Dehydrogenation Of MgH2 Research Articles

Synergistic effects of transition metal halides and activated carbon nanofibers on kinetics and reversibility of MgH2

MgH2 doped with transition metal halides (TiF4, NbF5, and ZrCl4) and activated carbon nanofibers (ACNF) for reversible hydrogen storage is prepared by ball milling technique. Transition metal halides provide catalytic effects for de/rehydrogenation kinetics, while ACNF benefits thermal conductivity and hydrogen permeability as well as prevents particle agglomeration during cycling. Significant reduction of onset and main dehydrogenation temperatures of MgH2 (ΔT = 243 and 158 °C, respectively) are achieved by doping with 5–10 wt % of NbF5, ACNF-TiF4 and ACNF-NbF5. During the 1st cycle, the latter samples liberate 4.7–5.0 wt % H2 within 1 h 30 min, whereas MgH2 doped with ACNF reaches only 1.5 wt % H2. The reaction between MgF2 and NbHx (x < 1) (MgH2-NbF5 and MgH2-ACNF-NbF5) during dehydrogenation results in the formation of new catalytic active species of Nb-F-Mg favoring kinetics. Upon four hydrogen release and uptake cycles, kinetics and reversibility within 1 h 30 min of MgH2-ACNF-NbF5 are preserved at 5.0 wt % H2, while those of MgH2-NbF5 and MgH2-ACNF-TiF4 decay to 4.4 wt % H2. Activation energy (EA) for dehydrogenation of MgH2 considerably decreases from 140.0 ± 10.2 to 37.8 ± 1.5 kJ/mol after doing with ACNF-NbF5. Superior performance of MgH2-ACNF-NbF5 to MgH2-NbF5 is due to synergistic effects of NbF5 and ACNF. In the case of MH2-ACNF-TiF4, the disappearance of active species benefiting kinetic properties and the formation of thermally stable TiH2 account for inferior hydrogen content reversible.

Enhancing hydrogen storage performances of MgH2 by Ni nano-particles over mesoporous carbon CMK-3

Nano-dispersed Ni particles over mesoporous carbon material CMK-3 (Ni/CMK-3) was fabricated by means of impregnation-reduction strategy using precursor NiCl2 · 6H2O, which is beneficial to improving the de/rehydrogenation performances of MgH2. The dehydrogenation onset temperature of MgH2–Ni/CMK-3 is significantly lowered by 170 K from that of pristine MgH2 (around 603 K). Totally 5.9 wt% of hydrogen absorption capacity is liberated within 1 h at a temperature of 423 K under a pressure of 3 MPa. This composite can absorb 3.9 wt% hydrogen even at a temperature of 328 K under 3 MPa H2. Activation energy values of both dehydrogenation (43.4 kJ mol−1) and rehydrogenation (37.4 kJ mol−1) for MgH2–Ni/CMK-3 are greatly enhanced from those of as-milled MgH2. Ni/CMK-3 also slightly destabilizes the dehydrogenation of MgH2 by 1.5 kJ mol The enhanced performances can be attributed to the synergistic effects of both destabilization and activation from nano-dispersed Ni particles.

Improved reversible dehydrogenation properties of MgH2 by the synergetic effects of graphene oxide-based porous carbon and TiCl3

In this study, we used a combination of graphene oxide-based porous carbon (GC) and titanium chloride (TiCl3) to improve the reversible dehydrogenation properties of magnesium hydride (MgH2). Examining the effects of GC and TiCl3 on the hydrogen storage properties of MgH2, the study found GC was a useful additive as confinement medium for promoting the reversible dehydrogenation of MgH2. And TiCl3 was an efficient catalytic dopant. A series of controlled experiments were carried out to optimize the sample preparation method and the addition amount of GC and TiCl3. In comparison with the neat MgH2 system, the MgH2/GC-TiCl3 composite prepared under optimized conditions exhibited enhanced dehydrogenation kinetics and lower dehydrogenation temperature. A combination of phase/microstructure/chemical state analyses has been conducted to gain insight into the promoting effects of GC and TiCl3 on the reversible dehydrogenation of MgH2. Our study found that GC was a useful scaffold material for tailoring the nanophase structure of MgH2. And TiCl3 played an efficient catalytic effect. Therefore, the remarkably improved dehydrogenation properties of MgH2 should be attributed to the synergetic effects of nanoconfinement and catalysis.

Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling

In this work, polyvinylpyrrolidone (PVP) coated Mg95Ni5 nano-composites were prepared by hydriding combustion synthesis (HCS) plus wet mechanical milling (WM) with tetrahydrofuran (THF) and donated as WM-x wt% PVP (x = 1, 3, 5 and 7) respectively. The phase compositions, microstructures and dehydriding property, as well as the co-effect of PVP and THF were investigated in detail. XRD results showed that the average crystal size of MgH2 in the milled Mg95Ni5 decreased from 23 nm without PVP to 18 nm with 1 wt% PVP. The peak temperature of dehydrogenation of MgH2 in the milled Mg95Ni5 decreased from 293.0 °C without THF to 250.4 °C with THF. The apparent activation energy for decomposition of MgH2 in WM-7 wt% PVP was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of milled Mg95Ni5 without THF and PVP. PVP and THF can facilitate the refinement of particle size during mechanical milling process. Attributed to small particle sizes and synergistic effect of PVP and THF, the composites exhibit markedly improved dehydriding properties.

Catalytic effect of nickel phthalocyanine on hydrogen storage properties of magnesium hydride: Experimental and first-principles studies

The high dehydrogenation temperature of magnesium hydride MgH2 is still the main obstacle to its practical application as a solid-state hydrogen storage medium. Using experimental and first-principles calculations approaches, we, for the first time, investigate the catalytic effect and mechanism of nickel phthalocyanine on the dehydrogenation properties of MgH2. The results display that a small amount of nickel phthalocyanine can promote MgH2 dehydrogenation at significantly decreased temperatures by more than 90 °C relative to milled pristine or graphene-added MgH2 system. However, the agglomeration of MgH2 particles is not evidently alleviated through nickel phthalocyanine addition. When MgH2 is milled with graphene firstly and then the obtained mixture is further milled with nickel phthalocyanine, the dehydrogenation properties and agglomeration of MgH2 particles can be synergistically improved to some extent. The first-principles calculations of dehydrogenation enthalpy and binding energy account for the experimental differences in catalysis and aggregation-resistance abilities of nickel phthalocyanine and graphene on MgH2 particles. Notably, the NiN4-inserted graphene is predicted to be an ideal additive for MgH2, which combines the synergetic catalysis-confinement effect of nickel phthalocyanine and graphene on MgH2 particles. Analysis of electronic structures reveals that the excellent catalytic effect of nickel phthalocyanine on MgH2 can be ascribed to the more electron transfer between nickel phthalocyanine and MgH2, which induces the significantly weakened bond strength of MgH and decreased dehydrogenation enthalpy of MgH2.

Evolution of reduced Ti containing phase(s) in MgH2/TiO2 system and its effect on the hydrogen storage behavior of MgH2

The present study aims to consolidate information regarding the catalytic reaction mechanism of MgH2/TiO2 hydrogen storage system. Reduction of titania, resulting to evolution of a monoxide rock salt nanocrystalline phase was identified by in-situ X-ray diffraction (XRD) study of a 3 mol% TiO2 additive mixed MgH2. Existence of homogeneously distributed Ti in a crystalline rock salt matrix was confirmed by combined analysis of X – ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and elemental chemical mapping techniques. Moreover, titanium in Ti3+ and Ti2+ valance states was confirmed by X - ray photoelectron spectroscopy (XPS). Differential scanning calorimetry (DSC) study proves that the reduced Ti oxide phases promote the low temperature dehydrogenation of MgH2. The present observations reiterate that the MgO rock salt layer existing in oxide additives loaded MgH2 is not necessarily a barrier layer that blocks the diffusion of hydrogen.

Role of chemical interaction between MgH2 and TiO2 additive on the hydrogen storage behavior of MgH2

The present study explores how the additive titania chemically reacts with magnesium hydride and influences the dehydrogenation of MgH2. Quantitative X − ray diffraction study of ball milled MgH2+xTiO2 (x=0.25, 0.33, 0.5 and 1) suggests that Ti substituted MgO is the main reaction product in all the product powders. Convincing evidence is obtained to conclude that Ti dissolution in MgO makes a dramatic behavioral change to MgO; passive MgO turns as an active in-built catalyst. The analysis correlating the dehydrogenation kinetics, composition of in-situ catalyst and sample durability suggests that effectiveness of Ti substituted MgO (MgxTiyOx+y) as a catalyst for MgH2 depends on the concentration of Ti in MgxTiyOx+y rock salt. These observations are immensely helpful for understanding the hydrogen desorption mechanism of metal oxide additives loaded MgH2 system.

Comments on \u201dEvidence of the hydrogen release mechanism in bulk MgH2\u201d

The effect of an electron beam induced dehydrogenation of MgH2 in the transmission electron microscope (TEM) is largely underestimated by Nogita et al., and led the authors to a misinterpretation of their TEM observations. Firstly, the selected area diffraction (SAD) pattern is falsely interpreted. A re-evaluation of the SAD pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only. Secondly, the transformation of the sample upon in-situ heating in the TEM cannot be ascribed to dehydrogenation, but is rather to be explained by the (nanoscale) Kirkendall effect, which leads to the formation of hollow MgO shells without any metallic Mg in their cores. Hence, the conclusions drawn from the TEM investigation are invalid, as the authors apparently have never studied MgH2.

Electron beam induced dehydrogenation of MgH2 studied by VEELS

AbstractNanosized or nanoconfined hydrides are promising materials for solid-state hydrogen storage. Most of these hydrides, however, degrade fast during the structural characterization utilizing transmission electron microscopy (TEM) upon the irradiation with the imaging electron beam due to radiolysis. We use ball-milled MgH2 as a reference material for in-situ TEM experiments under low-dose conditions to study and quantitatively understand the electron beam-induced dehydrogenation. For this, valence electron energy loss spectroscopy (VEELS) measurements are conducted in a monochromated FEI Titan3 80–300 microscope. From observing the plasmonic absorptions it is found that MgH2 successively converts into Mg upon electron irradiation. The temporal evolution of the spectra is analyzed quantitatively to determine the thickness-dependent, characteristic electron doses for electron energies of both 80 and 300 keV. The measured electron doses can be quantitatively explained by the inelastic scattering of the incident high-energy electrons by the MgH2 plasmon. The obtained insights are also relevant for the TEM characterization of other hydrides.

Enhancement of hydrogen desorption in magnesium hydride catalyzed by graphene nanosheets supported Ni-CeOx hybrid nanocatalyst

A new hybrid nanocatalyst Ni-CeOx/GNS (graphene nanosheets supported nanoscale Ni and CeOx, x = 1.69) of high surface area and porosity is synthesized by impregnation-reduction method. It is demonstrated that the Ni-CeOx/GNS nanocatalyst exhibits improved catalytic effect on the dehydrogenation performances of MgH2 compares to individual Ni/GNS, CeOx/GNS, or GNS. Differential scanning calorimetry (DSC) measurement proved that both the onset and peak desorption temperature of MgH2-5 wt% Ni-CeOx/GNS (abbreviated as MgNCG) nanocomposites can be decreased by 90.2 and 60.1 °C, respectively. Isothermal desorption kinetic test confirmed that the dehydrogenation kinetics of MgNCG nanocomposites is dozens of times higher than the pure milled MgH2, e.g. MgNCG nanocomposites can release 6.50 wt% H2 within 10 min at 300 °C with an initial hydrogen pressure of 5 KPa, superior to 0.16 wt% H2 desorbed by MgH2 under the same condition. Additionally, the activation energy (Ea) of MgNCG decreases significantly contrast with pure milled MgH2, it is proposed that the tri-component of Ni-CeOx/GNS nanocatalyst has a synergistic effect on the highly efficient dehydrogenation of MgH2.

The Effect of AlCl&lt;sub&gt;3&lt;/sub&gt; and Ti on Hydrogen Storage Performance of 4MgH&lt;sub&gt;2&lt;/sub&gt;-Li&lt;sub&gt;3&lt;/sub&gt;AlH&lt;sub&gt;6&lt;/sub&gt; System

The AlCl3/Ti co-doped 4MgH2-Li3AlH6 has been successfully synthesized using solid ball-milled method. The effect of AlCl3/Ti and different preparation conditions on reversible hydrogen storage of 4MgH2-Li3AlH6 were investigated. It was found that the formed Al3Ti and Al can improve the de/rehydriding performance. The onset temperature of the system dehydrogenation decreased to 58 °C, and it can release hydrogen of 7.2 wt.% at 400 °C. The hydriding of the system was found that the adsorption rate greatly increased from 0.02 to 0.35 wt.% min-1. The activation energy (Ea) of MgH2 dehydrogenation decreased from 147 to 113.7 kJ mol-1 by adding additive AlCl3 and Ti. The AlCl3/Ti improved the thermodynamic and kinetic performance of the 4MgH2-Li3AlH6 composite, which was attributed to the high catalytic activity of Al3Ti and Al.

Understanding the dehydrogenation process of MgH2 from the recombination of hydrogen atoms

An experimental investigation of the rate limiting steps for the desorption of magnesium hydrides is performed in this work to clarify the factors and corresponding mechanism determining the dehydrogenation process of MgH2. The effects of Mg nucleation and hydrogen atoms recombination on the desorption performance of MgH2 are investigated by ball milling MgH2 with Mg powders and Ni powders. The comparison between the effects of nucleate precursors and hydrogen atoms recombination is carried out by control experiment with metallographic observation and kinetic data analysis. The results show that the addition of Mg is mixed with MgH2 at the atomic scale and favors the nucleation of metals during dehydrogenation of MgH2, e.g., the activation energy is decreased by 10 kJ/mol with the addition of 10wt.%Mg. Ni improves the desorption kinetics to a great extent by accelerating the recombination of hydrogen atoms. The recombination of hydrogen atoms on the surface of newly formed metals limits the desorption process more considerably than the nucleation of Mg. The dehydrogenation kinetics of MgH2 can be significantly improved with the addition of nucleate precursors Mg and catalyst Ni powders.

Enhanced hydrogen storage properties of MgH2 co-catalyzed with K2NiF6 and CNTs.

The composite of MgH2/K2NiF6/carbon nanotubes (CNTs) is prepared by ball milling, and its hydrogenation properties are studied for the first time. MgH2 co-catalyzed with K2NiF6 and CNTs exhibited an improvement in the onset dehydrogenation temperature and isothermal de/rehydrogenation kinetics compared with the MgH2-K2NiF6 composite. The onset dehydrogenation temperature of MgH2 doped with 10 wt% K2NiF6 and 5 wt% CNTs is 245 °C, which demonstrated a reduction of 25 °C compared with the MgH2 + 10 wt% K2NiF6 composite. In terms of rehydrogenation kinetics, MgH2 doped with 10 wt% K2NiF6 and 5 wt% CNTs samples absorbed 3.4 wt% of hydrogen in 1 min at 320 °C, whereas the MgH2 + 10 wt% K2NiF6 sample absorbed 2.6 wt% of hydrogen under the same conditions. For dehydrogenation kinetics at 320 °C, the MgH2 + 10 wt% K2NiF6 + 5 wt% CNTs sample released 3.3 wt% hydrogen after 5 min of dehydrogenation. By contrast, MgH2 doped with 10 wt% K2NiF6 released 3.0 wt% hydrogen in the same time period. The apparent activation energy, Ea, for the dehydrogenation of MgH2 doped with 10 wt% K2NiF6 reduced from 100.0 kJ mol-1 to 70.0 kJ mol-1 after MgH2 was co-doped with 10 wt% K2NiF6 and 5 wt% CNTs. Based on the experimental results, the hydrogen storage properties of the MgH2/K2NiF6/CNTs composite is enhanced because of the catalytic effects of the active species of KF, KH and Mg2Ni that are formed in situ during dehydrogenation, as well as the unique structure of CNTs.

Constant rate thermal analysis of a dehydrogenation reaction

The kinetic analysis of constant rate thermal analysis (CRTA) curves obtained for MgH2 dehydrogenation under high vacuum reveals that the reaction obeys first-order kinetics.

Structure, thermal analysis and dehydriding kinetic properties of Na1−xLixMgH3 hydrides

NaMgH3 hydride with perovskite structure has been synthesized by high-energy ball milling, the maximum hydrogen-desorbed amount of which is 3.42 wt.% at 638 K. Two decomposition steps have been detected for perovskite-type NaMgH3 hydride, calculated values of activation energy for the two steps are 180.25 ± 8.25 kJ/mol and 156.23 ± 18.54 kJ/mol by Kissinger method. In comparison with NaMgH3 hydride, Li0.5Na0.5MgH3 hydride has better dehydriding kinetic properties and higher hydrogen-desorbed amount (4.11 wt.%) due to partial replacement of Na by Li. LiMgH3 hydride with perovskite structure cannot be synthesized by milling of the mixture of LiH and MgH2 hydrides. However, the maximum hydrogen-desorbed amount of this milled mixture is 5.54 wt.% at 638 K, this may suggest that LiH is a good catalyst for dehydrogenation of MgH2, but further research is needed.

Reaction kinetic behaviour with relation to crystallite/grain size dependency in the Mg–Si–H system

An empirical understanding of the relationship between crystallite size and reaction kinetics for the dehydrogenation of MgH2 in the presence of Si was determined. MgH2 was combined with Si under different conditions to obtain varying crystallite sizes of both reactants. Thermal analysis and isothermal desorption were undertaken to obtain reaction kinetic information and therefore determine activation energies as well as the rate limiting step for each of the different crystallite sizes. It was found that there is a strong correlation between crystallite size and activation energy for the growth of the Mg2Si phase, however, any correlation between the nucleation (of Mg2Si) activation energy was less evident. Direct measurements of kinetic behaviour from a manometric Sieverts apparatus showed that initial reaction kinetics were fastest when MgH2 was mixed with Si nanoparticles, however, this sample was not able to fully desorb. Data from the Sieverts measurements were then used with well-known theoretical models to determine the rate limiting step of the reaction. The three dimensional Carter–Valensi (or contracting volume) diffusion model could be used to describe the rate limiting step for most of the reactions. These results have led to a proposed mechanism for the formation of Mg2Si during the decomposition reaction.

Dehydrogenation Reaction Pathway of the LiBH4–MgH2 Composite under Various Pressure Conditions

This paper investigates dehydrogenation reaction behavior of the LiBH4–MgH2 composite at 450 °C under various hydrogen and argon back-pressure conditions. While the individual decompositions of LiBH4 and MgH2 simultaneously occur under 0.1 MPa H2, the dehydrogenation of MgH2 into Mg first takes place and subsequent reaction between LiBH4 and Mg into LiH and MgB2 after an incubation period under 0.5 MPa H2. Under 1 MPa H2, enhanced dehydrogenation kinetics for the same reaction pathway as that under 0.5 MPa H2 is obtained without the incubation period. However, the dehydrogenation reaction is significantly suppressed under 2 MPa H2. The formation of Li2B12H12 as an intermediate product during dehydrogenation seems to be responsible for the incubation period. The degradation in hydrogen capacity during hydrogen sorption cycles is not prevented with the dehydrogenation under 1 MPa H2, which effectively suppresses the formation of Li2B12H12. The overall dehydrogenation behavior under argon pressure conditions...

Enhanced joint catalysis of YH2/Y2O3 on dehydrogenation of MgH2

This work presents the in-situ preparation of YH2/Y2O3 nanocomposite and its catalytic effect on the dehydrogenation of MgH2. The YH2 was formed by the hydrogenation of original Mg93Y7 alloy powder, the in-situ formed YH2 particles of ca. 50nm were highly dispersed on the surface of MgH2 particles. The Y2O3 was obtained due to accident oxidization of alloy in the kinetic measurement. Compared with the YH2 catalyzed MgH2, the YH2/Y2O3 catalyzed MgH2 showed better hydrogen storage kinetic properties, especially the dehydrogenation rate was dramatically increased by 3 times. This result demonstrates the feasibility of massive production of efficient composite catalysts for the Mg-base hydrogen storage alloys by controllable hydrogenation and oxidization treatment.

An experimental survey of additives for improving dehydrogenation properties of magnesium hydride

The use of a wide range of additives has been known as an important method for improving hydrogen storage properties of MgH2. There is a lack of a standard methodology, however, that can be used to select or compare the effectiveness of different additives. A systematic experimental survey was carried out in this study to compare a wide range of additives including transitions metals, transition metal oxides, hydrides, intermetallic compounds, and carbon materials, with respect to their effects on dehydrogenation properties of MgH2. MgH2 with various additives were prepared by using a high-energy-high-pressure planetary ball milling method and characterized by using thermogravimetric analysis (TGA) techniques. The results showed that additives such as Ti and V-based metals, hydride, and certain intermetallic compounds have strong catalytic effects. Additives such as Al, In, Sn, Si showed minor effects on the kinetics of the dehydrogenation of MgH2, while exhibiting moderate thermodynamic destabilizing effects. In combination, MgH2 with both kinetic and thermodynamic additives, such as the MgH2–In–TiMn2 system, exhibited a drastically decreased dehydrogenation temperature.

Catalytic effect of nano-sized ScH2 on the hydrogen storage of mechanically milled MgH2

The hydrogen storage properties of ball milled xMgH2/(1−x)ScH2 (x=0.65–1) samples including capacity, kinetics, thermodynamics and cycling stability, were investigated. The effect of ScH2 catalyst content and ball milling duration on the kinetics of MgH2 dehydrogenation were studied. It was found that the optimal content of the catalyst ScH2 was ca. 12mol.%, which gave an activation energy (Ea) value of 62±5kJmol−1 and a hydrogen storage capacity of 5.8±0.1wt.% for the sample. There was no loss in kinetics in the ScH2 catalysed MgH2 system even after cycling at relatively high temperatures up to 450°C, and the nano-sized ScH2 (ca. 70nm) formed during ball milling remained after (de)hydrogenation cycling. Typical MgH2 dehydrogenation enthalpy (76±1kJmol−1 (H2)) and entropy (138±2Jmol−1K−1 (H2)) values observed by the 0.65MgH2/0.35ScH2 sample demonstrated the effect of ScH2 was purely a catalytic improvement of the kinetics.