Synergistic Effects of Mg and N Cosubstitution on Enhanced

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Synergistic Effects of Mg and N Cosubstitution on Enhanced Dehydrogenation Properties of LiBH4: A First-Principles Study Zhuonan Huang,†,‡ Yuqi Wang,*,† Di Wang,† Fusheng Yang,§ Zhen Wu,§ Lan Zheng,† Xiaolong Han,† Le Wu,† and Zaoxiao Zhang§ †

School of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Shaanxi 721013, P. R. China § School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, P. R. China J. Phys. Chem. C Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/11/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A metal + nonmetal (Mg + N) partial cosubstitution method for improving the dehydrogenation properties of lithium borohydride (LiBH4), which is a potential solid hydrogen storage material, is proposed. A detailed analysis of the electronic structure, charge density redistribution, and dehydrogenation properties reveals that the cosubstitution has a more positive effect on several properties, including the thermal stability, hydrogen dissociation energy, and dehydrogenation temperature, than the single substitution of Mg or N. When Mg and N are codoped into LiBH4, its formation enthalpy increases from −0.332 to −0.293 eV·atom−1 and its thermal stability decreases. Moreover, density functional theory calculations of the Mg + N cosubstituted system show that its hydrogen dissociation energy is the lowest and the onset dehydrogenation temperature is reduced to 160.5 °C, indicating that Mg + N cosubstitution can significantly promote the dehydrogenation thermodynamic performance of LiBH4 materials.

1. INTRODUCTION Many efforts have been focused on discovering suitable materials with high hydrogen storage capacities to meet the requirements of commercial vehicles powered by proton exchange membrane fuel cells.1−3 Complex light metal hydrides are interesting materials for mobile hydrogen storage because of their high gravimetric and volumetric hydrogen densities.4−6 For example, LiBH4 contains 18.5 wt % H.7−9 However, the slow dehydriding kinetics and high dissociation temperature of LiBH4 currently limit its practical applications.10,11 Consequently, many strategies have been employed to improve the dynamic and kinetic properties of LiBH4 in the last decade. First, Züttel et al.12 experimentally demonstrated that mixing 75% LiBH4 with 25% SiO2 can reduce the initial hydrogen release temperature to 200 °C, but the catalytic mechanism is still unknown. Later, Yu et al.13 showed that oxide additives, such as Fe2O3, V2O5, Nb2O5, TiO2, and SiO2, can effectively reduce the dehydriding temperature. The studied oxides destabilized LiBH4 in the following order: Fe2O3 > V2O5 > Nb2O5 > TiO2 > SiO2. The LiBH4−Fe2O3 sample with a mass ratio of 1:2 released 6 wt % hydrogen below 200 °C. Moreover, LiBH4 can be modified by ball milling with halides. For example, catalysts such as TiF3 and TiCl3 led to a considerable enhancement in the thermodynamics and kinetics.14 In © XXXX American Chemical Society

particular, TiF3 improves the kinetics of hydrogen release from LiBH4 more than TiCl3. This result is attributed to the possible substitution of the hydrogen in LiBH4 and LiH by F anions to generate LiBH4−xFx and LiH1−xFx during dehydrogenation. Fang et al.15 prepared a 3LiBH4/MnF2 composite by mechanically milling a mixture of LiBH4 and MnF2. On the basis of the thermodynamics, decomposition kinetics, and released hydrogen purity, the 3LiBH4/MnF2 composite appears to be a promising material for hydrogen storage because partially substituting Mn2+ for Li+ and F− for H− weakens the B−H bonds. In addition to halides, metals, and nonmetal oxides, which can enhance the kinetics of LiBH4, nonmetals can also play a significant role in promoting its dehydrogenation properties. Pendolino et al.16 studied the decomposition of a LiBH4 + B system by differential scanning calorimetry and thermogravimetry. They proposed that adding boron might be an effective way to reduce the dehydrogenation temperature of LiBH4, resulting in a decomposition temperature of 150 °C. Although these experimental studies have shown that incorporating some additives into LiBH4 can lower the Received: August 23, 2018 Revised: December 27, 2018 Published: December 31, 2018 A

DOI: 10.1021/acs.jpcc.8b08198 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C decomposition temperature and enhance the kinetics, the atomistic mechanisms of the decomposition and dehydrogenation processes and the roles of the additives are still unknown. Therefore, in recent years, many theoretical and computational studies have been performed to reveal the mechanisms by which additives affect the dehydrogenation of LiBH4. Van de Walle and co-workers17,18 reported first-principles studies of native defects and transition-metal impurities in LiBH4. They identified several transition-metal impurities that effectively enhance the kinetics of LiBH4. Their comprehensive results are in qualitative agreement with experimental observations. Yin et al.19 theoretically studied the decomposition reactions of undoped and F anion-doped LiBH4 by density functional theory (DFT) calculations. The calculated results revealed that the tuning of the thermodynamics by F anions is even more pronounced in the borohydride than in an aluminohydride. Wang et al.20 also investigated the synergistic effects of coexisting Li-vacancies and Ti on the dehydrogenation properties of the LiBH4(010) surface via first-principles calculations. The results indicated the existence of strong H−H, B−B, and Ti−B bonds and a weakened B−H bond in the transition states at the Ti−VLi surface, which lowered the kinetic barrier. Current research focuses on substituting a single element into LiBH4. However, a few reports of substituting the Li + B atoms with codoped metal + nonmetal atoms have appeared in the literature. Of the different metal or metal-containing additives, magnesium, which has the advantage of being light, improves the hydrogen release behavior of LiBH4.21−25 Moreover, amine complexes of many metal borohydrides, such as Mg(BH4)2·2NH3,26 Ca(BH4)2·2NH3,27 and LiBH4· NH3,28 were recently developed for hydrogen storage and were shown to have high hydrogen capacities and low decomposition temperatures. However, the main disadvantage of this type of system is that an undesirable release of NH3 usually occurs during the decomposition process. For these reasons, Mg + N codoping was investigated to improve the dehydrogenation properties of LiBH4 in this paper. Firstprinciples DFT calculations of pure and Mg + N codoped LiBH4 were performed. On the basis of a detailed analysis of the geometrical structures, electronic structures, bonding interactions between the atoms, and hydrogen dissociation energies of these systems, a specific atomic mechanism explaining the dehydrogenation of Mg + N codoped LiBH4 was proposed. An improved understanding of this underlying mechanism will aid in the rational design of modified materials for practical applications.

Figure 1. Optimized structures of the pure, Mg-substituted, Nsubstituted, and cosubstituted LiBH4. (a) 1 × 2 × 2 LiBH4 supercell, (b) Mg-doped system, (c) N-doped system, and (d) Mg + N codoped system (green, orange, purple, blue, and white spheres denote Li, Mg, B, N, and H atoms, respectively).

between cycles was less than 10−6 eV, and the residual forces were less than 0.02 eV·Å−1. In addition, the total energy of hydrogen gas was calculated for a hydrogen molecule in a large box. This method enables the interactions between molecules in neighboring cells to be neglected. The Li16B16H64 supercell is illustrated in Figure 1a. A Mg atom was added to the 1 × 2 × 2 LiBH4 supercell (Li16B16H64) by replacing a Li atom, as shown in Figure 1b. For comparison with the metal substitution, a B atom was replaced by a N atom, as shown in Figure 1c. Finally, the structural model of Mg + N cosubstituted LiBH4 was constructed, as shown in Figure 1d.

3. RESULTS AND DISCUSSION 3.1. Crystal Structures. Soulié et al.33 reported that the crystal structure of LiBH4 is orthorhombic with the space group Pnma (no. 62) at room temperature and has lattice parameters of a = 7.179 Å, b = 4.437 Å, and c = 6.803 Å. The Li, B, H1, H2, and H3 atoms occupy the 4c (0.157, 0.25, 0.102), 4c (0.304, 0.25, 0.431), 4c (0.90, 0.25, 0.956), 4c (0.404, 0.25, 0.28), and 8d (0.172, 0.05, 0.428) sites, respectively. To verify the accuracy of the calculations, the lattice parameters of bulk LiBH4 were optimized first. The calculated crystal lattice parameters are a = 7.264 Å, b = 4.378 Å, and c = 6.649 Å, which are in agreement with the experimental values and other calculations.34,35 The calculated atomic positions are also in satisfactory agreement with the experimental data. The structures of Li16B16H64 (S0), Li15MgB16H64 (S1), Li16B15NH64 (S2), and Li15MgB15NH64 (S3) were fully relaxed,

2. COMPUTATIONAL DETAILS First-principles DFT calculations were performed using the Perdew−Burke−Ernzerhof generalized gradient approximation and the projector augmented wave method,29−31 as implemented in the VASP ab initio simulation package.32 The planewave basis set cutoff was set to 700 eV for pure LiBH4 and metal/nonmetal-doped LiBH4. A Gaussian smearing of 0.1 eV was employed during structure optimization. The Brillouin zone was sampled using a 4 × 6 × 4 Monkhorst−Pack k-point mesh for bulk LiBH4 (orthorhombic phase: 24 atoms/unit cell). For the calculations of systems with impurities, a 1 × 2 × 2 LiBH4 supercell (Figure 1a) consisting of 96 atoms (Li16B16H64) was constructed and a 4 × 4 × 3 k-point mesh was employed. Convergence with respect to self-consistent iterations was assumed when the total energy difference B


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energy of nitrogen gas was calculated using a nitrogen molecule in a large cubic unit cell of a = b = c = 20 Å. As shown in Table 1, the occupation energies for substitution with Mg, N, and Mg + N are 3.132, 0.312, and 3.805 eV, respectively, suggesting that the single substitution is easier than the cosubstitution. It is noteworthy that the occupation energy for substituting the B atom with N is extremely low, which indicates that the nitrogen atom can easily replace the boron atom. 3.2. Thermal Stability and Hydrogen Dissociation Energy. The thermal stability is an essential metric that reflects the properties of solid-state hydrogen storage materials and is usually evaluated by the enthalpy of formation. The enthalpies of formation (ΔH) of the pure, Mg-substituted, Nsubstituted, and Mg + N cosubstituted systems were obtained using eqs 4−7.

and the optimized lattice parameters (Rop), cell volumes (V), and occupation energies (Eoccu) of the pure and doped LiBH4 system are listed in Table 1. Generally, for the Mg-substituted, Table 1. Lattice Parameters (Rop), Cell Volumes (V), Occupation Energies (Eoccu), and Formation Enthalpies (ΔH) of the Pure, Mg-Substituted, N-Substituted, and Mg + N Cosubstituted Systems after the Geometry Optimization Rop (Å) crystals

a

b

c

V (Å3)

Eoccu (eV)

ΔH (eV·atom−1)

S0 S1 S2 S3

7.264 7.589 7.711 7.427

8.756 8.865 8.692 8.678

13.298 13.136 13.139 13.654

845.80 883.84 879.41 880.02

3.132 0.312 3.805

−0.332 −0.300 −0.329 −0.293

ΔHpure = 1/96[Epure − 16E Li − 16E B − 32E H2 ]

N-substituted, and cosubstituted systems, the optimized lattice parameters and cell volumes vary slightly, mostly by less than 5% (except the lattice parameters a for S2). For the Mgsubstituted system, the lattice parameters a and b relatively increase by approximately 4.5 and 1.24%, respectively, and the cell volume also increases by 4.5%. This expansion might be explained by the slightly larger atomic radius of the Mg atom (1.60 Å) compared to that of the Li atom (1.56 Å). For the Nsubstituted system, the three hydrogen atoms attached to the B atom bond to the N atom when the B atom is replaced by the N atom and the remaining hydrogen atom bonds to the neighboring Li atom (the Li1−H4 bond length is approximately 1.885 Å). Hence, the crystal volume increases. The occupation energy of the dopant reflects the degree of difficulty for the substitution and was calculated according to the following formula Eoccu = EMg‐sub‐Li − Epure + E Li − EMg

(1)

Eoccu = E N‐sub‐B − Epure + E B − 1/2E N2

(2)

Eoccu = Eco‐sub − Epure − EMg − 1/2E N2 + E Li + E B

(3)

(4)

ΔHMg ‐ doped = 1/96[EMg ‐ sub ‐ Li − 15E Li − 16E B − 32E H2 − EMg ]

(5)

ΔHN ‐ doped = 1/96[E N ‐ sub ‐ B − 16E Li − 15E B − 32E H2 − 1/2E N2 ]

(6)

ΔHMg + N co ‐ doped = 1/96[Eco ‐ sub − 15E Li − 15E B − 32E H2 − 1/2E N2 − EMg ]

(7)

On the basis of the calculations with eqs 4−7, the formation enthalpies of the pure, Mg-substituted, N-substituted, and Mg + N cosubstituted systems are −0.332, −0.300, −0.329, and −0.293 eV·atom−1, respectively, as shown in Table 1. The calculated formation enthalpy of pure LiBH4 is very close to experimental (−0.335 eV·atom−1) and theoretical (−0.338 eV· atom−1) values reported in the literature.34,37 In general, a negative formation enthalpy indicates an exothermic process. The more negative the formation enthalpy, the more stable the structure is. Therefore, Mg + N cosubstitution destabilizes the crystal structure more than Mg or N substitution. The effects of the dopant on the dehydrogenation behavior of LiBH4 were further investigated. The hydrogen dissociation energy (Ed) can be determined using the following formula

where EMg‑sub‑Li, EN‑sub‑B, and Eco‑sub are the total energies of the Mg-substituted, N-substituted, and cosubstituted systems, respectively, Epure is the total energy of pure LiBH4, and ELi, EMg, EB, and EN2 are the total energies of the Li atom, Mg atom, B atom, and nitrogen molecule, respectively. The bodycentered cubic, hexagonal close-packed, and α-boron structures were employed for Li, Mg, and B, respectively.36 The

Ed = Etot‐H − Etot + 1/2E H2

(8)

Table 2. Hydrogen Removal Energies (eV) in the Pure, Mg-Substituted, N-Substituted, and Mg + N Cosubstituted Systems

B−H bonds

B1

B2

N−H bonds

N3

H1a H1b H1c H1d H2a H2b H2c H2d H3a H3b H3c H4

Li16B16H64

Li15MgB16H64

Li16B15NH64

Li15MgB15NH64

2.407 2.352 2.352 2.352 2.353 2.352 2.407 2.352

−0.987 −0.933 −0.933 −0.932 0.274 0.331 0.143 0.272

2.297 2.350 2.250 2.326 2.254 2.230 2.302 2.250 1.531 1.531 1.531 1.532

−1.523 −1.614 −1.616 −1.537 −0.400 −0.487 −0.647 −0.434 −2.458 −2.223 −2.461 −0.613

C


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Figure 2. Total and partial densities of states for the lithium borohydride systems. The Fermi level is set to zero. (a) Li16B16H64, (b) Li15MgB16H64, (c) Li16B15NH64, and (d) Li15MgB15NH64.

where Etot‑H represents the total energy of the compound after the removal of one H atom and Etot is the total energy of the compound before the H atom is removed. The energy of the H2 molecule (EH2) is calculated to be −6.79756 eV. The hydrogen dissociation energies in the pure, Mg-substituted, Nsubstituted, and Mg + N cosubstituted systems are shown in Table 2. The hydrogen dissociation energy of pure LiBH4 is approximately 2.352 eV, which is close to the reported values of 2.3−2.4 eV.38,39 In general, the hydrogen dissociation

energies of the single and cosubstituted systems are all lower than those of pure LiBH4; these energies are −0.987 to 0.331 eV for the Mg-substituted system, 1.531−2.350 eV for the Nsubstituted system, and −2.461 to −0.4 eV for the Mg + N cosubstituted system. This computational work reveals that Mg + N cosubstitution reduces the hydrogen dissociation energy the most. The hydrogen dissociation energies of the (B1)H atoms are negative in the Mg-substituted system but positive in the N-substituted system. These results suggest that Mg-doped LiBH4 has a good dehydrogenation performance, which has been demonstrated in many reports.40,41 Moreover, for the Mg D


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The Journal of Physical Chemistry C + N cosubstituted system, the hydrogen dissociation energy of the (N)H atoms is approximately −2.4 eV and the hydrogen dissociation energies of the (B1)H and (B2)H atoms are −1.5 eV and −0.4 eV, respectively. These results indicate that the N−H bonds are weaker than the B−H bonds. In summary, Mg + N cosubstitution is the most effective at promoting hydrogen dissociation. 3.3. Electronic Density of States. To better understand the effects of the additives on the properties of LiBH4, the electronic properties were investigated by calculating the density of states (total DOS and partial DOS). Figure 2a−d shows the total and partial densities of states for pure, Mgsubstituted, N-substituted, and Mg + N cosubstituted LiBH4, respectively. In pure LiBH4 (Figure 2a), the total DOS is divided into three parts and the calculated band gap is approximately 7.07 eV, which is consistent with the band gap of 7.01 eV reported by Hoang and Van de Walle.17 The valence-band maximum mainly consists of B p and H s bonding states, whereas the conduction-band minimum predominantly consists of B p, H s, and Li s antibonding states. Moreover, significant hybridization between the B s and p and H s states is observed, which indicates strong interactions between the B and H atoms in pure LiBH4. As shown in Figure 2b, the total DOS of the optimized Mgsubstituted system is similar to that of pure LiBH4. However, a new band appears in the band gap because of the overlap of the Mg s and H s electrons, which leads to the formation of a very weak Mg−H bonds. When a B atom is replaced by an N atom, as shown in Figure 2c, the total DOS is divided into five parts and Li16B15NH64 still has a nonmetallic character with a calculated band gap of 5.30 eV. Furthermore, the hybridization of the N p and H s orbitals and the N p, H s, and Li s orbitals is clearly observed in the energy regions of −5.85 to −5.65 eV and −0.15 to 0 eV, respectively, indicating the existence of strong Li−H, Li−N, and N−H interactions. These results are consistent with the previous conclusion about the high hydrogen dissociation energy of Li16B15NH64. For Li15MgB15NH64 (Figure 2d), the partial and total densities of states reveal a stronger hybridization of the Mg s and H s orbitals in the vicinity of the Fermi level, whereas the bonding peaks between −5.53 and −8.03 eV are mainly dominated by the B p, N p, and H s orbitals. Furthermore, the gap between the B−H bonding and antibonding states in the Mg + N cosubstituted system is lower than that in the pure system; thus, the excitation of the B−H bonding electrons into the antibonding states requires less energy. This result suggests that the Mg + N cosubstituted system has a lower hydrogen dissociation energy. To further analyze the chemical bonding in these compounds, the charge distribution and charge transfer in pure and Mg + N cosubstituted LiBH4 were also examined. The total charge density ρ(r) and difference charge density Δρ(r) of the (010) surfaces of pure Li16B16H64 and Li15MgB15NH64 are shown in Figure 3a−d. Figure 3a,b shows the total charge density of the plane that includes the Li, B, H, Mg, and N atoms for the pure and Mg + N cosubstituted systems. As shown in Figure 3a, a strong interaction between the B and H atoms and weak interaction between Li and [BH4] are observed. This result reveals that the B and H atoms form a covalent bond, whereas Li and the [BH4] group form an ionic bond. When a Li atom is replaced by a Mg atom, the charge densities show that new Mg−B and Mg−H bonds form. The Mg−B and Mg−H bonds can help

Figure 3. Contour maps of the total and difference charge density in the (010) planes of the pure and Mg + N cosubstituted systems: (a) total density of the (010) plane for Li16B16H64, (b) total density of the (010) plane for Li15MgB15NH64, (c) charge density difference of the (010) plane for Li16B16H64, and (d) charge density difference of the (010) plane for Li15MgB15NH64.

weaken the interactions between the B and H atoms, thereby enhancing hydrogen release. The difference charge density Δρ(r) is defined as the difference between the total charge density of the solid and a superposition of the atomic charge densities with the same spatial coordinates as in the solid.42 Figure 3c,d shows the difference charge densities Δρ(r) of the (010) surfaces of pure Li16B16H64 and Li15MgB15NH64; the red and blue colors indicate the gain and loss, respectively, of electrons during bond formation. As shown in Figure 3c, the Li atoms play the role of an electron donor and the electrons are mainly distributed around the H atoms in pure Li16B16H64. This result suggests that the interactions between Li and B are ionic bonds, whereas the bonds between B and H are covalent. Therefore, pure Li16B16H64 has a high stability and hydrogen dissociation energy. Figure 3d shows the difference charge densities in the (010) plane of the Mg + N cosubstituted system. After the Li and B atoms are replaced by the Mg and N atoms, respectively, Mg−H and N−H bonds are observed. Because the interactive effects of the Mg−H and N−H bonds weaken the B−H bonds, hydrogen release is enhanced. This conclusion is also easily verified by the variation in the bond lengths in Li15MgB15NH64 (Figure 4b). After Li is replaced by the Mg atom, bonds are clearly formed between the Mg and H atoms, as shown by the obvious overlap of the electron clouds of the Mg and H atoms in Figure 3b. When the B atom is replaced by the N atom, the distance between the Li and N E


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and N atoms, respectively, the peak values of the B−H bonding decrease significantly. This result implies that Mg + N codoping of LiBH4 can reduce the temperature required to remove hydrogen. Both bonding and antibonding states are observed in the −COHP of the N and H atoms in Li15MgB15NH64; thus, the bonding interactions between the N and H atoms might be considered weak. This result is consistent with the calculated results, indicating that the hydrogen dissociation energies of the (N)H atoms are small. For comparison, the COHP analyses of the Mg- and Nsubstituted systems are shown in the Supporting Information (Figure S2). Figure S2 clearly shows that the B−H bond in the Mg-substituted system is weaker than that in the N-substituted system. This result implies that Mg doping of LiBH4 might lower its decomposition temperature. 3.5. Dehydrogenation Temperature. A high desorption temperature is a key factor hindering the practical application of metal borohydrides. In fact, the high dehydrogenation temperatures are consistent with the relatively high enthalpies of desorption. Therefore, the thermodynamics of the dehydrogenation reactions can be described by the standard equation of the Gibbs free-energy change ΔG45

Figure 4. Change of the bond length under the influence of cosubstitution of Mg and N atoms. (a) Li16B16H64 and (b) Li15MgB15NH64.

atoms (2.383 Å) is smaller than that between Li and BN (the B atom that is replaced by the N atom) atoms (2.547 Å), which indicates that the Li atom is likely to form a bond with the N atom. In the Mg + N cosubstituted system, the distance between the Mg and N atoms (3.086 Å) is too long for the formation of a Mg−N bond. For comparison, the total and difference charge densities of the (010) surfaces of the Mg- and N-substituted systems are shown in the Supporting Information (Figures S1a−d). Figure S1a shows that new Mg−B and Mg−H bonds are formed. At the same time, Figure S1b,d reveals strong interactions between the N and H atoms. 3.4. Crystal Orbital Hamilton Population Analysis. To evaluate the bond strength between the B and H atom, crystal orbital Hamilton population (COHP) analyses were performed using the LOBSTER program.43,44 −COHP calculations provide information about the bond strengths, with positive and negative values representing bonding and antibonding, respectively, as shown in Figure 5. This figure reveals that considerable covalent interactions between B and H occur in pure Li16B16H64. Additionally, because of the ionic bonding between Li and H, the −COHP of those atoms is rather small. After Li and B atoms are partially replaced by Mg

ΔG = ΔHR − T ΔS

(9)

where ΔHR and ΔS are the enthalpy and entropy changes, respectively, of the dehydrogenation reaction. When the dehydrogenation reaction is at equilibrium, ΔG = 0. Therefore, the dehydrogenation temperature can be calculated using the equation ΔHR = TdesΔS, where ΔHR is the energy change between the reactants and products. ΔHR =

∑ Etot(products) − ∑ Etot(reactants)

(10)

11,46−48

For pure LiBH4, many studies have shown that the dehydriding reaction and accompanying phase decomposition mainly proceed above 573 K according to the following reaction: LiBH4 → LiH + B +

3 H2 2

(11)

Figure 5. −COHP curves for the pure and Mg + N cosubstituted systems. The Fermi level is set to zero. (a) Li16B16H64 and (b) Li15MgB15NH64. F


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enthalpy of Mg + N cosubstituted LiBH4 is calculated to be 56.358 kJ·mol−1 H2, and the dehydriding temperature is approximately 160.5 °C, which is significantly lower than the hydrogen release temperature of 304.8 °C for pure LiBH4. There are two possible reasons for the decrease in the hydrogen release temperature of the Mg + N cosubstituted system. On the one hand, a N−H···H−B dihydrogen bond (H···H < 2.4 Å) exists in Li15MgB15NH64 because the distance between H3c and H2a is 2.32 Å. Many studies have shown that the hydridic (H−) and protic (H+) atoms bonded to boron and nitrogen atoms are crucial for H2 liberation.50,51 On the other hand, the Mg and N atoms in Li15MgB15NH64 exhibit synergistic effects. Guo et al.52 demonstrated that metal ions can increase the reactivity of H(N) to improve N−H···H−B recombination, which could release hydrogen. In the same way, using eqs 15 and 16, the dehydrogenation reaction enthalpy and onset dehydrogenation temperature are calculated to be 62.75 kJ·mol−1 H2 and 207.1 °C, respectively, for the Mg-substituted system and 70.36 kJ·mol−1 H2 and 265.3 °C, respectively, for the N-substituted system. Siegel et al.41 reported that the dehydrogenation reaction enthalpy of a LiBH4 + MgH2 mixed system is 65.6 kJ·mol−1 H2. Their result is basically consistent with the value calculated in this work; the slight difference is probably due to the different reactants. Hence, the calculated results clearly reveal that Mg + N cosubstitution might provide a novel approach for effectively making dehydrogenation easier, thereby improving the dehydrogenation properties of LiBH4.

in which 13.8 mass % of hydrogen is released. However, the pathway of the decomposition reaction of Mg + N cosubstituted LiBH4 must be predicted first. According to the previous analysis of the bonding effects in Mg + N cosubstituted LiBH4, new Mg−B and Mg−H bonds are formed after the cosubstitution of the Mg and N atoms. Meanwhile, Yang et al.40 demonstrated that LiBH4 could be destabilized by mixing it with Mg and MgH2, which results in the formation of metal boride products. The reaction equation is as follows: 2LiBH4 + Mg → 2LiH + MgB2 + 3H 2

(12)

2LiBH4 + MgH 2 → 2LiH + MgB2 + 4H 2

(13)

That is, the signature of the thermodynamically preferred reaction in both cases is the formation of the stable MgB2. Therefore, the dominant reaction products are theoretically predicted to be LiH, MgB2, and H2 in the Mg-doped LiBH4 system. For Mg + N cosubstituted LiBH4, the substituted nitrogen has three possible forms after the decomposition reaction of Li15MgB15NH64; it can combine with LiH to generate a separate LiNH2 lattice, bond with H to generate gaseous NH3, or bond with B to generate a BN lattice. However, according to the previous analysis of the bonding effects in Mg + N cosubstituted LiBH4, a Mg−N bond cannot form because of the large distance between Mg and N and a B−N bond cannot form for the same reason. In addition, NH3 was reported to be readily captured by LiH at very short contact times;49 therefore, gaseous NH3 does not exist as one of the products. In conclusion, the dehydrogenation reaction of Li15MgB15NH64 is predicted to be:

4. CONCLUSIONS In summary, ab initio DFT calculations were performed to explore the effects of Mg, N, and Mg + N cosubstitution on the crystal structure, electronic structure, hydrogen dissociation, and thermodynamics of lithium borohydride. The calculated results clearly reveal that Mg + N cosubstitution destabilizes the crystal structure more than Mg or N substitution because of the interactive effects of the Mg−H and N−H bonds that weaken the B−H bonds. Furthermore, the hydrogen dissociation energies (Mg + N cosubstituted system: −2.461 to −0.4 eV, Mg-substituted system: −0.987 to 0.331 eV, Nsubstituted system: 1.531−2.350 eV, and pure: 2.3−2.4 eV) demonstrate the significant thermodynamic destabilization that occurs after Mg + N codoping, which is mainly caused by the dual bonding effects of Mg−H and N−H that help weaken the stable BH4 groups, thereby promoting hydrogen dissociation. The calculated electronic structures and COHP analyses also show that the Mg + N modification tends to weaken the B−H bonding interactions. The onset dehydrogenation temperature of Mg + N cosubstituted LiBH4 is reduced to approximately 160.5 °C, which is significantly lower than the hydrogen release temperatures of 304.8 °C, 265.3 , and 207.1 °C for the pure LiBH4, N-substituted, and Mg-substituted systems, respectively. Therefore, metal + nonmetal cosubstitution is an effective method for improving the dehydrogenation performance of lithium borohydride.

Li15MgB15NH64 → 14LiH + MgB2 + 13B + LiNH 2 + 24H 2

(14)

Similarly, the dehydrogenation reactions of Li15MgB16H64 and Li16B15NH64 can be written as eqs 15 and 16, respectively: 49 H2 2

(15)

45 H2 2 47 → 15LiH + LiNH 2 + 15B + H2 2

(16)

Li15MgB16H64 → 15LiH + MgB2 + 14B +

Li16B15NH64 → 16LiH + NH3 + 15B +

The enthalpy of reaction pathway (11) for pure LiBH4 can be expressed as follows: ΔHR =

∑ Etot[(LiH) + (B) + 3/2(H2)] − ∑tot (LiBH4)

(17)

The dehydrogenation reaction enthalpy ΔHR is calculated to be 75.11 kJ·mol−1 H2 for pure LiBH4 (when the zero-point energy corrections are taken into account, the value is 62.9 kJ· mol−1 H2). During the heating process, the entropy change for the gases is much higher than that for the solid materials. Therefore, the entropy change is approximately 130.7 J·K−1· mol−1 (ΔS ≈ ΔS(H2)) for the H2 dissociation process. As a result, using the formula ΔHR = TdesΔS, the onset dehydrogenation temperature Tdes of LiBH4 is calculated to be approximately 304.8 °C, which is in agreement with the experimental value of 300 °C.11 The dehydrogenation reaction

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08198. Contour maps of the total and difference charge densities in the (010) planes of the Mg- and NG


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substituted systems and −COHP curves for the Mg- and N-substituted systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuqi Wang: 0000-0001-6230-0330 Zaoxiao Zhang: 0000-0002-0960-4308 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (no. 21276209), Natural Science Foundation of Shaanxi Province (no. 2017JM2033), and Postgraduate Technology Innovation Project of Northwestern University (no. YZZ17131).

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Synergistic Effects of Mg and N Cosubstitution on Enhanced

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