Hydrogen Sorption Kinetics Research Articles

First-principles study on impact of Mo doping on the formation enthalpy and electronic structure of Li2MgN2H2 material

The influence of Mo doping on the formation enthalpy and electronic structure of Li2MgN2H2 material, as well as the specific improvement mechanism of its hydrogen storage properties, were investigated by employing the first-principles calculation method. The calculation results exhibit that when Mo is doped into the Li2MgN2H2 material, it is more inclined to take up Mg lattice site at the 8c position. For the Li2MgN2H2 material, Mo doping can effectively reduce its formation enthalpy by ca. 91.16 kJ/mol, thus resulting in the reduction of its structural stability. Moreover, when the Li2MgN2H2 material is doped with Mo, its cell volume increases, and its band gap narrows obviously. Meanwhile, the strong force between Mo and N causes the bond strengths of N–H and Li–N to weaken significantly. The aforementioned factors are beneficial to the improvement of its hydrogen sorption kinetics. This work aims to provide theoretical guidance for further development of new efficient catalysts to promote the Li–Mg–N–H material's application.

Core-shell structured Ni@C based additive for magnesium hydride system towards efficient hydrogen sorption kinetics

Magnesium hydride is a potential candidate for storing hydrogen; however, due to its slow kinetics and stable thermodynamics it is difficult to use in real-world applications. In this work, we successfully synthesized different composites of Ni@C with MgH2 to improve its kinetics. Experimental results indicate better performance of all catalyzed samples as compared to the pristine MgH2. Especially, the addition of core-shell structured Ni@C enabled MgH2 to desorb hydrogen at 252 °C, which is 56 °C lower than the milled MgH2. The apparent activation energy of dehydrogenation decreased from 109.2 ± 4.1 kJ/mol (milled MgH2) to 83.1 ± 0.3 kJ/mol (catalyzed MgH2). This kinetic enhancement can be attributed to the synergistic effects of Ni, making dissociation and recombination for hydrogen easier, and the C species, inhibiting particle agglomeration. PCT measurement showed that catalysts did not alter the process thermodynamics. In the cycle stability test of NG3:10: 2, the hydrogen storage capacity was retained at 75 % after 20th cycle and only falls by about 1.6 wt percent. In addition, XRD and SEM demonstrated that the novel species of Ni@C and Mg2Ni/Mg2NiH4 together increased the absorption and dissociation of hydrogen.

Mixed Metal Amide-Hydride Solid Solutions for Energy Storage Applications

Metal amide-hydride materials have been extensively studied for usage in energy applications, especially as solid electrolytes for all-solid-state batteries and as hydrogen storage. In specific cases, the formation of mixed metal amide hydride solid solutions promotes fast hydrogen sorption kinetics and tune the thermodynamics, allowing hydrogen absorption/desorption below 150 °C in hydrogen storage systems. In addition, these intermediates, such as Li2(BH4)(NH2) or Li4(BH4)(NH2)3 are potentially ionic conductors that can be used as solid electrolytes for solid-state batteries’ applications. In this work, a series of M-N-H solid solution structures based on mixed MNH2-MH materials of Group 1 elements (M = K, Rb, Cs and their combinations), such as KNH2-KH, KNH2-RbH, RbNH2-KH, RbNH2-CsH, RbNH2-CsNH2, etc., are reported. The results obtained by experimental determination (e.g., ex-situ XRD / in-situ synchrotron powder X-ray diffraction, IR, 1H 2D solid-state MAS NMR, QENS) and theoretical calculations (e.g., molecular dynamics calculations) confirm the formation of mixed metal amide hydride solid solutions associated with an exchange between both anionic (NH2 - and H-) and cationic species (K, Rb and Cs). These structurally disordered solid solutions exhibit high ionic conductivity compared to the pure materials, e.g., mixed RbNH2-CsNH2 shows an ionic conductivity in the range of 4×10-5 S×cm-1 at 373 K compared to that of pure RbNH2 and CsNH2 (< 10-7 S×cm-1 at the same condition). Although this solid solution exhibits only moderate ionic conductivity, it offers the possibility of tuning the functional and physical properties of mixed metal amide-hydrides by structural modification.

Tailoring hierarchical pore structures in carbon scaffolds for hydrogen storage of nanoconfined magnesium

Various carbon scaffolds have been developed for the nanoconfinement of Mg/MgH2 to achieve the synergistic effects of nanosizing-induced kinetic improvement, Mg-scaffold interaction-induced catalytic enhancement, and mechanical stress-driven thermodynamic stability tuning for advanced hydrogen storage. Yet, systematic studies linking the chemo-structural features of the carbon scaffolds and the hydrogen storage properties of Mg/MgH2 nanoparticles within the scaffolds remain largely unexplored. Here, we investigate the pore-dependent Mg nanoconfinement and its hydrogen storage behavior using hollow carbon nanospheres (HCNs) as scaffolds, dictated by their distinct pore properties. Contrary to the conventional wisdom of nanoconfinement, the Mg nanoparticles confined by the HCN with large mesopores exhibit faster hydrogen sorption kinetics and improved thermodynamic characteristics than those with small mesopores. The detailed analyses reveal that the interplay between reversible mechanical stress upon hydrogenation and nanosizing-induced facile kinetics leads to the observed experimental results. Ab initio computations further show that compressive stress retards bulk diffusion while at the same time enhances the surface hydrogen desorption, suggesting the need for stress optimization within the composite. Our study provides a design guideline for Mg strain engineering and the subsequent carbon scaffold structures for hydrogen storage, possibly leading to advanced hydrogen storage of confined Mg.

Why Hydrogen Dissociation Catalysts do not Work for Hydrogenation of Magnesium.

Provision of atomic hydrogen by hydrogen dissociation catalysts only moderately accelerates the hydrogenation rate of magnesium. They shed light on this well-known but technically challenging fact through a combined approach using an unconventional surface science technique together with Density Functional Theory (DFT) calculations. The calculations demonstrate the drastic electronic structure changes during transformation of Mg to MgH2 , which make fractional hydrogen coverage on the surface, as well as substoichiometric hydrogen content in the bulk energetically unfavorable. Reflecting Electron Energy Loss Spectroscopy (REELS) is used to measure the surface and bulk plasmon during hydrogen sorption in magnesium. The measurements show that the hydrogenation proceeds via the growth of magnesium hydride without the presence of chemisorbed hydrogen on the metallic magnesium surface exactly as indicated by the calculations. This is due to the low stability of sub-stoichiometric amounts of chemisorbed H correlating with the unfavorable charge state of Mg. They are merely bound to the unchanged adjacent Mg layers, thereby explaining the failure of classical hydrogenation catalysts, which effectively only hydrogenate Mg in their direct vicinity. The acceleration of hydrogen sorption kinetics in Mg must affect the polarization in the interface between Mg and MgH2 during hydrogenation.

Effect of Yb2O3@C composite catalyst on hydrogen storage performance of Mg–La–Ni alloy

The present study successfully synthesized a Yb2O3@C composite catalyst using an enhanced chemical blow molding carbonization method. The characterization results showed that the catalyst was highly dispersed in the porous carbon matrix by Yb2O3 nanoparticles. And the catalyst has developed pore structure, high specific surface area and high defect density. Yb2O3@C was mixed with the Mg96La3Ni alloy through ball milling, and the impact of varying amounts of Yb2O3@C on the hydrogen storage performance of Mg96La3Ni was comprehensively investigated. The research findings indicate that the addition of Yb2O3@C notably refines the Mg-based alloy, leading to an increase in both specific surface area and the count of active sites. Furthermore, Yb2O3@C markedly enhances the hydrogen sorption kinetics of Mg96La3Ni, with the most substantial impact noted at a Yb2O3@C content of 3 wt %. Additionally, the incorporation of Yb2O3@C substantially decreases the dehydrogenation activation energy of the alloy, while leaving its thermodynamic properties unaffected. Yb2O3 nanoparticles and the carbon matrix work synergistically to promote both nucleation reactions and hydrogen diffusion. This study presents a novel strategy for enhancing the performance of hydrogen storage materials based on magnesium.

An insight into the catalytic mechanism of perovskite ternary oxide for enhancing the hydrogen sorption kinetics of MgH2

Solid-state hydrogen storage has emerged as an efficient and reliable technique to commercialize hydrogen energy on a large scale. Magnesium hydride (MgH2), amongst other materials, shows excellent hydrogen storage capability. However, it suffers from setbacks like sluggish kinetics and high thermodynamic stability. Several studies have shown that doping with suitable materials is an effective method to improve the sorption kinetics. Current work is concerned with doping of MgH2 with perovskite-type ternary metal oxide NaNbO3 at 5,10 & 15 wt% doping concentration via ball milling and study of its sorption properties. Thermal desorption mass spectra (TDMS) with thermogravimetry (TG) and Differential scanning calorimetry (DSC) validate the optimum doping concentration to be 10 wt% NaNbO3 in MgH2. According to the isothermal hydrogenation plots, 10 wt% catalyzed sample was capable of absorbing 5.29 wt% hydrogen in just 4.2 min at 150ºC which by far outperforms the 2 h milled MgH2 sample which under the same set of conditions absorbs only 0.67 wt% H2. The catalyst starts affecting the absorption rate right from the room temperature whereas the milled sample has minimal absorption throughout the experiment. At room temperature, the average rate of absorption grows by factor 7 which is staggeringly high. The Kissinger analysis reveals activation energy for hydrogen release as 73.12 kJ/mol for 10 wt% doped NaNbO3-MgH2 system while 137.13 kJ/mol for as milled MgH2. The pressure-temperature isotherm (PCI) at four different temperatures gives a quantitative measure of the thermodynamic stability of the system. To fully comprehend the catalytic process, XRD, SEM, and XPS analysis were conducted after each stage of experiment. XPS suggested possible reduction of Nb valance state from + 5 to + 2 due to surface reduction reaction which further accelerated the sorption kinetics due to the electron transfer process.

The influence of defects on hydrogen sorption from Mg–V thin films

In this paper hydrogen sorption properties of Mg–V multilayer thin films are studied. Thin films are synthesized by means of RF magnetron sputtering. Further modification of material is done using low energy H ion irradiation. The hydrogen sorption properties and kinetics are assessed using TOF-ERDA, in situ optical microscopy coupled with TDS, and TEM analysis. The results of TOF-ERDA indicate full hydrogenation of samples, although the presence of oxygen throughout the film is observed. It corresponds to the formation of MgO, also confirmed by EELS results with hydride plasmon peak hindered by MgO peak. Hydrogenation causes severe damage to the surface of the film and fragmentation of the V layer. TDS and optical analysis indicate lower desorption temperatures for thinner films. The desorption onset does not depend on defects concentration. The kinetic analysis further shows that the apparent activation energy for the thinner film is two times lower.

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH2), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH2. In this work, a literature review of the wide-ranging thermodynamic properties of CaH2 is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH2. The thermodynamic values of hydrogen release from both molten and solid CaH2 were determined as ΔHdes (molten CaH2) = 216 ± 10 kJ mol−1.H2, ΔSdes (molten CaH2) = 177 ± 9 J K−1 mol−1.H2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH2 of 947 ± 65 °C. Similarly, in the solid-state: ΔHdes (solid CaH2) = 172 ± 12 kJ mol−1.H2, ΔSdes (solid CaH2) = 144 ± 10 J K−1 mol−1.H2. Moreover, the activation energy of hydrogen release from CaH2 was also calculated using DSC analysis as Ea = 203 ± 12 kJ mol−1. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material.

Mechanical synthesis of Mg6Pd1-xAgx alloys and their hydrogen absorption capability

In the present paper, we report the result of interactions between mechanically alloyed Mg6Pd1−xAgx alloys and hydrogen. Five Mg6Pd1−xAgx (x = 0, 0.1, 0.3, 0.5, and 0.7) alloys were selected to determine the possibility of substituting palladium with silver and determining the ability of such alloys to interact with hydrogen. Their structures were studied with an X-ray diffraction (XRD) analysis. The XRD analysis confirmed the existence of a single Mg6Pd – type phase for samples with silver concentrations reaching x = 0.5. A higher silver content resulted in a two-phase mixture with a dominating ε Mg25.04Ag7.96 phase. The kinetics of the hydrogenation process were determined with a Sievert-type sorption analyzer. A partial substitution of palladium with silver in the Mg6Pd1−xAgx alloys made the hydrogen sorption kinetics slightly better than those of a pure Mg6Pd intermetallic phase and pure magnesium; however, the hydrogen content was lower. The hydride decomposition temperature and the synthesized hydride kinetic properties were investigated by differential scanning calorimetry (DSC) with thermogravimetric analysis (TGA). The influence of silver on the decomposition kinetics was noticeable.

Fast Hydrogen Sorption Kinetics in Mg-VCl3 Produced by Cryogenic Ball-Milling.

Hydrogen storage in Mg/MgH2 materials is still an active research topic. In this work, a mixture of Mg-15wt.% VCl3 was produced by cryogenic ball milling and tested for hydrogen storage. Short milling time (1 h), liquid N2 cooling, and the use of VCl3 as an additive produced micro-flaked particles approximately 2.5-5.0 µm thick. The Mg-15wt.% VCl3 mixture demonstrated hydrogen uptake even at near room-temperature (50 °C). Mg-15wt.% VCl3 achieved ~5 wt.% hydrogen in 1 min at 300 °C/26 bar. The fast hydriding kinetics is attributed to a reduction of the activation energy of the hydriding reaction (Ea hydriding = 63.8 ± 5.6 kJ/mol). The dehydriding reaction occurred at high temperatures (300-350 °C) and 0.8-1 bar hydrogen pressure. The activation energy of the dehydriding reaction is 123.11 ± 0.6 kJ/mol. Cryomilling and VCl3 drastically improved the hydriding/dehydriding of Mg/MgH2.

On the Catalytic Mechanism of 3d and 4d Transition-Metal-Based Materials on the Hydrogen Sorption Properties of Mg/MgH2

The slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability of the Mg–H bond are the two major limitations for the large-scale utilization of MgH2. In this review, we introduce the catalytic mechanism of 3d and 4d transition metal (TM) on the hydrogen sorption properties of Mg/MgH2. The relative contribution of interatomic interactions to the thermodynamic stability of the TM-substituted MgH2 system is discussed. A synergy effect between the electronegativity and the radius of the TM element is proposed to explain the charge transfer process between TM and H in the TM-substituted MgH2 system. The catalytic mechanism of TM nearby the surface of Mg is more complicated than that in the volume of Mg, as the surface-doped TM can experience more options for doping sites, leading to the hindrance effect and causing various contributions of the d band center to the dissociation of hydrogen molecules and the diffusion of hydrogen atoms nearby the surface of Mg. In terms of the catalytic mechanism of TM for hydrogen sorption kinetics of Mg/MgH2, we particularly focused on the “hydrogen pump” effect existing in the Mg–TM–H system. Other mechanisms, such as a possible catalytic mechanism of TM for the hydrogen sorption properties of nano-sized freestanding Mg/MgH2, were also presented.

Strategies to enhance hydrogen storage performances in bulk Mg-based hydrides

Bulk Mg-based hydrogen storage materials have the potential to provide a low-cost solution to diversify energy storage and transportation. Compared to nano powders which require handling and processing under hydrogen or an inert gas atmosphere, bulk Mg-based alloys are safer and are more oxidation resistant. Conventional methods and existing infrastructures can be used to process and handle these materials. However, bulk Mg alloys have smaller specific surface areas, resulting in slower hydrogen sorption kinetics, higher equilibrium temperatures, and enthalpies of hydride formation. This work reviews the effects of the additions of a list of alloying elements and the use of innovative processing methods, e.g., rapid solidification and severe plastic deformation processes, to overcome these drawbacks. The challenges, advantages, and weaknesses of each method and future perspectives for the development of Mg-based hydrogen storage materials are discussed.

Graphene-Supported Sc 2 O 3 /TiO 2 for Superior Catalysis on Hydrogen Sorption Kinetics of MgH 2

Complex metal oxide catalysts greatly accelerate the hydrogen sorption rates in the magnesium hydride system. In this study, the graphene-supported Sc 2 O 3 /TiO 2 catalyst is synthesized by means of a simple method, and a surprisingly synergetic effect of the Sc 2 O 3 -TiO 2 cocatalyst on the hydrogen storage performance of MgH 2 is observed. The MgH 2 -Sc 2 O 3 /TiO 2 @Gn composite starts to release hydrogen at 140 °C and reaches the peak dehydrogenation temperature at 239.9 °C. It absorbs 6.55 wt% of H 2 in 1 min and desorbs 5.71 wt% of H 2 in 10 min at 300 °C, showing excellent hydrogen absorption and desorption performance. Furthermore, with the decrease of the grain size and changes in the structure, the activity of the catalyst is greatly improved. The low-valent titanium and scandium and oxygen vacancies formed in the process of dehydrogenation facilitate hydrogen diffusion and electron transfer, and further improve the kinetic performance of the Mg/MgH 2 -Sc 2 O 3 /TiO 2 @Gn system. This study aims to provide insights into studying complex metal oxides as catalysts to improve hydrogen storage performance, and shed light on other catalysis-related research.

Hydrogen sorption kinetics and mechanism of Mg2Fe(1−x)NixH6

This work aims to optimize the content of Ni substitution for Fe in Mg2FeH6 to form the quaternary intermetallic hydrides of Mg2Fe(1−x)NixH6 with the best kinetics and reversibility. Different degrees of Ni substitution (x) are obtained from varying the mole ratios of Mg2FeH6:Mg2NiH4 used as starting materials. By increasing Mg2NiH4 contents, the degree of Ni substitution enhances from x = 0.26–0.47, which is beneficial to dehydrogenation kinetics and reversible hydrogen capacities. The sample with significant Ni substitution rather decompose into Mg2Ni than Mg, which Mg2Ni reacts with Fe to reproduce Mg2Fe(1−x)NixH6 upon rehydrogenation. During cycling, Ni substitution degree of all samples enhances to the optimized composition of x∼0.5, resulting in the kinetic improvement. Furthermore, the structures of Mg2Fe(1−x)NixH6 with different Ni-substituted contents are simulated and their dehydrogenation energies are calculated to explain the role of Ni substitution on the dehydrogenation of Mg2Fe(1−x)NixH6.

Critical role of hydrogen sorption kinetics in electrocatalytic CO2 reduction revealed by on-chip in situ transport investigations

Precise understanding of interfacial metal−hydrogen interactions, especially under in operando conditions, is crucial to advancing the application of metal catalysts in clean energy technologies. To this end, while Pd-based catalysts are widely utilized for electrochemical hydrogen production and hydrogenation, the interaction of Pd with hydrogen during active electrochemical processes is complex, distinct from most other metals, and yet to be clarified. In this report, the hydrogen surface adsorption and sub-surface absorption (phase transition) features of Pd and its alloy nanocatalysts are identified and quantified under operando electrocatalytic conditions via on-chip electrical transport measurements, and the competitive relationship between electrochemical carbon dioxide reduction (CO2RR) and hydrogen sorption kinetics is investigated. Systematic dynamic and steady-state evaluations reveal the key impacts of local electrolyte environment (such as proton donors with different pKa) on the hydrogen sorption kinetics during CO2RR, which offer additional insights into the electrochemical interfaces and optimization of the catalytic systems.

XPS Investigation on Improving Hydrogen Sorption Kinetics of the KSiH3 System by Using Zr-Based Catalysts.

The superior hydrogen storage properties makes the KSiH3 system a potential hydrogen storage material for practical applications. A theoretical capacity of 4.3 wt% bring this material to the front line of all the available hydrogen storage materials; however, the activation barrier of the reaction restricts the system to absorb and desorb hydrogen reversibly at elevated temperatures even if the thermodynamics suggest its room temperature operation. Several catalysts have already been tested to enhance its kinetic properties. In this work, the efforts were made to reduce the activation energy by using Zr-based catalysts to the KSi/KSiH3 system. The value of activation energy was found to be lowest (i.e., 87 kJ mol-1) for the ZrH2-added KSiH3 system. The mechanism of this improvement was investigated by using X-ray photoelectron spectroscopy (XPS).

Architecturally robust tubular nano-clay grafted Li0.9Ni0.5Co0.5O2-x/LiFeO2 nanocomposites: New implications for electrochemical hydrogen storage

To meet the demand for sustainable energy development, focusing on storage systems of clean energy like hydrogen is a priority. However, the utilization of new sorbent materials for an in-depth understanding of hydrogen storage mechanisms remains lacking. Herein, a novel composite electrode based on halloysite nanotube-Li0.9Ni0.5Co0.5O2-x/LiFeO2 (HNT-LNCO/LFO) nanostructures with superior electrochemical hydrogen storage behavior is developed via two-pot synthesis strategy, in which the HNT bridges with rich active sites serve as a natural clay support for LNCO/LFO electrocatalysts to equip improved hydrogen sorption kinetics through the spillover phenomena and redox coupling effect. Varied parameters, such as fuel types, the molar ratio of fuel to nitrates, the concentration of the LNCO/LFO (mg/ml), and solvent were optimized to prepare HNT-LNCO/LFO nanocomposites with desired purity, structure, and morphology. These physicochemical changes were realized by a combination of X-ray Powder Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), High Resolution Transmission Electron Microscopy (HR-TEM), and Brunauer-Emmett-Teller (BET) analyses. Furthermore, the electrochemical nature of the proposed nanocomposites were probed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry charge–discharge (CCD) process in KOH medium. The effect of conductive LNCO/LFO contents (3.0 %, 5.0 % and 7.0 %) on the HNT substrate was evaluated with the discharge efficiency and hydrogen spillover mechanism. As a consequence, the LNCO/LFO nanoparticles and pristine HNT structures exhibited the hydrogen storage capacities of 499.67 and 84.05 mAhg−1 after 15 cycles, respectively. However, the contribution of LNCO/LFO nanoparticles with weight percent of 3.0 %, 5.0 % and 7.0 % in tubular matrix can produce the discharge efficiencies of 309.33, 437.34 and 132.25 mAhg−1 in three electrode cells, respectively. This systematic work will result in alternative insights of the well-designed HNT-based nanocomposites for highly efficient energy storage applications.

Investigation on hydrogen storage properties of as-cast, extruded and swaged Mg–Y–Zn alloys

In this work, three different states of Mg-9.1Y-1.8Zn alloys including as-cast, extruded and swaged were prepared by semi-continuous casting, extrusion and swaging processes, respectively. Their compositions, microstructures and hydrogen storage properties were investigated. The results show that Mg-9.1Y-1.8Zn alloys in three different states are all composed of Mg and long-period stacking ordered (LPSO) phases. The LPSO phases occurs to break and decompose after hydrogenation and in-situ forms the YHχ(χ = 2,3) nano-hydrides. The nano-hydrides can be used as in-situ catalysts to improve the hydrogen storage properties of alloys. Meanwhile, many nanocrystalline grains appear in the core of alloy after swaging, and the average grain size ranges from 80 to 200 nm. The presence of nanocrystals may increase the specific surface area of alloy, facilitating the diffusion and absorption of hydrogen. Comparatively, the swaged alloy exhibits the largest hydrogen storage capacity and excellent hydrogen sorption kinetics relative to other states of alloys.

Structure and hydrogen sorption properties of Mg-Mg2Ni nanoparticles prepared by gas phase condensation

The aim of this work is to investigate the hydrogen sorption kinetics and thermodynamics of Mg-Ni nanoparticles at relatively low temperature in relation to their microstructure. To this purpose, Mg-Ni nanoparticles (20 at% Ni) were prepared by gas phase condensation employing two thermal vapour sources. In the as-prepared state, Mg and Ni are mixed within individual nanoparticles, but the intermetallic Mg2Ni compound is not fully formed. After keeping the nanoparticles at 150 °C for two hours under high vacuum or at a mild hydrogen pressure of 0.15 bar, the formation of a Mg-Mg2Ni or MgH2-Mg2NiH0.3 nanocomposite is observed. Subsequently, fast kinetics of hydrogen sorption are recorded at 150 °C with activation energy of 80±8 kJ/mol (absorption) and 60±6 kJ/mol (desorption). However, the maximum hydrogen storage capacity is limited to 2.5 wt% because the transformation from Mg2NiH0.3 to Mg2NiH4 does not take place at 150 °C even at pressures well above the expected thermodynamic equilibrium. Therefore, only the transformation Mg↔MgH2 contributes to the reversible storage capacity. The corresponding equilibrium pressure determined by pressure-composition isotherms of absorption and desorption at 150 °C is 7.5 mbar, very close to the extrapolated value for bulk Mg. The partial replacement of Ni with Fe does not significantly alter the thermodynamics and kinetics of hydrogen sorption. The structure and hydrogen sorption properties of Mg-Ni nanoparticles are compared to those of Mg-Ti nanoparticles prepared by a similar procedure.