(720f) Kinetic Mechanisms of Hydrogen Sorption in Nanocrystalline Magnesium (hydride)

Abstract

Magnesium
(hydride) is a very promising material for hydrogen storage with very high
gravimetric capacity (7.6 wt.%). However, its slow kinetics of hydrogen
absorption/desorption (sorption) hinders its application. Strategies such as
particle size reduction, catalyst addition are employed to improve the kinetics
of sorption. To device the strategies for improving the kinetics, it is
essential to unravel the mechanisms of hydrogen sorption. The kinetics of
hydrogen sorption in ball milled nanocrystalline magnesium (hydride) powders are
investigated using the popular Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation (α
= 1?exp(?ktⁿ)) at different temperatures (180 °C-400 °C). The exponent n indicates the
dimensionality of the growth of Mg (during desorption) or MgH₂
(during absorption). n decreases with time from a high value (>1) to
a negligible value (<0.5) suggesting that the growth of the phase (Mg or MgH₂)
decreases with time. The cross-sectional observation of these powders using SEM
and TEM revealed that the depth of the growing phase from the particle surface
increases with time rendering a core-shell structure during sorption. The
estimated hydride/metal interface velocity (U) for growth of these
phases shows two clear regimes of high and low velocities in the n>0.5
and n<0.5 regimes respectively. The former velocity is at least 2
orders of magnitude higher than the latter. The diffusion coefficients (D)
are also estimated using Fick's 2^nd law (transient) for hydrogen
diffusion through the magnesium hydride phase. The estimated diffusion
coefficients are at least 4 orders of magnitude higher in the n>0.5
regime than in those in the n<0.5 regime. Moreover, the diffusion
coefficients in the n<0.5 regime are closer to the expected values
from literature. The estimated activation energy for hydrogen diffusion through
hydride is 91 kJ/mol H and is close to the literature reported value of 100±10
kJ/mol H. The combined n-U-D analysis supported by the
microstructural examinations suggests that the growth of the magnesium or
hydride is governed by hydride/metal interfacial movement in the n>0.5
regime and by hydrogen diffusion through hydride in the n<0.5 regime.
The present analysis distinguishes growth by the interfacial velocity from that
by diffusion during hydrogen sorption in magnesium (hydride). The present
analysis contributes to the advancement of the understanding on the kinetic
mechanisms of hydrogen sorption of MgH₂.