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Reversible hydrogen adsorption in Li functionalized [1,1]paracyclophane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 2 9 4 0 e1 2 9 4 8
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Reversible hydrogen adsorption in Li functionalized [1,1]paracyclophane Rohit Y. Sathe, T.J. Dhilip Kumar* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, 140001, India
highlights
graphical abstract
Hydrogen adsorption mechanism in
Li
functionalized
[1,1]
paracyclophane. Reversible hydrogen adsorption of 8 H2/complex with 13.42 H wt%. Charge polarization mechanism of hydrogen adsorption. BOMD
simulations
prove
the
reversibility and thermal stability of the host. Host is a prospective hydrogen storage candidate fulfilling the DOE targets.
article info
abstract
Article history:
Hydrogen is a good alternative to replace fossil fuels in automobiles. Storage of hydrogen
Received 3 January 2020
for vehicular applications with high gravimetric density is a challenging task. The
Received in revised form
hydrogen sorption capacity of [1,1]paracyclophane functionalized with Li is investigated
12 February 2020
using density functional theory. Li functionalized [1,1]paracyclophane physisorbs 8 H2
Accepted 1 March 2020
achieving the maximum hydrogen weight percentage up to 13.42 %. All positive vibrational
Available online 1 April 2020
frequencies and a significant difference in the energy of frontier molecular orbitals confirm the stability and high absolute hardness of the host. Molecular dynamics simulations prove
Keywords:
the thermal stability and reversibility of hydrogen adsorption over Li functionalized [1,1]
Hydrogen adsorption
paracyclophane implying the ease of on-board reversible hydrogen storage. Our findings
Density functional theory
confirm that Li decorated [1,1]paracylophane is a good hydrogen storage material meeting
CHELPG analysis
the 2020 targets of DOE.
Born-oppenheimer molecular
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
dynamics [1,1]paracyclophane
* Corresponding author. E-mail address: [email protected] (T.J. Dhilip Kumar). https://doi.org/10.1016/j.ijhydene.2020.03.009 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 2 9 4 0 e1 2 9 4 8
Introduction The use of fossil fuels to run automobiles is increasing at an alarming rate all over the world. This is not only diminishing the exhaustible fuel sources but also contributing to the increased rates of worldwide air pollution [1e3]. Extensive research is going on to find an alternative fuel source for automobiles. Hydrogen, being the most abundant element in the universe and a good energy carrier, is a better option to replace fossil fuels. Hydrogen has a higher energy density (33 kW h kg1) than traditional fossil fuels. Hydrogen fuel cells produce water as a byproduct, therefore hydrogen-fueled vehicles are eco-friendly [1,4,5]. The main obstacle in using hydrogen as a vehicular fuel is the storage of hydrogen with high gravimetric or volumetric density. Hydrogen can be stored by physical or chemical modes for vehicular applications [1,6]. Physical modes include highpressure vessels to store hydrogen in the form of compressed gas and cryo-compressed storage, or at a very low temperature known as cryogenic storage. These methods include high processing cost and use of costly material like carbon fiber reinforced plastics. These techniques are not economical and safe as the hydrogen stored under extreme pressure poses the threat of explosion [7e10]. Chemical modes like storing hydrogen in metal hydrides, chemical hydrides, metal organic frameworks [11], BN based systems [12], conjugated microporous polymers [13e15], covalent organic frameworks [16e18], etc. are widely investigated. Many recent investigations on hydrogen storage properties of carbon nanostructures like hydrogen storage capacities of twodimensional carbon nitride [19] and defect-engineered C4N nanosheets [20] report the improved hydrogen weight percentage and reversibility under ambient conditions. Department of Energy, USA has specified a target for hydrogen storage materials having at least 7.5 % hydrogen weight percentage along with reversible thermokinetics below 100 atm pressure in the temperature range of 40 to 85 C to be achieved till year 2020 [21e23]. In this study [1,1]paracyclophane (PCP11) functionalized with Li metal are screened for their hydrogen storage capacity. Paracyclophanes contain a wide range of molecules, and they are named according to the arene substitution pattern [24,25]. “[1,1]” in [1,1]paracycplophane denotes that there is a single (-CH2-) moiety in each bridge linking the benzene rings in the paracyclophane. This bridge is so short that it puts a strain in the benzene rings due to which benzene rings in PCP11 lose the planarity [24]. This strain is also utilized for metal functionalization. Li is functionalized over each benzene ring of PCP11 because it is a lightweight alkali metal. Earlier reports of hydrogen storage systems functionalized with Li show that each Li atom can adsorb up to a maximum of three hydrogen molecules [26e32]. Reports based Li functionalized carbonbased nanostructures such as nanotubes [33e35], nanosheets, graphene [36,37], graphyne [38], graphdiyne [39], etc. show enhanced hydrogen weight percentage. Liu et al. in their study of Li dispersed nanotubes report the hydrogen uptake up to 13.45 % [33], while the experimental findings discussing the hydrogen adsorption over alkali-doped carbon nanotubes are further presented by Yang [34]. Wu et al. in their extensive
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ab-initio investigation, discuss the enhanced hydrogen adsorption over Li functionalized boron carbide nanotubes [35]. Their findings report hydrogen mass density up to 6.2 %. Ataca et al. report the maximum hydrogen weight percentage of 12.8 % over Li covered graphene sheet [36]. While the improved hydrogen weight percentage of 16 wt% has been reported by Zhou et al. [37]. Working on other allotropes of Li functionalized carbon like graphyne and graphdiyne, Sathe et al. [38] and Panigrahi et al. [39] have reported the hydrogen weight percentage up to 8.69 and 6.5 %, respectively. We also tried to functionalize Sc atoms over PCP11 but due to stronger binding energy of Sc over benzene and already available strain over benzene rings due to shorter eCH2- chains, Sc did not get functionalized over PCP11. Functionalized Li acts as an open metal site for hydrogen adsorption. An interaction between the delocalized p-electrons over the benzene rings of PCP11 and Li atoms results in Li functionalization. Li atoms interact with p-electrons of benzene ring without cluster formation. Employing the DFT calculations, the structural stability and hydrogen adsorption mechanism in Li functionalized PCP11 is studied. Our findings prove that each Li atom reversibly adsorbs 4 hydrogen molecules. We present the details of computation in section Computational details, results are thoroughly discussed in details in section Results and discussion. Findings of the study are concluded in section Conclusions.
Computational details The calculations are carried out using the Gaussian 09 [40] package with Minnesota 06 (M06) exchange-correlation functional [41] and 6-311G(d,p) basis set. Minnesota 06 is parameterized for non-covalent interactions, organometallics, and metal thermochemistry. As we have functionalized PCP11 with Li metal and Li exhibits charge polarization [41e44] during functionalization and hydrogen adsorption, M06 is used to study these weak interactions. Additionally, there are many recent ab-initio investigations of hydrogen storage properties of chemically functionalized hosts using Minnesota 06 functional [45e52]. Before metalatoms are functionalized, the aromaticity of PCP11 is checked with Nucleus Independent Chemical Shift (NICS) calculations [40]. At a distance of 1 A from the center of the benzene rings, NICS is found to be a negative maximum (8.6 ppm) [53,54]. Therefore, metal atoms are functionalized at this distance. We checked the energy of binding for the metal atoms with the PCP11. Following equation is used to calculate the binding energy, 1 Ebi ¼ ½EPCP112Li ð2ELi þ EPCP11 Þ 2
(1)
where Ebi is the binding energy, EPCP112Li is the energy of Li functionalized PCP11, ELi is the energy of Li atom, EPCP11 is the energy of PCP11. Further, the hydrogen molecules are sequentially loaded on these metal functionalized complexes. For all the resulting complexes, the vibrational frequencies are checked to confirm the stability of these complexes. All vibrational frequencies
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were found to be positive. The adsorption and desorption energies are calculated with the following equations [3,55e59], Ead ¼
1 nEðH2 Þ þ EðPCP112LiÞ EðPCP112LinH2 Þ n
(2)
where Ead is adsorption energy, n denotes the number of adsorbed hydrogen molecules, the total energy of PCP11e2Li complex is given by EðPCP112LiÞ , EðH2 Þ is energy of the H2 molecule, the total energy of the hydrogen adsorbed PCP11e2Li is denoted by EðPCP112LinH2 Þ . Ede ¼
1 nEðH2 Þ þ E½PCP112LiðnH2 2Þ EðPCP112LinH2 Þ 2
(3)
where Ede is desorption energy and the total energy of a complex preceding to PCP11e2Li-nH2 complex is given by E½PCP112LiðnH2 2Þ . Study of structural changes before and after adsorption is performed. The difference between frontier orbitals is calculated to study the stability of complexes in terms of reactivity and absolute hardness [60e63]. Changes in total electron density are mapped to determine the potential site for adsorption. Electrostatic potential maps are calculated over the total density using full SCF and “CHarges from ELectrostatic Potentials” (CHELPG) charge analysis is performed to confirm the charge transfer mechanism. To analyze the effect of temperature on the hydrogen saturated derivative of Li functionalized PCP11, BornOppenheimer molecular dynamics (BOMD) is carried out. Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional of the generalized gradient approximation (GGA) is employed for these simulations. It renders improved results by considering the change in total density during the calculation of exchange-correlation energy. This study is performed using DMol3 package employing the NVT ensemble and double numerical basis sets with polarization function (DNP). DNP is all-electron numerical basis function. It is generated as values on an atomic-centered spherical-polar mesh where the calculation of the numerical part is performed by solving the atomic DFT equations. It includes a detype polarization function on heavy atoms with additional sets of detype polarization functions on all atoms. DNP is proven to be capable of minimizing or even eliminating basis set superposition error (BSSE) [64] and hence is useful for studying reaction coordinates and weak non-covalent interactions which are of prime concern while studying hydrogen adsorption mechanisms. For studying the orbital hybridization, the projected density of states (PDOS) analysis is performed. Vienna Ab-initio package (VASP) [65,66]is used along with PBE functional. In given PDOS analysis, Fermi-level is set at zero.
Fig. 1 e Structure of optimized PCP11 along with the bond distance between adjacent C atoms (A).
rings is due to the very short bridges connecting these two rings which consist of only single (-CH2-) moiety. This induces strain on benzene rings and their inherent planarity is lost. This strain plays a crucial role in Li functionalization. Li functionalization further induces the strain on the benzene rings. Li functionalized PCP11 complex is shown in Fig. 2. The binding energy (Ebi) of Li with PCP11 is found to be 1.33 eV. As mentioned earlier, the already available strain on the benzene rings of PCP11 results in the stronger binding with the metal. This is the reason that functionalization of Sc onto the benzene rings of PCP11 (Ebi ¼ 1.83 eV) imparts further strain over these rings and the host structure collapses. In PCP11, the ‘pore size’ or the distance between two eCH2- units in bridges is 4.771 A which on metal functionalization increases to 4.842 A notifying the stress induced by metal
Results and discussion Li functionalization Structure of PCP11 is optimized before functionalizing the metal atoms. Optimized structure of PCP11 is shown in Fig. 1. There are two benzene rings (left and right) in PCP11. Both of these rings are slightly bent inward. Non-planarity of these
Fig. 2 e Optimized structure of Li functionalized PCP11. Numbers specify the distance between the atoms (A).
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functionalization. This is also evident from the distance between C atoms in benzene rings which increases from 1.403 A to 1.447 A. The distance of functionalized Li atoms from the center of the benzene rings increases from 1 A to 1.75 A after optimization of the complex. This is due to the increased electron density in confined space close to the benzene rings. This helps in functionalization of metal atoms and induces further stress on the benzene rings. The energy gap between frontier molecular orbitals of PCP11 is found to be 1.92 eV. After functionalization of Li, this energy gap is reduced to 1.67 eV. High values of energy gap between frontier molecular orbitals specify that the PCP11 and PCP11e2Li complexes are kinetically stable [60,67,68]. Light alkali metal Li is functionalized over benzene rings due to charge polarization mechanism. Charge transfer between the benzene rings to the Li atoms results in the functionalization of Li over PCP11. To prove the mechanism of Li functionalization, we studied the projected density of states (PDOS) for PCP11 and PCP11e2Li. As shown in Fig. 3, the contribution of C(p) and Li(s) orbitals can be seen in Fig. 3(II) as compared to Fig. 3(I). This clearly supports the aforementioned charge polarization mechanism in forming the
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PCP11e2Li complex. Most of the orbital hybridization peaks are observed near Fermi-level, stating the stability of the complexes.
H2 physisorption A single H2 is adsorbed over each Li in each successive step of hydrogen adsorption. This gives the freedom to examine the stability of each of the resulting complex and trends of adsorption and desorption energies. Sequential hydrogen adsorption also avoids the complexity caused by steric hindrance due to crowding of hydrogen molecules and allows systematic evaluation of the charge transfer. Fig. 4 shows the steps of hydrogen sorption over PCP11e2Li. Charge polarization leads to the adsorption of hydrogen molecules over functionalized Li atoms [42,43]. As charge transfer from the Li atoms polarizes nearby hydrogen molecules and holds them in the close vicinity of the metal atoms [69e73]. PCP11e2Li complex reaches the hydrogen saturation limit after adsorption of four hydrogen molecules over each Li with a maximum hydrogen weight percentage of 13.42 %. To confirm the orbital hybridization in adsorption of hydrogen over each Li, we performed the PDOS analysis. Fig. 5 shows the projected density of state in PCP11e2Lie2H2. As seen in Fig. 5, orbital hybridization from H(s), C(p), and Li(s) orbitals is observed in near-Fermi-region in the range of 10 to 5 eV. This explains the involvement of charge polarization mechanism as a results of transfer of electron density between pecomplex, functionalized Li atoms, and adsorbed H2 molecules.
Adsorption and desorption energies Analysis of adsorption energy and desorption energy provides strong evidence about the reversibility of the system while studying hydrogen sorption systems [74]. It is a widely observed trend that during sequential loading of hydrogen molecules over the host, the desorption energy decreases [55,75e78]. This trend signifies the reversibility of the system as the desorption energy for the hydrogen saturated complex is less. The trend of sorption energies for Li functionalized PCP11 is shown in Fig. 6. For saturated complex in Li functionalized PCP11, the desorption energy is 0.065 eV. Sorption energies gradually decrease during the sequential adsorption process. This is because the hydrogen molecules are polarized to be held in a physisorbed position. The hydrogen molecules desorb from the metal functionalized PCP11 on slight changes in the thermodynamic parameters.
Structural changes
Fig. 3 e Projected density of states in (I) PCP11 and (II) PCP11e2Li.
Tracking the structural changes before and after the hydrogen adsorption helps to understand the adsorption phenomena [3,76,79]. As the number of adsorbed H2 increases over the host, the distance between metal atoms (Li-Ha) and adsorbed hydrogen molecules increases. In Table 1, the mean distance between center of the benzene ring and Li atoms is given by Rc-Li, Li-Ha specifies the mean distance between Li atom and adsorbed hydrogen molecules, and the mean distance
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Fig. 4 e Hydrogen sorption over PCP11e2Li. Here, (I) PCP11e2Li, (II) PCP11e2Lie2H2, (III) PCP11e2Lie4H2, (IV) PCP11e2Lie6H2, (V) PCP11e2Lie8H2. between H atoms in the adsorbed H2 is given by Ha-Ha. Due to the presence of charge polarization mechanism between Li and delocalized p-electrons of the benzene ring, the distance between the ring center and Li atoms stays below 1.79 A throughout the process. The difference in the energy between frontier orbitals (HOMO and LUMO) can be used to predict the strength and stability of metal complexes [67,80]. Lower chemical reactivity and higher kinetic stability are usually in the direct correlation with a large HOMOeLUMO gap. Large difference between the energies of these orbitals implies that the electrons in HOMO will require higher energy to jump from HOMO to LUMO. This signifies the chemical or absolute hardness of the complex [81,82]. Chemical hardness is the resistance of the system for changing electron density over the system. The absolute hardness (denoted as h) is the resistance given complex to undergo change the electron distribution of the system [68,81,83]; vm vN nðrÞ
In the aforementioned equation, h denotes absolute hardness, m is the electronic chemical potential, and N denotes the total number of electrons in the system. It can be read as; at a fixed potential nðrÞ, the absolute hardness is the change in the electronic chemical potential of a given complex with respect to the total number of electrons in the system [84].
h¼
(4)
Fig. 6 e Trends of adsorption energy and desorption energy in Li functionalized PCP11.
Table 1 e Structural changes in Li functionalized PCP11 and its hydrogen adsorbed derivatives. Name of the
Fig. 5 e Projected density of states in PCP11e2Lie2H2.
Complex PCP11e2Li PCP11e2Lie2H2 PCP11e2Lie4H2 PCP11e2Lie6H2 PCP11e2Lie8H2
Avg.
Avg.
Avg.
Rc-Li ( A) 1.707 1.734 1.744 1.751 1.782
Li-Ha ( A) e 2.168 2.202 2.283 2.377
Ha-Ha ( A) e 0.755 0.755 0.752 0.752
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Fig. 7 e Electrostatic potential maps for (I) PCP11, (II) PCP11e2Li, (III) PCP11e2Lie2H2, (IV) PCP11e2Lie4H2, (V) 3. PCP11e2Lie6H2, (VI) PCP11e2Lie8H2; Units: e/A
The calculated hardness for hydrogen saturated Li functionalized PCP11 is 3.27 eV and electrophilicity is 0.57 eV. These values specify the high stability of the PCP11e2Lie8H2. The highest HOMOeLUMO gap is predicted for hydrogen saturated complex. Saturated complex PCP11e2Lie8H2 has HOMO-LUMO gap of 1.9 eV which signifies that saturated complex is a stable complex in terms of chemical hardness.
Electrostatic potential maps and CHELPG charge analysis Electrostatic potential maps are used as guidelines to understand the transfer of electron density over the complex and to identify the preferred location where further hydrogen molecules can be adsorbed. Fig. 7 shows the electrostatic potential maps for PCP11, metal functionalized PCP11, and its hydrogen adsorbed derivatives. It can be seen in Fig. 7(I) that there is a region of high electron density on the top of both the benzene rings (can be seen at the side of the complex) and there is a region of moderately low electron density over the gap between two benzene rings (can be seen at the front side of the complex). The electron-dense region disappears on the functionalization of Li atoms which can be seen from Fig. 7(II). Electron density over Li functionalized PCP11 changes as hydrogen molecules are adsorbed over these complexes as evident from Fig. 7(III)e(VI). A region of slightly higher electron density develops near the benzene rings while a region with less electron density develops near the hydrogen adsorption site (at the side of the complexes). This specifies that the complex attains saturation in terms of hydrogen adsorption. CHarges from ELectrostatic Potentials using a Grid based method (CHELPG) is employed to calculate the charges over each atom during sequential hydrogen adsorption. In CHELPG atomic charge calculation, molecular electrostatic potential (MESP) is produced from the atomic charges at a number of points around the molecule. In Fig. 8 it can be seen that there is a significant variation in CHELPG charges over C and Li atoms during the whole process of adsorption. Fig. 8 shows the variation in the CHELPG charges over each atom in the system during the course of subsequent adsorption. It can be seen that charges over the hydrogen atoms do not vary much signifying a weak physisorption. Due to the charge transfer between lithium and carbon atoms, there is a significant fluctuation in the charges of these atoms. It can be seen that C and Li atoms in the system have the transferable charge left even after the adsorption of 3rd hydrogen molecules over each Li atom. This explains the adsorption of 4th hydrogen molecule over each Li atom. The reason for the enhanced adsorption of hydrogen molecules is the
continuous charge transfer processes involved in the Dewarlike mechanism and charge polarization mechanism, as well as the ease of transfer of electron density between delocalized peelectrons and dihydrogen complexes through light alkali metal Li. Thus, Li enhances the sorption capacity of PCP11. Fig. 8 supports the above explanation of charge transfer from peelectrons to hydrogen molecules with Li acting as a mediator. Upon functionalization onto the PCP11 complex, Li atoms gain a positive charge, as electron density first get transferred to the partially filled seorbital of Li and then gets back-transferred to the benzene p-complex. As the hydrogen saturation is achieved, the charges initially try to get neutralized and further achieve a slight positive charge. With average negative charge values over carbon atoms and positive charge over Li atoms, hydrogen atoms manage to stay almost neutrally charged.
Molecular dynamics To study the desorption phenomena, Born-Oppenheimer molecular dynamics studies are performed in DMol3 package implying the PBE functional of Generalized Gradient Approximation (GGA). Saturated complex (PCP11e2Lie8H2) is subjected to the MD simulation at 373 K and 473 K. This is done to understand the effect of temperature on the complexes and the adsorbed H2 molecules over these complexes. Fig. 9 shows the snapshots of PCP11e2Lie8H2 at different time-steps when subjected to a temperature of 373 K. In
Fig. 8 e Average CHELPG charges over each atom in Li functionalized PCP11 and its hydrogen adsorbed derivatives.
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Fig. 9 e Snapshots of MD simulation at 373 K depicting the effect of temperature of PCP11e2Lie8H2 at (I) 1 ps (II) 2 ps, (III) 3 ps, (IV) 4 ps, and (V) 5 ps.
Fig. 10 e Time dependent variation in potential energy on PCP11e2Lie8H2 at 373 K and 473 K.
Fig. 9(I) it can be seen that the hydrogen molecules start moving away from the complex and as the time passes, hydrogen molecules move away from the vicinity of Li atoms. At 5 ps (Fig. 9(V)), it can be seen that most of the H2 molecules leave the complex and the PCP11e2Li complex does not collapse. The complex stays stable after all H2 molecules leave the complex. This signifies the on-board reversibility of this, while the simulations done at higher temperature emphasize the stability of this complex at higher temperatures. Fig. 10 elaborates the effect of temperature on the potential energy of the complex. Desorption pattern varies as the temperature is changed. With an increase in the temperature, overall molecular movement in the structure increases. At higher temperatures, the hydrogen molecules are desorbed earlier than that of a lower temperature. These results are crucial to further investigate the on-board reversible storage capacity of these complexes as the temperature variation results into the desorption of the hydrogen molecules keeping the host complex stable.
Conclusions We explored the hydrogen sorption capacity of PCP11 functionalized with Li metal. DFT calculations are performed with M06 exchange-correlation functional and 6-311G(d,p) basis set. It is found that PCP11e2Li adsorbs hydrogen in molecular form and the hydrogen weight percentage is found to be 13.42 %. The trend of adsorption and desorption energies shows that the adsorption is reversible. All positive vibrational frequencies for these complexes confirm the stability of these complexes. CHELPG analysis confirms the transfer and back-transfer of charges from the delocalized
peelectrons and adsorbed dihydrogen complexes with Li atoms acting as a mediator. Dewar-like mechanism and charge polarization mechanism are involved in the Li functionalization and the hydrogen adsorption over PCP11. HOMO-LUMO gap in the Li functionalized hydrogen saturated PCP11 complex is found to be high which confirms its absolute hardness and less reactivity. MD simulation studies confirm that the host reversibly adsorbs hydrogen molecules and stays stable at higher temperatures. Our findings prove that Li functionalized PCP11 is a prospective hydrogen storage candidate.
Acknowledgment Authors thank IIT Ropar for funding and high performance computing cluster facility. Financially support from the Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR Grant 01(2782)14/EMR-II) is highly appreciated.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.009.
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Reversible hydrogen adsorption in Li functionalized [1,1]paracyclophane Rohit Y. Sathe, T.J. Dhilip Kumar* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, 140001, India
highlights
graphical abstract
Hydrogen adsorption mechanism in
Li
functionalized
[1,1]
paracyclophane. Reversible hydrogen adsorption of 8 H2/complex with 13.42 H wt%. Charge polarization mechanism of hydrogen adsorption. BOMD
simulations
prove
the
reversibility and thermal stability of the host. Host is a prospective hydrogen storage candidate fulfilling the DOE targets.
article info
abstract
Article history:
Hydrogen is a good alternative to replace fossil fuels in automobiles. Storage of hydrogen
Received 3 January 2020
for vehicular applications with high gravimetric density is a challenging task. The
Received in revised form
hydrogen sorption capacity of [1,1]paracyclophane functionalized with Li is investigated
12 February 2020
using density functional theory. Li functionalized [1,1]paracyclophane physisorbs 8 H2
Accepted 1 March 2020
achieving the maximum hydrogen weight percentage up to 13.42 %. All positive vibrational
Available online 1 April 2020
frequencies and a significant difference in the energy of frontier molecular orbitals confirm the stability and high absolute hardness of the host. Molecular dynamics simulations prove
Keywords:
the thermal stability and reversibility of hydrogen adsorption over Li functionalized [1,1]
Hydrogen adsorption
paracyclophane implying the ease of on-board reversible hydrogen storage. Our findings
Density functional theory
confirm that Li decorated [1,1]paracylophane is a good hydrogen storage material meeting
CHELPG analysis
the 2020 targets of DOE.
Born-oppenheimer molecular
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
dynamics [1,1]paracyclophane
* Corresponding author. E-mail address: [email protected] (T.J. Dhilip Kumar). https://doi.org/10.1016/j.ijhydene.2020.03.009 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 2 9 4 0 e1 2 9 4 8
Introduction The use of fossil fuels to run automobiles is increasing at an alarming rate all over the world. This is not only diminishing the exhaustible fuel sources but also contributing to the increased rates of worldwide air pollution [1e3]. Extensive research is going on to find an alternative fuel source for automobiles. Hydrogen, being the most abundant element in the universe and a good energy carrier, is a better option to replace fossil fuels. Hydrogen has a higher energy density (33 kW h kg1) than traditional fossil fuels. Hydrogen fuel cells produce water as a byproduct, therefore hydrogen-fueled vehicles are eco-friendly [1,4,5]. The main obstacle in using hydrogen as a vehicular fuel is the storage of hydrogen with high gravimetric or volumetric density. Hydrogen can be stored by physical or chemical modes for vehicular applications [1,6]. Physical modes include highpressure vessels to store hydrogen in the form of compressed gas and cryo-compressed storage, or at a very low temperature known as cryogenic storage. These methods include high processing cost and use of costly material like carbon fiber reinforced plastics. These techniques are not economical and safe as the hydrogen stored under extreme pressure poses the threat of explosion [7e10]. Chemical modes like storing hydrogen in metal hydrides, chemical hydrides, metal organic frameworks [11], BN based systems [12], conjugated microporous polymers [13e15], covalent organic frameworks [16e18], etc. are widely investigated. Many recent investigations on hydrogen storage properties of carbon nanostructures like hydrogen storage capacities of twodimensional carbon nitride [19] and defect-engineered C4N nanosheets [20] report the improved hydrogen weight percentage and reversibility under ambient conditions. Department of Energy, USA has specified a target for hydrogen storage materials having at least 7.5 % hydrogen weight percentage along with reversible thermokinetics below 100 atm pressure in the temperature range of 40 to 85 C to be achieved till year 2020 [21e23]. In this study [1,1]paracyclophane (PCP11) functionalized with Li metal are screened for their hydrogen storage capacity. Paracyclophanes contain a wide range of molecules, and they are named according to the arene substitution pattern [24,25]. “[1,1]” in [1,1]paracycplophane denotes that there is a single (-CH2-) moiety in each bridge linking the benzene rings in the paracyclophane. This bridge is so short that it puts a strain in the benzene rings due to which benzene rings in PCP11 lose the planarity [24]. This strain is also utilized for metal functionalization. Li is functionalized over each benzene ring of PCP11 because it is a lightweight alkali metal. Earlier reports of hydrogen storage systems functionalized with Li show that each Li atom can adsorb up to a maximum of three hydrogen molecules [26e32]. Reports based Li functionalized carbonbased nanostructures such as nanotubes [33e35], nanosheets, graphene [36,37], graphyne [38], graphdiyne [39], etc. show enhanced hydrogen weight percentage. Liu et al. in their study of Li dispersed nanotubes report the hydrogen uptake up to 13.45 % [33], while the experimental findings discussing the hydrogen adsorption over alkali-doped carbon nanotubes are further presented by Yang [34]. Wu et al. in their extensive
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ab-initio investigation, discuss the enhanced hydrogen adsorption over Li functionalized boron carbide nanotubes [35]. Their findings report hydrogen mass density up to 6.2 %. Ataca et al. report the maximum hydrogen weight percentage of 12.8 % over Li covered graphene sheet [36]. While the improved hydrogen weight percentage of 16 wt% has been reported by Zhou et al. [37]. Working on other allotropes of Li functionalized carbon like graphyne and graphdiyne, Sathe et al. [38] and Panigrahi et al. [39] have reported the hydrogen weight percentage up to 8.69 and 6.5 %, respectively. We also tried to functionalize Sc atoms over PCP11 but due to stronger binding energy of Sc over benzene and already available strain over benzene rings due to shorter eCH2- chains, Sc did not get functionalized over PCP11. Functionalized Li acts as an open metal site for hydrogen adsorption. An interaction between the delocalized p-electrons over the benzene rings of PCP11 and Li atoms results in Li functionalization. Li atoms interact with p-electrons of benzene ring without cluster formation. Employing the DFT calculations, the structural stability and hydrogen adsorption mechanism in Li functionalized PCP11 is studied. Our findings prove that each Li atom reversibly adsorbs 4 hydrogen molecules. We present the details of computation in section Computational details, results are thoroughly discussed in details in section Results and discussion. Findings of the study are concluded in section Conclusions.
Computational details The calculations are carried out using the Gaussian 09 [40] package with Minnesota 06 (M06) exchange-correlation functional [41] and 6-311G(d,p) basis set. Minnesota 06 is parameterized for non-covalent interactions, organometallics, and metal thermochemistry. As we have functionalized PCP11 with Li metal and Li exhibits charge polarization [41e44] during functionalization and hydrogen adsorption, M06 is used to study these weak interactions. Additionally, there are many recent ab-initio investigations of hydrogen storage properties of chemically functionalized hosts using Minnesota 06 functional [45e52]. Before metalatoms are functionalized, the aromaticity of PCP11 is checked with Nucleus Independent Chemical Shift (NICS) calculations [40]. At a distance of 1 A from the center of the benzene rings, NICS is found to be a negative maximum (8.6 ppm) [53,54]. Therefore, metal atoms are functionalized at this distance. We checked the energy of binding for the metal atoms with the PCP11. Following equation is used to calculate the binding energy, 1 Ebi ¼ ½EPCP112Li ð2ELi þ EPCP11 Þ 2
(1)
where Ebi is the binding energy, EPCP112Li is the energy of Li functionalized PCP11, ELi is the energy of Li atom, EPCP11 is the energy of PCP11. Further, the hydrogen molecules are sequentially loaded on these metal functionalized complexes. For all the resulting complexes, the vibrational frequencies are checked to confirm the stability of these complexes. All vibrational frequencies
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were found to be positive. The adsorption and desorption energies are calculated with the following equations [3,55e59], Ead ¼
1 nEðH2 Þ þ EðPCP112LiÞ EðPCP112LinH2 Þ n
(2)
where Ead is adsorption energy, n denotes the number of adsorbed hydrogen molecules, the total energy of PCP11e2Li complex is given by EðPCP112LiÞ , EðH2 Þ is energy of the H2 molecule, the total energy of the hydrogen adsorbed PCP11e2Li is denoted by EðPCP112LinH2 Þ . Ede ¼
1 nEðH2 Þ þ E½PCP112LiðnH2 2Þ EðPCP112LinH2 Þ 2
(3)
where Ede is desorption energy and the total energy of a complex preceding to PCP11e2Li-nH2 complex is given by E½PCP112LiðnH2 2Þ . Study of structural changes before and after adsorption is performed. The difference between frontier orbitals is calculated to study the stability of complexes in terms of reactivity and absolute hardness [60e63]. Changes in total electron density are mapped to determine the potential site for adsorption. Electrostatic potential maps are calculated over the total density using full SCF and “CHarges from ELectrostatic Potentials” (CHELPG) charge analysis is performed to confirm the charge transfer mechanism. To analyze the effect of temperature on the hydrogen saturated derivative of Li functionalized PCP11, BornOppenheimer molecular dynamics (BOMD) is carried out. Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional of the generalized gradient approximation (GGA) is employed for these simulations. It renders improved results by considering the change in total density during the calculation of exchange-correlation energy. This study is performed using DMol3 package employing the NVT ensemble and double numerical basis sets with polarization function (DNP). DNP is all-electron numerical basis function. It is generated as values on an atomic-centered spherical-polar mesh where the calculation of the numerical part is performed by solving the atomic DFT equations. It includes a detype polarization function on heavy atoms with additional sets of detype polarization functions on all atoms. DNP is proven to be capable of minimizing or even eliminating basis set superposition error (BSSE) [64] and hence is useful for studying reaction coordinates and weak non-covalent interactions which are of prime concern while studying hydrogen adsorption mechanisms. For studying the orbital hybridization, the projected density of states (PDOS) analysis is performed. Vienna Ab-initio package (VASP) [65,66]is used along with PBE functional. In given PDOS analysis, Fermi-level is set at zero.
Fig. 1 e Structure of optimized PCP11 along with the bond distance between adjacent C atoms (A).
rings is due to the very short bridges connecting these two rings which consist of only single (-CH2-) moiety. This induces strain on benzene rings and their inherent planarity is lost. This strain plays a crucial role in Li functionalization. Li functionalization further induces the strain on the benzene rings. Li functionalized PCP11 complex is shown in Fig. 2. The binding energy (Ebi) of Li with PCP11 is found to be 1.33 eV. As mentioned earlier, the already available strain on the benzene rings of PCP11 results in the stronger binding with the metal. This is the reason that functionalization of Sc onto the benzene rings of PCP11 (Ebi ¼ 1.83 eV) imparts further strain over these rings and the host structure collapses. In PCP11, the ‘pore size’ or the distance between two eCH2- units in bridges is 4.771 A which on metal functionalization increases to 4.842 A notifying the stress induced by metal
Results and discussion Li functionalization Structure of PCP11 is optimized before functionalizing the metal atoms. Optimized structure of PCP11 is shown in Fig. 1. There are two benzene rings (left and right) in PCP11. Both of these rings are slightly bent inward. Non-planarity of these
Fig. 2 e Optimized structure of Li functionalized PCP11. Numbers specify the distance between the atoms (A).
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functionalization. This is also evident from the distance between C atoms in benzene rings which increases from 1.403 A to 1.447 A. The distance of functionalized Li atoms from the center of the benzene rings increases from 1 A to 1.75 A after optimization of the complex. This is due to the increased electron density in confined space close to the benzene rings. This helps in functionalization of metal atoms and induces further stress on the benzene rings. The energy gap between frontier molecular orbitals of PCP11 is found to be 1.92 eV. After functionalization of Li, this energy gap is reduced to 1.67 eV. High values of energy gap between frontier molecular orbitals specify that the PCP11 and PCP11e2Li complexes are kinetically stable [60,67,68]. Light alkali metal Li is functionalized over benzene rings due to charge polarization mechanism. Charge transfer between the benzene rings to the Li atoms results in the functionalization of Li over PCP11. To prove the mechanism of Li functionalization, we studied the projected density of states (PDOS) for PCP11 and PCP11e2Li. As shown in Fig. 3, the contribution of C(p) and Li(s) orbitals can be seen in Fig. 3(II) as compared to Fig. 3(I). This clearly supports the aforementioned charge polarization mechanism in forming the
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PCP11e2Li complex. Most of the orbital hybridization peaks are observed near Fermi-level, stating the stability of the complexes.
H2 physisorption A single H2 is adsorbed over each Li in each successive step of hydrogen adsorption. This gives the freedom to examine the stability of each of the resulting complex and trends of adsorption and desorption energies. Sequential hydrogen adsorption also avoids the complexity caused by steric hindrance due to crowding of hydrogen molecules and allows systematic evaluation of the charge transfer. Fig. 4 shows the steps of hydrogen sorption over PCP11e2Li. Charge polarization leads to the adsorption of hydrogen molecules over functionalized Li atoms [42,43]. As charge transfer from the Li atoms polarizes nearby hydrogen molecules and holds them in the close vicinity of the metal atoms [69e73]. PCP11e2Li complex reaches the hydrogen saturation limit after adsorption of four hydrogen molecules over each Li with a maximum hydrogen weight percentage of 13.42 %. To confirm the orbital hybridization in adsorption of hydrogen over each Li, we performed the PDOS analysis. Fig. 5 shows the projected density of state in PCP11e2Lie2H2. As seen in Fig. 5, orbital hybridization from H(s), C(p), and Li(s) orbitals is observed in near-Fermi-region in the range of 10 to 5 eV. This explains the involvement of charge polarization mechanism as a results of transfer of electron density between pecomplex, functionalized Li atoms, and adsorbed H2 molecules.
Adsorption and desorption energies Analysis of adsorption energy and desorption energy provides strong evidence about the reversibility of the system while studying hydrogen sorption systems [74]. It is a widely observed trend that during sequential loading of hydrogen molecules over the host, the desorption energy decreases [55,75e78]. This trend signifies the reversibility of the system as the desorption energy for the hydrogen saturated complex is less. The trend of sorption energies for Li functionalized PCP11 is shown in Fig. 6. For saturated complex in Li functionalized PCP11, the desorption energy is 0.065 eV. Sorption energies gradually decrease during the sequential adsorption process. This is because the hydrogen molecules are polarized to be held in a physisorbed position. The hydrogen molecules desorb from the metal functionalized PCP11 on slight changes in the thermodynamic parameters.
Structural changes
Fig. 3 e Projected density of states in (I) PCP11 and (II) PCP11e2Li.
Tracking the structural changes before and after the hydrogen adsorption helps to understand the adsorption phenomena [3,76,79]. As the number of adsorbed H2 increases over the host, the distance between metal atoms (Li-Ha) and adsorbed hydrogen molecules increases. In Table 1, the mean distance between center of the benzene ring and Li atoms is given by Rc-Li, Li-Ha specifies the mean distance between Li atom and adsorbed hydrogen molecules, and the mean distance
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Fig. 4 e Hydrogen sorption over PCP11e2Li. Here, (I) PCP11e2Li, (II) PCP11e2Lie2H2, (III) PCP11e2Lie4H2, (IV) PCP11e2Lie6H2, (V) PCP11e2Lie8H2. between H atoms in the adsorbed H2 is given by Ha-Ha. Due to the presence of charge polarization mechanism between Li and delocalized p-electrons of the benzene ring, the distance between the ring center and Li atoms stays below 1.79 A throughout the process. The difference in the energy between frontier orbitals (HOMO and LUMO) can be used to predict the strength and stability of metal complexes [67,80]. Lower chemical reactivity and higher kinetic stability are usually in the direct correlation with a large HOMOeLUMO gap. Large difference between the energies of these orbitals implies that the electrons in HOMO will require higher energy to jump from HOMO to LUMO. This signifies the chemical or absolute hardness of the complex [81,82]. Chemical hardness is the resistance of the system for changing electron density over the system. The absolute hardness (denoted as h) is the resistance given complex to undergo change the electron distribution of the system [68,81,83]; vm vN nðrÞ
In the aforementioned equation, h denotes absolute hardness, m is the electronic chemical potential, and N denotes the total number of electrons in the system. It can be read as; at a fixed potential nðrÞ, the absolute hardness is the change in the electronic chemical potential of a given complex with respect to the total number of electrons in the system [84].
h¼
(4)
Fig. 6 e Trends of adsorption energy and desorption energy in Li functionalized PCP11.
Table 1 e Structural changes in Li functionalized PCP11 and its hydrogen adsorbed derivatives. Name of the
Fig. 5 e Projected density of states in PCP11e2Lie2H2.
Complex PCP11e2Li PCP11e2Lie2H2 PCP11e2Lie4H2 PCP11e2Lie6H2 PCP11e2Lie8H2
Avg.
Avg.
Avg.
Rc-Li ( A) 1.707 1.734 1.744 1.751 1.782
Li-Ha ( A) e 2.168 2.202 2.283 2.377
Ha-Ha ( A) e 0.755 0.755 0.752 0.752
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Fig. 7 e Electrostatic potential maps for (I) PCP11, (II) PCP11e2Li, (III) PCP11e2Lie2H2, (IV) PCP11e2Lie4H2, (V) 3. PCP11e2Lie6H2, (VI) PCP11e2Lie8H2; Units: e/A
The calculated hardness for hydrogen saturated Li functionalized PCP11 is 3.27 eV and electrophilicity is 0.57 eV. These values specify the high stability of the PCP11e2Lie8H2. The highest HOMOeLUMO gap is predicted for hydrogen saturated complex. Saturated complex PCP11e2Lie8H2 has HOMO-LUMO gap of 1.9 eV which signifies that saturated complex is a stable complex in terms of chemical hardness.
Electrostatic potential maps and CHELPG charge analysis Electrostatic potential maps are used as guidelines to understand the transfer of electron density over the complex and to identify the preferred location where further hydrogen molecules can be adsorbed. Fig. 7 shows the electrostatic potential maps for PCP11, metal functionalized PCP11, and its hydrogen adsorbed derivatives. It can be seen in Fig. 7(I) that there is a region of high electron density on the top of both the benzene rings (can be seen at the side of the complex) and there is a region of moderately low electron density over the gap between two benzene rings (can be seen at the front side of the complex). The electron-dense region disappears on the functionalization of Li atoms which can be seen from Fig. 7(II). Electron density over Li functionalized PCP11 changes as hydrogen molecules are adsorbed over these complexes as evident from Fig. 7(III)e(VI). A region of slightly higher electron density develops near the benzene rings while a region with less electron density develops near the hydrogen adsorption site (at the side of the complexes). This specifies that the complex attains saturation in terms of hydrogen adsorption. CHarges from ELectrostatic Potentials using a Grid based method (CHELPG) is employed to calculate the charges over each atom during sequential hydrogen adsorption. In CHELPG atomic charge calculation, molecular electrostatic potential (MESP) is produced from the atomic charges at a number of points around the molecule. In Fig. 8 it can be seen that there is a significant variation in CHELPG charges over C and Li atoms during the whole process of adsorption. Fig. 8 shows the variation in the CHELPG charges over each atom in the system during the course of subsequent adsorption. It can be seen that charges over the hydrogen atoms do not vary much signifying a weak physisorption. Due to the charge transfer between lithium and carbon atoms, there is a significant fluctuation in the charges of these atoms. It can be seen that C and Li atoms in the system have the transferable charge left even after the adsorption of 3rd hydrogen molecules over each Li atom. This explains the adsorption of 4th hydrogen molecule over each Li atom. The reason for the enhanced adsorption of hydrogen molecules is the
continuous charge transfer processes involved in the Dewarlike mechanism and charge polarization mechanism, as well as the ease of transfer of electron density between delocalized peelectrons and dihydrogen complexes through light alkali metal Li. Thus, Li enhances the sorption capacity of PCP11. Fig. 8 supports the above explanation of charge transfer from peelectrons to hydrogen molecules with Li acting as a mediator. Upon functionalization onto the PCP11 complex, Li atoms gain a positive charge, as electron density first get transferred to the partially filled seorbital of Li and then gets back-transferred to the benzene p-complex. As the hydrogen saturation is achieved, the charges initially try to get neutralized and further achieve a slight positive charge. With average negative charge values over carbon atoms and positive charge over Li atoms, hydrogen atoms manage to stay almost neutrally charged.
Molecular dynamics To study the desorption phenomena, Born-Oppenheimer molecular dynamics studies are performed in DMol3 package implying the PBE functional of Generalized Gradient Approximation (GGA). Saturated complex (PCP11e2Lie8H2) is subjected to the MD simulation at 373 K and 473 K. This is done to understand the effect of temperature on the complexes and the adsorbed H2 molecules over these complexes. Fig. 9 shows the snapshots of PCP11e2Lie8H2 at different time-steps when subjected to a temperature of 373 K. In
Fig. 8 e Average CHELPG charges over each atom in Li functionalized PCP11 and its hydrogen adsorbed derivatives.
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Fig. 9 e Snapshots of MD simulation at 373 K depicting the effect of temperature of PCP11e2Lie8H2 at (I) 1 ps (II) 2 ps, (III) 3 ps, (IV) 4 ps, and (V) 5 ps.
Fig. 10 e Time dependent variation in potential energy on PCP11e2Lie8H2 at 373 K and 473 K.
Fig. 9(I) it can be seen that the hydrogen molecules start moving away from the complex and as the time passes, hydrogen molecules move away from the vicinity of Li atoms. At 5 ps (Fig. 9(V)), it can be seen that most of the H2 molecules leave the complex and the PCP11e2Li complex does not collapse. The complex stays stable after all H2 molecules leave the complex. This signifies the on-board reversibility of this, while the simulations done at higher temperature emphasize the stability of this complex at higher temperatures. Fig. 10 elaborates the effect of temperature on the potential energy of the complex. Desorption pattern varies as the temperature is changed. With an increase in the temperature, overall molecular movement in the structure increases. At higher temperatures, the hydrogen molecules are desorbed earlier than that of a lower temperature. These results are crucial to further investigate the on-board reversible storage capacity of these complexes as the temperature variation results into the desorption of the hydrogen molecules keeping the host complex stable.
Conclusions We explored the hydrogen sorption capacity of PCP11 functionalized with Li metal. DFT calculations are performed with M06 exchange-correlation functional and 6-311G(d,p) basis set. It is found that PCP11e2Li adsorbs hydrogen in molecular form and the hydrogen weight percentage is found to be 13.42 %. The trend of adsorption and desorption energies shows that the adsorption is reversible. All positive vibrational frequencies for these complexes confirm the stability of these complexes. CHELPG analysis confirms the transfer and back-transfer of charges from the delocalized
peelectrons and adsorbed dihydrogen complexes with Li atoms acting as a mediator. Dewar-like mechanism and charge polarization mechanism are involved in the Li functionalization and the hydrogen adsorption over PCP11. HOMO-LUMO gap in the Li functionalized hydrogen saturated PCP11 complex is found to be high which confirms its absolute hardness and less reactivity. MD simulation studies confirm that the host reversibly adsorbs hydrogen molecules and stays stable at higher temperatures. Our findings prove that Li functionalized PCP11 is a prospective hydrogen storage candidate.
Acknowledgment Authors thank IIT Ropar for funding and high performance computing cluster facility. Financially support from the Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR Grant 01(2782)14/EMR-II) is highly appreciated.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.009.
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