Hydrogen storage properties of Mg2CoH5 prepared by reactive mechanical alloying

F. C. GENNARI; G. O. MEYER; I. GONZÁLEZ FERNÁNDEZ

Río de Janeiro, Brasil

Workshop; Nanomat 2006, III Workshop on Metastable and Nanostructured Materials; 2006

COPPE-UFRJ

Despite their low kinetics reaction at suitable temperatures, the study of magnesium based hydrides is motivated by the interest in their technological use in hydrogen storage devices. Among them, compounds such as Mg2NiH4, Mg2CoH5 and Mg2FeH6 cannot be easily produced as single-phase materials. In particular, tetragonal Mg2CoH5 cannot be synthesized from binary Mg-Co alloys because the corresponding intermetallic parent does not exist as a stable system. In this work, nanocrystalline Mg2CoH5 (hydrogen storage capacity of 4.5 wt.%) was successfully prepared by reactive mechanical alloying (RMA) after milling the stoichiometric mixture of 2Mg-Co under hydrogen atmosphere at a pressure of 5 atm. The evolution of the phases, their thermal stability and the microstructural modifications as a function of the milling time were analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). After 100 h of RMA the nanocomposite obtained consists of 50 wt% of Mg2CoH5. Hydrogen absorption/desorption kinetics was studied using modified Sievert volumetric equipment. High hydrogen absorption rates were measured in the temperature range of 150 to 350ºC.  Decomposition of Mg2CoH5 hydride started at about 200 ºC, i.e. more than 150 ºC below previous observation on samples of the same system prepared by sintering. Observed enhancement of hydrogen reaction and storage properties can be correlated with microstructural characteristics of the hydride produced by RMA.