Scanning electron microscopy of partially de-hydrogenated MgH2 powders

Evidence of the hydrogen release mechanism in bulk MgH2

“…al. 25 have confirmed the difficulty of Mg nucleation in MgH 2 , and the presence of some retained Mg phase is likely to accelerate dehydriding kinetics. If there were no pre-existing Mg grains, surface nucleation would be easier than nucleation within the volume due to the volume change.…”

Experimentally Observed Nucleation and Growth Behavior of Mg/MgH2 during De/Hydrogenation of MgH2/Mg: A Review

“…It should also be noted that the spatial distribution of any pre-existing Mg nuclei, and in particular, the distance of these nuclei from the free surface, will play an important role in determining the "nucleation and growth" behavior and, thereby, the desorption rate of the bulk samples. It has been confirmed that for pure MgH 2 without pre-existing Mg grains, surface nucleation of Mg (e.g., Figures 2, 3b,d and 4f) would be easier than nucleation of Mg within the volume due to the volume change [55,65,66], which leads to more strain for nucleation of Mg within the volume than that for surface nucleation of Mg, as can be seen in Figure 5a.This implies that the presence of the Mg phase on the surface of MgH 2 matrix can facilitate the elimination of the nucleation barrier for the formation of the Mg phase from the MgH 2 matrix [57,67]. However, there is a great deal of debate surrounding the situation in the presence of pre-existing Mg grains within the volume (Figure 5b).…”

“…It should also be noted that the spatial distribution of any pre-existing Mg nuc and in particular, the distance of these nuclei from the free surface, will play an import role in determining the "nucleation and growth" behavior and, thereby, the desorpt rate of the bulk samples. It has been confirmed that for pure MgH2 without pre-exist Mg grains, surface nucleation of Mg (e.g., Figures 2, 3b,d and 4f) would be easier t nucleation of Mg within the volume due to the volume change [55,65,66], which lead According to the above discussion, the "nucleation and growth" behavior during the dehydrogenation of pure MgH 2 can be summarized as follows: (1) At low temperatures, the rate of dehydrogenation of MgH 2 is slow due to the low nucleation rate. Thus, the rate-limiting step at this stage is nucleation; (2) As the temperature rises to a certain value, metallic Mg nucleates instantaneously at the free surfaces of particles; (3) Decomposition of MgH 2 and growth of Mg at the Mg-MgH 2 boundary (the movement of the inter-phase boundary), which is derived from the free energy difference between atoms in adjacent grains [55]; (4) The hydrogen atoms generated at the boundary diffuse through Mg to the surface of Mg. For ion-covalent hydrides, both ways (through metal or semiconductor) are possible.…”

“…However, there is a great deal of debate surrounding the situation in the presence of pre-existing Mg grains within the volume (Figure 5b). Some results showed more favorable growth from within than nucleation and growth from the surface in the presence of pre-existing Mg grains within the volume (e.g., Figure 3a,c) [55,65]. In contrast, there is another opinion suggesting that when the surface of the powders is completely covered with the MgH 2 phase, the observable dehydrogenation process most probably begins with the nucleation of the hcp-Mg on the surface of the hydrogenated powders as a result of the extremely slow rate of growth of the hcp-Mg phase left in the core of the particles owing to the low diffusivity of hydrogen through the MgH 2 phase [66].…”

Reply to ‘Comments on “Evidence of the hydrogen release mechanism in bulk MgH2”’

“…For comparison, literature reported results for hydrogen desorption of MgH 2 are in the range of 320–450 °C dependent on the presence of various catalysts (e.g. NaNH 2 4, Fe5, FeCl 3 6 or graphite7, respectively) and the heating rate. In the UHV-TEM the temperature range of the transformation we observed was between 400–455 °C, consistent within reason for the transformation of MgH 2 to Mg.…”