Improved hydrogen storage properties of LiBH4 doped Li–N–H system

Remarkable improvement of hydrogen sorption properties of Li–N–H system has been obtained by doping with a small amount of LiBH₄. The starting and ending temperatures of hydrogen desorption shift to lower temperatures and the release of NH₃ is obviously restrained by 10 mol% LiBH₄ doping. The kinetics of hydrogen desorption and absorption of Li–N–H system became faster by the addition of LiBH₄. About 4 wt.% H₂ can be released within 30 min and ∼4.8 wt.% H₂ can be reabsorbed within 2 min by LiBH₄ doped sample at 250 °C, while only 1.44 wt.% H₂ is released and 2.1 wt.% is reabsorbed for pure Li–N–H system. The quaternary hydride (LiNH₂)_x(LiBH₄)_(1−x) formed by the reaction between LiBH₄ and LiNH₂ may contribute to the enhancement of the hydrogen sorption performances by yielding a ionic liquid phase and transferring LiNH₂ from solid state to molten state with a weakened N–H bond.     

Hydrogen storage properties of destabilized MgH2–Li3AlH6 system

To improve the dehydrogenation properties of MgH₂, a novel hydrogen storage system, MgH₂–Li₃AlH₆, is prepared by mechanochemical milling. Three physical mixtures containing different mole ratios (1:4, 1:1 and 4:1) of MgH₂ and Li₃AlH₆ are studied and there exists a mutual destabilization effect between the components. The last mixture shows a capacity of 6.5 wt% H₂ with the lowest starting temperature of dehydrogenation (170 °C). First, Li₃AlH₆ decomposes into Al, LiH and H₂, and then the as-formed Al can easily destabilize MgH₂ to form the intermetallic compound Mg₁₇Al₁₂ at a temperature of 235 °C, which is about 180 °C lower than the decomposition temperature of pristine MgH₂. Finally, the residual MgH₂ undergoes a self-decomposition whose apparent activation energy has been reduced by about 22 kJ mol⁻¹ compared with pristine MgH₂. At a constant temperature of 250 °C, the mixture can dehydrogenate completely under an initial vacuum and rehydrogenate to form MgH₂ under 2 MPa H₂, showing good cycle stability after the first cycle with a capacity of 4.5 wt% H₂. The comparison between 4 MgH₂ + Li₃AlH₆ and 4 MgH₂ + LiAlH₄ mixtures is also investigated.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

Hydrogen storage properties and mechanisms of the Mg(BH4)2–NaAlH4 system

Hydrogen storage properties and mechanisms of the combined Mg(BH₄)₂–NaAlH₄ system were investigated systematically. It was found that during ball milling, the Mg(BH₄)₂–xNaAlH₄ combination converted readily to the mixture of NaBH₄ and Mg(AlH₄)₂ with a metathesis reaction. The post-milled samples exhibited an apparent discrepancy in the hydrogen desorption behavior with respect to the pristine Mg(BH₄)₂ and NaAlH₄. Approximately 9.1 wt% of hydrogen was released from the Mg(BH₄)₂–2NaAlH₄ composite milled for 24 h with an onset temperature of 101 °C, which is lowered by 105 and 139 °C than that of NaAlH₄ and Mg(BH₄)₂, respectively. At initial heating stage, Mg(AlH₄)₂ decomposed first to produce MgH₂ and Al with hydrogen release. Further elevating operation temperatures gave rise to the reaction between MgH₂ and Al and the self-decomposition of MgH₂ to release more hydrogen and form the Al_0.9Mg_0.1 solid solution and Mg. Finally, NaBH₄ reacted with Mg and partial Al_0.9Mg_0.1 to liberate all of hydrogen and yield the resultant products of MgAlB₄, Al₃Mg₂ and Na. The dehydrogenated sample could take up ∼6.5 wt% of hydrogen at 400 °C and 100 atm of hydrogen pressure through a more complicated reaction process. The hydrogenated products consisted of NaBH₄, MgH₂ and Al, indicating that the presence of Mg(AlH₄)₂ is significantly favorable for reversible hydrogen storage in NaBH₄ at moderate temperature and hydrogen pressure.     

Hydrogen storage properties of MgH2 mechanically milled with α and β SiC

To understand the influence of various crystallographic phases on hydrogen storage properties, ball milling of MgH₂ with hexagonal (α) and cubic (β) SiC have been performed. Structural characterization of all samples has been done by X-ray diffraction (XRD) analysis, particle size analysis and scanning electron microscopy (SEM). Investigation of hydrogen desorption properties of prepared nanocomposites has been done using temperature programmed desorption (TPD) technique. Despite the results of structural and morphological characterization of obtained nanocomposites are very similar, TPD spectra show significant differences regarding existence of intermediate temperature peak. In the sample milled with hexagonal SiC this peak originates both from H₂ and H₂O, while in the sample milled with cubic phase it only comes from H₂O. Both samples exhibit low temperature H₂ peak at 385 K.     

Remarkable decrease in dehydrogenation temperature of Li–B–N–H hydrogen storage system with CoO additive

Cobalt monoxide (CoO) was introduced into the Li–B–N–H system as a catalyst precursor, and the hydrogen desorption behavior of the LiBH₄–2LiNH₂–xCoO (x = 0–0.20) composites was investigated. It was observed that the majority of hydrogen desorption from the CoO-added sample occurred simultaneously with the melting of α-Li₄BN₃H₁₀. Moreover, the 0.05CoO-added sample exhibited optimized dehydrogenation properties, desorbing 9.9 wt% hydrogen completely with an onset temperature of 100 °C and exhibiting a decrease of more than 120 °C in the onset dehydrogenation temperature with respect to that of the additive-free sample. The activation energy of hydrogen desorption for the 0.05CoO-added sample was reduced by 30%. XAFS measurements showed that the CoO additive was first reduced chemically to metallic Co during the initial stage of thermal dehydrogenation, and the newly produced metallic Co acted as the catalytic active species in favor of the creation of B–N bonding. More importantly, approximately 1.1 wt% of hydrogen could be recharged into the fully dehydrogenated 0.05CoO-added sample at 350 °C and a hydrogen pressure of 110 atm, which represents much better performance than that exhibited by the pristine sample.     

Hydrogen storage properties of the Mg–Ti–H system prepared by high-energy–high-pressure reactive milling

Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen content. In the present work, we investigated the hydrogen release/uptake properties of the Mg–Ti–H system. Samples were prepared from the mixtures of MgH₂ and TiH₂ in molar ratios of 7:1 and 4:1 using a high-energy-high-pressure (HEHP) mechanical ball-milling method under 13.8 MPa hydrogen pressure. Thermogravimetric analysis (TGA) showed that a relatively large amount of hydrogen (5.91 and 4.82 wt.%, respectively, for the above two samples) was released between 126 and 313 °C while temperature was increased at a heating rate of 5 °C min⁻¹ under an argon flow. The onset dehydrogenation temperature of these mixtures, which is 126 °C, is much lower than that of MgH₂ alone, which is 381 °C. The activation energy of dehydrogenation was 71 kJ mol⁻¹, which is much smaller than that of as-received MgH₂ (153 kJ mol⁻¹) or as-milled MgH₂ (96 kJ mol⁻¹). Furthermore, the hydrogen capacity and the dehydrogenation temperature remained largely unchanged over five dehydrogenation and rehydrogenation cycles.     

Comprehensive hydrogen storage properties and catalytic mechanism studies of 2LiBH4–MgH2 system with NbF5 in various addition amounts

In the present work, the role of NbF₅ addition amount in affecting the comprehensive hydrogen storage properties (dehydrogenation, rehydrogenation, cycling performance, hydrogen capacity) of 2LiBH₄–MgH₂ system as well as the catalytic mechanism of NbF₅ have been systematically studied. It is found that increasing the addition amount of NbF₅ to the 2LiBH₄–MgH₂ system not only results in dehydrogenation temperature reduction and hydriding–dehydriding kinetics enhancement but also leads to the de/rehydrogenation capacity loss. Compared with other samples, 2LiBH₄–MgH₂ doping with NbF₅ in weight ratios of 40:4 exhibits superior comprehensive hydrogen storage properties, which can stably release ∼8.31 wt.% hydrogen within 2.5 h under 4 bar H₂ and absorb ∼8.79 wt.% hydrogen within 10 min under 65 bar H₂ at 400 °C even up to 20 cycling. As far as we know, this is the first time that excellent reversibility as high as 20 cycles without obvious degradation tendency in both of hydrogen capacity and reaction rate has been achieved in the 2LiBH₄–MgH₂ system. The further experimental study reveals that the highly catalytic effects of NbF₅ on the 2LiBH₄–MgH₂ system are derived from the reaction between NbF₅ and LiBH₄, which provides a fundamental insight into the catalytic mechanism of NbF₅.     

Study on the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 ternary-hydride system

In this paper, we report the hydrogen storage properties and reaction mechanism of NaAlH₄–MgH₂–LiBH₄ (1:1:1) ternary-hydride system prepared by ball milling. It was found that during ball milling, the NaAlH₄/MgH₂/LiBH₄ combination converted readily to the mixture of LiAlH₄/MgH₂/NaBH₄ and there is a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the system, which corresponds to the decomposition of LiAlH₄, MgH₂, and NaBH₄, respectively. The onset dehydrogenation temperature of MgH₂ in this system is observed at around 275 °C, which is over 55 °C lower from that of as-milled MgH₂. Meanwhile, NaBH₄-relevant decomposition showed significant improvement, starts to release hydrogen at 370 °C, which is reduced by about 110 °C compared to the as-milled NaBH₄. The second and third steps decomposition enthalpy of the system were determined by differential scanning calorimetry measurements and the enthalpies were changed to be 61 and 100 kJ mol⁻¹ H₂ respectively, which are smaller than that of MgH₂ and NaBH₄ alone. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂ and NaBH₄ in the composite was reduced to 96.85 and 111.74 kJ mol⁻¹ respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including Li–Mg, Mg–Al, and Mg–Al–B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway.     

Rehydrogenation and cycle studies of LiBH4–CaH2 composite

Rehydrogenation behavior of 6LiBH₄ + CaH₂ composite with NbF₅ has been studied between 350 and 500 °C after dehydrogenation at 450 °C. The composite exhibits the best rehydrogenation feature at 450 °C in terms of the overall rehydrogenation rate and the amount of absorbed hydrogen. It is found that about 9 wt% hydrogen is absorbed at 450 °C for 12 h. Up to 10 dehydrogenation–hydrogenation cycles have been carried out for the composite. It is demonstrated that 6LiBH₄ + CaH₂ with 15 wt% NbF₅ maintains a reversible hydrogen storage capacity of about 6 wt% at 450 °C after a slight degradation between the 1st and 5th cycles. The addition of NbF₅ seems to improve the cycle properties by retarding microstructural coarsening during cycles.     

Effects of MoS2 addition on the hydrogen storage properties of 2LiBH4–MgH2 systems

A 2LiBH₄–MgH₂–MoS₂ composite was prepared by solid-state ball milling, and the effects of MoS₂ as an additive on the hydrogen storage properties of 2LiBH₄–MgH₂ system together with the corresponding mechanism were investigated. As shown in the TG–DSC and MS results, with the addition of 20 wt.% of MoS₂, the onset dehydrogenation temperature is reduced to 206 °C, which is 113 °C lower than that of the pristine 2LiBH₄–MgH₂ system. Meanwhile, the total dehydrogenation amount can be increased from 9.26 wt.% to 10.47 wt.%, and no gas impurities such as B₂H₆ and H₂S are released. Furthermore, MoS₂ improves the dehydrogenation kinetics, and lowers the activation energy (E_a) 34.49 kJ mol⁻¹ of the dehydrogenation reaction between Mg and LiBH₄ to a value lower than that of the pristine 2LiBH₄–MgH₂ sample. According to the XRD test, Li₂S and MoB₂ are formed by the reaction between LiBH₄ and MoS₂, which act as catalysts and are responsible for the improved hydrogen storage properties of the 2LiBH₄–MgH₂ system.     

Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage

Two composite hydrogen storage materials based on Mg₂FeH₆ were investigated for the first time. The Mg₂FeH₆–LiBH₄ composite of molar ratio 1:5 showed a hydrogen desorption capacity of 5.6 wt.% at 370 °C, and could be rehydrogenated to 3.6 wt.% with the formation of MgH₂, as the material was heated to 445 °C and held at this temperature. The Mg₂FeH₆–LiNH₂ composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of 4.3 wt.% and released hydrogen at 100 °C lower then the Mg₂FeH₆–LiBH₄ composite, but this mixture could not be rehydrogenated. Compared to neat Mg₂FeH₆, both composites show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at these low temperatures. In particular, Mg₂FeH₆–LiNH₂ exhibits a much lower desorption temperature than neat Mg₂FeH₆, but only Mg₂FeH₆–LiBH₄ re-absorbs hydrogen.     

The hydrogen storage properties and reaction mechanism of the NaAlH4 + Ca(BH4)2 composite system

The hydrogen storage properties and reaction mechanism of the combined NaAlH₄ + Ca(BH₄)₂ (2:1) composite system was investigated in the present study. Analyses show that after 6 h of milling, the NaAlH₄ + Ca(BH₄)₂ combination fully converted to the mixture of Ca(AlH₄)₂ + NaBH₄, and a metathesis reaction occurred between the hydrides. Four major dehydrogenation stages were observed in the system, which corresponds to the decomposition of Ca(AlH₄)₂, CaAlH₅, CaH₂ and NaBH₄, respectively. The onset desorption temperature of the composite system is reduced to 125 °C, which is much lower than a unary component of NaAlH₄ and Ca(BH₄)₂. The de/rehydrogenation kinetics of the composite system had improve at a higher temperature. From the Kissinger plot, the apparent activation energies for the decomposition of CaAlH₅ and NaBH₄ in the composite system were reduced to 142.9 and 146.5 kJ/mol, respectively. It is believed that the formation of AlCa, AlB and CaB alloys during the dehydrogenation process is responsible for the distinct reduction in the onset desorption temperature and kinetics enhancement of the 2NaAlH₄ + Ca(BH₄)₂ composite system.     

Hydrogen sorption properties of Li–N–F–H pellets in laboratory and small tank scales

We propose the improvement of de/rehydrogenation kinetics and reversibility of LiNH₂–LiH system via F substitution for H in LiNH₂. Slight content of LiF–TiH₂ composite is milled with LiNH₂–3LiH and the obtained mixture is compacted into the pellet. By compositing LiNH₂ with excess LiH and compaction, NH₃ emission poisoning fuel cell catalysts and degrading hydrogen capacity can be prevented. The formation of LiNH_(2-x)F_x achieved from F substitution in LiNH₂ significantly enhances kinetic properties, for example, the 1st dehydrogenation at 280 °C completes within 5 min with the capacity of 3.6 wt % H₂. Although LiNH_(2-x)F_x cannot be recovered, good kinetics and reversibility upon five de/absorption cycles of LiNH₂–LiH (up to 3.0 wt % H₂ within 20 min) are preserved due to catalytic effects of LiF. Phase compositions and hydrogen capacities in laboratory and tank scales are comparable. This suggests the maintained de/rehydrogenation performance of Li–N–F–H pellets even after upscaling.     

Destabilization of the LiNH2–LiH hydrogen storage system by aluminum incorporation

The lithium amide–lithium hydride system (LiNH₂–LiH) is one of the most attractive light-weight materials for hydrogen storage. In an effort to improve its hydrogen sorption kinetics, the effect of 1 mol% AlCl₃ addition to LiNH₂–LiH system was systematically investigated by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis and hydrogen volumetric measurements. It is shown that Al³⁺ is incorporated into the LiNH₂ structure by partial substitution of Li⁺ forming a new amide in the Li–Al–N–H system, which is reversible under hydriding/dehydriding cycles. This new substituted amide displays improved hydrogen storage properties with respect to LiNH₂–LiH. In fact, a stable hydrogen storage capacity of about 4.5–5.0 wt% is observed under cycling and is completely desorbed in 30 min at 275 °C for the Li–Al–N–H system. Moreover, the concurrent incorporation of Al³⁺ and the presence of LiH are effective for mitigating the ammonia release. The results reveal a common reaction pathway for LiNH₂–LiH and LiNH₂–LiH plus 1 mol% AlCl₃ systems, but the thermodynamic properties are changed by the inclusion of Al³⁺ in the LiNH₂ structure. These findings have important implications for tailoring the properties of the Li–N–H system.     

Catalytic effect of MgFe2O4 on the hydrogen storage properties of Na3AlH6–LiBH4 composite system

The effect of MgFe₂O₄ on the hydrogen storage properties of the composite Na₃AlH₆4LiBH₄ was studied for the first time, where it was found that MgFe₂O₄ addition decreased the onset desorption temperature of Na₃AlH₆4LiBH₄. Hydrogen (～9.5 wt%) was released in three stages and the dehydrogenation temperatures were reduced to 80 °C, 350 °C, and 430 °C for the first, second, and third stage, respectively. The absorption kinetics of Na₃AlH₆4LiBH₄ was also significantly improved due to the catalytic effect of MgFe₂O₄. Using Kissinger analysis, the apparent activation energies of decomposition of the Li₃AlH₆ and NaBH₄ stages in Na₃AlH₆4LiBH₄-10 wt% MgFe₂O₄ were calculated to be 72 and 141 kJ/mol, respectively. These values were considerably lower than the corresponding values for the undoped composite. X-ray diffraction analysis revealed the formation of new products such as MgO and Fe during the heating process. Our results suggest that MgFe₂O₄ enhanced the hydrogen storage properties of Na₃AlH₆4LiBH₄ through the formation of active species, such as MgO and Fe.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Effect of SWNTs on the reversible hydrogen storage properties of LiBH4–MgH2 composite

Single-walled carbon nanotubes (SWNTs) were mechanically milled with LiBH₄/MgH₂ mixture, and examined with respect to its effect on the reversible dehydrogenation properties of the Li–Mg–B–H system. Experimental results show that the addition of SWNTs results in an enhanced dehydriding rate and improved cyclic stability of the LiBH₄/MgH₂ composite. For example, the LiBH₄/MgH₂ composite with 10 wt% purified SWNTs additive can release nearly 10 wt% hydrogen within 20 min at 450 °C, with an average dehydriding rate over 2 times faster than that of the neat LiBH₄/MgH₂ sample. Based on the results of phase analysis and a series of designed experiments, the mechanism underlying the observed property improvement was discussed.     

Enhanced hydrogen storage properties of TiN–LiAlH4 composite

The hydrogen storage properties of LiAlH₄ doped efficient TiN catalyst were systematically investigated. We observe that TiN catalyst enhances the dehydrogenation kinetics and decreases the dehydrogenation temperature of LiAlH₄. The dehydrogenation behaviors of 2%TiN–LiAlH₄ are investigated using temperature programmed desorption (TPD), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Interestingly, the onset hydrogen desorption temperature of 2%TiN–LiAlH₄ sample gets lowered from 151.0 °C to 90.0 °C with a faster kinetics, and the dehydrogenation rate reached a maximum value at 137.2 °C. By adding a small amount of as-prepared TiN, approximately 7.1 wt% of hydrogen can be released from the LiAlH₄ at 130 °C. Interestingly, the result of the FTIR indicates that the 2%TiN–LiAlH₄ maybe restore hydrogen under 5.5 MPa hydrogen. Moreover, 2%TiN–LiAlH₄ displayed a substantially reduced activation energy for LiAlH₄ dehydrogenation.