Ti-Mn based C14 Laves phase intermetallic compound was modified by substituting alloying elements such as Zr, V and Ni in order to design a high capacity MH electrode for Ni/MH rechargeable battery and its gaseous and electrochemical hydrogenation characteristics were investigated to find role of specific alloying element on the hydrogenation characteristics. Degradation mechanism was studied on the $AB_2$ type metal hydride, because its discharge capacity was largely decreased with charging/discharging cycles, initially. And then prevention method for the degradation was suggested from the discussion of degradation mechanism with respect to Ti and Zr based $AB_2$ type metal hydride electrode.
When V was substituted in Ti-Mn binary system, the crystal structure was maintained as C14 Laves phase at a composition of $Ti_{0.2}V_{0.4}Mn_{0.4}$ and $Ti_{0.4}V_{0.2}Mn_{0.4}$ and equilibrium pressure decreased below 1 atm without decreasing hydrogen storage capacity considerably. It was found that Ni should be included in Ti-V-Mn alloy in order to hydrogenate it electrochemically in KOH electrolyte by using the linear polarization method, because Ni had a catalytic effect on the charge transfer reaction in the electrochemical hydrogenation of MH negative electrode. But the substitution of Ni for Mn in Ti-V-Mn system caused the increase of equilibrium pressure above 1atm and decrease of hydrogen storage capacity due to the formation of Ti-rich BCC phase and the decrease of lattice volume. Zr was able to increase the reversible hydrogen storage capacity of Ti-V-Mn-Ni alloy without considerable change of hydrogenation properties owing to stabilizing the C14 Laves phase. The electrochemical discharge capacities of Ti-Zr-V-Mn-Ni system were in the range of 350 - 464 mAh/g and among them $Ti_{0.8}Zr_{0.2}V_{0.5}Mn_{0.5}Ni_{1.0}$ alloy had C14 Laves single phase and very high electrochemical discharge capacity of 460mAh/g.
Ti-Zr-V-Mn-Ni alloy was abruptly degraded within 10 cycles. It was found that a passive film was formed at the surface of alloy powder in the beginning and Mn whisker precipitated at the passive film in the end of degradation from the analysis of S.E.M morphologies for degraded electrode. The passive film was identified as Ti-dioxide $(TiO_2)$ from the analysis of A.E.S depth profile and X.R.D patterns. And contact and interfacial reaction resistance for the electrochemical hydrogenation reaction increased with formation and growth of $TiO_2$ from the analysis of E.I.S spectra. Accordingly, the degradation of Ti-based $AB_2$ type metal hydrides was caused by the formation and growth of $TiO_2$ film on the surface of metal hydride powder.
In contrast to Ti-based $AB_2$ type metal hydride, Zr-based metal hydride electrodes have long cycle-life. So that, it was necessary that Zr-based $AB_2$ type high capacity metal hydride was designed. In the Zr-Ti-V-Mn-Ni-X (X= Fe, Cr), $Zr_{0.8}Ti_{0.2}V_{0.36}Mn_{0.2}Fe_{0.27}Cr_{0.27}Ni_{0.9}$ alloy had high discharge capacity of 355mAh/g and did not degrade up to 225 cycles. From the I.C.P and A.E.S analyses, it was found that the surface of alloy powder became Ni-enriched region as Zr and V element dissolved up to 100cycles and then, thin Zr-oxide was formed at the surface as Zr dissolved slowly after 100 cycles in the KOH electrolyte. In the case of Zr-based metal hydride electrode, contact resistance did not increase with cycling and interfacial reaction resistance decreased up to 100 cycles and slowly increased after 100 cycles from the analysis of E.I.S spectra. Accordingly, the good cycle-life characteristics of Zr-based $AB_2$ type metal hydride is attributed to the formation of Ni-enriched region and thin Zr-oxide on the surface of metal hydride alloy powder in KOH solution.
It was concluded that cycle life of $AB_2$ type metal hydride electrode was depend on the stability of oxides formed on the surface of alloy in KOH solution and its thickness.
