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

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

Synthesis of BaFe12O19 by solid state method and its effect on hydrogen storage properties of MgH2

Previous studies have shown that ferrites give a positive effect in improving the hydrogen sorption properties of magnesium hydride (MgH₂). In this study, another ferrite, i.e., BaFe₁₂O₁₉, has been successfully synthesised via the solid state method, and it was milled with MgH₂ to enhance the sorption kinetics. The result showed that the MgH₂ + 10 wt% BaFe₁₂O₁₉ sample started to release hydrogen at about 270 °C which is about 70 °C lower than the as-milled MgH₂. The doped sample was able to absorb hydrogen for 4.3 wt% in 10 min at 150 °C, while as-milled MgH₂ only absorbed 3.5 wt% of hydrogen under similar conditions. The desorption kinetic results showed that the doped sample released about 3.5 wt% of hydrogen in 15 min at 320 °C, while the as-milled MgH₂ only released about 1.5 wt% of hydrogen. From the Kissinger plot, the apparent activation energy of the BaFe₁₂O₁₉-doped MgH₂ sample was 115 kJ/mol which was lower than the milled MgH₂ (141 kJ/mol)_. Further analyses demonstrated that MgO, Fe and Ba or Ba-containing contribute to the improvement by serving as active species, thus enhancing the MgH₂ for hydrogen storage.     

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

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

Hydrogen sorption,kinetics, reversibility,and reaction mechanisms of MgH2-xLiBH4 doped with activated carbon nanofibers for reversible hydrogen storage based laboratory powder and tank scales

De/rehydrogenation kinetics and reversibility of MgH₂ are improved by doping with activated carbon nanofibers (ACNF) and compositing with LiBH₄. Via doping with 5 wt % ACNF, hydrogen absorption of Mg to MgH₂ (T = 320 °C and p(H₂) = 50 bar) increases from 0.3 to 4.5 wt % H₂. Significant reduction of onset dehydrogenation temperature of MgH₂ to 340 °C (ΔT = 70 °C as compared with pristine MgH₂) together with 6.8–8.2 wt % H₂ can be obtained by compositing Mg-5 wt. % ACNF with LiBH₄ (LiBH₄:Mg mole ratios of 0.5:1, 1:1, and 2:1). During dehydrogenation of Mg-rich composites (0.5:1 and 1:1 mol ratios), the formation of MgB₂ and Mg_0.816Li_0.184 implying the reaction between LiBH₄ and MgH₂ favors kinetic properties and reversibility, while the composite with 2:1 mol ratio shows individual dehydrogenation of LiBH₄ and MgH₂. For up-scaling to hydrogen storage tank (～120 times greater sample weight than laboratory scale) of the most suitable composite (1:1 mol ratio), de/rehydrogenation kinetics and hydrogen content released at all positions of the tank are comparable and approach to those from laboratory scale. Due to high purity (100%) and temperature of hydrogen gas from hydride tank, the performance of single proton exchange membrane fuel cell enhances up to 30% with respect to the results from compressed gas tank.     

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.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C，which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

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.     

Determination of kinetic parameters and hydrogen desorption characteristics of MgH2-10 wt% (9Ni–2Mg–Y) nano-composite

Hydrogen desorption kinetic parameters of MgH₂ compounds were measured and compared with published gas solid reaction models. The compounds investigated in this study were as-received MgH₂, ball milled MgH₂, and MgH₂ ball milled with 9Ni–2Mg–Y catalyst compound. It was determined that different models were necessary to fit the hydrogen desorption data collected at different temperatures on the same sample, indicating that desorption mechanisms changed with respect to temperature. Addition of (9Ni–2Mg–Y) alloy as a catalyst to MgH₂ increased the hydrogen desorption capacity of MgH₂ from zero (for as-received MgH₂) to about 5 wt% at 350 °C within 500 s. The activation energy value was determined as 187 kJ/mol H₂ for the as-received MgH₂, 137 kJ/mol H₂ for 20 h ball milled MgH₂, and 62 kJ/mol H₂ for 20 h ball milled MgH₂-10 wt% (9Ni–2Mg–Y) nano-composite by the Arrhenius and Kissinger methods. Moreover, the integral heat of H₂ desorption for the MgH₂-10 wt% (9Ni–2Mg–Y) nano-composite was measured to be about 78 ± 0.5 kJ/mol H₂ by adsorption micro-calorimetry consistent with the results of the Arrhenius and Kissinger methods.     

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.     

The enhanced de/re‐hydrogenation performance of MgH2 with TiH2 additive

Catalytic effects of TiH₂ on hydrogenation/dehydrogenation kinetics of MgH₂ were investigated in this study. The TG analysis showed that the addition of the x wt% TiH₂ exhibited lower onset temperature of 160°C which is 100°C and 190°C lower than as‐milled and as‐received MgH₂. The dehydrogenation and hydrogenation kinetics were significantly improved compared with the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from ?137.13 to ?77.58 kJ/mol by the addition of TiH₂. XRD and XPS results showed that the phase of TiH₂ remained same during the dehydrogenation without any intermediate formation confirming its role as catalyst.     

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.     

Improved de/re-hydrogenation properties of MgH2 catalyzed by graphene templated Ti–Ni–Fe nanoparticles

The present investigation deals with the excellent catalytic effect of graphene templated Ti–Ni–Fe nanoparticles (Ti–Ni–Fe@Gr) on de/re-hydrogenation characteristics of MgH₂. The catalytic effect of Ti–Ni–Fe@Gr on MgH2 has also been compared with Ti@Gr, Ni@Gr, and Fe@Gr. It has been found that Ti–Ni–Fe@Gr lowers the onset desorption temperature up to 252 °C with improved kinetics and cyclability for the hydrogen release and absorption from MgH₂. The presence of a multivalence environment around Mg/MgH₂ has been analyzed by XPS analysis which gives the evidence of possible electronic exchange between the catalyst and Mg/MgH₂ during de-/rehydrogenation. Since Mg/MgH₂ and Ti–Ni–Fe are both anchored on graphene template, agglomeration detrimental to cycling is not possible. Thus negligible degradation of 0.22 wt% has been observed even after 24 cycles of de/re-hydrogenation.     

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.     

Effects of REF3 (RE = Y,La, Ce) additives on dehydrogenation properties of LiAlH4

The catalytic effects of rare earth fluoride REF₃ (RE = Y, La, Ce) additives on the dehydrogenation properties of LiAlH₄ were carefully investigated in the present work. The results showed that the dehydrogenation behaviors of LiAlH₄ were significantly altered by the addition of 5 mol% REF₃ through ball milling. The destabilization ability of these catalysts on LiAlH₄ has the order: CeF₃>LaF₃>YF₃. For instance, the temperature programmed desorption (TPD) analyses showed that the onset dehydrogenation temperature of CeF₃ doped LiAlH₄ was sharply reduced by 90 °C compared to that of pristine LiAlH₄. Based on differential scanning calorimetry (DSC) analyses, the dehydriding activation energies of the CeF₃ doped LiAlH₄ sample were 40.9 kJ/mol H₂ and 77.2 kJ/mol H₂ for the first and second dehydrogenation stages, respectively, which decreased about 40.0 kJ/mol H₂ and 60.3 kJ/mol H₂ compared with those of pure LiAlH₄. In addition, the sample doped with CeF₃ showed the fastest dehydrogenation rate among the REF₃ doped LiAlH₄ samples at both 125 °C and 150 °C during the isothermal desorption. The phase changes in REF₃ doped LiAlH₄ samples during ball milling and dehydrogenation were examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF₃ were proposed.     

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.     

A novel complex oxide TiVO3.5 as a highly active catalytic precursor for improving the hydrogen storage properties of MgH2

Herein, we demonstrate the successful preparation of a novel complex transition metal oxide (TiVO_3.5) by oxidizing a solid-solution MXene (Ti_0.5V_0.5)₃C₂ at 300 °C and its high activity as a catalyst precursor in the hydrogen storage reaction of MgH₂. The prepared TiVO_3.5 inherits the layered morphology of its MXene precursor, but the layer surface becomes very coarse because of the presence of numerous nanoparticles. Adding a minor amount of TiVO_3.5 remarkably reduces the dehydrogenation and hydrogenation temperatures of MgH₂ and enhances the reaction kinetics. The 10 wt% TiVO_3.5-containing sample exhibits optimal hydrogen storage properties, as it desorbs approximately 5.0 wt% H₂ in 10 min at 250 °C and re-absorbs 3.9 wt% H₂ in 5 s at 100 °C and under 50 bar of hydrogen pressure. The apparent activation energy is calculated to be approximately 62.4 kJ/mol for the MgH₂-10 wt% TiVO_3.5 sample, representing a 59% reduction in comparison with pristine MgH₂ (153.8 kJ/mol), which reasonably explains the remarkably reduced dehydrogenation operating temperature. Metallic Ti and V are detected after ball milling with MgH₂; they are uniformly dispersed on the MgH₂ matrix and act as actual catalytic species for the improvement of the hydrogen storage properties of MgH₂.     

Assessment of hydrogen storage property of CaMgBH system using NMR and thermal analysis techniques

Thermal dehydrogenation of Ca(BH₄)₂ and Ca(BH₄)₂2MgH₂ composite has been investigated, and the results were compared. The Ca((BH₄)₂ dehydrogenated in two steps between 325 °C and 500 °C as per the reactions Ca(BH₄)₂ = CaH₂ + 2B + 3H₂, and Ca(BH₄)₂ = 1/3CaB₆ + 2/3CaH₂ + 10/3H₂. The partial dehydrogenation of CaH₂ also takes place during the second step dehydrogenation according to the reaction CaH₂ + 6B = CaB₆ + H₂. The completion of second step dehydrogenation requires a temperature of higher than 500 °C. The activation energies corresponding to these steps were found to be 149 ± 8 kJ/mol and 162 ± 10 kJ/mol, respectively. The Ca(BH₄)₂2MgH₂ composite dehydrogenates in a single step as per the reaction: Ca(BH₄)₂ + 3MgH₂ = CaMg₂ + 2B + 7H₂ + Mg. The dehydrogenation of Ca(BH₄)₂2MgH₂ started at 340 °C and completed before 450 °C. The activation energy of Ca(BH₄)₂2MgH₂ dehydrogenation was found to be 180 ± 8 kJ/mol.     

Remarkable hydrogen storage properties and mechanisms of the shell–core MgH2@carbon aerogel microspheres

Carbon aerogel (CA) microspheres with highly crumpled graphene–like sheets surface and network internal structure have been successfully prepared by an inverse emulsion polymerization routine, subsequently ball milled with Mg powder to fabricate Mg@CA. The Mg change into MgH₂ phases, decorating on the surface of the CA forming MgH₂@CA microspheres composite after the hydrogenation process at 400 °C. The MgH₂@CA microspheres composite displays MgH₂–CA shell–core structure and shows enhanced hydrogenation and dehydrogenation rates. It can quickly uptake 6.2 wt% H₂ within 5 min at 275 °C and release 4.9 wt% H₂ within 100 min at 350 °C, and the apparent activation energy for the dehydrogenation is decreased to 114.8 kJ mol^?1. The enhanced sorption kinetics of the composite is attributed to the effects of the in situ formed MgH₂ NPs during the hydrogenation process and the presence of CA. The nanosized MgH₂ could reduce the hydrogen diffusion distance, and the CA provides the sites for nucleation and prevents the grains from agglomerating. This novel method of in situ producing MgH₂ NPs on zero–dimensional architecture can offer a new horizon for obtaining high performance materials in the hydrogen energy storage field.     

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.     

Enhanced hydrogen storage performances of LiBH4 modified with three-dimensional porous fluorinated graphene

Novel fluorinated graphene (FG) nano-sheets with three-dimensional (3D) porous structure were synthesized by one-pot hydrothermal reaction, and then ball milled with LiBH₄ to prepare the hydrogen storage composite material. The LiBH₄ with 20 wt.% FG composite begins to release hydrogen at 204 °C, 120 °C lower than that of pure LiBH₄. Moreover, it can release 3.45 wt.% hydrogen at 400 °C within 1000 s, which is 2.57 times faster than pure LiBH₄. The reversibility of the LiBH₄–FG composite also has been enhanced, its absorption capacity still reaches 78.6% of initial hydrogen uptake at the 4th cycle. According to the phase composition analyses, F^– can partially substitute the anionic H in LiBH₄ or LiH, resulting in a favorable thermodynamic modification. Additionally, the activation energy (E_a) of hydrogen desorption of LiBH₄ is reduced from 181.80 kJ/mol to 130.87 kJ/mol. The remarkably improved hydrogen storage performances of LiBH₄ are largely attributed to the combined effects of the nano-modifying and the function of F anion of the FG.