Magnesium vacancies and hydrogen doping in MgH2 for improving gravimetric capacity and desorption temperature

Performing an ab initio analysis, we inspect the effect of magnesium vacancies and hydrogen doping on the magnesium hydride (MgH₂). The Korringa – Kohn – Rostoker method integrated with the coherent potential approximation is used to perform our calculations. In particular, we find that the gravimetric capacity of MgH₂ increases from 7.658 to 9.816 wt% when the concentrations of magnesium vacancies and hydrogen dopant atoms increase from 0 to 10%. Concretely, the results reveal that the magnesium vacancies and the hydrogen doping have a beneficial effect on the hydrogen storage properties of the hydride by decreasing its desorption temperature and stability. This decrease can be explained on the one hand by the diminution of the number of Mg atoms that establish strong bonds with H atoms, and on the other hand by using the density of states, which indicates that when the concentrations increase, the Mg and H states shift to the conduction band. We also obtain that the value of the desorption temperature can be controlled by varying the concentrations of magnesium vacancies and hydrogen dopant atoms from 4.2 to 5.8% in order to reach the optimum range 289–393 K for the practical use of fuel cell vehicles.     

Role of vacancy defects on the dehydrogenation properties of the ternary hydride ZrNiH3: Ab-initio insights

ZrNi is considered a promising candidate for hydrogen storage and nickel-metal hydride rechargeable batteries (Ni-MH). The effect of creating zirconium and nickel vacancy defects on the dehydrogenation properties of ZrNiH₃ is investigated by means of first-principles calculations. The results indicate that nickel vacancy is energetically more favorable to form in ZrNiH₃ than zirconium vacancy, because of the lesser formation energy of Ni-vacancy. For both Zr and Ni vacancy defects, the formation enthalpy decreases with increasing the concentration of vacancy and, vice versa. In particular, it is found that with ~2.4% of zirconium vacancy defects or with ~4.5% of nickel vacancy defects in ZrNiH₃, the formation enthalpy is around - 40 kJ/mol.H₂, which is recommended by the U.S. Department of Energy (DOE). It is worth noting also that with slightly higher vacancy defects ~2.8 of Zr-vacancy or ~5.3% of Ni-vacancy in ZrNiH₃, it becomes harder to store hydrogen in these systems without cooling. Moreover, the density of states (DOS) analysis indicates that the stability of ZrNiH₃ decreases with increasing Zr-vacancy and Ni-vacancy concentrations, through the shrinkage in the size of the total DOS and shifting in the valence bands near to Fermi level.     

Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

The hydrogen absorption and desorption properties of a MgH₂ – 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH₂. Nb ethoxide reacts with MgH₂ during heating, releasing C₂H₆ and H₂, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH₂ without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H₂ to 94 kJ/mol H₂, and from 176 kJ/mol H₂ to 75 kJ/mol H₂, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbH_x achieved by the dispersion of the liquid additive.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

Room temperature conversion of Mg to MgH2 assisted by low fractions of additives

In recent works, it was noticed that Mg/MgH₂ mixed with additives by high energy ball milling allows temperature reductions of H₂ absorption/desorption without necessarily changing thermodynamic properties. Thus, the objective of this work was to investigate which additives, mixed in low fractions with MgH₂ powder would act as efficient hydrogen absorption/desorption catalysts at low temperatures, mainly at room temperature (RT). MgH₂ mixtures with 2 mol% additives (Fe, Nb₂O₅, TiAl and TiFe) were prepared by high energy reactive ball milling (RM). MgH₂–TiFe mixture showed the best results, both during desorption at 330 °C and absorption at RT. The hydrogen absorption was ≈ 2.67 wt% H₂ in 1 h and ≈ 4.44 wt% H₂ in 16 h (40% and 67% of maximum theoretical capacity, respectively). The MgH₂–TiFe superior performance was attributed to the hydrogen attraction by the created high energy interfaces and strong TiFe catalytic action facilitating the H₂ flow during Mg/MgH₂ reactions.     

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.     

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.     

Understanding the dehydrogenation properties of MgH2 catalysed by Na3AlF6

The catalytic effect of Na₃AlF₆ on the dehydrogenation properties of the MgH₂ with X wt% (X = 5, 10, 20 and 50) have been investigated by ball milling technique. Based on the temperature-programme-desorption result, the addition of 10 wt% Na₃AlF₆ to the MgH₂ has demonstrated the best dehydrogenation properties performance. The dehydrogenation temperature of the un-doped MgH₂ has experienced a reduction for about 60 °C after doped with 10 wt% Na₃AlF₆. The dehydrogenation kinetics also has been improved with the addition of 10 wt% Na₃AlF₆. Based on the Kissinger analysis, it was observed that the apparent activation energy of MgH₂ desorption is remarkably decreased from 158 kJ/mol to 129 kJ/mol with the addition of 10 wt% Na₃AlF₆. Meanwhile, the formations of new species, the NaMgF₃, the NaF and the AlF₃ in the doped composite after the de/rehydrogenation processes are found in the X-ray diffraction analysis. These new species are expected to act as the active species that probably contributes to enhance the dehydrogenation properties of MgH₂.     

Enhanced hydrogen properties of MgH2 by Fe nanoparticles loaded hollow carbon spheres

In the present investigation, we have reported the synergistic effect of Fe nanoparticles and hollow carbon spheres composite on the hydrogen storage properties of MgH₂. The onset desorption temperature for MgH₂ catalyzed with hollow carbon spheres and Fe nanoparticle (MgH₂-Fe-HCS) system has been observed to be 225.9 °C with a hydrogen storage capacity of 5.60 wt %. It could be able to absorb 5.60 wt % hydrogen within 55 s and desorb 5.50 wt % hydrogen within 12 min under 20 atm H₂ pressure at 300 °C. The desorption activation energy of MgH₂-Fe-HCS has been found to be 84.9 kJ/mol, whereas the desorption activation energies for as received MgH₂, Hollow carbon sphere catalyzed MgH₂ and Fe catalyzed MgH₂ are found to be 130 kJ/mol, 103 kJ/mol, and 94.2 kJ/mol respectively. MgH₂-Fe-HCS composite lowered the change in enthalpy of hydrogen desorption from MgH₂ by 20.02 kJ/mol as compared to pristine MgH₂. MgH₂-Fe-HCS shows better cyclability up to 24 cycles of hydrogenation and dehydrogenation of MgH₂. The mechanism for the better catalytic action of Fe and HCS on MgH₂ has also been discussed.     

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.     

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.     

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 single– and co– substitution of Ti and Fe on vacancy formation and dehydrogenation from MgH2 (110) surface: Ab initio study

In this paper, we studied the effect of single- and co-substitution of Ti and Fe on vacancy formation and hydrogen desorption on MgH₂ (110) surface through first principles calculations. In the surface, we calculated and compared total electronic energies for all possible co-substitution systems in order to determine the preferential Ti and Fe co-substitution site. Substitution energy showed that Ti and Fe co-substitution is more energetically favourable than single-substitution of Ti or Fe in the surface. The bond length, DOS and Bader atomic charge were estimated in order to understand bonding mechanism between atoms. We also calculated formation energy for H, Mg and complex Mg-H vacancies and hydrogen desorption energies. More importantly, it was found that improvement in vacancy formation and hydrogen desorption becomes more obvious in Ti and Fe co-substitution system than single-substitution system. Our results indicated that Ti and Fe co-substitution in MgH₂ (110) surface could be more effective method to improve poor hydrogen desorption properties of MgH₂ than Ti or Fe single-substitution.     

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.     

Effects of SnO2 on hydrogen desorption of MgH2

The hydrogen desorption properties of Magnesium Hydride (MgH₂) ball milled with cassiterite (SnO₂) have been investigated by X-ray powder diffraction and thermal analysis. Milling of pure MgH₂ leads to a reduction of the desorption temperature (up to 60 K) and of the activation energy, but also to a reduction of the quantity of desorbed hydrogen, referred to the total MgH₂ present, from 7.8 down to 4.4 wt%. SnO₂ addition preserves the beneficial effects of grinding on the desorption kinetics and limits the decrease of desorbed hydrogen. Best tradeoff – activation energy lowered from 175 to 148 kJ/mol and desorbed hydrogen, referred to the total MgH₂ present, lowered from 7.8 to 6.8 wt% – was obtained by co-milling MgH₂ with 20 wt% SnO₂.     

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.     

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.     

Remarkable hydrogen properties of MgH2 via combination of an in-situ formed amorphous carbon

Carbon-based materials have been proposed as an ideal medium to reduce the reaction energy barriers and improve the (de)hydrogenation kinetics of magnesium-based hydrogen storage material (MgH₂) in term of their excellent dispersion. However, tedious preparation process and uneven distribution of carbon restrict the application. Therefore, in this paper, we cover MgH₂ by in-situ formed amorphous carbon via a facile approach of co-sintering Mg with fluorene followed by hydriding combustion and ball milling processes, named as MgH₂-carbonization product of fluorene (MgH₂-CPF). As a result, the MgH₂-CPF composite prepared at 823 K initially dehydrogenates at 557 K, 94 K lower than the as-milled MgH₂ (651 K). Meanwhile, the composite can release 5.67 wt% H₂ within 1000 s at 623 K. Even at a lower temperature of 423 K, the MgH₂-CPF composite still reabsorbs 5.62 wt% H₂ within 3600 s, while the as-milled Mg can hardly absorb hydrogen under a same condition. Furthermore, by addition of CPF, the apparent activation energy of the system is decreased from 161.2 kJ/mol to 87.2 kJ/mol. Our finding suggests that the carbon layer can keep the MgH₂ from aggregation, promote hydrogen transport and improve the efficiency of hydrogen absorption and desorption.     

Facile synthesized Fe nanosheets as superior active catalyst for hydrogen storage in MgH2

Reversible hydrogen storage in MgH₂ under mild conditions is a promising way for the realization of “Hydrogen Economy”, in which the development of cheap and highly efficient catalysts is the major challenge. Herein, A two-dimensional layered Fe is prepared via a facile wet-chemical ball milling method and has been confirmed to greatly enhance the hydrogen storage performance of MgH₂. Minor addition of 5 wt% Fe nanosheets to MgH₂ decreases the onset desorption temperature to 182.1 °C and enables a quick release of 5.44 wt% H₂ within 10 min at 300 °C. Besides, the dehydrogenated sample takes up 6 wt% H₂ in 10 min under a hydrogen pressure of 3.2 MPa at 200 °C. With the doping of Fe nanosheets, the apparent activation energy of the dehydrogenation reaction for MgH₂ is reduced to 40.7 ± 1.0 kJ mol⁻¹. Further ab initio calculations reveal that the presence of Fe extends the Mg–H bond length and reduces its bond strength. We believe that this work would shed light on designing plain metal for catalysis in the area of hydrogen storage and other energy-related issues.     

Boosting low-temperature de/re-hydrogenation performances of MgH2 with Pd-Ni bimetallic nanoparticles supported by mesoporous carbon

Bimetallic Pd-Ni nano-particles supported by a mesoporous carbon material CMK-3 (denoted as Pd₃₀Ni₇₀/CMK-3) were synthesized through solution impregnation and hydrogen reduction methods. Among those hierarchical Ni-Pd nano-particles, majorly large ones (>10 nm) are dispersed over the surface of CMK-3, while a litter small ones (<10 nm) are embedded into the pores. It significantly improves the de/re-hydrogenation performances of MgH₂ at low temperature. The onset desorption temperature of MgH₂-Pd₃₀Ni₇₀/CMK-3 is lowered by 150 K from that of pristine MgH₂ (above 593 K). About 6 wt% hydrogen could be released during its decomposition below 561 K. Noticeably, MgH₂-Pd₃₀Ni₇₀/CMK-3 is capable of releasing 1.3 wt% H₂ even at 373 K. 4 wt% hydrogen can be absorbed at 343 K under a hydrogen pressure of 3 MPa within 18000 s. Activation energy values of both hydrogen decomposition (65.9 kJ mol⁻¹) and absorption (78.9 kJ mol⁻¹) for MgH₂-Pd₃₀Ni₇₀/CMK-3 are greatly improved from those of as-milled MgH₂. Thermal stability of the composite system is remarkably destabilized by 4.3 kJ mol H₂⁻¹ from pristine MgH₂ according to pressure-composition isotherm curves and van't Hoff plots. The enhanced performances can be ascribed to the synergistic effects of both destabilization and catalysis from nano-dispersed Pd and Ni particles, respectively.