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.     

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).     

Releasing 9.6 wt% of H2 from Mg(NH2)2–3LiH–NH3BH3 through mechanochemical reaction

Ball milling the mixture of Mg(NH₂)₂, LiH and NH₃BH₃ in a molar ratio of 1:3:1 results in the direct liberation of 9.6 wt% H₂ (11 equiv. H), which is superior to binary systems such as LiH–AB (6 equiv. H), AB–Mg(NH₂)₂ (No H₂ release) and LiH–Mg(NH₂)₂ (4 equiv. H), respectively. The overall dehydrogenation is a three-step process in which LiH firstly reacts with AB to yield LiNH₂BH₃ and LiNH₂BH₃ further reacts with Mg(NH₂)₂ to form LiMgBN₃H₃. LiMgBN₃H₃ subsequently interacts with additional 2 equivalents of LiH to form Li₃BN₂ and MgNH as well as hydrogen.     

Why the ball to powder ratio (BPR) is insufficient for describing the mechanical ball milling process

The ball to powder ratio (BPR) is a processing parameter that is frequently used in both mechanical (ball) milling and mechanical alloying. A number of recent studies provided the BPR as a principal milling parameter while neglecting other parameters, such the vial volume, the diameter and quantity of milling balls and the powder mass. In this experiment, different batches of magnesium hydride powder were milled using varying ball size, powder mass, and other parameters and a constant BPR. The hydrogen desorption properties (i.e., differential scanning calorimeter) and phase evolution (i.e., XRD phase analysis) of the milled powders were subsequently investigated. The obtained results demonstrated that the BPR cannot be provided as a single processing parameter. The DSC curves obtained during decomposition with a scanning rate of 5 °C/min revealed significant differences in desorption peak temperature among the samples milled using the same BPR. Additionally, XRD patterns revealed that the crystallite size after milling varied, suggesting that differences existed in the effectiveness of the milling process.     

Hydrogen desorption from alloys Mg–Cu(–KCl): Cu catalysis in detail

Effect of chemical composition of Mg-xCu based alloys (x = 9.94–58.00 wt %) modified by KCl upon their hydrogen storage performance was studied. Kinetic curves and pressure-concentration isotherms were measured in the ranges up to 60 bar and 388 °C, respectively. It was observed that desorption rate dc/dt is not significantly influenced by the composition. Unknown Cu-rich phase was detected that has shown a catalytic effect on desorption from a mixture with other phases. Activation energy of hydrogen desorption decreased with increasing x from 180 kJ/mol down to 98 kJ/mol. Average hydride dissociation enthalpy, ΔH, for the lowest plateau was 75 kJ/mol which is equal to literature value for pure Mg. Slightly lover average value, 67 kJ/mol was obtained for the second plateau and ΔH for the third one decreased from 70 kJ/mol for the lowest to 49 kJ/mol for the highest x.     

Microstructure and activation characteristics of Mg–Ni alloy modified by multi-walled carbon nanotubes

An Mg–6 wt% Ni alloy was fabricated by a casting technique and the drilled chips ball-milled by high energy ball milling to be examined for their hydrogenation modified with multi-walled carbon nanotubes (MWCNTs). The activation characteristics of ball-milled alloy are compared with those of the materials obtained by ball milling with 5 wt% MWCNTs for 0.5, 1, 2, 5 and 10 h. MWCNTs enhanced the absorption kinetics considerably in all cases. The hydrogen content of the modified powder with MWCNTs reached maximum hydrogen capacity within 2 min of exposure to hydrogen at 370 °C and 2 MPa pressure. X-ray diffraction analysis provided evidence that no carbon-containing phase was formed during milling. However, milling with MWCNTs reduced the crystallite size, even if the milling was carried out for only an hour. The rate-controlling steps of the hydriding reactions at different milling times were determined by fitting the respective kinetic equations. Evidence is provided that nucleation and growth of hydrides are accelerated drastically by a homogenous distribution of MWCNTs on the surface of the ball-milled powders. We show that MWCNTs are very effective at promoting the hydriding/dehydriding kinetics, as well as in increasing the hydrogen capacity of the magnesium alloy.     

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.     

Hydriding behavior of Mg-50 wt% ZrCrFe composite Prepared by high energy ball milling

Magnesium hydride has a high theoretically storage capacity, which amounts to 7.6 wt%. It is therefore a promising candidate for hydrogen storage applications. However, its major drawback is its high desorption temperature of well over 300 °C, which is related to the high stability of the Mg–H bonds and expressed in the high enthalpy of hydride formation (77 kJ/mol). The preparation of Mg composites with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties of Mg. Thus we prepared Mg-50 wt% ZrCrFe alloy composite by high energy ball milling under argon atmosphere. X-ray diffraction (XRD) studies on the composite before and after hydriding cycles suggest no intermetalic phase is formed between Mg and the elements of the alloy. The morphological studies carried on by Scanning Electron Microscope (SEM) technique suggest that the alloy particles are homogeneously distributed throughout the Mg surface. A particle reduction after hydrogenation is also visible. Hydriding/dehydriding properties of the composites are investigated by PCT measurements using a dynamic system. The maximum hydrogen capacity for this composite is found to be 4.5 wt%. The reaction kinetics have also been recorded in a temperature range from RT to 300 °C and the thermodynamic parameters calculated from Van’t Hoff plot. From the results it is found that the alloy reacts with hydrogen also when cooled to room temperature while at higher temperature it works as catalyst.     

Catalytic effect of halide additives ball milled with magnesium hydride

The influence of various halide additives milled with magnesium hydride (MgH₂) on its decomposition temperature was studied. The optimum amount of halide additive and milling conditions were evaluated.     

Hydriding/dehydriding properties of MgH2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH₂/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg₂Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

Effect of milling intensity on the formation of LiMgN from the dehydrogenation of LiNH2-MgH2 (1:1) mixture

Metal-N-H systems have recently attracted considerable attention as alternative hydrogen storage materials to traditional metal hydrides. In this work, the reactions of the mixture LiNH₂-MgH₂ (1:1) during different mechanical milling processes and the subsequent dehydrogenation reaction were investigated by using TGA, XRD and FT-IR in order to determine an optimal condition for the formation of pure LiMgN. High-energy milling (SPEX mill) and low-energy milling (rolling jar) techniques were used in this work. The results demonstrated that monolithic LiMgN can be produced using the low-energy ball milling technique. The hydrogenation properties of the as-prepared LiMgN were investigated by a Sieverts’ type instrument. In contrast, multiple reactions including the metathesis reaction between LiNH₂ and MgH₂ and release of H₂ and/or NH₃ took place during high-energy milling using the SPEX mill, which resulted in complicated and unexpected reactions during the subsequent dehydrogenation experiments. Consequently, the dehydrogenated products from the high-energy milled samples consisted of multi-phase mixtures.     

Effect of nano-structured Ta2C on hydrogen absorption−desorption kinetics of Mg−H2 system

High purity Ta₂C was successfully prepared and the hydrogen absorption−desorption kinetic properties of MgH₂−10 wt% Ta₂C composites were investigated systematically. It was found that the hydrogen absorption of Mg−10 wt% Ta₂C (20 nm) composite takes about 5 min to reach saturation at 573 K, and its hydride fully desorbs hydrogen within 15 min at 623 K. These kinetic properties are much better than those of the undoped Mg and MgH₂ prepared under the same condition, respectively. The improvement in the hydrogen storage kinetics is ascribed to the catalytic effect of Ta₂C and its inhibition role in crystallite growth.