The effect of non-stoichiometry on the hydrogen storage properties of Ti-substituted AB₂ alloys

Pressure–composition isotherms have been obtained for Ti_0.1Zr_0.9Mn_0.9V_0.1 Fe_0.5Ni_0.5, (Ti_0.1Zr_0.9)_1.1Mn_0.9V_0.1Fe_0.5Ni_0.5, Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55 having the C14 type hexagonal structure in the temperature and pressure ranges 30–100 °C and 0.1–50 bar using a pressure reduction method in order to find the effect of non-stoichiometry on hydrogen storage properties. The powder x-ray diffractograms of the alloys show the formation of C14 single phase; the lattice constants and unit cell volumes of these alloys have been calculated. Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 has a hydrogen storage capacity of 3.5 hydrogen atoms/fu at 20 bar and room temperature. The dependences of the thermodynamics of the dissolved hydrogen in these alloy hydrides in the temperature range 30–100 °C on the hydrogen concentration have shown the existence of α, α+β and β phase regions as seen in the isotherms. The effect of non-stoichiometry on the kinetics of hydrogen absorption in these alloys has been studied at 50 °C. The powder x-ray diffractograms of the alloy hydrides show that these alloys do not undergo any structural change upon hydrogenation and that there is only a volume expansion of around 25%. The activation energy and diffusion coefficient of dissolved hydrogen in Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Ni_0.5 have been calculated from the kinetics of hydrogen absorption measurements at different temperatures. The activation energy is E_a = 0.17 eV in the α+β phase region and E_a = 0.22 eV in the β phase region and the diffusion coefficient D = 2.7 × 10⁻¹¹ cm² s⁻¹ in the β phase at 30 °C.