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Alkali metal silanides α-MSiH3: A family of complex hydrides for solid-state hydrogen storage
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 e9
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Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage J. Zhang a,*, S. Yan a, H. Qu a, X.F. Yu a, P. Peng b a
Hunan Provincial Key Laboratory of Safety Design and Reliability Technology for Engineering Vehicle, Changsha University of Science and Technology, Changsha 410114, China b College of Materials Science and Engineering, Hunan University, Changsha 410082, China
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abstract
Article history:
The alkali metal silanides a-MSiH3 appear to be a promising family of complex hydrides for
Received 2 February 2017
solid-state hydrogen storage. Herein the structural, energetic and electronic properties of
Received in revised form
a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investi-
3 March 2017
gated for the first time by using first-principles calculations method based on density
Accepted 6 March 2017
functional theory. The structural parameters of a-MSiH3 and MSi including lattice con-
Available online xxx
stants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder
Keywords:
diffraction. The calculations of formation enthalpy show that a-KSiH3, a-RbSiH3 and a-
Alkali metal silanides
CsSiH3 silanides are easier to be synthetized relative to a-LiSiH3 and a-NaSiH3, which in-
Hydrogen storage materials
terprets well the lower thermostabilities of experimental a-LiSiH3 and a-NaSiH3. Never-
First-principles calculations
theless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The
Formation enthalpy
calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of
Hydrogen desorption enthalpy
a-MSiH3 silanides along the decomposition path of a-MSiH3/MSi þ H2 are gradually
Electronic structure
enhanced in the order of a-CsSiH3, a-RbSiH3, a-KSiH3, a-NaSiH3 and a-LiSiH3, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, a-KSiH3 (~4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH3 complex, which constitutes the ionic bonding between them. The bonding within SiH3 complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH3 mainly dominate the thermostabilities and dehydrogenation properties of a-MSiH3 silanides. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is as an ideal energy carrier for the fuel cell applications including stationary, mobile and portable power
applications. The safe, effective and affordable hydrogen storage is a key challenge in developing hydrogen economy [1]. Solid materials-based hydrogen storage through reversible formation of hydrides is one of the most promising methods
* Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2017.03.132 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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due to their unique safety and high energy densities compared to pressurised gaseous and cryogenically liquefied hydrogen [2,3]. In the past few decades, a variety of solid-state hydrogen storage materials have been explored and developed. Among these materials, the light metal hydrides and complex metal hydrides are considered to be the suitable hydrogen storage media for their high volumetric and gravimetric hydrogen capacities. For example, MgH2 and LiBH4 are the two typical representatives, which are widely researched for their high hydrogen storage densities (7.6 wt%/110 kg m3 for MgH2 and 18.5 wt%/121 kg m3 for LiBH4) [4e6]. However, these hydrides are always suffering from high temperature requirement, slow kinetics and irreversibility. Although several progresses have been achieved in enhancing the hydrogen storage performance of metal hydrides by alloying, nanoscaling, catalyzing, or compositing, the synergetic improvement in the operating temperature, reversibility and cyclic stability is barely satisfactory [7]. Recently, some multielement alloys or their hydrides such as M-Be-H (M ¼ Li, Na) [8], Zr-Fe-V [9], RE-Mg-Ni (RE ¼ Rare Metals) [10,11], Nb4M0.9Si1.1 (M ¼ Co, Ni) [12] have been investigated as the hydrogen storage or negative electrode materials of nickel-metal hydride (Ni/MH) batteries. In particular, the alkali metal silanides a-MSiH3 (M ¼ Li, Na, K, Rb, Cs) and MSi Zintl phases are paid increasing attention for their reversible hydrogen storage capability near ambient conditions [13e20]. The excellent thermodynamic characteristics of a-MSiH3 silanides originate from their low entropy variation [13]. For example, a-KSiH3 has been reported as a promising hydrogen storage material. It has high hydrogen storage capacity of 4.29 wt% and reasonable formation enthalpy DH ¼ 28 kJ/ mol$H2, which allows hydrogen sorption at 100 C [14e16]. Moreover, a-KSiH3 can be synthesized by direct solidegas reaction between KSi Zintl phase and hydrogen. Similar to aKSiH3, a-RbSiH3 and a-CsSiH3, with relatively lower hydrogen storage densities of 2.59 wt% and 1.83 wt%, have been reported to be cubic isolated solid-state phases and they can also be synthesized through direct hydrogenation of RbSi and CsSi Zintl phases [17]. Different from the three silanides above, aLiSiH3 and a-NaSiH3, with relatively higher hydrogen storage densities of 7.89 wt% and 5.56 wt%, present the lower
thermostability. They are difficult to be synthesized by the direct hydrogenation of NaSi and LiSi. Some studies have shown that NaSi disproportionates to NaH and Na8Si46 clathrate phase, and LiSi forms LiH and Si under hydrogen pressure [18,20]. Thus, the stabilization of a-LiSiH3 and a-NaSiH3 with high hydrogen storage densities is still a great challenge. Although the alkali metal silanides a-MSiH3 are proposed as a promising family of hydrogen storage materials, their formation abilities, thermostabilities and dehydrogenation properties have not been studied yet from an energy point of view. Moreover, the detailed structural parameters of a-MSiH3 and the complicated bonding characteristics within silanides have not been well investigated due to the difficulty in determining hydrogen atomic positions by using experimental methods. In this work, first-principles calculations method based on density functional theory is employed to conduct a comprehensive study on the structural, energetic and electronic properties of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) for the first time. The results will contribute to understanding the formation abilities, thermostabilities and dehydrogenation properties of a-MSiH3 silanides. Furthermore, some underlying mechanisms associated with the thermostabilities and dehydrogenation properties are also explored from the perspective of electronic structures, which are expected to be the theoretical guidances for designing high performance a-MSiH3 complex hydrides for solid-state hydrogen storage.
Computational models and method At room temperature, the alkali metal silanides a-MSiH3 (M ¼ Li, Na, K, Rb, Cs) exhibit the cubic NaCl-type arrangement of alkali metal M and Si atoms with the space group of Fm-3m, as shown in Fig. 1 [13]. The pyramidal SiH3 complexes are distributed in random orientations in the crystal structure. The unit cell of each silanide all contains 4 formula units and the M, Si and H atoms are located at the 4c, 4c and 96k sites, respectively [17]. Their experimental structural parameters including lattice constants and atomic positions are listed in Table 1. Due to the difficulty in synthesizing a-LiSiH3
Fig. 1 e The crystal cell models of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Table 1 e The calculated equilibrium lattice constants, atomic positions and total energies per formula unit (f.u.) of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs). Material Lattice constant (A) Atomic position Total energy K-point Cal.
Exp.
a-LiSiH3 (Fm-3m, cubic)
a ¼ 6.5690
e
a-NaSiH3 (Fm-3m, cubic)
a ¼ 6.7892
e
a-KSiH3 (Fm-3m, cubic)
a ¼ 7.2008
a ¼ 7.2245a
a-RbSiH3 (Fm-3m, cubic)
a ¼ 7.5616
a ¼ 7.4860a
a-CsSiH3 (Fm-3m, cubic)
a ¼ 7.9462
a ¼ 7.8320a
LiSi (I41/a, tetragonal) NaSi (C2/c, monoclinic)
a ¼ 9.1450 c ¼ 5.4574 a ¼ 11.9744 b ¼ 6.5333 c ¼ 11.0178 b ¼ 117.4591 a ¼ 13.7119
a ¼ 9.354b c ¼ 5.746b a ¼ 12.190c b ¼ 6.550c c ¼ 11.180c b ¼ 119 c a ¼ 12.620c
RbSi (P-43n, cubic)
a ¼ 13.1601
a ¼ 13.040c
CsSi (P-43n, cubic)
a ¼ 13.7286
a ¼ 13.500c
KSi (P-43n, cubic)
a b c
Cal.
Exp.
Li (4c): 0, 0, 0 Si (4c): 0.4698, 0.5192, 0.5012 H (96k): 0.4835, 0.6549, 0.7338 Na (4c):0, 0, 0 Si (4c): 0.4653, 0.5296, 0.5116 H (96k): 0.5289, 0.6491, 0.7052 K (4c): 0, 0, 0 Si (4c): 0.4798, 0.5187, 0.4808 H (96k): 0.5642, 0.6449, 0.6261 Rb (4c): 0, 0, 0 Si (4c): 0.4807, 0.5169, 0.4824 H (96k): 0.5610, 0.6380, 0.6228 Cs (4c): 0, 0, 0 Si (4c): 0.4815, 0.5137, 0.4878 H (96k): 0.5597, 0.6310, 0.6179 Li (16f): 0.1350, 0.8903, 0.1309 Si (16f): 0.1472, 0.8862, 0.6049 Na1 (8f): 0.3550, 0.6735, 0.3577 Na2 (8f): 0.6356, 0.9074, 0.4507 Si1 (8f): 0.4355, 0.1944, 0.3115 Si2 (8f): 0.5959, 0.4524, 0.3571 K1 (8e): 0.3338, 0.3338, 0.3338 K2 (24i): 0.3373, 0.1398, 0.0662 Si1 (8e): 0.0680, 0.0680, 0.0680 Si2 (24i): 0.0628, 0.3171, 0.4261 Rb1 (8e): 0.3327, 0.3327, 0.3327 Rb2 (24i): 0.3363, 0.1386, 0.0655 Si1 (8e): 0.0653, 0.0653, 0.0653 Si2 (24i): 0.0595, 0.3148, 0.4288 Cs1 (8e): 0.3336, 0.3336, 0.3336 Cs2 (24i): 0.3382, 0.1394, 0.0666 Si1 (8e): 0.0625, 0.0625, 0.0625 Si2 (24i): 0.0583, 0.3115, 0.4322
e e e e e e K (4c): 0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5632, 0.6365, 0.6365a Rb (4c): 0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5616, 0.6327, 0.6327a Cs (4c):0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5616, 0.6273, 0.6273a Li (16f): 0.0810, 0.8860, 0.0602b Si (16f): 0.0111, 0.9522, 0.5937b Na1 (8f): 0.351, 0.662, 0.358c Na2 (8f): 0.632, 0.900, 0.455c Si1 (8f): 0.440, 0.210, 0.314c Si2 (8f): 0.597, 0.463, 0.357c K1 (8e): 0.331, 0.331, 0.331c K2 (24i): 0.336, 0.141, 0.064c Si1 (8e): 0.068, 0.068, 0.068c Si2 (24i): 0.063, 0.318, 0.427c Rb1 (8e): 0.331, 0.331, 0.331c Rb2 (24i): 0.336, 0.141, 0.004c Si1 (8e): 0.066, 0.066, 0.066c Si2 (24i): 0.061, 0.316, 0.429c Cs1 (8e): 0.331, 0.331, 0.331c Cs2 (24i): 0.336, 0.141, 0.062c Si1 (8e): 0.063, 0.063, 0.063c Si2 (24i): 0.059, 0.315, 0.431c
(eV/f.u.) 345.0022
444
1459.3143
444
934.7379
333
814.7721
333
705.2695
333
297.4331
335
1411.6314
243
886.7409
222
766.6876
222
657.1339
222
Reference [17]. Reference [21]. Reference [22].
and a-NaSiH3 and the lackage of their structural parameters, the unit cells of a-LiSiH3 and a-NaSiH3 are both constructed by adopting the structural data of a-KSiH3. After dehydrogenation of a-MSiH3 along the decomposition path of a-MSiH3/MSi þ H2, the obtained MSi Zintl phases exhibit different crystal structures from a-MSiH3. For LiSi phase, it presents a tetragonal structure with the space of I41/a [21]. Its unit cell contains 16 formula units and the Li and Si atoms are located at the 16f and 16f sites, respectively. For NaSi phase, it has a monoclinic structure with the space of C2/c. Its unit cell also contains 16 formula units and two different kinds of Na and Si atoms are located at the 8f (Na1 and Na2) and 8f (Si1 and Si2) sites, respectively. For MSi (M ¼ K, Rb, Cs) phases, they all belong to the cubic structure with the space group of P43n [22]. Their unit cells all contain 32 formula units. Similar to NaSi phase, there are also two different kinds of M (M ¼ K, Rb, Cs) and Si atoms which are located at the 8e (M1), 24i (M2), 8e (Si1) and 24i (Si2) sites, respectively. The experimental structural parameters of MSi Zintl phases are also listed in Table 1. In this work, all calculations are performed using firstprinciples plane-wave pseudopotential method through the
CASTEP software package [23]. Ultrasoft pseudopotentials [24] in reciprocal space are used for the core-valence interactions. The generalized gradient approximation (GGA) in PerdeweBurkeeErnzerhof (PBE) format [25] is adopted as the exchange-correction functional. The plane wave cutoff energy with 500 eV and k sampling with different k-point mesh (see Table 1) in Brillouin zone [26] are employed for the geometry relaxation and electronic structures calculations of a-MSiH3 silanides and MSi Zintl phases. The pseudo atomic orbitals used are H 1s1, Si 3s23p2, Li 1s22s1, Na 2s22p63s1, K 3s23p64s1, Rb 4s24p65s1 and Cs 5s25p66s1, respectively. The convergence criteria for energy, force, stress and displacement are A, 0.05 GPa and 0.001 Å, respectively. 1.0 105 eV/atom, 0.03 eV/
Results and discussion Structural parameter To examine the accuracy of computational parameters selected in this work, some testing calculations on the
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Table 2 e The calculated equilibrium lattice constants, bond length and total energies per formula unit (f.u.) of solid bcc-Li, bcc-Na, bcc-K, bcc-Rb, bcc-Cs, fcc-Si and gaseous H2. Material Structural parameter ( A) Total K-point Cal. bcc-Li bcc-Na bcc-K bcc-Rb bcc-Cs fcc-Si H2 a b
a a a a a a d
¼ 3.4354 ¼ 4.1869 ¼ 5.2745 ¼ 5.6543 ¼ 6.1400 ¼ 5.4633 ¼ 0.7525
Exp. a a a a a a d
¼ 3.491a ¼ 4.225a ¼ 5.225a ¼ 5.585a ¼ 6.045a ¼ 5.430a ¼ 0.741b
energy (eV/f.u.) 189.9765 1304.2709 779.2330 659.2672 549.6685 107.3328 31.6977
8 6 6 4 4 8 2
8 6 6 4 4 8 2
8 6 6 4 4 8 2
Reference [27]. Reference [28].
structural parameters of solid bcc-Li, bcc-Na, bcc-K, bcc-Rb, bcc-Cs, fcc-Si and gaseous H2 are performed. The calculated results are listed in Table 2. It can be seen that the equilibrium lattice constants of solid elements and bond length of H2 molecule are in good agreement with experimental data [27,28]. The maximum deviation is only 1.59% (a value of bccLi), suggesting that the present calculations are precise enough to represent the ground-state properties of a-MSiH3 silanides and MSi Zintl phases.
Formation enthalpy The formation enthalpy is an important thermodynamic parameter of materials [29]. It is the best aid to establish whether theoretically predicted phases are likely to form and be stable, and such data may also serve as guidances for possible synthesis routes of materials. Commonly, the formation enthalpy is always negative, and the lower enthalpy value will correspond to the stronger formation ability and the high thermostability of synthesized materials. In order to investigate the formation ability and thermostability of aMSiH3 silanides and MSi Zintl phases, their formation enthalpies DHform are calculated by using Eqs. (1) and (2), respectively [30],
DHform ða MSiH3 Þ ¼Etot ða MSiH3 Þ Esolid tot ðMÞ 3 Esolid tot ðSiÞ Etot ðH2 Þ 2
(1)
solid DHform ðMSiÞ ¼ Etot ðMSiÞ Esolid tot ðMÞ Etot ðSiÞ
(2)
where Etot ða MSiH3 Þ and Etot ðMSiÞ are the total energies per formular unit of a-MSiH3 silanides and MSi Zintl phases, respectively. and Esolid tot ðSiÞ are the total energies per formular unit of solid M alkali metals and Si, respectively. Etot ðH2 Þ refers to the total energy of single gaseous H2 molecule. The calculated formation enthalpies of a-MSiH3 and MSi are plotted in Fig. 2. It can be seen that these enthalpy values are all negative, meaning structural stabilities of a-MSiH3 and MSi from a thermodynamics point of view. For a-MSiH3 silanides as shown in Fig. 2(a), their enthalpy values become gradually negative in the order of a-LiSiH3, a-NaSiH3, a-KSiH3, a-RbSiH3 and a-CsSiH3, suggesting the increasing thermostability. Comparatively, the values of a-KSiH3, a-RbSiH3 and a-CsSiH3 are much lower that those of a-LiSiH3 and a-NaSiH3, which indicates that a-KSiH3, a-RbSiH3 and a-CsSiH3 are easier to be synthetized and more stable relative to the other two silanides. The higher formation enthalpies of a-LiSiH3 and aNaSiH3 calculated here interpret well the lower thermostabilities and difficulty in synthesizing the two silanides reported by previous experiments [18,20]. As for MSi Zintl phases, the change of enthalpy values are different from that of a-MSiH3 silanides, as shown in Fig. 2(b). It can be found that the formation enthalpies of MSi become gradually negative in the order of NaSi, RbSi, LiSi, CsSi and KSi. Apparently, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. By contrast, KSi is the most stable Zintl phase with the lowest formation enthalpy of 16.8937 kJ/mol per formular unit.
Hydrogen desorption enthalpy Similar to the formation enthalpy, the hydrogen desorption enthalpy is also an important thermodynamic parameter used to identify and classify hydrogen storage materials. It determines the heat of the overall dehydrogenation reaction
Fig. 2 e The calculated formation enthalpies of a-MSiH3 silanides (a) and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) (b). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 3 e The calculated hydrogen desorption enthalpies (a) and theoretical hydrogen storage capacities (b) of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs).
of hydrogen storage materials, which in turn affects their dehydrogenation temperature [31]. In order to evaluate the dehydrogenation abilities of a-MSiH3 silanides, their hydrogen desorption enthalpies DHdes are further calculated by using Eq. (3) [32], 3 DHdes ða MSiH3 Þ ¼ Etot ðMSiÞ þ Etot ðH2 Þ Etot ðMSiH3 Þ 2
(3)
where the implications of Etot ðMSiH3 Þ, Etot ðMSiÞ and Etot ðH2 Þ are the same as those in Eqs. (1) and (2) above. The calculated results are plotted in Fig. 3(a). Evidently, the enthalpies values are all positive, suggesting the endothermic process for the dehydrogenation of each silanide. Notablely, the hydrogen desorption enthalpy of a-KSiH3 is calculated as 28.9729 kJ/ mol$H2, which is very close to the experimental (22.6 kJ/ mol$H2) and calculated data (28.1 kJ/mol$H2) reported by Chotard et al. [16]. According to the Van't Hoff equation eq lnðpH2 =p0H2 Þ ¼ ðDH0 =RTÞ þ ðDS0 =RÞ [33], it can be deduced that the dehydrogenation temperature T of hydrogen storage maeq terials at a given hydrogen equilibrium pressure pH2 exhibits a linear relationship with the hydrogen desorption enthalpy DH0 . Namely, the less hydrogen desorption enthalpy will correspond to the lower dehydrogenation temperature of hydrogen storage materials. Thus, the dehydrogenation temperature of a-MSiH3 silanides is gradually decreased in the order of a-CsSiH3, a-RbSiH3, a-KSiH3, a-NaSiH3 and a-LiSiH3. Although a-LiSiH3 and a-NaSiH3 can theoretically dehydrogenate at a relatively lower temperature, the two silanides have been reported to be difficult to form due to the disproportionation of LiSi and NaSi when hydrogenation [18,20]. Therefore, a-KSiH3 should be the most promising alkali metal silanide for reversible hydrogen storage by comprehensively considering its hydrogen storage capacity (4.29 wt%, see Fig. 3(b)), thermostability and dehydrogenation ability.
Electronic structure Density of states To disclose the bonding characteristics within a-MSiH3 silanides and understand their thermostabilities and dehydrogenation properties, the total and partial densities of states of
each silanide are calculated and plotted in Fig. 4(a)e(e), respectively. In these figures, the Fermi level (EF) is set as zero and used as a reference. It can be seen that these silanides all exhibit the semiconducting behaviors due to the discontinuities of total densities of states near EF with the energy gap less than 4 eV. The obvious Si (3s), Si (3p) and H (1s) orbitals hybridizing in the valence band below EF can be seen, which contributes the strong covalent bonding interactions between Si and H atoms. There is hardly any valence orbitals hybridizing between M and Si/H atoms below EF for a-MSiH3 silanides except for a-CsSiH3, meaning the main ionic bonding interactions between M and SiH3 complex. As far as a-CsSiH3 is concerned, the Cs (5p), Si (3s) and H (1s) orbitals hybridizing in the energy range between 10 eV and 7 eV can be observed as shown in Fig. 4(e), revealing some covalent bonding interactions between Cs and SiH3 apart from the ionic bonding ones. The total densities of states of five a-MSiH3 silanides are further compared as shown in Fig. 4(f). It can be seen that there are three main bonding peaks below EF which originate from the simultaneous contributions of Si and H valence orbitals for each silanide. It is notable that the heights of the three bonding peaks are gradually increased in the order of a-LiSiH3, a-NaSiH3, a-KSiH3, a-RbSiH3 and a-CsSiH3, which is just consistent with the change order of the formation enthalpies and hydrogen desorption enthalpies of five aMSiH3 silanides, as shown in Figs. 2(a) and 3(a). This indicates that the thermostabilities and dehydrogenation stabilities of a-MSiH3 silanides are closely associated with the covalent bonding interactions between Si and H atoms within a-MSiH3. Namely, the weaker covalent bonding interactions between Si and H will lead to the lower thermostabilities as well as the stronger dehydrogenation abilities of a-MSiH3 silanides.
Charge density Further analysis of the charge density distribution of a-MSiH3 silanides and charge transfer between atoms is performed. Due to the similarity of bonding characteristics of five a-MSiH3 silanides, a-KSiH3 is taken as an example and its total charge density r(r) and difference charge density Dr(r) are plotted in Fig. 5(a) and (b), respectively. The difference charge density
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 4 e The calculated total and partial densities of states of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs) (a)e(e) and the comparison of total densities of states (f).
Dr(r) is defined as the difference between the total charge density of the solid and a superposition of atomic charge density with the same spatial coordinates as in the solid [34]. It can be seen from Fig. 5(a) that the charges are mainly
distributed around the SiH3 complex. There is an obvious overlapping of electron cloud between Si and H within SiH3 complex, while nearly no overlapping of electron cloud between K and SiH3 can be observed. This suggests that there are
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 5 e The calculated valence (a) and difference charge densities (b) of a-KSiH3 silanide.
strong bonding interactions between Si and H, while there are weak ones between K and SiH3 complex. From Fig. 5(b), it can be also seen that there is a significant charge transfer leading to K positively and SiH3 units negatively charged, which results in the ionic bonding interactions between K and SiH3. Within SiH3 complex, it can be also found that there is a partial charge transfer from Si to H, indicating that, besides the covalent bonding interactions between Si and H, there is also a certain degree of ionic bonding ones.
Bond order and ionicity From the analysis of the density of states and charge density above, it is evident that the H atoms have a stronger bonding with Si relative to M atoms. In order to understand well the correlation among the bonding characteristics, thermostabilities and dehydrogenation properties of a-MSiH3, the bonding interactions between H and Si atoms are further systematically investigated according to the Mulliken population analysis [35] including the ionicity of each atoms and bond order between atoms within a-MSiH3. The results are plotted in Fig. 6. From Fig. 6(a), it is apparent that the ionicities of M atoms are always positive, whereas the ionicities of Si and H
atoms are always negative, meaning the charge transfer from M atoms towards Si and H atoms. As explained earlier, the bond order is the overlap population of electrons between atoms, and it is a measure of the strength of covalent bonding between atoms [36]. Fig. 6(b) shows the bond orders calculated between Si and surrounding H atoms within a-MSiH3. It is found that the HeSi bond orders are always positive, indicating that there are strong covalent bonding interactions between Si and H. Also, the covalent bonding interactions between H and Si within a-KSiH3, a-RbSiH3 and a-CsSiH3 are relatively stronger than those within a-LiSiH3 and a-NaSiH3. Due to the characteristics of SiH3 units with mixed ioniccovalent bonds between H and Si atoms, we further estimate the amount of transferred charges between H and Si atoms by calculating the ionicity differences DI0(I0[H]I0[Si]) [34], where I0[H] and I0[Si] are the ionicity of H and Si atoms, respectively. The results are also plotted in Fig. 6(b), It can be seen that the DI0(I0[H]I0[Si]) in a-KSiH3, a-RbSiH3 and aCsSiH3 are relatively lower than those in a-LiSiH3 and aNaSiH3, suggesting that few charge transfer from Si to H in aKSiH3, a-RbSiH3 and a-CsSiH3 relative to the other two systems. Based on the comprehensive analysis including the
Fig. 6 e The calculated ionicity (a), bond order of HeSi and ionicity difference DI0(I0[H]¡I0[Si]) (b) of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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bond order, ionicity differences, formation enthalpy and hydrogen desorption enthalpy, we can conclude again that the thermostabilities and dehydrogenation properties of aMSiH3 silanides are mainly dominated by the covalent bonding interactions between H and Si. The weaker covalent bonding interactions between Si and H will result in the lower thermostabilities and the stronger dehydrogenation abilities. This conclusion is just consistent with that induced from densities of states above.
Conclusions Using first-principles calculations method based on density functional theory, the structural, energetic and electronic properties of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) are systematically investigated for the first time. Some key conclusions are summarized as the following. 1) The structural parameters of a-MSiH3/MSi including lattice constants and atomic positions are determined through geometry optimization, which are close to the experimental data analysed from X-ray and neutron powder diffraction. 2) a-KSiH3, a-RbSiH3 and a-CsSiH3 silanides are easier to be synthetized relative to a-LiSiH3 and a-NaSiH3, whereas LiSi, KSi and CsSi Zintl phases are easier to be formed relative to NaSi and RbS. 3) The dehydrogenation abilities of a-MSiH3 silanides are gradually enhanced in the order of a-CsSiH3, a-RbSiH3, aKSiH3, a-NaSiH3 and a-LiSiH3, and a-KSiH3 (~4.29 wt%) is approved to be the most promising alkali metal silanide for reversible hydrogen storage. 4) There mainly exhibit the ionic bonding interactions between M and SiH3 complex in a-MSiH3 silanides. Within SiH3, there are mixed ionic-covalent bonding interactions between H and Si atoms. The covalent bonding interactions between H and Si atoms mainly dominate the thermostabilities and dehydrogenation properties of aMSiH3 silanides.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51401036), the Hunan Provincial Natural Science Foundation of China (No. 17JJ2263) and the Science Research Project of Hunan Province Office of Education (No. 16K001).
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] Zhu M, Wang H, Ouyang LZ, Zeng MQ. Composite structure and hydrogen storage properties in Mg-based alloys. Int J Hydrogen Energy 2006;31:251e7. [4] Ouyang LZ, Yang XS, Zhu M, Liu JW, Dong HW, Sun DL, et al. Enhanced hydrogen storage kinetics and stability by synergistic effects of in Situ formed CeH2.73 and Ni in CeH2.73MgH2-Ni nanocomposites. J Phys Chem C 2014;118:7808e20. [5] Sadhasivam T, Kim HT, Jung S, Roh SH, Park JH, Jung HY. Dimensional effects of nanostructured Mg/MgH2 for hydrogen storage applications: a review. Renew Sust Energy Rev 2017;72:523e34. [6] Shao J, Xiao XZ, Fan XL, Zhang LT, Li SQ, Ge HW, et al. Lowtemperature reversible hydrogen storage properties of LiBH4: a synergetic effect of nanoconfinement and nanocatalysts. J Phys Chem C 2014;118:11252e60. [7] Wang H, Lin HJ, Cai WT, Ouyang LZ, Zhu M. Tuning kinetics and thermodynamics of hydrogen storage in light metal element based systems-A review of recent progress. J Alloy Compd 2016;658:280e300. [8] Rehmat B, Rafiq MA, Javed Y, Irshad Z, Ahmed N, Mirza SM. Elastic properties of perovskite-type hydrides LiBeH3 and NaBeH3 for hydrogen storage. Int J Hydrogen Energy 2017. http://dx.doi.org/10.1016/j.ijhydene.2017.01.109. [9] Cao ZJ, Ouyang LZ, Wang H, Liu JW, Sun LX, Felderhoff M, et al. Development of Zr-Fe-V alloys for hybrid hydrogen storage system. Int J Hydrogen Energy 2016;41:11242e53. [10] Cao ZJ, Ouyang LZ, Li LL, Lu YS, Wang H, Liu JW, et al. Enhanced discharge capacity and cycling properties in highsamarium, praseodymium/neodymium-free, and low-cobalt A2B7 electrode materials for nickel-metal hydride battery. Int J Hydrogen Energy 2015;40:451e5. [11] Ouyang LZ, Cao ZJ, Li LL, Wang H, Liu JW, Min D, et al. Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling. Int J Hydrogen Energy 2014;39:12765e72. € m J, Zlotea C, Latroche M, Sahlberg M. Hydrogen[12] Angstro sorption properties of Nb4M0.9Si1.1 (M¼Co, Ni) hydrides. Int J Hydrogen Energy 2015;40:2692e7. [13] Kranak VF, Lin YC, Karlsson M, Mink J, Norberg ST, € ussermann U. Structural and vibrational properties of Ha Silyl (SiH 3 ) anion in KSiH3 and RbSiH3: new insight into Si-H interactions. Inorg Chem 2015;54:2300e9. [14] Jain A, Ichikawa T, Yamaguchi S, Miyaoka H, Kojima Y. Catalytic modification in dehydrogenation properties of KSiH3. Phys Chem Chem Phys 2014;16:26163e7. mencon D, Chotard JN. Catalyzed KSiH3 [15] Janot R, Tang WS, Cle as a reversible hydrogen storage material. J Mater Chem A 2016;4:19045e52. [16] Chotard JN, Tang WS, Raybaud P, Janot R. Potassium silanide (KSiH3): a reversible hydrogen storage material. Chem Eur J 2011;17:12302e9. [17] Tang WS, Chotard JN, Raybaud P, Janot R. Enthalpy-entropy compensation effect in hydrogen storage materials: striking example of alkali silanides MSiH3 (M¼K, Rb, Cs). J Phys Chem C 2014;118:3409e19. [18] Tang WS, Chotard JN, Janot R. Synthesis of single-phase LiSi by ball-milling: electrochemical behavior and hydrogenation properties. J Electrochem Soc 2013;160:A1232e40. [19] Jain A, Miyaoka H, Ichikawa T, Kojima Y. Tailoring the absorption-desorption properties of KSiH3 compound using nano-metals (Ni, Co, Nb) as catalyst. J Alloy Compd 2015;645:S144e7. [20] Tang WS, Chotard JN, Raybaud P, Janot R. Hydrogenation properties of KSi and NaSi Zintl phases. Phys Chem Chem Phys 2012;14:13319e24. [21] Evers J, Oehinger G, Sextl G. LiSi, a unique Zintl phasealthough stable, it long evaded synthesis. Eur J Solid State Inorg Chem 1997;34:773e84.
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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 e9
[22] Villars P, Calvert LD. Pearson's handbook of crystallographic data for intermetallic phases. Metals Park, Ohio: ASM; 1985. [23] Segall MD, Lindanp LD, Probert MJ, Pickard CJ, Hasnip PJ, Clark SJ, et al. First-principles simulation: ideas, illustrations and the CASTEP code. J Phys Condens Matter 2002;14:2717e44. [24] Vanderbilt D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys Rev B 1990;41:7892e5. [25] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [26] Fischer T, Almlof J. General methods for geometry and wave function optimization. J Phys Chem 1992;96:9768e74. [27] Kittel C. Introduction to solid state physics, Ch. 13. New York: John Wiley and Sons, Inc; 1986. [28] Fukai Y. The metal-hydrogen system. Berlin: SpringerVerleg; 1993. [29] Takahashi Y, Yukawa H, Morinaga M. Alloying effects on the electronic structure of Mg2Ni intermetallic hydride. J Alloy Compd 1996;242:98e107.
9
[30] Song Y, Guo ZX. Electronic structure, stability and bonding of the Li-N-H hydrogen storage system. Phys Rev B 2006;74:195120e6. [31] Zhang J, Huang YN, Peng P, Mao C, Shao YM, Zhou DW. Firstprinciples study on the dehydrogenation properties and mechanisms of Al-doped Mg2NiH4. Int J Hydrogen Energy 2011;36:5375e82. [32] van Setten MJ, de Wijs GA, Brocks G. Ab initio study of the effects of transition metal doping of Mg2NiH4. Phys Rev B 2007;76:075125e32. [33] Zhang J, Zhou YC, Ma ZS, Sun LQ, Peng P. Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principle calculations. Int J Hydrogen Energy 2013;38:3661e9. [34] Peles A, Alford JA, Ma Z, Yang L, Chou MY. First-principles study of NaAlH4 and Na3AlH6 complex hydrides. Phys Rev B 2004;70:165105e11. [35] Mulliken RS. Electronic population analysis on LCAO-MO molecular wave functions. J Chem Phys 1955;23:1833e40. [36] Matumura T, Yukawa H, Morinaga M. Alloying effects on the electronic structures of VH2 and V2H. J Alloy Compd 1999;284:82e8.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage J. Zhang a,*, S. Yan a, H. Qu a, X.F. Yu a, P. Peng b a
Hunan Provincial Key Laboratory of Safety Design and Reliability Technology for Engineering Vehicle, Changsha University of Science and Technology, Changsha 410114, China b College of Materials Science and Engineering, Hunan University, Changsha 410082, China
article info
abstract
Article history:
The alkali metal silanides a-MSiH3 appear to be a promising family of complex hydrides for
Received 2 February 2017
solid-state hydrogen storage. Herein the structural, energetic and electronic properties of
Received in revised form
a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investi-
3 March 2017
gated for the first time by using first-principles calculations method based on density
Accepted 6 March 2017
functional theory. The structural parameters of a-MSiH3 and MSi including lattice con-
Available online xxx
stants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder
Keywords:
diffraction. The calculations of formation enthalpy show that a-KSiH3, a-RbSiH3 and a-
Alkali metal silanides
CsSiH3 silanides are easier to be synthetized relative to a-LiSiH3 and a-NaSiH3, which in-
Hydrogen storage materials
terprets well the lower thermostabilities of experimental a-LiSiH3 and a-NaSiH3. Never-
First-principles calculations
theless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The
Formation enthalpy
calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of
Hydrogen desorption enthalpy
a-MSiH3 silanides along the decomposition path of a-MSiH3/MSi þ H2 are gradually
Electronic structure
enhanced in the order of a-CsSiH3, a-RbSiH3, a-KSiH3, a-NaSiH3 and a-LiSiH3, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, a-KSiH3 (~4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH3 complex, which constitutes the ionic bonding between them. The bonding within SiH3 complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH3 mainly dominate the thermostabilities and dehydrogenation properties of a-MSiH3 silanides. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is as an ideal energy carrier for the fuel cell applications including stationary, mobile and portable power
applications. The safe, effective and affordable hydrogen storage is a key challenge in developing hydrogen economy [1]. Solid materials-based hydrogen storage through reversible formation of hydrides is one of the most promising methods
* Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2017.03.132 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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due to their unique safety and high energy densities compared to pressurised gaseous and cryogenically liquefied hydrogen [2,3]. In the past few decades, a variety of solid-state hydrogen storage materials have been explored and developed. Among these materials, the light metal hydrides and complex metal hydrides are considered to be the suitable hydrogen storage media for their high volumetric and gravimetric hydrogen capacities. For example, MgH2 and LiBH4 are the two typical representatives, which are widely researched for their high hydrogen storage densities (7.6 wt%/110 kg m3 for MgH2 and 18.5 wt%/121 kg m3 for LiBH4) [4e6]. However, these hydrides are always suffering from high temperature requirement, slow kinetics and irreversibility. Although several progresses have been achieved in enhancing the hydrogen storage performance of metal hydrides by alloying, nanoscaling, catalyzing, or compositing, the synergetic improvement in the operating temperature, reversibility and cyclic stability is barely satisfactory [7]. Recently, some multielement alloys or their hydrides such as M-Be-H (M ¼ Li, Na) [8], Zr-Fe-V [9], RE-Mg-Ni (RE ¼ Rare Metals) [10,11], Nb4M0.9Si1.1 (M ¼ Co, Ni) [12] have been investigated as the hydrogen storage or negative electrode materials of nickel-metal hydride (Ni/MH) batteries. In particular, the alkali metal silanides a-MSiH3 (M ¼ Li, Na, K, Rb, Cs) and MSi Zintl phases are paid increasing attention for their reversible hydrogen storage capability near ambient conditions [13e20]. The excellent thermodynamic characteristics of a-MSiH3 silanides originate from their low entropy variation [13]. For example, a-KSiH3 has been reported as a promising hydrogen storage material. It has high hydrogen storage capacity of 4.29 wt% and reasonable formation enthalpy DH ¼ 28 kJ/ mol$H2, which allows hydrogen sorption at 100 C [14e16]. Moreover, a-KSiH3 can be synthesized by direct solidegas reaction between KSi Zintl phase and hydrogen. Similar to aKSiH3, a-RbSiH3 and a-CsSiH3, with relatively lower hydrogen storage densities of 2.59 wt% and 1.83 wt%, have been reported to be cubic isolated solid-state phases and they can also be synthesized through direct hydrogenation of RbSi and CsSi Zintl phases [17]. Different from the three silanides above, aLiSiH3 and a-NaSiH3, with relatively higher hydrogen storage densities of 7.89 wt% and 5.56 wt%, present the lower
thermostability. They are difficult to be synthesized by the direct hydrogenation of NaSi and LiSi. Some studies have shown that NaSi disproportionates to NaH and Na8Si46 clathrate phase, and LiSi forms LiH and Si under hydrogen pressure [18,20]. Thus, the stabilization of a-LiSiH3 and a-NaSiH3 with high hydrogen storage densities is still a great challenge. Although the alkali metal silanides a-MSiH3 are proposed as a promising family of hydrogen storage materials, their formation abilities, thermostabilities and dehydrogenation properties have not been studied yet from an energy point of view. Moreover, the detailed structural parameters of a-MSiH3 and the complicated bonding characteristics within silanides have not been well investigated due to the difficulty in determining hydrogen atomic positions by using experimental methods. In this work, first-principles calculations method based on density functional theory is employed to conduct a comprehensive study on the structural, energetic and electronic properties of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) for the first time. The results will contribute to understanding the formation abilities, thermostabilities and dehydrogenation properties of a-MSiH3 silanides. Furthermore, some underlying mechanisms associated with the thermostabilities and dehydrogenation properties are also explored from the perspective of electronic structures, which are expected to be the theoretical guidances for designing high performance a-MSiH3 complex hydrides for solid-state hydrogen storage.
Computational models and method At room temperature, the alkali metal silanides a-MSiH3 (M ¼ Li, Na, K, Rb, Cs) exhibit the cubic NaCl-type arrangement of alkali metal M and Si atoms with the space group of Fm-3m, as shown in Fig. 1 [13]. The pyramidal SiH3 complexes are distributed in random orientations in the crystal structure. The unit cell of each silanide all contains 4 formula units and the M, Si and H atoms are located at the 4c, 4c and 96k sites, respectively [17]. Their experimental structural parameters including lattice constants and atomic positions are listed in Table 1. Due to the difficulty in synthesizing a-LiSiH3
Fig. 1 e The crystal cell models of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Table 1 e The calculated equilibrium lattice constants, atomic positions and total energies per formula unit (f.u.) of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs). Material Lattice constant (A) Atomic position Total energy K-point Cal.
Exp.
a-LiSiH3 (Fm-3m, cubic)
a ¼ 6.5690
e
a-NaSiH3 (Fm-3m, cubic)
a ¼ 6.7892
e
a-KSiH3 (Fm-3m, cubic)
a ¼ 7.2008
a ¼ 7.2245a
a-RbSiH3 (Fm-3m, cubic)
a ¼ 7.5616
a ¼ 7.4860a
a-CsSiH3 (Fm-3m, cubic)
a ¼ 7.9462
a ¼ 7.8320a
LiSi (I41/a, tetragonal) NaSi (C2/c, monoclinic)
a ¼ 9.1450 c ¼ 5.4574 a ¼ 11.9744 b ¼ 6.5333 c ¼ 11.0178 b ¼ 117.4591 a ¼ 13.7119
a ¼ 9.354b c ¼ 5.746b a ¼ 12.190c b ¼ 6.550c c ¼ 11.180c b ¼ 119 c a ¼ 12.620c
RbSi (P-43n, cubic)
a ¼ 13.1601
a ¼ 13.040c
CsSi (P-43n, cubic)
a ¼ 13.7286
a ¼ 13.500c
KSi (P-43n, cubic)
a b c
Cal.
Exp.
Li (4c): 0, 0, 0 Si (4c): 0.4698, 0.5192, 0.5012 H (96k): 0.4835, 0.6549, 0.7338 Na (4c):0, 0, 0 Si (4c): 0.4653, 0.5296, 0.5116 H (96k): 0.5289, 0.6491, 0.7052 K (4c): 0, 0, 0 Si (4c): 0.4798, 0.5187, 0.4808 H (96k): 0.5642, 0.6449, 0.6261 Rb (4c): 0, 0, 0 Si (4c): 0.4807, 0.5169, 0.4824 H (96k): 0.5610, 0.6380, 0.6228 Cs (4c): 0, 0, 0 Si (4c): 0.4815, 0.5137, 0.4878 H (96k): 0.5597, 0.6310, 0.6179 Li (16f): 0.1350, 0.8903, 0.1309 Si (16f): 0.1472, 0.8862, 0.6049 Na1 (8f): 0.3550, 0.6735, 0.3577 Na2 (8f): 0.6356, 0.9074, 0.4507 Si1 (8f): 0.4355, 0.1944, 0.3115 Si2 (8f): 0.5959, 0.4524, 0.3571 K1 (8e): 0.3338, 0.3338, 0.3338 K2 (24i): 0.3373, 0.1398, 0.0662 Si1 (8e): 0.0680, 0.0680, 0.0680 Si2 (24i): 0.0628, 0.3171, 0.4261 Rb1 (8e): 0.3327, 0.3327, 0.3327 Rb2 (24i): 0.3363, 0.1386, 0.0655 Si1 (8e): 0.0653, 0.0653, 0.0653 Si2 (24i): 0.0595, 0.3148, 0.4288 Cs1 (8e): 0.3336, 0.3336, 0.3336 Cs2 (24i): 0.3382, 0.1394, 0.0666 Si1 (8e): 0.0625, 0.0625, 0.0625 Si2 (24i): 0.0583, 0.3115, 0.4322
e e e e e e K (4c): 0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5632, 0.6365, 0.6365a Rb (4c): 0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5616, 0.6327, 0.6327a Cs (4c):0, 0, 0a Si (4c): 0.5, 0.5, 0.5a H (96k): 0.5616, 0.6273, 0.6273a Li (16f): 0.0810, 0.8860, 0.0602b Si (16f): 0.0111, 0.9522, 0.5937b Na1 (8f): 0.351, 0.662, 0.358c Na2 (8f): 0.632, 0.900, 0.455c Si1 (8f): 0.440, 0.210, 0.314c Si2 (8f): 0.597, 0.463, 0.357c K1 (8e): 0.331, 0.331, 0.331c K2 (24i): 0.336, 0.141, 0.064c Si1 (8e): 0.068, 0.068, 0.068c Si2 (24i): 0.063, 0.318, 0.427c Rb1 (8e): 0.331, 0.331, 0.331c Rb2 (24i): 0.336, 0.141, 0.004c Si1 (8e): 0.066, 0.066, 0.066c Si2 (24i): 0.061, 0.316, 0.429c Cs1 (8e): 0.331, 0.331, 0.331c Cs2 (24i): 0.336, 0.141, 0.062c Si1 (8e): 0.063, 0.063, 0.063c Si2 (24i): 0.059, 0.315, 0.431c
(eV/f.u.) 345.0022
444
1459.3143
444
934.7379
333
814.7721
333
705.2695
333
297.4331
335
1411.6314
243
886.7409
222
766.6876
222
657.1339
222
Reference [17]. Reference [21]. Reference [22].
and a-NaSiH3 and the lackage of their structural parameters, the unit cells of a-LiSiH3 and a-NaSiH3 are both constructed by adopting the structural data of a-KSiH3. After dehydrogenation of a-MSiH3 along the decomposition path of a-MSiH3/MSi þ H2, the obtained MSi Zintl phases exhibit different crystal structures from a-MSiH3. For LiSi phase, it presents a tetragonal structure with the space of I41/a [21]. Its unit cell contains 16 formula units and the Li and Si atoms are located at the 16f and 16f sites, respectively. For NaSi phase, it has a monoclinic structure with the space of C2/c. Its unit cell also contains 16 formula units and two different kinds of Na and Si atoms are located at the 8f (Na1 and Na2) and 8f (Si1 and Si2) sites, respectively. For MSi (M ¼ K, Rb, Cs) phases, they all belong to the cubic structure with the space group of P43n [22]. Their unit cells all contain 32 formula units. Similar to NaSi phase, there are also two different kinds of M (M ¼ K, Rb, Cs) and Si atoms which are located at the 8e (M1), 24i (M2), 8e (Si1) and 24i (Si2) sites, respectively. The experimental structural parameters of MSi Zintl phases are also listed in Table 1. In this work, all calculations are performed using firstprinciples plane-wave pseudopotential method through the
CASTEP software package [23]. Ultrasoft pseudopotentials [24] in reciprocal space are used for the core-valence interactions. The generalized gradient approximation (GGA) in PerdeweBurkeeErnzerhof (PBE) format [25] is adopted as the exchange-correction functional. The plane wave cutoff energy with 500 eV and k sampling with different k-point mesh (see Table 1) in Brillouin zone [26] are employed for the geometry relaxation and electronic structures calculations of a-MSiH3 silanides and MSi Zintl phases. The pseudo atomic orbitals used are H 1s1, Si 3s23p2, Li 1s22s1, Na 2s22p63s1, K 3s23p64s1, Rb 4s24p65s1 and Cs 5s25p66s1, respectively. The convergence criteria for energy, force, stress and displacement are A, 0.05 GPa and 0.001 Å, respectively. 1.0 105 eV/atom, 0.03 eV/
Results and discussion Structural parameter To examine the accuracy of computational parameters selected in this work, some testing calculations on the
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Table 2 e The calculated equilibrium lattice constants, bond length and total energies per formula unit (f.u.) of solid bcc-Li, bcc-Na, bcc-K, bcc-Rb, bcc-Cs, fcc-Si and gaseous H2. Material Structural parameter ( A) Total K-point Cal. bcc-Li bcc-Na bcc-K bcc-Rb bcc-Cs fcc-Si H2 a b
a a a a a a d
¼ 3.4354 ¼ 4.1869 ¼ 5.2745 ¼ 5.6543 ¼ 6.1400 ¼ 5.4633 ¼ 0.7525
Exp. a a a a a a d
¼ 3.491a ¼ 4.225a ¼ 5.225a ¼ 5.585a ¼ 6.045a ¼ 5.430a ¼ 0.741b
energy (eV/f.u.) 189.9765 1304.2709 779.2330 659.2672 549.6685 107.3328 31.6977
8 6 6 4 4 8 2
8 6 6 4 4 8 2
8 6 6 4 4 8 2
Reference [27]. Reference [28].
structural parameters of solid bcc-Li, bcc-Na, bcc-K, bcc-Rb, bcc-Cs, fcc-Si and gaseous H2 are performed. The calculated results are listed in Table 2. It can be seen that the equilibrium lattice constants of solid elements and bond length of H2 molecule are in good agreement with experimental data [27,28]. The maximum deviation is only 1.59% (a value of bccLi), suggesting that the present calculations are precise enough to represent the ground-state properties of a-MSiH3 silanides and MSi Zintl phases.
Formation enthalpy The formation enthalpy is an important thermodynamic parameter of materials [29]. It is the best aid to establish whether theoretically predicted phases are likely to form and be stable, and such data may also serve as guidances for possible synthesis routes of materials. Commonly, the formation enthalpy is always negative, and the lower enthalpy value will correspond to the stronger formation ability and the high thermostability of synthesized materials. In order to investigate the formation ability and thermostability of aMSiH3 silanides and MSi Zintl phases, their formation enthalpies DHform are calculated by using Eqs. (1) and (2), respectively [30],
DHform ða MSiH3 Þ ¼Etot ða MSiH3 Þ Esolid tot ðMÞ 3 Esolid tot ðSiÞ Etot ðH2 Þ 2
(1)
solid DHform ðMSiÞ ¼ Etot ðMSiÞ Esolid tot ðMÞ Etot ðSiÞ
(2)
where Etot ða MSiH3 Þ and Etot ðMSiÞ are the total energies per formular unit of a-MSiH3 silanides and MSi Zintl phases, respectively. and Esolid tot ðSiÞ are the total energies per formular unit of solid M alkali metals and Si, respectively. Etot ðH2 Þ refers to the total energy of single gaseous H2 molecule. The calculated formation enthalpies of a-MSiH3 and MSi are plotted in Fig. 2. It can be seen that these enthalpy values are all negative, meaning structural stabilities of a-MSiH3 and MSi from a thermodynamics point of view. For a-MSiH3 silanides as shown in Fig. 2(a), their enthalpy values become gradually negative in the order of a-LiSiH3, a-NaSiH3, a-KSiH3, a-RbSiH3 and a-CsSiH3, suggesting the increasing thermostability. Comparatively, the values of a-KSiH3, a-RbSiH3 and a-CsSiH3 are much lower that those of a-LiSiH3 and a-NaSiH3, which indicates that a-KSiH3, a-RbSiH3 and a-CsSiH3 are easier to be synthetized and more stable relative to the other two silanides. The higher formation enthalpies of a-LiSiH3 and aNaSiH3 calculated here interpret well the lower thermostabilities and difficulty in synthesizing the two silanides reported by previous experiments [18,20]. As for MSi Zintl phases, the change of enthalpy values are different from that of a-MSiH3 silanides, as shown in Fig. 2(b). It can be found that the formation enthalpies of MSi become gradually negative in the order of NaSi, RbSi, LiSi, CsSi and KSi. Apparently, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. By contrast, KSi is the most stable Zintl phase with the lowest formation enthalpy of 16.8937 kJ/mol per formular unit.
Hydrogen desorption enthalpy Similar to the formation enthalpy, the hydrogen desorption enthalpy is also an important thermodynamic parameter used to identify and classify hydrogen storage materials. It determines the heat of the overall dehydrogenation reaction
Fig. 2 e The calculated formation enthalpies of a-MSiH3 silanides (a) and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) (b). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 3 e The calculated hydrogen desorption enthalpies (a) and theoretical hydrogen storage capacities (b) of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs).
of hydrogen storage materials, which in turn affects their dehydrogenation temperature [31]. In order to evaluate the dehydrogenation abilities of a-MSiH3 silanides, their hydrogen desorption enthalpies DHdes are further calculated by using Eq. (3) [32], 3 DHdes ða MSiH3 Þ ¼ Etot ðMSiÞ þ Etot ðH2 Þ Etot ðMSiH3 Þ 2
(3)
where the implications of Etot ðMSiH3 Þ, Etot ðMSiÞ and Etot ðH2 Þ are the same as those in Eqs. (1) and (2) above. The calculated results are plotted in Fig. 3(a). Evidently, the enthalpies values are all positive, suggesting the endothermic process for the dehydrogenation of each silanide. Notablely, the hydrogen desorption enthalpy of a-KSiH3 is calculated as 28.9729 kJ/ mol$H2, which is very close to the experimental (22.6 kJ/ mol$H2) and calculated data (28.1 kJ/mol$H2) reported by Chotard et al. [16]. According to the Van't Hoff equation eq lnðpH2 =p0H2 Þ ¼ ðDH0 =RTÞ þ ðDS0 =RÞ [33], it can be deduced that the dehydrogenation temperature T of hydrogen storage maeq terials at a given hydrogen equilibrium pressure pH2 exhibits a linear relationship with the hydrogen desorption enthalpy DH0 . Namely, the less hydrogen desorption enthalpy will correspond to the lower dehydrogenation temperature of hydrogen storage materials. Thus, the dehydrogenation temperature of a-MSiH3 silanides is gradually decreased in the order of a-CsSiH3, a-RbSiH3, a-KSiH3, a-NaSiH3 and a-LiSiH3. Although a-LiSiH3 and a-NaSiH3 can theoretically dehydrogenate at a relatively lower temperature, the two silanides have been reported to be difficult to form due to the disproportionation of LiSi and NaSi when hydrogenation [18,20]. Therefore, a-KSiH3 should be the most promising alkali metal silanide for reversible hydrogen storage by comprehensively considering its hydrogen storage capacity (4.29 wt%, see Fig. 3(b)), thermostability and dehydrogenation ability.
Electronic structure Density of states To disclose the bonding characteristics within a-MSiH3 silanides and understand their thermostabilities and dehydrogenation properties, the total and partial densities of states of
each silanide are calculated and plotted in Fig. 4(a)e(e), respectively. In these figures, the Fermi level (EF) is set as zero and used as a reference. It can be seen that these silanides all exhibit the semiconducting behaviors due to the discontinuities of total densities of states near EF with the energy gap less than 4 eV. The obvious Si (3s), Si (3p) and H (1s) orbitals hybridizing in the valence band below EF can be seen, which contributes the strong covalent bonding interactions between Si and H atoms. There is hardly any valence orbitals hybridizing between M and Si/H atoms below EF for a-MSiH3 silanides except for a-CsSiH3, meaning the main ionic bonding interactions between M and SiH3 complex. As far as a-CsSiH3 is concerned, the Cs (5p), Si (3s) and H (1s) orbitals hybridizing in the energy range between 10 eV and 7 eV can be observed as shown in Fig. 4(e), revealing some covalent bonding interactions between Cs and SiH3 apart from the ionic bonding ones. The total densities of states of five a-MSiH3 silanides are further compared as shown in Fig. 4(f). It can be seen that there are three main bonding peaks below EF which originate from the simultaneous contributions of Si and H valence orbitals for each silanide. It is notable that the heights of the three bonding peaks are gradually increased in the order of a-LiSiH3, a-NaSiH3, a-KSiH3, a-RbSiH3 and a-CsSiH3, which is just consistent with the change order of the formation enthalpies and hydrogen desorption enthalpies of five aMSiH3 silanides, as shown in Figs. 2(a) and 3(a). This indicates that the thermostabilities and dehydrogenation stabilities of a-MSiH3 silanides are closely associated with the covalent bonding interactions between Si and H atoms within a-MSiH3. Namely, the weaker covalent bonding interactions between Si and H will lead to the lower thermostabilities as well as the stronger dehydrogenation abilities of a-MSiH3 silanides.
Charge density Further analysis of the charge density distribution of a-MSiH3 silanides and charge transfer between atoms is performed. Due to the similarity of bonding characteristics of five a-MSiH3 silanides, a-KSiH3 is taken as an example and its total charge density r(r) and difference charge density Dr(r) are plotted in Fig. 5(a) and (b), respectively. The difference charge density
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 4 e The calculated total and partial densities of states of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs) (a)e(e) and the comparison of total densities of states (f).
Dr(r) is defined as the difference between the total charge density of the solid and a superposition of atomic charge density with the same spatial coordinates as in the solid [34]. It can be seen from Fig. 5(a) that the charges are mainly
distributed around the SiH3 complex. There is an obvious overlapping of electron cloud between Si and H within SiH3 complex, while nearly no overlapping of electron cloud between K and SiH3 can be observed. This suggests that there are
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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Fig. 5 e The calculated valence (a) and difference charge densities (b) of a-KSiH3 silanide.
strong bonding interactions between Si and H, while there are weak ones between K and SiH3 complex. From Fig. 5(b), it can be also seen that there is a significant charge transfer leading to K positively and SiH3 units negatively charged, which results in the ionic bonding interactions between K and SiH3. Within SiH3 complex, it can be also found that there is a partial charge transfer from Si to H, indicating that, besides the covalent bonding interactions between Si and H, there is also a certain degree of ionic bonding ones.
Bond order and ionicity From the analysis of the density of states and charge density above, it is evident that the H atoms have a stronger bonding with Si relative to M atoms. In order to understand well the correlation among the bonding characteristics, thermostabilities and dehydrogenation properties of a-MSiH3, the bonding interactions between H and Si atoms are further systematically investigated according to the Mulliken population analysis [35] including the ionicity of each atoms and bond order between atoms within a-MSiH3. The results are plotted in Fig. 6. From Fig. 6(a), it is apparent that the ionicities of M atoms are always positive, whereas the ionicities of Si and H
atoms are always negative, meaning the charge transfer from M atoms towards Si and H atoms. As explained earlier, the bond order is the overlap population of electrons between atoms, and it is a measure of the strength of covalent bonding between atoms [36]. Fig. 6(b) shows the bond orders calculated between Si and surrounding H atoms within a-MSiH3. It is found that the HeSi bond orders are always positive, indicating that there are strong covalent bonding interactions between Si and H. Also, the covalent bonding interactions between H and Si within a-KSiH3, a-RbSiH3 and a-CsSiH3 are relatively stronger than those within a-LiSiH3 and a-NaSiH3. Due to the characteristics of SiH3 units with mixed ioniccovalent bonds between H and Si atoms, we further estimate the amount of transferred charges between H and Si atoms by calculating the ionicity differences DI0(I0[H]I0[Si]) [34], where I0[H] and I0[Si] are the ionicity of H and Si atoms, respectively. The results are also plotted in Fig. 6(b), It can be seen that the DI0(I0[H]I0[Si]) in a-KSiH3, a-RbSiH3 and aCsSiH3 are relatively lower than those in a-LiSiH3 and aNaSiH3, suggesting that few charge transfer from Si to H in aKSiH3, a-RbSiH3 and a-CsSiH3 relative to the other two systems. Based on the comprehensive analysis including the
Fig. 6 e The calculated ionicity (a), bond order of HeSi and ionicity difference DI0(I0[H]¡I0[Si]) (b) of a-MSiH3 silanides (M ¼ Li, Na, K, Rb, Cs). Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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bond order, ionicity differences, formation enthalpy and hydrogen desorption enthalpy, we can conclude again that the thermostabilities and dehydrogenation properties of aMSiH3 silanides are mainly dominated by the covalent bonding interactions between H and Si. The weaker covalent bonding interactions between Si and H will result in the lower thermostabilities and the stronger dehydrogenation abilities. This conclusion is just consistent with that induced from densities of states above.
Conclusions Using first-principles calculations method based on density functional theory, the structural, energetic and electronic properties of a-MSiH3 silanides and MSi Zintl phases (M ¼ Li, Na, K, Rb, Cs) are systematically investigated for the first time. Some key conclusions are summarized as the following. 1) The structural parameters of a-MSiH3/MSi including lattice constants and atomic positions are determined through geometry optimization, which are close to the experimental data analysed from X-ray and neutron powder diffraction. 2) a-KSiH3, a-RbSiH3 and a-CsSiH3 silanides are easier to be synthetized relative to a-LiSiH3 and a-NaSiH3, whereas LiSi, KSi and CsSi Zintl phases are easier to be formed relative to NaSi and RbS. 3) The dehydrogenation abilities of a-MSiH3 silanides are gradually enhanced in the order of a-CsSiH3, a-RbSiH3, aKSiH3, a-NaSiH3 and a-LiSiH3, and a-KSiH3 (~4.29 wt%) is approved to be the most promising alkali metal silanide for reversible hydrogen storage. 4) There mainly exhibit the ionic bonding interactions between M and SiH3 complex in a-MSiH3 silanides. Within SiH3, there are mixed ionic-covalent bonding interactions between H and Si atoms. The covalent bonding interactions between H and Si atoms mainly dominate the thermostabilities and dehydrogenation properties of aMSiH3 silanides.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51401036), the Hunan Provincial Natural Science Foundation of China (No. 17JJ2263) and the Science Research Project of Hunan Province Office of Education (No. 16K001).
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] Zhu M, Wang H, Ouyang LZ, Zeng MQ. Composite structure and hydrogen storage properties in Mg-based alloys. Int J Hydrogen Energy 2006;31:251e7. [4] Ouyang LZ, Yang XS, Zhu M, Liu JW, Dong HW, Sun DL, et al. Enhanced hydrogen storage kinetics and stability by synergistic effects of in Situ formed CeH2.73 and Ni in CeH2.73MgH2-Ni nanocomposites. J Phys Chem C 2014;118:7808e20. [5] Sadhasivam T, Kim HT, Jung S, Roh SH, Park JH, Jung HY. Dimensional effects of nanostructured Mg/MgH2 for hydrogen storage applications: a review. Renew Sust Energy Rev 2017;72:523e34. [6] Shao J, Xiao XZ, Fan XL, Zhang LT, Li SQ, Ge HW, et al. Lowtemperature reversible hydrogen storage properties of LiBH4: a synergetic effect of nanoconfinement and nanocatalysts. J Phys Chem C 2014;118:11252e60. [7] Wang H, Lin HJ, Cai WT, Ouyang LZ, Zhu M. Tuning kinetics and thermodynamics of hydrogen storage in light metal element based systems-A review of recent progress. J Alloy Compd 2016;658:280e300. [8] Rehmat B, Rafiq MA, Javed Y, Irshad Z, Ahmed N, Mirza SM. Elastic properties of perovskite-type hydrides LiBeH3 and NaBeH3 for hydrogen storage. Int J Hydrogen Energy 2017. http://dx.doi.org/10.1016/j.ijhydene.2017.01.109. [9] Cao ZJ, Ouyang LZ, Wang H, Liu JW, Sun LX, Felderhoff M, et al. Development of Zr-Fe-V alloys for hybrid hydrogen storage system. Int J Hydrogen Energy 2016;41:11242e53. [10] Cao ZJ, Ouyang LZ, Li LL, Lu YS, Wang H, Liu JW, et al. Enhanced discharge capacity and cycling properties in highsamarium, praseodymium/neodymium-free, and low-cobalt A2B7 electrode materials for nickel-metal hydride battery. Int J Hydrogen Energy 2015;40:451e5. [11] Ouyang LZ, Cao ZJ, Li LL, Wang H, Liu JW, Min D, et al. Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling. Int J Hydrogen Energy 2014;39:12765e72. € m J, Zlotea C, Latroche M, Sahlberg M. Hydrogen[12] Angstro sorption properties of Nb4M0.9Si1.1 (M¼Co, Ni) hydrides. Int J Hydrogen Energy 2015;40:2692e7. [13] Kranak VF, Lin YC, Karlsson M, Mink J, Norberg ST, € ussermann U. Structural and vibrational properties of Ha Silyl (SiH 3 ) anion in KSiH3 and RbSiH3: new insight into Si-H interactions. Inorg Chem 2015;54:2300e9. [14] Jain A, Ichikawa T, Yamaguchi S, Miyaoka H, Kojima Y. Catalytic modification in dehydrogenation properties of KSiH3. Phys Chem Chem Phys 2014;16:26163e7. mencon D, Chotard JN. Catalyzed KSiH3 [15] Janot R, Tang WS, Cle as a reversible hydrogen storage material. J Mater Chem A 2016;4:19045e52. [16] Chotard JN, Tang WS, Raybaud P, Janot R. Potassium silanide (KSiH3): a reversible hydrogen storage material. Chem Eur J 2011;17:12302e9. [17] Tang WS, Chotard JN, Raybaud P, Janot R. Enthalpy-entropy compensation effect in hydrogen storage materials: striking example of alkali silanides MSiH3 (M¼K, Rb, Cs). J Phys Chem C 2014;118:3409e19. [18] Tang WS, Chotard JN, Janot R. Synthesis of single-phase LiSi by ball-milling: electrochemical behavior and hydrogenation properties. J Electrochem Soc 2013;160:A1232e40. [19] Jain A, Miyaoka H, Ichikawa T, Kojima Y. Tailoring the absorption-desorption properties of KSiH3 compound using nano-metals (Ni, Co, Nb) as catalyst. J Alloy Compd 2015;645:S144e7. [20] Tang WS, Chotard JN, Raybaud P, Janot R. Hydrogenation properties of KSi and NaSi Zintl phases. Phys Chem Chem Phys 2012;14:13319e24. [21] Evers J, Oehinger G, Sextl G. LiSi, a unique Zintl phasealthough stable, it long evaded synthesis. Eur J Solid State Inorg Chem 1997;34:773e84.
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132
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 e9
[22] Villars P, Calvert LD. Pearson's handbook of crystallographic data for intermetallic phases. Metals Park, Ohio: ASM; 1985. [23] Segall MD, Lindanp LD, Probert MJ, Pickard CJ, Hasnip PJ, Clark SJ, et al. First-principles simulation: ideas, illustrations and the CASTEP code. J Phys Condens Matter 2002;14:2717e44. [24] Vanderbilt D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys Rev B 1990;41:7892e5. [25] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [26] Fischer T, Almlof J. General methods for geometry and wave function optimization. J Phys Chem 1992;96:9768e74. [27] Kittel C. Introduction to solid state physics, Ch. 13. New York: John Wiley and Sons, Inc; 1986. [28] Fukai Y. The metal-hydrogen system. Berlin: SpringerVerleg; 1993. [29] Takahashi Y, Yukawa H, Morinaga M. Alloying effects on the electronic structure of Mg2Ni intermetallic hydride. J Alloy Compd 1996;242:98e107.
9
[30] Song Y, Guo ZX. Electronic structure, stability and bonding of the Li-N-H hydrogen storage system. Phys Rev B 2006;74:195120e6. [31] Zhang J, Huang YN, Peng P, Mao C, Shao YM, Zhou DW. Firstprinciples study on the dehydrogenation properties and mechanisms of Al-doped Mg2NiH4. Int J Hydrogen Energy 2011;36:5375e82. [32] van Setten MJ, de Wijs GA, Brocks G. Ab initio study of the effects of transition metal doping of Mg2NiH4. Phys Rev B 2007;76:075125e32. [33] Zhang J, Zhou YC, Ma ZS, Sun LQ, Peng P. Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principle calculations. Int J Hydrogen Energy 2013;38:3661e9. [34] Peles A, Alford JA, Ma Z, Yang L, Chou MY. First-principles study of NaAlH4 and Na3AlH6 complex hydrides. Phys Rev B 2004;70:165105e11. [35] Mulliken RS. Electronic population analysis on LCAO-MO molecular wave functions. J Chem Phys 1955;23:1833e40. [36] Matumura T, Yukawa H, Morinaga M. Alloying effects on the electronic structures of VH2 and V2H. J Alloy Compd 1999;284:82e8.
Please cite this article in press as: Zhang J, et al., Alkali metal silanides a-MSiH3: A family of complex hydrides for solid-state hydrogen storage, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.132