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 vertical bar ΔH vertical bar 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 vertical bar ΔH vertical bar of γ hydride is smaller than that of vertical bar ΔH vertical bar 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 vertical bar ΔH vertical bar value approaches to zero