Mg – Ni – Cu mixtures for hydrogen storage: a kinetic study

Mg-based ternary mixtures (Mg wt% in the range 40% - 80%) containing increasing amount (up to 30 wt %) of Ni and Cu were prepared by ball milling (BM) under Ar for 16 h and subsequently activated at high temperature (623 K) by charging/discharging cycles at H2 pressure of 50 bar/0.7 bar. The work aims to analyze the influence of the mixtures’ composition on the storage properties (H2 intake and sorption kinetics) and to describe the role played by an ad-hoc activation in reaching these same properties. The storage capacity of the mixtures decreases by decreasing the Mg starting content, the H2 active phases being ‘‘free Mg’’ and the ‘‘bonded Mg’’ intermetallic compounds Mg2Ni and Mg2Cu. After full activation (3 charging/ discharging runs), ‘‘free Mg’’ hydrogenates 10 times quicker than the ‘‘bonded Mg phases’’, while the discharging of both ‘‘free’’ and ‘‘bonded’’ Mg hydrides takes place simultaneously with similar kinetics. The best kinetic performance is shown by the samples with Mg = 60 wt% and 70 wt% and the highest Ni content (30% and 20% respectively), with sorption rates up to 7 times higher than those of the pure Mg/ MgH2 system.