Study on the gaseous and electrochemical hydrogen storage properties of as-milled CeMgNi-based alloys

For gaining further insight into the involvement of the gaseous and electrochemical hydrogen storage properties of CeMg₁₂-type alloys, partial substitution and ball milling were both used to synthesize the nanocrystalline and amorphous CeMg₁₁Ni + x wt.% Ni (x = 100, 200) samples. This research aims at elucidating the functional roles of Ni content and milling time on samples' structure and hydrogen storage performance. X-Ray diffraction and high-resolution transmission electron microscope were used to reveal the micro constructions of alloys. To determine the gaseous hydrogen storage property, Sievert's apparatus and a thermal gravity analysis bonded with a H₂ probe were adopted. The dehydrogenation activation energy was computed in the Kissinger method. The electrochemical performances of the as-milled samples were measured through a constant current system. Further researches showed that the electrochemical performance of as-milled samples had been dramatically improved by increasing Ni content. With milling duration lengthens, the gaseous hydrogen absorption capacity, gaseous hydriding rate and high rate discharge capability of samples reached the maximal values, but electrochemical discharge capacity and gaseous dehydriding rate always increased. The dehydrogenation activation energy decrease resulted by improving Ni percent and milling duration was deemed as the cause of the excellent gaseous kinetics of samples.     

Improved hydrogen storage kinetics of nanocrystalline and amorphous Pr-Mg-Ni-based PrMg12-type alloys synthesized by mechanical milling

In this paper, the nanocrystalline and amorphous PrMg₁₁Ni + x wt.% Ni (x = 100, 200) alloys were synthesized by mechanical milling. The gaseous and electrochemical hydrogen storage performances were studied in detail. The results reveal that increasing Ni content facilitates the glass forming of the alloys, and it significantly improves the gaseous and electrochemical hydrogen storage kinetics performance. Furthermore, milling time varying significantly affects the hydrogen storage properties of the alloys. The hydrogen capacity of the alloys first increases and then decreases with milling time prolonged. The hydriding rate and high-rate discharge ability (HRD) of the as-milled alloys have maximum values with milling time varying. But dehydriding rate always increases with milling time prolonged. The improved gaseous hydrogen storage kinetics of alloys are convinced to be ascribed to a reduction in hydrogen desorption activation energy caused by increasing Ni content and prolonging milling time.     

Dehydrogenation properties of two phases of LiNH2BH3

Lithium amidoborane (LiNH₂BH₃) is known as one of the most prospective hydrogen storage materials. In this paper, the differences between two allotropes (α-LiNH₂BH₃ and β-LiNH₂BH₃) of LiNH₂BH₃ in the dehydrogenation properties was reported for the first time. A series of mixtures of α-LiNH₂BH₃/β-LiNH₂BH₃ with different mass ratios were prepared by ball milling for different time and the contents of two phases in samples were determined with Rietveld's method. The thermal decomposition behaviors of samples were investigated by DSC. It shows that the initial dehydrogenation temperature of samples decreases with the content of α-LiNH₂BH₃ phase increasing. The initial dehydrogenation temperature of α-LiNH₂BH₃ is about 61 °C, which is approximately 15 °C lower than that of β-LiNH₂BH₃. Dehydrogenation kinetic analysis shows that α-LiNH₂BH₃ has the lower activation energy (157 kJ mol⁻¹) and higher rate (k = 1.422 × 10¹ min⁻¹) than that of β-LiNH₂BH₃ (272 kJ mol⁻¹ and 1.023 × 10⁻¹ min⁻¹, respectively). It is suggested that α-LiNH₂BH₃ is more supportive for hydrogen desorption. It gives a critical clue on exploring the dehydrogenation mechanism of lithium amidoborane. Moreover, the significant decrease of desorption temperature will shine a light on on-board hydrogen storage systems.     

Hydrogen storage property of as-milled La7RE3Mg80Ni10 (RE = Sm,Ce) alloys

Element replacement and mechanical milling are considered as the most effective ways to improve Mg-based alloys in their hydrogen storage performance. The as-milled La₇RE₃Mg₈₀Ni₁₀ (RE = Sm, Ce) alloys were prepared in this experiment by introducing both element replacement (replacing La by Ce or Sm partially) and mechanical milling technologies. The influence made by different replacing elements on the structure and hydrogen storage property of La₇RE₃Mg₈₀Ni₁₀ (RE = Sm, Ce) alloys was investigated in detail. X-ray diffraction, transmission electron microscope, automatic Sievert apparatus, thermogravimetry and differential scanning calorimetry were used to investigate the experimental alloys. The experiment reveals that a nanocrystalline and amorphous structure appears after mechanical milling. Moreover, comparing with the RE = Sm alloy, the RE = Ce alloy has a superior hydrogen desorption property, including larger hydrogen absorption capacity, faster hydriding/dehydriding rate, lower onset hydrogen desorption temperature, and lower dehydrogenation activation energy.     

Hydrogen storage behaviour of Cr- and Mn-doped Mg2Ni alloys fabricated via high-energy ball milling

This paper describes the efficient preparation of an Mg₂Ni alloy for hydrogen storage via high-energy ball milling mechanical alloying for 2 h. The degree of alloy amorphisation increases with increasing ball-milling time. Ball milling for 4 h affords partially amorphous alloys exhibiting the best hydrogen storage performance. Partial substitution of Ni with Cr and Mn improves the hydrogen absorption/desorption thermodynamics, kinetics and cycling performance of the alloy. Specifically, partial Mn substitution improves the cycling performance and reduces the activation energy of the hydrogen desorption reaction, effectively improving the hydrogen desorption kinetic performance. Mg₂Ni_0.8Mn_0.2 shows the best cycling and hydrogen absorption/desorption kinetic performances. Partial Cr substitution reduces the entropy and enthalpy changes of the hydrogen absorption/desorption reaction and effectively reduces the temperature of the initial hydrogen absorption/desorption reaction. In particular, Mg₂Ni_0.9Cr_0.1 shows the best thermodynamic performance.     

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.     

Hydrogen activation and storage properties of laves phase Ti1-xScxMn1.6V0.4 alloys

Hydrogen activation, storage properties and associated crystal structures of Ti_1-xSc_xMn_1.6V_0.4 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) alloys are investigated by hydrogenation and XRD. The unit-cells of alloys and hydrides expand with Sc content and hydrogen concentration. Minor addition of Sc significantly improves hydrogen activation and storage properties. The plateau pressure decreases, whereas the sloping factor and relative partial molar enthalpy for hydrogenation increase with Sc content. The activation properties strongly depend on the particle sizes. The bulk samples can easily be activated by implementing a hydrogen-induced cracking mechanism, which avoids removal of protective oxide films and compensates the lack of metallic B-metal in catalysis of hydrogen dissociation. Samples with smaller particle sizes are difficult to activate. The unusual particle size effect is interpreted by activation kinetics, and attributed to the high oxygen binding ability of B-metals and their contribution to the surface oxide films.     

Hydrogen storage properties of Zr2Co crystalline and amorphous alloys

To further explore the application feasibility of Zr₂Co alloy in tritium-related fields, hydrogenation/dehydrogenation properties of this material of crystalline or amorphous structure, prepared by arc melting or melt spinning, were studied by pressure-composition temperature measurement, X-ray diffraction, differential scanning calorimeter, thermal desorption spectroscopy. It was found that the two kinds of Zr₂Co alloys can absorb hydrogen in a close full concentration of ~9 mmol/g, and may have similar equilibrium hydrogen pressure in the order of 10^?6 Pa at room temperature. In their hydrogenated samples various hydrides were observed to form, including ZrH₂, Zr₂CoH₅, ZrCoH₃ and an amorphous one with gradually decreasing general thermostability. The amorphous alloy exhibited easier hydrogen induced disproportionation caused by highly stable ZrH₂ and much slower hydrogen absorption kinetics. This disproportionation behavior of the crystalline alloy was found to be entirely suppressed by changing heating process. The results firmly indicate that crystalline Zr₂Co alloy could be more favorable for tritium treatment due to very low equilibrium pressure and the feasibility of eliminating the disproportionation.     

Improvement of hydrogen storage properties of Mg by catalytic effect of Al-containing phases in Mg-Al-Ti-Zr-C powders

The hydrogen storage (HS) properties and structures of ball-milled (BM) Mg-Ti-Al-Zr-C powders prepared under various milling conditions were investigated. The additions Ti, Zr, Al and C improved HS performance of Mg-based materials. The beneficial effect can be explained by catalysis of particles rich in Al, Ti and Zr located on the surface of Mg grains. The particles provide effective pathways for the hydrogen diffusion from/into the re/forming MgH₂. The morphological and microstructural characteristics were investigated by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and by X-ray diffraction (XRD). The hydrogen sorption was measured by Sieverts method. The various preparation processes of powders resulted in two phases: Mg₁₇Al₁₂, Mg_1.95Al_0.05. It was found, that mainly these phases had a strong positive effect on HS properties of studied powders. Both phases increased desorption/absorption equilibrium pressure. Improvement of desorption kinetics of powder containing phase Mg₁₇Al₁₂ was more expressive than powders with of Mg_1.95Al_0.05.     

Preparation of TiO2 nanofibers,porous nanotubes,RGO/TiO2 composite by electrospinning and hydrothermal synthesis and their applications in improving the electrochemical hydrogen storage properties of Co2B material

Co₂B hydrogen storage material was prepared via a high temperature solid phase process. The TiO₂ nanofibers (TiO₂–NF) and TiO₂ porous nanotubes (TiO₂-NT) with different size, structure and morphology were fabricated by electrospinning and hydrothermal synthesis. In order to improve the conductivity, the reduced graphene oxide/TiO₂ nanotubes composite (RGO/TiO₂-NT) was synthesized by an alkaline hydrothermal process. The three-dimensional porous TiO₂ nanotubes were attached to the two-dimensional RGO and formed a uniform dispersion. For the purpose of improving the electrochemical performance of Co₂B, composites of Co₂B doped with TiO₂–NF, TiO₂-NT and RGO/TiO₂-NT were manufactured by ball milling. Ultimately, all the composite electrodes showed higher discharge capacities than ordinary Co₂B. Among them, Co₂B modified with RGO/TiO₂-NT exhibited the highest discharge capacity (691.4 mAh/g). TiO₂-NT with large specific surface area and unique tubular porous structure can offer sufficient electrochemical active sites to anchor hydrogen and improve the electrocatalytic activity of Co₂B, meanwhile, the RGO component in RGO/TiO₂-NT with excellent electrical conduction can further provide fast channels for charger transfer during the charging/discharging processes. Moreover, the corrosion resistance, HRD and kinetics performance of Co₂B were also enhanced after doping of TiO₂–NF, TiO₂-NT and RGO/TiO₂-NT.     

Improvement on hydrogen storage thermodynamics and kinetics of the as-milled SmMg11Ni alloy by adding MoS2

In this experiment, the Mg-based hydrogen storage alloys SmMg₁₁Ni and SmMg₁₁Ni + 5 wt.% MoS₂ (named SmMg₁₁Ni-5MoS₂) were prepared by mechanical milling. By comparing the structures and hydrogen storage properties of the two alloys, it could be found that the addition of MoS₂ has brought on a slight change in hydrogen storage thermodynamics, an obvious decrease in hydrogen absorption capacity, an obvious catalytic action on hydrogen desorption reaction, and a lowered onset desorption temperature from 557 to 545 K. Additionally, the addition of MoS₂ could dramatically improve the alloy in its hydrogen absorption and desorption kinetics. To be specific, the hydrogen desorption times of 3 wt.% H₂ at 593, 613, 633 and 653 K were measured to be 1488, 683, 390 and 192 s respectively for the SmMg₁₁Ni alloy, which were reduced to 938, 586, 296 and 140 s for the MoS₂ catalyzed SmMg₁₁Ni alloy at identical conditions. The activation energies of the alloys with and without MoS₂ for hydrogen desorption are 87.89 and 100.31 kJ/mol, respectively. The 12.42 kJ/mol decrease is responsible for the ameliorated hydrogen desorption kinetics by adding catalyst MoS₂.     

Hydrogen generation from the hydrolysis of LaMg12H27 ball-milled with LiH

The LaMg₁₂H₂₇ (LMH) and LMH-5 wt% LiH samples were prepared by different ball milling times, and the hydrogen generation performances of samples were investigated and compared. X-ray diffraction and scanning electron microscopy techniques were adopted to elucidate the performance improvement mechanisms. For the LMH, with the increase of ball milling time, the hydrogen generation yield and kinetics can be improved significantly. This may be caused by the change in particle size and crystallite size of the LMH sample after ball milling. However, for the LMH-5 wt% LiH, with the increase of the ball milling time, the initial kinetics increases firstly, and then decreases. This may be due to the gradual movement of LiH from the surface to the interior of LMH and to the covering of LMH after ball milling.     

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).     

Hydrogen storage properties of ultrahigh pressure Mg12NiY alloys with a superfine LPSO structure

Ultrahigh pressure (UP) plays a crucial role in modifying structures and properties of functional materials. The effects of UP treatment (4 GPa) on phase transition and hydrogen storage properties of Mg₁₂NiY alloys has been investigated at the temperature range of 800–1300 °C. The results show that the dimension of 18R-type long period stacking ordered (LPSO) structure in the Mg₁₂NiY sample after UP treatment at 1300 °C is two orders of magnitude smaller than that in the as-cast sample. The hydrogen storage capacity, kinetics and cycle properties of Mg₁₂NiY alloys are concurrently improved after UP treatment. The two-step reaction process is confirmed during hydrogenation process by combining cycle testing and in-situ transmission electron microscopy (TEM) observations. The reasons for high hydrogen storage properties are mainly related to three aspects: the increased volume fraction of high angle interfaces between LPSO phase and matrix, the reduction of hydrogen diffusion distance, and the low energy barrier of hydrogen diffusion in the interior of superfine LPSO structures.     

Effect of (Ti0.35V0.65)0.86Fe0.14Hy on synthesis and hydrogen storage properties of NaAlH4

The (Ti_0.35V_0.65)_0.86Fe_0.14H_y powder was prepared by melting, annealing and H₂-assisted-crushed method to avoid passivation. Then [(Ti_0.35V_0.65)_0.86Fe_0.14H_y]_x/100-NaAlH₄ composite system were synthesized using a two-step in-situ-milling method with the proportion of n (NaH):n (Al):n (Graphene):n (alloy) = 100:100:5:x (x = 2,5,8). It was found that lattice distortion had occurred on the alloy after 190hindividually milling, and the hydrogen storage capacity had decreased significantly to 1.10 wt%. However, after long-term composite milling, the alloy could still reduce the hydrogen pressure required for the synthesis of NaAlH₄, besides it could effectively reduce the hydriding/dehydriding temperature and improve the kinetic properties. This may due to the alloy's ability to dissociate H₂ and transfer H at room temperature, thereby enhancing the opportunity for direct contact between the matrix and H. In this study, x = 5 was the optimal alloy addition ratio, its dehydrogenation capacity at the 1st cycle reached 5.04 wt%; and at the 2nd and subsequent cycles, it remained rather stable at 4.40 wt%.     

Iron based catalyst for the improvement of the sorption properties of KSiH3

Recently, silanides (MSiH₃) have been proposed as the possible hydrogen storage materials due to their hydrogen storage properties. Among these silanides, KSiH₃ has been considered as leading contender due to its high hydrogen storage capacity i.e. 4.3 wt% and suitable thermodynamic parameters. It can absorb and desorb hydrogen reversibly at near ambient temperature, however, a high activation barrier slows down its kinetics. To enhance its kinetic properties, several catalysts have been attempted so far. Nb₂O₅ has been proven as leading catalyst with significant improvement. In the present work, Fe based catalysts were chosen due to their suitability for hydrogen storage materials. Among all the studied catalysts in this work, Fe₂O₃ was found to be the most effective catalyst, reducing the activation energy down to 75 kJ mol⁻¹ from 142 kJ mol⁻¹ for pristine KSi.     

Hydrogen storage in TiZrNbFeNi high entropy alloys,designed by thermodynamic calculations

The hydrogen storage properties of the novel equiatomic TiZrNbFeNi and non-equiatomic Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅ high entropy alloys (HEAs) were studied. These alloys were designed with the aid of thermodynamic calculations using the CALPHAD method due to their tendency to form single C14 Laves phase, a phase desirable for room-temperature hydrogen storage. The alloys, which were synthesized by arc melting, showed a dominant presence of C14 Laves phases with the (Zr, Ti)₁(Fe, Ni, Nb, Ti)₂ constitution and small amounts of cubic phases (<1.4 wt%), in good agreement with the thermodynamic predictions. Hydrogen storage properties, examined at room temperature without any activation procedure, revealed that a maximum hydrogen storage capacity was reached for the equiatomic alloy in comparison to the non-equiatomic alloy (1.64 wt% vs 1.38 wt%) in the first cycle; however, the non-equiatomic alloy presented superior reversibility of 1.14 wt% of hydrogen. Such differences on reversibility and capacity among the two alloys were discussed based on the chemical fluctuations of hydride-forming and non-hydride-forming elements, the volume per unit cell of the C14 Laves phases and the distribution of valence electrons.     

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.     

Hydrogen storage in organosilicon ionic liquids

New structures of ionic liquids have been studied as liquid organic hydrogen carriers. The hydrogenation of organosilicon compounds containing a carbazole fragment was carried out for the first time. The N–(CH₂)₃–Si fragment in the composition of the hydrogen carrier based on organosilicon moieties was found to be more suitable than the N–CH₂–Si bond. Ionic liquids with a carbazole fragment containing silicon atoms were synthesized for the first time. It was possible to significantly reduce the melting point of such ionic liquids by introducing an organosilicon linker between carbazole nitrogen and imidazolium ion. The synthesized ionic liquid is a liquid at room temperature. Hydrogenation-dehydrogenation experiments demonstrated that such ionic liquids are thermally stable up to at least 220 °C, and nearly quantitative conversion can be achieved. Both the hydrogenation and dehydrogenation processes were carried out using the same Pd/C catalyst. The theoretical total gravimetric hydrogen capacities for synthesized ionic liquids are 2.05% and 1.58%.     

Mechanochemical synthesis and dehydrogenation properties of Yb(AlH4)3

The facile synthesis of ytterbium tetrahydroaluminate Yb(AlH₄)₃ is conducted by a mechanochemical procedure under hydrogen atmosphere for the first time. Results show that the synthesized Yb(AlH₄)₃ remains as an amorphous state. The thermal decomposition of Yb(AlH₄)₃ goes through a four-stage pathway with several amorphous intermediate phases during the process. The first dehydrogenation step of Yb(AlH₄)₃ presents a relatively low apparent activation energy of 99.6 kJ mol^?1, and ninety percent of the hydrogen from this stage can be liberated within 20 min at 160 ^°C. Rehydrogenation tests above 160 ^°C and 14 MPa hydrogen pressure demonstrate the unsuccessful rehydrogenations of the first decomposition step due to the formation of a thermodynamically more stable compound YbHCl.