Improved hydrogen storage properties of MgH 2 catalyzed with K 2 NiF 6 ✩

Abstract

In this study, the hydrogen storage properties of MgH 2 - X wt% K 2 NiF 6 ( 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 MgH 2 +5 wt% K 2 NiF 6 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 157 ºC compared with the as-milled and as-received MgH 2 . In addition, the de/absorption kinetics of the MgH 2 +5 wt% K 2 NiF 6 were also improved significantly compared to the un-doped MgH 2 . The apparent activation energy for hydrogen desorption exhibited the decrement from 167.0 kJ/mol for as-milled MgH 2 to 111.0 kJ/mol with the addition of 5 wt% K 2 NiF 6 . Moreover, the X-ray diffraction spectra displayed the formation of new phases of KF, KH, Mg 2 Ni and Mg 2 NiH 4 by doping the K 2 NiF 6 with MgH 2 after the dehydrogenation and rehydrogenation processes. The scanning electron microscope results revealed that MgH 2 doped with 5 wt% K 2 NiF 6 demonstrated the smallest particle size compared to the as-received and as-milled MgH 2 . It is believed that the formation of in situ active species of KF, KH, and Mg 2 Ni could provide a synergetic catalytic effect in enhancing the hydrogen sorption properties of MgH 2 .