Reversible hydrogen storage in Mg2CoH5 prepared by a combined milling-sintering procedure

Mg2CoH5 hydride was synthesized in a two step procedure: milling a 2Mg–Co mixture under inert atmosphere at room temperature followed by sintering at 410 °C under 6.0 MPa of hydrogen pressure for one week. Under these conditions hydride formation first occurs in only 20 h, which is much faster than previously reported experiments. This improvement in the synthesis procedure can be associated with better intermixing of Mg–Co and refinement of the microstructure obtained during milling stage. The microstructure, structure and thermal behavior of the phases formed during the 2Mg–Co processing were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). From a combination of XRD and DSC measurements, the transition from tetragonal to cubic structure of Mg2CoH5 at 203 °C and its decomposition at 390 °C with subsequent MgCo intermetallic formation were identified. Measurements of equilibrium and kinetics properties of the reaction with hydrogen were performed using modified Sieverts-type equipment. Two plateaus were identified in the absorption pressure-composition isotherms, which were associated to the formation of Mg2CoH5 and Mg6Co2H11 hydrides. Hydrogenation and dehydrogenation reactions were reversible on cycling, with fast absorption rate in the range 250–400 °C.