$AB_2$ type Zr-based Laves phase alloys have been studied for potential application as negative electrode in a Ni/MH battery, because of their large discharge capacity and relatively good durability for charging-discharging cycling.
However, They have a serious disadvantage of poor activation behavior in KOH solution. Several researches have been done to solve this problem resulting from oxidation or hydroxidation on alloy surface.
These researches can be divided into two fields. One is related to surface treatment, which is eliminating process of surface oxide film, the other is alloy design method.
But, surface treatment method require additional process in electrode fabrication. And that is usually performed under hot temperature atmosphere, which do severe damage to property of cathode.
Alloy designing is much easier method for Zr-based alloy activation without such a shortcoming. alloy design method has focused on phase controlling to make multi-phase microstructure. In that case, its second phase acts as catalytic site for hydrogenation reaction.
Zr-Cr-Ni-La(Ce), Zr-V-Mn-Ni-La(Ce), Zr-Sc-V-Mn-Ni are example of multi-phase alloy having exellent activation property. But, in veiw of discharge capacity. It is no more than 290mAh/g.
In case of $Zr_0.23V_0.3Mn_0.2Ni_0.3$, it shows good performance in activation and discharge capacity is all but 315mAh/g, and activation mechanism has not been known. So, we were in search of ellucidating of its mechanism, on basis of that knowledge, developed Zr-based alloy with both high discharge capacity and good activation property.
Using morphological and electrochemical analysis, we could find that surface morphology and electocatalytic activity change during immersion in KOH solution. V-rich second phases are selectively corroded and dissolved to become Ni-rich phases. Resulting from these surface reaction in KOH solution, self-hydrogen charging occur through Ni phase. However, that alloy have poor cyclic durability because of such a corrosion mechanism. Finally we developed durable alloys by substitution alloying element.
