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.

LiAlH₄ and CaCl₂ were employed for mechano-chemical activation synthesis (MCAS) of Ca(AlH₄)₂ and LiCl hydride composite. After short ball milling time, their X-ray diffraction (XRD) peaks are clearly observed. After ball milling for a longer duration than 0.5 h, the CaAlH₅ diffraction peaks are observed which indicates that Ca(AlH₄)₂ starts decomposing during ball milling into CaAlH₅+Al+1.5H₂. It is estimated that less than 1 wt % H₂ was mechanically dehydrogenated in association with decomposition reaction. After 2.5 h of ball milling, no Ca(AlH₄)₂ diffraction peaks were observed on XRD patterns which suggests that Ca(AlH₄)₂ was decomposed. Thermal behavior of ball milled powders, which was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), indicates that a certain fraction of Ca(AlH₄)₂ could have been disordered/amorphized during ball milling being undetectable by XRD. The apparent activation energy for the decomposition of Ca(AlH₄)₂ and CaAlH₅ equals 135 kJ/mol and 183 kJ/mol, respectively.