Effect of additive distribution in H 2 absorption and desorption kinetics in MgH 2 milled with NbH 0.9 or NbF 5

Abstract This paper presents a comparative study of H 2 absorption and desorption in MgH 2 milled with NbF 5 or NbH 0.9 . The addition of NbF 5 or NbH 0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH 2 with 7 mol.% of NbF 5 absorbs 60% of its hydrogen capacity at 250 °C in 30 s, whereas the mixture with 7 mol.% of NbH 0.9 takes up 48%, and MgH 2 milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF 5 finishes in 10 min, whereas the mixture with NbH 0.9 only desorbs 50% of its hydrogen content, and MgH 2 without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH 0.9 , a phase observed in the hydrogen cycled MgH 2  + NbF 5 and MgH 2  + NbH 0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF 5 -added material is attributed to the combination of smaller size and enhanced distribution of NbH 0.9 with more favorable microstructural characteristics. The addition of NbF 5 also produces the formation of Mg(H x F 1-x ) 2 solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.