Hydrogen occupation in Ti4M2Oy compounds (M = Fe, Co, Ni, Cu, and y = 0, 1) and their hydrogen storage characteristics

Abstract

The hydrogen sorption properties of Ti4M2Oy compounds (M = Fe, Co, Ni, Cu or their mixture and y = 0, 1) were studied to assess their utility as room-temperature hydrogen storage materials. The main parameter controlling the hydrogen sorption property is the energy of hydrogen incorporation into the compounds, which was evaluated by density functional theory total-energy calculations. Energetics provides basic information on the sequence of hydrogen filling into the available interstitial sites, and when the minimum H–H distance is also considered, a more sensible prediction of the site occupation becomes possible. The calculation results suggested that between Ti4Fe2O and Ti4Ni2O, Ti4Ni2O was a better candidate for room-temperature hydrogen storage. An almost-single-phase Ti4Ni2O compound was successfully synthesized starting from TiO2, Ti, and Ni using the arc-melting method. Ti4Ni2O stored 1.3 wt% of hydrogen under 7 MPa of H2 pressure at 30 °C. The storage properties of Ti4Ni2O were modified by the partial substitution of Fe, Co, and Cu for Ni. The partial substitution did not improve the usable capacity, but the hydrogen absorption–desorption characteristics demonstrated that the equilibrium hydrogen pressure could be precisely controlled via composition change.