Systematic investigation on hydrogen storage properties of RNi5 (R: rare earth) intermetallic compounds with multi-plateau

We investigated the pressure–composition (P–C) isotherms of several binary RNi5 (R=La, Ce, Pr, Nd, Sm and Gd) intermetallic compounds to clarify the correlation of the hydrogen storage properties between multi-plateaux. It was found that all RNi5 compounds have similar P–C isotherms with two well-separated plateaux except for CeNi5, indicating that two hydrides with compositions of RNi5H3–4 and RNi5H6–7 are formed. With increasing the atomic number of R element in the RNi5 compounds, the unit cell volume decreases due to the decrease of the radius of the R element. This influences the absolute enthalpies of dehydrogenation |ΔH| of both hydrides (β and γ) to decrease, which consequently leads to the increase of the second plateau pressure as well as the first plateau. Contrary to LaNi5–H2 system, the |ΔH| of γ hydride is smaller than that of |ΔH| of β hydride for the other systems, which indicates that the pressure gap between two plateau pressures expands with decreasing temperature. The second plateau tends to disappear with large atomic number of R element because the |ΔH| value approaches to zero.