Comparative study on the structure and electrochemical hydriding properties of MgTi, Mg0.5Ni0.5Ti and MgTi0.5Ni0.5 alloys prepared by high energy ball milling

MgTi, Mg0.5Ni0.5Ti and MgTi0.5Ni0.5 alloys doped with 10 wt.% Pd were prepared by high energy ball milling and evaluated as hydrogen storage electrodes for Ni–MH batteries. X-ray diffraction analyses indicated that the Mg0.5Ni0.5Ti and MgTi0.5Ni0.5 alloys could be monophased or composed of a nanoscale mixture of MgTi + NiTi and MgTi + MgNi phases, respectively. Their hydrogen storage characteristics were investigated electrochemically in KOH electrolyte. No activation step was observed during the cycling of the Mg–Ti–Ni electrodes in contrast to that observed with the MgTi electrode. The highest hydrogen discharge capacity was obtained with the MgTi0.5Ni0.5 electrode (536 mAh g−1) compared to 401 and 475 mAh g−1 for the Mg0.5Ni0.5Ti and MgTi electrodes, respectively. The ternary Mg–Ti–Ni alloys showed a better cycle life with an average capacity decay rate per cycle lower than 1.5% compared to ∼7% for the binary MgTi electrode. The Mg–Ni–Ti electrodes also displayed a much higher discharge rate capability than the binary MgTi electrode, especially with the Mg0.5Ni0.5Ti electrode. The origin of this was established on the basis of the anodic polarization curves, where a substantial decrease of the concentration overpotential (reflecting a higher hydrogen diffusivity) was observed for the Mg0.5Ni0.5Ti electrode.