Abstract
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.
Highlights
We present a systematic study of designing an optimal composition of V-substituted TiFe alloy, exhibiting P2 /P1 ratio close to unity, maximizing the usable capacity under the target pressure range of 0.1–1 MPa
The composition of Ti51 Fe49 was chosen as the starting composition instead of Ti50 Fe50 because P2 of Ti50 Fe50 is higher than 1 MPa at 30 ◦ C [1], outside the target pressure range in this study
When comparing the plateau pressures, we focused on the desorption plateau pressures because the pressure hysteresis in Figure 7 was apparently affected by the composition and the absorption plateau pressures were less systematically changed by the composition change
Summary
The room-temperature hydrogen-storage alloy TiFe is the forerunner in the quest for a suitable medium owing to the abundance of constituting elements and appreciable hydrogen-storage capacity of 1.9 wt.% H [1,2] near ambient temperature and pressure. The two major bottlenecks must be overcome to commercialize TiFe alloy as a room-temperature hydrogen-storage material. The initial hydrogenation, or activation, should be carried out at room temperature. A high-temperature activation procedure was usually involved for activating pure TiFe [1]. Usable capacity under a practical operation pressure range, e.g., 1 MPa for hydrogen absorption and 0.1 MPa for desorption, must be increased. Hydrogenation of TiFe proceeds in two steps, sequentially forming TiFeH monohydride and TiFeH2 dihydride. The ratio of equilibrium pressure for each step, P2 /P1
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