) all H atoms and Ca atom in 3H2/Ca/BC2N, (d) all H atoms and Ca atom in 4H2/Ca/BC2N Fermi level is set to zero.
Metal atom
Maximum number of H2
BE (eV)
wt.%
Li Na K Ca Al
3 4 4 4 3
0.24 0.14 0.17 0.26 0.18
9.68 10.00 8.33 8.25 7.69
tals and Ca d orbitals also hybridize around Fermi level. The repulsive potential of hydrogen electrons pushes Ca s orbital into high energy state, the Kubas interaction becomes dominant for H2 adsorption, leading to larger adsorption energies [31]. The above results explain the small adsorption energy of the first hydrogen molecule and the increasing order in the adsorption energy of H2 on Ca/BC2N as shown in Fig. 4. For the cases of hydrogen adsorption on metal atoms that are adsorbed on both sides of BC2N, the maximum number of adsorbed H2, the corresponding average adsorption energy and the hydrogen storage capacity are listed in Table 2. We first pay attention to the Li2/BC2N system. When the first two H2 molecules are introduced to this system, the H–H bond length elongated slightly from 0.750 Å of the isolated molecule to 0.797 and 0.793 Å respectively (see Fig. 7). The average Ead is 0.23 eV/H2. Step by step, we add additional H2 molecules close to the Li atoms. Finally, each Li atom can adsorb three H2 molecules with an average adsorption energy of 0.24 eV/H2, and the hydrogen storage capacity is up to 9.68 wt.%. For the Ca2/BC2N system, each Ca atom can catch four H2 molecules. The average adsorption energy and the hydrogen storage capacity are 0.26 eV/H2 and 8.25 wt.%, respectively. To minimize the adatom–adatom interaction, we test the hydrogen storage behavior of one Li adatom adsorbed on a (4 4) supercell. It is found that the binding energy of Li is 2.10 eV/Li which is larger than that in the (2 2) supercell (1.83 eV/Li). As a result, the Li atom dispersed sparsely is more stable. The maximum number of adsorbed H2 molecules per adsorbed Li atom is five for the (4 4) supercell and four for the (2 2) supercell. The average binding energy of H2 is 0.20 eV/H2 which is larger than that in the system of Li decorated (2 2) supercell (0.19 eV/H2). Although the gravimetric density of H2 is lower, Li adsorption at lower coverage on the BC2N has its advantages: the structure is stable which leads to a higher hydrogen binding energy. For hydrogen binding, the generalized gradient approximation (GGA) [32] and LDA are known to provide a lower and an upper bound of a more accurate energy obtained with highly correlated methods such as MP2 or coupled-cluster with singles and doubles and perturbative triples correction [33]. For a cross check, we repeat the binding energy of H2 in the system 2H2/2Li/BC2N with the GGA. Here the effect of van der Waals (vdW) interactions is included explicitly by using the empirical correction scheme of
Fig. 7. The optimized configuration of (a) 2H2/2Li/BC2N, (b) 4H2/2Li/BC2N, (c) 6H2/ 2Li/BC2N.
Y.S. Wang et al. / Computational Materials Science 60 (2012) 181–185
Grimme (DFT + D2) [34], as implemented by Bucko et al. for periodic systems [35]. The calculated average binding energy of H2 is 0.23 eV/H2 which is the same as that with the LDA. Therefore, we only give the LDA results in this paper. Given the strength of the metal-host interaction discussed above, other metal atoms such as Na, K and Al are also considered as adatoms to capture hydrogen molecules. The results reveal that Na and K atoms each one can adsorb four H2 molecules with the average binding energy of 0.14 and 0.17 eV/H2. The corresponding hydrogen capacities are 10.0 and 8.33 wt.%, respectively. For the last system of Al2/BC2N, with three H2 molecules adsorption on each Al atom, the hydrogen capacity can reach up to 7.69 wt.% with an average adsorption energy of 0.18 eV/H2. 4. Conclusion In conclusion, metal-decorated BC2N complexes are investigated by first-principles calculations for H2 adsorption. Our calculations indicate that several metals such as Li, Na, Ca, and Al are adsorbed above and below T1 site, and K adsorbed T3 site stably and are able to adsorb multiple H2 molecules. In the case of Li decorated BC2N layer, the clustering of adsorbed Li atoms is hindered by the Coulomb repulsion. Through the polarization mechanism and the orbital hybridization each Li atom can capture three H2 molecules. The hydrogen capacity can reach up to 9.68 wt.% with an average adsorption energy of 0.24 eV/H2, which is likely for the system to operate under ambient thermodynamic conditions. Other atoms (i.e. Na, K, Ca, Al) decorated BC2N layer can also store hydrogen in molecular form with gravimetric density of 7.69– 10.0 wt.%. The resulting average adsorption energies are in the range of 0.14–0.26 eV/H2, which falls within the ideal hydrogen storage binding range. Acknowledgements We thank Professor Z.X. Guo for helpful discussions. The work was support partly by the NSF of China (Grant Nos. 10974182,
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10874154), partly by Innovation Scientists and Technicians Troop Construction Projects (ISTTCP) of Henan Province, and partly by Program for Science and Technology Innovation Talents in University (HASTIT) of Henan Province. The calculations were performed on the High Performance Clusters of Zhengzhou University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
L. Schlapbach, A. Züttel, Nature 414 (2001) 353. B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. A. Züttel, Mater. Today 6 (2003) 24. J.A. Turner, Science 305 (2004) 972. S.K. Bhatia, A.L. Myers, Langmuir 22 (2006) 1688. C. Ataca, E. Aktürk, S. Ciraci, H. Ustunel, Appl. Phys. Lett. 93 (2008) 043123. Q. Sun, P. Jena, Q. Wang, M. Marquez, J. Am. Chem. Soc. 128 (2006) 9741. K.R.S. Chandrakumar, S.K. Ghosh, Nano Lett. 8 (2008) 13. W. Liu, Y.H. Zhao, Y. Li, Q. Jiang, E.J. Lavernia, J. Phys. Chem. C 113 (2009) 2028. C. Ataca, E. Aktürk, S. Ciraci, Phys. Rev. B 79 (2009) 041406(R). H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Nano Lett. 10 (2010) 793. H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Phys. Rev. B 80 (2009) 115412. M. Yoon, S. Yang, C. Hicke, E. Wang, D. Geohegan, Z. Zhang, Phys. Rev. Lett. 100 (2008) 206806. T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. E. Durgun, S. Ciraci, W. Zhou, T. Yildirim, Phys. Rev. Lett. 97 (2006) 226102. M. Li, J. Li, Q. Sun, Y. Jia, J. Appl. Phys. 108 (2010) 064326. H. Lee, W.I. Choi, J. Ihm, Phys. Rev. Lett. 97 (2006) 056104. A.K. Geim, K.S. Novoselov, Nature Mater. 6 (2007) 183. M. Kawaguchi, Adv. Mater. 9 (1997) 615. J.P. Nicolich, F. Hofer, G. Brey, R. Riedel, J. Am. Ceram. Soc. 84 (2001) 279. A.Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Phys. Rev. B 39 (1989) 1760. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. I. Cabria, M.J. López, J.A. Alonso, J. Chem. Phys. 123 (2005) 204721. M. Yoon, S. Yang, E. Wang, Z. Zhang, Nano Lett. 7 (2007) 2578. C. Kittel, Introduction to Solid State Physics, eighth ed., Wiley, New York, 2005. Z.M. Ao, F.M. Peeters, Phys. Rev. B 81 (2010) 205406. Q. Sun, Q. Wang, P. Jena, Y. Kawazoe, J. Am. Chem. Soc. 127 (2005) 14582. G. Henkelman, A. Arnaldsson, H. Jonsson, Comput. Mater. Sci. 36 (2006) 354. H. An, C.S. Liu, Z. Zeng, C. Fan, X. Ju, Appl. Phys. Lett. 98 (2011) 173101. G. Kim, S. Jhi, S. Lim, N. Park, Phys. Rev. B 79 (2009) 155437. J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. Y. Gao, X.C. Zeng, J. Phys.: Condens. Matter 19 (2007) 386220. S. Grimme, J. Comput. Chem. 27 (2006) 1787. T. Bucˇko, J. Hafner, S. Lebègue, J.G. Ángyán, J. Phys. Chem. A 114 (2010) 11814.
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Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Metal decorated monolayer BC2N for hydrogen storage Yu Sheng Wang a,b, Peng Fei Yuan a, Meng Li a, Wei Fen Jiang b, Qiang Sun a, Yu Jia a,⇑ a b
Center of Clean Energy and Quantum Structures, and School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450052, China College of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou, Henan 450011, China
a r t i c l e
i n f o
Article history: Received 4 November 2011 Received in revised form 11 March 2012 Accepted 14 March 2012 Available online 19 April 2012 Keywords: Hydrogen storage Nanostructure Binding energy
a b s t r a c t Using first-principles density functional calculations, we show that metal decorated monolayer BC2N can serve as a high-capacity hydrogen storage medium with the gravimetric density of 7.69–10.0 wt.%. In particular, Li and Ca decorated BC2N are systematically investigated and found to be feasible for hydrogen storage without metal clustering owing to the Coulomb repulsion between the metal adatoms. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of H2 molecules and the average adsorption energies are 0.24 and 0.26 eV/H2, respectively. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Because of its abundant, renewable, pollution-free, nontoxic properties, and packing more energy per unit mass than any other fuel, hydrogen has been actively considered as the best substitute for fossil fuels in the future [1–4]. However, realizing hydrogen economy has substantial difficulties to overcome. Among these, the most difficult challenge is to find materials that can store hydrogen with large gravimetric and volumetric density and operate under ambient thermodynamic conditions. The US Department of Energy (DOE) has set the gravimetric density of 9 wt.% in 2015 for usable specific energy from H2. To achieve this goal, the storage materials should consist of only light elements. Another important criterion is the binding energy of the adsorbed H2 molecules should ideally be about 0.2 eV/H2 [5]. If the binding energy is too high, release of the hydrogen will be difficult at moderate operating temperatures, while if the binding energy is too weak, storage of the hydrogen will be ineffective. Current hydrogen storage technologies, such as cryogenic liquid, high-pressure gas cells, lowtemperature adsorbates, metal hydrides and chemical storage, cannot meet all the industry requirements. In recent years, metal-decorated nanostructures have widely attracted attention as potential hydrogen storage medias due to their light weights and large surface areas, which have achieved better results than the corresponding pristine substrate materials. For example, coating carbon nanostructures (i.e. graphenes, carbon fullerenes, carbon nanotubes) with alkaline metals (AMs) [6–9] and
⇑ Corresponding author. Tel./fax: +86 3716776118. E-mail address: [email protected] (Y. Jia). 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.03.028
alkaline-earth metals (AEMs) [10–13] can adsorb H2 with high gravimetric densities through static Coulomb interactions. On the other hand, transition metal (TM) decorated materials have H2 adsorption energies in the range of 0.3–0.8 eV/H2 via the Kubas mechanism [14–17]. However, TM tends to cluster due to its high cohesive energy, and the storage capability falls short of theoretical predictions. Moreover, the dissociation of the first H2 molecule added to the TM atoms will also lower the gravimetric density [15,16]. Hence the challenge to find appropriate H2 sorbents is far from being solved. Since carbon nanostructures have attracted rapidly growing research interest because of its various intriguing properties and potential applications in future electronics [18], the synthesis of new carbon-based compounds has also stimulated considerable interest for some years. A possibility is the formation of the BxCyNz compounds where carbon atoms of graphite are partially substituted by boron and nitrogen atoms and there has been some success in preparing BxCyNz compositions [19,20]. These compounds have been the focus of intensive studies recently because of the relevance of such materials to graphene, which is a two-dimensional network sp2-hybridized carbon atom. In addition, the BC2N thin films have been synthesized using chemical vapor deposition method with BCl3 and CH3CN as the starting materials. The atomic arrangement and electronic structure of the monolayer structures have been studied extensively [20,21]. Previous studies prove that metal decorated graphene can be used as good hydrogen storage material. The BC2N sheet has graphene-like structure which inspires us to consider whether metal decorated monolayer BC2N is also efficient hydrogen storage medium. So far as I know, no relevant research has been concentrated on the hydrogen storage of BC2N.
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B 4.98 Å
C N
T3
T2 T1
B2
B1 B3
B4
Fig. 1. The unit cell and optimized geometry for BC2N layer.
In this article, we conduct theoretical studies of high-capacity hydrogen storage media consisting of monolayer BC2N and metal atoms. As a prototype structure of BC2N sheets we consider the geometry which has inversion symmetry, each C atom has a C, B, and N atom nearest neighbor, while B (N) atom has two C atoms and one N (B) atom as nearest neighbors (see Fig. 1). Metals considered here include AMs (Li, Na, and K), AEMs (Be, Mg, and Ca), TMs (Ti, V, Cr, and Fe) and Al. It is found that the adsorption of metal atoms (except K) above and below the hollow site of boron carbon hexagon in BC2N resulting in stable configurations. AMs, Ca and Al as adatoms should be feasible for hydrogen storage. Each metal atom can bind up to 3–4 H2 molecules, respectively, corresponding to storage capacities of 7.69–10.0 wt.%. The average adsorption energies of H2 molecules on Li, Na, K, Ca and Al decorated BC2N are 0.24, 0.14, 0.17, 0.26 and 0.18 eV/H2, respectively, which are in the required energy window allowing hydrogen adsorption and desorption feasible at ambient temperatures and pressures. 2. Computational method All our calculations are carried out using first-principles method based on density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [22,23]. Prior works have proven that the physisorption results obtained from local density approximation (LDA) are in substantial agreement with experiments [24,25]. Therefore, LDA with projector-augmented wave pseudopotentials is used. The supercell contains four C atoms, two B atoms and two N atoms in our calculations. The energy cutoff for the plane-wave basis set is 450 eV with a vacuum layer thickness larger than 15 Å. The Brillouin zone of the supercell is sampled by 4 4 1 k-points within the Monkhorst–Pack scheme. The number of k-points in Brillouin zone is carefully tested by using the larger k points, 6 6 1, thus to ensure that the changes of total energy are less than 103 eV in our calculations. All the obtained structures are optimized without any symmetry constraints until the force on each atom is less than 102 eV/ Å and the total energy changes are less than 104 eV. 3. Results and discussion Our discussion begins with the binding of single metal atoms on BC2N sheet. There are seven different adsorption sites as shown in Fig. 1, which are the hollow center of the B–C hexagon (T1), the hollow center of the B–C–N hexagon (T2), the hollow center of the C–N hexagon (T3), the bridge of C–C bond (B1), the bridge of B–C (B2), the bridge of C–N (B3), and the bridge of B–N (B4). After full relaxation, it is found that the bridge sites are energetically unfavorable compared to T sites for all metals. The T1 site is energetically favorable for most bond metal atoms except K which prefers T3 site. The calculated binding energies (BEs) and the corresponding cohesive energies of the respective metal atoms are shown in Fig. 2. It is clear from Fig. 2a that the BEs of AMs and Ca are higher than the corresponding cohesive energies of bulk metals, which imply a rel-
Fig. 2. (a) Calculated metal binding energies on a BC2N layer and the corresponding experimental cohesive energies of the respective metal atoms including AMs, AEMs and Al (solid bars) (Ref. [26]). The distances between metal atoms and BC2N layer are denoted by solid squares (b) TMs.
ative stability of atomic metal atoms against clustering. While the BEs of Be, Mg and Al are lower than the corresponding cohesive energies. The BEs are correlated with the metal–BC2N distance (dMB) as shown in Fig. 2a. It turns out that, for the atoms in the same group, the smaller dMB is, the larger BE is. Fig. 2b displays the BEs and the corresponding cohesive energies of TMs. The BEs are little than the corresponding cohesive energies for all the TMs. That is to say, TMs tend to cluster. Hence, we next study the hydrogen storage of metal decorated BC2N only including Li, Na, K, Ca and Al (the BE of Al is comparable to the corresponding cohesive energy). Moreover, the BEs of the same metal atoms placed on the T1 site of BC2N sheet are consistently higher than that on top of the hexagon of a pure graphene sheet [6,10,27]. Meanwhile, in the case of metal decorated BC2N sheet, the metal–metal interaction is indeed negligible owing to the large distance of about 4.98 Å (see Fig. 1). These results indicate that the metal–BC2N interaction is strong and then stable. It is well known that the diffusion of metal atoms on the substrate might lead to clustering [28], which will limit the hydrogen uptake. To further confirm the stability of metal atoms on BC2N sheet, as a prototype system we test the total energy as the Li atom moves on the substrate. As shown in Fig. 3, there are four Li atoms on a (2 2) cell of BC2N. When the bottom-left Li atom moves through the positions from M0 to M6, its z coordinate is optimized whereas the remaining three Li atoms are fully relaxed. Beyond the position M4 of this Li atom, the Coulomb repulsion will push the bottomright Li atom to maintain a distance with it. The variation of the energy with the positions of Li atoms is indicated in Fig. 3b. It is found that the calculated diffusion barrier from M0 to M4 is 0.491 eV. For the position M5, z coordinate of the bottom right Li atom is optimized while two other coordinates keep unchangeably. After full relaxation, the vertical distance between the bottom-left Li and the BC2N sheet dLi-sheet increases from 1.676 Å to 3.723 Å while the distance between two bottom Li atoms dLi–Li is 2.784 Å. Similarly, in the case of position M6, dLi-sheet is 4.270 Å and dLi–Li is 2.852 Å. Thus, the original structure is energetically favorable owing to the repulsion between metal Li atoms, which can effectively hinder the mobility of metal on the substrate. Therefore, the long
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(b)
(a) Li
M 0 M1 M2 M3 M4 M 5 M6 Fig. 3. (a) Top view of the (2 2) cell of Li/BC2N complex. (b) The variation in total energy as the bottom-left Li atom moves through the positions from M0 to M6 indicated in (a).
distance of metal–metal, the relative strong bonding between the metal atoms and the BC2N layer, and the Coulomb repulsion between the metal atoms lead to the prevention of metal clustering on the surface. We consider next the double-sided adsorption of metal atoms in order to increase the available surface area for hydrogen storage. As shown in Fig. 1, there are three possible sites, i.e. T1, T2, and T3, for the second metal atom to be positioned on the other side of the BC2N layer. In fact, according to the calculated results, the two metal atoms prefer to occupy the T1 sites but on opposite sides of the BC2N layer except K atoms which prefer to locate on opposite T3 sites. The corresponding BEs of the respective metal atoms are also shown in Fig. 2. Compared to the systems with the single metal adatoms, the BEs are almost no changes. Therefore, each metal atom, such as AMs, Ca and Al, can stay on its site stably as well as the strong binding itself. We now enter the next phase of our calculation, namely, to study H2 adsorption on metal-decorated BC2N. The average adsorption energy Ead per H2 to the system is defined as the following term: Ead ¼ ½Etotal EmþBC2 N nEH2 =n, where n indicates the number of H2 molecules. Etotal, EmþBC2 N and EH2 are the energies of the considered system, the metal-decorated BC2N and hydrogen molecules, respectively. By this definition, the positive value of Ead corresponds that the adsorption is exothermic and hence stable. As prototype systems, we study in detail about the hydrogen storage properties of Li/BC2N and Ca/BC2N. Fig. 4 shows the average H2 adsorption energy on the Li(Ca) decorated BC2N. Each Li adatom can bind four H2 molecules while each Ca adatom binds five H2 molecules. There is a significant difference in adsorption energy of H2 between Li/BC2N and Ca/BC2N. The Ead in Li decorated BC2N is found to lie in the range of 0.19–0.26 eV/H2 while Ead in Ca decorated BC2N is in the range of 0.06–0.27 eV/H2. Table 1 lists the average bond length of H–H (dH–H) (Å), the average distance between Li(Ca) atom and H2 molecule (dLi–H/dCa–H) of these two systems with different numbers of H2. It is evident from Table 1 that
all the bond lengths of H2 are longer than that in the isolated molecular state (0.750 Å). In the case of hydrogen molecules adsorption on the Li decorated BC2N, the average bond length of dH–H increases from 0.787 Å (for the one H2 adsorption case) to 0.788 Å (for the two H2 adsorption case), and then decreases to 0.778 Å (for the four H2 adsorption case). This is in accord with the fact that the average adsorption energy of H2 to the Li/BC2N system first increase from 0.24 to 0.26 eV/H2 and then decrease to 0.19 eV/H2, as shown in Fig. 4. As the number of adsorbed H2 molecules increases from two to four, the average distance between Li atom and H2 is slightly increased from 1.937 Å to 2.458 Å. This finding also corroborates the decrease of the average adsorption energy of H2. For the system of nH2/Ca/BC2N, the average dH–H increases from 0.782 Å (for the one H2 adsorption case) to 0.824 Å (for the four H2 adsorption case), and then decreases to 0.813 Å (for the five H2 adsorption case). While the average dCa–H decreases from 2.501 Å to 2.291 Å then increases to 2.413 Å. Correspondingly, the average adsorption energy of H2 first increase from 0.06 to 0.28 eV/H2 and then decrease to 0.26 eV/H2. In order to investigate the magnitude of charges transferred from the adatoms to BC2N, we performed a Bader charge analysis on these systems [29]. It is found that each Li adatom donates 0.95 valence electrons to BC2N, therefore, Li atom becomes ionized and then binds H2 through static Coulomb interaction [7,30]. To further understand the adsorption mechanism, the PDOS of the nH2/Li/BC2N (n = 1, 2) systems and the three-dimensional electron charge density difference for H2/Li/BC2N are plotted in Fig. 5. As shown in Fig. 5a, the peak of Li 2s (2p) orbitals around 9.28 eV hybridizes with the H s orbitals, resulting in a electron transfer between H s orbitals and Li 2s (2p) orbitals in the H2/Li/BC2N system. From the different charge density inserted in Fig. 5a, we can clearly find that there is an induced dipole for the adsorbed hydrogen molecule, with electron density accumulation on the side close to the Li/BC2N and depletion on the side away from the Li atom. Previous studies have shown that the band broadening of the molecular level of H2 below the Fermi energy indicates a significant H2–H2 interaction that in turn increases its adsorption energy to the substrate [10]. The same mechanism is found in the 2H2/Li/BC2N system where the band broadening of about 8 eV in Fig. 5b. As a
Table 1 Average bond length of H–H (dH–H) (Å), bond length between metal atom (M = Li and Ca) and H2 molecule (dM–H) (Å). The number of H2
Fig. 4. Calculated H2 adsorption energy on the Li(Ca) decorated BC2N. The optimized configurations of hydrogen molecules adsorbed on Ca decorated BC2N are shown as inserts.
1 2 3 4 5
Li
Ca
dH–H (Å)
dLi–H (Å)
dH–H (Å)
dCa–H (Å)
0.787 0.788 0.781 0.778
1.918 1.937 2.237 2.458
0.782 0.804 0.827 0.824 0.813
2.501 2.370 2.282 2.291 2.413
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Y.S. Wang et al. / Computational Materials Science 60 (2012) 181–185 Table 2 The maximum number of adsorbed H2, the average H2 binding energy (BE) and the corresponding hydrogen gravimetric density of H2 adsorbed on the double-sided decorated BC2N with different metal atoms (Li, Na, K, Ca, Al).
Fig. 5. The PDOS for (a) all H atoms and Li atom in H2/Li/BC2N, (b) all H atoms and Li atom in 2H2/Li/BC2N. Fermi level is set to zero. The isosurface of charge density difference with an isovalue of 0.002 e/Å3 for the system of H2/Li/BC2N is shown as an insert. Red and blue colors of the isosurface represent electron accumulation and deletion regions after H2 adsorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
result, the nature of interaction between H2 molecules and Li-decorated BC2N relates not only to the polarization mechanism but also to the orbital hybridization. This phenomenon is similar to the cases of Li-doped B2C graphene [30]. It is important to note that the adsorption energies of the first two H2 molecules in Li/BC2N are larger than those in Ca/BC2N. In the case of Ca-decorated BC2N, Ca adatom is also ionized. Again, the Ca ion adsorbs H2 molecules through static Coulomb interaction. The Bader charge analysis shows that 0.71 electrons are transferred from Ca adatom to the BC2N sheet which is little than that of Li atom in Li/BC2N system. This is the reason why the Li ion catches first two H2 molecules more strongly than the Ca ion. In addition, the first and the second H2 molecules prefer to be parallel to the BC2N layer (see Fig. 5), hence the average binding energy is generally small [10]. In contrast, the average binding energy for three or more H2 molecules which are titled around Ca adatom is larger. Fig. 6 gives the PDOS of the nH2/Ca/BC2N (n = 1–4) systems. Similarly, the orbital hybridization of H s orbitals and Ca s (d) orbitals is also found in H2/Ca/BC2N system (see Fig. 6a). In the case of 2H2/Ca/BC2N, the H s orbitals located far below the Fermi level are broadened and split to two peaks indicating the H2–H2 interaction is significant. Consequently, both the polarization mechanism and the orbital hybridization contribute to the adsorption of the H2 molecules to Ca/ BC2N. As more H2 are adsorbed on Ca, the band broadening of the H s orbitals becomes more extraordinary. On the one hand, these is orbital hybridization between the H s orbitals and Ca d (p) orbitals far below the Fermi level. On the other hand, H s orbi-
Fig. 6. The PDOS for (a) all H atoms and Ca atom in H2/Ca/BC2N, (b) all H atoms and Ca atom in 2H2/Ca/BC2N, (c) all H atoms and Ca atom in 3H2/Ca/BC2N, (d) all H atoms and Ca atom in 4H2/Ca/BC2N Fermi level is set to zero.
Metal atom
Maximum number of H2
BE (eV)
wt.%
Li Na K Ca Al
3 4 4 4 3
0.24 0.14 0.17 0.26 0.18
9.68 10.00 8.33 8.25 7.69
tals and Ca d orbitals also hybridize around Fermi level. The repulsive potential of hydrogen electrons pushes Ca s orbital into high energy state, the Kubas interaction becomes dominant for H2 adsorption, leading to larger adsorption energies [31]. The above results explain the small adsorption energy of the first hydrogen molecule and the increasing order in the adsorption energy of H2 on Ca/BC2N as shown in Fig. 4. For the cases of hydrogen adsorption on metal atoms that are adsorbed on both sides of BC2N, the maximum number of adsorbed H2, the corresponding average adsorption energy and the hydrogen storage capacity are listed in Table 2. We first pay attention to the Li2/BC2N system. When the first two H2 molecules are introduced to this system, the H–H bond length elongated slightly from 0.750 Å of the isolated molecule to 0.797 and 0.793 Å respectively (see Fig. 7). The average Ead is 0.23 eV/H2. Step by step, we add additional H2 molecules close to the Li atoms. Finally, each Li atom can adsorb three H2 molecules with an average adsorption energy of 0.24 eV/H2, and the hydrogen storage capacity is up to 9.68 wt.%. For the Ca2/BC2N system, each Ca atom can catch four H2 molecules. The average adsorption energy and the hydrogen storage capacity are 0.26 eV/H2 and 8.25 wt.%, respectively. To minimize the adatom–adatom interaction, we test the hydrogen storage behavior of one Li adatom adsorbed on a (4 4) supercell. It is found that the binding energy of Li is 2.10 eV/Li which is larger than that in the (2 2) supercell (1.83 eV/Li). As a result, the Li atom dispersed sparsely is more stable. The maximum number of adsorbed H2 molecules per adsorbed Li atom is five for the (4 4) supercell and four for the (2 2) supercell. The average binding energy of H2 is 0.20 eV/H2 which is larger than that in the system of Li decorated (2 2) supercell (0.19 eV/H2). Although the gravimetric density of H2 is lower, Li adsorption at lower coverage on the BC2N has its advantages: the structure is stable which leads to a higher hydrogen binding energy. For hydrogen binding, the generalized gradient approximation (GGA) [32] and LDA are known to provide a lower and an upper bound of a more accurate energy obtained with highly correlated methods such as MP2 or coupled-cluster with singles and doubles and perturbative triples correction [33]. For a cross check, we repeat the binding energy of H2 in the system 2H2/2Li/BC2N with the GGA. Here the effect of van der Waals (vdW) interactions is included explicitly by using the empirical correction scheme of
Fig. 7. The optimized configuration of (a) 2H2/2Li/BC2N, (b) 4H2/2Li/BC2N, (c) 6H2/ 2Li/BC2N.
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Grimme (DFT + D2) [34], as implemented by Bucko et al. for periodic systems [35]. The calculated average binding energy of H2 is 0.23 eV/H2 which is the same as that with the LDA. Therefore, we only give the LDA results in this paper. Given the strength of the metal-host interaction discussed above, other metal atoms such as Na, K and Al are also considered as adatoms to capture hydrogen molecules. The results reveal that Na and K atoms each one can adsorb four H2 molecules with the average binding energy of 0.14 and 0.17 eV/H2. The corresponding hydrogen capacities are 10.0 and 8.33 wt.%, respectively. For the last system of Al2/BC2N, with three H2 molecules adsorption on each Al atom, the hydrogen capacity can reach up to 7.69 wt.% with an average adsorption energy of 0.18 eV/H2. 4. Conclusion In conclusion, metal-decorated BC2N complexes are investigated by first-principles calculations for H2 adsorption. Our calculations indicate that several metals such as Li, Na, Ca, and Al are adsorbed above and below T1 site, and K adsorbed T3 site stably and are able to adsorb multiple H2 molecules. In the case of Li decorated BC2N layer, the clustering of adsorbed Li atoms is hindered by the Coulomb repulsion. Through the polarization mechanism and the orbital hybridization each Li atom can capture three H2 molecules. The hydrogen capacity can reach up to 9.68 wt.% with an average adsorption energy of 0.24 eV/H2, which is likely for the system to operate under ambient thermodynamic conditions. Other atoms (i.e. Na, K, Ca, Al) decorated BC2N layer can also store hydrogen in molecular form with gravimetric density of 7.69– 10.0 wt.%. The resulting average adsorption energies are in the range of 0.14–0.26 eV/H2, which falls within the ideal hydrogen storage binding range. Acknowledgements We thank Professor Z.X. Guo for helpful discussions. The work was support partly by the NSF of China (Grant Nos. 10974182,
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10874154), partly by Innovation Scientists and Technicians Troop Construction Projects (ISTTCP) of Henan Province, and partly by Program for Science and Technology Innovation Talents in University (HASTIT) of Henan Province. The calculations were performed on the High Performance Clusters of Zhengzhou University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
L. Schlapbach, A. Züttel, Nature 414 (2001) 353. B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. A. Züttel, Mater. Today 6 (2003) 24. J.A. Turner, Science 305 (2004) 972. S.K. Bhatia, A.L. Myers, Langmuir 22 (2006) 1688. C. Ataca, E. Aktürk, S. Ciraci, H. Ustunel, Appl. Phys. Lett. 93 (2008) 043123. Q. Sun, P. Jena, Q. Wang, M. Marquez, J. Am. Chem. Soc. 128 (2006) 9741. K.R.S. Chandrakumar, S.K. Ghosh, Nano Lett. 8 (2008) 13. W. Liu, Y.H. Zhao, Y. Li, Q. Jiang, E.J. Lavernia, J. Phys. Chem. C 113 (2009) 2028. C. Ataca, E. Aktürk, S. Ciraci, Phys. Rev. B 79 (2009) 041406(R). H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Nano Lett. 10 (2010) 793. H. Lee, J. Ihm, M.L. Cohen, S.G. Louie, Phys. Rev. B 80 (2009) 115412. M. Yoon, S. Yang, C. Hicke, E. Wang, D. Geohegan, Z. Zhang, Phys. Rev. Lett. 100 (2008) 206806. T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 (2005) 175501. E. Durgun, S. Ciraci, W. Zhou, T. Yildirim, Phys. Rev. Lett. 97 (2006) 226102. M. Li, J. Li, Q. Sun, Y. Jia, J. Appl. Phys. 108 (2010) 064326. H. Lee, W.I. Choi, J. Ihm, Phys. Rev. Lett. 97 (2006) 056104. A.K. Geim, K.S. Novoselov, Nature Mater. 6 (2007) 183. M. Kawaguchi, Adv. Mater. 9 (1997) 615. J.P. Nicolich, F. Hofer, G. Brey, R. Riedel, J. Am. Ceram. Soc. 84 (2001) 279. A.Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Phys. Rev. B 39 (1989) 1760. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. I. Cabria, M.J. López, J.A. Alonso, J. Chem. Phys. 123 (2005) 204721. M. Yoon, S. Yang, E. Wang, Z. Zhang, Nano Lett. 7 (2007) 2578. C. Kittel, Introduction to Solid State Physics, eighth ed., Wiley, New York, 2005. Z.M. Ao, F.M. Peeters, Phys. Rev. B 81 (2010) 205406. Q. Sun, Q. Wang, P. Jena, Y. Kawazoe, J. Am. Chem. Soc. 127 (2005) 14582. G. Henkelman, A. Arnaldsson, H. Jonsson, Comput. Mater. Sci. 36 (2006) 354. H. An, C.S. Liu, Z. Zeng, C. Fan, X. Ju, Appl. Phys. Lett. 98 (2011) 173101. G. Kim, S. Jhi, S. Lim, N. Park, Phys. Rev. B 79 (2009) 155437. J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. Y. Gao, X.C. Zeng, J. Phys.: Condens. Matter 19 (2007) 386220. S. Grimme, J. Comput. Chem. 27 (2006) 1787. T. Bucˇko, J. Hafner, S. Lebègue, J.G. Ángyán, J. Phys. Chem. A 114 (2010) 11814.