First Hydrogenation Kinetics and Properties of Mechanically Treated Pure Mg

In this research, different Mg chips were produced via a mechanical chipping method in air to explore their potential as hydrogen storage materials. The production of fine magnesium chips was achieved with a laboratory-scale milling device, while coarse magnesium chips were fabricated by using a standard milling machine. The investigation of the first hydrogenation, at 300 °C under 50 atm, reveals that the fine chips require a shorter activation period of turbomolecular vacuum evacuation at 300 °C and exhibit a higher hydrogen absorption rate than the coarse chips. The activation stage anneals the dislocations formed during milling; therefore, they do not contribute to the hydrogenation. The enhanced performances of the fine chips are attributed to their larger specific surface area, smaller thickness, and the presence of macrostructural defects. Surface analysis of the outermost layers of the Mg chips, before and after activation procedures, sheds light on the surface reactions and thermal processes enabling the initiation of hydrogen absorption in Mg chips. The first hydrogen absorption kinetics appear to be characterized by a low-dimensional nucleation and growth mechanism. Apparent activation energies for absorption and desorption, determined using the Avrami and Kissinger equations, respectively, fall within the reported range for absorption and are lower than those reported for desorption. The mechanical chipping method in air is a simpler alternative to ball milling for producing Mg chips for hydrogen storage applications but requires an activation period before hydrogenation.

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