ELECTROCHEMICAL PERFORMANCES OF ZRM2 (M=V, CR, MN, NI) LAVES PHASES AND THE RELATION TO MICROSTRUCTURES AND THERMODYNAMICAL PROPERTIES

Zr-based AB(2) (B=V, Cr, Mn) Laves phases are studied for their abilit y to reversibly store hydrogen and are potential negative electrode ma terials in Ni-MH batteries. The hydrides formed are too stable for ele ctrochemical use and their thermodynamical properties can be adapted b y Ni substitution on the B sublattice. However, measured electrochemic al reversible capacities are very low compared to solid-gas results. T his phenomenon is related to passivation and surface corrosion in elec trolytic medium. To overcome this problem, the role of secondary phase s has been studied following two routes: precipitation of controlled a mounts of Zr-Ni binary intermetallic compounds or addition of a rare e arth (R) element leading to precipitation of RNi compound in the matri x. The consequences on the electrochemical behaviour are discussed in terms of the microstructure of the alloys which lead to composite elec trodes by taking advantage of the bulk properties of the Laves phase a nd the surface properties of the secondary phase(s).