The improved Hydrogen Storage Performances of the Multi-Component Composite : 2 Mg ( NH 2 ) 2 – 3 LiH – LiBH

2Mg(NH2)2–3LiH–LiBH4 composite exhibits an improved kinetic and thermodynamic properties in hydrogen storage in comparison with 2Mg(NH2)2–3LiH. The peak temperature of hydrogen desorption drops about 10 K and the peak width shrinks about 50 K compared with the neat 2Mg(NH2)2–3LiH. Its isothermal dehydrogenation and re-hydrogenation rates are respectively 2 times and 18 times as fast as those of 2Mg(NH2)2–3LiH. A slope desorption region with higher equilibrium pressure is observed. By means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) analyses, the existence of Li2BNH6 is identified and its roles in kinetic and thermodynamic enhancement are discussed.

Hydrogenation/dehydrogenation processes of Li2Mg(NH)2 were probed under the pressure composition isotherms conditions by ex-situ X-ray diffraction (XRD) [17] and in-situ neutron measurements [23].During the hydrogenation process, Li2Mg(NH)2 is first converted to Li2Mg2(NH)3, LiNH2 and LiH at a low pressure slope, and then form Mg(NH2)2 and 2LiH at the high pressure plateau.In the dehydrogenation, Mg(NH2)2 and 2LiH take the reverse reaction route and go back to Li2Mg(NH)2.
The reactions can be described as follows: Considerable works have been carried out to lower down the operating temperature and improve the dehydrogenation kinetics of Mg(NH2)2-2LiH composite by doping additives, such as alkali metal compounds [24][25][26][27] and metal borohydrides [28,29].The addition of KH significantly reduced the operating temperature from ca. 523 to 380 K, while the equilibrium pressure was ca.0.2 MPa.Rb based compounds were found to be more effective for improving the kinetic properties of Mg(NH2)2-2LiH composite than the potassium based additives, and the active species was identified to be RbH [24,30].Yang et al. [31] reported that a multi-component composite of LiNH2, MgH2 and LiBH4 with a molar ratio of 2:1:1 exhibited enhanced low-temperature desorption kinetics and a significant reduction in ammonia liberations.As a kind of self-catalyzing hydrogen storage material, LiBH4 catalyzed the dehydrogenation of 2LiNH2-MgH2 composite to form Li2Mg(NH)2, furthermore Li2Mg(NH)2 could react with LiBH4 to release more hydrogen.Following this work, Hu et al. [29] doped 10 mol% LiBH4 in Mg(NH2)2-2LiH composite and found the doping of LiBH4 not only improved the kinetics but also reduced the heat of dehydrogenation from 40 kJ/mol-H2 of Mg(NH2)2-2LiH composite to 36.5 kJ/mol-H2 of Mg(NH2)2-2LiH-0.1LiBH4composite.Other borohydrides, such as Mg(BH4)2 and Ca(BH4)2 [32][33][34], have similar effects as LiBH4 in Mg(NH2)2-2LiH composite because they convert to LiBH4 after a metathesis reaction.Li et al. [35] introduced 5 mol% LiBr to 2LiNH2-MgH2 composite and found certain effects on its thermodynamics and kinetics.In Cao's recentwork [36], thermodynamic properties of the dehydrogenation of 2Mg(NH2)2-3LiH composite were improved by stabilizing the dehydrogenated product, LiNH2, in Reaction (1).LiNH2 reacts with LiBH4, LiI and LiBr exothermically and forms more stable compounds, i.e., Li4(NH2)3BH4, Li3(NH2)2I, and Li2(NH2)Br; the reactions are as follows: The additions of LiBH4, LiI and LiBr noticeably reduced the heat of dehydrogenation from 40 kJ/mol-H2 of 2Mg(NH2)2-3LiH composite to 35.8, 33.3 and 31.9 kJ/mol-H2, respectively, which suggests that hydrogen release at 0.1 MPa equilibrium pressure can be thermodynamically allowed at 337, 333 and 320 K, respectively.Quaternary complex hydrides, such as Li2BNH6 [37] and Li4BN3H10 [38], were found by ball milling or heating the mixtures of LiNH2 and LiBH4 with corresponding molar ratios.If with more LiBH4 to facilitate the formation of Li2BNH6, Reaction (5) can be rewritten as Reaction (6): Herein we report the modification of 2Mg(NH2)2-3LiH system by adding a different ratio of LiBH4.The dehydrogenation and re-hydrogenation performances of Mg(NH2)2-2LiH-1LiBH4 composite were investigated by Thermogravimetry coupled differential thermal analysis (TG-DTA) measurement, temperature programmed desorption-mass spectroscopy (TPD-MS), isothermal volumetric release, and soak.We found that the increase in the amount of LiBH4 improved the kinetics of 2Mg(NH2)2-3LiH composite.
An interesting phenomenon was observed that there was a higher pressure slope in pressure composition isotherm (PCI) measurements of 2Mg(NH2)2-3LiH-LiBH4 composite than 2Mg(NH2)2-3LiH-1/3LiBH4 composite [36], which reveals the change of thermodynamic properties.Samples at this stage were analyzed by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and magic angle spinning nuclear magnetic resonance (MAS NMR).

The hydrogen Desorption Properties of 2Mg(NH2)2-3LiH-LiBH4 Composite
TG curve of 2Mg(NH2)2-3LiH-LiBH4 composite (T2) is shown in Figure 1a.The thermal desorption of this sample exhibits two main steps, one is from 400 K to 450 K and the other is from 475 K to 575 K.The weight losses of the first step and the second step are 3.63 wt% and 6.24 wt%, respectively.Moreover, the first-step weight loss is near the theoretical loss of H2 (3.7 wt%) deduced from Reaction (6).The DTA curve of T2 gives two main endothermic signals corresponding to the two desorption steps in TG curve.The small endothermic peak in Figure 1 at 363 K can be assigned to the phase transformation of LiBH4 in starting materials.
The kinetic desorption behaviors of 2Mg(NH2)2-3LiH (Tp) and 2Mg(NH2)2-3LiH-LiBH4 (T2) composites were investigated by TPD-MS too.H2 and NH3 signals are shown in Figure 2a,b, respectively.As shown in Figure 2b, the byproduct ammonia can be effectively inhibited in T2 sample during desorption process, and the two-step dehydrogenation of T2 sample is corresponding to the result of TG.In the first dehydrogenation step of T2 sample, 3.63 wt% hydrogen can be released according to the thermogravimetry analysis, which is near the weight loss of 3.7 wt% deduced from Reaction (6).The first dehydrogenation step of T2 sample occurred at the temperature of 400 K, and finished at 460 K.The first dehydrogenation peak temperature of T2 sample lowers ca.10 K compared with that of Tp sample (462 K), and the peak width shrinks ca.50 K from that of Tp samples.Such a sharp dehydrogenation peak reflects a fast rate of dehydrogenation near the peak temperature.Furthermore, the dehydrogenation activation energies (Ea) of T2 sample and Tp sample were determined by the Kissinger's method [36].Through the calculation, the activation energy (Ea) of T2 sample is 109 kJ/mol, which is lower than that of Tp (127 kJ/mol).Isothermal dehydrogenation and re-hydrogenation of T2 and Tp samples at 416 K were performed.Figure 3 (more detailed information is showed in Figure S1) shows that desorption and absorption kinetics of T2 sample are accelerated remarkably in comparison with the Tp sample.In 100 min T2 sample releases ca.80% hydrogen, while Tp sample only releases 40% hydrogen, where the total amount of hydrogen is calculated according to Reaction (6) and Reaction (1), respectively.The tangent slopes of the initial linear parts also indicate that the dehydrogenation rate of the T2 sample is ca. 2 times as fast as that of Tp sample.In the re-hydrogenation test, ca.80% hydrogen can be charged back to T2 sample within 50 min, while under the same condition, only ca.20% hydrogen can be soaked in Tp sample.And the tangent slopes of the initial linear parts also show that T2 is ca.18 times as fast as that of the Tp sample.XRD patterns of T2 samples after dehydrogenation and re-hydrogenation at 460 K were collected for structural analyses.As shown in Figure 4a, the dehydrogenation products of T2 sample are Li2Mg2(NH)3 and Li4BN3H10, which is somehow out of our initial expectation.Li2BNH6 has a hexagonal structure, and melts at ~365 K. Li4BN3H10 has a body-centered cubic structure, and melts at ~465 K. Li2BNH6 is less stable than Li4BN3H10 at temperatures above the melting temperature and decomposes to Li4BN3H10 and LiBH4 [39].However, we did not observe crystalline LiBH4 phase in XRD patterns in Figure 4a, which may become amorpous during this process.After the following re-hydrogenation, Mg(NH2)2 and LiH are regenarated and Li4BN3H10 still remains (seen in Figure 4a).The melting of Li4BN3H10 and/or Li2BNH6 may create a unique reaction environment, allowing interface reaction and mass transport of Reaction ( 1) to proceed at a faster rate [40].Figure 4b shows the FTIR spectra of dehydrogenation and re-hydrogenation samples.The symmetric and asymmetric N-H stretching vibrations of Li4BN3H10 are at 3243 and 3302 cm −1 , while the symmetric and asymmetric N-H stretching vibrations of LiNH2 are at 3257 and 3310 cm −1 .Corresponding to the results of XRD characterization, the vibrations of Li4BN3H10 can be found in the dehydrogenation and re-hydrogenation products of T2 sample.Isothermal dehydrogenation/re-hydrogenation processes of T2 sample are not fully reversible due to the kinetic and/or thermodynamic reasons.Li2Mg2(NH)3 or MgNH is found in the re-hydrogenated sample, which exhibits the vibration at 3197 cm −1 .

Pressure-Composition-Isotherm (PCI) Dehydrogenation at High Pressure
Pressure-Composition-Isotherm (PCI) dehydrogenation curves at 473 K of 2Mg(NH2)2-3LiH (Tp), 2Mg(NH2)2-3LiH-LiBH4 (T2) and 2Mg(NH2)2-3LiH-1/3LiBH4 (T4) samples are shown in Figure 5.It can be seen that the equilibrium plateau pressures of T2 and T4 composites at 473 K are almost the same.However, in the region where the amount of H2 desorption is between 0 and 1.2 wt% (2 hydrogen atoms), the dehydrogenation curves of T2 and T4 at the higher pressure show certain differences (see the insert figure), i.e., there is a higher pressure slope of the T2 (2Mg(NH2)2-3LiH-LiBH4) composite than T4 (2Mg(NH2)2-3LiH-1/3LiBH4) composite [36].The sloping curve indicates that the constitution of the material changes along with the hydrogen desorption.T2 samples with different amount of hydrogen desorption are characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and magic angle spinning nuclear magnetic resonance (MAS NMR).XRD patterns of T2 after ball milling and desorbing different amount of hydrogen at 460 K are showed in Figure 6, the pattern of fresh-made sample confirms the presence of starting materials Mg(NH2)2, LiH and LiBH4.When T2 desorbs 0.25 wt% hydrogen, the peaks of LiBH4 disappear and new peaks assignable to Li4BN3H10 are visible.As the amount of desorption is up to 0.58 wt%, peaks belonging to Li2BNH6 and Li4BN3H10 appear at the same time.However, with the increasing amount of released hydrogen, the peaks of Li4BN3H10 become stronger, the peaks of Li2BNH6 are getting weaker and finally disappear upon 1.4 wt% hydrogen is desorbed.Because diffraction peaks of Li2BNH6 are weak and some of them are overlapped with those of Li4BN3H10, FTIR are preformed synchronously.FTIR spectra of N-H stretching vibrations from the corresponding samples are shown in Figure 7.The B-H stretching frequencies (vibrations from 2000 to 2600 cm −1 ) were also investigated (Figure S2), but due to the sensitivity of the equipment of FT-IR, there is not much useful information that can be gained.In Figure 7, the post-milled composite gives typical N-H stretching vibrations at 3327 and 3273 cm −1 assigned to Mg(NH2)2, which means Mg(NH2)2 does not react with LiBH4 during the ball milling process.During the PCI dehydrogenation, LiNH2 produced by Reaction (1) is immediately combined with the nearby LiBH4, which causes that the typical N-H stretching vibrations at 3301 and 3258 cm −1 assigned to LiNH2 are not observed.Furthermore, the spectral signals at 3243 and 3301 cm −1 , which are assigned to the characteristic vibrations of Li4BN3H10, suggest that LiNH2 reacts with LiBH4 to form Li4BN3H10 at the beginning of PCI dehydrogenation.It is likely that the LiNH2-LiBH4 interface reaction produces the Li2BNH6 layer and Li4BN3H10 layer.However, the possibility of melting and decomposition of Li2BNH6 to Li4BN3H10 and LiBH4 may also exist during the PCI measures.The vibrations at 3246 cm −1 and 3294 cm −1 assigned to Li2BNH6 appear after the dehydrogenation of 0.58 wt%, which corresponds well with the XRD measurement.The solid-state phase of Li2BNH6 may originate from the quenching of the melts.The vibrations of Li2BNH6 are generally weak and broad, and companied with those of Li4BN3H10.Li2BNH6 finally disappears upon desorption of 1.4 wt% hydrogen.We tentatively propose that the formation of Li2BNH6 and Li2Mg2(NH)3 might be the reason for the high pressure slope in the initial dehydrogenation step.
As shown in Figure 8, 11 B MAS NMR spectra of T2 samples at the different dehydrogenation degree can be fitted into three species, i.e., LiBH4 (−41.62 ppm), Li4BN3H10 (−39.81 ppm) and Li2BNH6 (−37.88 ppm).Figure 8a shows that the peak of 11 B signal gradually becomes asymmetric and shifts to the lower chemical shift in pace with the increasing degree of dehydrogenation of T2 sample.After the weight loss of hydrogen reached 0.58 wt%, the board peak starts to appear the signal of Li2BNH6 (−37.88 ppm), which exhibits strongest intensity when desorption weight of hydrogen comes up to 0.94 wt%, plotted in Figure 8b.To summarize the tendency of the peak shift, Figure 8c reveals that the signal area of Li4BN3H10 increases and the signal peak of LiBH4 decreases with the dehydrogenation.
The signal of Li2BNH6 can be distinguished upon releasing 0.58 wt% hydrogen and it soon becomes invisible upon releasing 1.4 wt% hydrogen, which is consistent with the findings of XRD and FT-IR results.