Kinetics of dehydrogenation of the Mg–Ti–H hydrogen storage system

Among the proposed hydrogen storage systems, magnesium alloys have proved to be promising since they are rechargeable with high hydrogen capacities (theoretically up to 7.6 wt.%), reversibility and low costs. Small particle size, which can be achieved by milling, and small amounts of transition-metal compounds as catalysts result in increased hydrogen release/uptake kinetics. In this work, we developed the rate expression for the dehydrogenation of milled 7MgH 2/TiH 2, 10MgH 2/TiH 2, and MgH 2 samples. The complete rate expressions, together with the values of activation energy and other rate parameters, were determined for the three milled samples by analyzing data obtained from non-isothermal thermogravimetric analysis (TGA). The MgH 2 doped with TiH 2 by high-energy milling displayed substantially reduced apparent activation energy of 107–118 kJ/mol and significantly faster kinetics, compared with 226 kJ/mol for similarly milled MgH 2 without TiH 2 doping. ► The addition of TiH 2 significantly increases the dehydrogenation rate of MgH 2. ► Complete rate expressions for the dehydrogenation of milled 7MgH 2/TiH 2, 10MgH 2/TiH 2 and MgH 2 were formulated. ► The apparent activation energy of dehydrogenation of TiH 2 was 118 kJ/mol and that of 10MgH 2 was 107 kJ/mol compared with 226 kJ/mol without the addition of TiH 2. ► A procedure for obtaining the rate expression from non-isothermal dehydrogenation data was described. ► The important effect of the positive enthalpy of dehydrogenation on the temperature effect of dehydrogenation rate was explained quantitatively.