ABSORPTION-DESORPTION CHARACTERISTICS OF HYDROGEN FOR MISCHMETAL-NICKEL-COBALT ALLOYS

Mischmetal (Mm)-nickel-cobalt alloys MmNi5-xCo2(x = 1.0~4.0) have been found to have the same hexagonal structure as those of LaNi3 and MmNi3. They react readily with hydrogen to form hydrides of MmNi6.0Co1.9H6.3, MmNi3.0Co2.0H6.3, MmNi3.0Co2.8H8.2, MmNi2.0Co3.0H5.2, and MmNi1.6Co1.6H3.3 (hydrogen content: 1.4, 1.4, 1.2, 1.1 and 0.8wt%) under 60 atm hydrogen pressure at room temperature. Dissociation pressures of these hydrides are dependent on the content of cobalt, which substitutes partially for nickel, and log p decreases gradually with increasing content of cobalt. An enthalpy change — 8.4 kcal/mol H2 in the hydride formation is given for MmNi2.3Co2.3, and the dissociation pressure at 30°C yields 3.0 atm. The absorption rate of hydrogen for MmNi3-xCox(x=1.0~4.0) is getting lower than that for MmNi5 with increasing at, while the desorption rate of hydrogen remains constant when taking x >1.0; For MmNi2.3Co2.5 the hydrogen absorption-desorption cycles are repeated 30 times, but the variation in the hydrogen absorption-desorption ability has hardly been noticed. Mm0.5Ca0.5 Ni2.5Co2.5 reacts with hydrogen to form the hydride of Mm0.5Ca0.5 Ni2.5Co2.5H2.3 (hydrogen content: 1.1 wt%). The enthalpy change -8.3 kcal/mol H2 in the hydride formation is obtained, and the dissociation pressure of hydride at 30°C yields 3.8 atm. The hydrides of Mm0.5Ca0.5 Ni2.5Co2.5 and Mm2.3Cao.5Ni2.3Co2.5 suited as a stationary hydrogen storage material. © 1979, The Chemical Society of Japan. All rights reserved.