Hydrogen storage properties of nanostructured 2MgH2[sbnd]Co powders: The effect of high-pressure compression

Abstract

In this study, MgH2 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 2MgH2Co powder and the 2MgH2Co 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 2MgH2Co compressed pellet is mainly due to the decomposition of Mg2CoH5 while it is the dehydriding of MgH2 for the milled 2MgH2Co powder. Pressure composition absorption isotherms of the 2MgH2Co powder and 2MgH2Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg6Co2H11 and Mg2CoH5 hydride phases. For the compressed 2MgH2Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H2/55.70±3.34 kJ/mol H2 for Mg2CoH5 and -81.89±10.39 kJ/mol H2/74.47±5.27 kJ/mol H2 for Mg6Co2H11. In contrast, hydrogenation/dehydrogenation enthalpies of Mg2CoH5 and Mg6Co2H11 mechanically milled 2MgH2Co powder were −73.98±10.1 kJ/mol H2/71.67±1.38 kJ/mol H2 and -96.86±8.73 kJ/mol H2/89.95±10.81 kJ/mol H2, respectively. Fast hydrogenation was observed in the dehydrided 2MgH2Co 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 2MgH2Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH2Co compressed pellet (64.34 kJ-mol−1 H2) is lower than the mechanically milled 2MgH2Co powder (73.74 kJ-mol−1 H2). 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.