Dehydriding Properties Research Articles

Exploring the influence of Single-Walled carbon nanotubes substituted Mg–Ti alloy for hydriding and dehydriding properties

Currently, researchers are giving much more attention to carbon allotropes for hydrogen storage applications. SWCNT-(Mg–Ti) composites have been synthesized by the ball milling technique. The XRD patterns confirmed the solid solution formation of Ti in Mg. The substitution of SWCNT in Mg–Ti intermetallic alloy increases the dislocation density to 8.2x10−3nm−2 and reduces the crystal size from 46 to 28 nm and lattice strain of 0.54 to 0.28 %. The Raman analysis confirmed the existence of SWCNTs by presenting D and G bands and also confirmed the vibrations of Mg–Ti alloy. After the substitution of SWCNT, the exothermic peak reduced from 232 °C to 210 °C. The activation energy is also reduced after the substitution of SWCNT; it drastically reduces the activation energy to 66.59 kJ/mol for Mg67Ti29SWCNT4 alloy among the prepared samples. The impedance spectroscopy shows a higher diffusion coefficient of 5.2488x10−11 cm2s−1 for SWCNT substituted Mg–Ti alloy compared to pristine alloy. The hydriding/dehydriding studies were observed on the developed electrode material and Mg67Ti29SWCNT4 delivered a maximum storage capacity of 826 mAhg−1.

Improvement in Hydriding and Dehydriding Features of Mg-TaF5-VCl3 Alloy by Adding Ni and x wt% MgH2 (x = 1, 5, and 10) Together with TaF5 and VCl3.

In our previous work, TaF5 and VCl3 were added to Mg, leading to the preparation of samples with good hydriding and dehydriding properties. In this work, Ni was added together with TaF5 and VCl3 to increase the reaction rates with hydrogen and the hydrogen-storage capacity of Mg. The addition of Ni together with TaF5 and VCl3 improved the hydriding and dehydriding properties of the TaF5 and VCl3-added Mg. MgH2 was also added with Ni, TaF5, and VCl3 and Mg-x wt% MgH2-1.25 wt% Ni-1.25 wt% TaF5-1.25 wt% VCl3 (x = 0, 1, 5, and 10) were prepared by reactive mechanical milling. The addition of MgH2 decreased the particle size, lowered the temperature at which hydrogen begins to release rapidly, and increased the hydriding and dehydriding rates for the first 5 min. Adding 1 and 5 wt% MgH2 increased the quantity of hydrogen absorbed for 60 min, Ha (60 min), and the quantity of hydrogen released for 60 min, Hd (60 min). The addition of MgH2 improved the hydriding–dehydriding cycling performance. Among the samples, the sample with x = 5 had the highest hydriding and dehydriding rates for the first 5 min and the best cycling performance, with an effective hydrogen-storage capacity of 6.65 wt%.

Development of Magnesium-Based Material with Hydrogen-Storage Capacity of 7 wt.

TiCl₃ was chosen as an additive to increase hydriding and dehydriding rates of Mg. In our previous works, we found that the optimum percentage of additives that improved the hydriding and dehydriding features of Mg was approximately ten. Specimens consisting of 90 wt% Mg and 10 wt% TiCl₃ (named Mg-10TiCl₃) were prepared by high-energy ball milling in hydrogen. The specimens' hydriding and dehydriding properties were then studied. Mg-10TiCl₃ had an effective hydrogenstorage capacity (the quantity of hydrogen absorbed in 60 min) of approximately 7.2 wt% at 593 K under 12 bar H₂ at the second cycle. After high-energy ball milling in hydrogen, Mg-10TiCl₃ contained Mg, β-MgH₂, and small amounts of γ-MgH₂ and TiH1.924. TiH1.924 remained undercomposed even after dehydriding at 623 K in a vacuum for 2 h. The hydriding and dehydriding properties of Mg-10TiCl₃ were compared with those of other specimens such as Mg-10Fe₂O₃, Mg-10NbF5, and Mg-5Fe₂O₃-5Ni, for which the hydrogen-storage properties were previously reported.

Ni-Doped Carbon Nanotube-Mg(BH4)2 Composites for Hydrogen Storage

Borohydride magnesium is widely studied as a promising hydrogen storage material due to its high gravimetric and volumetric density (14.9 wt % and 112 g/L, respectively). Different types of catalysts play a good catalytic role in enhancing the hydrogen desorption rates in borohydride systems. In the present paper, multiwalled carbon nanotubes supported Ni (Ni/MWCNTs) were synthesized by a chemical reduction method, and the catalyst was introduced into Mg(BH4)2 to enhance the dehydrogenation kinetics. The effect of Ni/MWCNTs on the microstructural evolution and hydrogen storage properties of Mg(BH4)2 is studied. The Mg(BH4)2-5wt% Ni/MWCNTs (95M@5NiMWCNTs) sample exhibits the best hydrogen storage performance; it starts to release hydrogen at 93 °C, which is 113 °C lower than that of the primary Mg(BH4)2 sample. Moreover, it takes only 3600 s to desorb hydrogen completely at 300 °C, showing a significant improvement on hydrogen desorption kinetics. In comparison with the primary Mg(BH4)2 sample, the dehydriding apparent activation energy of the 95M@5Ni3MWCNTs2 sample is significantly lowered to 119.5 kJ mol–1. Transmission electron microscopy analysis exhibits that the striking improvement of dehydriding properties of Mg(BH4)2 is mainly due to the synergistic catalysis of Ni and MWCNTs. This work provides a new method for designing and synthesizing high-performance catalysts in borohydride hydrogen storage materials, which is potentially utilized as a hydrogen supplier for hydrogen fuel cells.

Synthesis of core-shell α-AlH3@Al(OH)3 nanocomposite with improved low-temperature dehydriding properties by mechanochemical mixing and ionic liquid treatment

Nano α-AlH3 is a promising H2 storage material, with high gravimetric capacity and dehydriding temperature. However, its instability at ambient conditions and relatively slow dehydriding kinetics hinder its applications. This work reports a simple and environmentally friendly synthesis of a novel core-shell α-AlH3@Al(OH)3 nanocomposite with 1.2 nm thick shell. First, α-AlH3/LiCl nanocomposite was synthesized using a mechanochemical method, during which, α-AlH3 was stabilized using imidazolium ionic liquid (IL). After IL application and consequent rinsing with water, α-AlH3@Al(OH)3 composite formed. Its dehydriding kinetics was significantly faster (with the activation energy equal to 52.6 kJ/mol) relative to the pure α-AlH3. At 140 °C, the α-AlH3@Al(OH)3 released 10.0 wt% of its original hydrogen content within 1000 s. For comparison, the same amount of hydrogen was released by the commercial α-AlH3 only after 3600 s. The presence of the Al(OH)3 shell stabilized the α-AlH3 and resulted in absolutely no decomposition of the AlH3 in the α-AlH3@Al(OH)3 composite under ambient conditions for 7 days. Synthesis of this novel core-shell α-AlH3@Al(OH)3 nanocomposite is a new and very promising strategy to improve the stability and the hydrogen storage kinetics simultaneously.

Structural and Hydrogen Storage Properties of Mg60-Ni40 and Mg80-Ni20 Alloys Prepared by Planar Flow Casting

This study was carried out to better understand the chemical composition, microstructure, and hydrogen properties of Mg binary alloys. In this study, Mg60-Ni40 and Mg80-Ni20 (wt.%) alloy ribbons with different microstructures have been produced by the melt spinning method. The structural, hydriding, and dehydriding properties of the alloys were evaluated. The phase constitutions and microstructures were characterized by XRD and SEM studies. Microstructural grain sizes measured for Mg60-Ni40 alloy ribbons were in the range of 1-5 µm, and the alloy contained finely dispersed Mg and Mg2Ni phases. In the case of Mg80-Ni20 alloy ribbons, the microstructural grain sizes were measured as less than 1 µm. The hydrogen absorption and desorption capacities and kinetics of these two alloys were found to be highly dependent on microstructure and phase properties. Also, the amount of Ni content affected both the hydrogen storage capacities and kinetics for both of the Mg-Ni binary alloys. The increasing amount of Ni resulted in reduced hydrogen absorption capacity. The Mg60-Ni40 and Mg80-Ni20 alloy ribbons demonstrated superior hydrogenation absorption capacity at 350 °C and reversibly absorbed about 4.62 and 5.51 (wt.%) mass hydrogen, respectively.

Improvement in the Hydrogen-Storage Characteristics of Magnesium Hydride by Grinding with Sodium Alanate and Transition Metals in a Hydrogen Atmosphere

In this work, MgH2 was used as a starting material instead of Mg. The sample was prepared by grinding MgH2 with sodium alanate and transition metals in a hydrogen atmosphere. Its hydriding and dehydriding properties were measured followed by X-ray diffraction (XRD) analyses and observations of its microstructure. Activation was not required for the 86MgH2 + 10Ni + 2NaAlH4 + 2Ti sample. At the first cycle (n = 1), the sample absorbed 4.96, 5.28 and 5.36 wt% H for 10, 15 and 60 min, respectively, at 593 K in 12 bar H2, showing that the sample absorbed quite a large amount of hydrogen for 60 min (nearly 5.5 wt% H). The initial hydriding rate increased as the temperature increased from 423 K to 553 K and decreased from 553 K to 593 K. The sample showed quite high hydriding rates at relatively low temperatures 423 K (at n = 1) and 473 K (at n = 2) in 12 bar H2, compared with those of other metallic element(s) or compound(s)-added Mg or MgH2 alloys, absorbing 2.89 wt% H for 5 min, 2.97 wt% H for 10 min, and 3.31 wt% H for 60 min at 473 K.

Role of NiMn9.3Al4.0Co14.1Fe3.6 alloy on dehydrogenation kinetics of MgH2

Abstract Hydriding and dehydriding properties of MgH 2 - x wt. % NiMn 9.3 Al 4.0 Co 14.1 Fe 3.6 ( x = 10, 25, 50) nanocomposites have been investigated in present work. Doped alloy was prepared by arc melting method and ball milled with MgH 2 to get nanocomposites. Onset temperature as low as 180 °C was observed for MgH 2 -50 wt% system which is 80 °C lower than the as-milled MgH 2 giving 131.34 KJ/mol activation energy. Structural analysis shows tetragonal, orthorhombic and monoclinic phases for MgH 2, Al 60 Mn 11 Ni 4 and Mg 2 NiH 4 . Morphology by SEM were undertaken to investigate the effect of hydrogenation on nanostructured alloy. DSC studies show a single broad exothermic peak in the temperature range 48 °C–353 °C after alloy addition in MgH 2 . These results indicate that the hydrogenation properties of MgH 2 nanocomposite have been improved compared to the as-milled MgH 2 .

Development of a Mg-Based Alloy with a Hydrogen-Storage Capacity of 7 wt% by Adding a Polymer CMC via Transformation-Involving Milling

The addition of CMC (Carboxymethylcellulose, Sodium Salt) may improve the hydriding and dehydriding properties of Mg since it has a relatively low melting point and the melting of CMC during transformation-involving milling may put the milled samples in good states to absorb and release hydrogen rapidly. Samples with compositions of 95 wt% Mg + 5 wt% CMC (named Mg-5CMC) and 90 wt% Mg + 10 wt% CMC (named Mg-10CMC) were made using transformation-involving milling. Mg-5CMC was activated in about 3 hydriding-dehydriding cycles. After activation, Mg-5CMC had a larger amount of hydrogen absorbed in 60 min, Ha (60 min), than Mg-10CMC and milled Mg. At the fourth cycle (CN=4), Mg-5CMC had a very high beginning hydriding rate (1.45 wt% H/min) and Ha (60 min) (7.38 wt% H), showing that the activated Mg-5CMC has an effective hydrogen-storage capacity of about 7.4 wt% at 593 K in hydrogen of 12 bar at CN=4. Mg-5CMC after transformation-involving milling contained Mg and very small amounts of β- MgH2 and MgO, and Mg-5CMC dehydrogenated at 593 K in hydrogen of 1.0 bar at the 4th cycle contained Mg and tiny amounts of β-MgH2 and MgO, with no evidence of the phases related to CMC. The milling of Mg with CMC in hydrogen is believed to introduce defects and cracks and lessen the particle size. To the best of our knowledge, this study is the first in which a polymer CMC is added to Mg by transformation-involving milling to improve the hydriding and dehydriding properties of Mg.

Effect of Al* generated in situ in hydriding on the dehydriding properties of Mg-Al alloys prepared by hydriding combustion synthesis and mechanical milling

In this work, the effects of Al* generated in situ in hydriding on the dehydriding of Mg-Al alloys prepared by Hydriding Combustion Synthesis (HCS) and Mechanical Milling (MM) were studied. Thermal analysis showed that the peak dehydriding temperature of MgH2 was reduced by 25 °C (or 11 °C) when 20 at% Al was added in HCS (or MM) process, even though, the same dehydriding reaction of 17MgH2 + 12Al*/Al → Al12Mg17 + 17H2 was observed. Isothermal dehydriding at 300 °C for 3 h demonstrated that it took 81 min for HCS+MM-MgH2 to desorb 50% of hydrogen, while only 24 min was required for HCS+MM-Mg80Al*20. The apparent activation energy for dehydriding of MgH2 was reduced from 144.3 ± 7.19 kJ/mol for HCS+MM-MgH2 to 118.8 ± 6.73 kJ/mol for HCS+MM-Mg90Al*10. The scanning electron microscopy/energy dispersive spectroscopy and X-ray photoelectron spectroscopy analyses revealed that the Al* in situ generated from the hydriding of Al12Mg17 in HCS would significantly decrease dehydriding temperature and improve dehydriding kinetics of the MgH2-Al composites because of a uniform distribution, the shortening reaction diffusion length, providing unoxidized surfaces to react with MgH2 and the high chemical activity of Al*.

Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling

In this work, polyvinylpyrrolidone (PVP) coated Mg95Ni5 nano-composites were prepared by hydriding combustion synthesis (HCS) plus wet mechanical milling (WM) with tetrahydrofuran (THF) and donated as WM-x wt% PVP (x = 1, 3, 5 and 7) respectively. The phase compositions, microstructures and dehydriding property, as well as the co-effect of PVP and THF were investigated in detail. XRD results showed that the average crystal size of MgH2 in the milled Mg95Ni5 decreased from 23 nm without PVP to 18 nm with 1 wt% PVP. The peak temperature of dehydrogenation of MgH2 in the milled Mg95Ni5 decreased from 293.0 °C without THF to 250.4 °C with THF. The apparent activation energy for decomposition of MgH2 in WM-7 wt% PVP was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of milled Mg95Ni5 without THF and PVP. PVP and THF can facilitate the refinement of particle size during mechanical milling process. Attributed to small particle sizes and synergistic effect of PVP and THF, the composites exhibit markedly improved dehydriding properties.

One-step uniform growth of magnesium hydride nanoparticles on graphene

A simple solvent-free method to synthesize MgH2 nanoparticles (MgH2 NPs) uniformly grown on graphene nanosheets (GNs) has been reported in this paper. Based on the formation of MgH2 by di-n-butylmagnesium ((C4H9)2Mg) thermal decomposition under hydrogen pressure, the GNs were added as matrix to hinder the agglomeration and growing of MgH2 NPs. The fabricated MgH2/GNs nanocomposites, in which MgH2 NPs were homogenously growing in the graphene matrix, have been synthesized by the favorable adsorption energy between (C4H9)2Mg and GNs. Resulting from the one-step solvent-free route, the generated MgH2 NPs shows high hydrogen capacity and steady hydriding and dehydriding properties, without the interference of the synthetic medium. At the same time, the size of the fabricated MgH2 NPs can be controlled by adjusting the mass ratio of MgH2 to graphene, the various hydrogen pressure and temperature. Attributed to smaller size effect, well uniform distribution of high density MgH2 NPs, and the agglomeration blocking ability of graphene, the MgH2/GNs-40wt% exhibits the favorite hydrogen storage performance.

Significant improvement in the dehydriding properties of perovskite hydrides, NaMgH3, by doping with K2TiF6

In this study, perovskite hydrides (NaMgH3) were synthesized in the presence and absence of a dopant, e.g., K2TiF6, by high-energy ball milling. Clear lattice distortion was observed in K2TiF6-doped NaMgH3 as compared to pristine NaMgH3. The doped sample exhibited better dehydriding properties, and the onset desorption temperature significantly decreased from 580 K to 328 K. The maximum amount of desorbed hydrogen was approximately 3.8 wt% at 638 K, and approximately 90% of hydrogen was desorbed within 20 min. Differential scanning calorimetry results indicated that the activation energy for the two steps in the decomposition of NaMgH3 decreases by doping with K2TiF6. This result suggested that the lattice distortion and low decomposition energy of the NaMgH3 phase, caused by the introduction of the K2TiF6 dopant, result in improved dehydriding properties for NaMgH3.

Enhancing the dehydriding properties of perovskite-type NaMgH3 by introducing potassium as dopant

NaMgH3 hydride is a potential candidate for hydrogen storage due to its high gravimetric and volumetric H densities (6 wt. % and 88 kg/m3). Perovskite-type NaMgH3 and Na0.9K0.1MgH3 hydrides have been synthesized by a high-energy ball milling method in this work. The replacement of Na by 10 at.% of K results in a slight increase of lattice parameters of NaMgH3 phase and the decrease of activation energy of decomposition reactions. In comparison with NaMgH3 hydride, Na0.9K0.1MgH3 hydride presented better dehydriding kinetic properties, the onset desorption-temperature of which is greatly decreased from 581 K to 430 K, the hydrogen-desorbed amount is about 3.0 wt. % within 6 min at 638 K, and the maximum amount is 5.25 wt. % (about 87.5% of the theoretical value) after 40 min. Further theoretical calculations, based on DFT, suggest that higher formation enthalpy and s-p-d hybridized orbitals (instead of s-p hybridized orbitals) in Na0.9K0.1MgH3 hydride maybe contribute to the improved dehydriding properties by reducing its structure stability.

Electrochemical hydriding and thermal dehydriding properties of nanostructured hydrogen storage MgNi26 alloy

The MgNi26 alloy was prepared by three different methods of gravity casting (GC), mechanical alloying (MA) and rapid solidification (RS). All samples were electrochemically hydrided in a 6 mol/L KOH solution at 80 °C for 240 min. The structures and phase compositions of the alloys were studied using optical microscopy and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. A temperature-programmed desorption technique was used to measure the absorbed hydrogen and study the dehydriding process. The content of hydrogen absorbed by the MgNi26-MA (approximately 1.3%, mass fraction) was 30 times higher than that of the MgNi26-GC. The MgNi26-RS sample absorbed only 0.1% of hydrogen. The lowest temperature for hydrogen evolution was exhibited by the MgNi26-MA. Compared with pure commercial MgH2, the decomposition temperature was reduced by more than 200 °C. The favourable phase and structural composition of the MgNi26-MA sample were the reasons for the best hydriding and dehydriding properties.

Improved Dehydriding Properties of NaMgH<sub>3</sub> Perovskite Hydride by Addition of Graphitic Carbon Nitride

NaMgH3 perovskite hydride and graphitic carbon nitride (g-C3N4) powder with multi-layer structure have been synthesized by high-energy ball-milling and heating-decomposition method respectively. In comparison with pristine NaMgH3 hydride, the as–milled NaMgH3+5wt.% g-C3N4 (NMH-5CN) composite presents better dehydriding kinetic properties and lower onset decomposition temperature. The onset decomposition temperature of NMH-5CN composite can be decreased about 87K, the hydrogen-desorbed amount is about 3.2 wt. % within 40min, and 1.4 wt. % within 10min at 638K, and the Kissinger analysis demonstrated that the activation energy (ΔE) of the first and second dehydrogenation step for NMH-5CN can be reduced about 40 kJ mol-1 and 22.4 kJ mol-1, respectively. The results suggest that g-C3N4 is an effective catalyst for the improvement of dehydriding properties of NaMgH3 perovskite hydride.

Effects of Milling and Hydriding-Dehydriding Cycling on the Hydrogen-Storage Behaviors of a Magnesium-Nickel-Tantalum Fluoride Alloy

In this study, Ni and TaF5 were picked as additives to enhance the hydriding and dehydriding rates of Mg. A sample with a composition of 80 wt% Mg+14 wt% Ni+6 wt% TaF5 (named Mg-14Ni-6TaF5) was prepared by reactive mechanical grinding. Its hydriding and dehydriding properties were then examined and the effects of milling and hydriding-dehydriding cycling were investigated. To activate the prepared sample, two hydriding-dehydriding cycles were required at 573 K and 593 K. At the second cycle, the sample absorbed 5.74 wt% H for 60 min under 12 bar H2 at 573 K. At 623 K, the activated Mg-14Ni-6TaF5 absorbed 2.53 wt% H for 2.5 min, 4.20 wt% H for 10 min, and 5.31wt% H for 60 min under 12 bar H2, and desorbed 2.01 wt% H for 2.5 min, 4.76 wt% H for 15 min, and 4.83 wt% H for 60 min under 1.0 bar H2. The initial hydriding rate increased as the temperature increased from 423 K to 573 K (due to the predominance of the acceleration of the thermally activated process) and then decreased from 573 K to 623 K due to the predominance of the decrease in the driving force for the hydriding reactionunder 12 bar H2.

In-situ synchrotron X-ray diffraction investigation of the hydriding and dehydriding properties of a cast Mg–Ni alloy

In previous research, it has been demonstrated that the hydrogenation kinetics of Mg–Ni alloys is significantly improved by elemental Na doping. To clarify the underlying mechanisms in terms of crystallographic phase changes during the reactions, in-situ hydrogen absorption and desorption as a function of temperature under constant hydrogen atmospheres of 2MPa and 0.2MPa, respectively, was studied. The experiments were performed at the Powder Diffraction beamline of the Australian Synchrotron facility with a gas flow cell and a hot air blower. This study showed that the hydrogenation of Na-doped Mg–Ni alloys can be identified at a temperature as low as 260°C via an interface-controlled nucleation and growth mechanism without any prior activation whilst the dehydrogenation occurred at about 370°C. The sequence of phase transformations associated with these reactions as well as associated expansion properties of individual phases during hydrogen sorption reactions as provided by the high resolution X-ray diffraction data are discussed.

Enhancement of the Hydriding and Dehydriding Rates of Mg by Adding TiCl3 and Reactive Mechanical Grinding

A sample with a composition of 95 wt% Mg-5 wt% TiCl3 (named Mg-5TiCl3) was prepared by reactive mechanical grinding, and its hydriding and dehydriding properties were examined. The activation of Mg-5TiCl3 was not required. Mg-5TiCl3 had an effective hydrogen-storage capacity of about 6.2 wt%. At the first cycle (n=1), the sample absorbed 5.77 wt% H for 5 min, 6.07 wt% H for 10 min, 6.17 wt% H for 30 min, and 6.19 wt% H for 60 min at 573 K under 12 bar H2. At n=1, the sample desorbed 0.03 wt% H for 2.5 min, 0.13 wt% H for 30 min, and 0.35 wt% H for 60 min at 573 K under 1.0 bar H2. Mg-5TiCl3 after reactive mechanical grinding contained Mg, β-MgH2, γ-MgH2, and TiH1.924. The XRD pattern of Mg-5TiCl3 dehydrided at the 4th hydriding-dehydriding cycle at 423-573 K contained β-MgH2, Mg, MgO, and TiH1.924. Mg-5TiCl3 had a higher initial hydriding rate and a larger quantity of hydrogen absorbed for 60 min than Mg-5TaF5, Mg-10Fe2O3, Mg-10MnO, and Mg, the hydrogen-storage properties of which were previously reported.(Received February 12, 2014)

Effect of Zn and Zr addition on the synthesis of an AlH3/MgCl2 nanocomposite and its de-hydriding properties

This paper presents the preliminary findings of the effects of 3d transition metals on the synthesis of an AlH3/MgCl2 nanocomposite and its de-hydriding properties.