Abstract
Ultrafine MgH2 nanocrystalline powders were prepared by reactive ball milling of elemental Mg powders after 200 h of high-energy ball milling under a hydrogen gas pressure of 50 bar. The as-prepared metal hydride powders were contaminated with 2.2 wt. % of FeCr-stainless steel that was introduced to the powders upon using stainless steel milling tools made of the same alloy. The as-synthesized MgH2 was doped with previously prepared TiC nanopowders, which were contaminated with 2.4 wt. % FeCr (materials of the milling media), and then ball milled under hydrogen gas atmosphere for 50 h. The results related to the morphological examinations of the fabricated nanocomposite powders beyond the micro-and nano-levels showed excellent distributions of 5.2 wt. % TiC/4.6 wt. % FeCr dispersoids embedded into the fine host matrix of MgH2 powders. The as-fabricated nanocomposite MgH2/5.2 wt. % TiC/4.6 wt. % FeCr powders possessed superior hydrogenation/dehydrogenation characteristics, suggested by the low value of the activation energy (97.74 kJ/mol), and the short time required for achieving a complete absorption (6.6 min) and desorption (8.4 min) of 5.51 wt. % H2 at a moderate temperature of 275 °C under a hydrogen gas pressure ranging from 100 mbar to 8 bar. van’t Hoff approach was used to calculate the enthalpy (∆H) and entropy (∆S) of hydrogenation for MgH2, which was found to be −72.74 kJ/mol and 112.79 J/mol H2/K, respectively. Moreover, van’t Hoff method was employed to calculate the ΔH and ΔS of dehydrogenation, which was found to be 76.76 kJ/mol and 119.15 J/mol H2/K, respectively. This new nanocomposite system possessed excellent absorption/desorption cyclability of 696 complete cycles, achieved in a cyclic-life-time of 682 h.
Highlights
Hydrogen storage is one of the key enabling technologies for realization of hydrogen energy economy [1]
We have investigated the effect of FeCr contamination introduced to the MgH2 powders upon ball milling in the long term on improving the hydrogenation/dehydrogenation properties of the metal hydride phase
Nanocrystalline MgH2 powders were synthesized by reactive ball milling of pure Mg powders, using a high-energy ball mill operated at 250 rpm under 50 bar of hydrogen atmosphere
Summary
Hydrogen storage is one of the key enabling technologies for realization of hydrogen energy economy [1]. In contrast to the traditional gas–solid hydrogenation process, which is achieved at temperatures far above room temperature, an attractive method—so-called reactive ball milling (RBM) [2,3]—was developed in the 1990s to conduct the exothermic reactions between the gas- and metallic solid phases at almost room temperature. This relatively new process has been considered as a powerful tool for fabrication of different nanocrystalline metallic nitrides and hydrides [4]. The RBM process has become a common technique successfully used for preparing nanocrystalline metal hydrides, including magnesium hydride (MgH2) and their composite powders [1,6]
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