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High operating temperatures and sluggish kinetics are major obstacles for practical applications of MgH2 as a solid hydrogen carrier. Introducing nanoscaled high-activity catalysts has been effective in improving the hydrogen cycling of MgH2. However, it remains still unclear that between nanoparticle size and morphology, which one is the decisive factor of the catalytic activity of a given catalyst. In this work, we studied this topic by taking nanostructured niobium oxide (Nb2O5) as a representative sample. Five types of Nb2O5 catalytic additives with different morphologies and nanosizes were synthesized, and their catalytic activities were compared with commercial microparticles. Our results unambiguously demonstrate that the catalytic activity of Nb2O5 is determined by the primary particle size rather than the morphology and structure because the ultrasmall Nb2O5 nanoparticles that measured ∼5 nm in size enable dehydrogenation of MgH2 starting at 165 °C after one-cycle activation. The smaller nanoparticle sizes not only enhance the reactivity of Nb2O5 but also lead to more uniform dispersion when ball-milled with MgH2, which enables in situ formation of more homogeneous and finer Nb-based active species and therefore much higher catalytic activity. This important insight will guide the design and optimization of novel high-activity catalysts for hydrogen cycling of MgH2 and other hydrogen storage materials. [ABSTRACT FROM AUTHOR]

• A unique structural catalyst of Ni and carbon composite is developed for MgH 2. • Ultrafine Ni particles are uniformly dispersed on porous hollow carbon nanospheres. • The composite with Ni loading up to 90 wt% shows highly-effective catalysis to MgH 2. • The catalyzed MgH2 system shows a low peak dehydrogenation temperature of 242 °C. • A reversible capacity of 6.4 wt% achieves after 50 cycles at a moderate cyclic regime. Magnesium hydride (MgH 2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign; however, it suffers from high dehydrogenation temperature and slow sorption kinetics. Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH 2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles (NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH 2. With an addition of 5 wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH 2 are lowered to 190 and 242 °C, respectively. 6.2 wt% H 2 is rapidly released within 30 min at 250 °C. The amount of H 2 that the dehydrogenation product can absorb at a low temperature of 150 °C in only 250 s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active, reacting with MgH 2 and forming nanosized Mg 2 Ni/Mg 2 NiH 4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the (de)hydrogenation kinetics of MgH 2. A composite catalyst with high-loading and ultrafine Ni nanoparticles uniformly dispersed on porous hollow carbon nanospheres was developed, which shows highly-effective and durable catalytic activity towards the de-/hydrogenation properties, and especially kinetics of MgH 2. [Display omitted] [ABSTRACT FROM AUTHOR]

Summary: High operation temperatures and slow reaction kinetics are major obstacles to use MgH2 as a solid hydrogen store. We report here the synthesis of Nb2O5 hollow spheres (o‐Nb2O5) with wall thickness of approximately 50 nm and mossy surfaces using a facile hydrothermal and calcination process, which showed high activity in catalysis of MgH2 for hydrogen storage. The dehydrogenation onset temperature of MgH2 was decreased to 195°C with 7 wt% of o‐Nb2O5. More than 5.5 wt% H2 can be desorbed at 300°C within 5 minutes. Hydrogen re‐absorption starts even at 25°C and reaches 5.6 wt% within 5 minutes at 200°C. Practical hydrogen capacity stabilizes at 5.8 wt% after 10 cycles of hydrogen uptake/release. The o‐Nb2O5 was found to be reduced in situ by MgH2 to low‐valence Nb species during the initial dehydrogenation process, which functions as an active catalyst and leads to the enhanced dehydrogenation kinetics. [ABSTRACT FROM AUTHOR]

Critical limitations in applying MgH2 as a hydrogen-storage medium include the high H2 desorption temperature and slow reaction kinetics. In this study, we synthesized hierarchical porous TiNb2O7 spheres in micrometer scale built with 20–50 nm nanospheres, which showed stable activity to catalyze hydrogen storage in MgH2 as precursors. The addition of 7 wt.% TiNb2O7 in MgH2 reduced the dehydrogenation onset temperature from 300 to 177 °C. At 250 °C, approximately 5.5 wt.% H2 was rapidly released in 10 min. Hydrogen uptake was detected even at room temperature under 50 bar hydrogen; 4.5 wt.% H2 was absorbed in 3 min at 150 °C, exhibiting a superior low-temperature hydrogenation performance. Moreover, nearly constant capacity was observed from the second cycle onward, demonstrating stable cyclability. During the ball milling and initial de/hydrogenation process, the high-valent Ti and Nb of TiNb2O7 were reduced to the lower-valent species or even zero-valent metal, which in situ created multivalent multielement catalytic surroundings. A strong synergistic effect was obtained for hybrid oxides of Nb and Ti by density functional theory (DFT) calculations, which largely weakens the Mg-H bonding and results in a large reduction in kinetic barriers for hydrogen storage reactions of MgH2. Our findings may guide the further design and development of high-performance complex catalysts for the reversible hydrogen storage of hydrides. [ABSTRACT FROM AUTHOR]

Hydrogen can play an important role in the development of a sustainable energy system. However, storing hydrogen in a safe, efficient and economical manner remains a huge challenge. Light metal hydrides have attracted considerable attention for hydrogen storage owing to their high gravimetric and volumetric hydrogen densities. However, the strong covalent and/or ionic bonds between metal atoms and hydrogen result in slow kinetics, poor reversibility, and temperatures too high for dehydrogenation, hence delaying their practical large–scale applications. Considerable efforts have been toward tailoring the thermodynamic and kinetic properties of light metal hydride–based hydrogen storage materials for performance improvement, with the fabrication of nanoscale particles being a key and effective strategy. This review covers the preparation methods and hydrogen storage performance of nanostructured light metal hydrides. The physical and chemical properties and hydrogen storage behaviors of reversible light metal hydrides are first summarized, including MgH 2 , borohydrides, aluminum hydrides, amide–hydride systems, and hydride composites. The second section focuses on the research progress in nanostructuring for enhancing the reversible hydrogen storage properties of these hydrides. Finally, the main challenges and the future research prospects are discussed. The combination of nanostructuring and nanocatalysis can significantly enhance the performance of these hydrides and make them practical hydrogen carriers. [Display omitted] • Physical and chemical properties of light metal hydrides are discussed. • Research advances in nanostructured light metal hydrides are summarized. • Breakthroughs in nanoscaled MgH 2 and LiBH 4 are highlighted. • Challenges and the future research directions are discussed. [ABSTRACT FROM AUTHOR]

A novel dual-cation/anion complex hydride (Li 2 Mg(BH 4 ) 2 (NH 2 ) 2 ), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH 4 NH 2 and Li 2 BH 4 NH 2 . The prepared Li 2 Mg(BH 4 ) 2 (NH 2 ) 2 crystallizes in a triclinic structure, and the [NH 2 ] and [BH 4 ] groups remain intact within the structure. Upon heating, the prepared Li 2 Mg(BH 4 ) 2 (NH 2 ) 2 decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li 2 Mg(BH 4 ) 2 (NH 2 ) 2 first decomposes to LiMgBN 2 , LiBH 4 , BN, LiH and MgBNH 8 at 100–250 °C, and then, the newly formed MgBNH 8 reacts with LiH to form Mg, LiBH 4 and BN at 250–340 °C. Finally, the decomposition of LiBH 4 releases hydrogen and generates LiH and B at 340–450 °C. [ABSTRACT FROM AUTHOR]

Sodium alanate (NaAlH4) has attracted intense interest as a prototypical high-density hydrogen-storage material. However, poor reversibility and slow kinetics limit its practical applications. Herein, a nanocrystalline ZrO2@C catalyst was synthesized by using Uio-66(Zr) as a precursor and furfuryl alcohol (FA) as a carbon source. The as-synthesized ZrO2@C exhibits good catalytic activity for the dehydrogenation and hydrogenation of NaAlH4. The NaAlH4-7 wt % ZrO2@C sample released hydrogen starting from 126 °C and reabsorbed it starting from 54 °C, and these temperatures are lower by 71 and 36 °C, respectively, relative to pristine NaAlH4. At 160 °C, approximately 5.0 wt % of hydrogen was released from the NaAlH4-7 wt % ZrO2@C sample within 250 min, and the dehydrogenation product reabsorbed approximately 4.9 wt % within 35 min at 140 °C and 100 bar of hydrogen. The catalytic function of the Zr-based active species is believed to contribute to the significantly reduced operating temperatures and enhanced kinetics. [ABSTRACT FROM AUTHOR]

Cesium hydride (CsH) was successfully synthesized by ball milling Cs under a hydrogen atmosphere of 50 bar at 50 °C. The effect of the prepared CsH on the hydrogen storage properties of the Mg(NH 2 ) 2 -2LiH system was systematically investigated. The Mg(NH 2 ) 2 -2LiH-0.08CsH composite exhibited optimal hydrogen storage properties because it reversibly stores approximately 4.62 wt% hydrogen with a dehydrogenation onset temperature of 70 °C via a two-stage reaction. At 150 °C, approximately 80% of the reversible capacity was quickly released from the Mg(NH 2 ) 2 -2LiH-0.08CsH composite within 100 min. The fully dehydrogenated CsH-containing sample began to absorb hydrogen at 55 °C and took up approximately 4.58 wt% hydrogen at 130 °C. A cycling analysis indicated that the CsH-containing Mg(NH 2 ) 2 -2LiH system exhibited good reversible hydrogen storage abilities. Detailed mechanistic studies revealed that during the initial heating process, CsH gradually reacted with Mg(NH 2 ) 2 to afford CsMg(NH)(NH 2 ), and CsH acted as a catalyst to reduce the activation energy barrier of the first dehydrogenation step. As the operating temperature increased, CsMg(NH)(NH 2 ) as a reactant participated in a second dehydrogenation step to decrease the desorption enthalpy change. This behavior reasonably explains the significantly improved hydrogen storage properties of the CsH-containing Mg(NH 2 ) 2 -2LiH system. [ABSTRACT FROM AUTHOR]

Solid-state hydrogen storage using various materials is expected to provide the ultimate solution for safe and efficient on-board storage. Complex hydrides have attracted increasing attention over the past two decades due to their high gravimetric and volumetric hydrogen densities. In this account, we review studies from our lab on tailoring the thermodynamics and kinetics for hydrogen storage in complex hydrides, including metal alanates, borohydrides and amides. By changing the material composition and structure, developing feasible preparation methods, doping high-performance catalysts, optimizing multifunctional additives, creating nanostructures and understanding the interaction mechanisms with hydrogen, the operating temperatures for hydrogen storage in metal amides, alanates and borohydrides are remarkably reduced. This temperature reduction is associated with enhanced reaction kinetics and improved reversibility. The examples discussed in this review are expected to provide new inspiration for the development of complex hydrides with high hydrogen capacity and appropriate thermodynamics and kinetics for hydrogen storage. [ABSTRACT FROM AUTHOR]

The Mg(BH 4 ) 2 ·2NH 3 – x NaAlH 4 ( x = 0–4) combination systems were prepared by ball milling, and the reversible hydrogen storage behavior and its mechanisms were investigated and discussed. Combining NaAlH 4 with Mg(BH 4 ) 2 ·2NH 3 significantly reduces the operating dehydrogenation temperatures and effectively suppresses the emission of NH 3 by-products. The dehydrogenation onset temperature of the Mg(BH 4 ) 2 ·2NH 3 –2NaAlH 4 system is lowered to ca. 70 °C, which is much lower than the onset temperatures of either Mg(BH 4 ) 2 ·2NH 3 or NaAlH 4 . In addition, ammonia emission from Mg(BH 4 ) 2 ·2NH 3 is thoroughly suppressed by the addition of NaAlH 4 , leading to approximately 11.3 wt% hydrogen released upon heating to 570 °C. Further investigations revealed that at the initial heating stage, Mg(BH 4 ) 2 ·2NH 3 first reacts with NaAlH 4 to produce NaBH 4 , Al 3 Mg 2 , Mg, Al 0.95 Mg 0.05 , BN, Na and AlN along with the release of hydrogen. Further increasing the operation temperature gives rise to a chemical reaction between NaBH 4 , AlN and Mg that liberates all of the hydrogen and yields the resultant products of MgAlB 4 , BN, Na and Al 3 Mg 2 . The dehydrogenated products can take up ∼3.5 wt% of hydrogen at 450 °C and 100 bar of hydrogen pressure, exhibiting a partial reversibility for hydrogen storage. [ABSTRACT FROM AUTHOR]

A nanocrystalline CeO2@C-containing NaAlH4 composite is successfully synthesized in situ by hydrogenating a NaH-Al mixture doped with CeO2@C. Compared with NaAlH4, the as-prepared CeO2@C-containing NaAlH4 composite, with a minor amount of excess Al, exhibits significantly improved hydrogen storage properties. The dehydrogenation onset temperature of the hydrogenated [NaH-Al-7 wt% CeO2@C]-0.04Al sample is 77°C lower than that of the pristine sample because of a reduced kinetic barrier. More importantly, the dehydrogenated sample absorbs ~4.7 wt% hydrogen within 35 min at 100 °C and 10 MPa of hydrogen. Compositional and structural analyses reveal that CeO2 is converted to CeH2 during ball milling and that the newly formed CeH2 works with the excess of Al to synergistically improve the hydrogen storage properties of NaAlH4. Our findings will aid in the rational design of novel catalyst- doped complex hydride systems with low operating temperatures, fast kinetics, and long-term cyclability. [ABSTRACT FROM AUTHOR]

Herein, an initial attempt to understand the relationships between hydrogen storage properties, reaction pathways, and material compositions in LiBH4- x Mg(AlH4)2 composites is demonstrated. The hydrogen storage properties and the reaction pathways for hydrogen release from LiBH4- x Mg(AlH4)2 composites with x=1/6, 1/4, and 1/2 were systematically investigated. All of the composites exhibit a four-step dehydrogenation event upon heating, but the pathways for hydrogen desorption/absorption are varied with decreasing LiBH4/Mg(AlH4)2 molar ratios. Thermodynamic and kinetic investigations reveal that different x values lead to different enthalpy changes for the third and fourth dehydrogenation steps and varied apparent activation energies for the first, second, and third dehydrogenation steps. Thermodynamic and kinetic destabilization caused by the presence of Mg(AlH4)2 is likely to be responsible for the different hydrogen desorption/absorption performances of the LiBH4- x Mg(AlH4)2 composites. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights: [•] A composite system composed of Mg(BH4)2⋅2NH3 and MgH2 was prepared. [•] Ammonia release from Mg(BH4)2⋅2NH3 is fully depressed in the presence of MgH2. [•] ∼13.9wt% H2 is released from Mg(BH4)2⋅2NH3–MgH2 initiated at 70°C. [•] The dehydrogenated sample can take up 3.7wt% H2 at 450°C and 100bar. [ABSTRACT FROM AUTHOR]

A significant decrease in the dehydrogenation temperature of Mg(AlH4)2 was achieved by low-energy ball milling with TiF4. Approximately 8.0 wt% of hydrogen was released from the Mg(AlH4)2-0.025TiF4 sample with an on-set temperature of 40 °C, which represents a decrease of 75 °C relative to pristine Mg(AlH4)2. In contrast to the three-step reaction for pristine Mg(AlH4)2, hydrogen desorption from the TiF4-doped sample involves a two-step process because the Ti-based species participates in the dehydrogenation reaction. The presence of TiF4 alters the nucleation and growth of the dehydrogenation product, significantly decreasing the activation energy barrier of the first step in the dehydrogenation of Mg(AlH4)2. Further hydrogenation measurements revealed that the presence of the Ti-based species was also advantageous for hydrogen uptake, as the on-set hydrogenation temperature was only 100 °C for the dehydrogenated TiF4-doped sample, compared with 130 °C for the additive-free sample. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The introduction of RbF into the Mg(NH2)2-2 LiH system significantly decreased its (de-)hydrogenation temperatures and enhanced its hydrogen-storage kinetics. The Mg(NH2)2-2 LiH-0.08 RbF composite exhibits the optimal hydrogen-storage properties as it could reversibly store approximately 4.76 wt % hydrogen through a two-stage reaction with the onset temperatures of 80 °C for and 55 °C for . At 130 °C, approximately 70 % of hydrogen was rapidly released from the 0.08 RbF-doped sample within 180 min, and the fully dehydrogenated sample could absorb approximately 4.8 wt % of hydrogen at 120 °C. Structural analyses revealed that RbF reacted readily with LiH to convert to RbH and LiF owing to the favorable thermodynamics during ball-milling. The newly generated RbH participated in the following reaction, consequently resulting in a decrease in the reaction enthalpy change and activation energy. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogen storage properties and mechanisms of the Ca(BH4)2-added 2LiNH2–MgH2 system were systematically investigated. The results showed that the addition of Ca(BH4)2 pronouncedly improved hydrogen storage properties of the 2LiNH2–MgH2 system. The onset temperature for dehydrogenation of the 2LiNH2–MgH2–0.3Ca(BH4)2 sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH2–MgH2–0.1Ca(BH4)2 sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH4)2 and LiNH2 firstly occurred to convert to Ca(NH2)2 and LiBH4, and then, the newly developed LiBH4 reacted with LiNH2 to form Li4(BH4)(NH2)3. Upon heating, the in situ formed Ca(NH2)2 and Li4(BH4)(NH2)3 work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH4)2-added 2LiNH2–MgH2 system. [Copyright &y& Elsevier]

Abstract: The decreased dehydrogenation temperature and improved dehydrogenation kinetics were achieved by high-energy ball milling Mg(AlH4)2. The particle size, grain size, microstrain and lattice distortion of the post-milled samples, i.e., from macro- to micro-scale, were systematically characterized by means of SEM and XRD measurements. The results indicated that the high-energy ball milling process led to not only a decrease in the particle size and grain size but also an increase in the microstrain and lattice distortion, which provides a synergetic effect of the thermodynamics and kinetics on lowering the dehydrogenation temperatures of the post-milled Mg(AlH4)2 samples. From the kinetic point of view, the refinement of the particles and grains shortens the diffusion distance, and the increase of the microstrain and lattice distortion enhances the diffusivity, which work together to decrease the apparent activation energy for hydrogen desorption. Besides, the presence of microstrain and lattice distortion increased the free energy of the post-milled samples, which was released by recovery and recrystallization processes upon heating. This offers more heat release during the first-step dehydrogenation, consequently leading to thermodynamically decline in dehydrogenation temperatures of the post-milled samples. Such a finding provides insights into the mechanistic understanding on decreased dehydrogenation temperature and improved dehydrogenation kinetics of the post-milled metal hydrides as hydrogen storage materials. [Copyright &y& Elsevier]

An ammonia-redistribution strategy for synthesizing metal borohydride ammoniates with controllable coordination number of NH3 was proposed, and a series of magnesium borohydride ammoniates were easily synthesized by a mechanochemical reaction between Mg(BH4)2 and its hexaammoniate. A strong dependence of the dehydrogenation temperature and purity of the released hydrogen upon heating on the coordination number of NH3 was elaborated for Mg(BH4)2⋅ x NH3 owing to the change in the molar ratio of Hδ+ and Hδ−, the charge distribution on Hδ+ and Hδ−, and the strength of the coordinate bond N:→Mg2+. The monoammoniate of magnesium borohydride (Mg(BH4)2⋅NH3) was obtained for the first time. It can release 6.5 % pure hydrogen within 50 minutes at 180 °C. [ABSTRACT FROM AUTHOR]

Hydrogen storage properties and mechanisms of the Ca(BH4)2-doped Mg(NH2)2-2 LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH4)2 and LiH readily occurs to yield CaH2 and LiBH4 during ball milling. The Mg(NH2)2-2 LiH-0.1 Ca(BH4)2 composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt % of H2 with an onset temperature of about 90 °C for dehydrogenation and 60 °C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt % of H2 is rapidly desorbed from the Mg(NH2)2-2 LiH-0.1 Ca(BH4)2 composite within 100 minutes at 140 °C, and rehydrogenation can be completed within 140 minutes at 105 °C and 100 bar H2. In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH2)2-2 LiH-0.1 Ca(BH4)2 composite are decreased by about 16.5 % and 28.1 %, respectively, and thus are responsible for the lower operating temperature and the faster dehydrogenation/hydrogenation kinetics. The fact that the hydrogen storage performances of the Ca(BH4)2-doped sample are superior to the individually CaH2- or LiBH4-doped samples suggests that the in situ formed CaH2 and LiBH4 provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH2)2-2 LiH system. [ABSTRACT FROM AUTHOR]

Abstract: Mg(AlH4)2 submicron rods with 96.1% purity have been successfully synthesized in a modified mechanochemical reaction process followed by Soxhlet extraction. ∼9.0 wt% of hydrogen is released from the as-prepared Mg(AlH4)2 at 125–440 °C through a stepwise reaction. Upon dehydriding, Mg(AlH4)2 decomposes first to generate MgH2 and Al. Subsequently, the newly produced MgH2 reacts with Al to form a Al0.9Mg0.1 solid solution. Finally, further reaction between the Al0.9Mg0.1 solid solution and the remaining MgH2 gives rise to the formation of Al3Mg2. The first step dehydrogenation is a diffusion-controlled reaction with an apparent activation energy of ∼123.0 kJ/mol. Therefore, increasing the mobility of the species involved in Mg(AlH4)2 will be very helpful for improving its dehydrogenation kinetics. [Copyright &y& Elsevier]

Abstract: 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 Al0.9Mg0.1 solid solution and Mg. Finally, NaBH4 reacted with Mg and partial Al0.9Mg0.1 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. [Copyright &y& Elsevier]

Abstract: The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH2)2–2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH2)2–2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH2)2–2LiH–0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH2)2 and LiH to convert to NaH, LiNH2 and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH2)2–2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems. [Copyright &y& Elsevier]

Abstract: 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 ∼5.08wt% of hydrogen, equivalent to 8.65moles 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 ∼4moles 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. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen-storage properties and mechanisms of a novel Li–Al–N ternary system were systematically investigated by a series of performance evaluation and structural examinations. It is found that ca. 5.2 wt% of hydrogen is reversibly stored in a Li3N–AlN (1:1) system, and the hydrogenated product is composed of LiNH2, LiH, and AlN. A stepwise reaction is ascertained for the dehydrogenation of the hydrogenated Li3N–AlN sample, and AlN is found reacting only in the second step to form the final product Li3AlN2. The calculation of the reaction enthalpy change indicates that the two-step dehydrogenation reaction is more thermodynamically favorable than any one-step reaction. Further investigations exhibit that the presence of AlN in the LiNH2–2LiH system enhances the kinetics of its first-step dehydrogenation with a 10% reduction in the activation energy due likely to the higher diffusivity of lithium and hydrogen within AlN. [Copyright &y& Elsevier]

Abstract: Hydrogen storage in a quaternary Mg–Ca–Li–N–H system has been investigated by XRD, FT-IR, TPD and PCI. Results showed that hydrogen desorption occurred at temperature about 50°C, and peaked at ∼178°C under a heating rate of 2°Cmin−1. The reversibility of hydrogen storage was observed and the maximum hydrogen storage capacity of about 2.7wt.% was reached at 220°C and 75atm. However, the plateau pressure for hydrogen desorption is relatively low (∼0.3atm). Structural identifications reveal that a new phase with tetragonal structure positioned at 31.8° and 51.3° of XRD peaks was generated during hydrogen absorption/desorption. [Copyright &y& Elsevier]

In this work, we report an effective synthetic strategy to obtain LiBH 4 featuring low-temperature and highly reversible hydrogen cycling. This is achieved by a unique nanocomposite structure where LiBH 4 nanoparticles of 5–10 nm on graphene are decorated by Ni nanocrystals of 2–4 nm. The prepared LiBH 4 nanocomposite reversibly desorbs and absorbs ~9.2 wt% hydrogen at 300 °C with a stable cyclability for up to 100 cycles, superior to all the literature results reported so far. The decisive factor affecting the hydrogen cycling is the reactivity of boron toward hydrogen. The formation of stable B 12 H 12 2- cluster during hydrogen cycling has been successfully prevented. The synergetic effects of nanostructuring and nanocatalysis lead to efficient formation of BH 4 ¯ during hydrogenation and elemental boron during dehydrogenation. This breakthrough sheds light on new strategies to explore borohydride family for practical hydrogen storage applications. A unique nanocomposite of Ni nanocrystal-decorated LiBH 4 nanoparticles smaller than 10 nm anchored on graphene delivers a reversible hydrogen storage capacity of 9.2 wt% at 300 °C with a stable cyclability for 100 cycles. [Display omitted] ● A novel one-pot solvothermal process is proposed for the fabrication of LiBH 4 nanostructures. ● 5–10 nm LiBH 4 nanoparticles decorated by Ni nanocrystals on graphene are successfully synthesized. ● The LiBH 4 nanocomposite stores reversibly 9.2 wt% H 2 at 300 °C and 100 bar H 2. ● Formation of volatile B 2 H 6 and stable Li 2 B 12 H 12 during hydrogen cycling is prevented. [ABSTRACT FROM AUTHOR]

MgH2 is one of the most promising hydrogen storage materials due to its high hydrogen storage capacity and favorable reversibility, but it suffers from stable thermodynamics and poor dynamics. In the present work, an intersected Y2O3/NiO hybrid with spherical hollow structure is synthesized. When introduced to MgH2 via ball-milling, the Y2O3/NiO hollow spheres are crushed into ultrafine particles, which are homogenously dispersed in MgH2, showing a highly effective catalysis. With an optimized addition of 10 wt% of the hybrid, the initial dehydrogenation peak temperature of MgH2 is reduced to 277 °C, lowered by 109 °C compared with that of the bare MgH2, which is further reduced to 261 °C in the second cycle. There is ca. 6.6 wt% H2 released at 275 °C within 60 min. For the fully dehydrogenation product, hydrogenation initiates at almost room temperature, and a hydrogenation capacity of 5.9 wt% is achieved at 150 °C within 150 min. There is still 5.2 wt% H2 desorbed after 50 cycles at a moderate cyclic condition, corresponding to the capacity retention of 79.2%. The crystal structure and morphology of the Y2O3/NiO hybrid is well preserved during cycling, showing long-term catalysis to the hydrogen storage of MgH2. The Y2O3/NiO hybrid also inhibits the agglomeration of MgH2 particles during cycling, favoring the cyclic stability. [ABSTRACT FROM AUTHOR]

Review: [emphasis on metal amides, alanates and borohydrides; 45 refs. [ABSTRACT FROM AUTHOR]

Abstract: A two-step ball-milling method has been provided to synthesize Mg(BH4)2 using NaBH4 and MgCl2 as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH4)2 is then combined with LiAlH4 by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH4)2 and LiAlH4 during milling, forming Mg(AlH4)2 and LiBH4. Mg(BH4)2 is excessive and remains in the ball-milled product when the molar ratio of Mg(BH4)2 to LiAlH4 is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH4)2 or LiAlH4. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH4)2 and LiAlH4. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH4)2 in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed. [Copyright &y& Elsevier]

10–20 nm-sized N-doped Nb 2 O 5 nanorods with superior catalytic activity for hydrogen storage in MgH 2 was successfully prepared by a novel graphene-induced nucleation and growth process. • A novel graphene-guided formation of N-doped Nb 2 O 5 nanorods is reported. • 10–20 nm-sized N-doped Nb 2 O 5 nanorods are successfully obtained. • The N-Nb 2 O 5 @C-catalyzed MgH 2 starts releasing H 2 from 170 °C. • The N-Nb 2 O 5 @C-catalyzed Mg absorbs 6.0 wt% H 2 at 25 °C and 50 bar H 2. • A synergistic catalytic effect from graphene and NbN 0.9 O 0.1 is revealed. High operation temperatures and slow kinetics remain big challenges for using magnesium (Mg) as a practical hydrogen storage medium. In this work, a novel graphene-guided nucleation and growth process was developed for the preparation of N-doped Nb 2 O 5 nanorods that enable remarkably improved hydrogen storage properties of MgH 2. The nanorods were measured to be 10–20 nm in diameter. MgH 2 doped with 10 wt% of the nanorods released 6.2 wt% H 2 from 170 °C, which is 130 °C lower than additive-free MgH 2 , thanks to a 40% reduction in the kinetic barriers. About 5.5 wt% of H 2 was desorbed in isothermal dehydrogenation test at 175 °C. Reloading of hydrogen was notably completed at 25 °C under 50 atm of hydrogen pressure, which has not been reported before. Density functional theory (DFT) calculations demonstrate the extended bond lengths and weakened bond strengths of Mg-H or H-H when MgH 2 /H 2 adsorbs on the Nb-N-O/graphene model, consequently favouring lower operating temperatures and improved kinetics for hydrogen storage in MgH 2 catalyzed by the graphene-guided N-Nb 2 O 5 nanorods. Our findings provide useful insights in the design and preparation of high-performance catalysts of transition metals and rare metals for on-board hydrogen storage. [ABSTRACT FROM AUTHOR]

Summary: High operation temperatures and slow reaction kinetics are major obstacles to use MgH2 as a solid hydrogen store. We report here the synthesis of Nb2O5 hollow spheres (o‐Nb2O5) with wall thickness of approximately 50 nm and mossy surfaces using a facile hydrothermal and calcination process, which showed high activity in catalysis of MgH2 for hydrogen storage. The dehydrogenation onset temperature of MgH2 was decreased to 195°C with 7 wt% of o‐Nb2O5. More than 5.5 wt% H2 can be desorbed at 300°C within 5 minutes. Hydrogen re‐absorption starts even at 25°C and reaches 5.6 wt% within 5 minutes at 200°C. Practical hydrogen capacity stabilizes at 5.8 wt% after 10 cycles of hydrogen uptake/release. The o‐Nb2O5 was found to be reduced in situ by MgH2 to low‐valence Nb species during the initial dehydrogenation process, which functions as an active catalyst and leads to the enhanced dehydrogenation kinetics. [ABSTRACT FROM AUTHOR]

Abstract Herein, we demonstrate the successful preparation of a novel complex transition metal oxide (TiVO 3.5) by oxidizing a solid-solution MXene (Ti 0.5 V 0.5) 3 C 2 at 300 °C and its high activity as a catalyst precursor in the hydrogen storage reaction of MgH 2. The prepared TiVO 3.5 inherits the layered morphology of its MXene precursor, but the layer surface becomes very coarse because of the presence of numerous nanoparticles. Adding a minor amount of TiVO 3.5 remarkably reduces the dehydrogenation and hydrogenation temperatures of MgH 2 and enhances the reaction kinetics. The 10 wt% TiVO 3.5 -containing sample exhibits optimal hydrogen storage properties, as it desorbs approximately 5.0 wt% H 2 in 10 min at 250 °C and re-absorbs 3.9 wt% H 2 in 5 s at 100 °C and under 50 bar of hydrogen pressure. The apparent activation energy is calculated to be approximately 62.4 kJ/mol for the MgH 2 -10 wt% TiVO 3.5 sample, representing a 59% reduction in comparison with pristine MgH 2 (153.8 kJ/mol), which reasonably explains the remarkably reduced dehydrogenation operating temperature. Metallic Ti and V are detected after ball milling with MgH 2 ; they are uniformly dispersed on the MgH 2 matrix and act as actual catalytic species for the improvement of the hydrogen storage properties of MgH 2. Graphical abstract Image 1 Highlights • A complex oxide TiVO 3.5 is synthesized for catalyzing hydrogen release from MgH 2. • The Ti3+ and V4+ in TiVO 3.5 are reduced to the zero-valence state after milling. • The MgH 2 -10 wt% TiVO 3.5 sample exhibits improved hydrogen storage properties. • The presence of TiVO 3.5 reduces the dehydrogenation onset temperature by 70 °C. • The dehydrogenated MgH 2 -10 wt% TiVO 3.5 sample absorbs 3.9 wt% H 2 in 5 s at 100 °C. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • MgH 2 -10 wt% (Mg(AlH 4) 2 -0.5TiF 4) is designed and prepared by high-energy ball milling. • Operating temperatures of the sample are 100 °C lower than the pristine MgH 2. • Al 3 Ti, MgF 2 , Al are in situ formed from Mg(AlH 4) 2 -0.5TiF 4 precursor during milling. • Kinetic analysis reveals that the additive promotes both the nucleation and growth. • The sample exhibits excellent hydrogen storage reversibility. Abstract A mixture with the composition of MgH 2 –10 wt% (Mg(AlH 4) 2 -0.5TiF 4) (denoted as Mg-Al-Ti-F-doped MgH 2) is designed and prepared by high-energy ball milling under the hydrogen pressure of 60 bar for 12 h. During the milling, a chemical reaction occurs between Mg(AlH 4) 2 and TiF 4 to generate in situ Al 3 Ti, MgF 2 and Al, which work together to induce a reduction of greater than 100 °C in the dehydrogenation temperature. At 275 °C, the Mg-Al-Ti-F-doped MgH 2 rapidly releases 6.3 wt% H 2 within 10 min in an isothermal experiment, while no appreciable hydrogen release is observed for the pristine MgH 2 under identical conditions. The dehydrogenated Mg-Al-Ti-F-doped sample starts to take up hydrogen at room temperature with the 6.3 wt% amount of H 2 at 150 °C, which is greatly superior to that of the pristine sample. A comprehensive kinetic analysis reveals that the superior functionality of the in situ formed Al 3 Ti, MgF 2 and Al are mainly attributed to the following two factors: acting as nucleation centers and reducing the activation energy of growth. [ABSTRACT FROM AUTHOR]

Carbon-supported nanocrystalline TiO 2 (TiO 2 @C) shows good catalytic activity in the hydrogen storage reaction of MgH 2 . Adding a small amount of carbon-supported nanocrystalline TiO 2 remarkably reduces the dehydrogenation operating temperatures because the MgH 2 -10 wt% TiO 2 @C sample starts releasing H 2 at 205 °C, which is 95 °C lower than that of pristine MgH 2 . At 300 °C, the 10 wt% TiO 2 @C-containing sample rapidly releases 6.5 wt% hydrogen within 7 min. More importantly, the dehydrogenated 10 wt% TiO 2 @C-containing sample takes up hydrogen even at room temperature and under a hydrogen pressure of 50 bar, and approximately 6.6 wt% hydrogen is absorbed within 10 min at 140 °C. Kinetic measurements reveal a 30% and 50% reduction in the apparent activation energy of the dehydrogenation and hydrogenation of MgH 2 , respectively, with the presence of 10 wt% TiO 2 @C additive. Density functional theory calculations present the extended bond lengths and the reduced bond strengths for Mg-H bonding when MgH 2 adsorbs on the TiO 2 clusters, which is responsible for the reduced de-/hydrogenation temperatures of the TiO 2 @C-containing MgH 2 . [ABSTRACT FROM AUTHOR]

Cobalt hydroxide (Co(OH) 2 ) is introduced into the LiBH 4 /2LiNH 2 composite to improve its hydrogen storage properties. The 0.05Co(OH) 2 -containing sample exhibits a significantly reduced dehydrogenation temperature and improved hydrogen storage reversibility. Its dehydrogenation onset temperature is as low as 70 °C, representing the lowest onset dehydrogenation temperature for the known catalyst-added LiBH 4 /2LiNH 2 system. At 200 °C, approximately 9.1 wt% of hydrogen is rapidly released from the 0.05Co(OH) 2 -containing sample within 15 min, greatly superior to the pristine sample because nearly no hydrogen release from it is observed under identical conditions. More importantly, the dehydrogenated sample absorbs 1.1 wt% of hydrogen at 350 °C, achieving partial reversibility for hydrogen storage in the Li B N H system. Structural investigations reveal that the added Co(OH) 2 is first reduced to metallic Co during ball milling, and the Co thus formed in situ serves as the active catalyst in improving the hydrogen storage properties of the LiBH 4 /2LiNH 2 sample. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2–4 nm-sized TiC particles are fabricated by hydrogen-assisted one-pot calcination. • Ultrasmall TiC nanoparticles effectively improve H 2 storage properties of NaAlH 4. • The 7 wt% nano-TiC@C modified NaAlH 4 releases 5 wt% H 2 starting from 65 °C. • Full hydrogenation can be achieved at 30 °C under 100 bar H 2. • Ultrasmall TiC particles lead to highly reactive catalytic species. Sodium alanate, NaAlH 4 , has great potential as a hydrogen carrier but suffers from sluggish kinetics and poor reversibility caused by high energy barriers. Transition metal-based catalysts are especially effective in reducing kinetic energy barriers for hydrogen cycling of NaAlH 4. Herein, we demonstrate a facile fabrication of TiC nanoparticles with 2–4 nm in size supported on carbon (nano-TiC@C). The resultant product consists of approximately 49.2 wt% of TiC and 50.8 wt% of C, and exhibits high and stable catalytic activity for hydrogen storage process of NaAlH 4. The 7 wt% nano-TiC@C-containing NaAlH 4 releases 5 wt% H 2 starting from 65 °C and reabsorbs all released hydrogen at 30 °C under 100 bar H 2 , outperforming NaAlH 4 modified by commercial TiC nanoparticles (∼50 nm in size). The enhancement is related to the ultrasmall size and high reactive activity of as-synthesized TiC nanoparticles. Moreover, the weak electronegativity of C prevents the formation of Na-based by-products, which are often observed in oxide and halide-containing systems. This finding sheds light on how to design and synthesize high-performance catalytic additives for light-metal hydride-based hydrogen storage materials. [ABSTRACT FROM AUTHOR]

A MAX-phase carbide (Ti 3 AlC 2 ) with purity 99% was synthesized by first ball milling a mixture of Ti, Al and C and subsequently sintering at 1500 °C. The catalytic effects of the as-prepared Ti 3 AlC 2 on the hydrogen storage reaction of MgH 2 were evaluated for the first time. The results indicated that the MgH 2 -7 wt% Ti 3 AlC 2 sample had optimal hydrogen storage properties. The dehydrogenation onset temperature of the MgH 2 -7 wt% Ti 3 AlC 2 sample decreased to 205 °C, which is 70 °C lower than that of the as-milled pristine MgH 2 . While heating to 340 °C, the hydrogen desorption reached 6.9 wt%. The dehydrogenated MgH 2 -7 wt% Ti 3 AlC 2 sample absorbed 5.8 wt% hydrogen within 60 s at 150 °C, while the hydrogen uptake amount in dehydrogenated pristine MgH 2 was only 2.7 wt%, even after 2000 s. The apparent activation energy was calculated to be 104.7 kJ/mol for the MgH 2 -7 wt% Ti 3 AlC 2 sample, which is 50.4 kJ/mol lower than that of the pristine MgH 2 . The existence state of Ti 3 AlC 2 during dehydrogenation was also analyzed and discussed. [ABSTRACT FROM AUTHOR]

A porous Co 3 O 4 with a particle size of 1–3 µm was successfully prepared by heating Co-based metal organic frameworks MOF-74(Co) up to 500 °C in air atmospheric conditions. The as-prepared porous Co 3 O 4 significantly reduced the dehydrogenation temperatures of the LiBH 4 -2LiNH 2 system and improved the purity of the released hydrogen. The LiBH 4 -2LiNH 2 -0.05/3Co 3 O 4 sample started to release hydrogen at 140 °C and released hydrogen levels of approximately 9.7 wt% at 225 °C. The end temperature for hydrogen release was lowered by 125 °C relative to that of the pristine sample. Structural analyses revealed that the as-prepared porous Co 3 O 4 is in-situ reduced to metallic Co, which functions as an active catalyst, reducing the kinetic barriers and lowering the dehydrogenation temperatures of the LiBH 4 -2LiNH 2 system. More importantly, the porous Co 3 O 4 -containing sample exhibited partially improved reversibility for hydrogen storage in the LiBH 4 -2LiNH 2 system. [ABSTRACT FROM AUTHOR]

Sodium alanate (NaAlH4) has attracted intense interest as a prototypical high-density hydrogen-storage material. However, poor reversibility and slow kinetics limit its practical applications. Herein, a nanocrystalline ZrO2@C catalyst was synthesized by using Uio-66(Zr) as a precursor and furfuryl alcohol (FA) as a carbon source. The as-synthesized ZrO2@C exhibits good catalytic activity for the dehydrogenation and hydrogenation of NaAlH4. The NaAlH4-7 wt % ZrO2@C sample released hydrogen starting from 126 °C and reabsorbed it starting from 54 °C, and these temperatures are lower by 71 and 36 °C, respectively, relative to pristine NaAlH4. At 160 °C, approximately 5.0 wt % of hydrogen was released from the NaAlH4-7 wt % ZrO2@C sample within 250 min, and the dehydrogenation product reabsorbed approximately 4.9 wt % within 35 min at 140 °C and 100 bar of hydrogen. The catalytic function of the Zr-based active species is believed to contribute to the significantly reduced operating temperatures and enhanced kinetics. [ABSTRACT FROM AUTHOR]

A 2D Ti 3 C 2 MXene is introduced into NaAlH 4 to improve its hydrogen storage properties for the first time. In the presence of Ti 3 C 2 , the operating temperatures for hydrogen storage in NaAlH 4 are remarkably reduced, and the hydrogenation/dehydrogenation kinetics are significantly enhanced. The onset dehydrogenation temperature of the 7 wt% Ti 3 C 2 -containing NaAlH 4 sample is reduced to 100 °C, and hydrogen recharging starts at 50 °C. Approximately 4.7 wt% hydrogen is released from the NaAlH 4 -7 wt% Ti 3 C 2 sample within 100 min at 140 °C, and the dehydrogenated sample absorbs 4.6 wt% hydrogen within 60 min at 120 °C. However, pristine NaAlH 4 only absorbs 0.4 wt% hydrogen under identical conditions. Further cycling measurements show significantly improved cycling stability for the Ti 3 C 2 -containing NaAlH 4 ; the hydrogenation/dehydrogenation behaviour remains nearly constant after 10 cycles. XRD and XPS analyses reveal that the Ti 3 C 2 reacts with NaAlH 4 and is reduced to metallic Ti and Ti 3+ species, which are responsible for the lowered operating temperatures and improved dehydrogenation/hydrogenation kinetics. [ABSTRACT FROM AUTHOR]

A reactive hydride system of Ca(BH 4 ) 2 +Mg(AlH 4 ) 2 in a molar ratio of 1: 1 with improved dehydrogenation thermodynamics and kinetics compared with either initial constituent is obtained. The system shows a three-step dehydrogenation process, the temperature of which is lower than that of the main dehydrogenation of pure Ca(BH 4 ) 2 . There is 8.4 wt.% H 2 released when the system is heated to 330 °C, significantly higher than the corresponding value of 2.6 wt.% H 2 for pure Ca(BH 4 ) 2 . The rate of the main dehydrogenation of the Ca(BH 4 ) 2 +Mg(AlH 4 ) 2 system isothermally maintaining at 300 °C is 0.337 wt.% H 2 /min, which is 10 times faster than that of pure Ca(BH 4 ) 2 . The reversibility of the combined system is also improved compared with either of the starting constituents. A mechanism study reveals that the initially decomposed MgH 2 and Al from Mg(AlH 4 ) 2 prefer to react with each other, forming a Al(Mg) solid solution, which reacts further with Ca(BH 4 ) 2 , reducing the reaction enthalpy and activation energy of the system. Thus, the system is destabilized, resulting in improved overall hydrogen storage properties. [ABSTRACT FROM AUTHOR]

A combined Ca(BH 4 ) 2 +LiBH 4 system with improved hydrogen storage properties is obtained by the addition of a particulate LaMg 3 alloy assisted with ball milling in a H 2 atmosphere. It is found that LaMg 3 is mostly hydrogenated to symbiotic low crystalline MgH 2 and amorphous La-containing hydride of fine particles after the milling, which are homogenously embedded in the Ca(BH 4 ) 2 +LiBH 4 matrix. The multi-hydride system shows significant improvements in both hydrogen storage thermodynamics and kinetics. Particularly, the system with LaMg 3 addition in a molar ratio of 0.3 shows an overall superior hydrogen storage property. The main dehydrogenation starts from ca. 200 °C, which is 100 °C lower than the main dehydrogenation temperature of the pristine Ca(BH 4 ) 2 +LiBH 4 system. Re-hydrogenation initiates at a low temperature of ca. 150 °C, and the system maintains 70% of its first hydrogen desorption capacity after 5 cycles. The mechanism of the improved hydrogen storage properties of the combined system is discussed. [ABSTRACT FROM AUTHOR]

The ternary imide Li 2 Mg(NH) 2 is considered to be one of the most promising on-board hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%, favorable thermodynamic properties and good cycling stability. In this work, Li 2 Mg(NH) 2 was synthesized by dynamically dehydrogenating a mixture of Mg(NH 2 ) 2 –2LiH up to 280 °C under different gas (Ar and H 2 ) and pressures (0–9.0 bar). The crystal structure of Li 2 Mg(NH) 2 was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li 2 Mg(NH) 2 and the dehydrogenation/rehydrogenation properties of the Mg(NH 2 ) 2 –2LiH system strongly depend on the gas back pressure in the dehydrogenation process due to the effect of the pressure on the dehydrogenation kinetics. This study provides a new approach for improving the hydrogen storage properties of the amide–hydride systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • A valid strategy to in-situ introduce highly active additive to LiBH 4 is employed. • TiO is in-situ introduced into LiBH 4 and the LiBH 4 -0.06TiO system is obtained. • Enhanced kinetics and favorable cycling stability are achieved for the system. • Li 3 BO 3 and TiH 2 are generated and act as catalysts for LiBH 4. • Dehydrogenation energy barrier is reduced and formation of Li 2 B 12 H 12 is inhibited. LiBH 4 is a promising candidate for solid state hydrogen storage, however, it still suffers from high hydrogen desorption temperature, harsh hydrogen absorption conditions, and poor reversibility, which hinder its practical development. In this paper, a novel synthetic strategy of heat treating a LiBH 4 and Ti(OEt) 4 mixture is employed to prepare LiBH 4 system with TiO in-situ introduced. With an optimized TiO content of 0.06 in molar fraction, the LiBH 4 -0.06TiO system shows onset and peak dehydrogenation temperatures of 240 °C and 340 °C, respectively, which are 140 °C and 90 °C lower than those of the pure LiBH 4. The LiBH 4 -0.06TiO system can rapidly release 9 wt% H 2 after dwelling at 400 °C for 10 min. The hydrogenation of the dehydrogenation product initiates at 150 °C, and a capacity of 9 wt% is reached after isothermal dwelling at 500 °C under 50 bar of H 2 for 100 min. The capacity retention of the system can reach 74.4% after 10 cycles, indicating a favorable reversibility. With the introduction of TiO, the apparent dehydrogenation activation energy of the system is evidently reduced, and the formation of Li 2 B 12 H 12 , a highly thermal stable intermediate phase, is greatly suppressed. In addition, the aggregation is evidently alleviated. All of these contribute to the enhanced dehydrogenation kinetics and reversibility. TiO reacts with LiBH 4 , forming Li 3 BO 3 and TiH 2 after the initial dehydrogenation, which play significant catalytic effect to LiBH 4. [ABSTRACT FROM AUTHOR]