High-temperature high-pressure reactive ball milling synthesis of Mg-Ni-based solid-state hydrogen storage materials

Abstract

High-temperature, high-pressure reactive ball milling (HTHPRBM) may become a revolutionary technique in mechanochemical synthesis since it allows mechanochemical processes to be conducted under both elevated temperature and pressure with constant control of the conditions. This study explores the possibility of the application of this novel method in the synthesis of solid-state hydrogen storage materials from Mg Ni system. The 2 Mg:Ni molar ratio mixture was processed in a planetary ball mill under hydrogen pressure and temperatures up to 325 °C, indicating that the temperature significantly affects reaction kinetics. The application of high temperatures enabled the reaction to be completed in approximately 40 min, even at the low rotational speed (250 rpm) of the mill. However, despite predictions, especially at lower temperatures, the process unfortunately favoured the formation of magnesium hydride instead of the ternary Mg 2 NiH 4 hydride. Increasing the milling temperature only caused the magnesium particles to decrease in size and undergo hydrogenation, whereas the nickel particles underwent cold welding and agglomeration. Hence, nickel particles act only as catalysts, improving MgH 2 formation instead of producing satisfactory amounts of magnesium‑nickel hydride. The magnesium nickel hydride Mg 2 NiH 4 was present only in samples synthesised above 250 °C. • High-temperature, high-pressure reactive planetary ball milling is applied for the synthesis of Mg 2 NiH 4 . • Elevated temperature promotes the formation of MgH 2 and Mg 2 NiH 4 mixture. • Lack of strain hardening at elevated temperature prevents refinement of nickel particles and causes agglomeration.