Hydrogen storage properties of nanostructured 2MgH_2-Co powders: The effect of high-pressure compression

摘要

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 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 CoH5 while it is the dehydriding of MgH2 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 Mg6Co2H11 and Mg2CoH5 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 Mg2CoH5 and -81.89 +/- 10.39 kJ/mol H-2/74.47 +/- 5.27 kJ/mol H-2 for Mg6Co2H11. In contrast, hydrogenation/dehydrogenation enthalpies of Mg2CoH5 and Mg6CO2H11 mechanically milled 2MgH(2)-Co powder were -73.98 +/- 10.1 kJ/mol Hz/ 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 degrees 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. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.