Hydrogen Storage in Silicon Carbide Nanotubes by Lithium Doping

ARTICLE pubs.acs.org/JPCC

Hydrogen Storage in Silicon Carbide Nanotubes by Lithium Doping Xiao Wang and K. M. Liew* Department of Building and Construction, City University of Hong Kong, Kowloon, Hong Kong SAR ABSTRACT: Using ab initio density-functional theory, we have studied the interaction of hydrogen molecules (H2) with a single lithium (Li)-doped silicon carbide nanotube (SiCNT). The hydrogen molecule physisorbs on a pure SiCNT with a binding energy of about 0.086 eV. However, the binding energy rises to 0.211 eV when H2 binds to a Li-adsorbed SiCNT. The increase in binding is due to the charge transfer from Li to the nanotube. Up to four H2 molecules can be attached to a Liadsorbed SiCNT with an average binding energy of 0.165 eV, which is close to the lowest requirement proposed by the U.S. Department of Energy, and it indicates that this system is a good storage medium for H2.

1. INTRODUCTION With the rapid consumption of fossil energy resources and increasing environmental pollution, there is an urgent need to develop alternatives that are environmentally friendly and renewable energy sources and carriers. Hydrogen is a rich resource that exists in the gaseous state and by far is the best alternative fuel due to its unique qualities of being nonpolluting and renewable and to its high thermal energy efficiency. The application of hydrogen as a fuel is still very limited because of the lack of economic ways for its storage. Carbon-based materials, such as carbon nanotubes, have attracted much attention as a storage medium for molecular hydrogen due to their high surface area and light weight.1-3 However, experimental studies have shown that, at room temperature and under ambient pressure, the hydrogen-storage capability and the binding energy of H2 with pristine carbon nanotubes are both too small, which may not exceed 1 wt % and 0.030 eV (1 eV ≈ 23.06 kcal/mol), respectively.4-7 Attempts have thus been made to improve the binding of H2 with functionalized carbon nanotubes. Previous experimental and theoretical studies have shown that doping transition-metal atoms or alkali atoms on carbon nanotubes can appreciably enhance the adsorption energy of H2.8-16 Yildirim and Ciraci, for example, studied a titanium (Ti)-decorated carbon nanotube surface and found that up to 8 wt % of hydrogen could be absorbed on the Ti-doped carbon nanotube if the Ti atoms could be uniformly coated on the nanotube surface.12 Sun et al., however, showed that Ti prefers to cluster on fullerene C60 surfaces and is thus not effective in increasing the binding of H2 to the Ti-doped fullerene.17 They further studied a lithium (Li)dispersed fullerene system and found that Li atoms rather prefer to stay as atoms on these surfaces because the binding energy between Li and C60 is slightly higher than the cohesive energy of Li clustering. Hydrogen molecules bind to the Li on the carbon nanotube surface with a binding energy of 0.075 eV/H2 and a capability of 13 wt %.18 Cabria et al. studied the interaction of H2 r 2011 American Chemical Society

with Li-doped graphene and carbon nanotubes using the firstprinciples density functional method and found that the binding energy of H2 with the Li-doped system was greater than that with a pure carbon system.14 In fact, the Li-doping surface modification technique becomes a novel research focus in hydrogen storage studies recently. Huang et al. reported that a Li-decorated benzene molecule could be used as high-capacity hydrogen storage media near room temperature from their first-principles calculations.19 Some theoretical and experimental studies have also found that doping the metal-organic frameworks (MOFs) with Li is a good strategy to enhance the hydrogen storage performance.20-23 Silicon carbide nanotubes (SiCNTs), which were first synthesized in 2001,24 have great potential for applications in highpowered and high-temperature electronics due to the high reactivity of the exterior surface, which facilitates sidewall decoration and stability against oxidation in air at high temperatures.25-27 Theoretical calculations show that the binding energy of H2 increases by approximately 20% with SiCNTs compared with that with pure carbon nanotubes due to the partially heteropolar binding nature of the Si-C bonds.28 This makes SiCNTs more suitable for hydrogen storage. In addition, experimental and theoretical results14,18 suggest that Li-doped carbon nanostructures not only improve the molecular hydrogen interaction with the material but also give a higher capability. Hence, it would be interesting to study the interaction of Li atoms with SiCNTs. In the study reported here, we performed first-principles simulations of the interaction of hydrogen molecules with systems formed by a Li impurity doped onto a (5,5) SiCNT and compared the results with those for the interaction with a pure SiCNT without Li doping. The simulations show that the hydrogen adsorption on the Li-doped SiCNT due to molecular physisorption is similar to Received: July 14, 2010 Revised: October 29, 2010 Published: February 10, 2011 3491

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that on a Li-doped carbon nanotube,14,29,30 but with a much greater binding energy.

2. MODEL SYSTEM AND COMPUTATIONAL METHODS In this work, the unrestricted spin-polarized density functional theory (DFT) calculations are carried out by using the Dmol31-33 package to study the interactions between a single Li atom and a single-walled SiCNT. This method has been widely used in theoretical calculations of carbon-based nanotube systems, including their functionalization with metal atoms.34-37 The all-electron calculations and a double numerical basis set plus polarization functional (DNP) are adopted.31 The generalized gradient approximation (GGA) with the Perdew-BurkeErnzerhof (PBE) correction is utilized throughout the paper.38 The positions of all the atoms are relaxed until all the force components are smaller than 8 meV/Å. To clarify the electronic nature of the Li-doped SiCNT, the Hirshfeld charge39 analysis is computed and discussed. A single-walled SiCNT (5,5), in which the chiral indexes (n,m) have the same meaning as those for a carbon nanotube, is constructed according to the principle that Si and C atoms are placed alternatively such that there are no adjacent atoms of the same type. The main body of this SiCNT consists of 45 carbon and 45 silicon atoms along the longitudinal direction. Hydrogen atoms are added to saturate the dangling bonds at the ends of the SiCNT, as shown in Figure 1. After full structural optimization, the average Si-C bond length of the SiCNT is about 1.796 Å, which is in good agreement with other ab initio results.26,40-42 The contribution of the zero-point vibrational energy (ZPE) to the total energy was not evaluated in this study. 3. RESULTS AND DISCUSSION First, we compute the interaction of a H2 molecule with a pure SiCNT. We have optimized the structure of a H2 molecule on the nanotube surface and define the binding energy as EðH2 Þ ¼ Etotal ðSiCNTÞ þ Etotal ðsingle H2 Þ - Etotal ðSiCNT with H2 Þ The binding energy is 0.086 eV, which is much higher than that with a pure carbon nanotube of 0.025-0.030 eV.7 Because of the strong 2p potential of the C atom, the valence charge is strongly accumulated around the C atom, resulting in large asymmetry in charge distribution, displaying features of typical ionic bindings. The charge transfer from Si to C is about 0.331 e for this SiCNT, as revealed from the Hirshfeld analysis, close to the value of 0.450 e obtained by Mulliken population analysis calculated by Zhao et al.26 The point charges on the walls of the SiCNT induce a dipole on the hydrogen molecule, which results in more efficient binding.28 We next studied the stable configurations of a single Li atom on a (5,5) SiCNT. Two kinds of structures are considered: internal and external. The binding energy (Eb) of the Li atom encapsulated by or adsorbed onto the SiCNT system is obtained from the following expression Eb ¼ EðSiCNTÞ þ EðLiÞ - EðLi þ SiCNTÞ where E(SiCNT) and E(Li) are the total energy of the pure SiCNT and a free Li atom. E(Li þ SiCNT) represents the total energy of the Li encapsulated by or adsorbed onto the SiCNT

Figure 1. Different binding sites of a single Li atom adsorbed onto a (5,5) single-walled SiCNT. Gray, yellow, and white balls denote carbon, silicon, and hydrogen atoms, respectively.

system. When the Li atom is encapsulated in the nanotube, after optimization, it retains its initial position (at the center of the tube), and the calculated binding energy is 0.55 eV. The Hirshfeld charge analysis indicates that about 0.149 electrons are transferred from the Li atom to the SiCNT. For the situation in which the Li atom is adsorbed onto the outside of the nanotube, we consider five different initial adsorption sites, as depicted in Figure 1. The five sites are (1) on top of the carbon atom (C), (2) on top of the silicon atom (S), (3) in the center of the hexagonal ring (H), (4) on top of the zigzag Si-C bond (Z), and (5) on top of the bridge Si-C bond (B). Two stable configurations at the H and C sites are obtained. Upon geometry optimization, a Li atom initially located at an S, Z, or B site spontaneously moved to a C or H site. The C site is the most energetically favorable site, with Eb = 1.41 eV. Table 1 shows the Li atom absorption energies, the shortest C-Li and Si-Li bond lengths, and the Hirshfeld charges for a Li atom on the C and H sites or in the center of the SiCNT. From the density of states projected to different atoms (PDOS) of the (5,5) SiCNT shown in Figure 2, it is obvious that the highest occupied molecular levels (HOMOs) and the lowest unoccupied molecular levels (LUMOs) are highly localized to C and Si atoms, respectively. This is coincident with the Hirshfeld results that the charge is strongly accumulated around C atoms. When a Li atom adsorbs at the exterior of the tube, because of the high electronegativity of C atoms, the Li atom prefers to bond with the C atom, rather than the Si atom.42 The Li atom denotes its 2s electrons, breaking the related Si-C π-π bonds nearby and resulting in a local state close to the LUMO, which can be observed in the DOS of the Li-doped SiCNT system (see Figure 3). Furthermore, the Si-C bond lengths in the SiCNT involving Li adsorption on the C site are 1.803 and 1.911 Å, respectively, which are longer than those in the perfect SiCNT (1.798 and 1.794 Å). To further understand the bonding characteristics, the deformation electron density of Li doped onto the C site of the SiCNT is calculated. Figure 4 represents the distribution of the deformation electron density in cross-section and shows that the charge accumulation on the C atom nearest the Li atom increases, whereas the charge depletion of the Si atom that is positioned nearest the Li atom decreased and the charge of the Li atom became depleted. The Hirshfeld charge analysis confirms this result by indicating that 0.383 e electrons are transferred from the Li 2s to the SiCNT surface, resulting in a partially cationic Li atom and the strengthened polarization between the Si-C bonds, especially the nearest one. The same charge transformation phenomenon is also observed when the Li is doped on the H site, as shown in Table 1. The Li 3492

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Table 1. Binding Energy, the Shortest C-Li and Si-Li Bond Lengths, and Hirshfeld Charges (in e Units) of the (5,5) SiCNT Doped with Li Li and H2 at the same exterior side of the SiCNT Eb (eV)

DC-Li

DSi-Li

QH

C

1.41

2.086

2.525

0.383

H

1.27

2.313

2.365

0.358

site

Li and H2 at opposite sides of the SiCNT (Li atom is encapsulated by the SiCNT) site

Eb (eV)

DC-Li

DSi-Li

QH

center

0.55

4.339

4.274

0.149

Figure 4. Charge density redistribution plot for atoms in a cross section of a Li-doped (5,5) SiCNT. The charge is accumulated in white regions and depleted in dark gray regions.

Figure 2. PDOS of the SiCNT projected to C (solid line) and Si (dotted line). The Fermi level is set to zero, as indicated by the vertical dotted line.

Figure 5. Optimized structure of a Li-doped SiCNT with one hydrogen molecule adsorbed.

Figure 3. DOS of a Li-decorated SiCNT. The Fermi level is set to zero, as indicated by the vertical dotted line.

atom adsorbs on the H site of the SiCNT with 0.358 e electrons that are transferred to the surface of the SiCNT.

Now we focus on the interaction between the H2 molecule and the Li atom adsorbed onto a SiCNT. Taking a Li adsorbed on the C site of the SiCNT system as an example, when one H2 molecule was introduced to this system, we have studied various hydrogen-binding sites near the Li atom: (a) on top of the Li atom, (b) on top of the Si-C bond along the zigzag direction, (c) on top of the Si-C bond along the bridge direction, (d) on top of the Si atom; and (e) at the center of the hexagonal ring (along the tube axis and perpendicular to the tube axis). For each binding site, we take two orientations of the H2 molecule: with the molecular axis perpendicular and parallel to the nanotube surface, respectively. For the most stable configuration of H2, the adsorbed molecule has a H-H bond length of 0.756 Å, as shown in Figure 5, which is slightly larger than the value of 0.749 Å for a free H2 molecule. The equilibrium Li-H distances are found to be 2.054 and 2.069 Å, respectively, which are very close to the value (2.040 Å) found in the case of a free Liþ ion interacting with H2.43 The shortest Li-C distance extends to 2.095 Å, and 3493

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the nearest Si-C bond length almost remains unchanged. The corresponding binding energy is obtained to be 0.211 eV, which is comparable to that in LiþH2 of 0.250 eV43 and higher than that in a Li-doped carbon nanotube.7,14 The binding of hydrogen in molecular form is governed by the same charge polarization mechanism that happened in the Li-doped CNT system,7,14,44,45 which can be proved by the increased dipole moment of the entire system. When the H2 molecule is adsorbed onto the Lidoped SiCNT, the dipole moment of the entire system is increased by 0.58 debye. However, it is observed that the two hydrogen atoms are asymmetrically “side on” adsorbed onto Li caused by the nearest negatively charged C atom. This can also be verified qualitatively from the Hiresheld charge analysis of the hydrogen in the Li-doped SiCNT. There is little charge (0.050 and 0.047 e) transferred from the two hydrogen atoms to Li (0.094 e added), and the charge of the nearest Si-C bond of SiCNT has almost no change (only 0.003 e added). Like the

interaction between H2 and a Li-doped SiCNT system on the C site described above, we also studied the interaction between H2 and a Li-doped SiCNT system on the H site. After examination of different initial configurations of the H2 molecules, a stable configuration with H2 molecules almost symmetrically adsorbed on Li can be obtained. It is found that the binding energy is about 0.208 eV, which is a little bit smaller than that of H2 on a Li-doped

Table 2. Average Adsorption Energy per H2 on a Li-Doped SiCNT H2 adsorbed Li-doped SiCNT

average adsorption energy per H2

complex

(eV)

1 H2@Li-SiCNT 2 H2@Li-SiCNT

0.211 0.202

3 H2@Li-SiCNT

0.179

4 H2@Li-SiCNT

0.164

Figure 6. Optimized structure of a Li-doped SiCNT with different numbers of hydrogen molecules adsorbed: (A) two H2, (B) three H2, and (C) four H2 (the four H2 molecules are numbered 1-4).

Figure 7. Average DOS plots per atom for the 4 H2@Li-SiCNT system. The four H2 molecules are labeled 1-4, as shown in Figure 6C. The Fermi level is set to zero and indicated by a short dotted line. 3494

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charge analysis shows that the charge transferred from the Li atom to the nanotube is 0.149 electrons, but remains located near the Li atom, as can be seen in Figure 8. Thus, when the molecule is adsorbed onto the SiCNT encapsulating a Li atom, it has a physisorption binding energy similar to that of the pure SiCNT case, as it does not feel the charge transferred by the Li atom to the nanotube.

Figure 8. Charge density redistribution plot for a (5,5) SiCNT encapsulating a Li atom.

SiCNT of the C site that could be rationalized due to the fact that Li on this system has only a little reduced localized charge (see Table 1). The above results show that H2 interacts with Li adsorbed onto the C site of the SiCNT and that it is the most energetically favorable site. For this reason, we increased the number of H2 molecules close to the Li atom in the system, and the calculated results predict that up to four H2 molecules can be adsorbed onto the Li atom in the Li-doped complex. Even the average adsorption energy per H2 decreases as the number of H2 increases (see Table 2). The binding energy per H2 in a 4 H2@Li-SiCNT system is 0.165 eV, which is close to the lowest requirement (0.020 eV per H2) proposed by the U.S. Department of Energy.46-48 In the case of multiple H2 adsorptions, several initial configurations of the H2 molecules are examined to search for the lowest-energy configuration, and the optimized structures are shown in Figure 6A-C. In Figure 6C, the four H2 molecules are numbered 1-4 to facilitate the discussion that follows. From the figures, it can be observed that the configurations of the attached H2 molecules are almost symmetrical. To further understand the electronic hybridization behavior, the DOS plots are determined and shown in Figure 7 for the 4 H2@Li-SiCNT system. The PDOS of H2 molecules 1 (H1) and 2 (H2) are almost the same with both main peaks being located at -8.14 eV. The main peaks for H2 molecules 3 (H3) and 4 (H4) occur at about -7.96 eV. Figure 7a-e shows that the peaks of Li located at -8.09 eV hybridize with the H in the lowerenergy range, where the σ bonding of the H2 molecule overlaps the Li 2s orbital. On the other hand, the bands of Li interact with those of C in the higher-energy range with a peak that can be observed around -2.24 eV (see Figure 7e,f). This suggests that Li acts as a “bridge” in this reaction and interacts with the molecule and the SiCNT simultaneously. This result rationalizes the observation that Li dopants have a strong influence in hydrogen storage systems. The Li atom partially donates its valence electron to the SiCNT, which strengthens the polarization of the Si-C bond, and binds H2 in molecular form due to the dipole caused by the polarization.28 This is confirmed by the Hirshfeld charge analysis, which reveals that every Li in the 4 H2@Li-SiCNT system carries a charge of þ0.235 e. To complete the study, the adsorption of a H2 molecule onto a SiCNT encapsulating a Li atom is also examined. The interaction energy is similar to that obtained for adsorption onto a pure SiCNT. Actually, the binding energy is a little smaller due to the curvature of the nanotube (Eb = 0.082 eV). The Hirshfeld

4. CONCLUSION In conclusion, we have carried out DFT calculations of the adsorption of a single Li atom onto a SiCNT and the physisorption of hydrogen molecules on pure and Li-doped SiCNTs. Geometric analysis shows that the Li atom prefers to be adsorbed at the top of the C atom. The calculations predict that the physisorption binding energies with the Li-adsorbed SiCNT are about 2.5 times larger than those with a pure SiCNT. Up to four H2 molecules can be attached to the Li-adsorbed SiCNT with an average binding energy of 0.165 eV. The Hirshfeld charge analysis shows that the Li atom partially donates its valence electron to the SiCNT, which strengthens the polarization between the Si-C bonds and enhances the interaction between the hydrogen molecules and the Liadsorbed surface. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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