Excellent synergistic catalytic mechanism of in-situ formed nanosized Mg2Ni and multiple valence titanium for improved hydrogen desorption properties of magnesium hydride

Nowadays, catalytic doping has been regarded as one of the most promising and effective methods to improve the sluggish kinetics of magnesium hydride (MgH₂). Herein, we synthesized Ni/TiO₂ nanocomposite with the particle sizes about 20 nm by an extremely facile solvothermal method. Then, the Ni/TiO₂ nanocomposite was doped into MgH₂ to enhance its reversible hydrogen storage properties. A remarkably enhancement of de/rehydrogenation kinetics of MgH₂ can be achieved by doped with Ni/TiO₂ nanocomposite, compared to that solely doped with Ni or TiO₂ nanoparticles. The hydrogen desorption peak temperature of MgH₂Ni/TiO₂ is 232 °C, which is 135.4 °C lower than that of ball-milled MgH₂ (367.4 °C). Moreover, the MgH₂Ni/TiO₂ can desorb 6.5 wt% H₂ within 7 min at 265 °C and absorb ∼5 wt% H₂ within 10 min at 100 °C. In particular, the apparent activation energy of MgH₂Ni/TiO₂ is obviously decreased from 160.5 kJ/mol (ball-milled MgH₂) to 43.7 ± 1.5 kJ/mol. Based on the analyses of microstructure evolution, it is proved that metallic Ni particles can react with Mg easily to form fine Mg₂Ni particles after dehydrogenation, and the in-situ formed Mg₂Ni will transform into Mg₂NiH₄ in the subsequent rehydrogenation process. The significantly improved hydrogen absorption/desorption properties of MgH₂Ni/TiO₂ can be ascribed to the synergistic catalytic effect of reversible transformation of Mg₂Ni/Mg₂NiH₄ which act as “hydrogen pump”, and the multiple valence titanium compounds (Ti^4+/3+/2+) which promote the electrons transfer of MgH₂/Mg.     

MgCNi3 prepared by powder metallurgy for improved hydrogen storage properties of MgH2

With the depletion of global energies resources, improvement of hydrogen storage properties of materials like MgH₂ is of great interest for future efficient renewable resources. In this study, a novel antiperovskite MgCNi₃ was synthesized by powder metallurgy then introduced into Mg to fabricate MgMgCNi₃ composite. The hydrogen storage properties of the obtained MgMgCNi₃ composite were evaluated. MgMgCNi₃ showed a high capacity of hydrogen storage and fast kinetics of hydrogen uptake/release at relatively low temperatures. About 4.42 wt% H₂ was absorbed within 20 min at 423 K, and 4.81 wt% H₂ was reversibly released within 20 min at 593 K. By comparison, milled MgH₂ absorbed only 0.99 wt% H₂ and hardly underwent any hydrogen evolution under the same conditions. In addition, MgMgCNi₃ composite showed outstanding cycling stability, with hydrogen absorption capacity retention rates reaching 98% after ten cycles at 623 K. The characterization analyses revealed that MgCNi₃ and Mg formed Mg₂NiH₄ hydride and carbonaceous material during hydrogenation, where Mg₂NiH₄ induced dehydrogenation of MgH₂ and carbon played a dispersive role during the composite reaction. Both features synergistically benefited the hydrogen storage properties of MgH₂.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Trimesic acid-Ni based metal organic framework derivative as an effective destabilizer to improve hydrogen storage properties of MgH2

Herein, a new type of trimesic acid-Ni based metal organic framework (TMA-Ni MOF) was synthesized and then, its derivative Ni@C was introduced into MgH₂ as destabilizer through high energy ball milling to prepare a Mg–Ni@C–H composite. X-ray diffraction analyses indicate the formation of Mg₂Ni/Mg₂NiH₄ as major phases after dehydrogenation/rehydrogenation of the composite, respectively. Two hydrogen absorption plateaus are observed in the Mg–Ni@C–H composite, corresponding to the hydrogenation of Mg and Mg₂Ni, with the enthalpy change values of −75.8 and −52.3 kJ mol⁻¹ H₂ respectively. Thus, it can be concluded that a destabilization effect is brought by Ni@C on thermodynamic properties of MgH₂. In addition, the hydriding/dehydriding kinetics of MgH₂ is notably accelerated with the addition of Ni-based MOF derivative. The activation energy values of both hydrogen absorption and desorption are significantly lowered down with the assistance of Ni@C. Moreover, stable hydrogen de/absorption capacity and kinetics are remained during 25 cycles of high-rate re/dehydrogenation, which can be ascribed to the carbon-wrapped structure of the composite, with which the aggregation of the nanosized particles can be evidently avioded.     

Zinc substituted MgH2 - a potential material for hydrogen storage applications

The search for efficient materials for onboard hydrogen storage applications is an emerging research field. Using density functional calculations, we demonstrate Zn substituted MgH₂ as a potential material for hydrogen storage. We predicted the ground state crystal structure of ZnH₂ which is found to be Pna2₁ (orthorhombic) structure with meta-stable behavior. The structural phase stability and phase transition of Mg_1−xZn_xH₂ systems have been analyzed. The H site energy of Mg_1−xZn_xH₂ systems is calculated to understand the hydrogen desorption process. Our calculations suggest that Zn substitution reduces the stability of MgH₂, thereby it may reduce the decomposition temperature of MgH₂. The band structure and density of states calculations reveal that the Mg_1−xZn_xH₂ systems are insulators. The chemical bonding behavior of Mg_1−xZn_xH₂ systems is established as iono-covalent in nature. Moreover, Zn substitution in MgH₂ induces disproportionate MgH bonds which could also contribute the reduction in the decomposition temperature as well as H sorption kinetics.     

Hydrogen storage properties of nanostructured 2MgH2Co powders: The effect of high-pressure compression

In this study, MgH₂ and Co powders were mechanically milled in the molar ratio 2:1 and compressed to hard-packed cylindrical pellets. The microstructure, phase changes, and hydrogen storage properties of the mechanically milled 2MgH₂Co powder and the 2MgH₂Co compressed pellet were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and synchronous thermal (DSC/TG) analyses. Dehydrogenation of the 2MgH₂Co compressed pellet is mainly due to the decomposition of Mg₂CoH₅ while it is the dehydriding of MgH₂ for the milled 2MgH₂Co powder. Pressure composition absorption isotherms of the 2MgH₂Co powder and 2MgH₂Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg₆Co₂H₁₁ and Mg₂CoH₅ hydride phases. For the compressed 2MgH₂Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H₂/55.70±3.34 kJ/mol H₂ for Mg₂CoH₅ and -81.89±10.39 kJ/mol H₂/74.47±5.27 kJ/mol H₂ for Mg₆Co₂H₁₁. In contrast, hydrogenation/dehydrogenation enthalpies of Mg₂CoH₅ and Mg₆Co₂H₁₁ mechanically milled 2MgH₂Co powder were −73.98±10.1 kJ/mol H₂/71.67±1.38 kJ/mol H₂ and -96.86±8.73 kJ/mol H₂/89.95±10.81 kJ/mol H₂, respectively. Fast hydrogenation was observed in the dehydrided 2MgH₂Co compressed pellet with about 2.75 wt% absorbed in less than 1 min at 300 °C and a maximum hydrogen storage capacity of 4.43 wt% (2.32 wt% for the 2MgH₂Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH₂Co compressed pellet (64.34 kJ-mol⁻¹ H₂) is lower than the mechanically milled 2MgH₂Co powder (73.74 kJ-mol⁻¹ H₂). The results show that mechanical milling followed by high-pressure compression is an efficient method for the synthesis of Mg-based complex hydrides with superior hydrogen sorption properties.     

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg₂Ni, Mg₂Co as well as a small amount of Mg₁₂Ce, and convert into the MgH₂–CeH_2.73-Mg₂NiH₄–Mg₂CoH₅ composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH₂, Mg₂Ni/Mg₂NiH₄ and Mg₆Co₂H₁₁/Mg₂CoH₅. Among them, the transformation between Mg₂Ni and Mg₂NiH₄ triggers the “spill-over” effect which promote the decomposition of MgH₂ phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg₆Co₂H₁₁ to Mg₂CoH₅ phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H₂ within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H₂ within 60 min at the temperature of 260 °C.     

Tailoring the hydrogen absorption desorption's dynamics of MgMgH2 system by titanium suboxide doping

Titanium suboxide (TiO) is one of the best catalysts which improved the hydrogen absorption-desorption property of MgH₂ Mg system. The TiO catalyzed Mg MgH₂ have shown a remarkably reduced apparent activation energy and enhanced the hydrogen absorption-desorption kinetics. The X-ray photoelectron spectroscopy (XPS) analysis has indicated that the oxidation state of Ti in TiO remains unchanged during ball milling and hydrogen absorption-desorption of TiO-doped-MgH₂. The X-ray diffraction (XRD) analysis further confirms the XPS result. The TiO has shown the excellent catalytic effect on the MgMgH₂ system which remarkably reduced the hydrogen absorption-desorption temperatures.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

Hydrogen storage properties of Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys containing LPSO phases

In this work, the as-cast Mg-rich Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys are prepared by the semi-continuous casting method, and their hydrogen storage performance and catalytic mechanisms are investigated by experimental and first-principles calculations approaches. The results show that the LPSO phases decompose and in-situ form the RE(Gd/Y)H_x(x = 2,3) nano-hydrides upon hydrogenation. These nano-hydrides not only serve as the in-situ catalysts to promote the hydrogen ab/desorption of Mg matrix, but also present the pinning effect to inhibit the growth of Mg/MgH₂ grains during hydrogenation and dehydrogenation. Comparatively, the two alloys exhibit the similar hydrogen absorption kinetics, while the hydrogen desorption kinetics of Mg_98.5Gd₁Zn_0.5 is superior to that of Mg_98.5Gd_0.5Y_0.5Zn_0.5. The first-principles calculations reveal that the GdH₂ and YH₂ hydrides exhibit different catalytic effects on weakening the bond strength of H–H within H₂ and Mg–H within MgH₂, which interprets well the differences in the hydrogen ab/desorption kinetics between Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys.     

Highly dispersed metal nanoparticles on TiO2 acted as nano redox reactor and its synergistic catalysis on the hydrogen storage properties of magnesium hydride

In this paper, we have synthesized highly dispersed Co metal nanoparticles with the particle size about 5–10 nm on TiO₂ (25–50 nm) for the first time through an extremely facile solvothermal method. It is supposed that the synthesized Co/TiO₂ composite can combine the catalytic advantage of both Co and TiO₂, exhibiting the superior catalytic effect on the hydrogen de/absorption properties of MgH₂. The experimental data confirmed the above supposition and demonstrated that Co/TiO₂ additive highly enhances the hydrogen de/absorption kinetics of MgH₂ as compared to separate Co or TiO₂ additive. Specifically, the MgH₂Co/TiO₂ composite begins to desorb hydrogen at about 190 °C with a low apparent activation energy of 77 kJ/mol. Besides, the MgH₂Co/TiO₂ composite has a desorption peak temperature of 235.2 °C, which is 53.2, 94.2 and 132.2 °C lower than that of MgH₂TiO₂ (288.4 °C), MgH₂Co (329.4 °C) and ball-milled MgH₂ (367.4 °C). Moreover, MgH₂Co/TiO₂ composite also exhibits low temperature rehydrogenation properties, which can absorb 6.07, 5.56 and 4.24 wt% H₂ within 10 min at the temperature of 165, 130 and 100 °C, respectively. It is supposed that such excellent hydrogen desorption properties and low desorption energy barrier of MgH₂Co/TiO₂ composite are mainly ascribed to the novel synergistic catalytic effects of Co and TiO₂. Herein, we propose a novel catalytic mechanism and think that Co/TiO₂ acts as “nano redox reactor”, which can facilitate the dissociation and recombination process of hydrogen, thus reducing the reaction energy barrier and enhancing the de/rehydrogenation of MgH₂.     

Electronic structure and stability of new FCC magnesium hydrides Mg7MH16 and Mg6MH16 (M = Ti,V, Nb): An ab initio study

MgH₂ is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH₂ with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg₇TiH_x, Mg_6.5NbH_x and Mg₆VH_x of Ca₇Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH₂. Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self-consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties.     

Fast hydrogen absorption/desorption kinetics in reactive milled Mg-8 mol% Fe nanocomposites

This study aims to better understand the Fe role in the hydrogen sorption kinetics of Mg–Fe composites. Mg-8 mol% Fe nanocomposites produced by high energy reactive milling (RM) for 10 h resulted in MgH₂ mixed with free Fe and a low fraction of Mg₂FeH₆. Increasing milling time to 24 h allowed formation of a high fraction of Mg₂FeH₆ mixed with MgH₂. The hydrogen absorption/desorption behavior of the nanocomposites reactive milled for 10 and 24 h was investigated by in-situ synchrotron X-ray diffraction, thermal analyses and kinetics measurements in Sieverts-type apparatus. It was found that both 10 and 24 h milled nanocomposites presents extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions, i.e., 300–350 °C under 10 bar H₂ for absorption and 0.13 bar H₂ for desorption. Nanocomposites with MgH₂, low Fe fraction and no Mg₂FeH₆ are suggested to be the most appropriate solution for hydrogen storage under the mild conditions studied.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

Fabrication and optical property improvement of gasochromic switchable mirror based on Pd/MgNb2O5 thin film

Recently, the application of magnesium-based gasochromic switchable mirror has been baffled by the low optical dynamic range due to the insufficient conversion between Mg and MgH₂. Based on the excellent catalytic property of Nb₂O₅ for the magnesium-hydrogen reaction, we fabricated fluorocarbon (FC)/Pd/MgNb₂O₅ switchable films via magnetron sputtering and low-temperature inductively coupled plasma chemical vapor deposition technologies in this study, and the optical performance and microstructure of the films were investigated. The results show that the FC/Pd/Mg-3 mol% Nb₂O₅ film exhibits the higher dynamic ranges of the luminous (49.4%) and solar transmittance (54.7%), compared with most of reported Mg-based alloys and MgTiO₂ systems. The addition of Nb₂O₅ and covering a FC layer on the Pd/Mg film could effectively accelerate the switching response and improve such optical properties as transmittance at transparent state and dynamic range. The microstructural analysis reveals that the Nb₂O₅ and appropriate amount of ternary MgNbO phase in the Mg-based layer are supposed to facilitate rapid and sufficient hydrogen absorption and desorption. Furthermore, the high-valence Nb ions scattering around Mg atoms may act as a transfer station for catalyzing the reversible reaction between Mg and H. The excellent optical modulation performance and enhanced hydrophobicity of the FC/Pd/MgNb₂O₅ films signify an application prospect for gasochromic switchable mirror.     

Mg@C60 nano-lamellae and its 12.50 wt% hydrogen storage capacity

The design and synthesis of new hydrogen storage materials with high capacity are the prerequisite for extensive hydrogen energy application which can be achieved by multi-site hydrogen storage. Herein, a Mg@C₆₀ nano-lamellae structure with multiple hydrogen storage sites has been prepared through a simple ball-milling process in which Mg nanoparticles (∼5 nm) are homogeneously dispersed on C₆₀ nano-lamellae. The as-obtained C₆₀/Mg nano-lamellae displays an excess hydrogen uptake of 12.50 wt% at 45 bar, which is far higher than the theoretical value (7.60 wt%) of metal Mg and the US Department of Energy (DOE) target (5.50 wt%, 2020 year), also the experimental values reported by now. The enhanced hydrogen storage mainly comes from several storage sites: MgH₂, H_x–C₆₀ (CH chemical bonding), H₂@C₆₀ (the endohedral H₂ in C₆₀). Interestingly, the hybridization of Mg and C₆₀ not only facilitate the dissociation of H₂ molecules to form CH bonding with C₆₀, but also promote the deformation of C₆₀ and access H₂ molecules into the cavity of C₆₀. This work provides new insight into the underlying chemistry behind the high hydrogen storage capacities of a new class of hydrogen storage materials, fullerene/alkaline-earth metals nanocomposites.     

Hydrogen storage properties of core-shell structured Mg@TM (TM = Co,V) composites

Target improving the hydrogen sorption properties of Mg, core-shell structured Mg@TM (TM = Co, V) composites were synthesized via an approach combining arc plasma method and electroless plating. The core-shell structures with the MgH₂ core and V or Co containing hydride shells for hydrogenated Mg@TM particles were observed through HAADF-STEM and HRTEM techniques. The measured hydrogenation enthalpy (ΔH_abs = ?70.02 kJ/mol H₂) and activation energy (E_a = 67.66 kJ/mol H₂) of the ternary Mg@Co@V composite were lower than those of binary composites and the pure Mg powder. In addition, the onset dehydrogenation temperature for the hydrogenated ternary composite measured from DSC was 323 °C, about 60 °C lower than that of pure MgH₂. On one hand, these improved properties can be attributed to the core-shell structure which may introduce more contacts between catalysts and Mg, thus providing more nucleation sites for hydrogen sorption. On the other hand, the co-effect of MgCo hydrides (Mg₂CoH₅&Mg₃CoH₅) acting as “hydrogen pump” and V₂H accelerating the dissociation of H₂ might also contribute to the improved hydrogen sorption properties of Mg.     

Superior catalysis of NbN nanoparticles with intrinsic multiple valence on reversible hydrogen storage properties of magnesium hydride

Magnesium hydride, as a potential solid state hydrogen carrier has attracted great attention around the world especially in the energy storage domain due to the high hydrogen storage capacity and the good cycling stability. But kinetic and thermodynamic barriers also impede the practical application and development of MgH₂. Nanoscale catalysts are deemed to be the most effective measure to overcome the kinetic barrier and lower the temperature required for hydrogen release in MgH₂. NbN nanoparticles (~20 nm) with intrinsic Nb³⁺-N and Nb⁵⁺-N were prepared using the molten salt method and used as catalysts in the MgH₂ system. It is found that the NbN nanoparticles exhibit a superior catalytic effect on de/rehydrogenation kinetics for the MgH₂/Mg system. About 6.0 wt% hydrogen can be liberated for the MgH₂+5NbN sample within 5 min at 300 °C, and it takes 12 min to desorb the same amount of hydrogen at 275 °C. Meanwhile, the MgH₂+5NbN sample can absorb 6.0 wt% hydrogen within 16 min at 150 °C, and absorb 5.0 wt% hydrogen within 24 min even at 100 °C. Particularly, the catalyzed samples exhibit excellent hydrogen absorption/desorption kinetic stability. After multiple cycles, there is no kinetic attenuation and the hydrogen capacity remains at about 6.0 wt%. It is demonstrated that the NbN nanoparticles with intrinsic multiple valence can be the critical effect in improving the hydrogen storage kinetics of MgH₂. The stability of Nb₄N₃ phase and Nb³⁺-N and Nb⁵⁺-N valence states can ensure a stable catalytic effect in the system.     

Effects of trimesic acid-Ni based metal organic framework on the hydrogen sorption performances of MgH2

A metal-organic framework based on Ni (II) as metal ion and trimasic acid (TMA) as organic linker was synthesized and introduced into MgH₂ to prepare a Mg-(TMA-Ni MOF)-H composite through ball-milling. The microstructures, phase changes and hydrogen storage behaviors of the composite were systematically studied. It can be found that Ni ion in TMA-Ni MOF is attracted by Mg to form nano-sized Mg₂Ni/Mg₂NiH₄ after de/rehydrogenation. The hydriding and dehydriding enthalpies of the Mg-MOF-H composite are evaluated to be −74.3 and 78.7 kJ mol⁻¹ H₂, respectively, which means that the thermodynamics of Mg remains unchanged. The absorption kinetics of the Mg-MOF-H composite is improved by showing an activation energy of 51.2 kJ mol⁻¹ H₂. The onset desorption temperature of the composite is 167.8 K lower than that of the pure MgH₂ at the heating rate of 10 K/min. Such a significant enhancement on the sorption kinetic properties of the composite is attributed to the catalytic effects of the nanoscale Mg₂Ni/Mg₂NiH₄ derived from TMA-Ni MOF by providing gateways for hydrogen diffusion during re/dehydrogenation processes.