ELECTROCHEMICAL AND SURFACE-PROPERTIES OF ZR(VXNI1-X)2 ALLOYS AS HYDROGEN-ABSORBING ELECTRODES IN ALKALINE ELECTROLYTE

Multicomponent Zr-Ni-based alloys of the nominal composition ZrNi2 have some promising properties as electrode materials in reversible metal hydride batteries. We have prepared a series of pseudobinary Zr(VxNi1-x)2 alloys in small samples of 6 g by levitation melting. Their bulk properties were then characterized by means of X-ray diffraction, hydrogen storage capacity and plateau pressure measurements. The kinetics were studied by means of high rate dischargeability and exchange current densities. The cycle life of pressed powder electrodes was tested with two different compacting materials, copper and nickel. The surface composition of the alloy grains was analysed by means Of X-Tay photoelectron spectroscopy (XPS) before introduction into the electrolyte, after activation and in the corroded state. For alloys of the cubic C15 (CU2Mg) structure the lattice parameter a depends linearly on the vanadium content. The hydrogen storage capacity reaches a maximum of about 350 mA h g-1 at the composition Zr(V0.25Ni0.75)2. Also, the stability of the hydrides depends strongly on the vanadium content, the room temperature plateau pressure rising to about 0.5 bar for Zr(V0.15Ni0.85)2- It does not follow the behaviour expected from the relative hydrogen affinities of the pure components (Zr > V > Ni). Changes in the crystal and/or electronic structure seem to be the reason. Under high rate discharge (120 mA g-1) we have found that alloys of the composition Zr(VxNi1-x)2 with 0.1 less-than-or-equal-to x less-than-or-equal-to 0.25 retain about 80% of their capacity. One factor affecting the rate of the discharge reaction is the charge transfer, which is measured by the exchange current density. The exchange current density is highest for Zr(V0.2Ni0.8)2. Values of 80 and 40 mA g-1 were determined for nickel- and copper-compacted electrodes respectively. Furthermore, nickel-compacted electrodes activate faster. However, they also corrode faster owing to the instability of nickel in the applied potential range. XPS analysis showed that activation of an electrode in an electrolyte causes the dissolution of oxides of vanadium from the surface layers and therefore an enrichment of nickel. The degradation process might involve oxidation of all the alloy elements, dissolution of the soluble oxides and/or loss of grain contact by the formation of electrically non-conducting oxides.