Papers by Keyword: Hydrogen Storage

Abstract: The most attractive way to storage hydrogen safely and economically is in metal hydrides. In particular, magnesium has attracted much interest since their hydrogen capacity exceeds that of known metal hydrides. One of the approaches to improve the kinetic is addition of metal oxide. In this paper, we tried to improve the hydrogen absorption properties of Mg. The effect of transition oxides, such as Nb2O5 on the kinetics of the Mg hydrogen absorption reaction was investigated. MgHx-Nb2O5 composites have been synthesized by hydrogen induced mechanical alloying. The powder synthesized was characterized by XRD, SEM, EDX, BET and simultaneous TG/DSC analysis. The hydrogenation behaviors were evaluated by using an automatic Sievert’s type PCT apparatus. Absorption/desorption kinetics and PCI of MgHx catalyzed with 5wt.%Nb2O5(as-received), 5wt.%Nb2O5(30min. milled) are determined at 423, 473, 523, 573, 623K.

Abstract: The diffusion of hydrogen has been investigated in the AB2 Laves phase Compounds TiCr1.78 and TiCr1.4Mn0.4, by absorption techniques. It has been found that H at temperatures higher than 700 K diffuses through the classical over-barrier mechanism, while at low temperature (around 100 K) the diffusion is governed by phonon-assisted tunnelling. The activation energy for classical hopping is rather high and increases with the substitution of Mn for Cr. In the range of H contents nH investigated (nH=H/Me0.03) the chemical diffusion coefficient does not depend on the H concentration.

Abstract: Powders of elemental Mg, Zn, Al and Ag were milled in order to produce nanocrystalline alloys with nominal composition Mg98M2 (M=Zn, Al and Ag). Pure Mg was also mechanically milled without any additions. Single-phase nanocrystalline (crystal size 24-26 nm) Mg98M2 alloys were produced after 216 ks of milling. A passivity procedure was followed immediately after milling, by gradually exposing the alloy powders to air (~ 12 hrs). After this procedure, the mechanically alloyed powders were kept under argon atmosphere before being hydrided at 200 and 300 °C under 0.5 and 3 MPa P for 10 min. Previously milled (~ 1.5 years before) and passivated powder alloys (stored in air and referred to as “AE” samples) were also hydrided under the same conditions. No hydriding was observed in the as-received Mg powders (crystal size >> 100 nm), but the as-milled, passivated nanocrystalline alloys were partially hydrided (even the AE samples). The amounts of the MgH2 phase in the hydrided samples were larger in the Ar-stored than in the AE samples under all hydriding conditions. The possible role of MgO and Mg hydroxides, as well as of the alloying elements, on the hydriding behavior of the nanostructured, mechanically alloyed powder alloys is discussed.

Abstract: Ammonia borane(BH3NH3) is a promising hydrogen storage material because of its high gravimetric (19.6 wt% H2) and volumetric hydrogen density with an accompanying moderate decomposition temperature. Previously reported structures determined by using x-ray and neutron diffraction on hydrides show differences in bond lengths and atomic coordination. Here, the crystal structures of fully and half deuterated ammonia borane were investigated as a function of temperature using powder neutron diffraction. The neutron diffraction patterns show a significant difference due to large difference in the scattering length of D and H. It is evident that an order-disorder phase transition occurs around 225 K for all compounds. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature disordered tetragonal structure with space group I/4mm at about 225K. The differential scanning calorimetry studies confirm this phase transformation and also indicate that all compounds melt and decompose at above 370 K. The c cell parameter remains unchanged in the orthorhombic phase from 16 K to 200K and increases liaa nearly above 225K. As the temperature is increased, the BH3-NH3 groups start to reorient along the c axis, and the D/H atoms become disordered, leading to the tetragonal phase transition around 225K.

Abstract: FeTi intermetallic powders are very promising media for reversible hydrogen storage. However, difficult activation treatments including annealing at elevated temperatures in high pressure H2 gas atmosphere are mandatory. In the present work nanostructured FeTi powders were produced and activated in situ at room temperature using mechanical alloying/milling (MA/MM) of pure metallic constituents, Fe and Ti, added with sodium borohydride. The resultant powders, FeTiHx, already H2 pre-charged, absorbed a significant amount of H2 but require optimization for reversible absorption/desorption. This system has one of the highest volumetric storage capacities and can be produced at low cost. Several parameters of the as-milled powders were controlled. The phase constitution of the reaction products was characterized by X-ray diffraction and scanning electron microscopy and the absorption isotherms of the activated powders were determined.

Abstract: Recently, there is interest in triggering shape recovery of shape-memory polymers(SMPs) by novel non-external heating. In this paper, many hard works have been carried out to make SMP induced by solution. The main challenge in the development of such polymer systems is the conversion of solution-induced effects at the molecular level to macroscopic movement of working pieces. This paper presents a systematic study on the effects of solution on the glass transition temperature (Tg). The results reveal that the hydrogen bonding of shape memory polymer (SMP) was aroused by the absorbed solution that significantly reduces Tg of polymer. The mechanism behind it is solution firstly intenerates polymeric materials till the Tg of polymer lowered down to the temperature of ambient, then hydrogen bonding interaction improves the flexibility of polymeric macro-molecular chains. Thus, the shape memory effect (SME) can undergo solution-driven shape recovery. In addition, it provides a new approach that the SMP can be induced by applying non-energy stimulus. The Dynamic Mechanical Analyzer (DMA) results reveal that the modulus of polymer was softened gradually with immersion time increasing. The experimental result is approximate to the theory.

Abstract: We studied the atomic-level structure of a model Mg-MgH2 interface by means of the Car-Parrinello molecular dynamics method (CPMD). The interface was characterized in terms of total energy calculations, and an estimate of the work of adhesion was given, in good agreement with experimental results on similar systems. Furthermore, the interface was studied in a range of temperatures of interest for the desorption of hydrogen. We determined the diffusivity of atomic hydrogen as a function of the temperature, and give an estimate of the desorption temperature.

Abstract: Kinetics of hydrogen desorption from Mg2NiH4 was studied. Experimental material was prepared by two techniques – by melting and casting and by ball-milling and compacting into pellets. Experimental materials were hydrogen charged at elevated temperature and pressure. The pellets were charged in two different regimes resulting in structures with high fraction of twinned low-temperature phase LT2 and with low fraction of LT2. It was made an attempt to measure diffusion coefficients of hydrogen and its temperature dependence both in high-temperature (HT) and in low-temperature (LT) phases of Mg2NiH4. The measurement was carried out in temperature interval from 449 K to 576 K by the volumetric method. It was found that the LT2 slows-down the desorption rate considerably.

Abstract: Hydrogen is suggested as a promising fuel of the near future for the utilization in automotive and mobile applications. Therefore, safe and effective hydrogen storage systems need to be developed. One of the possibilities, suitable especially for mobile applications, is the storage of hydrogen in the form of light-metal hydrides. In this work we studied microstructure and hydrogen absorption and desorption kinetics in selected Mg-Ni alloys. Hydrogen saturation was carried out by the cathodic polarization in alkaline water-based solution. It was confirmed that hydrogen could be stored in the Mg2Ni intermetallic phase forming Mg2NiH0.3 phase using this technology. MgH2 hydride is also formed when the temperature of 90 °C is applied. The total content of hydrogen in the material after saturation is approx. 0.7 wt. % according to the thermogravimetry analysis. This low value is caused probably by the surface oxidation, blocking further hydrogen diffusion. Thermal hydrogen desorption tests showed that the Mg2NiH0.3 phase is able to release hydrogen even at temperatures lower than 100 °C.

Abstract: Zircaloy is commonly applied as structural element in nuclear reactors owing to the gamma radiation transparency of Zr. One of the research interests in Zr-Nb alloys is related to its behavior in H2O-rich environments due to hydrogen embrittlement. In the present work the microstructural evolution (crystallite size and microhardness), crystallography and hydrogenation behavior (after milling) due to mechanical alloying (MA) are studied for the Zr-Nb5%at and pure Zr. The MA study of Zr and the Zr-Nb system showed that frequency of rotation in a planetary mill and alloy composition play a major role on the evolution of crystallite size and microhardness. Nb addition was found to induce a partial allotropic transformation of the Zr structure (α→ω) during MA. Indeed, for milling experiments with significant Fe contamination the formation of an fcc phase was observed. Further, MA for extended times (over 5 hours) was found to reduce the hydrogen absorption capacity of Zr and the Zr-Nb system studied.