Hydrogen driven vehicles with fuel cells offer the most environmentally friendly automotive technology. Among the known metals and alloys with potential use in hydrogen storage, magnesium is generally regarded as the most promising because of its high capacity (7.6 wt.%) and low cost. Mechanical (ball) milling has proved to be simple and very effective technique for synthesizing Mg-related materials exhibited sufficiently fast hydriding/dehydriding kinetics for technical applications.

Microstructure and morphology of Mg/MgH₂ powder (or compacts) are crucial material parameters for fast sorption kinetics. However, to our knowledge, there are only a few works in the literature about microstructure and morphology of MgH₂ powder after processing and cycling hydrogen loading. One of that reasons is the behavior of this hydride under the electron beam. MgH₂ starts to release hydrogen as soon as the electrons interact with thin sample and decomposed to Mg. The microstructural and structural investigations were performed using scanning electron microscope (FE SEM) equipped with duo-STEM detector and transmission electron microscope (TEM). This investigations were carried out on nanocrystalline MgH₂ without and with Cr₂O₃ nanopowder as a catalyst.

Morphological investigations of mechanical (ball) milled MgH₂ powders showed a MgH₂ particle size of 0.5-10 μm. The bright field STEM images of milled MgH₂ exhibited a homogeneous crystallite size distribution with average crystallite size smaller than 100 nm. The brittle nano-Cr₂O₃ particles are embedded by the MgH₂ matrix, forming a nanocomposite structure. The particle size of Cr₂O₃ was found to be 10-300 nm. TEM investigations showed the presence of small fraction of MgO phase not only in the surface of MgH₂ particles but also within the micrometric particles. These oxide regions are formed probably by aggregation of passivated MgH₂ particles during the ball milling.