(541f) 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.