Mechanistic insight into hydrogen release from ball-milled titanium hydride: Roles of particle size, surface oxide layers, and ball-milling atmosphere

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Titanium and its alloys are extensively applied in the aerospace, biomedical, and energy industries owing to their unique physical and chemical properties. Titanium hydride (TiH 2 ), a simple metal hydride capable of reversibly absorbing and releasing hydrogen, has been utilized for hydrogen storage; additionally, it is considered an excellent catalyst because of its active hydrogen content. Consequently, understanding hydrogen migration within the Ti phase and hydrogen desorption is critical. In this study, we investigated the mechanisms governing H 2 release from TiH 2 by systematically analyzing the effects of particle size, surface oxide layers, and ball-milling atmosphere. Temperature-programmed desorption measurements and in situ transmission electron microscopy (TEM) observations reveal that reductions in the crystallite and grain sizes significantly lower the onset and first-stage H 2 -release temperatures, even in the presence of an oxide layer, by promoting surface-controlled desorption. Meanwhile, the surface oxide layer modestly increases the desorption barrier. TEM analyses clarify that the oxide layer is amorphous below 400 °C and is reduced and/or crystallized at higher temperatures, correlating with the second and third desorption stages. These stages are governed by coupled decomposition, phase transformation, and bulk hydrogen diffusion processes rather than surface-limited reactions. These findings provide a comprehensive understanding of the interplay between surface and bulk effects on H 2 release from TiH 2 and offer valuable insights for optimizing titanium-based hydrogen storage materials.