Role of Elemental Substitution on Hydrogen Incorporation in Characteristic Ti/Fe-Based Oxides

Elemental substitution is well-known to benefit hydrogen
storage
in the intermetallic compound TiFe; however, the impact of these substituents
on the native and passivating surface oxide has not been well-established.
Here, we perform density functional theory calculations on several
characteristic oxides that are likely to form on exposed TiFe surfaces,
specifically, TiO2, FeO, TiFeO3, and Ti4Fe2O. In TiO2 and TiFeO3,
additional acceptor species substituting for Ti can increase the solubility
of protonic hydrogen (Hi+), likely enhancing hydrogen permeation
of the oxide layer. In FeO, few dopants will act as acceptors, and
hydrogen will incorporate as substitutional HO+, which is likely to be less mobile
and will hinder hydrogen permeation. Finally, we find that Ti4Fe2O, which is thermodynamically metastable, may
be stabilized by defects under O-poor conditions, although the presence
of certain substitutional elements can destabilize it. In general,
Mn, V, and Cr are among the best substituents for enhancing hydrogenation
through the oxide layer. Our results help to explain what features
make additional elements particularly effective for enabling reversible
hydrogen storage in TiFe, and they can guide further technological
development of high-performing TiFe-based alloys for future energy
applications.