Degradation by water vapor of MmNi₅-based hydrogen-absorbing alloys with different compositions

The effects of water concentration in hydrogen and the composition of MmNi₅-based hydrogen-absorbing alloys on their absorbing-desorbing properties were investigated. Absorption-desorption cycling tests of hydrogen containing 660 ppm and 1600 ppm H₂O were carried out for two kinds of MmNi₅-based hydrogen-absorbing alloys. The hydrogen absorbing-desorbing fraction of MmNi_4.04Co_0.60Mn _0.31Al_0.05 with a high content of Ce, where the fraction is defined as the amount of hydrogen undergoing absorption-desorption relative to the maximum amount, decreased significantly, because of a decrease in the rate of hydrogen absorption-desorption. On the other hand, that Of MmNi _4.23Co_0.51Mn_0.12 with a low content of Ce did not greatly decrease. Although the rate of hydrogen absorption Of MmNi _4.23Co_0.51Mn_0.12 decreased in the early cycles, it reactivated with an increase of the number of cycles. Degradation by water vapor leads to differences in the hydrogen absorbing-desorbing fraction because of the different contents of Ce. With a low content of Ce, MmNi ₅-based hydrogen-absorbing alloys have a high resistance to degradation by water vapor. Therefore, when MmNi₅-based hydrogen-absorbing alloys are applied to electric-load leveling systems, they should be chosen according to their Mm composition.