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.     

Improved hydrogen storage properties of MgH2 using transition metal sulfides as catalyst

In this study, some transition metal sulfides (TiS₂, NbS₂, MoS₂, MnS, CoS₂ and CuS) are used as catalyst to enhance the hydrogen storage behaviors of MgH₂. The MgH₂-sulfide composites with different sulfides addition are prepared by ball-milling. The phase composition and hydrogen storage properties are studied in detail. The results confirm that all these sulfides can significantly increase the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂–TiS₂ has the best hydrogenation and dehydrogenation kinetics, followed by the MgH₂–NbS₂, MgH₂–MnS, MgH₂–MoS₂, MgH₂–CoS₂, MgH₂–CuS and MgH₂. Also, the onset dehydrogenation temperature of the MgH₂–TiS₂ is about 204 °C, which is lower about 126 °C than that of the MgH₂. The dehydrogenation activation energy can be reduced to 50.8 kJ mol^?1 when doping TiS₂ in MgH₂. The beneficial catalytic effects of the sulfides can be ascribed to the in-situ formation of MgS, TiH₂, NbH, Mo, Mn, Mg₂CoH₅ and MgCu₂ phases.     

Effects of KNbO3 catalyst on hydrogen sorption kinetics of MgH2

The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ternary compound catalyst such as K₂NiF₆, the influence of KNbO₃ on hydrogen storage properties of MgH₂ has been investigated for the first time. The MgH₂ - KNbO₃ composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH₂ and MgH₂-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH₂ with other Nb-contained catalysts such as Nb, NbO and Nb₂O₃, the desorption rate was almost the same as the one registered for as-milled MgH₂. The analysis of differential scanning calorimetry (DSC) showed that MgH₂-KNbO₃ composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH₂ without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol⁻¹ for this material, while for the as-milled MgH₂ was about 165 ± 2.0 kJ mol⁻¹. It is believed that Nb-ternary oxide catalyst (KNbO₃) showed a good catalytic effect and enhance the sorption kinetics of MgH₂.     

Enhanced hydrogen desorption kinetics and cycle durability of amorphous TiMgVNi3-doped MgH2

MgH₂ is considered as a promising hydrogen storage material for on-board applications. In order to improve hydrogen storage properties of MgH₂, the amorphous TiMgVNi₃-doped MgH₂ is prepared by ball milling under hydrogen atmosphere. It is found that the catalytic (Ti,V)H₂ and Mg₂NiH₄ nanoparticles are in situ formed after activation. As a result, the amorphous TiMgVNi₃-doped MgH₂ exhibits enhanced dehydrogenation kinetics (the activation energy for hydrogen desorption is 94.4 kJ mol^?1 H₂) and superior cycle durability (the capacity retention rate is up to 92% after 50 cycles). These results demonstrate that the in situ formation of highly dispersed catalytic nanoparticles from an amorphous phase is an effective pathway to enhance hydrogen storage properties of MgH₂.     

Enhancement of hydrogen sorption properties of MgH2 with a MgF2 catalyst

Effect of a MgF₂ catalyst, prepared by ball-milling, on the hydrogen desorption ability of commercial MgH₂ was investigated. When MgH₂ was catalyzed with a MgF₂ composite, it exhibited good cyclability and sharp faceting, with a small grain size (around 10 nm), which differs from those of pure MgH₂. The addition of the MgF₂ catalyst suggests that the F anion could significantly contribute to the cyclability of Mg particles and aid in the inhibition of MgH₂ grain growth.     

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.     

Excellent catalysis of MoO3 on the hydrogen sorption of MgH2

To improve the hydrogen sorption kinetics of MgH₂, the MoO₃ nanobelts were added into MgH₂ by mechanical milling, leading to fine distribution of MoO₃ in the MgH₂ matrix. Compared to uncatalyzed MgH₂, the hydriding and dehydriding rates of MoO₃-catalyzed MgH₂ were significantly improved. The MgH₂ doped with 2 mol% MoO₃ exhibited fast dehydrogenation without activation, and the initial dehydrogenation amount of 5 wt% could be reached within 900 s at 300 °C. The dehydrogenation apparent activation energy is decreased down to 114.7 kJ/mol. The excellent catalytic effect of MoO₃ originates from its specific role as fast hydrogen diffusion pathways. In the hydrogenation process, the MoO₃ transformed to MoO₂, resulting in the fading of catalytic activity.     

Multiple improvements of hydrogen sorption and their mechanism for MgH2 catalyzed through TiH2@Gr

The present investigation reports the effect of TiH₂ templated over graphene (TiH₂@Gr) on the hydrogen sorption characteristics of MgH₂/Mg. The as synthesized TiH₂@Gr leads to significant effect on sorption in MgH₂ by the following effects: the first is dehydrogenation of MgH₂–TiH₂@Gr, which leads to the conversion of some part of TiH₂ into TiH_1.924. TiH₂ together with TiH_1.924 works as a better catalyst than TiH₂ alone. The second is ball-milling of TiH₂@Gr, which produces defective graphene, which also works as co-catalyst. The third is anchoring of TiH₂ on graphene, which does not allow the catalyst to agglomerate. The catalytic effect of TiH₂@Gr on MgH₂ is found to be better than Ti@Gr and TiO₂@Gr. The onset desorption temperature for MgH₂–TiH₂@Gr is ~204 °C, which is 31 °C and 36 °C lower than MgH₂–Ti@Gr, MgH₂–TiO₂@Gr respectively. The better catalytic behavior of TiH₂@Gr also persists during de/re-hydrogenation kinetics and cycling study of MgH₂. The feasible mechanism for superior catalytic for TiH₂@Gr on MgH₂ has been put forward.     

MnFe2O4 nanopowder synthesised via a simple hydrothermal method for promoting hydrogen sorption from MgH2

The effects of MnFe₂O₄ nanopowder synthesised via a simple ‘hydrothermal’ method on the hydrogen storage properties of MgH₂ are investigated for the first time. The particle size of the as-synthesised MnFe₂O₄ nanoparticles is determined to be about 10 nm. We observe that MnFe₂O₄ catalyst decreases the decomposition temperature of MgH₂ and enhances the sorption kinetics. Interestingly, the onset hydrogen desorption temperature of 10 wt% MnFe₂O₄-doped MgH₂ sample gets lowered from 350 °C to 240 °C with faster kinetics, and the sample shows an average dehydrogenation rate 8–9 times faster than that of the as-milled MgH₂ sample. By adding 10 wt% of as-prepared MnFe₂O₄ to MgH₂, approximately 5.5 wt% hydrogen can be absorbed in 10 min at 200 °C. In contrast, the un-doped MgH₂ sample absorbed only 4.0 wt% hydrogen in the same period of time. From the Kissinger analysis, the apparent activation energy for hydrogen released in the MnFe₂O₄-added MgH₂ composite is found to be 108.42 kJ/mol, which is much lower than the activation energy for hydrogen released in the as-milled MgH₂ (146.57 kJ/mol). It is believed that the in situ formed Fe particle and Mn-containing phases together play a synergistic role in remarkably improving MgH₂ storage properties.     

Effects of TiF3 addition on the hydrogen storage properties of 4MgH2 + Cd composite

In this paper, the best performance of the MgH₂ destabilized system with different ratios of Cd (1:1, 2:1, 3:1 and 4:1) have been studied for the first time. Remarkable enhancements on the onset dehydrogenation temperature, as well as the isothermal de/rehydrogenation kinetics were shown by the 4MgH₂ + Cd composite. In order to improve the hydrogen storage properties of the 4MgH₂ + Cd, TiF₃ was added and its catalytic effects were investigated. Temperature programmed dehydrogenation result had revealed that the onset dehydrogenation temperature was improved once the 10 wt% TiF₃ was incorporated into the 4MgH₂ + Cd system. The absorption and desorption kinetics were also improved compared to the un-doped 4MgH₂ + Cd composite system. The scanning electron microscope result had displayed that the 4MgH₂ + Cd + 10 wt% TiF₃ had the smallest particle size compared to the pure and the ball-milled MgH₂, as well as the 4MgH₂ + Cd composite system. The X-ray diffraction results had demonstrated the formation of an intermediate compound, Mg₃Cd, which was formed during the heating process. For the TiF₃-doped sample, it is reasonable to conclude that the in-situ formed TiH₂ and F-containing species play a synergetic role to encourage interactions between the MgH₂ and the Cd and thus further ameliorate the performances of the hydrogen storage of 4MgH₂ + Cd composite system.     

The effect of ball-milling gas environment on the sorption kinetics of MgH2 with/without additives for hydrogen storage

A study of the effect of the ball-milling gas environment on the kinetic enhancement of MgH₂ with different additives was conducted using argon and hydrogen. The as-sourced MgH₂ was milled for 20 h and then milled for a further 2 h after adding 1–2 mol% of one of the additives titanium isopropoxide, niobium oxide or carbon buckyballs, varying the gas environment for both ball-milling stages. The milling environment had little or no effect on the desorption kinetics in most cases. However, in some cases, the absorption uptake differed by up to 2 wt%, depending on the gas used_. This effect was not consistent among the composite samples surveyed, demonstrating the importance of reporting all information about the ball-milling processes used, including the gas environment.     

Effect of NaH/MgB2 ratio on the hydrogen absorption kinetics of the system NaH + MgB2

In this work the effect of the ratio of starting reactants on the hydrogen absorption reaction of the system xNaH + MgB₂ is investigated. At a constant hydrogen pressure of 50 bar, depending on the amount of NaH present in the system NaH + MgB₂, different hydrogen absorption behaviors are observed. For two system compositions: NaH + MgB₂ and 0.5NaH + MgB₂, the formation of NaBH₄ and MgH₂ as only crystalline hydrogenation products is achieved. The relation between the ratio of the starting reactants and the obtained hydrogenation products is discussed in detail.     

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 in Ca-decorated,B-substituted metal organic framework

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H₂ binding energy to Ca. The Kubas interaction between H₂ molecules and Ca added in the MOF gives rise to a large number of bound H₂'s (8H₂'s per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.     

Enhancing hydrogen storage performance of MgH2 through the addition of BaMnO3 synthesized via solid-state method

Currently, magnesium hydride (MgH₂) as a solid-state hydrogen storage material has become the subject of major research owing to its good reversibility, large hydrogen storage capacity (7.6 wt%) and affordability. However, MgH₂ has a high decomposition temperature (>400 °C) and slow desorption and absorption kinetics. In this work, BaMnO₃ was synthesized using the solid-state method and was used as an additive to overcome the drawbacks of MgH₂. Interestingly, after adding 10 wt% of BaMnO₃, the initial desorption temperature of MgH₂ decreased to 282 °C, which was 138 °C lower than that of pure MgH₂ and 61 °C lower than that of milled MgH₂. For absorption kinetics, at 250 °C in 2 min, 10 wt% of BaMnO₃-doped MgH₂ absorbed 5.22 wt% of H₂ compared to milled MgH₂ (3.48 wt%). Conversely, the desorption kinetics also demonstrated that 10 wt% of BaMnO₃-doped MgH₂ samples desorbed 5.36 wt% of H₂ at 300 °C within 1 h whereas milled MgH₂ only released less than 0.32 wt% of H₂. The activation energy was lowered by 45 kJ/mol compared to that of MgH₂ after the addition of 10 wt% of BaMnO₃. Further analyzed by using XRD revealed that the formation of Mg_0·9Mn_0·1O, Mn₃O₄ and Ba or Ba-containing enhanced the performance of MgH₂.     

Ternary transition metal alloy FeCoNi nanoparticles on graphene as new catalyst for hydrogen sorption in MgH2

The present investigation deals with the synthesis of ternary transition metal alloy nanoparticles of FeCoNi and graphene templated FeCoNi (FeCoNi@GS) by one-pot reflux method and there use as a catalyst for hydrogen sorption in MgH₂. It has been found that the MgH₂ catalyzed by FeCoNi@GS (MgH₂: FeCoNi@GS) has the onset desorption temperature of ~255 °C which is 25 °C and 100 °C lower than MgH₂ catalyzed by FeCoNi (MgH₂: FeCoNi) (onset desorption temperature 280 °C) and the ball-milled (B.M) MgH₂ (onset desorption temperature 355 °C) respectively. Also MgH₂: FeCoNi@GS shows enhanced kinetics by absorbing 6.01 wt% within just 1.65 min at 290 °C under 15 atm of hydrogen pressure. This is much-improved sorption as compared to MgH₂: FeCoNi and B.M MgH₂ for which hydrogen absorption is 4.41 wt% and 1.45 wt% respectively, under the similar condition of temperature, pressure and time. More importantly, the formation enthalpy of MgH₂: FeCoNi@GS is 58.86 kJ/mol which is 19.26 kJ/mol lower than B.M: MgH₂ (78.12 kJ/mol). Excellent cyclic stability has also been found for MgH₂: FeCoNi@GS even up to 24 cycles where it shows only negligible change from 6.26 wt% to 6.24 wt%. A feasible catalytic mechanism of FeCoNi@GS on MgH₂ has been put forward based on X-ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), and microstructural (electron microscopic) studies.     

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.     

Improved hydrogen storage properties of MgH2 by the addition of KOH and graphene

Mg hydride is a competitive candidates for hydrogen storage based on its high gravimetric hydrogen capacity and accessibility. In this study, a small amount of KOH and graphene were added into MgH₂ by high energy ball milling. MgH₂ doped with both KOH and graphene has a greatly improved hydrogen storage performance. The existence of graphene and the in-situ formed KMgH₃ and MgO decreased activation energy of MgH₂ to 109.89 ± 6.03 kJ/mol. The both KOH and graphene doped sample has a reversibly capacity of 5.43 wt % H₂ and can released H₂ as much as 6.36 times and 1.84 times faster than those of undoped sample and only KOH doped sample at 300 °C, respectively. The addition of graphene not only can provide more “H diffusion channels”, but also can disperse the catalyst.     

Effect of Ni/tubular g-C3N4 on hydrogen storage properties of MgH2

Additive doping is one of the effective methods to overcome the shortcomings of MgH₂ on the aspect of relatively high operating temperatures and slow desorption kinetics. In this paper, hollow g-C₃N₄ (TCN) tubes with a diameter of 2 μm are synthesized through the hydrothermal and high-temperature pyrolysis methods, and then nickel is chemically reduced onto TCN to form Ni/TCN composite at 278 K. Ni/TCN is then introduced into the MgH₂/Mg system by means of hydriding combustion and ball milling. The MgH₂–Ni/TCN composite starts to release hydrogen at 535 K, which is 116 K lower than the as-milled MgH₂ (651 K). The MgH₂–Ni/TCN composite absorbs 5.24 wt% H₂ within 3500 s at 423 K, and takes up 3.56 wt% H₂ within 3500 s, even at a temperature as low as 373 K. The apparent activation energy (E_a) of the MgH₂ decreases from 161.1 to 82.6 kJ/mol by the addition of Ni/TCN. Moreover, the MgH₂–Ni/TCN sample shows excellent cycle stability, with a dehydrogenation capacity retention rate of 98.0% after 10 cycles. The carbon material enhances sorption kinetics by dispersing and stabilizating MgH₂. Otherwise, the phase transformation between Mg₂NiH₄ and Mg₂NiH_0.3 accelerates the re/dehydrogenation reaction of the composite.