Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg2Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg2Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH2 and Mg2NiH4 – a certain amount of Mg2NiH0.3 was always present. Similar as Mg2NiH4 phase, Mg2NiH0.3 islands were located on the surface of MgH2 grains. Mg2NiH0.3 transformed into Mg2NiH4 at the expense of hydrogen from an adjoining MgH2 grain. In such a way, a clean double layer (Mg)–Mg2NiH4 was formed, acting as a gate for easy hydrogen desorption from MgH2. It was found that the Mg2NiH4 phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg2NiH4 desorption bridges and enrichment of Mg2NiH4 on LT1 during the ageing facilitated onset of rapid hydrogen desorption.

► Phase changes during incubation time of H desorption from MgNi–H were investigated.
► Mechanism of retarded H-desorption from H-charged Mg–Ni alloys was found.
► Internal hydridation of residual Mg2NiH0.3 provides easy-desorption bridges for H.