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Reversible solid‐state hydrogen storage in metal hydrides is a key technology for pollution‐free energy conversion systems. Herein, the LiBH2–MgH2 composite system with and without ScCl3 additive is investigated using synchrotron‐ and neutron‐radiation‐based probing methods that can be applied to characterize such lightweight metal–hydrogen systems from nanoscopic levels up to macroscopic scale. Combining the results of neutron‐ and photon‐based methods allows a complementary insight into reaction paths and mechanisms, complex interactions between the hydride matrix and additive, hydrogen distribution, material transport, structural changes, and phase separation in the hydride matrix. The gained knowledge is of great importance for development and optimization of such novel metal‐hydride‐based hydrogen storage systems with respect to future applications. [ABSTRACT FROM AUTHOR]

CO2 emissions have been continuously increasing during the last half of the century with a relevant impact on the planet and are the main contributor to the greenhouse effect and global warming. The development of new technologies to mitigate these emissions poses a challenge. Herein, the recycling of CO2 to produce CH4 selectively by using Mg2FeH6 and Mg2NiH4 complex hydrides as dual conversion promoters and hydrogen sources has been demonstrated. Magnesium-based metal hydrides containing Fe and Ni catalyzed the hydrogenation of CO2 and their total conversion was obtained at 400 °C after 5 h and 10 h, respectively. The complete hydrogenation of CO2 depended on the complex hydride, H2 : CO2 mol ratio, and experimental conditions: temperature and time. For both hydrides, the activation of CO2 on the metal surface and its subsequent capture resulted in the formation of MgO. Investigations on the Mg2FeH6–CO2 system indicated that the main process occurs via the reversed water–gas shift reaction (WGSR), followed by the methanation of CO in the presence of steam. In contrast, the reduction of CO2 by the Mg-based hydride in the Mg2NiH4–CO2 system has a strong contribution to the global process. Complex metal hydrides are promising dual promoter-hydrogen sources for CO2 recycling and conversion into valuable fuels such as CH4. [ABSTRACT FROM AUTHOR]

The hydrogen storage properties of 6Mg(NH 2) 2 9LiH- x (LiBH 4) (x = 0, 0.5, 1, 2) system and the role of LiBH 4 on the kinetic behaviour and the dehydrogenation/hydrogenation reaction mechanism were herein systematically investigated. Among the studied compositions, 6Mg(NH 2) 2 9LiH 2LiBH 4 showed the best hydrogen storage properties. The presence of 2 mol of LiBH 4 improved the thermal behaviour of the 6Mg(NH 2) 2 9LiH by lowering the dehydrogenation peak temperature nearly 25 °C and by reducing the apparent dehydrogenation activation energy of about 40 kJ/mol. Furthermore, this material exhibited fast dehydrogenation (10 min) and hydrogenation kinetics (3 min) and excellent cycling stability with a reversible hydrogen capacity of 3.5 wt % at isothermal 180 °C. Investigations on the reaction pathway indicated that the observed superior kinetic behaviour likely related to the formation of Li 4 (BH 4)(NH 2) 3. Studies on the rate-limiting steps hinted that the sluggish kinetic behaviour of the 6Mg(NH 2) 2 9LiH pristine material are attributed to an interface-controlled mechanism. On the contrary, LiBH 4 -containing samples show a diffusion-controlled mechanism. During the first dehydrogenation reaction, the possible formation of Li 4 (BH 4)(NH 2) 3 accelerates the reaction rates at the interface. Upon hydrogenation, this 'liquid like' of Li 4 (BH 4)(NH 2) 3 phase assists the diffusion of small ions into the interfaces of the amide-hydride matrix. • Reaction kinetics of 6Mg(NH 2) 2 -9LiH- x (LiBH 4) (x = 0, 0.5, 1, 2) system. • Reduction in the dehydrogenation apparent activation energies. • Excellent cycling properties over 20 cycle. • Rate-limiting processes and the role of Li 4 (BH 4)(NH 2) 3. [ABSTRACT FROM AUTHOR]

The 6Mg(NH2)2–9LiH–LiBH4 composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H2. However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816–15822). In this work, Li3N and YCl3 have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H2 was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H2 pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H2 pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed. [ABSTRACT FROM AUTHOR]

The use of fossil fuels as an energy supply becomes increasingly problematic from the point of view of both environmental emissions and energy sustainability. As an alternative, hydrogen is widely regarded as a key element for a potential energy solution. However, differently from fossil fuels such as oil, gas, and coal, the production of hydrogen requires energy. Alternative and intermittent renewable energy sources such as solar power, wind power, etc., present multiple advantages for the production of hydrogen. On the one hand, the renewable sources contribute to a remarkable reduction of pollutants released to the air and on the other hand, they significantly enhance the sustainability of energy supply. In addition, the storage of energy in form of hydrogen has a huge potential to balance an effective and synergetic utilization of renewable energy sources. In this regard, hydrogen storage technology is a key technology towards the practical application of hydrogen as "energy carrier". Among the methods available to store hydrogen, solid-state storage is the most attractive alternative from both the safety and the volumetric energy density points of view. Because of their appealing hydrogen content, complex hydrides and complex hydride-based systems have attracted considerable attention as potential energy vectors for mobile and stationary applications. In this review, the progresses made over the last century on the synthesis and development of tetrahydroborates and tetrahydroborate-based systems for hydrogen storage purposes are summarized. [ABSTRACT FROM AUTHOR]

We report for the first time the formation of a metal amide–hydride solid solution. The dissolution of KH into KNH2 leads to an anionic substitution, which decreases the interaction among NH2− ions. The rotational properties of the high temperature polymorphs of KNH2 are thereby retained down to room temperature. [ABSTRACT FROM AUTHOR]

Designing safe, compact and high capacity hydrogen storage systems is the key step towards introducing a pollutant free hydrogen technology into a broad field of applications. Due to the chemical bonds of hydrogen–metal atoms, metal hydrides provide high energy density in safe hydrogen storage media. Reactive hydride composites (RHCs) are a promising class of high capacity solid state hydrogen storage systems. Ca(BH4)2 + MgH2 with a hydrogen content of 8.4 wt% is one of the most promising members of the RHCs. However, its relatively high desorption temperature of ∼350 °C is a major drawback to meeting the requirements for practical application. In this work, by using NbF5 as an additive, the dehydrogenation temperature of this RHC was significantly decreased. To elucidate the role of NbF5 in enhancing the desorption properties of the Ca(BH4)2 + MgH2 (Ca-RHC), a comprehensive investigation was carried out via manometric measurements, mass spectrometry, Differential Scanning Calorimetry (DSC), in situ Synchrotron Radiation-Powder X-ray Diffraction (SR-PXD), X-ray Absorption Spectroscopy (XAS), Anomalous Small-Angle X-ray Scattering (ASAXS), Scanning and Transmission Electron Microscopy (SEM, TEM) and Nuclear Magnetic Resonance (NMR) techniques. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogen sorption behavior of the Mg2FeH6–MgH2 hydride system is investigated via in-situ synchrotron and laboratory powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), particle size distribution (PSD) and volumetric techniques. The Mg2FeH6–MgH2 hydride system is obtained by mechanical milling in argon atmosphere followed by sintering at high temperature and hydrogen pressure. In-situ SR-PXD results show that upon hydriding MgH2 is a precursor for Mg2FeH6 formation and remained as hydrided phase in the obtained material. Diffusion constraints preclude the further formation of Mg2FeH6. Upon dehydriding, our results suggest that MgH2 and Mg2FeH6 decompose independently in a narrow temperature range between 275 and 300 °C. Moreover, the decomposition behavior of both hydrides in the Mg2FeH6–MgH2 hydride mixture is influenced by each other via dual synergetic-destabilizing effects. The final hydriding/dehydriding products and therefore the kinetic behavior of the Mg2FeH6–MgH2 hydride system exhibits a strong dependence on the temperature and pressure conditions. [Copyright &y& Elsevier]

In this paper, a gas-to-power (GtoP) system for power outages is digitally modeled and experimentally developed. The design includes a solid-state hydrogen storage system composed of TiFeMn as a hydride forming alloy (6.7 kg of alloy in five tanks) and an air-cooled fuel cell (maximum power: 1.6 kW). The hydrogen storage system is charged under room temperature and 40 bar of hydrogen pressure, reaching about 110 g of hydrogen capacity. In an emergency use case of the system, hydrogen is supplied to the fuel cell, and the waste heat coming from the exhaust air of the fuel cell is used for the endothermic dehydrogenation reaction of the metal hydride. This GtoP system demonstrates fast, stable, and reliable responses, providing from 149 W to 596 W under different constant as well as dynamic conditions. A comprehensive and novel simulation approach based on a network model is also applied. The developed model is validated under static and dynamic power load scenarios, demonstrating excellent agreement with the experimental results. [ABSTRACT FROM AUTHOR]

In recent years, the use of selected additives for improving the kinetic behavior of the system 2LiH + MgB2 (Li-RHC) has been investigated. As a result, it has been reported that some additives (e.g., 3TiCl3·AlCl3), by reacting with the Li-RHC components, form nanostructured phases (e.g., AlTi3) possessing peculiar microstructural properties capable of enhancing the system's kinetic behavior. The effect of in-house-produced AlTi3 nanoparticles on the hydrogenation/dehydrogenation kinetics of the 2LiH + MgB2 (Li-RHC) system is explored in this work, with the aim of reaching high hydrogen storage performance. Experimental results show that the AlTi3 nanoparticles significantly improve the reaction rate of the Li-RHC system, mainly for the dehydrogenation process. The observed improvement is most likely due to the similar structural properties between AlTi3 and MgB2 phases which provide an energetically favored path for the nucleation of MgB2. In comparison with the pristine material, the Li-RHC doped with AlTi3 nanoparticles has about a nine times faster dehydrogenation rate. The results obtained from the kinetic modeling indicate a change in the Li-RHC hydrogenation reaction mechanism in the presence of AlTi3 nanoparticles. [ABSTRACT FROM AUTHOR]

The hybrid hydrogen storage method consists of the combination of both solid-state metal hydrides and gas hydrogen storage. This method is regarded as a promising trade-off solution between the already developed high-pressure storage reservoir, utilized in the automobile industry, and solid-state storage through the formation of metal hydrides. Therefore, it is possible to lower the hydrogen pressure and to increase the hydrogen volumetric density. In this work, we design a non-stoichiometric AB2 C14-Laves alloy composed of (Ti0.9Zr0.1)1.25Cr0.85Mn1.1Mo0.05. This alloy is synthesized by arc-melting, and the thermodynamic and kinetic behaviors are evaluated in a high-pressure Sieverts apparatus. Proper thermodynamic parameters are obtained in the range of temperature and pressure from 3 to 85 °C and from 15 to 500 bar: ΔHabs. = 22 ± 1 kJ/mol H2, ΔSabs. = 107 ± 2 J/K mol H2, and ΔHdes. = 24 ± 1 kJ/mol H2, ΔSdes. = 110 ± 3 J/K mol H2. The addition of 10 wt.% of expanded natural graphite (ENG) allows the improvement of the heat transfer properties, showing a reversible capacity of about 1.5 wt.%, cycling stability and hydrogenation/dehydrogenation times between 25 to 70 s. The feasibility for the utilization of the designed material in a high-pressure tank is also evaluated, considering practical design parameters. [ABSTRACT FROM AUTHOR]

In this work, the possibility of creating a polymer-based adaptive scaffold for improving the hydrogen storage properties of the system 2LiH+MgB2+7.5(3TiCl3·AlCl3) was studied. Because of its chemical stability toward the hydrogen storage material, poly(4-methyl-1-pentene) or in-short TPXTM was chosen as the candidate for the scaffolding structure. The composite system was obtained after ball milling of 2LiH+MgB2+7.5(3TiCl3·AlCl3) and a solution of TPXTM in cyclohexane. The investigations carried out over the span of ten hydrogenation/de-hydrogenation cycles indicate that the material containing TPXTM possesses a higher degree of hydrogen storage stability. [ABSTRACT FROM AUTHOR]

Hydrogen technology has become essential to fulfill our mobile and stationary energy needs in a global low–carbon energy system. The non-renewability of fossil fuels and the increasing environmental problems caused by our fossil fuel–running economy have led to our efforts towards the application of hydrogen as an energy vector. However, the development of volumetric and gravimetric efficient hydrogen storage media is still to be addressed. LiBH4 is one of the most interesting media to store hydrogen as a compound due to its large gravimetric (18.5 wt.%) and volumetric (121 kgH2/m3) hydrogen densities. In this review, we focus on some of the main explored approaches to tune the thermodynamics and kinetics of LiBH4: (I) LiBH4 + MgH2 destabilized system, (II) metal and metal hydride added LiBH4, (III) destabilization of LiBH4 by rare-earth metal hydrides, and (IV) the nanoconfinement of LiBH4 and destabilized LiBH4 hydride systems. Thorough discussions about the reaction pathways, destabilizing and catalytic effects of metals and metal hydrides, novel synthesis processes of rare earth destabilizing agents, and all the essential aspects of nanoconfinement are led. [ABSTRACT FROM AUTHOR]

Lithium borohydride (LiBH4) and sodium borohydride (NaBH4) were synthesized via mechanical milling of LiBO2, and NaBO2 with Mg–Al-based waste under controlled gaseous atmosphere conditions. Following this approach, the results herein presented indicate that LiBH4 and NaBH4 can be formed with a high conversion yield starting from the anhydrous borates under 70 bar H2. Interestingly, NaBH4 can also be obtained with a high conversion yield by milling NaBO2·4H2O and Mg–Al-based waste under an argon atmosphere. Under optimized molar ratios of the starting materials and milling parameters, NaBH4 and LiBH4 were obtained with conversion ratios higher than 99.5%. Based on the collected experimental results, the influence of the milling energy and the correlation with the final yields were also discussed. [ABSTRACT FROM AUTHOR]

In this work, we investigated the influence of the K2Mn(NH2)4 additive on the hydrogen sorption properties of the Mg(NH2)2 + 2LiH (Li–Mg–N–H) system. The addition of 5 mol% of K2Mn(NH2)4 to the Li–Mg–N–H system leads to a decrease of the dehydrogenation peak temperature from 200 °C to 172 °C compared to the pristine sample. This sample exhibits a constant hydrogen storage capacity of 4.2 wt.% over 25 dehydrogenation/rehydrogenation cycles. Besides that, the in-situ synchrotron powder X-ray diffraction analysis performed on the as prepared Mg(NH2)2 + 2LiH containing K2Mn(NH2)4 indicates the presence of Mn4N. However, no crystalline K-containing phases were detected. Upon dehydrogenation, the formation of KH is observed. The presence of KH and Mn4N positively influences the hydrogen sorption properties of this system, especially at the later stage of rehydrogenation. Under the applied conditions, hydrogenation of the last 1 wt.% takes place in only 2 min. This feature is preserved in the following three cycles. [ABSTRACT FROM AUTHOR]

Several different milling parameters (additive content, rotation velocity, ball-to-powder ratio, degree of filling, and time) affect the hydrogen absorption and desorption properties of a reactive hydride composite (RHC). In this paper, these effects were thoroughly tested and analyzed. The milling process investigated in such detail was performed on the 2LiH-MgB2 system doped with TiCl3. Applying an upgraded empirical model, the transfer of energy to the material during the milling process was determined. In this way, it is possible to compare the obtained experimental results with those from processes at different scales. In addition, the different milling parameters were evaluated independently according to their individual effect on the transferred energy. Their influence on the reaction kinetics and hydrogen capacity was discussed and the results were correlated to characteristics like particle and crystallite size, specific surface area, presence of nucleation sites and contaminants. Finally, an optimal value for the transferred energy was determined, above which the powder characteristics do not change and therefore the RHC system properties do not further improve. [ABSTRACT FROM AUTHOR]

Mg2FeH6 is regarded as potential hydrogen and thermochemical storage medium due to its high volumetric hydrogen (150 kg/m3) and energy (0.49 kWh/L) densities. In this work, the mechanism of formation of Mg2FeH6 under equilibrium conditions is thoroughly investigated applying volumetric measurements, X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and the combination of scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HR-TEM). Starting from a 2Mg:Fe stoichiometric powder ratio, thorough characterizations of samples taken at different states upon hydrogenation under equilibrium conditions confirm that the formation mechanism of Mg2FeH6 occurs from elemental Mg and Fe by columnar nucleation of the complex hydride at boundaries of the Fe seeds. The formation of MgH2 is enhanced by the presence of Fe. However, MgH2 does not take part as intermediate for the formation of Mg2FeH6 and acts as solid-solid diffusion barrier which hinders the complete formation of Mg2FeH6. This work provides novel insight about the formation mechanism of Mg2FeH6. [ABSTRACT FROM AUTHOR]

Reaction kinetic behaviour and cycling stability of the 2LiBH4–MgH2 reactive hydride composite (Li-RHC) are experimentally determined and analysed as a basis for the design and development of hydrogen storage tanks. In addition to the determination and discussion about the properties; different measurement methods are applied and compared. The activation energies for both hydrogenation and dehydrogenation are determined by the Kissinger method and via the fitting of solid-state reaction kinetic models to isothermal volumetric measurements. Furthermore, the hydrogen absorption–desorption cycling stability is assessed by titration measurements. Finally, the kinetic behaviour and the reversible hydrogen storage capacity of the Li-RHC are discussed. [ABSTRACT FROM AUTHOR]

Thermodynamic and heat transfer properties of the 2LiBH4-MgH2 composite (Li-RHC) system are experimentally determined and studied as a basis for the design and development of hydrogen storage tanks. Besides the determination and discussion of the properties, different measurement methods are applied and compared to each other. Regarding thermodynamics, reaction enthalpy and entropy are determined by pressure-concentration-isotherms and coupled manometric-calorimetric measurements. For thermal diffusivity calculation, the specific heat capacity is measured by high-pressure differential scanning calorimetry and the effective thermal conductivity is determined by the transient plane source technique and in situ thermocell. Based on the results obtained from the thermodynamics and the assessment of the heat transfer properties, the reaction mechanism of the Li-RHC and the issues related to the scale-up for larger hydrogen storage systems are discussed in detail. [ABSTRACT FROM AUTHOR]

NaAlH 4 was melt infiltrated within a CO 2 activated carbon aerogel, which had been preloaded with TiCl 3 . Nanoconfinement was verified by Small Angle X-Ray Scattering (SAXS) and the nature of the Ti was investigated with Anomalous SAXS (ASAXS) and X-Ray Absorption Near Edge Structure (XANES) to determine its size and chemical state. The Ti is found to be in a similar state to that found in the bulk Ti-doped NaAlH 4 system where it exists as Al 1− x Ti x nanoalloys. Crystalline phases exist within the carbon aerogel pores, which are analysed by in-situ Powder X-Ray Diffraction (PXD) during hydrogen cycling. The in-situ data reveals that the hydrogen release from NaAlH 4 and its hydrogen uptake occurs through the Na 3 AlH 6 intermediate when confined at this size scale. The hydrogen capacity from the nanoconfined NaAlH 4 is found to initially be much higher in this CO 2 activated aerogel compared with previous studies into unactivated aerogels. [ABSTRACT FROM AUTHOR]

Highlights: [•] Nanoconfined 2LiBH4–MgH2–0.13TiCl4 was simply prepared by solution impregnation and melt infiltration. [•] Up to two times faster desorption kinetics as compared with nanoconfined 2LiBH4–MgH2. [•] Significant low onset dehydrogenation temperature (T= 140°C). [•] New reactive phase formations during de/rehydrogenation. [ABSTRACT FROM AUTHOR]

Abstract: Nanoconfinement of 2LiBH4–MgH2–TiCl3 in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl3 by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH4–MgH2 via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl3 impregnation, for example, nanoconfined 2LiBH4–MgH2–TiCl3 requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH4–MgH2 (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H2 storage capacity (3.6–3.75 wt. % H2) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH4–MgH2–TiCl3. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH4 and MgH2 after rehydrogenation using FTIR and SR-PXD techniques, respectively. [Copyright &y& Elsevier]

Hydrogen technology has become essential to fulfill our mobile and stationary energy needs in a global low-carbon energy system. The non-renewability of fossil fuels and the increasing environmental problems caused by our fossil fuel-running economy have led to our efforts towards the application of hydrogen as an energy vector. However, the development of volumetric and gravimetric efficient hydrogen storage media is still to be addressed. LiBH 4 is one of the most interesting media to store hydrogen as a compound due to its large gravimetric (18.5 wt.%) and volumetric (121 kgH 2 /m 3 ) hydrogen densities. In this review, we focus on some of the main explored approaches to tune the thermodynamics and kinetics of LiBH 4 : (I) LiBH 4 + MgH 2 destabilized system, (II) metal and metal hydride added LiBH 4 , (III) destabilization of LiBH 4 by rare-earth metal hydrides, and (IV) the nanoconfinement of LiBH 4 and destabilized LiBH 4 hydride systems. Thorough discussions about the reaction pathways, destabilizing and catalytic effects of metals and metal hydrides, novel synthesis processes of rare earth destabilizing agents, and all the essential aspects of nanoconfinement are led., Competing Interests: The authors declare no conflict of interest.

CO2 emissions have been continuously increasing during the last half of the century with a relevant impact on the planet and are the main contributor to the greenhouse effect and global warming. The development of new technologies to mitigate these emissions poses a challenge. Herein, the recycling of CO2 to produce CH4 selectively by using Mg2FeH6 and Mg2NiH4 complex hydrides as dual conversion promoters and hydrogen sources has been demonstrated. Magnesium-based metal hydrides containing Fe and Ni catalyzed the hydrogenation of CO2 and their total conversion was obtained at 400 °C after 5 h and 10 h, respectively. The complete hydrogenation of CO2 depended on the complex hydride, H2 : CO2 mol ratio, and experimental conditions: temperature and time. For both hydrides, the activation of CO2 on the metal surface and its subsequent capture resulted in the formation of MgO. Investigations on the Mg2FeH6-CO2 system indicated that the main process occurs via the reversed water-gas shift reaction (WGSR), followed by the methanation of CO in the presence of steam. In contrast, the reduction of CO2 by the Mg-based hydride in the Mg2NiH4-CO2 system has a strong contribution to the global process. Complex metal hydrides are promising dual promoter-hydrogen sources for CO2 recycling and conversion into valuable fuels such as CH4.

The 6Mg(NH 2 ) 2 -9LiH-LiBH 4 composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H 2 . However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816-15822). In this work, Li 3 N and YCl 3 have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H 2 was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H 2 pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H 2 pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed.

We report for the first time the formation of a metal amide-hydride solid solution. The dissolution of KH into KNH 2 leads to an anionic substitution, which decreases the interaction among NH 2 - ions. The rotational properties of the high temperature polymorphs of KNH 2 are thereby retained down to room temperature.

Designing safe, compact and high capacity hydrogen storage systems is the key step towards introducing a pollutant free hydrogen technology into a broad field of applications. Due to the chemical bonds of hydrogen-metal atoms, metal hydrides provide high energy density in safe hydrogen storage media. Reactive hydride composites (RHCs) are a promising class of high capacity solid state hydrogen storage systems. Ca(BH4)2 + MgH2 with a hydrogen content of 8.4 wt% is one of the most promising members of the RHCs. However, its relatively high desorption temperature of ∼350 °C is a major drawback to meeting the requirements for practical application. In this work, by using NbF5 as an additive, the dehydrogenation temperature of this RHC was significantly decreased. To elucidate the role of NbF5 in enhancing the desorption properties of the Ca(BH4)2 + MgH2 (Ca-RHC), a comprehensive investigation was carried out via manometric measurements, mass spectrometry, Differential Scanning Calorimetry (DSC), in situ Synchrotron Radiation-Powder X-ray Diffraction (SR-PXD), X-ray Absorption Spectroscopy (XAS), Anomalous Small-Angle X-ray Scattering (ASAXS), Scanning and Transmission Electron Microscopy (SEM, TEM) and Nuclear Magnetic Resonance (NMR) techniques.