The role of Fe during hydride formation in the Mg-Fe system: a Mössbauer investigation.

BAUM, LORENA; MEYER, MARCOS; MENDOZA ZÉLIS, LUIS

Foz do Iguassu, Brasil

Conferencia; XIV International Conference on Hyperfine Interactions; 2007

Mg and its alloys are theoretically among the best candidates for hydrogen storage in the form of solid hydrides due to their high volumetric and gravimetric capacity (MgH2 contains 110 kg H2/m3) but their sorption kinetics must be considerable improved in order to become of practical use. The reduction of grain sizes to the nanometer scale and the addition of small quantities of transition metals, like Fe, as catalysts, have demonstrated to be useful for that purpose. Furthermore, Mg combines with some transition metals and hydrogen, to form complex ternary hydrides with very high volumetric hydrogen content. A noteworthy case is that of Mg2FeH6, containing 150 kg H2/m3, although Mg and Fe are practically immiscible and do not form , in the absence of a third element, any compound. When Fe-catalyzed Mg is cyclically H2 charged and discharged, the ternary hydride could form, affecting not only the net charge capacity but also the system microstructure and thus the sorption kinetics. Mechanical milling is beeing used, both to refine the Mg matrix and to disperse the selected metallic catalyst. When Mg-Fe mixtures are milled in H2 atmosphere the formation of the ternary compound was also observed, even for minute Fe addition. By selecting the milling time and the starting powder mixture composition, the relative fraction of MgH2, Fe and Mg2FeH6 as well as the sample microstructure can be varied and thus the sorption properties. With this purpose we have prepared a series of Mg/Fe samples milled in H2 for different times and characterized them with X-ray diffraction and Mössbauer spectroscopy, among other techniques. The Fe environment was carefully analyzed in order to understand the role of Fe in the catalytic process and the conditions for the formation of the ternary hydride.