Optimizing Hydrogen Storage Pathways in Ti–Al Alloys through Controlled Oxygen Addition

Abstract

In the present study, we aimed to destabilize the Ti–Al system with nonmetallic oxygen. The synthesis of α-(Ti, Al)[O] starting from TiO₂, Ti, and Al was carried out through the arc melting method, resulting in three different oxygen content levels, 3.4, 10, and 20 at%. The room temperature activation of α-(Ti, Al)[O] was not successful, and the activation was performed at 300°C under 5 MPa H₂ pressure. The structural changes after hydrogenation (maximum absorption capacity of 3.74 wt% hydrogen) arose from the transformation of α-(Ti, Al)[O] to cubic (Ti, Al)[O]H_x (c-(Ti, Al)[O]H_x); nonetheless, they recovered their original lattice parameters, which are meaningfully larger than those of α-Ti, after dehydrogenation. The hydrogen storage capacities for various α-(Ti, Al)[O] compositions generally decreased with increasing oxygen (3.4 and 10 at%) and aluminum content in the alloy. In contrast, for the compositions with a higher oxygen content of 20 at%, the hydrogen storage capacity slightly increased as the Al concentration increased: Ti_0.790Al_0.010O_0.200 absorbed 2.91 wt% hydrogen, whereas Ti_0.767Al_0.033O_0.200 absorbed 3.04 wt% hydrogen. The thermogravimetric analysis showed that samples with 20 at% O released hydrogen at lower temperatures even though the major phase after hydrogenation is c-(Ti, Al)[O]H_x regardless of the oxygen content.