Preparation of Hydrogen Storage Alloys from the Oxides by Calcium Co-Reduction in Molten CaCl2

The cost reduction of the hydrogen storage alloys is one of keys for practical application. As shown in Fig.1, the conventional procedure needs many heat cycles from the raw materials, i.e., oxide ores. A new process is studied to synthesize the hydrogen storage alloy directly from its oxide mixture. This method is operated in the molten CaCl2 and it combines the calcium co-reduction and the in situ dissolution of the by-product CaO, as shown in Fig.2. Because of a strong reducibility of Ca, the stable oxide of TiO2 can be reduced to Ti with only 500 ppm oxygen in a step. The dissolution of CaO is useful to enhance the reduction, deoxidation and alloying. The residual Ca and CaCl2 are easily soluble in water and environmentally friendly. The binary alloy consisting of 30 mol%Ti-70 mol%V and the intermetallic compound of TiCr2 were chosen as the examples, aiming at the ternary Ti-V-Cr alloy in future. The well-homogenized Ti-V solid solution and -TiCr2 were obtained from the mechanical mixture of refractive V2O3 and TiO2, and of Cr2O3 and TiO2, respectively. The molten CaCl2 was indispensable in both cases to complete the reaction. The melt of V2O5 covered the residual TiO2 and it was preferentially reduced to V particles, as shown in Fig.3(a), which prohibited the subsequent reduction and alloying. However, the usage of refractive V2O3 could promote the co-reduction, decrease the oxygen content and produce a good quality of powder with the homogeneous concentration. Morphology of the obtained powders was commonly coral-like slightly sintered fine particles, as shown in Fig.3(b). Their wide surface area seems suitable for the hydrogen absorption. The narrow stoichiometric compound TiCr2 could be also formed in the molten CaCl2. A tiny amount of Ti and Cr coexisted with the low-temperature phase, -TiCr2, although the reaction temperature and time were varied. These alloy powders showed the hydrogen storage capacity after the normal activation treatment, as shown in Fig.4. Fig.3 SEM images of the powder obtained (a) from TiO2+V2O5 and (b) from TiO2+V2O3.