Effect of Metal-Substitution on Hydrogen Storage Properties for Mischmetal-Nickelf Alloys

Abstract

A study was made on the characteristics of the hydrogen absorption and desorption of Mm_1-x⋅AxNi₅ and MmNi₅ B_y of which Mm or. Ni in MmNi₅ were partially substituted by the metals (A and B). The dissociation pressure of, Mm_1-xAx Ni₅ (A: Al, B, Cu, Mn and Si)hydrides was higher than that of NmNi₅ hydride, whereas, the dissociation pressure of Mm⋅Ni₅_B_y (B: Al, Co, Cr, Cu, Fe, Mn, Si, Ti, V and Zn) hydrides depended on the con tent'of the metals, which substituted partially for nickel, and log P decreased in proportion to y. The dissociation pressure of MmNi_5-yB_y hydrides decreased in the following order: Mn> Al>Cr>Si>Fe>Co>Cu. A linear correlation was found between log P and the cell volum e of the alloy for the _5-yB_y. The trend of lower pressures along with the Mn and Al contents were in agreement with the cell volume correlation observed in other AB, compounds. MmNi_4.5B_0.5(B: Al, Cr, Mn and Si) and MmNi_2.5Co_2.5 could easily be activated and react fairly readily with hydrogen to form the hydrides MmNi_4.5Al_0.5H_4.9, MmNi_2.5Co_2.5H_5.2, Mm⋅MmNi_4.5Cr_0.5H_6.6 and, MmNi_4.5Si_0.5H_3.8 (hydrogen contents: 1.2, 1.2, 1.4, 1.5 and 0.9 wt%, respectively) under a hydrogen pressure of 60 atm at room temperature. 'The enthalpy changes in hydride formation determined from the dissociation isotherms for MmNi_4.5Al_0.5-H, MmNi_2.5 Cro_2.5-H and MmNi_4.5Mno_0.5-H systems were -5.5, -8.4, -6.1 and -4.2 kcal/mol H₂, respectively. These values were smaller than or nearl y the same as those of LaNi₅ and MmNi₅. The dissociation pressures were 2.5, 2.0, 4.8 and 2.0 atm at 20°C, respectively. The apparent activation energy of hydrogen desorption for these alloys were smaller than those of LaNi₅ and MmNi₅. For MmNi_4.5, Al_0.5, MmNi_2.5Co_2.5, MmNi_4.5Cr_0.5, and MmNi_4.5Mn_0.5 the hydrogen absorption-desorption cycles were repeated 500 times but no variation in the hydrogen absorption-desorption capacity was observed. The cyclic studies with MmNi_4.5Mn_0.5 showed that the spalling occurred more rapidly than with MmNi_2.5Co_2.5 and MmNi_4.5Cr_0.5