Improved hydrogen storage properties of MgH2 catalyzed with K2NiF6

Abstract

In this study, the hydrogen storage properties of MgH2-X wt% K2NiF6 (X=5, 10, 15, 20, and 50) were investigated for the first time. From the analysis of the onset desorption temperature and isothermal de/absorption kinetics, it was shown that MgH2+5 wt% K2NiF6 sample has the best performance. The 5 wt% doped sample started to release hydrogen at about 260°C, which was a reduction of about 95°C and 157ºC compared with the as-milled and as-received MgH2. In addition, the de/absorption kinetics of the MgH2+5 wt% K2NiF6 were also improved significantly compared to the un-doped MgH2. The apparent activation energy for hydrogen desorption exhibited the decrement from 167.0kJ/mol for as-milled MgH2 to 111.0kJ/mol with the addition of 5 wt% K2NiF6. Moreover, the X-ray diffraction spectra displayed the formation of new phases of KF, KH, Mg2Ni and Mg2NiH4 by doping the K2NiF6 with MgH2 after the dehydrogenation and rehydrogenation processes. The scanning electron microscope results revealed that MgH2 doped with 5 wt% K2NiF6 demonstrated the smallest particle size compared to the as-received and as-milled MgH2. It is believed that the formation of in situ active species of KF, KH, and Mg2Ni could provide a synergetic catalytic effect in enhancing the hydrogen sorption properties of MgH2.