Abstract
Magnesium hydride, despite the decomposition temperature being incompatible with the operating temperature of a typical PEM cell, is still considered a prospective material for hydrogen storage. Hence, this paper presents new aspects of the influence of milling time on the structural changes and temperature of MgH2 decomposition, with particular emphasis on the changes taking place in the first few seconds of the milling process. This paper presents qualitative and quantitative changes in the powder particle morphology determined using scanning electron microscopy (SEM) and infrared particle size analysis (IPS) systems. The crystallographic structure of the powders in the initial state and after mechanical milling was characterized by X-ray diffraction. The decomposition temperature and activation energy were determined by the differential scanning calorimetry (DSC). Changes in the activation energy and decomposition temperature were observed after only 1–2 min of the milling process. Two basic stages of the milling process were distinguished that impacted the MgH2 decomposition temperature, i.e., mechanical activation and a nanostructuring process. The activation was associated with the initial stage of particle size reduction and an increase in the fraction of fresh chemically active powder particle surfaces. On the other hand, the nanostructuring process was related to an additional decrease in the MgH2 decomposition temperature.
Highlights
The dynamic development of the hydrogen economy creates a real alternative to conventional energy carriers
Depending on the type of ball mill, the typical milling time for magnesium or magnesium hydride ranges from 15 min to 20 h for high-energy mills [9,11,24,25,30,42] and from 20 to 150 h for low-energy mills. [1,15]
The thermal stability of magnesium hydride, expressed by the decomposition temperature, was related to the technological parameter, which is the milling time, and to structural changes expressed by changing the size of the powder particles and the size of the crystallites
Summary
The dynamic development of the hydrogen economy creates a real alternative to conventional energy carriers. Hydrides in solid form are a prospective method of storing hydrogen, and this approach has been dynamically developing in recent decades. Hydrides in solid form, which include metal hydrides [1,2,3], hydrides based on intermetallic phases [4,5] and chemical compounds with hydrogen (complex metal hydrides) [6,7], are characterized by a much higher volume capacity than that of hydrogen in compressed or liquefied form. Hydrides show no imperfections that characterize the other two systems, such as high-pressure safety considerations, high compression costs, large evaporation losses, safety and condensation costs. A high purity of the hydrogen, which is released from hydrides in solid form, appears to be very important because it enables a direct fuel cell supply [8]
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