Hydrogen Absorption and Desorption Behavior of Magnesium Hydride: Incubation Period and Reaction Mechanism

Abstract

The hydrogen absorption and desorption reactions of pure MgH₂ were investigated by pressure-time measurements using a Sieverts’ type instrument in the temperature and pressure ranges of 653–683 K and 0.5–1.7 MPa, respectively. The absorption and desorption behaviors were analyzed using a fraction of the reaction product during the hydrogen absorption and desorption. The fraction was evaluated based on the amount of absorbed and desorbed hydrogen.
The hydrogen absorption of pure Mg immediately occurs when the thermodynamic condition in which the reaction can proceed is reached at 653–683 K, but the hydrogen desorption does not start immediately when it can thermodynamically proceed at the same temperatures. Incubation periods were observed and had varied values in the range from 0.15 to 1.5 ks under the above-mentioned pressure and temperature conditions.
In order to clarify the hydrogen desorption mechanism, the data obtained were analyzed by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation. The obtained values of the Avrami exponents varied from 3 to 0.6 with the increasing fraction of Mg. The hydrogen desorption process was classified into four stages based on the KJMA plots of the hydrogen desorption curves of MgH₂ measured in this study. These values indicated that the Mg nuclei generate and three-dimensional grow during the initial stage, then the growth is restricted to a two- or one-dimensional.