Hydrogen occupation in Ti₄M₂O_y compounds (M = Fe, Co, Ni, Cu, and y = 0, 1) and their hydrogen storage characteristics

The hydrogen sorption properties of Ti₄M₂O_y 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.