Abstract:

The correlation between structural factors and hydrogen storage capacity of nonstoichiometric TiMn_x (x = 1.4, 1.5, 1.6, 1.7) alloys was investigated systematically. The capacities of TiMn_x alloys were different with changing the x value, although all the alloys show a C14-type Laves phase. The TiMn_1.4 alloy shows 0.65% (w, mass fraction) capacity and a sloping plateau with hysteresis, and the TiMn_1.5 alloy exhibits the highest capacity of 1.2% (w) and a flat plateau. By further increasing the x value, the hydrogen storage capacity was reduced significantly, such as approximately 0.30% (w) and zero for the alloys with x = 1.6, 1.7, respectively. Structural analysis shows that the Ti atoms partially occupy the Mn(2a) site in the nonstoichiometric TiMn_x alloys, and the increasing occupation factor (g) exhibits a negative effect on the volume of tetrahedron interstitials for hydrogen storage. The key factors for changing the volume of tetrahedron interstitials are the bond lengths of Ti―Ti and Mn(2a)―Mn(2a), where Mn(2a)―Mn(2a) bonds play a dominant role in improving the hydrogen storage capacity.

Key words: Hydrogen storage alloy, Laves phase, Tetrahedral interstitial, Nonstoichiometrics, Atom occupation