Dehydrogenation Mechanisms in Nanocrystalline Magnesium Hydride

Magnesium hydride is a promising material for on-board hydrogen storage due to its high gravimetric capacity (7.6 wt. %). However, its applicability is limited by slow dehydrogenation kinetics. Dehydrogenation mechanisms involve kinetics of several phenomena and the morphological changes of metal/hydride phase.

Dehydrogenation kinetics consists of nucleation and growth of magnesium phase by interfacial movement and/or by diffusion of H-atom through the hydride/Mg phase. Kinetics is studied using Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation: α=1-exp(-ktⁿ), where α: converted fraction of Mg phase, k: kinetic factor involving nucleation density (N_o), growth velocity (U) and growth dimensionality (n) of Mg phase. n represents the morphological manifestation of Mg phase which can initially nucleate and grow as isolated colonies in 3D (n≈3). Eventual impingement of the neighbouring Mg colonies, can arrest growth in the lateral dimensions (i.e. n<3). n and k along with other kinetic factors can explain the dehydrogenation mechanisms.

Dehydrogenation mechanism was investigated by performing isothermal experiments at 320-400 °C. The dehydrogenation curves show two distinct stages, namely, the incubation stage and post incubation stage. The incubation stage experiences slower kinetics and is observed predominantly at lower temperatures. Dehydrogenation mechanisms in the incubation and post incubation stages are studied by dividing the entire dehydrogenation curve into several small segments and applying JMAK analysis. This analysis yields discrete values of n and k which vary with time and their variation trends are different in the incubation and post incubation stages.

During the incubation stage, n increases from a negligible (n=0.01) to a significant (n=2.24) value. As n reflects the average growth dimensionality of the Mg phase, its increase can be attributed to the increase in number of growing Mg colonies with time. The activation energies for nucleation (10 kJ/mol) and growth (213 kJ/mol) are estimated from N_o and U values, respectively. The activation energy of nucleation is one order lower than that for growth suggesting that Mg nucleation is fast while its growth is slow in the incubation stage.

In the post incubation stage, n values decrease with time from a high value (n>1) to a negligible value (n<0.50). The decreasing n can be attributed to the changing growth morphology of Mg phase during dehydrogenation. Microscopic observations (Dark Field TEM) and the estimation of volume fraction of Mg/MgH₂ phases shows that Mg phase nucleates and grows mainly near the particle surface and partly in the interior. During this stage, the interface velocity (U) changes by ~1 order. The estimated activation energies for nucleation and growth in post incubation stage are 55 kJ/mol and 152 kJ/mol respectively. This indicates that Mg phase growth limits dehydrogenation in the post induction stage.

The overall activation energy for dehydrogenation estimated using Raman spectroscopy is 158 kJ/mol. This is close to the activation energies for growth in both the incubation and post incubation stages, suggesting Mg phase growth as the controlling step during dehydrogenation of MgH₂.