MECHANICALLY MILLED MG COMPOSITES FOR HYDROGEN STORAGE - THE RELATIONSHIP BETWEEN MORPHOLOGY AND KINETICS

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 desorp tion kinetics. This problem can be overcome by mechanically milling Mg alloys with other phases to catalyze the hydriding and dissociation r eactions. We have investigated composites formed by mechanically milli ng La2Mg17 with LaNi5. The hydrogen absorption and desorption rates of these composites were maximized by the addition of 40 wt. % LaNi5. Th e kinetics for this composition proved far superior to those of the ba se La2Mg17 component. It absorbed 95% of its full hydrogen capacity (3 .7 wt. %H-2) in 27 a at 250 degrees C and desorbed the same quantity o f hydrogen in 4 min. Under the same conditions pure La2Mg17 took 32 mi n to absorb and 3 h to desorb 95% of its full hydrogen capacity (5.0 w t. % H-2). Understanding the mechanisms behind the improved kinetics o f these composites is critical for the development of better hydrogen storage materials. It is known that, after a few hydrogen absorption-d esorption cycles at high temperatures (300 degrees C), the composites are transformed into a very fine powder matrix (grain size similar to 1 mu m) of La, Mg and Mg2Ni. The enhanced properties of this new compo site are due to changes in its microscopic morphology and catalytic in teractions between the three new component phases. SEM, Microprobe and XPS depth profiling have been used to investigate the complex morphol ogy of the composite particles. 'Full kinetics' and 'incremental kinet ics' measurements were performed to compare the absorption and desorpt ion 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 mate rials. (C) 1998 Elsevier Science S.A.