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.     

Synergetic effect of reactive ball milling and cold pressing on enhancing the hydrogen storage behavior of nanocomposite MgH2/10 wt% TiMn2 binary system

Intermetallic TiMn₂ compound was employed for improving the de/rehydrogenation kinetics behaviors of MgH₂ powders. The metal hydride powders, obtained after 200 h of reactive ball milling was doped with 10 wt% TiMn₂ powders and high-energy ball milled under pressurized hydrogen of 70 bar for 50 h. The cold-pressing technique was used to consolidate them into 36-green buttons with 12 mm in diameter. During consolidation, the hard TiMn₂ spherical powders deeply embedded into MgH₂ matrix to form homogeneous nanocomposite bulk material. The apparent activation energies of hydrogenation and dehydrogenation for the fabricated buttons were 19.3 kJ/mol and 82.9 kJ/mol, respectively. The present MgH₂/10 wt% TiMn₂ nanocomposite binary system possessed superior hydrogenation/dehydrogenation kinetics at 225 °C to absorb/desorb 5.1 wt% hydrogen at 10 bar/200 mbar H₂ within 100 s and 400 s, respectively. This new system revealed good cyclability of achieving 414 cycles within 600 h continuously without degradations. For the present study, the consolidated buttons were used as solid-state hydrogen storage for feeding proton-exchange membrane fuel cell through a house made Ti-reactor at 250 °C. This nanocomposite system possessed good capability for providing the fuel cell with hydrogen flow at an average rate of 150 ml/min. The average current and voltage outputs were 3 A and 5.5 V, respectively.     

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

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

Enhanced hydrogen-storage properties of MgH2 by Fe–Ni catalyst modified three-dimensional graphene

High dehydrogenation temperature and slow dehydrogenation kinetics impede the practical application of magnesium hydride (MgH₂) serving as a potential hydrogen storage medium. In this paper, Fe–Ni catalyst modified three-dimensional graphene was added to MgH₂ by ball milling to optimize the hydrogen storage performance, the impacts and mechanisms of which are systematically investigated based on the thermodynamic and kinetic analysis. The MgH₂+10 wt%Fe–Ni@3DG composite system can absorb 6.35 wt% within 100 s (300 °C, 50 atm H₂ pressure) and release 5.13 wt% within 500 s (300 °C, 0.5 atm H₂ pressure). In addition, it can absorb 6.5 wt% and release 5.7 wt% within 10 min during 7 cycles, exhibiting excellent cycle stability without degradation. The absorption-desorption mechanism of MgH₂ can be changed by the synergistic effects of the two catalyst materials, which significantly promotes the improvement of kinetic performance of dehydrogenation process and reduces the hydrogen desorption temperature.     

Excellent synergistic catalytic mechanism of in-situ formed nanosized Mg2Ni and multiple valence titanium for improved hydrogen desorption properties of magnesium hydride

Nowadays, catalytic doping has been regarded as one of the most promising and effective methods to improve the sluggish kinetics of magnesium hydride (MgH₂). Herein, we synthesized Ni/TiO₂ nanocomposite with the particle sizes about 20 nm by an extremely facile solvothermal method. Then, the Ni/TiO₂ nanocomposite was doped into MgH₂ to enhance its reversible hydrogen storage properties. A remarkably enhancement of de/rehydrogenation kinetics of MgH₂ can be achieved by doped with Ni/TiO₂ nanocomposite, compared to that solely doped with Ni or TiO₂ nanoparticles. The hydrogen desorption peak temperature of MgH₂Ni/TiO₂ is 232 °C, which is 135.4 °C lower than that of ball-milled MgH₂ (367.4 °C). Moreover, the MgH₂Ni/TiO₂ can desorb 6.5 wt% H₂ within 7 min at 265 °C and absorb ∼5 wt% H₂ within 10 min at 100 °C. In particular, the apparent activation energy of MgH₂Ni/TiO₂ is obviously decreased from 160.5 kJ/mol (ball-milled MgH₂) to 43.7 ± 1.5 kJ/mol. Based on the analyses of microstructure evolution, it is proved that metallic Ni particles can react with Mg easily to form fine Mg₂Ni particles after dehydrogenation, and the in-situ formed Mg₂Ni will transform into Mg₂NiH₄ in the subsequent rehydrogenation process. The significantly improved hydrogen absorption/desorption properties of MgH₂Ni/TiO₂ can be ascribed to the synergistic catalytic effect of reversible transformation of Mg₂Ni/Mg₂NiH₄ which act as “hydrogen pump”, and the multiple valence titanium compounds (Ti^4+/3+/2+) which promote the electrons transfer of MgH₂/Mg.     

Superior catalysis of NbN nanoparticles with intrinsic multiple valence on reversible hydrogen storage properties of magnesium hydride

Magnesium hydride, as a potential solid state hydrogen carrier has attracted great attention around the world especially in the energy storage domain due to the high hydrogen storage capacity and the good cycling stability. But kinetic and thermodynamic barriers also impede the practical application and development of MgH₂. Nanoscale catalysts are deemed to be the most effective measure to overcome the kinetic barrier and lower the temperature required for hydrogen release in MgH₂. NbN nanoparticles (~20 nm) with intrinsic Nb³⁺-N and Nb⁵⁺-N were prepared using the molten salt method and used as catalysts in the MgH₂ system. It is found that the NbN nanoparticles exhibit a superior catalytic effect on de/rehydrogenation kinetics for the MgH₂/Mg system. About 6.0 wt% hydrogen can be liberated for the MgH₂+5NbN sample within 5 min at 300 °C, and it takes 12 min to desorb the same amount of hydrogen at 275 °C. Meanwhile, the MgH₂+5NbN sample can absorb 6.0 wt% hydrogen within 16 min at 150 °C, and absorb 5.0 wt% hydrogen within 24 min even at 100 °C. Particularly, the catalyzed samples exhibit excellent hydrogen absorption/desorption kinetic stability. After multiple cycles, there is no kinetic attenuation and the hydrogen capacity remains at about 6.0 wt%. It is demonstrated that the NbN nanoparticles with intrinsic multiple valence can be the critical effect in improving the hydrogen storage kinetics of MgH₂. The stability of Nb₄N₃ phase and Nb³⁺-N and Nb⁵⁺-N valence states can ensure a stable catalytic effect in the system.     

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.     

Improvement of hydrogen storage properties in MgH2 catalysed by K2NbF7

The catalytic effects of K₂NbF₇ on the hydrogen storage properties of MgH₂ have been studied for the first time. MgH₂ + 5 wt% K₂NbF₇ has reduced the onset dehydrogenation temperature to 255 °C, which is 75 °C lower than the as-milled MgH₂. For the rehydrogenation kinetic, at 150 °C, MgH₂ + 5 wt% K₂NbF₇ absorbs 4.7 wt% of hydrogen in 30 min whereas the as-milled MgH₂ only absorbs 0.7 wt% of hydrogen under similar condition. For the dehydrogenation kinetic, at 320 °C, the MgH₂ + 5 wt% K₂NbF₇ is able to release 5.2 wt% of hydrogen in 5.6 min as compared to 0.3 wt% by the as-milled MgH₂ under similar condition. Comparatively, the E_a value of MgH₂ + 5 wt% K₂NbF₇ is 96.3 kJ/mol, which is 39 kJ/mol lower compared to the as-milled MgH₂. The MgF₂, the KH and the Nb that are found after the heating process are believed to be the active species that have improved the system properties. It is concluded that the K₂NbF₇ is a good catalyst to improve the hydrogen storage properties of MgH₂.     

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.     

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.     

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.     

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

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

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.     

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.     

ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism

Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH₂, yet this catalysis could be enhanced further by supporting NPs on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently doped into MgH₂ to remarkably improve its hydrogen storage properties. The MgH₂-Co@CNTs starts to obviously release hydrogen at 267.8 °C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 °C within 15 min. For absorption, the MgH₂-Co@CNTs uptakes 6.15 wt% H₂ at 250 °C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH₂-Co@CNTs can be well preserved during the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based on XRD, TEM and EDS results, the catalytic mechanism of MgH₂-Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg₂Co to Mg₂CoH₅, and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg₂Co/Mg₂CoH₅ can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH₂. Meanwhile, the carbon pieces coated on the surfaces of MgH₂ particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH₂ NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH₂-Co@CNTs system.     

Remarkable kinetics of novel Ni@CeO2–MgH2 hydrogen storage composite

Magnesium hydroxide (MgH₂) has excellent reversibility and high capacity, and is one of the most promising materials for hydrogen storage in practical applications. However, it suffers from high dehydrogenation temperature and slow sorption kinetics. Rare earth hydrides and transition metals can both significantly improve the de/hydrogenation kinetics of MgH₂. In this work, MgH₂–Mg₂NiH₄–CeH_2.73 is in-situ synthesized by introducing Ni@CeO₂ into MgH₂. The unique coating structure of Ni@CeO₂ facilitates homogeneous distribution of synergetic CeH_2.73 and Mg₂NiH₄ catalytic sites in subsequent ball milling process. The as-fabricated composite MgH₂-10 wt% Ni@CeO₂ powders possess superior hydrogenation/dehydrogenation characteristics, absorbing 4.1 wt% hydrogen within 60 min at 100 °C and releasing 5.44 wt% H₂ within 10 min at 350 °C. The apparent activation energy of MgH₂-10 wt% Ni@CeO₂ is determined to be 84.8 kJ/mol and it has favorable hydrogen cycling stability with almost no decay in capacity after 10 cycles.     

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.     

Studies on de/rehydrogenation characteristics of nanocrystalline MgH2 co-catalyzed with Ti,Fe and Ni

In the present study, we have investigated the combined effect of different transition metals such as Ti, Fe and Ni on the de/rehydrogenation characteristics of nano MgH₂. Mechanical milling of MgH₂ with 5 wt% each of Ti, Fe and Ni for 24 h at 12 atm of H₂ pressure lead to the formation of nano MgH₂-Ti5Fe5Ni5. The decomposition temperature of nano MgH₂-Ti5Fe5Ni5 is lowered by 90 °C as compared to nano MgH₂ alone. It is also found that the nano MgH₂-Ti5Fe5Ni5 absorbs 5.3 wt% within 15 min at 270 °C and 12 atm hydrogen pressures. However, nano MgH₂ reabsorbs only 4.2 wt% under identical condition. An interesting result of the present study is that mechanical milling of MgH₂ separately with Fe and Ni besides refinement in particle size also leads to the formation of alloys Mg₂NiH₄ and Mg₂FeH₆ respectively. On the other hand, when MgH₂ is mechanically milled together with Ti, Fe and Ni, the dominant result is the formation of nano particles of MgH₂. Moreover the activation energy for dehydrogenation of nano MgH₂ co-catalyzed with Ti, Fe and Ni is 45.67 kJ/mol which is 35.71 kJ/mol lower as compared to activation energy of nano MgH₂ (81.34 kJ/mol). These results are one of the most significant in regard to improvement in de/rehydrogenation characteristics of known MgH₂ catalyzed through transition metal elements.     

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

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

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.