Microstructural evolution and improved hydrogenation–dehydrogenation kinetics of nanostructured melt-spun Mg–Ni–Mm alloys

▶ Nanocrystalline microstructures synthesised by the melt-spinning technique. ▶ The hydrides obtained by Reactive Ball Milling process. ▶ The microstructural evolution of the melt-spun ribbons and hydrides studied by TEM. ▶ The effect of nanostructuring on the hydrogen storage properties. The microstructural evolution of as-quenched ribbons and ball-milled hydrides of the Mg–10Ni–2Mm alloy was studied by TEM. These studies showed a refinement of the microstructures during the applied processing and a nucleation of MmMg 12 intermetallic at the grain boundaries of Mg and Mg 2Ni. The interface between MmMg 12 and Mg 2Ni is semi-coherent, with an ordered repetition of the consistent atomic arrangements. The kinetics of H-absorption/desorption is improved due to the fast hydrogen diffusion in the nanograins, thus, providing paths for H-exchange. TEM studies showed (a) stability of the nano-sized grains in the ball-milled Cu-1000 (the surface velocity of the copper wheel: 1000 rpm) sample that underwent cycling of hydrogen desorption and absorption during heating to 350 °C; (b) formation of MmH 3− x hydride from MmMg 12 and its preferential location at grain boundaries of MgH 2. Clearly, MmH 3− x and Mg 2NiH 4 act as nucleation centres to initiate the formation of MgH 2, thus, promoting hydrogen absorption by the Mg alloys. Pressure–composition–temperature diagrams show the presence of two plateaux, Mg–MgH 2 and Mg 2Ni–Mg 2NiH 4. The MgH 2 plateau showed no hysteresis and practically no slope, while the plateau for Mg 2NiH 4 exhibited both a pronounced hysteresis and a slope, particularly for the nanocrystalline sample. The maximum hydrogen storage capacity of the nanocrystalline sample was higher than that of the microcrystalline one.