Remarkable catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen sorption properties of MgH2

In the present study, the catalytic effect of Ni and ZrO₂ nanoparticles on the hydrogen absorption and desorption properties of MgH₂ has been investigated. The MgH₂ nanocomposites were prepared by high-energy ball-milling. The morphology, phase structure, thermal behavior, and hydrogen storage properties of the materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), and the pressure-composition temperature (PCT) methods. ZrO₂ and Ni nanoparticles were homogenously dispersed into the MgH₂ matrix. The calculated apparent activation energy for dehydrogenation was 63.4 kJ/mol, which was decreased by 80.1 kJ/mol compared to that of as-milled MgH₂. As a result, MgH₂+5 wt.%Ni+5 wt.%ZrO₂ demonstrated improved dehydrogenation and hydrogenation kinetics at 310 °C. The MgH₂+5 wt.%Ni+5 wt.%ZrO₂ sample released about 6.83 wt.% and absorbed about 6.10 wt.% in less than 30 min. Therefore, the co-catalysis of Ni and ZrO₂ significantly enhances the hydrogenation and dehydrogenation properties of MgH₂.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.     

Enhanced hydrogen storage properties of 4MgH2 + LiAlH4 composite system by doping with Fe2O3 nanopowder

In this paper, the hydrogen storage properties and reaction mechanism of the 4MgH₂ + LiAlH₄ composite system with the addition of Fe₂O₃ nanopowder were investigated. Temperature-programmed-desorption results show that the addition of 5 wt.% Fe₂O₃ to the 4MgH₂ + LiAlH₄ composite system improves the onset desorption temperature to 95 °C and 270 °C for the first two dehydrogenation stage, which is lower 40 °C and 10 °C than the undoped composite. The dehydrogenation and rehydrogenation kinetics of 5 wt.% Fe₂O₃-doped 4MgH₂ + LiAlH₄ composite were also improved significantly as compared to the undoped composite. Differential scanning calorimetry measurements indicate that the enthalpy change in the 4MgH₂–LiAlH₄ composite system was unaffected by the addition of Fe₂O₃ nanopowder. The Kissinger analysis demonstrated that the apparent activation energy of the 4MgH₂ + LiAlH₄ composite (125.6 kJ/mol) was reduced to 117.1 kJ/mol after doping with 5 wt.% Fe₂O₃. Meanwhile, the X-ray diffraction analysis shows the formation of a new phase of Li₂Fe₃O₄ in the doped composite after the dehydrogenation and rehydrogenation process. It is believed that Li₂Fe₃O₄ acts as an actual catalyst in the 4MgH₂ + LiAlH₄ + 5 wt.% Fe₂O₃ composite which may promote the interaction of MgH₂ and LiAlH₄ and thus accelerate the hydrogen sorption performance of the MgH₂ + LiAlH₄ composite system.     

Effects of nano size mischmetal and its oxide on improving the hydrogen sorption behaviour of MgH2

This paper reports the catalytic effects of mischmetal (Mm) and mischmetal oxide (Mm-oxide) on improving the dehydrogenation and rehydrogenation behaviour of magnesium hydride (MgH₂). It has been found that 5 wt.% is the optimum catalyst (Mm/Mm-oxide) concentration for MgH₂. The Mm and Mm-oxide catalyzed MgH₂ exhibits hydrogen desorption at significantly lower temperature and also fast rehydrogenation kinetics compared to ball-milled MgH₂ under identical conditions of temperature and pressure. The onset desorption temperature for MgH₂ catalyzed with Mm and Mm-oxide are 323 °C and 305 °C, respectively. Whereas the onset desorption temperature for the ball-milled MgH₂ is 381 °C. Thus, there is a lowering of onset desorption temperature by 58 °C for Mm and by 76 °C for Mm-oxide. The dehydrogenation activation energy of Mm-oxide catalyzed MgH₂ is 66 kJ/mol. It is 35 kJ/mol lower than ball-milled MgH₂. Additionally, the Mm-oxide catalyzed dehydrogenated Mg exhibits faster rehydrogenation kinetics. It has been noticed that in the first 10 min, the Mm-oxide catalyzed Mg (dehydrogenated MgH₂) has absorbed up to 4.75 wt.% H₂ at 315 °C under 15 atmosphere hydrogen pressure. The activation energy determined for the rehydrogenation of Mm-oxide catalyzed Mg is ∼62 kJ/mol, whereas that for the ball-milled Mg alone is ∼91 kJ/mol. Thus, there is a decrease in absorption activation energy by ∼29 kJ/mol for the Mm-oxide catalyzed Mg. In addition, Mm-oxide is the native mixture of CeO₂ and La₂O₃ which makes the duo a better catalyst than CeO₂, which is known to be an effective catalyst for MgH₂. This takes place due to the synergistic effect of CeO₂ and La₂O₃. It can thus be said that Mm-oxide is an effective catalyst for improving the hydrogen sorption behaviour of MgH₂.     

Desorption kinetics of lithium amide/magnesium hydride systems at constant pressure thermodynamic driving forces

Lithium amide and magnesium hydride are lightweight materials with high hydrogen-holding capacities and thus they are of interest for hydrogen storage. In the present work mixtures with initial molar compositions of (LiNH₂ + MgH₂) and (2LiNH₂ + MgH₂) were ball milled with and without the presence of 3.3 mol% potassium hydride dopant. Temperature programmed desorption, TPD, analyses of the mixtures showed that the potassium hydride doped samples had lower onset temperatures than their corresponding pristine samples. The dehydrogenation kinetics of the doped and pristine mixtures was compared at 210 °C. In each case a constant pressure thermodynamic driving force was applied in which the ratio of the plateau pressure to the applied hydrogen pressure was set at 10. Under equivalent conditions, the (LiNH₂ + MgH₂) mixture desorbed hydrogen about 4 times faster than the (2LiNH₂ + MgH₂) mixture. The addition of potassium hydride dopant was found to have a 25-fold increase on the desorption rates of the (2LiNH₂ + MgH₂) mixture, however it had almost no effect on the desorption rates of the (LiNH₂ + MgH₂) mixture. Activation energies were determined by the Kissinger method. Results showed the potassium hydride doped mixtures to have lower activation energies than the pristine mixtures.     

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.     

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.     

Kinetic rate-limiting steps in dehydrogenation of Li-N-H and Li-Mg-N-H systems - Effects of elemental Si and Al

The study is focused on the (LiNH₂ + LiH) and the (2LiNH₂ + MgH₂) hydrogen storage systems. Efforts were made to achieve an in-depth understanding of the dehydrogenation mechanisms following the introduction of elemental Si and Al to both systems. A variety of analytical instruments were employed, such as Temperature-Programmed Desorption, Mass Spectrometry, X-ray diffraction (XRD) and InfraRed spectroscopy. Unlike the effect of elemental Al in the (LiNH₂ + LiH) system, which showed no promising improvement in the dehydrogenation, a significant kinetic improvement in the (LiNH₂ + LiH) system was achieved upon addition of elemental Si. Kinetic improvement by elemental Si was described as a result of the LiH destabilisation through the formation of a Li₂Si phase and an increase in H⁻ anion concentration. On the other hand, once elemental Al is added to the (2LiNH₂ + MgH₂) system, the overall dehydrogenation kinetics of the system is delayed through the formation of a LiAl Phase. The results suggest that dehydrogenation mechanism in both the (LiNH₂ + LiH) and the (2LiNH₂ + MgH₂) systems are identical. Dehydrogenation reaction starts with the electrostatic interaction of the oppositely charged hydrogen atoms in amide and hydride and proceeds by mass transfer of reactant species across the product layer at the later stage of the dehydrogenation. However, it was particularly identified that each system has a unique kinetic rate limiting step. Dehydrogenation kinetics seems to be controlled by the diffusion of the H⁻ anion in the (LiNH₂ + LiH) system but by the diffusion of the Li⁺ cation in the (2LiNH₂ + MgH₂) system.     

Effect of LiBH4 on hydrogen storage property of MgH2

The effect of lithium borohydride (LiBH₄) on the hydriding/dehydriding kinetics and thermodynamics of magnesium hydride (MgH₂) was investigated. It was found that LiBH₄ played both positive and negative effects on the hydrogen sorption of MgH₂. With 10 mol.% LiBH₄ content, MgH₂–10 mol.% LiBH₄ had superior hydrogen absorption/desorption properties, which could absorb 6.8 wt.% H within 1300 s at 200 °C under 3 MPa H₂ and completed desorption within 740 s at 350 °C. However, with the increasing amount of LiBH₄, the hydrogenation/dehydrogenation kinetics deteriorated, and the starting desorption temperature increased and the hysteresis of the pressure-composition isotherm (PCI) became larger. Our results showed that the positive effect of LiBH₄ was mainly attributed to the more uniform powder mixture with smaller particle size, while the negative effect of LiBH₄ might be caused by the H–H exchange between LiBH₄ and MgH₂.     

Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2

In this study, the hydrogen storage properties of MgH₂ with the addition of K₂TiF₆ were investigated for the first time. The temperature-programmed desorption results showed that the addition of 10 wt% K₂TiF₆ to the MgH₂ exhibited a lower onset desorption temperature of 245 °C, which was a decrease of about 105 °C and 205 °C compared with the as-milled and as-received MgH₂, respectively. The dehydrogenation and rehydrogenation kinetics of 10 wt% K₂TiF₆-doped MgH₂ were also significantly improved compared to the un-doped MgH₂. The results of the Arrhenius plot showed that the activation energy for the hydrogen desorption of MgH₂ was reduced from 164 kJ/mol to 132 kJ/mol after the addition of 10 wt% K₂TiF₆. Meanwhile, the X-ray diffraction analysis showed the formation of a new phase of potassium hydride and titanium hydride together with magnesium fluoride and titanium in the doped MgH₂ after the dehydrogenation and rehydrogenation process. It is reasonable to conclude that the K₂TiF₆ additive doped with MgH₂ played a catalytic role through the formation of active species of KH, TiH₂, MgF₂ and Ti during the ball milling or heating process. It is therefore proposed that this newly developed product works as a real catalyst for improving the hydrogen sorption properties of MgH₂.     

ZrO2@Nb2CTx composite as the efficient catalyst for Mg/MgH2 based reversible hydrogen storage material

While Mg/MgH₂ system has a high hydrogen storage capacity, its sluggish hydrogen desorption rate has hindered practical applications. Herein, we report that the hydrogen absorption and desorption kinetics of Mg/MgH₂ system can be significantly improved by using the synergetic effect between Nb₂CT_x MXene and ZrO₂. The catalyst of Nb₂CT_x MXene loading with ZrO₂ (ZrO₂@Nb₂CT_x) is successfully synthesized, and the dehydrogenation activation energy of MgH₂ becomes as low as 60.0 kJ/mol H₂ when ZrO₂@Nb₂CT_x is used as the catalyst, which is far smaller than the case of ZrO₂ (94.8 kJ/mol H₂) and Nb₂CT_x MXene (125.6 kJ/mol H₂). With the addition of ZrO₂@Nb₂CT_x catalyst, MgH₂ can release about 6.24 wt.% and 5.69 wt.% of hydrogen within 150 s at 300 °C and within 900 s even at 240 °C, respectively. Moreover, it realizes hydrogen absorption at room temperature, which can uptake 2.98 wt.% of hydrogen within 1800 s. The catalytic mechanism analysis demonstrates that the in-situ formed nanocomposites can weaken the Mg–H bonding and provide more hydrogen diffusion channels, enabling the dissociation and recombination of hydrogen under milder reaction conditions.     

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.     

Enhanced hydrogen sorption properties of Ni and Co-catalyzed MgH2

MgH₂ is one of the most promising hydrogen storage materials due to its high capacity and low cost. In an effort to develop MgH₂ with a low dehydriding temperature and fast sorption kinetics, doping MgH₂ with NiCl₂ and CoCl₂ has been investigated in this paper. Both the dehydrogenation temperature and the absorption/desorption kinetics have been improved by adding either NiCl₂ or CoCl₂, and a significant enhancement was obtained in the case of the NiCl₂ doped sample. For example, a hydrogen absorption capacity of 5.17 wt% was reached at 300 °C in 60 s for the MgH₂/NiCl₂ sample. In contrast, the ball-milled MgH₂ just absorbed 3.51 wt% hydrogen at 300 °C in 400 s. An activation energy of 102.6 kJ/mol for the MgH₂/NiCl₂ sample has been obtained from the desorption data, 18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH₂/CoCl₂, which also exhibits an enhanced kinetics, and of the pure MgH₂ sample, respectively. In addition, the enhanced kinetics was observed to persist even after 9 cycles in the case of the NiCl₂ doped MgH₂ sample. Further kinetic investigation indicated that the hydrogen desorption from the milled MgH₂ is controlled by a slow, random nucleation and growth process, which is transformed into two-dimensional growth after NiCl₂ or CoCl₂ doping, suggesting that the additives reduced the barrier and lowered the driving forces for nucleation.     

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.     

Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications

The present study compares the dehydrogenation kinetics of (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ systems and their vulnerabilities to the NH₃ emission problem. The (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixture. On the basis of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system are discussed in light of the newly proposed reaction mechanism.     

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.     

The effect of milling energy input and molar ratio on the dehydrogenation and thermal conductivity of the (LiNH2 + nMgH2) (n = 0.5, 0.7, 0.9, 1.0, 1.5 and 2.0) nanocomposites

Hydride nanocomposites in the (LiNH₂ + nMgH₂) system have been synthesized by ball milling with varying input of milling energy injected into powder particles, Q_TR (kJ/g). The grain (crystallite) size of LiNH₂ and MgH₂ decreases rapidly with increasing Q_TR up to approximately 150–200 kJ/g and subsequently more or less saturates at the value of 10–20 nm. For the injected energy Q_TR ≈ 250–350 kJ/g the specific surface area (SSA) increases from the initial 2.4 m²/g for powder mixtures before milling to 30–37 m²/g for nanocomposites after milling. After injecting Q_TR ≈ 550 kJ/g there is a further increase of SSA to 52 m²/g which is over 20-fold increase of SSA from its initial value. That clearly indicates that a profound reduction of particle size has occurred. The hydride phases formed during ball milling with relatively low Q_TR are identified as a-Mg(NH₂)₂ (amorphous magnesium imide) and LiH. The ball milled (LiNH₂ + nMgH₂) nanocomposite system with n = 0.5–0.9 can effectively desorb about 4–5 wt.% H₂ with a reasonable rate at the temperature range close to 200 °C. Within a low temperature range up to ∼250 °C, regardless of the molar ratio n and the injected energy Q_TR the thermal desorption of the (LiNH₂ + nMgH₂) nanocomposites occurs without any release of ammonia, NH₃. For all molar ratios, n, the hydride nanocomposites are fully reversible at 175 °C under a relatively mild pressure of 50 bar H₂. The quantity of H₂ desorbed decreases with increasing molar ratio n, due to increasing fraction of inactive, retained MgH₂. However, at 125 °C the dehydrogenation rate is very sluggish and the quantity of released H₂ is minimal. At the temperature range lower than ∼250 °C dehydrogenation of ball milled nanocomposites occurs through formation of the Li₂Mg(NH)₂ hydride phase. The value of the measured dehydrogenation enthalpy change of 46.7 kJ/molH₂ is relatively low and apparently, it is not responsible for sluggish dehydrogenation at 125 °C. The measurements of thermal conductivity for non-milled powders and ball milled nanocomposites show a dramatic reduction of thermal conductivity after ball milling. It seems that this could be a principal factor responsible for such a low dehydrogenation rate at low temperatures.     

Improved hydrogen desorption properties of exfoliated graphite and graphene nanoballs modified MgH2

Magnesium hydride is a leading hydrogen storage material with high hydrogen content, however, suffers with sluggish kinetics. Several methods have been adopted to improve its kinetics, out of which, the addition of catalyst is an impressive way. Carbon materials have shown their promises as catalyst for several hydrogen storage materials. The present work is devoted to investigating the catalytic effects of exfoliated graphite and graphene nanoballs on dehydrogenation kinetics of MgH₂. The lowest onset temperature of 282 °C is observed for graphene nanoballs modified MgH₂ system. Exfoliated graphite mixed MgH₂ desorbed hydrogen at onset temperature 301 °C which is also less than the dehydrogenation temperature of pure MgH₂ (410 °C). The dehydrogenation kinetics has significantly improved by the addition of these catalysts as compared to the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from 170 (pure MgH₂) to 136 ± 2 and 140 ± 2 kJ/mol by the addition of exfoliated graphite and graphene nanoballs, respectively. The XRD results confirmed the presence of MgH₂ after milling with exfoliated graphite and graphene nanoballs that indicates that there are no reactions during the milling thus both the additives are effective to improve the dehydrogenation as a catalyst.