Kinetics and mechanism of MgH2 hydrolysis in MgCl2 solutions

In the present work we systematically studied the hydrolysis of magnesium hydride in MgCl₂ aqueous solutions, which was used as a process promotor. The initial hydrolysis rate, the pH of the reaction mixture, and the overall reaction yield are all found to be linearly dependent of the logarithm of MgCl₂ concentration. The phase-structural and elemental compositions of the formed precipitates showed that they do not contain chlorine ions and solely consist of Mg(OH)₂. The size of the Mg(OH)₂ crystallites increased with increasing content of MgCl₂ in the aqueous solution.The best agreement between the observed and modelled hydrolysis kinetics was achieved by applying a pseudo-homogeneous model that describes the process rate as increasing with H⁺ ions concentration. The deposition of Mg(OH)₂ which is impermeable to water and blocks the surface of the remaining MgH₂ however simultaneously and partially suspends this reaction. We therefore propose a mechanism of MgH₂ hydrolysis in the presence of MgCl₂ that is based on the comparison of the kinetic dependencies, variations of solutions pH and the structural and elemental analysis data for the solid deposits formed during the interaction. We furthermore define the kinetic model of the process, and the equation that describes the variation in pH of solutions containing chloride salts. Hydrolysis efficiency increased with increased relative MgCl₂ amount; the best performance being achieved for the stoichiometric ratio MgH₂+0.7MgCl₂ (MgCl₂/MgH₂ weight ratio of 12.75/100). This provided a hydrogen yield of 1025 mL (H₂)/g MgH₂. Maximum hydrogen yield peaked at 89% of the theoretical H₂ generation capacity, and was achieved within 150 min of hydrolysis start, 35% of hydrogen being released in the first 10 min after start, the hydrogen generation rate being as high as 800 mL min⁻¹·g⁻¹ MgH₂.     

MgH2-based nanocomposites prepared by short-time high energy ball milling followed by cold rolling: A new processing route

A new processing route consisting of a short-time high energy ball milling (HEBM) step followed by cold rolling (CR) to produce MgH₂-based nanocomposites was investigated. Samples of pure MgH₂ and MgH₂-mixtures containing 2 mol% of FeF₃ were processed under air atmosphere and their effects on the microstructure and the hydrogen storage properties were studied in detail. X-ray Diffraction (XRD) analysis on all samples revealed crystallite sizes in the nanometer range for the beta-MgH₂ and FeF₃ phases (beta (beta): ∼8–13 nm and FeF₃: ∼16–21 nm). No extra crystallite size reduction was observed for the HEBM + CR samples in comparison with those only cold rolled. Scanning electron microscopy (SEM) evaluation together with elemental composition analysis indicated a finer size distribution of additive particles and also a more intimate level of mixing for the HEBM + CR mixture than for the mixture obtained only by CR. These features were associated with the HEBM step applied before the CR. These characteristics led to a lower hydrogen desorption temperatures and enhanced desorption kinetics behavior for the HEBM + CR samples in comparison with the CR samples.     

Hydrogenation properties of MgH2- x wt% AC (x=0, 5, 10, 15) nanocomposites

Magnesium is considered as a promising candidate for hydrogen storage due to its high storage capacity (theoretical value ~ 7.6 wt%). Nanocomposites of Magnesium hydride and activated charcoal (AC) were prepared using ball milling method. These nanocomposites were characterized by XRD, TGA, DSC and SEM techniques. The TGA analysis show that the MgH₂-5 wt% AC nanocomposite exhibits dehydrogenation capacity of 7.45 wt% (which is very close to the storage capacity of MgH₂) and starts release of hydrogen at 140 °C temperature. The results from the Kissinger plot from DSC result showed that the activation energy for hydrogen desorption of MgH₂ with 5 wt% AC was reduced compared to those of as-received.     

Production of hydrogen from magnesium hydrides hydrolysis

Magnesium hydride could be considered as a good candidate for the hydrolysis reaction because it can be produced at a relatively low cost. However, this reaction is incomplete and very slow because of the formation of a magnesium hydroxide layer on the surface of MgH₂ particles. In order to overcome this problem, various treatments such as ball milling with or w/o additives, addition of acids, ultrasounds and increase of temperature, have been tried. Different characterization methods such as XRD, BET, particle size, SEM, etc. have been used to explain the effects of the treatments cited above on the improvement of the kinetics and the yield of the MgH₂ hydrolysis reaction.     

Hydrogen storage in carbon/Mg nanocomposites synthesized by ball milling

Carbon nanocomposites obtained by ball milling of graphite and magnesium with organic additives (benzene or cyclohexane) under different conditions have been studied with the aim of preparing novel hydrogen storage materials. It has been proved by thermal desorption spectrometry (TDS) and neutron diffraction measurements that the hydrogen taken up by the nanocomposites exists in at least two states; the one is the hydrogen strongly associated with the carbon component and the other the hydride in the magnesium component. The ball milling resulted in the generation of large amounts of dangling carbon bonds in graphite, which acted as active sites to take up the hydrogen. When _D2${\mathrm{D}}_2$ gas was brought into contact with such composites, the isotope exchange reaction with the hydrogen in the magnesium hydride occurred at 453 K, and not with the hydrogen associated with the carbon. The properties of such hydrogen taken up were also discussed from the standpoint of isotope effects.     

Enhancement of hydrogen sorption properties of MgH2 with a MgF2 catalyst

Effect of a MgF₂ catalyst, prepared by ball-milling, on the hydrogen desorption ability of commercial MgH₂ was investigated. When MgH₂ was catalyzed with a MgF₂ composite, it exhibited good cyclability and sharp faceting, with a small grain size (around 10 nm), which differs from those of pure MgH₂. The addition of the MgF₂ catalyst suggests that the F anion could significantly contribute to the cyclability of Mg particles and aid in the inhibition of MgH₂ grain growth.     

Effect of ball milling strategy (milling device for scaling-up) on the hydrolysis performance of Mg alloy waste

Ball milling strategy is of prime importance on the hydrolysis performance of Mg alloy waste. The effect of milling device (e.g. Fritsch Pulverisette 6 (P6) and Australian Uni-Ball-II (UB)), milling atmosphere (H₂ and Ar), milling time, nature of the additives graphite and AlCl₃ and synergetic effect by chronological or simultaneous addition were examined. An equivalence between both mills was established and it was shown that the process with the UB is 10 times longer than that with the P6 to acquire a similar material. Mg alloy milled without additives in the P6 under Ar for 10 h improves the hydrolysis performance. Using a single additive, the best hydrolysis performances are obtained with graphite (yield of 95% of total capacity reached in 5 minutes) due to the formation of a protective graphite layer. By incorporating both additives sequentially, the best material, from the hydrogen production point of view, was Mg alloy milled with G for 2 h and then with AlCl₃ for 2 extra hours (full hydrolysis in 5 minutes). Mg alloy milled with the P6 were compared to those milled with the UB. Mg alloy milled with graphite or with sequential addition of G and AlCl₃ under Ar generated more than 90% of their total capacity. Our results confirm that laboratory-milling strategy can be scaled-up to industrial scale.     

Boosting low-temperature de/re-hydrogenation performances of MgH2 with Pd-Ni bimetallic nanoparticles supported by mesoporous carbon

Bimetallic Pd-Ni nano-particles supported by a mesoporous carbon material CMK-3 (denoted as Pd₃₀Ni₇₀/CMK-3) were synthesized through solution impregnation and hydrogen reduction methods. Among those hierarchical Ni-Pd nano-particles, majorly large ones (>10 nm) are dispersed over the surface of CMK-3, while a litter small ones (<10 nm) are embedded into the pores. It significantly improves the de/re-hydrogenation performances of MgH₂ at low temperature. The onset desorption temperature of MgH₂-Pd₃₀Ni₇₀/CMK-3 is lowered by 150 K from that of pristine MgH₂ (above 593 K). About 6 wt% hydrogen could be released during its decomposition below 561 K. Noticeably, MgH₂-Pd₃₀Ni₇₀/CMK-3 is capable of releasing 1.3 wt% H₂ even at 373 K. 4 wt% hydrogen can be absorbed at 343 K under a hydrogen pressure of 3 MPa within 18000 s. Activation energy values of both hydrogen decomposition (65.9 kJ mol⁻¹) and absorption (78.9 kJ mol⁻¹) for MgH₂-Pd₃₀Ni₇₀/CMK-3 are greatly improved from those of as-milled MgH₂. Thermal stability of the composite system is remarkably destabilized by 4.3 kJ mol H₂⁻¹ from pristine MgH₂ according to pressure-composition isotherm curves and van't Hoff plots. The enhanced performances can be ascribed to the synergistic effects of both destabilization and catalysis from nano-dispersed Pd and Ni particles, respectively.     

Nanostructured hydrogen storage materials prepared by high-energy reactive ball milling of magnesium and ferrovanadium

Hydrogen storage nanocomposites prepared by high energy reactive ball milling of magnesium and vanadium alloys in hydrogen (HRBM) are characterised by exceptionally fast hydrogenation rates and a significantly decreased hydride decomposition temperature. Replacement of vanadium in these materials with vanadium-rich Ferrovanadium (FeV, V80Fe20) is very cost efficient and is suggested as a durable way towards large scale applications of Mg-based hydrogen storage materials. The current work presents the results of the experimental study of Mg–(FeV) hydrogen storage nanocomposites prepared by HRBM of Mg powder and FeV (0–50 mol.%). The additives of FeV were shown to improve hydrogen sorption performance of Mg including facilitation of the hydrogenation during the HRBM and improvements of the dehydrogenation/re-hydrogenation kinetics. The improvements resemble the behaviour of pure vanadium metal, and the Mg–(FeV) nanocomposites exhibited a good stability of the hydrogen sorption performance during hydrogen absorption – desorption cycling at T = 350 °C caused by a stability of the cycling performance of the nanostructured FeV acting as a catalyst. Further improvement of the cycle stability including the increase of the reversible hydrogen storage capacity and acceleration of H₂ absorption kinetics during the cycling was observed for the composites containing carbon additives (activated carbon, graphite or multi-walled carbon nanotubes; 5 wt%), with the best performance achieved for activated carbon.     

Effect of ball milling in presence of additives (Graphite,AlCl3, MgCl2 and NaCl) on the hydrolysis performances of Mg17Al12

In most of the Mg–Al alloys, Al forms with Mg the intermetallic compound Mg₁₇Al₁₂. In order to understand the hydrogen production from the Mg–Al alloys waste by the hydrolysis reaction in “model” seawater (i.e. 3.5 wt % NaCl), hydrolysis with Mg₁₇Al₁₂ was investigated. The effect of ball milling time, the nature of the additives (graphite, NaCl, MgCl₂ and AlCl₃) and the synergetic effects of both graphite and AlCl₃ were investigated. It has been established that increasing ball milling time up to 5 h is necessary to activate the intermetallic and to decrease sufficiently its crystallites and particles size. On one hand, the presence of AlCl₃ provides the best hydrolysis performance (14% of the theoretical hydrogen volume in 1 h). On the other hand, the mixture obtained by simultaneous addition of graphite and AlCl₃ shows the best hydrolysis performances with 16% of the theoretical H₂ volume reached in 1 h.     

Kinetic performance of hydrogen generation enhanced by AlCl3 via hydrolysis of MgH2 prepared by hydriding combustion synthesis

Magnesium hydride (MgH₂) is a very promising hydrogen storage material and it has been paid more and more attention on the application of supplying hydrogen on-board because the theoretical hydrogen yield is up to 1703 mL/g when it reacts with water. However, the hydrolysis reaction is inhibited rapidly by the passivation layer of Mg(OH)₂ formed on the surface of MgH₂. This paper reports that high purity MgH₂ (～98.7 wt%) can be readily obtained by the process of hydriding combustion synthesis (HCS) and the hydrogen generation via hydrolysis of the as-prepared HCSed MgH₂ can be dramatically enhanced by the addition of AlCl₃ in hydrolysis solutions. An excellent kinetics of hydrogen generation of 1487 mL/g in 10 min and 1683 mL/g in 17 min at 303 K was achieved for the MgH₂-0.5 M AlCl₃ system, in which the theoretical hydrogen yield (1685 mL/g) of the HCSed product was nearly reached. The mechanism of the hydrolysis kinetics enhancement was demonstrated by the generation of a large amounts of H⁺ from the Al³⁺ hydrolysis and the pitting corrosion (Cl^?) of the Mg(OH)₂ layer wrapped on the surface of MgH₂ even at a low temperature. In addition, the apparent activation energies for the MgH₂ hydrolysis in the 0.1 M AlCl₃ and 0.5 M AlCl₃ solutions are decreased to 34.68 kJ/mol and 21.64 kJ/mol, respectively, being far superior to that of reported in deionized water (58.06 kJ/mol). The results suggest that MgH₂ + AlCl₃ system may be used as a promising hydrogen generation system in practical application of supplying hydrogen on-board.     

Isothermal activation,thermodynamic and hysteresis of MgH2 hydrides catalytically modified by high-energy ball milling with MWCNTs and TiF3

MgH₂-M (M = MWCNTs or TiF₃ or both of them) composites prepared by high-energy ball milling (HEBM) are used in this work to illustrate the effect of catalysts on isothermal activation, thermodynamic and hysteresis of MgH₂ hydrides. The phase compositions, microstructures, particle morphologies and distributions of MgH₂ with catalysts have been evaluated. The isothermal synergetic catalytic-activation and dehydrogenation effect of MWCNTs and TiF₃ evaluated by P-C-T give the evidences that the addition of catalysts is an effective strategy to destabilize MgH₂ and reduce the dehydrogenation temperatures. The isothermal activation process can be remarkly accelerated by adding MWCNTs or TiF₃. It's worthnoting that fast initial absorption rate and high hydrogenation capacity are obtained for modified MgH₂ when adding MWCNTs coupling with TiF₃ and no special activation treatment is needed. The activation effect is mainly attributed to the large contraction/expansion stresses caused by accelerated catalytic desorption/absorption cycles. The catalytic effect on thermodynamic is mainly attributed to electronic exchange reactions with hydrogen molecules during the dissociation-absorption or recombination-desorption process. The thermodynamic hysteresis of catalyzed MgH₂ is also investigated to evaluate the energy consumption and estimate the efficiency of the isothermal hydrogen absorption/desorption process. A probable synergetic catalytic-activation mechanism is probed.     

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.     

Hydrogen production from hydrolysis of magnesium wastes reprocessed by mechanical milling under air

Magnesium-based wastes were reprocessed by mechanical milling under air atmosphere and used to produce hydrogen by hydrolysis on a laboratory scale. The evolution of the material during reprocessing and the generation of hydrogen in a 0.6 M MgCl₂ aqueous solution at 24 °C are reported. The morphology, microstructure and phase abundance change with milling time. During mechanical processing, (i) particle size and crystallite size reduce, (ii) microstrain accumulates in the material, (iii) Al dissolves in Mg, (iv) the amount of Mg₁₇Al₁₂ (β-phase) increases and (v) small quantities of Fe from the milling tools are incorporated in the material. By hydrolysis, hydrogen yields in the 70–90% range after 30 min of reaction have been obtained, depending on milling time. Reactants are not exhausted during the hydrolysis reaction in the saline solution, due to the formation of a Mg(OH)₂ layer that produces a passivating effect. Higher generation has been observed for larger particles and for materials reprocessed for longer milling times. Reaction kinetics also improves with milling time, with faster rates observed for the smaller particles. The shape of the hydrolysis curves can be fitted with a model that corresponds to a reaction limited by a three dimensional geometric contraction process. Mg₁₇Al₁₂ and Fe favor hydrogen production by acting as micro-galvanic cathodes during the reaction.     

Achieving both high hydrogen capacity and low decomposition temperature of the metastable AlH3 by proper ball milling with TiB2

AlH₃ is a metastable hydride with a high hydrogen density of 10.1 wt% and it can release hydrogen at a low temperature of 150–200 °C. Many additives (e.g., NbF₅, TiF₃, etc.) introduced by ball milling can significantly reduce the decomposition temperature of AlH₃, but often simultaneously decrease the available hydrogen capacity. In this work, TiB₂ was introduced by ball milling to improve the decomposition performance of AlH₃. AlH₃ + x wt% TiB₂ (x = 2.5, 5, 7.5, 10) composites were prepared by ball milling, and the milling conditions were optimized. It was shown that the decomposition performance of the AlH₃ + 2.5 wt% TiB₂ ball milled at 225 rpm for 108 min is the best. The onset decomposition temperature is 78 °C, which is 60 °C lower than that of pure AlH₃. The decomposition is terminated at 130 °C with 8.5 wt% of hydrogen is obtained. In addition, 5.3 wt% of hydrogen can be released within 200 min at constantly 80 °C. Under the same conditions, ball-milled AlH₃ can hardly release any hydrogen. The activation energy calculated by the Kissinger's method is 86 kJ mol^?1, which was 28 kJ mol^?1 lower than that of ball-milled AlH₃. Catalytic mechanism study reveals that the Al₂O₃ layers on the surface of AlH₃ will interact with TiB₂ to form Al–Ti–B solid solution, resulting in lattice distortion. Through lattice activation, the decomposition kinetics of AlH₃ is improved. This work provides an efficient strategy to achieve both high hydrogen capacity and low decomposition temperature of metastable AlH₃ by proper ball milling with metal borides.     

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.     

Improvement of hydrogen dehydrogenation performance of lithium amide pyrolysis by ball milling with magnesium

Li–N–H system is one of the promising candidates for hydrogen storage. However, with the heat treatment, LiNH₂ can release not only H₂ but N₂ and NH₃. To improve the purity and amount of H₂ in the products, LiNH₂–Mg composite is prepared by ball milling. In the composite, Mg can absorb the gaseous byproducts, e.g. NH₃ and N₂, effectively when the LiNH₂ pyrolysis. Finally, H is desorbed in the form of H₂ and N is absorbed by Mg and forms Mg₃N₂. Besides, LiNH₂ reacts with Mg directly to form LiMgN and release H₂. After re-hydrogenation of the composite, H₂ is stored into Li₂MgN₂H₂ and LiH, thus the cyclic mechanism is different with the first dehydrogenation of the LiNH₂–Mg composite. First-principles calculation shows that the NH₃ molecule can be captured by Mg when it is adsorbed on top and bridge positions. After captured by Mg, the molecular structure of NH₃ is changed and the barrier energy of NH₃ dissociation is dramatically reduced. In addition, the barrier energy of NH₃ and N₂ dissociation on Mg (0001) plane is lower than that of H₂, which means NH₃ and N₂ can be absorbed by Mg before Mg react with H₂.     

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.     

Effects of SiC nanoparticles with and without Ni on the hydrogen storage properties of MgH2

In this work, MgH₂–SiC–Ni was prepared by magneto-mechanical milling in hydrogen atmosphere. Scanning electron microscope mapping images showed a homogeneous dispersion of both Ni and SiC among MgH₂ particles. Based on the differential scanning calorimetry traces, the temperature of desorption is reduced by doping MgH₂ with SiC and Ni. Hydrogen absorption/desorption behaviour of the samples was investigated by Sievert's method at 300 °C, and the results showed that both capacity and kinetics were improved by adding SiC and Ni. The hydrogen desorption kinetic investigation indicated that for pure MgH₂, the rate-determining step is surface controlled and recombination, while for the MgH₂–SiC–Ni sample it is controlled as described by the Johnson–Mehl–Avrami 3D model (JMA 3D).     

The recent development on MgH2 system by 16 wt% nickel addition and particle size reduction through ball milling: A noticeable hydrogen capacity up to 5 wt% at low temperature and pressure

The addition of nickel as catalyst and particle size reduction through milling process is considered as the best approach for MgH₂ properties to be thermodynamically and kinetically more favorable. The recent development in MgH₂ is done by adding Ni (14–16 wt%) and particle size reduction through simple mechanical milling at a large sample of 100 g. The research sequences are easy to adopt for the implementation and sustainable research of MgH₂. Starting from the moisture test to the milling process, continue with particle size distribution (PSD) for the milled sample and final moisture test after PSD. Some critical finding from our research includes high capacity storage of Mg₈₄:Ni₁₆ above 5 wt% H₂ within 20 min at 573 K, the effect of moisture content on system performance and the different effect of carbon (C) in the system at specific temperature and pressure that may have.