Hydrogen storage properties of nanostructured 2MgH2Co powders: The effect of high-pressure compression

In this study, MgH 2 and Co powders were mechanically milled in the molar ratio 2:1 and compressed to hard-packed cylindrical pellets. The microstructure, phase changes, and hydrogen storage properties of the mechanically milled 2MgH 2 Co powder and the 2MgH 2 Co compressed pellet were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and synchronous thermal (DSC/TG) analyses. Dehydrogenation of the 2MgH 2 Co compressed pellet is mainly due to the decomposition of Mg 2 CoH 5 while it is the dehydriding of MgH 2 for the milled 2MgH 2 Co powder. Pressure composition absorption isotherms of the 2MgH 2 Co powder and 2MgH 2 Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg 6 Co 2 H 11 and Mg 2 CoH 5 hydride phases . For the compressed 2MgH 2 Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H 2 /55.70±3.34 kJ/mol H 2 for Mg 2 CoH 5 and -81.89±10.39 kJ/mol H 2 /74.47±5.27 kJ/mol H 2 for Mg 6 Co 2 H 11 . In contrast, hydrogenation/dehydrogenation enthalpies of Mg 2 CoH 5 and Mg 6 Co 2 H 11 mechanically milled 2MgH 2 Co powder were −73.98±10.1 kJ/mol H 2 /71.67±1.38 kJ/mol H 2 and -96.86±8.73 kJ/mol H 2 /89.95±10.81 kJ/mol H 2 , respectively. Fast hydrogenation was observed in the dehydrided 2MgH 2 Co compressed pellet with about 2.75 wt% absorbed in less than 1 min at 300 °C and a maximum hydrogen storage capacity of 4.43 wt% (2.32 wt% for the 2MgH 2 Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH 2 Co compressed pellet (64.34 kJ-mol −1 H 2 ) is lower than the mechanically milled 2MgH 2 Co powder (73.74 kJ-mol −1 H 2 ). The results show that mechanical milling followed by high-pressure compression is an efficient method for the synthesis of Mg-based complex hydrides with superior hydrogen sorption properties .