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Hydrogen storage in scandium doped small boron clusters (BnSc2, n=3–10): A density functional study
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Hydrogen storage in scandium doped small boron clusters (BnSc2, n¼3e10): A density functional study Shakti S. Ray, Smruti R. Sahoo, Sridhar Sahu* High Performance Computing Lab, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, 826004, India
article info
abstract
Article history:
In this work, we present the hydrogen adsorption capacity of Sc doped small boron clusters
Received 10 August 2018
(BnSc2, n ¼ 3e10) using density functional theory. Almost no structural change was
Received in revised form
observed in the host clusters after hydrogen adsorption. Stabilities of the studied clusters
27 November 2018
were confirmed by various reactivity parameters such as hardness (h), electrophilicity (u),
Accepted 13 December 2018
and electronegativity (c). The average adsorption energies was found in the range of 0.08
Available online 7 February 2019
e0.19 eV/H2 inferring physisorption process, and the fact is also supported by the average A. All the clusters distance from Sc to H2 molecules which was in the range of 2.13 Å-2.60
Keywords:
were found to have gravimetric density satisfying the target set by the U.S. Department of
Density functional theory
Energy (US-DOE) (5.5 wt% by 2020). From Bader's topological analysis, it was confirmed that
Boron cluster
the nature of interaction was likely to be somewhat closed shell type. ADMP molecular
NBO
dynamics simulations study was performed at different temperatures to understand the
AIMALL
adsorption and dissociation of H2 from the complexes.
ADMP
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, being considered as a sustainable energy carrier, has received a lot of attention in the area of energy research because of its higher energy density, high efficiency and environmental friendliness [1e4]. However, the major bottleneck for the industrial applications of hydrogen energy is to develop cost-effective hydrogen storage carriers. Hydrogen storage as gas under high pressure or as liquid at cryogenic temperatures has inherent problems associated with safety, cost and capacity [5]. Further, developing new storage material with high volumetric and gravimetric densities (5.5 wt% and >40 kg/m3 by, as proposed by U.S. Department of Energy
(US-DOE)) with fast kinetics as well as favorable thermodynamics has also been quite a challenging task [6,7]. Hence, considerable efforts are devoted toward developing new solidstate storage materials. The storage materials bind the hydrogen atoms, basically, through three processes [1,8]. In chemisorption process the binding energy is in the range of 2e4 eV like chemical hydrides [9,10]. However, the major drawbacks in chemical hydrides are their slow dehydrogenation kinetics and desorption at high temperature due to metal-hydrogen binding [11,12]. On the other hand, for the materials, like nanostructured carbon, hydrogen molecules are weakly bonded with a binding energy in the range of few meV. This is regarded as physisorption process [13,14]. The third form of adsorption process with
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Sahu). https://doi.org/10.1016/j.ijhydene.2018.12.109 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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binding energy in the 0.1e0.8 eV range is also proposed which is intermediate between physisorption and chemisorption. Metal-decorated carbon-based nano-clusters and their derivates in which the metal atoms bind hydrogen in molecular form with intermediate binding energy, belong to the third form of adsorption [15e18]. For example, Sun et al. theoretically predicted that the Li12C60 could store up to 60H2 at most with binding energy of 0.075 eV/H2 resulting in high gravimetric density of 13 wt% [19]. Similar work has also been reported by Yoon et al. [20]. Nanostructured clusters have emerged as a new class of materials beacuse of their riveting reaction kinetics, thermodynamics and catalytic behavior [21,22]. A bagful of literature are found focusing on the hydrogen storage materials based on nanostructured clusters. For example, alkali metal and carbon fullerene terminated carbon chains was theoretically investigated by Ju et al. who calculated gravimetric density up to 10 wt% [23]. A similar theoretical study was performed by Chai et al. for lithiated carbon chain who found the gravimetric density of about 17.9 wt% above the temperature of liquid nitrogen [7]. Alkali and alkaline-earth metal decorated C24 fullerene was investigated by Cheng et al. who reported hydrogen storage capacity up to 12.7 wt% with adsorption energy varying in the range 0.13e0.19 eV/H2 [24]. Similar study on Li, Na doped C60 was also reported by Ren et al. [25]. Kiser et al. reported using both experimental and theoretical studies, Cs-doped C60 cluster could adsorb a maximum of 67H2 leading to a gravimetric density of 8.9 wt% [26]. Similarly, synthesis and characterization of Li-doped C60 for reversible hydrogen storage was carried out by Teprovich et al. who reported that the complex could adsorb up to 5% wt of H2 at the onset temperature of 270 degree Celsius [27]. Titanium decorated C120 nanotours was theoretically investigated by Chavez et al. who reported the hydrogen storage efficiency up to 5.6 wt% [28]. In addition, hydrogen storage in carbonaceous and other clusters were also widely reported in the literature [29e33]. Since last few years boron clusters, alongside carbonaceous materials, have also been considered as a promising candidate for hydrogen storage. But only a handful of work investigating hydrogen storage in boron nanoclusters has been reported so far. For example, Dong et al. investigated yitrium doped B80 fullerene that could bind 71H2 molecule resulting in the gravimetric density up to 6.85 wt% [34]. Tang et al. and Lu et al. investigated Ti and alkali and alkalineearth atom decorated B38 fullerene for hydrogen storage and found gravimetric densities up to 7.44 wt % and 6.47 wt % respectively [35,36]. Similarly, Liu et al. and Tang et al. theoretically investigated Sc and M-decorated (M ¼ Fe, Co, Ni) B38 fullerenes and found the gravimetric densities more than 7.0 wt % [37,38]. Wang et al. theoretically explored the hydrogen uptake capacity of Li decorated B24 cluster and found that the cluster can hold 16H2 molecules giving rise to 9.24 wt% of gravimetric density [39]. Recently, hydrogen trapping capacity of sodium coated hexagonal B36 cluster was explored by Ye et al. who reported that the cluster could trap up 10H2 molecules leading to 4.4 wt % of H2 with a binding energy of 0.25 eV/H2 [40]. Similar studies were also performed by Leu et al. and Jiang et al. [41,42]. Wang et al. investigated H2 storage capacity of hollow lithium decorated
B60 and found that the cage could uptake maximum up to 28H2 with an average binding energy of 0.33 eV/H2 leading to 8.19 wt% of gravimetric density [43]. Besides, Tang et al. performed molecular dynamics simulations to study the hydrogen storage capacity of alkali, alkaline-earth and transition metal-doped smallest boron cage (B28) and reported that it could provide up to 5.5 wt % of H2 desorbable at room temperature [44]. Yan et al. theoretically studied B40 fullerene doped with alkali metals those could give rise to gravimetric density up to 8.8 wt% of H2 at room temperature with an intermediate binding energy [45]. Similarly, Zhang et al. reported that Ca doped B40 could hold up to 30H2 molecules with adsorption energy of 0.177 eV/H2 leading to gravimetric density of 8.11 wt% [46]. Tang et al. investigated the hydrogen storage capacity of Scandium doped B40 cluster using molecular dynamic simulation and reported that the cluster could store maximum up to 6H2 per Sc atom giving rise to maximum gravimetric density of 8.3 wt% with average adsorption energy of 0.33e0.58 eV [47]. Li terminated linear boron chain was studied by Chai et al. TAO-DFT method and reported that the Li2Bn system could give rise to maximum of 17.0 wt % gravimetric density with a binding energy of 20e40 kJ/mol per H2 [48]. Studies related to other boron sheet uthors [49e51]. and monolayers are reported by various a Besides, hydrogen storage in other boron-based materials were also reported by many others [52e55]. However, considering hydrogen storage in small boron clusters (Bn, n < 20), only a few works have been reported so far. Pati et al. investigated the hydrogen storage capacity of metal doped B9 cluster and found that it could store hydrogen via chemisorption [56]. Guo et al. studied hydrogen storage properties of B6Tiþ 3 and found that it could store maximum up to 10H2 molecules resulting in the gravimetric density of 8.82 wt% [57]. Ju et al. investigated Mg doped small boron clusters Bn, (n ¼ 2e7) and observed that as compared to the three-dimensional structural boron clusters, the planar Bn, (n ¼ 4e7) clusters are more suitable candidates for H2 adsorption [58]. Wang et al. investigated the hydrogen trapping efficiency of B5V3 cluster and observed that the cluster could store up to 7H2 molecules with an average adsorption energy of 0.60 eV/H2 giving rise to 7.0 wt% of gravimetric density [59]. Similarly, Wen et al. reported the hydrogen storage properties of B6Na8 cluster and found that the cluster could store up to 32H2 molecule with adsorption energy in the range of 0.17e0.19 eV [60]. Recently, Hydrogen storage efficiency of binary alloy ScBn cluster was reported by Huang et al. who found that it could store up to 9.11 wt% of hydrogen molecule [61]. In this work, we report the adsorption of molecular hydrogen on the scandium doped small boron cluster BnSc2 (n ¼ 3e10) using density functional theory.
Computational details All the structures were optimized with and without H2 molecules using Perdew-Burke-Ernzerhof (PBE) hybrid exchange functional and employing 6e311þþG(d,p) basis set within the framework of density functional theory (DFT). PBE hybrid functional has been widely used by various authors to
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successfully investigate the hydrogen storage capacity of different clusters [61]. All the optimizations were accomplished without any imaginary harmonic frequencies. In addition, molecular dynamics simulations were performed to investigate the stability as well as the reversibility of adsorbed hydrogen molecules on the Sc doped boron clusters using atom centered density matrix propagation (ADMP). During the ADMP-MD simulation velocity scaling method were used to maintain the temperature. The time step (Dt) of ADMP-MD simulation is set at 0.1fs. All the calculations were carried out using Gaussian 09 computational program [62]. In addition, the nature of the interaction between the hydrogen molecules and sorption centers of the host cluster was explored using Bader's theory of atoms in molecules using AIMALL computational packages [63]. In order to get a more elaborate picture of the nature of the type of bonding, natural bond orbital (NBO) analysis was performed using the NBO 3.1 implemented in Gaussian 09 program [64]. Moreover, the electronic density of state (DOS) of the studied complexes was calculated using GaussSum program [65]. The stability and reactivity of the scandium decorated boron clusters and their H2 trapped analogos can be analyzed by investigating their, global reactivity descriptors, such as hardness (h), electrophilicity (u) and electronegativity (c) [66e69]. In addition, the kinetic stabilities of the scandium decorated boron clusters are obtained by calculating the energy gap (Eg) between their highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Now, chemical hardness, h is defined as, h¼
IA 2
(1)
where I and A are vertical ionization potential and electron affinity calculated using the Koopman's Theorem [70]. Again, electronegativity, c is expressed as c¼
IþA 2
(2)
Similarly, the electrophilicity index can be defined as u¼
c2 2h
(3)
The average hydrogen adsorption energy without zeropoint energy correction (Eads), is obtained using the following equation: Eads ¼ ½fEðHostÞ þ nEðH2 Þg EðComplexÞ=n
(4)
here EðComplexÞ, EðH2 Þ, EðHostÞ is the total electronic energies without zero point energy correction of hydrogenated complex and hydrogen molecule and host cluster respectively. Moreover, the formation energies (Eform) was obtained using the following equation: Eform ¼ ½EðComplexÞ fnEðH2 Þ þ EðHostÞg
(5)
here EðComplexÞ, EðH2 Þ, EðHostÞ is the total electronic energies without zero point energy correction of hydrogenated complex and hydrogen molecule and host cluster respectively. The hydrogen storage gravimetric density was calculated by following equation:
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H2 ðwt%Þ ¼
MH2 100 MH2 þ MHost
(6)
where MH2 indicates the mass of total number of adsorbed H2 molecules and MHost indicates the mass of Sc doped small boron cluster.
Results and discussion In this section, we present our calculations of adsorption of molecular H2 on scandium (Sc) doped small boron clusters, BnSc2, n ¼ 3 10: Before hydrogen adsorption, we optimized the geometries of bare BnSc2 clusters employing the methodology mentioned in the computational section, and all the stable geometries of host BnSc2 clusters and those adsorbed with H2 molecules are presented in Figs. 1 and 2 respectively. The optimized geometrical parameters of the studied clusters are listed in Table 1. We find that our optimized geometrical data for host BnSc2, ðn ¼ 3 10Þ clusters fairly agree with those reported by Wu et al. [71]. For example, the SceSc bond lengths in the cases of B3Sc2, B4Sc2, and B5Sc2, are found to be 3.09 A, 2.90 A, and 2.87 A which are in good agreement with those reported earlier [71]. Moreover, nominal changes in the geometries of the host clusters is observed after H2 adsorption. For instance, almost no change in BeB bond lengths is observed before and after the H2 adsorption in all the clusters A is except B10Sc2 in which a small change of the order of 0.09 noted. Similarly, in the cases of B3Sc2, B6Sc2, and B10Sc2 clusters no significant change is observed in BeSc bond lengths before and after H2 adsorption, whereas a change in the range of 0.02e0.07 A is noticed for the clusters B4Sc2, B5Sc2, B7Sc2, B8Sc2, and B9Sc2. Similarly, an elongation in SceSc bond lengths in the range of 0.01e0.15 A is noted in the host clusters after H2 adsorption. The insignificant changes in the geometries of host clusters after interaction with H2 molecules infer the adsorption process to be physisorption type. Moreover, the average distance between the adsorbent Sc atom and molecular hydrogen lies in the range of 2.13 A - 2.28 A. The distance slightly increase from B3Sc2-2H2 to B6Sc2-8H2 and then decreases onwards. The HeH distance in all the complexes are found to fall in the range of 0.76 A - 0.80 A and the elongation in HeH as compared to the bond length of the isolated H2 molecule (0.74 Å), is due to the charge transfer from Sc atom to H2 molecule confirming non-classical Kubas-Niu-Rao-Jena type interaction between Sc and H2 and inferring the process to be physisorption [72]. Stability is one of the important parameters to focus while studying the hydrogen storage mechanisms in different clusters. This stability factor can be investigated by various reactivity parameters such as hardness (h), electrophilicity (u), and electronegativity (c). In Table 2, we provide all these reactivity parameters computed at PBE/6e311þþG(d,p) level of theory. In all the studied clusters except B6Sc2, the value of h is found to increase with H2 adsorption, while the electrophilicity index, u decrease. For instance, the value of h is found be maximum (0.95 eV) for B10Sc2-6H2 and minimum (0.21 eV) for B6Sc2-8H2, and as expected, minimum u (8.27 eV) and maximum u (27.65 eV) are found for B10Sc2-6H2 and B6Sc2-8H2
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Fig. 1 e Optimized structures of small boron cluster (BnSc2,n ¼ 3e10) at PBE/6e311þþG(d,p) level of theory.
complexes respectively. Hence, following maximum hardness and minimum electrophilicity principle, the results obtained theoretically confirms the enhanced stability of the investigated clusters [73,74]. However, in the case of B6Sc2 we notice a reverse trend for h and u. Further, the value of c is found to decrease with the adsorption of H2 molecules in the complexes being maximum (4.11eV) for B10Sc2-6H2 and minimum (2.85eV) for B3Sc2-2H2 complexes., In addition, to investigate the kinetic stability of the hydrogenated clusters we also computed their HOMO-LUMO energy gaps (Eg) and the values are listed in Table 2. For all the clusters, except B6Sc2, the value of Eg increases upon H2 adsorption confirming the stability of the complexes. Moreover, the large Eg reduces the reactivity of
the clusters causing slightly increase in the bond length thus inferring better stabilization. The variation of Eg with the number of hydrogen molecules per clusters is presented in Fig. 3. It can be seen from the figure that there is an increase in Eg with an increase in the number of hydrogen molecules adsorbed which is an indication of stabilization of the complexes. Hence, the enhanced stabilities of the investigated BnSc2 (n ¼ 3e10) clusters gives a theoretical insight that clusters can be used as promising hydrogen storage materials. Furthermore, we also calculated the average adsorption energy (Eads ) at the same computational methodology, and the values are provided in Table 2, and the variation of adsorption energy with number of H2 molecules per cluster are presented
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Fig. 2 e Optimized geometries of hydrogen trapped Sc doped boron clusters at PBE/6e311þþG(d,p) level.
in Fig. 4. It is observed that the value of Eads falls in the range of 0.08e0.19 eV/H2, which confirms the process of reversible hydrogen physisorption mechanism [1]. We find that the value of Eads decreases with more number of H2 molecules trapped in a cluster. However, we do not observe any systematic variation in Eads with consecutive addition of H2 molecules. For example, in B7Sc2, we find a gradual decrease in
Eads up to eight H2 molecules, whereas in the case of B8Sc2, Eads increase up to the addition of 4H2 molecules and then decreases. Moreover, it is observed that, with an increase in number, the H2 molecules are adsorbed in a nonuniform manner and secondly the distance to the H2 molecules from the substrate also increases due to the steric interaction between the adsorbed H2 molecules. All these factors are
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Table 1 e Average bond lengths between Boron atoms (Be B), Boron Scandium (BeSc), Scandium Hydrogen (SceH), Scandium Scandium(Sc-Sc) and Hydrogen Hydrogen (He H) in A. Complexes
B3Sc2 B3Sc2-2H2 B4Sc2 B4Sc2-4H2 B5Sc2 B5Sc2-4H2 B6Sc2 B6Sc2-8H2 B7Sc2 B7Sc2-8H2 B8Sc2 B8Sc2-8H2 B9Sc2 B9Sc2-6H2 B10Sc2 B10Sc2-6H2
BeB
BeSc
SceSc
SceH
HeH
( A)
( A)
( A)
( A)
( A)
1.575 1.575 1.576 1.568 1.547 1.549 1.595 1.599 1.567 1.570 1.544 1.547 1.602 1.603 1.630 1.541
1.965 1.965 1.745 1.813 1.708 1.725 1.679 1.685 1.580 1.607 1.511 1.549 1.594 1.620 2.241 2.252
2.130
0.807
2.184
0.790
2.243
0.787
2.608
0.762
2.403
0.768
2.264
0.766
2.285
0.772
2.220
0.771
3.09 3.24 2.90 3.00 2.87 2.93 3.35 3.36 3.15 3.21 2.96 3.09 3.11 3.16 3.31 3.38
(3.11) (2.92) (2.88) (3.38) (3.18) (2.98) (3.14)
Fig. 3 e Variation of HOMO-LUMO gap with number of H2 moleclules per cluster of hydrogen trapped Sc decorated small boron cluster at PBE/6e311þþG(d,p) levels of theory.
(3.36)
Values in the parenthesis are the Scandium Scandium(Sc-Sc) bond lengths reported by Wu et al. [71].
responsible for the reduction of Eads . Besides, though the value of Eads for adsorption of the first H2 molecule on the complexes lies in range of 0.12 eVe0.21 eV concluding physisorption process, however with the increase in number of H2 molecule, the mean inter-molecular interaction among the H2 molecules also affect the adsorption energy. In Table 2, we also list the values of formation energy (Eform) calculated using Eq. (5). Negative values of Eform indicate the stability of the studied clusters [75]. It is found that the formation energy decrease (more negative) with subsequent addition of H2 molecule, however, no specific correlation is noted. Besides, we also calculate the differential adsorption energy of all the clusters and the results are provided in Table S3 in the Supplementary Information. The value of differential adsorption energy is found in the range of 0.24 to 0.03 eV.
A qualitative picture of the adsorption mechanism in the studied complex can be found by analyzing the average natural bond orbital (NBO) charges on adsorbent and the adsorbed H2 molecules. The computed values of the NBO charges are listed in Table 3. It is noted that the distortions, though very nominal, in the clusters (mainly observed via BeSc bond lengths) can be attributed to the charge distribution or transfer between B and Sc before and after H2 adsorption. For example, the BeSc bond length change of 0.027 A is noted in B7Sc2 which is due to the charge difference of 0.90 unit before and after the adsorption, whereas in B5Sc2, the BeSc bond length change by 0.017 A can be attributed to the charge variation of 0.44 unit. Though the total amount of charge on adsorbed H2 molecules increases with the number of H2, however, no specific correlation is observed. The reason might be the asymmetric arrangement of the H2 molecules around the Sc adsorbents. However, it is noted that both Sc atoms of the complexes adsorb the same number of H2 molecules by donating an equal amount of charges. For instance, the charge
Table 2 e Calculated Hardness (h), electronegativity (c), Electrophilicity Index (u), HOMO¡LUMO Energy Gap (Eg) and average adsorption energy of scandium doped small boron cluster, as well as hydrogen trapped complexes. Complexes B3Sc2 B3Sc2-2H2 B4Sc2 B4Sc2-4H2 B5Sc2 B5Sc2-4H2 B6Sc2 B6Sc2-8H2 B7Sc2 B7Sc2-8H2 B8Sc2 B8Sc2-8H2 B9Sc2 B9Sc2-6H2 B10Sc2 B10Sc2-6H2
h (eV)
u (eV)
c (eV)
Eg (eV)
0.44 0.47 0.28 0.28 0.43 0.59 0.32 0.21 0.15 0.29 0.08 0.41 0.70 0.81 0.81 0.95
9.29 8.40 24.20 24.06 17.21 11.72 22.38 27.65 49.97 22.12 98.12 18.48 10.85 8.77 9.94 8.27
2.86 2.85 3.70 3.67 3.85 3.71 3.79 3.45 3.91 3.62 4.11 3.90 3.91 3.70 4.03 3.98
0.88 0.90 0.56 0.56 0.86 1.18 0.64 0.41 0.36 0.59 0.17 0.82 1.41 1.61 1.63 1.91
Eads(eV/H2)
Eform (eV)
0.13
0.26
0.18
0.72
0.19
0.77
0.08
0.68
0.12
1.01
0.14
1.14
0.18
1.08
0.19
1.19
wt% e 3.19 e 5.67 e 5.30 e 9.43 e 8.87 e 8.37 e 6.06 e 5.75
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Fig. 4 e Variation of average adsorption energy with number of adsorbed H2 molecules per cluster.
Table 3 e NBO charges for BnSc2, n ¼ 3e10 clusters before and after H2 adsorption at PBE/6e311þþG(d,p) level of theory, and the values in the parenthesis are NBO charges before H2 adsorption. Complexes B3Sc2 B4Sc2 B5Sc2 B6Sc2 B7Sc2 B8Sc2 B9Sc2 B10Sc2
B 0.22 0.31 0.28 0.36 0.21 0.16 0.13 0.08
(0.24) (0.46) (0.38) (0.38) (0.33) (0.28) (0.15) (0.19)
Sc1
Sc2
H2
0.53 (0.61) 0.46 (0.94) 0.52 (0.96) 0.75 (1.51) 0.28 (1.18) 0.04 (1.15) 0.13 (0.41) 0.07 (0.99)
0.53 (0.61) 0.46 (0.94) 0.52 (0.96) 0.75 (1.51) 0.28 (1.18) 0.04 (1.15) 0.13 (0.41) 0.07 (0.99)
0.10 0.25 0.36 0.64 1.10 1.39 0.94 1.04
on both the Sc atoms in B6Sc2 before and after the H2 adsorption are 1.51 and 0.75, respectively and similarly, the charge on Sc atoms in B7Sc2 is 1.18 and 0.28, respectively. Although, the H2 molecules are added sequentially to the host clusters, however, we observe that the investigated clusters such as B3Sc2, B4Sc2, B5Sc2, B6Sc2, B7Sc2, B8Sc2, B9Sc2 and B10Sc2 can accommodate a maximum up to 2, 4, 4, 8, 8, 8, 6, and 6 number of H2 molecules, which means that the number of H2 adsorption does not show any specific relation with the number of boron atoms. For example, B6Sc2, B7Sc2, B8Sc2 can adsorb maximum up to 8H2 molecules each whereas B9Sc2 and B10Sc2 adsorb maximum up to 6H2. However, in all the cases, an equal number of H2 molecules are adsorbed on both the Sc atoms. The computed gravimetric densities of the studied clusters are provided in Table 2. We can see that the gravimetric density of all the studied clusters, except B3Sc2 are found in the range of 5.30 wt % 9.43 wt% which is quite remarkable in accordance with the target set by US DOE (5.5 wt% by 2020) [7]. The complexes B6Sc2, B7Sc2, and B8Sc2 are found to have adsorbing up to 8H2 molecules giving rise to comparatively high gravimetric densities of 9.43 wt %, 8.87 wt%, and 8.37 wt% respectively which bespeaks the fact that these clusters can be promoted as potential candidates for hydrogen storage. In addition, the range of binding energies is also favorable for Kubas type of
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interactions and hence can be more suitable for room temperature hydrogen storage [11]. According to Bader's quantum theory of atoms in molecules (QTAIM), the bond critical point (BCP) are generally the saddle points of the electron density (r), and appear between any two chemically bonded nuclei [76]. In order to describe the relative decrease or increase of charge accumulation at the bonding sites, we examined electron density (r) and its Laplacian V2 r at the BCPs which can give a qualitative knowledge about nature of bonding interaction. These parameters are calculated using AIMALL computational package and are listed in Table 4. It can be seen that, the calculated r < 0:20 with positive V2 r at the bond critical point of SceH, suggests that H2 molecule is more likely to have somewhat closed shell type of interaction with the Sc atoms. As proposed by Cremer et al. the nature of interaction can also be analyzed by calculating the total energy density (H BCP ) at the BCP [77,78]. We find (Table 4) the values of H BCP to be negative (in the ranges of 0.008 to 0.03 for SceH) in all the Sc doped boron clusters which suggest that potential energy density dominates over the kinetic energy density at the BCP [77,78]. For a detailed comparative study of electronic structures, along with frontier molecular orbital (FMOs), partial density of states (PDOS) on each fragment of the investigated BnSc2 clusters are calculated at PBE/6e311þþG(d,p) level of theory. The value of full width at half maxima (FWHM) is fixed at 0.3 eV. All the PDOS pictures are presented in Fig. 5, and the molecular orbitals (HOMO and LUMO) are provided in the Supplementary Information. From Fig. 5, Figs. S3 and S4 it can be observed that, in most of the investigated clusters, the delocalization of LUMOs are generally larger than that of the HOMOs, except B7Sc2, B8Sc2, and B9Sc2, where HOMOs delocalization are slightly more than LUMOs. From the PDOS spectrum we found that, in the B3Sc2 cluster, the contributions of HOMOs from the Sc1, Sc2 atoms are 29%, 29% respectively. Similarly, in B7Sc2 and B10Sc2 complexes, the contributions of HOMOs from the Sc1, Sc2 are 35%, 35% and 14%, 14%, respectively. This indicates that both the Sc atoms in the complexes have almost equal contributions to the HOMOs (and LUMOs) inferring the fact that both Sc atoms can adsorb an equal number of H2 molecules. In order to explore the adsorption and desorption process of H2 molecules at different temperatures, ADMP molecular dynamics simulations have been performed for all the studied complexes. We performed the thermostatic simulation at 1 atm pressure and five different temperature i.e 0K, 30K, 70K, 200K and 300K for 100fs time steps. The trajectories during the simulations are presented in Fig. 6, and the corresponding snapshots at different time steps are provided in Fig. S5. It is observed that within 80fs for 0K, 57fs for 30K, 70fs for 70K and with 50fs for 200K and 300K respectively, the first H2 molecule flies away from the B3Sc2 cluster, and all H2 molecule get desorbed at 100fs for all the considered temperature.For B4Sc2, the first H2 molecule starts moving away from the metal center at 0K, 30K, 200K and 300K at around 55fs, 60fs, 30fs, and 40fs respectively, and at end of simulation, it is unable to hold any H2 molecule at 200K and 300K, whereas at 0K, 30K, and 70K it can hold up to three H2 molecules at 100fs. It is noted that after complete desorption the original structure of B4Sc2 cluster is almost recovered without any
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Table 4 e Average number of electrons (N) in Sc/H atoms, electron density (r)in a.u., V2 r, Total energy density (H BCP of (Sc,H) and (Sc,B). Complex
N
rScH
V2 rScH
rScB
V2 rScB
B3Sc2-2H2 B4Sc2-4H2 B5Sc2-4H2 B6Sc2-8H2 B7Sc2-8H2 B8Sc2-8H2 B9Sc2-6H2 B10Sc2-6H2
1.97/1.08 1.96/1.03 1.95/1.02 1.94/1.00 1.94/1.00 1.94/1.01 1.94/1.05 1.94/1.03
0.0303 0.028 0.025 0.0124 0.0173 0.023 0.022 0.025
0.1321 0.1119 0.097 0.0390 0.066 0.094 0.089 0.102
0.071 0.063 0.059 0.060 0.051 0.041 0.034 0.052
0.080 0.100 0.091 0.107 0.093 0.073 0.091 0.088
H
BCPScH
0.03 0.02 0.02 0.008 0.01 0.02 0.02 0.02
BCP ) in a.u at
H
BCPScB
0.04 0.04 0.03 0.04 0.05 0.03 0.02 0.02
Fig. 5 e Partial density of state(PDOS) plot for first(Sc1) and second(Sc2) atoms in hydrogen trapped BnSc2, (n ¼ 1e3) complexes at PBE/6e311þþG(d,p) level. A fermi energy is set to zero energy and indicated by the blue dashed line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
distortion. Hence, B4Sc2 can be treated as a potential H2storage material. Similar observation in the case C6Li6 cluster was also reported for by Giri et al. who found that at lower temperatures the cluster could bind 6H2 molecules, whereas at higher temperatures it witnessed complete desorption without any distortion in its original optimized structure [79]. Similarly, Okamoto et al. studied adsorption-desorption dynamics of Ti2eC2H4 complex and reported that the adsorption and desorption process relied on the theoretical
methodologies [80]. B5Sc2 can hold up to two H2 molecule for 100 fs at all studied temperatures. Similarly, within 25fs and 27fs first H2 molecule of B6Sc2 and B7Sc2 starts moving away from the metal center and at 100fs they can hold 3H2 molecules with an average HeH distance of 0.75 A respectively for all considered temperatures. Similar results are observed for B8Sc2 and B9Sc2 clusters. In the case of B10Sc2 cluster, four H2 molecules get desorbed at 100fs and only 2H2 molecule retained with the host cluster with an average HeH distance
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Fig. 6 e Potential energy trajectories of hydrogen trapped BnSc2, n ¼ 3e10 at 0K,30K,70K, 200K and 300K temperatures.
of 0.75 A at all the considered temperatures. Moreover, it is observed that during ADMP simulation, all the cluster adsorb hydrogen with molecular form except the B5Sc2, in which one H2 molecule undergoes dissociative adsorption. During the simulation, negligible deformation in host clusters and change in the alignment of H2 molecule around the metal center is observed.
Conclusion In conclusion, the hydrogen storage mechanism in Sc doped small boron clusters BnSc2, (n ¼ 3e10) has been studied using density functional theory. A maximum of 8H2 can be accomodate by B6Sc2, B7Sc2 and B8Sc2 with gravimetric
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densities of 9.43, 8.87, 8.37 wt% respectively. Furthermore 6H2 got adsorbed in B9Sc2, B10Sc2 resulting the gravimetric density of 6.06 and 5.75 wt% respectively. The adsorption energy was found to fall in the range 0.08e0.19 eV/H2 which infers that the process was physisorption kind. Stability of the clusters was checked by calculating reactivity parameters such as hardness (h), electronegetivity (c) and electrophilicity index (u). It was observed that hardness increased when more number of hydrogen molecule were accommodated while the electrophilicity followed a decreasing trend with the addition of hydrogen molecules inferring the stability of the systems. Topological analysis using Bader's quantum theory of atoms in molecules (QTAIM) concluded the interaction between H2 and Sc atom to be somewhat closed shell type with r < 0:20 a.u and positive V2 9 at bond critical point (BCP) of (Sc, H). ADMP molecular dynamics simulations study was performed at five different temperatures such as 0K, 30K, 70K, 200K, and 300K. It was noted that for B6Sc2 to B9Sc2 up to 3H2 were attached to the metal centers at all the considered temperatures. Desorption of H2 molecules from the complexes support the applicability of these systems for hydrogen storage.
Acknowledgement We acknowledge the financial support from Science & Engineering Research Board (SERB), DST, India under grant no. EMR/2014/000141. Authors also acknowledge Indian Institute of Technology (Indian School of Mines), Dhanbad for providing support and other research facilities.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.109.
references
[1] Jena P. Materials for hydrogen storage: past, present, and future. J Phys Chem Lett 2011;2(3):206e11. [2] Niemann MU, Srinivasan SS, Phani AR, Kumar A, Goswami DY, Stefanakos EK. Nanomaterials for hydrogen storage applications: a review. J Nanomater 2008;2008:950967. [3] Dresselhaus MS, Thomas IL. Alternative energy technologies. Nature 2001;414:332e7. [4] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32(9):1121e40. [5] Meng S, Kaxiras E, Zhang Z. Metaldiboride nanotubes as highcapacity hydrogen storage media. Nano Lett 2007;7:663e7. [6] Tavhare P, Titus E, Chaudhari A. Boron substitution effect on adsorption of H2 molecules on organometallic complexes. Int J Hydrogen Energy 2019;44:345e53. [7] Seenithurai S, Chai JD. Effect of Li termination on the electronic and hydrogen storage properties of linear carbon chains: a TAO-DFT study. Sci Rep 2017;7:4966.
[8] Schlapbach L, Zuttel A. Hydrogen-storage for mobile applications. Nature 2001;414:353e8. [9] Orimo SI, Nakamori Y, Eliseo JR, Zuttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107:4111e32. [10] Grochala W, Edwards PP. Thermal decomposition of the noninterstitial hydrides for storage and production of hydrogen. Chem Rev 2004;104:1283e315. [11] Bora PL, Singh A. New insight into designing metallacarborane based room temperature hydrogen storage media. J Chem Phys 2013;139:164319. [12] Singh AK, Sadrzadeh A, Yakobson BI. Metallacarboranes: towards promising hydrogen storage metal-organic framework. J Am Chem Soc 2010;132:14126e9. [13] Dillion AC, Jones KM, Bekkedahl TA, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386:377e9. [14] Yurum Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [15] Yildirim T, Inguez J, Ciraci S. Molecular and dissociative adsorption of multiple hydrogen molecules on transition metal decorated C60. Phys Rev B 2005;72:153403. [16] Sun Q, Wang Q, Jena P, Kawazoe Y. Clustering of Ti on a C60 surface and its effect on hydrogen storage. J Am Chem Soc 2005;127:14582e3. [17] Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501. [18] Tang CM, Weihua SC, Kang ZJ, He X, Zhang Z. Transition metal Ti coated porous fullerene C24B24: potential material for hydrogen storage. Int J Hydrogen Energy 2015;40: 16271e7. [19] Sun Q, Jena P, Wang Q, Marquez M. First-principles study of hydrogen storage on Li12C60. J Am Chem Soc 2006;28:9741e5. [20] Yoon M, Yang S, Hicke C, Wang E, Geohegan D, Zhang Z. Calcium as the superior coating metal in functionalization of carbon fullerenes for high-capacity hydrogen storage. Phys Rev Lett 2008;100:206806. [21] Lang SM, Bernhardt TM. Gas phase metal cluster model systems for heterogeneous catalysis. Phys Chem Chem Phys 2012;14:9255e69. [22] Schwarz H. Doping effects in cluster-mediated bond activation. Angew Chem Int Ed 2015;54:10090e100. [23] Liu CS, An H, Guo LJ, Zeng Z, Ju X. Theoretical realization of cluster-assembled hydrogen storage materials based on terminated carbon atomic chains. J Chem Phys 2011;134:024522. [24] Zhang Y, Cheng X. Hydrogen storage property of alkali and alkaline-earth metal atom decorated C24 fullerene: a DFT study. Chem Phys 2018;505:26e33. [25] Ren HJ, Cui CX, Li XJ, Liu YJ. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerene. Int J Hydrogen Energy 2017;42:312e32. [26] Kaiser A, Renzler M, Kranabetter L, Shwarzler M, Parajuli R, Echt O, Scheier P. On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies. Int J Hydrogen Energy 2017;42:3078e86. [27] Teprovich JA, Wellons MS, Lascola R, Hwang SS, Ward PA, Compton RN, Zidan R. Synthesis and characterization of a lithium-doped fullerene (Lix-C60-Hy) for reversible hydrogen storage. Nano Lett 2012;12:582e9. [28] Chavez EL, Castaneda YP, Quiroz AG, Alvarado FC, Gongora JD, Gonjalez LJ. Ti-decorated C120 nanotorus: a new molecular structure for hydrogen storage. Int J Hydrogen Energy 2017;42:30237e42. [29] Long J, Li J, Nan F, Yin S, Li J, Cen W. Tailoring the thermostability and hydrogen storage capacity of Li
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 0 1 9 e6 0 3 0
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
decorated carbon materials by Heteroatom doping. Appl Surf Sci 2018;435:1065e71. Ren H, Cui CX, Li XJ, Liu YJ. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerenes. Int J Hydrogen Energy 2017;42:312e21. pez GE. Stability of Nguyen B, Cruz A, McGregor D, Lo molecular hydrogen inside C720 fullerene: a path integral Monte Carlo study. Phys Lett A 2017;381:298e300. Ruse E, Buzaglo M, Pri-Bar I, Shunak L, Nadiv R, Pevzner S, Mendelson OS, Skripnyuk VM, Rabkin E, Regev O. Hydrogen storage kinetics: the graphene nanoplatelet size effect. Carbon 2018;130:369e76. Bhattacharya A, Bhattacharya S. Exploring N-rich phases in LixNy clusters for hydrogen storage at nanoscale. J Phys Chem Lett 2015;6:3726e30. Wu WW, Tian ZY, Dong SL. Yttrium-dispersed B80 fullerenes: promising materials for hydrogen storage. Europhys Lett 2015;109:56004. Liu P, Zhang H, Cheng X, Tang Y. Ti-decorated B38 fullerene: a high capacity hydrogen storage material. Int J Hydrogen Energy 2016;41:19123e8. Lu QLa, Huang SG, Li YD, Wan JG, Luo QQ. Alkali and alkalineearth atom-decorated B38 fullerenes and their potential for hydrogen storage. Int J Hydrogen Energy 2015;40:13022e8. Liu P, Liu f, Wang Q, Ma Q. DFT simulation on hydrogen storage property over Sc decorated B38 fullerene. Int J Hydrogen Energy 2018;43:19540e6. Liu P, Zhang H, Cheng X, Tang Y. Transition metal atom Fe, Co, Ni decorated B38 fullerene: potential material for hydrogen storage. Int J Hydrogen Energy 2017;42:15256e61. Song N, Lv J, Wang Y. B24 cluster as promising material for lithium storage and hydrogen storage applications. Comput Mater Sci 2013;77:31e4. Ye XJ, Teng ZW, Yang XL, Liu CS. Na-coated hexagonal B36 as superior hydrogen storage materials. J Saudi Chem Soc 2018;22:84e9. Liu CS, Wang X, Ye XJ, Yan X, Zeng Z. Curvature and ionization-induced reversible hydrogen storage in metalized hexagonal B36. J Chem Phys 2014;141:194306. Du J, Sun X, Zhang L, Zhang C, Jiang G. Hydrogen storage of Li4&B36 cluster. Sci Rep 2018;8:1940. Wang J, Wei ZJ, Zhao HY, Liu Y. Hollow Li20B60 cage: stability and hydrogen storage. Sci Rep 2016;6:24500. https://doi.org/ 10.1038/srep24500. Si L, Tang C. The reversible hydrogen storage abilities of metal Na (Li, K, Ca, Mg, Sc, Ti, Y) decorated all-boron cage B28. Int J Hydrogen Energy 2017;42:16611e9. Liu CS, Ye XJ, Wang X, Yan X. Metalized B40 fullerene as a novel material for storage and optical detection of hydrogen: a first-principles study. RSC Adv 2016;6:56907e12. Zhang Y, Cheng X. A novel hydrogen storage medium of Cacoated B40: first principles study. Int J Hydrogen Energy 2018;43:15338e47. Tang C, Zhang X. The hydrogen storage capacity of Sc atoms decorated porous boron fullerene B40: a DFT study. Int J Hydrogen Energy 2016;41:16992e9. Seenithurai S, Chai JD. Electronic and hydrogen storage properties of Li-terminated linear boron chains studied by TAO-DFT. Sci Rep 2018;8:13538. Tang X, Gu Y, Kou L. Theoretical investigation of calciumdecorated b12 boron sheet for hydrogen storage. Chem Phys Lett 2018;695:211e5. Zhang F, Chen R, Zhang W. Ti- decorated boron monolayer : a promising material for hydrogen storage. RSC Adv 2016;6:12925e31. Wang J, Du Y, Sun L. Ca-decorated novel boron sheet: a potential hydrogen storage medium. Int J Hydrogen Energy 2016;41:5276e8.
6029
[52] Bai H, Bai B, Zhang L, Huang W, Mu YW, Zhai HZ, Li SD. Lithium-decorated borospherene B40: a promising hydrogen storage medium. Sci Rep 2016;6:35518. [53] Li L, Zhang H, Cheng X. The high hydrogen storage capacities of Li-decorated borophene. Comput Mater Sci 2017;137:119e24. [54] Srinivasu K, Ghosh SK. An ab initio investigation of hydrogen adsorption in Li-doped closo-boranes. J Phys Chem C 2010;115:1450e6. [55] Srinivasu K, Ghosh SK. Theoretical studies on hydrogen adsorption properties of lithium decorated diborane (B2H4Li) and diboryne (B2H2Li2). Int J Hydrogen Energy 2011;36:15681e8. [56] Banerjee S, Periyasamy G, Pati SK. Density functional theoretical investigation on structure, optical response and hydrogen adsorption properties of B9/metaleB9 clusters. Phys Chem Chem Phys 2013;15:8303. [57] Guo C, Wang C. Computational investigation of hydrogen storage on B6Tiþ 3 . Int J Hydrogen Energy 2018;43:1658e66. [58] Wu YY, Xu SY, Zhao FQ, Ju XH. Adsorption and dissociation of H2 on Bn and MgBn (n = 2e7) clusters: a DFT investigation. J Cluster Sci 2014;26. https://doi.org/10.1007/s10876-014-07914. [59] Guo C, Wang C. Computational investigation of hydrogen storage on B5V3. Mol Phys 2018. https://doi.org/10.1080/ 00268976.2018.1423710. [60] Tang C, Wang Z, Zhang X, Wen N. The hydrogen storage properties of Na decorated small boron cluster B6Na8. Chem Phys Lett 2016;661:161e7. [61] Huang H, Wu Ba, Gao Q, Li P, Yang X. Structural, electronic and spectral properties referring to hydrogen storage capacity in binary alloy ScBn (n¼1-12) clusters. Int J Hydrogen Energy 2017;42:21086e95. [62] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 09, Revision E.01. Wallingford CT: Gaussian, Inc; 2013. [63] Keith TA. AIMALL (version 11.02.27, standard). Overland Park,KS, USA: TK Gristmill Software; 2011. [64] Glendening ED, Reed AE, Carpenter JE and Weinhold F, NBO 3.1 [65] O'Boyle NM, Tenderholt Al, Langner KM. cclib: a library for package-independent computational chemistry algorithm. J Comput Chem 2008;29:839e45. [66] Parr RG. Absolute hardness: companion parameter to absolute electronegativity. J Am Chem Soc 1983;105:7512e6. [67] Sen KD, Jorqenson CK. Electronegativity; structure and bonding, vol. 66. Berlin: Springer; 1987. [68] Parr RG, Szentpaly LV, Liu S. Electrophilicity index. J Am Chem Soc 1999;121:1922e4. [69] Chatraj PK. Chemical reactivity theory: a density functional view. Boca Raton,FL: Taylor and Francis. CRC Press; 2009. ¨ ber die Zuordnung von Wellenfunktionen [70] Koopmans TA. U und Eigenwerten zu den Einzelnen Elektronen Eines Atoms. Physica 1934;1:104e13. [71] Jia J, Li X, Li Y, Ma L, Wu HS. Density functional theory investigation on the structure and stability of Sc2Bn ( n ¼ 1e10) clusters. Comp and Theo Chem 2014;1027:128e34. [72] Kumar S, Kumar TJD. Electronic structure calculation oh hydrogen storage in lithium -decorated metal-Graphyne framework. Appl Mater Interface 2017;9:28659e66. [73] Chamorro E, Chattaraj PK, Fuentealba P. Variation of the electrophilicity index along the reaction path. J Phys Chem A 2003;107:7068e72. [74] Parr RG, Chattaraj PK. Principle of maximum hardness. J Am Chem Soc 1991;113:1854e5. [75] Yao LF, Li K, Zhou NG. First-principles study of Mn adsorption on Al4C3 (0001) surface. Appl Surf Sci 2016;363:168e72.
6030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 0 1 9 e6 0 3 0
[76] Bader RFW. Atoms in molecules.A quantum theory. Oxford: Clarendon press; 1990. [77] Cremer D, Kraka E. Chemical bonds without bonding electron density d does the difference electron-density analysis suffice for a description of the chemical bond? Int Ed Engl 1984;23:627. [78] Espinosa E, Alkorta I, Elguero J, Molins E. From weak to strong interactions: a comprehensive analysis of the topological
and energetic properties of the electron density distribution involving XeH FeY systems. J Chem Phys 2002;117:5529. [79] Giri S, Lund F, Nunez SA, Toro-Labbe A. Can star like C6Li6 be treated as a potential H2 storage material? J Phys Chem C 2013;117:5544e51. [80] Okamoto Y. Can Ti2-C2H4 complex adsorb H2 molecules? J Phys Chem C 2008;112:17721e5.
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Hydrogen storage in scandium doped small boron clusters (BnSc2, n¼3e10): A density functional study Shakti S. Ray, Smruti R. Sahoo, Sridhar Sahu* High Performance Computing Lab, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, 826004, India
article info
abstract
Article history:
In this work, we present the hydrogen adsorption capacity of Sc doped small boron clusters
Received 10 August 2018
(BnSc2, n ¼ 3e10) using density functional theory. Almost no structural change was
Received in revised form
observed in the host clusters after hydrogen adsorption. Stabilities of the studied clusters
27 November 2018
were confirmed by various reactivity parameters such as hardness (h), electrophilicity (u),
Accepted 13 December 2018
and electronegativity (c). The average adsorption energies was found in the range of 0.08
Available online 7 February 2019
e0.19 eV/H2 inferring physisorption process, and the fact is also supported by the average A. All the clusters distance from Sc to H2 molecules which was in the range of 2.13 Å-2.60
Keywords:
were found to have gravimetric density satisfying the target set by the U.S. Department of
Density functional theory
Energy (US-DOE) (5.5 wt% by 2020). From Bader's topological analysis, it was confirmed that
Boron cluster
the nature of interaction was likely to be somewhat closed shell type. ADMP molecular
NBO
dynamics simulations study was performed at different temperatures to understand the
AIMALL
adsorption and dissociation of H2 from the complexes.
ADMP
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, being considered as a sustainable energy carrier, has received a lot of attention in the area of energy research because of its higher energy density, high efficiency and environmental friendliness [1e4]. However, the major bottleneck for the industrial applications of hydrogen energy is to develop cost-effective hydrogen storage carriers. Hydrogen storage as gas under high pressure or as liquid at cryogenic temperatures has inherent problems associated with safety, cost and capacity [5]. Further, developing new storage material with high volumetric and gravimetric densities (5.5 wt% and >40 kg/m3 by, as proposed by U.S. Department of Energy
(US-DOE)) with fast kinetics as well as favorable thermodynamics has also been quite a challenging task [6,7]. Hence, considerable efforts are devoted toward developing new solidstate storage materials. The storage materials bind the hydrogen atoms, basically, through three processes [1,8]. In chemisorption process the binding energy is in the range of 2e4 eV like chemical hydrides [9,10]. However, the major drawbacks in chemical hydrides are their slow dehydrogenation kinetics and desorption at high temperature due to metal-hydrogen binding [11,12]. On the other hand, for the materials, like nanostructured carbon, hydrogen molecules are weakly bonded with a binding energy in the range of few meV. This is regarded as physisorption process [13,14]. The third form of adsorption process with
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Sahu). https://doi.org/10.1016/j.ijhydene.2018.12.109 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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binding energy in the 0.1e0.8 eV range is also proposed which is intermediate between physisorption and chemisorption. Metal-decorated carbon-based nano-clusters and their derivates in which the metal atoms bind hydrogen in molecular form with intermediate binding energy, belong to the third form of adsorption [15e18]. For example, Sun et al. theoretically predicted that the Li12C60 could store up to 60H2 at most with binding energy of 0.075 eV/H2 resulting in high gravimetric density of 13 wt% [19]. Similar work has also been reported by Yoon et al. [20]. Nanostructured clusters have emerged as a new class of materials beacuse of their riveting reaction kinetics, thermodynamics and catalytic behavior [21,22]. A bagful of literature are found focusing on the hydrogen storage materials based on nanostructured clusters. For example, alkali metal and carbon fullerene terminated carbon chains was theoretically investigated by Ju et al. who calculated gravimetric density up to 10 wt% [23]. A similar theoretical study was performed by Chai et al. for lithiated carbon chain who found the gravimetric density of about 17.9 wt% above the temperature of liquid nitrogen [7]. Alkali and alkaline-earth metal decorated C24 fullerene was investigated by Cheng et al. who reported hydrogen storage capacity up to 12.7 wt% with adsorption energy varying in the range 0.13e0.19 eV/H2 [24]. Similar study on Li, Na doped C60 was also reported by Ren et al. [25]. Kiser et al. reported using both experimental and theoretical studies, Cs-doped C60 cluster could adsorb a maximum of 67H2 leading to a gravimetric density of 8.9 wt% [26]. Similarly, synthesis and characterization of Li-doped C60 for reversible hydrogen storage was carried out by Teprovich et al. who reported that the complex could adsorb up to 5% wt of H2 at the onset temperature of 270 degree Celsius [27]. Titanium decorated C120 nanotours was theoretically investigated by Chavez et al. who reported the hydrogen storage efficiency up to 5.6 wt% [28]. In addition, hydrogen storage in carbonaceous and other clusters were also widely reported in the literature [29e33]. Since last few years boron clusters, alongside carbonaceous materials, have also been considered as a promising candidate for hydrogen storage. But only a handful of work investigating hydrogen storage in boron nanoclusters has been reported so far. For example, Dong et al. investigated yitrium doped B80 fullerene that could bind 71H2 molecule resulting in the gravimetric density up to 6.85 wt% [34]. Tang et al. and Lu et al. investigated Ti and alkali and alkalineearth atom decorated B38 fullerene for hydrogen storage and found gravimetric densities up to 7.44 wt % and 6.47 wt % respectively [35,36]. Similarly, Liu et al. and Tang et al. theoretically investigated Sc and M-decorated (M ¼ Fe, Co, Ni) B38 fullerenes and found the gravimetric densities more than 7.0 wt % [37,38]. Wang et al. theoretically explored the hydrogen uptake capacity of Li decorated B24 cluster and found that the cluster can hold 16H2 molecules giving rise to 9.24 wt% of gravimetric density [39]. Recently, hydrogen trapping capacity of sodium coated hexagonal B36 cluster was explored by Ye et al. who reported that the cluster could trap up 10H2 molecules leading to 4.4 wt % of H2 with a binding energy of 0.25 eV/H2 [40]. Similar studies were also performed by Leu et al. and Jiang et al. [41,42]. Wang et al. investigated H2 storage capacity of hollow lithium decorated
B60 and found that the cage could uptake maximum up to 28H2 with an average binding energy of 0.33 eV/H2 leading to 8.19 wt% of gravimetric density [43]. Besides, Tang et al. performed molecular dynamics simulations to study the hydrogen storage capacity of alkali, alkaline-earth and transition metal-doped smallest boron cage (B28) and reported that it could provide up to 5.5 wt % of H2 desorbable at room temperature [44]. Yan et al. theoretically studied B40 fullerene doped with alkali metals those could give rise to gravimetric density up to 8.8 wt% of H2 at room temperature with an intermediate binding energy [45]. Similarly, Zhang et al. reported that Ca doped B40 could hold up to 30H2 molecules with adsorption energy of 0.177 eV/H2 leading to gravimetric density of 8.11 wt% [46]. Tang et al. investigated the hydrogen storage capacity of Scandium doped B40 cluster using molecular dynamic simulation and reported that the cluster could store maximum up to 6H2 per Sc atom giving rise to maximum gravimetric density of 8.3 wt% with average adsorption energy of 0.33e0.58 eV [47]. Li terminated linear boron chain was studied by Chai et al. TAO-DFT method and reported that the Li2Bn system could give rise to maximum of 17.0 wt % gravimetric density with a binding energy of 20e40 kJ/mol per H2 [48]. Studies related to other boron sheet uthors [49e51]. and monolayers are reported by various a Besides, hydrogen storage in other boron-based materials were also reported by many others [52e55]. However, considering hydrogen storage in small boron clusters (Bn, n < 20), only a few works have been reported so far. Pati et al. investigated the hydrogen storage capacity of metal doped B9 cluster and found that it could store hydrogen via chemisorption [56]. Guo et al. studied hydrogen storage properties of B6Tiþ 3 and found that it could store maximum up to 10H2 molecules resulting in the gravimetric density of 8.82 wt% [57]. Ju et al. investigated Mg doped small boron clusters Bn, (n ¼ 2e7) and observed that as compared to the three-dimensional structural boron clusters, the planar Bn, (n ¼ 4e7) clusters are more suitable candidates for H2 adsorption [58]. Wang et al. investigated the hydrogen trapping efficiency of B5V3 cluster and observed that the cluster could store up to 7H2 molecules with an average adsorption energy of 0.60 eV/H2 giving rise to 7.0 wt% of gravimetric density [59]. Similarly, Wen et al. reported the hydrogen storage properties of B6Na8 cluster and found that the cluster could store up to 32H2 molecule with adsorption energy in the range of 0.17e0.19 eV [60]. Recently, Hydrogen storage efficiency of binary alloy ScBn cluster was reported by Huang et al. who found that it could store up to 9.11 wt% of hydrogen molecule [61]. In this work, we report the adsorption of molecular hydrogen on the scandium doped small boron cluster BnSc2 (n ¼ 3e10) using density functional theory.
Computational details All the structures were optimized with and without H2 molecules using Perdew-Burke-Ernzerhof (PBE) hybrid exchange functional and employing 6e311þþG(d,p) basis set within the framework of density functional theory (DFT). PBE hybrid functional has been widely used by various authors to
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successfully investigate the hydrogen storage capacity of different clusters [61]. All the optimizations were accomplished without any imaginary harmonic frequencies. In addition, molecular dynamics simulations were performed to investigate the stability as well as the reversibility of adsorbed hydrogen molecules on the Sc doped boron clusters using atom centered density matrix propagation (ADMP). During the ADMP-MD simulation velocity scaling method were used to maintain the temperature. The time step (Dt) of ADMP-MD simulation is set at 0.1fs. All the calculations were carried out using Gaussian 09 computational program [62]. In addition, the nature of the interaction between the hydrogen molecules and sorption centers of the host cluster was explored using Bader's theory of atoms in molecules using AIMALL computational packages [63]. In order to get a more elaborate picture of the nature of the type of bonding, natural bond orbital (NBO) analysis was performed using the NBO 3.1 implemented in Gaussian 09 program [64]. Moreover, the electronic density of state (DOS) of the studied complexes was calculated using GaussSum program [65]. The stability and reactivity of the scandium decorated boron clusters and their H2 trapped analogos can be analyzed by investigating their, global reactivity descriptors, such as hardness (h), electrophilicity (u) and electronegativity (c) [66e69]. In addition, the kinetic stabilities of the scandium decorated boron clusters are obtained by calculating the energy gap (Eg) between their highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Now, chemical hardness, h is defined as, h¼
IA 2
(1)
where I and A are vertical ionization potential and electron affinity calculated using the Koopman's Theorem [70]. Again, electronegativity, c is expressed as c¼
IþA 2
(2)
Similarly, the electrophilicity index can be defined as u¼
c2 2h
(3)
The average hydrogen adsorption energy without zeropoint energy correction (Eads), is obtained using the following equation: Eads ¼ ½fEðHostÞ þ nEðH2 Þg EðComplexÞ=n
(4)
here EðComplexÞ, EðH2 Þ, EðHostÞ is the total electronic energies without zero point energy correction of hydrogenated complex and hydrogen molecule and host cluster respectively. Moreover, the formation energies (Eform) was obtained using the following equation: Eform ¼ ½EðComplexÞ fnEðH2 Þ þ EðHostÞg
(5)
here EðComplexÞ, EðH2 Þ, EðHostÞ is the total electronic energies without zero point energy correction of hydrogenated complex and hydrogen molecule and host cluster respectively. The hydrogen storage gravimetric density was calculated by following equation:
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H2 ðwt%Þ ¼
MH2 100 MH2 þ MHost
(6)
where MH2 indicates the mass of total number of adsorbed H2 molecules and MHost indicates the mass of Sc doped small boron cluster.
Results and discussion In this section, we present our calculations of adsorption of molecular H2 on scandium (Sc) doped small boron clusters, BnSc2, n ¼ 3 10: Before hydrogen adsorption, we optimized the geometries of bare BnSc2 clusters employing the methodology mentioned in the computational section, and all the stable geometries of host BnSc2 clusters and those adsorbed with H2 molecules are presented in Figs. 1 and 2 respectively. The optimized geometrical parameters of the studied clusters are listed in Table 1. We find that our optimized geometrical data for host BnSc2, ðn ¼ 3 10Þ clusters fairly agree with those reported by Wu et al. [71]. For example, the SceSc bond lengths in the cases of B3Sc2, B4Sc2, and B5Sc2, are found to be 3.09 A, 2.90 A, and 2.87 A which are in good agreement with those reported earlier [71]. Moreover, nominal changes in the geometries of the host clusters is observed after H2 adsorption. For instance, almost no change in BeB bond lengths is observed before and after the H2 adsorption in all the clusters A is except B10Sc2 in which a small change of the order of 0.09 noted. Similarly, in the cases of B3Sc2, B6Sc2, and B10Sc2 clusters no significant change is observed in BeSc bond lengths before and after H2 adsorption, whereas a change in the range of 0.02e0.07 A is noticed for the clusters B4Sc2, B5Sc2, B7Sc2, B8Sc2, and B9Sc2. Similarly, an elongation in SceSc bond lengths in the range of 0.01e0.15 A is noted in the host clusters after H2 adsorption. The insignificant changes in the geometries of host clusters after interaction with H2 molecules infer the adsorption process to be physisorption type. Moreover, the average distance between the adsorbent Sc atom and molecular hydrogen lies in the range of 2.13 A - 2.28 A. The distance slightly increase from B3Sc2-2H2 to B6Sc2-8H2 and then decreases onwards. The HeH distance in all the complexes are found to fall in the range of 0.76 A - 0.80 A and the elongation in HeH as compared to the bond length of the isolated H2 molecule (0.74 Å), is due to the charge transfer from Sc atom to H2 molecule confirming non-classical Kubas-Niu-Rao-Jena type interaction between Sc and H2 and inferring the process to be physisorption [72]. Stability is one of the important parameters to focus while studying the hydrogen storage mechanisms in different clusters. This stability factor can be investigated by various reactivity parameters such as hardness (h), electrophilicity (u), and electronegativity (c). In Table 2, we provide all these reactivity parameters computed at PBE/6e311þþG(d,p) level of theory. In all the studied clusters except B6Sc2, the value of h is found to increase with H2 adsorption, while the electrophilicity index, u decrease. For instance, the value of h is found be maximum (0.95 eV) for B10Sc2-6H2 and minimum (0.21 eV) for B6Sc2-8H2, and as expected, minimum u (8.27 eV) and maximum u (27.65 eV) are found for B10Sc2-6H2 and B6Sc2-8H2
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Fig. 1 e Optimized structures of small boron cluster (BnSc2,n ¼ 3e10) at PBE/6e311þþG(d,p) level of theory.
complexes respectively. Hence, following maximum hardness and minimum electrophilicity principle, the results obtained theoretically confirms the enhanced stability of the investigated clusters [73,74]. However, in the case of B6Sc2 we notice a reverse trend for h and u. Further, the value of c is found to decrease with the adsorption of H2 molecules in the complexes being maximum (4.11eV) for B10Sc2-6H2 and minimum (2.85eV) for B3Sc2-2H2 complexes., In addition, to investigate the kinetic stability of the hydrogenated clusters we also computed their HOMO-LUMO energy gaps (Eg) and the values are listed in Table 2. For all the clusters, except B6Sc2, the value of Eg increases upon H2 adsorption confirming the stability of the complexes. Moreover, the large Eg reduces the reactivity of
the clusters causing slightly increase in the bond length thus inferring better stabilization. The variation of Eg with the number of hydrogen molecules per clusters is presented in Fig. 3. It can be seen from the figure that there is an increase in Eg with an increase in the number of hydrogen molecules adsorbed which is an indication of stabilization of the complexes. Hence, the enhanced stabilities of the investigated BnSc2 (n ¼ 3e10) clusters gives a theoretical insight that clusters can be used as promising hydrogen storage materials. Furthermore, we also calculated the average adsorption energy (Eads ) at the same computational methodology, and the values are provided in Table 2, and the variation of adsorption energy with number of H2 molecules per cluster are presented
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Fig. 2 e Optimized geometries of hydrogen trapped Sc doped boron clusters at PBE/6e311þþG(d,p) level.
in Fig. 4. It is observed that the value of Eads falls in the range of 0.08e0.19 eV/H2, which confirms the process of reversible hydrogen physisorption mechanism [1]. We find that the value of Eads decreases with more number of H2 molecules trapped in a cluster. However, we do not observe any systematic variation in Eads with consecutive addition of H2 molecules. For example, in B7Sc2, we find a gradual decrease in
Eads up to eight H2 molecules, whereas in the case of B8Sc2, Eads increase up to the addition of 4H2 molecules and then decreases. Moreover, it is observed that, with an increase in number, the H2 molecules are adsorbed in a nonuniform manner and secondly the distance to the H2 molecules from the substrate also increases due to the steric interaction between the adsorbed H2 molecules. All these factors are
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Table 1 e Average bond lengths between Boron atoms (Be B), Boron Scandium (BeSc), Scandium Hydrogen (SceH), Scandium Scandium(Sc-Sc) and Hydrogen Hydrogen (He H) in A. Complexes
B3Sc2 B3Sc2-2H2 B4Sc2 B4Sc2-4H2 B5Sc2 B5Sc2-4H2 B6Sc2 B6Sc2-8H2 B7Sc2 B7Sc2-8H2 B8Sc2 B8Sc2-8H2 B9Sc2 B9Sc2-6H2 B10Sc2 B10Sc2-6H2
BeB
BeSc
SceSc
SceH
HeH
( A)
( A)
( A)
( A)
( A)
1.575 1.575 1.576 1.568 1.547 1.549 1.595 1.599 1.567 1.570 1.544 1.547 1.602 1.603 1.630 1.541
1.965 1.965 1.745 1.813 1.708 1.725 1.679 1.685 1.580 1.607 1.511 1.549 1.594 1.620 2.241 2.252
2.130
0.807
2.184
0.790
2.243
0.787
2.608
0.762
2.403
0.768
2.264
0.766
2.285
0.772
2.220
0.771
3.09 3.24 2.90 3.00 2.87 2.93 3.35 3.36 3.15 3.21 2.96 3.09 3.11 3.16 3.31 3.38
(3.11) (2.92) (2.88) (3.38) (3.18) (2.98) (3.14)
Fig. 3 e Variation of HOMO-LUMO gap with number of H2 moleclules per cluster of hydrogen trapped Sc decorated small boron cluster at PBE/6e311þþG(d,p) levels of theory.
(3.36)
Values in the parenthesis are the Scandium Scandium(Sc-Sc) bond lengths reported by Wu et al. [71].
responsible for the reduction of Eads . Besides, though the value of Eads for adsorption of the first H2 molecule on the complexes lies in range of 0.12 eVe0.21 eV concluding physisorption process, however with the increase in number of H2 molecule, the mean inter-molecular interaction among the H2 molecules also affect the adsorption energy. In Table 2, we also list the values of formation energy (Eform) calculated using Eq. (5). Negative values of Eform indicate the stability of the studied clusters [75]. It is found that the formation energy decrease (more negative) with subsequent addition of H2 molecule, however, no specific correlation is noted. Besides, we also calculate the differential adsorption energy of all the clusters and the results are provided in Table S3 in the Supplementary Information. The value of differential adsorption energy is found in the range of 0.24 to 0.03 eV.
A qualitative picture of the adsorption mechanism in the studied complex can be found by analyzing the average natural bond orbital (NBO) charges on adsorbent and the adsorbed H2 molecules. The computed values of the NBO charges are listed in Table 3. It is noted that the distortions, though very nominal, in the clusters (mainly observed via BeSc bond lengths) can be attributed to the charge distribution or transfer between B and Sc before and after H2 adsorption. For example, the BeSc bond length change of 0.027 A is noted in B7Sc2 which is due to the charge difference of 0.90 unit before and after the adsorption, whereas in B5Sc2, the BeSc bond length change by 0.017 A can be attributed to the charge variation of 0.44 unit. Though the total amount of charge on adsorbed H2 molecules increases with the number of H2, however, no specific correlation is observed. The reason might be the asymmetric arrangement of the H2 molecules around the Sc adsorbents. However, it is noted that both Sc atoms of the complexes adsorb the same number of H2 molecules by donating an equal amount of charges. For instance, the charge
Table 2 e Calculated Hardness (h), electronegativity (c), Electrophilicity Index (u), HOMO¡LUMO Energy Gap (Eg) and average adsorption energy of scandium doped small boron cluster, as well as hydrogen trapped complexes. Complexes B3Sc2 B3Sc2-2H2 B4Sc2 B4Sc2-4H2 B5Sc2 B5Sc2-4H2 B6Sc2 B6Sc2-8H2 B7Sc2 B7Sc2-8H2 B8Sc2 B8Sc2-8H2 B9Sc2 B9Sc2-6H2 B10Sc2 B10Sc2-6H2
h (eV)
u (eV)
c (eV)
Eg (eV)
0.44 0.47 0.28 0.28 0.43 0.59 0.32 0.21 0.15 0.29 0.08 0.41 0.70 0.81 0.81 0.95
9.29 8.40 24.20 24.06 17.21 11.72 22.38 27.65 49.97 22.12 98.12 18.48 10.85 8.77 9.94 8.27
2.86 2.85 3.70 3.67 3.85 3.71 3.79 3.45 3.91 3.62 4.11 3.90 3.91 3.70 4.03 3.98
0.88 0.90 0.56 0.56 0.86 1.18 0.64 0.41 0.36 0.59 0.17 0.82 1.41 1.61 1.63 1.91
Eads(eV/H2)
Eform (eV)
0.13
0.26
0.18
0.72
0.19
0.77
0.08
0.68
0.12
1.01
0.14
1.14
0.18
1.08
0.19
1.19
wt% e 3.19 e 5.67 e 5.30 e 9.43 e 8.87 e 8.37 e 6.06 e 5.75
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Fig. 4 e Variation of average adsorption energy with number of adsorbed H2 molecules per cluster.
Table 3 e NBO charges for BnSc2, n ¼ 3e10 clusters before and after H2 adsorption at PBE/6e311þþG(d,p) level of theory, and the values in the parenthesis are NBO charges before H2 adsorption. Complexes B3Sc2 B4Sc2 B5Sc2 B6Sc2 B7Sc2 B8Sc2 B9Sc2 B10Sc2
B 0.22 0.31 0.28 0.36 0.21 0.16 0.13 0.08
(0.24) (0.46) (0.38) (0.38) (0.33) (0.28) (0.15) (0.19)
Sc1
Sc2
H2
0.53 (0.61) 0.46 (0.94) 0.52 (0.96) 0.75 (1.51) 0.28 (1.18) 0.04 (1.15) 0.13 (0.41) 0.07 (0.99)
0.53 (0.61) 0.46 (0.94) 0.52 (0.96) 0.75 (1.51) 0.28 (1.18) 0.04 (1.15) 0.13 (0.41) 0.07 (0.99)
0.10 0.25 0.36 0.64 1.10 1.39 0.94 1.04
on both the Sc atoms in B6Sc2 before and after the H2 adsorption are 1.51 and 0.75, respectively and similarly, the charge on Sc atoms in B7Sc2 is 1.18 and 0.28, respectively. Although, the H2 molecules are added sequentially to the host clusters, however, we observe that the investigated clusters such as B3Sc2, B4Sc2, B5Sc2, B6Sc2, B7Sc2, B8Sc2, B9Sc2 and B10Sc2 can accommodate a maximum up to 2, 4, 4, 8, 8, 8, 6, and 6 number of H2 molecules, which means that the number of H2 adsorption does not show any specific relation with the number of boron atoms. For example, B6Sc2, B7Sc2, B8Sc2 can adsorb maximum up to 8H2 molecules each whereas B9Sc2 and B10Sc2 adsorb maximum up to 6H2. However, in all the cases, an equal number of H2 molecules are adsorbed on both the Sc atoms. The computed gravimetric densities of the studied clusters are provided in Table 2. We can see that the gravimetric density of all the studied clusters, except B3Sc2 are found in the range of 5.30 wt % 9.43 wt% which is quite remarkable in accordance with the target set by US DOE (5.5 wt% by 2020) [7]. The complexes B6Sc2, B7Sc2, and B8Sc2 are found to have adsorbing up to 8H2 molecules giving rise to comparatively high gravimetric densities of 9.43 wt %, 8.87 wt%, and 8.37 wt% respectively which bespeaks the fact that these clusters can be promoted as potential candidates for hydrogen storage. In addition, the range of binding energies is also favorable for Kubas type of
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interactions and hence can be more suitable for room temperature hydrogen storage [11]. According to Bader's quantum theory of atoms in molecules (QTAIM), the bond critical point (BCP) are generally the saddle points of the electron density (r), and appear between any two chemically bonded nuclei [76]. In order to describe the relative decrease or increase of charge accumulation at the bonding sites, we examined electron density (r) and its Laplacian V2 r at the BCPs which can give a qualitative knowledge about nature of bonding interaction. These parameters are calculated using AIMALL computational package and are listed in Table 4. It can be seen that, the calculated r < 0:20 with positive V2 r at the bond critical point of SceH, suggests that H2 molecule is more likely to have somewhat closed shell type of interaction with the Sc atoms. As proposed by Cremer et al. the nature of interaction can also be analyzed by calculating the total energy density (H BCP ) at the BCP [77,78]. We find (Table 4) the values of H BCP to be negative (in the ranges of 0.008 to 0.03 for SceH) in all the Sc doped boron clusters which suggest that potential energy density dominates over the kinetic energy density at the BCP [77,78]. For a detailed comparative study of electronic structures, along with frontier molecular orbital (FMOs), partial density of states (PDOS) on each fragment of the investigated BnSc2 clusters are calculated at PBE/6e311þþG(d,p) level of theory. The value of full width at half maxima (FWHM) is fixed at 0.3 eV. All the PDOS pictures are presented in Fig. 5, and the molecular orbitals (HOMO and LUMO) are provided in the Supplementary Information. From Fig. 5, Figs. S3 and S4 it can be observed that, in most of the investigated clusters, the delocalization of LUMOs are generally larger than that of the HOMOs, except B7Sc2, B8Sc2, and B9Sc2, where HOMOs delocalization are slightly more than LUMOs. From the PDOS spectrum we found that, in the B3Sc2 cluster, the contributions of HOMOs from the Sc1, Sc2 atoms are 29%, 29% respectively. Similarly, in B7Sc2 and B10Sc2 complexes, the contributions of HOMOs from the Sc1, Sc2 are 35%, 35% and 14%, 14%, respectively. This indicates that both the Sc atoms in the complexes have almost equal contributions to the HOMOs (and LUMOs) inferring the fact that both Sc atoms can adsorb an equal number of H2 molecules. In order to explore the adsorption and desorption process of H2 molecules at different temperatures, ADMP molecular dynamics simulations have been performed for all the studied complexes. We performed the thermostatic simulation at 1 atm pressure and five different temperature i.e 0K, 30K, 70K, 200K and 300K for 100fs time steps. The trajectories during the simulations are presented in Fig. 6, and the corresponding snapshots at different time steps are provided in Fig. S5. It is observed that within 80fs for 0K, 57fs for 30K, 70fs for 70K and with 50fs for 200K and 300K respectively, the first H2 molecule flies away from the B3Sc2 cluster, and all H2 molecule get desorbed at 100fs for all the considered temperature.For B4Sc2, the first H2 molecule starts moving away from the metal center at 0K, 30K, 200K and 300K at around 55fs, 60fs, 30fs, and 40fs respectively, and at end of simulation, it is unable to hold any H2 molecule at 200K and 300K, whereas at 0K, 30K, and 70K it can hold up to three H2 molecules at 100fs. It is noted that after complete desorption the original structure of B4Sc2 cluster is almost recovered without any
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Table 4 e Average number of electrons (N) in Sc/H atoms, electron density (r)in a.u., V2 r, Total energy density (H BCP of (Sc,H) and (Sc,B). Complex
N
rScH
V2 rScH
rScB
V2 rScB
B3Sc2-2H2 B4Sc2-4H2 B5Sc2-4H2 B6Sc2-8H2 B7Sc2-8H2 B8Sc2-8H2 B9Sc2-6H2 B10Sc2-6H2
1.97/1.08 1.96/1.03 1.95/1.02 1.94/1.00 1.94/1.00 1.94/1.01 1.94/1.05 1.94/1.03
0.0303 0.028 0.025 0.0124 0.0173 0.023 0.022 0.025
0.1321 0.1119 0.097 0.0390 0.066 0.094 0.089 0.102
0.071 0.063 0.059 0.060 0.051 0.041 0.034 0.052
0.080 0.100 0.091 0.107 0.093 0.073 0.091 0.088
H
BCPScH
0.03 0.02 0.02 0.008 0.01 0.02 0.02 0.02
BCP ) in a.u at
H
BCPScB
0.04 0.04 0.03 0.04 0.05 0.03 0.02 0.02
Fig. 5 e Partial density of state(PDOS) plot for first(Sc1) and second(Sc2) atoms in hydrogen trapped BnSc2, (n ¼ 1e3) complexes at PBE/6e311þþG(d,p) level. A fermi energy is set to zero energy and indicated by the blue dashed line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
distortion. Hence, B4Sc2 can be treated as a potential H2storage material. Similar observation in the case C6Li6 cluster was also reported for by Giri et al. who found that at lower temperatures the cluster could bind 6H2 molecules, whereas at higher temperatures it witnessed complete desorption without any distortion in its original optimized structure [79]. Similarly, Okamoto et al. studied adsorption-desorption dynamics of Ti2eC2H4 complex and reported that the adsorption and desorption process relied on the theoretical
methodologies [80]. B5Sc2 can hold up to two H2 molecule for 100 fs at all studied temperatures. Similarly, within 25fs and 27fs first H2 molecule of B6Sc2 and B7Sc2 starts moving away from the metal center and at 100fs they can hold 3H2 molecules with an average HeH distance of 0.75 A respectively for all considered temperatures. Similar results are observed for B8Sc2 and B9Sc2 clusters. In the case of B10Sc2 cluster, four H2 molecules get desorbed at 100fs and only 2H2 molecule retained with the host cluster with an average HeH distance
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Fig. 6 e Potential energy trajectories of hydrogen trapped BnSc2, n ¼ 3e10 at 0K,30K,70K, 200K and 300K temperatures.
of 0.75 A at all the considered temperatures. Moreover, it is observed that during ADMP simulation, all the cluster adsorb hydrogen with molecular form except the B5Sc2, in which one H2 molecule undergoes dissociative adsorption. During the simulation, negligible deformation in host clusters and change in the alignment of H2 molecule around the metal center is observed.
Conclusion In conclusion, the hydrogen storage mechanism in Sc doped small boron clusters BnSc2, (n ¼ 3e10) has been studied using density functional theory. A maximum of 8H2 can be accomodate by B6Sc2, B7Sc2 and B8Sc2 with gravimetric
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densities of 9.43, 8.87, 8.37 wt% respectively. Furthermore 6H2 got adsorbed in B9Sc2, B10Sc2 resulting the gravimetric density of 6.06 and 5.75 wt% respectively. The adsorption energy was found to fall in the range 0.08e0.19 eV/H2 which infers that the process was physisorption kind. Stability of the clusters was checked by calculating reactivity parameters such as hardness (h), electronegetivity (c) and electrophilicity index (u). It was observed that hardness increased when more number of hydrogen molecule were accommodated while the electrophilicity followed a decreasing trend with the addition of hydrogen molecules inferring the stability of the systems. Topological analysis using Bader's quantum theory of atoms in molecules (QTAIM) concluded the interaction between H2 and Sc atom to be somewhat closed shell type with r < 0:20 a.u and positive V2 9 at bond critical point (BCP) of (Sc, H). ADMP molecular dynamics simulations study was performed at five different temperatures such as 0K, 30K, 70K, 200K, and 300K. It was noted that for B6Sc2 to B9Sc2 up to 3H2 were attached to the metal centers at all the considered temperatures. Desorption of H2 molecules from the complexes support the applicability of these systems for hydrogen storage.
Acknowledgement We acknowledge the financial support from Science & Engineering Research Board (SERB), DST, India under grant no. EMR/2014/000141. Authors also acknowledge Indian Institute of Technology (Indian School of Mines), Dhanbad for providing support and other research facilities.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.109.
references
[1] Jena P. Materials for hydrogen storage: past, present, and future. J Phys Chem Lett 2011;2(3):206e11. [2] Niemann MU, Srinivasan SS, Phani AR, Kumar A, Goswami DY, Stefanakos EK. Nanomaterials for hydrogen storage applications: a review. J Nanomater 2008;2008:950967. [3] Dresselhaus MS, Thomas IL. Alternative energy technologies. Nature 2001;414:332e7. [4] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32(9):1121e40. [5] Meng S, Kaxiras E, Zhang Z. Metaldiboride nanotubes as highcapacity hydrogen storage media. Nano Lett 2007;7:663e7. [6] Tavhare P, Titus E, Chaudhari A. Boron substitution effect on adsorption of H2 molecules on organometallic complexes. Int J Hydrogen Energy 2019;44:345e53. [7] Seenithurai S, Chai JD. Effect of Li termination on the electronic and hydrogen storage properties of linear carbon chains: a TAO-DFT study. Sci Rep 2017;7:4966.
[8] Schlapbach L, Zuttel A. Hydrogen-storage for mobile applications. Nature 2001;414:353e8. [9] Orimo SI, Nakamori Y, Eliseo JR, Zuttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107:4111e32. [10] Grochala W, Edwards PP. Thermal decomposition of the noninterstitial hydrides for storage and production of hydrogen. Chem Rev 2004;104:1283e315. [11] Bora PL, Singh A. New insight into designing metallacarborane based room temperature hydrogen storage media. J Chem Phys 2013;139:164319. [12] Singh AK, Sadrzadeh A, Yakobson BI. Metallacarboranes: towards promising hydrogen storage metal-organic framework. J Am Chem Soc 2010;132:14126e9. [13] Dillion AC, Jones KM, Bekkedahl TA, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386:377e9. [14] Yurum Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [15] Yildirim T, Inguez J, Ciraci S. Molecular and dissociative adsorption of multiple hydrogen molecules on transition metal decorated C60. Phys Rev B 2005;72:153403. [16] Sun Q, Wang Q, Jena P, Kawazoe Y. Clustering of Ti on a C60 surface and its effect on hydrogen storage. J Am Chem Soc 2005;127:14582e3. [17] Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501. [18] Tang CM, Weihua SC, Kang ZJ, He X, Zhang Z. Transition metal Ti coated porous fullerene C24B24: potential material for hydrogen storage. Int J Hydrogen Energy 2015;40: 16271e7. [19] Sun Q, Jena P, Wang Q, Marquez M. First-principles study of hydrogen storage on Li12C60. J Am Chem Soc 2006;28:9741e5. [20] Yoon M, Yang S, Hicke C, Wang E, Geohegan D, Zhang Z. Calcium as the superior coating metal in functionalization of carbon fullerenes for high-capacity hydrogen storage. Phys Rev Lett 2008;100:206806. [21] Lang SM, Bernhardt TM. Gas phase metal cluster model systems for heterogeneous catalysis. Phys Chem Chem Phys 2012;14:9255e69. [22] Schwarz H. Doping effects in cluster-mediated bond activation. Angew Chem Int Ed 2015;54:10090e100. [23] Liu CS, An H, Guo LJ, Zeng Z, Ju X. Theoretical realization of cluster-assembled hydrogen storage materials based on terminated carbon atomic chains. J Chem Phys 2011;134:024522. [24] Zhang Y, Cheng X. Hydrogen storage property of alkali and alkaline-earth metal atom decorated C24 fullerene: a DFT study. Chem Phys 2018;505:26e33. [25] Ren HJ, Cui CX, Li XJ, Liu YJ. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerene. Int J Hydrogen Energy 2017;42:312e32. [26] Kaiser A, Renzler M, Kranabetter L, Shwarzler M, Parajuli R, Echt O, Scheier P. On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies. Int J Hydrogen Energy 2017;42:3078e86. [27] Teprovich JA, Wellons MS, Lascola R, Hwang SS, Ward PA, Compton RN, Zidan R. Synthesis and characterization of a lithium-doped fullerene (Lix-C60-Hy) for reversible hydrogen storage. Nano Lett 2012;12:582e9. [28] Chavez EL, Castaneda YP, Quiroz AG, Alvarado FC, Gongora JD, Gonjalez LJ. Ti-decorated C120 nanotorus: a new molecular structure for hydrogen storage. Int J Hydrogen Energy 2017;42:30237e42. [29] Long J, Li J, Nan F, Yin S, Li J, Cen W. Tailoring the thermostability and hydrogen storage capacity of Li
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 0 1 9 e6 0 3 0
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
decorated carbon materials by Heteroatom doping. Appl Surf Sci 2018;435:1065e71. Ren H, Cui CX, Li XJ, Liu YJ. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerenes. Int J Hydrogen Energy 2017;42:312e21. pez GE. Stability of Nguyen B, Cruz A, McGregor D, Lo molecular hydrogen inside C720 fullerene: a path integral Monte Carlo study. Phys Lett A 2017;381:298e300. Ruse E, Buzaglo M, Pri-Bar I, Shunak L, Nadiv R, Pevzner S, Mendelson OS, Skripnyuk VM, Rabkin E, Regev O. Hydrogen storage kinetics: the graphene nanoplatelet size effect. Carbon 2018;130:369e76. Bhattacharya A, Bhattacharya S. Exploring N-rich phases in LixNy clusters for hydrogen storage at nanoscale. J Phys Chem Lett 2015;6:3726e30. Wu WW, Tian ZY, Dong SL. Yttrium-dispersed B80 fullerenes: promising materials for hydrogen storage. Europhys Lett 2015;109:56004. Liu P, Zhang H, Cheng X, Tang Y. Ti-decorated B38 fullerene: a high capacity hydrogen storage material. Int J Hydrogen Energy 2016;41:19123e8. Lu QLa, Huang SG, Li YD, Wan JG, Luo QQ. Alkali and alkalineearth atom-decorated B38 fullerenes and their potential for hydrogen storage. Int J Hydrogen Energy 2015;40:13022e8. Liu P, Liu f, Wang Q, Ma Q. DFT simulation on hydrogen storage property over Sc decorated B38 fullerene. Int J Hydrogen Energy 2018;43:19540e6. Liu P, Zhang H, Cheng X, Tang Y. Transition metal atom Fe, Co, Ni decorated B38 fullerene: potential material for hydrogen storage. Int J Hydrogen Energy 2017;42:15256e61. Song N, Lv J, Wang Y. B24 cluster as promising material for lithium storage and hydrogen storage applications. Comput Mater Sci 2013;77:31e4. Ye XJ, Teng ZW, Yang XL, Liu CS. Na-coated hexagonal B36 as superior hydrogen storage materials. J Saudi Chem Soc 2018;22:84e9. Liu CS, Wang X, Ye XJ, Yan X, Zeng Z. Curvature and ionization-induced reversible hydrogen storage in metalized hexagonal B36. J Chem Phys 2014;141:194306. Du J, Sun X, Zhang L, Zhang C, Jiang G. Hydrogen storage of Li4&B36 cluster. Sci Rep 2018;8:1940. Wang J, Wei ZJ, Zhao HY, Liu Y. Hollow Li20B60 cage: stability and hydrogen storage. Sci Rep 2016;6:24500. https://doi.org/ 10.1038/srep24500. Si L, Tang C. The reversible hydrogen storage abilities of metal Na (Li, K, Ca, Mg, Sc, Ti, Y) decorated all-boron cage B28. Int J Hydrogen Energy 2017;42:16611e9. Liu CS, Ye XJ, Wang X, Yan X. Metalized B40 fullerene as a novel material for storage and optical detection of hydrogen: a first-principles study. RSC Adv 2016;6:56907e12. Zhang Y, Cheng X. A novel hydrogen storage medium of Cacoated B40: first principles study. Int J Hydrogen Energy 2018;43:15338e47. Tang C, Zhang X. The hydrogen storage capacity of Sc atoms decorated porous boron fullerene B40: a DFT study. Int J Hydrogen Energy 2016;41:16992e9. Seenithurai S, Chai JD. Electronic and hydrogen storage properties of Li-terminated linear boron chains studied by TAO-DFT. Sci Rep 2018;8:13538. Tang X, Gu Y, Kou L. Theoretical investigation of calciumdecorated b12 boron sheet for hydrogen storage. Chem Phys Lett 2018;695:211e5. Zhang F, Chen R, Zhang W. Ti- decorated boron monolayer : a promising material for hydrogen storage. RSC Adv 2016;6:12925e31. Wang J, Du Y, Sun L. Ca-decorated novel boron sheet: a potential hydrogen storage medium. Int J Hydrogen Energy 2016;41:5276e8.
6029
[52] Bai H, Bai B, Zhang L, Huang W, Mu YW, Zhai HZ, Li SD. Lithium-decorated borospherene B40: a promising hydrogen storage medium. Sci Rep 2016;6:35518. [53] Li L, Zhang H, Cheng X. The high hydrogen storage capacities of Li-decorated borophene. Comput Mater Sci 2017;137:119e24. [54] Srinivasu K, Ghosh SK. An ab initio investigation of hydrogen adsorption in Li-doped closo-boranes. J Phys Chem C 2010;115:1450e6. [55] Srinivasu K, Ghosh SK. Theoretical studies on hydrogen adsorption properties of lithium decorated diborane (B2H4Li) and diboryne (B2H2Li2). Int J Hydrogen Energy 2011;36:15681e8. [56] Banerjee S, Periyasamy G, Pati SK. Density functional theoretical investigation on structure, optical response and hydrogen adsorption properties of B9/metaleB9 clusters. Phys Chem Chem Phys 2013;15:8303. [57] Guo C, Wang C. Computational investigation of hydrogen storage on B6Tiþ 3 . Int J Hydrogen Energy 2018;43:1658e66. [58] Wu YY, Xu SY, Zhao FQ, Ju XH. Adsorption and dissociation of H2 on Bn and MgBn (n = 2e7) clusters: a DFT investigation. J Cluster Sci 2014;26. https://doi.org/10.1007/s10876-014-07914. [59] Guo C, Wang C. Computational investigation of hydrogen storage on B5V3. Mol Phys 2018. https://doi.org/10.1080/ 00268976.2018.1423710. [60] Tang C, Wang Z, Zhang X, Wen N. The hydrogen storage properties of Na decorated small boron cluster B6Na8. Chem Phys Lett 2016;661:161e7. [61] Huang H, Wu Ba, Gao Q, Li P, Yang X. Structural, electronic and spectral properties referring to hydrogen storage capacity in binary alloy ScBn (n¼1-12) clusters. Int J Hydrogen Energy 2017;42:21086e95. [62] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 09, Revision E.01. Wallingford CT: Gaussian, Inc; 2013. [63] Keith TA. AIMALL (version 11.02.27, standard). Overland Park,KS, USA: TK Gristmill Software; 2011. [64] Glendening ED, Reed AE, Carpenter JE and Weinhold F, NBO 3.1 [65] O'Boyle NM, Tenderholt Al, Langner KM. cclib: a library for package-independent computational chemistry algorithm. J Comput Chem 2008;29:839e45. [66] Parr RG. Absolute hardness: companion parameter to absolute electronegativity. J Am Chem Soc 1983;105:7512e6. [67] Sen KD, Jorqenson CK. Electronegativity; structure and bonding, vol. 66. Berlin: Springer; 1987. [68] Parr RG, Szentpaly LV, Liu S. Electrophilicity index. J Am Chem Soc 1999;121:1922e4. [69] Chatraj PK. Chemical reactivity theory: a density functional view. Boca Raton,FL: Taylor and Francis. CRC Press; 2009. ¨ ber die Zuordnung von Wellenfunktionen [70] Koopmans TA. U und Eigenwerten zu den Einzelnen Elektronen Eines Atoms. Physica 1934;1:104e13. [71] Jia J, Li X, Li Y, Ma L, Wu HS. Density functional theory investigation on the structure and stability of Sc2Bn ( n ¼ 1e10) clusters. Comp and Theo Chem 2014;1027:128e34. [72] Kumar S, Kumar TJD. Electronic structure calculation oh hydrogen storage in lithium -decorated metal-Graphyne framework. Appl Mater Interface 2017;9:28659e66. [73] Chamorro E, Chattaraj PK, Fuentealba P. Variation of the electrophilicity index along the reaction path. J Phys Chem A 2003;107:7068e72. [74] Parr RG, Chattaraj PK. Principle of maximum hardness. J Am Chem Soc 1991;113:1854e5. [75] Yao LF, Li K, Zhou NG. First-principles study of Mn adsorption on Al4C3 (0001) surface. Appl Surf Sci 2016;363:168e72.
6030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 0 1 9 e6 0 3 0
[76] Bader RFW. Atoms in molecules.A quantum theory. Oxford: Clarendon press; 1990. [77] Cremer D, Kraka E. Chemical bonds without bonding electron density d does the difference electron-density analysis suffice for a description of the chemical bond? Int Ed Engl 1984;23:627. [78] Espinosa E, Alkorta I, Elguero J, Molins E. From weak to strong interactions: a comprehensive analysis of the topological
and energetic properties of the electron density distribution involving XeH FeY systems. J Chem Phys 2002;117:5529. [79] Giri S, Lund F, Nunez SA, Toro-Labbe A. Can star like C6Li6 be treated as a potential H2 storage material? J Phys Chem C 2013;117:5544e51. [80] Okamoto Y. Can Ti2-C2H4 complex adsorb H2 molecules? J Phys Chem C 2008;112:17721e5.