Influence of amorphization on electrode performances of AB2 type hydrogen storage alloys

It is generally accepted that the electrochemical discharge capacity of a hydrogen storage alloy is closely related to its crystal characteristics. From our experiment, the discharge capacity of AB(2) type Laves phase hydrogen storage alloy ZrCr0.4Mn0.2V0.1Ni1.3 decreased sharply from 324 mAh g(-1) to 25 mAh g(-1) after amorphization by mechanically milling the crystalline (as-cast) alloy with stainless steel balls. We believe that for the as-cast alloy, hydrogen atoms stored in the tetrahedral interstitial sites of Zr2B2 (B=Cr, V or Ni) all contribute to its electrochemical capacity. However for the amorphous alloy only hydrogen atoms in low-energy Zr,B and Zr, tetrahedral interstitial sites contribute to its electrochemical capacity. We also believe that only half of the total sites in the crystalline or amorphous alloys can absorb and desorb hydrogen atoms reversibly, and that the process of ball milling manages to lower the barrier potential for the hydrogen discharge reaction by about 60 kT (mol H)(-1). We resolve the phenomenon into the finer particles including abundant hydrogen diffusion channels and the formation of a looser tetrahedral interstitial structure in the metal glass. (C) 1998 Elsevier Science S.A.