- Email: [email protected]
Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field
Accepted Manuscript Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field Leila Saedi, Elham Alipour, Zahra Javanshir, Vahid Vahabi PII:
S1093-3263(18)30705-8
DOI:
https://doi.org/10.1016/j.jmgm.2018.12.004
Reference:
JMG 7287
To appear in:
Journal of Molecular Graphics and Modelling
Received Date: 16 September 2018 Revised Date:
26 November 2018
Accepted Date: 7 December 2018
Please cite this article as: L. Saedi, E. Alipour, Z. Javanshir, V. Vahabi, Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field, Journal of Molecular Graphics and Modelling (2019), doi: https://doi.org/10.1016/j.jmgm.2018.12.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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T-Graphene as a verestile material for hydrogen storage.
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Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field
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Leila Saedi,a* Elham Alipour,b Zahra Javanshir,c Vahid Vahabid
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Department of Chemistry, East Tehran Branch, Islamic Azad University, Tehran, Iran Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran c Department of Chemistry, Faculty of Sciences, Ahar Branch, Islamic Azad University, Ahar, Iran d Department of Chemistry, College of Sciences, Central Tehran Branch, Islamic Azad University, Tehran, Iran
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b
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* Email: [email protected]
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Abstract In the present work, we have studied a new allotrope of graphene, denoted as T-graphene (TG)
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flake as a versatile material in hydrogen storage. Recently, the metallic character of TG has been revealed. Our results show that the Li-decoration has a significant effect on the electronic properties of TG flake. Our density functional theory (DFT) calculations exhibit that the energy band gap of TG flake is decreased by decorating of the Li atom. Hydrogen adsorption on Li-
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decorated TG flake (Li/TG) under the influence of different external electric fields (EFs) is also
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explored by DFT calculations. We found that the hydrogen adsorption on the Li/TG increases when the positive EF is applied. Our results also show that the adsorption energy of the hydrogen on the Li/TG can be gradually enhanced by increasing the applied positive external EF along with the charge transfer direction. Moreover, Li atom in the Li/TG shows the high hydrogen capacity up to six H2 molecules. On the other hand, the H2 adsorption on the Li/TG is
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remarkably decreased by applying the negative EFs to the Li/TG. Therefore, the H2 adsorption/release procedure on the Li/TG are reversible and can be tuned by applying the appropriate EFs. Our study exhibits that the Li/TG is a promising material for reversible
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adsorption and release of H2.
Keywords: T-graphene; Hydrogen storage; Li decoration; External electric field; Density functional theory.
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1. Introduction Carbon can form several allotropes with sp; sp2 and sp3 hybridizations. The carbon allotropes
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represent diverse physical properties which depends on their hybridization. For example, the diamond with sp3 hybridized carbon atoms shows a hard insulator character but graphene with sp2 hybridized carbon atoms exhibits the metallic nature. With various properties of the carbon allotropes, the carbon-based materials can be used in numerous fields, such as field-effect
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transistors [1,2], transparent conducting electrodes [3,4], Hall-effect sensors [5,6], super-
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capacitor [7], nanoelectronics [8,9], energy storage [10], cutting tools [11], electrodes [12–14], lubricants [15], hydrogen storage media [16,17] and metal-free catalysis [18,19]. These large range applications of carbon-based materials, motivate many researchers to search for designing novel carbon allotropes by means of theoretical as well as experimental methods. In this respect, a novel two dimensional (2D) carbon allotrope, known as tetragonal graphene (TG), has been
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recently predicted theoretically by Liu et al. [20]. The TG possesses tetragonal symmetry as compared to the hexagonal symmetry of graphene. Liu et al. [20] have revealed that the planer TG is dynamically stable. They also demonstrated that the electron beam radiation [20, 21] and
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locating carbon atoms on Ni (111) surface [20] may provide the necessary requirements for the
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synthesis of the TG. Wang et al. [22] studied the electronic properties of 2D TG nanosheets as well as 1D TG nanoribbons based on phonon mode analysis [23]. They revealed that the 1D nanoribbons have the higher density of states (DOS) at the Fermi level as compared to 2D TG nanosheets. Structural, mechanical, and electronic properties of TG have been recently studied by functional tight-binding (DFTB) method [24, 25]. Motivated by the previous studies on the newly predicted TG nanomaterial, in the present work, we have explored the Li-decorated TG (Li/TG) as a promising material for hydrogen storage.
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The search for renewable energy resources is a very active research field with considerable potential for industrial applications. Hydrogen, with large resources and non-pollution, has been exhibited as an excellent candidate in renewable energy resources [26-28]. However, finding
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feasible strategies and highly efficient materials for hydrogen storage is still a big challenge [29]. Recently, the applications of carbon-based nanomaterials such as carbon nanotubes, fullerenes, and graphene in hydrogen storage have been extensively investigated [30-36]. However, the H2
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adsorption on the pristine carbon-based nanomaterials is too weak [37]. On the other side, decorating carbon-based nanomaterials with transition metals [38-40], alkaline metals [41, 42],
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or alkaline earth metals [43] has been suggested as excellent strategies to improve the hydrogen storage in these nanomaterials. In this respect, in the present work, we investigate the hydrogen adsorption on the Li/TG under different EFs using DFT calculations. First, we explore the stability of different decoration sites for Li atom on the TG flake. Next, we investigate the
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structural and electronic properties of single H2 adsorption on the Li/TG under different EFs. Finally, we explore the multi-H2 adsorption on the Li/TG to investigate the hydrogen storage capacity of the Li/TG.
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2. Computational details
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All quantum chemical calculations were performed using the general theoretical and computational method based on all-electrons DFT with the generalized gradient approximation (GGA) in Perdew- Burke-Ernzerhof (PBE) [44] functional form and the 6-31+G(d) basis set, as implemented in GAUSSIAN 09 [45]. Also, we adopted a DFT + D (D stands for dispersion) approach with the Grimme’s vdW correction in our calculations [46]. Further, the natural bond orbital analysis (NBO) [47] was utilized to compute the given charge transfer in this work. The
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binding energy (Ebin) of a Li on the TG flake and the adsorption energy (Eads) for the H2 adsorbed
Ebin=E(Li/TG)-E(Li)-E(TG)
(1)
Eads=E(H2-Li/TG)-E(H2)-E(Li/TG)
(2)
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on the Li/TG systems are computed using the following equations
where, E (Li/TG) is the total energy of the Li/TG system, E (Li) and E (TG) are the total energies
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of a Li atom and the TG fake, respectively. Also, E(H2-Li/TG) and E(H2) are the total energies of the H2-Li/TG complexes and isolated H2 molecule, respectively. Also, for applying the
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appropriate EFs, the keywords “Field = Z + n” and “Field = Z-n” (n > 0) [48, 49] were used for imposing positive and negative EFs, respectively, on the H2-Li/TG. 3. Results and discussion
3.1. Pristine TG flake and Li/TG systems
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We have selected the planer TG fake model similar to the planer TG nanosheets studied in Refs. [50-52]. The optimized structure of the TG flake is represented in Fig. 1. Structurally, there are two kinds of C-C bonds in TG flake: the bonds in square rings and bonds in connecting square
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rings. The bond lengths of the optimized TG flake are also represented in Fig. 1(a).
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Fig. 1. Optimized structure (a), DOS plot (b), and HOMO and LUMO distributions (c) of the pristine TG flake. The dashed line in DOS plot shows the TG’s Fermi energy. To avoid boundary effects, atoms at the open ends of the TG flake were saturated with hydrogen atoms. In this respect, the C-C bonds in the edges of TG flake are elongated compared to the bond lengths in the center of flake (1.47 Å and 1.39 Å). In Fig. 1 (b), we have presented the DOS
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of the pristine TG flake. From the DOS plot, it is obvious that the pristine TG flake has metallic character. In fact, the pristine TG flake with HOMO and LUMO energies at -4.06 eV and -3.84 eV, respectively, and HOMO-LUMO energy gap (Eg) of 0.22 eV exhibits metallic character.
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This is in agreement with the previous studies on the TG flake [50-52]. Structurally, the TG flake
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model has two types of carbon rings: the small square ring (R1) consisting of 4 C atoms and the large eight-member ring (R2) (Fig. 1 (a)). Our frontier molecular orbital (FMO) analysis shows (Fig. 1 (c)) that the HOMO and LUMO orbitals are more distributed on the square rings of the pristine TG flake. Here, we examined all the Li decoration sites on the TG flake. Our DFT calculations show that the stable configuration in which the Li atom is located on top of R1 hollow site (Fig. 2).
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Fig. 2. Optimized configuration and DOS plot of the Li/TG.
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Fig. 2, shows that the Li atom locates above the TG flake at a distance of 1.76 Å. The calculated Ebin of the optimized structure of Li/TG with Li atom decorated on the square ring is about 45.53 kcal/mol. On the other side, the Ebin of Li decorated on the eight-membered rings of TG is 37.01 kcal/mol. Ideally, the binding energies for the catalytic activity of catalytic activity of highperformance adsorbents should be in a range of 45–50 kcal mol−1 [53, 54]. Accordingly, the Li/TG with
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Li atom decorated on the square ring shows more stability compared to the Li/TG with Li atom decorated on the eight-member ring. The calculated C-C bond length of the square ring in TG flake, around
the Li atom (1.49 Å), is larger than that in pristine TG flake (1.46 Å). In order to reveal the
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correlation between the electronic property and the nature of binding in Li/TG, we calculate the
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partial density of state (PDOS) of Li/ TG, as shown in Fig. 2. As shown in this figure, in the high energy regions of the DOS of Li/TG, the DOS generates from the contributions of Li and TG orbitals. On the other hand, in the low energy regions, the DOS mostly generates from the TG orbitals. In fact, the DOS of Li/TG (Fig. 2) closely matches that of pristine TG flake (Fig. 1), except for the contribution from Li atom in high energy regions. We found that the HOMO and LUMO levels of Li/TG shift towards the higher and lower energies, respectively, as compared to those of pristine TG flake. Accordingly, the Eg of the Li/TG exhibits a lower value (0.14 eV) as 7
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compared to the pristine TG flake (0.22 eV). As shown in Fig. 1, the Fermi level of the pristine TG flake located at -3.95 eV. After the Li decorating, the Fermi level approaches to the LUMO region (-3.88 eV). This shift to lower energies (Fig. 2) leads to n-type semiconducting behavior
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in the Li/TG system [55-58]. The natural charge on the Li atom represent a large charge transfer (0.925e) from the Li atom to the TG flake; thus there is a strong electrostatic interaction between the Li and the TG.
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3.2. Hydrogen adsorption on Li/TG
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We systematically investigate the H2 adsorption on the Li/TG to explore its reactivity evolution toward the hydrogen molecule. Moreover, we examined both end-on (H2 axis aligns perpendicular to the Li/TG) and site-on (H2 axis aligns parallel to the Li/TG) configurations, and
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found that the side-on configuration is more stable for the H2-Li/TG complex, as shown in Fig. 3.
Fig. 3. The Top (a) and side (b) views and molecular electrostatic potential (MEP) map of the H2-Li/TG complex. The H2 bond length in the Li/TG system is elongated (0.77 Å) from that in the isolated gas phase (0.75 Å). In fact, enhancement of the H-H bond in the adsorbed H2 molecule exhibits strong interaction between the H2 and Li/TG. We also performed the molecular electrostatic potential (MEP) analysis of the H2-Li/TG complex (Fig. 3). The MEP analysis shows a large overlapping
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between the orbitals of the Li/TG and the H2 molecule. Moreover, the depletion of charge density on the hydrogen indicates the polarization of the H2 on the Li/TG. The Eads value of the H2 adsorbed on the Li/TG shows (-4.06 kcal/mol) the strong interaction between H2 molecule
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and Li/TG (Fig. 3). To reveal how the EFs, affect the H2 adsorption on the Li/TG, the H2-Li/TG complex has been optimized under the influence of different EFs. In this respect, H2-Li/TG complex has been optimized under the influence of EFs with magnitudes ranging from −50 ×
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10−4 a.u. to 50 × 10−4 a.u. All the external EFs have been applied parallel to the charge transfer direction (Li/TG → H2) in both positive EFs (F > 0) and negative EFs (F < 0), with the direction
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of the Li/TG → H2 and Li/TG ← H2, respectively (Fig. 4).
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Fig. 4. Cartesian axis and the direction of the positive and negative EFs (F, in 10−4 a.u.) in the H2-Li/TG.
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Our results show that the H2-Li/TG represents relatively large Eads without any EF (Fig. 3). However, one exciting question emerges: Can the Eads of the H2 on the Li/TG be increased by applying the appropriate EFs? The Li/TG can be used to reversible H2 storage media by applying the different EFs? To answer these questions, the Eads of the H2-Li/TG under the different EFs have been calculated and listed in Table 1. Table 1. The Eads with the EFs for the H2-Li/TG. Electric field (×10-4 a.u.)
Eads Electric field (kcal/mol) (×10-4 a.u.) 9
Eads (kcal/mol)
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-2.17 -2.10 -1.42 -0.99 -0.86
10 20 30 40 50
-5.34 -6.39 -8.47 -10.42 -12.71
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-10 -20 -30 -40 -50
To better represent the results, the correlation between the Eads values of the H2-Li/TG and the
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different EFs is represented in Fig. 5.
Electric Field (×104 a.u.) -50 -40 -30 -20 -10
-4.00 -7.00
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30
40
50
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Eads (kcal mol-1)
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Fig. 5. Dependence of the applied EFs (in 10−4 a.u.) on the Eads of H2-Li/TG As shown in Table 1 and Fig. 5, the Eads of H2 on the Li/TG gradually increase along with the
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applied EF changing from −50 ×10−4 to 50 × 10−4 a.u., representing that the Eads of H2 on the Li/TG can be controlled by changing the magnitude and direction of an applied EF. Specifically, the field-free Eads of H2-Li/TG (-4.06 kcal/mol) is considerably enhanced to -12.71 kcal/mol, via applying an EF of 50 × 10−4 a.u. along the charge transfer direction, whereas it is decreased (0.86 kcal/mol) under the EF of −50 × 10−4 a.u. against the charge transfer direction. In fact, on Li/TG, the Li atom is polarized in the applied positive EF. The electrostatic interaction between the hydrogen molecule and polarized Li makes the Li atom favored for hydrogen adsorption. The 10
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distance between the hydrogen molecule and the Li is decreased by the polarization interaction induced by the applied positive EF and the polar bond on Li/TG. Concurrently, the H-H bond length increases remarkably with increasing the positive EF. In the absence of an EF, the H-H
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bond length in the H2-Li/TG is 0.77 Å (Fig. 3). The H-H bond length becomes 0.84 Å when the EF reaches 50×10-4 a.u., increased nearly by 10%. On the other hand, the H-H bond length as well as the distance between the H2 molecule and Li atom in the H2-Li/TG, decreases and
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increases, respectively, with increasing the magnitude of negative EF. In fact, when the negative EFs were applied to the Li/TG, the Eads values of hydrogen molecule on the Li/TG (Eads=-0.86
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kcal/mol) are remarkably decreased as compared to the H2 adsorbed on the Li/TG in the absence of an EF (Table 1 and Fig. 5). Therefore, the H2 adsorption/release procedure on the Li/TG are reversible and can be easily tuned by applying the appropriate EFs.
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3.3. The capacity of hydrogen storage on Li/TG
We investigated the average Eads of the hydrogen molecules on the Li/TG by imposing an EF of 50×10−4 a.u., by increasing the number of adsorbed hydrogen molecules (Fig. 6). We found that
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the Li/TG can adsorb up to six hydrogen molecules with the average Eads slightly decreasing from -12.71 kcal/mol (number of H2 on the Li/TG, n=1) to -3.11 kcal/mol (n=6). This reduction
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in Eads is mainly a result of the steric hindrance effect among the adsorbed hydrogen molecules. Our calculations show that a further increase in the number of hydrogen leads to some H2 molecules moving far away from the Li/TG during the geometry optimization and the decrease in the Eads; thus, we define n=6 as a likely saturation for full hydrogen coverage on the Li/TG under the influence of the EF (Fig. 6).
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Fig. 6. The most stable configurations of Li/TG at full hydrogen coverage. The average adsorption energy of hydrogen molecules on the Li/TG at different hydrogen coverages (n=1-6 (a-f)) are also given.
At full hydrogen coverage, the average Eads of H2 molecules on Li/TG (-3.11 kcal/mol per
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molecule) is still much higher than the optimal Eads for the H2 on high-performance adsorbents (4~ -14 kcal/mol) [59, 60]. In addition to the Eads values, the gravimetric density of the H2 adsorbed on the Li/TG flake is explored to estimate the maximum storage capacity of the H2
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molecule. The gravimetric density of H2 is calculated from the equation
where M(H2) is the molecular weight of H2 and M(complex) is the weight of the adsorbed complex system. Experimental studies of hydrogen uptakes on the surface of the pristine graphene sheet are about 0.4 wt% and 0.2 wt%, at 77 K and 1 bar [61, 62]. Our results show that the Li/TG has increased the uptake capacities of the adsorbent efficiently. For nH2-Li/TG systems (n=1-6), hydrogen uptakes are about 0.24 wt% (n=1), 0.48 wt% (n=2), 0.72 wt% (n=3), 0.95 wt% (n=4), 1.19 wt% (n=5) and 1.43 wt% (n=6), respectively. In this respect, the Li/TG 12
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flake shows larger up taken of weight percentage of hydrogen than reported results for pristine graphene.
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4. Conclusion DFT calculations are carried out to study the Li/TG application in hydrogen storage. Our calculations revealed that the Li atom decorated on the hollow site of a square ring is more
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preferable that the 8-member ring of the TG flake. After a single hydrogen molecule adsorption on Li/TG, it is found that the Eads is -4.61 kcal/mol. In order to explore the effects of the external
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EFs on the Eads of H2 on the Li/TG, the positive and negative EFs are applied. It is shown that the H2 and Li atom are polarized under the positive external EFs. It can be seen that the Eads values increase dramatically with the increase in positive EFs intensity. Our results show that the Li/TG under positive EF can adsorb up to six hydrogen molecules. On the contrary, the Eads values of
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hydrogen molecule on the Li/TG remarkably decreased when the negative EFs were applied to the Li/TG, as compared to the H2 adsorbed on the Li/TG in the absence of an EF. Our results propose that the Li/TG is a promising material as a reversible hydrogen adsorption media. The
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direction.
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outcomes of the current study are expected to motivate active research attempts to search in this
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The stability and electronic properties of Li/T-graphene (TG) were studied. The H2 adsorption on the surface of Li/TG has been investigated. H2 adsorption/release processes can be easily controlled by applying electric fields.
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• • •
S1093-3263(18)30705-8
DOI:
https://doi.org/10.1016/j.jmgm.2018.12.004
Reference:
JMG 7287
To appear in:
Journal of Molecular Graphics and Modelling
Received Date: 16 September 2018 Revised Date:
26 November 2018
Accepted Date: 7 December 2018
Please cite this article as: L. Saedi, E. Alipour, Z. Javanshir, V. Vahabi, Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field, Journal of Molecular Graphics and Modelling (2019), doi: https://doi.org/10.1016/j.jmgm.2018.12.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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T-Graphene as a verestile material for hydrogen storage.
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Reversible hydrogen adsorption on Li-decorated T-graphene flake: The effect of electric field
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Leila Saedi,a* Elham Alipour,b Zahra Javanshir,c Vahid Vahabid
a
Department of Chemistry, East Tehran Branch, Islamic Azad University, Tehran, Iran Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran c Department of Chemistry, Faculty of Sciences, Ahar Branch, Islamic Azad University, Ahar, Iran d Department of Chemistry, College of Sciences, Central Tehran Branch, Islamic Azad University, Tehran, Iran
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b
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* Email: [email protected]
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Abstract In the present work, we have studied a new allotrope of graphene, denoted as T-graphene (TG)
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flake as a versatile material in hydrogen storage. Recently, the metallic character of TG has been revealed. Our results show that the Li-decoration has a significant effect on the electronic properties of TG flake. Our density functional theory (DFT) calculations exhibit that the energy band gap of TG flake is decreased by decorating of the Li atom. Hydrogen adsorption on Li-
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decorated TG flake (Li/TG) under the influence of different external electric fields (EFs) is also
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explored by DFT calculations. We found that the hydrogen adsorption on the Li/TG increases when the positive EF is applied. Our results also show that the adsorption energy of the hydrogen on the Li/TG can be gradually enhanced by increasing the applied positive external EF along with the charge transfer direction. Moreover, Li atom in the Li/TG shows the high hydrogen capacity up to six H2 molecules. On the other hand, the H2 adsorption on the Li/TG is
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remarkably decreased by applying the negative EFs to the Li/TG. Therefore, the H2 adsorption/release procedure on the Li/TG are reversible and can be tuned by applying the appropriate EFs. Our study exhibits that the Li/TG is a promising material for reversible
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adsorption and release of H2.
Keywords: T-graphene; Hydrogen storage; Li decoration; External electric field; Density functional theory.
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1. Introduction Carbon can form several allotropes with sp; sp2 and sp3 hybridizations. The carbon allotropes
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represent diverse physical properties which depends on their hybridization. For example, the diamond with sp3 hybridized carbon atoms shows a hard insulator character but graphene with sp2 hybridized carbon atoms exhibits the metallic nature. With various properties of the carbon allotropes, the carbon-based materials can be used in numerous fields, such as field-effect
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transistors [1,2], transparent conducting electrodes [3,4], Hall-effect sensors [5,6], super-
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capacitor [7], nanoelectronics [8,9], energy storage [10], cutting tools [11], electrodes [12–14], lubricants [15], hydrogen storage media [16,17] and metal-free catalysis [18,19]. These large range applications of carbon-based materials, motivate many researchers to search for designing novel carbon allotropes by means of theoretical as well as experimental methods. In this respect, a novel two dimensional (2D) carbon allotrope, known as tetragonal graphene (TG), has been
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recently predicted theoretically by Liu et al. [20]. The TG possesses tetragonal symmetry as compared to the hexagonal symmetry of graphene. Liu et al. [20] have revealed that the planer TG is dynamically stable. They also demonstrated that the electron beam radiation [20, 21] and
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locating carbon atoms on Ni (111) surface [20] may provide the necessary requirements for the
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synthesis of the TG. Wang et al. [22] studied the electronic properties of 2D TG nanosheets as well as 1D TG nanoribbons based on phonon mode analysis [23]. They revealed that the 1D nanoribbons have the higher density of states (DOS) at the Fermi level as compared to 2D TG nanosheets. Structural, mechanical, and electronic properties of TG have been recently studied by functional tight-binding (DFTB) method [24, 25]. Motivated by the previous studies on the newly predicted TG nanomaterial, in the present work, we have explored the Li-decorated TG (Li/TG) as a promising material for hydrogen storage.
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The search for renewable energy resources is a very active research field with considerable potential for industrial applications. Hydrogen, with large resources and non-pollution, has been exhibited as an excellent candidate in renewable energy resources [26-28]. However, finding
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feasible strategies and highly efficient materials for hydrogen storage is still a big challenge [29]. Recently, the applications of carbon-based nanomaterials such as carbon nanotubes, fullerenes, and graphene in hydrogen storage have been extensively investigated [30-36]. However, the H2
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adsorption on the pristine carbon-based nanomaterials is too weak [37]. On the other side, decorating carbon-based nanomaterials with transition metals [38-40], alkaline metals [41, 42],
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or alkaline earth metals [43] has been suggested as excellent strategies to improve the hydrogen storage in these nanomaterials. In this respect, in the present work, we investigate the hydrogen adsorption on the Li/TG under different EFs using DFT calculations. First, we explore the stability of different decoration sites for Li atom on the TG flake. Next, we investigate the
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structural and electronic properties of single H2 adsorption on the Li/TG under different EFs. Finally, we explore the multi-H2 adsorption on the Li/TG to investigate the hydrogen storage capacity of the Li/TG.
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2. Computational details
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All quantum chemical calculations were performed using the general theoretical and computational method based on all-electrons DFT with the generalized gradient approximation (GGA) in Perdew- Burke-Ernzerhof (PBE) [44] functional form and the 6-31+G(d) basis set, as implemented in GAUSSIAN 09 [45]. Also, we adopted a DFT + D (D stands for dispersion) approach with the Grimme’s vdW correction in our calculations [46]. Further, the natural bond orbital analysis (NBO) [47] was utilized to compute the given charge transfer in this work. The
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binding energy (Ebin) of a Li on the TG flake and the adsorption energy (Eads) for the H2 adsorbed
Ebin=E(Li/TG)-E(Li)-E(TG)
(1)
Eads=E(H2-Li/TG)-E(H2)-E(Li/TG)
(2)
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on the Li/TG systems are computed using the following equations
where, E (Li/TG) is the total energy of the Li/TG system, E (Li) and E (TG) are the total energies
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of a Li atom and the TG fake, respectively. Also, E(H2-Li/TG) and E(H2) are the total energies of the H2-Li/TG complexes and isolated H2 molecule, respectively. Also, for applying the
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appropriate EFs, the keywords “Field = Z + n” and “Field = Z-n” (n > 0) [48, 49] were used for imposing positive and negative EFs, respectively, on the H2-Li/TG. 3. Results and discussion
3.1. Pristine TG flake and Li/TG systems
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We have selected the planer TG fake model similar to the planer TG nanosheets studied in Refs. [50-52]. The optimized structure of the TG flake is represented in Fig. 1. Structurally, there are two kinds of C-C bonds in TG flake: the bonds in square rings and bonds in connecting square
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rings. The bond lengths of the optimized TG flake are also represented in Fig. 1(a).
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Fig. 1. Optimized structure (a), DOS plot (b), and HOMO and LUMO distributions (c) of the pristine TG flake. The dashed line in DOS plot shows the TG’s Fermi energy. To avoid boundary effects, atoms at the open ends of the TG flake were saturated with hydrogen atoms. In this respect, the C-C bonds in the edges of TG flake are elongated compared to the bond lengths in the center of flake (1.47 Å and 1.39 Å). In Fig. 1 (b), we have presented the DOS
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of the pristine TG flake. From the DOS plot, it is obvious that the pristine TG flake has metallic character. In fact, the pristine TG flake with HOMO and LUMO energies at -4.06 eV and -3.84 eV, respectively, and HOMO-LUMO energy gap (Eg) of 0.22 eV exhibits metallic character.
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This is in agreement with the previous studies on the TG flake [50-52]. Structurally, the TG flake
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model has two types of carbon rings: the small square ring (R1) consisting of 4 C atoms and the large eight-member ring (R2) (Fig. 1 (a)). Our frontier molecular orbital (FMO) analysis shows (Fig. 1 (c)) that the HOMO and LUMO orbitals are more distributed on the square rings of the pristine TG flake. Here, we examined all the Li decoration sites on the TG flake. Our DFT calculations show that the stable configuration in which the Li atom is located on top of R1 hollow site (Fig. 2).
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Fig. 2. Optimized configuration and DOS plot of the Li/TG.
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Fig. 2, shows that the Li atom locates above the TG flake at a distance of 1.76 Å. The calculated Ebin of the optimized structure of Li/TG with Li atom decorated on the square ring is about 45.53 kcal/mol. On the other side, the Ebin of Li decorated on the eight-membered rings of TG is 37.01 kcal/mol. Ideally, the binding energies for the catalytic activity of catalytic activity of highperformance adsorbents should be in a range of 45–50 kcal mol−1 [53, 54]. Accordingly, the Li/TG with
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Li atom decorated on the square ring shows more stability compared to the Li/TG with Li atom decorated on the eight-member ring. The calculated C-C bond length of the square ring in TG flake, around
the Li atom (1.49 Å), is larger than that in pristine TG flake (1.46 Å). In order to reveal the
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correlation between the electronic property and the nature of binding in Li/TG, we calculate the
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partial density of state (PDOS) of Li/ TG, as shown in Fig. 2. As shown in this figure, in the high energy regions of the DOS of Li/TG, the DOS generates from the contributions of Li and TG orbitals. On the other hand, in the low energy regions, the DOS mostly generates from the TG orbitals. In fact, the DOS of Li/TG (Fig. 2) closely matches that of pristine TG flake (Fig. 1), except for the contribution from Li atom in high energy regions. We found that the HOMO and LUMO levels of Li/TG shift towards the higher and lower energies, respectively, as compared to those of pristine TG flake. Accordingly, the Eg of the Li/TG exhibits a lower value (0.14 eV) as 7
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compared to the pristine TG flake (0.22 eV). As shown in Fig. 1, the Fermi level of the pristine TG flake located at -3.95 eV. After the Li decorating, the Fermi level approaches to the LUMO region (-3.88 eV). This shift to lower energies (Fig. 2) leads to n-type semiconducting behavior
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in the Li/TG system [55-58]. The natural charge on the Li atom represent a large charge transfer (0.925e) from the Li atom to the TG flake; thus there is a strong electrostatic interaction between the Li and the TG.
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3.2. Hydrogen adsorption on Li/TG
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We systematically investigate the H2 adsorption on the Li/TG to explore its reactivity evolution toward the hydrogen molecule. Moreover, we examined both end-on (H2 axis aligns perpendicular to the Li/TG) and site-on (H2 axis aligns parallel to the Li/TG) configurations, and
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found that the side-on configuration is more stable for the H2-Li/TG complex, as shown in Fig. 3.
Fig. 3. The Top (a) and side (b) views and molecular electrostatic potential (MEP) map of the H2-Li/TG complex. The H2 bond length in the Li/TG system is elongated (0.77 Å) from that in the isolated gas phase (0.75 Å). In fact, enhancement of the H-H bond in the adsorbed H2 molecule exhibits strong interaction between the H2 and Li/TG. We also performed the molecular electrostatic potential (MEP) analysis of the H2-Li/TG complex (Fig. 3). The MEP analysis shows a large overlapping
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between the orbitals of the Li/TG and the H2 molecule. Moreover, the depletion of charge density on the hydrogen indicates the polarization of the H2 on the Li/TG. The Eads value of the H2 adsorbed on the Li/TG shows (-4.06 kcal/mol) the strong interaction between H2 molecule
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and Li/TG (Fig. 3). To reveal how the EFs, affect the H2 adsorption on the Li/TG, the H2-Li/TG complex has been optimized under the influence of different EFs. In this respect, H2-Li/TG complex has been optimized under the influence of EFs with magnitudes ranging from −50 ×
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10−4 a.u. to 50 × 10−4 a.u. All the external EFs have been applied parallel to the charge transfer direction (Li/TG → H2) in both positive EFs (F > 0) and negative EFs (F < 0), with the direction
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of the Li/TG → H2 and Li/TG ← H2, respectively (Fig. 4).
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Fig. 4. Cartesian axis and the direction of the positive and negative EFs (F, in 10−4 a.u.) in the H2-Li/TG.
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Our results show that the H2-Li/TG represents relatively large Eads without any EF (Fig. 3). However, one exciting question emerges: Can the Eads of the H2 on the Li/TG be increased by applying the appropriate EFs? The Li/TG can be used to reversible H2 storage media by applying the different EFs? To answer these questions, the Eads of the H2-Li/TG under the different EFs have been calculated and listed in Table 1. Table 1. The Eads with the EFs for the H2-Li/TG. Electric field (×10-4 a.u.)
Eads Electric field (kcal/mol) (×10-4 a.u.) 9
Eads (kcal/mol)
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-2.17 -2.10 -1.42 -0.99 -0.86
10 20 30 40 50
-5.34 -6.39 -8.47 -10.42 -12.71
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-10 -20 -30 -40 -50
To better represent the results, the correlation between the Eads values of the H2-Li/TG and the
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different EFs is represented in Fig. 5.
Electric Field (×104 a.u.) -50 -40 -30 -20 -10
-4.00 -7.00
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-10.00 -13.00
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Eads (kcal mol-1)
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Fig. 5. Dependence of the applied EFs (in 10−4 a.u.) on the Eads of H2-Li/TG As shown in Table 1 and Fig. 5, the Eads of H2 on the Li/TG gradually increase along with the
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applied EF changing from −50 ×10−4 to 50 × 10−4 a.u., representing that the Eads of H2 on the Li/TG can be controlled by changing the magnitude and direction of an applied EF. Specifically, the field-free Eads of H2-Li/TG (-4.06 kcal/mol) is considerably enhanced to -12.71 kcal/mol, via applying an EF of 50 × 10−4 a.u. along the charge transfer direction, whereas it is decreased (0.86 kcal/mol) under the EF of −50 × 10−4 a.u. against the charge transfer direction. In fact, on Li/TG, the Li atom is polarized in the applied positive EF. The electrostatic interaction between the hydrogen molecule and polarized Li makes the Li atom favored for hydrogen adsorption. The 10
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distance between the hydrogen molecule and the Li is decreased by the polarization interaction induced by the applied positive EF and the polar bond on Li/TG. Concurrently, the H-H bond length increases remarkably with increasing the positive EF. In the absence of an EF, the H-H
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bond length in the H2-Li/TG is 0.77 Å (Fig. 3). The H-H bond length becomes 0.84 Å when the EF reaches 50×10-4 a.u., increased nearly by 10%. On the other hand, the H-H bond length as well as the distance between the H2 molecule and Li atom in the H2-Li/TG, decreases and
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increases, respectively, with increasing the magnitude of negative EF. In fact, when the negative EFs were applied to the Li/TG, the Eads values of hydrogen molecule on the Li/TG (Eads=-0.86
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kcal/mol) are remarkably decreased as compared to the H2 adsorbed on the Li/TG in the absence of an EF (Table 1 and Fig. 5). Therefore, the H2 adsorption/release procedure on the Li/TG are reversible and can be easily tuned by applying the appropriate EFs.
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3.3. The capacity of hydrogen storage on Li/TG
We investigated the average Eads of the hydrogen molecules on the Li/TG by imposing an EF of 50×10−4 a.u., by increasing the number of adsorbed hydrogen molecules (Fig. 6). We found that
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the Li/TG can adsorb up to six hydrogen molecules with the average Eads slightly decreasing from -12.71 kcal/mol (number of H2 on the Li/TG, n=1) to -3.11 kcal/mol (n=6). This reduction
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in Eads is mainly a result of the steric hindrance effect among the adsorbed hydrogen molecules. Our calculations show that a further increase in the number of hydrogen leads to some H2 molecules moving far away from the Li/TG during the geometry optimization and the decrease in the Eads; thus, we define n=6 as a likely saturation for full hydrogen coverage on the Li/TG under the influence of the EF (Fig. 6).
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Fig. 6. The most stable configurations of Li/TG at full hydrogen coverage. The average adsorption energy of hydrogen molecules on the Li/TG at different hydrogen coverages (n=1-6 (a-f)) are also given.
At full hydrogen coverage, the average Eads of H2 molecules on Li/TG (-3.11 kcal/mol per
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molecule) is still much higher than the optimal Eads for the H2 on high-performance adsorbents (4~ -14 kcal/mol) [59, 60]. In addition to the Eads values, the gravimetric density of the H2 adsorbed on the Li/TG flake is explored to estimate the maximum storage capacity of the H2
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molecule. The gravimetric density of H2 is calculated from the equation
where M(H2) is the molecular weight of H2 and M(complex) is the weight of the adsorbed complex system. Experimental studies of hydrogen uptakes on the surface of the pristine graphene sheet are about 0.4 wt% and 0.2 wt%, at 77 K and 1 bar [61, 62]. Our results show that the Li/TG has increased the uptake capacities of the adsorbent efficiently. For nH2-Li/TG systems (n=1-6), hydrogen uptakes are about 0.24 wt% (n=1), 0.48 wt% (n=2), 0.72 wt% (n=3), 0.95 wt% (n=4), 1.19 wt% (n=5) and 1.43 wt% (n=6), respectively. In this respect, the Li/TG 12
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flake shows larger up taken of weight percentage of hydrogen than reported results for pristine graphene.
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4. Conclusion DFT calculations are carried out to study the Li/TG application in hydrogen storage. Our calculations revealed that the Li atom decorated on the hollow site of a square ring is more
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preferable that the 8-member ring of the TG flake. After a single hydrogen molecule adsorption on Li/TG, it is found that the Eads is -4.61 kcal/mol. In order to explore the effects of the external
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EFs on the Eads of H2 on the Li/TG, the positive and negative EFs are applied. It is shown that the H2 and Li atom are polarized under the positive external EFs. It can be seen that the Eads values increase dramatically with the increase in positive EFs intensity. Our results show that the Li/TG under positive EF can adsorb up to six hydrogen molecules. On the contrary, the Eads values of
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hydrogen molecule on the Li/TG remarkably decreased when the negative EFs were applied to the Li/TG, as compared to the H2 adsorbed on the Li/TG in the absence of an EF. Our results propose that the Li/TG is a promising material as a reversible hydrogen adsorption media. The
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direction.
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outcomes of the current study are expected to motivate active research attempts to search in this
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The stability and electronic properties of Li/T-graphene (TG) were studied. The H2 adsorption on the surface of Li/TG has been investigated. H2 adsorption/release processes can be easily controlled by applying electric fields.
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