Hydrogen absorption-desorption kinetics of MmNi 5 and related compounds

Abstract

A study was made to determine the effects of aluminum and iron substitution on the reaction kinetics of MmNi 5 ( Mm = mischmetal). Each sample used in this study i.e. MmNi 5, MmNi 4.5 A1 0.5 and MmNi 4.85Fe 0.15, was mixed with a heat buffer so that nearly isothermal conditions could be maintained. Experiments were also designed so that the ratio of the equilibrium plateau pressure to the opposing pressure was the same for each sample. This was done to ensure that the thermodynamic driving force acting on each sample was the same. In addition, a sufficiently large number of absorption-desorption cycles were initially done on each sample for reproducible results to be obtained. The relative reaction rates among the various samples studied were found to change with the temperatures and pressure ratios that were chosen. Thus it is very important that any comparison of the reaction kinetics of metal hydrides should also include information about the temperature intervals used and the activation energies of each sample, as well as quantitative expressions showing how the reaction rates change with the imposed pressure ratio. At 25 °C and a pressure ratio of 2, reaction rates were found to be in the following order: MmNi 5 > MmNi 4.5 Al 0.5 ≈ LaNi 5 > MmNi 4.85 Fe 0.15 Thus it was concluded that aluminum and iron substitution has a deleterious effect on the reaction kinetics of MmNi 5 hydride. Of the materials studied, MmNi 4.5Al 0.5 seems to be the most practical material for hydrogen storage. This is because it has thermodynamic and kinetic properties similar to those of LaNi 5 but at a cheaper price.