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Abstract:
The search for technologically and economically viable storage solutions for hydrogen fuel would benefit
greatly from research strategies that involve systematic property tuning of potential storage materials via atomiclevel
modification. Here, we use first-principles density-functional theory to investigate theoretically the structural
and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic
hydrogen. Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al
or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films.We calculated
the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and we included
the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite
temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for
hydrogen absorption or desorption. The formation enthalpies of metal hydrides are greatly increased in thin
films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where
the chemical potential of free H2 is very high). Metal doping increases the thermodynamic stabilities of the
hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption
from H2 gas. In particular, with Na alloying, hydrogen can be absorbed and/or desorbed at experimentally
accessible T and P conditions