Effects of VF₄on the hydriding cycling at 373 K and dehydriding of Mg₉₉Ni prepared by hydriding combustion synthesis and mechanical milling (HCS+MM)

The catalytic effects of VF₄on 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+VF₄composite was around 50 K lower than that of the HCS+MM-Mg₉₉Ni composite. The hydrogen desorption kinetics of the HCS+MM-Mg₉₉Ni+VF₄composite 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+VF₄composites 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+VF₄composite was determined to be 95.62 kJ/mol H₂, 39.1% less than that of as-received MgH₂. Mechanism analysis indicated that the VH_0.91phase probably formed from the mechanical milling contributed to the enhanced dehydrogenation and hydriding cycling properties of HCS+MM-Mg₉₉Ni.