Kinetics Of MgH2 Research Articles

Introducing 2D layered WS2 and MoS2 as an active catalyst to enhance the hydrogen storage properties of MgH2

The current work describes how 2D layered materials, such as MoS2 and WS2, might improve the de/re-hydrogenation kinetics of MgH2. In the presence of WS2 catalyst, desorption of MgH2 begins at 277 °C, with a hydrogen storage capacity of 5.95 wt%, while the onset desorption temperature of MgH2 catalyzed by MoS2 is 330 °C. In just 1.3 min at 300 °C under 13 atm hydrogen pressure, the MgH2-WS2 absorbed approximately 3.72 wt% of hydrogen, and in 20 min at 300 °C under 1 atm hydrogen pressure, it desorbed around ∼5.57 wt% of hydrogen. To ensure the cyclic stability up to 25 cycles of de/re-hydrogenation of the catalyzed MgH2, a continuously 25 cycles of dehydrogenation (under 1 atm hydrogen pressure at 300 °C) and rehydrogenation (under 13 atm hydrogen pressure at 300 °C) were carried out. As a result, MgH2-WS2 exhibits superior cyclic stability than MgH2–MoS2. In addition, with the de/re-hydrogenation kinetics, MgH2-WS2 has a lower reaction activation energy (∼117 kJ/mol) than to other catalyzed and pristine samples. Conversely, the thermodynamical parameters, specifically the change in enthalpy of MgH2, are unaffected by addition of these layered WS2 and MoS2 catalysts.

Synergistic Effects of In‐Situ‐Formed Multivalent Titanium and Mg2Ni for Improving the Hydrogen‐Storage Properties of MgH2

AbstractMagnesium hydride (MgH2) has gained significant consideration as a cost‐effective solid‐state hydrogen‐storage material with a substantial hydrogen capacity. However, its practical applications are impeded by its poor kinetics and high thermal stability. In this study, we employed a hydrothermal method and high‐temperature calcination to prepare well‐dispersed NiTiO3 nanoparticles smaller than 20 nm. Next, we investigated the effect of the NiTiO3 nanoparticles for educating the hydrogen storage performance of MgH2. The synergistic catalytic effect of multivalent titanium and Mg2Ni/Mg2NiH4 enhanced hydrogen storage performance of the MgH2 system. The addition of 5 wt % NiTiO3 lowered the initial hydrogen evolution temperature of MgH2 to 194 °C. After 12 minutes at 275 °C, the 5 wt % NiTiO3‐MgH2 composites released 6.1 wt % of hydrogen. The dehydrogenation activation energy (Ea) of the 5 wt % NiTiO3‐MgH2 composites decreased significantly to 38.78±2.1 kJ/mol. Furthermore, hydrogen uptake of 5 wt % NiTiO3‐MgH2 at 125 °C reaches 5.1 wt % in 300 s under hydrogen pressure of 20 bar. These findings introduce an innovative concept for preparing high active catalysts for improving the dehydrogenation/hydrogenation kinetics of MgH2.

Synergetic catalysis of Ni@C@CeO2 for driving ab/desorption of MgH2 at moderate temperature

Multiple catalysts have exhibited high activity on improving the hydrogen storage performance of MgH2. Herein, the hydrogen ab/desorption kinetics of MgH2 is significantly improved by using Ni@C@CeO2 as the catalyst. The results show that 10 wt%-Ni@C@CeO2 doped MgH2 can absorb 4.51 wt% hydrogen within 60 min under low temperature 75 °C. Furthermore, the MgH2-Ni@C@CeO2 composites release approximately 4.88 wt% H2 within 10 min at 325 °C. Moreover, the composites show excellent cycling performance, with negligible decrease in hydrogen storage capacity, hydrogen uptake rate and hydrogen release rate after ten cycles at 350 °C. The Rietveld refinement of X-ray diffraction and transmittance electron microscopy measurements reveal that Mg2NiH4/Mg2Ni and CeH2.73 phases are in-situ formed. The mutual conversion of “Mg2Ni/Mg2NiH4” during hydrogen ab/desorption is a well-known “hydrogen pump” effect, which improvs the hydrogen storage performance of MgH2. The presence of CeH2.73 accelerate the conversion of Mg2Ni/Mg2NiH4, which we call ‘facilitated hydrogen pump’ effect. This attempt paves a potential way to achieve high performance Mg-based composites hydrogen storage materials via collaborative action between Ni and Ce species.

Superior catalytic action of high-entropy alloy on hydrogen sorption properties of MgH2

Magnesium hydride (MgH2) is the mostly used material for solid-state hydrogen storage. However, their slow kinetics and highly unfavorable thermodynamics make them unsuitable for the practical applications. The current study describes the unusual catalytic action of a new class of catalyst, a high-entropy alloy (HEA) of Al20Cr16Mn16Fe16Co16Ni16 and its leached version, on the de/re-hydrogenation properties of MgH2. The onset desorption temperature of MgH2 was reduced significantly from 360 °C (for ball-milled MgH2) to 338 °C when it was catalyzed with a leached HEA-based catalyst. On the other hand, a fast de/re-hydrogenation kinetics of MgH2 was observed during the addition of leached HEA-based catalyst. It absorbed ∼6.1 wt% of hydrogen in just 2 min at a temperature of 300 °C under 10 atm hydrogen pressure and desorbed ∼5.4 wt% within 40 min. At moderate temperatures and low pressure, the HEA-based catalyst reduced desorption temperatures and improved re-hydrogenation kinetics. Even after 25 cycles of de/re-hydrogenation, the storage capacity of MgH2 catalyzed with the leached version of HEA degrades negligibly.

Synergistic catalytic effects of Ni MOF catalyst and RE alloying on the hydrogen absorption/desorption kinetics of MgH2

The Mg90Ce5Y5 + x wt. % Ni MOF (x = 0, 3) composites were prepared successfully and we investigated the synergistic catalytic effects of Ni MOF catalyst doping and RE alloying for the hydrogen storage properties of pure Mg. The N3 sample absorbs about 3.05 wt. % H2 at 473 K within 2 min, while the pure Mg almost does not initiates reaction at the same condition. The Edes of N3 only is calculated as 69.0 ± 3.9 kJ/mol and much lower than pure MgH2 (160 kJ/mol). The improved hydrogen storage kinetics is considered that the CeH2.73 and YH2 phases provide nucleation sites and channels of diffusion inside the particle and the extra Ni active positions, defects and cracks facilitate H2 dissociation/recombination, generated by Ni MOF on the particle surface.

Remarkable low-temperature hydrogen cycling kinetics of Mg enabled by VH nanoparticles

Nanoscaled catalysts have attracted much more attention due to their more abundant active sites and better dispersion than their bulky counterparts. In this work, VHx nanoparticles smaller than 10 nm in average size are successfully synthesized by a simple solid-state ball milling coupled with THF washing process, which are proved to be highly effective in enhancing the hydrogen absorption/desorption kinetics of MgH2 at moderate temperatures. The nano-VHx-modified MgH2 releases hydrogen from 182 °C, which is 88 °C lower than additive-free MgH2. The release of hydrogen amounts to 6.3 wt.% H within 10 min at 230 °C and 5.6 wt.% H after 30 min at 215 °C with initial vacuum. More importantly, the dehydrogenated MgH2+10 wt.% nano-VHx rapidly absorbs 5.2 wt.% H within 3 min at 50 °C under 50 bar H2. It even takes up 4.3 wt.% H within 30 min at room temperature (25 °C) under 10 bar H2, exhibiting superior hydrogenation kinetics to most of the previous reports. Mechanistic analyses disclose the reversible transformation between V and V-H species during the hydrogen desorption-absorption process. The homogeneously distributed V-based species is believed to act as hydrogen pump and nucleation sites for MgH2 and Mg, respectively, thus triggering fast hydrogenation/dehydrogenation kinetics.

Remarkable kinetics of novel Ni@CeO2–MgH2 hydrogen storage composite

Magnesium hydroxide (MgH2) has excellent reversibility and high capacity, and is one of the most promising materials for hydrogen storage in practical applications. However, it suffers from high dehydrogenation temperature and slow sorption kinetics. Rare earth hydrides and transition metals can both significantly improve the de/hydrogenation kinetics of MgH2. In this work, MgH2–Mg2NiH4–CeH2.73 is in-situ synthesized by introducing Ni@CeO2 into MgH2. The unique coating structure of Ni@CeO2 facilitates homogeneous distribution of synergetic CeH2.73 and Mg2NiH4 catalytic sites in subsequent ball milling process. The as-fabricated composite MgH2-10 wt% Ni@CeO2 powders possess superior hydrogenation/dehydrogenation characteristics, absorbing 4.1 wt% hydrogen within 60 min at 100 °C and releasing 5.44 wt% H2 within 10 min at 350 °C. The apparent activation energy of MgH2-10 wt% Ni@CeO2 is determined to be 84.8 kJ/mol and it has favorable hydrogen cycling stability with almost no decay in capacity after 10 cycles.

Improved hydrogen storage characteristics of magnesium hydride using dual auto catalysts (MgF2+CsH)

This study discusses the improvement in the hydrogen sorption properties of MgH2 with dual auto-catalysts, MgF2 and CsH. The auto-catalysts are formed due to the reaction between MgH2 and CsF during the dehydrogenation reaction of MgH2. It has been observed that MgF2 and CsH not only improve MgH2's hydrogen sorption properties, also aids its positive thermodynamic tuning, which is favourable for hydrogen storage. The on-set desorption temperature of MgH2 catalysed by MgF2+CsH is 249 °C, which is 106 °C lower than that of ball-milled MgH2 without any additives measured under identical measurement conditions. The catalysts helped in improving both the de/rehydrogenation kinetics of MgH2. The MgH2 catalysed by MgF2+CsH released 4.73 wt % H2 in 15 min at 300 °C. Furthermore, its initial re-hydrogenation rate under isothermal conditon at 300 °C is 4.62 wt % H2 in 5 min. The catalysed sample exhibits negligible hydrogen storage degradation of 0.39 wt % H2 after 25 de/re-hydrogenation cycles. Using the Kissinger method, the activation energy of MgH2 catalysed by MgF2+CsH was estimated to be 98.1 ± 0.5 kJmol-1. From the Van't Hoff plot, the decomposition and formation enthalpies of MgH2 were determined to be 66.6 ± 1.1 kJmol-1 and 63.1 ± 1.2 kJmol-1, respectively. From the experimental observation, a feasible mechanism for the de/re-hydrogenation behaviour of MgH2 with MgF2+CsH is proposed.

Two-dimensional vanadium carbide for simultaneously tailoring the hydrogen sorption thermodynamics and kinetics of magnesium hydride

Magnesium hydride (MgH 2 ) is a potential material for solid-state hydrogen storage. However, the thermodynamic and kinetic properties are far from practical application in the current stage. In this work, two-dimensional vanadium carbide (V 2 C) MXene with layer thickness of 50−100 nm was fist synthesized by selectively HF-etching the Al layers from V 2 AlC MAX phase and then introduced into MgH 2 to improve the hydrogen sorption performances of MgH 2 . The onset hydrogen desorption temperature of MgH 2 with V 2 C addition is significantly reduced from 318 °C for pure MgH 2 to 190 °C, with a 128 °C reduction of the onset temperature. The MgH 2 + 10 wt% V 2 C composite can release 6.4 wt% of H 2 within 10 min at 300 °C and does not loss any capacity for up to 10 cycles. The activation energy for the hydrogen desorption reaction of MgH 2 with V 2 C addition was calculated to be 112 kJ mol −1 H 2 by Arrhenius's equation and 87.6 kJ mol −1 H 2 by Kissinger's equation. The hydrogen desorption reaction enthalpy of MgH 2 + 10 wt% V 2 C was estimated by van't Hoff equation to be 73.6 kJ mol −1 H 2 , which is slightly lower than that of the pure MgH 2 (77.9 kJ mol −1 H 2 ). Microstructure studies by XPS, TEM, and SEM showed that V 2 C acts as an efficient catalyst for the hydrogen desorption reaction of MgH 2 . The first-principles density functional theory (DFT) calculations demonstrated that the bond length of Mg− H can be reduced from 1.71 Å for pure MgH 2 to 2.14 Å for MgH 2 with V 2 C addition, which contributes to the destabilization of MgH 2 . This work provides a method to significantly and simultaneously tailor the hydrogen sorption thermodynamics and kinetics of MgH 2 by two-dimensional MXene materials.

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.

Differences in the heterogeneous nature of hydriding/dehydriding kinetics of MgH2 - TiH2 nanocomposites

Fine powder dispersions of MgH2 - TiH2 composites were obtained through different manufacturing routes using reactive mechanical grinding. The as prepared powder dispersions were subjected to hydrogen sorption kinetics and the results clearly revealed a heterogeneous kinetic nature already reported in nanocrystalline MgH2. The differences observed in the heterogeneous kinetic of isothermal kinetics reveal a strong dependence on grain boundaries microstructure of the prepared hydrides. The basic findings described here suggest the need for studies to understand how the superposition of complex mechanisms are involved in kinetics regarding the microstructure.

Catalytic Effect of Facile Synthesized TiH1.971 Nanoparticles on the Hydrogen Storage Properties of MgH2.

Catalytic doping plays an important role in enhancing the hydrogen storage performance of MgH2, while finding an efficient and reversible catalyst remains to be a great challenge in enhancing the de/rehydrogenation properties of MgH2. Herein, a bidirectional nano-TiH1.971 catalyst was prepared by a wet chemical ball milling method and its effect on hydrogen storage properties of MgH2 was studied. The results showed that all the TiH1.971 nanoparticles were effective in improving the de/rehydrogenation kinetics of MgH2. The MgH2 composites doped with TiH1.971 could desorb 6.5 wt % H2 in 8 min at 300 °C, while the pure MgH2 only released 0.3 wt % H2 in 8 min and 1.5 wt % H2 even in 50 min. It was found that the smaller the size of the TiH1.971 particles, the better was the catalytic effect in promoting the performance of MgH2. Besides, the catalyst concentration also played an important role and the 5 wt %-c-TiH1.971 modified system was found to have the best hydrogen storage performance. Interestingly, a significant hydrogen absorption amount of 4.60 wt % H2 was evidenced for the 5 wt %-c-TiH1.971 doped MgH2 within 10 min at 125 °C, while MgH2 absorbed only 4.11 wt% hydrogen within the same time at 250 °C. The XRD results demonstrated that the TiH1.971 remained stable in cycling and could serve as an active site for hydrogen transportation, which contributed to the significant improvement of the hydrogen storage properties of MgH2.

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.

Mechanically-Induced Catalyzation of MgH2 Powders with Zr2Ni-Ball Milling Media

Magnesium hydride (MgH2) holds immense promises as a cost-effective hydrogen storage material that shows excellent storage capacity suitable for fuel cell applications. Due to its slow hydrogen charging/discharging kinetics and high apparent activation energy of decomposition, MgH2 is usually doped with one or more catalytic agents to improve its storage capacity. So often, milling the metal hydride with proper amounts of catalyst leads to heterogeneous distribution of the catalytic agent(s) in MgH2 matrix. The present work proposes a cost-effective process for doping Mg powders with Zr2Ni particles upon ball milling the powders with Zr2Ni-balls milling media under pressurized hydrogen. Fine Zr2Ni particles were gradually eroded from the balls and homogeneously embedded into the milled powders upon increasing the ball milling time. As a result, these fine hard intermetallic particles acted as micro-milling media and leading to the reduction the Mg/MgH2 powders. Meanwhile, Zr2Ni eroded particles possessed excellent heterogeneous catalytic effect for improving the hydrogenation/dehydrogenation kinetics of MgH2. This is implied by the short time required to absorb (425 s)/desorb (700 s) 6.2 wt% H2 at 200 °C and 225 °C, respectively. The as-milled MgH2 with Zr2Ni balls possessed excellent cyclability, indexed by achieving continuous 646 cycles in 985.5 h (~1.5 cycle per hour) without serious degradation.

Metallic glassy Ti2Ni grain-growth inhibitor powder for enhancing the hydrogenation/dehydrogenation kinetics of MgH2.

Because of its high thermal stability and poor hydrogenation/dehydrogenation kinetics, magnesium hydride (MgH2) requires mechanical treatment and/or doping with catalytic agents(s) to understand the decomposition temperature and accelerate the gas uptake/release kinetics. Whereas all catalytic species used for this purpose are crystalline materials, in this paper use of titanium nickel (Ti2Ni) metallic glassy (MG) nanopowders for enhancing the hydrogenation/dehydrogenation kinetics behavior of MgH2 powders is reported. In the present research, MG-Ti2Ni ribbons, prepared using a melt spinning technique were snipped into small pieces and then cryo-milled under a flow of liquid nitrogen to obtain submicron-powders (500 nm). The as-prepared MgH2 powders were doped with 10 wt% of the glassy powder and then cryo-milled for 25 h. The structural and morphological analysis indicated that the cryo-milling process succeeded in maintaining the short-range order structure of MG-Ti2Ni, and in reducing the MgH2 grain size to the nanolevel. The results showed that the as-prepared nanocomposite powders obtained after 25 h of cryo-milling decomposed at 283 °C, with an apparent activation energy of 87.3 kJ mol−1. The MgH2/10 wt% MG-Ti2Ni nanocomposite powders were cold rolled into thin strips, using a cold rolling technique. These cold rolled strips possessed excellent morphological characteristics, shown by the homogeneous distribution of the MgH2 spherical particles (10 nm in diameter) in the glassy Ti2Ni matrix. Furthermore, the hydrogenation/dehydrogenation kinetics measured at 225 °C were very fast, as indicated by the short time (400 s) required to uptake/release 5.7 wt% H2. At this temperature, the system possessed good life-time cycling performance – achieving 84 continuous cycles within 30 h without failure or degradation.

Improved hydrogen storage properties of MgH2 by the addition of TiCN and its catalytic mechanism

The hydrogen storage properties of MgH2–x wt%TiCN (x = 5, 10, 15) composites were systematically investigated and the results show that the addition of TiCN can effectively improve the de/rehydrogenation kinetics of MgH2. Taken the onset dehydrogenation temperature and isothermal de/rehydrogenation kinetics into consideration, the MgH2–10 wt% TiCN composite was shown to have the best performance. It was found that the MgH2–10 wt% TiCN composite could release 4 wt% H2 in 17.3 min at 300 °C while the as-synthesized MgH2 could not release any hydrogen under the same condition. Besides, the MgH2–10 wt% TiCN composite could absorb 4.63 wt% H2 under 3.2 MPa hydrogen pressure at 300 °C within 20 s. Compared with as-synthesized MgH2, the activation energy of the MgH2–10 wt% TiCN composite was significantly decreased from 183.76 ± 10 to 106.82 ± 5 kJ/mol. X-ray diffraction analysis revealed that the TiCN remained stable during the ball milling and the following de/rehydrogenation cycle. The catalytic mechanism was also proposed that the TiCN particles absorbed on MgH2 not only served as charge transfer centers and accelerated the hydrogen incorporation and dissociation rate but also provided more diffusion channels for hydrogen, which contributed to the good de/rehydrogenation properties of MgH2.

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 (Ti4+/3+/2+) which promote the electrons transfer of MgH2/Mg.

ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism

Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH2, yet this catalysis could be enhanced further by supporting NPs on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently doped into MgH2 to remarkably improve its hydrogen storage properties. The MgH2-Co@CNTs starts to obviously release hydrogen at 267.8 °C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 °C within 15 min. For absorption, the MgH2-Co@CNTs uptakes 6.15 wt% H2 at 250 °C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH2-Co@CNTs can be well preserved during the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based on XRD, TEM and EDS results, the catalytic mechanism of MgH2-Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg2Co to Mg2CoH5, and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg2Co/Mg2CoH5 can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH2. Meanwhile, the carbon pieces coated on the surfaces of MgH2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH2-Co@CNTs system.

Advancement in the Hydrogen Absorbing and Releasing Kinetics of MgH2 by Mixing with Small Percentages of Zn(BH4)2 and Ni

Zn(BH4)2 made in our former investigation and Ni were mixed with MgH2 to promote the hydrogen absorption and release features of Mg. A 96 w/o MgH2 + 2 w/o Ni + 2 w/o Zn(BH4)2 sample [named MgH2–4NZ] was prepared by milling in a planetary ball mill in a hydrogen atmosphere. The proportion of the additive was small (4 w/o) in order to increase hydrogen absorbing and releasing rates without majorly sacrificing the hydrogen-storage capacity. The hydrogen absorption and release features of the MgH2–4NZ were inspected in detail and compared with those of 99 w/o MgH2 + 1 w/o Zn(BH4)2 [named MgH2–1Z] and 95 w/o MgH2 + 2.5 w/o Ni + 2.5 w/o Zn(BH4)2 [named MgH2–5NZ] samples. The activation of the MgH2–4NZ was not required. The MgH2–4NZ had a useful hydrogen-storage capacity (the quantity of hydrogen absorbed after 60 min) of about 5.5 w/o at the first cycle. At the first cycle, the MgH2–4NZ absorbed 3.84 w/o hydrogen after 5 min and 5.47 w/o hydrogen after 60 min at 593 K in 12 bar hydrogen. The MgH2–4NZ had a higher releasing rate, larger amounts of hydrogen absorbed and released after 60 min, and a better cycling capability than the MgH2–1Z. Staying of Ni (as Mg2Ni) and a larger amount of Zn among particles is believed to have led to the better cycling capability of the MgH2–4NZ.

Metallic Glassy V45Zr20Ni20Cu10Al3Pd2 Alloy Powders for Superior Hydrogenation/Dehydrogenation Kinetics of MgH2

Nanocrystalline MgH2 powders with an average grain size of 7 nm in diameter were synthesized via reactive ball milling (RBM) approach, using a high energy ball mill operated under a high hydrogen pressure of 50 bar. The as-synthesized MgH2 powders obtained after 200 h of RBM showed slow hydrogenation/dehydrogenation kinetics. In order to improve the kinetics behaviors of this binary metal hydride system, multicomponent-metallic glassy V45Zr20Ni20Cu10Al3Pd2 alloy powders fabricated by mechanical alloying under liquid nitrogen were used as a heterogeneous catalytic agent. For the purpose of the present study, the MgH2 powders were doped with 10 wt.% of the metallic glassy powders and ball-milled for 50 h. The results have shown that the decomposition temperature of as-doped MgH2 with 10 wt.% of V45Zr20Ni20Cu10Al3Pd2 powders was 308.9 °C, being far below than that one measured for pure MgH2 (416 °C). This new nanocomposite system possessed outstanding properties, indexed by the fast kinetics of absorption and desorption of 5.5 wt.% H2 within 200 s at a rather low temperature (180 °C).