TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

We utilized transmission electron microscopy (TEM) analysis, with a cryogenically cooled sample stage, to detail the microstructure of partially transformed pure and titanium fluoride-catalyzed magnesium hydride powder during hydrogenation cycling. The TiF₃-catalyzed MgH₂ powder demonstrated excellent hydrogen storage kinetics at various temperatures, whereas the uncatalyzed MgH₂ showed significant degradation in both kinetics and capacity. TEM analysis on the partially hydrogen absorbed and partially desorbed pure Mg(MgH₂) revealed a large fraction of particles that were either not transformed at all or were completely transformed. On the other hand, in the MgH₂+TiF₃ system it was much easier to identify regions with both the hydride and the metal phase coexisting in the same particle. This enabled us to establish the metal hydride orientation relationship (OR) during hydrogen absorption. The OR was determined to be (1 1 0)MgH₂ || (?1 1 0 ?1)Mg and ?1 1 1]MgH₂ || 0 1 ?1 1]Mg. During absorption the number density of the hydride nuclei does not show a dramatic increase due the presence of TiF₃. Conversely, during desorption the TiF₃ catalyst substantially increases the number of the newly formed Mg crystallites, which display a strong texture correlation with respect to the parent MgH₂ phase. Titanium fluoride also promotes extensive twinning in the hydride phase.