HYDRIDING PROPERTIES OF MMNI5 SYSTEM WITH ALUMINUM, MANGANESE AND TIN SUBSTITUTIONS

The hydrogen storage properties of annealed MmNi5-xMx (M=Al, Mn and Sn) systems containing Indian mischmetal (Mm) (composition (wt.%): Ce, 43; La, 23; Nd, 18; Pr, 5; Sm, 3; Fe, 8) were studied using desorption pressure-composition isotherms. The compositions studied were MmNi4.6Al0.4, MmNi4.2Al0.8, MmNi4.6Mn0.4, MmNi4.2Mn0.8 and MmNi4.6Sn0.4. The partial substitution of Ni in MmNi5 by Al, Mn and Sn led to a decrease in the hydride formation plateau pressure. The decrease in the plateau pressure was enhanced with increasing Al and Mn substitution. The enthalpy DELTAH and entropy DELTAS of hydride formation of these compounds were obtained. The enthalpy of hydriding decreased with increasing substitution. The variation in the enthalpy of hydriding and plateau pressure with Al, Mn and Sn substitution was explained by considering the increase in the unit cell volume of MmNi5 caused by substitution. The hydrogen storage capacity of MmNi5 was unaffected by Al and Mn substitution whereas Sn substitution led to a decrease in capacity.