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We employ a ternary eutectic Mg 76.87 Ni 12.78 Y 10.35 alloy to achieve a fine eutectic structure with long-period stacking ordered (LPSO) structure, and investigate the corresponding hydrogen storage behavior. The as-cast Mg 76.87 Ni 12.78 Y 10.35 alloy is composed of Mg 15 NiY, Mg 2 Ni, MgNi 4 Y and Mg phases. A high density of stacking faults and 14H-type LPSO structures form in the Mg 15 NiY phase. This fine ternary eutectic structure has shown significant improvement in promotion of the activation and following hydrogen absorption kinetics compared with as-cast non-eutectic structure. Pressure-Composition-Temperature (PCT) curves demonstrate two major pressure platforms due to the presence of Mg 15 NiY and Mg 2 Ni in our Mg–Ni–Y alloys. Nanosized Mg 2 NiH x , MgH 2 and YH x are in situ generated from the decomposition of LPSO structure during hydrogenation. The fine ternary eutectic structure and dispersed nanocatalysts from decomposition of LPSO structure synergistically facilitate the activation and hydrogen absorption kinetics of ternary eutectic Mg–Ni–Y alloy. Image 1 • A fine ternary eutectic Mg–Ni–Y hydrogen storage alloy is designed and prepared. • A 14H-type long-period stacking ordered (LPSO) structure is induced in this alloy. • In situ dispersed nanocatalysts are produced in LPSO during hydrogenation. • Activation and kinetics are improved by ternary eutectic microstructure with LPSO. • Hydrogenation mechanism of ternary eutectic Mg–Ni–Y alloy with LPSO is proposed. [ABSTRACT FROM AUTHOR]

Ternary eutectic Mg 76.87 Ni 12.78 Y 10.35 (at. %) ribbons with mixed amorphous and nanocrystalline phases were prepared by melt spinning. The microstructures of the melt-spun, hydrogenated and dehydrogenated samples were examined and compared by X-ray diffraction and transmission electron microscopy. The amorphous structure transforms into a thermally stable nanocrystalline structure with a grain size of about 5 nm during hydrogen ab/desorption cycles. The Mg, Mg 2 Ni and phases with Y in the melt-spun state transform into MgH 2 , Mg 2 NiH 4 , Mg 2 NiH 0.3 , YH 2 and YH 3 after hydrogenation, and transform back to Mg, Mg 2 Ni and YH 2 upon subsequent dehydrogenation. The reaction enthalpy (ΔH) and entropy (ΔS) of the higher plateau pressure corresponding to Mg 2 Ni hydride formation are −53.25 kJ mol−1 and −107.74 J K−1 mol−1, respectively. The amorphous/nanocrystalline structure effectively reduces the enthalpy and entropy of Mg 2 Ni hydride formation, but has little effect on Mg. The activation energy for dehydrogenation of the hydrogenated ribbons is 69 kJ mol−1. This suggests that Mg–Ni–Y with ternary eutectic composition can form an amorphous/nanocrystalline structure by melt spinning, and this nanostructure efficiently improves the thermodynamics and kinetics for hydrogen storage. • Ternary eutectic Mg–Ni–Y and melt spinning are used to maximize the amorphous. • Amorphous Mg–Ni–Y becomes nanocrystalline structure with a grain size of only 5 nm. • Nanocrystalline structure is stable in hydrogenation and dehydrogenation processes. • Low enthalpy and activation energy are achieved in this nanostructured alloy. [ABSTRACT FROM AUTHOR]