Abstract
One practical solution for utilizing hydrogen in vehicles with proton-exchange fuel cells membranes is storing hydrogen in metal hydrides nanocrystalline powders. According to its high hydrogen capacity and low cost of production, magnesium hydride (MgH2) is a desired hydrogen storage system. Its slow hydrogenation/dehydrogenation kinetics and high thermal stability are the major barriers restricting its usage in real applications. Amongst the several methods used for enhancing the kinetics behaviors of MgH2 powders, mechanically milling the powders with one or more catalyst species has shown obvious advantages. Here we are proposing a new approach for gradual doping MgH2 powders with Ni particles upon ball milling the powders with Ni-balls milling media. This proposed is-situ method showed mutually beneficial for overcoming the agglomeration of catalysts and the formation of undesired Mg2NiH4 phase. Moreover, the decomposition temperature and the corresponding activation energy showed low values of 218 °C and 75 kJ/mol, respectively. The hydrogenation/dehydrogenation kinetics examined at 275 °C of the powders milled for 25 h took place within 2.5 min and 8 min, respectively. These powders containing 5.5 wt.% Ni performed 100-continuous cycle-life time of hydrogen charging/discharging at 275 °C within 56 h without failure or degradation.
Highlights
Owing to the dramatic global environmental changes associated with man-made carbon dioxide emissions and the huge consumption of the limited resources of fossil fuels, developing alternate energy sources is important for a sustainable future
X-ray diffraction (XRD) and field emission-high resolution transmission electron microscope (FE-HRTEM) techniques were employed to investigate the structural changes of hcp-Mg powders upon reactive ball milling technique (RBM) under a hydrogen gas pressure (50 bar), using Ni-balls as milling media
After 12.5 h of RBM time, new Bragg-peaks corresponding to β-MgH2 (PDF file# 03-065-3365) and γ-MgH2 (PDF file# 00-035-1184) were appeared, implying elemental mapping of Mg and Ni for a different sample obtained after 25 h of RBM a mixture of MgH2 doped with 5.5 wt.% Ni nanoparticles are presented in (d), (e) and (f), respectively
Summary
Owing to the dramatic global environmental changes associated with man-made carbon dioxide emissions and the huge consumption of the limited resources of fossil fuels, developing alternate energy sources is important for a sustainable future. It is agreed that mechanically-induced doping of MgH2 with the abrasive powders of hard phases such as carbides, oxides, intermetallic and metallic glassy alloys materials lead to fast grain refining of the MgH2 upon releasing the crystalline stored energy, leading to refine the MgH2 grains along their grain boundaries where superfine grains are formed. Such desirable fine grains with their short-distance grain boundaries always facilitate short diffusion path[20], leading to fast diffusion of the hydrogen atoms[21,22,23]. Our synthesized nanocomposite MgH2/5.5 wt.% Ni composite powders revealed fast hydrogenation/dehydrogenation processes, taking place at moderate temperature and low value of activation energy (75 kJ/mol)
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