Single-step preparation and hydrogenation of single walled carbon nanotubes-titanium dioxide composite

The composite material made up of carbon nanotubes (CNTs) and metal oxide nanostructures have been investigated for hydrogen storage application. The present experimental work deals with the investigation of hydrogen storage in single walled carbon nanotubes-titanium dioxide (SWCNTs-TiO₂) composite. The SWCNTs-TiO₂ composite has been made by depositing the pellet containing the mixture of SWCNTs and TiO₂ using electron beam (EB) evaporation technique in hydrogen ambient. The preparation and hydrogenation of the SWCNTs-TiO₂ composite have been done in a single-step. The characterization results expose that the deposition of SWCNTs with TiO₂ material is possible using EB evaporation technique without any significant structural decomposition of SWCNTs. The amount of hydrogen incorporated is found to be 3.2 wt.%, and it is attributed to both the synergetic action of SWCNTs-TiO₂ nanostructures and the method of preparation. The stored hydrogen is found to be released completely in the temperature range of 120–215 °C.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Hydrolysis of AlLi/NaBH4 system promoted by Co powder with different particle size and amount as synergistic hydrogen generation for portable fuel cell

Hydrogen generation from the hydrolysis of aluminum lithium/sodium borohydride (referred to as AlLi/NaBH₄) system activated by Co powder with different particle size and amount was evaluated in this paper. The designed aluminum–lithium–cobalt (referred to as Al–Li–Co/NaBH₄) systems including Al-5 wt% Li-50 wt% nano Co, Al-7.5 wt% Li-25 wt% nano Co, Al-5 wt% Li-50 wt% micro Co, and Al-7.5 wt% Li-25 wt% micro Co had 100% hydrogen yield at 323 K. The hydrogen generation rates of these systems were regulated by Co species, Co amount, as well as consecutive runs of NaBH₄ hydrolysis. The underlying activation mechanism, including the formation of Al_0.94Co_1.06 alloy and highly active and stable Co-based catalyst has been elaborated in this study. Experimental data present an inexpensive and highly efficient hydrogen source for portable fuel cell.     

Hydrogen generation from the ball milled composites of sodium and lithium borohydride (NaBH4/LiBH4) and magnesium hydroxide (Mg(OH)2) without and with the nanometric nickel (Ni) additive

The composites of (NaBH₄+2Mg(OH)₂) and (LiBH₄+2Mg(OH)₂) without and with nanometric Ni (n-Ni) added as a potential catalyst were synthesized by high energy ball milling. The ball milled NaBH₄-based composite desorbs hydrogen in one exothermic reaction in contrast to its LiBH₄-based counterpart which dehydrogenates in two reactions: an exothermic and endothermic. The NaBH₄-based composite starts desorbing hydrogen at 240 °C. Its ball milled LiBH₄-based counterpart starts desorbing at 200 °C. The latter initially desorbs hydrogen rapidly but then the rate of desorption suddenly decelerates. The estimated apparent activation energy for the NaBH₄-based composite without and with n-Ni is equal to 152 ± 2.2 and 157 ± 0.9 kJ/mol, respectively. In contrast, the apparent activation energy for the initial rapid dehydrogenation for the LiBH₄-based composite is very low being equal to 47 ± 2 and 38 ± 9 kJ/mol for the composite without and with the n-Ni additive, respectively. XRD phase studies after volumetric isothermal dehydrogenation tests show the presence of NaBO₂ and MgO for the NaBH₄-based composite. For the LiBH₄-based composite phases such as MgO, Li₃BO₃, MgB₂, MgB₆ are the products of the first exothermic reaction which has a theoretical H₂ capacity of 8.1 wt.%. However, for reasons which are not quite clear, the first reaction never goes to full completion even at 300 °C desorbing ∼4.5 wt.% H₂ at this temperature. The products of the second endothermic reaction for the LiBH₄-based composite are MgO, MgB₆, B and LiMgBO₃ and the reaction has a theoretical H₂ capacity of 2.26 wt.%. The effect of the addition of 5 wt.% nanometric Ni on the dehydrogenation behavior of both the NaBH₄-and LiBH₄-based composites is rather negligible. The n-Ni additive may not be the optimal catalyst for these hydride composite systems although more tests are required since only one n-Ni content was examined.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Comprehensive hydrogen storage properties and catalytic mechanism studies of 2LiBH4–MgH2 system with NbF5 in various addition amounts

In the present work, the role of NbF₅ addition amount in affecting the comprehensive hydrogen storage properties (dehydrogenation, rehydrogenation, cycling performance, hydrogen capacity) of 2LiBH₄–MgH₂ system as well as the catalytic mechanism of NbF₅ have been systematically studied. It is found that increasing the addition amount of NbF₅ to the 2LiBH₄–MgH₂ system not only results in dehydrogenation temperature reduction and hydriding–dehydriding kinetics enhancement but also leads to the de/rehydrogenation capacity loss. Compared with other samples, 2LiBH₄–MgH₂ doping with NbF₅ in weight ratios of 40:4 exhibits superior comprehensive hydrogen storage properties, which can stably release ∼8.31 wt.% hydrogen within 2.5 h under 4 bar H₂ and absorb ∼8.79 wt.% hydrogen within 10 min under 65 bar H₂ at 400 °C even up to 20 cycling. As far as we know, this is the first time that excellent reversibility as high as 20 cycles without obvious degradation tendency in both of hydrogen capacity and reaction rate has been achieved in the 2LiBH₄–MgH₂ system. The further experimental study reveals that the highly catalytic effects of NbF₅ on the 2LiBH₄–MgH₂ system are derived from the reaction between NbF₅ and LiBH₄, which provides a fundamental insight into the catalytic mechanism of NbF₅.     

Hydrogen storage performance of 5LiBH4 + Mg2FeH6 composite system

The hydrogen storage properties of 5LiBH₄ + Mg₂FeH₆ reactive hydride composites for reversible hydrogen storage were investigated by comparing with the 2LiBH₄ + MgH₂ composite in the present work. The dehydrogenation pathway and reaction mechanism of 5LiBH₄ + Mg₂FeH₆ composite were also investigated and elucidated. The self-decomposition of Mg₂FeH₆ leads to the in situ formation of Mg and Fe particles on the surface of LiBH₄, resulting in a well dispersion between different reacting phases. The formation of FeB is observed during the dehydrogenation of 5LiBH₄ + Mg₂FeH₆ composite, which might supplies nucleation sites of MgB₂ during the dehydrogenation process, but is not an ascendant catalyst for the self-decomposition of LiBH₄. And FeB can also transform to the LiBH₄ and Fe by reacting with LiH and H₂ during the rehydrogenation process. The dehydrogenation capacity for 5LiBH₄ + Mg₂FeH₆ composite still gets to 6.5 wt% even after four cycles. The X-ray diffraction analyses reveal the phase transitions during the hydriding and dehydriding cycle. The formed FeB in the composite maintains a nanostructure after four hydriding-dehydriding cycles. The loss of hydrogen storage capacity and de-/rehydrogenation kinetics can be attributed to the incomplete generation of Mg₂FeH₆ during the rehydrogenation process.     

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.     

Nanocrystallites-forming hierarchical porous Ni/Al2O3–TiO2 catalyst for dehydrogenation of organic chemical hydrides

Dehydrogenation of organic chemical hydrides has been improved by reconstructing the catalyst in the form of hierarchical porous structure nanocatalyst, in which the economical Ni was adopted as catalytic component and nano Al₂O₃–TiO₂ hybrid composite as support. The Al₂O₃–TiO₂ composite was prepared by spontaneous self-assembly of nano Al₂O₃ and TiO₂ aggregates by hydrolysis of tetra-n-butyl-titanate under continuous agitation. The multi-scaled distribution of Al₂O₃–TiO₂ aggregates with hierarchy could be observed in dynamic light scattering spectrometer. The aggregates are comprised of nano-sized γ-Al₂O₃ and anatase TiO₂ crystallites with sizes of about 5 and 7 nm, respectively. The surface modulation by TiO₂ could be verified in FTIR Spectra. The migration of Ti species and crystallite growth were hindered by the Al₂O₃ skeleton and the hierarchical porous structure was sustained during the thermal related process. The multi-scaled distributed pores were confirmed by both TEM analysis and N₂ adsorption results. The results of dehydrogenation experiments showed that the hierarchical porous structure nano Ni/Al₂O₃–TiO₂ exhibited superior catalytic performance to Ni/Al₂O₃ with the optimum conversion of 99.9% at 400 °C, while the catalyst of Ni/Al₂O₃ exhibited only 16.5% under the same condition.     

Kinetic properties of La2Mg17–x wt.% Ni (x = 0–200) hydrogen storage alloys prepared by ball milling

The as-cast La₂Mg₁₇ with different amount of Ni powders were mixed through ball milling to produce a new type of La₂Mg₁₇–x wt.% Ni (x = 50, 100, 150, 200) alloy. The microstructures of the alloys were characterized by XRD technique, the results show that the crystal structure transfers to amorphous one with the increasing amount of Ni powders. La₂Mg₁₇–50 wt.% Ni alloy reaches the highest hydrogen absorption capacity of 5.13 wt.% at 300 °C under 2 MPa hydrogen pressure due to its amorphous structure. Furthermore, La₂Mg₁₇–50 wt.% Ni alloy expresses fast hydriding kinetics and absorbs 4.99 wt.% hydrogen gas in 200 s. The hydrogen desorption ability described as discharge capacity during electrochemical reaction is fade next to La₂Mg₁₇–200 wt.% Ni alloy, attributed to the less Mg₂NiH₄ with lower enthalpies and easier to release H₂. The maximum discharge capacity of La₂Mg₁₇–200 wt.% Ni alloy reaches to exciting 980.90 mAh/g, while the La₂Mg₁₇ alloy is only 18.10 mAh/g with inconspicuous improvement of cycle stability. These dramatic difference in electrochemical performance reflect the consequence of sluggish dehydriding process of La₂Mg₁₇–50 and 100 wt.% Ni alloys again. Whereas La₂Mg₁₇–200 wt.% Ni alloy has lower resistance both on alloy surface and in the bulk.     

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.     

Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

Hydrogen generation from Al/NaBH4 hydrolysis promoted by Li-NiCl2 additives

On-demand hydrogen generation from solid-state Al/NaBH₄ hydrolysis activated by Li-NiCl₂ additives are elaborated in the present paper. Hydrogen generation amount and rate can be regulated by changing Al/NaBH₄ weight ratio, Li and NiCl₂ amount, hydrolytic temperature, etc. The optimized Al−10 wt.% Li−15 wt.% NiCl₂/NaBH₄ mixture (weight ratio of 1:1) yields 1778 ml hydrogen/1 g mixture with 100% efficiency within 50 min at 323 K. The improved hydrolytic performance comes from the effect of Li-NiCl₂ additives, which decrease aluminum particle size in the milling process and produce the catalytic promoter BNi₂/Al(OH)₃ in the hydrolytic process. Compared with the conventional reaction of Al and NaBH₄ in water, there is an interaction of Al/NaBH₄ hydrolysis which improves the hydrolytic kinetics of Al/NaBH₄ via the catalytic effect of hydrolysis by-products Al(OH)₃, BNi₂, and NaBO₂. The Al/NaBH₄ mixture may be applied as a portable hydrogen generation material. Our experimental data lay a foundation for designing practical hydrogen generators.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

Hydrogenation/dehydrogenation in MgH2-activated carbon composites prepared by ball milling

A systematic investigation was performed on the hydrogen storage behaviors of ball-milled MgH₂-activated carbon (AC) composites. Differential Scanning Calorimetry (DSC) measurement on the desorption temperature was carried out and indicated that the onset and peak temperatures both decreased with increasing AC adding amount, for example, the desorption peak temperature shifted from 349 °C for 1 wt% AC to 316 °C for 20 wt% AC. Furthermore, it is noted that the hydrogen absorption capacity and hydriding kinetics of the composites were also dependent on the adding amount of AC, and the optimum condition could be achieved by mechanical milling of MgH₂ with 5 wt% AC. The Mg-5wt%AC composite can absorb about 6.5 wt% hydrogen within 7 min at 300 °C and 6.7 wt% within 2 h at 200 °C, respectively. It is also demonstrated that MgH₂-5wt% AC exhibited good hydrogen desorption property that could release 6.5 wt% at 330 °C within 30 min. X-ray diffraction patterns (XRD) and transmission electron microscopy (TEM) observations revealed that the grain size of the synthesized composites decreased with increasing AC amount. This may contribute to the improvement of hydrogen storage in MgH₂-AC composites.     

Characterizations of composite NaNH2-NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH₂-NaBH₄ (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na₃(NH₂)₂BH₄, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na₃(NH₂)₂BH₄. TG-DTA demonstrates that the phase change temperature of the composite NaNH₂-NaBH₄ (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na₃BN₂; while above 400 °C, the previous by-product Na₃BN₂ continues to decompose so as to give metal Na gradually.     

Al–Li3AlH6: A novel composite with high activity for hydrogen generation

A novel material for hydrogen generation with high capacity of H₂ generation has been successfully prepared by ball milling the mixture of Al and home-made fresh Li₃AlH₆ powder. Its theoretical capacity of hydrogen released is higher than that of pure Al. Results obtained have shown conversion efficiency of Al–Li₃AlH₆ composite can be close to 100% by increasing the content of Li₃AlH₆. When the content of Li₃AlH₆ is 20 wt%, the maximum hydrogen generation rate and hydrogen yield are 2737.6 mL g⁻¹ min⁻¹ and 1513.1 mL g⁻¹, respectively, at room temperature. By XRD, SEM analyses and reaction heat measurements, it demonstrates that the additive Li₃AlH₆ can provide an additional source of H₂ and an alkaline environment (LiOH) as well as additional heat to promote the Al/H₂O reaction. Therefore, the Al–Li₃AlH₆ composite has a very high activity and high capacity of hydrogen released.     

Co-effects of Tm2O3 and porous silica on reversible hydrogen storage in NaAlH4

The co-effects of lanthanide oxide Tm₂O₃ and porous silica on the hydrogen storage properties of sodium alanate are investigated. NaAlH₄-Tm₂O₃ (10 wt%) and NaAlH₄-Tm₂O₃ (10 wt%)-porous SiO₂ (10 wt%) are prepared by the ball milling method, and their hydrogen desorption/re-absorption capacities are compared. Dehydrogenation process was performed at 150 °C under vacuum and rehydrogenation was performed at 150 °C for 4 h under ∼9 MPa in highly pure hydrogen. The results show that Tm₂O₃ has a catalytic effect on the hydrogen desorption and re-absorption of NaAlH₄. The hydrogen desorption capacity of Tm₂O₃ single-doped NaAlH₄ is 4.6 wt%, higher than that of undoped NaAlH₄ (4.3 wt%). During the dehydrogenation process, NaAlH₄ is completely decomposed and no intermediate product Na₃AlH₆ is detected. The addition of porous silica improves the dehydrogenation performance of NaAlH₄. Tm₂O₃ and porous silica co-doped NaAlH₄ could release a maximum hydrogen amount of 4.7 wt%, higher than that of undoped NaAlH₄ and Tm₂O₃ single-doped NaAlH₄. Moreover, porous silica improves the reversibility of hydrogen storage in NaAlH₄.     

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.     

Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

Both kinetics and thermodynamics properties of MgH₂ are significantly improved by the addition of Mg(AlH₄)₂. The as-prepared MgH₂–Mg(AlH₄)₂ composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH₂-relevant desorption is 32.3 kJ mol⁻¹ H₂ in the MgH₂–Mg(AlH₄)₂ composite, obviously decreased than that of pure MgH₂. The dehydriding mechanism of MgH₂–Mg(AlH₄)₂ composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH₄)₂ decomposes and produces active Al^∗. Subsequently, the in-situ formed Al^∗ reacts with MgH₂ and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH₂ and Al^∗, under 3 MPa H₂ pressure at 300 °C for 5 h.