Effects of VF4 on the hydriding cycling at 373 K and dehydriding of Mg99Ni prepared by hydriding combustion synthesis and mechanical milling (HCS+MM)

Abstract

The catalytic effects of VF4 on the hydriding cycling properties at low temperature (373 K) and dehydrogenation of Mg99Ni prepared by hydriding combustion synthesis (HCS) and mechanical milling (MM) have been systematically investigated. The onset dehydrogenation temperature of the HCS+MM-Mg99Ni+VF4 composite was around 50 K lower than that of the HCS+MM-Mg99Ni composite. The hydrogen desorption kinetics of the HCS+MM-Mg99Ni+VF4 composite was largely accelerated, desorbing 5.26 wt% hydrogen within 1800 s at 523 K. In contrast, only 2.71 wt% hydrogen was desorbed under the same condition for the HCS+MM-Mg99Ni. Moreover, the HCS+MM-Mg99Ni+VF4 composites could desorb completely 6.50 wt% hydrogen at 533 K in 1800 s and at 543 K in 1300 s, respectively. The hydriding cycling property of the HCS+MM-Mg99Ni at low temperature was studied and it was improved obviously by the additive VF4, so that the composite could reach its saturated hydriding capacity of 5.57 wt% with a high activity at 373 K within 50 s at the fifth hydriding cycle. Additionally, the dehydrogenation activation energy of the HCS+MM-Mg99Ni+VF4 composite was determined to be 95.62 kJ/mol H2, 39.1% less than that of as-received MgH2. Mechanism analysis indicated that the VH0.91 phase probably formed from the mechanical milling contributed to the enhanced dehydrogenation and hydriding cycling properties of HCS+MM-Mg99Ni.