Study of Mg2CoH5 prepared by a combined milling-sintering procedure

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

Lahaina, Maui, Hawaii

Simposio; MH2006-International Symposium on Metal-Hydrogen Systems; 2006

International Symposium on Metal-Hydrogen Systems

The ternary Mg2CoH5 hydride is worth studying because of its high gravimetric (4.4 wt%) and volumetric (about 100 kg H2. m-3) hydrogen storage capacity, better absorption-desorption kinetics than MgH2 and low cost. Due to the absence of its parent intermetallic compound (Mg2Co), the synthesis method is an open issue. The Mg2CoH5 hydride was synthesized in a two step procedure: a 2Mg-Co powder mixture was mechanical alloyed under inert atmosphere at room temperature, pressed to form a pellet and heated at 450°C under 5MPa hydrogen pressure for 1 week, following a procedure reported [1]. We observed the hydrogenation of the sample in only 20 hours. This improvement in the hydride formation can be associated with the enlargement of the milling stage. The microstructure and Mg-Co distribution was studied by scanning electron microscopy (SEM) on resin-mounted and polished samples. The formed phases after each step were identified by X-ray diffraction (XRD). The thermal behavior was investigated by differential scanning calorimetry (DSC). From XRD and DSC measurements the transition from tetragonal to cubic structure of Mg2CoH5 at 202°C and the decomposition of the hydride starting at 390°C were observed. The hydriding-dehydriding reaction was studied by pressure-composition isotherms and absorption/desorption kinetics up to 400°C in the range of 0 to 5MPa. Two plateaus were identified in the P-x isotherms and associated to the formation of Mg2CoH5 and Mg6Co2H11 hydrides.