Design of V-Substituted TiFe-Based Alloy for Target Pressure Range and Easy Activation.

Titanium iron (TiFe) alloy is a room-temperature hydrogen-storage material, and it absorbs hydrogen via a two-step process to form TiFeH and then TiFeH2. The effect of V addition in TiFe alloy was recently elucidated. The V substitution for Ti sublattice lowers P2/P1 ratio, where P1 and P2 are the equilibrium plateau pressure for TiFe/TiFeH and TiFeH/TiFeH2, respectively, and thus restricts the two-step hydrogenation within a narrow pressure range. The focus of the present investigation was to optimize the V content such that maximum usable storage capacity can be achieved for the target pressure range: 1 MPa for absorption and 0.1 MPa for desorption. The effect of V substitution at selective Ti or Fe sublattices was closely analyzed, and the alloy composition Ti46Fe47.5V6.5 displayed the best performance with ca. 1.5 wt.% of usable capacity within the target pressure range. At the same time, another issue in TiFe-based alloys, which is a difficulty in activation at room temperature, was solved by Ce addition. It was shown that 3 wt.% Ce dispersion in TiFe alloy imparted to it easy room-temperature (RT) activation properties.

Hydrogenation Properties of TiFe Doped with Zirconium.

The goal of this study was to optimize the activation behaviour of hydrogen storage alloy TiFe. We found that the addition of a small amount of Zr in TiFe alloy greatly reduces the hydrogenation activation time. Two different procedural synthesis methods were applied: co-melt, where the TiFe was melted and afterward re-melted with the addition of Zr, and single-melt, where Ti, Fe and Zr were melted together in one single operation. The co-melted sample absorbed hydrogen at its maximum capacity in less than three hours without any pre-treatment. The single-melted alloy absorbed its maximum capacity in less than seven hours, also without pre-treatment. The reason for discrepancies between co-melt and single-melt alloys was found to be the different microstructure. The effect of air exposure was also investigated. We found that the air-exposed samples had the same maximum capacity as the argon protected samples but with a slightly longer incubation time, which is probably due to the presence of a dense surface oxide layer. Scanning electron microscopy revealed the presence of a rich Zr intergranular phase in the TiFe matrix, which is responsible for the enhanced hydrogenation properties of these Zr-doped TiFe alloys.