Activation behavior of an AB2 type metal hydride alloy for NiMH batteries

• Activation of (TiZr) (VNiMnCr)2 Laves phase alloy was studied by galvanostatic cycling.
• Alloy with coarse particle size activates faster due to formation of fresh surfaces.
• Milled powders activate faster but capacity is adversely affected by extended milling.
• Hot alkaline treatment leads to fast activation yielding a capacity of 390 mAh/g.
• This high capacity was attributed to surface roughness raising hydrogen pressure locally.

Activation behavior of an AB2, namely (Ti0.36Zr0.64) (V0.15Ni0.58Mn0.20Cr0.07)2 Laves phase alloy, was investigated with regards to; particle size, ball milling and hot alkaline treatments. Galvanostatic cycling in open cells showed that an untreated alloy initially had almost no capacity, but reached a value of 220 mAh/g after 14 cycles. Experiments with different particle sizes showed that coarse particles activate faster yielding an improved capacity. In terms of activation more pronounced effect was obtained with boiling the alloy powder in a hot KOH solution. A capacity in excess of 300 mAh/g is reached in the first cycle after a 20 min treatment. The capacity was highest after 80 min, yielding a value of 390 mAh/g well above that expected from the gas-phase storage in the alloy. This was attributed to the formation of rough surface in the powder, which may stabilize hydrogen bubbles allowing pressures above 1 atm to be reached locally in the surface.