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Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH 2), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH 2. In this work, a literature review of the wide-ranging thermodynamic properties of CaH 2 is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH 2. The thermodynamic values of hydrogen release from both molten and solid CaH 2 were determined as Δ H des (molten CaH 2) = 216 ± 10 kJ mol−1.H 2 , Δ S des (molten CaH 2) = 177 ± 9 J K−1 mol−1.H 2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH 2 of 947 ± 65 °C. Similarly, in the solid-state: Δ H des (solid CaH 2) = 172 ± 12 kJ mol−1.H 2 , Δ S des (solid CaH 2) = 144 ± 10 J K−1 mol−1.H 2. Moreover, the activation energy of hydrogen release from CaH 2 was also calculated using DSC analysis as E a = 203 ± 12 kJ mol−1. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material. • The thermodynamics for hydrogen desorption of calcium hydride were determined. • This is the first study in over 60 years to determine the thermal properties. • The kinetics of hydrogen release were measured using the Kissinger method. • The melting point of CaH 2 was measured under a hydrogen atmosphere. • A literature review of previous thermodynamic studies on CaH 2 is included. [ABSTRACT FROM AUTHOR]

Metal hydrides have been demonstrated as energy storage materials for thermal battery applications. This is due to the high energy density associated with the reversible thermochemical reaction between metals and hydrogen. Magnesium iron hydride (Mg 2 FeH 6) is one such material that has been identified as a thermal energy storage material due to its reversible hydrogenation reaction at temperatures between 400 and 600 °C. This study demonstates an automated thermal battery prototype containing 900 g of Mg 2 FeH 6 as the thermal energy storage material with pressurised water acting as the heat transfer fluid to charge and discharge the battery. The operating conditions of the system were optimised by assessing the ideal operating temperature, flow rate of the heat transfer fluid, and hydrogen pressures. Overall, excellent cyclic energy storage reversibility was demonstrated between 410 and 450 °C with a maximum energy capacity of 1650 kJ which is 87% of the theoretical value (1890 kJ). [Display omitted] • A prototype thermal battery was developed using Mg 2 FeH 6 as the thermal energy storage material. • The automated battery consisted of a heat transfer fluid system to charge and discharge the material. • Complete optimisation of the operating conditions of the system was established. • Excellent cycle reversibility was demonstrated between 410 and 450 °C. [ABSTRACT FROM AUTHOR]

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]

Metal hydrides have received much attention due to the flourishing concept of a hydrogen economy. In addition, recent studies of metal hydride materials have shown that technological implementation of these compounds not only lies in the storage of hydrogen, but for a range of multi-functional applications; including, smart optical windows, energy storage materials in fuel cells for both stationary and mobile devices, and as thermal energy storage materials for concentrating solar thermal plants. This review concentrates on the molecular structures, thermodynamic properties and other physical properties of the complexes of [NiH 4 ] 4− , [FeH 6 ] 4− , [CoH 5 ] 4− and [RuH 6 ] 4− . The synthesized derivatives of these compounds have also been reviewed to give a full overview on the advancement of these systems and will allow for a fresh prospective for future studies to fully understand the physical and chemical nature of these complex transition metal hydrides. [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]

Abstract: Aluminum hydride (alane; AlH3) has been identified as a leading hydrogen storage material by the US Department of Energy. With a high gravimetric hydrogen capacity of 10.1 wt.%, and a hydrogen density of 1.48 g/cm3, AlH3 decomposes cleanly to its elements above 60 °C with no side reactions. This study explores in detail the thermodynamic and spectroscopic properties of AlH3; in particular the α, α′ and γ polymorphs, of which α′-AlH3 is reported for the first time, free from traces of other polymorphs or side products. Thermal analysis of α-, α′-, and γ-AlH3 has been conducted, using DSC and TGA methods, and the results obtained compared with each other and with literature data. All three polymorphs were investigated by 1H MAS-NMR spectroscopy for the first time, and their 27Al MAS-NMR spectra were also measured and compared with literature values. AlH3·nEt2O has also been studied by 1H and 27Al MAS-NMR spectroscopy and DSC and TGA methods, and an accurate decomposition pathway has been established for this adduct. [Copyright &y& Elsevier]

Abstract: The structure of AlH3·2NMe3 has been investigated by single-crystal X-ray diffraction over the range of 296–173K. Over this temperature range a phase change is observed from Cmca to Pbcm where the methyl groups convert from a statistically disordered conformation to adopt a mutually eclipsed conformation at lower temperatures. Measurement of the unit cell dimensions shows a decrease in the lengths of the a and b axes, and an increase in that of the c axis as the temperature is lowered, with inflections apparent between 223 and 233K in the region of the phase change. Low-temperature DSC measurements reveal the change from Pbcm to Cmca to occur at 218.3K, with an enthalpy of 107.7Jmol−1. The molecular structure of AlH3·2NMe3 is compared with those of related amine adducts of Group 13 hydrides, either measured experimentally or calculated using DFT methods. 1H, 13C and 27Al NMR spectroscopy has also been utilized to characterize AlH3·2NMe3 and its 1:1 counterpart AlH3·NMe3. [Copyright &y& Elsevier]

Hydrogen and ammonia are both potential energy carriers being considered for large scale energy transport. Sodium borohydride (NaBH 4) has been widely applied as a potential solid-state hydrogen storage material. It can produce hydrogen gas and sodium metaborate (NaBO 2) after hydrolysis with water at room temperature. The regeneration of NaBH 4 from NaBO 2 could significantly reduce the cost of NaBH 4 and enable its wide-spread industrial use. In this work, we demonstrate a simple method for regenerating NaBH 4 from NaBO 2 ·4H 2 O using Mg 2 N 3 , which combines NaBH 4 production with NH 3 gas production in a single step at room temperature. NaBH 4 is successfully synthesized from NaBO 2 ·4H 2 O using the Mg 3 N 2 reducing agent via ball milling under a hydrogen atmosphere. NaBH 4 is formed at a 76.6% yield by planetary ball milling at 600 rpm under 40 bar hydrogen for 12 h. NH 3 gas is also formed, which can be easily separated from the solid products. Therefore, this one-step process could produce two different types of carbon-free hydrogen carriers suitable for energy export from renewable sources. • One step process to store hydrogen in NaBH 4 and NH 3. • Lower cost for NaBH 4 regeneration. • Mg 3 N 2 can be considered as a promising reductant for simultaneous synthesis of NaBH 4 and NH 3. • The hydrogen pressure have a effect on the ball milling. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage offers a cost-effective and efficient approach for storing thermal energy at high temperature (∼1100 °C) for concentrated solar power and large-scale long duration energy storage. SrCO 3 is a potential candidate as a thermal energy storage material due to its high energy density of 205 kJ/mol of CO 2 during reversible CO 2 release and absorption. However, it loses cyclic capacity rapidly due to sintering. This study determined that the cyclic capacity of SrCO 3 was enhanced by the addition of either reactive SrTiO 3 or inert SrZrO 3 , where the molar ratios of SrCO 3 to SrZrO 3 were varied from 1:0.125 to 1:1. Thermogravimetric analysis over 15 CO 2 sorption cycles demonstrated that both materials retained ∼80 % of their maximum cyclic capacity on the milligram scale. Repeated measurements using gram scale samples revealed a decrease in maximum capacity to 11 % using a sample of SrCO 3 – 0.5 SrZrO 3 over 53 cycles, while the use of SrTiO 3 additives allowed for the retention of 80 % maximum capacity over 55 cycles. These findings highlight the potential of reactive additives in enhancing the performance of thermochemical energy storage systems, while providing valuable insights for the development of cost-effective materials. [Display omitted] • SrCO 3 is a cost-effective thermochemical energy storage material. • TGA shows both SrZrO 3 and SrTiO 3 additives increase cyclic capacity up to ∼80 %. • SrTiO 3 reduces sintering whereas SrZrO 3 does not prevent sintering. [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]

• Carbonation reaction kinetics of limestone-based CaCO 3 /Al 2 O 3 mixture is estimated. • Numerical modelling of CaCO 3 based reactor for thermal storage application is performed. • The influence of operating parameters on the reactor's performance is examined. • The effect of incorporating graphite fin into the CaCO 3 reactor is studied. This study conducts kinetic analyses of the carbonation reaction of CaCO 3 (doped with Al 2 O 3) as well as parametric analyses of the performance of a thermochemical reactor, which can act as a thermal battery. Kinetic measurements of CO 2 release and absorption were carried out using thermogravimetric analysis (TGA) at 815, 830 and 845 °C on a CaCO 3 /Al 2 O 3 sample that had been previously cycled over 500 times. The rapid reaction kinetics revealed that the Avrami nucleation growth model with exponent 3 fits well to explain the carbonation reaction. The numerical study considered a cylindrical reactor with a height and diameter of 100 mm. According to numerical analysis, at an applied CO 2 pressure of 1 bar, increasing the thermal conductivity of the reactor bed from 1.33 to 5 W/m.K increases the rate of carbonation reaction by 74%. When the applied CO 2 pressure is increased from 1 to 2 bar, the performance of the reactor bed with thermal conductivity of 1.33 W/m.K improves by 42%; however, when the applied CO 2 pressure is increased from 2 to 3 bar, the performance improves by only 18%. Additionally, when the boundary temperature of the reactor was lowered by 30 °C, performance was enhanced by 43% at an applied CO 2 pressure of 1 bar. This study also examined the effect of using a graphite fin as a heat extraction system. The graphite fin allowed for more rapid heat extraction and increased the carbonation reaction by 44% in the reactor bed with poor thermal conductivity (1.33 W/m.K) but had no effect in the reactor with modest thermal conductivity of (5 W/m.K) due to its ability to already transfer heat effectively to the reactor shell. [ABSTRACT FROM AUTHOR]

The inherent nature concerning the intermittency of concentrating solar power (CSP) plants can be overcome by the integration of efficient thermal energy storage (TES) systems. Current CSP plants employ molten salts as TES materials although metal hydrides (MH) have proven to be more efficient due to their increased operating temperatures. Nonetheless, the heat exchange between the MH bed and the heat transfer medium used to operate a heat engine is a critical factor in the overall efficiency of the TES system. In this work, a computational study is carried out to investigate the performance of a magnesium hydride TES packed bed using a multiphase (boiling) medium instead of single-phase heat absorption methods. The boiling heat transfer behaviour is simulated by using the Eulerian two-fluid framework. The simulations are conducted at a transient state using SST- k -ω Reynolds-Averaged Navier-Stokes equations. It is observed that, unlike the single-phase heat collection method, the multiphase heat absorption method maintains a constant temperature in the heat transfer fluid throughout the reactor. Consequently, a higher temperature gradient is realised between the MH bed and heat transfer fluid (HTF), leading to improvements in the overall reaction rate of the hydrogenation process. • Performance enhancement of a magnesium hydride reactor due to multiphase heat absorption was studied. • Boiling heat transfer behaviour was simulated using Eulerian two-fluid framework. • The percentage of improvement in reactor performance was variable at different reacted fractions. • Dry out condition was critical in the reactor efficiency. [ABSTRACT FROM AUTHOR]

Metal hydrides are regarded as one of the most attractive options for thermal energy storage (TES) materials for concentrated solar thermal applications. Improved thermal performance of such systems is vitally determined by the effectiveness of heat exchange between the metal hydride and the heat transfer fluid (HTF). This paper presents a numerical study supported by experimental validation on a magnesium hydride reactor fitted with a helical coil heat exchanger for enhanced thermal performance. The model incorporates hydrogen absorption kinetics of ball-milled magnesium hydride, with titanium boride and expanded natural graphite additives obtained by Sievert's apparatus measurements and considers thermal diffusion within the reactor to the heat transfer fluid for a realistic representation of its operation. A detailed parametric analysis is carried out, and the outcomes are discussed, examining the ramifications of hydrogen supply pressure and its flow rate. The study identifies that the enhancement of thermal conductivity in magnesium hydride has an insignificant impact on current reactor performance. • 3-D validation of MgH 2 reactor with a helical coil heat exchanger was performed. • Reaction kinetics was found out by using Sievert's apparatus. • Effect of operating parameters on MH reactor performance was studied. • Addition of 20% ENG into MH reactor, under the study, proved ineffective. [ABSTRACT FROM AUTHOR]

Abstract Thermochemical energy storage materials have advantage of much higher energy densities compared to latent or sensible heat storage materials. Metal hydrides show good reversibility and cycling stability combined with high enthalpies. They can be used for short and long-term heat storage applications and can increase the overall flexibility and efficiency of solar thermal energy production. Metal hydrides with working temperatures less than 500 °C were in the focus of research and development over the last years. For the new generation of solar thermal energy plants new hydrides materials with working temperatures above 600 °C must be developed and characterized. In addition to thorough research on new metal hydrides, the construction and engineering of heat storage systems at these high temperatures are challenging. Corrosion problems, hydrogen embrittlement and selection of heat transfer fluids are significant topics for future research activities. Highlights • Metal hydrides are described concerning their potential for heat storage at different temperatures. • For future thermochemical energy storage useful for the next generation of solar power plants new metal hydrides with working temperatures above 600 °C must be developed. • In this regard challenges of engineering, corrosion, hydrogen embrittlement and heat transfer must be solved. [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]

In solid-state hydrogen storage in light metal hydrides, nanoconfinement and the use of catalysts represent promising solutions to overcoming limitations such as poor reversibility and slow kinetics. In this work, the morphology and hydrogen desorption kinetics of NaAlH 4 melt-infiltrated into a previously developed Ti-based doped porous Al scaffold is analysed. Small-angle X-ray scattering and scanning electron microscopy analysis of low NaAlH 4 loading in the porous Al scaffold has revealed that mesopores and small macropores are filled first, leaving the larger macropores/voids empty. Temperature-programmed desorption experiments have shown that NaAlH 4 -infiltrated porous Al scaffolds show a higher relative H 2 release, with respect to NaAlH 4 + TiCl 3 , in the temperature range 148–220 °C, with the temperature of H 2 desorption trending to bulk NaAlH 4 with increasing scaffold loading. The Ti-based catalytic effect is reproduced when the dopant is present in the scaffold. Further work is required to increase the mesoporous volume in order to enhance the nanoconfinement effect. [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]