- Email: [email protected]
Confinement effects on structural, electronic properties and dehydrogenation thermodynamics of LiBH4
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Confinement effects on structural, electronic properties and dehydrogenation thermodynamics of LiBH4 Chuan Liu a, Shengli Zhang a, Peng Wang b, Shiping Huang a,*, Huiping Tian b a
Division of Molecule and Materials Simulation, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China b Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
article info
abstract
Article history:
Confinement effect on the structural, electronic and thermodynamic properties of LiBH4 is
Received 3 February 2013
investigated by density functional theory. The thermodynamically and dynamically stable
Received in revised form
confinement structure is testified to be g-LiBH4@C31Ti according to the adsorption energy
15 April 2013
and vibrational frequency calculations. The tridentate structure formed by [BH4] and Liþ
Accepted 16 April 2013
in the unconfined LiBH4 changes into bidentate structure in g-LiBH4@C31Ti. We
Available online 22 May 2013
observe that both the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of g-LiBH4@C31Ti shift to high energy level and the splits of
Keywords:
DOS peaks occur at the states of H 1s, B 2p, and Li 2p orbitals. Different from the
Hydrogen storage
first-step decomposition reaction of LiBH4, the one for g-LiBH4@C31Ti changes into
Ti-doped graphene
2LiBH4@C31Ti / 2LiH þ 2B@C31Ti þ 3H2. Moreover, the reaction enthalpy for the first-step
Density functional theory
decomposition reaction of g-LiBH4@C31Ti decreases to 5.864 eV, which is smaller than that
Thermodynamic properties
(17.204 eV) of LiBH4. According to the hydrogen removal energy calculations, we observe that the confinement effects make the removal of the first and second hydrogen atoms in g-LiBH4@C31Ti easy. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Developing safe, convenient, and highly efficient hydrogen storage materials is the key to realize the use of hydrogen as an energy carrier for automotive applications [1]. Recently, complex light metal hydrides such as LiH, MgH2, NaAlH4, and LiBH4 have received widespread attention as solid-state hydrogen storage materials due to their higher gravimetric and volumetric hydrogen capacities compared to conventional metal hydrides [2e6]. Unfortunately, the unsatisfactory
thermodynamics and kinetics of hydrogen release and uptake in these complex light metal hydrides restrict their utilization as hydrogen storage materials at reasonable operating temperatures and pressure. For instance, as one kind of complex light metal hydrides, LiBH4 holds great prospects with a high hydrogen content of 18.4 wt%. The decomposition of LiBH4 occurs in two steps:
2LiBH4 /2LiH þ 2B þ 3H2 ;
* Corresponding author. Fax: þ86 10 64427616. E-mail address: [email protected] (S. Huang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.088
(1)
8368
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
2LiH/2Li þ H2 :
(2)
However, the decomposition temperature for the first step occurs at around 400 C while that for the second step is unexpectedly as high as 950 C. Both the decomposition temperatures for the two steps are too high for the practical applications. Furthermore, the reversibility of hydrogen storage for LiBH4 has been observed to take place only at 600 C under 350 atm H2 [7,8]. In addition, the hydrogen absorption and desorption mechanism of LiBH4 were reported, and Li2B12H12 was theoretically [9] and experimentally [10] proposed as reaction intermediate. The hydriding (dehydriding) process of LiBH4 including the Li2B12H12 intermediate is as follows: 1 5 13 3 LiBH4 4 Li2 B12 H12 þ LiH þ H2 4LiH þ B þ H2 : 12 6 12 2
(3)
Up to now, many methods have been reported to address these issues of the unsatisfactory thermodynamics, kinetics, and reversibility of complex light metal hydrides [11e17]. An alternative and promising approach is confining the complex light metal hydrides within porous scaffold hosts. Experimentally, it is found that by confinement within porous scaffold hosts, the kinetic limitations of these complex light metal hydrides can be decreased along with the reduction of the diffusion distances [17e25]. In addition to kinetic properties, the thermodynamic stability of these complex light metal hydrides is also influenced significantly by the surface energy [26]. Another advantage of confinement is that the sintering of particles of complex light metal hydrides can be inhibited [27]. Theoretically, the results of density functional theory (DFT) calculations also suggest that the confinement of these complex light metal hydrides within porous scaffold hosts is beneficial to the hydrogen sorption/desorption processes [28e30]. In view of previous experimental and theoretical efforts [24e28], we believe that it is interesting and necessary to study the effect of porous scaffold hosts on the structural stability and the electronic properties of the complex light metal hydrides. Finding out stable configurations of complex light metal hydridesporous scaffold hosts and the feasible decomposition reactions for the stable configurations are highly desirable by DFT calculations. Very recently, carbon materials loaded with titaniumcontaining compounds exhibit good catalytic properties in improving the thermodynamics and kinetics of hydrogen storage materials [31,32]. Bhattacharya et al. [33] and Li et al. [34] reported that the Ti-doped graphene shows relatively higher hydrogen storage capacity than graphene. Thus, we use the Ti-doped graphene, which will play a role of the support generating confinement. Previous papers reported that the formation of a transition-metal cluster is an overwhelming tendency when transition metals are adsorbed onto the surface of carbon-based nanostructures [35,36]. However, Bhattacharya et al. [33] demonstrated that the insertion of Ti atom into the vacancy would result in a strong covalent bonding between Ti atom and graphene sheet, which prevents the possibility of metal clustering. In the report of Li et al. [34], they reported that Ti-doped graphene by firstly introducing a vacancy does not form Ti clusters at moderate concentrations.
What is more, to study the interaction of LiBH4 with the Tidoped graphene, we build a model of LiBH4 cluster with a single LiBH4 formula unit. In the previous reports, very small clusters can mimic the properties of their crystals if the clusters are characterized by strong covalent or ionic bonds [24,37e40]. Berseth et al. [24] demonstrated that the calculated AleH bond lengths in the NaAlH4 cluster and the energy needed to remove one H atom from NaAlH4 cluster agree well with those in NaAlH4 crystal. Thus, they suggested that a NaAlH4 cluster may serve as a model when calculating the properties of NaAlH4 interacting with carbon nanostructures. The calculated BeH bond lengths (1.20e1.25 A) in LiBH4 cluster agree well with that (1.23 A) in LiBH4 crystal. The hydrogen removal energy for LiBH4 cluster is 2.240 eV and that for its crystal is 2.238 eV. Thus, a LiBH4 cluster is used as a model when calculating the properties of LiBH4 interacting with Tidoped graphene. In this work, we report the confinement effect generated from Ti-doped graphene on the structural, electronic and thermodynamic properties of LiBH4 by DFT calculations. First, the thermodynamically and dynamically stable confinement structure of LiBH4 adsorbed onto Ti-doped graphene is investigated. Then, we have systematically studied the electronic properties of the confined LiBH4 system in the aspects of density of states, difference charge density, electron localization function, and Bader atomic charge. Finally, the reaction enthalpies for the candidate decomposition pathways and the hydrogen removal energies for the confined LiBH4 system are analyzed in detail.
2.
Computational details
All first-principles calculations are carried out based on density functional theory [41,42] by using the projector augmented wave (PAW) [43] method as implemented in the Vienna Ab initio Simulation Package (VASP) [44]. The generalized gradient approximation (GGA) based calculations have been performed with the PerdeweBurkeeErnzerhof (PBE) [45] exchange-correlation potential. The k-point mesh is generated by the MonkhorstePack [46] method, and a 4 4 1 kpoint mesh is used for sampling the Brillouin zone for geometry optimization, while a 9 9 1 k-point mesh is used for the electronic structure calculations. For high precision calculations, a kinetic cutoff energy of 550 eV is used for the plane wave basis for all systems. Besides, for obtaining the optimized ground state geometries, the conjugate gradient algorithm is used until the residual forces is within 0.02 eV/ A and the self-consistent total energy converges to within 1 104 eV. The system of a 4 4 supercell of graphene (32 C atoms) with a single doping atom (Ti atom) substituting a C atom is chosen as the host of confinement, as shown in Fig. 1. The Ti-doped graphene is set up by firstly removing one central carbon atom from the graphene to form a vacancy and then doping one Ti atom at the vacancy. The thickness of the vacuum layer along Z direction is set as 15 A for the sake of preventing any unphysical interaction between repeated images. The total energy and electronic properties calculations of LiBH4, LiH, H2, B, and Li are accomplished by placing them in a
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
8369
Fig. 1 e The optimized structures of graphene sheet and Ti-doped graphene sheet: (a) graphene sheet, (b) Ti-doped graphene sheet. cubic box with a side length of 15 A. For all these systems, gamma point is used for sampling the Brillouin zone. We firstly optimize Ti-doped graphene, and then adsorb the LiBH4 cluster onto Ti-doped graphene to perform the following structural optimization and properties calculations. The LiBH4 cluster is obtained from LiBH4 crystal. All calculations are performed under the condition of periodic boundary.
3.
Results and discussions
3.1.
Ti-doped graphene and confined LiBH4
3.1.1.
Ti-doped graphene
The optimized graphene and Ti-doped graphene are plotted in Fig. 1. Ti-doped graphene retains the planar form, following the pattern of pristine graphene. The bonds in the Ticontained six-membered ring in Ti-doped graphene are divided into three types, namely L1, L2, and L3, as shown in Fig. 1b. The calculated bond length of TieC bond (marked as L1) is 1.77 A. The CeC bond that is closer to Ti atom (marked as L2) is found to be 1.36 A, and the value is smaller than that (1.42 A) in pristine graphene. However, the CeC bond that is farther from Ti atom (marked as L3) is found to be 1.45 A, a little longer than that (1.42 A) in pristine graphene.
3.1.2. Thermodynamically and dynamically stable confined LiBH4 system As to the confinement of LiBH4 onto Ti-doped graphene, initial structural models from different orientations and positions are taken into account in this work. After structure optimizations for all of the trial geometries, three types of confinement structures are obtained, which are named as a-LiBH4@C31Ti, b-LiBH4@C31Ti, and g-LiBH4@C31Ti, as plotted in Fig. 2. The increasing of energy in attaching LiBH4 to Ti-doped graphene is called the adsorption energy (DE ), and the defined adsorption energy is written as follows:
DE ¼ EðLiBH4 @C31 TiÞ EðLiBH4 Þ EðC31 TiÞ;
(4)
where E(LiBH4@C31Ti), E(LiBH4), and E(C31Ti) represent the total energies of LiBH4@C31Ti, LiBH4, and C31Ti, respectively. Based on the definition of adsorption energy, a positive value of DE indicates that the process of confining LiBH4 onto Ti-doped graphene is an endothermic reaction. According to Eq. (4), the calculated adsorption energies for a-LiBH4@C31Ti, b-LiBH4@C31Ti, and g-LiBH4@C31Ti are 7.587, 7.257, and 8.325 eV, respectively. The negative value implies that the processes of forming these three types of structures are exothermic reactions. Hence, all of these three types of confinement structures are thermodynamically favorable structures. To ensure that these three types of confinement structures are dynamically stable structures, we also calculate the harmonic vibrational frequencies [47] of LiBH4 on Ti-doped graphene surface. Unfortunately, only gLiBH4@C31Ti exhibits positive vibrational frequencies along with the range from 67.29 to 2561.92 cm1, which demonstrates that g-LiBH4@C31Ti is a dynamically stable structure. Both a-LiBH4@C31Ti and b-LiBH4@C31Ti exhibit negative vibrational frequencies, lying in the range of 47.00 to 0 cm1 and 65.01 to 0 cm1; the negative vibrational frequencies imply that a-LiBH4@C31Ti and b-LiBH4@C31Ti are dynamically unstable structures. On the basis of the above analyses of adsorption energies and harmonic vibrational frequencies, the thermodynamically favorable and dynamically stable confinement structure is g-LiBH4@C31Ti. Thus, all of the following structural, electronic, and thermodynamic properties calculations are based on the g-LiBH4@C31Ti structure.
3.1.3.
Structure of the stable confined LiBH4
Depending on whether two or three hydrogen atoms from [BH4] anion are toward Liþ or Tiþ cations, we distinguish them as bidentate or tridentate structure. In addition, in order to clearly distinguish the four different hydrogen atoms, we have labeled them as H1, H2, H3, and H4, as shown in Fig. 2. In g-LiBH4@C31Ti, as shown in Fig. 2c, [BH4] anion is mainly
8370
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
Fig. 2 e Three different structures of LiBH4@C31Ti systems after optimization and the optimized structure of LiBH4: (a) aLiBH4@C31Ti structure, (b) b-LiBH4@C31Ti structure, (c) g-LiBH4@C31Ti structure, (d) optimized LiBH4.
adsorbed onto Tiþ cation while Liþ cation is adsorbed onto the graphene surface. [BH4] anion changes into bidentate structure with Liþ in g-LiBH4@C31Ti from the tridentate structure in the unconfined LiBH4, and the bond lengths for LieH1 and LieH3 elongate to 1.99 and 1.98 A from 1.83 to 1.84 A. For the BeH bonds in [BH4] anion, the bond length of BeH2 bond elongates to 1.25 A from 1.20 A while that of BeH4 bond shrinks to 1.20 A from 1.25 A. Ti atom binds with [BH4] anion via strong tridentate TieH bonds, and the calculated TieH bond lengths for TieH1, TieH2, and TieH3 bond are 2.41, 2.10, and 2.32 A, respectively. Apart from the confined LiBH4, Tidoped graphene also suffers a significant structural change due to the impacts of [BH4] anion, which can be shown on the protruding out of Ti atom from graphene sheet at a distance of 1.72 A. Excepting for the Ti atom, the C atoms around Ti atom also protrude out of graphene sheet. The new TieC bonds are separated into two groups based on the bond lengths of the new TieC bonds. The first group is the TieC bond that is near Li atom, and the bond length of the new TieeC bond prolongs to 1.99 A. The second group is the two TieC bonds that are far away from Li atom, and the bond lengths of the two new TieC bonds prolong to 1.96 A. The difference of bond lengths between the two groups of TieC bonds can be attributed to the repulsive interaction between Ti and Li atoms.
3.2.
Electronic properties of confined LiBH4 system
3.2.1.
Electronic density of states
For better understanding the relationship between the electronic structure and the nature of the bonding in g-LiBH4@C31Ti, the calculated total and partial density of states (DOSs) of g-LiBH4@C31Ti are plotted in Fig. 3a. Fermi energy is set to zero for all of the DOS plots. The calculated band-gap (from the top of valence band to the bottom of conduction band) for g-LiBH4@C31Ti is 0.570 eV, which demonstrates that gLiBH4@C31Ti exhibits semiconductor property. As can be clearly seen from Fig. 3a, the total DOS under Fermi level mainly generates from C 2s, C 2p, and Ti 3d orbitals along with partial contribution of H 1s, B 2s, and B 2p orbitals. It is worth to mention that a series of peaks can be observed in the range from 7.500 eV to Fermi level in the partial DOS of Li atom,
which can be attributed to the hybridization between C 2p and Li 2p orbitals. In addition, the orbital hybridization between Ti 3d and B 2p orbitals can be found in the range of 5.349 to 4.208 eV, and the orbital hybridization between Ti 3d and H2 1s orbitals can be found in the range of 5.349 to 4.208 eV and 3.162 to 2.687 eV. The hybridizations between Ti 3d and B 2p orbitals and between Ti 3d and H2 1s orbitals indicate that the interaction between Ti atom and the BH4 unit is weak covalent interaction. In order to better understand the effect of Ti-doped graphene on LiBH4, we compare the total and partial DOSs of g-LiBH4@C31Ti and the unconfined LiBH4, as shown in Fig. 3b. Significantly, we observe that the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of g-LiBH4@C31Ti shift to high energy level. The result suggests that the g-LiBH4@C31Ti configuration is much more stable than the unconfined LiBH4 [48]. Interestingly, the split of DOS peaks of H 1s, B 2p, and Li 2p orbitals is also found in the DOS of g-LiBH4@C31Ti. The comparison of the energy with maximum DOS value in the region (remarked yellow in Fig. 3) for H 1s, B 2p, Li 2s, and Li 2p orbitals between the unconfined LiBH4 and g-LiBH4@C31Ti are listed in Table 1. It can be seen that the energies with maximum DOS value for 1s orbital of H1, H2, H3, H4 atoms; 2p orbital of B atom; and 2s, 2p orbitals of Li atom in the unconfined LiBH4 are all located at 0.976 eV. However, the peaks for all the orbitals in g-LiBH4@C31Ti become more complex than that in the unconfined LiBH4, which demonstrates that the orbital hybridizations of H, B, and Li atoms change in g-LiBH4@C31Ti. The orbitals located at 4.684 eV are H1 1s, H2 1s, B 2px, Li 2py, and Li 2pz orbitals, demonstrating that these orbitals strongly hybridize at the value of 4.684 eV. The orbitals located at 4.303 eV are H1 1s, H3 1s, H4 1s, B 2pz, Li 2py, and Li 2pz, indicating that these orbitals strongly hybridize at the value of 4.303 eV. The orbitals located at 4.779 eV are H3 1s, B 2py, Li 2s, and Li 2px orbitals, illustrating that these orbitals strongly hybridize. In fact, the complex orbital hybridizations of H, B, and Li atoms in the g-LiBH4@C31Ti can be attributed to the effect of Ti atom. From the inset of the partial DOS of Ti atom in Fig. 3, we can see that three maximum states are located at the energies of 4.779, 4.684, and 4.303 eV.
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
8371
Fig. 3 e Total and partial density of states for g-LiBH4@C31Ti and LiBH4: (a) for g-LiBH4@C31Ti, (b) for LiBH4. In all of the DOS plots, “0” marks the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2.2. Difference charge density and electron localization function The difference charge density (DCD) for g-LiBH4@C31Ti is also calculated for analyzing the charge transfer before and after the confinement of LiBH4, as shown in Fig. 4. Here, the DCD is defined as the difference of the self-consistent charge density
Table 1 e The comparison of the energy with maximum DOS value in the region (remarked yellow in Fig. 3) for H 1s, B 2p, Li 2s, and Li 2p orbitals between the unconfined LiBH4 and g-LiBH4@C31Ti. Orbital
Energy with maximum DOS value/(eV) LiBH4
H1 1s H2 1s H3 1s H4 1s B2px B 2py B 2pz Li 2s Li 2px Li 2py Li 2pz
0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976
g-LiBH4@C31Ti 4.684, 4.684 4.779, 4.303 4.684 4.779 4.303 4.779 4.779 4.684, 4.684,
4.303 4.303
4.303 4.303
between the confined and the unconfined system, and the equation for DCD can be written as DQscf ¼ Qscf ðLiBH4 @C31 TiÞ Qscf ðLiBH4 Þ Qscf ðC31 TiÞ:
(5)
where Qscf (LiBH4@C31Ti), Qscf (LiBH4), and Qscf (C31Ti) represent the self-consistent charge densities for g-LiBH4@C31Ti, LiBH4, and C31Ti, respectively. A positive DQscf indicates charge density gain for g-LiBH4@C31Ti compared with those of LiBH4 and C31Ti. From the DCD, we observe that the DCDs for H, B, Li, and Ti atoms are all positive and they are in the order of HDCD (close to 1.5) > TiDCD (close to 1.0) > BDCD (close to 0.5) > LiDCD (a little higher than 0). The positive DCDs demonstrate the decreasing of electrons of H, B, Li, and Ti atoms compared with those of the unconfined LiBH4 system. The transfer of electrons from BH4 to graphene sheet can be confirmed by the negative DCDs, which are intensively distributed around the C atoms near Ti atom. For analyzing electron localization and chemical bonds, the electron localization function (ELF) [49e52] calculation is performed, as plotted in Fig. 5. In order to simply express the electron localization measure, the value of ELF is defined as a function of ratio to vary in the interval [0, 1]. Based on the definition of ELF, ELF ¼ 1.0 implies a completely electron localized region, ELF ¼ 0.5 corresponds to the homogeneous electron gas at any density, while ELF ¼ 0.0 indicates a
8372
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
delocalized region. In the ELF of g-LiBH4@C31Ti, we observe that the ELF value around H atom is close to 1.0, which indicates that electrons are strongly localized around H atoms. However, the ELF around B atom is less than 0.1, which demonstrates that electrons around B atom are delocalized. The characteristic of covalent bonding between B and H atoms can be obtained from the homogeneous electron gas between them along with ELF value of nearly 0.5. The ELF value around Li atom is nearly equal to 0. This fact can be attributed to the lost of valence electrons of Li atom, which can also be confirmed based on the following Bader atomic charge calculation. It can be found that the interactions between Li atom and the BH4 unit and between Li atom and graphene sheet are mainly ionic interactions. In addition, the ionic interaction can also be found between Ti atom and graphene sheet. However, the ELF overlap between Ti atom and the BH4 unit illustrates that there exists covalent interaction between Ti atom and the BH4 unit. Fig. 4 e The calculated difference charge density plot for g-LiBH4@C31Ti, the inset displays the slice used for the difference charge density calculation.
3.2.3.
Bader atomic charge
According to the Bader’s theory of atoms in molecules [53e55], we calculate and analyze the Bader atomic charges of LiBH4 and g-LiBH4@C31Ti, as presented in Table 2. We observe that the calculated Bader atomic charges for H atoms are all negative (with a unit of jej) while those for B, Li, and Ti atoms are all positive, demonstrating that H atoms get electrons while B, Li, and Ti atoms lose electrons. The calculated total Bader atomic charge for the unconfined LiBH4 is zero. However, in the confined system of g-LiBH4@C31Ti, the calculated total Bader atomic charge of LiBH4 changes into 0.107 jej. The difference of Bader atomic charge for BH4 unit between the confined and the unconfined system is 0.113 jej while that for Li atom is only 0.006 jej. Hence, the change of the Bader atomic charge of LiBH4 can be mainly attributed to the change of the Bader atomic charge of BH4 unit. The calculated difference of Bader atomic charge for Ti atom between the confined and the unconfined system is 0.004 jej, and the value is smaller than that for the BH4 unit. This characteristic indicates that the electrons are transferred from the BH4 unit to the graphene sheet. The result is also in good agreement with that obtained in the difference charge density calculation. The difference of Bader atomic charge for H4 between the confined and the unconfined system is found to be 0.104 jej. The value is nearly equal to the difference of Bader atomic charge for the whole BH4 unit,
Table 2 e Calculated Bader atomic charges (in units of jej) for H, B, Li, Ti atoms and graphene sheet. Atom/group
Fig. 5 e The electron localization function plot for g-LiBH4@C31Ti. ELF [ 1.0 (in red color) implies a completely electron localized region, ELF [ 0.5 (in green color) corresponds to the homogeneous electron gas at any density, while ELF [ 0.0 (in blue color) indicates a delocalized region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H1 H2 H3 H4 B Li Ti Graphene
Bader atomic charges (jej) LiBH4/C31Ti
g-LiBH4@C31Ti
0.613 0.551 0.610 0.618 1.512 0.880 1.476 1.475
0.608 0.575 0.610 0.514 1.540 0.874 1.480 1.587
8373
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
demonstrating that the decrease of electrons of the BH4 unit is mainly due to the decrease of electrons of H4 atom.
3.3.
Atom
Thermodynamics and decomposition pathways
The pathways of the decomposition reaction for the unconfined LiBH4 have been given in the first section of this work. For the confined LiBH4, two different candidate decomposition pathways are supposed as 2LiBH4 @C31 Ti/2LiH@C31 Ti þ 2B þ 3H2 ;
(6)
2LiH@C31 Ti/2Li@C31 Ti þ H2 ;
(7)
or 2LiBH4 @C31 Ti/2LiH þ 2B@C31 Ti þ 3H2 ;
(8)
2LiH/2Li þ H2 :
(9)
For all systems, the raw data in the total energy calculations are used for the reaction enthalpy calculations. For the unconfined LiBH4, the reaction enthalpy calculations are based on the Eqs. (1) and (2). For the confined LiBH4, the reaction enthalpies are calculated based on the Eqs. (6) and (7) or Eqs. (8) and (9). The corresponding results are listed in Table 3. For the unconfined LiBH4, the calculated reaction enthalpy is 17.204 eV for its first-step decomposition reaction and 0.483 eV for its second-step decomposition reaction. Due to the effect of Ti-doped graphene, the calculated reaction enthalpies based on Eqs. (6) and (7) are 15.991 and 0.899 eV, and the calculated reaction enthalpies based on Eqs. (8) and (9) are 5.864 and 0.483 eV. The results indicate that the decomposition pathways for the confined LiBH4 expressed as Eqs. (8) and (9) are much more thermodynamically favorable than those expressed as Eqs. (6) and (7). The significant and favorable energy decrease implies that the hydrogen desorption process of LiBH4 obtains obvious improvement when LiBH4 is confined into Ti-doped graphene.
3.4.
Hydrogen removal energy
Table 3 e Calculated reaction enthalpies for the unconfined LiBH4 and the g-LiBH4@C31Ti system. DH (eV)
Reaction
LiBH4 g-LiBH4@C31Ti 17.204 0.483 15.991 0.895 5.864 0.483
Average hydrogen removal energy (eV) LiBH4
g-LiBH4@C31Ti
2.240 1.749 1.797
1.581 1.483 1.833
a
F-H S-Ha T-Ha
a F-H, S-H and T-H represent the first hydrogen atom, the second hydrogen atom and the third hydrogen atom that are removed from LiBH4 or g-LiBH4@C31Ti system, respectively.
remove one hydrogen atom from it and relax the hydrogen removed structure. The difference of the total energy between the hydrogen removed and the hydrogen unremoved structure is defined as hydrogen removal energy. The defined hydrogen removal energy can be expressed as DE ¼E LiBHn1 @ðC31 TiÞm þ m H E LiBHn @ðC31 TiÞm ; ðn ¼ 2; 3; 4; m ¼ 0; 1Þ
(10)
From Table 4 when removing the first H atom, the average hydrogen removal energy for g-LiBH4@C31Ti is 1.581 eV, which is smaller than that (2.240 eV) for the unconfined LiBH4 by 0.659 eV. The result illustrates that the confinement effect benefits the removal of the first hydrogen. When removing the second H atom, the calculated average hydrogen removal energy for g-LiBH3@C31Ti decreases to 1.483 eV. The value is smaller than that (1.749 eV) for the unconfined LiBH3 by 0.266 eV. The reduced hydrogen removal energy demonstrates that the confinement effect also benefits the removal of the second hydrogen, although the removal of the second hydrogen is not easier than that of the first hydrogen. When removing the third H atom, the calculated hydrogen removal energy for g-LiBH2@C31Ti increases to 1.833 eV. The value is larger than that (0.797 eV) of the unconfined LiBH2 by 0.036 eV, illustrating that the confinement effect does not benefit the removal of the third H atom.
4.
To explore the effect of Ti-doped graphene on the hydrogen removal process of LiBH4, we therefore compare the hydrogen removal energy for the confined and the unconfined LiBH4. The calculated results are listed in Table 4. The method to calculate hydrogen removal energy is that we firstly calculate the total energy of hydrogen contained system, and then we
2LiBH4 / 2LiH þ 2B þ 3H2 2LiH / 2Li þ H2 2LiBH4@C31Ti / 2LiH@C31Ti þ 2B þ 3H2 2LiH@C31Ti / 2Li@C31Ti þ H2 2LiBH4@C31Ti / 2LiH þ 2B@C31Ti þ 3H2 2LiH / 2Li þ H2
Table 4 e Hydrogen removal energies for the unconfined LiBH4 and g-LiBH4@C31Ti.
Conclusions
First-principles DFT calculations have been performed to study the confinement effect on LiBH4. The thermodynamically favorable and dynamically stable confinement structure is g-LiBH4@C31Ti according to the adsorption energy and harmonic vibrational frequency calculations. Confining LiBH4 onto Ti-doped graphene is a chemisorption process. The interaction between Li atom and the graphene sheet is ionic interaction, and BH4 unit is adsorbed onto Ti atom by weak covalent interaction. The shift of the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals is observed in the DOS of g-LiBH4@C31Ti. Due to the effect of Ti atom, the split of the DOS peaks occurs at the states of H 1s, B 2p, and Li 2p orbitals. The difference charge density and Bader atomic charge calculations confirm that electrons are transferred from LiBH4 to graphene sheet. A feasible decomposition pathway for g-LiBH4@C31Ti is proposed, as expressed by Eqs. (8) and (9). The calculated reaction enthalpy based on Eq. (8) is 5.864 eV, which is lower than that (17.204 eV) based on Eq. (1)
8374
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
by 11.340 eV. In addition, the calculated hydrogen removal energies of LiBH4 and g-LiBH4@C31Ti demonstrate that the confinement effect is beneficial to the removal of the first and the second hydrogen atom from g-LiBH4@C31Ti.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant 21076007) and the National Basic Research Program of China (grant no. 2010CB732301). This project is supported by “Chemical Cloud Computing” of Beijing University of Chemical Technology.
references
[1] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8. [2] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32:1121e40. [3] Orimo S, Nakamori Y, Eliseo JR, Zu¨ttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107:4111e32. [4] Vegge T. Equilibrium structure and Ti-catalyzed H2 desorption in NaAlH4 nanoparticles from density functional theory. Phys Chem Chem Phys 2006;8:4853e61. [5] Filinchuk YE, Yvon K, Meisner GP, Pinkerton FE, Balogh MP. On the composition and crystal structure of the new quaternary hydride phase Li4BN3H10. Inorg Chem 2006;45:1433e5. [6] Barkhordarian G, Jensen TR, Doppiu S, Bo¨senberg U, Borgschulte A, Gremaud R, et al. Formation of Ca(BH4)2 from hydrogenation of CaH2þMgB2 composite. J Phys Chem C 2008;112:2743e9. [7] Orimo S, Nakamori Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloys Compd 2005;404:427e30. [8] Wellons MS, Berseth PA, Zidan R. Novel catalytic effects of fullerene for LiBH4 hydrogen uptake and release. Nanotechnology 2009;20:204022. [9] Ohba N, Miwa K, Aoki M, Noritake T, Towata SI, Nakamori Y, et al. First-principles study on the stability of intermediate compounds of LiBH4. Phys Rev B 2006;74:075110. [10] Friedrichs O, Remhof A, Hwang S-J, Zu¨ttel A. Role of Li2B12H12 for the formation and decomposition of LiBH4. Chem Mater 2010;22:3265e8. [11] Au M, Jurgensen AR, Spencer WA, Anton DL, Pinkerton FE, Hwang SJ, et al. Stability and reversibility of lithium borohydrides doped by metal halides and hydrides. J Phys Chem C 2008;112:18661e71. [12] Pinkerton FE, Meisner GP, Meyer MS, Balogh MP, Kundrat MD. Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8. J Phys Chem B 2005;109:6e8. [13] Ozolins V, Majzoub EH, Wolverton C. First-principles prediction of thermodynamically reversible hydrogen storage reactions in the LieMgeCaeBeH system. J Am Chem Soc 2009;131:230e7. [14] Sudik A, Yang J, Halliday D, Wolverton C. Hydrogen storage properties in (LiNH2)2eLiBH4e(MgH2)X mixtures (X¼0.0e1.0). J Phys Chem C 2008;112:4384e90. [15] Xu J, Yu X, Ni J, Zou Z, Li Z, Yang H. Enhanced catalytic dehydrogenation of LiBH4 by carbon-supported Pd nanoparticles. Dalton Trans 2009:8386e91.
[16] Yu XB, Grant DM, Walker GS. Dehydrogenation of LiBH4 destabilized with various oxides. J Phys Chem C 2009;113:17945e9. [17] Vajo JJ. Influence of nano-confinement on the thermodynamics and dehydrogenation kinetics of metal hydrides. Curr Opin Solid State Mater Sci 2011;15:52e61. [18] Biener J, Stadermann M, Suss M, Worsley MA, Biener MM, Rose KA, et al. Advanced carbon aerogels for energy applications. Energy Environ Sci 2011;4:656e67. [19] Adelhelm P, de Jongh PE. The impact of carbon materials on the hydrogen storage properties of light metal hydrides. J Mater Chem 2011;21:2417e27. [20] Wu C, Cheng HM. Effects of carbon on hydrogen storage performances of hydrides. J Mater Chem 2010;20:5390e400. [21] Liu X, Peaslee D, Jost CZ, Majzoub EH. Controlling the decomposition pathway of LiBH4 via confinement in highly ordered nanoporous carbon. J Phys Chem C 2010;114:14036e41. [22] Gross AF, Vajo JJ, Van Atta SL, Olson GL. Enhanced hydrogen storage kinetics of LiBH4 nanoporous carbon scaffolds. J Phys Chem C 2008;112:5651e7. [23] Ngene P, van den Berg R, Verkuijlen MHW, de Jong KP, de Jongh PE. Reversibility of the hydrogen desorption from NaBH4 by confinement in nanoporous carbon. Energy Environ Sci 2011;4:4108e15. [24] Berseth PA, Harter AG, Zidan R, Blomqvist A, Moyse´s Arau´jo C, Scheicher RH, et al. Carbon nanomaterials as catalysts for hydrogen uptake and release in NaAlH4. Nano Lett 2009;9:1501e5. [25] Xiong R, Sang G, Yan X, Zhang G, Ye X. Improvement of the hydrogen storage kinetics of NaAlH with Ti-loaded highordered mesoporous carbons (Ti-OMCs) by melt infiltration. J Mater Chem 2012;22:17183e9. [26] Fichtner M. Properties of nanoscale metal hydrides. Nanotechnology 2009;20:204009. [27] Vajo JJ, Salguero TT, Gross AF, Skeith SL, Olson GL. Thermodynamic destabilization and reaction kinetics in light metal hydride systems. J Alloys Compd 2007;446:409e14. [28] Kodi Pandyan R, Seenithurai S, Mahendran M. Hydrogen storage in MgH2 coated single walled carbon nanotubes. Int J Hydrogen Energy 2011;36:3007e15. [29] Liang JJ, Paul Kung W-C. Confinement of MgeMgH2 systems into carbon nanotubes changes hydrogen sorption energetics. J Phys Chem B 2005;109:17,837e17,841. [30] Scheicher RH, Li S, Moyses Araujo C, Blomqvist A, Ahuja R, Jena P. Nanotechnology 2011;22:335e401. [31] Xiong RJ, Sang G, Yan XY, Zhang GH, Ye XQ, Jiang CL, et al. Improvement of the hydrogen storage kinetics of NaAlH4 with nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) by melt infiltration. Int J Hydrogen Energy 2012;37:10222e8. [32] Xiong RJ, Sang G, Yan XY, Zhang GH, Ye XQ. Improvement of the hydrogen storage kinetics of NaAlH4 with Ti-loaded high-ordered mesoporous carbons (Ti-OMCs) by melt infiltration. J Mater Chem 2012;22:17183e9. [33] Bhattacharya A, Bhattacharya S, Majumder C, Das GP. Transition-metal decoration enhanced room-temperature hydrogen storage in a defect-modulated graphene sheet. J Phys Chem C 2010;114:10297e301. [34] Li S, Zhao HM, Jena P. Ti-doped nano-porous graphene: a material for hydrogen storage and sensor. Front Phys 2011;6:204e8. [35] Durgun E, Ciraci S, Yildirim T. Functionalization of carbonbased nanostructures with light transition-metal atoms for hydrogen storage. Phys Rev B 2008;77:085405. [36] Sun Q, Wang Q, Jena P, Kawazoe Y. Clustering of Ti on a C60 surface and its effect on hydrogen storage. J Am Chem Soc 2005;127:14582e3. [37] Chen ZY, Castleman JAW. Growth of titanium nitride: from clusters to microcrystals. J Chem Phys 1993;98:231e5.
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
[38] Xia P, Bloomfield LA. Accommodation of two excess electrons in sodium chloride cluster anions. Phys Rev Lett 1993;70:1779e82. [39] Reddy BV, Jena P. Signature of crystalline order in ultrasmall metal oxide clusters. Chem Phys Lett 1998;288:253e60. [40] Sun Q, Rao BK, Jena P, Stolic P, Kim YD, Gantefor G, et al. Appearance of bulk properties in small tungsten oxide clusters. J Chem Phys 2004;121:9417e22. [41] Hohenberg P, Kohn W. In homogeneous electron gas. Phys Rev 1964;136:B864e71. [42] Kohn W, Sham LJ. Self-consistent equations including exchange and correlation effects. Phys Rev 1965;140:A1133e8. [43] Blo¨chl PE. Projector augmented-wave method. Phys Rev B 1994;50:17953e79. [44] Kresse G, Furthmu¨ller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996;6:15e50. [45] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [46] Monkhorst HJ, Pack JD. Special points for brillouin-zone integrations. Phys Rev B 1976;13:5188e92. [47] Barman S, Sen P, Das GP. Ti-decorated doped silicon fullerene: a possible hydrogen-storage material. J Phys Chem C 2008;112:19963e8.
8375
[48] Ao ZM, Jiang Q, Zhang RQ, Tan TT, Li S. Al doped graphene: a promising material for hydrogen storage at room temperature. J Appl Phys 2009;105:074307. [49] Becke AD, Edgecombe KE. A simple measure of electron localization in atomic and molecular systems. J Chem Phys 1990;92:5397e403. [50] Savin A, Jepsen O, Flad J, Andersen OK, Preuss H, von Schnering HG. Electron localization in solid state structures of the elements: the diamond structure. Angew Chem Int Ed Engl 1992;31:187e8. [51] Silvi B, Savin A. Classification of chemical bonds based on topological analysis of electron localization functions. Nature 1994;371:683e6. [52] Liu C, Zhang SL, Huang SP, Wang P, Tian HP. Structure, electronic characteristic and thermodynamic properties of K2ZnH4 hydride crystal: a first-principles study. J Alloys Compd 2013;549:30e7. [53] Bader RFW. A quantum theory of molecular structure and its applications. Chem Rev 1991;91:893e928. [54] Henkelman G, Arnaldsson A, Jonsson H. A fast and robust algorithm for Bader decomposition of charge density. Comput Mater Sci 2006;36:354e60. [55] Sanville E, Kenny SD, Smith R, Henkelman G. An improved grid-based algorithm for Bader charge allocation. J Comput Chem 2007;28:899e908.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Confinement effects on structural, electronic properties and dehydrogenation thermodynamics of LiBH4 Chuan Liu a, Shengli Zhang a, Peng Wang b, Shiping Huang a,*, Huiping Tian b a
Division of Molecule and Materials Simulation, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China b Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
article info
abstract
Article history:
Confinement effect on the structural, electronic and thermodynamic properties of LiBH4 is
Received 3 February 2013
investigated by density functional theory. The thermodynamically and dynamically stable
Received in revised form
confinement structure is testified to be g-LiBH4@C31Ti according to the adsorption energy
15 April 2013
and vibrational frequency calculations. The tridentate structure formed by [BH4] and Liþ
Accepted 16 April 2013
in the unconfined LiBH4 changes into bidentate structure in g-LiBH4@C31Ti. We
Available online 22 May 2013
observe that both the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of g-LiBH4@C31Ti shift to high energy level and the splits of
Keywords:
DOS peaks occur at the states of H 1s, B 2p, and Li 2p orbitals. Different from the
Hydrogen storage
first-step decomposition reaction of LiBH4, the one for g-LiBH4@C31Ti changes into
Ti-doped graphene
2LiBH4@C31Ti / 2LiH þ 2B@C31Ti þ 3H2. Moreover, the reaction enthalpy for the first-step
Density functional theory
decomposition reaction of g-LiBH4@C31Ti decreases to 5.864 eV, which is smaller than that
Thermodynamic properties
(17.204 eV) of LiBH4. According to the hydrogen removal energy calculations, we observe that the confinement effects make the removal of the first and second hydrogen atoms in g-LiBH4@C31Ti easy. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Developing safe, convenient, and highly efficient hydrogen storage materials is the key to realize the use of hydrogen as an energy carrier for automotive applications [1]. Recently, complex light metal hydrides such as LiH, MgH2, NaAlH4, and LiBH4 have received widespread attention as solid-state hydrogen storage materials due to their higher gravimetric and volumetric hydrogen capacities compared to conventional metal hydrides [2e6]. Unfortunately, the unsatisfactory
thermodynamics and kinetics of hydrogen release and uptake in these complex light metal hydrides restrict their utilization as hydrogen storage materials at reasonable operating temperatures and pressure. For instance, as one kind of complex light metal hydrides, LiBH4 holds great prospects with a high hydrogen content of 18.4 wt%. The decomposition of LiBH4 occurs in two steps:
2LiBH4 /2LiH þ 2B þ 3H2 ;
* Corresponding author. Fax: þ86 10 64427616. E-mail address: [email protected] (S. Huang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.088
(1)
8368
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
2LiH/2Li þ H2 :
(2)
However, the decomposition temperature for the first step occurs at around 400 C while that for the second step is unexpectedly as high as 950 C. Both the decomposition temperatures for the two steps are too high for the practical applications. Furthermore, the reversibility of hydrogen storage for LiBH4 has been observed to take place only at 600 C under 350 atm H2 [7,8]. In addition, the hydrogen absorption and desorption mechanism of LiBH4 were reported, and Li2B12H12 was theoretically [9] and experimentally [10] proposed as reaction intermediate. The hydriding (dehydriding) process of LiBH4 including the Li2B12H12 intermediate is as follows: 1 5 13 3 LiBH4 4 Li2 B12 H12 þ LiH þ H2 4LiH þ B þ H2 : 12 6 12 2
(3)
Up to now, many methods have been reported to address these issues of the unsatisfactory thermodynamics, kinetics, and reversibility of complex light metal hydrides [11e17]. An alternative and promising approach is confining the complex light metal hydrides within porous scaffold hosts. Experimentally, it is found that by confinement within porous scaffold hosts, the kinetic limitations of these complex light metal hydrides can be decreased along with the reduction of the diffusion distances [17e25]. In addition to kinetic properties, the thermodynamic stability of these complex light metal hydrides is also influenced significantly by the surface energy [26]. Another advantage of confinement is that the sintering of particles of complex light metal hydrides can be inhibited [27]. Theoretically, the results of density functional theory (DFT) calculations also suggest that the confinement of these complex light metal hydrides within porous scaffold hosts is beneficial to the hydrogen sorption/desorption processes [28e30]. In view of previous experimental and theoretical efforts [24e28], we believe that it is interesting and necessary to study the effect of porous scaffold hosts on the structural stability and the electronic properties of the complex light metal hydrides. Finding out stable configurations of complex light metal hydridesporous scaffold hosts and the feasible decomposition reactions for the stable configurations are highly desirable by DFT calculations. Very recently, carbon materials loaded with titaniumcontaining compounds exhibit good catalytic properties in improving the thermodynamics and kinetics of hydrogen storage materials [31,32]. Bhattacharya et al. [33] and Li et al. [34] reported that the Ti-doped graphene shows relatively higher hydrogen storage capacity than graphene. Thus, we use the Ti-doped graphene, which will play a role of the support generating confinement. Previous papers reported that the formation of a transition-metal cluster is an overwhelming tendency when transition metals are adsorbed onto the surface of carbon-based nanostructures [35,36]. However, Bhattacharya et al. [33] demonstrated that the insertion of Ti atom into the vacancy would result in a strong covalent bonding between Ti atom and graphene sheet, which prevents the possibility of metal clustering. In the report of Li et al. [34], they reported that Ti-doped graphene by firstly introducing a vacancy does not form Ti clusters at moderate concentrations.
What is more, to study the interaction of LiBH4 with the Tidoped graphene, we build a model of LiBH4 cluster with a single LiBH4 formula unit. In the previous reports, very small clusters can mimic the properties of their crystals if the clusters are characterized by strong covalent or ionic bonds [24,37e40]. Berseth et al. [24] demonstrated that the calculated AleH bond lengths in the NaAlH4 cluster and the energy needed to remove one H atom from NaAlH4 cluster agree well with those in NaAlH4 crystal. Thus, they suggested that a NaAlH4 cluster may serve as a model when calculating the properties of NaAlH4 interacting with carbon nanostructures. The calculated BeH bond lengths (1.20e1.25 A) in LiBH4 cluster agree well with that (1.23 A) in LiBH4 crystal. The hydrogen removal energy for LiBH4 cluster is 2.240 eV and that for its crystal is 2.238 eV. Thus, a LiBH4 cluster is used as a model when calculating the properties of LiBH4 interacting with Tidoped graphene. In this work, we report the confinement effect generated from Ti-doped graphene on the structural, electronic and thermodynamic properties of LiBH4 by DFT calculations. First, the thermodynamically and dynamically stable confinement structure of LiBH4 adsorbed onto Ti-doped graphene is investigated. Then, we have systematically studied the electronic properties of the confined LiBH4 system in the aspects of density of states, difference charge density, electron localization function, and Bader atomic charge. Finally, the reaction enthalpies for the candidate decomposition pathways and the hydrogen removal energies for the confined LiBH4 system are analyzed in detail.
2.
Computational details
All first-principles calculations are carried out based on density functional theory [41,42] by using the projector augmented wave (PAW) [43] method as implemented in the Vienna Ab initio Simulation Package (VASP) [44]. The generalized gradient approximation (GGA) based calculations have been performed with the PerdeweBurkeeErnzerhof (PBE) [45] exchange-correlation potential. The k-point mesh is generated by the MonkhorstePack [46] method, and a 4 4 1 kpoint mesh is used for sampling the Brillouin zone for geometry optimization, while a 9 9 1 k-point mesh is used for the electronic structure calculations. For high precision calculations, a kinetic cutoff energy of 550 eV is used for the plane wave basis for all systems. Besides, for obtaining the optimized ground state geometries, the conjugate gradient algorithm is used until the residual forces is within 0.02 eV/ A and the self-consistent total energy converges to within 1 104 eV. The system of a 4 4 supercell of graphene (32 C atoms) with a single doping atom (Ti atom) substituting a C atom is chosen as the host of confinement, as shown in Fig. 1. The Ti-doped graphene is set up by firstly removing one central carbon atom from the graphene to form a vacancy and then doping one Ti atom at the vacancy. The thickness of the vacuum layer along Z direction is set as 15 A for the sake of preventing any unphysical interaction between repeated images. The total energy and electronic properties calculations of LiBH4, LiH, H2, B, and Li are accomplished by placing them in a
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
8369
Fig. 1 e The optimized structures of graphene sheet and Ti-doped graphene sheet: (a) graphene sheet, (b) Ti-doped graphene sheet. cubic box with a side length of 15 A. For all these systems, gamma point is used for sampling the Brillouin zone. We firstly optimize Ti-doped graphene, and then adsorb the LiBH4 cluster onto Ti-doped graphene to perform the following structural optimization and properties calculations. The LiBH4 cluster is obtained from LiBH4 crystal. All calculations are performed under the condition of periodic boundary.
3.
Results and discussions
3.1.
Ti-doped graphene and confined LiBH4
3.1.1.
Ti-doped graphene
The optimized graphene and Ti-doped graphene are plotted in Fig. 1. Ti-doped graphene retains the planar form, following the pattern of pristine graphene. The bonds in the Ticontained six-membered ring in Ti-doped graphene are divided into three types, namely L1, L2, and L3, as shown in Fig. 1b. The calculated bond length of TieC bond (marked as L1) is 1.77 A. The CeC bond that is closer to Ti atom (marked as L2) is found to be 1.36 A, and the value is smaller than that (1.42 A) in pristine graphene. However, the CeC bond that is farther from Ti atom (marked as L3) is found to be 1.45 A, a little longer than that (1.42 A) in pristine graphene.
3.1.2. Thermodynamically and dynamically stable confined LiBH4 system As to the confinement of LiBH4 onto Ti-doped graphene, initial structural models from different orientations and positions are taken into account in this work. After structure optimizations for all of the trial geometries, three types of confinement structures are obtained, which are named as a-LiBH4@C31Ti, b-LiBH4@C31Ti, and g-LiBH4@C31Ti, as plotted in Fig. 2. The increasing of energy in attaching LiBH4 to Ti-doped graphene is called the adsorption energy (DE ), and the defined adsorption energy is written as follows:
DE ¼ EðLiBH4 @C31 TiÞ EðLiBH4 Þ EðC31 TiÞ;
(4)
where E(LiBH4@C31Ti), E(LiBH4), and E(C31Ti) represent the total energies of LiBH4@C31Ti, LiBH4, and C31Ti, respectively. Based on the definition of adsorption energy, a positive value of DE indicates that the process of confining LiBH4 onto Ti-doped graphene is an endothermic reaction. According to Eq. (4), the calculated adsorption energies for a-LiBH4@C31Ti, b-LiBH4@C31Ti, and g-LiBH4@C31Ti are 7.587, 7.257, and 8.325 eV, respectively. The negative value implies that the processes of forming these three types of structures are exothermic reactions. Hence, all of these three types of confinement structures are thermodynamically favorable structures. To ensure that these three types of confinement structures are dynamically stable structures, we also calculate the harmonic vibrational frequencies [47] of LiBH4 on Ti-doped graphene surface. Unfortunately, only gLiBH4@C31Ti exhibits positive vibrational frequencies along with the range from 67.29 to 2561.92 cm1, which demonstrates that g-LiBH4@C31Ti is a dynamically stable structure. Both a-LiBH4@C31Ti and b-LiBH4@C31Ti exhibit negative vibrational frequencies, lying in the range of 47.00 to 0 cm1 and 65.01 to 0 cm1; the negative vibrational frequencies imply that a-LiBH4@C31Ti and b-LiBH4@C31Ti are dynamically unstable structures. On the basis of the above analyses of adsorption energies and harmonic vibrational frequencies, the thermodynamically favorable and dynamically stable confinement structure is g-LiBH4@C31Ti. Thus, all of the following structural, electronic, and thermodynamic properties calculations are based on the g-LiBH4@C31Ti structure.
3.1.3.
Structure of the stable confined LiBH4
Depending on whether two or three hydrogen atoms from [BH4] anion are toward Liþ or Tiþ cations, we distinguish them as bidentate or tridentate structure. In addition, in order to clearly distinguish the four different hydrogen atoms, we have labeled them as H1, H2, H3, and H4, as shown in Fig. 2. In g-LiBH4@C31Ti, as shown in Fig. 2c, [BH4] anion is mainly
8370
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
Fig. 2 e Three different structures of LiBH4@C31Ti systems after optimization and the optimized structure of LiBH4: (a) aLiBH4@C31Ti structure, (b) b-LiBH4@C31Ti structure, (c) g-LiBH4@C31Ti structure, (d) optimized LiBH4.
adsorbed onto Tiþ cation while Liþ cation is adsorbed onto the graphene surface. [BH4] anion changes into bidentate structure with Liþ in g-LiBH4@C31Ti from the tridentate structure in the unconfined LiBH4, and the bond lengths for LieH1 and LieH3 elongate to 1.99 and 1.98 A from 1.83 to 1.84 A. For the BeH bonds in [BH4] anion, the bond length of BeH2 bond elongates to 1.25 A from 1.20 A while that of BeH4 bond shrinks to 1.20 A from 1.25 A. Ti atom binds with [BH4] anion via strong tridentate TieH bonds, and the calculated TieH bond lengths for TieH1, TieH2, and TieH3 bond are 2.41, 2.10, and 2.32 A, respectively. Apart from the confined LiBH4, Tidoped graphene also suffers a significant structural change due to the impacts of [BH4] anion, which can be shown on the protruding out of Ti atom from graphene sheet at a distance of 1.72 A. Excepting for the Ti atom, the C atoms around Ti atom also protrude out of graphene sheet. The new TieC bonds are separated into two groups based on the bond lengths of the new TieC bonds. The first group is the TieC bond that is near Li atom, and the bond length of the new TieeC bond prolongs to 1.99 A. The second group is the two TieC bonds that are far away from Li atom, and the bond lengths of the two new TieC bonds prolong to 1.96 A. The difference of bond lengths between the two groups of TieC bonds can be attributed to the repulsive interaction between Ti and Li atoms.
3.2.
Electronic properties of confined LiBH4 system
3.2.1.
Electronic density of states
For better understanding the relationship between the electronic structure and the nature of the bonding in g-LiBH4@C31Ti, the calculated total and partial density of states (DOSs) of g-LiBH4@C31Ti are plotted in Fig. 3a. Fermi energy is set to zero for all of the DOS plots. The calculated band-gap (from the top of valence band to the bottom of conduction band) for g-LiBH4@C31Ti is 0.570 eV, which demonstrates that gLiBH4@C31Ti exhibits semiconductor property. As can be clearly seen from Fig. 3a, the total DOS under Fermi level mainly generates from C 2s, C 2p, and Ti 3d orbitals along with partial contribution of H 1s, B 2s, and B 2p orbitals. It is worth to mention that a series of peaks can be observed in the range from 7.500 eV to Fermi level in the partial DOS of Li atom,
which can be attributed to the hybridization between C 2p and Li 2p orbitals. In addition, the orbital hybridization between Ti 3d and B 2p orbitals can be found in the range of 5.349 to 4.208 eV, and the orbital hybridization between Ti 3d and H2 1s orbitals can be found in the range of 5.349 to 4.208 eV and 3.162 to 2.687 eV. The hybridizations between Ti 3d and B 2p orbitals and between Ti 3d and H2 1s orbitals indicate that the interaction between Ti atom and the BH4 unit is weak covalent interaction. In order to better understand the effect of Ti-doped graphene on LiBH4, we compare the total and partial DOSs of g-LiBH4@C31Ti and the unconfined LiBH4, as shown in Fig. 3b. Significantly, we observe that the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of g-LiBH4@C31Ti shift to high energy level. The result suggests that the g-LiBH4@C31Ti configuration is much more stable than the unconfined LiBH4 [48]. Interestingly, the split of DOS peaks of H 1s, B 2p, and Li 2p orbitals is also found in the DOS of g-LiBH4@C31Ti. The comparison of the energy with maximum DOS value in the region (remarked yellow in Fig. 3) for H 1s, B 2p, Li 2s, and Li 2p orbitals between the unconfined LiBH4 and g-LiBH4@C31Ti are listed in Table 1. It can be seen that the energies with maximum DOS value for 1s orbital of H1, H2, H3, H4 atoms; 2p orbital of B atom; and 2s, 2p orbitals of Li atom in the unconfined LiBH4 are all located at 0.976 eV. However, the peaks for all the orbitals in g-LiBH4@C31Ti become more complex than that in the unconfined LiBH4, which demonstrates that the orbital hybridizations of H, B, and Li atoms change in g-LiBH4@C31Ti. The orbitals located at 4.684 eV are H1 1s, H2 1s, B 2px, Li 2py, and Li 2pz orbitals, demonstrating that these orbitals strongly hybridize at the value of 4.684 eV. The orbitals located at 4.303 eV are H1 1s, H3 1s, H4 1s, B 2pz, Li 2py, and Li 2pz, indicating that these orbitals strongly hybridize at the value of 4.303 eV. The orbitals located at 4.779 eV are H3 1s, B 2py, Li 2s, and Li 2px orbitals, illustrating that these orbitals strongly hybridize. In fact, the complex orbital hybridizations of H, B, and Li atoms in the g-LiBH4@C31Ti can be attributed to the effect of Ti atom. From the inset of the partial DOS of Ti atom in Fig. 3, we can see that three maximum states are located at the energies of 4.779, 4.684, and 4.303 eV.
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
8371
Fig. 3 e Total and partial density of states for g-LiBH4@C31Ti and LiBH4: (a) for g-LiBH4@C31Ti, (b) for LiBH4. In all of the DOS plots, “0” marks the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2.2. Difference charge density and electron localization function The difference charge density (DCD) for g-LiBH4@C31Ti is also calculated for analyzing the charge transfer before and after the confinement of LiBH4, as shown in Fig. 4. Here, the DCD is defined as the difference of the self-consistent charge density
Table 1 e The comparison of the energy with maximum DOS value in the region (remarked yellow in Fig. 3) for H 1s, B 2p, Li 2s, and Li 2p orbitals between the unconfined LiBH4 and g-LiBH4@C31Ti. Orbital
Energy with maximum DOS value/(eV) LiBH4
H1 1s H2 1s H3 1s H4 1s B2px B 2py B 2pz Li 2s Li 2px Li 2py Li 2pz
0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976 0.976
g-LiBH4@C31Ti 4.684, 4.684 4.779, 4.303 4.684 4.779 4.303 4.779 4.779 4.684, 4.684,
4.303 4.303
4.303 4.303
between the confined and the unconfined system, and the equation for DCD can be written as DQscf ¼ Qscf ðLiBH4 @C31 TiÞ Qscf ðLiBH4 Þ Qscf ðC31 TiÞ:
(5)
where Qscf (LiBH4@C31Ti), Qscf (LiBH4), and Qscf (C31Ti) represent the self-consistent charge densities for g-LiBH4@C31Ti, LiBH4, and C31Ti, respectively. A positive DQscf indicates charge density gain for g-LiBH4@C31Ti compared with those of LiBH4 and C31Ti. From the DCD, we observe that the DCDs for H, B, Li, and Ti atoms are all positive and they are in the order of HDCD (close to 1.5) > TiDCD (close to 1.0) > BDCD (close to 0.5) > LiDCD (a little higher than 0). The positive DCDs demonstrate the decreasing of electrons of H, B, Li, and Ti atoms compared with those of the unconfined LiBH4 system. The transfer of electrons from BH4 to graphene sheet can be confirmed by the negative DCDs, which are intensively distributed around the C atoms near Ti atom. For analyzing electron localization and chemical bonds, the electron localization function (ELF) [49e52] calculation is performed, as plotted in Fig. 5. In order to simply express the electron localization measure, the value of ELF is defined as a function of ratio to vary in the interval [0, 1]. Based on the definition of ELF, ELF ¼ 1.0 implies a completely electron localized region, ELF ¼ 0.5 corresponds to the homogeneous electron gas at any density, while ELF ¼ 0.0 indicates a
8372
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
delocalized region. In the ELF of g-LiBH4@C31Ti, we observe that the ELF value around H atom is close to 1.0, which indicates that electrons are strongly localized around H atoms. However, the ELF around B atom is less than 0.1, which demonstrates that electrons around B atom are delocalized. The characteristic of covalent bonding between B and H atoms can be obtained from the homogeneous electron gas between them along with ELF value of nearly 0.5. The ELF value around Li atom is nearly equal to 0. This fact can be attributed to the lost of valence electrons of Li atom, which can also be confirmed based on the following Bader atomic charge calculation. It can be found that the interactions between Li atom and the BH4 unit and between Li atom and graphene sheet are mainly ionic interactions. In addition, the ionic interaction can also be found between Ti atom and graphene sheet. However, the ELF overlap between Ti atom and the BH4 unit illustrates that there exists covalent interaction between Ti atom and the BH4 unit. Fig. 4 e The calculated difference charge density plot for g-LiBH4@C31Ti, the inset displays the slice used for the difference charge density calculation.
3.2.3.
Bader atomic charge
According to the Bader’s theory of atoms in molecules [53e55], we calculate and analyze the Bader atomic charges of LiBH4 and g-LiBH4@C31Ti, as presented in Table 2. We observe that the calculated Bader atomic charges for H atoms are all negative (with a unit of jej) while those for B, Li, and Ti atoms are all positive, demonstrating that H atoms get electrons while B, Li, and Ti atoms lose electrons. The calculated total Bader atomic charge for the unconfined LiBH4 is zero. However, in the confined system of g-LiBH4@C31Ti, the calculated total Bader atomic charge of LiBH4 changes into 0.107 jej. The difference of Bader atomic charge for BH4 unit between the confined and the unconfined system is 0.113 jej while that for Li atom is only 0.006 jej. Hence, the change of the Bader atomic charge of LiBH4 can be mainly attributed to the change of the Bader atomic charge of BH4 unit. The calculated difference of Bader atomic charge for Ti atom between the confined and the unconfined system is 0.004 jej, and the value is smaller than that for the BH4 unit. This characteristic indicates that the electrons are transferred from the BH4 unit to the graphene sheet. The result is also in good agreement with that obtained in the difference charge density calculation. The difference of Bader atomic charge for H4 between the confined and the unconfined system is found to be 0.104 jej. The value is nearly equal to the difference of Bader atomic charge for the whole BH4 unit,
Table 2 e Calculated Bader atomic charges (in units of jej) for H, B, Li, Ti atoms and graphene sheet. Atom/group
Fig. 5 e The electron localization function plot for g-LiBH4@C31Ti. ELF [ 1.0 (in red color) implies a completely electron localized region, ELF [ 0.5 (in green color) corresponds to the homogeneous electron gas at any density, while ELF [ 0.0 (in blue color) indicates a delocalized region. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H1 H2 H3 H4 B Li Ti Graphene
Bader atomic charges (jej) LiBH4/C31Ti
g-LiBH4@C31Ti
0.613 0.551 0.610 0.618 1.512 0.880 1.476 1.475
0.608 0.575 0.610 0.514 1.540 0.874 1.480 1.587
8373
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
demonstrating that the decrease of electrons of the BH4 unit is mainly due to the decrease of electrons of H4 atom.
3.3.
Atom
Thermodynamics and decomposition pathways
The pathways of the decomposition reaction for the unconfined LiBH4 have been given in the first section of this work. For the confined LiBH4, two different candidate decomposition pathways are supposed as 2LiBH4 @C31 Ti/2LiH@C31 Ti þ 2B þ 3H2 ;
(6)
2LiH@C31 Ti/2Li@C31 Ti þ H2 ;
(7)
or 2LiBH4 @C31 Ti/2LiH þ 2B@C31 Ti þ 3H2 ;
(8)
2LiH/2Li þ H2 :
(9)
For all systems, the raw data in the total energy calculations are used for the reaction enthalpy calculations. For the unconfined LiBH4, the reaction enthalpy calculations are based on the Eqs. (1) and (2). For the confined LiBH4, the reaction enthalpies are calculated based on the Eqs. (6) and (7) or Eqs. (8) and (9). The corresponding results are listed in Table 3. For the unconfined LiBH4, the calculated reaction enthalpy is 17.204 eV for its first-step decomposition reaction and 0.483 eV for its second-step decomposition reaction. Due to the effect of Ti-doped graphene, the calculated reaction enthalpies based on Eqs. (6) and (7) are 15.991 and 0.899 eV, and the calculated reaction enthalpies based on Eqs. (8) and (9) are 5.864 and 0.483 eV. The results indicate that the decomposition pathways for the confined LiBH4 expressed as Eqs. (8) and (9) are much more thermodynamically favorable than those expressed as Eqs. (6) and (7). The significant and favorable energy decrease implies that the hydrogen desorption process of LiBH4 obtains obvious improvement when LiBH4 is confined into Ti-doped graphene.
3.4.
Hydrogen removal energy
Table 3 e Calculated reaction enthalpies for the unconfined LiBH4 and the g-LiBH4@C31Ti system. DH (eV)
Reaction
LiBH4 g-LiBH4@C31Ti 17.204 0.483 15.991 0.895 5.864 0.483
Average hydrogen removal energy (eV) LiBH4
g-LiBH4@C31Ti
2.240 1.749 1.797
1.581 1.483 1.833
a
F-H S-Ha T-Ha
a F-H, S-H and T-H represent the first hydrogen atom, the second hydrogen atom and the third hydrogen atom that are removed from LiBH4 or g-LiBH4@C31Ti system, respectively.
remove one hydrogen atom from it and relax the hydrogen removed structure. The difference of the total energy between the hydrogen removed and the hydrogen unremoved structure is defined as hydrogen removal energy. The defined hydrogen removal energy can be expressed as DE ¼E LiBHn1 @ðC31 TiÞm þ m H E LiBHn @ðC31 TiÞm ; ðn ¼ 2; 3; 4; m ¼ 0; 1Þ
(10)
From Table 4 when removing the first H atom, the average hydrogen removal energy for g-LiBH4@C31Ti is 1.581 eV, which is smaller than that (2.240 eV) for the unconfined LiBH4 by 0.659 eV. The result illustrates that the confinement effect benefits the removal of the first hydrogen. When removing the second H atom, the calculated average hydrogen removal energy for g-LiBH3@C31Ti decreases to 1.483 eV. The value is smaller than that (1.749 eV) for the unconfined LiBH3 by 0.266 eV. The reduced hydrogen removal energy demonstrates that the confinement effect also benefits the removal of the second hydrogen, although the removal of the second hydrogen is not easier than that of the first hydrogen. When removing the third H atom, the calculated hydrogen removal energy for g-LiBH2@C31Ti increases to 1.833 eV. The value is larger than that (0.797 eV) of the unconfined LiBH2 by 0.036 eV, illustrating that the confinement effect does not benefit the removal of the third H atom.
4.
To explore the effect of Ti-doped graphene on the hydrogen removal process of LiBH4, we therefore compare the hydrogen removal energy for the confined and the unconfined LiBH4. The calculated results are listed in Table 4. The method to calculate hydrogen removal energy is that we firstly calculate the total energy of hydrogen contained system, and then we
2LiBH4 / 2LiH þ 2B þ 3H2 2LiH / 2Li þ H2 2LiBH4@C31Ti / 2LiH@C31Ti þ 2B þ 3H2 2LiH@C31Ti / 2Li@C31Ti þ H2 2LiBH4@C31Ti / 2LiH þ 2B@C31Ti þ 3H2 2LiH / 2Li þ H2
Table 4 e Hydrogen removal energies for the unconfined LiBH4 and g-LiBH4@C31Ti.
Conclusions
First-principles DFT calculations have been performed to study the confinement effect on LiBH4. The thermodynamically favorable and dynamically stable confinement structure is g-LiBH4@C31Ti according to the adsorption energy and harmonic vibrational frequency calculations. Confining LiBH4 onto Ti-doped graphene is a chemisorption process. The interaction between Li atom and the graphene sheet is ionic interaction, and BH4 unit is adsorbed onto Ti atom by weak covalent interaction. The shift of the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals is observed in the DOS of g-LiBH4@C31Ti. Due to the effect of Ti atom, the split of the DOS peaks occurs at the states of H 1s, B 2p, and Li 2p orbitals. The difference charge density and Bader atomic charge calculations confirm that electrons are transferred from LiBH4 to graphene sheet. A feasible decomposition pathway for g-LiBH4@C31Ti is proposed, as expressed by Eqs. (8) and (9). The calculated reaction enthalpy based on Eq. (8) is 5.864 eV, which is lower than that (17.204 eV) based on Eq. (1)
8374
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
by 11.340 eV. In addition, the calculated hydrogen removal energies of LiBH4 and g-LiBH4@C31Ti demonstrate that the confinement effect is beneficial to the removal of the first and the second hydrogen atom from g-LiBH4@C31Ti.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant 21076007) and the National Basic Research Program of China (grant no. 2010CB732301). This project is supported by “Chemical Cloud Computing” of Beijing University of Chemical Technology.
references
[1] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8. [2] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32:1121e40. [3] Orimo S, Nakamori Y, Eliseo JR, Zu¨ttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107:4111e32. [4] Vegge T. Equilibrium structure and Ti-catalyzed H2 desorption in NaAlH4 nanoparticles from density functional theory. Phys Chem Chem Phys 2006;8:4853e61. [5] Filinchuk YE, Yvon K, Meisner GP, Pinkerton FE, Balogh MP. On the composition and crystal structure of the new quaternary hydride phase Li4BN3H10. Inorg Chem 2006;45:1433e5. [6] Barkhordarian G, Jensen TR, Doppiu S, Bo¨senberg U, Borgschulte A, Gremaud R, et al. Formation of Ca(BH4)2 from hydrogenation of CaH2þMgB2 composite. J Phys Chem C 2008;112:2743e9. [7] Orimo S, Nakamori Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloys Compd 2005;404:427e30. [8] Wellons MS, Berseth PA, Zidan R. Novel catalytic effects of fullerene for LiBH4 hydrogen uptake and release. Nanotechnology 2009;20:204022. [9] Ohba N, Miwa K, Aoki M, Noritake T, Towata SI, Nakamori Y, et al. First-principles study on the stability of intermediate compounds of LiBH4. Phys Rev B 2006;74:075110. [10] Friedrichs O, Remhof A, Hwang S-J, Zu¨ttel A. Role of Li2B12H12 for the formation and decomposition of LiBH4. Chem Mater 2010;22:3265e8. [11] Au M, Jurgensen AR, Spencer WA, Anton DL, Pinkerton FE, Hwang SJ, et al. Stability and reversibility of lithium borohydrides doped by metal halides and hydrides. J Phys Chem C 2008;112:18661e71. [12] Pinkerton FE, Meisner GP, Meyer MS, Balogh MP, Kundrat MD. Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8. J Phys Chem B 2005;109:6e8. [13] Ozolins V, Majzoub EH, Wolverton C. First-principles prediction of thermodynamically reversible hydrogen storage reactions in the LieMgeCaeBeH system. J Am Chem Soc 2009;131:230e7. [14] Sudik A, Yang J, Halliday D, Wolverton C. Hydrogen storage properties in (LiNH2)2eLiBH4e(MgH2)X mixtures (X¼0.0e1.0). J Phys Chem C 2008;112:4384e90. [15] Xu J, Yu X, Ni J, Zou Z, Li Z, Yang H. Enhanced catalytic dehydrogenation of LiBH4 by carbon-supported Pd nanoparticles. Dalton Trans 2009:8386e91.
[16] Yu XB, Grant DM, Walker GS. Dehydrogenation of LiBH4 destabilized with various oxides. J Phys Chem C 2009;113:17945e9. [17] Vajo JJ. Influence of nano-confinement on the thermodynamics and dehydrogenation kinetics of metal hydrides. Curr Opin Solid State Mater Sci 2011;15:52e61. [18] Biener J, Stadermann M, Suss M, Worsley MA, Biener MM, Rose KA, et al. Advanced carbon aerogels for energy applications. Energy Environ Sci 2011;4:656e67. [19] Adelhelm P, de Jongh PE. The impact of carbon materials on the hydrogen storage properties of light metal hydrides. J Mater Chem 2011;21:2417e27. [20] Wu C, Cheng HM. Effects of carbon on hydrogen storage performances of hydrides. J Mater Chem 2010;20:5390e400. [21] Liu X, Peaslee D, Jost CZ, Majzoub EH. Controlling the decomposition pathway of LiBH4 via confinement in highly ordered nanoporous carbon. J Phys Chem C 2010;114:14036e41. [22] Gross AF, Vajo JJ, Van Atta SL, Olson GL. Enhanced hydrogen storage kinetics of LiBH4 nanoporous carbon scaffolds. J Phys Chem C 2008;112:5651e7. [23] Ngene P, van den Berg R, Verkuijlen MHW, de Jong KP, de Jongh PE. Reversibility of the hydrogen desorption from NaBH4 by confinement in nanoporous carbon. Energy Environ Sci 2011;4:4108e15. [24] Berseth PA, Harter AG, Zidan R, Blomqvist A, Moyse´s Arau´jo C, Scheicher RH, et al. Carbon nanomaterials as catalysts for hydrogen uptake and release in NaAlH4. Nano Lett 2009;9:1501e5. [25] Xiong R, Sang G, Yan X, Zhang G, Ye X. Improvement of the hydrogen storage kinetics of NaAlH with Ti-loaded highordered mesoporous carbons (Ti-OMCs) by melt infiltration. J Mater Chem 2012;22:17183e9. [26] Fichtner M. Properties of nanoscale metal hydrides. Nanotechnology 2009;20:204009. [27] Vajo JJ, Salguero TT, Gross AF, Skeith SL, Olson GL. Thermodynamic destabilization and reaction kinetics in light metal hydride systems. J Alloys Compd 2007;446:409e14. [28] Kodi Pandyan R, Seenithurai S, Mahendran M. Hydrogen storage in MgH2 coated single walled carbon nanotubes. Int J Hydrogen Energy 2011;36:3007e15. [29] Liang JJ, Paul Kung W-C. Confinement of MgeMgH2 systems into carbon nanotubes changes hydrogen sorption energetics. J Phys Chem B 2005;109:17,837e17,841. [30] Scheicher RH, Li S, Moyses Araujo C, Blomqvist A, Ahuja R, Jena P. Nanotechnology 2011;22:335e401. [31] Xiong RJ, Sang G, Yan XY, Zhang GH, Ye XQ, Jiang CL, et al. Improvement of the hydrogen storage kinetics of NaAlH4 with nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) by melt infiltration. Int J Hydrogen Energy 2012;37:10222e8. [32] Xiong RJ, Sang G, Yan XY, Zhang GH, Ye XQ. Improvement of the hydrogen storage kinetics of NaAlH4 with Ti-loaded high-ordered mesoporous carbons (Ti-OMCs) by melt infiltration. J Mater Chem 2012;22:17183e9. [33] Bhattacharya A, Bhattacharya S, Majumder C, Das GP. Transition-metal decoration enhanced room-temperature hydrogen storage in a defect-modulated graphene sheet. J Phys Chem C 2010;114:10297e301. [34] Li S, Zhao HM, Jena P. Ti-doped nano-porous graphene: a material for hydrogen storage and sensor. Front Phys 2011;6:204e8. [35] Durgun E, Ciraci S, Yildirim T. Functionalization of carbonbased nanostructures with light transition-metal atoms for hydrogen storage. Phys Rev B 2008;77:085405. [36] Sun Q, Wang Q, Jena P, Kawazoe Y. Clustering of Ti on a C60 surface and its effect on hydrogen storage. J Am Chem Soc 2005;127:14582e3. [37] Chen ZY, Castleman JAW. Growth of titanium nitride: from clusters to microcrystals. J Chem Phys 1993;98:231e5.
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 3 8 ( 2 0 1 3 ) 8 3 6 7 e8 3 7 5
[38] Xia P, Bloomfield LA. Accommodation of two excess electrons in sodium chloride cluster anions. Phys Rev Lett 1993;70:1779e82. [39] Reddy BV, Jena P. Signature of crystalline order in ultrasmall metal oxide clusters. Chem Phys Lett 1998;288:253e60. [40] Sun Q, Rao BK, Jena P, Stolic P, Kim YD, Gantefor G, et al. Appearance of bulk properties in small tungsten oxide clusters. J Chem Phys 2004;121:9417e22. [41] Hohenberg P, Kohn W. In homogeneous electron gas. Phys Rev 1964;136:B864e71. [42] Kohn W, Sham LJ. Self-consistent equations including exchange and correlation effects. Phys Rev 1965;140:A1133e8. [43] Blo¨chl PE. Projector augmented-wave method. Phys Rev B 1994;50:17953e79. [44] Kresse G, Furthmu¨ller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996;6:15e50. [45] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [46] Monkhorst HJ, Pack JD. Special points for brillouin-zone integrations. Phys Rev B 1976;13:5188e92. [47] Barman S, Sen P, Das GP. Ti-decorated doped silicon fullerene: a possible hydrogen-storage material. J Phys Chem C 2008;112:19963e8.
8375
[48] Ao ZM, Jiang Q, Zhang RQ, Tan TT, Li S. Al doped graphene: a promising material for hydrogen storage at room temperature. J Appl Phys 2009;105:074307. [49] Becke AD, Edgecombe KE. A simple measure of electron localization in atomic and molecular systems. J Chem Phys 1990;92:5397e403. [50] Savin A, Jepsen O, Flad J, Andersen OK, Preuss H, von Schnering HG. Electron localization in solid state structures of the elements: the diamond structure. Angew Chem Int Ed Engl 1992;31:187e8. [51] Silvi B, Savin A. Classification of chemical bonds based on topological analysis of electron localization functions. Nature 1994;371:683e6. [52] Liu C, Zhang SL, Huang SP, Wang P, Tian HP. Structure, electronic characteristic and thermodynamic properties of K2ZnH4 hydride crystal: a first-principles study. J Alloys Compd 2013;549:30e7. [53] Bader RFW. A quantum theory of molecular structure and its applications. Chem Rev 1991;91:893e928. [54] Henkelman G, Arnaldsson A, Jonsson H. A fast and robust algorithm for Bader decomposition of charge density. Comput Mater Sci 2006;36:354e60. [55] Sanville E, Kenny SD, Smith R, Henkelman G. An improved grid-based algorithm for Bader charge allocation. J Comput Chem 2007;28:899e908.