DFT studies of hydrogen storage properties of Mg0.75Ti0.25

Research highlights: → Stability and electronic structures of four different types of Mg0.75Ti0.25 alloys and their hydrides. → Hydrogenation energy of Mg0.75Ti0.25Hx as a function of hydrogen concentration → Hydrogen induced structural rearrangement of the intermetallic Mg0.75Ti0.25. → Partial dehydrogenation might improve the reversibility of the material for hydrogen storage. - Abstract: Absorption energies of hydrogen in Mg0.75Ti0.25 alloys as a function of the hydrogen concentration were calculated using Density Functional Theory. Four types of structures of alloys and their hydrides including TiAl3, ZrAl3, AuCu3, and segregated types of structures were considered. The stability of the configurations, and the structural and electronic bonding properties were studied. The hydrogenation properties depend highly on the structure of the alloys. The ordered alloys have very similar properties to that of pure Mg. For segregated alloys, the hydrogenation properties can be divided to Ti-like, ordered alloy-like and Mg-like from low to high hydrogen concentration. The formation energies show that for the four structures, segregated Mg0.75Ti0.25 is favored for alloys, whereas TiAl3 type of Mg0.75Ti0.25H2 are favored for hydrides. Therefore hydrogen induced structural rearrangement of the intermetallic structures of the Mg0.75Ti0.25 might occur upon hydrogen cycling. For the non-homogenous Mg-Ti-H system, further phase segregation of Ti in Mg might occur. Partial dehydrogenation with some hydrogen remaining in the Ti-rich region may improve reversibility.