Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.     

Hydrogen storage properties of Mg–Ni–C system hydrogen storage materials prepared by hydriding combustion synthesis and mechanical milling

Mg–Ni–C composite hydrogen storage materials were prepared by first ball milling the powder mixtures of carbon aerogel and nano-Ni, and then mixed with magnesium powder followed by hydriding combustion synthesis (HCS). The HCS product was further treated by mechanical milling for 10 h. The effect of Ni/C ratio on the structures and hydrogen absorption/desorption properties of the materials were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure–composition–temperature (PCT) measurements. It is found that 90Mg–6Ni–4C system shows the best hydriding/dehydriding properties, which absorbs hydrogen at a saturated capacity of 5.23 wt.% within 68 s at 373 K and desorbs 3.74 wt.% hydrogen within 1800 s at 523 K. Moreover, the dehydriding onset temperature of the system is 430 K, which is 45 K lower than that of 90Mg–10Ni system or 95 K lower than that of 90Mg–10C system. The improved hydriding/dehydriding properties are related greatly to the Ni/C ratio and the structures of the composite systems.     

Hydrogen storage characteristics of the nanocrystalline and amorphous Mg–Nd–Ni–Cu-based alloys prepared by melt spinning

Nanocrystalline and amorphous Mg–Nd–Ni–Cu-based (Mg₂₄Ni₁₀Cu₂)_100−xNd_x (x = 0–20) alloys were prepared by melt spinning and their structures as well as hydrogen storage characteristics were investigated. The analysis of XRD, TEM and SEM linked with EDS reveal that all the as-cast alloys hold a multiphase structure, containing Mg₂Ni-type major phase as well as some secondary phases Mg₆Ni, Nd₅Mg₄₁ and NdNi, whose amounts clearly grow with Nd content rising. Furthermore, the as-spun Nd-free alloy displays an entire nanocrystalline structure whereas the as-spun Nd-added alloys have a mixed structure of nanocrystalline and amorphous, moreover, the amorphization degree of the alloys visibly increases with Nd content rising, implying that the addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy. The addition of Nd results in a slight decrease in the hydrogen absorption capacity of the as-cast and spun alloys, but it significantly enhances their hydrogen storage kinetics and hydriding/dehydriding cycle stability of the alloy. In order to reveal the capacity degradation mechanism of the as-spun alloy, the structure evolution of the nanocrystalline and amorphous alloys during the hydriding–dehydriding cycles was investigated. It is found that the root causes of leading to the capacity degradation of the nanocrystalline and amorphous alloys are nanocrystalline coarsening, crystal defect decreasing and amorphous phase crystallizing.     

Structure and hydrogen storage performances of La–Mg–Ni–Cu alloys prepared by melt spinning

The (Mg₂₄Ni₁₀Cu₂)_100-xLa_x(x = 0, 5, 10, 15, 20) alloys were prepared adopting the method of melt spinning technology. Adding La brings on the formation of secondary phases of La₂Mg₁₇ and LaMg₃, while it does not change the major phase of Mg₂Ni. Originally, there already have nanocrystals and amorphous structures in the experimental alloys, and the addition of La is more conducive to the formation of glass. With adding La in as-spun alloys, the gaseous hydrogen absorption capacity was significantly reduced, but it markedly improved their hydriding rates. Adding La and melt spinning considerably enhanced the dehydriding rate, the reason for which is the decrease of activation energy incurred by adding La and melt spinning. In addition, the discharge capacity of the alloys were able to reach a maximum value during La content varying, and it obviously increased with spinning rate rising.     

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H₂ in 600 s and desorb 3.9 wt.% H₂ in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H₂, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.     

Synthesis and hydrogen storage characteristics of Mg–B–H compounds by a gas–solid reaction

Magnesium borohydride [Mg(BH₄)₂] is an attractive complex hydride for hydrogen storage. In this study, attempts to synthesize Mg(BH₄)₂ were carried out by a solid–gas reaction through MgH₂ and B₂H₆ in the absence of a liquid medium. The source of B₂H₆ was obtained by heating a mixture of NaBH₄ and ZnCl₂. The profile of pressure versus temperature indicated that the absorption kinetics of B₂H₆ by MgH₂ were slow. Structural analysis confirmed the formation of Mg–B–H compounds. The reaction products presented two-step hydrogen release during heating. A small amount of hydrogen could be released from the as-synthesized Mg–B–H compounds at a low temperature of 215 °C. The slow reaction kinetics were significantly affected by the surface conditions of the MgH₂ powders.     

Hydrogen storage properties of Mg–Ce–Ni nanocomposite induced from amorphous precursor with the highest Mg content

The effect of Ce and Ni contents on the glass-forming ability (GFA) of Mg–Ce–Ni system in the Mg-rich corner of Mg–Ce–Ni system is revealed. Ce is more advantageous for the GFA of Mg-rich Mg–Ce–Ni system than Ni, and the lowest Ce content is ∼5 at.% to obtain the fully amorphous alloy. Amorphous alloy with the highest Mg content, Mg₉₀Ce₅Ni₅, was obtained by melt-spinning. With the amorphous alloy as precursor, nanostructure multi-phases compositae was prepared by crystallizing it in hydrogenation process. The compositae with reversible hydrogen storage capacity of 5.3 wt.% shows much faster kinetics and lower MgH₂ desorption activation energy than those of induction-melt Mg₉₀Ce₅Ni₅ alloy. Both in situ formed nanosized Mg₂Ni and CeH_2.73 act as effective catalysts and significantly improve the hydrogen storage properties of MgH₂.     

Enhanced electrochemical and hydrogen storage properties of La–Mg–Ni-based alloy electrode using a Ni and N co-doped reduced graphene oxide nanocomposite as a catalyst

To enhance the electrochemical property of a La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy, a three-dimensional (3D) reduced graphene oxide (rGO)-supported nickel and nitrogen co-doped (Ni–N@rGO) nanocomposite is fabricated by an impregnation method and introduced into the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy. The results show that the reversible hydrogen storage property and the comprehensive electrochemical performance of the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy are enhanced effectively when it is modified by the Ni–N@rGO nanocomposite. The high-rate dischargeability values at a discharge current density of 1500 mA g⁻¹ for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy and Ni–N@rGO-modified samples are 0.0% and 70.5%, respectively. Additionally, the anodic peak currents for the unmodified alloy electrode is 892 mA g⁻¹. Under the catalytic action of the Ni–N@rGO nanocomposite, the value increases to 2307 mA g⁻¹, which is 2.59 times larger than that of unmodified samples. The results also indicate that the diffusion ability of the hydrogen atom in the alloy electrode body enhances significantly when modified by the Ni–N@rGO nanocomposite. The hydrogen diffusion coefficient for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy electrode increases from 3.93 × 10⁻¹⁰ cm² s⁻¹ to 6.15 × 10⁻¹⁰ cm² s⁻¹ when is modified by Ni–N@rGO nanocomposite. These improvements in the comprehensive electrochemical properties are mainly attributed to the excellent electrochemical activity and conductivity of the Ni–N@rGO nanocomposite.     

Nanocrystalline structure and electrochemical hydrogen storage properties of the as-milled Mg–V–Ni–Fe–Zn-based materials

Among the electrode materials for Ni-MH batteries, the Mg alloy electrodes such as MgNi, Mg₂Ni, REMg₁₂, La₂Mg₁₇ are considered the most suitable anode materials due to their high discharge capacity and low cost. However, the poor electrochemical cycling stability prevents its practical application. In this paper, Mg_50-xV_xNi₄₅Fe₃Zn₂ (x = 0, 1, 2, 3, 4) + 50 wt% Ni alloys were prepared by partially replacing Mg with V and using mechanical ball milling techniques with amorphous and nanocrystalline structures. Electrochemical tests showed that the ball-milled alloy had good electrochemical uptake and release performance. The maximum release performance is achieved in the first cycle. After that, the discharge level and cycle stability increased significantly with increasing ball grinding time and V content.     

Formation and hydrogen storage properties of in situ prepared Mg–Cu alloy nanoparticles by arc discharge

Mg–Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg₂Cu, MgCu₂ and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg₂Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.     

Hydrogen storage properties of Mg–Ni nanoparticles

The aim of this work is to investigate metal–hydride transformation in Magnesium (Mg) nanoparticles decorated by Nickel (Ni). The samples were synthesized by Inert Gas Condensation: Mg single crystal nanoparticles were deposited on a metal substrate and subsequently their surface was exposed to evaporation of Ni. Structural analysis was made by Synchrotron Radiation Powder X-ray Diffraction and thermodynamic measurements by Sieverts apparatus. Ni decoration significantly improves the hydrogen release and uptake kinetics of the nanoparticles. The results connect the formation of Mg₂Ni and Mg₂NiH₄ phases to the enhancement of hydrogen sorption properties.     

The effect of CuI–PbI2 nanocomposite fabricated by the sonochemical route on electrochemical hydrogen storage characteristics

Hydrogen storage is an essential technique for developing hydrogen technology and electrochemical cells in stable energy, transport, and portable power. Hydrogen holds the maximum specific power of all fuels; nevertheless, its low ambient temperature density occurs in a lower energy density; therefore, there is a need to develop advanced storage procedures that own the potential for greater energy density. Therefore, this research incorporates the fabrication of novel nanocomposite (CuI–PbI₂) by the sonochemical method, architectural, morphological observations, and relevant electrochemical hydrogen storage features. The electrochemical features with different morphology have revealed 515 and 585 mAh/g discharge capacity for bulk and nano-sized samples after 15 cycles, respectively.     

Electrochemical hydrogen storage property of Co–S alloy prepared by ball-milling method

A series of Co–S alloys were synthesized by means of ball milling of Co and S powders at different hours and investigated as the negative material for Ni/MH batteries. The structures and surface configuration of the alloys were characterized by XRD and TEM. The electrochemical measurements demonstrated that the Co–S particles showed excellent electrochemical reversibility and considerably high charge–discharge capacity. Among the alloys, the Co–S alloy milled 20 h showed relatively high discharge capacity and excellent cycling stability at discharge current density 25 mA/g. Its highest discharge capacity was about 350 mAh/g and remained 300 mAh/g after 100 cycles, the capacity retention rate was about 86%. The hydrogen storage mechanism was studied by XRD and TPD measurements.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Structural characterization and hydrogen storage properties of MgH2–Mg2CoH5 nanocomposites

Mixtures of XMg–Co containing different amounts of Mg (X = 2, 3 and 7) were reactive milled under hydrogen atmosphere. 2Mg–Co only formed the Mg₂CoH₅ complex hydride, while the mixtures 3Mg–Co and 7Mg–Co formed different contents of Mg₂CoH₅ and MgH₂. Their structural features and hydrogen storage properties were analyzed by different techniques. In-situ synchrotron X-ray diffraction, combined with thermal analysis techniques, (differential scanning calorimetry, thermal gravimetric analysis and quadrupole mass spectrometer) was carried out to observe the behavior of the MgH₂–Mg₂CoH₅ mixtures during the first H-desorption. It was found that the presence of the Mg₂CoH₅ complex hydride has a beneficial effect on the first H-desorption of the MgH₂. Additionally, after first desorption, conventional hydrogenation under high pressure and high temperature of 3Mg–Co and 7Mg–Co samples led to the formation of the Mg₆Co₂H₁₁ complex hydride. The presence of Mg₆Co₂H₁₁ considerably impaired the desorption properties of the nanocomposites.     

Hydrogen storage properties of the Mg–Ti–H system prepared by high-energy–high-pressure reactive milling

Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen content. In the present work, we investigated the hydrogen release/uptake properties of the Mg–Ti–H system. Samples were prepared from the mixtures of MgH₂ and TiH₂ in molar ratios of 7:1 and 4:1 using a high-energy-high-pressure (HEHP) mechanical ball-milling method under 13.8 MPa hydrogen pressure. Thermogravimetric analysis (TGA) showed that a relatively large amount of hydrogen (5.91 and 4.82 wt.%, respectively, for the above two samples) was released between 126 and 313 °C while temperature was increased at a heating rate of 5 °C min⁻¹ under an argon flow. The onset dehydrogenation temperature of these mixtures, which is 126 °C, is much lower than that of MgH₂ alone, which is 381 °C. The activation energy of dehydrogenation was 71 kJ mol⁻¹, which is much smaller than that of as-received MgH₂ (153 kJ mol⁻¹) or as-milled MgH₂ (96 kJ mol⁻¹). Furthermore, the hydrogen capacity and the dehydrogenation temperature remained largely unchanged over five dehydrogenation and rehydrogenation cycles.     

Combinatorial search for hydrogen storage alloys: Mg–Ni and Mg–Ni–Ti

A combinatorial study was carried out for hydrogen storage alloys involving processes similar to those normally used in their fabrication. The study utilized a single sample of combined elemental (or compound) powders which were milled and consolidated into a bulk form and subsequently deformed to heavy strains. The mixture was then subjected to a post annealing treatment, which brings about solid state reactions between the powders, yielding equilibrium phases in the respective alloy system. A sample, comprising the equilibrium phases, was then pulverized and screened for hydrogen storage compositions. X-ray diffraction was used as a screening tool, the sample having been examined both in the as processed and the hydrogenated state. The method was successfully applied to Mg–Ni and Mg–Ni–Ti yielding the well known Mg₂Ni as the storage composition. It is concluded that a partitioning of the alloy system into regions of similar solidus temperature would be required to encompass the full spectrum of equilibrium phases.     

Synthesis,characterization and investigation of the electrochemical hydrogen storage properties of CuO–CeO2 nanocomposites synthesized by green method

Pure CuO–CeO₂ nanocomposites were synthesized by simple thermal decomposition method in presence of various Cu salts as a copper source and fructose as a green capping agent. In this study, the effect of various parameters such as the type of copper sources, temperature and time of reaction on the morphology and the particles size were studied. The products were characterized via X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), N2 adsorption (BET), vibrating sample magnetometer (VSM), and infrared spectrum (FT-IR). The optical property of the nanocomposite was examined via UV–vis (DRS) spectroscopy and the band gap was calculated to 3 eV. Also, the hydrogen storage capacity of CuO–CeO₂ nanocomposites and CeO₂ nanoparticles were investigated via chronopotentiometry method for the first time. The discharge capacity of CeO₂ nanoparticles and CuO–CeO₂ nanocomposites in 1 mA current and 20 cycles obtained 2150 and 2450 mAh/g, respectively.     

Microstructural evolution of ball-milled Mg–Ni powder during hydrogen sorption

Ball-milling of Mg₇₅Ni₂₅ powder blends were carried out in a SPEX-8000 shaker mill. The morphology and microstructure of the milled powders were studied by scanning electron microscopy and X-ray diffraction, respectively. The dehydrogenation process of the sample milled for 10 h was stopped at different hydrogen contents (25, 50 and 75 percent of the maximum capacity) in a Sieverts' type apparatus, in order to achieve partially desorbed states. For comparison, the fully hydrided (100 percent) and the fully dehydrided (0 percent) states were also obtained. Convolutional multiple whole profile fitting analysis of the corresponding X-ray powder diffractograms was carried out in order to monitor the evolution of microstructural parameters during desorption, such as average coherent crystallite size and size distribution of two hydrides (Mg₂NiH₄ and Mg₂NiH_0.3) that nucleate during the hydrogenation of Mg–Ni powders. The desorption induced changes in the relative amount of the hydride phases were also quantified.