Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures

In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi₅ utilizing the hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the effect of hydrogen pressure on the ignition temperature and the initial activation of hydrogenation. In the experiments, La₂O₃, Ni, and Ca were dry-mixed, and then heated at 0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from bulk to powder with hydrogen pressure. This was probably caused by volume expansion due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The results of this research can be applied as an innovative production route for LaNi₅ without the conventional melting of La and Ni.     

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.     

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.     

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.     

Effects of NbF5 addition on the hydrogen storage properties of LiAlH4

We investigated the effects of NbF₅ addition by ball milling on the hydrogen storage properties of LiAlH₄. Pressure-composition-temperature (PCT) experiments showed that addition of 0.5 and 1 mol% NbF₅ in LiAlH₄ improves the onset desorption temperature and results in little decrease in hydrogen capacity, with approximately 7.0 wt% released by 188 °C. Isothermal dehydriding kinetics measurements indicated that the NbF₅-doped sample shows an average dehydrogenation rate 5–6 times faster than that of the as-received LiAlH₄ sample. In the x-ray diffraction results, there are distinct peaks of Al and LiH that appear after desorption. There is no peak of NbF₅ before or after desorption. Desorption kinetics measurements indicated that the activation energy, E_A, for LiAlH₄ + 1 mol% NbF₅ is about 67 kJ/mol for first reaction stage and about 77 kJ/mol for second reaction stage. The desorption process was further characterised by differential scanning calorimetry, and the possible mechanism of the effects of NbF₅ addition is discussed.     

Improving hydrogen storage properties of MgH2 by addition of alkali hydroxides

In this study, alkali hydroxides (NaOH, LiOH, KOH) were attempted as new-type catalytic additives to improve the hydrogen storage properties of MgH₂ by ball milling. It is shown that the additives readily react with the MgH₂ to form perovskite-structured NaMgH₃ and KMgH₃, but not the LiMgH₃. Although the in-situ formed perovskite hydrides remain stable during hydriding/dehydriding cycles, both NaMgH₃ and KMgH₃ show predominant catalytic role on the hydriding and dehydriding of MgH₂. In contrast, the LiOH modified MgH₂ presents much inferior sorption kinetics because of the absence of LiMgH₃. The catalytic mechanism of perovskite hydrides can be explained by the high hydrogen mobility in the perovskite structure. This study shows that such cheap hydroxides could be used as efficient catalysts to improve the hydrogen storage properties of high-capacity metal hydrides and complex hydrides.     

Self-ignition combustion synthesis of LaNi5 utilizing hydrogenation heat of metallic calcium

This paper describes self-ignition combustion synthesis (SICS) of LaNi₅ in a pressurized hydrogen atmosphere using metallic calcium as both the reducing agent and the heat source. In this study, the effects of hydrogen on the ignition temperature and the hydrogenation properties of the products were mainly examined. In the experiments, La₂O₃, Ni, and Ca were dry-mixed in the molar ratio of 1:10:6 and then heated up at a hydrogen pressure of 1.0 MPa until the ignition due to the hydrogenation of calcium. For the sake of comparison, the same experiments were performed in a normal argon atmosphere. The results showed that the ignition temperature was drastically lowered by hydrogen; it was only 600 K in the case of hydrogen as compared to 1100 K in the case of argon. The products also exhibited high initial activity and hydrogen storage capacity of 1.54 mass%. The proposed method offers many benefits for using cost-effective rare-earth oxide, saving productive time and energy, improving initial activity of the product and applying to any AB₅-type hydrogen storage alloy.     

Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

The preparation of Co9S8 and CoS2 nanoparticles by a high energy ball-milling method and their electrochemical hydrogen storage properties

Two different Co–S compounds with enhanced hydrogen storage properties, Co₉S₈ and CoS₂, were prepared by ball-milling mixtures of Co metal and S powder. X-ray diffractometry, scanning electron microscopy and transmission electron microscopy were used to show that specific molar ratios of Co:S and ball-milling speeds and times result in pure Co₉S₈ and CoS₂, thus overcoming a long-standing inability to obtain pure Co–S compounds via ball-milling. A galvanostatic charge–discharge process and cyclic voltammetry measurements showed that the as-obtained Co₉S₈ and CoS₂ nanoparticles have enhanced electrochemical hydrogen storage capacities of 1.79 and 1.57 wt% hydrogen, respectively, which are higher than those previously reported. In addition, based on the corresponding X-ray photoelectron spectroscopy and cyclic voltammetry measurements, a new electrochemical hydrogen storage mechanism for the two Co–S compounds was proposed and discussed.     

NiB nanoparticles: A new nickel-based catalyst for hydrogen storage properties of MgH2

In this paper, amorphous NiB nanoparticles were fabricated by chemical reduction method and the effect of NiB nanoparticles on hydrogen desorption properties of MgH₂ was investigated. Measurements using temperature-programmed desorption system (TPD) and volumetric pressure–composition isotherm (PCI) revealed that both the desorption temperature and desorption kinetics have been improved by adding 10 wt% amorphous NiB. For example, the MgH₂–10 wt%NiB mixture started to release hydrogen at 180 °C, whereas it had to heat up to 300 °C to release hydrogen for the pure MgH₂. In addition, a hydrogen desorption capacity of 6.0wt% was reached within 10 min at 300 °C for the MgH₂–10 wt%NiB mixture, in contrast, even after 120 min only 2.0 wt% hydrogen was desorbed for pure MgH₂ under the same conditions. An activation energy of 59.7 kJ/mol for the MgH₂/NiB composite has been obtained from the desorption data, which exhibits an enhanced kinetics possibly due to the additives reduced the barrier and lowered the driving forces for nucleation. Further cyclic kinetics investigation using high-pressure differential scanning calorimetry technique (HP-DSC) indicated that the composite had good cycle stability.     

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.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Recoverable Ni2Al3 nanoparticles and their catalytic effects on Mg-based nanocomposite during hydrogen absorption and desorption cycling

The Mg-3.9 wt% Ni₂Al₃ nanocomposite is produced by hydrogen plasma-metal reaction method. The particle size of Mg is in range of 40–160 nm with an average size of 90 nm. The Ni₂Al₃ nanoparticles (NPs) of about 9 nm uniformly disperse on the surface of Mg NPs and in situ transform to Mg₂NiH_0.3 and Al after hydrogen absorption process. Surprisingly, the Mg₂NiH_0.3 and Al can recover to the initial state of Ni₂Al₃ after hydrogenation/dehydrogenation cycle. The Mg-Ni₂Al₃ nanocomposite shows enhanced hydrogen sorption rate and storage capacity. It can quickly uptake 6.4 wt% H₂ within only 10 min at 573 K, and release 6.1 wt% H₂ within 10 min at 623 K. The apparent activation energies for hydrogenation and dehydrogenation are calculated to be 55.4 and 115.7 kJ mol^?1 H₂. The enhanced hydrogen storage performances of the Mg-Ni₂Al₃ nanocomposite are attributed to both the nanostructure of Mg and the catalytic effects of Ni₂Al₃ NPs.     

Effect of ball milling time on the hydrogen storage properties of TiF3-doped LiAlH4

In the present work, the catalytic effect of TiF₃ on the dehydrogenation properties of LiAlH₄ has been investigated. Decomposition of LiAlH₄ occurs during ball milling in the presence of 4 mol% TiF₃. Different ball milling times have been used, from 0.5 h to 18 h. With ball milling time increasing, the crystallite sizes of LiAlH₄ get smaller (from 69 nm to 43 nm) and the dehydrogenation temperature becomes lower (from 80 °C to 60 °C). Half an hour ball milling makes the initial dehydrogenation temperature of doped LiAlH₄ reduce to 80 °C, which is 70 °C lower than as-received LiAlH₄. About 5.0 wt.% H₂ can be released from TiF₃-doped LiAlH₄ after 18 h ball milling in the range of 60 °C–145 °C (heating rate 2 °C min⁻¹). TiF₃ probably reacts with LiAlH₄ to form the catalyst, TiAl₃. The mechanochemical and thermochemical reactions have been clarified. However, the rehydrogenation of LiAlH₄/Li₃AlH₆ can not be realized under 95 bar H₂ in the presence of TiF₃ because of their thermodynamic properties.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

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     

Effect of Mg content on structure,hydrogen storage properties and thermal stability of melt-spun Mgx(LaNi3)100−x alloys

Amorphous Mg_x(LaNi₃)_100−x (x = 40, 50, 60, 70) alloys with ribbon shape (5 mm wide, 0.2 mm thick) have been prepared by rapid solidification, using a melt-spinning technique. Their microstructure, hydrogen storage properties and thermal stability were studied by means of XRD, SEM, PCTPro2000 and DSC analysis, respectively. The results indicated that when Mg_x(LaNi₃)_100−x alloys have been hydrogenated at 573 K under 2 MPa hydrogen pressure, LaH₃ phase is formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH₄ phase formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH_0.3 phase formed in the case of x (x = 40, 50), and MgH₂ phase formed in the case of x = 70. Experimental data of hydrogen desorption kinetics, tested at 523 K, 573 K and 623 K, are in good agreement with Avrami–Erofeev equation. The maximum hydrogen absorption capacity is 2.71 wt.% for Mg₇₀(LaNi₃)₃₀ and 2.35 wt.% for Mg₇₀(LaNi₃)₃₀, the increase of hydrogen desorption capacity is in the order of x = 70 > x = 60 > x = 50 > x = 40. Based on DSC analysis, the activation energies for dehydrogenation of these samples are calculated to be 122 ± 2 kJ/mol (x = 40) > 101 ± 3 kJ/mol (x = 50) > 84 ± 5 kJ/mol (x = 60) > 64 ± 3 kJ/mol (x = 70), which are in agreement with the results of hydrogen desorption kinetics.     

The effect of additives on the hydrogen storage properties of NaAlH4

The effect of Ti, Co, Ni and LaCl₃ on hydrogen release of NaAlH₄ was studied by pressure-content-temperature (PCT) equipment. The result showed that the sample doped with 3 mol% LaCl₃ presented the largest amount of hydrogen release. Increasing the amount of LaCl₃ from 1 to 6 mol% caused such marked changes in behavior that the amount and rate of hydrogen release increased first and then decreased. In addition, the study on the rehydrogenation temperature of NaAlH₄ doped with 3 mol% LaCl₃ showed that the doped sample during the first rehydrogenation cycle carried out in PCT at 110 °C under 8 MPa after being discharged of hydrogen at 270 °C presented the largest amount of hydrogen release.