Thermodynamic and corrosion study of Sm1-xMgxNiy (y = 3.5 or 3.8) compounds forming reversible hydrides

AB5 compounds (A = rare earth, B = transition metal) have been widely studied as anodes for Ni-MH applications. However, they have reached their technical limitations and the search for new promising materials with high capacity is foreseen. ABy compounds (2 < y < 5) are good candidates. They are made by stacking [AB5] and [A2B4] units along the c crystallographic axis. The latter unit allows a large increase in capacity, while the [AB5] unit provides good cycling stability. Consequently, the AB3.8 composition (i.e. A5B19 with three [AB5] for one [A2B4]) is expected to exhibit better cycling stability than the AB3.5 (i.e. A2B7 with two [AB5] for one [A2B4]). Furthermore, substitution of rare earth by light magnesium improves both the capacity and cycling stability. In this paper, we compare the hydrogenation and corrosion properties of two binary compounds, SmNi3.5 and SmNi3.8, and two pseudo-binary ones, (Sm, Mg)Ni3.5 and (Sm, Mg)Ni3.8. A better solid-gas cycling stability is highlighted for the binary SmNi3.8. The pseudo-binary compounds also exhibit higher cycling stability than the binary ones. Furthermore, their resistance to corrosion was investigated.