Hydrogen sorption performance of Mg – Ni – C – TiO2 mixtures

Mg–Ni–C–TiO2 mixtures with fixed Mg: Ni: C ratio (80.75 wt%: 14.25 wt%: 5 wt%) and TiO2 content increasing from 0.5 wt% to 2.0 wt% were prepared by high energy ball milling (BM) under Ar for 2 different times (45 min and 90 min) and characterized with respect to their kinetic and thermodynamic hydrogen sorption properties. The aim of the work was to study the influence of the TiO2 presence on the activation behavior and the sorption properties of the Mg: Ni: C ternary system, studied in detail in a previous paper. When TiO2 is added to the mixtures, the hydrogen active phases remain “free Mg” and Mg2Ni, that fully hydrogenate during the charging runs, with formation of -MgH2 and Mg2NiH4, but the number of charging/discharging cycles needed for the sorption activation (performed at 623 K and 35 bar/0.9 bar hydrogen pressure) is reduced from 10 to 7. Moreover, the addition of TiO2: (a) does not lead to a decrease in the gravimetric capacity with respect to the ternary system (6.4 wt% H2), suggesting that the oxide increases the sorption efficiency of the active phases; (b) does not affect the equilibrium pressures of the Mg/MgH2 and Mg2Ni/Mg2NiH4 systems; (c) leads to a small decrease in the hydrides starting dehydrogenation temperatures (5 K and 4 K respectively at H2 pressure = 0.9 bar) and to a very small increase (1 kJ/mol H2) in the desorption enthalpy; (d) is responsible of an appreciable increase in the sorption rates (up to 10 times for absorption and to 2.7 times for desorption).