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

PIGHIN, SANTIAGO A.; COCO, BRUNO; TROIANI, HORACIO; CASTRO, FACUNDO J.; URRETAVIZCAYA, GUILLERMINA

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2018

0360-3199

This paper presents a comparative study of H2 absorption and desorption in MgH2 milledwith NbF5 or NbH0.9. The addition of NbF5 or NbH0.9 greatly improves hydriding anddehydriding kinetics. After 80 h of milling the mixture of MgH2 with 7 mol.% of NbF5 absorbs60% of its hydrogen capacity at 250 C in 30 s, whereas the mixture with 7 mol.% ofNbH0.9 takes up 48%, and MgH2 milled without additive only absorbs 2%. At the sametemperature, hydrogen desorption in the mixture with NbF5 finishes in 10 min, whereasthe mixture with NbH0.9 only desorbs 50% of its hydrogen content, and MgH2 withoutadditive practically does not releases hydrogen. The kinetic improvement is attributed toNbH0.9, a phase observed in the hydrogen cycled MgH2 þ NbF5 and MgH2 þ NbH0.9 materials,either hydrided or dehydrided. The better kinetic performance of the NbF5-addedmaterial is attributed to the combination of smaller size and enhanced distribution ofNbH0.9 with more favorable microstructural characteristics. The addition of NbF5 alsoproduces the formation of Mg(HxF1-x)2 solid solutions that limit the practically achievablehydrogen storage capacity of the material. These undesired effects are discussed.© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved