A density functional theory study of hydrogen occupation in VNiTi alloys used for dense metal membranes

• Large distance between H atoms in VNiTi is favoured for sites of the same symmetry.
• Simultaneous occupation of T and O sites in VNiTi alloys is energetically feasible.
• Variation of alloy composition influences the site preference for H occupation.
• Increase of the Ti:Ni ratio by V = const increases the hydrogen absorption affinity.

Attempting to further the development of non-noble dense metal membranes for H2 separation we conduct a density functional theory study of hydrogen occupancy in V-based alloys with Ni and Ti substitutional solutes. Clusters consisting of 19 quasi-randomly coordinated metal atoms are built to model body-centred cubic VNi and VNiTi alloys with different stoichiometry. The total energy of the target systems is calculated using spatially localised functions. The disposition of a pair of hydrogen atoms within the metal lattice is explored and the binding energy in both tetrahedral and octahedral interstices is evaluated. Large spatial distance between absorbed H atoms is favoured for each of the interstitial sites, rejecting the idea of H clustering in the investigated solid solutions. Moreover, simultaneous occupation of both tetrahedral and octahedral interstices is found to be energetically feasible despite the common believe for solely tetrahedral occupancy in metals with body-centred cubic structure. Nonetheless, the most favourable absorption site depends on the solute concentration in the V-based alloys. Calculations of the binding energy using cluster models with different metal atomic ratio provide information on the hydrogen absorption affinity as a function of alloy composition. Enhancement of the absorption affinity with added Ti until certain limit is found, while Ni solutes influence this property in the opposite direction. The applied methodology can be used further in high-throughput calculations to screen various metal alloys for hydrogen separation membranes.