The Ti-based metal hydride electrode for Ni-MH rechargeable batteries

The pressure-composition isotherms and electrochemical characteristics of titanium-based hydrogen storage alloys have been studied for various compositions. In the Ti-Zr-V-Mn-Ni system, Ti0.2Zr0.05V0.4Mn0.35-xNix (x = 0.1-0.25) alloys were found to have a large hydrogen storage capacity (greater than 1.75 wt.% g(-1) alloy) and high discharge capacity (350-440 mA h g(-1)). Their structure was confirmed to be multi-phase and composed of an AB(2)-type C14 Laves phase matrix and V-rich b.c.c. second phase by using X-ray diffraction, scanning electron microscope and electron microprobe analyses. In order to identify the contribution of each phase to the high discharge capacity of multi-phase alloys, these two phases were prepared separately and their hydrogen storage capacities were investigated. It was found that the V-rich b.c.c. second phase was hardly hydrogenated in KOH electrolyte, though its theoretical hydrogen storage capacity was as high as 1.93 wt.% H g(-1) alloy as determined from the pressure-composition isotherms in the solid-gas reaction. Its potential hydrogen storage capacity was able to be utilized by the presence of a C14 matrix phase which had a catalytic activity for the charge-transfer reaction in KOH electrolyte.