Synthesis and characterization of nanocrystalline magnesium-based hydrogenstorage alloy electrode materials

Nanocrystalline Mg2Ni-type hydrogen storage alloys were synthesized using t he powder metallurgical technique and mechanical milling with nickel powder under an argon atmosphere. Changes in phase structures with mechanical tre atment were studied by X-ray diffraction (XRD) and transmission electron mi croscopy (TEM) with selected-area electron diffraction (SED). As expected, the phase structure of the synthesized material changes with milling time, from crystallite to nanocrystallite and amorphous. The electrochemical beha vior of these materials with various phase structures in alkaline media, su ch as electrochemical capacity, high rate dischargeability and cycle life, were investigated and compared. The Mg2Ni type alloy electrode with nanocry stalline structure showed superior discharge characteristics compared with those with crystallite and amorphous structure. It was also found that the electrochemical hydriding/dehydriding behavior of Mg2Ni-type hydrogen stora ge alloys has been further improved by a new approach of particle inlay dev eloped in this study. The analysis of the electrochemical impedance spectro scopy (EIS) reveals that for the nanocrystalline Mg2Ni type alloy electrode , both the charge transfer resistance of the hydrogen redox reaction on the surface and the diffusion resistance of the hydrogen atom in the bulk allo y were greatly reduced by mechanical treatment with nickel addition. The ra te-determining step of the electrode reaction changed with the electrode na nocrystalline structure. (C) 1999 Elsevier Science Ltd. All rights reserved .