Hydrogen Storage Properties of Nanocrystalline Mg<sub>2</sub>Ni<sub>x</sub>Cu<sub>1-x</sub> Synthesized by Mechanical Alloying

Abstract

There is an increasing interest in developing Mg-based nanocrystalline hydrogen storage materials by mechanical alloying because Mg and its alloys have high storage capacity and their slow kinetics can be improved significantly by ball milling. However, their hydrides are still too stable for most practical uses. Improvement of the thermodynamics of Mg 2 Ni-based materials can be achieved by partial substitution. Nanocrystalline Mg 2 Ni x Cu 1-x (x-1, 0.95, 0.85, 0.75, 0.65, 0.5, 0.4) powders were prepared by mechanical alloying elemental powders of Mg, Ni and Cu. The ternary alloys have the same crystal structure as Mg 2 Ni and the nanostructures are stable below 623K. The hydrogen storage behaviors of these powders were investigated. The absorption rate at high temperature (573K) and hydrogen storage capacity do not change much with partial substitution of Ni by Cu. The pressure-composition-temperature (PCT) curves show two plateaus in the composition range of 0.95>x≥0.5. X-ray analyses indicate that the ternary alloys decompose on hydrogenation. Partial absorption to the end of the first plateau leads to the formation of nickel rich Mg 2 (Ni,Cu)H 4 which coexists with a residual copper rich Mg 2 (Cu,Ni). Some traces of Mg(Cu,Ni) 2 are also observed on the pattern. After full hydrogenation, a mixture of three phases Mg 2 (Ni,Cu)H 4 , MgH 2 and Mg(Cu,Ni) 2 was found. In conventional Mg 2 Cu, this kind of disproportionation reaction is usually slow and not always reversible. Here we found that the reaction is fast and fully reversible when the copper content is not too high (x>0.75). This is most likely related to the particular nanostructure and the partial substitution of Cu by Ni in the Mg 2 (Cu,Ni) alloy.