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

Nanostructured materials for hydrogen storage with a composition of Mg85Ni15−xMx (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 MgH2 and Mg2NiH4 formation, respectively, compared to the binary alloy; resulting in some desorption of H2 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.