Studies of the effect of Hf doping on the electrochemical performance of C15 Laves type metal hydride battery anode alloys

This paper focuses on the studies of the electrochemical performance of arc-melted Hf-modified Ti-Zr based AB₂ Laves type metal hydride battery anode alloys with composition Hf_xZr_24-xTi_6.5V_3.9Mn_22.2Fe_3.8Ni₃₈La_0.3Sn_0.3 (x = 1–4). The phase composition and the morphology of the alloys were characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Metal-gas interactions and electrochemical performances were studied by building PCT diagrams and by electrochemical tests performed in a 9 N KOH electrolyte solution at ambient temperature when using the half-cells in a three-electrode assembly. All studied alloys were two-phase and contained the Laves phase intermetallics of C15 (major constituent) and C14 (minor constituent) types. An increase in Hf content caused a growth of the abundance of the C14 phase and was accompanied by a decrease in the electrochemical discharge capacities, exchange current densities, and H diffusion rates as compared to the Hf-free alloy. Among the Hf-containing alloys, an introduction of 2 wt% Hf into the alloy's composition resulted in a higher H diffusion rate, while the highest H storage capacity has been reached for the alloy with the lowest, 1 wt%, content of Hf. The surface of the alloys contained catalysing hydrogen exchange metallic nickel clusters with the highest enrichment observed for the 2 wt% Hf alloy. An influence of Hf on the hydrogen storage and electrochemical performance can be understood in terms of the modification of the intrinsic properties of the alloys following a substitution (Zr/Ti) → Hf as C15 type alloys show the fastest hydrogen diffusion rates while a gradually increasing content of the hexagonal C14 type in the Hf-containing alloys slows down the H diffusion process.