Effect of Mg shell on MgH₂ dehydrogenation by morphological and mathematical analysis

Magnesium (Mg) formation during dehydrogenation of nanocrystalline magnesium hydride (MgH₂), exhibiting a bimodal particle size distribution (4 ± 1 and 16 ± 8 μm) at 320 and 350°C, was investigated using X-ray diffraction patterns, cross-sectional scanning electron microscopy (SEM) and transmission electron microscopy. Magnesium hydride dehydrogenation consists of an initial period of slow hydrogen gas (H₂) release (incubation period). Following incubation, hydrogen release increases significantly. SEM images of the dehydrogenating magnesium hydride particles are graphically divided into several concentric annuli. The radially outward flux of hydrogen released within the particles was estimated by developing a quantitative approach. During incubation, the absence of a magnesium shell necessitates slower hydrogen atom diffusion through magnesium hydride as depicted by the flux estimation. Beyond incubation, the enhanced hydrogen release is due to the presence of a magnesium shell. Thus, the effect of the absence/presence of a magnesium shell on magnesium hydride dehydrogenation was studied using the estimated fluxes.