Interaction of zirconia with magnesium hydride and its influence on the hydrogen storage behavior of magnesium hydride

This study demonstrates how zirconia additive transforms to zirconium hydride and substantially lowers the dehydrogenation temperature of magnesium hydride. We prepared MgH₂+xZrO₂ (x = 0.125 and 0.5) powder samples reacted for 15 min, 1 h, 5 h, 10 h, 15 h, 20 h and 25 h, and monitored the phase changes at each stage of the reaction. Differential scanning calorimetry (DSC) study provides the first crucial evidence regarding the chemical transformation of zirconia. Subsequently, detailed additional sample testing by X-ray diffraction (XRD), energy dispersive x-ray spectroscopy and confocal Raman microscopy provide strong supports that low temperature dehydrogenation of magnesium hydride is a result of formation of an active in situ product (zirconium hydride). This observation is validated by the negative Gibbs free energy values obtained for the formation of zirconium hydride over a broad working temperature range of 0–600 °C. Scanning electron microscopy (SEM) results prove the high dispersion of tiny nanoparticles all across the surface after the chemical interaction between MgH₂ and ZrO₂ and atomic force microscopy (AFM) study further proves that objects with grain sizes of ～10 nm are abundant throughout the scanned surfaces. These observations reiterate that better metal oxide additives interact with MgH₂ and results to the evolution of highly active insitu nanocatalysts.     

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.     

Excellent catalytic effect of LaNi5 on hydrogen storage properties for aluminium hydride at mild temperature

The catalytic effect of rare-earth hydrogen storage alloy is investigated for dehydrogenation of alane, which shows a significantly reduced onset dehydrogenation temperature (86 °C) with a high-purity hydrogen storage capacity of 8.6 wt% and an improved dehydrogenation kinetics property (6.3 wt% of dehydrogenation at 100 °C within 60 min). The related mechanism is that the catalytic sites on the surface of the hydrogen storage alloy and the hydrogen storage sites of the entire bulk phase of the hydrogen storage reduce the dehydrogenation temperature of AlH₃ and improve the dehydrogenation kinetic performance of AlH₃. This facile and effective method significantly improves the dehydrogenation of AlH₃ and provides a promising strategy for metal hydride modification.     

Agent-free synthesis of graphene oxide/transition metal oxide composites and its application for hydrogen storage

Graphene oxide (GO) wrapped transition metal oxide composite materials were synthesized by a very simple route without any additional agents and the hydrogen adsorption properties of the materials were investigated. The morphologies of GO/V₂O₅ and GO/TiO₂ were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that single- or few-layered GO sheets wrapped throughout the V₂O₅ and TiO₂ particles. According to X-ray photoelectron spectroscopy (XPS), the C–OH species of GO and the surface-adsorbed oxygen of the transition metal oxide bond together via a dehydration reaction. The wrapping phenomenon of GO causes the enhancement of hydrogen storage capacity at liquid nitrogen temperature (77 K) compared with those of the pristine transition metal oxides and GO. The enhancement of hydrogen storage capacity of GO-wrapped transition metal oxide composite materials results from the existence of interspaces between the transition metal oxide particles and the thin GO layers.     

Synthetical catalysis of nickel and graphene on enhanced hydrogen storage properties of magnesium

Reduced graphene-oxide-supported nickel (Ni@rGO) nanocomposite catalysts were synthesized, and incorporated into magnesium (Mg) hydrogen storage materials with the aim of improving the hydrogen storage properties of these materials. The experimental results revealed that the catalytic effect of the Ni@rGO nanocomposite on Mg was more effective than that of single nickel (Ni) nanoparticles or graphene. When heated at 100 °C, the Mg–Ni and Mg–Ni@rGO composites absorbed 4.70 wt% and 5.48 wt% of H₂, respectively, whereas the pure Mg and Mg@rGO composite absorbed almost no hydrogen. The addition of the Ni@rGO composite as a catalyst yielded significant improvement in the hydrogen storage property of the Mg hydrogen storage materials. The apparent activation energy of the pure Mg sample (i.e., 163.9 kJ mol⁻¹) decreased to 139.7 kJ mol⁻¹ and 123.4 kJ mol⁻¹, respectively, when the sample was modified with single rGO or Ni nanoparticles. Under the catalytic action of the Ni@rGO nanocomposites, the value decreased further to 103.5 kJ mol⁻¹. The excellent hydrogen storage properties of the Mg–Ni@rGO composite were attributed to the catalytic effects of the highly surface-active Ni nanoparticles and the unique structure of the composite nanosheets.     

Superior catalysis of NbN nanoparticles with intrinsic multiple valence on reversible hydrogen storage properties of magnesium hydride

Magnesium hydride, as a potential solid state hydrogen carrier has attracted great attention around the world especially in the energy storage domain due to the high hydrogen storage capacity and the good cycling stability. But kinetic and thermodynamic barriers also impede the practical application and development of MgH₂. Nanoscale catalysts are deemed to be the most effective measure to overcome the kinetic barrier and lower the temperature required for hydrogen release in MgH₂. NbN nanoparticles (~20 nm) with intrinsic Nb³⁺-N and Nb⁵⁺-N were prepared using the molten salt method and used as catalysts in the MgH₂ system. It is found that the NbN nanoparticles exhibit a superior catalytic effect on de/rehydrogenation kinetics for the MgH₂/Mg system. About 6.0 wt% hydrogen can be liberated for the MgH₂+5NbN sample within 5 min at 300 °C, and it takes 12 min to desorb the same amount of hydrogen at 275 °C. Meanwhile, the MgH₂+5NbN sample can absorb 6.0 wt% hydrogen within 16 min at 150 °C, and absorb 5.0 wt% hydrogen within 24 min even at 100 °C. Particularly, the catalyzed samples exhibit excellent hydrogen absorption/desorption kinetic stability. After multiple cycles, there is no kinetic attenuation and the hydrogen capacity remains at about 6.0 wt%. It is demonstrated that the NbN nanoparticles with intrinsic multiple valence can be the critical effect in improving the hydrogen storage kinetics of MgH₂. The stability of Nb₄N₃ phase and Nb³⁺-N and Nb⁵⁺-N valence states can ensure a stable catalytic effect in the system.     

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.     

Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.     

Highly dispersed metal nanoparticles on TiO2 acted as nano redox reactor and its synergistic catalysis on the hydrogen storage properties of magnesium hydride

In this paper, we have synthesized highly dispersed Co metal nanoparticles with the particle size about 5–10 nm on TiO₂ (25–50 nm) for the first time through an extremely facile solvothermal method. It is supposed that the synthesized Co/TiO₂ composite can combine the catalytic advantage of both Co and TiO₂, exhibiting the superior catalytic effect on the hydrogen de/absorption properties of MgH₂. The experimental data confirmed the above supposition and demonstrated that Co/TiO₂ additive highly enhances the hydrogen de/absorption kinetics of MgH₂ as compared to separate Co or TiO₂ additive. Specifically, the MgH₂Co/TiO₂ composite begins to desorb hydrogen at about 190 °C with a low apparent activation energy of 77 kJ/mol. Besides, the MgH₂Co/TiO₂ composite has a desorption peak temperature of 235.2 °C, which is 53.2, 94.2 and 132.2 °C lower than that of MgH₂TiO₂ (288.4 °C), MgH₂Co (329.4 °C) and ball-milled MgH₂ (367.4 °C). Moreover, MgH₂Co/TiO₂ composite also exhibits low temperature rehydrogenation properties, which can absorb 6.07, 5.56 and 4.24 wt% H₂ within 10 min at the temperature of 165, 130 and 100 °C, respectively. It is supposed that such excellent hydrogen desorption properties and low desorption energy barrier of MgH₂Co/TiO₂ composite are mainly ascribed to the novel synergistic catalytic effects of Co and TiO₂. Herein, we propose a novel catalytic mechanism and think that Co/TiO₂ acts as “nano redox reactor”, which can facilitate the dissociation and recombination process of hydrogen, thus reducing the reaction energy barrier and enhancing the de/rehydrogenation of MgH₂.     

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.     

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid.     

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.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

First-principles studies concerning optimization of hydrogen storage in nanoporous reduced graphite oxide

By means of Density Functional Theory (DFT) calculations we investigated the optimal pore size for reduced graphite oxide (GOH) to favor hydrogen storage and to prevent oxygen interference. The interlayer distance of GOH is found to increase with oxygen content, given by the number of hydroxyl groups. Four types of GOHs were considered, with O/C ratio within a 0.09–0.38 range. In the case of the highest O/C ratio considered, 0.38, a spontaneous redox-reaction between hydroxyl groups delivering a water molecule and an epoxy group was found. Thus, GOHs with high O/C ratio are not recommended for hydrogen storage. In these materials the absorption energy values of hydrogen is in the range of −0.2 and −0.5 eV/molecule, that is within the values expected to allow an efficient storage. The best GOH for hydrogen storage was found to be that with a 0.09 O/C ratio since it has the largest void space and adequate absorption energy, −0.52 eV/molecule. On the other hand, oxygen absorption energy is lower in absolute value than that of hydrogen, which favors absorption of the latter, thus creating adequate conditions for its storage without oxygen interference.     

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.     

Facile synthesis and hydrogen storage application of nitrogen-doped carbon nanotubes with bamboo-like structure

This paper reports a facile method for the preparation of nitrogen-doped carbon nanotubes (N-doped CNTs) that shows enhanced hydrogen storage capacity. The synthesis method involves simple pyrolysis of melamine using FeCl₃ as catalyst in tube furnace. The materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, elemental analysis, Raman spectroscopy, and nitrogen adsorption–desorption analysis. The results indicated that the prepared N-doped CNTs have a bamboo-like structure with thin compartment layers. The nitrogen doping concentration, specific surface area, and total pore volume of the N-doped CNTs were determined to be 1.5 at%, 135 m²/g, and 0.38 cm³/g, respectively. The hydrogen adsorption measurements at 77 K showed that the N-doped CNTs exhibits gravimetric hydrogen uptake of 0.21 wt% at 1 bar and 1.21 wt% at 7 bar. At room temperature, hydrogen uptake as high as 0.17 wt% at 298 K and 19 bar is achieved, which is among the highest data reported for the N-doped carbon materials under the same condition.     

Hydrogen storage evaluation based on investigations of the catalytic properties of metal/metal oxides in electrospun carbon fibers

In order to investigate the catalytic capacity of metals and metal oxides based on electrospun carbon fibers for improving hydrogen storage, electrospinning and heat treatments were carried out to obtain metal/metal oxide-embedded carbon fibers. Although the fibers were treated with the same activation procedure, they had different pore structures, due to the nature of the metal oxide. When comparing the catalytic capacity of metal and metal oxide, metal exhibits better performance as a catalyst for the improvement of hydrogen storage, when considering the hydrogen storage system. When a metal oxide with an m.p. lower than the temperature of heat treatment was used, the metal oxide was changed to metal during the heat treatment, developing a micropore structure. The activation process produced a high specific surface area of up to 2900 m²/g and a pore volume of up to 2.5 cc/g. The amount of hydrogen adsorption reached approximately 3 wt% at 100 bar and room temperature.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

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.