Abstract
We investigated the pressure–composition ( P– C) isotherms of several binary RNi 5 (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 RNi 5 compounds have similar P– C isotherms with two well-separated plateaux except for CeNi 5, indicating that two hydrides with compositions of RNi 5H 3–4 and RNi 5H 6–7 are formed. With increasing the atomic number of R element in the RNi 5 compounds, the unit cell volume decreases due to the decrease of the radius of the R element. This influences the absolute enthalpies of dehydrogenation |Δ H| 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 LaNi 5–H 2 system, the |Δ H| of γ hydride is smaller than that of |Δ H| 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 |Δ H| value approaches to zero.
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