Electronic Structure Calculations of Hydrogen Storage in Lithium

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Electronic Structure Calculations of Hydrogen Storage in Lithium Decorated Metal-Graphyne Framework Sandeep Kumar, and Thogluva Janardhanan Dhilip Kumar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09893 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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ACS Applied Materials & Interfaces

Electronic Structure Calculations of Hydrogen Storage in Lithium Decorated Metal-Graphyne Framework Sandeep Kumar and Thogluva Janardhanan Dhilip Kumar∗ Department of Chemistry Indian Institute of Technology Ropar Rupnagar 140001, India

ABSTRACT Porous metal-graphyne framework (MGF) made up of graphyne linker decorated with Lithium has been investigated for hydrogen storage. Applying density functional theory spin-polarized generalized gradient approximation with the Perdew-Burke-Ernzerhof functional containing Grimme’s diffusion parameter with double numeric polarization basis set, the structural stability, and physicochemical properties have been analyzed. Each linker binds two Li atoms over the surface of graphyne linker forming MGF-Li8 by Dewar coordination. On saturation with hydrogen, each Li atom physisorbs 3 H2 molecules resulting MGF-Li8 -H24 . H2 and Li interact by charge polarization mechanism leading to elongation in average H-H bond length indicating physisorption. Sorption energy decreases gradually from ≈0.4 eV to 0.20 eV on H2 loading. Molecular dynamics simulations and computed sorption energy range indicate the high reversibility of H2 in the MGF-Li8 framework with the hydrogen storage capacity of 6.4 wt %. The calculated thermodynamic practical hydrogen storage at room temperature makes Li decorated MGF system a promising hydrogen storage material. Keywords: Hydrogen storage; Dewar coordination; Charge polarization; Molecular dynamics; Density functional theory

1.

INTRODUCTION

Dwindling supplies of presently available fossil fuels, and their adverse effects on the environment due to CO2 emissions have raised interest in the development of sustainable, clean fuels such as hydrogen1 for vehicular applications. In the present scenario, there is an urgent need for materials to store sustainable fuels like hydrogen1–3 and to remove greenhouse gases from the atmosphere.4,5 The sustainable hydrogen fuel is considered as a “fuel of future” because it is clean, light, has ample source of energy, and produces only water after its combustion reaction.6–9 The storage of hydrogen in the form of compressed and liquefied hydrogen gas are not commercially viable because these strategies consume a large amount of energy as compared with chemical energy stored in hydrogen.8,10 According to the U.S. department of energy’s (DOE) 2015 storage target the desired material for hydrogen storage requires a gravimetric density of 5.5 wt % at 233-358 K and 3-100 bar for automobile applications.11 A variety of different porous materials has been studied for hydrogen storage experimentally and theoretically.7,12 Nanoporous materials have been investigated for hydro-

∗ Electronic

address: [email protected]

gen storage at ambient temperature and pressure conditions in a nondissociative manner.13–16 Different porous materials are proposed to explore their hydrogen storage properties viz. metal-organic frameworks (MOFs),16–20 porous aromatic frameworks (PAFs),21–24 covalent organic frameworks (COFs),25–29 polyhedral oligomeric silsesquioxane (POSS) frameworks,30,31 zeolites,32–34 metal clusters35–39 and some other structures.40–44 The arduous investigation of hydrogen storage materials from a decade is still needed to explore materials for potential hydrogen storage, which completes all the requirements set by the U.S. DOE.11 Metal-organic frameworks (MOFs) are an exciting, highly tunable crystalline hybrid nanoporous materials constituted by inorganic and organic molecular building blocks with high surface area, that provides the binding sites for guest gas molecules.13–16 The exceptional tunability of MOFs are providing the number of molecular building block combinations,45 and many of these combinations of MOFs are being the most studied hydrogen storage material.46 In designing metal graphyne framework (MGF), graphyne is used as a linker in the metal organic framework (MOF). In MGF, magnesium oxide (Mg4 O) is used as a metal node making Mg4 O-graphyne framework. Graphyne is a 2D carbon allotrope, with sp and sp2 hybrid carbon atoms, first predicted by Baughman, Eckhardt, and Kertesz.47 Graphyne consists of carbon networks of

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benzene ring joined together by acetylenic linkages (-CC≡C-C-) instead of (-C-C=C-C-) in graphene. There are four different types of graphyne identified viz. α, β, γ, and 6,6,12-graphynes.48,49 Several studies have shown that metal decorated γ-graphyne is used for hydrogen storage with high gravimetric density.50–55 Guo et al.51 reported Li-decorated graphyne has maximum hydrogen loading capacity with 18.6 wt % . In present work, the viability of hydrogen storage properties in Li decorated newly designed metal graphyne framework (MGF) is investigated. Graphyne linker is decorated with Li metals. By applying first principle DFT calculations the structural stability of MGF and hydrogen adsorption on Li decorated MGF have been studied. This paper is organized as follows: The details of the computational methods used in this work are given in section 2. In section 3, the results and discussions of hydrogen loading capacity of Li decorated MGF are presented. Finally, summary and conclusions are given in section 4.

using the equation, Eb =

1 [EM GF -Li8 − (8ELi + EM GF )] 8

where, EM GF -Li8 , ELi and EM GF are total energies of MGF-Li8 , single Li metal and MGF, respectively. The average H2 adsorption energy, Ead are determined by using the following equation, Ead =

1 [EM GF -Li8 + nEH2 − EM GF -Li8 -nH2 ] n

(2)

where n being the number of hydrogen molecules, EH2 is the energy of H2 molecule and EM GF -Li8 -nH2 is the total energy of n number of hydrogen molecules adsorbed on MGF-Li8 . To determine the desorption energy of sequentially adsorbed hydrogen molecules, the following equation is used, Ede = EH2 + EM GF -Li8 -(n−1)H2 − EM GF -Li8 -nH2

2.

(1)

(3)

COMPUTATIONAL DETAILS

The calculations are carried out by density functional theory (DFT) as implemented in the DMOL3 package.57,58 The spin polarized generalized gradientcorrected approximation (GGA) and Perdew-BurkeErnzerhof (PBE) exchange-correlation functional with double numeric basis set including p-polarization function (DNP) is used for geometry optimization and physicochemical properties calculation. Dispersioncorrected DFT (DFT-D) is applied for the evaluation of weak van der Waals (vdW) interaction. All the geometries are optimized without imposing any symmetry constraints. To further improve the accuracy in determining weak interactions using the BroydenFletcher-Goldfarb-Shanno (BFGS) algorithm with 10−5 Ha as an energy convergence threshold value, 2 × 10−3 Ha/˚ A for gradient convergence and 5 × 10−3 ˚ A used for maximum displacement. Ab initio methods accurately evaluate weak interaction though typically scale poorly with system size, which makes them computationally expensive for such systems. The local density approximation (LDA) or GGA based DFT scale well with the system size. Among GGAs, PBE is reported to be more successful over other GGAs.59,60 Energy attributes viz. Li binding energy, the H2 adsorption and desorption energies, frontier orbital energy gaps, and structural attributes viz. variation in bond lengths, electrostatic potential maps, Hirshfeld charges, and thermodynamics of practical hydrogen storage are computed and analyzed. The Li binding energy in MGF-Li8 is calculated by

in which EM GF -Li8 -(n−1)H2 is the total energy of preceding H2 adsorbed on MGF-Li8 -nH2 . Molecular dynamics (MD) simulations are performed to examine the stability and reversibility of the adsorbed hydrogen in Li decorated MGF. MD simulations of fully hydrogen trapped Li decorated MGF are carried out based on Born-Oppenheimer Molecular Dynamics61 (BOMD). Γ-point sampling is used with a 5 ps run time and 1 fs time-step. MD simulations are performed in the canonical NVT ensemble using Nos´e thermostat with different simulation temperature ranges from 150 to 373 K. The thermodynamics of practical hydrogen storage capacity using chemical potential, µ, has been estimated at the low temperature-high pressure adsorbing condition and high temperature-low pressure desorbing conditions.

3.

RESULTS AND DISCUSSION

The geometry of graphyne linker is optimized and then computed nucleus independent chemical shifts (NICS)62,63 values to determine the most preferential site for Li decoration based on aromaticity of graphyne linker. Geometry optimization and NICS values of graphyne is calculated by DFT with Becke’s three parameter LeeYoung-Parr functional (B3LYP) and Pople’s 6-311G(d,p) basis set with Gaussian-09 program.64 The calculated NICS value is found to be -16.40 ppm in the hollow center of linker. The negative NICS value shows the aromaticity of graphyne linker which is preferred site for Li decoration.


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TABLE 1: The average bond distance between Li and center of the graphyne linker of MGF (Li-Gc ), Li and physisorbed hydrogen distance (Li-Hp ) and physisorbed hydrogen distance (Hp -Hp ). All the bond distance values are measured in angstrom (˚ A). System Li-Gc Li-Hp Hp -Hp MGF-Li8 0.953 MGF-Li8 -8H2 1.105 2.035 0.756 MGF-Li8 -16H2 1.111 2.048 0.754 MGF-Li8 -24H2 1.187 2.088 0.753

FIG. 1: Optimized structure of metal-graphyne framework (MGF) system. Atom colors: Mg-green; C-dark grey; Hwhite; O-red

3.1.

Structural properties of hydrogen saturation in MGF-Li8

observed that on complete saturation each Li adsorbed three H2 molecules in MGF-Li8 making MGF-Li8 -24H2 complex. The H2 adsorption pathway of MGF-Li8 is shown in Figures 2(b-d). On introduction of the first H2 molecule on each Li in MGF-Li8 the distance between center of graphyne linker (Gc ) and Li is increased from 0.953 to 1.105 ˚ A and the distance between Li and adsorbed molecular hydrogen is found to be 2.035 ˚ A with elongated H-H distance of 0.756 ˚ A compared to free H2 distance from 0.741 ˚ A. Introduction of the second and the third H2 molecule to the system the distance between Gc and Li is further increased to 1.111 and 1.187 ˚ A, respectively, and the distance between Li and H2 molecules (Li-Hp ) is increased marginally to ≈2.048 ˚ A.

Using the first-principles calculations, Mg4 OGraphyne-framework, MGF, is optimized and the resulting structure is shown in the Figure 1. The stability of MGF has been determined by calculating the vibrational frequencies of optimized MGF and found that the structure is a stationary point with real energy minimum and without any imaginary frequency in the vibrational spectra. The calculated frequencies and optimized coordinates are provided in supporting information. The distance between the two metal nodes diagonally is found to be 24.455 ˚ A while 17.333 ˚ A for adjacent metal nodes. To the optimized framework, lithium is decorated on the graphyne ring of the MGF and the geometry is optimized. The optimized structure decorated with eight Li atoms, MGF-Li8 , is shown in the Figure 2(a). Li metal is used in its ground state configuration and total eight Li atoms bind with MGF. Each graphyne linker binds with two Li atoms through Dewar coordination56 in which π electrons of graphyne linker coordinated with Li, with average distance between Li-graphyne π complex (Li-Gc ) is 0.953 ˚ A. Li decoration on the graphyne results in expansion of the framework with the distance between two metal nodes diagonally increased to 24.539 ˚ A while adjacent nodes increased to 17.374 ˚ A.

It can be inferred from the Figures 2(b-d) sequential H2 introduction to the Li-decorated graphyne framework the distance between two diagonal metal nodes decreases from 24.519 ˚ A to 24.502 ˚ A and then to ˚ 24.455 A from H2 free framework distance of 24.539 ˚ A (Figure 2(a)). Similarly, the distance between two adjacent metal nodes decreases from 17.367 ˚ A to 17.350 ˚ A and then to 17.315 ˚ A from initial H2 free framework distance of 17.374 ˚ A. This clearly indicates that the charge transfer is taking place from graphyne to Li and then to H2 resulting in the compression of the framework.

In the optimized MGF-Li8 structure, H2 molecules are introduced sequentially on each Li and the structure is optimized. After H2 adsorption in MGF-Li8 the geometrical changes evaluated in MGF-Li8 -nH2 system. It is

In MGF-Li8 -24H2 the gravimetric hydrogen wt % is found to be 4.5 which can further be increased by introducing H2 molecules in the pore space of MGF. The measured pore size of MGF is ≈24.5 ˚ A which remains con-

The average distance between H−H of adsorbed H2 molecules is weakened and close to ≈0.754 ˚ A after optimization compared to bare H2 molecule’s distance of 0.741 ˚ A, which reveals that all the hydrogen molecules are physisorbed in MGF-Li8 -nH2 . The physisorption of H2 explained by charge polarization mechanism65,66 proposed by Niu-Rao-Jena. The Li metal polarizes the H2 molecules and binds with H2 molecules in molecular form resulting in physisorption. The average bond distance between Li and center of the graphyne linker (Li-Gc ), Li and physisorbed hydrogen distance (Li-Hp ) and physisorbed hydrogen distance (Hp -Hp ) in ˚ A are provided in table 1.



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FIG. 2: Hydrogen adsorption pathway of optimized structures of (a) MGF-Li8 (b) MGF-Li8 -8H2 (c) MGF-Li8 -16H2 and (d) MGF-Li8 -24H2 systems. Atom colors: Mg- green; C-dark grey; H-white; O-red; Li-purple.

stant up to full saturation of hydrogen. The large pore size of MGF provides a host site for introducing more number of H2 molecules. After geometry optimization, it is seen that total 12 H2 molecules can be accommodated in its pore space making MGF-Li8 -36H2 with 6.4 hydrogen wt %. The average H-H distance of free hydrogen in pore space of MGF is elongated to 0.750 ˚ A. The optimized geometry of MGF-Li8 -36H2 is given in Figure 3. 3.2.

Energetic parameters

The calculated binding energy per Li atom of MGFLi8 computed using equation (1) is found to be 2.90 eV, indicating that Li atom binds strongly with π electrons of graphyne linker of MGF. The HOMO-LUMO energy gap (Eg ) is calculated to examine the kinetic stability of MGF-Li8 and their hydrogen trapped analogues. The HOMO-LUMO energy gap, (Eg ) is found to be 0.64, 0.66, 0.69, 0.67 and 0.67 eV for MGF, MGF-Li8 , MGF-Li8 -8H2 , MGF-Li8 -16H2 and MGF-Li8 -24H2 ,

respectively. The Eg increases on Li decoration of MGF and the introduction of a first hydrogen molecule to the MGF-Li8 shows the stability increases. On addition of the second and the third H2 molecule to the system, the energy gap shows the high stability of MGF-Li8 -24H2 . To understand the interaction of adsorbed H2 molecules to the Li decorated MGF, the average H2 adsorption energy, Ead is calculated by using equation (2). The sequential desorption energy, Ede , is calculated by using equation (3), to examine the reversibility of H2 molecules adsorbed on MGF-Li8 . The calculated Ead and Ede is shown in Figure 4. The adsorption energy for first hydrogen molecule adsorbed on MGF-Li8 is found to be 0.36 eV. On further addition of the second and the third H2 molecules in each Li resulting in MGF-Li8 16H2 and MGF-Li8 -24H2 in the system the adsorption energy decreases and is found to be 0.30 and 0.27 eV, respectively. Low adsorption energy values indicate that the H2 molecules are physisorbed (weakly bonded) to the MGF-Li8 -nH2 (where n = 8, 16 and 24). The adsorption energy for remaining 12 H2 molecules in MGF-Li8 -nH2


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cate that physisorbed hydrogen molecules can be easily desorbed from the system, which makes MGF-Li8 -nH2 (where n = 8, 16 and 24) is highly reversible hydrogen storage material.

3.3.

FIG. 3: Optimized structure of MGF-Li8 -36H2 with hydrogen loading capacity 6.4 wt%. Atom colors: Mg- green; C-dark grey; H-white; O-red; Li-purple.

FIG. 4: Average hydrogen adsorption and sequential desorption energy of MGF-Li8 -nH2 (where n = 8, 16 and 24).

(where n = 25 - 36 H2 ) are also calculated and found to be 0.25 eV for the 25th H2 and 0.22 eV for the 36th H2 molecule. The adsorption energy decreases sequentially on the introduction of H2 molecules in pore space of MGF indicating that the H2 molecules are adsorbed on the MGF-Li8 -nH2 (n = 25 - 36 H2 ). The calculated values are provided in supporting information. The calculated desorption energy shows similar behavior as average adsorption energy. The desorption energy values for first, second and third H2 molecules are 0.36, 0.25 and 0.20 eV, respectively. The low desorption energy values indi-

Electrostatic potentials and Hirshfeld charge analysis of MGF-Li8 -nH2

Electrostatic potential (ESP) maps of MGF-Li8 framework and hydrogen trapped MGF-Li8 -nH2 (n = 8, 16, 24) have been obtained and top and bottom views of charge evolution of ESP maps are shown in Figure 5. In ESP plots, the blue and red colors indicate aggregation and depletion of electron charges. In the top view of ESP map of MGF-Li8 shows that Li has electron deficient (red) as compared to the metal node (blue-green). On introduction of the first H2 molecule to the MGFLi8 , the color of Li changes red to yellowish red indicating that the charge transfer takes place from Li to H2 . Similarly, on the introduction of second and third H2 molecules on MGF-Li8 -nH2 , the color of Li changes from yellow to yellowish-green indicating further charge transfer from Li to H2 . From the bottom view, it can be noticed that there is a change of graphyne linker color from dark blue to light blue when a number of H2 molecules adsorbed on Li metal, indicating charge transfer from linker to Li metal. Li atom, in turn, polarizes and binds the H2 molecules weakly resulting in molecular bonding as explained by charge polarization mechanism.65,66 The charge transfer mechanism further quantified by Hirshfeld charge analysis. Population analysis have been performed by computing the Hirshfeld charges to understand the H2 binding with the Li decorated on the metal-graphyne framework. The average Hirshfeld charges for C atoms of graphyne linker, Li atoms before and after H2 adsorption are calculated and is shown in Figure 6. The Hirshfeld charges for carbon of graphyne linker is found to be -22 millielectron units (meu) in MGF which increases on H2 loading in MGF-Li8 up to -15 meu shows that the electronic charge transfer takes place from graphyne linker to Li. The Hirshfeld charges for Li metal calculated before the H2 adsorption is found to be 320 meu which decreases after sequential addition of H2 attaining 220 meu due to charge transfer from graphyne to Li. The Hirshfeld charges for sequentially trapped H2 molecules are calculated and found to be positive near zero and decreases negatively as more H2 are adsorbed to Li metal due to charge polarization of Li.

3.4.

Molecular dynamics simulations of Hydrogen saturated MGF-Li8 system

In order to explore the reversibility of the hydrogen in Li decorated MGF, the MD simulations of MGF-Li8 -



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FIG. 5: Top and bottom views of electrostatic potential map of (a) MGF-Li8 (b) MGF-Li8 -8H2 (c) MGF-Li8 -16H2 and (d) MGF-Li8 -24H2 systems. The units are in e/˚ A3 .

evolution up to 2 ps, two H2 molecules remains with each Li while for prolonged evolution, another 8 H2 molecules get desorbed from MGF-Li8 -24H2 system forming MGFLi8 -8H2 . MD simulations at 373 K shows that all the H2 molecules are desorbed from MGF-Li8 -24H2 , which makes MGF-Li8 system suitable for hydrogen storage. It is observed that Mg4 O-graphyne framework (MGF) is stable and Li stays near to the graphyne linker over the entire range of temperature even up to 373 K. It is also observed that no cluster formation of Li at higher temperature of 373 K. MD simulations clearly shows that all the H2 molecules are reversibly adsorbed making the MGFLi8 system a promising candidate for hydrogen storage material.

FIG. 6: Hirshfeld charges of Li, C atom of graphyne linker and physisorbed hydrogen (Hp ) of MGF-Li8 -nH2 system (where n = 8, 16 and 24).

24H2 have been performed over a temperature range of 150, 200, 300 and 373 K using BOMD. The snap shots of MD simulations are given in Figure 7. The recommended hydrogen desorption temperature for potential hydrogen storage material should be ≈ 358 K. It is already confirmed that all the hydrogen molecules are physisorbed in MGF-Li8 -24H2 system. It is found that all the H2 molecules hold up to 150 K. At elevated temperature 200 K, the Li-H2 binding strength becomes weaker and 8 H2 molecules are desorbed from MGF-Li8 -24H2 , one hydrogen molecule from each Li. Since the first H2 molecule comes out at 200 K makes MGF-Li8 system suitable for hydrogen storage below room temperature. At 300 K, for

3.5.

Thermodynamically usable hydrogen capacity

A quantitative picture of H2 adsorption at different temperature (T ) and pressure (P ) using the thermodynamic properties of the system to know usable H2 capacity have been estimated. The H2 occupation number binding at each Li atom at equilibrium can be obtained using the equation: PNmax

ngn exp[n(µ − Ead )/kB T ] f = Pn=0 Nmax n=0 gn exp[n(µ − Ead )/kB T ]

(4)

where Nmax is the maximum binding number of H2 molecules to each Li atom, n is the number of H2 molecules adsorbed, gn is the configurational degeneracy for a given n, kB is the Boltzmann constant, Ead is the adsorption energy of H2 to Li-graphyne and µ is the H2 gas phase chemical potential at given T and P obtained


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FIG. 7: Snap-shots of MD simulations of MGF-Li8 -24H2 at the temperatures of (a) 150 K, (b) 200 K, (c) 300 K, and (d) 373 K.

using the equation:

4.

CONCLUSIONS

µ = µideal − 0.00015(T − 186.5) + 0.00065[(log10 P − 0.5)2 − 0.25] (5) The phonon contribution to the entropy is ignored in this estimation and further only one H2 configuration is considered. The H2 gas chemical potential µ as a function of T and P is taken from the experimental values.67,68 The µideal is -0.21 eV for 30 atm and 298 K resulting in µ of -0.22 eV while µideal is -0.36 eV for 3 atm and 373 K with µ of -0.38 eV. The occupation number, f , is calculated to be 2.10 at 30 atm and 298 K, and f is 0.36 which is close to zero at 3 atm and 383 K. Here 30 atm and 298 K is provisionally chosen as the adsorbing condition and 3 atm and 373 K as the desorbing condition. Thermodynamically usable occupation number of H2 to Li-graphyne at adsorption condition is calculated to be 2 H2 per Li atom, corresponding to a practical usable capacity of 3.5 wt % which will increase to 5.5 wt % considering additional H2 loading in pore-volume of the framework.

In this study, the porous MGF is built with graphyne linker decorated with Li. By applying first-principles electronic structure calculations and MD simulations using GGA-PBE functional with DNP basis the hydrogen sorption efficiency of Li-decorated MGF framework and their stability has been studied. Li metal atoms bind with MGF making MGF-Li8 . Each Li atom in MGF-Li8 is found to hold maximum three H2 molecules by physisorption resulting in MGF-Li8 -24H2 system. The physisorbed H2 molecules and Li bonded through charge polarization mechanism with increased average H-H bond distance to ≈0.754 ˚ A. The calculated average adsorption and desorption energies for adsorbed H2 molecules are found to be low and decreases with increasing number of hydrogen molecules revealing the high reversibility of hydrogen from Li decorated MGF. The ESP plots and Hirshfeld charge analysis elucidate the charge transfer mechanism of physisorbed hydrogen molecules.



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The BOMD simulations performed in canonical NVT ensemble using N´ose thermostat signifies the stability of Li decorated system and reversibility of H2 molecules from MGF-Li8 -24H2 . The MD simulations show that the H2 molecules are trapped to the Li-decorated MGF at low temperature and at a higher temperature of 373 K all the H2 molecules are released. The MGF-Li8 skeletal system remains stable via 5 ps BOMD simulations at 373 K while Li clustering is not observed. The hydrogen wt % for MGF-Li8 -36H2 system have the gravimetric density of 6.4 making it a potential storage material with respect to higher wt %, stability, and reversibility. This work can be extended to study the effect of coadsorption of O2 and other competitive gas molecules in influencing the hydrogen storage in Li decorated MGF material.69

[5]

[6] [7]

[8] [9]

ACKNOWLEDGEMENTS [10]

This work is financially supported by the Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR No. 01(2782)14/EMR-II). SK thanks SERB, India for Junior Research Fellowship. Interdisciplinary research project for Hydrogen Storage under Renewable and Clean Energy from IIT Ropar is grate fully acknowledged. The authors thank IIT Ropar for high performance computing cluster facility.

[12]

Supporting Information Available

[13]

[11]

The optimized geometry of graphyne linker, Li decorated and hydrogen loaded graphyne linker structures are shown in Figures S1-S5. Vibrational frequencies are provided in Table S1. Adsorption energy of MGF-Li8 -nH2 (n = 25 - 36) is provided in Table S2. The internal coordinates of all optimized geometries of graphyne linker, the Li decorated and hydrogen loaded analogous and the MGF, MGF-Li8 , MGF-Li8 -nH2 (n = 8, 16, 24) are provided in Tables S3-S13 as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Table 1

The average bond distance between Li and center of the graphyne linker of MGF (LiGc ), Li and physisorbed hydrogen distance (Li-Hp ) and physisorbed hydrogen distance (Hp -Hp ). All the bond distance values are measured in angstrom (˚ A).

Figure 1.

Optimized structure of MGF system. Atom colors: Mg-green; C-dark grey; Hwhite; O-red.

Figure 2.

Hydrogen adsorption pathway of optimized structures of (a) MGF-Li8 (b) MGF-Li8 8H2 (c) MGF-Li8 -16H2 and (d) MGF-Li8 24H2 systems. Atom colors: Mg- green; Cdark grey; H-white; O-red; Li-purple.

Figure 3.

Optimized structure of MGF-Li8 -36H2 with hydrogen loading capacity 6.4 wt%. Atom colors: Mg- green; C-dark grey; Hwhite; O-red; Li-purple.

Figure 4.

Average hydrogen adsorption and sequential desorption energy of MGF-Li8 -nH2 (where n = 8, 16 and 24)

Figure 5.

Top and bottom views of electrostatic potential map of MGF, MGF-Li8 and MGFLi8 -nH2 systems (n = 8, 16 and 24). The units are in e/˚ A3 .

Figure 6.

Hirshfeld charges of Li, C atom of graphyne linker and physisorbed hydrogen (Hp ) of MGF-Li8 -nH2 system (where n = 8, 16 and 24).

Figure 7.

Snap-shots of MD simulations of MGF-Li8 24H2 at the temperatures of (a) 150 K, (b) 200 K, (c) 300 K, and (d) 373 K.


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49x44mm (300 x 300 DPI)


Electronic Structure Calculations of Hydrogen Storage in Lithium

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