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.     

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 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.     

Hydrogen storage property improvement of La–Y–Mg–Ni alloy by ball milling with TiF3

Ball milling the powders of Mg-based alloys with transition metal compounds is effective for improving their hydrogen storage performances. In this experiment, the alloys of La_1.7Y_0.3Mg₁₆Ni + x wt.% TiF₃ (x = 0–10) were prepared through mechanical milling technology. XRD, SEM, HRTEM and granulometry were used to measure the composition and microstructure of alloys. The isothermal hydrogen storage property was measured by a Sievert apparatus. The results reveal that the TiF₃ additive in ball-milled samples transforms into MgF₂ and TiH₂ after the first hydrogen absorption. Adding TiF₃ enhances the crystallinity and reduces the average particle and crystallite sizes of alloys, which is beneficial to accelerating hydriding and dehydriding kinetics. Adding 7 wt% TiF₃ into alloy decreases the dehydrogenation activation energy from 72.2 to 64.0 kJ/mol and improves the hydrogen absorption rate at low temperatures, absorbing 3.50 wt% H in 0.5 min at 323 K.     

Light-weight NaNH2–NaBH4 hydrogen storage material synthesized via liquid phase ball milling

As a light-weight and low-cost hydrogen storage composite, NaNH₂–NaBH₄ (molar ratio of 2:1) was prepared by a liquid phase ball milling (LPBM) method under the co-protection of argon and cyclohexane. The structure evolution and the thermal decomposition performance of the as-prepared sample were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermo gravimetric-differential thermal analysis (TG-DTA), respectively. It is found that the interaction of NaNH₂ with NaBH₄ is enhanced by LPBM, thus causes a preferred orientation for the crystal structure of NaNH₂, and the red-shifts of the N–H stretching vibration and the B–H stretching vibration. In addition, the as-prepared NaNH₂–NaBH₄ (2/1) can achieve a low activation energy of 76.4 kJ mol⁻¹ during the main decomposition stage, which is only 47.9% of that of the one synthesized via a solid state ball milling (SSBM) method, and is very close to that of the Co–B catalyst promoted one. This indicates the LPBM method is an efficient way to get high-performance NaNH₂–NaBH₄, whose thermal decomposition kinetics can be greatly improved without any catalyst.     

Characteristics of electrochemical hydrogen storage using Ti–Fe based alloys prepared by ball milling

In this paper, the TiFe-based master alloy Ti_1.04Fe_0.7Ni_0.1Zr_0.1Mn_0.1Pr_0.06 was fabricated by conventional induction melting with high purity helium as the protective gas. After that, the as-cast specimens were mechanically milled with nickel powders to synthesize the as-milled Ti_1.04Fe_0.7Ni_0.1Zr_0.1Mn_0.1Pr_0.06 + 10 wt.% Ni composites with excellent electrochemical characteristics. The master alloy is composed of TiFe, Ti₂Fe and Pr phases, which has a typical crystal structure. Mechanically milling the master alloy with nickel powder leads to the reductions of the grain size and particle size, even forming amorphous structure. The experimental results showed that the specimens after ball-milling treatment can be used to hydriding and dehydriding by electrochemistry, getting the maximal discharge capacity in the first cycle, and no activation was required. The discharge capacity of the as-milled composites declined from 264.2 to 133.6 mAh/g with the milling duration extending from 5 to 30 h. The electrochemical kinetics markedly declined with prolonging milling duration. However, the electrochemical cycling stability of the specimens reduced firstly and then increased with the prolongation of grinding duration.     

Ti–Fe based alloys prepared by ball milling for electrochemical hydrogen storage

In this study, the Ti_1.04Fe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_0.06 composite was prepared by using vacuum induction melting under inert atmosphere. Then, the specimen was milled with 5 wt% Ni powders for 10–40 h to realize the general improvements in hydrogenation performance. The phase component was determined and the morphology and microscopic structure were observed using XRD, SEM and HRTEM, respectively. The electrochemical properties of the alloys were studied. The results showed that the as-milled specimens got the maximal discharge capacity without any activation. It reached 305 mAh/g for the 30 h milling specimen, which was better than the other specimens. Besides, ball milling can enhance the electrochemical cyclic stability of the experimental alloys. The capacity retention rate (S₁₀₀) increased from 57.6 to 70.2% after 100 charging and discharging cycles with increasing milling duration from 10 to 40 h. The high rate discharge ability of the 30 h milling specimen had the maximal value of 92.8%.     

Hydrogen storage thermodynamics and kinetics of RE–Mg–Ni-based alloys prepared by mechanical milling

The nanocrystalline/amorphous NdMg₁₁Ni + x wt.% Ni (x = 100, 200) composite hydrogen storage alloys were synthesized by ball milling, and the effects of Ni content and milling time on the hydrogen storage thermodynamics and dynamics of the alloys were systematically investigated. The results reveal that the variation of the Ni content has a slight effect on the thermodynamic properties of the alloys, but it significantly improves their absorption and desorption kinetics performance. The variation of the milling time clearly affects the hydrogen storage properties of the alloys. Hydrogen absorption capacity and hydrogen absorption saturation ratio have maximum values with milling time varying. But hydrogen desorption ratio always increases with milling time prolonging. It is found that the hydrogen desorption activation energy of the alloys clearly decreases with increasing Ni content and milling time, which is responsible for the improved hydrogen desorption kinetics of the alloys.     

Hydrogen storage properties and mechanisms of the Mg(BH4)2–NaAlH4 system

Hydrogen storage properties and mechanisms of the combined Mg(BH₄)₂–NaAlH₄ system were investigated systematically. It was found that during ball milling, the Mg(BH₄)₂–xNaAlH₄ combination converted readily to the mixture of NaBH₄ and Mg(AlH₄)₂ with a metathesis reaction. The post-milled samples exhibited an apparent discrepancy in the hydrogen desorption behavior with respect to the pristine Mg(BH₄)₂ and NaAlH₄. Approximately 9.1 wt% of hydrogen was released from the Mg(BH₄)₂–2NaAlH₄ composite milled for 24 h with an onset temperature of 101 °C, which is lowered by 105 and 139 °C than that of NaAlH₄ and Mg(BH₄)₂, respectively. At initial heating stage, Mg(AlH₄)₂ decomposed first to produce MgH₂ and Al with hydrogen release. Further elevating operation temperatures gave rise to the reaction between MgH₂ and Al and the self-decomposition of MgH₂ to release more hydrogen and form the Al_0.9Mg_0.1 solid solution and Mg. Finally, NaBH₄ reacted with Mg and partial Al_0.9Mg_0.1 to liberate all of hydrogen and yield the resultant products of MgAlB₄, Al₃Mg₂ and Na. The dehydrogenated sample could take up ∼6.5 wt% of hydrogen at 400 °C and 100 atm of hydrogen pressure through a more complicated reaction process. The hydrogenated products consisted of NaBH₄, MgH₂ and Al, indicating that the presence of Mg(AlH₄)₂ is significantly favorable for reversible hydrogen storage in NaBH₄ at moderate temperature and hydrogen pressure.     

Hydrogen storage properties of destabilized MgH2–Li3AlH6 system

To improve the dehydrogenation properties of MgH₂, a novel hydrogen storage system, MgH₂–Li₃AlH₆, is prepared by mechanochemical milling. Three physical mixtures containing different mole ratios (1:4, 1:1 and 4:1) of MgH₂ and Li₃AlH₆ are studied and there exists a mutual destabilization effect between the components. The last mixture shows a capacity of 6.5 wt% H₂ with the lowest starting temperature of dehydrogenation (170 °C). First, Li₃AlH₆ decomposes into Al, LiH and H₂, and then the as-formed Al can easily destabilize MgH₂ to form the intermetallic compound Mg₁₇Al₁₂ at a temperature of 235 °C, which is about 180 °C lower than the decomposition temperature of pristine MgH₂. Finally, the residual MgH₂ undergoes a self-decomposition whose apparent activation energy has been reduced by about 22 kJ mol⁻¹ compared with pristine MgH₂. At a constant temperature of 250 °C, the mixture can dehydrogenate completely under an initial vacuum and rehydrogenate to form MgH₂ under 2 MPa H₂, showing good cycle stability after the first cycle with a capacity of 4.5 wt% H₂. The comparison between 4 MgH₂ + Li₃AlH₆ and 4 MgH₂ + LiAlH₄ mixtures is also investigated.     

Hydrogen storage properties of nanocrystalline and amorphous Pr–Mg–Ni-based alloys synthesized by mechanical milling

To improve the hydrogen storage properties of PrMg₁₂ alloy, one Mg atom was replaced by one Ni atom in the alloy. And then the PrMg₁₁Ni alloy was milled with two or three times Ni powers. The as-milled PrMg₁₁Ni + x wt.% Ni (x = 0, 100, 200) (PrMg₁₁Ni-xNi (x = 0, 100, 200)) alloys possess a nanocrystalline and amorphous structure. The gaseous hydrogen storage properties were tested by Sievert apparatus and Differential scanning calorimetry (DSC) connected with a H₂ detector. The results indicate that increasing Ni content have obvious improvement effects on the gaseous hydrogen storage kinetics and thermodynamics for both the hydrogenation and dehydrogenation reaction. Furthermore, the variation of milling time also has an obvious influence on gaseous hydrogen storage properties of the alloys. The hydrogen storage capacity firstly increases and decreases again with prolonging milling time, and the peak values are 5.859 wt.% for the x = 100 alloy and 6.164 wt.% for the x = 200 alloy. The hydrogen absorption rates have similar trends, but the hydrogen desorption rates always increase. The hydrogen desorption activation energy markedly lowers with Ni content and milling time increasing, which is the real reason for the improvement of hydrogen desorption kinetics.     

Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS + MM)

The effect of Cu-doping on the hydrogen storage properties of Mg₉₅Ni₅Cu_x (x = 0, 0.5, 1, 2) prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. For dehydriding properties, the dehydriding temperature onset decreases from 450 K for Mg₉₅Ni₅ to 420 K for Mg₉₅Ni₅Cu₂. Additionally, the activation energy for dehydriding decreases from 116 kJ/mol for Mg₉₅Ni₅ to 98 kJ/mol for Mg₉₅Ni₅Cu₂, indicating that the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen absorption capacity of Mg₉₅Ni₅Cu₂ at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM pretreatment before HCS. The factors for the improvement of the hydrogen storage properties are discussed in this paper.     

Microstructural characteristics and hydrogen storage properties of the Mg–Ni–TiS2 nanocomposite prepared by a solution-based method

Magnesium hydride is considered as a promising solid-state hydrogen storage material due to its high hydrogen capacity. How to improve hydrogen desorption kinetics of MgH₂ is one of key issues for its practical applications. In this study, we synthesize a Mg–Ni–TiS₂ composite through a solution-based synthetic strategy. In the as-prepared composite, the co-precipitated Mg and Ni nanoparticles are highly dispersed on TiS₂ nanosheets. As a result, the activation energy for hydrogen desorption decreases to 79.4 kJ mol⁻¹. Meanwhile, the capacity retention rate is kept at the level of 98% and only slight kinetic deterioration is caused after fifty hydrogenation-dehydrogenation cycles. Further investigation indicates that the superior hydrogen desorption kinetics is attributed to the synergistically catalytic effect of the in situ formed Mg₂NiH₄ and TiH₂, and the remained TiS₂. The excellent cycle stability is related not only to the inhibition effect of the secondary phases on powder agglomeration and crystallite growth of Mg and MgH₂ but also to the prevention effect of MgS and TiS₂ on redistribution of catalytic Mg₂NiH₄ and TiH₂ nanoparticles during cycling. This work introduces a feasible approach to develop Mg-based hydrogen storage materials.     

Hydrogen storage properties of Nb–Hf–Ni ternary alloys

The hydrogen storage properties of Nb_xHf_(1−x)/2Ni_(1−x)/2 (x = 15.6, 40) alloys were investigated with respect to their hydrogen absorption/desorption, thermodynamic, and dynamic characteristics. The PCT curves show that all the specimens can absorb hydrogen at 303 K, 373 K, 423 K, 473 K, 523 K, 573 K, and 673 K, but they couldn't desorb hydrogen below 373 K. The maximum hydrogen absorption capacity reaches 1.23 wt.% for Nb_15.6Hf_42.2Ni_42.2 and 1.48 wt.% for Nb₄₀Hf₃₀Ni₃₀ at 303 K at a pressure of 3 MPa. When the temperature was increased, the hydrogen absorption capacities significantly decreased. However, the hydrogen equilibrium pressure increased. When the temperature exceeded 523 K, the hydrogen equilibrium pressure disappeared. When niobium content was increased, the kinetic properties of hydrogen absorption/desorption improved. The results from the microstructure analysis show that both alloys consist of the BCC Nb-based solid solution phase, the B_f-HfNi intermetallic phase, and the eutectic phase {B_f-HfNi + BCC Nb-based solid solution}. When the Nb content was increased, the volume fraction and Nb content in the Nb-based solid solution phase increased. Thus, the improved kinetics is related to the increase in the primary BCC Nb-based solid solution in the Nb₄₀Hf₃₀Ni₃₀ alloy. The kinetic mechanisms of hydrogen absorption/desorption in these two alloys are found to obey the chemical reaction mechanism at all temperatures tested.     

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.     

Hydrogen storage properties of Li–Ca–N–H system with different molar ratios of LiNH2/CaH2

In this work, dehydrogenation and rehydrogenation of three LiNH₂/CaH₂ samples with LiNH₂/CaH₂ molar ratio of 2/1, 3/1 and 4/1 were systematically investigated. Remarkable differences were observed in the temperature dependence of hydrogen desorption and subsequent absorption. LiNH₂/CaH₂ in a molar ratio of 2/1 transforms to ternary imide Li₂Ca(NH)₂ after desorbing about 4.5 wt.% H₂ at 350 °C. And it has a reversible hydrogen storage capacity of 2.7 wt.% at 200 °C. As for the LiNH₂/CaH₂ mixture in a molar ratio of 4/1, it transforms to a new compound with a composition of Li₄CaN₄H₆ after being dehydrogenated at 350 °C. The rehydrogenation of both LiCa(NH)₂ and Li₄CaN₄H₆ gives LiNH₂, LiH and the solid solution of 2CaNH–Ca(NH₂)₂.     

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.     

Hydrogen storage properties of 2Mg–Fe mixtures processed by hot extrusion: Influence of the extrusion ratio

Hot extrusion processing was employed to produce 2Mg–Fe bulk mixtures for hydrogen storage. 2Mg–Fe powder mixtures were prepared by high-energy ball milling. These mixtures were cold pressed into cylindrical pre-forms, which were then processed by hot extrusion (at 300 °C) to produce bulks. In this work, we analyzed the influence of the extrusion ratio (3/1, 5/1 and 7/1) on the sorption properties of the bulks. The nanometric grain size remained unaltered after all hot extrusion conditions. More porous bulks were produced at an extrusion ratio of 3/1. In the first cycle of hydrogenation, the sample processed at 3/1 absorbed more hydrogen (4 wt% of H) than the precursor powders (3 wt% of H). The results showed that the desorption temperature of bulks were very similar to that of 2Mg–Fe powders, which is good considering the lower surface area of bulks, and that samples with Fe in excess presented lower desorption temperatures.     

A numerical study on the effects of H2 addition in non-premixed turbulent combustion of C3H8–H2–N2 mixture using a steady flamelet approach

This study has been implemented in two sections. At first, the turbulent jet flame of DLR-B is simulated by combining the k–ε turbulence model and a steady flamelet approach. The DLR-B flame under consideration has been experimentally investigated by Meier et al. who obtained velocity and scalar statistics. The fuel jet composition is 33.2% H₂, 22.1% CH₄ and 44.7% N₂ by volume. The jet exit velocity is 63.2 m/s resulting in a Reynolds number of 22,800. Our focus in the first part is to validate the developed numerical code. Comparison with experiments showed good agreement for temperature and species distribution. At the second part, we exchanged methane with propane in the fuel composition whilst maintaining all other operating conditions unchanged. We investigated the effect of hydrogen concentration on C₃H₈–H₂–N₂ mixtures so that propane mole fraction extent is fixed. The hydrogen volume concentration rose from 33.2% up to 73.2%. The achieved consequences revealed that hydrogen addition produces elongated flame with increased levels of radiative heat flux and CO pollutant emission. The latter behavior might be due to quenching of CO oxidation process in the light of excessive cold air downstream of reaction zone.     

Effect of C (graphite) doping on the H2 sorption performance of the Mg–Ni storage system

Binary Mg–Ni mixtures and ternary Mg–Ni–C (graphite) samples with fixed proportions of metals (Mg 85%–Ni 15% by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for t_BM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg–Ni system.