Hydrogenation of Magnesium Nanoblades: The Effect of Concentration Dependent Hydrogen Diffusion

By assuming the H diffusion coefficient and H adsorption rate to be exponentially and linearly dependent on H concentration, a physical model is developed to predict the hydrogenation process of Mg nanoblades. The predicted H uptake curves agree well with the experimental data from V-coated Mg nanoblades. The obtained H diffusion coefficients in MgH_x between Mg and MgH₂ have nearly three orders of magnitude variation. The characteristic time of H surface adsorption is longer than that of H diffusion in Mg but shorter than that in MgH₂ for 100 nm thick nanoblades. Thus, as it proceeds, the hydrogenation process gradually changes from surface reaction-limited to diffusion-limited. In both one- and two-dimensional simulations, it is shown that a hydride shell is not formed during hydrogenation. In contrast, a hydride core is formed during dehydrogenation. The strong (exponential) concentration dependence of H diffusion coefficient throws profound influence on the stability and instability of a diffusion front, i.e., a H diffusion front in hydrogenation, and a H-vacancy diffusion front in dehydrogenation. In the latter case, the front tends to corrugate forming islands when the H₂ release rate is high.