Hydrogen absorption kinetics of the catalyzed MgH₂ by niobium oxide

The hydrogen absorption kinetics of magnesium hydride (MgH₂) composite doped with 1 mol% Nb₂O₅ prepared by ball milling was examined under various temperatures and pressures. The composite after dehydrogenation at 200 °C absorbs gaseous hydrogen of ∼4.5 mass% within 15 s even at room temperature under 1.0 MPa hydrogen pressure or at 0 °C under 3.0 MPa, and finally their capacities reach up to 5 mass%. At 150 and 250 °C, a large amount of hydrogen gas of more than 5.0 mass% is absorbed within 30 s and their capacity reach up to 5.7 mass% under 1.0 MPa. Interestingly, the absorption kinetics of the catalyzed Mg shows two unusual behaviors in the initial reaction stage of the time scale within 30 s. One is that the kinetics decreases with increase in the temperature from 150 to 250 °C under any pressures (0.2, 1.0 and 3.0 MPa). The other is that the amount of hydrogen absorption drastically increases with increase in the initial pressure from 1.0 to 3.0 MPa at 0 °C or from 0.2 to 1.0 MPa at room temperature (∼20 °C). These behaviors may be explained by taking into account heat generation of Mg due to fast hydrogen uptake in such a short time.