Thermodynamic
Destabilization of Magnesium Hydride
Using Mg-Based Solid Solution Alloys
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Abstract
Thermodynamic destabilization of
magnesium hydride is a difficult
task that has challenged researchers of metal hydrides for decades.
In this work, solid solution alloys of magnesium were exploited as
a way to destabilize magnesium hydride thermodynamically. Various
elements were alloyed with magnesium to form solid solutions, including:
indium (In), aluminum (Al), gallium (Ga), and zinc (Zn). Thermodynamic
properties of the reactions between the magnesium solid solution alloys
and hydrogen were investigated. Equilibrium pressures were determined
by pressure–composition–isothermal (PCI) measurements,
showing that all the solid solution alloys that were investigated
in this work have higher equilibrium hydrogen pressures than that
of pure magnesium. Compared to magnesium hydride, the enthalpy (Δ<i>H</i>) of decomposition to form hydrogen and the magnesium alloy
can be reduced from 78.60 kJ/(mol H<sub>2</sub>) to 69.04 kJ/(mol
H<sub>2</sub>), and the temperature of 1 bar hydrogen pressure can
be reduced to 262.33 °C, from 282.78 °C, for the decomposition
of pure magnesium hydride. Further, <i>in situ</i> XRD analysis
confirmed that magnesium solid solutions were indeed formed after
the dehydrogenation of high-energy ball-milled MgH<sub>2</sub> with
the addition of the solute element(s). XRD results also indicated
that intermetallic phases of Mg with the solute elements were present
along with MgH<sub>2</sub> in the rehydrogenated magnesium solid solution
alloys, providing a reversible hydrogen absorption/desorption reaction
pathway. However, the alloys were shown to have lower hydrogen storage
capacity than that of pure MgH<sub>2</sub>