Abstract : In the frame of the research on intermetallic compounds for nickel-hydride battery application, attention is focused on AB2 alloys so called Laves phases.
ZrCr2, for example, exhibits a large hydrogen capacity but a too low hydride formation equilibrium pressure for practical application. Moreover, the electrolyte (concentrated potassium hydroxide) involves surface passivation through blocking electrochemical mechanisms.
First, the study is devoted to the adjustment of the hydride thermodynamic properties regarded to electrochemical application. This was achieved on ZrCr2 by substitution of chromium by nickel. In a second part surface properties are modified by the precipitation of secondary phases in equilibrium with the Laves phase.
The Zr-Ni-Cr phase diagram has been determined at 1000°C in the ZrCr2-ZrNi-Ni-Cr region and has revealed large solubility of nickel in C14 and C15 Laves phases as well as equilibrium of these phases with three Zr-Ni intermetallic compounds. The knowledge of this diagram has allowed the synthesis of biphasic alloys where the Laves phase of constant composition is in equilibrium with a secondary phase at variable rate.
The study of hydrogenation properties of Laves phases has shown the possibility of adjusting the equilibrium pressure of its hydrides to electrochemical application keeping high hydrogen capacity. The hydrogenation properties of secondary phases were also characterised.
Finally, it appeared that secondary phases precipitation with increasing rate in the Laves phase matrix produces a spectacular increase of electrochemical capacities. Such an effect is interpreted by the modification of the surface in contact with electrolyte and by the diffusion of hydrogen from electrolyte to the Laves phase through the secondary phase when present.