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Fundamental Study of Reversible Hydrogen Storage in Ti and Li Functionalized Calix[4]arene Sandeep Kumar, and Thogluva Janardhanan Dhilip Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12306 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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The Journal of Physical Chemistry

Fundamental Study of Reversible Hydrogen Storage in Ti and Li Functionalized Calix[4]arene Sandeep Kumar and T. J. Dhilip Kumar∗ Department of Chemistry Indian Institute of Technology Ropar Rupnagar 140001, India

(Dated: April 3, 2017)

Abstract Hydrogen is the most promising candidate for a sustainable energy source in the transport sector. However, the storage of hydrogen is a major problem. Calix[4]arene (CX) is functionalized with Ti and Li metals on the delocalized π electrons of benzene rings and the metal functionalized system is studied for hydrogen storage efficiency by applying density functional theory using the M06 hybrid functional and 6-311G(d,p) basis set. The calculated binding energy indicates Ti coordinates with CX strongly while Li coordinates weakly and the binding of CX and metal is through Dewar mechanism. On saturation with hydrogen, each Ti atom traps 4 H2 molecules while each Li atom traps 3 H2 molecules on CX. Hydrogen molecules are adsorbed on the metal atoms by KubasNiu-Rao-Jena interaction. The global reactivity index obtained for the system obeys maximum hardness and minimum electrophilicity principle. Molecular dynamics simulations are performed using spin-polarized generalized gradient approximation with Perdew-Burke-Ernzerhof functional including Grimme diffusion parameter on H2 saturated systems. The dissociation of H2 molecules in Ti functionalized CX system begins from 273 K, while all the H2 molecules are desorbed by 473 K. The storage capacity is found to be 8.7 wt. % for Ti and 10.1 wt. % for Li functionalized CX. When the Ti atom is intercalated between the two CX moieties, the storage capacity does not reduce significantly. This study reveals that the Ti functionalized CX is a potential reversible hydrogen storage material.

∗

Electronic address: [email protected]

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INTRODUCTION

Due to the rapid growth of world population and increased living standards, there is an increase in energy demand. Presently, the world meets its energy needs mainly using carbon-based fossil fuels such as natural gas, coal and oil. The transport sector mainly depends on petroleum-based fossil fuels. These fossil fuels are decreasing and have adverse effects on the environment due to increasing CO2 content in the atmosphere which results in global warming. Thus carbon-based fossil fuels need to be replaced by alternative clean energy sources. The most plentiful and simplest element of the universe is hydrogen, can be considered as a possible solution1 for the sustainable energy because it is a clean, renewable, light, non-polluting and abundant source of energy2 with high calorific value compared to other sources of energy.3 The main problem in making hydrogen technology commercially available is the efficient production of hydrogen and the storage. Nowadays hydrogen is stored in high pressure tanks in the form of compressed gas. It, however, requires a large volume and the tank should be made from composite material, which is expensive.4 The drawback of storing hydrogen in liquid form is that it requires very low temperature to remain in liquid form under pressure which can be dangerous. The use of solid materials is the best alternative for the storage of hydrogen.5

The ideal solid hydrogen storage system should exhibit high hydrogen reversibility and stability. Moreover, the hydrogen interaction with the material should be intermediate between physisorption and chemisorption6 and should fulfill the ultimate storage target of 7.5 wt. % by 2020 set by the U.S. Department of Energy (DOE).7 Various materials from last two decades have already been studied for hydrogen storage to reveal their hydrogen storage properties, like metal-organic frameworks (MOF),8–17 fullerenes,18–21 metal hydrides,22–29 clathrates,30–33 isoreticular metal-organic frameworks (IRMOFs),34 covalent-organic frameworks,35–39 zeolites,40 alanates,41 metal clusters42–45 and so on. But none of those materials meets all the targets set by the U.S. DOE at ambient conditions. To accomplish the target, metal functionalized Calix[4]arene has been proposed as the potential storage material. Macrocyclic compound CX is constructed by linking benzene residues connected via methylene groups at ortho positions. Such packing mode results in basket shape forming a relatively large cavity. The cavity inside and outside possesses

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electron rich character defined by aromatic rings sufficient for binding guest metal atoms.

In the present study, for the first time, Calix[4]arene (CX) is functionalized from outside the ring with Ti and Li metals as the host site for the hydrogen storage. The CX system has delocalized π-electrons due to the presence of resonating electron cloud above and below the aromatic ring. The metal atoms bind with π-electrons of benzene ring from outside the CX without clustering. On the metal functionalized CX system, hydrogen molecules are saturated on the metal atoms and their structural, energetic and dynamical properties have been obtained. By applying density functional theory (DFT) calculations, the structural stability of CX and hydrogen trapping efficiency of metal functionalized CX have been studied. Energy attributes such as metal binding energy, the H2 adsorption and desorption energies, frontier orbital energy gaps, and conceptual density functional parameters of hardness and philicity have been computed. The structural attributes, namely, variation in bond lengths, Hirshfeld charges and electrostatic potential maps are obtained and analyzed. 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 metal functionalized CX are presented. Finally, summary and conclusions are given in section 4.

2.

COMPUTATIONAL DETAILS

The molecular geometries of CX, Ti and Li metal functionalized derivatives and their hydrogen trapped analogs are optimized by using Gaussian 09 suite of program package46 with the first principles DFT. In this study, hybrid meta exchange-correlation M06 functional and 6-311G(d,p) basis set has been used to optimize all the geometries. M06 functional is parameterized for metals and non-metals and is best suited for studying non-covalent interactions.47 All the metal atoms are used in their ground state configurations.

The binding energy of metal functionalized CX is calculated by using the following equation, Eb =

1 [ECX -M4 − (4EM + ECX )] 4

(1)

where ECX -M4 is the total energy of a CX-M4 system, ECX is the energy of CX and EM is 3



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the energy of isolated metals.

The stability and reactivity of the metal functionalized CX and their H2 trapped analogs are studied using conceptual DFT48 by calculating global reactivity descriptors, such as hardness,49 electrophilicity50 and electronegativity51

Electrophilicity, ω, is defined as ω=

χ2 2η

(2)

where χ is electronegativity and η is hardness. By using Koopman’s theorem52 vertical ionization potential (I) and electron affinity (A) are calculated, where η is expressed as η =I −A

(3)

I +A 2

(4)

and χ is expressed as χ=

The average H2 adsorption energy, Ead , of metal functionalized CX with the H2 trapped analogs are calculated by using the following equation Ead =

1 [ECX -M4 + nEH2 − ECX -M4 -nH2 ] n

(5)

where ECX -M4 is the total energy of CX-M4 , EH2 is the energy of H2 molecule and ECX -M4 -nH2 is the total energy of a maximum number of hydrogen adsorbed on CX functionalized by metals where n being number of hydrogen molecules.

Desorption energy of sequentially trapped hydrogen molecule is calculated by using the equation, Ede = EH2 + ECX -M4 -(n−1)H2 − ECX -M4 -nH2

(6)

where ECX -M4 -(n−1)H2 is the total energy of preceding hydrogen adsorbed on CX-M4 -nH2 .

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The kinetic stability of metal functionalized CX is obtained by calculating energy gap (Eg ) between highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Molecular dynamics (MD) of hydrogen saturated metal functionalized CX are performed based on Born-Oppenheimer Molecular Dynamics53 (BOMD) DFT method within the spin-polarized generalized gradient approximation (GGA) method of Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional set using the DMOL3 package.54,55 MD simulations are performed with double numeric polarization basis set in the canonical NVT ensemble using N´ose thermostat. MD simulations are done at various temperature range by using Γ-point sampling with a 5 ps run time and 1 fs time-step.

3.

RESULTS AND DISCUSSION

The optimized structure of CX is shown in Figure 1. The nucleus independent chemical shifts (NICS(1)) index56,57 are computed to study the aromaticity of CX and the NICS(1) index is found to be -10.29 ppm. The negative NICS(1) index indicates the aromatic nature

FIG. 1: Optimized structure of CX system. Atom colors: C-dark grey; H-white; O-red

of CX. Aromatic CX is functionalized with Ti and Li metals. Interaction of Ti and Li metal atoms on the CX can take place at all four benzene rings of CX from outside and inside the cavity. When metals are introduced from inside the cavity, it results in metal clustering due to steric hindrance. Therefore, metals are introduced outside the cavity and their hydrogen sorption properties are obtained. The frontier HOMO and LUMO orbitals plot of CX are shown in Figure S1 of supporting information. 5



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3.1. 3.1.1.

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Hydrogen adsorption on Ti functionalized CX (CX-Ti4 system) Structural changes in CX-Ti4 before and after hydrogen trapping

The optimized geometry of CX is functionalized with transition metal Ti. Ti atoms are bonded with π-electron cloud of benzene rings of CX from outside the ring by Dewar coordination.58 The average distance between Ti metal and center of the benzene ring (Rc ) is found to be 1.694 ˚ A. Binding energy for the CX-Ti4 system is calculated and is found to be 2.73 eV. High binding energy value indicates a strong interaction between the transition metal, Ti and benzene ring. Molecular hydrogen is introduced sequentially on each metal in CX-Ti4 and found that each Ti atom adsorbs maximum of 4 hydrogen molecules on complete saturation resulting in CX-Ti4 -16H2 . The optimized geometries of Ti functionalized CX (CX-Ti4 ) with the sequential H2 trapped analogs in cyclic form are shown in Figure 2.

In CX-Ti4 -16H2 it is observed that first hydrogen molecule is chemisorbed where the H2 molecule splits into 2 H atoms and the remaining three hydrogen molecules are physisorbed on the Ti metal in the system. The Ti metal and hydrogen form Kubas63–66 type bonding in CX-Ti4 system with the first H2 molecule, where H2 molecule donates electrons to the empty d-orbitals of Ti atom, which reverse transfer electrons to the anti-bonding orbital of the H2 molecules resulting in chemisorption. Both the H atoms are attached to the Ti having Ti-H bond length 1.743 ˚ A. The introduction of second, third and fourth H2 molecule results in physisorption on CX-Ti4 . Physisorption can be explained by charge polarization mechanism59,60 proposed by Niu et al., where the electronic charge on the metal atom, generated due to the charge transfer from CX, polarizes the H2 molecule which results in the near molecular bonding of H2 . The physisorbed H2 bond distance is found to be ≈0.80 ˚ A as compared to bare H2 molecule distance (0.741 ˚ A), indicating the adsorption is physisorption. Since both the mechanisms are operating in adsorbing hydrogen, it can be called as Kubas-Niu-Rao-Jena interaction. The various structural parameters are provided in Table 1.

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FIG. 2: Optimized structures of (a) CX-Ti4 (b) CX-Ti4 -4H2 (c) CX-Ti4 -8H2 (d) CX-Ti4 -12H2 and (e) CX-Ti4 -16H2 systems indicating adsorption and desorption cycle. Atom colors: C-dark grey; H-white; O-red; Ti-grey.

3.1.2.

Energetic and conceptual DFT parameters

To investigate the stability of the CX-Ti4 system, reactivity parameters like hardness, electrophilicity and electronegativity have been calculated. The hardness (η) increases while electrophilicity (ω) decreases as the number of H2 molecules increases in the CX-Ti4 system, which shows the stability of the system increases according to the maximum hardness61 and minimum electrophilicity62 principle. The hardness is maximum for CX-Ti-16H2 (5.45 eV) and minimum for CX-Ti4 (0.87 eV). However, the electrophilicity of CX-Ti4 system shows decreasing trend with trapping of H2 molecules. CX-Ti4 has maximum electrophilicity (3.85 eV) while CX-Ti4 -16H2 has minimum electrophilicity (0.75 eV). Electronegativity (χ) of the CX-Ti4 system generally decreases on the introduction of H2 molecules. Electronegativity

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TABLE 1:

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The bond distance between metal and center of the CX ring (M-Rc ), metal

and chemisorbed hydrogen distance (M-Hc ), metal and physisorbed hydrogen distance (M-Hp ), chemisorbed hydrogen distance (Hc -Hc ) and physisorbed hydrogen distance (Hp -Hp ). All the bond distances are average values measured in ˚ A. System

M-Rc M-Hc M-Hp Hc -Hc Hp -Hp

CX-Ti4

1.694

-

CX-Ti4 -4H2 1.667 1.743

-

-

-

-

2.851

-

CX-Ti4 -8H2 1.741 1.738 1.918 2.763 0.801 CX-Ti4 -12H2 1.806 1.743 1.916 3.060 0.801 CX-Ti4 -16H2 1.824 1.741 1.942 3.226 0.799

TABLE 2: Calculated hardness (η), electrophilicity (ω), electronegativity (χ) and HOMO-LUMO energy gap (E g ) of all hydrogen trapped CX-Ti4 systems. The hydrogen wt % is also provided. Parameters are in eV units. System CX-Ti4

η

ω

χ

Eg wt%

0.87 3.85 2.58 1.57

-

CX-Ti4 -4H2 4.59 1.78 4.08 2.58 5.16 CX-Ti4 -8H2 4.79 1.37 3.62 2.94 6.37 CX-Ti4 -12H2 4.90 1.21 3.44 2.99 7.55 CX-Ti4 -16H2 5.45 0.75 2.86 3.46 8.70

of Ti functionalized CX is 2.58 eV which increases on trapping of the first H2 molecule in the CX-Ti4 system and is found to be 4.08 eV. The introduction of succeeding three hydrogen molecules to the system lead to a gradual decrease in electronegativity and is found to be 3.62, 3.44 and 2.86 eV for second, third and fourth H2 molecule, respectively. Further to investigate the kinetic stability of the CX-Ti4 system, the HOMO-LUMO energy gap (Eg ) has been calculated. The Eg increases gradually on the addition of hydrogen molecule in the CX-Ti4 system. In the CX-Ti4 system, Eg is found to be 1.57 eV. The energy gap of CX-Ti4 -16H2 is 3.46 eV. The increase in energy gap implies that the stability of H2 trapped system increases. The various energy parameters are provided in Table 2. The HOMO-LUMO orbital diagram of CX-Ti4 and its H2 adsorbed systems CX-Ti4 -nH2

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FIG. 3: The average adsorption and sequential desorption energy of CX-Ti4 -nH2 (n = 4, 8, 12 and 16) system.

(n = 4, 8, 12, 16) are given in Figure S2 in supporting information. The hydrogen wt % is calculated to be 8.7 for the hydrogen saturated CX-Ti4 -16H2 system.

The average hydrogen adsorption energy, Ead , and sequential desorption energy, Ede , has been calculated by equations (5) and (6), respectively, using M06/6-311G(d,p) and shown in Figure 3. The adsorption energy for first to fourth hydrogen molecule on Ti metal in CX-Ti4 -nH2 is calculated and is found to be 1.60, 1.13, 0.97 and 0.80 eV, respectively. The adsorption energy decreases on increasing the number of H2 molecules in the CX-Ti4 -nH2 . The desorption energy also shows a similar trend like adsorption energy for CX-Ti4 -16H2 . The desorption energy for the first hydrogen molecule is found to be 1.60 eV and for a second, third and fourth hydrogen molecule is found to be 0.67, 0.63 and 0.31 eV, respectively. The Ede decreases on increasing the number of hydrogen molecules in the system. The high desorption energy for the first adsorbed hydrogen molecule in the CX-Ti4 -nH2 system due to chemisorption. The physisorbed hydrogen molecules can be easily desorbed due to the lower desorption energy of the system. The lower value of adsorption and desorption energies reveal the high reversibility of H2 in the CX-Ti4 system. The adsorption and desorption energies have also been obtained using the GGA-PBE functional with double numeric polaization (DNP) basis sets. Both the energies decreases from 1.1 eV to 0.7 eV as a number of hydrogen 9



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molecules increases in the system. The non-local functional under-predicts the adsorption energy. The comparison of adsorption and desorption energies data are provided in the Table S1 of supporting information.

3.1.3.

CX-Ti4 -nH2 Electrostatic potentials and Hirshfeld charge analysis

The ESP maps of CX-Ti4 and CX-Ti4 -nH2 (n = 4, 8, 12 and 16) have been obtained and their top and bottom views of charge evaluation are shown in Figure 4. In CX-Ti4 , the bottom view shows high electron density (red) while the top view shows low electron density (green). In CX-Ti4 , when the first H2 molecule is trapped, the electron density variation can be seen as a change in color from red to green which signifies a decrease in electron density and in top, color changes from yellow green to blue green indicating the decrease in electron density while on the top corners, the yellow color represents high electron density due to electron transfer from H2 to empty d-orbital of metal and from metal via back donation to the anti-bonding molecular orbital of H2 molecules resulting in chemisorption. Further on an introduction of second, third and fourth hydrogen molecule, top view shows the high electron density due to color changes from dark green to yellow green indicating that these hydrogen molecules are physisorbed on Ti metal centers by charge transfer while the bottom view shows low electron density (blue-green).

It is observed from adsorption and desorption energy calculations that the first H2 molecule is chemisorbed in Ti functionalized CX and remaining three H2 molecules are physisorbed. Hirshfeld charge analysis also confirms the same behavior of the system. Hirshfeld charges are calculated on the Ti metal atoms before and after the H2 adsorption and also for each H2 molecule in the CX-Ti4 -nH2 system (where n = 4, 8, 12 and 16). The average Hirshfeld charge on Ti metal is found to be 0.22 electron units (e.u.) in the CX-Ti4 system, which decreases to -0.05 e.u. after the first hydrogen is chemisorbed (n = 4) and decreases to -0.09 e.u. after the remaining 3 H2 molecules are physisorbed. The charge gained by the Ti metal with the H2 adsorption shows that charge transfer takes place from H2 to Ti metal. On sequential H2 adsorption, the average Hirshfeld charge for chemisorbed hydrogen atoms (Hc ) decreases from -0.21 to -0.17 e.u. The first hydrogen molecule which is chemisorbed on Ti metal has maximum negative Hirshfeld charge, while it decreases with 10


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FIG. 4: Top and bottom views of charge generation of ESP plots of CX-Ti4 and CX-Ti4 -nH2 systems (n = 4, 8, 12 and 16). The units are in e/˚ A3 .

increase in the number of physisorbed H2 molecules. The Hirshfeld charges for physisorbed H2 (Hp ) molecules are found to be positive and almost close to zero for sequential adsorption of H2 molecules to the system which confirms that the charge transfer is from H2 to Ti metal.

3.1.4.

MD simulations of hydrogen saturated CX-Ti4 system

MD simulations of CX-Ti4 -16H2 have been performed at four different temperatures namely 200, 273, 373 and 473 K to examine the stability and reversibility of the fully H2 saturated system. The snap-shots of MD simulations of the CX-Ti4 -16H2 system at all four temperatures are shown in Figure 5. The MD simulations show that the hydrogen remains trapped in the Ti functionalized CX system up to 200 K and on increasing the temperature from 200 K to 273 K the first H2 dissociates from the CX-Ti4 -nH2 system. Further, MD simulations at 373 K, shows that 2 physisorbed hydrogen molecules are desorbed from the system while the fourth chemisorbed hydrogen bond distance elongates from the Ti metal of the CX-Ti4 -nH2 system. Overall, 12 H2 molecules are released from the CX-Ti4 -16H2 system on 5 ps time scale evolution at 373 K. When the MD simulations performed at 473 K, it is observed that all 16 H2 molecules including both chemisorbed and physisorbed are released from the Ti functionalized CX. At higher temperature, it is also observed that the 11



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FIG. 5: Snap-shots of MD simulation of CX-Ti4 -16H2 at the temperatures of (a) 200 K, (b) 273 K, (c) 373 K, and (d) 473 K.

CX system is stable and no clustering effect of Ti is observed.

3.1.5.

Ti sandwich between two CX systems

Several studies have shown that hydrogen can be stored in Ti metal atom sandwiched between two rings.19,67–69 Chen and coworkers reported a comprehensive theoretical study of NH3 , H2 and O2 adsorption on Ti-benzene complexes [Ti(Bz2 ) and Ti2 (Bz)2 ].67 They have reported that the interaction of H2 and Ti, sandwiched between two benzene rings is physisorption and with two Ti metal atoms, out of one is always exposed and adsorb hydrogen in a dissociative manner (chemisorbed). Tang et al68,69 described the hydrogen storage in Ti decorated benzene-Ti-graphene sandwich type structures, with physisorption of 3 H2 molecules on Ti. To address the self-assembly of CX in a real system, the Ti sandwiching 12


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FIG. 6: Two benzene rings of two CX are bonded with Ti as sandwich type structure

2 CX moieties have been geometry optimized and shown in Figure 6. The calculations are performed without any symmetry constraints with GGA-PBE functional including Grimme diffusion function and DNP basis set. The H2 molecules are introduced in every Ti atom including sandwiched metal. It is observed that 4 H2 molecules are adsorbed in free Ti while 3 H2 molecules are physisorbed in the sandwiched Ti resulting in CX-Ti3 -12H2 -Ti-3H2 -CXTi3 -12H2 complex and shown in Figure 7. The H-H bond distance is elongated to 0.831 ˚ A from 0.741 ˚ A for the free H2 . The average adsorption energy is found to be ≈0.8 eV. Also, as the H2 molecules are physisorbed, the storage will be reversible. The hydrogen wt. % with one sandwiched Ti between two CX moieties is found to be 8.30 while in the single CX the hydrogen wt. % is 8.70. Therefore, in a real material when these CX molecules come together the ability to store hydrogen will be not affected significantly. A similar study with Li sandwiching 2 CX moieties is attempted and the structure failed to geometry optimize indicating only Ti intercalate the CX moieties. Thus the Ti metal functionalized CX exhibits excellent hydrogen storage properties.

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FIG. 7: Two benzene rings of two CX are bonded with Ti as sandwich type structure with 8.3 hydrogen wt. %

3.2. 3.2.1.

Hydrogen adsorption of CX functionalized with Li Structural changes in CX-Li4 before and after hydrogen trapping

The optimized geometries of Li functionalized CX system (CX-Li4 ) with the sequential H2 trapped analogs in cyclic form are shown in Figure S3 in supporting information. The structural changes after the adsorption of H2 molecules in the CX-Li4 system are analyzed. CX is functionalized by introducing 4 Li metal atoms resulting in CX-Li4 in which the metal atoms bind with π cloud of benzene ring resulting Li-benzene π-complex by Dewar coordination58 with an average distance of 2.079 ˚ A between Li metal (M) and center of the ring (Rc ). Then, molecular hydrogen is introduced on each metal atom and found that CX-Li4 adsorbs maximum of 3 H2 molecules on each Li. It is observed that all three H2 molecules are physisorbed on the Li metal atoms. The H2 physisorption can be explained by charge polarization mechanism59,60 proposed by Niu et al., where the charge on the Li atom, generated due to the charge transfer from CX which polarizes the H2 molecule resulting in molecular bonding. Out of three hydrogen molecules, the first H2 molecule binds with Li metal at 1.964 ˚ A distance, the second H2 molecule binds at 2.023 ˚ A and the third H2 14


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molecule binds with Li metal at a distance of 2.061 ˚ A. The average distance between H−H molecules remains constant at ≈0.76 ˚ A after optimization indicating physisorption. The distance between Rc and the Li atom increases on the addition of H2 molecules to the CX-Li4 system. The average bond distance between metal and center of the CX ring (MRc ), metal and physisorbed hydrogen distance (M-Hp ) and physisorbed hydrogen distance (Hp -Hp ) are provided in Table S2 in supporting information.

3.2.2.

Energetic and conceptual DFT parameters

Binding energy per Li atom of CX-Li4 is computed and is found to be 0.27 eV, which shows that the metal atom interacted with the π electrons of benzene rings of CX. The stability of the CX-Li4 has been investigated by computing the reactivity parameters such as hardness, electrophilicity and electronegativity.

The calculated hardness and

electrophilicity for the CX-Li4 and its hydrogen trapped analogs indicate that the stability of the CX-Li4 system increases according to the maximum hardness and minimum electrophilicity principle. There is a considerable increase in hardness from CX-Li4 (1.88 eV) to CX-Li4 -12H2 (3.06 eV). However, the electrophilicity decreases with the adsorption of H2 molecules on CX-Li4 . CX-Li4 -12H2 has minimum electrophilicity (0.36 eV) while CX-Li4 has maximum (1.02 eV). Electronegativity decreases with sequentially added H2 molecules to the CX-Li4 . The values of hardness, electrophilicity and electronegativity are given in Table S3 in supporting information. The frontier orbitals energy gap (Eg ) of CX-Li4 and their hydrogen loaded systems is calculated and provided in Table S3. The Eg for CX-Li4 is found to be 0.58 eV and increases with the number of H2 molecules introduced sequentially. with 0.78 eV for the CX-Li4 -4H2 and CX-Li4 -8H2 systems. The energy gap for CX-Li4 -12H2 reaches to 0.81 eV. The increase in Eg with the introduction of H2 molecules shows that the stability of CX-Li4 increases with the addition of H2 molecules in the system. The HOMO-LUMO orbital diagrams of CX-Li4 and their H2 adsorbed systems, CX-Li4 -nH2 (n = 4, 8, 12), are shown in Figure S4 in supporting information. The maximum hydrogen wt. % is calculated to be 10.15 for the hydrogen saturated CX-Li4 -12H2 system.

The average hydrogen adsorption energy, Ead and sequential desorption energy, Ede , has been calculated by equations (5) and (6), respectively, using M06/6-311G(d,p) and shown 15



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in Figure S5 of supporting information. The adsorption energy for all the H2 molecules adsorbed on each Li metal is found close to 0.16 eV. The desorption energy also shows a similar trend as adsorption energy. The physisorbed hydrogen molecules can be easily desorbed due to its lower desorption energy. The lower values of adsorption and desorption energies reveal the high reversibility of the molecular H2 in the CX-Li4 -12H2 system. The adsorption and desorption energies have also been obtained using the GGA-PBE/DNP level. Both the energies are in the range of 0.3 eV. The non-local functional over-predicts the adsorption and desorption energies and their comparison data are provided in the Table S4 of supporting information. Li sandwiching 2 CX moieties is attempted and the structure failed to geometry optimize indicating only Ti intercalate the CX moieties.

3.2.3.

MD simulations of hydrogen saturated CX-Li4 system

The MD simulations of CX-Li4 -12H2 have been performed at four different temperatures of 150, 200, 273 and 300 K. The snap-shots of resulting structures are shown in Figure S6 in supporting information. Up to 150 K, CX-Li4 -12H2 holds all the H2 molecules and at the elevated temperature of 200 K, H2 molecules start desorbing from the CX-Li4 -12H2 system. At 273 K, almost all H2 molecules are desorbed. MD simulations shows that all the H2 molecules are desorbed at 300 K, which makes the CX-Li4 system suitable for hydrogen storage slightly below room temperature since H2 molecules is already leaving at 200 K.

4.

SUMMARY AND CONCLUSIONS

The hydrogen storage properties of the d-block Ti and s-block Li metals functionalized CX system has been studied using the first principles M06/6-311G(d,p) level of theory. The metals are functionalized from outside of each benzene ring of CX. Ti metal strongly binds to the CX through Dewar coordination. On saturation with the H2 molecule, each Ti functionalized CX is found to adsorb 4 H2 molecules with 8.7 wt % hydrogen. Li functionalized CX adsorbs 3 H2 molecules by physisorption with 10.15 wt % hydrogen storage capacity. The physisorbed H2 molecules and metals form Kubas interaction with low adsorption and desorption energies with an average H-H bond length elongated to 0.80 ˚ A and 0.76 ˚ A for Ti and Li metals, respectively. In the Ti functionalized CX, the first hydrogen molecule

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is chemisorbed and the remaining three H2 molecules are physisorbed.

In Ti, due to

charge polarization mechanism, H2 binding results in physisorption. Low sorption energies enhance reversibility of the Ti and Li functionalized CX. The increment in HOMO-LUMO energy gap on sequentially adsorbed H2 molecules on Ti and Li functionalized CX indicates increased stability of the system.

The global reactivity parameters computed for the hydrogen adsorbed CX-Ti4 and CX-Li4 shows the high stability obeying maximum hardness and minimum electrophilicity principles. MD simulations using the GGA-PBE functional including Grimme diffusion function with DNP basis of fully hydrogen adsorbed analogs are performed at a range of temperatures. Simulations of CX-Ti4 with H2 are carried out at 200, 273, 373 and 473 K while for CX-Li4 with hydrogen are done at 150, 200, 273 and 300 K. H2 molecules are released between 273 473 K in Ti functionalized CX system while desorbed at low temperature in Li functionalized CX system. This fundamental study reveals that the Ti functionalized CX is more favorable as compared to the Li functionalized CX exhibiting excellent hydrogen storage properties in stability, optimal hydrogen wt % and reversibility. When the Ti atom is intercalated between the two CX moieties resulting in CX-Ti3 -12H2 -Ti-CX-Ti3 -12H2 complex, the hydrogen wt. % is found to be 8.3 indicating high storage capacity even in the sandwich structures.

ACKNOWLEDGEMENTS

This work is financially supported by the Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR Grant No.

01(2782)14/EMR-II). Interdisciplinary research

project for Hydrogen Storage under Renewable and Clean Energy from IIT Ropar is gratefully acknowledged. The calculations are carried out in IIT Ropar using High Performance Computing cluster facility.

Supporting Information Available

Calculated adsorption and desorption energies, HOMO-LUMO gaps, structural, energetic, ESP parameters and MD snap-shots of CX-Li system are provided as supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Table 1

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The bond distance between metal and center of the CX ring (M-Rc ), metal and chemisorbed hydrogen distance (M-Hc ), metal and physisorbed hydrogen distance (M-Hp ), chemisorbed hydrogen distance (Hc -Hc ) and physisorbed hydrogen distance (Hp -Hp ). All the bond distances are average values measured in ˚ A.

Table 2

Calculated hardness (η), electrophilicity (ω), electronegativity (χ) and HOMOLUMO energy gap (E g ) of all hydrogen trapped CX-Ti4 . The hydrogen wt % is also provided. The energy parameters are in eV units.

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Figure 1.

Optimized structure of CX. Atom colors: C-dark grey; H-white; O-red.

Figure 2.

Optimized structures of (a) CX-Ti4 (b) CX-Ti4 -4H2 (c) CX-Ti4 -8H2 (d) CXTi4 -12H2 and (e) CX-Ti4 -16H2 indicating adsorption and desorption cycle. Atom colors: C-dark grey; H-white; O-red; Ti-grey.

Figure 3.

The average adsorption and sequential desorption energy of CX-Ti4 -nH2 (n = 4, 8, 12 and 16) system.

Figure 4.

Top and bottom views of charge generation of ESP plots of CX-Ti4 and CX˚3 . Ti4 -nH2 (n = 4, 8, 12 and 16). The units are in e/A

Figure 5.

Snap-shots of MD simulations of CX-Ti4 -16H2 at the temperatures of (a) 200 K, (b) 273 K, (c) 373 K, and (d) 473 K.

Figure 6.

Two benzene rings of two CX are bonded with Ti as sandwich type structure.

Figure 7.

Two benzene rings of two CX are bonded with Ti as sandwich type structure with 8.3 hydrogen wt. %.

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Fundamental Study of Reversible Hydrogen ... - ACS Publications

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