Synergetic effects in electrochemical properties of ZrVxNi4.5−x (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) metal hydride alloys

The structure, gaseous storage, and electrochemical properties of a series of ZrVxNi4.5−x (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) metal hydride alloys were studied. As the V-content in the alloy increased, the main phase shifted from ZrNi5/cubic Zr2Ni7 to ZrNi5/monoclinic Zr2Ni7/ZrNi9 and then to monoclinic Zr2Ni7 only. Other secondary phases, such as ZrNi, Zr3Ni5, VNi2 and VNi3, were also observed. The microstructures of the alloys with relatively higher V-content (x ⩾ 0.3) were similar: 70% monoclinic Zr2Ni7 and 30% VNi2. Maximum hydrogen storage capacities measured in the gaseous phase study were low (<0.05 H/M) and decreased with the increase in V-content due to the increase in equilibrium hydrogen pressure. However, the equivalent hydrogen storage capacities converted from the electrochemical discharge capacities that were measured in the half-cell configuration were 5–15 times higher. The largest discharge capacity measured, 177 mA h g−1, was observed from the alloy with the highest V-content (ZrV0.5Ni4.0). The phenomenon of the electrochemical capacity being larger than its corresponding maximum gaseous phase storage capacity was attributed to the decrease in equivalent hydrogen pressure in the electrochemical environment. Two hypotheses were proposed for the lowering in equilibrium hydrogen pressure: first, the creation of an activated surface in KOH and secondly, the synergetic effect from neighboring secondary phases. Although the discharge capacity and surface catalytic capability could be improved through further composition optimization (done with the incorporation of other modifying elements), the bulk hydrogen diffusion coefficients measured from the ZrVxNi4.5−x system were the highest among all the available metal hydride alloys in rechargeable nickel/metal hydride battery applications. The improvement in bulk hydrogen diffusion coefficient makes ZrVxNi4.5−x system attractive for applications requiring extremely high power density, such as hybrid electric vehicles.

► Microstructure, PCT, and electrochemical properties of Zr(VNi)4.5 were reported.
► Zr2Ni7 is the main storage phase. Other phases include ZrNi5, ZrNi9, VNi2, and VNi3.
► Two structures, monoclinic and cubic, of Zr2Ni7 were found.
► High proton bulk diffusion is observed and recommended for high-power application.
► 5–15 Times higher discharge capacity than the PCT capacity was reported.