Catalytic effects of TiO2 on hydrogen storage thermodynamics and kinetics of the as-milled Mg-based alloy

Abstract

The addition of transition elements as catalyst in the ball-milling process is reckoned as a very efficient method to ameliorate the hydrogen storage property of magnesium based materials. In this study, mechanical grinding was employed to prepare the Mg-based alloys La7Ce3Mg80Ni10 + x wt% TiO2 (x = 0–10) (named La7Ce3Mg80Ni10 -xTiO2 (x = 0–10)). The microstructure characterization of as-milled alloys was achieved with the help of XRD, SEM and TEM. The isothermal and non-isothermal hydrogen storing kinetics was measured by Sievert apparatus, DSC and TGA with H2 detector. Hydrogen desorption activation energies were evaluated by Arrhenius and Kissinger methods. According to the result, the as-milled alloys possess an amorphous and nanocrystalline structure. Besides, the particle size of alloy containing TiO2 obviously diminishes compared with the TiO2-free alloy, which suggests that the additive TiO2 enhances the efficiency of mechanical milling. Moreover, the as-milled (x = 5) alloy has the optimal activation property, hydrogenation and dehydrogenation kinetics, and it absorbs 4 wt% H2 in 45 s at 473 K and 3 MPa, desorbing 3 wt% H2 in 172 s at 573 K and 1 × 10−4 MPa. The effect of TiO2 content on the thermodynamics property of alloys is slight, and the absolute value of the dehydrogenation enthalpy change (ΔH) of the as-milled (x = 5) alloy is 73.03 kJ/mol. In addition, the as-milled (x = 5) alloy has the minimum dehydrogenation apparent activation energy (57.4 kJ/mol), which also explains why the as-milled (x = 5) alloy exhibits the optimal hydrogen storage property.