Hydrogen trapping in metal alloys used in gas separation membranes

Dense metal membranes are of particular interest for H2 production of high purity. Palladium and Pd-based membranes are well known for this purpose. However, the high cost and limited availability of Pd prevent their use in large scale applications. Major efforts are focused on the design and optimization of novel low cost alloys, such as non-palladium alloys, exhibiting high H2 permeability, metal abundance, thermal stability and embrittlement resistance. The assessment of fundamental properties of these materials is of crucial importance for the screening of non-Pd based alloys. The prediction of basic quantities via computational approaches shows various advantages, for example it decreases the number of long experiments. In this frame, ab-initio methodologies can play an important role due to their independence of fitting and adjustable parameters. Thereupon, the purpose of this study is to analyse the H2 trapping process in V-Ni-Ti alloys using an ab-initio approach in the framework of DFT. V-Ni-Ti alloys have been experimentally proved as promising alternative of the Pd alloys [1-3]. Thus, an ab-initio investigation on the H2 trapping as function of metal composition gives important knowledge that can be used as inputs in other simulations (such as periodic and slab calculations) or to carry out alloy's screening. Since studies on H2 trapping process in V-Ni-Ti are absent, this work intents to remedy this shortcoming. Atomic clusters were adopted to describe the target alloys. This approach has been used in a previous study on H2 trapping in Nb-Mo alloys [4,5], giving results in qualitative agreement with the available experimental data [6]. Then, the nanoscale clusters were used to estimate the most favourable arrangements of the hydrogen atoms in the unit cells of the addressed alloys and the corresponding trapping energies (i.e. the energy difference between empty and hydrogen filled clusters). V85Ni15 was used as benchmark. Starting from this binary alloy, ternary alloys were defined in two different ways: first keeping the concentration of V constant whereas Ni atoms were replaced by Ti; secondly, keeping the Ni concentration constant and varying the concentrations of V and Ti. The ab-initio calculations of trapping energies were performed for all hydrogen absorption sites: tetrahedral (T), octahedral (O) and sites displaced from the T and O. The calculations showed that with respect to the V85Ni15, the hydrogen atoms are stabilized in some sites of the ternary alloys whereas in other sites the H atoms are destabilized. The most stable absorption sites for hydrogen were then determined. The trapping energies, referred to all the absorption sites in the bcc cell, were statistically analysed using Boltzmann distribution and average trapping energies were obtained. The average energies for V-Ni and V-Ni-Ti are approximately similar if two H atoms per unit cell are considered. Thus, the hydrogen trapping analysis was extended considering higher concentration of H atoms per unit cell to complete the description of the trapping process. Finally, the complete description of this process allows to define an atomic scale descriptor useful in the screening of other metal alloys for membranes.