Characterization of Mg–20 wt% Ni–Y hydrogen storage composite prepared by reactive mechanical alloying

Mg–20 wt% Ni–Y composite was successfully prepared by reactive mechanical alloying (RMA). X-ray diffraction (XRD) measurement showed that both MgH₂ and Mg₂NiH₄ co-exist in the milled composite. The composite exhibits excellent hydrogen sorption kinetics and does not need activation on the first hydrogen storage process. It can absorb 3.92 and 5.59 wt% hydrogen under 3.0 MPa hydrogen pressure at 293 and 473 K in 10 min, respectively, and desorb 4.67wt% hydrogen at 523 K in 30 min under 0.02 MPa hydrogen pressure. The equilibrium desorption pressure of the composite are 0.142, 0.051 and 0.025 MPa at 573, 543 and 523 K, respectively. The differential scanning calorimetry (DSC) measurement showed that dehydrogenation of Mg–20 wt% Ni–Y composite was depressed about 100 K comparing to that of milled pure MgH₂. It is deduced that both the catalysis effect of Mg₂Ni and YH₃ distributed in Mg substrate and the crystal defects formed by RMA are the main reason for improving hydrogen sorption kinetics of the Mg–20 wt% Ni–Y composite.     

Hydrogen sorption in MgH2-based composites: The role of Ni and LiBH4 additives

In the present work we investigate the hydrogen sorption properties of composites in the MgH₂–Ni, MgH₂–Ni–LiH and MgH₂–Ni–LiBH₄ systems and analyze why Ni addition improve hydrogen sorption rates while LiBH₄ enhance the hydrogen storage capacity. Although all composites with Ni addition showed significantly improved hydrogen storage kinetics compared with the pure MgH₂, the fastest hydrogen sorption kinetics is obtained for Ni-doped MgH₂. The formation of Mg₂Ni/Mg₂NiH₄ in Ni-doped MgH₂ composite and its microstructure allows to uptake 5.0 wt% of hydrogen in 25 s and to release it in 8 min at 275 °C. In the MgH₂–Ni–LiBH₄ composite, decomposition of LiBH₄ occurs during the first dehydriding leading to the formation of diborane, which has a Ni catalyst poison effect via the formation of a passivating boron layer. A combination of FTIR, XRD and volumetric measurements demonstrate that the formation of MgNi₃B₂ in the MgH₂–Ni–LiBH₄ composite happens in the subsequent hydriding cycle from the reaction between Mg₂Ni/Mg₂NiH₄ and B. Activation energy analysis demonstrates that the presence of Ni particles has a catalytic effect in MgH₂–Ni and MgH₂–Ni–LiH systems, but it is practically nullified by the addition of LiBH₄. The beneficial role of LiBH₄ on the hydrogen storage capacity of the MgH₂–Ni–LiBH₄ composite is discussed.     

Hydrogen storage in rapidly solidified and crystallized Mg–Ni-(Y,La)–Pd alloys

Amorphous-crystalline composite ribbons of quaternary Mg–Ni–(Y,La)–Pd alloys are produced via rapidly solidification and used as precursors for creating nanocrystalline hydrogen storage materials. The resulting materials demonstrate relatively high hydrogen capacity of around 4.5 mass% H and excellent absorption/desorption kinetics at 573 K. Additionally, the alloys demonstrate reversible hydrogen storage at 473 K. A composition of Mg₈₅Ni₁₀Y_2.5Pd_2.5 fully absorbs and desorbs 4.6 mass% H in 90 min. The cyclability of the quaternary alloys demonstrates good stability, with little loss in maximum capacity through 8–10 cycles. This has been attributed to the improved stability of the nanocrystalline structure attained via the Y and La additions. Thermodynamically, the enthalpy of the hydrogen absorption reaction is reduced by 5 kJ/mol in the quaternary alloys, compared to Mg-MgH₂; while the entropy of reaction is also reduced.     

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg20Ni10−xCox (x = 0–4) alloys prepared by melt-spinning

In order to improve the hydriding and dehydriding performances of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂₀Ni_10−xCo_x (x = 0–4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. Thermal stability of the as-spun alloys was researched by DSC. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results showed that no amorphous phase formed in the as-spun Co-free alloy, but the as-spun alloys containing Co showed certain amount of amorphous phase. The hydrogen absorption capacities of the as-cast alloys first increase and then decrease with the variety of Co content. The hydrogen desorption capacities of as-cast and spun alloys rise with increasing Co content. The rapid quenching significantly improved the hydrogenation and dehydrogenation capacities and the kinetics of the alloys. When the quenching rate increased from 0 (as-cast was defined as spinning rate of 0 m/s) to 30 m/s, the hydrogen absorption capacity of the alloys (x = 0) at 200 °C and 1.5 MPa in 20 min rose from 1.39 to 3.12 wt%, and from 1.91 to 2.96 wt% for the alloy (x = 4). The hydrogen desorption capacity of the alloy (x = 0) in 20 min increased from 0.19 to 0.89 wt%, and from 1.39 to 2.15 wt% for the alloy (x = 4).     

Local and crystal structure of Mg1.9Al0.1Ni hydrogen storage alloys during hydrogen absorption–desorption cycling

The evolution of crystal structure and chemical state of Mg_1.9Al_0.1Ni alloy during hydrogen absorption–desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). We research the hydrogen storage capacity of the Mg_1.9Al_0.1Ni by the H/D kinetic curves. The H/D kinetic curves indicate that the hydrogen storage capacity increased with the increased cycles and the samples were activated after 10 cycles have the maximum hydrogen storage capacity. The local structure of Ni atoms was studied by extended X-ray absorption fine structure (EXAFS). The EXAFS results indicate the Ni–Ni bonds distance has no obviously change with the cycles increasing, whereas the Ni–Mg bond lengths increase, and the Ni–Mg bond lengths are longer obviously than before 10 cycles whereas it has no obviously change after 10 cycles.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Effects of Ni,Fe2O3, and CNT addition by reactive mechanical grinding on the reaction rates with H2 of Mg-based alloys

Ni, Fe₂O₃, and CNT were added to Mg. The content of the additives was about 20 wt % with that of Fe₂O₃ 6 wt%. The contents of about 20 wt % additives and 6 wt% Fe₂O₃ are known optimum ones to improve the reaction rates of Mg with H₂. Samples with compositions of 80 wt% Mg–14 wt% Ni–6 wt% Fe₂O₃ (named as Mg–14Ni–6Fe₂O₃), and 78 wt% Mg–14 wt% Ni–6 wt% Fe₂O₃–2 wt% CNT (named as Mg–14Ni–6Fe₂O₃–2CNT) were prepared by reactive mechanical grinding. The hydriding and dehydriding properties of these samples were then measured, and the effects of Ni, Fe₂O₃, and CNT addition on the hydriding and dehydriding rates of Mg-based alloys were investigated by comparing their hydrogen-storage properties with those of pure Mg and Mg–10 wt% Fe₂O₃.     

Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage

Two composite hydrogen storage materials based on Mg₂FeH₆ were investigated for the first time. The Mg₂FeH₆–LiBH₄ composite of molar ratio 1:5 showed a hydrogen desorption capacity of 5.6 wt.% at 370 °C, and could be rehydrogenated to 3.6 wt.% with the formation of MgH₂, as the material was heated to 445 °C and held at this temperature. The Mg₂FeH₆–LiNH₂ composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of 4.3 wt.% and released hydrogen at 100 °C lower then the Mg₂FeH₆–LiBH₄ composite, but this mixture could not be rehydrogenated. Compared to neat Mg₂FeH₆, both composites show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at these low temperatures. In particular, Mg₂FeH₆–LiNH₂ exhibits a much lower desorption temperature than neat Mg₂FeH₆, but only Mg₂FeH₆–LiBH₄ re-absorbs hydrogen.     

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.     

Effects of the preparative parameters of hydriding combustion synthesis on the properties of Mg–Ni–C as hydrogen storage material

Magnesium may be the most promising solid-state hydrogen storage material owing to its high storage capacity (7.6 wt%) and highest volumetric density (2 times of liquid H₂). On the other hand, suffers from its sluggish absorption/desorption characteristics. In the present study, the simple/cost-effective hydriding combustion synthesis (HCS) was used to prepare highly-active Mg-based-samples. The preparative parameters of HCS were varied, and its effects on the micro-structural and hydrogen storage properties were determined. The results and its analysis showed that the simple HCS process possesses a multifaceted dependence on a range of experimental factors and affect the final product. The estimated dependence enabled us to explain the combined effect of individual experimental factors on the prepared samples. The Mg–Ni–C sample prepared at 610 °C with 6%wt-nano-Ni and 4 wt%-multi-walled-CNTs as reactants, resulted in sample with a surface area as high as 19.01 m²/g and a desorption capacity of 5.77 wt%, highlighting the promising characteristics of HCS to prepare highly-active Mg-based-materials.     

Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling

This work deals with the assessment of the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. Samples are prepared from Mg_xFe (x: 2, 3 and 15) elemental powder mixtures via low energy ball milling under hydrogen atmosphere at room temperature. The highest yield is obtained with Mg₁₅Fe after 150 h of milling (90 wt% of MgH₂). The thermodynamic characterization carried out between 523 and 673 K shows that the obtained Mg–Fe–H hydride systems have similar thermodynamic parameters, i.e. enthalpy and entropy. However, in equilibrium conditions, Mg₁₅Fe has higher hydrogen capacity and small hysteresis. In dynamic conditions, Mg₁₅Fe also shows better hydrogen capacity (4.85 wt% at 623 K absorbed in less than 10 min and after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability in comparison to the other studied compositions. From hydrogen uptake rate measurements performed at 573 and 623 K, the rate-limiting step of the hydrogen uptake reaction is determined by fitting particle kinetic models. According to our results, the hydrogen uptake is diffusion controlled, and this mechanism does not change with the Mg–Fe proportion and temperature.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

Superior dehydrogenation performance of nanoscale lithium borohydride modified with fluorographite

A significant enhancement in the dehydrogenation performance of LiBH₄ is achieved by modifying with fluorographite (FGi). In-depth investigations show that the dehydrogenation thermodynamics and kinetics of LiBH₄ are strongly improved by ball milling LiBH₄ with FGi. The ball-milled LiBH₄–FGi (mass ratio of 1:1) composite starts to release hydrogen without impurity gas at around 180 °C, and obtains a hydrogen desorption capacity of 7.2 wt% below 200 °C in seconds, which is improved dramatically compared with pristine ball-milled LiBH₄. Microscopic morphology indicates that numerous ∼90 nm spots formed on the surface of FGi. Based on the microstructure analyses combined with hydrogen storage performances, the prominent effect of FGi is largely attributed to the nano-modifying effect and the exothermic reaction between LiBH₄ and FGi during the dehydrogenation process. Furthermore, partial reversibility of the LiBH₄–FGi composite has been demonstrated and the mechanism underlying the cycling capacity loss is discussed. The use of FGi may shed light on future study on searching for new strategies to improve both the thermodynamics and kinetics of light-metal complex hydrides.     

Hydrogen storage properties of Mg–TM–La (TM = Ti,Fe, Ni) ternary composite powders prepared through arc plasma method

For the first time, Mg based Mg–Transition metal (TM) –La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg–TM–La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La₂O₃ introduced during passivation. Based on the Pressure–Composition–Temperature measurements, the hydrogenation enthalpies of Mg are determined to be −68.7 kJ/mol H₂ for Mg–Ti–La powder, −72.9 kJ/mol H₂ for Mg–Fe–La powder and −82.1 kJ/mol H₂ for Mg–Ni–La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg–TM–La composites when compared to those in the binary Mg–TM or Mg–RE composites. This is especially true for the Mg–Ni–La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg₂Ni and La₂O₃. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage.     

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Hydrogen-storage properties of Mg–23.5Ni–(0 and 5)Cu prepared by melt spinning and crystallization heat treatment

Mg–23.5 wt% Ni and Mg–23.5 wt% Ni–5 wt% Cu alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys absorbed 4.34 and 4.84 wt% H, respectively, at 573 K under 12 bar H₂ for 60 min. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys desorbed 4.27 and 4.81 wt% H, respectively, at 573 K under 1.0 bar H₂ for 30 min. The hydriding rates of the alloys are quite high, even at 473 K, while the dehydriding rates of the samples at 473 K are nearly zero.     

Kinetic properties of La2Mg17–x wt.% Ni (x = 0–200) hydrogen storage alloys prepared by ball milling

The as-cast La₂Mg₁₇ with different amount of Ni powders were mixed through ball milling to produce a new type of La₂Mg₁₇–x wt.% Ni (x = 50, 100, 150, 200) alloy. The microstructures of the alloys were characterized by XRD technique, the results show that the crystal structure transfers to amorphous one with the increasing amount of Ni powders. La₂Mg₁₇–50 wt.% Ni alloy reaches the highest hydrogen absorption capacity of 5.13 wt.% at 300 °C under 2 MPa hydrogen pressure due to its amorphous structure. Furthermore, La₂Mg₁₇–50 wt.% Ni alloy expresses fast hydriding kinetics and absorbs 4.99 wt.% hydrogen gas in 200 s. The hydrogen desorption ability described as discharge capacity during electrochemical reaction is fade next to La₂Mg₁₇–200 wt.% Ni alloy, attributed to the less Mg₂NiH₄ with lower enthalpies and easier to release H₂. The maximum discharge capacity of La₂Mg₁₇–200 wt.% Ni alloy reaches to exciting 980.90 mAh/g, while the La₂Mg₁₇ alloy is only 18.10 mAh/g with inconspicuous improvement of cycle stability. These dramatic difference in electrochemical performance reflect the consequence of sluggish dehydriding process of La₂Mg₁₇–50 and 100 wt.% Ni alloys again. Whereas La₂Mg₁₇–200 wt.% Ni alloy has lower resistance both on alloy surface and in the bulk.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.     

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.     

Hydrogen storage by Mg-based nanocomposites

Hydrogen storage materials research is entered to a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities and the electrochemical discharge capacities of the Mg₂(Ni, Cu)-, LaNi₅-, ZrV₂-type nanocrystalline alloys and Mg₂Ni/LaNi₅-, Mg₂Ni/ZrV₂-type nanocomposites have been measured. The electronic properties of the Mg₂Ni_1-xCu_x, LaNi₅ and ZrV₂ alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)₂Ni (50 wt%)-La(Ni,Mn,Al,Co)₅ (50 wt%) and (Mg,Mn)₂Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)_2.4 (25 wt%) materials releases 1.65 wt% and 1.38 wt% hydrogen at 25 °C, respectively. The strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence hydrogenation properties of Mg-based nanocomposities.