Article pubs.acs.org/JPCC
Hydrogen Adsorption and Storage in Heteroatoms (B, N) Modified Carbon-Based Materials Decorated with Alkali Metals: A Computational Study Hsien-Wei Huang, Han-Ju Hsieh, I-Hsiang Lin, Yu-Jhe Tong, and Hsin-Tsung Chen* Department of Chemistry and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City 32023, Taiwan ABSTRACT: The hydrogen adsorption and storage capacity on various carbon-based ring materials decorated with alkali metal ions have been systematically investigated by using quantum chemistry calculations. To improve the bonding ability between H2 molecules and carbon-ring based molecular complexes, we also explain the strategy of modifying the carbon ring materials by substituting carbon element to boron or nitrogen element (up to three atoms). Our calculations show that the Li+-, Na+-, and K+decorated carbon-based molecular complexes with B- and N-substitution enhance the hydrogen storage capacity. The Mulliken charge analysis is performed to illustrate the interaction between H2 and M+@carbon-ring complex. The number of binding H2 molecules on the M+-carbon-ring complexes (M+ = Li+, Na+, and K+) depends on ionic radii of the metal cations. It is found that corresponding gravimetric density are predicted to be 11.21−13.95 and 10.42−13.24 wt % for the B- and N-substituted complexes, respectively.
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INTRODUCTION Hydrogen has been considered as a promising clean energy carrier due to its high energy density, lightweight, and zero pollution.1,2 Regarding the use of hydrogen as a fuel for the zero-emission vehicle, one well-known problem is effective storage of hydrogen.3 Hydrogen can be stored in many ways.4,5 First, hydrogen is stored in liquid form or compressed in gaseous form by mechanical technologies. Metal hydrides, for example, LiAlH4, LiBH4, LiNH2, Li2NH, NaAlH4, NaBH4, and Mg(BH4)2 and other hydrogen-rich compounds, such as NH3, NH3BH3, ethanol, and so on6−8 via chemical methods where hydrogen is bond chemically in the forms offer a second way for hydrogen storage. Third is the adsorption method, in which, hydrogen atom is incorporated directly into the interstitial sites of the bulk crystals.9−11 The fourth one includes hydrogen storage technologies on the basis of the physisorption of H2 molecules onto the highly fractured surface of nanostructured or microporous materials such as activated carbon, fullerenes, carbon nanotubes, graphene, mesoporous silica, zeolites, metal−organic frameworks (MOFs), covalent−organic frameworks (COFs), and clathrates belong to this category.5,12−16 The objective of this work is to develop novel and lightweight promising materials belong to the latest category that can improve their hydrogen storage capacity than existing materials. It is well-known that the nanostructured carbon-based materials, for example, fullerene, carbon nanotube, graphene, and so on, have high surface area, lightweight, and chemically stable properties. However, unsubstituted carbon-based materials can only bind one hydrogen molecule on each side of carbon ring with the binding energy of 4−5 kJ/mol.17,18 One of the effective strategies to solve these problems is to modify carbon ring with other elements. It was evidenced that alkali metal positive ions and transition-metal can nondissociatively bind several H2 molecules via electrostatic forces.19−21 Rao and © XXXX American Chemical Society
Jena reported that a Li cation can adsorb H2 molecules up to six with the binding energy of 0.202 eV (4.66 kcal/mol).20 Further theoretical study done by Sun et al. showed that the Li-coated C60 fullerene, in which 12 lithium atoms are capped onto the 12 pentagons of C60, can bind 5 hydrogen molecules per Li atom, resulting in 60 H2 molecules adsorbed on Li12C60 with the binding energy of 0.075 eV (1.72 kcal/mol) per H2 molecule. They also reported that Ca32C60 can absorb up to 62 H2 molecules in two layers by first-principles calculations.22 Ti12C60 is another candidate materials for hydrogen storage, but 12 titanium atoms tend to form a small cluster which depresses its storage capacity.23 It is expected that the Li-doped graphene is superior to a Li-doped carbon nanotube because its both sides might be readily employed to ensure efficient hydrogen storage.24−28 It was found that the adsorption of hydrogen is significantly weakened by the strong electrostatic interaction between the adsorbed Li cations.24,26,27 However, the electrostatic interaction between the adsorbed Li cations adsorbed on a well-defined porous graphene can be lower leading to improvement of adsorption of hydrogen molecules.29 The other strategy to enhance the capacity of hydrogen storage is modifying the carbon rings by the substitution of carbon atoms in the carbon rings with boron or nitrogen. Boron or nitrogen substitutions in benzene and other cyclic hydrocarbons30 have been an active and productive field of study for more than 50 years and applications in nanoscience,31 electronic materials,32 and pharmacology.33 Viswanathan et al.34−36 and Yang et al.37,38 have investigated the hydrogen storage in heteroatom (nitrogen and boron) substituted carbon nanotube and microporous carbon, respectively, and Received: February 11, 2015 Revised: March 15, 2015
A
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
The adsorption energies were calculated by ΔEads = E[nH2complex] − E[(n − 1)H2-complex] − E[H2], where E[nH2complex], E[(n − 1)H2-complex], and E[H2] are the calculated electronic energies of nH2 molecules adsorbed on the complex, (n − 1)H2 molecules adsorbed on the complex, and one H2 molecule, respectively. The average adsorption energies were calculated by ΔEave = {E[nH2-complex] − E[complex] − E[nH2]}/n, where E[nH2-complex], E[complex], and E[nH2] are the energies of nH2 molecules adsorbed on the complex, the bare complex, and nH2 molecules, respectively. The molecular dynamics (MD) simulations using canonical NVT ensemble with Nosé thermostat50 were carried out to verify the stability of the complexes and the hydrogen capability on the related complexes. The simulations were done at three different temperatures (200, 300, and 373 K) with 1 fs per time-step until time reached 1 ps.
demonstrated that B- and N-substituted carbon-based materials have a higher hydrogen storage capacity compared to the pure carbon materials experimentally. In addition, they also studied hydrogen atom adsorption (chemical adsorption) on B- and N-substituted carbon-based materials using theoretical calculations. The theoretical predictions on the effects of B- and N-substitution are consistent with the experimental results. Yang et al.37,38 also showed that doping noble metals (Ru and Pt) on B- and N-substituted microporous carbon enhances the hydrogen storage capacity. Kim et al. showed that the hydrogen storage may be improved by the B-substitution in the C36 fullerene.39 Further, the boron-doped carbon systems with Li metals coating in carbon nanotube has been studied with regard to hydrogen storage.40,41 Hydrogen storage capacity in aromatic carbon ring molecular materials decorated with alkali or alkaliearth metals has been theoretically studied by Bodrenko et al.42 In addition, Gopalsamy and Subramanian carried out a density functional theory calculation on the hydrogen storage capacity of alkali and alkali-earth metal ions doped carbon based materials such as cubane, cyclohexane, and adamantine.43 In the present work, we chose the carbon ring, benzene, as the studying model. In addition, we extensively attempt to modify the carbon ring by B and N substitution decorated with lighter and cheaper metal (Li, Na, and K) and carry out a comprehensive analysis of the hydrogen storage capacity of the several carbon-based molecular complexes such as B- and N-substituted materials from a theoretical point of view.
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RESULTS AND DISCUSSION The optimized configurations of benzene, B- and N-doped carbon-ring are depicted in Figure 1. The calculated C−C bond length in the benzene is 1.396 Å, which is in good agreement with experimental value.51 It was found that the B-doped carbon-ring becomes slightly distorted, whereas the N-doped carbon-ring retains its planarity. The mode of interaction of alkali metal cations (Li+, Na+, K+) with these carbon-ring materials is represented in Figure 2. The binding energies for alkali metal cations on B- and N-doped carbon-rings are summarized in Table 2. The calculated distances between the cations and the host materials are listed in Table 3. The favorite adsorption position of the metal cations on benzene and Bdoped carbon-ring is found to be the hollow center of the hexagon while the metal cations are found likely to bind to nitrogen atom of N-doped carbon ring, as shown in Figure 2. The calculated adsorption energy (−35.29 kcal/mol) for a Li+ on the benzene is very close to that (38.05 kcal/mol) on a (3 × 3) graphene (7.38 Å × 7.38 Å). As seen in Table 2, the interaction between the metal cations and the B-doped or N-doped carbon ring is stronger than that between the metal cations and benzene except for the cases of 3B and 3N substitutions, indicating that the B-doped or N-doped carbon ring can enhance the interaction. It should be noticed that the Li−Li, Na−Na, and K−K bonding energy (−15.7, −10.5, and −7.5 kcal/mol) is much smaller than the metal−carbon ring binding energy (see Table 2), which avoids the formation of metal cluster on the substrate. In addition, the interaction varies as Li+-carbon-ring > Na+-carbon-ring > K+-carbon-ring, which is in reverse order as the trend of the ionic radius of the corresponding metal cation. On the contrary, the distance, listed in Table 3, between the metal cation and the ring varies as Li+-carbon-ring < Na+-carbon-ring < K+-carbon-ring, which is in good agreement with the variation in the ionic radius of the corresponding metal cation. One should note that lighter metal cations of the same charge are bound stronger and located closer to the ring. The binding of a single H2 molecule to M+-carbon-ring complex has been considered. Figure 3 shows the optimized geometries of H2@carbon-ring-M+ complexes (depict M+ = Li+ only). The optimization of a single hydrogen molecule on the M+-carbon-ring complex does not lead to its dissociation as seen in Figure 3. The calculated H−H lengths are 0.743, 0.740, and 0.739 Å with adsorption energies of −4.64, −2.55, and −1.53 kcal/mol for H2@benzene-Li+, H2@benzene-Na+, H2@ benzene-K+, respectively. The H−H distance is predicted to be
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COMPUTATIONAL METHODS Geometry optimizations and energies for all complexes have been carried out by Becke’s three-parameter hybrid functional as varied by Lee, Yang, and Parr (B3LYP)44,45 and secondorder Møller−Plesset (MP2)46−48 levels with the split-valence 6-311++G(2d,2p) basis set as implemented in the Gaussian 09 program.49 Harmonic vibration frequency calculations for all complexes at the same levels were performed; all the complexes reported were found to be minima on their potential energy surface. As seen in Table 1, in test calculations on the Table 1. Calculated Adsorption Energy (kcal/mol) of a Hydrogen Molecule Adsorbed on a Benzene at Various Methods
a
MP2a
sasis set method
B3LYP
MP2
6-311++g(2d,2p)
−0.04
−1.31
cc-pVTZ
0.00
−1.15
−1.00 (6-311G**) −1.16
aug-cc-pVTZ
−0.03
−1.60
−1.59
MP2b
−1.17 (TZVPP) −1.39
Ref 17. bRef 18.
interaction energy of a hydrogen molecule with benzene, the calculated energy at the MP2/6-311++G(2d,2p) is −1.31 kcal/mol, which is well in agreement with values of −1.15 ∼ −1.16, −1.39−1.60, and −1.17 kcal/mol at the MP2/cc-pVTZ, MP2/cc-pVTZ, and MP2/TZVPP level,17,18 respectively, whereas the calculation results of B3LYP/6-311++G(2d,2p) strongly underestimate the interaction energy. This is because the DFT method does not consider dispersive forces, but the MP2 method comprises both weakly overlapping densities and dispersion interaction. Thus, we chose MP2 with the 6-311++G(2d,2p) basis set to investigate the interaction between H2 molecules and the complexes. B
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Topological structures of the model nanoring molecules. Gray, white, blue, and pink color balls represent carbon, hydrogen, nitrogen, and boron, respectively.
Table 3. Calculated Distance (Å) between M+ and CarbonBased Ring Materials Li+
Figure 2. Mode of interaction of alkali metal cations with nanoring molecules. (a) M+-benzene, (b) M+-B-doped carbon ring, and (c) M+N-doped carbon ring, where M+ = Li+, Na+, and K+.
Table 2. Calculated Binding Energy (kcal/mol) for M+ (Li+, Na+, K+) on Various Carbon-Based Ring Materials Li+
Na+
K+
carbon ring
B3LYP
MP2
B3LYP
MP2
B3LYP
MP2
3B 2B-p 2B-o 2B-m 1B benzene 1N 2N-m 2N-o 2N-p 3N
−32.98 −41.96 −45.21 −40.50 −39.33 −37.03 −45.49 −39.59 −55.69 −37.70 −33.14
−30.15 −39.43 −42.96 −38.67 −37.45 −35.29 −43.09 −37.02 −53.57 −35.84 −30.50
−23.32 −27.42 −29.69 −26.62 −25.43 −23.47 −31.91 −27.00 −40.53 −25.36 −21.56
−20.21 −25.16 −27.54 −24.80 −23.64 −21.85 −29.45 −24.48 −38.55 −23.38 −19.06
−14.05 −18.77 −20.36 −18.14 −17.19 −15.60 −21.96 −17.79 −30.71 −16.41 −13.20
−14.36 −20.65 −22.50 −20.25 −19.45 −17.99 −22.21 −17.91 −31.71 −16.89 −13.21
0.743 Å, which is elongated slightly by 0.07 Å compared to that (0.736 Å) of a calculated gas-phase H2 molecule for the H2@Bdoped carbon-ring-Li+ complexes. For the H2@B-doped carbon-ring-Na+ complexes and H2@B-dpoed carbon-ring-K+ complexes, the H−H distance is calculated to be from 0.741 and 0.739 Å, respectively. The calculated adsorption energy is observed to increase in the order: H2@B-doped carbon-ring-K+
Na+
K+
carbon ring
B3LYP
MP2
B3LYP
MP2
B3LYP
MP2
3B 2B-p 2B-o 2B-m 1B benzene 1N 2N-m 2N-o 2N-p 3N
2.436 2.366 2.342 2.390 2.343 2.317 1.911 1.927 1.927 1.932 1.945
2.501 2.408 2.387 2.434 2.382 2.349 1.949 1.967 1.965 1.969 1.986
3.684 2.820 2.798 2.843 2.797 2.774 2.305 2.322 2.329 2.328 2.346
3.720 2.885 2.862 2.910 2.859 2.829 2.355 2.380 2.381 2.381 2.408
4.065 3.245 3.228 3.260 3.224 3.203 2.709 2.745 2.699 2.751 2.787
3.938 3.221 3.204 3.235 3.193 3.163 2.724 2.760 2.704 2.763 2.803
Figure 3. Optimized configurations of H2@carbon-ring-Li+ complexes at MP2/6-311++G(2d,2p) level. (a) H2@benzene-Li+, (b) H2@Bdoped carbon-ring-Li+, and (c) H2@N-doped carbon-ring-Li+.
(−1.50 ∼ −1.48 kcal/mol) < H2@B-doped carbon-ring-Na+ (−2.50 ∼ −2.46 kcal/mol) < H2@B-doped carbon-ring-Li+ C
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 4. Calculated Adsorption Energies (kcal/mol) and Distance (Å) of H2@Carbon-Ring-M+ Complexes at MP2/ 6-311++G(2d,2p) Level Li+
ion
Na+ b
Table 6. Calculated Mulliken Charges over Alkali Metal Cations of Carbon-Ring-M+ and H2@Carbon-Ring-M+ Systems at MP2/6-311++G(2d,2p) Level
K+
Li+
Na+
K+
complex
Eadsa
d(H−H)
Eads
d(H−H)
Eads
d(H−H)
carbon ring
before
after
before
after
before
after
3B 2B 1B benzene 1N 2N 3N
−4.56 −4.49 −4.33 −4.64 −4.45 −4.61 −4.79
0.743 0.743 0.743 0.743 0.743 0.743 0.744
−2.48 −2.46 −2.50 −2.55 −2.43 −2.53 −2.63
0.741 0.741 0.741 0.740 0.740 0.741 0.741
−1.49 −1.48 −1.50 −1.53 −1.40 −1.46 −1.52
0.739 0.739 0.739 0.739 0.739 0.739 0.739
C6H6 C5BH5 C4B2H4 C3B3H3 C5NH5 C4N2H4 C3N3H3
0.752 0.714 0.711 0.716 0.757 0.741 0.721
0.662 0.652 0.644 0.615 0.599 0.553 0.501
0.882 0.846 0.842 0.826 0.929 0.924 0.918
0.812 0.776 0.760 0.712 0.843 0.829 0.815
0.953 0.923 0.921 0.917 0.987 0.981 0.983
0.910 0.875 0.873 0.860 0.955 0.945 0.944
a
Eads is the adsorption energy of a single H2 molecule adsorbed on carbon-ring-M+ complex. bd(H−H) is the distance of a single adsorbed H2 molecule.
0.9−1.0 kcal/mol (4−5 kJ/mol) is introduced to exclude weakly bound H2 molecules which may be easily desorbed from complex by thermal energy. This is also the interaction energy of a H2 molecule to the pristine six-membered carbon ring (benzene). The optimized structures of nH2@carbon-ring-Li+, nH2@carbon-ring-Na+, and nH2@carbon-ring-K+ complexes are depicted in Figures 4, 5, and 6, respectively. One should note that we only show the results of meta-2B-doped ring-M+ (or meta-2N-doped ring-M+) since the H2 adsorption behavior is the same for para- and ortho-2B-doped ring-M+ (or para- and ortho-2N-doped ring-M+). As seen in Table 7, the maximum number of H2 molecules adsorbed on the Li+@benzene, Na+@ benzene, and K+@benzene complexes are three, five, and seven, respectively. The average distance between the H2 molecule and the metal cation is predicted to be 2.223, 2.628, 3.079 Å (see Table 5) for the 3H2@benzene-Li+, 5H2@benzene-Na+, and 7H2@benzene-K+, respectively. The corresponding average adsorption energies are −3.31, −2.08, and −1.52 kcal/mol per hydrogen molecule. By substituting one carbon atom (or two carbon atoms) with boron, a maximum of 3H2, 5H2, and 7H2 molecules are adsorbed on 1B-doped carbon-ring-Li+ (or 2Bdoped carbon-ring-Li+), 1B-doped carbon-ring-Na+ (or 2Bdoped carbon-ring-Na+), and 1B-doped carbon-ring-K+ (or 2B-doped carbon-ring-K+). One should note that 3B-doped carbon-ring-Li+, 3B-doped carbon-ring-Na+, and 3B-doped carbon-ring-K+ complexes can store with five, six, and nine H2 molecules. The average distance between the H2 molecule and the metal cation is predicted to be 2.241 (2.217 and 2.267), 2.645 (2.612 and 2.665), and 3.064 (3.071 and 3.085) Å for the 3H2@1B-doped ring-Li+ (3H2@2B-doped ring-Li+ and 5H2@ 3B-doped ring-Li+), 5H2@1B-doped ring-Na+ (5H2@2B-doped ring-Na+ and 6H2@3B-doped ring-Na+), and 7H2@1B-doped ring-K+ (7H2@2B-doped ring-K+ and 9H2@3B-doped ring-K+),
(−4.56 ∼ −4.33 kcal/mol), see Table 4. As shown in Table 5, the average distance between the H2 molecule and the metal cation is predicted to decrease in the following order: H2@Bdoped carbon-ring-K+ (3.001−3.012 Å) > H2@B-doped carbon-ring-Na+ (2.515−2.523 Å) > H2@B-doped carbonring-Li+ (2.057−2.062 Å). Similar to H2@B-doped carbon-ringM+ complexes, The predicted H−H distance varies from 0.743 to 0.744 Å, 0.740 to 0.741 Å, and 0.739 Å for the H2@N-doped carbon-ring-Li+, H2@N-doped carbon-ring-Na+, H2@N-doped carbon-ring-K+, respectively. As shown in Table 4, the calculated adsorption energy is observed to increase in the order: H2@N-doped carbon-ring-K+ (−1.52 ∼ −1.40 kcal/mol) < H2@N-doped carbon-ring-Na+ (−2.63 ∼ −2.43 kcal/mol) < H2@N-doped carbon-ring-Li+ (−4.79 ∼ −4.45 kcal/mol). The average distance between the H2 molecule and the metal cation is predicted to decrease in the order: H2@N-doped carbonring-K+ (3.011−3.031 Å) > H2@N-doped carbon-ring-Na+ (2.513−2.528 Å) > H2@N-doped carbon-ring-Li+ (2.075− 2.085 Å), as summarized in Table 5. The Mulliken charge analysis is carried out for the above complexes to examine the interaction between H2 and M+@carbon-ring complex. The calculated Mulliken charges are listed in Table 6. As shown in Table 6, the charge of the metal cations decreases after the adsorption of a hydrogen molecule. This clearly points out that the metal cation polarizes the H2 molecule and the charge transfer occurs from the H2 molecule to the metal cation resulting in the elongation of H−H bond length and H2 physisorption. We then explored the hydrogen storage capacity of the considered modified complexes. The threshold energy of
Table 5. Calculated Distance (Å) at between Adsorbed H2 Molecule and M+ of nH2@Carbon-Ring-M+ at MP2/6-311++G(2d,2p) Level Li+
Na+
K+
complex
d(I−H2)a
d*(I−H2)b
d(I−H2)a
d*(I−H2)b
d(I−H2)a
d*(I−H2)b
nH2@3B-doped carbon ring nH2@2B-doped carbon ring nH2@1B-doped carbon ring nH2@benzene nH2@1N-doped carbon ring nH2@2N-doped carbon ring nH2@3N-doped carbon ring
2.062 2.059 2.057 2.056 2.085 2.080 2.075
2.267 2.217 2.241 2.223 2.276 2.260 2.257
2.523 2.517 2.515 2.510 2.528 2.521 2.513
2.665 2.612 2.645 2.628 2.673 2.738 2.734
3.012 3.006 3.001 2.993 3.031 3.021 3.011
3.085 3.071 3.064 3.079 3.095 3.108 3.061
a
d(I−H2) is the distance from the cation to a single adsorbed H2 molecule. bd*(I−H2) is the average distance from the cation to each of the adsorbed H2 molecules. D
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. Optimized configurations of nH2@carbon-ring-Li+ complexes at MP2/6-311++G(2d,2p) level, where n = the maximum number of adsorbed H2 molecules.
Figure 5. Optimized configurations of nH2@carbon-ring-Na+ complexes at MP2/6-311++G(2d,2p) level, where n = the maximum number of adsorbed H2 molecules.
between the maximum number of nH2 molecule and the metal cation is computed to be 2.276 (2.260), 2.673 (2.738), and 3.095 (3.108) for 5H2@1N-doped ring-Li+ (5H2@2Ndoped ring-Li+), 6H2@1N-doped ring-Na+ (7H2@2N-doped
respectively. The corresponding average adsorption energies are −3.13 (−3.18 and −3.33), −2.02 (−1.98 and −2.07), and −1.48 (−1.45 and −1.51) kcal/mol per hydrogen molecule. For the N substituted carbon-ring, the average distance E
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Optimized configurations of nH2@carbon-ring-K+ complexes at MP2/6-311++G(2d,2p) level, where n = the maximum number of adsorbed H2 molecules.
Their average adsorption energies are −3.38, −1.98, and −1.57 kcal/mol per hydrogen molecule. Notice that the adsorption of number of H2 molecules on the M+-carbon-ring complexes depends on ionic radii of the metal cations. To compare the ability of various complexes to store H2 molecules, we have calculated the hydrogen storage capacity in wt % defined as
Table 7. Gravimetric Density (wt%), Average Hydrogen Adsorption Energy (Eave, in kcal/mol), and Maximum Number of Adsorbed Hydrogen Molecules for Selected nH2@Carbon-Ring-M+ Complexes at MP2/6-311++G(2d,2p) Level borondoped
wt % (Eave), {H2} of nH2@carbon-ring-Li+
0B 1B 2B-m 3B borondoped
6.59 (−3.31) {3H2} 6.74 (−3.13) {3H2} 6.90 (−3.18) {3H2} 11.24 (−3.33) {5H2}
0B 1B 2B-m 3B borondoped
9.01 (−2.08) {5H2} 9.17 (−2.02) {5H2} 9.35 (−1.98) {5H2} 11.21 (−2.07) {6H2}
0B 1B 2B-m 3B
10.69 10.85 11.02 13.95
10.42 (−3.17) {5H2} 10.31 (−3.26) {5H2} 10.20 (−3.38) {5H2}
wt %, (Eave), {H2} of nH2@carbon-ring-Na+ 10.53 (−2.02) {6H2} 11.97 (−1.93) {7H2} 11.86 (−1.98) {7H2}
wt %, (Eave), {H2} of nH2@carbon-ring-K+ (−1.52) (−1.48) (−1.45) (−1.51)
{7H2} {7H2} {7H2} {9H2}
13.24 (−1.47) {9H2} 13.14 (−1.49) {9H2} 11.76 (−1.57) {8H2}
nitrogendoped 0N 1N 2N-m 3N nitrogendoped
c = MnH2 /(MnH2 + Mcomplex ) × 100%
Here, MnH2 is the molar mass of nH2 molecules, and Mcomplex is the molar mass of the complex. As shown in Table 7, the gravimetric densities range from 6.74 to 11.24, 9.17−11.21, and 10.85−13.95 wt % for the nH2@B-doped ring-Li+, nH2@Bdoped ring-Na+, and nH2@B-doped ring-K+ complexes and from 10.20 to 10.42, 10.53−11.86, and 11.76−13.24 wt % for the nH2@N-doped ring-Li+, nH2@N-doped ring-Na+, and nH2@N-doped ring-K+ complexes. Comparison of adsorption energy, maximum number of bound hydrogen molecules, and wt % of nH2@B-doped ringM+ and nH2@N-doped ring-M+ complexes shows that the 3B-doped ring-M+ complex much enhances the hydrogen storage capacity while the hydrogen storage capacity only slightly increases for the 1B-doped ring-M+ and 2B-doped ringM+ complexes. In the case of nH2@N-doped ring-M+ complex, all of N-substituted complexes effectively enhance hydrogen storage capacity of the corresponding complexes. To verify the stability and hydrogen storage in the above materials, molecular dynamics (MD) using canonical NVT ensemble with Nosé thermostat50 on the complex with highest hydrogen storage capacity is simulated. The recommended temperature for H2 delivery by the U.S. DOE is in the range of
0N 1N 2N-m 3N nitrogendoped 0N 1N 2N-m 3N
ring-Na+), and 9H2@1N-doped ring-K+ (9H2@2N-doped ringK+), respectively. The corresponding average adsorption energies are −3.17 (−3.26), −2.02 (−1.93), and −1.47 (−1.49) kcal/mol per hydrogen molecule. Notice that 3Ndoped ring-Li+, 3N-doped ring-Na+, and 3N-doped ring-K+ complexes can be hydrogenated with five, seven, and eight H2 molecules with the average distance of 2.257, 2.734, and 3.061. F
DOI: 10.1021/acs.jpcc.5b01416 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C −40−120 °C (233−393 K).52 Therefore, three different temperatures (200, 300, and 373 K) have been chosen for MD simulation. The complexes are found to be structurally stable even at 373 K. The results show that H2 remains bound to the cations (Li+, Na+, and K+) in the complexes at the low temperature of 200 K, while more than 75% of hydrogen is desorbed at the temperature of 300 K after 1 ps time. At the elevated temperature of 373 K, all adsorbed H2 are desorbed. The B- and N-substituted carbon ring decorated with Li+, Na+, and K+ cations exhibits promising hydrogen storage properties. Detailed MD simulations for large system such as MOFs with B- and N-substituted carbon-ring linker will be carried out in the future.
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CONCLUSION We have systematically studied the hydrogen adsorption and storage capacity on various carbon-based materials decorated with alkali metals by employing the MP2/6-311++g(2d,2p) level of theory. The complexes with positive charge metals exhibit better hydrogen adsorption process because of its better charge transfer and stronger interaction. Furthermore, through a comparison study on the hydrogen adsorption behavior of Li+-, Na+-, and K+-decorated carbon-based molecular complexes, the B- and N-substituted complexes enhance the hydrogen storage capacity of the corresponding complexes. The Mulliken charge analysis is carried out and points out that the metal cation polarizes the H2 molecule and the charge transfer occurs from the H2 molecule to the metal cation resulting in the elongation of H−H bond length and H2 physisorption. According to our calculations, the maximum hydrogen storage capacity can reach 11.21−13.95 and 10.42− 13.24 wt % for the B- and N-substituted complexes, respectively, it has already exceeded the target specified by U.S. Department of Energy with 9 wt %. The MD simulation shows that the B- and N-substituted carbon ring decorated with Li+, Na+, and K+ cations can be a promising materials for hydrogen storage. The results may be useful for extending the study of graphene-based system (2-D structure) and for developing promising linkers in metal−organic frameworks (3-D structure) for hydrogen storage.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Tel.: +886-3-265-3324. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology (MOST) and National Center for Theoretical Sciences (NCTS), Taiwan, for supporting this study, under Grant Numbers NSC 101-2113-M-033-009-MY3 and MOST 103-2632-M-033-001-MY3 and the use of CPUs at the National Center for High-Performance Computing in Taiwan. Besides, we deeply appreciated Professor M. C. Lin (from NCTU, Taiwan, and Emory University, U.S.A.) for his encouragement and instruction.
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