Ultrasonic irradiation on hydrolysis of magnesium hydride to enhance hydrogen generation

This paper describes the ultrasonic irradiation on the hydrolysis of magnesium hydride to enhance hydrogen generation; the effects of the ultrasonic frequency and the sample size on the hydrogen generation were mainly examined. In the experiments, three MgH₂ particle and nanofiber samples were soaked in distilled water and ultrasonically irradiated at frequencies of 28, 45, and 100 kHz. Then, the amount of hydrogen generated was measured. We found that the low frequency of ultrasonic irradiation and the relatively small sample size accelerated the hydrolysis reaction MgH₂ + 2H₂O = Mg(OH)₂ + 2H₂ + 277 kJ. In particular, the MgH₂ nanofibers exhibited the maximum hydrogen storage capacity of 14.4 mass% at room temperature at a frequency of 28 kHz (ultrasound irradiation). The results also experimentally validated that a combination of ultrasonic irradiation and MgH₂ hydrolysis is considerably effective for efficiently generating hydrogen without heating and adding any agent.     

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.     

Magnesium hydride based hydrogen chemical source: Development and application perspectives

In this paper, we studied the process of hydrolysis of magnesium hydride with water vapor and considered the possibility of creating a chemical source of hydrogen based on this process. To study the hydrolysis reaction of magnesium hydride powder at temperatures above 100 °C, an experimental setup with a quartz tube — a reactor 300 mm long — was designed. The mass and volume of a single powder load in a quartz reactor was 65 g and 130 ± 2 cm³, respectively. The length of the powder zone along the axis of the reactor was approximately 10 cm.Based on the data obtained during the experiment, it can be distinguished that the length of the reaction zone is greatest at the beginning and at the end of the hydrolysis process and is approximately 5–6 cm. In the middle of the hydrolysis process, for the time interval from 3000 to 5000 s, the reaction zone is the smallest - about 4–5 cm. The proposed design of the hydrolysis reactor and the experimental setup also made it possible to study the peculiarities of the hydrolysis of magnesium hydride powder with water vapor; magnesium, the composition of the reaction products and the controllability of the generation of a stream of hydrogen.     

Effect of hydride nucleation rate on the hydrogen capacity of Mg

The hydrogen capacity of magnesium is usually below its theoretical value of 7.6 wt.% hydrogen. Based on the model that hydrogen capacity is reached when the hydride colonies/grains nucleated on the surface of Mg powders impinge on each other, we designed two hydrogenation methods in order to shed light on the effect of hydride nucleation rate on the hydrogen capacity of a commercial pure magnesium powder. We have demonstrated that increasing the nucleation rate significantly reduces the hydrogen capacity. It is elucidated that at a high nucleation rate, the complete coverage of the powder surface is reached at a smaller volume fraction of magnesium hydride. The results of the analysis of the hydrogenation curves using the Johnson–Mehl–Avrami (JMA) equation revealed three stages of hydrogenation. The modification of the nucleation rate was found to affect stages 1 and 2, where the nucleation and growth of the hydride take place, most significantly.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Generation of hydrogen from magnesium hydride oxidation in water in presence of halides

We have performed a systematic analysis of magnesium hydride oxidation by water in presence of several transition, non-transition metals and ammonia halides. It was observed that the most effective catalysts of the process are ammonia chloride and bromide, magnesium chloride and a few mixed salts: magnesium chloride mixed with aqueous solutions of ammonium, copper or potassium chlorides. It was observed that solution amount impacts the reaction rate, but not the overall hydrogen yield. The fastest MgH₂ oxidation (120 min to completion; ~100% hydrogen yield) was observed when it was activated by the mixture of MgCl₂ and NH₄Cl).     

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.     

Enhancement of hydrogen sorption in magnesium hydride using expanded natural graphite

Magnesium hydrogenation reaction being exothermic and limited by heat removal, the thermal conductivity of ball-milled magnesium hydride (BM MgH₂) powders has to be improved. The compression of BM MgH₂ associated to Expanded Natural Graphite (ENG) to form compacted disks has been investigated. Using BM MgH₂ without ENG, its compression reduces the porosity and increases its volumetric hydrogen storage capacity. Incorporating ENG before compression drastically improves the thermal conductivity in the direction normal to compression axis. Moreover, the thermal conductivity increases linearly with ENG content, and can be adjusted to fulfill the loading time requirements. The thermodynamic properties and intrinsic sorption kinetics remain unchanged. However, both compression and ENG incorporation reduce the hydrogen permeability, especially in the direction parallel to the compression axis, which imposes a limit to the disk thickness. A small-size instrumented tank has been loaded with either pure BM MgH₂ powder or with disks having different ENG contents. The results obtained for both cases are compared.     

Catalysis derived from flower-like Ni MOF towards the hydrogen storage performance of magnesium hydride

Herein, a novel flower-like Ni MOF with good thermostability is introduced into MgH₂ for the first time, and which demonstrates excellent catalytic activity on improving hydrogen storage performance of MgH₂. The peak dehydrogenation temperature of MgH₂-5 wt.% Ni MOF is 78 °C lower than that of pure MgH₂. Besides, MgH₂-5 wt.% Ni MOF shows faster de/hydrogenation kinetics, releasing 6.4 wt% hydrogen at 300 °C within 600 s and restoring about 5.7 wt% hydrogen at 150 °C after dehydrogenation. The apparent activation energy for de/hydrogenation reactions are calculated to be 107.8 and 42.8 kJ/mol H₂ respectively, which are much lower than that of MgH₂ doped with other MOFs. In addition, the catalytic mechanism of flower-like Ni MOF is investigated in depth, through XRD, XPS and TEM methods. The high catalytic activity of flower-like Ni MOF can be attributed to the combining effect of in-situ generated Mg₂Ni/Mg₂NiH₄, MgO nanoparticles, amorphous C and remaining layered Ni MOF. This research extends the knowledge of elaborating efficient catalysts via MOFs in hydrogen storage materials.     

Hydrogen storage property of sandwiched magnesium hydride nanoparticle thin film

Hydrogen sorption property of magnesium (Mg) in the form of sandwiched Pd/Mg/Pd films is investigated. Pulsed laser deposition method was applied to deposit the samples consisting of films of nanoparticles. The enthalpy of formation of MgH₂ was found to be −68 kJ/mol H₂ for films with nanoparticle size on the order of 50 nm, which is smaller than the value for bulk MgH₂ and may be explained by the concept of excess volume.     

Iron and niobium based additives in magnesium hydride: Microstructure and hydrogen storage properties

In this study, powder mixtures of MgH₂ + 2 mol.% X, with X = Nb, Nb₂O₅, NbF₅, Fe, Fe₂O₃, FeF₃, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF₃) in the MgH₂ matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH₂ sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH₂, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH₂ particle size. DSC analysis revealed that fluorides (NbF₅, FeF₃) are much more effective than oxides (Nb₂O₅, Fe₂O₃) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb₂O₅ is more efficient than Fe₂O₃. Finally, the best results for desorption kinetics were observed for the fluorides: NbF₅ and FeF₃ (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb₂O₅, Fe₂O₃ and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample.     

Effect of V,Nb, Ti and graphite additions on the hydrogen desorption temperature of magnesium hydride

In this study the effects of mechanical milling with 5 wt.% of additives (V, Nb, Ti and Graphite) on the hydrogen desorption temperature of the magnesium hydride (MgH₂) were studied. The powder mixtures were mechanically milled for 2 h. X-ray diffraction (XRD), scanning electron microscope (SEM), and optical microscope (OM) techniques were used for the structural and morphological characterization of powders. Differential scanning calorimeter (DSC) was used to investigate the effects of the mechanical milling with additives on the hydrogen desorption temperature of the magnesium hydride powder. DSC results show that the hydrogen desorption temperatures of mechanically milled MgH₂ with additives are depressed about ∼40–50 °C compared with that of as-received MgH₂. The particle size analysis results indicate that decrease of the particle size of powders leads to a decrease of the hydrogen desorption temperature. Moreover, increasing specific surface area can also contribute to a decrease on the hydrogen desorption temperature.     

Experimental and numerical study of a magnesium hydride tank

A small-scale experimental magnesium hydride tank was designed and tested to illustrate the feasibility of hydrogen storage in magnesium hydride. A prototype of the tank was filled with 123 g of previously ball-milled and doped MgH₂. About 80 nl of hydrogen can be reversibly stored at a pressure less than 1 MPa. However, owing to the fact that the heat of a reaction limits the absorption and desorption processes, these latter are slowed down. To have a better understanding of the heat and mass transfers in the tank, a numerical model was developed using the Fluent software. The numerical simulations of hydrogen sorption are found to be in good agreement with the experimental results. During desorption, as an example, the reaction occurs locally and progresses from the tank walls towards the core.     

Scaled-up production of a promising Mg-based hydride for hydrogen storage

The feasibility of scaling up the production of a Mg-based hydride as material for solid state hydrogen storage is demonstrated in the present work. Magnesium hydride, added with a Zr–Ni alloy as catalyst, was treated in an attritor-type ball mill, suitable to process a quantity of 0.5–1 kg of material. SEM–EDS examination showed that after milling the catalyst was well distributed among the magnesium hydride crystallites. Thermodynamic and kinetic properties determined by a Sievert's type apparatus showed that the semi-industrial product kept the main properties of the material prepared at the laboratory scale. The maximum amount of stored hydrogen reached values between 5.3 and 5.6 wt% and the hydriding and dehydriding times were of the order of few minutes at about 300 °C.     

Effects of Ti-based catalysts on hydrogen desorption kinetics of nanostructured magnesium hydride

In the present work, the synergetic effect of Ti-based catalysts (TiH₂ and TiO₂ particles) on hydrogen desorption kinetics of nanostructured magnesium hydride was investigated. Nanostructured 84 mol% MgH₂–10%mol TiH₂–6%mol TiO₂ nanocomposite powder was prepared by high-energy ball milling and subjected to thermal analyses. Evaluation of the absorption/desorption properties revealed that the addition of the Ti-based catalysts significantly improved the hydrogen storage performance of MgH₂. A decrease in the decomposition temperature (as high as 100 °C) was attained after co-milling of MgH₂ with the Ti-based catalysts. Meanwhile, solid-state chemical reactions between MgH₂ and TiO₂ nanoparticles during co-milling slightly decreased the maximum hydrogen capacity. It was also found that formation of micro-cracks at the particle surfaces during thermal cycling enhanced the H-kinetics. Isothermal and non-isothermal thermal analysis revealed that the addition of Ti-catalysts reduced the decomposition activation energy of MgH₂ by 20–30 kJ/mol.     

Performance analysis of a high-temperature magnesium hydride reactor tank with a helical coil heat exchanger for thermal storage

Metal hydrides are regarded as one of the most attractive options for thermal energy storage (TES) materials for concentrated solar thermal applications. Improved thermal performance of such systems is vitally determined by the effectiveness of heat exchange between the metal hydride and the heat transfer fluid (HTF). This paper presents a numerical study supported by experimental validation on a magnesium hydride reactor fitted with a helical coil heat exchanger for enhanced thermal performance. The model incorporates hydrogen absorption kinetics of ball-milled magnesium hydride, with titanium boride and expanded natural graphite additives obtained by Sievert's apparatus measurements and considers thermal diffusion within the reactor to the heat transfer fluid for a realistic representation of its operation. A detailed parametric analysis is carried out, and the outcomes are discussed, examining the ramifications of hydrogen supply pressure and its flow rate. The study identifies that the enhancement of thermal conductivity in magnesium hydride has an insignificant impact on current reactor performance.     

Scaling up effects of Mg hydride in a temperature and pressure-controlled hydrogen storage device

A research program addressed to evaluate the magnesium hydride storage scaling up effects is being developed by CESI RICERCA, Milano, and the Hydrogen Group of Padova University. A storage device containing 500 g of magnesium hydride powder (manufactured by Venezia Tecnologie S.p.A. using high-energy ball milling) has been designed and tested in different operating conditions. A number of absorption and desorption cycles at different temperatures and pressures has been carried out in order to see if the results are comparable with laboratory data obtained on small amounts (fractions of grams) of powder samples. A sensible performance degradation that reduced the overall storage capacity of about 50% has been noticed after 20 cycles, presumably due to local powder heating, fragmentation and subsequent compaction. Further tests on a smaller tank equipped also with a porous baffle gave useful indications for the design of an improved large hydrogen reservoir.     

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.     

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.