Thermodynamic particularities of some CeNi5-based metal hydride systems with high dissociation pressure

The thermodynamic behaviour of the CeNi5-H-2, Ce-0.8 La0.2Ni5-H-2 and Ce0.8La0.2Ni4.7Cu0.3-H-2 systems during a few activation absorption-desorption cycles was studied by means of high gaseous pressure technique. P-C isotherms in the temperature range from 273 to 353 K were measured and thermodynamic parameters for hydride formation and decomposition reactions were calculated. For all studied systems the same hydrogen capacity (6.5-6.8 H atoms per AB(5) unit at 1000 atm and 296 K) and the same isotherm shape with a long nearly horizontal plateau and very large hysteresis were obtained. The change in alloy composition in the range CeNi5 --> Ce0.8La0.2Ni5 --> Ce0.8La0.2Ni4.7Cu0.3 led to an increase in plateau length and a decrease in hysteresis factor P-abs/P-des. The hysteresis factor also decreased considerably after two or three activation cycles, by 4-8 times as compared with the first cycle. This phenomenon was connected not only with an absorption pressure decrease but also with a significant increase in desorption pressure. The relative change P-1/P-act in equilibrium pressure during the activation did not depend on alloy composition for the absorption reaction, but in the case of desorption it decreased with partial substitution of Ce and Ni by La and Cu correspondingly.