Electrochemical Behavior of a Nanostructured La1.25Gd0.25Mg0.5Ni7 Hydrogen Storage Material Modified with Magnetron Sputtered Nickel

Abstract

A nanocrystalline La1.25Gd0.25Mg0.5Ni7 hydrogen storage material with a dominant A2B7 phase was obtained by mechanical alloying (MA) and heat-treatment (1123 K for 0.5 h). The concentration of the AB5 phase is 6.4 at.% and is marginally lower than that in the Gd-free parent La1.5Mg0.5Ni7 alloy. The powder fraction of 20–50 μm was encapsulated with thin (0.31 μm thick), amorphous nickel using magnetron sputtering. Electrochemical multicycling allowed the determination of the corrosion kinetic parameters, as well as the exchange current density of a H2O/H2 system and the effective diffusivity of hydrogen. Partial substitution of Gd for La in the La1.5Mg0.5Ni7 nanocrystalline parent alloy increases the electrode discharge capacity (up to 16%) but somewhat deteriorates its activation properties. Gd addition does not affect the corrosion degradation resistance, although the corrosion properties are weakened after particle encapsulation with the amorphous nickel coating. Finally, in spite of the Gd-modified La1.5Mg0.5Ni7 nanocrystalline alloy revealing a 10% lower exchange current density of the H2O/H2 system compared to the parent alloy, this property can be considerably improved (by >50%) by Ni particle encapsulation. The hydrogen diffusivity of the Gd-modified material is faster compared to the parent material but is slowed down by the Ni coating.