Extensive investigations have been carried out for developing a Zr-based Laves phase alloy with high capacity and high rate-capability for electrochemical application. After some careful selection of ZrMn2 Laves phase alloys to have suitable hydrogen equilibrium pressure and to be discharged in KOH solution, the $ZrMn_{0.5}V_{0.5}N_{1.4}$ alloys met the above requirements reasonably well. The hydrogen storage performance and discharge characteristics of $ZrMn_{0.5}V_{0.5}N_{1.4}$ (y = 0.0, 0.2, 0.4 and 0.6) alloys were determined for variable Ni content, which is believed to have some influence on the rate capability. As the Ni content in the alloy increased, the hydrogen storage capacity decreased and the hydrogen equilibrium pressure increased. However, it was unexpectedly found that the rate-capability of these alloys decreased with increasing Ni content. In order to analyze the above phenomena, the reaction surface area, surface reaction kinetics and surface morphologies were examined by the BET method, measurements of the exchange current and SEM respectively. The decrease of reaction surface area with increasing Ni content in this alloy explains that the major factor affecting the rate capability is the reaction surface area and not the specific surface catalytic activity. After comparison of discharge characteristics such as discharge capacity and rate capability of $ZrMn_{1-x}V_{x}N_{1.4+y}$ (x = 0.5, 0.7; y = 0.0, 0.2, 0.4 and 0.6) for variable Mn to V ratio, it is found that the more Mn rich alloy showed higher rate capability due to a larger reaction surface. On the basis of the above results, the alloy composition was optimized for a Mn to Ni ratio of 0.7 : 1.2, i.e. for ZrMn0.7V0.5Ni1.2. In order to increase the discharge capacity of the ZrMn0.7V0.5Ni1.2 alloy, Ti was partially substituted for Zr and the stoichiometry was also changed. This change also increases the rate-capability. After such careful alloy design reached by substitution and changing stoichiometry, $Zr_{0.9}Ti_{0.1}(Mn_{0.7}V_{0.5} Ni_{1.4})_{0.92}$ alloys with high capacity and high rate-capability have been developed. This alloy has a discharge capacity of 394 mAh/g at 0.25C discharge rate and shows high rate-capability equaling that of commercialized AB5 type alloys.
Systematic work has been done on investigating the inner cell pressure characteristics of sealed type Ni-MH battery in which $Zr_{0.9}Ti_{0.1}(Mn_{0.7}V_{0.5}Ni_{1.4})_{0.92}$ alloys is used as anode. The inner pressure of this type Ni-MH battery increases more rapidly with the charge/discharge cycling than that of the other type Ni-MH battery with commercialized $AB_{5}$-type alloy. The increase of inner cell pressure in the sealed type Ni/MH battery with above Zr-Ti-Mn-V-Ni alloy is mainly due to the accumulation of oxygen gas during charge/discharge cycling. The accumulation of oxygen is due to the low rate of oxygen recombination on the MH electrode surface during charge/discharge cycling. The difference in oxygen recombination rate between $AB_{5}$-type electrode and Zr-Ti-Mn-V-Ni alloy electrode is caused by the difference in electrode reaction surface area resulting from different particle size after their activation and the difference in surface catalytic activity for oxygen recombination reaction, respectively. After EIS analysis, it is identified that the surface catalytic activity affects more dominantly on the oxygen recombination reaction than the reaction surface area. In order to suppress the inner cell pressure of Ni-MH battery with Zr-Ti-Mn-V-Ni alloy anode, it is suggested that the surface catalytic activity for oxygen recombination should be improved.
In order to improve the surface catalytic activity of Zr-Ti-Mn-V-Ni alloy which were closely related to the inner pressure behavior in a sealed cell the electrode was fabricated by mixing alloy with Cu powder. By replacing carbon black with Cu powder in 50% the inner cell pressure rarely increases with cycles, that is the best inner cell pressure behavior. This is due to the active gas recombination reaction. After measuring the surface area of electrode and surface catalytic activity, it is found that the surface catalytic activity for oxygen recombination reaction is much more improved in 50% Cu powder-mixed electrode. This phenomenon is due to the thin Cu layer on the Zr-Ti-Mn-V-Ni alloy surface which is formed by alternate dissolution and precipitation reactions of mixed-Cu powder during cycling. This thin Cu layer has the role of preventing the alloy surface from oxidation and enhnace the possibility of the presence of metallic Ni, which is believed to have a high catalytic activity for oxygen recombination reaction, on MH surface. Therefore, the inner pressure of the cell with a Zr-Ti-Mn-V-Ni alloy is decreased to a level equaling to that with the commercial $AB_{5}$-type alloy
