Search

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]

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]

Addition of a minor quantity of CoO significantly reduces the dehydrogenation temperature, accelerates the dehydrogenation rate and increases the hydrogen purity of the LiBH4·NH3–3LiH system. The LiBH4·NH3–3LiH–0.1CoO sample exhibits optimal dehydrogenation properties because it releases 8.5 wt% of hydrogen below 250 °C, which is approximately 90 °C lower than that of the pristine sample. At 200 °C, approximately 8.0 wt% of hydrogen is released from the LiBH4·NH3–3LiH–0.1CoO sample within 100 min, whereas only 4.1 wt% is released from the pristine sample under identical conditions. The EXAFS analyses reveal that upon heating, CoO is first reduced to metallic Co at 130 °C and then partially combines with B to form a Co–B species. The in situ formed Co and Co–B are finely dispersed in the dehydrogenated intermediates, and they play critical roles as active catalysts in favour of breaking the B–H bonds of the Li–B–N–H species. This effectively decreases the thermodynamic and kinetic barriers of the dehydrogenation reaction of the LiBH4·NH3–3LiH system. [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 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]

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: Metal–N–H-based materials are of particular interest as a group of new complex hydrides owing to their potential applications in hydrogen storage. A variety of metal–N–H-based systems have been developed so far for their hydrogen storage performances. This review deals with the Li–Mg–N–H-based combination systems which are widely recognized as one of the most promising hydrogen storage media for practical applications. The emphasis is on the structural characteristics of the lithium/magnesium amides/imides/nitrides, the hydrogen storage properties determined by the material compositions, the thermodynamics and kinetics of the hydrogen storage process, and the reaction mechanisms for de-/hydrogenation of the Li–Mg–N–H combination systems. The challenges and direction in further improving the hydrogen storage performances of the Li–Mg–N–H-based combination systems are pointed out as well. [Copyright &y& Elsevier]

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: A new ternary Metal–N–H sample of Mg–Ca–N–H has been prepared by mechanical ball milling—Mg(NH2)2 and CaH2 (1:1) in a planetary ball mill machine. The structure and hydrogen desorption properties were investigated. The results show that pure H2 gas can be released during ball milling. After 72h ball milling, ∼3.8H atoms (∼3.88wt%) were found to be detached from the starting material, which is higher than most of the traditional metal hydrides. Moreover, volumetric release and soak tests on the sample collected after 12h of ball milling show that hydrogen desorption starts at a temperature around 50°C. DSC measurement on the post-12h milled sample reveals that the overall heat effect of hydrogen desorption is ∼28.2kJ/mol H2, implying that Mg–Ca–N–H system could be a potential lower temperature hydrogen storage system. In addition, the high-pressure release testing shows that 3.0wt% of hydrogen can be desorbed from the post-12h milled sample at a temperature of 380°C although 80bars of hydrogen was applied, indicating that the Mg–Ca–N–H possesses high equilibrium desorption pressure. However, recharging the sample with hydrogen is rather difficult, which may be due to the high kinetic barrier. [Copyright &y& Elsevier]

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]

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]

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]

Abstract: The hydrogen storage in metal–N–H systems is reviewed. Exemplary systems including Li–N–H, Mg–N–H, Li–Mg–N–H and Li–Al–N–H are highlighted. Analyses and discussions are focused on the thermodynamics and kinetics of the respective systems. [Copyright &y& Elsevier]