Hydrogen absorption-desorption characteristics of misch metal-nickel-silicon alloys

TLDR: The hydrogen absorption and desorption characteristics of misch metal-nickel-silicon alloys were investigated in this article, where the same hexagonal structure as LaNi5 and MmNi5 was used to form the hydrides MmNis5−ySiy.

Abstract: The hydrogen absorption and desorption characteristics of misch metal-nickel-silicon alloys were investigated. The alloys MmNis5−ySiy (Mm denotes misch metal and 0.4 ⩽ y ⩽ 0.8) were found to have the same hexagonal structure as LaNi5 and MmNi5, and they reacted readily with hydrogen to form the hydrides MmNi4.6 Si0.45H4.35, MmNi4.5 Si0.5H3.8, MmNi4.4Si0.6H2.1 and MmNi4.2Si0.8H3.1 (hydrogen contents of 1.0 wt.%, 0.9 wt.%, 0.5 wt.% and 0.7 wt.% respectively) under a hydrogen pressure of 60 atm at room temperature. The dissociation pressures of these hydrides were dependent on the silicon content (the silicon partially substitutes for nickel), and the value of log P became lower than that for MmNi5 hydride as y was increased. The enthalpy changes on hydride formation as determined from the dissociation isotherms for the MmNi4.5Si0.5-H and MmNi4.2Si0.8-H systems were −6.6 kcal (mol H2)−1 and −9.0 kcal (mol H2)−1 respectively; the value for the former was smaller than that for LaNi5, and the value for the latter was greater than those for LaNi5 and MmNi5. The dissociation pressures of these hydrides at 20 °C were 7.5 atm and 1.5 atm respectively. The desorption rate of hydrogen for MmNi4.2Si0.8 was larger than those for LaNi5 and MmNi5, and a value of 10.9–12.0 kcal mol−1 was obtained for the apparent activation energy of hydrogen desorption. The hydrides of MmNi4.6Si0.4, MmNi4.5Si0.5 and MmNi4.5Si0.8 have properties which make them suitable as stationary hydrogen storage materials.