Investigation on kinetics mechanism of hydrogen absorption in the La2Mg17-based composites

A new model has been successfully used to investigate the hydrogen absorption kinetics mechanism of La₂Mg₁₇-based composites. The results indicate that different preparation conditions lead to different rate-controlling steps during hydrogen absorption process. For La₂Mg₁₇–LaNi₅ composite synthesized by the method of melting, the rate-controlling step is the surface penetration of hydrogen atoms, which does not change by addition agent (LaNi₅). However, mechanical milling can change the rate-limiting steps of hydriding reaction in the La₂Mg₁₇–LaNi₅ composite from surface penetration to diffusion of hydrogen in the hydride layer. With the enhancement of milling intensity, the rate-controlling step in La_1.8Ca_0.2Mg₁₄Ni₃ alloy changes from surface penetration to diffusion. In addition, the activation energies of hydrogen absorption for La₂Mg₁₇−20 wt%LaNi₅ and La_1.8Ca_0.2Mg₁₄Ni₃ are obtained by this model.     

Hydrogen sorption properties of MgH2/NaBH4 composites

The hydrogen sorption properties of magnesium hydride–sodium borohydride composites prepared by means of high-energy ball milling under Ar atmosphere were investigated. Mutual influence of milling time and the content of NaBH₄ were studied. Microstructural and morphological analyses were carried out using X-ray Diffraction (XRD), laser scattering measurements and Scanning Electron Microscopy (SEM), while kinetic analysis and cycling were performed in a Sievert's volumetric apparatus. It has been shown that low content of NaBH₄ and short milling time are beneficial for hydrogen sorption kinetics.     

Hydrogen desorption properties of MgH2/LiAlH4 composites

The hydrogen desorption properties of MgH₂–LiAlH₄ composites obtained by mechanical milling for different milling times have been investigated by Thermal Desorption Spectroscopy (TDS) and correlated to the sample microstructure and morphology analysed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The MgH₂–LiAlH₄ composites show improved hydrogen desorption properties in comparison with both as-received and ball-milled MgH₂. Mixing of MgH₂ with small amount of LiAlH₄ (5 wt.%) using short mechanical milling (15 min) shifts, in fact, the hydrogen desorption peak to lower temperature than those observed with both as-received and milled MgH₂ samples. Longer mixing times of the MgH₂–LiAlH₄ composites (30 and 60 min) reduce the catalytic activity of the LiAlH₄ additive as revealed by the shift of the hydrogen desorption peak to higher temperatures.     

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.     

Enhanced hydrogen storage properties of LiBH4 modified by NbF5

In this work, the hydriding–dehydriding properties of the LiBH₄–NbF₅ mixtures were investigated. It was found that the dehydrogenation and reversibility properties of LiBH₄ were significantly improved by NbF₅. Temperature-programed dehydrogenation (TPD) showed that 5LiBH₄–NbF₅ sample started releasing hydrogen from as low as 60 °C, and 4 wt.% hydrogen could be obtained below 255 °C. Meanwhile, ∼7 wt.% H₂ could be reached at 400 °C in 20LiBH₄–NbF₅ sample, whereas pristine LiBH₄ only released ∼0.7 wt.% H₂. In addition, reversibility measurement demonstrated that over 4.4 wt.% H₂ could still be released even during the fifth dehydrogenation in 20LiBH₄–NbF₅ sample. The experimental results suggested that a new borohydride possibly formed during ball milling the LiBH₄–NbF₅ mixtures might be the source of the active effect of NbF₅ on LiBH₄.     

Synthesis and hydriding properties of Li2Mg(NH)2

The phase pure Li₂Mg(NH)₂ has been synthesized via a dehydriding treatment of a ball milled 2LiNH₂ + MgH₂ mixture. This phase pure Li₂Mg(NH)₂ has been utilized to investigate its hydriding kinetics at the temperature range 180-220 °C. It is found that the hydriding process of Li₂Mg(NH)₂ is very sluggish even though it has favorable thermodynamic properties for near the ambient temperature operation. Holding at 200 °C for 10 h only results in 3.75 wt.% H₂ uptake. The detailed kinetic analysis reveals that the hydriding process of Li₂Mg(NH)₂ is diffusion-controlled. Thus, this study unambiguously indicates that the future direction to enhance the hydriding kinetics of this promising hydrogen storage material system should be to minimize the diffusion distance and increase the diffusion rate.     

Improved hydrogen sorption performance of NbF5-catalysed NaAlH4

The effect of NbF₅ on the hydrogen sorption performance of NaAlH₄ has been investigated. It was found that the dehydrogenation/hydrogenation properties of NaAlH₄ were significantly enhanced by mechanically milling with 3 mol% NbF₅. Differential scanning calorimetry results indicate that the ball-milled NaAlH₄-0.03NbF₅ sample lowered the completion temperature for the first two steps dehydrogenation by 71 °C compared to the pristine NaAlH₄ sample. Isothermal hydrogen sorption measurements also revealed a significant enhancement in terms of the sorption rate and capacity, in particular, at reduced operation temperatures. The apparent activation energy for the first-step and the second-step dehydrogenation of the NaAlH₄-0.03NbF₅ sample is estimated to be 88.2 kJ/mol and 102.9 kJ/mol, respectively, by using Kissinger’s approach, which is much lower than for pristine NaAlH₄, indicating the reduced kinetic barrier. The rehydrogenation kinetics of NaAlH₄ was also improved with 3 mol% NbF₅ doping, absorbing ∼1.7 wt% hydrogen at 150 °C for 2 h under ∼5.5 MPa hydrogen pressure. In contrast, no hydrogen was absorbed by the pristine NaAlH₄ sample under the same conditions. The formation of Na₃AlH₆ was detected by X-ray diffraction on the rehydrogenated NaAlH₄-0.03NbF₅ sample. Furthermore, the structural changes in the NbF₅-doped NaAlH₄ sample after ball milling and the hydrogen sorption were carefully examined, and the active species and mechanism of catalysis in NbF₅-doped NaAlH₄ are discussed.     

Catalytic mechanism of Nb2O5 and NbF5 on the dehydriding property of Mg95Ni5 prepared by hydriding combustion synthesis and mechanical milling

The catalytic mechanism of Nb₂O₅ and NbF₅ on the dehydriding property of Mg₉₅Ni₅ prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. It was shown that NbF₅ was more efficient than Nb₂O₅ in improving the dehydriding property. In particular, the dehydriding temperature onset decreases from 460 K for Mg₉₅Ni₅ to 450 K for Mg₉₅Ni₅with 2.0 at.% Nb₂O₅, whereas it decreases to 410 K for that with 2.0 at.% NbF₅. By means of X-ray diffraction and X-ray photoelectron spectroscopy, it was confirmed that the interaction between the Nb ions and the H atoms and that between the anions (O²⁻ or F⁻) and Mg²⁺ existed in Mg₉₅Ni₅ doped with Nb₂O₅ or NbF_5. Further, the pressure–concentration-isotherms analysis clarified that these interactions destabilized the Mg–H bonding, and that NbF₅ had a better effect on the destabilization of the Mg–H bonding than Nb₂O₅ contributing to the better dehydriding property of (Mg₉₅Ni₅)2.0−NbF₅.     

Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles

2LiBH₄/MgH₂ system is a representative and promising reactive hydride composite for hydrogen storage. However, the high desorption temperature and sluggish desorption kinetics hamper its practical application. In our present report, we successfully introduce CoNiB nanoparticles as catalysts to improve the dehydrogenation performances of the 2LiBH₄/MgH₂ composite. The sample with CoNiB additives shows a significant desorption property. Temperature programmed desorption (TPD) measurement demonstrates that the peak decomposition temperatures of MgH₂ and LiBH₄ are lowered to be 315 °C and 417 °C for the CoNiB-doped 2LiBH₄/MgH₂. Isothermal dehydrogenation analysis demonstrates that approximately 10.2 wt% hydrogen can be released within 360 min at 400 °C. In addition, this study gives a preliminary evidence for understanding the CoNiB catalytic mechanism of 2LiBH₄/MgH₂     

Tuning the de/hydriding thermodynamics and kinetics of Mg by mechanical alloying with Sn and Zn

Mg₉₅Sn₃Zn₂ alloy was prepared by mechanical alloying. The phase constituents and phase transition were analyzed by X-ray diffraction (XRD) method. The microstructure was characterized by scanning electron microscope (SEM). The hydrogen storage properties were evaluated in detail by the measurements of isothermal hydrogen absorption and desorption, and pressure-composition isotherms (PCI) using the Sieverts method. The addition of Zn benefits to extend the solubility of Sn in the Mg lattice, as a result supersaturated Mg(Sn, Zn) ternary solid solution was synthesized by mechanical alloying, which decomposed to MgH₂, Sn and MgZn₂ in the hydrogenating process. The in situ formed nanostructure Mg₂Sn and MgZn₂ have positive effects on the hydrogen absorption and desorption of Mg. Mg₉₅Sn₃Zn₂ alloy showed significantly improved kinetics with lowered hydrogen absorption and desorption activation energies of 38.1 kJ/mol and 86.6 kJ/mol respectively, and exhibited a reduced dehydriding enthalpy of 67.0 ± 1.9 kJ/(mol·H₂).     

Enhanced hydrogen sorption properties of Ni and Co-catalyzed MgH2

MgH₂ is one of the most promising hydrogen storage materials due to its high capacity and low cost. In an effort to develop MgH₂ with a low dehydriding temperature and fast sorption kinetics, doping MgH₂ with NiCl₂ and CoCl₂ has been investigated in this paper. Both the dehydrogenation temperature and the absorption/desorption kinetics have been improved by adding either NiCl₂ or CoCl₂, and a significant enhancement was obtained in the case of the NiCl₂ doped sample. For example, a hydrogen absorption capacity of 5.17 wt% was reached at 300 °C in 60 s for the MgH₂/NiCl₂ sample. In contrast, the ball-milled MgH₂ just absorbed 3.51 wt% hydrogen at 300 °C in 400 s. An activation energy of 102.6 kJ/mol for the MgH₂/NiCl₂ sample has been obtained from the desorption data, 18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH₂/CoCl₂, which also exhibits an enhanced kinetics, and of the pure MgH₂ sample, respectively. In addition, the enhanced kinetics was observed to persist even after 9 cycles in the case of the NiCl₂ doped MgH₂ sample. Further kinetic investigation indicated that the hydrogen desorption from the milled MgH₂ is controlled by a slow, random nucleation and growth process, which is transformed into two-dimensional growth after NiCl₂ or CoCl₂ doping, suggesting that the additives reduced the barrier and lowered the driving forces for nucleation.     

Measurement and modelling of kinetics of hydrogen sorption by LaNi5 and two related pseudobinary compounds

The hydriding/dehydriding rates and the pressure–composition isotherms were measured for LaNi5, LaNi_4.85Al_0.15 and LaNi_4.75Fe_0.25 under quasi-isothermal and variable pressure conditions. Isothermal conditions were obtained by reducing the thermal time constant of the experimental device. Empirical rate equations to describe the sorption reaction kinetics were derived. These rates are expressed as a function of temporal composition, saturated composition, temperature, applied pressure and essentially the initial operating conditions which were not considered in most of all the previous studies related to the reaction kinetics of metal hydrides. Besides, the rate equations presented in this work can be integrated easily in the numerical models that predict dynamic flow and heat and mass transfer within realistic metal–hydrogen devices. This paper also discusses the effects of Fe and Al as substituents for Ni on P–C isotherms and reaction rates of LaNi₅ alloy.     

Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride

In present paper, different sizes of CeO₂ nanoparticles were synthesized by ball milling and their effect on the absorption kinetics and decomposition temperature of MgH₂ was studied. It was found that a small amount of admixing of the above said catalysts with MgH₂ exhibits improved hydrogen storage properties. Among these different sizes of CeO₂ nanoparticles, 2 weight % admixed CeO₂ with a particle size of ∼10–15 nm led to decrease in desorption temperature by ∼50 K. Moreover, it also shows 1.5 times better absorption kinetics with respect to pure MgH₂. The samples were characterized using SEM, TEM and XRD techniques. The hydrogenation/dehydrogenation properties were measured by gas reaction controller.     

Studies on dehydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with vanadium and vanadium based catalysts (V,V2O5 and VCl3)

In the present study, we have investigated the effect of vanadium and its compounds (V, V₂O₅ and VCl₃) on desorption characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture. The hydrogen storage characteristics of 1:2 Mg(NH₂)₂/LiH mixture gets enhanced with admixing of V, V₂O₅ and VCl₃ separately. The VCl₃ has been found to be the most effective followed by V and V₂O₅. The activation energy for dehydrogenation process of 1:2 Mg(NH₂)₂/LiH mixture with and without catalyst has been evaluated using a method suggested by Ozawa et al. [25]. Based on the experimental results, schematic reaction scheme for decomposition of Mg(NH₂)₂ in the presence of VCl₃ has also been proposed.     

The effects of Ti-based additives on the kinetics and reactions in LiH/MgB2 hydrogen storage system

In this work we investigated the effect of Ti, TiH₂, TiB₂, TiCl₃, and TiF₃ additives on the hydrogen de/re-sorption kinetics and reaction pathways of LiH/MgB₂ mixture. From high pressure differential scanning calorimeter (HP-DSC) measurements it was found that these additives all effectively decrease the onset temperature of hydrogenation. The isothermal hydrogenation/dehydrogenation measurements suggest that Ti, TiH₂, and TiB₂ can significantly improve the hydrogen sorption kinetics of LiH/MgB₂ mixture. The absorption kinetics of TiF₃ and TiCl₃ doped samples are slower than the baseline (2LiH-MgB₂ without additive), but their desorption kinetics are faster than the baseline and other additives doped systems. X-ray diffraction (XRD) analysis reveals that the additive Ti in LiH/MgB₂ actively participates in both hydrogenation and dehydrogenation process, which can be regarded as an effective additive of this system.     

Changes in kinetic parameters of decomposition of MgH2 destabilized by irradiation with C ions

In attempt to improve desorption behaviour of MgH₂, the influence of well-defined structural changes induced within a thin surface layer of MgH₂ have been investigated. The defects were induced by 30 keV C²⁺ ions irradiation using different fluencies ranging from 10¹²–10¹⁶ ions/cm². The hydrogen desorption properties were investigated by thermal desorption spectroscopy analysis (TDS), while kinetics parameters were deduced using non-isothermal kinetic approach. The existence of multiple TDS peaks and different curve shapes indicate difference in desorption mechanism. To understand changes in the rate limiting step, shapes of all desorption peaks have been analyzed using different kinetic models. Regarding the irradiated sample, the function based on Avrami–Erofeev model with n=4$n=4$ gives the best fit over θ   range from 0.3 to 0.8 while for untreated sample the best fit is obtained for Avrami–Erofeev model with n=3$n=3$. The change in mechanism can be attributed to the different way of nuclei growth.     

Effect of iron on the hydriding properties of the Mg6Pd hydrogen storage system

The effects of iron addition on the hydriding properties of Mg₆Pd were investigated. It was found that 15 min of co-milling Mg₆Pd and Fe in a high energy mill was sufficient to improve the sorption kinetics. Rietveld refinement of neutron powder diffraction patterns indicated that the presence of iron drastically modifies the system’s micro-structure. A possible link between structure and hydrogen storage properties is discussed.     

Hydrogen storage performance of LiBH4+1/2MgH2 composites improved by Ce-based additives

LiBH₄+1/2MgH₂ is a promising reactive hydride composite for hydrogen storage. In the present study, three Ce-based additives were used as catalysts to enhance the hydrogen storage performance of LiBH₄+1/2MgH₂ composites. The composites with Ce additives demonstrated significantly improved dehydrogenation kinetics and cyclic stability compared with the pure composite. X-ray diffraction and scanning electron microscopy analyses clearly revealed the phase transitions and morphological evolution during the hydriding-dehydriding cycling. The composites with Ce-based additives displayed stable nanostructures, in contrast to the rapid microstructural deterioration in the uncatalyzed composite. The CeB₆ formed in the composites had a particle size of 10 nm after five cycles. It may act as the nucleus for MgB₂ formation during dehydrogenation and thus account for the structural and performance stability of the composites upon cycling.     

Comparative studies of reaction rates of NH3 with MgH2 and LiH

The reaction rate of MgH₂ with NH₃ is studied using a two-layered structure containing a top MgH₂ layer and a bottom LiNH₂ layer. Quantification of the effluent gas composition from the two-layered structure indicates substantial NH₃ emission, while the X-ray diffraction analysis reveals little formation of the reaction products between MgH₂ and NH₃. In contrast, the study of the two-layered structure containing a top LiH layer and a bottom LiNH₂ layer reveals that the reaction between LiH and NH₃ is much faster than that between MgH₂ and NH₃.     

Reversible de-/hydriding characteristics of a novel Mg18In1Ni3 alloy

The present work demonstrates the reversible hydrogen storage properties of the ternary alloy Mg₁₈In₁Ni₃, which is prepared by ball-milling Mg(In) solid solution with Ni powder. The two-step dehydriding mechanism of hydrogenated Mg₁₈In₁Ni₃ is revealed, namely the decomposition of MgH₂ is involved with different intermetallic compounds or Ni, which leads to the formation of Mg₂Ni(In) solid solution or unknown ternary Mg–In–Ni alloy phase. As a result, the alloy Mg₁₈In₁Ni₃ shows improved thermodynamics in comparison with pure Mg. The Ni addition also results in the kinetic improvement, and the minimum desorption temperature is reduced down to 503 K, which is a great decrease comparing with that for Mg–In binary alloy. The composition and microstructure of Mg–In–Ni ternary alloy could be further optimized for better hydrogen storage properties.