Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

Influence of different amounts of FeCl3 on decomposition and hydrogen sorption kinetics of MgH2

In the present work, the hydrogen storage properties of MgH₂-X wt.% FeCl₃ (X = 5, 10, 15 and 20) are investigated experimentally. It is found that the MgH₂ + 10 wt.% FeCl₃ sample exhibits the best comprehensive hydrogen storage properties, in terms of the onset dehydrogenation temperature, the hydrogen amounts de/reabsorbed as well as the relative de/rehydrogenation rates. The onset dehydrogenation temperature of the 10 wt.% FeCl₃-doped MgH₂ sample is reduced by about 90 °C compared to the as-milled MgH₂, and the sorption kinetics measurements indicate that the FeCl₃-doped sample displays an average dehydrogenation rate 5–6 times faster than that of the undoped MgH₂ sample. Higher levels of doping introduce negative effects, such as lower capacity and slower absorption/desorption rates compared to samples with lower FeCl₃ doping levels. The apparent activation energy for hydrogen desorption is decreased from 166 kJ•mol⁻¹ for as-milled MgH₂ to 130 kJ•mol⁻¹ by the addition of 10 wt.% FeCl₃. It is believed that the improvement of the MgH₂ sorption properties in the MgH₂/FeCl₃ composite is due to the catalytic effects of the in-situ generated Fe species and MgCl₂ that are formed during the heating process.     

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.     

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.     

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.     

CNTs decorated with CoFeB as a dopant to remarkably improve the dehydrogenation/rehydrogenation performance and cyclic stability of MgH2

The chain-like carbon nanotubes (CNTs) decorated with CoFeB (CoFeB/CNTs) prepared by oxidation-reduction method is introduced into MgH₂ to facilitate its hydrogen storage performance. The addition of CoFeB/CNTs enables MgH₂ to start desorbing hydrogen at only 177 °C. Whereas pure MgH₂ starts hydrogen desorption at 310 °C. The dehydrogenation apparent activation energy of MgH₂ in CoFeB/CNTs doped-MgH₂ composite is only 83.2 kJ/mol, and this is about 59.5 kJ/mol lower than that of pure MgH₂. In addition, the completely dehydrogenated MgH₂−10 wt% CoFeB/CNTs sample can start to absorb hydrogen at only 30 °C. At 150 °C and 5 MPa H₂, the MgH₂ in CoFeB/CNTs doped-MgH₂ composite can absorb 6.2 wt% H₂ in 10 min. The cycling kinetics can remain rather stable up to 20 cycles, and the hydrogen storage capacity retention rate is 98.5%. The in situ formation of Co₃MgC, Fe, CoFe and B caused by the introduction of CoFeB/CNTs can provide active and nucleation sites for the dehydrogenation/rehydrogenation reactions of MgH₂. Moreover, CNTs can provide hydrogen diffusion pathways while also enhancing the thermal conductivity of the sample. All of these can facilitate the dehydrogenation/rehydrogenation performance and cyclic stability 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.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

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.     

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.     

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.     

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.     

Fluorographene nanosheets enhanced hydrogen absorption and desorption performances of magnesium hydride

Fluorographene (FG), which inherits the properties of graphene and fluorographite (FGi), was successfully fabricated through a simple sonochemical exfoliation route in N-methyl-2-pyrrolidone (NMP) and then MgH₂-FG composite was prepared by ball milling. The dehydrogenation and rehydrogenation performances of MgH₂-FG composite were investigated systematically comparing with as-received MgH₂ and MgH₂-G composite. It is found that the as-prepared FG exhibited a significant catalytic effect on the dehydrogenation and rehydrogenation properties of MgH₂. The MgH₂-FG composite can uptake 6.0 wt% H₂ in 5 min and release 5.9 wt% H₂ within 50 min at 300 °C, while the as-received MgH₂ uptakes only 2.0 wt% H₂ in 60 min and hardly releases hydrogen at the same condition. The hydrogen storage cycling kinetics in the first 10 cycles remains almost the same, indicating the excellent reversibility of the MgH₂-FG composite. SEM analysis shows that the particle size of MgH₂-FG composite was ∼200 nm, much smaller than that of as-received MgH₂ (∼20 μm). TEM observations show that MgH₂ particles were embedded in FG layers during ball milling. The dehydrogenation apparent activation energy for the MgH₂ is reduced from 186.3 kJ mol⁻¹ (as-received MgH₂) to 156.2 kJ mol⁻¹ (MgH₂-FG composite). The catalytic mechanism has been proposed that F atoms in FG serve as charge-transfer sites and accelerate the rate of hydrogen incorporation and dissociation, consequently enhance the dehydrogenation and rehydrogenation properties of MgH₂-FG composite. Furthermore, the FG can inhibit the sintering and agglomeration of MgH₂ particle, thus it improves the cycling dehydrogenation and rehydrogenation of MgH₂-FG composite.     

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.     

Dual-cation K2TaF7 catalyst improves high-capacity hydrogen storage behavior of MgH2

MgH₂ has been extensively regarded as a low-cost hydrogen storage material with high gravimetric hydrogen capacity of approximately 7.6 wt%. However, the hydrogen release and absorption kinetics in MgH₂ still needs further improving. For the first time, the catalytic impacts of a new dual-cation metal fluoride K₂TaF₇ upon the hydrogen storage characteristics of MgH₂ have been investigated in this work. With only 1 wt% K₂TaF₇ dopant, the initial dehydrogenation temperature of MgH₂ was lowered by about 130 °C, releasing more than 7.3 wt% hydrogen totally. The desorption activation energy of MgH₂ + 1 wt% K₂TaF₇ composite was decreased to 107.2 ± 1.2 kJ mol^?1. Besides, at 190 °C, the dehydrogenated MgH₂ + 1 wt% K₂TaF₇ sample could absorb 6.56 wt% H₂, while pristine MgH₂ re-absorbed only 3.45 wt% H₂. Further studies revealed that K₂TaF₇ could react with MgH₂ during dehydrogenation and produce symbiotic hydrides KMgH₃ and TaH_0.8, which could play the role of hydrogen pumps during hydrogen release and uptake. The cooperative catalysis between the hydrogen pump effect and the active interface in the multi-hydride area significantly enhanced the reversible hydrogen storage in the MgH₂+1 wt% K₂TaF₇ composite. This study provides new thinking for novel catalysts to elevate the hydrogen storage performance of MgH₂.     

Synergistic enhancement of hydrogen storage properties in MgH2 using LiNbO3 catalyst

Extensive researches are being conducted to improve the high dehydrogenation temperature and sluggish hydrogen release rate of magnesium hydride (MgH₂) for better industrial application. In this study, LiNbO₃, a catalyst composed of alkali metal Li and transition metal Nb, was prepared through a direct one-step hydrothermal synthesis, which remarkably improved the hydrogen storage performance of MgH₂. With the addition of 6 wt% LiNbO₃ in MgH₂, the initial dehydrogenation temperature decreases from 300 °C to 228 °C, representing a drop of almost 72 °C compared to milled MgH₂. Additionally, the MgH₂-6 wt.% LiNbO₃ composite can quickly release 5.45 wt% of H₂ within 13 min at 250 °C, and absorbed about 3.5 wt% of H₂ within 30 min at 100 °C. It is also note that LiNbO₃ shows better catalytic effect compared to solely adding Li₂O or Nb₂O₅. Furthermore, the activation energy of MgH₂-6 wt.% LiNbO₃ decreased by 44.37% compared to milled MgH₂. The enhanced hydrogen storage performance of MgH₂ is attributed to the in situ formation of Nb-based oxides in the presence of LiNbO₃, which creates a multielement and multivalent chemical environment.     

Effect of Ni and SAPO-34 co-additive on enhancing hydrogen storage performance of MgH2

The hydrogen storage performances of MgH₂ improved by the addition of Ni and SAPO-34 were studied in detail. The mixture of MgH₂ with Ni and SAPO-34 was a physical reaction as shown by the X-ray diffraction (XRD) results. The SAPO-34 and Ni were uniformly distributed on the surface of MgH₂. The thermodynamic and kinetic properties of 90MgH₂/5Ni/5SAPO-34 were investigated by differential scanning calorimetry (DSC) and pressure-composition-isothermal (PCI) methods. The results showed that the dehydrogenation activation energy of 90MgH₂/5Ni/5SAPO-34 decreased by 64.3 kJ/mol compared with that of MgH₂. In addition, the relationship between the value of dehydrogenation heat and hydrogen content was also investigated by in-situ calorimetry. The enthalpy value of each sample in the dehydrogenation processes were calculated by in-situ calorimetry measurement. The dehydrogenation enthalpies of as-milled MgH₂ and 90MgH₂/5Ni/5SAPO-34 were 63.2 kJ/mol H₂ and 53.6 kJ/mol H₂, respectively. Thus, the co-doping of Ni and SAPO-34 contributed significantly to decrease the thermodynamic stability and improve the hydrogen sorption kinetic properties 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.     

Effect of adding different percentages of HfCl4 on the hydrogen storage properties of MgH2

Magnesium hydride (MgH₂) is the best candidate material to store hydrogen in the solid-state form owing to its advantages such as good reversibility, high hydrogen storage capacity (7.6 wt%), low raw material cost and abundance in the earth. Nevertheless, slow desorption/absorption kinetics and high thermodynamic stability are two issues that have constrained the commercialization of MgH₂ as a solid-state hydrogen storage material. So, to boost the desorption/absorption kinetics and to alter the thermodynamics of MgH₂, hafnium tetrachloride (HfCl₄) was used as a catalyst in this study. Different percentages of HfCl₄ (5, 10, 15 and 20 wt%) were added to MgH₂ and their catalytic influences on the hydrogen storage properties of MgH₂ were investigated. Results showed that the 15 wt% HfCl₄-doped MgH₂ sample was the best composite to enhance the hydrogen storage performance of MgH₂. The onset decomposition temperature of the 15 wt% HfCl₄-doped MgH₂ composite was decreased by ~75 °C compared to as-milled MgH₂. Meanwhile, the desorption/absorption kinetic measurements showed an improvement compared to the undoped MgH₂. From the Kissinger analysis, the apparent dehydrogenation activation energy was 167.0 kJ/mol for undoped MgH₂ and 102.0 kJ/mol for 15 wt% HfCl₄-doped MgH₂. This shows that the HfCl₄ addition reduced the activation energy of the hydrogen decomposition of MgH₂. The desorption enthalpy change calculated by the van't Hoff equation showed that the addition of HfCl₄ to MgH₂ did not affect the thermodynamic properties. Scanning electron microscopy showed that the size of the MgH₂ particles decreased and there was less agglomeration after the addition of HfCl₄. It is believed that the decrease in the particle size and in-situ generated MgCl₂ and Hf-containing species had synergistic catalytic effects on enhancing the hydrogen storage properties of the HfCl₄-doped MgH₂ composite.     

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.