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.     

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.     

Enhanced hydrogen reversibility of nanoconfined LiBH4–Mg(BH4)2

This study shows the hydrogen desorption kinetics and reversible hydrogen storage properties of 0.55LiBH₄–0.45Mg(BH₄)₂ melt-infiltrated in different nanoporous carbon aerogels with different BET surface areas of 689 or 2660 m²/g and pore volumes of 1.21 or 3.13 mL/g. These investigations clearly show a significantly improved hydrogen storage capacity after four cycles of hydrogen release and uptake for bulk 0.55LiBH₄–0.45Mg(BH₄)₂ and infiltrated in carbon aerogel and the high surface area scaffold, where 22, 36 and 58% of the initial hydrogen content remain after four cycles of hydrogen release and uptake, respectively. Nanoconfinement in high surface area carbon aerogel appears to facilitate hydrogen release illustrated by release of 13.3 wt% H₂ (93%) and only 8.4 wt% H₂ (58%) from bulk hydride in the first cycle using the same physical condition. Notably, nanoconfinement also appear to have a beneficial effect on hydrogen uptake, since 8.3 wt% H₂ (58%) is released from the high surface area scaffold and only 3.1 wt% H₂ (22%) from the bulk sample during the fourth hydrogen release.     

Hydrogen storage properties and reaction mechanisms of K2Mn(NH2)4–8LiH system

Hydrogen storage properties of K₂Mn(NH₂)₄–8LiH were investigated by considering its de/re-hydrogenation properties and reaction mechanisms. Experimental results show that the dehydrogenated K₂Mn(NH₂)₄–8LiH can be almost re-hydrogenated completely at 230 °C and 50 bar of H₂ with a hydrogenation rate more than 1.0 wt%/min. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and FTIR investigations reveal that during ball milling K₂Mn(NH₂)₄ reacts with LiH to form LiNH₂ and K–Mn-species1 which is probably a K–Mn-containing hydride. The ball milled sample releases hydrogen in a multi-step reaction with the formation of K₃MnH₅ and K–Mn-species2 as intermediates and Li₂NH, Mn₃N₂ and MnN as final products. The full hydrogenated products are LiH, LiNH₂, and K–Mn-species2. The K–Mn-species2 may play a critical role for the fast hydrogeneration. This work indicates that transition metal contained amide-hydride composite holds potentials for hydrogen storage.     

Reversible dehydrogenation of Mg(BH4)2–LiH composite under moderate conditions

The Mg(BH₄)₂-xLiH (0.1 ≤ x ≤ 0.8) composites which exhibit favorable dehydrogenation and encouraging reversibility are experimentally investigated. LiH additive reduces the onset temperature for dehydrogenation to 150 °C. And hydrogen release exceeds 10 wt.% from the new binary material below 250 °C. Furthermore, rehydrogenation results show that 3.6 wt.% hydrogen can still be recharged after twenty cycles at 180 °C. It should be emphasized that the long-term reversibility of borohydride under 200 °C is long overdue. TPD, PCT, and high-pressure DSC measurements are used to characterize the improvements in thermodynamic and kinetic ways. In addition, FT-IR and NMR studies indicate that the composite has a significant synergistic effect during (de)hydrogenation processes. This work suggests that controlled cation stoichiometry combined with doping by metal Li⁺ subvalent to Mg²⁺ facilitate the formation of polyborane intermediates [B₃H₈]⁻ and [B₂H₆]²⁻. They improve the dehydrogenation properties and make the material reversible under mild conditions.     

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH₄–Mg(BH₄)₂ (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH₄, Mg(BH₄)₂ or pristine LiBH₄–Mg(BH₄)₂, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co⁺ which may serve as the functional species in catalyzing the dehydrogenation of the composite.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Improved dehydrogenation properties of the combined Mg(BH4)2·6NH3–nNH3BH3 system

A combined strategy via mixing Mg(BH₄)₂·6NH₃ with ammonia borane (AB) is employed to improve the dehydrogenation properties of Mg(BH₄)₂·6NH₃. The combined system shows a mutual dehydrogenation improvement in terms of dehydrogenation temperature and hydrogen purity compared to the individual components. A further improved hydrogen liberation from the Mg(BH₄)₂·6NH₃–6AB is achieved with the assistance of ZnCl₂, which plays a crucial role in stabilizing the NH₃ groups and promoting the recombination of NH^δ+?HB^δ−. Specifically, the Mg(BH₄)₂·6NH₃–6AB/ZnCl₂ (with a mole ratio of 1:0.5) composite is shown to release over 7 wt.% high-pure hydrogen (>99 mol%) at 95 °C within 10 min, thereby making the combined system a promising candidate for solid hydrogen storage.     

Reversible hydrogen storage behavior of LiBH4–Mg(OH)2 composites

The dehydrogenation/hydrogenation properties of LiBH₄-xMg(OH)₂ were systematically investigated. The results show that the LiBH₄-0.3Mg(OH)₂ composite possesses optimal dehydrogenation properties: approximately 9.6 wt% of hydrogen is released via a stepwise reaction with an onset temperature of 100 °C. In the range of 100–250 °C, a chemical reaction between LiBH₄ and Mg(OH)₂ first occurs to give rise to the generation of LiMgBO₃, MgO and H₂. From 250 to 390 °C, the newly developed LiMgBO₃ reacts with LiBH₄ to form MgO, Li₃BO₃, LiH, B₂O₃ and Li₂B₁₂H₁₂ with hydrogen release. From 390 to 450 °C, the decomposition of LiBH₄ and Li₂B₁₂H₁₂ proceeds to release additional hydrogen and to form LiH and B. A further hydrogenation experiment indicates that the dehydrogenated LiBH₄-0.3Mg(OH)₂ sample can take up 4.7 wt% of hydrogen at 450 °C and 100 bar of hydrogen with good cycling stability, which is superior to the pristine LiBH₄.     

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.     

Thermal dehydrogenation behaviors and mechanisms of Mg(BH4)2∙6NH3-xLiH combination systems

A reactive composite of Mg(BH₄)₂⋅6NH₃-xLiH is prepared, and the effects of the LiH content on the dehydrogenation/hydrogenation properties of the material are investigated. The results show that the presence of LiH with x = 3 reduces the onset dehydrogenation temperature of Mg(BH₄)₂⋅6NH₃ from 130 °C to 80 °C in TPD mode. Approximately 14.3 wt% hydrogen is released from the Mg(BH₄)₂⋅6NH₃-6LiH composite with distinctly reduced ammonia evolution while heating to 340 °C. Upon heating, Mg(BH₄)₂⋅6NH₃ first reacts with LiH to form Mg(NH₂)₂, Li₃BN₂H₈ and LiBH₄ with the release of H₂ and the evolution of a minor amount of NH₃. The newly formed Mg(NH₂)₂ then reacts with LiH to produce H₂ and Li₂Mg(NH)₂. Further elevating the operating temperature induces chemical reactions between Li₂Mg(NH)₂, LiBH₄ and Li₃BN₂H₈, causing the release of additional H₂ and production of Li₃BN₂, LiMgBN₂ and LiH. The dehydrogenated sample at 210 °C absorbs 2.2 wt% of hydrogen, exhibiting partial reversibility for hydrogen storage.     

Effects of compaction pressure and graphite content on hydrogen storage properties of Mg(NH2)2–2LiH hydride

Preparation of hydride–graphite compacts serves as an effective method to improve the volumetric hydrogen storage density and the effective thermal conductivity for light complex hydrides. This paper presents the effects of compaction pressure and expanded natural graphite (ENG) content on the hydrogen storage properties of the Mg(NH₂)₂–2LiH–0.07KOH compacts. The results show that the hydrogen desorption kinetics of the 1st sorption cycle decreases with the increase of the compaction pressure. However, the compacts exhibit the similar hydrogen desorption kinetics and capacities from the 2nd sorption cycles on regardless of the compaction pressure. The ENG addition significantly enhances the desorption kinetics because of the improvement of the heat transfer performance of the hydride. Furthermore, the volumetric hydrogen storage density of the hydride reaches 47 g/L after the compaction at 365 MPa, but it reduces by increasing the ENG content.     

Hydrogen storage properties of Mg–Ni nanoparticles

The aim of this work is to investigate metal–hydride transformation in Magnesium (Mg) nanoparticles decorated by Nickel (Ni). The samples were synthesized by Inert Gas Condensation: Mg single crystal nanoparticles were deposited on a metal substrate and subsequently their surface was exposed to evaporation of Ni. Structural analysis was made by Synchrotron Radiation Powder X-ray Diffraction and thermodynamic measurements by Sieverts apparatus. Ni decoration significantly improves the hydrogen release and uptake kinetics of the nanoparticles. The results connect the formation of Mg₂Ni and Mg₂NiH₄ phases to the enhancement of hydrogen sorption properties.     

Hydrogen storage properties of Li–Ca–N–H system with different molar ratios of LiNH2/CaH2

In this work, dehydrogenation and rehydrogenation of three LiNH₂/CaH₂ samples with LiNH₂/CaH₂ molar ratio of 2/1, 3/1 and 4/1 were systematically investigated. Remarkable differences were observed in the temperature dependence of hydrogen desorption and subsequent absorption. LiNH₂/CaH₂ in a molar ratio of 2/1 transforms to ternary imide Li₂Ca(NH)₂ after desorbing about 4.5 wt.% H₂ at 350 °C. And it has a reversible hydrogen storage capacity of 2.7 wt.% at 200 °C. As for the LiNH₂/CaH₂ mixture in a molar ratio of 4/1, it transforms to a new compound with a composition of Li₄CaN₄H₆ after being dehydrogenated at 350 °C. The rehydrogenation of both LiCa(NH)₂ and Li₄CaN₄H₆ gives LiNH₂, LiH and the solid solution of 2CaNH–Ca(NH₂)₂.     

Synergetic effect of C (graphite) and Nb2O5 on the H2 sorption properties of the Mg–MgH2 system

Ternary Mg–Nb₂O₅ – graphitic C mixtures (molar ratio % = 97.5:0.5:2.0) were prepared by high-energy ball milling (BM) under Ar for different times (from 0.25 h to 4 h) and thoroughly characterized by manometric, calorimetric, X-ray powder diffraction, and scanning electron microscopy analyses. The aims of the work were: - to assess the effect of the simultaneous presence of the two dopants on the reactivity and the sorption properties of the Mg–MgH₂ system; - to study the influence of the milling time on the performance of the mixtures.     

Reversible hydrogenation/dehydrogenation performances of the Na2LiAlH6–Mg(NH2)2 system

Complex hydrides and Metal–N–H-based materials have attracted considerable attention due to their high hydrogen content. In this paper, a novel amide–hydride combined system was prepared by ball milling a mixture of Na₂LiAlH₆–Mg(NH₂)₂ in a molar ratio of 1:1.5. The hydrogen storage performances of the Na₂LiAlH₆–1.5Mg(NH₂)₂ system were systematically investigated by a series of dehydrogenation/hydrogenation evaluation and structural analyses. It was found that a total of ∼5.08 wt% of hydrogen, equivalent to 8.65 moles of H atoms, was desorbed from the Na₂LiAlH₆–1.5Mg(NH₂)₂ combined system. In-depth investigations revealed that the variable milling treatments resulted in the different dehydrogenation reaction pathways due to the combination of Al and N caused by the energetic milling. Hydrogen uptake experiment indicated that only ∼4 moles of H atoms could be reversibly stored in the Na₂LiAlH₆–1.5Mg(NH₂)₂ system perhaps due to the formation of AlN and Mg₃N₂ after dehydrogenation.     

Hydrogen storage of the Mg–C composites

The effect of different kinds of carbon on the hydrogen sorption kinetics by magnesium–carbon composites was analyzed. To prepare magnesium-based composites by ball milling, graphite and carbon nanomaterials (hereinafter CNM) obtained by the electroexplosion technique were used. Phase composition and structure state of the as-milled and hydrogenated magnesium–carbon and magnesium–nickel–carbon composites have been investigated. It was found the crystallite size in the Mg–CNM composite is smaller in comparison with the magnesium–graphite and magnesium–graphite–nickel mixtures. The CNM additives to magnesium essentially improve the hydrogen sorption kinetics. It results in a reduction of hydrogen sorption temperature. The noticeable hydrogen absorption took place already at a temperature of 363 K. The hydrogen capacity was about 5 wt% for magnesium ball milled with CNM additives.     

Effect of C (graphite) doping on the H2 sorption performance of the Mg–Ni storage system

Binary Mg–Ni mixtures and ternary Mg–Ni–C (graphite) samples with fixed proportions of metals (Mg 85%–Ni 15% by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for t_BM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg–Ni system.     

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.     

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.