Hydrogen absorption kinetics of V4Cr4Ti alloy prepared by aluminothermy

The hydrogen absorption kinetics of V4Cr4Ti alloy, synthesized by aluminothermy process has been investigated in the temperature range of 373–773 K. The obtained hydrogen absorption kinetic curves were linearly fitted using a series of mechanism function to reveal the kinetics parameter and reaction mechanism. Nucleation and growth, one dimensional diffusion and three-dimensional diffusion processes are the intrinsic rate limiting steps of hydrogen absorption at 373 K. It was found that nucleation and growth processes disappear between 413 K–473 K. However at higher temperatures (>473 K), nucleation and growth as well as one dimensional diffusion process disappear. In the temperature ranges investigated (473 K–773 K), three-dimensional diffusion process was the intrinsic rate limiting step. The apparent activation energy was calculated using Arrhenius equation and found to be 6.1 kJ/mol. This value appears to be relatively higher which can be attributed to the presence of aluminium, which has blocked the absorption sites and increased the activation energy.     

Hydrogen solid solution thermodynamics of V1−xAlx (x: 0, 0.18, 0.37, 0.52) alloys

Thermodynamic parameters such as enthalpy and entropy of the vanadium–hydrogen solid solution are investigated as a function of aluminum content using hydrogen solubility data and the Sievert's constant. The enthalpy decreases with increase in the aluminum content. Entropy shows anomalous behavior as it first increases with the aluminum content for V_1−xAl_x (x: 0, 0.18, 0.37) but then substantially decreases for V_0.48Al_0.52. The lattice parameters and the electrical resistivity of the alloys are calculated to explain the mechanical and electronic effects on the thermodynamic parameters. It is found that the electrical resistivity of vanadium systematically decreases and the lattice constant increases with increase in aluminum content. The hardness of the alloys increases with aluminum which indicates that aluminum hardens the vanadium by simple solid solution effect. The variation of enthalpy and entropy is explained on the basis of change in Fermi energy level of the host matrix vanadium, the strong bonding nature of V–Al in the alloy and increased activity of hydrogen due to aluminum in the alloy.     

Experiment assessment of hydrogen production from activated aluminum alloys in portable generator for fuel cell applications

An experiment assessment of hydrogen production from activated aluminum alloy in portable hydrogen generator for fuel cell applications was investigated. The optimum hydrogen capacity of the high–reactive Al–Bi–NaCl alloys (the abbreviation of milled material of aluminum, bismuth and NaCl particles) is about 9–9.4 wt.%, meeting the targets (9 wt.%) of the US Department of Energy in 2015. Hydrogen production rate can be controlled via controlling the water flow rate in the generator, being 1.369–6.198 L hydrogen/min while the water flow rate ranges in 5–20 mL/min. The larger water flow rate often leads to higher temperature and results in unsafety in the generator as the hydrolysis reaction of aluminum alloy and water releases 15 kJ/g heat. However, the heat problem can be successfully eliminated by using effective cooling stytles, which enable the maximum temperature of Al–H₂O mixture (the abbreviation of hydrolysis products of aluminum alloy in water) controlled less than 474 K even though the water flow rate is 20 mL/min. Therefore, the experiment results show that the portable hydrogen generator from aluminum alloy could supply the CO₂–free, high hydrogen capacity and safe hydrogen for fuel cell applications.     

Enhancement of hydrogen generation rate in reaction of aluminum with water

The aim of this investigation is to enhance hydrogen generation rate in aluminum–water reaction by improving the activity of aluminum particles and using the heat released during the reaction. This was accomplished by developing fresh surfaces by milling aluminum particles together with salt. Salt particles not only serve as nano-millers, but also surround activated particles and prevent re-oxidation of bare surfaces in the air. Therefore, the activated powder can be easily stored for a long time. Immersing the powder in warm water, the salt covers are washed away and hydrogen begins to release at a high rate until efficiency of 100% is achieved. The rate of reaction depends crucially on initial temperature of water. Hence, the mass of water was reduced to employ released energy to increase water temperature and, consequently, to increase hydrogen production rate. The optimum value of salt-to-aluminum mole ratio for achieving high activation, air-storage capability and 100% efficiency was obtained to be 2. When immersed in water, at initial temperatures of 55 and 70 °C, the powder lead to average hydrogen generation rate of ∼101 and ∼210 ml/min per 1 g of Al, respectively. To increase the rate of corrosion, three different alloys/composites of aluminum were prepared by mechanical alloying and activated with optimum salt-to-aluminum mole ratio. The alloys/composites formed galvanic cells after being immersed in water. In the case of aluminum–bismuth alloy, the average hydrogen generation rate increased to ∼287 and ∼713 ml/min per 1 g of Al, respectively.     

Material properties and empirical rate equations for hydrogen sorption reactions in 2 LiNH2–1.1 MgH2–0.1 LiBH4–3 wt.% ZrCoH3

2 LiNH₂–1.1 MgH₂–0.1 LiBH₄–3 wt.% ZrCoH₃ is a solid state hydrogen storage material with a hydrogen storage capacity of up to 5.3 wt.%. As the material shows sufficiently high desorption rates at temperatures below 200 °C, it is used for a prototype solid state hydrogen storage tank with a hydrogen capacity of 2 kWh_el that is coupled to a high temperature proton exchange membrane fuel cell. In order to design an appropriate prototype reactor, model equations for the rate of hydrogen sorption reactions are required. Therefore in the present study, several material properties, like bulk density and thermodynamic data, are measured. Furthermore, isothermal absorption and desorption experiments are performed in a temperature and pressure range that is in the focus of the coupling system. Using experimental data, two-step model equations have been fitted for the hydrogen absorption and desorption reactions. These empirical model equations are able to capture the experimentally measured reaction rates and can be used for model validation of the design simulations.     

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg20Ni10−xCox (x = 0–4) alloys prepared by melt-spinning

In order to improve the hydriding and dehydriding performances of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂₀Ni_10−xCo_x (x = 0–4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. Thermal stability of the as-spun alloys was researched by DSC. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results showed that no amorphous phase formed in the as-spun Co-free alloy, but the as-spun alloys containing Co showed certain amount of amorphous phase. The hydrogen absorption capacities of the as-cast alloys first increase and then decrease with the variety of Co content. The hydrogen desorption capacities of as-cast and spun alloys rise with increasing Co content. The rapid quenching significantly improved the hydrogenation and dehydrogenation capacities and the kinetics of the alloys. When the quenching rate increased from 0 (as-cast was defined as spinning rate of 0 m/s) to 30 m/s, the hydrogen absorption capacity of the alloys (x = 0) at 200 °C and 1.5 MPa in 20 min rose from 1.39 to 3.12 wt%, and from 1.91 to 2.96 wt% for the alloy (x = 4). The hydrogen desorption capacity of the alloy (x = 0) in 20 min increased from 0.19 to 0.89 wt%, and from 1.39 to 2.15 wt% for the alloy (x = 4).     

Absorption and desorption of H by NbMo alloys at high temperature

Absorption and desorption of hydrogen have been investigated in Nb₉₅Mo₅ and Nb₈₀Mo₂₀ alloys over wide temperature ranges. On continuous heating H desorption from Nb₉₅Mo₅ was found to take place between 800 and 1000 K and from Nb₈₀Mo₂₀ between 900 and 1100 K. The observed increase in the desorption temperature with increasing Mo content has been attributed to a higher stability of the Mo and NbMo oxides with respect to those of Nb. The solid–gas reaction during absorption was first order and the rate limiting process consisted in the penetration of H atoms through surface oxides. At high temperatures and in the presence of H the oxides are expected to become permeable to H due to the reduction of higher valence to lower valence oxides. The values of the activation energy for H diffusion within the oxide films were 0.82 ± 0.04 eV for Nb₉₅Mo₅ and 1.1 ± 0.1 eV for Nb₈₀Mo₂₀. The thicknesses of the oxide films estimated from the absorption data were of the order of 1 μm.     

Improving the hydrogen absorption properties of commercial Mg–Zn–Zr alloy

Commercial alloy ZK60 (Mg-6 wt%Zn-0.8 wt% Zr) was used as a hydrogen-storage material to study the effect of cold rolling, ball milling, and plus graphite additives on hydrogen-storage characteristics, hydrogen absorption–desorption behavior, and the related microstructural change of the alloy. Experimental results showed that cold-rolled alloy could not be activated easily. Even after ball milling for 20 h and hydrogen absorption–desorption cycling for 10 times, no saturated hydrogen absorption was observed for cold-rolled alloy. In contrast, alloys with 5 wt% graphite additives could be easily activated after the first hydrogen absorption–desorption cycle, and a saturated hydrogen absorption of 6.9 wt% was obtained after absorption–desorption cycling for five times. A hydrogen absorption of 5.52 wt%, equivalent to 80% of the saturated absorption amount, was measured in 5 min, showing a hydrogen absorption rate of 1.104 wt%/min. The sample reached saturation in 30 min.     

Hydrogen storage properties of Ti2−xCrVMx (M = Fe,Co, Ni) alloys

In this work, quaternary alloys having compositions Ti_1.9CrVM_0.1 and Ti_1.8CrVM_0.2 (M = Fe, Co and Ni) have been studied in detail for their structural aspects and hydrogen absorption–desorption properties. All the alloys form bcc phase solid solutions and after hydrogen absorption the structures change to fcc. The pressure composition isotherms, hydrogen storage capacities and hydrogen absorption kinetics were studied using Sievert's type of volumetric setup. The Ti_1.9CrVFe_0.1, Ti_1.9CrVCo_0.1 and Ti_1.9CrVNi_0.1 alloys are found to absorb maximum 3.80, 3.68 and 3.91 wt.% of hydrogen respectively; whereas, Ti_1.8CrVCo_0.2 and Ti_1.8CrVNi_0.2 alloys show 3.52 and 3.67 wt.% of hydrogen at room temperature. All the alloys show fast hydrogen absorption kinetics at the room temperature. From differential scanning calorimetric measurements, it has been found that Fe, Ni and Co substitution in place of Ti decreased the hydrogen desorption temperature drastically compared to the parent alloy.     

Study on hydrogen storage properties of LaNi3.8Al1.2−xMnx alloys

Effects of the Mn substitution on microstructures and hydrogen absorption/desorption properties of LaNi_3.8Al_1.2−xMn_x (x = 0.2, 0.4, 0.6) hydrogen storage alloys were investigated. The pressure-composition (PC) isotherms and absorption kinetics were measured in a temperature range of 433 K ≤ T ≤ 473 K by the volumetric method. XRD analyses showed that with the increase of the Mn content in the LaNi_3.8Al_1.2−xMn_x alloys, the lattice parameter a was decreased, c increased and the unit cell volume V reduced. It was found that the absorption/desorption plateau pressure was increased and the hydrogen storage capacity was enhanced with the increase of Mn content. The absorption/desorption plateau pressure of the alloys was linearly changed with the Mn content x and the lattice parameter a, while the hydrogen storage capacity was linearly increased with the increase of c/a ratio. It was also found that the slope factor S_f was closely correlated with the lattice strain of the alloys.     

Investigation on hydrogen production using multicomponent aluminum alloys at mild conditions and its mechanism

A series of Al alloys with low melting point metals Ga, In, Sn as alloy elements were fabricated using mechanical alloying method. The phase compositions and morphologies of different Al alloys were characterized by XRD and SEM techniques. The reaction of the Al alloys with water for hydrogen evolving at mild conditions (at room temperature in neutral water) was studied. The results showed that there were no hydrogen yields for binary Al–Ga, Al–In, Al–Sn and the ternary Al–Ga–Sn alloys. The hydrogen yields were observed for Al–Ga–In and Al–In–Sn ternary alloys. The Al–In–Sn alloys showed an even faster hydrogen generation rate and higher yields than Al–Ga–In alloys. Based on the ternary Al–Ga–In and Al–In–Sn system, the hydrogen production property of quaternary Al–Ga–In–Sn was greatly improved. The hydrogen conversion efficiency of the optimized Al–3%Ga–3%In–5%Sn alloy was nearly 100% in tap water. The highest hydrogen generation rate reached 1560 mL/g min in distilled water or deionized water. It was suggested that both the embrittlement of Al by liquid Ga–In–Sn eutectic and the active points formed by intermetallic compounds In₃Sn and InSn₄ may be attributed to the high activity of Al–Ga–In–Sn alloys at room temperature.     

Hydrogen transport properties of several vanadium-based binary alloys

Vanadium-based alloys are an emerging alternative to palladium alloys for use in hydrogen-selective alloy membranes. The tendency of vanadium to embrittle, due to its high hydrogen absorption, means it lacks the robustness required for industrial hydrogen separation applications. Alloying vanadium with certain elements reduces hydrogen absorption, but also influences the diffusivity of hydrogen through the bulk material. Consequently, diffusivity and absorption data must be decoupled in order to fully evaluate the influence of various alloying additions on the hydrogen transport properties of vanadium alloys. To address this need, the hydrogen transport properties of V–Al (V₉₅Al₅, V₉₀Al₁₀, V₈₅Al₁₅, V₈₀Al₂₀, V₇₅Al₂₅, expressed as atom%) and V–Cr (V₉₅Cr₅, V₉₀Cr₁₀, V₈₅Cr₁₅) alloys have been compared through a series of absorption and flux measurements. Pd-coated alloy disks were formed from arc melted and sectioned ingots, and each alloy was subjected to a microstructural analyses and a detailed examination of hydrogen absorption and permeation properties. Additions of Al and Cr reduce the hydrogen absorption and diffusivity of vanadium, with V–Cr alloys exhibiting the greatest hydrogen diffusivity for a given hydrogen feed pressure. The diffusivity of each alloy showed strong concentration dependence. Diffusivity-concentration results have been overlayed with an isoflux curve corresponding to a target flux of 1.0 mol m⁻² s⁻¹, enabling prediction of the thickness and pressure required to achieve this target flux target for a given alloy.     

Electrochemical hydriding of Mg–Ni–Mm (Mm = mischmetal) alloys as an effective method for hydrogen storage

In this work, the electrochemical hydriding method was used for storing hydrogen in four binary Mg–Ni (Ni content from 15 to 34 wt.%) alloys and one ternary Mg–26Ni–12Mm alloy. Both the as-cast and powdered alloys were hydrided in a 6 M KOH solution at 80 °C for 120–480 min. The structures and phase compositions of the alloys, both before and after hydriding, were studied using optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Differential scanning calorimetry and mass spectrometry were used to study the dehydriding process. In the case of as-cast alloys, the best combination of hydriding parameters (maximum hydrogen concentration on surface; depth of hydrogen penetration) was achieved in the Mg–26Ni alloy. In the case of powdered alloys, the Mg–34Ni alloy absorbed the highest amount of hydrogen, nearly 4.5 wt.%. The only hydride formed during hydriding was the MgH₂ hydride. The results of the mass spectrometry analysis reveal a significant thermodynamic destabilization of magnesium hydride due to Ni and Mm. The decomposition temperature of MgH₂ was reduced by more than 200 °C. The results are discussed in relation to the electronic structure and atomic size of the alloying elements and the structural variations in the alloys.     

Study of hydrogen production and storage based on aluminum–water reaction

The rate and yield of hydrogen production from the reaction between activated aluminum and water has been investigated. The effect of different parameters such as water–aluminum ratio, water temperature and aluminum particle size and shape was studied experimentally. The aluminum activation method developed in-house involves 1%–2.5% of lithium-based activator which is diffused into the aluminum particles, enabling sustained reaction with tap water or sea water at room temperature. Hydrogen production rates in the range of 200–600 ml/min/g Al, at a yield of about 90%, depending on operating parameters, were demonstrated. The work further studied the application in proton exchange membrane (PEM) fuel cells in order to generate green electric energy, demonstrating theoretical specific electric energy storage that can exceed batteries by 10–20 folds.     

Hydrogen storage in rapidly solidified and crystallized Mg–Ni-(Y,La)–Pd alloys

Amorphous-crystalline composite ribbons of quaternary Mg–Ni–(Y,La)–Pd alloys are produced via rapidly solidification and used as precursors for creating nanocrystalline hydrogen storage materials. The resulting materials demonstrate relatively high hydrogen capacity of around 4.5 mass% H and excellent absorption/desorption kinetics at 573 K. Additionally, the alloys demonstrate reversible hydrogen storage at 473 K. A composition of Mg₈₅Ni₁₀Y_2.5Pd_2.5 fully absorbs and desorbs 4.6 mass% H in 90 min. The cyclability of the quaternary alloys demonstrates good stability, with little loss in maximum capacity through 8–10 cycles. This has been attributed to the improved stability of the nanocrystalline structure attained via the Y and La additions. Thermodynamically, the enthalpy of the hydrogen absorption reaction is reduced by 5 kJ/mol in the quaternary alloys, compared to Mg-MgH₂; while the entropy of reaction is also reduced.     

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V₄₈Fe₁₂Ti_15-xCr₂₅Al_x (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.     

Development of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption pressures for hybrid hydrogen storage vessel application

Three series of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption plateau pressures for hybrid hydrogen storage vessel application were prepared by induction levitation melting, as well as their crystallographic characteristics and hydrogen storage properties were investigated. The results show that all of the alloys were determined as a single phase of C14-type Laves structure. As the Fe content in the TiCr_1.9−xMn_0.1Fe_x (x = 0.4–0.6) alloys increases, the hydrogen absorption and desorption plateau pressures increase, and the hydrogen storage capacity and plateau slope factor decrease respectively. The same trends are observed when increasing the Mn content in the TiCr_1.4−yMn_yFe_0.6 (y = 0.1–0.3) alloys, except for the plateau slope factor. Compared with the stoichiometric TiCr_1.1Mn_0.3Fe_0.6 alloy, the titanium super-stoichiometric Ti_1+zCr_1.1Mn_0.3Fe_0.6 (z = 0.02, 0.04) alloys have larger hydrogen storage capacities and lower hydrogen desorption plateau pressures. Among the studied alloys, Ti_1.02Cr_1.1Mn_0.3Fe_0.6 has the best overall properties for hybrid hydrogen storage application. Its hydrogen desorption pressure at 318 K is 41.28 MPa, its hydrogen storage capacity is 1.78 wt.% and its dissociation enthalpy (ΔH_d) is 16.24 kJ/mol H₂.     

Numerical modeling of hydrogen absorption measurements in Ni-catalyzed Mg

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this work, hydrogen absorption in Ni-catalyzed magnesium was measured in a wide range of pressure (500 kPa–5000 kPa) and temperature (498 K–573 K). Using this information, a model for the absorption kinetics and thermal behavior of the hydrogen storage system was proposed. This model could be used in the design of Ni-catalyzed magnesium storage tanks and other applications. It considers the independent contribution of three variables: temperature, pressure and reacted fraction to estimate the hydrogen absorption rate. An activation energy for the process was estimated and the value obtained (92 kJ/mol) was concordant with previous values reported in the literature.     

Substitutional V–Pd alloys for the membranes permeable to hydrogen: Hydrogen solubility at 150–400 °C

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V-xPd alloys (x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–10⁶) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10⁻⁴ to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys.