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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.

The potential of high-temperature metal hydrides (HTMHs) as thermal energy storage materials means that accurate assessment of their properties is required on the laboratory scale. Above ≃ 450 °C, the hydrogen permeability through sample cell reactors (SCRs) used in experiments to characterise HTMHs can have an appreciable impact on the data. In this work, the thermodynamic and kinetic properties of TiHx have been measured at temperatures up to 725 °C as a test of different practical methods for limiting the effects of hydrogen permeability and solubility in SCRs during high-temperature characterisation of metal hydrides. Aluminium-coated stainless steel and α-SiC were used to construct SCRs with reduced hydrogen permeability using commercially available techniques. When steel SCRs are the only practical option, guidelines have been developed to choose experimental conditions that minimise the effect on the data collected due to their hydrogen permeability and solubility. A method has also been developed to correct collected data for the hydrogen permeability and solubility of SCRs.

Concentrating solar power (CSP) plants require thermal energy storage (TES) systems to produce electricity during the night and periods of cloud cover. The high energy density of high-temperature metal hydrides (HTMHs) compared to state-of-the-art two-tank molten salt systems has recently promoted their investigation as TES systems. A common challenge associated with high-temperature metal hydride thermal energy storage systems (HTMH TES systems) is storing the hydrogen gas until it is required by the HTMH to generate heat. Low-temperature metal hydrides can be used to store the hydrogen but can comprise a significant proportion of the overall system cost and they also require thermal management, which increases the engineering complexity. In this work, the potential of using a hydrogen compressor and large-scale underground hydrogen gas storage using either salt caverns or lined rock caverns has been assessed for a number of magnesium- and sodium-based hydrides: MgH2, Mg2FeH6, NaMgH3, NaMgH2F and NaH. Previous work has assumed that the sensible heat of the hydrogen released from the HTMH would be stored in a small, inexpensive regenerative material system. However, we show that storing the sensible heat of the hydrogen released would add between US$3.6 and US$7.5/kWhth to the total system cost for HTMHs operating at 565 °C. If the sensible heat of released hydrogen is instead exploited to perform work then there is a flow-on cost reduction for each component of the system. The HTMHs combined with underground hydrogen storage all have specific installed costs that range between US$13.7 and US$26.7/kWhth which is less than that for current state-of-the-art molten salt heat storage. Systems based on the HTMHs Mg2FeH6 or NaH have the most near term and long term potential to meet SunShot cost targets for CSP thermal energy storage. Increasing the operating temperature and hydrogen equilibrium pressure of the HTMH is the most effective means to reduce costs further.

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.

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.

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%.

Hydrogen is touted as one of the solutions for future energy requirements. Metal hydrides typically studied for their high hydrogen capacity can be used as a thermal storage system by taking advantage of their endothermic/exothermic reactions with hydrogen. This allows the harnessing of solar energy by utilising thermal storage to alleviate its intermittent nature. Lithium amide (LiNH 2 ), imides (Li 2 NH) and nitrides (Li 3 N) have been widely studied for their hydrogen storage at relatively low operating temperatures, typically suited for mobile applications. However, little work has been done involving the imide to nitride reaction of lithium-based materials due to their high temperature range. The following techniques were used to characterise this system: temperature programmed desorption (TPD), temperature programmed photographic analysis (TPPA), x-ray diffraction (XRD) and pressure-composition-temperature (PCT) measurements. TPD results revealed that only a single-step reaction occurred between 100 and 600 °C. TPPA revealed that having a molten solid solution of the sample, depreciated the reversibility of hydrogen absorption and desorption. The molten sample behaved quite vigorously in TPPA measurements and consequently blocked sample filters and sintering sample cell walls; creating engineering problems at higher temperatures. The results revealed that for reactions involving Li 2 NH and lithium hydride (LiH), the temperature range required for thermal storage is above the melting point of the system. The diffusion and absorption of hydrogen through stainless steel would also occur in the sample cells used, resulting in further problems. The reaction pathway of Li 2 NH and LiH was also found to be far more complex than generally reported: XRD revealed that the expected final product of Li 3 N could not be identified, instead a lithium imide-nitride hydride phase (Li 4‑2 x N 1‑ x H 1‑ x (NH) x ) was identified as the final product of this system. PCT measurements were conducted to identify the kinetic and thermodynamics of this system, but because of the molten solid-solution problem, an accurate result could not be obtained.

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 multi-stage Al production through a carbosulphidation of Al2O3, followed by a sodiothermic reduction of Al2S3, was proposed. In Stage-1, alumina (or bauxite) is reduced to Al2S3 in the presence of carbon and H2S. In Stage-2, Al2S3 is reduced to Al through reactions with Na or NaH. The thermodynamic analysis predicted Al2S3 to be the main intermediate Al-compound when H2S is reacted with Al2S3 and C at 1000-2000°C at 1 atm. Al2S3 formation was predicted to be low at 1100-1300°C at 1 atm (0.1 moles/mole Al2O3) but increased with increasing temperature (0.96 moles/mole Al2O3 at 1800°C). The thermodynamic analysis of sodiothermic reduction predicted that Al metal can be extracted from Al2S3 below 800°C at 1 atm. The Na2S produced can be hydrolysed to form H2S and NaOH. H2S can be re-used and the Na can be reproduced from NaOH and put back into the process.

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.

Altering the decomposition pathway of potassium alanate, KAlH4, with aluminium sulfide, Al2S3, presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH(4)-Al2S3 during heating under dynamic vacuum began at 185 degrees C, 65 degrees C lower than for pure KAlH4, and released 71% of the theoretical hydrogen content below 300 degrees C via several unknown compounds. The major hydrogen release event, centred at 276 degrees C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 angstrom, = 122.88 degrees) and hexagonal (a = 10.079, c = 7.429 angstrom) unit cells, respectively. Unlike the 6NaAlH(4)-Al2S3 system, the 6KAlH(4)-Al2S3 system did not have M3AlH6 (M = alkali metal) as one of the intermediate decomposition products nor were the final products M2S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 degrees C. The measured enthalpy of hydrogen absorption (H-abs) was in the range -44.5 to -51.1 kJ mol(-1) H-2, which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H-2 release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties.

Altering the decomposition pathway of potassium alanate, KAlH4, with aluminium sulfide, Al2S3, presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH4-Al2S3 during heating under dynamic vacuum began at 185 °C, 65 °C lower than for pure KAlH4, and released 71% of the theoretical hydrogen content below 300 °C via several unknown compounds. The major hydrogen release event, centred at 276 °C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 Å, β = 122.88°) and hexagonal (a = 10.079, c = 7.429 Å) unit cells, respectively. Unlike the 6NaAlH4-Al2S3 system, the 6KAlH4-Al2S3 system did not have M3AlH6 (M = alkali metal) as one of the intermediate decomposition products nor were the final products M2S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 °C. The measured enthalpy of hydrogen absorption (ΔHabs) was in the range -44.5 to -51.1 kJ mol-1 H2, which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H2 release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties.

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage” different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications, The research for the lab-scale compressor is part of the activities of SCCER HaE, which is financially supported by Innosuisse - Swiss Innovation Agency . The authors thank the Alexander von Humboldt Foundation in the frame of the post-doctoral fellowship of Dr. J. Puszkiel (No. 1187279 STP ) as well as the European Union for their funding of projects STORHY (contract Nr. SES6-CT-2004-502667 , FP6-2002-Energy-1, 6.1.3.2.2), NESSHY (contract Nr. 518271 , FP6-2004-Energy-3, 6.1.3.2.2) and the EU Horizon 2020 /RISE project HYDRIDE4MOBILITY. Financial support from the S02 and KP8 S05), the European Union's Seventh Framework Programme ( FP7/2007e2013 ) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement no. 256653 (SSH2S), from the European Fuel Cells and Hydrogen Joint Undertaking in the framework of BOR4STORE (Grant agreement no. 303428 ), from the Australian Research Council for grants LP120101848 , LP150100730 , and LE0989180 , The Innovation Fund Denmark (project HyFill-Fast), DST within Hydrogen South Africa/HySA programme (projects KP3 National Research Foundation/NRF of South Africa , incentive funding grant number 109092 and the Research Council of Norway (project 285147 ) is thankfully acknowledged

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.

In this paper, experimental investigation on the carbosulphidation of alumina (Al2O3) for aluminium (Al) production and the sodiothermic reduction of aluminium sulphide (Al2S3) are presented. Hydro...

The cohesive or atomization energy of an ionic solid is the energy required to decompose the solid into its constituent independent gaseous atoms at 0 K, while its lattice energy, Upot, is the energy required to decompose the solid into its constituent independent gaseous ions at 0 K. These energies may be converted into enthalpies at a given temperature by the addition of the small energies corresponding to integration of the heat capacity of each of the constituents. While cohesive energies/enthalpies are readily calculated by thermodynamic summing of the formation energies/enthalpies of the constituents, they are also currently intensively studied by computational procedures for the resulting insight on the interactions within the solid. In supporting confirmation of their computational results, authors generally quote "experimental" cohesive energies which are, in fact, simply the thermodynamic sums. However, these "experimental" cohesive energies are quoted in many different units, atom-based or calorimetric, and on different bases such as per atom, per formula unit, per oxide ion, and so forth. This makes comparisons among materials very awkward. Additionally, some of the quoted values are, in fact, lattice energies which are distinctly different from cohesive energies. We list large numbers of reported cohesive energies for binary halides, chalcogenides, pnictogenides, and Laves phase compounds which we bring to the same basis, and identify a number as incorrectly reported lattice energies. We also propose that cohesive energies of higher-order ionic solids may also be estimated as thermodynamic enthalpy sums.

An empirical understanding of the relationship between crystallite size and reaction kinetics for the dehydrogenation of MgH 2 in the presence of Si was determined. MgH 2 was combined with Si under different conditions to obtain varying crystallite sizes of both reactants. Thermal analysis and isothermal desorption were undertaken to obtain reaction kinetic information and therefore determine activation energies as well as the rate limiting step for each of the different crystallite sizes. It was found that there is a strong correlation between crystallite size and activation energy for the growth of the Mg 2 Si phase, however, any correlation between the nucleation (of Mg 2 Si) activation energy was less evident. Direct measurements of kinetic behaviour from a manometric Sieverts apparatus showed that initial reaction kinetics were fastest when MgH 2 was mixed with Si nanoparticles, however, this sample was not able to fully desorb. Data from the Sieverts measurements were then used with well-known theoretical models to determine the rate limiting step of the reaction. The three dimensional Carter–Valensi (or contracting volume) diffusion model could be used to describe the rate limiting step for most of the reactions. These results have led to a proposed mechanism for the formation of Mg 2 Si during the decomposition reaction.

The addition of Group IV elements of Si, Ge or Sn to Mg-based hydrides has led to the successful destabilisation of MgH 2 or NaMgH 3 , resulting in hydrogen release at lower temperatures. This is the first time a direct comparison has been made with all the samples prepared and characterised using identical conditions. Pure MgH 2 desorbs hydrogen at a pressure of 1 bar at 282 °C, a temperature too high for typical mobile applications. The addition of Group IV metals to MgH 2 causes the formation of intermetallic compounds (Mg 2 Si, Mg 2 Ge and Mg 2 Sn) upon hydrogen release. Theoretical calculations show promising thermodynamic equilibrium conditions for these systems. Experimentally, these conditions were difficult to achieve, however, hydrogen desorption results show that Ge has the most significant effect in allowing low temperature hydrogen release, followed by Sn, then Si. It was found that Si also has a beneficial effect on NaMgH 3 , reducing the desorption temperature.

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.

The hydrogen storage properties of eutectic melting 0.68LiBH4–0.32Ca(BH4)2 (LiCa) as bulk and nanoconfined into a high surface area, SBET = 2421 ± 189 m2/g, carbon aerogel scaffold, with an average pore size of 13 nm and pore volume of Vtot = 2.46 ± 0.46 mL/g, is investigated. Hydrogen desorption and absorption data were collected in the temperature range of RT to 500 °C (ΔT/Δt = 5 °C/min) with the temperature then kept constant at 500 °C for 10 h at hydrogen pressures in the range of 1–8 and 134–144 bar, respectively. The difference in the maximum H2 release rate temperature, Tmax, between bulk and nanoconfined LiCa during the second cycle is ΔTmax ≈ 40 °C, which over five cycles becomes smaller, ΔTmax ≈ 10 °C. The high temperature, Tmax ≈ 455 °C, explains the need for high temperatures for rehydrogenation in order to obtain sufficiently fast reaction kinetics. This work also reveals that nanoconfinement has little effect on the later cycles and that nanoconfinement of pure LiBH4 has a strong effect in o...

The Mg–Si–H system is economically favorable as a hydrogen storage medium for renewable energy systems while moving toward sustainable energy production. Hydrogen desorption from MgH2 in the presence of Si is achievable, forming magnesium silicide (Mg2Si). However, absorbing hydrogen into Mg2Si remains problematic due to severe kinetic limitations. The objective of this study is to reduce these kinetic limitations by synthesizing Mg2Si nanoparticles to limit the migration distance for magnesium atoms from the Mg2Si matrix to produce MgH2 and Si, thus improving the reversibility of the Mg–Si–H system. Mg2Si nanoparticles were synthesized using a reduction reaction undertaken by solid–liquid mechanochemical ball milling. Particle size was controlled by adding a reaction buffer (lithium chloride) to the starting reagents to restrict particle growth during milling. The reaction buffer was removed from the nanoparticles using tetrahydrofuran and small-angle X-ray scattering revealed an average Mg2Si particle s...

Silicon nanoparticles have been synthesised using mechanochemical ball milling and an inert salt buffer to limit the growth and control the size of the Si particles produced. The solid–liquid metathesis reaction used silicon tetrachloride and lithium with LiCl as the buffer to generate Si nanoparticles. Once the LiCl was removed, X-ray amorphous Si was identified using electron energy loss spectra, at 99 eV and energy filtered transmission electron microscopy. The morphological analysis showed spherical like particles with an average size between 10–30 nm depending on the amount of salt buffer phase added to the reactants. This synthesis method can be used to produce very small Si particles in tuneable sizes for a wide range of applications.

Rare earth metal borohydrides have been proposed as materials for solid state hydrogen storage because of their reasonably low temperature of decomposition. New synthesis methods{} which provide halide free yttrium and gadolinium borohydride{} are presented using dimethyl sulfide and new solvates as intermediates. The solvates M(BH4)3S(CH3)2 (M = Y or Gd) are transformed to [small alpha] Y(BH4)3 or Gd(BH4)3 at [similar]140 [degree]C as verified by thermal analysis. The monoclinic structure of Y(BH4)3S(CH3)2{} space group P21/c{} a = 5.52621(8){} b = 22.3255(3){} c = 8.0626(1) A and [small beta] = 100.408(1)[degree]{} is solved from synchrotron radiation powder X ray diffraction data and consists of buckled layers of slightly distorted octahedrons of yttrium atoms coordinated to five borohydride groups and one dimethyl sulfide group. Significant hydrogen loss is observed from Y(BH4)3 below 300 [degree]C and rehydrogenation at 300 [degree]C and p(H2) = 1550 bar does not result in the reformation of Y(BH4)3{} but instead yields YH3. Moreover{} composites systems Y(BH4)3 LiBH4 1 : 1 and Y(BH4)3 LiCl 1 : 1 prepared from as synthesised Y(BH4)3 are shown to melt at 190 and 220 [degree]C{} respectively.

A simplified techno-economic model has been used as a screening tool to explore the factors that have the largest impact on the costs of using metal hydrides for concentrating solar thermal storage. The installed costs of a number of paired metal hydride concentrating solar thermal storage systems were assessed. These comprised of magnesium-based (MgH2, Mg2FeH6, NaMgH3, NaMgH2F) high-temperature metal hydrides (HTMH) for solar thermal storage and Ti1.2Mn1.8H3.0 as the low-temperature metal hydride (LTMH) for hydrogen storage. A factored method approach was used for a 200 MWel power plant operating at a plant capacity factor (PCF) of 50% with 7 hours of thermal storage capacity at full-load. In addition, the hydrogen desorption properties of NaMgH2F have been measured for the first time. It has a practical hydrogen capacity of 2.5 wt% (2.95 wt% theoretical) and desorbs hydrogen in a single-step process above 478 °C and in a two-step process below 478 °C. In both cases the final decomposition products are NaMgF3, Na and Mg. Only the single-step desorption is suitable for concentrating solar thermal storage applications and has an enthalpy of 96.8 kJ mol−1 H2 at the midpoint of the hydrogen desorption plateau. The techno-economic model showed that the cost of the LTMH, Ti1.2Mn1.8H3.0, is the most significant component of the system and that its cost can be reduced by increasing the operating temperature and enthalpy of hydrogen absorption in the HTMH that, in turn, reduces the quantity of hydrogen required in the system for an equivalent electrical output. The result is that, despite the fact that the theoretical thermal storage capacity of NaMgH2F (1416 kJ kg−1) is substantially lower than the theoretical values for MgH2 (2814 kJ kg−1), Mg2FeH6 (2090 kJ kg−1) and NaMgH3 (1721 kJ kg−1), its higher enthalpy and operating temperature leads to the lowest installed cost of the systems considered. A further decrease in cost could be achieved by utilizing metal hydrides with yet higher enthalpies and operating temperatures or by finding a lower cost option for the LTMH.

The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600 °C, under both vacuum and hydrogen (H2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type) and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250 °C, anhydrous Li2B12H12 begins to decompose to a substoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85-7.00 Å, dependent on the amount of H released. Aging Pa-3 Li2B12H12 under 450 °C/125 bar H2 pressure for 24 h produces a previously unobserved well-crystallized β-Li2B12H12 polymorph, and a nanocrystalline γ-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (Tm = 280 °C) initially results in the formation of LiH and γ-Li2B12H12. The γ-Li2B12H12 polymorph then decomposes to a substoichiometric Li2B12H(12-x) composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600 °C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H(12-x) suggests the H poor B12H(12-x) pseudo-icosahedra remain isolated and are not polymerized. Li2B12H(12-x) is persistent to at least 600 °C under vacuum, with no LiH formation observable and only a ca. d = 7.00 Å halo remaining. By 650 °C, Li2B12H(12-x) is finally decomposed, and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the β- and γ-Li2B12H12 polymorphs and substoichiometric Li2B12H(12-x).

A simple mechanical milling and annealing process has been used to synthesize CaNi5-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi5 phase. Secondary phases, including Ca2Ni7 and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi5 phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi5. Fairly flat plateau regions are retained for all compositions except the CaNi4.8Mo0.2 composition where a Ca2Ni7 phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi4.9Mo0.1 alloy. CaNi4.9Al0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi4.8Mo0.2 composition.

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.

Recently, there has been increasing interest in thermal energy storage (TES) systems for concentrated solar power (CSP) plants, which allow for continuous operation when sunlight is unavailable. Thermochemical energy storage materials have the advantage of much higher energy densities than latent or sensible heat materials. Furthermore, thermochemical energy storage systems based on metal hydrides have been gaining great interest for having the advantage of higher energy densities, better reversibility, and high enthalpies. However, in order to achieve higher efficiencies desired of a thermal storage system by the US Department of Energy, the system is required to operate at temperatures >600 °C. Operation at temperatures >600 °C presents challenges including material selection, hydrogen embrittlement and permeation of containment vessels, appropriate selection of heat transfer fluids, and cost. Herein, the technical difficulties and proposed solutions associated with the use of metal hydrides as TES materials in CSP applications are discussed and evaluated.

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.

Lithium hydride destabilised with aluminium, LiH–Al (1 : 1 mole ratio) was systematically studied and its suitability as a thermal energy storage system in Concentrating Solar Power (CSP) applications was assessed. Pressure composition isotherms (PCI) measured between 506 °C and 652 °C were conducted to investigate the thermodynamics of H2 release. Above the peritectic temperature (596 °C) of LiAl, PCI measurements were not consistently reproducible, possibly due to the presence of a molten phase. However, below 596 °C, the hydrogen desorption enthalpy and entropy of LiH–Al was ΔHdes = 96.8 kJ (mol H2)−1 and ΔSdes = 114.3 J (K mol H2)−1, respectively LiH(s) at 956 °C, ΔHdes = 133.0 kJ (mol H2)−1 and ΔSdes = 110.0 J (K mol H2)−1. Compared to pure LiH, the Li–Al–H system has a reduced operating temperature (1 bar H2 pressure at T ∼ 574 °C) that, combined with favourable attributes such as high reversibility, good kinetics and negligible hysteresis, makes the Li–Al–H system a potential candidate for solar thermal energy storage applications. Compared to pure LiH, the addition of Al can reduce the cost of the raw materials by up to 44%. This cost reduction is insufficient for next generation CSP but highlights the potential to improve the properties and cost of high temperature hydrides via destabilisation.

We have studied the complex decomposition mechanism of cubic γ-Mg(BH4)2 (Ia3d, a = 15.7858(1) A) by in-situ synchrotron X-ray diffraction, temperature-programmed desorption, visual observation of the melt, and Fourier transform infrared (FTIR) spectroscopy. The decomposition and release of hydrogen proceeds through eight distinct steps, including two polymorphic transitions before melting, with a new e-Mg(BH4)2 phase at ca. 150 °C. After melting, strong changes in sample color from yellow to brown to gray are consistent with the unknown Mg–B–H phase(s) (that diffract with high d-spacing halos) in the sample changing from an average composition of MgB2H5.3 at 325 °C, to MgB2.9H3.2 at 350 °C, and to MgB4.0H3.7 by 450 °C. From 350 to 450 °C, the crystalline Mg proportion increases. No combination of previously assigned anionic BnHm species (including MgB12H12 and Mg(B3H8)2) can account for the average composition of the unknown proportion of the sample. This is supported by FTIR spectra showing an absence o...

CaNi5–based alloys have been synthesized by mechanical alloying followed by isothermal annealing. The formation of the CaNi5 structure occurred when the milled powders were heated at 800 °C under vacuum for 3 h. The abundance of CaNi5 phase in the alloys ranges from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi5 whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption plateau regions are narrowed compared to those of pure CaNi5. Substitution of Zr into the Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where a flat plateau is maintained. The reaction enthalpy ΔH for both absorption and desorption are directly proportional to the unit cell volume of the alloys. The hydrogen storage capacity of all alloys rapidly decays for the first 50 cycles at 85 °C followed by a more gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200 cycles is in the range of 65–75% of the initial capacity.

The MgH2 + 0.02Ti-additive system (additives = 35 nm Ti, 50 nm TiB2, 40 nm TiC

Increased reliance on solar energy conversion technologies will necessarily constitute a major plank of any forward global energy supply strategy. It is possible that solar photovoltaic (PV) technology and concentrating solar thermal (CST) power technology will play roughly equal, but complementary roles by 2050. The ability to increase reliance on CST power technology during this period, however, will be constrained by a number of factors: the large plant sizes dictated by economies of scale, the high associated transmission infrastructure investment cost, and the limitations of current thermal energy storage technologies. Thus, solar technology's main midterm role is seen to be as hybrid solar thermal power plant. The development of low-cost, high-temperature, high-energy density thermal energy storage systems is needed to enable CST plants to be dispatchable and accelerate the deployment of this technology. Thermochemical storage has the best potential to achieve these energy storage requirements and a brief overview of thermochemical energy storage options for CST plants points to high-temperature metal-hydride thermochemical heat energy storage systems. Hydrogen storage systems offer the highest energy storage capacity per volume and are therefore the most likely candidates for achieving the goal of fully dispatchable CST plants. A number of high-temperature metal-hydride thermochemical solar energy storage systems have been proposed and a small number of these systems are currently being investigated and developed. A key component of this work is matching the thermochemical metal-hydride system with a suitable “low-temperature” hydrogen storage material to produce systems that are self-regulating. A summary of the development status of these systems suggests that, despite the technical challenges associated with high-temperature thermochemical energy storage systems, their potential advantages are now seeing development occurring. Although in the early stages, their commercialisation could be fast tracked.

Magnesium silicide (Mg2Si) has attracted interest as a hydrogen storage material due to favorable thermodynamics (ΔHdesorption = 36 kJ/mol H2) for room temperature operation. To date, direct hydriding of Mg2Si under hydrogen gas to form MgH2 and Si has only been attempted at low pressure and has been hindered by poor kinetics of absorption. In this paper we study the dehydrogenation reaction with in-situ neutron powder diffraction and present results of our attempts to hydrogenate Mg2Si under both hydrogen and deuterium gas up to temperatures of 350 °C and pressures of 1850 bar. Even under these extreme absorption conditions Mg2Si does not absorb any measureable quantity of hydrogen or deuterium.

The Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H2, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H2 and −99.8 ± 0.2 J/mol H2/K for absorption and 25.4 ± 0.1 kJ/mol H2 and 108.5 ± 0.2 J/mol H2/K for desorption, indicating the presence of a significant hysteresis effect.

Ordered microporous carbons containing dispersed platinum nanoparticles were fabricated and chosen as suitable models to investigate micro-structure development and hydrogen transport properties of zeolite-templated carbons. X-ray photoelectron spectroscopy analysis revealed that the enhanced heat of adsorption is related to the narrow micro-channels templated from the zeolite and the presence of certain CO groups on the carbon. The lack of a well-defined and intense rotational transition line and the persistent broad H2 recoil spectrum in neutron scattering results suggests a distribution of binding sites. Most interestingly, hydrogen diffusion occurs on two time scales, consisting of a fast liquid-like jump diffusion on the timescale of picoseconds along with an even faster bulk-like diffusion. The liquid-like motion is characterized by a diffusion constant of (2.1 ± 0.3) × 10−8 m2/s with an activation energy of ca. 77 K; both values indicate somewhat lower mobility than similar dynamics of H2 on nanotubes, activated carbon XC-72, or Grafoil, yet greater mobility than that of bulk liquid. These unusual characteristics for hydrogen in carbons are believed to arise from the network of narrow pores in this zeolite-templated image of the zeolite. In fact, the diffusion constants of the templated carbons are extremely similar to those measured for zeolite 13X.

Ball-milled sodium amide and magnesium hydride (NaNH2:MgH2 = 1:1 molar ratio) desorbs 3.3 wt % of hydrogen between 70 and 335 °C with three desorption events. X-ray diffraction indicates that the hydrogen desorption is associated with two unidentified magnesium-containing phases. Fourier transform infrared spectroscopy indicates that these two phases correspond to an imide and nitride, respectively. Analysis of the desorption products shows a large excess of NaH and MgH2 and that optimizing the starting reagents will increase both the kinetics and the amount of hydrogen desorbed from the system below 165 °C.

Magnesium hydride nanoparticles were synthesized within a carbon aerogel (CA) scaffold using a dibutylmagnesium precursor. The synthesis reaction was tracked using small-angle X-ray scattering (SAXS) to analyze the structural evolution during MgH2 formation. The CA/MgH2 composite was also investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to provide a better representation of the physical system. The CA has a large quantity of 2 nm pores as shown by nitrogen adsorption data. Both SAXS and TEM investigations confirm that MgH2 does form within the 2 nm pores but XRD proves that there is also a significant quantity of larger MgH2 particles within the system. Variations between hydrogen desorption isotherms from the CA/MgH2 composite and bulk MgH2 are detected that are indicative of changes in the decomposition properties of the small fraction of 2 nm MgH2 nanoparticles within the CA/MgH2 composite, changes which match theoretical predictions.

Carbon xerogels were prepared from the sol-gel polymerisation of resorcinol with furfural followed by carbonisation and activation, and their hydrogen storage properties were studied. Acetic acid was used as a catalyst to adjust the initial pH value of the resorcinol-furfural solution. The acetic acid facilitates the condensation reaction by reducing the gelation time dramatically from a few days to several hours. The effect of precursor acidity on the final gels are chemically distinguishable (by 13C CPMAS NMR) from the acetic acid catalysed organic gels compared with uncatalysed gel. The connectivity of the primary carbon particles and particle size are related to the degree of acidity, as were found from SEM photographs. The pore structures were studied by small angle X-ray scattering and nitrogen adsorption techniques after carbonisation and activation in order to determine the effect of precursor pH variables on the resulting samples. A catalysed carbon xerogel prepared with pH ∼ 4.8 has a surface area of 1924 ± 26 m2 g−1, a micropore volume of 0.86 ± 0.08 cm3 g−1 and a hydrogen uptake of 4.65 wt.% at 77 K and 3.9 MPa, showing it to be a promising material for hydrogen storage.

Carbon aerogels doped with nanoscaled Co particles were prepared by first coating activated carbon aerogels using a wet-thin layer coating process. The resulting metal-doped carbon aerogels had a higher surface area (∼1667 m2 g−1) and larger micropore volume (∼0.6 cm3 g−1) than metal-doped carbon aerogels synthesised using other methods suggesting their usefulness in catalytic applications. The hydrogen adsorption behaviour of cobalt doped carbon aerogel was evaluated, displaying a high ∼4.38 wt.% H2 uptake under 4.6 MPa at −196 °C. The hydrogen uptake capacity with respect to unit surface area was greater than for pure carbon aerogel and resulted in ∼1.3 H2 (wt. %) per 500 m2 g−1. However, the total hydrogen uptake was slightly reduced as compared to pure carbon aerogel due to a small reduction in surface area associated with cobalt doping. The improved adsorption per unit surface area suggests that there is a stronger interaction between the hydrogen molecules and the cobalt doped carbon aerogel than for pure carbon aerogel.

A mechanochemical synthesis process has been used to synthesise alane (AlH 3 ) nanoparticles. The alane is synthesised via a chemical reaction between lithium alanate (LiAlH 4 ) and aluminium chloride (AlCl 3 ) at room temperature within a ball mill and at 77 K within a cryogenic mill. The reaction product formed consists of alane nanoparticles embedded within a lithium chloride (LiCl) by-product phase. The LiCl is washed with a solvent resulting in alane nanoparticles which are separated from the by-product phase but are kinetically stabilised by an amorphous particle surface layer. The synthesis of a particular alane structural phase is largely dependent on the milling conditions and two major phases (α, α′) as well as two minor phases (β, γ) have been identified. Ball milling at room temperature can also provide enough energy to allow alane to release hydrogen gas and form aluminium metal nanoparticles. A comparison between XRD and hydrogen desorption results suggest a non-crystalline AlH 3 phase is present in the synthesised samples.

A number of MCM-41 samples were produced with different ageing times and zinc dopant levels. Hydrogen adsorption experiments performed at 77 K on pure MCM-41 samples showed a maximum excess wt% uptake of 2.01%. Samples were doped with Zn in the Si:Zn ratio of 50:1, 25:1 and 10:1. These samples showed an increase in d -spacing in their XRD patterns, suggestive of Zn incorporation into the structure but resulting in a decrease in long range pore ordering and surface area with increasing Zn content. Two of the three doped samples showed an increase in H 2 adsorbed per unit surface area suggestive of increased interaction strength. However, definitive confirmation is difficult due to degradation of the pore structure and surface area. Adsorption at room temperature was minimal, being 0.1 wt% or below.

The hydrogen uptake characteristics of a mechanically milled Ti–Mg–Ni sample processed in accord with that of Lomness et al. [Hydrogen uptake characteristics of mechanically alloyed mixtures of Ti–Mg–Ni. Int J Hydrogen Energy 2002; 27: 915–20] has been shown to rapidly absorb 2.5 wt % of hydrogen at 296 K . This is significantly less than the 11 wt % presented by Lomness et al. but occurs at a lower temperature. X-ray diffraction shows that our ball-milled sample comprises low crystallinity elemental Ti, Mg and Ni which converts to Ti 2 Ni , Ti and Mg during temperature cycling. Examination of the hydrided sample shows the presence of Ti 2 NiH x ( x > 0.5 ) , TiH y ( 0.7 y 2 ) and MgH 2 . Unidentified minor phases are also present.