Catalysis derived from flower-like Ni MOF towards the hydrogen storage performance of magnesium hydride

Herein, a novel flower-like Ni MOF with good thermostability is introduced into MgH₂ for the first time, and which demonstrates excellent catalytic activity on improving hydrogen storage performance of MgH₂. The peak dehydrogenation temperature of MgH₂-5 wt.% Ni MOF is 78 °C lower than that of pure MgH₂. Besides, MgH₂-5 wt.% Ni MOF shows faster de/hydrogenation kinetics, releasing 6.4 wt% hydrogen at 300 °C within 600 s and restoring about 5.7 wt% hydrogen at 150 °C after dehydrogenation. The apparent activation energy for de/hydrogenation reactions are calculated to be 107.8 and 42.8 kJ/mol H₂ respectively, which are much lower than that of MgH₂ doped with other MOFs. In addition, the catalytic mechanism of flower-like Ni MOF is investigated in depth, through XRD, XPS and TEM methods. The high catalytic activity of flower-like Ni MOF can be attributed to the combining effect of in-situ generated Mg₂Ni/Mg₂NiH₄, MgO nanoparticles, amorphous C and remaining layered Ni MOF. This research extends the knowledge of elaborating efficient catalysts via MOFs in hydrogen storage materials.     

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–CeH_2.73 is in-situ synthesized by introducing Ni@CeO₂ into MgH₂. The unique coating structure of Ni@CeO₂ facilitates homogeneous distribution of synergetic CeH_2.73 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.     

Synergistic effect of rGO supported Ni3Fe on hydrogen storage performance of MgH2

Bimetallic catalysts possess unique physical and chemical properties that distinct from the individual, which offer the opportunity to ameliorate the hydrogen storage properties of MgH₂. Herein, a Ni₃Fe catalyst homogeneously loaded on the surface of reduced graphene oxide (Ni₃Fe/rGO) was prepared based on layered double hydroxide (LDH) precursor. The novel Ni₃Fe/rGO nano-catalyst was subsequently doped into MgH₂ to improve its hydrogen storage performance. The MgH₂-5 wt.% Ni₃Fe/rGO composite requires only 100 s to reach 6 wt% hydrogen capacity at 100 °C, while for MgH₂ doped with 5 wt% Ni₃Fe, Ni/rGO and Fe/rGO all require more than 500 s to uptake 3 wt% hydrogen under the same condition. The onset dehydrogenation temperature of the MgH₂-5 wt.% Ni₃Fe/rGO composite is about 185 °C, much lower than that of the MgH₂ doped with 5 wt% Ni₃Fe (205 °C), Ni/rGO (210 °C) and Fe/rGO (250 °C), and it can release H₂ completely even in 1000 s at 275 °C. Besides, the MgH₂-5 wt% Ni₃Fe/rGO displays the lowest dehydrogenation apparent activation energy of 59.3 kJ/mol calculated by Kissinger equation. The synergetic effect attributing to rGO, in-situ formed active species of Mg₂Ni and Fe is in charge of the excellent catalytic effect on hydrogen storage behavior of MgH₂. Meanwhile, this study supplies innovative insights to design high efficiency catalysts based on the LDH precursor.     

Graphene-anchored Ni6MnO8 nanoparticles with steady catalytic action to accelerate the hydrogen storage kinetics of MgH2

Transition metal-based oxides have been proven to have a substantial catalytic influence on boosting the hydrogen sorption performance of MgH₂. Herein, the catalytic action of Ni₆MnO₈@rGO nanocomposite in accelerating the hydrogen sorption properties of MgH₂ was investigated. The MgH₂ + 5 wt% Ni₆MnO₈@rGO composites began delivering H₂ at 218 °C, with about 2.7 wt%, 5.4 wt%, and 6.6 wt% H₂ released within 10 min at 265 °C, 275 °C, and 300 °C, respectively. For isothermal hydrogenation at 75 °C and 100 °C, the dehydrogenated MgH₂ + 5 wt% Ni₆MnO₈@rGO sample could absorb 1.0 wt% and 3.3 wt% H₂ in 30 min, respectively. Moreover, as compared to addition-free MgH₂, the de/rehydrogenation activation energies for doped MgH₂ composites were lowered to 115 ± 11 kJ/mol and 38 ± 7 kJ/mol, and remarkable cyclic stability was reported after 20 cycles. Microstructure analysis revealed that the in-situ formed Mg₂Ni/Mg₂NiH₄, Mn, MnO₂, and reduced graphene oxide synergically enhanced the hydrogen de/absorption properties of the Mg/MgH₂ system.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of ball-milled Mg90Y1.5Ce1.5Ni7 alloy

In this work, the Mg₉₀Y_1.5Ce_1.5Ni₇ sample is successfully prepared by combining the vacuum induction melting and the mechanical milling. The phase composition and microstructure characteristics are studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The hydrogenated sample is composed of MgH₂, Mg₂NiH₄, CeH_2.73 phases, whereas only the MgH₂ and Mg₂NiH₄ phases are decomposed during dehydrogenation. The hydrogen storage properties of Mg₉₀Y_1.5Ce_1.5Ni₇ samples are measured by semi-automatic Sievert type apparatus. It is found that the samples could be fully activated within three cycles of absorption and dehydrogenation, with a reversible hydrogen storage capacity of about 5.6 wt%. Also, the “optimal hydrogenation temperature” is reduced to 200 °C, and the dehydrogenation activation energy is calculated to be 68.2 kJ/mol and 65.8 kJ/mol by using the Arrhenius and Kissinger equations, respectively. This work provides a scientific approach to promote the practical application of Mg-based alloy.     

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.     

Synthesis of low-cost biomass charcoal-based Ni nanocatalyst and evaluation of their kinetic enhancement of MgH2

In this study, a low-cost biomass charcoal (BC)-based nickel catalyst (Ni/BC) was introduced into the MgH₂ system by ball-milling. The study demonstrated that the Ni/BC catalyst significantly improved the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂ + 10 wt% Ni/BC-3 composite starts to release hydrogen at 187.8 °C, which is 162.2 °C lower than the initial dehydrogenation temperature of pure MgH₂. Besides, 6.04 wt% dehydrogenation can be achieved within 3.5 min at 300 °C. After the dehydrogenation is completed, MgH₂ + 10 wt% Ni/BC-3 can start to absorb hydrogen even at 30 °C, which achieved the absorption of 5 wt% H₂ in 60 min under the condition of 3 MPa hydrogen pressure and 125 °C. The apparent activation energies of dehydrogenation and hydrogen absorption of MgH₂ + 10 wt% Ni/BC-3 composites were 82.49 kJ/mol and 23.87 kJ/mol lower than those of pure MgH₂, respectively, which indicated that the carbon layer wrapped around MgH₂ effectively improved the cycle stability of hydrogen storage materials. Moreover, MgH₂ + 10 wt% Ni/BC-3 can still maintain 99% hydrogen storage capacity after 20 cycles. XRD, EDS, SEM and TEM revealed that the Ni/BC catalyst evenly distributed around MgH₂ formed Mg₂Ni/Mg₂NiH₄ in situ, which act as a “hydrogen pump” to boost the diffusion of hydrogen along with the Mg/MgH₂ interface. Meanwhile, the carbon layer with fantastic conductivity enormously accelerated the electron transfer. Consequently, there is no denying that the synergistic effect extremely facilitated the hydrogen absorption and desorption kinetic performance of MgH₂.     

Facilitating de/hydrogenation by long-period stacking ordered structure in Mg based alloys

We propose a simple strategy to effectively improve the hydrogenation and dehydrogenation kinetics of Mg based hydrogen storage alloys. We designed and prepared an Mg_91.9Ni_4.3Y_3.8 alloy consisting of a large quantity of long-period stacking ordered (LPSO) phases. A type of highly dispersed multiphase nanostructure, which can markedly promote the de/hydrogenation kinetics, has been obtained utilizing the decomposition of LPSO phases at first a few of hydrogenation reactions. The fine structures of LPSO phases and the microstructural evolutions of the alloy during hydrogenation and dehydrogenation reactions were in detail characterized by means of transmission electron microscopy (TEM). The LPSO phases transformed to MgH₂, Mg₂NiH₄, and YH₃ after the first hydrogenation. The highly dispersed nanostructure at macro and micro (nano) scale range remains even after several de/hydrogenation cycles. The alloy shows excellent hydrogen storage properties and its reversible hydrogen absorption/desorption capacities are about 5.8 wt% at 300 °C. Particularly, the alloy exhibits very fast dehydrogenation kinetics. The dehydrogenated sample can release approximately 5 wt% hydrogen at 300 °C within 200 s and 5.5 wt% within 600 s. We elucidate the structural mechanism of the alloy with outstanding hydrogen storage performance.     

Interfacial engineering of nickel/vanadium based two-dimensional layered double hydroxide for solid-state hydrogen storage in MgH2

As a high-density solid-state hydrogen storage material, magnesium hydride (MgH₂) is promising for hydrogen transportation and storage. Yet, its stable thermodynamics and sluggish kinetics are unfavorable for that required for commercial application. Herein, nickel/vanadium trioxide (Ni/V₂O₃) nanoparticles with heterostructures were successfully prepared via hydrogenating the NiV-based two-dimensional layered double hydroxide (NiV-LDH). MgH₂ + 7 wt% Ni/V₂O₃ presented more superior hydrogen absorption and desorption performances than pure MgH₂ and MgH₂ + 7 wt% NiV-LDH. The initial discharging temperature of MgH₂ was significantly reduced to 190 °C after adding 7 wt% Ni/V₂O₃, which was 22 and 128 °C lower than that of 7 wt% NiV-LDH modified MgH₂ and additive-free MgH₂, respectively. The completely dehydrogenated MgH₂ + 7 wt% Ni/V₂O₃ charged 5.25 wt% H₂ in 20 min at 125 °C, while the hydrogen absorption capacity of pure MgH₂ only amounted to 4.82 wt% H₂ at a higher temperature of 200 °C for a longer time of 60 min. Moreover, compared with MgH₂ + 7 wt% NiV-LDH, MgH₂ + 7 wt% Ni/V₂O₃ shows better cycling performance. The microstructure analysis indicated the heterostructural Ni/V₂O₃ nanoparticles were uniformly distributed. Mg₂Ni/Mg₂NiH₄ and metallic V were formed in-situ during cycling, which synergistically tuned the hydrogen storage process in MgH₂. Our work presents a facile interfacial engineering method to enhance the catalytic activity by constructing a heterostructure, which may provide the mentality of designing efficient catalysts for hydrogen storage.     

Microstructure and hydrogen storage properties of melt-spun Mg-Cu-Ni-Y alloys

Microstructural and hydrogen storage properties of three nanocrystalline melt-spun Mg-base alloys (Mg₉₀Cu_2.5Ni_2.5Y₅, Mg₈₅Cu₅Ni₅Y₅ and Mg₈₀Cu₅Ni₅Y₁₀) have been investigated in view of their application as reversible hydrogen storage materials. The activation procedure and the hydrogen sorption kinetics of these alloys were studied by thermogravimetry at different temperatures in the range from 100 °C to 380 °C. It has been found that these alloys can reach reversible gravimetric hydrogen storage densities of up to 4.8 wt.%-H₂. Even at a low temperature of 100 °C, the hydrogenation kinetics of the investigated alloys is rather high in the range of 1.5 wt.%-H₂ per hour. In the hydrogenated state, these alloys consist of MgH₂, high temperature Mg₂NiH₄, Mg₂NiH_0.3, YH₂, YH₃ as well as MgCu₂. The presence of MgCu₂ indicates the reaction of Mg₂Cu with hydrogen. After repeated hydrogenation/dehydrogenation the preservation of a nanocrystalline grain structure has been confirmed by scanning electron microscopy, energy-filtered and conventional transmission electron microscopy. Additionally, the distribution of hydrogen in the hydrogenated sample was mapped by means of electron energy loss spectroscopy.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Ternary Mg₈₆Y₁₀Ni₄ alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH₂, Mg₂NiH₄, YH₂, and a small amount of YH₃. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg₈₆Y₁₀Ni₄ alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH₂ transformation in the Mg₈₆Y₁₀Ni₄ alloy were slightly higher than that of pure Mg/MgH₂.     

Boosting the hydrogenation activity of dibenzyltoluene catalyzed by Mg-based metal hydrides

Hydrogenation of dibenzyltoluene (DBT) is of great significance for the application in liquid organic hydrogen carriers (LOHCs). We successfully develop Mg-based metal hydrides (Mg₂NiH₄, MgH₂, and LaH₃) reactive ball-milling for the hydrogenation of DBT. Mg-based metal hydrides milled with 500 min exhibit the best catalytic activity, the hydrogen uptake of DBT can reach 4.63 wt% at the first 4 h and finally achieve 5.70 wt% through 20 h, which is the first time to use hydrogen storage material as a catalyst for the hydrogenation of DBT. The excellent catalytic hydrogenation performance of Mg-based metal hydrides mostly originates from numerous catalytic activity centers formed at the surfaces of Mg₂NiH₄ nanoparticles in the MgH₂ matrix. Inspired by this mechanism, more general metal hydrides can be explored for catalyzing the hydrogenation of LOHCs. The new application of Mg-based metal hydrides is beneficial to developing efficient LOHC based hydrogen storage systems and offers novel insights to hydride-based catalysts.     

Synergistic catalytic effect of SrTiO3 and Ni on the hydrogen storage properties of MgH2

Study on the synergistic catalytic effect of the SrTiO₃ and Ni on the improvement of the hydrogen storage properties of the MgH₂ system has been carried out. The composites have been prepared using ball milling method and comparisons on the hydrogen storage properties of the MgH₂ – Ni and MgH₂ – SrTiO₃ composites have been presented. The MgH₂ – 10 wt% SrTiO₃ – 5 wt% Ni composite is found to has a decomposition temperature of 260 °C with a total decomposition capacity of 6 wt% of hydrogen. The composite is able to absorb 6.1 wt% of hydrogen in 1.3 min (320 °C, 27 atm of hydrogen). At 150 °C, the composite is able to absorb 2.9 wt% of hydrogen in 10 min under the pressure of 27 atm of hydrogen. The composite has successfully released 6.1 wt% of hydrogen in 13.1 min with a total dehydrogenation of 6.6 wt% of hydrogen (320 °C). The apparent activation energy, E_a, for decomposition of SrTiO₃-doped MgH₂ reduced from 109.0 kJ/mol to 98.6 kJ/mol after the addition of 5 wt% Ni. The formation of Mg₂Ni and Mg₂NiH₄ as the active species help to boost the performance of the hydrogen storage properties of the MgH₂ system. Observation of the scanning electron microscopy images suggested the catalytic role of the SrTiO₃ additive is based on the modification of composite microstructure.     

Hydrogenation properties of MgH2- x wt% AC (x=0, 5, 10, 15) nanocomposites

Magnesium is considered as a promising candidate for hydrogen storage due to its high storage capacity (theoretical value ~ 7.6 wt%). Nanocomposites of Magnesium hydride and activated charcoal (AC) were prepared using ball milling method. These nanocomposites were characterized by XRD, TGA, DSC and SEM techniques. The TGA analysis show that the MgH₂-5 wt% AC nanocomposite exhibits dehydrogenation capacity of 7.45 wt% (which is very close to the storage capacity of MgH₂) and starts release of hydrogen at 140 °C temperature. The results from the Kissinger plot from DSC result showed that the activation energy for hydrogen desorption of MgH₂ with 5 wt% AC was reduced compared to those of as-received.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

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.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.     

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.     

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.