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 7MgH2/TiH2, 10MgH2/TiH2, and MgH2 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 MgH2 doped with TiH2 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 MgH2 without TiH2 doping.

► The addition of TiH2 significantly increases the dehydrogenation rate of MgH2. ► Complete rate expressions for the dehydrogenation of milled 7MgH2/TiH2, 10MgH2/TiH2 and MgH2 were formulated. ► The apparent activation energy of dehydrogenation of TiH2 was 118 kJ/mol and that of 10MgH2 was 107 kJ/mol compared with 226 kJ/mol without the addition of TiH2. ► 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.