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.     

Studies on dehydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with vanadium and vanadium based catalysts (V,V2O5 and VCl3)

In the present study, we have investigated the effect of vanadium and its compounds (V, V₂O₅ and VCl₃) on desorption characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture. The hydrogen storage characteristics of 1:2 Mg(NH₂)₂/LiH mixture gets enhanced with admixing of V, V₂O₅ and VCl₃ separately. The VCl₃ has been found to be the most effective followed by V and V₂O₅. The activation energy for dehydrogenation process of 1:2 Mg(NH₂)₂/LiH mixture with and without catalyst has been evaluated using a method suggested by Ozawa et al. [25]. Based on the experimental results, schematic reaction scheme for decomposition of Mg(NH₂)₂ in the presence of VCl₃ has also been proposed.     

Enhancing hydrogen storage properties of the Mg/MgH2 system by the addition of bis(tricyclohexylphosphine)nickel(II) dichloride

This is a first report on the use of the bis(tricyclohexylphosphine)nickel (II) dichloride complex (abbreviated as NiPCy3) into MgH₂ based hydrogen storage systems. Different composites were prepared by planetary ball-milling by doping MgH₂ with (i) free tricyclohexylphosphine (PCy3) without or with nickel nanoparticles, (ii) different NiPCy3 contents (5–20 wt%) and (iii) nickel and iron nanoparticles with/without NiPCy3. The microstructural characterization of these composites before/after dehydrogenation was performed by TGA, XRD, NMR and SEM-EDX. Their hydrogen absorption/desorption kinetics were measured by TPD, DSC and PCT. All MgH₂ composites showed much better dehydrogenation properties than the pure ball-milled MgH₂. The hydrogen absorption/release kinetics of the Mg/MgH₂ system were significantly enhanced by doping with only 5 wt% of NiPCy3 (0.42 wt% Ni); the mixture desorbed H₂ starting at 220 °C and absorbed 6.2 wt% of H₂ in 5 min at 200 °C under 30 bars of hydrogen. This remarkable storage performance was not preserved upon cycling due to the complex decomposition during the dehydrogenation process. The hydrogen storage properties of NiPCy3-MgH₂ were improved and stabilized by the addition of Ni and Fe nanoparticles. The formed system released hydrogen at temperatures below 200 °C, absorbed 4 wt% of H₂ in less than 5 min at 100 °C, and presented good reversible hydriding/dehydriding cycles. A study of the different storage systems leads to the conclusion that the NiPCy3 complex acts by restricting the crystal size growth of Mg/MgH₂, catalyzing the H₂ release, and homogeneously dispersing nickel over the Mg/MgH₂ surface.     

Facile synthesized Fe nanosheets as superior active catalyst for hydrogen storage in MgH2

Reversible hydrogen storage in MgH₂ under mild conditions is a promising way for the realization of “Hydrogen Economy”, in which the development of cheap and highly efficient catalysts is the major challenge. Herein, A two-dimensional layered Fe is prepared via a facile wet-chemical ball milling method and has been confirmed to greatly enhance the hydrogen storage performance of MgH₂. Minor addition of 5 wt% Fe nanosheets to MgH₂ decreases the onset desorption temperature to 182.1 °C and enables a quick release of 5.44 wt% H₂ within 10 min at 300 °C. Besides, the dehydrogenated sample takes up 6 wt% H₂ in 10 min under a hydrogen pressure of 3.2 MPa at 200 °C. With the doping of Fe nanosheets, the apparent activation energy of the dehydrogenation reaction for MgH₂ is reduced to 40.7 ± 1.0 kJ mol⁻¹. Further ab initio calculations reveal that the presence of Fe extends the Mg–H bond length and reduces its bond strength. We believe that this work would shed light on designing plain metal for catalysis in the area of hydrogen storage and other energy-related issues.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

Study of the synthesis of PMMA-Mg nanocomposite for hydrogen storage application

The nano metallic-based material has received the particular attention of scientists in H₂ storage. Herein, an efficient air-stable nano metallic magnesium (Mg)-Polymethyl methacrylate (PMMA) system, in which methyl magnesium chloride (MeMgCl) as organic Mg precursor is in-situ reduced to metallic Mg particles (Mg NPs) by lithium naphthalene (Li-naphthalene) in soluble PMMA/THF system, exhibits an excellent H₂ storage performance and do not require harsh operation condition. In order to form well-distributed Mg NPs (co. 5 nm) in PMMA gel framework, it is an important procedure to mix Mg ion and Li-naphthalene completely, as well the restriction effect of polymer molecular chain. The synthesized mechanism of nanocomposite and the optimal reaction conditions were ascertained by designing a series of experiments. Notably, PMMA can not confined the size of metallic Mg by blending method, and the mixed β/γ-Mg presents nearly no ability to adsorb hydrogen. Here, the air stable Mg NPs is in-situ reduced in PMMA can be reacted with H₂, and O₂ and H₂O molecules can not be infiltrated into PMMA. The correlation between the size of Mg NPs, the amount of PMMA and hydrogen storage performance for the PMMA-Mg NPs composite (PMC) is studied. We have found that hydrogen storage capacity of PMC could be enhanced as decreasing the size of Mg NPs by adjusting the amount of PMMA. The Mg NPs in PMMA might release the higher amount of H₂ at below 300 °C with a rapid absorption/desorption kinetics than the reported material in literature. The obtained nanocomposite are able to deliver dense hydrogen in demanding environments.     

Enhanced hydrogen storage by a variable temperature process

We present a simple method of variable temperature process that can potentially enhance the hydrogen storage properties of a large variety of solid state materials. In this approach, hydrogen gas is first introduced at about room temperature, which is followed by a gradual increase to a preset maximum temperature value, T_max. Using this approach, we investigated hydrogen absorption properties of vertically aligned arrays of magnesium nanotrees and nanoblades fabricated by glancing angle deposition (GLAD) technique, and conventional Mg thin film. Weight percentage (wt%) storage values were measured by quartz crystal microbalance (QCM). After exposing Mg samples to H₂ at 30 bar and 30 °C, dynamic absorption measurements were conducted as the temperature was increased from 30 °C to maximum values of T_max = 100, 200, and 300 ^°C all within 150 min. QCM measurements revealed that variable temperature method results in significant improvements in hydrogen storage values over the ones obtained by conventional constant temperature process. At a low effective temperature T_eff = 165 °C (T_max = 300 °C), we achieved storage values of 6.19, 4.76, and 2.79 wt% for Mg nanotrees, nanoblades, and thin film, respectively.     

Facile synthesis of nickel-vanadium bimetallic oxide and its catalytic effects on the hydrogen storage properties of magnesium hydride

To improve the dehydrogenation/hydrogenation performance of magnesium hydride (MgH₂), a nickel-vanadium bimetallic oxide (NiV₂O₆) was prepared by a simple hydrothermal method using ammonium metavanadate and nickel nitrate as raw materials. This oxide was used to improve the hydrogen storage performance of MgH₂. NiV₂O₆ reacted with Mg to form Mg₂Ni and V₂O₅; Mg₂Ni and V₂O₅ played an important role in improving the hydrogen storage properties of MgH₂. The NiV₂O₆-doped MgH₂ had an excellent hydrogen absorption and desorption kinetics performance, and it could absorb 5.59 wt% of hydrogen within 50 min at 150 °C and release about 5.3 wt% of hydrogen within 12 min. The apparent activation energies for the dehydrogenation and hydrogenation of MgH₂-NiV₂O₆ were 92.9 kJ mol^?1 and 24.9 kJ mol^?1, respectively. These were 21.7% and 66.3% lower than those of MgH_2, respectively. The mechanism analysis demonstrated that the improved kinetic properties of MgH₂ resulted from the heterogeneous catalysis of vanadium and nickel.     

Catalysis and evolution on cycling of nano-structured magnesium multilayer thin films

This paper explores the hydrogen cycling properties of Mg/Cr and Mg/V multilayer thin films and studies the effect of chromium and vanadium transition metal catalysts on the cycling properties of thick magnesium coatings. Two transition-metal catalysed magnesium-based multilayer PVD coatings are compared with a non-catalysed magnesium control sample. The (micro-)structural evolution of the thin film coatings into fine, flakey powders is studied in-depth using XRD, SEM and TEM and the hydrogen storage properties of all three materials are assessed using volumetric, gravimetric and calorimetric methods focussing on the effect of the microstructure and composition of the coatings on the hydrogen storage kinetics. It was found that the chromium-catalysed coating had the most favourable hydrogen storage kinetics with an activation energy for the dehydrogenation reaction of 65.7 ± 2.5 kJ mol⁻¹ and a hydrogen capacity of 6.1 ± 0.3 wt%. The mechanism of the dehydrogenation reaction of the catalysed samples was studied using the CV and JMAK kinetic models and it was found that the catalyst material influenced not only the hydrogen storage kinetics but also the mechanism of the reaction.     

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.     

Magnesium vacancies and hydrogen doping in MgH2 for improving gravimetric capacity and desorption temperature

Performing an ab initio analysis, we inspect the effect of magnesium vacancies and hydrogen doping on the magnesium hydride (MgH₂). The Korringa – Kohn – Rostoker method integrated with the coherent potential approximation is used to perform our calculations. In particular, we find that the gravimetric capacity of MgH₂ increases from 7.658 to 9.816 wt% when the concentrations of magnesium vacancies and hydrogen dopant atoms increase from 0 to 10%. Concretely, the results reveal that the magnesium vacancies and the hydrogen doping have a beneficial effect on the hydrogen storage properties of the hydride by decreasing its desorption temperature and stability. This decrease can be explained on the one hand by the diminution of the number of Mg atoms that establish strong bonds with H atoms, and on the other hand by using the density of states, which indicates that when the concentrations increase, the Mg and H states shift to the conduction band. We also obtain that the value of the desorption temperature can be controlled by varying the concentrations of magnesium vacancies and hydrogen dopant atoms from 4.2 to 5.8% in order to reach the optimum range 289–393 K for the practical use of fuel cell vehicles.     

Air exposure improving hydrogen desorption behavior of Mg–Ni-based hydrides

It is well established that H₂O and O₂ have an inauspicious influence on hydrogen reactivity of hydrogen storage alloys. In this work, an unexpected improvement of the desorption behavior was discovered by just exposing the magnesium rich Mg–Ni hydrides into the air for a certain period. Upon an exposure duration of 4 months, the dehydrogenation peak and onset temperature were sharply lowered by 150 °C and 130 °C. Furthermore, the air-exposed sample could quickly absorb 3.08 wt% H₂ and desorb 2.81 wt% H₂ within 400 s at 300 °C. Besides the refinement of the powders due to the spontaneous hydrolysis reaction, the in-situ formed magnesium hydroxide layer and Ni are thought to be responsible for the remarkable improvement. This work gives interesting insights that the self-generating surface passivation is not necessarily harmful in the solid-state hydrogen storage area, especially for the cases where active sites of catalysis are present.     

Synthesis of MgH2 using autocatalytic effect of MgH2

We describe the synthesis of MgH₂ using autocatalytic effect of MgH₂. The MgH₂ was synthesized by ball milling Mg with 5 wt% of MgH₂. The ball milling was carried out at different pressures of 15, 30 and 45 atm of H₂ followed by heat treatment (heating under vacuum and annealing at 30 atm of H₂ pressure (at 350 °C) for 10 h). It has been found that the MgH₂ synthesized using 30 atm of H₂ pressure during ball milling, followed by heat treatment and annealing (MgH₂30BM) is the optimum material as it has lowest desorption temperature (325 °C) faster rehydrogenation kinetic (6.60 wt% within 30 min at 300 °C and 20 atm hydrogen pressure). Also MgH₂30BM maintains the storage capacity of more than 6.00 wt% (loss of 0.6 wt%) after 10 cycles of de/re-hydrogenation. The as synthesized MgH₂ has superior de/re-hydrogenation properties and is ∼4 times cheaper as compared to MgH₂ procured from the chemical company like Alfa-Aesar. It is to be mentioned that under above mentioned temperature and pressure conditions the stand alone Mg (without having MgH₂ as catalyst) doesnot at all converts to MgH₂. The present study opens the gateway for economical synthesis of MgH₂ at large scale.     

Microstructural evolution and hydrogen storage properties of a Ni-modified Mg15Al alloy

On the basis of modification of transition metals on Mg-Al hydrogen storage alloys, Mg15Al5Ni alloy with Ni content of 5 wt% has been prepared by high energy ball mill. The results show that Ni particles uniformly distribute on the surface of particles, while several Ni particles are embedded inside alloy particles. These Ni particles tend to redistribute after hydrogenation. The phase composition analysis reveals the formation of stable Al₃Ni₂ phase in Ni-modified alloy after hydrogenation. The hydrogen absorption performance of Mg15Al5Ni alloy has been improved by introducing Ni, which can absorb 4.36 wt% hydrogen within 5 min at 350 °C. Meanwhile, the activation properties of Mg15Al5Ni alloy can be obviously deteriorated due to the addition of Ni. However, uniformly distributed Al₃Ni₂ nanocrystals with grain sizes around 10 nm hinder grain growth of hydrides, ameliorating hydrogenation kinetics of Mg15Al5Ni alloy. Besides, the modified effect of Ni on hydrogenation kinetics of Mg15Al5Ni alloy has been also discussed in this work.     

Remarkable synergistic effects of Mg2NiH4 and transition metal carbides (TiC,ZrC, WC) on enhancing the hydrogen storage properties of MgH2

Nanostructuring and catalyzing are effective methods for improving the hydrogen storage properties of MgH₂. In this work, transition-metal-carbides (TiC, ZrC and WC) are introduced into Mg–Ni alloy to enhance its hydrogen storage performance. 5 wt% transition-metal-carbide containing Mg₉₅Ni₅ (atomic ratio) nanocomposites are prepared by mechanical milling pretreatment followed by hydriding combustion synthesis and mechanical milling process, and the synergetic enhancement effects of Mg₂NiH₄ and transition-metal-carbides are investigated systematically. Due to the inductive effect of Mg₂NiH₄ and charge transfer effect between Mg/MgH₂ and transition-metal-carbides, Mg₉₅Ni₅-5 wt.% transition-metal-carbide samples all exhibit excellent hydrogen storage kinetic at moderate temperature and start to release hydrogen around 216 °C. Among them, 2.5 wt% H₂ (220 °C) and 4.7 wt% H₂ (250 °C) can be released from the Mg₉₅Ni₅-5 wt.% TiC sample within 1800 s. The unique mosaic structure endows the Mg₉₅Ni₅-5 wt.% TiC with excellent structural stability, thus can reach 95% of saturated hydrogen capacity within 120 s even after 10 cycles of de-/hydrogenation at 275 °C. And the probable synergistic enhancement mechanism for hydrogenation and dehydrogenation is proposed.     

Magnesium promoted hydrocalumite derived nickel catalysts for ethanol steam reforming

Hydrocalumite derived nickel (Ni) catalysts with different loading of magnesium (Mg) (7.5/10/15 wt%, as promoters) were for the first time prepared and tested for ethanol steam reforming (ESR) in this work. The catalytic performances of different Mg promoted catalysts were mainly evaluated in the temperature range between 550 and 700 °C as determined by thermodynamic simulation. Experimental results showed that the optimal reaction temperature was 650 °C in terms of the hydrogen yields for these ESR catalysts, especially for 15Ni7.5Mg/HCa which presented a remarkable catalytic performance. Its hydrogen yields reached 90% while ethanol was almost fully converted at 650 °C. Based on the characterization results, it's believed that 15Ni7.5Mg/HCa with a certain amount of Mg loading can get the smallest Ni⁰ crystallite sizes, better H₂ reducibility and suitable basicities on strong basic sites. The catalytic performances of ESR catalysts were mainly related to the Ni⁰ crystallite size, reducibility and basicity for the prepared hydrocalumites derived Ni catalysts, and 15Ni7.5Mg/HCa could be considered as one of the best catalysts for ESR.     

Halide-free Grignard reagents for the synthesis of superior MgH2 nanostructures

Grignard reagents can provide a simple path to generate, through their thermal decomposition, magnesium (Mg) and/or its hydride (MgH₂). However, existing compounds lack the ability to lead to “adequate” MgH₂ structures to enable effective hydrogen storage. Herein, we report on the possibility to tune Grignard reagent's chemical structure, i.e. number of β-hydrogen, and the activation energy for their thermal decomposition to lead to Mg/MgH₂. For example, di-tert-butylmagnesium with nine β-hydrogen was able to decompose at a very low temperature of 167 °C to generate MgH₂/Mg, which is 100 °C lower than the temperature required to generate MgH₂ from di-n-butylmagnesium, i.e. the only compound known to date. More remarkably, the MgH₂ synthesized from the di-tert-butylmagnesium precursor was able to release hydrogen from 100 °C. These promising hydrogen storage properties are credited to the formation of the metastable γ-MgH₂ phase, which is believed to result from the structural defects generated upon the thermal decomposition of di-tert-butylmagnesium.     

Development of Ca–Mg–H2–ZrCl4 composite for hydrogen storage applications

Both CaH₂ and MgH₂ are good candidate for the development of hydrogen storage materials because of their high hydrogen storage capacity. However, both the hydrides are quite stable thermodynamically and required high temperature for hydrogen sorption process. The MgH₂–CaH₂ composite could show the favourable hydrogen sorption reaction because of Ca–Mg intermetallic formation. The idea motivated to perform the experiments starting with these metal hydrides. It has been found that the hydrogen sorption reaction kinetics improved substantially. The dihydrogen product has shown a few intermetallic of magnesium and calcium. The hydrogen sorption temperature and pressure of the alloy was remarkably improved by the doping with ZrCl₄ as a catalyst. The activation energy and the thermodynamic parameters of un-catalyzed and catalyzed alloy were studied. Present studied indicated that the CaH₂–MgH₂–ZrCl₄ could be a potential candidate for the mobile hydrogen storage system.     

Interfacial engineering of nickel/vanadium based two-dimensional layered double hydroxide for solid-state hydrogen storage in MgH2

As a high-density solid-state hydrogen storage material, magnesium hydride (MgH₂) is promising for hydrogen transportation and storage. Yet, its stable thermodynamics and sluggish kinetics are unfavorable for that required for commercial application. Herein, nickel/vanadium trioxide (Ni/V₂O₃) nanoparticles with heterostructures were successfully prepared via hydrogenating the NiV-based two-dimensional layered double hydroxide (NiV-LDH). MgH₂ + 7 wt% Ni/V₂O₃ presented more superior hydrogen absorption and desorption performances than pure MgH₂ and MgH₂ + 7 wt% NiV-LDH. The initial discharging temperature of MgH₂ was significantly reduced to 190 °C after adding 7 wt% Ni/V₂O₃, which was 22 and 128 °C lower than that of 7 wt% NiV-LDH modified MgH₂ and additive-free MgH₂, respectively. The completely dehydrogenated MgH₂ + 7 wt% Ni/V₂O₃ charged 5.25 wt% H₂ in 20 min at 125 °C, while the hydrogen absorption capacity of pure MgH₂ only amounted to 4.82 wt% H₂ at a higher temperature of 200 °C for a longer time of 60 min. Moreover, compared with MgH₂ + 7 wt% NiV-LDH, MgH₂ + 7 wt% Ni/V₂O₃ shows better cycling performance. The microstructure analysis indicated the heterostructural Ni/V₂O₃ nanoparticles were uniformly distributed. Mg₂Ni/Mg₂NiH₄ and metallic V were formed in-situ during cycling, which synergistically tuned the hydrogen storage process in MgH₂. Our work presents a facile interfacial engineering method to enhance the catalytic activity by constructing a heterostructure, which may provide the mentality of designing efficient catalysts for hydrogen storage.     

Enhancement of H2 uptake due to morphological modulation in vanadium pentoxide foam

The high pressure H₂ sorption isotherms for vanadium pentoxide foam (VOF) were obtained at a liquid nitrogen temperature. The enhancement of hydrogen storage capacity occurred in as-prepared VOF (∼1.0 wt%) in contrast to that in pristine vanadium pentoxide (∼0.2 wt%). The maximum capacity of hydrogen storage (∼2.0 wt%) was achieved by thermal annealing at T_a = 623 K. The enhancement of hydrogen storage in VOF is attributed to the morphological modulation by thermal annealing.