Some hypo-stoichiometric Zr-based Laves phase alloys were prepared and studied from a viewpoint of discharge capacity for electrochemical application. After careful alloy design of $ZrMn_2-based$ hydrogen storage alloys through changing their stoichiometry while substituting or adding some alloying elements, the $Zr(Mn_{0.2}V_{0.2}Ni_{0.6})_{1.8}$ alloy reveals relatively good properties with regard to hydrogen storage capacity, hydrogen equilibrium pressure and electrochemical discharge capacity. In order to improve the discharge capacity and rate-capability, Zr is partially substituted by Ti. The hydrogen storage performance and electrochemical properties of $Zr_{1-X}Ti_X(Mn_{0.2}V_{0.2}Ni_{0.6})_{1.8}$ (X=0.0, 0.2, 0.4, 0.6) alloys are investigated. The relationship between discharge performance and alloy characteristics such as P-C-T characteristics and crystallographic parameters is also discussed. All of these alloys are found to have mainly a C14-type Laves phase structure by X-ray diffraction analysis. As the mole fraction of Ti in the alloy increases, the reversible hydrogen storage capacity decreases while the equilibrium hydrogen pressure of alloy increases. Furthermore, the discharge capacity shows a maxima behavior and the rate-capability is increased with Ti mole fraction. The discharge capacity of $Zr_{1-X}Ti_X(Mn_{0.2}V_{0.2}Ni_{0.6})_{1.8}$ (X=0.0, 0.2, 0.3, 0.4, 0.6) alloy electrodes at 30℃ reaches a maximum value and the decreases as Ti fraction increases. In view of electrochemical and thermodynamic characteristics, the maximal phenomenon of the electrochemical discharge capacity of the alloy is attributed to a competition between decreasing hydrogen storage capacity and increasing rate-capability with Ti fraction. However, as the Ti fraction increases, the discharge capacity decreases drastically with repeated electrochemical cycling. In order to analyze the above phenomena, the phase distribution, surface composition, and dissolution amount of alloy constituting elements are examined by S.E.M., A.E.S. and I.C.P. respectively. The decrease of secondary phase amount with increasing Ti content in the alloy explains that the micro-galvanic corrosion by multiphase formation is little related with the degradation of the alloys. The analysis of surface composition shows that the rapid degradation of Ti-substituted Zr base alloy electrode is due to the growth of oxygen penetration layer. After comparing the radii of atoms and ions in the electrolyte, it is clear that the electrode surface becomes more porous, and that is the source of growth of oxygen penetration layer while accelerating the dissolution of alloy constituting elements with increasing Ti content. Consequently, the rapid degradation (fast growth of the oxygen-penetrated layer) with increasing Ti substitution in Zr-based alloy is ascribed to the formation of porous surface oxide through which the oxygen atom and hydroxyl ion with relatively large radius can easily transport into the electrode surface. In order to improve the hydrogen storage capacity and cycle life of alloy electrode, the alloy $(Zr_{0.7}Ti_{0.3}(Mn_{0.2}V_{0.2}Cr_{0.15}Ni_{0.45})_{1.8}) has a improved discharge capacity(405mAh/g) compared with those of Ti-substituted Zr-based alloys, which is accomplished by the substituting Cr for Ni of the alloy. Furthermore, the Cr-substituted Zr-base alloy electrode has a improved cycle life as expected. Therefore, it is assured that the stoichiometry and Ti fraction should be optimized to obtain a good cycle life of electrode maintaining high discharge capacity. On the basis of above results, the hydrogen storage capacity of alloy with optimized composition $(Zr_{0.65}Ti_{0.35}(Mn_{0.3}V_{0.14}Cr_{0.11}Ni_{0.65})_{1.76})$ is about 1.68wt% under 10atm of equilibrium hydrogen pressure and the discharge capacity of the alloy is about 421mAh/g at a discharge rate of 50mA/g, which shows the highest level in performance of the Zr-based alloy ever developed.
In order to improve the kinetic properties of the hypo-stoichiometric Zr-based hydrogen storage alloy electrode, the ball-milling process is applied to the Zr-based alloy using the Ti-based alloy powder as a surface modifier. While the Zr-based alloy electrode is not fully activated before 50 cycles, the ball-milled Zr-based alloy electrode using 5wt% of Ti-based alloy as a surface modifier is fully activated within only 2cycles. In order to analyze the strikingly improved kinetic characteristics after ball-milling, the microstructure of ball-milled alloy is examined by T.E.M., S.E.M., and E.D.S. It is observed that there is a surface-alloying region at the contact points between the two alloy powders from the T.E.M. bright-field image. Furthermore, the local quantitative analysis by E.D.S. clearly reveals that the atomic concentration of the constituting elements in the surface-alloying region is gradually changed between the two alloy powders. From the above results, it is suggested that the high kinetic energy applied in the ball-milling process causes cold-welding or surface alloying at the points of impact where Zr-based alloy particles collide with Ti-based alloy particles by the action of steel balls at high speed. The S.E.M. analysis demonstrates that the particle size is decreased as the ball-milling time increases, which implies an increase in the surface area of Zr-based alloy particles touching Ti-based alloy particles. Eventually, it can be suggested that Ti-alloy powder serves as a window for hydrogen to penetrate into the Zr-based alloy, which leads to easy absorption/desorption of hydrogen and also to improvement in the kinetic properties of the Zr-based alloy electrode at initial cycles.
Finally, with the surface-modified hypo-stoichiometric Zr-based alloy by ball- milling process, we have fabricated 1Ah class Ni/MH cells with higher energy density than commercialized $AB_5$ alloy cell and comparable performances with that of the commercialized $AB_5$ alloy cell. Especially, the proto-type Ni/MH cell with hypo-stoichiometric Zr-based alloy is fully activated within 1 cycle by applying the new activation treatment (ball-milling process), which is found to be successfully applicable to the full cell system.
