Thermodynamic insights on H migration and stability of hydrides in over-stoichiometric Ti doped Ti-Fe-Mn hydrogen storage alloy: A theoretical calculation and experimental study

TiFe-based hydrogen storage alloy shows great application potential due to the high capacity and fast reaction rate at room temperature. However, the thermodynamic mechanism remains unclear that resulting in difficulties in optimization of hydrogen storage properties. In this study, Ti_1.0Fe_0.8Mn_0.2 and Ti_1.1Fe_0.8Mn_0.2 alloys are designed, and phase analysis by both theoretical and experimental strategies are performed. The corresponding special quasi-random structure (SQS) models are constructed with over-stoichiometric Ti occupying Fe-site element positions. The experimental results show that over-stoichiometric Ti doping reduces the H₂ plateau pressure from 0.413 MPa to 0.322 MPa at 30℃. The Ti_1.1Fe_0.8Mn_0.2 alloy exhibits higher electron activity evidenced by the density of states, and higher energy is required for hydrogen desorption indicated by hydrogen vacancy formation energy analysis. The calculation results are well in agreement with the experiment results. The thermodynamic properties are caused by weakened anti-site Ti-H bonds and strengthened Fe-H bond, which collectively enhance the overall hydrogen bonding strength in hydride. Furthermore, it is proposed that in the solid solution state, hydrogen atoms preferentially occupy the 2c sites. As hydrogenation proceeds and the alloy transitions from a hydrogen solid solution to a hydride phase, hydrogen atoms preferentially occupy the 2a and 4e sites rather than the stable 2c sites. This occupation follows the sequence 2a → 4e → 2c during TiFeH and TiFeH₂ hydride formation, thereby minimizing the total system energy.