MgH2 Particles Research Articles

Evolution of Nitrogen-Coordinated Metal Single Atoms Toward Single-Atom Alloys on MgH₂ as Efficient and Stable Hydrogen Spillover Catalysts.

M-N-C catalysts with nitrogen-coordinated metal single-atom active sites have demonstrated high activity for hydrogen storage materials, but their stability in this application remains uncertain. This study addresses this issue by using nickel phthalocyanine (NiPc) molecules on MgH₂ particles as a model system. It is found that the N-coordinated high-valence Ni single atoms in the NiN₄ active site are unstable in the reducing environment of hydrogen storage, spontaneously evolving into zero-valence Ni, forming a Ni₁-Mg single-atom alloy (SAA). The Ni₁-Mg SAA exhibits remarkable stability in catalyzing Mg hydrogen storage reactions. Furthermore, it demonstrates comprehensive catalytic activity for each step of hydrogen absorption and desorption from Mg, surpassing the efficiency of the NiN₄ active site, especially in the critical steps of hydrogenation and dehydrogenation. Overall, the catalytic performance of Ni₁-Mg SAA is superior to most known nickel-based catalysts. This evolutionary process is also observed in FePc, CoPc, and tetraphenylporphyrin nickel (Ni-TPP), suggesting that this reducing transformation is a universal phenomenon for MN₄-type active sites in hydrogen storage catalysis.

Intensifying Hydrogen Evolution in Solid–Liquid MgH2 Hydrolysis Reaction by a High Shear Reactor

AbstractThe reactor is very critical to intensify the reaction rate controlled by mass transfer. Solid magnesium hydride (MgH2) shows great advantages in hydrogen storage; however, poor liquid–solid hydrolysis kinetics limit its application. Various chemical reactors were explored and are used to improve the hydrolysis efficiency. Results show that the mixing style could affect the surface coating behavior. Specifically, the higher temperature and mixing strength could promote the MgH2 hydrolysis. Furthermore, induced crystallization could effectively relieve coating and strengthen the hydrolysis, especially at the high mixing level. The result indicated that the mass transfer distance between crystal seed and formed MgH2 particles played an important role in MgH2 hydrolysis.

Microstructure and hydrogen storage properties of MgH2/MIL-101(Cr) composite

In this paper, a new hydrogen storage composite based on magnesium hydride and metal-organic framework MIL-101(Cr) (an acronym for Material Institute Lavoisier) has been mechanically synthesized and investigated. The hydrogen sorption and desorption behavior in the composite was investigated for the temperature and pressure ranges of (593−653) K and (0−3) MPa, respectively. According to the pressure-composition-temperature isotherms of hydrogen sorption/desorption, the MgH2–5 wt%MIL-101(Cr) composite starts to absorb hydrogen at a lower pressure at 593 K. In addition, the hydrogen release peak for composite shifts to lower temperatures by about 140 K compared to the pure milled MgH2 during temperature programmed desorption at a heating rate of 6 K/min. The activation energy of hydrogen desorption from the composite is 120 ± 2 kJ/mol, which is 36% lower than the activation energy of hydrogen desorption from magnesium hydride (189 ± 2 kJ/mol). The enthalpy of hydrogen absorption was found to be 73 kJ/mol H2 and 60 kJ/mol H2 for milled MgH2 and MgH2–5 wt%MIL-101(Cr) composites, respectively. It is showed that the MIL-101(Cr) MOF structures decompose during ball milling and the chromium oxide nanoparticles form a core-shell structure with MgH2 particles. Thus, composite has better sorption and desorption properties than pure magnesium/magnesium hydride due to the nanoconfinement of MgH2. The chromium oxide nanoparticles act as active sites on the surface of the magnesium particles and facilitate the dissociative chemisorption or recombinative desorption of hydrogen. The main regularities of the phase transition in the magnesium-hydrogen system for the composite and magnesium hydride during dehydrogenation have been studied for temperatures in the range of 298–750 K. The in situ analysis of the phase transitions during the dehydrogenation of MgH2 and MgH2–5 wt%MIL-101(Cr) composite indicates a pronounced catalytic effect of the addition of MIL-101(Cr) to the Mg/MgH2. Based on the experimental results and ab initio calculations, a mechanism for the sorption and desorption of hydrogen by the composite has been proposed.

Application of nitrogen-doped graphene-supported titanium monoxide as a highly active catalytic precursor to improve the hydrogen storage properties of MgH2

MgH2 has broad prospects in energy storage applications; however, its poor thermodynamic properties and slow hydrogen absorption and desorption rates are unsuitable for commercial needs. In the present work, TiO@N-C (denoted as TTONC), a highly active catalytic precursor, was employed to improve the hydrogen storage properties of MgH2. The TTONC-catalyzed MgH2 could uptake hydrogen at room temperature, and the onset dehydrogenation temperature of TTONC-catalyzed MgH2 was 94 °C lower than that of pristine MgH2 (∼300 °C). Dehydrogenated TTONC-catalyzed MgH2 had a capacity retention rate of 95.2% after 50 hydrogen absorption-desorption cycles. The dehydrogenation and hydrogenation apparent activation energies of MgH2-TTONC were 90.5 kJ.mol−1 and 52.7 kJ.mol−1, respectively. The catalytic mechanism analysis revealed that the in situ formed Ti generated TiH2 in the hydrogenation process, and Ti/TiH2 acted as a hydrogen pump to diffuse hydrogen atoms in the hydrogen absorption/desorption process. The formation of stable CN layers with carbon structural defects on the surface of MgH2 inhibited the fragmentation and agglomeration of MgH2 particles, and they served as nucleation sites to enhance hydrogen diffusion. Hence, the dehydrogenation/hydrogenation properties and cyclic stability of MgH2 were greatly enhanced by the addition of TTONC.

Effect of MOF-derived carbon–nitrogen nanosheets co-doped with nickel and titanium dioxide nanoparticles on hydrogen storage performance of MgH2

Magnesium hydride (MgH2) is considered one of the most ideal materials for hydrogen storage. However, the practical applications of MgH2 are greatly hindered by its high operating temperature and poor hydrogen storage kinetics. In the present work, carbon–nitrogen nanosheets co-doped with disk-like nickel and titanium dioxide nanoparticles ((Ni, TiO2)CN) were prepared using metal–organic frameworks (MOFs) as precursors to improve the hydrogen storage performance of MgH2. The as-synthesized ((Ni, TiO2)CN)-doped MgH2 system manifested excellent hydrogen storage properties at low temperatures and could absorb 5.08 wt% of hydrogen in 100 min at 40 °C and 6.17 wt% of hydrogen in 10 min at 125 °C. The dehydrogenation activation energy of the ((Ni, TiO2)CN)-doped MgH2 system was obtained as 83.1 kJ∙mol−1, which was considerably lower than that of pure MgH2. The mechanism analysis revealed that Ni reacted with Mg to form Mg2Ni coating around Mg, the in situ Mg2Ni and its hydride (Mg2NiH4) acted as a “hydrogen pump” to drive hydrogen diffusion and dissociation. And the mutual reversible change in polyvalent Ti ions could promote the transfer of charge, which was conducive to accelerating the formation of Mg/MgH2. Additionally, stable CN layers not only could inhibit the fragmentation and agglomeration of MgH2 particles, but also could make the active substance dispersed more evenly; thus, the cycle stability and hydrogen absorption and desorption kinetics of MgH2 was greatly enhanced under the synergistic catalysis of in situ formed Mg2Ni/Mg2NiH4, polyvalent titanium, and N-C nanosheets. The findings of this work could provide a new strategy to improve the properties of Mg-based hydrogen storage materials.

Liquid Channels Built-In Solid Magnesium Hydrides for Boosting Hydrogen Sorption

Realizing rapid and stable hydrogen sorption at low temperature is critical for magnesium-based hydrogen storage materials. Herein, liquid channels are built in magnesium hydride by introducing lithium borohydride ion conductors as an efficient route for improving its hydrogen sorption. For instance, the 5 wt% LiBH4-doped MgH2 can release about 7.1 wt.% H2 within 40 min at 300 °C but pure MgH2 only desorbs less than 0.7 wt.% H2, and more importantly it delivers faster desorption kinetics with more than 10 times enhancement to pure MgH2. The hydrogen absorption capacity of LiBH4-doped MgH2 can still be well kept at approximately 7.2 wt.% without obvious capacity degradation even after six absorption and desorption cycles. This approach is not only through building ion transfer channels as a hydrogen carrier for kinetic enhancement but also by inhibiting the agglomeration of MgH2 particles to obtain stable cyclic performance, which brings further insights to promoting the hydrogen ab-/desorption of other metal hydrides.

The phase transitions behavior and defect structure evolution in magnesium hydride/single-walled carbon nanotubes composite at hydrogen sorption-desorption processes

The development of hydrogen storage methods is one of the important areas of research in the field of hydrogen energy, while metal hydrides and composites based on them are the most preferred candidates for this. In this paper, one of the composite hydrogen storage materials based on magnesium hydride and single-walled carbon nanotubes was considered. The hydrogen sorption and desorption behavior in composite have been investigated for temperatures and pressures ranges (593−653) K and (0−3) MPa, respectively. Using scanning electron microscopy, it was shown that carbon nanotubes are homogeneously distributed on the surface of MgH2 particles and some of nanotubes are partially embedded in the bulk of MgH2. According to pressure-composition-temperature curves and Van’t Hoff plots it was determined that the enthalpy value of hydrogen absorption process was 69 KJ/mol H2 and 62 KJ/mol H2 for milled MgH2 and MgH2 with 5 wt% single-walled carbon nanotubes, respectively. The main regularities of phase transition in the magnesium-hydrogen system for composite and magnesium hydride during dehydrogenation have been researched for temperatures in the range of 298–750 K. The in situ analysis of the phase transitions during dehydrogenation of MgH2 and composite based on MgH2 and 5 wt% of single-walled carbon nanotubes in argon atmosphere indicates pronounced catalytic effect of carbon nanotubes on the hydrogen desorption. The thermally stimulated desorption curves showed that hydrogen begins to release from the composite where the MgH2 decomposition is minimal. An in situ Doppler broadening spectroscopy study revealed that this is caused by a significant changes magnesium defect structure as a result of the carbon nanotubes/nanoparticles embedding, and the low-temperature hydrogen release follows from the decomposition of associated defects (dislocation-hydrogen or vacancy-hydrogen complexes) as well as its effective diffusion to the surface.

Deflagration characteristics of freely propagating flames in magnesium hydride dust clouds

The flame propagation processes of MgH2 dust clouds with four different particle sizes were recorded by a high-speed camera. The dynamic flame temperature distributions of MgH2 dust clouds were reconstructed by the two-color pyrometer technique, and the chemical composition of solid combustion residues were analyzed. The experimental results showed that the average flame propagation velocities of 23 μm, 40 μm, 60 μm and 103 μm MgH2 dust clouds in the stable propagation stage were 3.7 m/s, 2.8 m/s, 2.1 m/s and 0.9 m/s, respectively. The dust clouds with smaller particle sizes had faster flame propagation velocity and stronger oscillation intensity, and their flame temperature distributions were more even and the temperature gradients were smaller. The flame structures of MgH2 dust clouds were significantly affected by the particle sinking velocity, and the combustion processes were accompanied by micro-explosion of particles. The falling velocities of 23 μm and 40 μm MgH2 particles were 2.24 cm/s and 6.71 cm/s, respectively. While the falling velocities of 60 μm and 103 μm MgH2 particles were as high as 15.07 cm/s and 44.42 cm/s, respectively, leading to a more rapid downward development and irregular shape of the flame. Furthermore, the dehydrogenation reaction had a significant effect on the combustion performance of MgH2 dust. The combustion of H2 enhanced the ignition and combustion characteristics of MgH2 dust, resulting in a much higher explosion power than the pure Mg dust. The micro-structure characteristics and combustion residues composition analysis of MgH2 dust indicated that the combustion control mechanism of MgH2 dust flame was mainly the heterogeneous reaction, which was affected by the dehydrogenation reaction.

Enhanced hydrogen storage kinetics of MgH2 by the synergistic effect of Mn3O4/ZrO2 nanoparticles.

As an ideal material for solid-state hydrogen storage, magnesium hydride (MgH2) has attracted enormous attention due to its cost-effectiveness, abundant resources, and outstanding reversibility. However, the high thermodynamics and poor kinetics of MgH2 still hinder its practical application. In this work, a simple stirring-hydrothermal method was used to successfully prepare bimetallic Mn3O4/ZrO2 nanoparticles, which were subsequently doped into MgH2 by mechanical ball milling to improve its hydrogen sorption performance. The MgH2 + 10 wt% Mn3O4/ZrO2 composite began discharging hydrogen at 219 °C, which was 111 °C lower compared to the as-synthesized MgH2. At 250 °C, the MgH2 + 10 wt% Mn3O4/ZrO2 composite released 6.4 wt% hydrogen within 10 min, whereas the as-synthesized MgH2 reluctantly released 1.4 wt% hydrogen even at 335 °C. Moreover, the dehydrogenated MgH2 + 10 wt% Mn3O4/ZrO2 sample started to charge hydrogen at room temperature. 6.0 wt% hydrogen was absorbed when heated to 250 °C under 3 MPa H2 pressure, and 4.1 wt% hydrogen was taken up within 30 min at 100 °C at the same hydrogen pressure. In addition, compared with the as-synthesized MgH2, the de/rehydrogenation activation energy values of the MgH2 + 10 wt% Mn3O4/ZrO2 composite were decreased to 64.52 ± 13.14 kJ mol-1 and 16.79 ± 4.57 kJ mol-1, respectively, which incredibly contributed to the enhanced hydrogen de/absorption properties of MgH2.

Ni-CNTs as an efficient confining framework and catalyst for improving dehydriding/rehydriding properties of MgH2

MgH2 is one of the most potential hydrogen storage material with a high gravimetric hydrogen storage capacity. however, its applications have been plagued by its high operating temperature and the relatively poor kinetics for releasing and absorbing hydrogen. In the present study, nanoconfinement or catalytic doping were introduced to improve de/rehydrogenation kinetic properties of MgH2. The MgH2@Ni-CNTs was fabricated by holding the MgH2 particles on the surface of the CNT-Ni. MgH2 supported by carbon nanotubes loaded with Ni nanoparticles (MgH2@Ni-CNTs) shows a superior H2 de/ab-sorption kinetics than that of MgH2. This nano-composite releases H2 at 250 °C with the dehydrogenation capacity of 7.29 wt% within 60 min. The dehydrogenation Ea of MgH2 in MgH2@Ni-CNTs is reduced to 74.8 kJ/mol by the confining and catalytic impact of Ni-CNTs. It also has a significant improvement on rehydriding kinetics, with a 7.20 wt% H2 absorption in 30 min even at a temperature as low as 200 °C. Moreover, the MgH2@Ni-CNTs also shows good cycling stability without distinct capacity decay after cycling ten times. Studies show that during dehydrogenation and hydrogenation processes, the CNTs-Ni act as confining framework and catalyst, can accelerate diffusion of hydrogen and prevent the Mg/MgH2 agglomeration. The application of this novel MgH2@Ni-CNTs extends the horizon of transition metallic catalyst and nanoconfinement, and offers a novel way to enhance the hydrogen storage property of the nanoconfined MgH2.

NaH doped TiO2 as a high-performance catalyst for Mg/MgH2 cycling stability and room temperature absorption

This paper presents the catalytic effect of NaH doped nanocrystalline TiO2 (designated as NaTiOxH) in the improvement of MgH2 hydrogen storage properties. The catalyst preparation involves ball milling NaH with TiO2 for 3 hr. The addition of 5 wt% NaTiOxH powder into MgH2 reduces its operating temperature to ∼185 °C, which is ∼110 °C lower than the additive-free as-milled MgH2. The composite remarkably desorbs ∼7.2 wt% H2 within 15 min at ∼290 °C and reabsorbs ∼4.5 wt% H2 in 45 min at room temperature under 50 bar H2. MgH2 dehydrogenation is activated at 57 kJ/mol by the catalyst. More importantly, the addition of 2.5 wt% NaTiOxH catalyst aids MgH2 to reversibly produce ∼6.1 wt% H2 upon 100 cycles within 475 hr at 300 °C. Microstructural investigation into the catalyzed MgH2 composite reveals a firm contact existing between NaTiOxH and MgH2 particles. Meanwhile, the NaTiOxH catalyst consists of catalytically active Ti3O5, and “rod-like” Na2Ti3O7 species liberated in-situ during preparation; these active species could provide multiple hydrogen diffusion pathways for an improved MgH2 sorption process. Furthermore, the elemental characterization identifies the reduced valence states of titanium (Ti<4+) which show some sort of reversibility consistent with H2 insertion and removal. This phenomenon is believed to enhance the mobility of Mg/MgH2 electrons by the creation and elimination of oxygen vacancies in the defective (TiO2-x) catalyst. Our findings have therefore moved MgH2 closer to practical applications.

Fast and stable hydrogen storage in the porous composite of MgH2 with Nb2O5 catalyst and carbon nanotube

The MgH2-Nb2O5-carbon nanotube (CNT) composite was fabricated to accommodate the volume change of particles during the hydrogen storage cycles by holding the MgH2-Nb2O5 particles within the sponge-like matrix of the CNT. This allowed for preservation of the composite structure and led to more stable hydrogen sorption properties during 20 cycles, as compared to without CNT. To investigate this effect of CNT on the cyclic stability of MgH2-Nb2O5, CNT and expanded graphite (EG) were added to MgH2-Nb2O5 via ball milling. The MgH2-Nb2O5-CNT powder showed stable cyclic performance, similar to the MgH2-Nb2O5-CNT composite, whereas the MgH2-Nb2O5-EG powder exhibited cyclic degradation similar to MgH2-Nb2O5. From SEM-EDS, it was found that the C/Mg ratio of the surface of the MgH2-Nb2O5-CNT powder was higher than that of the MgH2-Nb2O5-EG powder. Thus, the fibrous CNT on the surface of the MgH2 particles could be responsible for the greater cyclic stability of the MgH2-Nb2O5-CNT composite.

Enhanced Hydrogen Storage Performance of MgH2 by the Catalysis of a Novel Intersected Y2O3/NiO Hybrid

MgH2 is one of the most promising hydrogen storage materials due to its high hydrogen storage capacity and favorable reversibility, but it suffers from stable thermodynamics and poor dynamics. In the present work, an intersected Y2O3/NiO hybrid with spherical hollow structure is synthesized. When introduced to MgH2 via ball-milling, the Y2O3/NiO hollow spheres are crushed into ultrafine particles, which are homogenously dispersed in MgH2, showing a highly effective catalysis. With an optimized addition of 10 wt% of the hybrid, the initial dehydrogenation peak temperature of MgH2 is reduced to 277 °C, lowered by 109 °C compared with that of the bare MgH2, which is further reduced to 261 °C in the second cycle. There is ca. 6.6 wt% H2 released at 275 °C within 60 min. For the fully dehydrogenation product, hydrogenation initiates at almost room temperature, and a hydrogenation capacity of 5.9 wt% is achieved at 150 °C within 150 min. There is still 5.2 wt% H2 desorbed after 50 cycles at a moderate cyclic condition, corresponding to the capacity retention of 79.2%. The crystal structure and morphology of the Y2O3/NiO hybrid is well preserved during cycling, showing long-term catalysis to the hydrogen storage of MgH2. The Y2O3/NiO hybrid also inhibits the agglomeration of MgH2 particles during cycling, favoring the cyclic stability.

Achieving superior hydrogen storage properties of MgH2 by the effect of TiFe and carbon nanotubes

Multiple catalysts have exhibited high activity on improving the hydrogen storage performance of magnesium hydride. Herein, TiFe as a superior catalyst was successfully prepared, and then doped into MgH2 via ball milling to improve the de/rehydrogenation properties of MgH2 at low temperatures. Compared with as-prepared MgH2, the onset desorption temperature could be reduced to 175 °C after adding 15 wt%-TiFe to MgH2. Furthermore, the 10 wt%-TiFe doped MgH2 released approximately 6.5 wt% H2 within 10 min at 300 °C, and 5.3 wt% H2 could be rapidly absorbed at low temperature of 125 °C under hydrogen pressure of 3 MPa. Moreover, the additional doping of carbon nanotubes (CNTs) evenly distributed on the surface of MgH2 particles, enabling the MgH2 + 10 wt%-TiFe + 5 wt%-CNTs composite with outstanding cycling performance. The activation energy of hydrogenation for MgH2 was reduced from 72.5 ± 2.7 kJ/mol to 60.7 ± 8.0 kJ/mol after doping with TiFe and CNTs. However, the addition of TiFe and CNTs had limited influence on the thermodynamic properties of MgH2. The analysis of XRD, TEM, SEM and EDS indicated that uniformly distributed TiFe and CNTs served as active catalytic unit and aggregation preventer on enhancing the hydrogen storage properties of MgH2, respectively.

Improving the energy release characteristics of PTFE/Al by doping magnesium hydride

Magnesium hydride (MgH2) was doped into PTFE/Al to improve the energy release characteristics of the material system and strive for better application in military engineering. Five types of PTFE/Al/MgH2 reactive materials with different MgH2 content were prepared by molding sintering method. The dynamic mechanical properties of the materials were studied by performing split-Hopkinson pressure bar (SHPB) tests and scanning electron microscope characterizations. The thermal behavior, reaction energy, reaction process and reaction mechanism were systematically investigated by conducting thermogravimetry-differential scanning calorimetry tests, oxygen bomb calorimeter measurements, X-ray diffraction and SHPB tests. The results show that MgH2 particles less than 10% content contribute to heightening the dynamic mechanical properties of PTFE/Al system. The product Mg generated by decomposition of MgH2 can not only react with gas phase C2F4+ but also undergo a Grignard-type reaction with condensed PTFE. The reaction energy and ignition threshold of PTFE/Al/MgH2 reactive materials enhance monotonously as MgH2 content rose. With the increase of MgH2 content from 0% to 20%, the reaction time is prolonged as well as the reaction intensity is enhanced dramatically arising from the massive water vapour produced by the reaction between O2 and H2. The gaseous products generated can form a high pressure shortly after the reaction, which helps to elevate the damage effect of the PTFE/Al system.

Investigation of the reaction mechanism of the hydrolysis of MgH2 in CoCl2 solutions under various kinetic conditions

This paper reports kinetic investigation on dehydrogenation kinetics of magnesium hydride (MgH2) in aqueous solutions of cobalt chloride (CoCl2) under various conditions. For this aim, various CoCl2 solutions (2.5–10 wt%) as activator and hydrolysis temperatures (293–363 K) were tested for achieving active hydrogen production by breaking out passive surface. nucleation-growth and surface area approaches were used for investigation of dehydrogenation mechanism and samples characteristic features were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optimum activator concentration was determined as 6.25 wt% CoCl2 with fastest hydrogen production rate 18.55 mL min−1 g−1 with complete conversion of MgH2 to Mg(OH)2 at room temperature. The kinetic and thermodynamic assessments of dehydrogenation were deduced basing on power law kinetic models with Arrhenius and Eyring approaches. Experimental results dedicated that this approach provided practical and basic application for hydrogen generation by using macroscale MgH2 particles in presence of CoCl2 solution via inhibiting formation of passivation layer with 20 kJmol−1 apparent activation energy.

Effect of adding different percentages of HfCl4 on the hydrogen storage properties of MgH2

Magnesium hydride (MgH2) is the best candidate material to store hydrogen in the solid-state form owing to its advantages such as good reversibility, high hydrogen storage capacity (7.6 wt%), low raw material cost and abundance in the earth. Nevertheless, slow desorption/absorption kinetics and high thermodynamic stability are two issues that have constrained the commercialization of MgH2 as a solid-state hydrogen storage material. So, to boost the desorption/absorption kinetics and to alter the thermodynamics of MgH2, hafnium tetrachloride (HfCl4) was used as a catalyst in this study. Different percentages of HfCl4 (5, 10, 15 and 20 wt%) were added to MgH2 and their catalytic influences on the hydrogen storage properties of MgH2 were investigated. Results showed that the 15 wt% HfCl4-doped MgH2 sample was the best composite to enhance the hydrogen storage performance of MgH2. The onset decomposition temperature of the 15 wt% HfCl4-doped MgH2 composite was decreased by ~75 °C compared to as-milled MgH2. Meanwhile, the desorption/absorption kinetic measurements showed an improvement compared to the undoped MgH2. From the Kissinger analysis, the apparent dehydrogenation activation energy was 167.0 kJ/mol for undoped MgH2 and 102.0 kJ/mol for 15 wt% HfCl4-doped MgH2. This shows that the HfCl4 addition reduced the activation energy of the hydrogen decomposition of MgH2. The desorption enthalpy change calculated by the van't Hoff equation showed that the addition of HfCl4 to MgH2 did not affect the thermodynamic properties. Scanning electron microscopy showed that the size of the MgH2 particles decreased and there was less agglomeration after the addition of HfCl4. It is believed that the decrease in the particle size and in-situ generated MgCl2 and Hf-containing species had synergistic catalytic effects on enhancing the hydrogen storage properties of the HfCl4-doped MgH2 composite.

Improving hydrogen sorption performances of MgH2 through nanoconfinement in a mesoporous CoS nano-boxes scaffold

For the first time, structure-modified CoS-nanoboxes (CoS-NBs) scaffold as a multifunctional Supporting material has been developed by using a template-consumption method for nanoconfinement of MgH2 particles within its mesoporous structure (denoted as MgH2@CoS-NBs). Microstructural observations via transmission electron microscopy (TEM) reveal the uniform distribution of MgH2 nanocrystals within the mesoporous networks of CoS-NBs scaffold and the surface decoration of MgH2 crystals by in-situ formed MgS phase. The peculiar core-shell structured morphology of MgH2 nanocrystals and the “nano-size effect” notably enhances the thermodynamic properties of MgH2 through the reduction in hydriding and dehydriding enthalpies (−65.6 ± 1.1 and 68.1 ± 1.4 kJ mol−1 H2, respectively). Moreover, the improved hydrogen sorption kinetics can be attributed to the catalytic effect of MgS through reducing the intrinsic kinetic energy barriers for both hydriding and dehydriding (57.4 ± 2.2 and 120.8 ± 3.2 kJ mol−1 H2, respectively). The “nano-size effect” of nanoconfined Mg/MgH2 crystals, the catalyzing effect of MgS and the multifunctional role of CoS-NBs scaffold are likely to be synergistically contributed to the superior hydrogen sorption performances of MgH2@CoS-NBs composite, as compared to the pure MgH2.

Synergy between metallic components of MoNi alloy for catalyzing highly efficient hydrogen storage of MgH2

Catalysts play a critical role in improving the hydrogen storage kinetics in Mg/MgH2 system. Exploring highly efficient catalysts and catalyst design principles are hot topics but challenging. The catalytic activity of metallic elements on dehydrogenation kinetics generally follows a sequence of Ti>Nb>Ni>V>Co>Mo. Herein, we report a highly efficient alloy catalyst composed of low-active elements of Mo and Ni (i.e. MoNi alloy) for MgH2 particles. MoNi alloy nanoparticles show excellent catalytic effect, even outperforming most advanced Ti-based catalysts. The synergy between Mo and Ni elements can promote the break of Mg-H bonds and the dissociation of hydrogen molecules, thus significantly improves the kinetics of Mg/MgH2 system. The MoNi-catalyzed Mg/MgH2 system can absorb and release 6.7 wt.% hydrogen within 60 s and 10 min at 300 °C, respectively, and exhibits excellent cycling stability and low-temperature hydrogen storage performance. This study provides a strategy for designing efficient catalysts for hydrogen storage materials using the synergy of metal elements.

Crystallite growth characteristics of Mg during hydrogen desorption of MgH2

MgH2 is one of promising hydrogen storage materials, but Mg crystallites grow up very fast during hydrogen desorption, leading to the degradation of hydrogen storage properties. Therefore the growth behavior and mechanism of Mg crystallites during hydrogen desorption of nanocrystalline MgH2 were investigated in this work. It was found that the transformation from MgH2 to Mg occurred by the surface-controlled ‘nucleation and growth’ mechanism. After the instantaneous nucleation of Mg at free surfaces of MgH2 particles, Mg crystallites grew through three stages, namely one-dimensional, then two-dimensional and finally one-dimensional growths. In the second stage, Mg crystallites grew quickly as compared with other stages. After complete hydrogen desorption, the average Mg crystallite size in MgH2−10 wt% Pr3Al11 sample was smaller than that in pure MgH2 sample due to the presence of Pr3Al11.