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Grignard reagents can provide a simple path to generate, through their thermal decomposition, magnesium (Mg) and/or its hydride (MgH2). However, existing compounds lack the ability to lead to “adequate” MgH2 structures to enable effective hydrogen storage. Herein, we report on the possibility to tune Grignard reagent's chemical structure, i.e. number of β-hydrogen, and the activation energy for their thermal decomposition to lead to Mg/MgH2. For example, di-tert-butylmagnesium with nine β-hydrogen was able to decompose at a very low temperature of 167 °C to generate MgH2/Mg, which is 100 °C lower than the temperature required to generate MgH2 from di-n-butylmagnesium, i.e. the only compound known to date. More remarkably, the MgH2 synthesized from the di-tert-butylmagnesium precursor was able to release hydrogen from 100 °C. These promising hydrogen storage properties are credited to the formation of the metastable γ-MgH2 phase, which is believed to result from the structural defects generated upon the thermal decomposition of di-tert-butylmagnesium.

Borohydride (LiBH4, NaBH4 and KBH4) nanoparticles were successfully synthesized by a modified ternary anti-precipitation method. A co-solvent was introduced to facilitate the formation of a microemulsion and modify the metastable zone to favour the precipitation of borohydride particles. In this process, the stabilising surfactant's carbon chain length was found to provide an additional mean to further tune particle size due to the resulting steric hindrance. The method reported here finally provides an effective and simple mean to prepare nanosized borohydride particles for hydrogen storage purposes, while minimising the amount of stabilising surfactant that can contaminate the hydrogen release.

NaBH4 is a promising material for hydrogen storage because of its high hydrogen density (10.8 mass %). However, the sluggish hydrogen kinetics and poor thermodynamics of the dehydrogenation process...

Sodium borohydride (NaBH4) is a promising hydrogen storage material due to its potential for reversible hydrogen storage with high hydrogen capacity (10.8 mass%). However, the temperature at which hydrogen can be released from NaBH4 is too high (> 500 °C) for practical use and the reversible release of hydrogen is only feasible under severe conditions of temperature and pressure. Nanosizing is an effective strategy to improve the hydrogen properties of NaBH4. In this context, control over the particle size of NaBH4 has become important. Herein, we report on a solvent evaporation method for the synthesis of isolated NaBH4 nanoparticles with varying mean particle sizes and morphologies. In particular, the effects of surfactants’ carbon chain lengths, their steric hindrance and functional groups in facilitating the stabilisation of NaBH4 nanoparticles of various sizes and shapes were investigated. Longer linear carbon chains bearing hard type functional groups were found to favour the growth of smaller NaBH4 nanoparticles.

Polymer Electrolyte Membrane Fuel Cells (PEMFC) are often viewed as enablers of decarbonised energy systems since they transform hydrogen directly into electricity with water as the main by-product. The attraction for fuel cells is also in their versatility because they can be implemented across a wide range of applications from microelectronics to large scale power generation. Herein, we review recent progress on the design and fabrication of PEMFC with a special focus on their air-breathing planar configuration as this extends the possibility of PEMFC to thin and flexible designs. To date, the deployment of planar PEMFC highly depends upon scientific progress and technological solutions for cost reduction and long-term durability including the development of better proton conducting membranes and platinum-free catalysts to drive the oxygen reduction and hydrogen oxidation reactions efficiently. Long term durability is another challenge that can be addressed through the advancement of inexpensive and lightweight current collectors and highly efficient gas diffusion electrodes for better distribution of the reactants while maintaining an optimum hydration of the proton conducting membrane. Innovative fabrication methods for the various components of planar PEMFC as well as effective stack design and assembly are also critical for efficiency maximization, reproducibility and overall cost reduction.

Borohydrides are promising candidates for hydrogen storage and generation assuming these can be tuned to reversibly deliver hydrogen under practical conditions. Nanostructuring of borohydrides has the potential to enable such control but this has remained elusive due to the lack of approaches to effectively prepare and stabilize borohydride nanostructures. Herein, a simple and straightforward method to assemble NaBH4, as a model borohydride, into sphere, cube, and bar‐like shapes using tetrabutylammonium bromide (TBAB), octadecylamine (ODA), and tridecanoic acid (TDA), respectively, is demonstrated. More importantly, the shape of the NaBH4 nanostructures could be tuned from one morphology to another by simply adjusting the ratio of surfactants and this is found to lead to distinct hydrogen properties. In particular, remarkable shifts in the melting points and hydrogen desorption temperatures are observed depending on the NaBH4 morphologies, i.e., spheres, cubes, or bars. For example, for the NaBH4 spheres, hydrogen release starts at ≈50 °C compared to the >500 °C of bulk NaBH4 without any transition metal dopant or catalyst. Observation of such structure‐dependent relationships finally enables new avenues to deliberately tune borohydrides toward nanostructures with specific hydrogen properties otherwise difficult to control.

To date, hydrogen generation and storage are two separated processes. We report on a new concept where photocatalytically generated hydrogen is simultaneously stored in-situ within the material photo-generating hydrogen. To this aim, we successfully synthesised a “forest” of vertically aligned TiO2 nanotubes decorated with Pd nanoparticles acting as the hydrogen store. Upon illumination of TiO2, hydrogen was effectively generated and full storage of hydrogen within the Pd nanostructures was achieved within 100 min. This result demonstrates new avenues on the possibility of designing hybrid nanostructures for the effective use of hydrogen as an energy vector.

Titanium-iron (TiFe) is known to be a low-cost alloy that can be reactivated to nearly full hydrogen storage capacity after oxidation. However, this reactivation requires multiple heat treatments at high temperatures under vacuum even upon partial substitution of Fe with a small amount of manganese to form TiFe0.85Mn0.15. This process is cumbersome in the context of practical applications, and better strategies for the facile activation of TiFe are required. Herein, we report on an improvement in the reactivation strategy of TiFe0.85Mn0.15 by using a single heat treatment process after air oxidation of the alloy. After full oxidation of the material under air for 2 h, reactivation was achieved in a single step by heating the alloy to 300 °C under hydrogen pressure. During that process, the reduction of Fe was believed to lead to new reactive paths at the surface of the alloy facilitating the permeation of hydrogen. As a result, full cycling capability of the material was quickly re-gained, and the hydrogen capacity loss was of only 0.1 mass%.

AlH 3 (alane) is considered as an ideal one-off compact hydrogen storage material thanks to its high storage capacity and moderate decomposition temperature. However, application has been hindered by the lack of direct hydrogen reversibility, i.e. regeneration of AlH 3 directly from the reaction of spent Al with hydrogen under mild conditions. Herein, we investigated the effect of nanoconfinement on the properties of AlH 3 and the potential of this approach toward the direct release and uptake of hydrogen by nanosized Al. AlH 3 was confined within the mesopores of a high surface area graphite (HSAG). The decomposition of the nanoconfined AlH 3 was detected 50 °C lower than the bulk, showing some effect of nanoconfinement on the overall temperature for hydrogen release. Some hydrogen uptake was also observed at 150 °C and 60 bar hydrogen pressure, and this indicates that reversible release of hydrogen with alane under moderate conditions of pressure and temperature may be possible.

Hydrogen has been long known to provide a solution toward clean energy systems. With this notion, many efforts have been made to find new ways of storing hydrogen. As a result, decades of studies has led to a wide range of hydrides that can store hydrogen in a solid form. Applications of these solid-state hydrides are well-suited to stationary applications. However, the main challenge arises in making the selection of the Metal Hydrides (MH) that are best suited to meet application requirements. Herein, we discuss the current state-of-art in controlling the properties of room temperature (RT) hydrides suitable for stationary application and their long term behavior in addition to initial activation, their limitations and emerging trends to design better storage materials. The hydrogen storage properties and synthesis methods to alter the properties of these MH are discussed including the emerging approach of high-entropy alloys. In addition, the integration of intermetallic hydrides in vessels, their operation with fuel cells and their use as thermal storage is reviewed.

As the world is shifting toward an increased reliance on renewable energy, the need for effective and robust energy carriers is more than pressing. In this context, hydrogen has a key role to play. However, the storage of hydrogen in a cost-effective, safe, and compact manner is a bottleneck to the future hydrogen economy primarily due to the lack of incentives and technical difficulties in storing hydrogen. So far, materials capable of ambient hydrogen uptake/release have a low storage capacity while high-capacity hydrides that may offer more compact alternatives still rely on very high hydrogen release temperatures. This chapter summarizes the current potential of the solid-state hydrogen technology in the renewable energy sector and potential paths to engineer the next generation of materials along with their hydrogen thermodynamic and kinetic paths.

Hydrogen is often touted as the fuel of the future, but hydrogen is already an important feedstock for the chemical industry. This review highlights current means for hydrogen production and use, and the importance of progressing R&D along key technologies and policies to drive a cost reduction in renewable hydrogen production and enable the transition of chemical manufacturing toward green hydrogen as a feedstock and fuel. The chemical industry is at the core of what is considered a modern economy. It provides commodities and important materials, e.g., fertilizers, synthetic textiles, and drug precursors, supporting economies and more broadly our needs. The chemical sector is to become the major driver for oil production by 2030 as it entirely relies on sufficient oil supply. In this respect, renewable hydrogen has an important role to play beyond its use in the transport sector. Hydrogen not only has three times the energy density of natural gas and using hydrogen as a fuel could help decarbonize the entire chemical manufacturing, but also the use of green hydrogen as an essential reactant at the basis of many chemical products could facilitate the convergence toward virtuous circles. Enabling the production of green hydrogen at cost could not only enable new opportunities but also strengthen economies through a localized production and use of hydrogen. Herein, existing technologies for the production of renewable hydrogen including biomass and water electrolysis, and methods for the effective storage of hydrogen are reviewed with an emphasis on the need for mitigation strategies to enable such a transition.

Titanium-iron (TiFe) is known to be highly sensitive to oxidation, and this impacts its use in hydrogen storage applications. To investigate the ability to control the surface oxidation of TiFe-based alloys, TiFe0.85Mn0.15 was used as a base alloy due to its ease of activation and the addition of Cr investigated to facilitate its re-activation upon oxidation. Upon the formation of TiFe0.85Mn0.15Crx (x = 0.15 and 0.30) an improvement in reactivation was observed. After exposure of TiFe0.85Mn0.15Cr0.30 to air for 2 h, the alloy could be re-activated at 150 °C under a 3 MPa hydrogen pressure. In comparison, a temperature of 300 °C under the same hydrogen pressure was needed to re-activate TiFe0.85Mn0.15. This indicates that Cr addition provided to some extent resistance against deactivation. This is also reflected in the HP-DSC measurements where a partial oxide reduction was observed at 100 °C for the oxidised TiFe0.85Mn0.15Cr0.30 alloy as opposed to the 220 °C of TiFe0.85Mn0.15. The partially reduced species acted as catalytic sites for hydrogen activation.

The stability of magnesium and its hydride (Mg/MgH2) against moisture and oxygen can be improved by forming a multilayered structure with a protective polymeric layer. Herein, we report for the first time the fabrication of such a multilayered structure via an electrochemical deposition method consisting of the successive depositions of Mg on electropolymerised polypyrrole (PPy) films. The thickness of the PPy and the Mg layers was ∼1 μm, which is larger than the Mg/Pd multilayers prepared via physical deposition methods, owing to the higher surface roughness of electropolymerised films. However, such films displayed remarkable hydrogen storage properties. Hydrogenation of the PPy/Mg film was achieved at 100 °C and hydrogen release started from 125 °C with a peak at ∼215 °C. When covered by a second PPy layer, the hydrogen desorption temperature increased slightly to 230 °C. These hydrogen absorption and desorption temperatures are significantly lower than that of pristine Mg/MgH2 micron sized powders and this was achieved without any additional catalyst. Furthermore, such hydrogenated PPy/Mg/PPy film structures were found to be stable in air even after one week.

Precipitation of a LiBH4 solution into an antisolvent led to formation of nanoparticles in the size range of 2 to 18 nm. By direct deposition of these nanoparticles onto a nickel(II) phthalocyanine substrate, LiBH4 was destabilized and the hydrogen release temperature was dramatically reduced to 350 °C through a single step decomposition. Remarkably, upon hydrogen release and uptakes, the morphology of the material evolved to single crystal “plates”-like particles and a reversible hydrogen storage capacity of 3.2 mass% at 350 °C under 6 MPa H2 pressure was observed. As evident by X-ray photoelectron spectroscopy analysis, such an enhancement is believed to result from the effective electron transfer interplay between LiBH4, LiH, B, and the nickel(II) phthalocyanine, enabling a destabilization of LiBH4 and the facile rehydrogenation of LiH and B into LiBH4. This study thus reveals a novel approach to destabilize LiBH4 by the use of an “electron active” substrate.

Aluminium hydride (AlH3) is a promising hydrogen storage material due to its competitive hydrogen storage density and moderate decomposition temperature. However, there is no convenient way to prepare/regenerate AlH3 from (spent) Al by direct hydrogenation. Herein, we report on a novel approach to generate AlH3 from the decomposition of triethylaluminium (Et3Al) under mild hydrogen pressures (10 MPa) with the use of surfactants. With tetraoctylammonium bromide (TOAB), the synthesis led to the formation of nanosized AlH3 with the known α phase, and these nanoparticles released hydrogen from 40 °C instead of the 125 °C observed with bulk α-AlH3. However, when tetrabutylammonium bromide (TBAB) was used instead of TOAB, larger nanoparticles believed to be related to the formation of β-AlH3 were obtained, and these decomposed through a single exothermic process. Despite the possibility to form α-AlH3 under low conditions of temperature (180 °C) and pressure (10 MPa), TOAB stabilised AlH3 was found to be irreversible when subjected to hydrogen cycling at 150 °C and 7 MPa hydrogen pressure.

The concept of light activation for triggering hydrogen release or uptake in hydrogen storage materials was investigated with the aid of gold (Au) nanoparticles dispersed at the surface of typical hydrides including magnesium hydride (MgH2 ), lithium hydride (LiH) and sodium alanate (NaAlH4 ). Upon Xe lamp illumination, the overall temperature of the materials reached ca. 100 °C owing to the plasmonic heating effect of the Au nanoparticles. Direct-heat-driven hydrogen storage at the same temperature with a conventional electrical furnace was found to be less effective with a smaller fraction of hydrogen released from Au/LiH and no phase conversion for Au/NaAlH4 or for Au/Mg under hydrogen pressure. The better efficiency of the observed light-driven hydrogen storage is attributed to the higher temperature in the vicinity of the Au nanoparticles owing to their plasmonic effect. With further improvements, light-activated hydrogen storage could lead to an effective approach for hydrogen uptake and release from hydrides at low temperatures.

2,11,20,29-Tetra-tert-butyl-2,3-naphthalocyanine (TTBNc) was used as an alternative host to support magnesium (Mg) nanoparticles. After deposition and decomposition of dibutylmagnesium on TTBNc, Mg nanoparticles of around 4 nm supported on TTBNc were observed by TEM. These TTBNc stabilized Mg nanoparticles were found to absorb hydrogen at 100 °C and release hydrogen from 75 °C. The Mg-TTBNc material showed good hydrogen cycling properties and structural stability. Kinetic measurements showed fast hydrogen absorption within 2 min at 150 °C. The hydrogen desorption kinetics were slower at the same temperature but faster at 250 °C with 80% of the hydrogen desorbed within 1 h. Enthalpy and entropy for hydrogen uptake and release in Mg-TTBNc determined from PCT measurements were found to be of 52.7 ± 4.9 kJ mol−1 H2 and 107.8 ± 9.4 J K−1 mol−1 H2, respectively. These values are much lower than those of bulk Mg.

Magnesium nanoparticles with the mean particle size of ∼18 nm were synthesized through the thermal decomposition of di-n-butylmagnesium using Ni nanobelts as a catalyst and support with the aim of improving the hydrogen storage properties of magnesium by coupling nanosizing and catalysis. Full hydrogenation of the magnesium nanoparticles was achieved at a low temperature of 100 °C, and hydrogen release occurred at ∼230 °C. The material showed fast hydrogen absorption with good structural stability during cycling. Hydrogen desorption occurred in 200 min at 250 °C. These enhanced hydrogen storage properties were assigned to a lower activation energy (69.2 ± 2.5 kJ mol–1 H2), and the remarkably low enthalpy of hydrogenation (34.4 ± 5.4 kJ mol–1 H2) although partially compensated by a reduced entropy of 76.9 ±5.4 J K–1 mol–1 H2. The improvement of both kinetics and thermodynamics is believed to result from the coupling effects of nanosizing and catalysis.

Lightweight complex hydrides, M(BH4)n (M = Li, Na, Mg, and Ca; n = 1 for Li and Na, n = 2 for Mg and Ca), are believed to be promising hydrogen storage materials with extreme high hydrogen density up to 18.5 mass %. However, these materials suffer high dehydrogenation temperature, melting, and reversibility problems, which exclude them from the list of practical hydrogen storage systems. Herein, borohydrides (M(BH4)n–Ti, with M = M1 or M2 and n = 1 or 2), were modified with TiCl3 via a wet chemistry approach, and in some cases this led to the formation of solvent-stabilized nanoparticles. As a result of TiCl3 modification, the melting before hydrogen release was suppressed as evidenced by DSC and thermal microscopy observations. Furthermore, the hydrogen release temperature of M(BH4)n–Ti was significantly reduced. For example, the dehydrogenation temperature of NaBH4–Ti was reduced from 570 to 120 °C. Ti modification was also found to improve to some extent the reversibility of the doped materials. In par...

For achieving an economy based on renewable energy sources, effective solutions toward energy storage remain the main challenge. To date, there is no efficient systems to store electricity in large amounts. A promising solution is to accumulate energy in the form of hydrogen, which can then be conveniently stored and transported. However, compared to the volumetric energy density of fossil fuels, current technologies relying on hydrogen compression or liquefaction have major disadvantages, including low energy density and safety issues. The use of light materials forming hydrides could provide an alternative way to stock hydrogen with high volumetric energy densities. Herein, we present recent developments in the research for magnesium/graphene hybrid materials and their hydrogen-storage properties.

Effective solutions for the storage of energy are paramount to enable the transition toward decarbonized energy systems relying on widely abundant and recyclable resources. In this context, the use of hydrogen as the universal clean energy vector has always appeared as a prominent solution. The attraction for hydrogen resides in its high energy density, versatile means of production, and flexible end-use. Hydrogen can be utilised across a range of stationary and portable applications but the Holy Grail remains the development of high-density hydride materials for use in vehicles. In particular, magnesium and its hydride compounds provide a safe and efficient way to store hydrogen with high density; however these hydrides suffer from significant drawbacks in terms of kinetics and operating temperatures for hydrogen uptake/release. The properties of hydride materials can be tailored to meet set targets for practical operation, but this strongly depends upon the very early stages of the synthetic approaches utilised and their effectiveness in leading to the desired performances. Herein, with a special focus on magnesium based materials and the current understanding of their interaction with hydrogen, the various synthetic methods developed so far are reviewed and compared with the aim of guiding future developments toward practical hydrogen storage materials. The review is then extended to complex hydrides of magnesium with potentially more favourable thermodynamics.

Water, through a metal–water reaction, is an appealing candidate to store and release hydrogen (H2), in particular as a portable, easy to use energy storage source. However, the release of hydrogen from the reaction of water with light metals can be violent or be inhibited by the formation of an oxide layer as is the case with magnesium (Mg). For this reason, we studied the use of graphene powder (Gr) as a nano-support to control the reactivity of Mg in water. Nanofilms and nanoparticles of Mg have been grown directly on graphene sheets inducing an increase of Mg surface area and the formation of metallic nanostructures that enhance the responsiveness of metal in water. We surprisingly observed that the water–Mg/Gr reaction happens at room temperature with impressive H2 production (the Mg/Gr hydrogen gravimetric density is 3%). Therefore, the hydrogen generated from water has been successfully used in a fuel cell proving that Mg/Gr is a promising material for on-demand H2 generation.

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.

Lithium aluminium hydride (LiAlH 4 ) is a promising hydrogen storage material with a storage capacity of 10.6 mass % H 2 . However, its practical use is hampered by the lack of direct rehydrogenation routes. In this study, we report on the confinement of LiAlH 4 into the nanoporosity of a high surface area graphite resulting in a remarkable improvement of its hydrogen storage properties. Nanoconfined LiAlH 4 started hydrogen desorption near 135 °C and after full dehydrogenation at 300 °C limited rehydrogenation was observed at the same temperature and 7 MPa of hydrogen pressure. Rehydrogenation took place through the formation of Li 3 AlH 6 with some limited rehydrogenation back to LiAlH 4 indicating the existence of different (de)hydrogenation paths upon nanoconfinement as compared to the known dehydrogenation path of bulk LiAlH 4 .

Complex borohydrides of calcium (Ca(BH4)2) and magnesium (Mg(BH4)2) have gained increasing attention as promising hydrogen storage materials due to their high volumetric and gravimetric density. However, these borohydrides suffer from high desorption temperatures and poor hydrogen reversibility. In this study, Ca(BH4)2 and Mg(BH4)2 were confined within the porosity of Cu2S hollow spheres. Upon confinement, hydrogen was released at a temperature as low as 50 °C and full hydrogen release was completed by 300 °C. Upon cycling at 300 °C and 6 MPa H2 pressure, some hydrogen uptake of 0.9 and 0.5 mass% was achieved for these composite materials, i.e. Ca(BH4)2@Cu2S and Mg(BH4)2@Cu2S, respectively. Partial rehydrogenation was confirmed by FTIR proving the regeneration of borohydride phases. The nature of these improvements was studied through comparison to the physical mixture of borohydrides and Cu2S hollow spheres, and the results confirmed the effect of nanoconfinement in facilitating the destabilisation of Ca(BH4)2 and Mg(BH4)2. These results confirm that a well-designed nanoporous structure confining the decomposition products of borohydrides during hydrogen release could potentially lead to high reversible hydrogen storage material.

Upon hydrogen absorption, magnesium (Mg) spontaneously converts to the thermodynamically favoured β-MgH2 phase, but this leads to hydrogen release/uptake far from the desired low temperatures and fast kinetics. Conversion to the metastable γ-MgH2 phase should lead to improved hydrogen sorption properties. However, experimental verification of such a hypothesis remained distant. In this work, we report the electrochemical synthesis of nanosized Mg leading to the formation of a mixed γ/β hydride phase upon hydrogen absorption with a high γ-MgH2 content (29.6%). More remarkably, full release of hydrogen from these phases occurred at 200 °C only and upon hydrogen reabsorption at 100 °C, the γ/β-MgH2 mixture was restored. It was thus possible to determine for the first time the effect of γ-MgH2 on the kinetic and thermodynamic properties of the Mg/H2 reaction. The presence of γ-MgH2 was found to lead to a significant reduction of the apparent activation energy from 106.2 ± 4.0 to 69.1 ± 2.9 kJ mol−1 and a reduction of the Mg/H2 reaction enthalpy from 74.8 ± 1.0 to 57.7 ± 5.3 kJ mol−1 H2. However, this was accompanied by a concomitant decrease in entropy limiting the reduction of hydrogen desorption temperatures. γ-MgH2 can thus lead to improve hydrogen kinetics but to reach ambient hydrogen uptake and release ways to tune the enthalpy/entropy compensation effects have to be devised.

Sodium borohydride (NaBH4) is an attractive hydrogen carrier owing to its reactivity with water: it can generate 4 equivalents of H2 by hydrolysis (NaBH4 + 4H2O → NaB(OH)4 + 4H2). Since using NaBH4 in the solid state is the most favorable way to achieve a high gravimetric hydrogen storage capacity (theoretical maximum of 7.3 wt%), we have investigated the possibility of developing a core@shell nanocomposite (NaBH4@Ni) where a metallic nickel catalyst facilitating the hydrolysis is directly supported onto NaBH4 nanoparticles. Following our initial work on core–shell hydrides, the successful preparation of NaBH4@Ni has been confirmed by TEM, EDS, IR, XRD and XPS. During hydrolysis, the intimately combined Ni0 and NaBH4 allow the production of H2 at high rates (e.g. 6.1 L min−1 g−1 at 39 °C) when water is used in excess. After H2 generation, the spent fuel is composed of an aqueous solution of NaB(OH)4 and a nickel-based agglomerated material in the form of Ni(OH)2 as evidenced by TEM, XPS and XRD. The effective gravimetric hydrogen storage capacity of nanosized NaBH4@Ni has been optimized by adjusting the required amount of water for hydrolysis and an effective hydrogen capacity of 4.4 wt% has been achieved. This is among the best reported values.

LiH has great potential as a high capacity hydrogen storage material (12 wt.%), however its thermodynamic stability has so far precluded practical application. Temperatures near 700 °C are required for hydrogen release and uptake. Herein, we report on a novel method to realise hydrogen uptake and release under milder temperature conditions without using any catalyst or alloying. Through nanoconfinement within the pores (2–20 nm) of high surface area graphite (HSAG) LiH displayed remarkable hydrogen storage properties and was able to release 1.9 wt.% of hydrogen from 200 °C. Reversibility was also achieved under the moderate conditions of 300 °C and 6 MPa hydrogen pressure. This demonstrates that the properties of LiH are particle size dependent and thus leads to new possibilities to realise the potential of LiH as a practical high capacity hydrogen storage material.

LaNi5 nanoparticles of about 200 nm showed fast kinetics but no significant alteration in thermodynamics possibly because these nanoparticles were too large to cause detectable thermodynamic changes. For the first time, we successfully synthesised highly dispersed LaNi5 nanoparticles on carbon (nanoLaNi5/C) through a precipitation-reduction method. Carbon-supported LaNi5 nanoparticles have much smaller particle size (26 ± 8 nm) as compared to unsupported LaNi5 nanoparticles (214 ± 75 nm) (nanoLaNi5) prepared by the same method and bulk LaNi5 (35 ± 17 μm). The hydrogen storage properties of these three materials were investigated and correlated to particle sizes. The rate for hydrogen uptake and release was faster for the smallest particles, i.e. nanoLaNi5/C > nanoLaNi5 > bulk LaNi5. The highly dispersed LaNi5 on carbon reversibly absorbed 1.1 mass% hydrogen in less than one minute. More importantly, both enthalpy and entropy significantly decreased upon reduction of particle size down to 26 nm. The enthalpy of nanoLaNi5/C decreased by 5 kJ mol−1 H2 for absorption and 8 kJ mol−1 H2 for desorption, while the entropy decreased by 15 J K−1 mol−1 H2 for absorption and 19 J K−1 mol−1 H2 for desorption. Unlike magnesium hydride, nanosizing LaNi5 led to the thermodynamic stabilisation of its hydride.

Lithium is a material of choice for batteries, but it has also the potential to store energy with high density as a hydrogen storage material, i.e. via the formation of its hydride (LiH). However, the high thermodynamic stability of LiH has so far precluded the use of lithium as an effective hydrogen storage material owing the high temperature 700 °C for hydrogen release. Herein, we report on a novel method to enable the reversible storage of hydrogen with lithium under mild conditions of pressure (6 MPa) and temperature (350 °C). Through the catalytic hydrogenation of lithium, LiH particles were restricted to a few nanometres (

Magnesium hydride (MgH2) nanoparticles have been synthesized in the presence of MoS2 via a simple hydrogenolysis route, involving the decomposition of di-n-butylmagnesium. Remarkably using delaminated MoS2 instead of bulk MoS2 led to the formation of magnesium worm-like structures, which collapsed and recrystallized to some extent upon hydrogen cycling. Hydrogen release from these structures occurred at a relatively low temperature (300 °C), and MoS2 was found to effectively improve the hydrogen desorption kinetics of nanosized MgH2. It took less than 150 min to fully release hydrogen from MgH2 catalyzed by delaminated MoS2. In comparison, uncatalysed nanosized MgH2 particles required more than 300 min. Furthermore, MoS2 was found to significantly influence the thermodynamic properties of the Mg/MgH2 reaction, via the destabilization of the Mg–H bond. Compared to ball-milled MgH2, the MoS2 modified materials showed significantly higher hydrogen absorption plateau pressures (∼0.15 MPa). This demonstrates that pathways enabling the tuning of the hydrogen storage properties of nanosized MgH2 toward lower temperatures and fast kinetics may exist.

LaNi5 nanoparticles as small as 170 and 250 nm were prepared by combustion and precipitation methods followed by calcium hydride reduction, respectively. The formation of such small particles was due to the effective reduction of the nanoscale lanthanum/nickel oxide precursors at low temperatures (600 °C). LaNi5 nanoparticles were found to be stable and hydrogen cycling did not change their morphology but led to a significant decrease in crystallite size reflecting the formation of lattice defects. The reduction of particle size enhanced hydrogen kinetics with full desorption occurring in a few minutes. Particle size effects on thermodynamics were also investigated by measuring the equilibrium plateau pressure (Peq) of the LaNi5/H2 reaction at various temperatures. Nanosized LaNi5 showed a relatively large hysteresis between Peq for hydrogen absorption and desorption because of lattice defects. However, no significant changes in enthalpy and entropy were observed for these nanoparticles.

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.

As strong reducing agents, alkali metals including Li, Na and K enable the electroless synthesis of magnesium nanoparticles. In this investigation, we determined the effects of such alkali metals on the properties of magnesium at the nanoscale. The magnesium nanoparticles were successfully synthesised by directly reducing di-n-butylmagnesium with an excess amount of alkali-metal naphthalenide in order to coat individual magnesium nanoparticles within an alkali matrix. Upon heat treatment under hydrogen pressure a perovskite structure MMgH3 was formed on Na- and K-coated magnesium nanoparticles under mild conditions but was absent on the Li-coated materials leading to LiH only. These perovskite structures were found to have no significant influence on the thermodynamics of magnesium. Compared to ball-milled MgH2, these materials showed lower hydrogen absorption plateau pressure (∼0.05 MPa). However, the magnesium particles generated with potassium had fast hydrogen kinetics and low reaction enthalpy due to their small size. More remarkably these coated magnesium nanoparticles remained stable upon cycling which provides a new route to stabilise magnesium nanoparticles.

Herein, we report on a novel method for deposition of magnesium (Mg) nanoparticles at the surface of carbon materials. Through the suspension of carbon nanotubes (CNTs) in an electrolyte containing di-n-butylmagnesium as a precursor, Mg nanoparticles were effectively deposited at the surface of the CNTs as soon as these touched the working electrode. Through this process, CNTs supported Mg particles as small as 1 nm were synthesized and the distribution of the nanoparticles was found to be influenced by the concentration of the CNTs in the electrolyte. Hydrogenation of these nanoparticles at 100°C was found to lead to low temperature hydrogen release starting at 150°C, owing to shorter diffusion paths and higher hydrogen mobility in small Mg particles. However, these hydrogen properties drastically degraded as soon as the hydrogenation temperature exceeded 200°C and this may be related to the low melting temperature of ultrasmall Mg particles.

Magnesium nanoparticles as small as 7 nm have been synthesized by reducing di-n-buytlmagnesium with lithium and individually coated with elements including Co, Fe, Ni, Si, or Ti for further tuning of their thermodynamic and kinetic properties. Coating was achieved via a transmetalation process whereby precursors were directly deposited at the surface of the magnesium nanoparticles acting as a reducing agent. Studies of hydrogen storage properties showed that these coated magnesium nanoparticles were capable of absorbing hydrogen at low temperatures

Hydrogen is regarded as a promising energy carrier to substitute fossil fuels. However, storing hydrogen with high density remains a challenge. NaBH4 is a potential hydrogen storage material due to its high gravimetric hydrogen density (10.8 mass%), but the hydrogen kinetic and thermodynamic properties of NaBH4 are poor against the application needs. Nanosizing is an effective strategy to improve the hydrogen properties of NaBH4. In this context, we report on the direct synthesis of NaBH4 nanoparticles (~6&ndash, 260 nm) from the NaOCH3 precursor. The hydrogen desorption properties of such nanoparticles are reported as well as experimental conditions that lead to the synthesis of (Na2B12H12) free NaBH4 nanoparticles.

Magnesium hydride (MgH 2 ) with a range of morphologies has been synthesized via a simple hydrogenolysis route involving the decomposition of di-n-butylmagnesium. As the synthetic medium evolved from an inert atmosphere of argon to hydrogen pressure, the morphology shifted from rod like to small particles (25–170 nm). In cyclohexane, a solvent relative inert toward magnesium, smaller particles (15–50 nm) were formed. However in diethyl ether, which is more reactive toward magnesium, flakes organized in large microstructures were obtained. Remarkably in all cases β-MgH 2 was readily obtained with some residual carbon contamination. Hydrogen release from these structures occurred at a relatively low temperature (300 °C), with desorption kinetics faster or equivalent to that of ball milled magnesium. In particular, hydrogen desorption from the smallest particles of MgH 2 produced via the hydrogenolysis of di-n-butylmagnesium under hydrogen pressure or cyclohexane was impressive with the full desorption achieved in less than 10 min without any catalyst. These remarkable hydrogen storage properties are believed to result from an appropriate stabilization of the nanoparticles generated.

The kinetics of hydrogen absorption/desorption can be improved by decreasing particle size down to a few nanometres. However, the associated evolution of activation energy remains unclear. In an attempt to clarify such an evolution with respect to particle size, we electrochemically deposited Mg nanoparticles on a catalytic nickel and noncatalytic titanium substrate. At a short deposition time of 1 h, magnesium particles with a size of 68 ± 11 nm could be formed on the nickel substrate, whereas longer deposition times led to much larger particles of 421 ± 70 nm. Evaluation of the hydrogen desorption properties of the deposited magnesium nanoparticles confirmed the effectiveness of the nickel substrate in facilitating the recombination of hydrogen, but also a significant decrease in activation energy from 56.1 to 37.8 kJ·mol−1 H2 as particle size decreased from 421 ± 70 to 68 ± 11 nm. Hence, the activation energy was found to be intrinsically linked to magnesium particle size. Such a reduction in activation energy was associated with the decrease of path lengths for hydrogen diffusion at the desorbing MgH2/Mg interface. Further reduction in particle size to a few nanometres to remove any barrier for hydrogen diffusion would then leave the single nucleation and growth of the magnesium phase as the only remaining rate-limiting step, assuming that the magnesium surface can effectively catalyse the dissociation/recombination of hydrogen.

Owing to its high storage capacity (10.8 mass %), sodium borohydride (NaBH(4)) is a promising hydrogen storage material. However, the temperature for hydrogen release is high (500 °C), and reversibility of the release is unachievable under reasonable conditions. Herein, we demonstrate the potential of a novel strategy leading to high and stable hydrogen absorption/desorption cycling for NaBH(4) under mild pressure conditions (4 MPa). By an antisolvent precipitation method, the size of NaBH(4) particles was restricted to a few nanometers (30 nm), resulting in a decrease of the melting point and an initial release of hydrogen at 400 °C. Further encapsulation of these nanoparticles upon reaction of nickel chloride at their surface allowed the synthesis of a core--shell nanostructure, NaBH(4)@Ni, and this provided a route for (a) the effective nanoconfinement of the melted NaBH(4) core and its dehydrogenation products, and (b) reversibility and fast kinetics owing to short diffusion lengths, the unstable nature of nickel borohydride, and possible modification of reaction paths. Hence at 350 °C, a reversible and steady hydrogen capacity of 5 mass % was achieved for NaBH(4)@Ni; 80% of the hydrogen could be desorbed or absorbed in less than 60 min, and full capacity was reached within 5 h. To the best of our knowledge, this is the first time that such performances have been achieved with NaBH(4). This demonstrates the potential of the strategy in leading to major advancements in the design of effective hydrogen storage materials from pristine borohydrides.

The kinetics of hydrogen absorption/desorption in magnesium can be improved without any catalysis assistance and MgO was found to be more effective than the best catalyst reported so far, i.e., Nb2O5. Herein, a quantitative analysis of the hydrogen kinetics in magnesium modified with MgO was performed in order to identify possible rate controlling mechanisms. While hydrogen absorption was found to be diffusion controlled as commonly reported, hydrogen desorption evolved from nucleation and growth to an interface controlled process depending on the desorption temperature. Comparison with the effect of Nb2O5 indicates that similar rate limiting steps occur regardless of the oxide additive. These findings are reconciled by considering the tribological effect of solid oxide additives, as a correlation between oxides electronegativity and improvement in hydrogen kinetics was found. Such a correlation clearly highlights the mechanical effect of solid oxides in facilitating the grinding and stabilisation of small magnesium particles for efficient and fast hydrogen kinetics.

Hydrogen has long been proposed as a versatile energy carrier that could facilitate a sustainable energy future. For an energy economy centred around hydrogen to function, a storage method is required that is optimised for both portable and stationary applications and is compatible with existing hydrogen technologies. Storage by chemisorption in borohydride species emerges as a promising option because of the advantages of solid-state storage and the unmatched hydrogen energy densities that borohydrides attain. One of the most nuanced challenges limiting the feasibility of borohydride hydrogen storage is the irreversibility of their hydrogen storage reactions. This irreversibility has been partially attributed to the formation of stable dodecahydro-closo-dodecaborates (Mn=1,2B12H12) during the desorption of hydrogen. These dodecaborates have an interesting set of properties that are problematic in the context of borohydride decomposition but suggest a variety of useful applications when considered independently. In this review, dodecaborates are explored within the borohydride thermolysis system and beyond to present a holistic discussion of the most important roles of the dodecaborates in modern chemistry.

7 pp. Published in Journal of Alloys and Compounds Copyright © 2007 Elsevier B.V., Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH2 with and without 2 mol% Nb2O5 as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb2O5 additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics., Authors thank the financial support (project nos. HPRN-CT-2002-00208 and MRTN-CT-2006-035366) from the European Commission

This paper analyzes the improvement of MgH2 decomposition by the addition of nickel-based catalysts, mostly carbon-supported nickel catalysts. Our study shows that some experimental parameters such as the catalyst content, its composition, the method of preparation, and milling time strongly affect its activity toward MgH2 decomposition. We observe that carbon-supported nickel catalysts have superior performance than those just containing carbon and nickel physically mixed and, especially, than those containing only nickel. This emphasizes the role played by carbon and by the nickel−carbon interaction on the activity of such MgH2-based materials. A decrease, followed by differential scanning calorimetry (DSC), in the decomposition temperature of MgH2 of 150 °C can be achieved with some selected supported catalysts. Suitable selection of the catalyst preparation conditions leads to materials for which decomposition of 6.6 wt % hydrogen occurs in just 9 min at 300 °C, in comparison to 240 min required for u...

Hydrogen sorption properties were investigated for selected mixtures of MgH2 and carbon materials, including graphite, activated carbon, multi-walled carbon nanotubes (MWCNTs), carbon nanofibres (CNFs) and activated carbon fibres. The introduction of such carbon materials decreases the decomposition temperature of MgH2. The best results were achieved in the mixtures involving CNFs and MWCNTs, with metallic impurities, particularly nickel and iron, inherited from the original synthesis. The peak temperatures of MgH2 decomposition were reduced to 322 and 341 °C, respectively, compared with 360 °C for simply milled MgH2. Substantial improvement in the decomposition kinetics of MgH2 was achieved, particularly with CNF additions. The MgH2 decomposition was completed within 20 min at 300 °C with a 5 wt.% CNF addition, compared to 240 min for simply milled MgH2. The improved kinetics was maintained even after several hydriding–dehydriding cycles. X-ray diffraction and transmission electron microscopy reveal that some structural changes occur after cycling: there is an increase in particle size of the MgH2 phase, with separation between the magnesium and carbon phases, indicating a clear relationship between the decomposition temperature of MgH2 and its structure. It has also been shown that the presence of carbon materials prevents MgH2 particle growth which, in turn, enhances its decomposition.

Undesirable climate changes due to excessive anthropogenic CO2 emissions are of critical concern. Hydrogen as a clean energy carrier holds great promise in mitigating the problems. However, storing sufficient amount of hydrogen safely and practically poses large technical challenges, associated with materials properties that depend strongly on structure, chemistry and reaction path. Mechanical milling and chemical additions are effective in modifying various hydride systems. Considerable progresses have been achieved in improving thermodynamic and kinetic properties for hydrogen sorption. A final step to meet the technical challenges may rest with hybrid systems that can make use of modified physi- and chemi-sorptions, guided by computational simulations.

Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH 2 ) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb 2 O 5 when added in larger volume fraction. The effect of Nb 2 O 5 on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH 2 can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb 2 O 5 on MgH 2 sorption properties are rationalised in a new approach considering Nb 2 O 5 as a dispersing agent, which helps reduce MgH 2 particle size during milling.