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Lithium aluminium hydride (LiAlH 4 ) has long been identified as a viable hydrogen storage material, due to its high attainable theoretical gravimetric hydrogen capacity of 7.9 wt%. The main impediment to its viability for technical application is its limitation for regeneration. Recently, solvent-mediated regeneration has been achieved at room temperature using dimethyl-ether, Me 2 O, although the reaction pathway has not been determined. This in situ multinuclear NMR spectroscopy study ( 27 Al and 7 Li) has confirmed that the Me 2 O-mediated, direct synthesis of LiAlH 4 occurs by a one-step process in which LiAlH 4 · x Me 2 O is formed, and does not involve Li 3 AlH 6 or any other intermediates. Hydrogenation has been shown to occur below ambient temperatures (at 0 °C) for the first time, and the importance of solvate adducts formed during the process is demonstrated. [ABSTRACT FROM AUTHOR]

• The SrH 2 and 2Al system was studied for thermochemical energy storage applications. • Thermodynamic destablization was observed compared to pure strontium hydride. • The system reversibly stores H 2 at a temperature of 846 °C at 1 bar H 2 pressure. • After 50 cycles mild degradation in H 2 storage capacity was observed. • The material could be a thermal energy storage material upon further modification. [Display omitted] Thermal batteries are ideal for storing renewable energies or excess electricity from the grid. The most efficient thermal batteries utilize reversible thermochemical reactions where the heat produced during discharge drives a heat engine. Metal hydrides can be used as the thermal energy storage (TES) material in these batteries, since when heated, hydrogen is released in an endothermic process, charging the battery. When this hydrogen is reintroduced to the metal the metal hydride is reformed during the exothermic reaction (discharge). The optimal thermal battery would have a high operating temperature, low operating pressure and low material cost. SrH 2 could meet these demands except its operating temperature is above 1000 °C. Adding aluminum to strontium hydride causes thermal destablization allowing an operating temperature of 1 bar hydrogen at 846 ± 36 °C, providing ideal properties as a TES material. The SrH 2 -2Al system reacts in two stages with the second step exhibiting only a 32% reduction in capacity over 50 cycles. Pressure-composition isotherm analysis of the second step determined the thermodynamics of H 2 desorption to be Δ H des = 132 ± 2 kJ/mol H 2 and Δ S des = 118 ± 2 J/K/mol H 2. Further studies by scanning electron microscopy have determined changes in morphology over cyclic activity, while simultaneous thermal analysis and powder X-ray diffraction have identified the reaction pathways of the process. A cost analysis of the system has shown that a reduction in materials cost would enhance technological application of this material. [ABSTRACT FROM AUTHOR]

A new class of quaternary complex transition metal hydrides (Na 2 Mg 2 T H 8 ( T = Fe, Ru)) have been synthesized and their structures determined by combined synchrotron radiation X-ray and powder neutron diffraction. The compounds can be considered as a link between ionic and complex hydrides in terms of incorporating independently coordinated ionic and covalent hydrogen. These novel isostructural complex transition metal hydrides crystallize in the orthorhombic space group Pbam , where the octahedral complex hydride anion is surrounded by a cubic array of four Mg 2+ and four Na + cations, forming distinct two-dimensional layers. An intriguing feature of these materials is the distorted octahedral coordination of the isolated H − anions by four Na + and two Mg 2+ cations, which form layers between the transition metal containing layers. The vibrational modes of the H − anions and complex hydride anion are independently observed for the first time in a quaternary complex transition metal hydride system by Raman and IR spectroscopy. [ABSTRACT FROM AUTHOR]

Abstract Porous Al scaffolds were synthesised and melt-infiltrated with various eutectic metal borohydride mixtures (0.725LiBH 4 -0.275KBH 4, 0.68NaBH 4 -0.32KBH 4, 0.4NaBH 4 -0.6 Mg(BH 4) 2) to simultaneously act as both a confining framework and a reactive destabilising agent for H 2 release. The scaffolds were synthesised by sintering a pellet of NaAlH 4 /2 mol%TiCl 3 at 450 °C under dynamic vacuum. During the sintering process the sodium alanate (NaAlH 4) decomposed to Al metal. The vacuum applied at elevated temperature promoted the Na metal to vaporise and be extruded from the pellet. The pores of the resulting Al scaffold were created during removal of the H 2 and the Na from the body of the NaAlH 4 /2 mol%TiCl 3 pellet. According to the morphological observations carried out by a Scanning Electron Microscope (SEM), melt-infiltrated eutectic mixtures of metal borohydrides were highly dispersed into the porous scaffolds. Temperature Programmed Desorption (TPD) experiments, revealed that the melt-infiltrated samples exhibited faster H 2 desorption kinetics in comparison to bulk samples, with onset temperatures (T des) lower than the bulk by 150–250 °C. The as-synthesised porous Al scaffolds acted as a reactive containment vessel for these eutectic mixtures that simultaneously nanoconfined and destabilised the mixtures. Highlights • Porous scaffolds were fabricated by sintering a NaAlH 4 /2 mol%TiCl 3 pellet at 450 °C. • The pores formed an open network with thin interconnecting walls. • The porous Al scaffolds destabilised the infiltrated eutectic mixtures. • All melt-infiltrated samples started desorbing H 2 150–250 °C lower than the bulk. • The melt-infiltrated samples showed faster reaction kinetics to the bulk. [ABSTRACT FROM AUTHOR]

A novel approach for confining LiBH 4 within a porous aluminium scaffold was applied in order to enhance its hydrogen storage properties, relative to conventional techniques for confining complex hydrides. The porous aluminium scaffold was fabricated by sintering NaAlH 4, which was in the form of a dense pellet, under dynamic vacuum. The final product was a porous aluminium scaffold with the Na and H 2 having been removed from the initial pellet. This technique contributed to achieving highly dispersed LiBH 4 particles that were also destabilised by the presence of the aluminium scaffold. In this study, the effectiveness of this novel fabrication method of confined/destabilised LiBH 4 was extensively investigated, which aimed to simultaneously improve the hydrogen release at lower temperature and the kinetics of the system. These properties were compared with the properties of other confined LiBH 4 samples found in the literature. As-synthesised samples were characterised using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Nitrogen Adsorption measurements. The hydrogen storage capacity of all samples was analysed using temperature programmed desorption in order to provide a comprehensive survey of their hydrogen desorption properties. The porous aluminium scaffold has a wide pore size distribution with most of the porosity due to pores larger than 50 nm. Despite this the onset hydrogen desorption temperature (T des ) of the LiBH 4 infiltrated into the porous aluminium scaffold was 200 °C lower than that of bulk LiBH 4 and 100 °C lower than that of nanosized LiBH 4 . Partial cycling could be achieved below the melting point of LiBH 4 but the kinetics of hydrogen release decreased with cycle number. [ABSTRACT FROM AUTHOR]

The thermochemical energy storage properties of calcium hydride (CaH 2) destabilised with either silicon (Si) or Ca x Si y compounds at various molar ratios, were thoroughly studied by a combination of experimental and computer assisted thermodynamic calculations. Particularly, the destabilisation effect of Si on CaH 2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH 2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH 2 and Si forming Ca x Si y at varying temperatures, which was confirmed by in-situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and mass-spectroscopic measurements. The most suitable destabilisation reactions between CaH 2 and Si or Ca x Si y that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH 2 and CaSi system (in a 2:3 molar ratio of CaH 2 to CaSi) showed desirable operating conditions with a decomposition temperature of 747 ± 33 °C at a hydrogen pressure of 1 bar. Pressure composition isothermal measurements were conducted on this system to determine its practical enthalpy of decomposition to form Ca 5 Si 3. The calculated value (107.3 kJ mol−1 H 2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H 2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca 5 Si 3 phase. ga1 • Calcium hydride has been thermodynamically destabilised with silicon. • A multistep decomposition pathway was determined by theoretical calculations. • The experimental thermal decomposition pathway and thermodynamics were established. • The Ca–Si system is a viable thermal energy storage material. [ABSTRACT FROM AUTHOR]

CaH 2 is a metal hydride with a high energy density that decomposes around 1100 °C at 1 bar of H 2 pressure. Due to this high decomposition temperature, it is difficult to utilise this material as a thermal battery for the next generation of concentrated solar power plants, where the currently targeted operational temperature is between 600 and 800 °C. In this study, CaH 2 has been mixed with calcium halide salts (CaCl 2 , CaBr 2 and CaI 2) and annealed at 450 °C under 100 bar of H 2 pressure to form CaHCl, CaHBr and CaHI. These hydride-halide salts incur a thermodynamic destabilisation of their hydrogen release, compared to CaH 2 , so that they can operate between 600 and 800 °C within practical operating pressures (1–10 bar H 2) for thermochemical energy storage. The as-synthesised metal hydrides were studied by in-situ synchrotron X-ray diffraction, temperature programmed desorption and pseudo pressure composition isothermal analysis. Each of the calcium hydride-halide salts decomposed to form calcium metal and a calcium halide salt after hydrogen release. In comparison to pure CaH 2, their decomposition reactions were faster when heated up to 850 °C, and the experimental values of the desorbed hydrogen gas were very close to the theoretical ones. All samples after their decomposition showed signs of sintering, which hindered their rehydrogenation reaction. • CaH 2 was thermodynamically destabilised when mixed with calcium halide salts. • The new compounds exhibited faster reaction kinetics compared to CaH 2. • All three compounds exhibited equilibrium pressures between 1 and 10 bar at 650 °C. [ABSTRACT FROM AUTHOR]