Hydrogen storage in Sc and Li decorated metal-inorganic framework

Hydrogen is a versatile, clean, and efficient energy carrier considered as an ideal substitute for a future energy source in the automobile industry. ...
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Hydrogen storage in Sc and Li decorated metal-inorganic framework Sandeep Kumar, Madhu Samolia, and Thogluva Janardhanan Dhilip Kumar ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00034 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Hydrogen Storage in Sc and Li Decorated Metal-Inorganic Framework Sandeep Kumar, Madhu Samolia, and Thogluva Janardhanan Dhilip Kumar∗ Department of Chemistry Indian Institute of Technology Ropar Rupnagar 140001, India Hydrogen is a versatile, clean, and efficient energy carrier considered as an ideal substitute for a future energy source in the automobile industry. Metal-inorganic framework with borazocine (BN) linker resulting in metal-BN framework (MBF) has been studied for hydrogen storage. Borazocine (B4 N4 H8 ) is decorated with metals, M (Sc, Li) and studied the stability and hydrogen storage capacity. Density functional theory with generalized gradient approximation and Perdew-BurkeErnzerhof functional with double numeric polarized basis set augmented with p-function are used to explore the structural stability, hydrogen sorption kinetics of metal decorated MBF. It is observed that each Sc and Li physisorbed 4 and 3 H2 molecules, respectively. BN ring binds with metals (Sc and Li) by Dewar coordination while the metal atoms adsorb H2 molecules by Kubas-Niu-Rao-Jena mechanism. Molecular dynamics simulations show that the Sc decorated MBF system is stable and the adsorbed hydrogen is reversible at ambient conditions. The low sorption energies indicate that the Sc decorated MBF system is an ideal hydrogen storage material. The H2 storage capacity is found to be 7.80 and 8.25 wt % for Sc and Li decorated MBF, respectively. The high hydrogen wt % indicates that the metal decorated framework is a potential hydrogen storage material. Keywords: Metal-inorganic framework; Dewar coordination; Hydrogen adsorption-desorption; Kubas-NiuRao-Jena Interaction; Molecular dynamics; Desorption temperature

1.

INTRODUCTION

The growing energy demands require sustainable energy sources to avoid the depletion of fossil fuels and environmental pollution.1 Hydrogen has considered as an essential source of future energy, since it has significant advantages over petroleum-based fuels, because it has zero carbon emission, does not produce toxic pollutants, highly abundant element both by mass and number, and has very high energy content.2–4 To find out such materials which can effectively store hydrogen in a reversible manner at ambient conditions is one of the main challenge.4 The U.S. Department of Energy (DOE) has set ultimate 2020 hydrogen storage targets of 7.5 wt % at 233-358 K and 3-100 bar for vehicular applications.5 Hydrogen can be stored in materials in two forms, in molecular form as H2 molecule adsorbs weakly on a substrate (physisorption)6 and in atomic form as H atom adsorbs strongly with the substrate (chemisorption).7 In earlier reported systems such as silicene8 , stanene9,10 , and organic-molecule adsorbed Bi/Sb(111) film11 , the hydrogen binds strongly by covalent bonding on the substrate which requires high temperature to release the adsorbed hydrogen. H2 storage via physisorption is most promising route because it is highly reversible and follows fast kinetics under ambient conditions required by the DOE.7,12,13 Different types of porous materials have been studied experimentally and theoretically for H2 adsorption and desorption under ambient temperature and pressure

∗ Electronic

address: [email protected]

conditions. MOFs are nanoporous materials constructed by inorganic and organic molecular building blocks into crystalline frameworks with different topologies.14 Due to the crystallinity, exceptional tunability and porosity of nanoporous materials, researchers have tried to develop porous materials from over a decade for high hydrogen storage at ambient conditions.1,14–17 Among them, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs) have been widely studied for hydrogen and gas storage.18–24 However, the hydrogen storage capacity of MOFs and COFs are found to decrease drastically near room temperature because of their weak interaction with H2 molecules which results in low adsorption energy values not enough to meet the requirements for automobile applications.25–27 To increase the interaction between adsorbed H2 molecules and a host material, the functionalization of host material by incorporating functional groups in the host material or by metal decoration of host materials are being explored.28–30 The metal decorated MOFs, COFs, 2D sheets (graphene, graphyne), hydrogenated metal clusters are frequently studied for hydrogen storage.31–36 Alkali (Li, Na)37,38 and transition metals (Sc, Ti, V, Fe, Ni, and Pd)39,40 are well known for improving the H2 binding strength with metal site in host materials. Early transition metals such as Sc, Ti, and V are well studied for high hydrogen storage capacity at ambient temperature and pressure conditions.39,40 MOF NU-100 designed and studied theoretically for high hydrogen storage capacity of 9.95 wt % at 77 K, and then this MOF was synthesized and found the same structure and properties as their designed MOF structure.41 This motivated us to study the newly designed MBF decorated with different

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metals for hydrogen storage by using first-principles electronic structure calculations. In the present work, metal-borazocine framework (MBF) designed by replacing the linker in MOF by inorganic borazocine (BN) linker. The BN linker of MBF is decorated with metal atoms (Sc and Li) and their stability, physicochemical properties and hydrogen trapping efficiency of resulting metal (Sc and Li) decorated MBFs are studied. In MBF, magnesium oxide (Mg4 O) is used as a metal node and borazocine as linker resulting Mg4 Oborazocine framework. Borazocine linker is linear and symmetric with the molecular formula B4 N4 H8 . In MBF, the O atom of metal node binds directly with B atoms of borazocine located at 2, 6 positions. The structural stability of MBF and hydrogen adsorption on metal (Sc and Li) decorated MBF, hydrogen storage capacity of metal decorated MBF and Born-Oppenheimer Molecular dynamics (BOMD) simulations of hydrogen saturated metal decorated MBF have been studied. This paper is arranged as follows: The computational details are provided in section 2. The explanation of the results is given in section 3. Finally, summary and conclusions are provided in section 4.

2.

COMPUTATIONAL METHODOLOGY

Electronic structure calculations are performed by density functional theory (DFT) as implemented in the DMOL3 code.42,43 The generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE)44 exchange correlation functional with double numeric basis set (DNP) augmented with p-polarization function is applied. Dispersion-corrected DFT (DFT-D) scheme with Grimme’s correction function is used to evaluate the weak van der Waals (vdW) interaction.45 Geometries are fully optimized without imposing symmetry constraints in all the structures. The following parameters are used for calculations: Broyden-Fletcher-GoldfarbShanno (BFGS) algorithm is used to further improve the accuracy in evaluating weak interactions with 10−5 Ha as an energy convergence value, 2 × 10−3 Ha/˚ A for gradient convergence with thermal smearing of 0.005 Ha, the realspace global orbital cutoff radius of 4.7 ˚ A and maximum displacement 5 × 10−3 Ha/˚ A is used. Though selected ab initio methods accurately evaluate weak interactions but when the system size increases which lower its performance. The GGA based DFT calculations scale well with increased the number of atoms of the system. Energy parameters namely, metal binding energy, adsorption and desorption energy of hydrogen and structural changes such as variation in bond distance, and other parameters like electrostatic potential maps, Hirshfeld charge, BOMD simulations, and thermodynamics of usable hydrogen storage are analyzed. The binding energy of metal decorated MBF is deter-

mined as Eb =

1 [EM BF −M4 − (4EM + EM BF )] 4

(1)

where M = Sc and Li, EM BF −M4 , EM and EM BF is the energy of metal (Sc and Li) decorated MBF, energy of isolated metal and the energy of MBF. The average adsorption energy of hydrogen in metal decorated MBF is determined by using the equation Ead =

1 [EM BF −M4 −nH2 − (nEH2 + EM BF −M4 )] (2) n

where EM BF −M4 −nH2 , EH2 , and EM BF −M4 is total energy of n number of hydrogen molecules adsorbed on metal decorated MBF, energy of H2 molecule and total energy of metal decorated MBF. The desorption energy of sequentially adsorbed hydrogen molecules is calculated by using the equation Ed =

 1 nEH2 + EM BF −M4 −(n−4)H2 − EM BF −M4 −nH2 (3) n

where EM BF −M4 −(n−4)H2 is the energy of preceding hydrogen molecules adsorbed on MBF-M4 -nH2 .

The Van’t Hoff equation is used to describe the equilibrium between gas and condensed phase of H2 as,   ∆S ∆H + (4) ln Keq = − RT R P

where Keq = PHo2 , and PH2 , and Po are equilibrium and the reference pressure namely atmospheric pressure, and R, gas constant, T absolute temperature and ∆H is change in H2 enthalpy and ∆S is change in H2 entropy R from gas to condensed phase. By substituting kB = N and reorganize the above equation,   ∆H ∆S PH2 = Po exp − + (5) kB T N R Desorption temperature, TD can be determined by using Van’t Hoff equation46,47 as  −1  ∆S Ead − lnP (6) TD = kB R where Ead = ∆H N , P = 1 atm, and kB is the Boltzmann constant, R is the gas constant, ∆H is change in enthalpy and ∆S is change in H2 entropy. BOMD48 simulations of fully H2 saturated Sc and Li decorated systems are performed to examine the stability of the systems and reversibility of the adsorbed hydrogen in Sc and Li-decorated MBF systems. The time step is set to 1 fs and run time up to 5 ps for simulations. Nos´e thermostat in canonical NVT ensemble is used to control the temperature during simulations. BOMD simulations are performed with different temperature ranges for both the fully hydrogen saturated systems.

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to be -0.42 electron unit (e.u.) which decreases on the addition of H2 molecules on each Sc in the system. In MBF-Sc4 , the Hirshfeld charges on Sc is found to be 0.44 e.u. which increases with the adsorption of H2 molecules on each Sc in MBF-Sc4 -nH2 . This indicate that charge is gained by Sc metal from the BN ring of MBF. Hirshfeld charges for adsorbed hydrogen molecules is found near to be zero and no significant charge transfer is observed from adsorbed hydrogen molecules to Sc showing that all the hydrogen molecules are physisorbed. The average Hirshfeld charge on Li atoms in MBFLi4 , are found to be 0.41 electron unit (e.u.). Hirshfeld charges on Li increases on sequential adsorption of H2 in MBF-Li4 and found to be 0.28 e.u. for the MBFLi4 -12H2 indicate that charge gained by the Li atoms. Hirshfeld charges for the physisorbed H2 molecules decreases from 0.05 e.u. to 0.02 e.u. for the MBF-Li4 -4H2 and MBF-Li4 -12H2 , respectively. Hirshfeld charge analysis also signifies that electron density is enhanced on the Li and H2 molecules due to charge transfer from π electrons of BN ring during the H2 adsorption as shown in Figure 7.

3.5. Molecular Dynamics Simulations of MBF-Sc4 -16H2 and MBF-Li4 -12H2 systems

To explore the desorption mechanism of the adsorbed H2 from MBF-Sc4 -16H2 and MBF-Li4 -12H2 , BOMD simulations are performed at different temperatures and their snap-shots are given in Figure 8 and S5 in Supporting Information for MBF-Sc4 -16H2 and MBF-Li4 -12H2 , respectively. The recommended temperature for desorption of adsorbed hydrogen is 233 to 358 K for ideal hydrogen storage material. BOMD simulations of MBF-Sc4 -16H2 is performed at 300, 373 and 473 K. BOMD simulations at temperature up to 300 K shows that hydrogen start desorbing from the MBF-Sc4 -16H2 and one hydrogen molecule from each Sc is desorbed. At elevated temperature up to 373 K, BOMD simulations show that all the hydrogen molecules are desorbed from MBF-Sc4 -16H2 . At a higher temperature of 473 K, the MBF structure is stable and no clustering effect of Sc metal has been observed which is common in carbon nanotubes and transition metal decorated MOFs. BOMD simulations of MBF-Li4 -12H2 is performed over a temperature range of 120, 300 and 373 K. Simulations of MBF-Li4 -12H2 show that hydrogen molecules start desorbing at 120 K and all the hydrogen molecules are desorbed by 300 K. The simulations of MBF-Li4 -12H2 shows that Li decorated MBF can be used for hydrogen adsorption at below room temperature. BOMD simulations also confirm the stability of MBF system and it is observed that MBF system is stable even at 473 K.

3.6.

Desorption Temperature

Desorption temperature, TD , for adsorbed hydrogen molecules are calculated by equation (6). Calculated adsorption energy values are used to determine the desorption temperature. By using equilibrium pressure 1 atm and ∆S value is used from literature47 , for the determination of average desorption temperature TD . By using the adsorption energy of first and last adsorbed H2 molecule on each metal, we can also compute the highest (TDH ) and lowest (TDL ) temperature for onset and full desorption of H2 from the system. For MBF-Sc4 -16H2 , the calculated TD is found to be 334 K. The TDH and TDL are found to be 392 and 282 K, respectively, for MBF-Sc4 -16H2 . The calculated TD , TDH and TDL are found to be 148, 164 and 122 K respectively for the MBFLi4 -12H2 system. The desorption temperatures and MD simulations of both the hydrogen loaded metal decorated systems are correlating. 3.7.

Thermodynamic Usable Hydrogen Capacity

The promising hydrogen storage material should adsorb large number of hydrogen molecules and these hydrogen molecules should be desorbed at operating conditions. Thus, the usable H2 capacity has been calculated by using thermodynamic properties of metal (Sc and Li) decorated MBF at different temperature (T ) and pressure (P ) conditions. The H2 occupation (adsorption) number (f ) has been calculated by specifying the P and T at the time of adsorption and desorption of hydrogen molecules. The prevailing values of P and T for hydrogen adsorption are 30 atm and 298 K (high pressure at room temperature) while for desorption the P and T values are 3 atm and 373 K (low pressure at high temperature).46 The H2 occupation number is calculated by using the equation: PNmax ngn exp[n(µ − E)/kB T ] (7) f = Pn=0 Nmax n=0 gn exp[n(µ − E)/kB T ]

where Nmax is the maximum adsorbed number of H2 molecules on each Sc and Li atom in MBF-M4 (M = Sc and Li), n is the number of adsorbed H2 molecules, gn is the configurational degeneracy of n, kB is the Boltzmann constant, E is the adsorption and desorption energy of H2 molecules adsorbed in metal decorated systems. The adsorption and desorption energy used for hydrogen adsorption and desorption conditions of temperature and pressure. Chemical potential, µ of H2 in gas phase at given T and P determined by the following equation: µ = µideal − 0.00015(T − 186.5) + 0.00065[(log10 P − 0.5)2 − 0.25]

(8)

The chemical potential µ of H2 gas is taken from the experimental values as a function of T and P .46,47 The

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Supporting Information Available

Optimized geometries of MBF-Li4 and its H2 adsorbed systems are provided in Figure S1(a-d) and in Figure S2. The ESP plots for MBF-Sc4 -nH2 and MBF-Li4 -nH2 , are given in Figure S3, S4 and images of BOMD simulations of MBF-Li4 -12H2 system are given in Figure S5. The calculated adsorption energy for hydrogen molecules in pore space is given in Table S1 and S2 for MBF-Sc4 -nH2 and MBF-Li4 -nH2 . The internal coordinates of optimized geometries of MBF, MBF-Sc4 and MBF-Sc4 -nH2 (n = 4, 8, 12 and 16) and MBF-Li4 and MBF-Li4 -nH2 (n = 4, 8 and 12) are provided in Table S3- S13 as supporting information. REFERENCES [1] Gomez-Gualdron, D. A.; Colon, Y. J.; Zhang, X.; Wang, T. C.; Chen, Y.-S.; Hupp, J. T.; Yildirim, T.; Farha, O. K.; Zhang, J.; Snurr, R. Q. Evaluating Topologically Diverse Metal-Organic Frameworks for Cryo-Adsorbed Hydrogen Storage. Energy Environ. Sci., 2016, 9, 32793289. [2] Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Hydrogen Storage in Microporous Metal-Organic Frameworks. Science, 2003, 300, 1127-1129. [3] Schlapbach, L.; Z¨ uttel, A. Hydrogen-storage materials for mobile applications. Nature, 2001, 414, 353-358. [4] Graetz, J. New Approaches to Hydrogen Storage. Chem. Soc. Rev., 2009, 38, 73-82. [5] The U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy. Target Explanation Document: Onboard Hydrogen Storage for Light-Duty Fuel Cell Vehicles. https://energy.gov/sites/prod/files/2017/05/f34/fcto targets onboard hydro storage explanation.pdf, accessed on 09/01/2018. [6] Blomqvist, A.; Araujo, C. M.; Srepusharawoot, P.; Ahuja, R. Li-Decorated Metal-Organic Framework 5: A Route to Achieving a Suitable Hydrogen Storage Medium. Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 20173-20176. [7] Jena, P. Materials For Hydrogen Storage: Past, Present, and Future. J. Phys. Chem. Lett., 2011, 2, 206-211. [8] Zhang, C.-W.; Yan, S.-S. First-Principles Study of Ferromagnetism in Two-Dimensional Silicene with Hydrogenation. J. Phys. Chem. C, 2012, 116, 4163-4166. [9] Zhang, R.-W.; Zhang, C.-W.; Ji, W.-X.; Li, S.-S.; Hu, S.-J.; Yan, S.-S.; Li, P.; Wang, P.-J.; Li, F. EthynylFunctionalized Stanene Film: A Promising Candidate as Largegap Quantum Spin Hall Insulator. New J. Phys., 2015, 17, 083036. [10] Wang, Y.-P.; Ji, W.-X.; Zhang, C.-W.; Li, P.; Li, F.; Wang, P.-J.; Li, S.-S.; Yan, S.-S. Large-Gap Quantum Spin Hall State in Functionalized Dumbbell Stanene. Appl. Phys. Lett., 2016, 108, 073104. [11] Li, S.-S.; Ji, W.-X.; Hu, S.-J.; Zhang, C.-W.; Yan, S.-S. Materials Effect of Amidogen Functionalization on Quantum Spin Hall Effect in Bi/Sb(111) Films. ACS Appl. Mater. Interfaces, 2017, 9, 41443-41463.

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Table 1.

Metal (M = Sc and Li) and BN ring center distance (M-Rc ), physisorbed hydrogen and metal distance (M-Hp ), and distance between physisorbed hydrogen (Hp -Hp ) measured before and after the adsorption of H2 molecules in MBF-M4 . All are average distances given in ˚ A.

Figure 1.

Optimized structure of MBF. Green, red, blue, peach, and white color represents Mg, O, N, B, and H atoms, respectively.

Figure 2.

Optimized geometries of: (a) MBF-Sc4 (b) MBF-Sc4 -4H2 (c) MBF-Sc4 -8H2 (d) MBFSc4 -12H2 and (e) MBF-Sc4 -16H2 systems. Green, red, blue, peach, white, and gray color represents Mg, O, N, B, H and Sc atoms, respectively.

Figure 3.

Optimized geometry of MBF-Sc4 -26H2 with 7.8 wt % of hydrogen.

Figure 4.

Average hydrogen adsorption energy of MBF-Sc4 -nH2 (n = 4, 8, 12 and 16) and MBF-Li4 -nH2 (n = 4, 8, and 12) system.

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Figure 5.

Sequential desorption energy of MBF-Sc4 nH2 (n = 4, 8, 12 and 16) and MBF-Li4 -nH2 (n = 4, 8, and 12) system.

Figure 6.

Hirshfield charges for BN ring, Sc, and physisorbed hydrogen (Hp ) of MBF-Sc4 -nH2 system (n = 4, 8, 12 and 16).

Figure 7.

Hirshfield charges for BN ring, Li, and ph-

ysisorbed hydrogen (Hp ) of MBF-Li4 -nH2 system (n = 4, 8 and 12). Figure 8.

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Images of MD simulations of MBF-Sc4 16H2 at temperatures of (a) 300 K (b) 373 K and (c) 473 K.

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52x29mm (300 x 300 DPI)

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