Encapsulation of La1.5Mg0.5Ni7 nanocrystalline hydrogen storage alloy with Ni coatings and its electrochemical characterization

Nanocrystalline La1.5Mg0.5Ni7 alloy has been synthesized by mechanical alloying and heat treatment (850 °C) and a 20-50 μm fraction of its particles has been encapsulated with three kinds of Ni-based coatings: (i) electroless NiP, 1 μm thick, (ii) magnetron sputtered Ni, 0.087 μm thick and (iii) magnetron sputtered Ni, 0.29 μm thick. The electrochemical charge/discharge multicycling of powder composite electrodes has been carried out in view of material potential usefulness for Ni/MHx batteries. The nanocrystalline La1.5Mg0.5Ni7 alloy exhibits four times lower capacity fade as compared to a microcrystalline LaNi5 reference compound. Modification of La1.5Mg0.5Ni7 particle surface with comparatively thick layer of electroless NiP coating deteriorates electrode corrosion behavior and worsens kinetics of hydrogen electrosorption due to unsatisfactory adhesion of the coating to the Mg-containing substrate. Corrosion protection of the La1.5Mg0.5Ni7 nanomaterial by magnetron sputtered Ni films depends on average film thickness. Relatively thick (0.29 μm) sputtered Ni film limits corrosion degradation and stabilizes exchange current density of H2O/H2 system. Particle modification by Ni encapsulation does not affect the hydrogen diffusivity. Effective diffusion coefficient of hydrogen for La1.5Mg0.5Ni7 nanocrystalline material, irrespectively of surface modification, is close to 2·10−10 cm2s−1.