Papers by Keyword: Hydrogen-Storage Material

Abstract: The most promising hydrogen storage materials are perhaps complex metal hydrides. Thus, a plausible first step in the rehydrogenation mechanism is proposed by simulating the reversible hydrogen storage in Zr-doped NaAlH₄. It provides insight into the catalytic role of Zr atoms on an Al surface in the chemisorptions of molecular hydrogen. It is found that the diffusion of hydride species on Al-metallic phase and formation of Al hydride species is probably the key to syntheses the next products in the rehydrogenation reaction.

Abstract: Magnesium-based hydrogen-storage materials were prepared by reactive ball-milling under hydrogen atmosphere. It was shown that crystallitic carbon from anthracite carbonization was an effective milling aid for magnesium. Dispersive nano-particles about 20 to 60 nm were prepared from magnesium with 35 wt.% of crystallitic carbon additive by milling for 3 h under 1 MPa of hy-drogen atmosphere. The magnesium hydrided into MgH2 and the crystallitic carbon was endowed with C=CH₂ functional group during milling. The hydrogen-storage materials were used for the hy-drodesulfurization of CS₂ and thiophene, and H₂S and MgS yielded after reaction. To add molybdenum into the hydrogen-storage materials was in favor of the hydrogenation of sulfo-compounds.

Abstract: We carried out in-situ observation on the catalytic effect of Nb2O5 in MgH2 by using a high voltage transmission electron microscope (HVEM). We synthesized two kinds of samples, which were prepared by ball milling and by hand mixing. In milled sample, Nb2O5 was not confirmed from high resolution images, however, NbO was identified. As the temperature increased, the decomposition of MgH2 proceeded, while Mg formed and grew. It suggested that NbO had the catalytic effect to promote the dehydrogenation of MgH2. In mixed sample, which was prepared to clearly observe the boundary between the catalyst and Mg phase, it was revealed that the dehydrogenation started from the boundary of MgH2 and Nb2O5. This result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH2 through Mg phase to the boundary.

Abstract: Mg-rich alloys of the binary system Mg-Ni are prospective hydrogen-storage materials. In the present study, desorption characteristics of hydrided Mg2Ni intermetalic and hydrided Mg/Mg2Ni eutectic mixture were investigated. Structure of experimental materials during the hydrogenation was observed by SEM. Three modifications of (Mg2Ni)Hx (x ~ 4) were prepared differing in the ratio of two low-temperature phases f = LT2/LT1: with (i) f >1, (ii) f ~ 1 and with (iii) f <1. Evolution of the ratio f during hydrogen desorption was checked by XRD. It was found that the micro-twinned phase LT2 is not desirable in hydrogen-storage materials containing Mg2Ni intermetallic. Diffusion coefficient of hydrogen in LT2 is about 20 times lower than in LT1.

Abstract: In this paper we have studied from first-principles the effect of magnetism on the hydrogen-metal interaction and the binding properties of palladium with 3d-alloying atoms in the presence of vacancies induced during hydrogenation process. Our first-principles calculations were carried out by means of state of the art ab-initio method based on density functional theory and all-electron PAW-potentials. We have analyzed the changes of the atomic and electronic structures of palladium crystal induced by the presence of substitutional 3d-alloying atoms, interstitial hydrogen and structural defect (palladium vacancy). The obtained results have shown that magnetism can strongly affect the hydrogen-metal interaction in palladium based alloys. We have also demonstrated that the presence of vacancies in the palladium matrix can alter the interaction energy between hydrogen and alloying transition metal atoms.

Abstract: Mg-Ni alloys are perspective hydrogen-storage materials. In the present work, kinetics of hydrogen desorption was studied in Mg2NiH4 hydride and in hydrogenated eutectic (Mg/Mg2Ni)Hx. Time dependence of desorbed hydrogen was measured in temperature interval 552 –723 K and hydrogen diffusion coefficients were evaluated from obtained desorption curves. Results were interpreted in relation to phase composition investigated parallel by XRD. It was found that microtwinned low-temperature phase LT2 slows-down considerably the hydrogen desorption rate.

Abstract: Hydrogen storage in solid hydrides is the most attractive method of on-board hydrogen storage in fuel cell for cars. Mg metal exhibits a high-storage capacity by weight and has been considered a group of potentially attractive candidates for solid-state hydrogen storage. In this study, mechanochemical synthesis of nanocrystalline Mg-based hydrogen storage composites from various starting materials in specialized hydrogen ball mills has been achieved. The reactive synthesis process and the hydrogen desorption behaviors of the composite hydrides were investigated by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC). The results show that nano-sized MgH2 and Mg(AlH4)2 could be directly synthesized by pure Mg and pretreated Al powder, as well as Mg-Li-Al alloy powder. Alloying element Li could remarkably promote the synthesis of magnesium alanate, the product composite hydrides releasing 6.2wt% H2 through multi-step decompositions, of which the starting endothermic peaks are as low as 65°C.

Abstract: A good method to store hydrogen is in it atomic form in crystalline structure of metals at low pressure. Thanks to magnesium’s high hydrogen storage capacity, its low weight and its high natural abundance, it is an attractive material to develop hydrogen solid state storage. The production of Mg-based nanocomposites can enhance the kinetics of H-sorption of magnesium and the temperature of release of hydrogen. Transition metals as iron, which have important catalytic activity in hydrogen reactions with Mg, and the surface protective compound MgF2, are interesting additions for magnesium mixtures for hydrogen storage. In this work, Mg-based nanocomposites containing Fe and MgF2 were produced by reactive milling under hydrogen using the addition of FeF3, or directly MgF2 and Fe. The efficiency of centrifugal and planetary mill in MgH2 synthesis was compared. The phase evolution during different milling times (from 1 to 96 h) using the planetary was investigated. The different H-desorption behavior of selected milled mixtures was studied and associated with the different present phases in each case.

Abstract: In this work oxidized and oxide-free amorphous boron (a-B) powder and elemental Mg were used in an attempt to directly synthesize the Mg(BH4)2 complex hydride by controlled reactive mechanical alloying (CRMA) under hydrogen in a magneto-mill up to 200h. The particle size was refined to the 100-200nm range. Nanocrystalline MgH2 (~6nm crystallite size) was formed within the particles when an oxidized a-B is used. In contrast, a mixture of MgB2 and an amorphous hydride MgHx was formed when an oxide-free a-B was used. Differential scanning calorimetry (DSC) test up to 500°C produced a single endothermic heat event at 357.7°C due to hydrogen desorption. In addition, desorption conducted in a Sieverts-type apparatus revealed ~1.4wt.% of hydrogen release. The X-ray diffraction pattern after DSC test of the 200h milled sample made with oxide-free boron showed the presence of MgB2.