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Carbon doped boron nitride cages as competitive candidates for hydrogen storage materialsw H. Y. Wu,a X. F. Fan,a Jer-Lai Kuo*ac and Wei-Qiao Deng*b Received (in Cambridge, UK) 12th June 2009, Accepted 17th November 2009 First published as an Advance Article on the web 7th December 2009 DOI: 10.1039/b911503j By the incorporation of C atoms into (BN)12 fullerene, our theoretical investigation shows that carbon doped boron nitride cages (BNC) can achieve a high hydrogen storage amount of 7.43 wt%, and dehydrogenation of the corresponding BNC hydrides (BNCH) is thermodynamically favored for practical applications of hydrogen energy, making BNC competitive candidates for hydrogen storage materials.
As ideally expected, hydrogen is absorbed in the storage medium at a high pressure, low temperature condition (10 bar, 25 1C), and is released at a low pressure, high temperature condition (1 bar, 100 1C), which ensures the safe operation for a practical device.21 Thus, two thermodynamic restrictions for hydrogen storage materials, in terms of BNC, can be set up as follows:
Hydrogen fuel has been considered to be an alternative energy source, with a potential use for vehicles, personal electronics and other portable power applications.1–3 However, the production, storage and use of hydrogen, with the consideration of costs, still face a variety of challenges, which make the role of hydrogen in the sustainable energy future controversial.4 Nevertheless, niche applications of hydrogen fuel is still of great necessity in the real life. To facilitate the practical applications of hydrogen energy, it is required that a hydrogen storage material should reversibly store 6 wt% hydrogen under ambient conditions (1–10 bar and 0–100 1C), which can maintain fuel efficiency under reasonable device operating conditions. To achieve this challenging goal, a lot of candidates have been proposed, such as carbon-based materials,5–7 organic polymers,8,9 metal–organic frameworks10–12 and metal hydrides.13–15 However, no materials can meet the requirement thus far. Boron nitride cages (BNC), the analogues of fullerenes, have been successfully synthesized16–18 and observed to have the possibility of storing B3 wt% hydrogen gas.19 Theoretical prediction20 showed a H2 storage amount of B4.9 wt% can be achieved with 38 H2 molecules in B60N60. Compared with a carbon cluster, hydrogen is more stable in BN fullerenes. It indicates that BNC could be good candidates for hydrogen storage materials. However, research focusing on hydrogen storage properties of BNC is mainly from the aspect of physisorption and the chemisorption of hydrogen on BNC materials is rarely studied. Here, we present a computational study to investigate the hydrogen chemisorption property of C-doped BN fullerenes, a new class of BNC material. Our prediction indicates that the C-doped BN fullerene is a hydrogen storage material, whose hydrogenation and dehydrogenation reactions can be thermodynamically favored under ambient conditions.
BNC þH2 ! BNCH DG1 0;
10 bar; 25 C
a
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China. E-mail:
[email protected] c Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan. E-mail:
[email protected] w Electronic supplementary information (ESI) available: Detailed explanations of the calculated data in Table 1, and Fig. S1–2. See DOI: 10.1039/b911503j
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1 bar; 100 C
BNCH ! BNC þH2 DG2 0;
ð1Þ ð2Þ
where both Gibbs free energy changes of hydrogenation and dehydrogenation reactions are required to be negative. This helps us screen the appropriate BNC by analyzing the Gibbs free energy changes in the hydrogenation reaction of BNC and dehydrogenation of the corresponding BNC hydride (BNCH), in which hydrogen atoms are chemisorbed on the outside of the BNC. We choose (BN)12 (Th symmetry, as shown in Fig. S1 in ESIw), the smallest stable BN fullerene,22–24 as a representative to search the suitable BNC for hydrogen storage. Since all B sites are equivalent in (BN)12 under Th symmetry, by substituting C atoms for up to two B atoms, we get seven BNC structures in total, for investigation. As illustrated in the second column of Table 1, the seven BNC are: undoped (BN)12 fullerene (B12N12), 1C-doped (BN)12 (B11N12C), and five 2C-doped (BN)12 with different C–C spans (B10N12C2-1–B10N12C2-5). To determine whether these BNC are thermodynamically favored in the hydrogenation and dehydrogenation reactions, we carried out ab initio calculations using a hybrid density functional B3LYP method25 with the standard 6-31G* basis set, which has been found to be quite adequate for the present system.26 All the structures were fully optimized and found to be stable, which was examined by vibrational frequency calculations, with a frequency scaling factor of 0.96 adopted for thermal energy corrections.27 Here, all the calculations were performed with the Gaussian 03 program.28 Table 1 shows the computational results per mole of H2 molecules of the reaction. Te is the H2 release equilibrium temperature when Gibbs free energy change for the dehydrogenation reaction equals zero. The corresponding enthalpy and entropy changes, DHe and DSe, are obtained based on Ellingham approximation (see more detailed explanations in the ESI.w) For the (BN)12 molecule, the dehydrogenation reaction is endothermic, with DG, DHe and DSe all being positive. Though its free energy change for reaction (1) is negative, 6.29 kJ mol1 energy cost in reaction (2) makes the H2 release at 100 1C not thermodynamically favored for (BN)12. Chem. Commun., 2010, 46, 883–885 | 883
Table 1 Calculated thermodynamic data (DHe, DG, DG1, DG2, kJ mol1; DSe, J mol1 K1) per mole of H2 gas (average values of full hydrogenation) absorbed/released in the hydrogenation reaction of the BNC, and the maximum hydrogen content (wt%) of the corresponding BNCH. Te (1C) refers to the temperature when the Gibbs free energy change for the dehydrogenation reaction equals zero. The front views of investigated BNC molecules are shown BNC
DHe
DG
DSe
Te
DG1
DG2
wt%
B12N12
55.26
15.79
131.11
148.7
21.49
6.29
7.46
B11N12C
39.31
1.79
125.81
39.3
7.50
7.74
7.43
B10N12C2-1
81.78
38.68
138.68
320.1
44.39
29.12
6.84
B10N12C2-2
73.05
31.12
136.53
264.0
36.83
21.57
6.84
B10N12C2-3
23.74
11.90
118.14
72.5
6.19
21.53
7.41
B10N12C2-4
24.62
11.17
118.85
66.3
5.47
20.81
7.41
B10N12C2-5
23.47
12.24
118.23
75.0
6.53
21.91
7.41
With one C atom substituting for a B site on (BN)12, the free energy change DG is effectively lowered to 1.79 kJ mol1, indicating a much improved dehydrogenation process for B11N12C, which is predicted to dehydrogenate spontaneously at 39.3 1C according to our calculations. Moreover, both free energy changes of B11N12C for reaction (1) and (2) are negative, showing that B11N12C can reversibly store 7.43 wt% hydrogen under ambient conditions. This C-doping effect, previously reported for BN nanotubes,29,30 was attributed to the different electronic structures between C and B atoms. Since C atom has one more valence electron than B, B11N12C locally behaves electron-rich and possesses the electron donor property around the C atom. Consequently, the repulsion between the electrons of the H and the p-electrons of the B11N12C nanocage was 884 | Chem. Commun., 2010, 46, 883–885
enhanced, thus weakening the chemical bonds between the hydrogen and the nanocage. Ultimately, hydrogen was more easily released from B11N12C. To further realize its hydrogen storage property, the activation energy for the hydrogenation/dehydrogenation reaction of B11N12C was also estimated, as well as that of (BN)12 for comparison. The activation energy Ea for the hydrogenation reaction of B11N12C is estimated to be 2.14 eV, which is a little bit higher than that for hydrogen chemisorption on BN nanotubes (1.8–2.0 eV),31,32 and (BN)12 fullerene (1.83 eV), but lower than that for hexagonal BN sheets (B2.4 eV).32 For the dehydrogenation reaction of B11N12C, the activation energy Ea is about 2.40 eV, which is much lower than the activation energy of 5.66 eV for (BN)12 This journal is
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fullerene in the hydrogen release reaction. More discussions on the activation energy can be found in the ESI.w From the calculation results, we can see that the activation energy is around 2 eV for B11N12C, which is about half of the H2 binding energy (2.27 eV), indicating that further effort should be made to lower the activation barrier by using the appropriate catalyst, and this would be another interesting problem for our future work. Interestingly, as one more C atom is doped on the B site, two different effects are observed. On one hand, following the dehydrogenation-enhanced effect as discussed above, the free energy changes DG for B10N12C2-3–B10N12C2-5 are all negative, implying a thermodynamically favored H2 release process. But the penalty is that the binding of H atoms onto these BNC becomes difficult due to the positive values of DG1. On the other hand, when the two C atoms are too close as in the B10N12C2-1 (C–C: 2.06 A˚) and B10N12C2-2 (C–C: 2.57 A˚), the two neighboring N atoms (the two N atoms at the center of BNC front views in Table 1) will not absorb H atoms, because their excessive electrons transferred from the two C atoms greatly weaken the chemical bonds between the nanocage and the hydrogen. Therefore, only 22 H atoms can be attached to the BNC, resulting in an overall dehydrogenation-worsen effect for B10N12C2-1 and B10N12C2-2, whose free energy changes DG have larger positive values compared with that of (BN)12. We can draw a conclusion that no more potential BNC for hydrogen storage can be found, as more C atoms replace B atoms in the (BN)12 molecule (B12xN12Cx, x > 2). Furthermore, noticeably seen from Table 1, the entropy changes of the reactions are around 130 J mol1 K1, indicating that the entropy change of the dehydrogenation reaction is mainly due to the release of the hydrogen gas, and not much dependent on the structures of BNC molecules. We have also investigated the other two possible cases, where C atoms substitute for N atoms in (BN)12 fullerene or a B–N pair of (BN)12 is replaced by a C–C pair. Unfortunately, neither of them can provide good candidates for hydrogen storage materials. Since a C atom has one less valence electron than N, B12N12xCx (x = 1, 2) is an electrondeficient complex having the electron acceptor property. So the hydrogen chemisorption energy of BNC in the first case is increased, resulting in an even worse dehydrogenation property of the corresponding BNCH. However, this strategy can be applied to BN fullerenes with larger sizes than (BN)12, for large BN nanocages with a full H coverage have small H chemisorption energy. As for the second case, which can be viewed as a combination of (BN)12 and C24, the same trend as in the first case with respect to the dehydrogenation property is obtained, due to the high release equilibrium temperature of ca. 900 1C for C24 fullerene. In summary, by introducing C atoms into (BN)12 fullerene, full hydrogenation calculations show that the B11N12C compound can spontaneously store hydrogen at room temperature and 10 bar and release hydrogen at 100 1C and 1 bar, with the activation energy for hydrogenation/ dehydrogenation reaction being around 2 eV. The hydrogen storage capacity can reach up to 7.43 wt%. Our calculation
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concludes that B11N12C is a potential candidate for hydrogen storage materials. We hope that our research work will stimulate the experimental effort in this direction. This work was supported by Nanyang Technological University and Ministry of Education in Singapore under URC Grants (RG34/05, RG57/05, and ARC24/07, no. T206B1218RS) and Dalian Institute of Chemical Physics.
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