Reversible Metal-Hydride Transformation in Mg-Ti-H Nanoparticles at Remarkably Low Temperatures

We study the kinetics of hydrogen sorption in Mg-Ti-H nanoparticles prepared by gas phase condensation of mixed Mg-Ti vapors under a H-2-containing atmosphere. Four samples with different Ti contents from 14 to 63at.% Ti are examined in the 100-150 degrees C range. The hydrogen absorption kinetics coupled with the formation of MgH2 can be described by a nucleation and growth model. The activation energy is in the range 43-52 kJ/mol and the rate constant (at 150 degrees C) increases from 2710-3 s(-1) to 9210-3 s(-1) with increasing Ti content. Hydrogen desorption is well modeled by a sequence of surface-limited and contracting-volume kinetics, except at the highest Ti content where nucleation and growth is observed. The activation energy of surface-limited kinetics is approximate to 32 kJ /mol. The rate constant (at 150 degrees C) increases from 0.510-3 s(-1) to 1.210-3 s(-1) with the Ti content. These results open an unexplored kinetic window for Mg-based reversible hydrogen storage close to ambient temperature.