Hydrogen absorption and its influence on the thermal stability of Zr65Al10Ni10Cu15 amorphous alloy

The hydrogen absorption properties of Zr₆₅Al₁₀Ni₁₀Cu₁₅ amorphous alloy with a wide supercooled liquid region were evaluated using a Sieverts-type apparatus. The amorphous alloy absorbs 0.34, 0.80 and 0.85 wt.% hydrogen within 10, 6 and 5 min at 373, 473 and 523 K, respectively. According to Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, the hydrogen absorption activation energy of the amorphous alloy was 1.27 kJ mol⁻¹. The pressure–composition (P–C) isotherms of the amorphous Zr₆₅Al₁₀Ni₁₀Cu₁₅ alloy at 573, 623 and 673 K did not show a plateau, and the hydrogen absorption capacities were 0.8, 1.3 and 1.7 wt.%, respectively. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis demonstrated that the thermal stability of the amorphous alloy was improved with an enlarged supercooled liquid region after the hydrogen uptake below 473 K, but was decreased after the hydrogenation above 523 K. The alloy still kept the amorphous structure after hydrogenation at 573 K, and transformed into the crystalline phases of ZrH₂, ZrNi and AlCu after the hydrogenation at 673 K.     

Hydrogen storage of melt-spun amorphous Mg65Ni20Cu5Y10 alloy deformed by high-pressure torsion

Rapidly quenched amorphous Mg₆₅Ni₂₀Cu₅Y₁₀ metallic glass compacts were subjected to heavy shear deformation by high-pressure torsion until different amounts of ultimate strain. High-resolution X-ray diffraction analysis and scanning electron microscopy revealed that high-pressure torsion resulted in a deformation dependent microstructure. High-pressure calorimetry measurements revealed that hydrogen uptake in the fully amorphous alloy occurs at a significantly lower temperature compared to the fully crystallized state, while the amount of absorbed hydrogen increased considerably after heavy shear deformation due to the formation of Mg₂Ni crystals.     

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.     

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 desorption performance of high-energy ball milled Mg2NiH4 catalyzed by multi-walled carbon nanotubes coupling with TiF3

The Mg₂NiH₄ complex hydrides were synthesized by high-energy ball milling (HEBM) MgH₂ + Ni mixtures. Multi-walled carbon nanotubes (MWCNTs) or TiF₃ as catalysts were added and the catalytic-dehydrogenation behaviors were investigated. All prepared samples are characterized by X-ray diffraction (XRD) spectroscopy, scanning electron microscope (SEM) and differential scanning calorimetry (DSC) to acquire information of microstructure, phase compositions, surface and dehydrogenation properties. The results indicate that the method of adding catalysts by HEBM is reasonable and the hydrogen desorption property of Mg₂NiH₄ is improved by catalysts. It is worth noting that the dehydrogenation temperature (T_D) and the activation energy (E_a) of Mg₂NiH₄ catalyzed by MWCNTs coupling with TiF₃ are reduced to 230 °C (243.6 °C of Mg₂NiH₄) and 53.24 kJ/mol (90.13 kJ/mol of Mg₂NiH₄), respectively. The addition of proper catalysts is proved to be an effective strategy to decrease T_D and E_a of Mg₂NiH₄ hydrides.     

A comparative study on the microstructure and cycling stability of the amorphous and nanocrystallization Mg60Ni20La10 alloys

Amorphous and nanocrystalline Mg₆₀Ni₂₀La₁₀ alloys were prepared by melt-spun and crystallization of the amorphous alloy respectively. Microstructural evolution of the amorphous and crystallized (CA) alloys during hydrogenation/dehydrogenation cycles was studied and compared in the present work. The CA alloy exhibits homogeneous and fine (<50 nm) multiphase microstructure composed of LaMg₂Ni, Mg₂Ni and LaMgNi₄. The CA alloy has slightly lower hydrogenation ability but far excellent cycling stability compared with the amorphous alloy. The amorphous and CA alloys have identical phase constitution including Mg₂Ni and LaH₃ after cycling. While, microstructures of the two cycled alloys show dramatically distinct characters. Grain size of the cycled CA alloy is almost unchanged compared with the original alloy, which contributes to the better cycling stability. However, grain growth especially coarsening of Mg₂Ni is severe in the cycled amorphous alloy, leading to difficulty to dehydrogenation. The better coarsening resistance of the CA alloy is attributed to the crisscrossed distribution of Mg₂Ni and LaH₃ and well-matched interfacial configuration between Mg₂Ni and LaH₃ that (113)Mg₂Ni ??? ? (111)LaH₃. However, hydrogenation of the amorphous alloy leads to large and inhomogeneous microstructure which is ascribed to the preferential recrystallization and growth of Mg₂Ni, contributing to rapid degradation of the amorphous alloy. The present work illumines a potential way to prepare stable nanocrystalline by introducing secondary phase with interlaced microstructure and well-matched interfacial configuration using nanocrystallization method.     

Hydrogen absorption processes in Mg2Ni-based systems: Thermal and mechanochemical kinetics

Mg₂Ni/Ni and LaMg₂Ni alloy powders were exposed to hydrogen under isothermal and mechanical treatment conditions. In the former case, the amount of hydrogen absorbed tends with time to a final asymptotic value. Once such a value has been reached, further hydrogen absorption can be obtained only by submitting the powders to mechanical processing in the presence of hydrogen. Hydrogen absorption processes under isothermal and mechanical treatment conditions exhibited different kinetics and their rates have been compared on a phenomenological basis starting from kinetic evidences. It appeared that mechanical treatment enhances the rate of hydrogen absorption by four orders of magnitude as a consequence of a mix of surface area enlargement, temperature rise and local structural excitation processes.     

Hydrogen entry and absorption in ZrO2 coated iron studied by electrochemical permeation and desorption techniques

One (entry) side of iron membranes was coated with ZrO₂ by sol-gel method. Hydrogen permeation through and hydrogen desorption from the uncoated and ZrO₂ coated iron membranes were studied using the electrochemical detection of hydrogen. The coated and uncoated membranes were charged with hydrogen cathodically generated from 0.1 M NaOH under constant current. During the initial period, the effect of the ZrO₂ coating was insignificant. However, the coating quite efficiently prevented the iron surface become more active to hydrogen entry during a long-lasting charging. After cessation of hydrogen charging, the hydrogen desorption was measured at both sides of the membranes. The analysis of the hydrogen desorption rates enabled the determination of the total amounts of hydrogen and distinction its different forms: the diffusible and reversibly trapped hydrogen.     

Hydrogen storage properties of TiMn1.5V0.2-based alloys for application to fuel cell system

To meet the requirements of fuel cell power system for electric bike, the influence of partial substitution of Zr and Cr on hydrogen storage performance of TiMn_1.5V_0.2-based alloys is investigated first, and a hydrogen storage tank is then built using the developed TiMn_1.5V_0.2-based alloy as metal hydride bed and its hydrogen supply ability is further evaluated. It is found that for TiMn_1.5V_0.2-based alloys, the Zr substitution for Ti effectively reduces the plateau pressure but increases the plateau slope, while the partial substitution of Mn by Cr decreases the absorption plateau pressure, leading to a smaller hysteresis factor. After the optimization of components, 6 kg of Ti_0.95Zr_0.05Mn_1.4Cr_0.1V_0.2 alloy powder with 5 wt.% aluminum foam is mixed uniformly to form a metal hydride bed inside the tank. The measurements show that the tank releases up to 82 g of hydrogen to produce a 200 W fuel cell output for 300 min and has a stable cyclic capacity, indicating that hydrogen storage system of TiMn_1.5V_0.2-based alloys for fuel cell power system of electric bike is applicable.     

Effects of hydrogen on the nanomechanical properties of a bulk metallic glass during nanoindentation

In this reported work, the effects of hydrogen on the nanomechanical properties of a Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glass were investigated. The experimental results demonstrated that the nanohardness of the subject material was significantly reduced as the hydrogen content in glass increased, which was caused by the induced softening from the presence of hydrogen as observed by a decrease in the elastic modulus of the glass. The flow serration of the load-displacement on the glass during nanoindentation gradually became smooth when the hydrogen content was high, which was similar to the nanoindentation loading rate effect. The transition in the flow serration was a distinct physical phenomenon, suggesting a change of in the character of plasticity. A single shear band couldn't accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously. The electronic structure of the hydrogenated Zr₅₅Cu₃₀Ni₅Al₁₀ were measured by X-Ray photoemission spectroscopy (XPS). In comparison with their quaternary counterparts, the XPS spectra of the hydrogenated samples were characterized by a shift of the Zr and Al band to lower binding energies. These suggested that the presence of solute hydrogen atoms resulted in the occurrence of the valence electron transferring from the Zr 3d band to the ZrH bonding state, which would weaken the surrounding metallic glass.     

Effects of SnO2 on hydrogen desorption of MgH2

The hydrogen desorption properties of Magnesium Hydride (MgH₂) ball milled with cassiterite (SnO₂) have been investigated by X-ray powder diffraction and thermal analysis. Milling of pure MgH₂ leads to a reduction of the desorption temperature (up to 60 K) and of the activation energy, but also to a reduction of the quantity of desorbed hydrogen, referred to the total MgH₂ present, from 7.8 down to 4.4 wt%. SnO₂ addition preserves the beneficial effects of grinding on the desorption kinetics and limits the decrease of desorbed hydrogen. Best tradeoff – activation energy lowered from 175 to 148 kJ/mol and desorbed hydrogen, referred to the total MgH₂ present, lowered from 7.8 to 6.8 wt% – was obtained by co-milling MgH₂ with 20 wt% SnO₂.     

Hydrogen storage properties and microstructure of melt-spun Mg90Ni8RE2 (RE = Y, Nd, Gd)

For hydrogen storage applications a nanocrystalline Mg₉₀Ni₈RE₂ alloy (RE = Y, Nd, Gd) was produced by melt spinning. The microstructure in the as-cast, melt-spun and hydrogenated state was characterized by X-ray diffraction and electron microscopy. Its activation, hydrogenation/dehydrogenation properties and cycle stability were examined by thermogravimetry in the temperature range from 50 °C to 385 °C and pressures up to 30 bar H₂. It was found that the activated alloy can reach a reversible gravimetric hydrogen storage density of up to 5.6 wt.%-H. Furthermore, the reversible gravimetric hydrogen storage density increases with the number of hydrogenation/dehydrogenation cycles, while the dehydrogenation rate remained unchanged. This observation was attributed to the increase of the specific surface area of the ribbon due to cracking during repeated cycling. However, the microstructure of the hydrogenated alloy remained nanocrystalline throughout cycling.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Hydrogen sorption properties of V85Ni15

An alloy with nominal composition V₈₅Ni₁₅ was prepared in a vacuum induction melting apparatus. The chemical analysis of the sample showed a V (Ni) content of 82.63 (15.68) at% and small contaminations by carbon and aluminum. XRD measurements confirmed that the bcc Ni-V solid solution phase is the main constituent of the sample. However, a minor impurity (approximately 6.5 wt%) due to the precipitation of the sigma NiV_3-x phase is present in the sample.Hydrogen sorption measurements extending from 150 °C up to 400 °C in a wide pressure range (up to 90 bar) were performed. Upon hydrogenation below 250 °C, one observes the subsequent presence of an α phase and of two hydride phases with different values of dehydrogenation enthalpy: ΔH_dehydr = 21 ± 1 kJ/mol for 0.15 ≤ H/M ≤ 0.30 and ΔH_dehydr = 26 ± 2 kJ/mol for 0.53 ≤ H/M ≤ 0.59. Above 300 °C, there is no evidence of the formation of hydrides and only an α phase is present up to the maximum measured composition, with a hydrogenation enthalpy ΔH_hydr = 10.0 ± 0.5 kJ/mol.     

Hydrogen storage properties and thermal stability of V35Ti20Cr45 alloy by heat treatment

The crystal structure, microstructure, hydrogen storage properties and thermal stability of the as-cast and annealed V₃₅Ti₂₀Cr₄₅ alloys prepared by arc-melting were studied in this work. It was confirmed that the as-cast alloy is a body-centered cubic (bcc) single phase, while it consists of bcc main phase and C14-typed Laves secondary phase after annealed at 973 K for 72 h. As a result of the microstructure change, the activation performance and kinetic properties of the annealed alloy are improved greatly due to the catalysis of C14-typed Laves secondary phase in the annealed alloy. The kinetic mechanism of hydrogen absorption/desorption processes in the as-cast and annealed alloys was discussed using the Johnson-Mehl-Avrami (JMA) equation. Based on the plateau pressure data from pressure-composition-temperature (PCT) measurements with the Van't Hoff equation, the calculated formation enthalpies of the hydride formed in the as-cast and annealed alloys indicate that heat treatment results in lower thermal stability of the hydride in the as-cast alloy. Furthermore, using the Kissinger method with the peak temperatures from differential scanning calorimeter (DSC) measurements at different heating rates, the calculated activation energies of the dehydrogenation in the as-cast and annealed alloys suggest that heat treatment is very beneficial to improve hydrogen absorption/desorption capacities in the alloy.     

New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF₃. A nanocomposite of Mg_0.95In_0.05-5 wt% CeF₃ was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg_0.95In_0.05 solid solution is changed by the addition of CeF₃. Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite transforms to MgH₂, MgF₂ and intermetallic compounds of MgIn and CeIn₃ by hydrogenation. Upon dehydrogenation, MgH₂ reacts with the intermetallic compounds of MgIn and CeIn₃ forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn₃ impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol⁻¹⋅H₂, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol⁻¹⋅H₂, a reduction of ～73 kJ⋅mol⁻¹⋅H₂ relative to that for Mg_0.95In_0.05 solid solution. As a result, Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite can release ～57% H₂ in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.