Hydrogen storage measurements in novel Mg-based nanostructured alloys produced via rapid solidification and devitrification

Abstract

Nanostructured materials for hydrogen storage with a composition of Mg 85Ni 15− x M x (M = Y or La, x = 0 or 5) are formed by devitrification of amorphous and amorphous-nanocrystalline precursors produced by melt-spinning. All three compositions exhibit a maximum storage capacity of about 5 mass % H at 573 K. When ball-milled for 30 min in hexanes, the binary alloy can be activated (first-cycle hydrogen absorption) at 473 K. DSC experiments indicate that desorption in this sample begins at 525 K, compared to 560 K when the material is activated at 573 K; which indicates an improvement in the hydride reaction thermodynamics due to capillarity effects. Additions of Y and La improve the degradation in storage capacity observed during cycling of the binary alloy by slowing microstructural coarsening. Alloying with La also shows a decrease of about 8 kJ/mol and 5 kJ/mol in the enthalpy of reaction for MgH 2 and Mg 2NiH 4 formation, respectively, compared to the binary alloy; resulting in some desorption of H 2 at 473 K. The improved thermodynamics are discussed in terms of destabilization of the hydrides relative to new equilibrium phases introduced by alloying additions. The proposed hydriding reaction for the La-containing material is in agreement with previously reported experimental results.