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Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene
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 x x x ( 2 0 1 7 ) 1 e7
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Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene Vijayanand Kalamse a,c, Priyanka Tavhare b, Amol Deshmukh c, Rahul Krishna d, Elby Titus d, Ajay Chaudhari b,* a
Dept. of Physics, Science College, Nanded, 431605, India Department of Physics, The Institute of Science, FORT, Mumbai, 400 032, India c School of Physical Sciences, S. R. T. M. University, Nanded, 431606, India d Center for Automation and Technology (TEMA), Department of Mechanical Engineering, University of Aveiro, Portugal b
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abstract
Article history:
Interaction of molecular hydrogen with Li and Ti doped boron substituted naphthalene viz.
Received 16 November 2016
C6B4H8Ti2 and C6B4H8Li2 has been studied using density functional theory (DFT) method.
Received in revised form
The C6B4H8Li2 complex can interact with maximum of four hydrogen molecules, whereas
13 February 2017
three H2 molecules are adsorbed on C10H8Li2 complex. The C6B4H8Ti2 complex can interact
Accepted 15 February 2017
with maximum of eight hydrogen molecules. The gravimetric hydrogen uptake capacity of
Available online xxx
C6B4H8Ti2 and C6B4H8Li2 complex is found to be 6.85 and 5.55 wt % respectively, which is higher than that of unsubstituted C10H8Ti2 and C10H8Li2 complexes. The boron substitution
Keywords:
has significantly affected the hydrogen adsorption energies. The H2 adsorption energy and
Boron substituted naphthalene
Gibb's free energy corrected H2 adsorption energy are found to be more prominent after
Hydrogen storage
boron substitution. The C6B4H8Ti2 and C6B4H8Li2 complexes are more stable than the
Desorption temperature
respective unsubstituted C10H8Ti2 and C10H8Li2 complexes due to their higher binding
ADMP molecular dynamics
energies. According to the atom-centered density matrix propagation (ADMP) molecular dynamics simulations C6B4H8Li2 complex retain not a single adsorbed hydrogen molecule during the simulation at room temperature, whereas five hydrogen molecules at 300 K and eight at 100 K are remain absorbed on C6B4H8Ti2 complex. The C6B4H8Ti2 complex is found to be more promising material for hydrogen storage than C10B4H8Li2. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Despite several positive aspects of hydrogen (the energy carrier) as a fuel over hydrocarbon fuels, researchers are yet to succeed to solve its storage problem. To materialize the hydrogen economy [1] and make hydrogen as a fuel for the future. [2,3], its storage problem need to be overcome. Though
it is not simple [4], one major breakthrough in finding hydrogen storage material will not only solve the global energy crisis but also help in preserving conventional energy sources. It will make every nation self energy equipped. Traditional ways of storing hydrogen cannot be implemented in automotive fuel cells or directly in mobile applications. The flaws in the traditional ways of storing hydrogen and its use in mobile application have given a new challenge to the
* Corresponding author. E-mail address: [email protected] (A. Chaudhari). http://dx.doi.org/10.1016/j.ijhydene.2017.02.107 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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researchers to store hydrogen in some other ways. Solid state hydrogen storage [5] is one of the options to solve this problem. The important aspect in solid state storage of hydrogen is about the reversible adsorption and desorption process of hydrogen from the host material. Chemical hydrides and metal hydrides can be the best readily available options [6e10]. The problem related to hydrides is that detaching the atomic hydrogen from hydride is itself an energy consuming process. Moreover, hydrogen stored in molecular form is required in hydrogen powered fuel cells. So the efforts are directed in finding a material that can store maximum hydrogen in molecular form with favorable thermodynamic conditions for adsorption and desorption. The hydrogen uptake capacity of a material should also meet the target set by the department of energy (DOE) [11] for vehicular applications. Varieties of carbon based as well as inorganic materials have been tested for the hydrogen storage purpose. It has been observed that doping of metal atoms in these materials has significantly enhanced the hydrogen uptake capacity [12e16]. Recently doping of metal atoms in organic compounds and their hydrogen storage properties have been studied theoretically [17,18]. Moreover, these metal doped organic compounds can be used as linkers in metal organic frameworks which are also among the promising materials for hydrogen storage [19]. Previous theoretical reports have shown that boron substitution may tune up different properties related to the hydrogen storage [20]. Mono and di-boron substituted benzene ring functionalized with alkali as well as alkaline earth metals have been studied for the hydrogen storage purpose theoretically [21]. Multi-functionalized complexes have been tested for hydrogen storage [22,23]. Hydrogen uptake capacity of benzene is found to be increased after boron substitution [24]. Here we have studied the interaction of molecular hydrogen with Li and Ti doped boron substituted naphthalene viz. C6B4H8Ti2 and C6B4H8Li2 using density functional theory (DFT) method. We have reported different structural aspect as well as the nature of bonding in these newly modeled complexes. Different properties like hydrogen uptake capacity, hydrogen adsorption energies, temperature dependence of adsorption energies etc. of multi-functionalized boron substituted naphthalene have been studied. The stability and reactivity of these complexes have been explored by means of HOMO-LUMO gap and binding energies. The ADMP molecular dynamics simulations have also been performed at different temperatures to verify the hydrogen uptake capacity.
Computational details All the geometries are optimized using the 1997 hybrid functional of Perdew, Burke and Ernzerhof (PBE) [25] in combination with split valence 6-31G(d,p) basis set. The molecular dynamics (MD) simulations have been carried out using atom centered density matrix propagation (ADMP) [26]. The time step (Dt) of ADMPeMD was set at 0.2 fs and 0.1 fs for Li and Ti doped complex respectively. The temperature was maintained using the velocity scaling method during the MD simulations. All the calculations are performed using the Gaussian suite of programs [27].
The averaged adsorption energy without zero point energy correction (DE) is calculated as
DE ¼ {E[BSOM] þ (n E[H2])E[BSOM(nH2)]}/n
(1)
where E[X] is the total energy of X without zero point energy correction and BSOM (Boron Substituted Organometallic Complex) is either C6B4H8Li2 or C6B4H8Ti2. The averaged adsorption energy with zero point energy correction (DEZPE) is calculated as
DEZPE ¼ {EZPE[BSOM] þ (n EZPE[H2])EZPE[BSOM(nH2)]}/n
(2)
where EZPE[X] is the total energy of X with zero point energy correction. Similarly averaged adsorption energy with Gibbs free energy correction (DEG) is calculated as
DEG ¼ {EG[BSOM] þ (n EG[H2])EG[BSOM(nH2)]}/n
(3)
where EG[X] stands for the total energy of X with Gibbs free energy correction.
Results and discussion Structural analysis In this work, we have first substituted four boron atoms in place of carbon of naphthalene ring and optimized that geometry using PBEPBE/6-31G(D,P) method. As a consequence of boron substitution the ultimate molecular weight of the host material is slightly reduced. The optimized geometry of C6B4H8 can be seen in Fig. 1(a). Then we have doped C6B4H8 complex with two Li and two Ti atoms, one on each ring as shown in Fig. 1(b) and Fig. 2(a) respectively. Different bond lengths in C6B4H8Li2 and C6B4H8Ti2 before and after hydrogen adsorption are given in Table 1. The distance between two carbon atoms at the fusion of two rings viz. 2 Ce4C in bare C6B4H8 and C10H8 complex is 1.41 A and 1.44 A respectively. The distance 1Ce3C as well as 6Ce5C (which are terminal carbons) is 1.37 Å in the former and 0.05 A shorter than the latter. The average distance of any boron atom from its adjacent carbon is 1.56 A in C6B4H8, C6B4H8Li2 and C6B4H8Li2(4H2) complex. The 2Ce4C bond at the fusion in A and C6B4H8Li2 and C6B4H8Ti2 complex is elongated by 0.07 0.20 A respectively than that in the C6B4H8. But when maximum hydrogen molecules are interacted with C6B4H8Ti2 complex this bond (2Ce4C) gets shortened significantly by 0.17 A. There is no major change found in Ce Li, BeLi and LieLi bond lengths in C6B4H8Li2 complex even after interaction with four hydrogen molecules. But CeTi, Be Ti and TieTi bond lengths considerably elongated in C6B4H8Ti2(8H2) complex than that in C6B4H8Ti2 complex. All the hydrogen molecules are at average 2.30 A from corresponding Li atom in C6B4H8Li2(4H2) complex. In C6B4H8Ti2(8H2) complex, except two hydrogen molecules, rest six are at less than 2 A from corresponding Ti atom.
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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 x x x ( 2 0 1 7 ) 1 e7
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Fig. 1 e Optimized geometry of (a) C6B4H8 (b) C6B4H8Li2 and (c) C6B4H8Li2(4H2) complex.
Fig. 2 e Optimized geometry of (a) C6B4H8Ti2 and (b) C6B4H8Ti2(8H2) complex.
Gravimetric uptake capacity The keen interest is lying in the gravimetric uptake capacity of these two complexes with favorable thermodynamic conditions. It has been found that C6B4H8Li2 and C6B4H8Ti2 complexes interact with maximum of four and eight hydrogen molecules respectively with respective hydrogen uptake capacity of 5.55 and 6.85 wt %. This gravimetric uptake capacity is higher than that of unsubstituted C10H8Li2 (3.73 wt %) and C10H8Ti2 (6.72 wt %) complex. The H2 uptake capacity of C6B4H8Li2 and C6B4H8Ti2 complexes is up to the benchmark set by US DOE for vehicular applications. Thus substitution of boron has enhanced the uptake capacity in functionalized
naphthalene. We have also tried to study the interaction of nine and ten hydrogen molecules with C6B4H8Ti2 complex. It has been found that they are moving away from C6B4H8Ti2 complex in the optimization process and hence could not be adsorbed.
Adsorption energy calculations We have calculated the hydrogen adsorption energies for C6B4H8Li2(4H2) and C6B4H8Ti2(8H2) complexes which are given in Table 2. It is clear that the zero point energy correction to the adsorption energies cannot be ignored. Since the average LieH2 distance is 2.30 A, we have very weak hydrogen adsorption
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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in a C6B4H8, b C6B4H8Li2, c C6B4H8Ti2, d C6B4H8Li2(4H2) and e C6B4H8Ti2(8H2) complex. The Table 1 e Different bond lengths (A) labels assigned as a superscript to atoms are for the sake of reader's convenience. Assignments Bond length ( A) Assignments Bond length ( A) 1
Ce3C 2 Ce4C 5 Ce6C 3 Ce17B 1 Ce15B 4 Ce17B 2 Ce15B 2 Ce16B 4 Ce18B 5 Ce16B 6 Ce18B 1 Ce19Ti/20Li 3 Ce19Ti/20Li 4 Ce19Ti/20Li 2 Ce19Ti/20Li 15 Be19Ti/20Li 17 Be19Ti/20Li 2 Ce20Ti/19Li
a
b
c
d
e
1.37 1.41 1.37 1.55 1.55 1.57 1.57 1.57 1.57 1.55 1.55 e e e e e e e
1.39 1.48 1.39 1.57 1.57 1.55 1.55 1.55 1.55 1.57 1.57 2.24 1.24 2.13 2.13 2.23 2.23 2.13
1.47 1.61 1.47 1.50 1.50 1.58 1.58 1.58 1.58 1.50 1.50 2.23 2.23 2.11 2.11 2.33 2.33 2.11
1.39 1.47 1.39 1.56 1.56 1.55 1.55 1.55 1.55 1.56 1.56 2.26 2.26 2.14 2.15 2.25 2.24 2.15
1.42 1.47 1.43 1.54 1.52 1.56 1.58 1.58 1.57 1.52 1.53 2.43 2.37 2.35 2.28 2.39 2.44 2.30
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Ce20Ti/19Li 5 Ce20Ti/19Li 6 C_20Ti/19Li 16 Be20Ti/19Li 18 Be20Ti/19Li 19 Ti/Lie20Ti/Li 20 Lie2324H 20 Lie2122H 19 Lie2526H 19 Lie2728H 20 Tie2122H 20 Tie2324H 20 Tie25H 20 Tie26H 20 Tie2728H 19 Tie2930H 19 Tie3132H 19 Tie3334H 19 Tie3536H
a
b
c
d
e
e e e e e e e e e e e e e e e e e e e
2.13 2.24 2.24 2.23 2.23 3.24 e e e e e e e e e e e e e
2.11 2.23 2.23 2.33 2.33 2.66 e e e e e e e e e e e e e
2.14 2.26 2.26 2.25 2.24 3.21 2.31 2.30 2.31 2.30 e e e e e e e e e
2.36 2.37 2.36 2.39 2.42 2.92 e e e e 2.01 1.87 1.85 1.75 1.91 1.84 1.92 2.11 1.90
energies for the C6B4H8Li2(4H2) complex. Also the negative value of DEG (0.18 eV) for the C6B4H8Li2(4H2) complex indicates that the formation of this complex is endothermic. On the counterpart, the DE and DEZPE values for the C6B4H8Ti2(8H2) complex are 0.61 and 0.43 eV respectively and are well within the range of physisorption and chemisorption. This is one of the important requirements for ideal hydrogen storage. Also the positive value of DEG implies that the complex formation is thermodynamically favorable. It can be observed from Fig. 3 that the DEG values become more positive as temperature decreases. Moreover, the C6B4H8Ti2 complex can adsorb hydrogen molecules with positive DEG values over a wide range of temperature. The adsorbed hydrogen molecules on C6B4H8Ti2 complex can be easily desorbed by raising the temperature. The C10H8Li2(3H2) and C6B4H8Li2(4H2) have shown negative DEG values even at very low temperature scale.
Nature of bonding and binding energies The calculated binding energy for C6B4H8Li2 and C6B4H8Ti2 complex is 6.16 eV and 13.87 eV respectively, which indicates stronger binding of transition metal atom than an alkali metal atom to C6B4H8. Moreover, the calculated binding energy for C10H8Li2 and C10H8Ti2 complex is 1.94 eV and 10.30 eV respectively. This clearly indicates that the boron substitution
Table 2 e Calculated averaged adsorption energies per H2 molecule without and with zero point energy correction (DE and DEZPE) and that with Gibbs free energy correction (DEG) in eV at 298.15 K for C6B4H8Li2(4H2), C6B4H8Ti2(8H2) complexes. Complex
DE
DEZPE
DEG
C6B4H8Li2(4H2) C6B4H8Ti2(8H2) C10H8Ti2(8H2)
0.10 0.61 0.41
0.03 0.43 0.25
0.18 0.13 0.05
Fig. 3 e Temperature dependence of Gibbs free energy corrected H2 adsorption energy (DEG) values for C6B4H8Li2(4H2) and C6B4H8Ti2(8H2).
has increased the binding energies of multi-functionalized naphthalene complexes. Upon boron substitution, the Li and Ti atoms are bonded strongly to C6B4H8 complex than the unsubstituted material viz. C10H8. Natural electronic configuration reveals that in the formation of C6B4H8Li2 complex, the 2s orbital of both the Li atoms loses about 0.95 electrons to the 2p orbital of its own as well as to 2p orbital of boron atoms. When C6B4H8Li2 complex interacts with four hydrogen molecules, very few electrons are transferred between hydrogen molecules and Li atom. This is also confirmed from the longer LieH2 distances (See Table 1) and results in the lower adsorption energy for this complex. On the other hand, in C6B4H8Ti2 complex formation, the 4s orbital of each Ti atom loses 0.34 electrons to the 2p
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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orbital of boron. Due to this significant charge transfer, Ti binds strongly to the base and the complex has higher binding energy. On interaction of C6B4H8Ti2 complex with maximum hydrogen molecules, the 3d and 3p orbitals of Ti gain 0.33 and 0.58 electrons respectively.
Stability of the complexes The kinetic stability of C6B4H8Li2 and C6B4H8Ti2 complexes with successive addition of hydrogen molecules has been tested by means of a gap between highest occupied molecular orbital and lowest unoccupied molecular orbital i.e. HOMOLUMO gap. It is clear from Fig. 4 that the gap is almost constant in case of C6B4H8Li2 complex even after adsorption of hydrogen molecules. This indicates that C6B4H8Li2 has lesser tendency to interact with hydrogen molecules, which is also quite clear from its poor hydrogen adsorption energy. In case of C6B4H8Ti2, the gap increases with successive addition of H2 molecules and its maximum value is 1.83 eV for C6B4H8Ti2(8H2) complex. Larger the gap greater is the stability of a complex and hence less tendency of further reactivity. The HOMO-LUMO gap of C6B4H8Ti2 is lower than that for the C10H8Ti2 complex. It is found that the HOMO-LUMO gap for the C10H8Ti2 complex is slightly increased when first two hydrogen molecules are added. A sudden increase in the gap for the C10H8Ti2(4H2) and C10H8Ti2(6H2) complex is observed. This is due to the fact that in C10H8Ti2(nH2) complexes for n ¼ 2, one of the H2 molecules is adsorbed in dissociated (atomic) form, whereas for n ¼ 4 and n ¼ 6 two hydrogen molecules are adsorbed in diatomic form. The gap for the C10H8Ti2(nH2) complex in case of n ¼ 8 is found to be slightly decreased. This is due to the fact that all the H2 molecules for this complex are adsorbed in molecular form and no hydrogen molecule is dissociated. In case of C6B4H8Ti2(nH2) (where n ¼ 2,4,6,8) complexes, one of the hydrogen molecules is adsorbed in dissociated form in every complex. The dissociation of hydrogen molecules is
Fig. 4 e Energy gap between highest occupied and lowest unoccupied molecular orbital for C6B4H8Li2(nH2), C6B4H8Ti2(nH2) and C10H8Ti2(nH2) complexes with successive addition of H2 molecules.
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affecting the HOMO-LUMO gap. As far as C10H8Ti2(nH2) and C6B4H8Ti2(nH2) complexes are concerned, for n ¼ 4 and 6, two hydrogen molecules are dissociated in the former and one in the latter so the gap is higher for the former than the latter. Also, because no hydrogen molecule is dissociated in C10H8Ti2(8H2) complex, the gap is decreased whereas one of the hydrogen molecules is adsorbed in diatomic form in C6B4H8Ti2(8H2), so the gap is higher. All the complexes studied here have no imaginary vibrational frequency indicating that they are quantum mechanically stable structures.
ADMP-MD simulations The ADMP-MD simulations have been performed at 300 K and 100 K using the same method so as to compare the results that obtained from electronic structure calculations. Fig. 5 shows the trajectories of H2 molecules from the Li atom in C6B4H8Li2(4H2) complex at 300 K. It is very clear from Fig. 5 that not a single H2 molecule remain adsorbed on C6B4H8Li2 complex at 300 K. The weak interaction of hydrogen molecule with C6B4H8Li2 complex confirms the lower adsorption energies obtained for C6B4H8Li2(4H2) complex. The similar kind of trajectories are also obtained for the C10H8Li2(3H2) complex at 300 K. As can be seen from Fig. 6(a), during the simulations at 100 K, two H2 molecules have been desorbed from the host material and the distance is more than 5 A. The remaining A from respective Li two H2 molecules oscillate at about 2 atoms as shown in Fig. 6(b). The trajectories of H2 molecules from Ti atoms at 300 K and 100 K are shown in Fig. 7. These trajectories for C6B4H8Ti2(8H2) complex have shown that out of eight adsorbed hydrogen molecules, three have been desorbed from the C6B4H8Ti2 complex at room temperature. Out of the three desorbed molecules, one hydrogen molecule has started to move away from the metal atom at about 0.4 ps. So, according to ADMP-MD simulations, C6B4H8Ti2 complex can bind up to five hydrogen molecules at room temperature. This corresponds to its gravimetric H2 uptake of 4.40 wt %. The adsorption energy at 100 K is higher than that
Fig. 5 e Trajectories of time evolution of distance of H2 molecules from Li atom in C6B4H8Li2(4H2) complex at 300 K.
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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Fig. 6 e Trajectories of time evolution of distance of H2 molecules from Li atom in C6B4H8Li2(4H2) complex at 100 K.
at 298.15 K for the C10H8Ti2(8H2) complex. It indicates that the adsorption of H2 molecules at lower temperature is stronger with the host material. This is also confirmed when we performed ADMP-MD simulations at 100 K. It is found that all the eight hydrogen molecules are remain adsorbed at 100 K on C6B4H8Ti2 complex. These adsorbed hydrogen molecules can be desorbed by raising the temperature. Finally, we have calculated the desorption temperature for all the studied complexes here using the van't Hoff equation
TD ¼ (DEZPE/KB) (DS/Rln P)1
(4)
where Boltzmann constant KB ¼ 1.38 1023 J/K, the gas constant R ¼ 8.31 J/K Mol and the equilibrium pressure P ¼ 1 atm. We have used the value of DS from Ref. [28] and the zero point energy corrected adsorption energies of maximum H2 adsorbed complexes to calculate the desorption temperature. From the formula it is obvious that desorption temperature is in proportion to DEZPE and rest of the terms are merely constants. The approximate desorption temperature calculated for C6B4H8Li2(4H2), C6B4H8Ti2(8H2) and C10H8Ti2(8H2) complexes is 38.25 K, 777.83 K and 522.81 K respectively.
Conclusions The hydrogen uptake capacity of boron substituted functionalized naphthalene C6B4H8Ti2 and C6B4H8Li2 complex is found to be 6.85 and 5.55 wt % respectively using the DFT method. The boron substitution in C10H8Ti2 and C10H8Li2 complex has improved the hydrogen adsorption energies. Though C10H8Li2 complex after boron substation viz. C6B4H8Li2 interacts with four hydrogen molecules, the interaction is weaker compared to C6B4H8Ti2. The ADMP-MD simulations also reveal that the C6B4H8Li2 complex is inefficient for hydrogen storage. The ADMP-MD simulation results have shown that all the eight adsorbed hydrogen molecules remain adsorbed on C6B4H8Ti2 complex during the simulations at 100 K and retain its uptake capacity of 6.85 wt.% obtained by electronic structure calculations. At room temperature, the uptake capacity of C6B4H8Ti2 complex is 4.40 wt % according to ADMP-MD results. The binding energy of C6B4H8Li2/C6B4H8Ti2 complex is found to be larger than the respective unsubstituted C10H8Li2/C10H8Ti2 complex so that the degradation of a complex is difficult in the former than the latter. The transition metal doped complex is very sound material in all aspect than the alkali metal doped complex for hydrogen storage.
Fig. 7 e Trajectories of time evolution of distance of H2 molecules from Ti atom in C6B4H8Ti2(8H2) at (a) 300 K and (b) 100 K. Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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Acknowledgements Thanks to the S. R. T. M. University, Nanded, The Institute of science, Mumbai and C-DAC, Pune for providing necessary computing facility. The financial support from CSIR, New Delhi, India (Grant No. 03(1223)/12/EMR-II) is gratefully acknowledged.
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[15] Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94. 175501(1-4). [16] Hussain T, Pathak B, Maark TA, Araujo CM, Scheicher RH, Ahuja R. Ab initio study of lithium-doped graphane for hydrogen storage. Europhys Lett 2011;96. 27013(1-4). [17] Kalamse V, Wadnerkar N, Chaudhari A. Hydrogen storage in C2H4V and C2H4Vþ organometallic compounds. J Phys Chem C 2010;114:4704e9. [18] Kalamse V, Wadnerkar N, Deshmukh A, Chaudhari A. C2H2M (M¼Ti, Li) complex: a possible hydrogen storage material. Int J Hydrogen Energy 2012;37:3727e32. [19] Lotfi R, Saboohi Y. Effect of metal doping, boron substitution and functional groups on hydrogen adsorption of MOF-5: a DFT-D study. Comput Theor Chem 2014;1044:36e43. [20] Srinivasu K, Ghosh SK. Tuning the metal binding energy and hydrogen storage in alkali metal decorated MOF-5 through boron doping: a theoretical investigation. J Phys Chem C 2011;115:16984e91. [21] Bodrenko IV, Avdeenkov AV, Bessarabov DG, Bibikov AV, Nikolaev AV, Taran MD, et al. Hydrogen storage in aromatic carbon ring based molecular materials decorated with alkali or alkali-earth metals. J Phys Chem C 2012;116:25286e92. [22] Kalamse V, Wadnerkar N, Chaudhari A. Multi-functionalized naphthalene complexes for hydrogen storage. Energy 2013;49:469e74. [23] Kalamse V, Krishna R, Titus E, Chaudhari A. Boron substituted and un-substituted aromatic complexes as hydrogen storage media. Int J Hydrogen Energy 2012;41:11723e9. [24] Deshmukh A, Konda R, Kalamse V, Chaudhari A. Improved H2 uptake capacity of transition metal doped benzene by boron substitution. RSC Adv 2016;6:47033e42. [25] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [26] Schlegel HB, Iyengar SS, Li X, Millam JM, Voth GA, Scuseria GE, et al. Ab initio molecular dynamics: propagating the density matrix with Gaussian orbitals. III. Comparison with Born-Oppenheimer dynamics. J Chem Phys 2002;117:8694e704. [27] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 03. Wallingford: Gaussian Inc.; 2004. [28] Lide DR. CRC handbook of chemistry and physics. 75th ed. 1994. New York.
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Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene Vijayanand Kalamse a,c, Priyanka Tavhare b, Amol Deshmukh c, Rahul Krishna d, Elby Titus d, Ajay Chaudhari b,* a
Dept. of Physics, Science College, Nanded, 431605, India Department of Physics, The Institute of Science, FORT, Mumbai, 400 032, India c School of Physical Sciences, S. R. T. M. University, Nanded, 431606, India d Center for Automation and Technology (TEMA), Department of Mechanical Engineering, University of Aveiro, Portugal b
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abstract
Article history:
Interaction of molecular hydrogen with Li and Ti doped boron substituted naphthalene viz.
Received 16 November 2016
C6B4H8Ti2 and C6B4H8Li2 has been studied using density functional theory (DFT) method.
Received in revised form
The C6B4H8Li2 complex can interact with maximum of four hydrogen molecules, whereas
13 February 2017
three H2 molecules are adsorbed on C10H8Li2 complex. The C6B4H8Ti2 complex can interact
Accepted 15 February 2017
with maximum of eight hydrogen molecules. The gravimetric hydrogen uptake capacity of
Available online xxx
C6B4H8Ti2 and C6B4H8Li2 complex is found to be 6.85 and 5.55 wt % respectively, which is higher than that of unsubstituted C10H8Ti2 and C10H8Li2 complexes. The boron substitution
Keywords:
has significantly affected the hydrogen adsorption energies. The H2 adsorption energy and
Boron substituted naphthalene
Gibb's free energy corrected H2 adsorption energy are found to be more prominent after
Hydrogen storage
boron substitution. The C6B4H8Ti2 and C6B4H8Li2 complexes are more stable than the
Desorption temperature
respective unsubstituted C10H8Ti2 and C10H8Li2 complexes due to their higher binding
ADMP molecular dynamics
energies. According to the atom-centered density matrix propagation (ADMP) molecular dynamics simulations C6B4H8Li2 complex retain not a single adsorbed hydrogen molecule during the simulation at room temperature, whereas five hydrogen molecules at 300 K and eight at 100 K are remain absorbed on C6B4H8Ti2 complex. The C6B4H8Ti2 complex is found to be more promising material for hydrogen storage than C10B4H8Li2. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Despite several positive aspects of hydrogen (the energy carrier) as a fuel over hydrocarbon fuels, researchers are yet to succeed to solve its storage problem. To materialize the hydrogen economy [1] and make hydrogen as a fuel for the future. [2,3], its storage problem need to be overcome. Though
it is not simple [4], one major breakthrough in finding hydrogen storage material will not only solve the global energy crisis but also help in preserving conventional energy sources. It will make every nation self energy equipped. Traditional ways of storing hydrogen cannot be implemented in automotive fuel cells or directly in mobile applications. The flaws in the traditional ways of storing hydrogen and its use in mobile application have given a new challenge to the
* Corresponding author. E-mail address: [email protected] (A. Chaudhari). http://dx.doi.org/10.1016/j.ijhydene.2017.02.107 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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researchers to store hydrogen in some other ways. Solid state hydrogen storage [5] is one of the options to solve this problem. The important aspect in solid state storage of hydrogen is about the reversible adsorption and desorption process of hydrogen from the host material. Chemical hydrides and metal hydrides can be the best readily available options [6e10]. The problem related to hydrides is that detaching the atomic hydrogen from hydride is itself an energy consuming process. Moreover, hydrogen stored in molecular form is required in hydrogen powered fuel cells. So the efforts are directed in finding a material that can store maximum hydrogen in molecular form with favorable thermodynamic conditions for adsorption and desorption. The hydrogen uptake capacity of a material should also meet the target set by the department of energy (DOE) [11] for vehicular applications. Varieties of carbon based as well as inorganic materials have been tested for the hydrogen storage purpose. It has been observed that doping of metal atoms in these materials has significantly enhanced the hydrogen uptake capacity [12e16]. Recently doping of metal atoms in organic compounds and their hydrogen storage properties have been studied theoretically [17,18]. Moreover, these metal doped organic compounds can be used as linkers in metal organic frameworks which are also among the promising materials for hydrogen storage [19]. Previous theoretical reports have shown that boron substitution may tune up different properties related to the hydrogen storage [20]. Mono and di-boron substituted benzene ring functionalized with alkali as well as alkaline earth metals have been studied for the hydrogen storage purpose theoretically [21]. Multi-functionalized complexes have been tested for hydrogen storage [22,23]. Hydrogen uptake capacity of benzene is found to be increased after boron substitution [24]. Here we have studied the interaction of molecular hydrogen with Li and Ti doped boron substituted naphthalene viz. C6B4H8Ti2 and C6B4H8Li2 using density functional theory (DFT) method. We have reported different structural aspect as well as the nature of bonding in these newly modeled complexes. Different properties like hydrogen uptake capacity, hydrogen adsorption energies, temperature dependence of adsorption energies etc. of multi-functionalized boron substituted naphthalene have been studied. The stability and reactivity of these complexes have been explored by means of HOMO-LUMO gap and binding energies. The ADMP molecular dynamics simulations have also been performed at different temperatures to verify the hydrogen uptake capacity.
Computational details All the geometries are optimized using the 1997 hybrid functional of Perdew, Burke and Ernzerhof (PBE) [25] in combination with split valence 6-31G(d,p) basis set. The molecular dynamics (MD) simulations have been carried out using atom centered density matrix propagation (ADMP) [26]. The time step (Dt) of ADMPeMD was set at 0.2 fs and 0.1 fs for Li and Ti doped complex respectively. The temperature was maintained using the velocity scaling method during the MD simulations. All the calculations are performed using the Gaussian suite of programs [27].
The averaged adsorption energy without zero point energy correction (DE) is calculated as
DE ¼ {E[BSOM] þ (n E[H2])E[BSOM(nH2)]}/n
(1)
where E[X] is the total energy of X without zero point energy correction and BSOM (Boron Substituted Organometallic Complex) is either C6B4H8Li2 or C6B4H8Ti2. The averaged adsorption energy with zero point energy correction (DEZPE) is calculated as
DEZPE ¼ {EZPE[BSOM] þ (n EZPE[H2])EZPE[BSOM(nH2)]}/n
(2)
where EZPE[X] is the total energy of X with zero point energy correction. Similarly averaged adsorption energy with Gibbs free energy correction (DEG) is calculated as
DEG ¼ {EG[BSOM] þ (n EG[H2])EG[BSOM(nH2)]}/n
(3)
where EG[X] stands for the total energy of X with Gibbs free energy correction.
Results and discussion Structural analysis In this work, we have first substituted four boron atoms in place of carbon of naphthalene ring and optimized that geometry using PBEPBE/6-31G(D,P) method. As a consequence of boron substitution the ultimate molecular weight of the host material is slightly reduced. The optimized geometry of C6B4H8 can be seen in Fig. 1(a). Then we have doped C6B4H8 complex with two Li and two Ti atoms, one on each ring as shown in Fig. 1(b) and Fig. 2(a) respectively. Different bond lengths in C6B4H8Li2 and C6B4H8Ti2 before and after hydrogen adsorption are given in Table 1. The distance between two carbon atoms at the fusion of two rings viz. 2 Ce4C in bare C6B4H8 and C10H8 complex is 1.41 A and 1.44 A respectively. The distance 1Ce3C as well as 6Ce5C (which are terminal carbons) is 1.37 Å in the former and 0.05 A shorter than the latter. The average distance of any boron atom from its adjacent carbon is 1.56 A in C6B4H8, C6B4H8Li2 and C6B4H8Li2(4H2) complex. The 2Ce4C bond at the fusion in A and C6B4H8Li2 and C6B4H8Ti2 complex is elongated by 0.07 0.20 A respectively than that in the C6B4H8. But when maximum hydrogen molecules are interacted with C6B4H8Ti2 complex this bond (2Ce4C) gets shortened significantly by 0.17 A. There is no major change found in Ce Li, BeLi and LieLi bond lengths in C6B4H8Li2 complex even after interaction with four hydrogen molecules. But CeTi, Be Ti and TieTi bond lengths considerably elongated in C6B4H8Ti2(8H2) complex than that in C6B4H8Ti2 complex. All the hydrogen molecules are at average 2.30 A from corresponding Li atom in C6B4H8Li2(4H2) complex. In C6B4H8Ti2(8H2) complex, except two hydrogen molecules, rest six are at less than 2 A from corresponding Ti atom.
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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Fig. 1 e Optimized geometry of (a) C6B4H8 (b) C6B4H8Li2 and (c) C6B4H8Li2(4H2) complex.
Fig. 2 e Optimized geometry of (a) C6B4H8Ti2 and (b) C6B4H8Ti2(8H2) complex.
Gravimetric uptake capacity The keen interest is lying in the gravimetric uptake capacity of these two complexes with favorable thermodynamic conditions. It has been found that C6B4H8Li2 and C6B4H8Ti2 complexes interact with maximum of four and eight hydrogen molecules respectively with respective hydrogen uptake capacity of 5.55 and 6.85 wt %. This gravimetric uptake capacity is higher than that of unsubstituted C10H8Li2 (3.73 wt %) and C10H8Ti2 (6.72 wt %) complex. The H2 uptake capacity of C6B4H8Li2 and C6B4H8Ti2 complexes is up to the benchmark set by US DOE for vehicular applications. Thus substitution of boron has enhanced the uptake capacity in functionalized
naphthalene. We have also tried to study the interaction of nine and ten hydrogen molecules with C6B4H8Ti2 complex. It has been found that they are moving away from C6B4H8Ti2 complex in the optimization process and hence could not be adsorbed.
Adsorption energy calculations We have calculated the hydrogen adsorption energies for C6B4H8Li2(4H2) and C6B4H8Ti2(8H2) complexes which are given in Table 2. It is clear that the zero point energy correction to the adsorption energies cannot be ignored. Since the average LieH2 distance is 2.30 A, we have very weak hydrogen adsorption
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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in a C6B4H8, b C6B4H8Li2, c C6B4H8Ti2, d C6B4H8Li2(4H2) and e C6B4H8Ti2(8H2) complex. The Table 1 e Different bond lengths (A) labels assigned as a superscript to atoms are for the sake of reader's convenience. Assignments Bond length ( A) Assignments Bond length ( A) 1
Ce3C 2 Ce4C 5 Ce6C 3 Ce17B 1 Ce15B 4 Ce17B 2 Ce15B 2 Ce16B 4 Ce18B 5 Ce16B 6 Ce18B 1 Ce19Ti/20Li 3 Ce19Ti/20Li 4 Ce19Ti/20Li 2 Ce19Ti/20Li 15 Be19Ti/20Li 17 Be19Ti/20Li 2 Ce20Ti/19Li
a
b
c
d
e
1.37 1.41 1.37 1.55 1.55 1.57 1.57 1.57 1.57 1.55 1.55 e e e e e e e
1.39 1.48 1.39 1.57 1.57 1.55 1.55 1.55 1.55 1.57 1.57 2.24 1.24 2.13 2.13 2.23 2.23 2.13
1.47 1.61 1.47 1.50 1.50 1.58 1.58 1.58 1.58 1.50 1.50 2.23 2.23 2.11 2.11 2.33 2.33 2.11
1.39 1.47 1.39 1.56 1.56 1.55 1.55 1.55 1.55 1.56 1.56 2.26 2.26 2.14 2.15 2.25 2.24 2.15
1.42 1.47 1.43 1.54 1.52 1.56 1.58 1.58 1.57 1.52 1.53 2.43 2.37 2.35 2.28 2.39 2.44 2.30
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Ce20Ti/19Li 5 Ce20Ti/19Li 6 C_20Ti/19Li 16 Be20Ti/19Li 18 Be20Ti/19Li 19 Ti/Lie20Ti/Li 20 Lie2324H 20 Lie2122H 19 Lie2526H 19 Lie2728H 20 Tie2122H 20 Tie2324H 20 Tie25H 20 Tie26H 20 Tie2728H 19 Tie2930H 19 Tie3132H 19 Tie3334H 19 Tie3536H
a
b
c
d
e
e e e e e e e e e e e e e e e e e e e
2.13 2.24 2.24 2.23 2.23 3.24 e e e e e e e e e e e e e
2.11 2.23 2.23 2.33 2.33 2.66 e e e e e e e e e e e e e
2.14 2.26 2.26 2.25 2.24 3.21 2.31 2.30 2.31 2.30 e e e e e e e e e
2.36 2.37 2.36 2.39 2.42 2.92 e e e e 2.01 1.87 1.85 1.75 1.91 1.84 1.92 2.11 1.90
energies for the C6B4H8Li2(4H2) complex. Also the negative value of DEG (0.18 eV) for the C6B4H8Li2(4H2) complex indicates that the formation of this complex is endothermic. On the counterpart, the DE and DEZPE values for the C6B4H8Ti2(8H2) complex are 0.61 and 0.43 eV respectively and are well within the range of physisorption and chemisorption. This is one of the important requirements for ideal hydrogen storage. Also the positive value of DEG implies that the complex formation is thermodynamically favorable. It can be observed from Fig. 3 that the DEG values become more positive as temperature decreases. Moreover, the C6B4H8Ti2 complex can adsorb hydrogen molecules with positive DEG values over a wide range of temperature. The adsorbed hydrogen molecules on C6B4H8Ti2 complex can be easily desorbed by raising the temperature. The C10H8Li2(3H2) and C6B4H8Li2(4H2) have shown negative DEG values even at very low temperature scale.
Nature of bonding and binding energies The calculated binding energy for C6B4H8Li2 and C6B4H8Ti2 complex is 6.16 eV and 13.87 eV respectively, which indicates stronger binding of transition metal atom than an alkali metal atom to C6B4H8. Moreover, the calculated binding energy for C10H8Li2 and C10H8Ti2 complex is 1.94 eV and 10.30 eV respectively. This clearly indicates that the boron substitution
Table 2 e Calculated averaged adsorption energies per H2 molecule without and with zero point energy correction (DE and DEZPE) and that with Gibbs free energy correction (DEG) in eV at 298.15 K for C6B4H8Li2(4H2), C6B4H8Ti2(8H2) complexes. Complex
DE
DEZPE
DEG
C6B4H8Li2(4H2) C6B4H8Ti2(8H2) C10H8Ti2(8H2)
0.10 0.61 0.41
0.03 0.43 0.25
0.18 0.13 0.05
Fig. 3 e Temperature dependence of Gibbs free energy corrected H2 adsorption energy (DEG) values for C6B4H8Li2(4H2) and C6B4H8Ti2(8H2).
has increased the binding energies of multi-functionalized naphthalene complexes. Upon boron substitution, the Li and Ti atoms are bonded strongly to C6B4H8 complex than the unsubstituted material viz. C10H8. Natural electronic configuration reveals that in the formation of C6B4H8Li2 complex, the 2s orbital of both the Li atoms loses about 0.95 electrons to the 2p orbital of its own as well as to 2p orbital of boron atoms. When C6B4H8Li2 complex interacts with four hydrogen molecules, very few electrons are transferred between hydrogen molecules and Li atom. This is also confirmed from the longer LieH2 distances (See Table 1) and results in the lower adsorption energy for this complex. On the other hand, in C6B4H8Ti2 complex formation, the 4s orbital of each Ti atom loses 0.34 electrons to the 2p
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orbital of boron. Due to this significant charge transfer, Ti binds strongly to the base and the complex has higher binding energy. On interaction of C6B4H8Ti2 complex with maximum hydrogen molecules, the 3d and 3p orbitals of Ti gain 0.33 and 0.58 electrons respectively.
Stability of the complexes The kinetic stability of C6B4H8Li2 and C6B4H8Ti2 complexes with successive addition of hydrogen molecules has been tested by means of a gap between highest occupied molecular orbital and lowest unoccupied molecular orbital i.e. HOMOLUMO gap. It is clear from Fig. 4 that the gap is almost constant in case of C6B4H8Li2 complex even after adsorption of hydrogen molecules. This indicates that C6B4H8Li2 has lesser tendency to interact with hydrogen molecules, which is also quite clear from its poor hydrogen adsorption energy. In case of C6B4H8Ti2, the gap increases with successive addition of H2 molecules and its maximum value is 1.83 eV for C6B4H8Ti2(8H2) complex. Larger the gap greater is the stability of a complex and hence less tendency of further reactivity. The HOMO-LUMO gap of C6B4H8Ti2 is lower than that for the C10H8Ti2 complex. It is found that the HOMO-LUMO gap for the C10H8Ti2 complex is slightly increased when first two hydrogen molecules are added. A sudden increase in the gap for the C10H8Ti2(4H2) and C10H8Ti2(6H2) complex is observed. This is due to the fact that in C10H8Ti2(nH2) complexes for n ¼ 2, one of the H2 molecules is adsorbed in dissociated (atomic) form, whereas for n ¼ 4 and n ¼ 6 two hydrogen molecules are adsorbed in diatomic form. The gap for the C10H8Ti2(nH2) complex in case of n ¼ 8 is found to be slightly decreased. This is due to the fact that all the H2 molecules for this complex are adsorbed in molecular form and no hydrogen molecule is dissociated. In case of C6B4H8Ti2(nH2) (where n ¼ 2,4,6,8) complexes, one of the hydrogen molecules is adsorbed in dissociated form in every complex. The dissociation of hydrogen molecules is
Fig. 4 e Energy gap between highest occupied and lowest unoccupied molecular orbital for C6B4H8Li2(nH2), C6B4H8Ti2(nH2) and C10H8Ti2(nH2) complexes with successive addition of H2 molecules.
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affecting the HOMO-LUMO gap. As far as C10H8Ti2(nH2) and C6B4H8Ti2(nH2) complexes are concerned, for n ¼ 4 and 6, two hydrogen molecules are dissociated in the former and one in the latter so the gap is higher for the former than the latter. Also, because no hydrogen molecule is dissociated in C10H8Ti2(8H2) complex, the gap is decreased whereas one of the hydrogen molecules is adsorbed in diatomic form in C6B4H8Ti2(8H2), so the gap is higher. All the complexes studied here have no imaginary vibrational frequency indicating that they are quantum mechanically stable structures.
ADMP-MD simulations The ADMP-MD simulations have been performed at 300 K and 100 K using the same method so as to compare the results that obtained from electronic structure calculations. Fig. 5 shows the trajectories of H2 molecules from the Li atom in C6B4H8Li2(4H2) complex at 300 K. It is very clear from Fig. 5 that not a single H2 molecule remain adsorbed on C6B4H8Li2 complex at 300 K. The weak interaction of hydrogen molecule with C6B4H8Li2 complex confirms the lower adsorption energies obtained for C6B4H8Li2(4H2) complex. The similar kind of trajectories are also obtained for the C10H8Li2(3H2) complex at 300 K. As can be seen from Fig. 6(a), during the simulations at 100 K, two H2 molecules have been desorbed from the host material and the distance is more than 5 A. The remaining A from respective Li two H2 molecules oscillate at about 2 atoms as shown in Fig. 6(b). The trajectories of H2 molecules from Ti atoms at 300 K and 100 K are shown in Fig. 7. These trajectories for C6B4H8Ti2(8H2) complex have shown that out of eight adsorbed hydrogen molecules, three have been desorbed from the C6B4H8Ti2 complex at room temperature. Out of the three desorbed molecules, one hydrogen molecule has started to move away from the metal atom at about 0.4 ps. So, according to ADMP-MD simulations, C6B4H8Ti2 complex can bind up to five hydrogen molecules at room temperature. This corresponds to its gravimetric H2 uptake of 4.40 wt %. The adsorption energy at 100 K is higher than that
Fig. 5 e Trajectories of time evolution of distance of H2 molecules from Li atom in C6B4H8Li2(4H2) complex at 300 K.
Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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Fig. 6 e Trajectories of time evolution of distance of H2 molecules from Li atom in C6B4H8Li2(4H2) complex at 100 K.
at 298.15 K for the C10H8Ti2(8H2) complex. It indicates that the adsorption of H2 molecules at lower temperature is stronger with the host material. This is also confirmed when we performed ADMP-MD simulations at 100 K. It is found that all the eight hydrogen molecules are remain adsorbed at 100 K on C6B4H8Ti2 complex. These adsorbed hydrogen molecules can be desorbed by raising the temperature. Finally, we have calculated the desorption temperature for all the studied complexes here using the van't Hoff equation
TD ¼ (DEZPE/KB) (DS/Rln P)1
(4)
where Boltzmann constant KB ¼ 1.38 1023 J/K, the gas constant R ¼ 8.31 J/K Mol and the equilibrium pressure P ¼ 1 atm. We have used the value of DS from Ref. [28] and the zero point energy corrected adsorption energies of maximum H2 adsorbed complexes to calculate the desorption temperature. From the formula it is obvious that desorption temperature is in proportion to DEZPE and rest of the terms are merely constants. The approximate desorption temperature calculated for C6B4H8Li2(4H2), C6B4H8Ti2(8H2) and C10H8Ti2(8H2) complexes is 38.25 K, 777.83 K and 522.81 K respectively.
Conclusions The hydrogen uptake capacity of boron substituted functionalized naphthalene C6B4H8Ti2 and C6B4H8Li2 complex is found to be 6.85 and 5.55 wt % respectively using the DFT method. The boron substitution in C10H8Ti2 and C10H8Li2 complex has improved the hydrogen adsorption energies. Though C10H8Li2 complex after boron substation viz. C6B4H8Li2 interacts with four hydrogen molecules, the interaction is weaker compared to C6B4H8Ti2. The ADMP-MD simulations also reveal that the C6B4H8Li2 complex is inefficient for hydrogen storage. The ADMP-MD simulation results have shown that all the eight adsorbed hydrogen molecules remain adsorbed on C6B4H8Ti2 complex during the simulations at 100 K and retain its uptake capacity of 6.85 wt.% obtained by electronic structure calculations. At room temperature, the uptake capacity of C6B4H8Ti2 complex is 4.40 wt % according to ADMP-MD results. The binding energy of C6B4H8Li2/C6B4H8Ti2 complex is found to be larger than the respective unsubstituted C10H8Li2/C10H8Ti2 complex so that the degradation of a complex is difficult in the former than the latter. The transition metal doped complex is very sound material in all aspect than the alkali metal doped complex for hydrogen storage.
Fig. 7 e Trajectories of time evolution of distance of H2 molecules from Ti atom in C6B4H8Ti2(8H2) at (a) 300 K and (b) 100 K. Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107
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 x x x ( 2 0 1 7 ) 1 e7
Acknowledgements Thanks to the S. R. T. M. University, Nanded, The Institute of science, Mumbai and C-DAC, Pune for providing necessary computing facility. The financial support from CSIR, New Delhi, India (Grant No. 03(1223)/12/EMR-II) is gratefully acknowledged.
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Please cite this article in press as: Kalamse V, et al., Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.107