Additives in protic–hydridic hydrogen storage compounds: a molecular study

Abstract

Here we have studied the dehydrogenation energetics of protic–hydridic based hydrogen storage compounds. Various set of compounds are studied having different ratio of protic and hydridic hydrogens. Moreover, protic–hydridic based chemical additives (NH₃, BH₃ and NH₃BH₃) are added to protic–hydridic based hydrogen storage compounds to increase their hydrogen gravimetric density. Our study shows such chemical additives not only increase their hydrogen gravimetric density but also improve their dehydrogenation properties. Such practice turned out to be excellent as we not only generated some model compounds [LiBH₂NH₃ (ΔE_avg = 3.47 eV), LiNH₂BH₃·BH₃ (ΔE_avg = 3.75 eV), LiBH₂NH₃·BH₃ (ΔE_avg = 3.95 eV] but also compounds like LiNH₂BH₃:NH₃BH₃ (ΔE_avg = 3.85 eV), which has already been reported to be a promising hydrogen storage material. We find the dehydrogenation properties of the reported compounds are best when they have close to equal numbers of protic and hydridic hydrogens.

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1. Introduction

The serious environmental problems associated with fossil fuel burning are the foremost concern of this century. Therefore finding an alternative and sustainable energy source is very important which can replace the fossil fuel based energy.^1–3 The desirable energy source must be green, sustainable and renewable⁴ and have the least adverse effect on the environment. Hydrogen instead of carbon-based (fossil fuel) energy has enormous potential to be used as an uninterrupted and secure energy carrier which can fulfil the demand of global energy.^5–8 Moreover, it produces H₂O as a by-product after electrochemical or combustion processes which is suitable for mankind.⁹ But the biggest problem with hydrogen is its storage. To utilize H₂ as a globally acceptable fuel in vehicles and portable electronics, we require a solid on-board hydrogen storage material which can store a high gravimetric density of hydrogen and can deliver large amounts of hydrogen at moderate conditions.¹⁰ For this purpose several potential storage media have been developed such as carbon materials,^11–13 metal organic frameworks (MOFs),^14–17 metal hydrides,¹⁸ and complex hydrides.¹⁹ But these materials are not suitable for practical use in the sense that either these materials bind with hydrogen in the surface by weak dispersive interactions (physisorption) or through stronger chemical associations (chemisorption). The goal is therefore to develop some low-cost, light-weight solid chemical hydrides (i.e. the chemical storage of hydrogen) which have high hydrogen energy density with faster dehydrogenation kinetics. Recently, ammonia borane^20–22 (NH₃BH₃, AB for short) received unique attention as a promising hydrogen storage material due to its exceptionally high hydrogen density (19.6 wt%) with releasing the first equivalent of H₂ (i.e. 6.5 wt% of H₂) under moderate thermal condition (below 100 °C) having significant air stability. The better desorption properties of AB might be due to the equal combination of protic and hydridic hydrogens present in the system. However, for the maximum use of H₂ coming from AB needs higher temperature which release undesirable borazine as side product.²³ Similarly, there are many other examples of protic–hydridic based light weighted hydrogen storage materials such as (1) lithium borohydride ammoniate [Li(NH₃)_4/3BH₄],²⁴ (3) lithium borohydride ammonia borane [LiBH₄·NH₃BH₃],²⁵ (4) borohydride hydrazinates [LiBH₄·NH₂NH₂],²⁶ (5) Mg(BH₄)₂·NH₃,²⁷ (6) Mg(BH₄)₂·(NH₃BH₃)₂,²⁸ (7) Mg(BH₄)₂·(NH₃)₂·(NH₃BH₃),²⁸ (8) Al(BH₄)₃·6NH₃-4AB,²⁹ (9) Li₂Al(BH₄)₅(NH₃BH₃)₃·6NH₃ (ref. 29) and many more.^27,28 These materials drew great attention as a promising hydrogen storage material for their light weight, high gravimetric density and faster dehydrogenation. This opens up a new direction for hydrogen storage materials where the combination of protic and hydridic hydrogens is very important. Though the hydrogen dehydrogenation kinetics is very promising but still they are very far from the practical usages. Therefore the biggest challenge is to release the maximum number of hydrogen molecules at moderate thermal condition. For this several approaches have been adopted such as transition metals catalysis,²³ nano-scaffolding,^29,30 ionic liquids³¹ and additives.^24–28 Among all these, chemical additives having protic and hydridic hydrogens can be very promising because it not only increases their gravimetric density but can also improve their dehydrogenation properties.^32,33 Therefore, we have adopted chemical additives approach to lower the decomposition temperature and improve the dehydrogenation properties. As we all know light alkali metal doping^34,35 such as Li does not reduce the gravimetric density much but can improve the dehydrogenation kinetics significantly. Moreover, mixing light metal hydrides are reported to be very effective to improve their dehydrogenation properties.³⁶ So here we would like to study whether alkali metal doping with different additives can play a major role in increasing their gravimetric density as well as their dehydrogenation properties. For this, such a combination of ligands are chosen where the ratio of protic and hydridic hydrogen changes. So we have considered those set of ligands which has either protic, hydridic or both type of hydrogens. There are many possible compounds with such kind of hydrogens but initially we chose to study those set of compounds which are experimental characterized.²³ Then we have proposed some model compounds which can be very promising for future applications. Considering all these, the following series of compounds are chosen to see how mixing of certain additives could improve their dehydrogenation properties.

(I) NH₃, BH₃, NH₃BH₃.^{20–22,38–40}

(II) LiNH₂,^41,42 LiBH₂,^43,44 LiNH₂BH₃,^45–47 LiBH₂NH₃.⁴⁸

(III) LiNH₂:NH₃, LiBH₂:NH₃ LiNH₂BH₃:NH₃,⁴⁹ LiBH₂NH₃:NH₃.

(IV) LiNH₂:BH₃ LiBH₂:BH₃, LiNH₂BH₃:BH₃, LiBH₂NH₃:BH₃.

(V) LiNH₂:NH₃BH₃, LiBH₂:NH₃BH₃, LiNH₂BH₃:NH₃BH₃,^35,37 LiBH₂NH₃:NH₃BH₃.

In our work, we prefer the gas phase approach for the calculations of atomic and molecular dehydrogenation energy because it gives us the freedom to model any molecular structure which can be restricted for the bulk calculations. As we try to model some light weighted hydrogen storage material by introducing different kind of protic–hydridic based additives to improve their dehydrogenation properties, so the molecular approach is the most suitable one.

2. Computational details

The geometries of all the structures are fully optimized at B3LYP/6-311++G(d,p) using Gaussian 09 suit of programs.⁵⁰ This uses well known three parameter functional of Becke's including Hartree–Fock exchange contribution with a nonlocal correction for the exchange potential proposed by Becke,⁵¹ with nonlocal corrections for the correlation energy suggested by Lee, Young and Parr.⁵² The presence of stationary point is characterized by vibrational frequency calculation computing analytical Hessian matrix. Here we have calculated the atomic and molecular hydrogen liberation energy in each step of every compound. By this calculation we try to set a trend that how the atomic hydrogen desorption energy improves with the presence of dopant and additives. This type of calculation gives an idea that what type of dopant and additives are important for better dehydrogenation and encourage us to model some compounds which can be useful as a hydrogen storage materials. The atomic (ΔE_H) and molecular hydrogen (ΔE_H2) liberation energy is calculated using the following eqn (1) and (2).

ΔE_H = (E_SP + E_{atomic hydrogen}) − E_GS (1)

ΔE_H2 = (E_SP + E_{molecular hydrogen}) − E_GS (2)

Here E_SP denotes the single point energy of the optimized molecule after hydrogen removal and E_GS is the ground state energy of the optimized structure. The natural-bond orbital (NBO)⁵³ charge also calculated for all the molecules to understand their charge distributions during the de-hydrogenation process.

3. Results and discussion

The results and discussion part is divided into three sections. In our first section, we discuss the standard hydrogen storage compounds having protic (NH₃), hydridic (BH₃) and protic–hydridic (NH₃–BH₃) hydrogens. In our next section, the Li substituted protic (LiNH₂), hydridic (LiBH₂), and protic–hydridic compounds (LiNH₂BH₃ and LiBH₂NH₃) are discussed. In our last section, we have used protic, hydridic and protic–hydridic based chemical additives (NH₃, BH₃, and NH₂BH₃) on the Li-substituted protic (LiNH₂), hydridic (LiBH₂), and protic–hydridic compounds (LiNH₂BH₃ and LiBH₂NH₃) for increasing their hydrogen gravimetric density. The last section has been divided into three parts based on the three different type of chemical additives (NH₃, BH₃, and NH₂BH₃) used on the Li-substituted protic (LiNH₂), hydridic (LiBH₂), and protic–hydridic compounds (LiNH₂BH₃ and LiBH₂NH₃).

Therefore, we try to understand whether such combination of hydrogens could facilitate the hydrogen desorption or not? The successive hydrogen desorption energy is calculated to understand whether stability of the intermediate compounds play an important role to their dehydrogenation or not?

3.1. NH₃, BH₃, NH₃BH₃

We begin our analysis by calculating atomic hydrogen desorption energies of the first series of compounds having protic, hydridic and protic–hydridic hydrogens. The calculated hydrogen removal energies are listed in Table 1. Their optimized structures are shown in Schemes 1–3.

Table 1 The calculated atomic and molecular hydrogen removal energies (eV) are presented. Here ΔE_avg is the average hydrogen removal energy for the complete dehydrogenation

Compounds	H liberation steps (energy in eV)							H2 liberation steps (energy in eV)
	1^st	2^nd	3^rd	4^th	5^th	6^th	ΔEavg	1^st	2^nd	3^rd	ΔEavg
NH3	4.92	6.46	4.34				5.24	6.61			6.61
BH3	4.80	3.94	3.61				4.12	3.94			3.94
NH3BH3	4.68	2.03	4.81	2.99	5.10	6.42	4.33	2.76	3.98	6.76	4.50

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Scheme 1 Successive dehydrogenation energetics for NH₃.

Scheme 2 Successive dehydrogenation energetics for BH₃.

Scheme 3 Successive dehydrogenation energetics for NH₃BH₃, (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

3.1.1. Ammonia [NH₃, 21.42 wt%]. We have calculated the H removal energy of NH₃ and we find the successive N–H bond energies (Scheme 1) are 4.92, 6.46 and 4.34 eV respectively. Therefore first hydrogen removal energy is lower than the second one where as third one is the lowest. Interestingly, the molecular hydrogen liberation energy is very high (6.61 eV) for ammonia.

3.1.2. Borane [BH₃, 27.75 wt%]. Similarly, It is considered that BH₃ is the simplest compound of hydridic hydrogens. Here the atomic hydrogen liberation energy shows (Scheme 2) that the second hydrogen (3.94 eV) removal energy is lower than the first one (4.80 eV). It is interesting because in case of NH₃ (protic hydrogens) the second N–H bond energies were higher than the first one where as in case of hydridic hydrogens the second hydrogen removal is easier than the first one. Interestingly, in BH₃, the successive dehydrogenation energies are lessening for each steps. Even, the molecular hydrogen liberation energy (3.94 eV) of BH₃ is far lower than in NH₃ (6.61 eV).

3.1.3. Ammonia borane [NH₃BH₃, 19.44 wt%]. In ammonia borane (AB), it has both types of hydrogen: protic and hydridic. First the atomic hydrogen liberation energy is calculated in NH₃BH₃ and the calculated N–H and B–H bond energies (Table 1) are 4.92 and 4.68 eV respectively.

Therefore, the B–H bond energy is lower than the N–H bond energy. Interestingly, B–H bond energies in NH₃BH₃, are lower than in BH₃ (4.80 eV). The reason of lower H removal energy can be explained from their atomic charge distributions. The NBO charges on N atom of NH₃BH₃ and NH₃ are −0.837 and −1.049 respectively. Hence, N is less electronegative in NH₃BH₃ than in NH₃.

This might be due to the electron deficient nature of the B atom of BH₃ group. Similarly B is more electron negative (−0.151) in NH₃BH₃ than in (0.335) BH₃. Hence, in NH₃BH₃ the N is more electropositive than in NH₃ and B (−0.151) is more electro negative than in BH₃. The NBO charge differences (−0.685) between the N and B bond also decreases in AB.

The second hydrogen removal energy of AB is lower for the N–H (2.03 eV) bond than the B–H (4.68 eV) bond. Such a lower removal energy (2.03 eV) can be related to the combination of protic and hydridic hydrogens. The NBO charge analysis shows B is more electronegative (−0.176) in NH₂BH₃ than in NH₃BH₃. We find, if the protic hydrogen liberates first then it might be facilitating the hydridic hydrogen elimination so that the electro neutrality is maintained. This can be seen from their atomic charge distribution also. Moreover, after atomic hydrogen removal, the charge difference in N–B bond decreases to −0.516. In AB, the calculated H₂ molecular desorption energy is of 2.76 eV. Such dehydrogenation, gives a stable planner geometry of NH₂BH₂ with high negative charge on N (−0.996) and positive charge on B (0.422). This compound has been experimentally characterized and found to be an important intermediate for borazine formation.²³ So, for NH₂BH₂ (Scheme 3) the boron hydrogen removal energy (4.81 eV) is very high. This might be due to the extra stability gained by the planner NH₂BH₂ intermediate. But interestingly the next hydrogen is calculated to be released easily (2.99 eV) from the N centre. As in the previous step hydridic hydrogen was released, so to maintain the electron neutrality protic hydrogen can be released easily. We find the removal energy is quite high (5.10 and 6.42 eV) for the next two steps. This might be due to the fact that the negative charges on N and positive charges on B increases in the NHBH and NHB compounds. Therefore, the N–H and B–H bond energies can be lowered if we can induce more positive charges on the N centre and negative charges on the B centre respectively. Moreover, lower the charge differences in the N–B bond, facilitates the atomic hydrogen liberation process.

3.2. LiNH₂, LiBH₂, LiNH₂BH₃, LiBH₂NH₃

In our second series, we wanted to study the effect of alkali metal doping on the dehydrogenation properties of protic–hydridic compounds. Here we consider Li doping as Li is a very light weight metal and promising for improving their dehydrogenation properties. Therefore, Li substitution is done on the protic or hydridic hydrogens of NH₃, BH₃ and NH₃BH₃, respectively. All the optimized structures are shown in Schemes 4–7.

Scheme 4 Successive dehydrogenation energetics for LiNH₂.

Scheme 5 Successive dehydrogenation energetics for LiBH₂.

Scheme 6 Successive dehydrogenation energetics for LiNH₂BH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

Scheme 7 Successive dehydrogenation energetics for LiBH₂NH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

3.2.1. LiNH₂ (8.78 wt%). The first compound in this set is LiNH₂ having two protic hydrogens. This compound can be synthesized using various methods. The very first method used to synthesize was the gas phase method. Here, the reaction between lithium vapour and ammonia is carried out in the argon environment (Ar acting as a carrier gas).⁴² The most popular method for the preparation of LiNH₂ is the ball milling method by taking Li₃N as a starting material.⁴³

We also calculate the atomic hydrogen liberation energy of LiNH₂. The first and second hydrogen liberation energy (Scheme 4) of LiNH₂ is exactly same with the first and second liberation energy of NH₃ (4.92 eV). As Li is substituting one of the hydrogen of NH₃, so comparison can be made with the first hydrogen removal energy of LiNH₂ with the second hydrogen removal energies of NH₃. We find the second hydrogen removal energy (6.45 eV) of NH₃ is higher than the first one (4.92 eV) of LiNH₂. Therefore, Li is improving the dehydrogenation properties of NH₃. So our plan is to mix such kind of chemical compounds (like LiNH₂:NH₃) so that we not only increase their gravimetric density but also improves their dehydrogenation properties.

3.2.2. LiBH₂ (5.92 wt%). LiBH₂ can be synthesized by decomposing LiBH₄ at 320 °C–380 °C under 10 atm H₂ pressure.⁴⁴ The B–H bond energy (4.21 eV) of LiBH₂ is surprisingly lower than the N–H bond energy (4.92 eV) of LiNH₂ and B–H bond energy (4.80 eV) of BH₃. This might be due to the presence of Li atom in the vicinity of boron centre which weakens the B–H bond strength.

3.2.3. LiNH₂:BH₃ (13.58 wt%). The third compound in this series is LiNH₂BH₃ (LiAB). LiAB can be synthesized from the commercially available compounds like LiH and NH₃BH₃ (AB).⁴⁵

The structure of LiAB (Scheme 6) was optimized where we find Li is bridged between the N and B atoms. Our NBO charge calculation shows Li (0.837) is positively charged and bonded to negatively charged B (−0.189) and N (−1.139) atoms. The Li–N and Li–B bond distances are 1.89 Å and 1.98 Å respectively. So, Li is more closely bonded to N atom. Now, comparing the NBO charges on N (−0.837) and B (−0.151) in AB with N (−1.139) and B (−0.189) in LiAB, we find N is more electro-negative in LiAB.

We have calculated the N–H and B–H bond energy in the Li-doped ammonia borane (LiAB). The Li doping effect is clearly shown in the first atomic hydrogen liberation energy i.e. B–H bond breaking energy (4.50 eV) is lower than B–H bond energy (4.68 eV) in AB. Interestingly, the N–H bond energy (4.89 eV) of AB is not much changed compare to the N–H bond energy (4.92 eV) in NH₃BH₃. This might be due to the presence of positively charged Li (0.837), which induces more negative charges on N (−1.139) than in B (−0.189). Therefore, the first atomic hydrogen liberation is favourable from the B centre. The Li doping effect is visible for the second atomic hydrogen liberation also. We find N–H bond energy (2.65 eV) is very lower than the N–H bond energy (5.47 eV) in the NH₂BH₂. Similarly, the B–H bond energy is lower (4.59 eV) than the B–H bond energy (4.81 eV) in NH₂BH₂. So the atomic hydrogen liberation is more favourable from the N centre. The successive hydrogen removal energy (Scheme 6) for the third, fourth and fifth steps are 4.52, 2.64 and 4.98 eV respectively. Interestingly, in LiNH₂BH₃, the average hydrogen removal energy (Table 2) is 3.86 eV, which is much lower than the average hydrogen removal energy of NH₃BH₃ (4.33 eV).

Table 2 The calculated atomic and molecular hydrogen removal energies (eV) are presented. Here ΔE_avg is the average hydrogen removal energy for the complete dehydrogenation

Compounds	H liberation steps (energy in eV)						H2 liberation steps (energy in eV)
	1^st	2^nd	3^rd	4^th	5^th	ΔEavg	1^st	2^nd	3^rd	ΔEavg
LiNH2	4.92	6.45				5.69	6.42			6.42
LiBH2	4.21	4.76				4.49	3.83			3.83
LiNH2:BH3	4.50	2.65	4.52	2.64	4.98	3.86	4.12	3.48		3.80
LiBH2:NH3	2.58	2.65	4.52	2.64	4.98	3.47	2.25	3.48		2.86

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3.2.4. LiBH₂:NH₃ (13.58 wt%). The last compound of this series is LiBH₂NH₃. Here, Li doping was considered on the boron hydrogen of ammonia borane (AB). The optimized structure of LiBH₂NH₃ is presented in Scheme 7. In LiBH₂NH₃, the distance between the Li–B and Li–N are 2.25 Å and 3.36 Å respectively. Therefore here Li is directly bonded to the B atom. The NBO charge distribution shows Li (0.416) is positively charged and inducing more negative charges on B (−0.569) whereas N (−0.862) has similar charges as in AB. Therefore, in LiNH₂BH₃, the first atomic hydrogen liberation is easier from the N–H bond (2.58 eV). The calculated N–H bond energy (2.58 eV) is quite lower than the N–H bond energy in (4.92 eV) NH₃BH₃ and (4.89 eV) LiNH₂BH₃. In case of LiBH₂NH₃, after the first hydrogen removal, the energetic steps are similar as reported for LiNH₂BH₃. But here the average hydrogen removal energy is 3.47 eV which is quite lower than in (3.86 eV) LiNH₂BH₃. This is because the first hydrogen removal energy (2.58 eV) of LiBH₂NH₃ was lower than in the LiNH₂BH₃ (4.50 eV).

Therefore, Li doping highly influences the hydrogen removal energies of the protic and hydridic hydrogens and more importantly, the average hydrogen removal energies are lower for the compounds having more hydridic hydrogens.

3.3. LiNH₂:NH₃, LiBH₂:NH₃, LiNH₂BH₃:NH₃, LiBH₂NH₃:NH₃

So, from our first set of calculations, we realised mixing of protic and hydridic hydrogens could improve their dehydrogenation properties. Hence, we decided to study the following set of compounds LiNH₂:NH₃, LiBH₂:NH₃, LiNH₂BH₃:NH₃, LiBH₂NH₃:NH₃ where NH₃ used as an additive having protic hydrogens. So, that the ratio of protic and hydridic hydrogens are changed from one compound to other. All the optimized structures are presented in Schemes 8–10.

Scheme 8 Successive dehydrogenation energetics for LiNH₂:NH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

Scheme 9 Successive dehydrogenation energetics for LiNH₂BH₃:NH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

Scheme 10 Successive dehydrogenation energetics for LiBH₂NH₃:NH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

3.3.1. LiNH₂:NH₃ (12.50 wt%). LiNH₂:NH₃ is a model compound having protic hydrogens where NH₃ used as a chemical additive on LiNH₂. The first hydrogen liberation energy (4.66 eV) of LiNH₂:NH₃ is lower compare to the liberation energy of (4.92 eV) LiNH₂ and (4.92 eV) NH₃. If we compare the NBO charges on N1 and N2 (Scheme 8) in LiNH₂:NH₃, we find N2 is more positively charged than in N1 (−1.507). Therefore the H from the N2 (−1.085) (ammonia) is easier to release. Here also it shows higher the positive charges on N, easier to remove the hydrogen.

The second hydrogen can be liberated from any of the four equivalent hydrogens. The second hydrogen removal energy is higher 6.23 eV than the first one. On the other hand, the third (3.66 eV), fourth (3.39 eV) and fifth (2.22 eV) hydrogen desorption energy is quite low compare to the first two. Therefore, the trend of atomic H liberation is mainly depending on their charge distribution and structural rearrangements. The average energy needs to remove one H from LiNH₂:NH₃ is about 4.03 eV which is quite lower than in NH₃ (5.24 eV) and (5.69 eV) LiNH₂. Therefore, mixing of Li and NH₃ certainly improving their dehydrogenation properties.

3.3.2. LiBH₂:NH₃ (13.58 wt%). The NH₃ addition to LiBH₂ will give the structure of LiBH₂:NH₃ which is basically Li substituted hydrazine borane. Therefore, this compound has already been studied and discussed in the previous section.

3.3.3. LiNH₂BH₃:NH₃ (14.86 wt%). Here LiNH₂BH₃:NH₃ is studied as a model compound where NH₃ used as a chemical additive. In this compound the number of protic hydrogens are more than hydridic hydrogens. We have calculated the first atomic hydrogen liberation energy and we find the B–H bond energy is 4.49 eV same with the B–H bond energy (4.50 eV) of LiNH₂BH₃. Therefore, NH₃ addition does not change the B–H bond energy. The second hydrogen liberation is favourable from the N1–H (Scheme 9) side with removal energy of 2.63 eV. This might be due to the positive charge increases on the N1 (1.14) centre. The successive dehydrogenation energies from third to eight steps (Scheme 9) are 4.52, 2.57, 4.97, 6.51, 4.45, and 5.59 eV respectively. The average dehydrogenation energy is of 4.21 eV which is quite higher than in (3.86 eV) LiNH₂BH₃. Therefore, in this case, adding chemical additives such as NH₃ only increase the gravimetric density but not their dehydrogenation properties.

3.3.4. LiBH₂NH₃:NH₃ (14.86 wt%). In LiBH₂NH₃:NH₃, the ratio of protic and hydridic hydrogens is 3 : 1. The first hydrogen liberation energy (2.39 eV, Table 3) value is slightly lower than the (2.58 eV, Table 2) LiBH₂NH₃. But, from there on, the energetic will be same as in LiNH₂BH₃·NH₃. But as the first hydrogen removal energy is lower, the average hydrogen removal energy (4.20 eV) is better than in (4.46 eV) LiNH₂BH₃·NH₃. Therefore in this case chemical additive such as NH₃ is less helpful for improving their dehydrogenation properties (Table 3).

Table 3 The calculated atomic and molecular hydrogen removal energies (eV) are presented. Here ΔE_avg is the average hydrogen removal energy for the complete dehydrogenation

Compounds	H liberation steps (energy in eV)									H2 liberation steps (energy in eV)
	1^st	2^nd	3^rd	4^th	5^th	6^th	7^th	8^th	ΔEavg	1^st	2^nd	3^rd	4^th	ΔEavg
LiNH2:NH3	4.66	6.23	3.66	3.39	2.22				4.03	6.27	3.08			4.67
LiNH2BH3:NH3	4.49	2.63	4.52	2.57	4.97	6.51	4.45	5.59	4.46	4.07	3.32	6.71	5.28	4.84
LiBH2NH3:NH3	2.39	2.63	4.52	2.57	4.97	6.51	4.45	5.59	4.20	1.97	3.32	6.71	5.28	4.32

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3.4. LiNH₂:BH₃, LiBH₂:BH₃, LiNH₂BH₃:BH₃, LiBH₂NH₃:BH₃

Here we discuss another set of compounds where BH₃ used as chemical additive. Therefore, the ratio of hydridic hydrogens are more than the protic hydrogens.

3.4.1. LiNH₂:BH₃ (13.58 wt%). The BH₃ addition to LiNH₂ will give the structure of LiNH₂BH₃ which is basically Li substituted hydrazine borane. Therefore, this compound has already been studied and discussed in the previous section.

3.4.2. LiBH₂:BH₃ (14.88 wt%). The first compound of this set is LiBH₂:BH₃ having hydridic hydrogens only. The optimized structure of LiBH₂:BH₃ has B₂H₆ geometry where Li is taking one of the bridging hydrogen's positions. The successive dehydrogenation energies for the second to fifth steps are (Scheme 11) 5.12, 4.19, 5.83 and 5.30 eV respectively. The average atomic hydrogen removal energy is 4.85 eV which is higher than in (4.49 eV) LiBH₂ and (4.12 eV) BH₃. Therefore, chemical additive BH₃ could not improve the dehydrogenation properties of LiBH₂.

Scheme 11 Successive dehydrogenation energetics for LiBH₂:BH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

3.4.3. LiNH₂BH₃:BH₃ (15.79 wt%). Therefore, the average hydrogen removal energy is 3.75 eV which is low in comparison to (3.86 eV) LiNH₂BH₃ and (4.12 eV) BH₃. Therefore BH₃ could be a very promising chemical additive for compounds having both protic and hydridic hydrogens.

3.4.4. LiBH₂NH₃:BH₃ (15.79 wt%). In case of LiBH₂NH₃:BH₃, we have more hydridic hydrogens than protic. Similarly, we have studied LiNH₂BH₃:BH₃ where hydridic and protic hydrogen ratio is 2 : 1. The successive hydrogen removal steps are presented in Scheme 12. Here, we find N–H hydrogens are easier to remove than the B–H hydrogens.

Scheme 12 Successive dehydrogenation energetics for LiNH₂BH₃:BH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

This can be concluded by NBO charge calculation as the positive charge on N (−0.882) is more than in LiNH₂BH₃ as well as the negative charge on B1 (−0.172) is more than AB and LiAB.

The calculated dehydrogenation steps are presented in Scheme 13. The average hydrogen removal energy calculated to be 3.95 eV which is high in comparison to (3.47 eV, Table 2) LiBH₂NH₃ but low in compare to (4.12 eV, Table 1) BH₃.

Scheme 13 Successive dehydrogenation energetics for LiBH₂NH₃:BH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

In these set of calculations we find LiNH₂BH₃:BH₃ having best dehydrogenation properties (Table 4).

Table 4 The calculated atomic and molecular hydrogen removal energies (eV) are presented. Here ΔE_avg is the average hydrogen removal energy for the complete dehydrogenation

Compounds	H liberation steps (energy in eV)									H2 liberation steps (energy in eV)
	1^st	2^nd	3^rd	4^th	5^th	6^th	7^th	8^th	ΔEavg	1^st	2^nd	3^rd	4^th	ΔEavg
LiBH2:BH3	3.68	5.24	4.19	5.83	5.30				4.85	3.92	4.29			4.10
LiNH2BH3:BH3	4.60	2.53	4.56	3.04	4.41	3.36	4.33	3.21	3.75	4.09	3.78	2.83	2.74	3.36
LiBH2NH3:BH3	3.71	5.12	3.57	3.51	4.40	3.14	3.84	4.34	3.95	4.92	2.43	4.63	3.46	3.69

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3.5. LiNH₂:NH₃BH₃, LiBH₂:NH₃BH₃, LiNH₂BH₃:NH₃BH₃, LiBH₂NH₃:NH₃BH₃

Here in this series, NH₃BH₃ used as a chemical additive having equal number of protic and hydridic hydrogens.

3.5.1. LiNH₂:NH₃BH₃ (14.86 wt%). After structural relaxation, the structure of LiNH₂:NH₃BH₃ is converted into the geometry of LiNH₂BH₃:NH₃ which has been studied and discussed in the previous section.

3.5.2. LiBH₂:NH₃BH₃ (15.79 wt%). Similarly we could not optimize the geometry of LiBH₂:NH₃BH₃. After optimization the structure turned out to be LiBH₂NH₃:BH₃, already discussed in the previous section.

3.5.3. LiNH₂BH₃:NH₃BH₃ (16.25 wt%). In this series, the very first compound we have studied is LiNH₂BH₃:NH₃BH₃. This compound can be synthesized³⁵ by ball milling method. The reactant mixture (LiH and AB) are taken in 1 : 2 ratio into the milling machine.

Here we have almost equal number of protic and hydridic hydrogens but the hydrogen gravimetric density is as high as 16.2 wt%. The first atomic hydrogen is easy to remove from B2–H and the value is calculated to be 4.43 eV (Table 5). Similarly all the successive dehydrogenation steps are calculated and presented in Scheme 14. The average hydrogen removal energy is of 3.85 eV which is quite low in compare to NH₃BH₃ (4.33 eV, Table 1) but very much similar with LiNH₂BH₃ (3.86 eV, Table 2) which is already reported to be very promising material for hydrogen storage. This kind of low dehydrogenation value can also be explain by the low NBO charge difference between the N and B centre as discussed earlier. Therefore, addition of such kind of chemical additives (NH₃BH₃) is very much helpful and promising for the hydrogen storage application. It looks the dehydrogenation thermodynamics depend lot of the ratio of protic and hydridic hydrogens. In this particular case we found the protic and hydridic hydrogen ratio is almost equal and the dehydrogenation properties are very promising.

Scheme 14 Successive dehydrogen energetics for LiNH₂BH₃:NH₃BH₃ (here, H_N and H_B means hydrogen attached to nitrogen and boron atom respectively).

3.5.4. LiBH₂NH₃:NH₃BH₃ (16.25 wt%). The next material we tried to model in this series is LiBH₂NH₃:NH₃BH₃. Similarly, here NH₃BH₃ is added as a substituent in the vicinity of LiBH₂NH₃. Here also the protic and hydridic hydrogen ratio is almost equal as in LiNH₂BH₃:NH₃BH₃. But when we try to relax then it takes the geometry of LiNH₂BH₃:NH₃BH₃ (Table 5).

Table 5 The calculated atomic and molecular hydrogen removal energies (eV) are presented. Here ΔE_avg is the average hydrogen removal energy for the complete dehydrogenation

Compounds	1^st	2^nd	3^rd	4^th	5^th	6^th	7^th	8^th	9^th	10^th	11^th	ΔEavg
H liberation steps
LiNH2BH3:NH3BH3	4.43	2.34	4.58	2.99	4.46	2.76	4.44	2.96	4.48	3.50	5.45	3.85
H2 liberation steps
LiNH2BH3:NH3BH3	3.77	3.46	3.66	2.60	4.64							3.63

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4. Conclusions

In the present work, molecular level calculations are performed to understand the dehydrogenation properties of protic–hydridic (NH₃, BH₃ and NH₃BH₃) based hydrogen storage compounds. Li-substitution is done to improve their dehydrogenation properties. Protic–hydridic based chemical additives were added to improve their hydrogen storage density as well as their dehydrogenation properties. We find, the dehydrogenation properties of these compounds depend lot on their charge distribution. We find the hydrogen removal is easier if we could induce more positive charge on N and negative charge on B. Chemical additives, such as NH₃, BH₃ and NH₃BH₃, certainly increase their gravimetric density as well as improve their dehydrogenation properties. The dehydrogenation properties of the compound are best when they have close to equal numbers of the protic and hydridic hydrogens. The protic and hydridic hydrogens are easier to remove if more positive and negative charges could be induced on N and B-centre. Therefore, such mixing, led us to get some model compounds LiBH₂NH₃ (ΔE_avg = 3.47 eV), LiNH₂BH₃·BH₃ (ΔE_avg = 3.75 eV), LiBH₂NH₃·BH₃ (ΔE_avg = 3.95 eV) which not only improves their gravimetric density but also their dehydrogenation properties. Moreover, such practise turned out to be excellent as we could even model compounds like LiNH₂BH₃:NH₃BH₃ (ΔE_avg = 3.85 eV) which has already been experimentally characterized and reported to be a very promising hydrogen storage material. Therefore, the results we get through mixing of chemical additives are expected to develop a new generation of light-weighted hydrogen storage materials.

Acknowledgements

We thank IIT Indore for the lab and computing facilities. This work is supported by Council of Scientific and Industrial Research [CSIR, Grant number: 01(2723)/13/EMR(II)], New Delhi. A. M. thanks CSIR for the Senior Research Fellowship.

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Footnote

† Electronic supplementary information (ESI) available: We have tabulated the energetic for all the probable dehydrogenation steps for all compounds in Table S6. Table S7 lists NBO charge distributions for all the compounds. Full list of authors of Gaussian 09 reference (ref. 50) is given. See DOI: 10.1039/c4ra09778e

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