HYDROGEN STORAGE PROPERTIES OF MMNI5-XMNX SYSTEM BASED ON INDIAN MISCHMETAL

Intermetallics of composition MmNi5-xMnx (x = 0.4 and x = 0.8) using Indian mischmetal (Mm) have been synthesized and evaluated for hydrogen storage applications. They have the same hexagonal structure as CaCu5 and show a tendency towards hydride formation. The plateau pressure of hydriding is found to decrease with increasing Mn content. Annealing does not significantly affect the hydride formation pressure. The thermodynamic properties, e.g. enthalpy (DELTAH) and entropy (DELTAS) of hydriding have been estimated experimentally. The enthalpies of hydride formation are -23.78 and -30.77 kJ mol-1 H-2 for MmNi4.6Mno0.4 and MmNi4.2Mn0.8, respectively. The entropies of hydride formation are -103.14 and -110.68 J K-1 mol-1 H-2 for MmNi4.6 Mn0.4 and MmNi4.2Mn0.8, respectively. The theoretically estimated enthalpy values are -11.55 and -12.56 kJ mol-1 H-2 for MmNi4.6Mn0.4 and MmNi4.2Mn0.8, respectively. The decrease in the heat of hydride formation has been related to an increase in the volume of the unit cell of MmNi5-xMnx.