Hydrogenation Properties of RNi<SUB>5</SUB> (R: Rare Earth) Intermetallic Compounds with Multi Pressure Plateaux

Abstract

Some hydrogen storage materials with the hexagonal CaCu5-type crystal structure have two pressure plateaux during hydrogen absorption and desorption processes. To clarify the correlation of hydrogenation properties between two plateaux, we investigated the pressure-composition isotherms of several binary RNi5 (R = La, Ce, Pr, Nd and Sm) intermetallic compounds at pressures up to 35 MPa and at temperatures ranging from 196 to 423 K by use of a modified Sieverts’ method. We also determined the unit cell volume of the compounds to correlate the hydrogenation properties with the crystal structure. It was found that RNi5 compounds had qualitatively similar hydrogenation properties in that two well-separated plateaux indicated the existence of two hydrides with the compositions of RNi5H3–4 (β phase) and RNi5H6–7 (γ phase) except CeNi5. Similar to the lower plateau pressure, the higher plateau pressure increases with increasing the atomic number of R element or with decreasing the unit cell volume of these compounds. For LaNi5–H2 system, the pressure gap between two plateaux expands with increasing temperature, indicating that the absolute value of enthalpy of dehydrogenation (|ΔH|) of β hydride is smaller than that of γ hydride or |ΔH(β)|<|ΔH(γ)|. On the other hand, for other RNi5–H2 systems the pressure gap expands with decreasing temperature; that is, the |ΔH| values of both hydrides show the opposite tendency or |ΔH(β)|>|ΔH(γ)|. The maximum hydrogen content of each RNi5 compound exceeds six hydrogen atoms per formula unit and some compounds absorb the hydrogen to reach the hydride composition of RNi5H7 without new plateau.