Search Results (17)

In order to improve the hydrogen storage properties of Laves phase AB₂-type alloys, a series of Ti_1−xZr_xMn_1.0Cr_0.85Fe_0.1 (x = 0.1–0.5) alloys were prepared by arc melting. The effects of Zr content on microstructure and hydrogen storage properties was investigated in detail. Crystal structure characterizations confirmed that all the alloys exhibit a single-phase C14 Laves structure, and the lattice parameters increase with increasing Zr content. The hydrogen storage measurements of the alloys indicate that with increasing Zr content, the hydrogen storage capacity initially increases and then decreases. The hydrogen absorption and desorption measurements of the alloys were performed by a Sieverts-type apparatus. Pressure–composition–temperature (P-C-T) tests at various temperatures showed that all the alloys display sloped plateaus. Increasing Zr content results in a gradual decrease in hydrogen absorption and desorption plateau pressures. Moreover, these alloys exhibit varying degrees of hysteresis, which also becomes more pronounced with a rise in Zr content. In summary, the Ti_0.7Zr_0.3Mn_1.0Cr_0.85Fe_0.1 alloy demonstrates the best comprehensive hydrogen storage capacity. Further investigation on the cyclic performance of the Ti_0.7Zr_0.3Mn_1.0Cr_0.85Fe_0.1 alloy was conducted. It was found that the alloy particles undergo significant pulverization after hydrogenation cycles, but the alloy maintained good phase structure stability and hydrogen storage performance. Full article

High-energy ball milling for different durations was used to synthesize nanocrystalline Mg₆₀Ni₂₅Cu₁₀Ce₅ powders. The morphology and microstructure of the milled powders were investigated by scanning electron microscopy and X-ray diffraction, respectively. It was found that different milling times result in considerably different phase composition. The powder milled for 1 h is characterized by elemental Mg, Ni, Cu and Ce with some minor content of intermetallics. In total, 3 h milling promotes the intensive formation of intermetallic compounds, while 10 h of powder processing results in a partially amorphous state coupled with compound phases. Isothermal hydrogenation and dehydrogenation experiments were conducted in a Sieverts’-type apparatus. It was found that all powders absorb H₂ reversibly, while the shortest milling time provides the best overall capacity. Excellent kinetics without any activation cycle were obtained for the 3 h milled composite, releasing and absorbing 50% of the total hydrogen content within 120 s. Each kinetic measurement has satisfactorily been fitted by the Johnson–Mehl–Avrami function. X-ray diffraction analysis on the dehydrided powders confirmed the complete desorption. Full article

Classical kinetic models describing the hydrogen absorption of nanocrystalline metallic hydrides generally do not involve any parameter related to the change in the crystallite size during the hydrogenation at constant temperature. In the present investigation, ball-milled nanocrystalline Mg powders exhibiting lognormal crystallite size distribution have been subjected to hydrogen absorption in a Sievert-type apparatus. Partially absorbed states were achieved by interrupting the hydrogenation cycle at different hydrogen content, i.e., when 15%, 50%, and 90% of Mg powder transformed to MgH₂. The evolution of the characteristic size of the nucleating MgH₂ phase was determined from X-ray diffraction analysis. Considering the crystallite size distribution of the as-milled powder agglomerate as well as the growth during the isothermal hydrogenation process, a time-dependent multi-particle reaction function $\overline{{\propto }_{CV}}\left(t;R\left(t\right)\right)$ was developed. It was shown unambiguously for this case study that the measured hydrogen absorption curve of the ball-milled Mg powder shows the best correlation with this model when it is compared to classical kinetic functions or the previously developed multi-particle reaction function excluding the change in the average crystallite size during hydrogenation. Full article

Milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, MgH₂-50NaAlH₄, and MgH₂-2Ni-10NaAlH₄ samples were prepared by milling in a planetary ball mill in hydrogen atmosphere (reactive mechanical milling, RMM). Decomposition temperatures of milled MgH₂, NaAlH₄, MgH₂-10NaAlH₄, and MgH₂-30NaAlH₄ were examined in a Sieverts-type hydrogen absorption and release apparatus, in which the hydrogen pressures were kept nearly constant during hydrogen absorption or release. As the content of NaAlH₄ in the sample increased, the temperature at the highest peak in the ratio of increase in released hydrogen quantity to increase in temperature versus temperature curve decreased. Hydriding in 12 bar hydrogen and dehydriding in 1.0 bar hydrogen at 593 K of MgH₂-30NaAlH₄ are performed by the reversible reactions MgH₂ ⇔ Mg + H₂ and 17MgH₂ + 12Al ⇔ Mg₁₇Al₁₂ + 17H₂. MgH₂-30NaAlH₄ was the best Mg-based composite among Mg-based alloys in which an oxide, a halide, a fluoride, or a complex hydride was added, with a high hydrogen absorption rate for 2.5 min (2.20 wt% H/min) and a large effective hydrogen storage capacity (7.42 wt% H). Full article

High-energy ball milling was applied to produce nanocrystalline MgH₂-FeTi powder composites. In order to achieve a remarkable synergetic effect between the two materials, the amount of the FeTi catalyst was chosen to be 40 wt.%, 50 wt.% and 60 wt.%. The morphology and microstructure of the as-milled powders were characterized by scanning electron microscopy and X-ray diffraction, respectively. The evaluation of the diffraction profiles by the Convolutional Multiple Whole Profile fitting algorithm provided a detailed microstructural characterization of the coherently scattering α-MgH₂ crystallites. Differential scanning calorimetry experiments revealed two overlapping endotherms corresponding to the dehydrogenation of metastable γ-MgH₂ and stable α-MgH₂ hydrides. Isothermal hydrogen-sorption experiments were carried out in a Sieverts-type apparatus. It was established that the MgH₂-40 wt.% FeTi powder is capable of absorbing 5.8 wt.% hydrogen, while extraordinary absorption kinetics were observed for the MgH₂-50 wt.% FeTi alloy, i.e., 3.3 wt.% H₂ is absorbed after 100 s. Full article

The main key to the future transition to a hydrogen economy society is the development of hydrogen production and storage methods. Hydrogen energy is the energy produced via the reaction of hydrogen with oxygen, producing only water as a by-product. Hydrogen energy is considered one of the potential substitutes to overcome the growing global energy demand and global warming. A new study on CH₄ conversion into hydrogen and hydrogen storage was performed using a magnesium-based alloy. MgH₂-12Ni (with the composition of 88 wt% MgH₂ + 12 wt% Ni) was prepared in a planetary ball mill by milling in a hydrogen atmosphere (reaction-involved milling). X-ray diffraction (XRD) analysis was performed on samples after reaction-involved milling and after reactions with CH₄. The variation of adsorbed or desorbed gas over time was measured using a Sieverts’-type high-pressure apparatus. The microstructure of the powders was observed using a scanning transmission microscope (STEM) with energy-dispersive X-ray spectroscopy (EDS). The synthesized samples were also characterized using Fourier transform infrared (FT-IR) spectroscopy. The XRD pattern of MgH₂-12Ni after the reaction with CH₄ (12 bar pressure) at 773 K and decomposition under 1.0 bar at 773 K exhibited MgH₂ and Mg₂NiH₄ phases. This shows that CH₄ conversion took place, the hydrogen produced after CH₄ conversion was then adsorbed onto the particles, and hydrides were formed during cooling to room temperature. Ni and Mg₂Ni formed during heating to 773 K are believed to cause catalytic effects in CH₄ conversion. The remaining CH₄ after conversion is pumped out at room temperature. Full article

A high-energy ball-milling method was applied for different milling times (1 h, 3 h, and 10 h) to synthetize nanocrystalline MgH₂ powder samples catalyzed by Fe₂Ti. Morphology and microstructure of the powders were characterized by scanning electron microscopy and X-ray diffraction. The recorded diffraction profiles were evaluated by the convolutional multiple whole profile fitting algorithm, in order to determine microstructural parameters of the composites, such as average crystallite size and average dislocation density. Differential scanning calorimetry was performed to investigate the dehydrogenation characteristics of the alloys. It was obtained that there exists an optimal milling time (3 h) when desorption occurs at the lowest temperature. X-ray diffraction of partially dehydrided states confirmed a two-step H-release, including the subsequent desorption of γ-MgH₂ and α-MgH₂. The effect of milling time on the H-sorption performance was investigated in a Sievert-type apparatus. The best overall hydrogenation performance was obtained for the composite milled for 3 h. Full article

Thermal analysis methods have been used in many reports to determine the activation energy for hydride decomposition (dehydrogenation). In our preceding work, we showed that the dehydrogenation rate of Mg-5Ni samples obeyed the first-order law, and the Kissinger equation could thus be used to determine the activation energy. In the present work, we obtained the activation energy for dehydrogenation by applying data from a volumetric method to the Kissinger equation. The quantity of hydrogen released from hydrogenated Mg-5Ni samples and the temperature of the reactor were measured as a function of time at different heating rates (Φ) in a Sieverts-type volumetric apparatus. The values of dH_d/dt, the dehydrogenation rate, were calculated as time elapsed and the temperature (T_m) with the highest dH_d/dt was obtained. The values of dH_d/dT, the rate of increase in released hydrogen quantity (H_d) to temperature (T) increase, were calculated according to time, and the temperature (T_m) with the highest dH_d/dT was also obtained. In addition, the values of dT/dt, the rate of increase in temperature to time (t) increase, were calculated according to time, and the temperature (T_m) with the highest dH_d/dt was obtained. Φ and T_m were then applied to the Kissinger equation to determine the activation energy for dehydrogenation of Mg-5Ni samples. Full article

In the current study, Twin Parallel Channel Angular Extrusion (TPCAE) as a developed SPD processing technique is used to improve the hydrogen storage properties of AZ91 cast alloy. The processing is conducted at different temperatures, ranging from 340 °C down to 200 °C. The hydrogen absorption and desorption tests are conducted kinetically at three different temperatures, using a Sievert-type apparatus. Remarkable improvement in the absorption kinetic is achieved as a result of the TPCAE processing. A maximum absorption capacity of 6.1 wt.% within a time span of 2000 s is achieved for the sample with three passes of processing complemented at 250 °C. Also, the kinetic of dehydrogenation is improved significantly and complete desorption at 350 °C is achieved for all the processed samples within a time span of maximum 2500 s. By calculating the activation energy of hydrogenation and evaluating the microstructure changes, it is found that implementing sufficient thermomechanical work level along with applying the last pass of the process at lower temperature results in a reduction of the activation energy and improvement of the hydrogenation kinetic. Full article

We developed a method to determine the activation energy for hydride decomposition using a Sieverts-type apparatus and the Kissinger equation, not using thermal analysis methods. The quantity of hydrogen released from the sample and the temperature of the reactor were first measured as a function of time at different heating rates (Φ) in a Sieverts-type apparatus. The dehydriding rates were calculated according to time and the temperature T_m (at which the dehydriding rate was the highest). Φ and T_m were then applied to the Kissinger equation. The dehydriding rate of Mg-5Ni samples obeyed a first-order law, and the Kissinger equation could thus be used to determine the activation energy. On a heating rate of 3 K/min, the decomposition rate of hydride was the highest at 590.0 K. From a plot of ln (Φ/T_m²) versus 1/T_m, the obtained activation energy for hydride decomposition was 174 kJ/mole. Full article

The deterioration of hydrogen charging performances resulting from the surface chemical action of electrophilic gases such as CO₂ is one of the prevailing drawbacks of TiMn_1.52 materials. In this study, we report the effect of autocatalytic Pd deposition on the morphology, structure, and hydrogenation kinetics of TiMn_1.52 alloy. Both the uncoated and Pd-coated materials were characterized using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). XRD analyses indicated that TiMn_1.52 alloy contains C14-type Laves phase without any second phase, while the SEM images, together with a particle size distribution histogram, showed a smooth non-porous surface with irregular-shaped particles ranging in size from 1 to 8 µm. The XRD pattern of Pd-coated alloy revealed that C14-type Laves phase was still maintained upon Pd deposition. This was further supported by calculated crystallite size of 29 nm for both materials. Furthermore, a Sieverts-type apparatus was used to study the kinetics of the alloys after pre-exposure to air and upon vacuum heating at 300 °C. The Pd-coated AB₂ alloy exhibited good coating quality as confirmed by EDS with enhanced hydrogen absorption kinetics, even without activation. This is attributed to improved surface tolerance and a hydrogen spillover mechanism, facilitated by Pd nanoparticles. Vacuum heating at 300 °C resulted in removal of surface barriers and showed improved hydrogen absorption performances for both coated and uncoated alloys. Full article

Energy storage is one of the main challenges to address in the near future—in particular due to the intermittent energy produced by extensive renewable energy production plants. The use of hydrides for this type of energy storage has many positive aspects. Hydride-based systems consist of absorption and desorption reactions that are strongly exothermic and endothermic, respectively. Heat management in the design of hydrogen storage tanks is an important issue, in order to ensure high-level performance in terms of the kinetics for hydrogen release/uptake and reasonable storage capacity. When loose powder is used, material in the form of pellets should be considered in order to avoid detrimental effects including decreased cycling performance. Moreover, sustainable materials in large-scale hydrogen reactors could be recovered and reused to improve any life cycle analysis of such systems. For these reasons, magnesium hydride was used in this study, as it is particularly suitable for hydrogen storage due to its high H₂ storage capacity, reversibility and the low costs. Magnesium hydride was ball-milled in presence of 5 wt % Fe as a catalyst, then compacted with an uniaxial press after the addition of expanded natural graphite (ENG). The materials underwent 45 cycles in a Sievert’s type apparatus at 310 °C and eight bar, in order to study the kinetics and cycling stability. Scanning electron microscopy was used to investigate microstructural properties and failure phenomena. Together with Rietveld analysis, X-ray diffraction was performed for phase identification and structural information. The pellets demonstrated suitable cycling stability in terms of total hydrogen storage capacity and kinetics. Full article

Pinecones, a common biomass waste, has an interesting composition in terms of cellulose and lignine content that makes them excellent precursors in various activated carbon production processes. The synthesized, nanostructured, activated carbon materials show textural properties, a high specific surface area, and a large volume of micropores, which are all features that make them suitable for various applications ranging from the purification of water to energy storage. Amongst them, a very interesting application is hydrogen storage. For this purpose, activated carbon from pinecones were prepared using chemical activation with different KOH/precursor ratios, and their hydrogen adsorption capacity was evaluated at liquid nitrogen temperatures (77 K) at pressures of up to 80 bar using a Sievert’s type volumetric apparatus. Regarding the comprehensive characterization of the samples’ textural properties, the measurement of the surface area was carried out using the Brunauer–Emmett–Teller method, the chemical composition was investigated using wavelength-dispersive spectrometry, and the topography and long-range order was estimated using scanning electron microscopy and X-ray diffraction, respectively. The hydrogen adsorption properties of the activated carbon samples were measured and then fitted using the Langmuir/ Töth isotherm model to estimate the adsorption capacity at higher pressures. The results showed that chemical activation induced the formation of an optimal pore size distribution for hydrogen adsorption centered at about 0.5 nm and the proportion of micropore volume was higher than 50%, which resulted in an adsorption capacity of 5.5 wt% at 77 K and 80 bar; this was an increase of as much as 150% relative to the one predicted by the Chahine rule. Full article

A high-energy ball milling and subsequent high-pressure torsion method was applied to synthesize nanocrystalline magnesium samples catalyzed by TiO₂ or titanate nanotubes. The microstructure of the as-milled powders and the torqued bulk disks was characterized by X-ray diffraction. The recorded diffractograms have been evaluated by the convolutional multiple whole profile fitting algorithm, which provided microstructural parameters (average crystal size, crystallite size distribution, average dislocation density). The morphology of the nanotube-containing disks has been examined by high-resolution transmission electron microscopy. The effect of the different additives and preparation conditions on the hydrogen absorption behavior was investigated in a Sieverts’-type apparatus. It was found that the ball-milling route has a prominent effect on the dispersion and morphology of the titanate nanotubes, and the absorption capability of the Mg-based composite is highly dependent on these features. Full article

A detailed analysis of the dehydrogenation mechanism of LiBH₄/xLiAlH₄ (x = 0.5, 1, 2) composites was performed by thermogravimetry (TG), differential scanning calorimetry (DSC), mass spectral analysis (MS), powder X-ray diffraction (XRD) and scanning electronic microscopy (SEM), along with kinetic investigations using a Sievert-type apparatus. The results show that the dehydrogenation pathway of LiBH₄/xLiAlH₄ had a four-step character. The experimental dehydrogenation amount did not reach the theoretical expectations, because the products such as AlB₂ and LiAl formed a passivation layer on the surface of Al and the dehydrogenation reactions associated with Al could not be sufficiently carried out. Kinetic investigations discovered a nonlinear relationship between the activation energy (E_a) of dehydrogenation reactions associated with Al and the ratio x, indicating that the E_a was determined both by the concentration of Al produced by the decomposition of LiAlH₄ and the amount of free surface of it. Therefore, the amount of effective contact surface of Al is the rate-determining factor for the overall dehydrogenation of the LiBH₄/xLiAlH₄ composites. Full article