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 ability to reversibly store hydrogen and are potential negative electrode materials in Ni-MH batteries. The hydrides formed are too stable for electrochemical use and their thermodynamical properties can be adapted by Ni substitution on the B sublattice. However, measured electrochemical reversible capacities are very low compared to solid-gas results. This phenomenon is related to passivation and surface corrosion in electrolytic medium. To overcome this problem, the role of secondary phases has been studied following two routes: precipitation of controlled amounts of Zr-Ni binary intermetallic compounds or addition of a rare earth (R) element leading to precipitation of RNi compound in the matrix. The consequences on the electrochemical behaviour are discussed in terms of the microstructure of the alloys which lead to composite electrodes by taking advantage of the bulk properties of the Laves phase and the surface properties of the secondary phase(s).