Microstructural and Kinetic Evolution of Fe Doped MgH2 during H2 Cycling

Abstract

The effect of extended H 2 sorption cycles on the structure and on the hydrogen storage performances of MgH 2 powders with 5 wt% of Fe particle catalyst is reported. MgH 2 powders with and without Fe have been ball milled under Argon, the doped MgH 2 nanocomposite has been cycled under hydrogen pressure up to a maximum of 47 desorption and absorption cycles at 300 °C. After acceleration during the first 10 cycles, the kinetics behavior of doped MgH 2 is constant after extended cycling, in terms of maximum storage capacity and rate of sorption. The major effect of cycling on particle morphology is the progressive extraction of Mg from the MgO shell surrounding the powder particles. The Mg extraction from the MgO shell leaves the catalyst particles inside the hydride particles. Many empty MgO shells are observed in the pure ball milled MgH 2 upon cycling at higher temperature, suggesting that this enhancement of the extraction efficiency is related to the higher operating temperature which favors Mg diffusivity with respect to the H ion one.

In Situ Neutron Powder Diffraction of Li6C60 for Hydrogen Storage

“…24 For instance, for magnesium hydride formation n ~ 0.5 during the absorption process, 25 which is similar to that observed here for the bcc hydrofulleride formation. The process of magnesium hydride formation was found to be limited by the dissociation and diffusion of hydrogen in the system.…”

Mechanically-Induced Catalyzation of MgH2 Powders with Zr2Ni-Ball Milling Media

“…Reactive ball milling (RBM), which was investigated in the beginning of the 1990s [1,2] is a cost effective process, employed for conducting gas-solid reactions at ambient temperature, starting from pure metal and desired gas. Since then, RBM has been intensively used to produce large amounts of high quality MgH 2 nanocrystalline powders, starting from Mg metal and hydrogen gas [3][4][5].…”

On the degradation mechanisms of Mg hydride pellets for hydrogen storage in tanks

“…As far as the structural and morphological evolution at a microscopic level is concerned, it has been already ascertained [21] that the particle suffers a complex modification during the pellet cycling, where also the presence of a native oxide on the surface of the as milled sample can play an important role. Briefly, during the absorption reaction the surface oxide layer, which is always present owing to the high affinity of Mg with oxygen, behaves as a diffusion barrier separating the two reacting species, namely Mg inside the particle and hydrogen outside the particle.…”

“…On the other hand this effect produces empty oxide shells. In fact the extraction of metallic Mg from the particles by the above mechanism can be completed and after a few cycles the oxide crust, originally present at the powder particle surface results empty [19,21,22].…”