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Hydrated and anhydrous LiB11H14salts are prepared through a facile approach. Liquid-like Li+conductivity (10−2S cm−1) is observed for a-LiB11H14·(H2O)nat 60 °C. LiB11H14·2H2O is classified as a new class of ionic liquid as it melts near 70 °C.

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 (Mg2FeH6) 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 Mg2FeH6 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).

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.

Solid-state sodium batteries have attracted great attention owing to their improved safety, high energy density, large abundance and low cost of sodium compared to the current Li-ion batteries. Sodium-boranes have been studied as potential solid-state electrolytes and the search for new materials is necessary for future battery applications. Here, a facile and cost-effective solution-based synthesis of Na

The potential and research surrounding metal hydride (MH) based thermal energy storage is discussed, focusing on next generation thermo-chemical energy storage (TCES) for concentrated solar power. The site availability model to represent the reaction mechanisms of both the forward and backward MH reaction is presented, where this model is extrapolated to a small pilot scale reactor, detailing how a TCES could function/operate in a real-world setting using a conventional shell & tube reactor approach. Further, the important parameter of effective thermal conductivity is explored using an innovative multi-scale model, to providing extensive and relevant experimental data useful for reactor and system design. Promising high temperature MH material configurations may be tuned by either destabilisation, such as using additions to Ca and Sr based hydrides, or by stabilisation, such as fluorine addition to NaH, MgH2, or NaMgH3. This versatile thermodynamic tuning is discussed, including the challenges in accurately measuring the material characteristics at elevated temperatures (500 –700 °C). Attention to scale up is explored, including generic design and prototype considerations, and an example of a novel pilot-scale pillow-plate reactor currently in development; where materials used are discussed, overall tank design scope and system integration.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage

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.

Stable power generation from renewable energy requires the development of new materials that can be used for energy storage. A new reactive carbonate composite (RCC) based on SrCO3 is proposed as a material with high energy density for thermochemical energy storage. SrCO3–SrSiO3 can promote the thermodynamic destabilisation of SrCO3, making its operating temperature (700 °C) more suitable for concentrated solar thermal power applications. Utilising a eutectic mixture of salts as a catalyst, the reversible carbonation reaction achieves cycle stability of ∼80% of efficiency over multiple cycles. The high volumetric (1878 MJ m−3) and gravimetric energy density (500 kJ kg−1) of the RCC allows a compact thermal battery to be developed. Additionally, the low cost of SrCO3 makes the RCC a highly competitive alternative energy storage material in terms of cost and efficiency when compared to existing molten salt based energy storage technology.

Metal boron-hydrogen compounds are considered as promising solid electrolyte candidates for the development of all-solid-state batteries (ASSB), owing to the high ionic conductivity exhibited bycloso- andnido-boranes. In this study, an optimised low cost preparation method of MB11H14·(H2O)n, (M = Li and Na) and KB11H14is proposed and analysed. The formation of the B11H14−salt is pH-dependent, and H3O+competes with small ionic radii cations, such as Li+and Na+, to produce a hydronium salt of B11H14−, which forms B11H13OH−upon heating. The use of diethyl ether to extract B11H14−salt from the aqueous medium during synthesis is an important step to avoid hydrolysis of the compound upon drying. The proposed method of synthesis results in LiB11H14and NaB11H14coordinated with water, whereas KB11H14is anhydrous. Hydrated LiB11H14·(H2O)nand NaB11H14·(H2O)nexhibit exceptional ionic conductivities at 25 °C, 1.8 × 10−4S cm−1and 1.1 × 10−3S cm−1, respectively, which represent some of the highest solid-state Li+and Na+conductivities at room temperature. The salts also exhibit oxidative stability of 2.1 Vvs.Li+/Li and 2.6 Vvs.Na+/Na, respectively. KB11H14undergoes a reversible polymorphic structural transition to a metastable phase before decomposing. All synthesisednido-boranes decompose at temperatures greater than 200 °C.

Metal hydrides continue to vie for attention as materials in multiple technological applications including hydrogen storage media, thermal energy storage (TES) materials, and hydrogen compressors. ...

Fluorine-substituted sodium hydride is investigated for application as a thermal energy storage material inside thermal batteries. A range of compositions of NaHxF1–x (x = 0, 0.5, 0.7, 0.85, 0.95, ...

CaH2 has 20 times the energy density of molten salts and was patented in 2010 as a potential solar thermal energy storage material. Unfortunately, its high operating temperature (>1000 °C) and corrosivity at that temperature make it challenging to use as a thermal energy storage (TES) material in concentrating solar power (CSP) plants. To overcome these practical limitations, here we propose the thermodynamic destabilization of CaH2 with Zn metal. It is a unique approach that reduces the decomposition temperature of pure CaH2 (1100 °C at 1 bar of H2 pressure) to 597 °C at 1 bar of H2 pressure. Its new decomposition temperature is closer to the required target temperature range for TES materials used in proposed third-generation high-temperature CSP plants. A three-step dehydrogenation reaction between CaH2 and Zn (1 : 3 molar ratio) was identified from mass spectrometry, temperature-programmed desorption and in situ X-ray diffraction studies. Three reaction products, CaZn13, CaZn11 and CaZn5, were confirmed from in situ X-ray diffraction studies at 190 °C, 390 °C and 590 °C, respectively. The experimental enthalpy and entropy of the second hydrogen release reaction were determined by pressure composition isotherm measurements, conducted between 565 and 614 °C, as ΔHdes = 131 ± 4 kJ mol−1 H2 and ΔSdes = 151 ± 4 J K−1 mol−1 H2. Hydrogen cycling studies of CaZn11 at 580 °C showed sufficient cycling capacity with no significant sintering occurring during heating, as confirmed by scanning electron microscopy, demonstrating its great potential as a TES material for CSP applications. Finally, a cost comparison study of known destabilized CaH2 systems was carried out to assess the commercial potential.

Metal hydrides have demonstrated ideal physical properties to be the next generation of thermal batteries for solar thermal power plants. Previous studies have demonstrated that they already operate at the required operational temperature and offer greater energy densities than existing technology. Thermal batteries using metal hydrides need to store hydrogen gas released during charging, and so far, practical demonstrations have employed volumetric storage of gas. This practical study utilises a low temperature metal hydride, titanium manganese hydride (TiMn1.5Hx), to store hydrogen gas, whilst magnesium iron hydride (Mg2FeH6) is used as a high temperature thermal battery. The coupled system is able to achieve consistent energy storage and release cycles. With titanium manganese hydride operating at ambient temperature (20 °C), Mg2FeH6 has to operate between ∼350 °C and ∼500 °C to counteract the pressure hysteresis displayed by TiMn1.5 between hydrogen uptake and release. The results attest the high susceptibility of both materials to thermal issues, such as a requirement for large temperature offsets, in order for the battery to achieve full cycling capacity. An energy density of 1488 kJ kg−1 was experimentally attained for 40 g of Mg2FeH6 with a maximum operating temperature around 520 °C.

For renewable energy sources to replace fossil fuels, large scale energy storage is required and thermal batteries have been identified as a commercially viable option. In this study, a 3.2 kg prototype (0.82 kWh th) of the limestone-based CaCO 3-Al 2O 3 (16.7 wt%) thermochemical energy storage system was investigated near 900 °C in three different configurations: (i) CaCO 3 was thermally cycled between 850 °C during carbonation and 950 °C during calcination whilst activated carbon was utilised as a CO 2 gas storage material. (ii) The CaCO 3 temperature was kept constant at 900 °C while utilising the activated carbon gas storage method to drive the thermochemical reaction. (iii) A mechanical gas compressor was used to compress CO 2 into volumetric gas bottles to achieve a significant under/overpressure upon calcination/carbonation, i.e. ≤ 0.8 bar and > 5 bar, respectively, compared to the ∼1 bar thermodynamic equilibrium pressure at 900 °C. Scenarios (i) and (iii) showed a 64% energy capacity retention at the end of the 10th cycle. The decrease in capacity was partly assigned to the formation of mayenite, Ca 12Al 14O 33, and thus the absence of the beneficial properties of the expected Ca 5Al 6O 14 while sintering was also observed. The 316L stainless-steel reactor was investigated in regards to corrosion issues after being under CO 2 atmosphere above 850 °C for approximately 1400 h, and showed no significant degradation. This study illustrates the potential for industrial scale up of catalysed CaCO 3 as a thermal battery and provides a viable alternative to the calcium-looping process.

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.

Porous Al scaffolds were synthesised and melt-infiltrated with various eutectic metal borohydride mixtures (0.725LiBH4-0.275KBH4, 0.68NaBH4-0.32KBH4, 0.4NaBH4-0.6 Mg(BH4)2) to simultaneously act as both a confining framework and a reactive destabilising agent for H2 release. The scaffolds were synthesised by sintering a pellet of NaAlH4/2 mol%TiCl3 at 450 °C under dynamic vacuum. During the sintering process the sodium alanate (NaAlH4) 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 H2 and the Na from the body of the NaAlH4/2 mol%TiCl3 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 H2 desorption kinetics in comparison to bulk samples, with onset temperatures (Tdes) 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.

Metal hydrides present favourable thermal storage properties particularly due to their high energy density during thermochemical hydrogenation. For this purpose, sodium magnesium hydride (NaMgH3) has shown promising qualities that could lead to an industrialised application, but first requires to be examined on a lab-scale under realistic operating conditions. Herein, the cycling reversibility of NaMgH3 is undertaken on a 150 g scale with active heat extraction and delivery using superheated water vapour as the heat transfer fluid. The thermal and cycling properties of the hydride material are enhanced by addition of TiB2 and exfoliated natural graphite. Over 40 cycles the NaMgH3 showed minimal loss in capacity, but revealed difficulties in terms of thermal management to avoid local overheating, resulting in the production of undesired molten sodium metal. The temperature cycling showed a hydrogen flow culminating at 1 g h−1, which was insufficient to ensure thermal energy retrieval. The increase of the inlet hydrogen pressure has been shown to be instrumental in achieving an acceptable flow rate of 10 g h−1. Indeed, this design, despite high heat losses to the environment, was able to supply a third of the chemical energy available to the heat transfer fluid.

Hydrogen and ammonia have attracted attention as potential energy vectors due to their abundance and minimal environmental impact when used as a fuel source. To be a commercially viable alternative to fossil fuels, gaseous fuel sources must adhere to a wide range of standards specifying hydrogen delivery temperature, gravimetric capacity and cost. In this article, an ammonium chloride–metal hydride reaction cycle that enables the solid thermal decomposition products to be recycled using industrial processes is proposed. A range of metal hydrides and metal amides were reacted with ammonium chloride to determine the reaction pathways, products and overall feasibility of the cycle. The NH4Cl–MH (MH = metal hydride) and NH4Cl–MNH2 (MNH2 = metal amide) mixtures were heated to temperatures of up to 500 °C. The resulting products were experimentally characterised using temperature program desorption residual gas analysis, simultaneous differential scanning calorimetry and thermogravimetric analysis and in situ powder X-ray diffraction. Similar analysis was undertaken to determine the effect of catalyst addition to the starting materials. A maximum yield of 41 wt% of hydrogen and ammonia gas mixtures were released from the NH4Cl–MH materials at a maximum yield of 41 wt%. This exceptional gravimetric capacity allows for volumetric gas densities (363–657 kg m−3) that are much higher than pure NH3, H2 or metal hydride materials. Overall, this reaction cycle allows carbon-neutral regeneration of the starting materials, making it a potential sustainable energy option.

The thermal conductivity, thermal diffusivity and heat capacity of materials are all vital properties in the determination of the efficiency of a thermal system. However, the thermal transport properties of heat storage materials are not consistent across previous studies, and are strongly dependent on the sample composition and measurement method. A comprehensive analysis of thermal transport properties using a consistent preparation and measurement method is lacking. This study aims to provide the foundation for a detailed insight into thermochemical heat storage material properties with consistent measurement methods. The thermal transport properties of pelletised metal hydrides, carbonates and oxides were measured using the transient plane source method to provide the thermal conductivity, thermal diffusivity and heat capacity. This information is valuable in the development of energy storage and chemical processing systems that are highly dependent on the thermal conductivity of materials.

Closo-borates and derivatives thereof have shown great potential as electrolyte materials for all-solid-state batteries owing to their exceptional ionic conductivity and high thermal and chemical stability. However, because of the myriad of possible chemical modifications of the large, complex anion, only a fraction of closo-borate derivatives has so far been investigated as electrolyte materials. Here, the crystal structures, thermal properties, and ionic conductivities of M2B12(OH)12 (M = Li, Na, K, and Cs) are investigated with a focus on their possible utilization as new solid-state ion conductors for solid-state batteries. The compounds generally show rich thermal polymorphism, with eight identified polymorphs among the four dehydrated compounds. Both Li2B12(OH)12 and Na2B12(OH)12 undergo a first-order transition, in which the cation sublattices become disordered, resulting in an order of magnitude jump in ionic conductivity for Na2B12(OH)12. K2B12(OH)12 undergoes a second-order polymorphic transition driven by a change in the anion-cation interaction, with no evidence of dynamic disorder. The ionic conductivities of M2B12(OH)12 range from 1.60 × 10-8 to 5.97 × 10-5 S cm-1 at 250 °C for M = Cs and Li, respectively, showing decreasing conductivity with increasing cation size. Compared with the analogous M2B12H12 compounds, such relatively low conductivities are suggested to be a consequence of strong and directional anion-cation interactions, resulting in a more static anion framework.

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. SrH2 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 SrH2-2Al 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 H2 desorption to be ΔHdes = 132 ± 2 kJ/mol H2 and ΔSdes = 118 ± 2 J/K/mol H2. 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.

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 NaAlH4 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 NaAlH4 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 NaAlH4-infiltrated porous Al scaffolds show a higher relative H2 release, with respect to NaAlH4 + TiCl3, in the temperature range 148–220 °C, with the temperature of H2 desorption trending to bulk NaAlH4 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.

Complex transition metal hydrides have been identified as being materials for multi-functional applications holding potential as thermal energy storage materials, hydrogen storage materials and optical sensors. Na2Mg2NiH6 (2Na+·2Mg2+·2H−·[NiH4]4−) is one such material. In this study, the decomposition pathway and thermodynamics have been explored for the first time, revealing that at 225 °C, hydrogen desorption commences with two major decomposition steps, with maximum H2 desorption rates at 278 and 350 °C as measured by differential scanning calorimetry. The first step of decomposition results in the formation of Mg2NiHx (x < 0.3) and NaH, before these compounds decompose into Mg2Ni and Na, respectively. PCI analysis of Na2Mg2NiH6 has determined the thermodynamics of decomposition for the first step to have a ΔHdes and ΔSdes of 83 kJ mol−1 H2 and 140 J K−1 mol−1 H2, respectively. Hydrogen cycling of the first step has been achieved for 10 cycles without any significant reduction in hydrogen capacity, with complete hydrogen desorption within 20 min at 395 °C. Despite the relatively high cost of Ni, the ability to effectively store hydrogen reversibly at operational temperatures of 318–568 °C should allow this material to be considered as a thermal energy storage material.

A simple and cost-effective method for the synthesis of a porous Al scaffold has been optimised using only NaAlH4 and TiCl3. The starting materials were compacted into a pellet and sintered under dynamic vacuum to remove the Na and H2. The sintering conditions, such as vacuum level, temperature, and time, were the key factors that influenced both the extraction of Na and H2 from the pellet and the overall porosity. Quantitative phase analysis by X-ray diffraction revealed that after the sintering process, the as-prepared porous Al scaffold consisted primarily of Al. Morphological observations conducted by scanning electron microscopy showed that the scaffold exhibited an open network of pores with a small number of mesopores and no formation of micropores. The specific surface area of the scaffold was determined to be 7.9 ± 0.1 and 6.0 ± 0.5 m2/g by the Brunauer–Emmet–Teller method and from small-angle X-ray scattering measurements, respectively. The total porosity of the Al scaffold was 44.6%.

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.

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.

A novel porous Mg scaffold was synthesised and melt-infiltrated with LiBH4 to simultaneously act as both a confining framework and a destabilising agent for H2 release from LiBH4. This porous Mg scaffold was synthesised by sintering a pellet of NaMgH3 at 450 °C under dynamic vacuum. During the sintering process the multi-metal hydride, decomposed to Mg metal and molten Na. The vacuum applied in combination with the applied sintering temperature, created the ideal conditions for the Na to vaporise and to gradually exit the pellet. The pores of the scaffold were created by the removal of the H2 and Na from the body of the NaMgH3 pellet. The specific surface area of the porous Mg scaffold was determined by the Brunauer–Emmett–Teller (BET) method and from Small-Angle X-ray Scattering (SAXS) measurements, which was 26(1) and 39(5) m2 g−1 respectively. The pore size distribution was analysed using the Barrett–Joyner–Halenda (BJH) method which revealed that the majority of the pores were macropores, with only a small amount of mesopores present in the scaffold. The melt-infiltrated LiBH4 was highly dispersed in the porous scaffold according to the morphological observation carried out by a Scanning Electron Microscope (SEM) and also catalysed the formation of MgH2 as seen from the X-ray diffraction (XRD) patterns of the samples after the infiltration process. Temperature Programmed Desorption (TPD) experiments, which were conducted under various H2 backpressures, revealed that the melt-infiltrated LiBH4 samples exhibited a H2 desorption onset temperature (Tdes) at 100 °C which is 250 °C lower than the bulk LiBH4 and 330 °C lower than the bulk 2LiBH4/MgH2 composite. Moreover, the LiH formed during the decomposition of the LiBH4 was itself observed to fully decompose at 550 °C. The as-synthesised porous Mg scaffold acted as a reactive containment vessel for LiBH4 which not only confined the complex metal hydride but also destabilised it by significantly reducing the H2 desorption temperature down to 100 °C.

The thermochemical energy storage properties of calcium hydride (CaH2) destabilised with either silicon (Si) or CaxSiy 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 CaH2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH2 and Si forming CaxSiy 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 CaH2 and Si or CaxSiy that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH2 and CaSi system (in a 2:3 molar ratio of CaH2 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 Ca5Si3. The calculated value (107.3 kJ mol−1 H2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca5Si3 phase.

CaH2 is a metal hydride with a high energy density that decomposes around 1100 °C at 1 bar of H2 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, CaH2 has been mixed with calcium halide salts (CaCl2, CaBr2 and CaI2) and annealed at 450 °C under 100 bar of H2 pressure to form CaHCl, CaHBr and CaHI. These hydride-halide salts incur a thermodynamic destabilisation of their hydrogen release, compared to CaH2, so that they can operate between 600 and 800 °C within practical operating pressures (1–10 bar H2) 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 CaH2, 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.

A novel species of metal amidoborane ammoniate, [Al(NH 2 BH 3 ) 63− ][Al(NH 3 ) 63+ ] has been successfully synthesized in up to 95% via the one-step reaction of AlH 3 ·OEt 2 with liquid NH 3 BH 3 · n NH 3 (n = 1~6) at 0 °C. This solution based reaction method provides an alternative pathway to the traditional mechano-chemical ball milling methods, avoiding possible decomposition. MAS 27 Al NMR spectroscopy confirms the formulation of the compound as an Al(NH 2 BH 3 ) 63− complex anion and an Al(NH 3 ) 63+ cation. Initial dehydrogenation studies of this aluminum based M-N-B-H compound demonstrate that hydrogen is released at temperatures as low as 65 °C, totaling ~8.6 equivalents of H 2 (10.3 wt %) upon heating to 105 °C. This method of synthesis offers a promising route towards the large scale production of metal amidoborane ammoniate moieties. Keywords: aluminum; amidoborane; boranes; dehydrogenation; hydrogen storage; synthetic methods; Nuclear Magnetic Resonance Spectroscopy (NMR)

Complex transition metal hydrides have potential technological application as hydrogen storage materials, smart windows and sensors. Recent exploration of these materials has revealed that the incorporation of anionic hydrogen into these systems expands the potential number of viable complexes, while varying the countercation allows for optimisation of their thermodynamic stability. In this study, the optimised synthesis of Na2Mg2TH8 (T = Fe, Ru) has been achieved and their thermal decomposition properties studied by ex situ Powder X-ray Diffraction, Gas Chromatography and Pressure-Composition Isotherm measurements. The temperature and pathway of decomposition of these isostructural compounds differs considerably, with Na2Mg2FeH8 proceeding via NaMgH3 in a three-step process, while Na2Mg2RuH8 decomposes via Mg2RuH4 in a two-step process. The first desorption maxima of Na2Mg2FeH8 occurs at ca. 400 °C, while Na2Mg2RuH8 has its first maxima at 420 °C. The enthalpy and entropy of desorption for Na2Mg2TH8 (T = Fe, Ru) has been established by PCI measurements, with the ΔHdes for Na2Mg2FeH8 being 94.5 kJ mol(-1) H2 and 125 kJ mol(-1) H2 for Na2Mg2RuH8.

Magnesium borohydride (Mg(BH4)2) is an attractive candidate as a hydrogen storage material due to its high hydrogen content and predicted favorable thermodynamics. In this work we demonstrate reversible hydrogen desorption in partially decomposed Mg(BH4)2 which was ball milled together with 2 mol% Co-based additives. Powder X-ray diffraction and infrared spectroscopy showed that after partial decomposition at 285 °C, amorphous boron-hydride compounds were formed. Rehydrogenation at equivalent temperatures and hydrogen pressures of 120 bar yielded the formation of crystalline Mg(BH4)2 in the first cycle, and amorphous Mg(BH4)2 with other boron–hydrogen compounds upon the third H2 absorption. Reversibility was observed in the samples with and without Co-based additives, although the additives enhanced hydrogen desorption kinetics in the first cycle. X-ray absorption spectroscopy at Co K-edge revealed that all the additives, apart from Co2B, reacted during the first desorption to form new stable species.

Metal hydrides (MHs) are promising candidates as thermal energy storage (TES) materials for concentrated solar thermal applications. A key requirement for this technology is a high temperature heat transfer fluid (HTF) that can deliver heat to the MHs for storage during the day, and remove heat at night time to produce electricity. In this study, supercritical water was used as a HTF to heat a prototype thermochemical heat storage reactor filled with MgH2 powder during H2 sorption, rather than electrical heating of the MH reactor. This is beneficial as the HTF flows through a coil of tubing embedded within the MH bed and is hence in better contact with the MgH2 powder. This internal heating mode produces a more uniform temperature distribution within the reactor by increasing the heat exchange surface area and reducing the characteristic heat exchange distances. Moreover, supercritical water can be implemented as a heat carrier for the entire thermal energy system within a concentrating solar thermal plant, from the receiver, through the heat storage system, and also within a conventional turbine-driven electric power generation system. Thus, the total system energy efficiency can be improved by minimising HTF heat exchangers.

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.

Rare earth (RE) borohydrides have received considerable attention during the past ten years as possible hydrogen storage materials due to their relatively high gravimetric hydrogen density. This review illustrates the rich chemistry, structural diversity and thermal properties of borohydrides containing RE elements. In addition, it highlights the decomposition and rehydrogenation properties of composites containing RE-borohydrides, light-weight metal borohydrides such as LiBH4 and additives such as LiH.

The decomposition and rehydrogenation of γ-Mg(BH4)2 ball milled together with 2 mol % of Ni-based additives, Ninano, NiCl2, NiF2, and Ni3B, has been investigated during one hydrogen desorption–absorption cycle. Under the applied ball-milling conditions, no mechanochemical reactions between γ-Mg(BH4)2 and Niadd were observed. Hydrogen desorption carried out at temperatures of 220–264 °C resulted for all samples in partial decomposition of Mg(BH4)2 and formation of amorphous phases, as seen by powder X-ray diffraction (PXD). PXD analysis after rehydrogenation at temperatures of 210–262 °C and at pressures between 100 and 155 bar revealed increased fractions of crystalline β-Mg(BH4)2, indicating a partial reversibility of the composite powders. The highest amount of [BH4]− is formed in the composite containing Ni3B. Analysis by X-ray absorption spectroscopy performed after ball milling, after desorption, and after absorption shows that the Ni3B additive remains unaffected, whereas NiCl2 and NiF2 additives re...

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.

The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost for Concentrating Solar-thermal Power (CSP). We focus on the underlying technology that allows metal hydrides to function as Thermal Energy Storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room-temperature and as high as 1100 oC. The potential of metal hydrides for thermal storage is explored while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature thermal energy storage are also addressed.

Concentrating solar–thermal power (CSP) with thermal energy storage (TES) represents an attractive alternative to conventional fossil fuels for base-load power generation. Sodium alanate (NaAlH4) is a well-known sodium-based complex metal hydride but, more recently, high-temperature sodium-based complex metal hydrides have been considered for TES. This review considers the current state of the art for NaH, NaMgH3−x F x , Na-based transition metal hydrides, NaBH4 and Na3AlH6 for TES and heat pumping applications. These metal hydrides have a number of advantages over other classes of heat storage materials such as high thermal energy storage capacity, low volume, relatively low cost and a wide range of operating temperatures (100 °C to more than 650 °C). Potential safety issues associated with the use of high-temperature sodium-based hydrides are also addressed.

Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH4)3 and Pr(BH4)3 have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH4)3 confirmed the formation of two different Er(BH4)3 polymorphs: α-Er(BH4)3 with space group Pa[3 with combining macron], a = 10.76796(5) Å, and β-Er(BH4)3 in Pm[3 with combining macron]m with a = 5.4664(1) Å. A variety of Pr(BH4)3 phases were found after extraction with diethyl ether: α-Pr(BH4)3 in Pa[3 with combining macron] with a = 11.2465(1) Å, β-Pr(BH4)3 in Pm[3 with combining macron]m with a = 5.716(2) Å and LiPr(BH4)3Cl in I[4 with combining macron]3m, a = 11.5468(3) Å. Almost phase pure α-Pr(BH4)3 in Pa[3 with combining macron] with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH4)3 and Pr(BH4)3 proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH4 from the decomposed RE(BH4)3 samples.

Cl− to BH 4 − ion substitution was induced in NaBH4 through mechano-chemical reactions with all the first period transition metal chlorides and CdCl2. The products were identified by Rietveld refinement of powder X-ray and neutron diffraction data to be mainly Na(BH4)1−xClx. These possess cubic NaCl-type structures, with unit cell parameters between 5.7801(3) and 5.6576(2) A, and compositions ranging from x = 0.69 (with Cu) to 0.92 (with Zn). Infrared spectroscopy of selected samples confirms the substitution through a shift of the vibrational modes of the BH 4 − group towards higher wavenumbers. An observed shape change of the vibrational features from Lorentzian to Gaussian is related to the presence of the transition metal. There is no correlation between x and the thermal behavior of the samples. The lowest decomposition temperature is found for the Zn containing sample (103 °C), while Cd leads to the highest value (521 °C). Their behavior is related to the presence of NaZn(BH4)3 and metallic Cd in the samples, respectively.

The structure of AlH3·2NMe3 has been investigated by single-crystal X-ray diffraction over the range of 296–173 K. 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 233 K in the region of the phase change. Low-temperature DSC measurements reveal the change from Pbcm to Cmca to occur at 218.3 K, with an enthalpy of 107.7 J mol−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.