Hydrogen storage behavior in C15 Laves phase compound TiCr2 by first principles

Hydrogen storage in TiCr2 alloy of AB(2) type with C15 Laves phase structure has been studied using density functional theory and plane-wave pseudopotential technique. The H atom interacts stronger with Cr atoms than with Ti and tends to occupy the g (2A2B) site due to the largest interstitial space. The g sites together with e (1A3B) sites form a three-dimensional clathratelike network, providing a diffusion pathway for H atoms, and the energy barriers for the H diffusion along these paths were computed. Interaction between two interstitial H atoms inside the TiCr2 lattice can be considered as a screened Coulomb repulsion, which seems to control the maximum hydrogen content in the metal hydrides. The absorption energies and electronic structures of metal hydrides TiCr2Hx with x=0.5-12 have been computed, and the theoretical maximum hydrogen storage capacity (between 2.6 and 3.8 wt %) agrees with experiments. The effects of the lattice expansion due to hydrogen absorption of different amounts have also been investigated.