Kinetics tuning of TiFe alloys by elemental substitution: Enabling activation under moderate conditions

This study investigates the activation behavior of TiFe0.80-X0.20 (X = Co, Cu, Cr, Al) alloys to identify the most effective substitutions for improving hydrogen absorption kinetics. Activation was tested using two methods: a Sievert Volumetric Apparatus at room temperature and 64 bar of hydrogen, and high-pressure differential scanning calorimetry with 50 bar hydrogen under thermal cycles up to 400 °C. Activation properties were analyzed by assessing time for incubation and for full charging, that are influenced, respectively, by surface and bulk diffusion of hydrogen. In addition to phase composition, the choice of substituting elements directly affects diffusion-related parameters, such as hydrogen diffusion coefficients and activation energy, thereby influencing the kinetics of activation even in the absence of phase changes. Results showed that Cr-substituted alloys are rapidly activated, owing to the presence of TiCr2 compound, similarly to Al-containing alloys that absorbed hydrogen immediately. In contrast, Co- and Cu-substituted alloys required extended activation times, due to less quantity of secondary phases and limited diffusion channels. The results provide a comparative basis for alloy selection and design of TiFe-based materials for hydrogen storage systems operating under moderate conditions.