Mechanically milled Mg composites for hydrogen storage: the relationship between morphology and kinetics

Abstract

Magnesium based alloys are potentially the best materials for gaseous hydrogen storage due to their high capacity per weight. Unfortunately, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be overcome by mechanically milling Mg alloys with other phases to catalyze the hydriding and dissociation reactions. We have investigated composites formed by mechanically milling La 2Mg 17 with LaNi 5. The hydrogen absorption and desorption rates of these composites were maximized by the addition of 40 wt. % LaNi 5. The kinetics for this composition proved far superior to those of the base La 2Mg 17 component. It absorbed 95% of its full hydrogen capacity (3.7 wt. %H 2) in 27 s at 250°C and desorbed the same quantity of hydrogen in 4 min. Under the same conditions pure La 2Mg 17 took 32 min to absorb and 3 h to desorb 95% of its full hydrogen capacity (5.0 wt. % H 2). Understanding the mechanisms behind the improved kinetics of these composites is critical for the development of better hydrogen storage materials. It is known that, after a few hydrogen absorption–desorption cycles at high temperatures (300°C), the composites are transformed into a very fine powder matrix (grain size ∼1 μm) of La, Mg and Mg 2Ni. The enhanced properties of this new composite are due to changes in its microscopic morphology and catalytic interactions between the three new component phases. SEM, Microprobe and XPS depth profiling have been used to investigate the complex morphology of the composite particles. `Full kinetics' and `incremental kinetics' measurements were performed to compare the absorption and desorption rates of the main hydriding phases. These measurements demonstrate the relationships which exist between the component phases and their contribution to the excellent overall kinetics of these composite materials.