Monolithic nickel-doped molybdenum nitride improves hydrogen storage properties of MgH2

Abstract

Magnesium hydride has a high potential for use as a solid-state hydrogen storage material; however, its practical application is hampered by a number of drawbacks, including thermodynamic stability and slow hydrogen uptake/release kinetics. In this research we successfully prepared shell-like Ni/Mo2N by simple hydrothermal method and calcination, and the composite MgH2-6Ni/Mo2N was obtained by mechanical ball milling of 6%Ni/Mo2N with MgH2. A series of experiments demonstrated that the MgH2 composite material showed outstanding hydrogen adsorption/desorption performance. Specifically, MgH2-6Ni/Mo2N starts to release hydrogen at 186.3 °C, releases 5.92wt.% H2 in 30min at 265 °C, and absorbs 5.46wt.% H2 in 15min at 150 °C, and the activation energy of dehydrogenation decreases to 76.35kJ/mol.， In addition, calculations are carried out using kinetic modelling, the results indicate that MgH2-6Ni/Mo2N can be transformed from a surface permeation model with a slow hydrogen absorption rate to a diffusion model with a fast hydrogen absorption rate even at lower temperatures. Mechanistic analysis shows that Ni/Mo2N forms Mg2NiH4/Mg2Ni ‘hydrogen pump’ with MgH2 during hydrogenation/dehydrogenation, and the generated Mg2NiH4/Mg2Ni and Mo2N remain stable during hydrogenation/dehydrogenation, which improves the hydrogen storage performance of MgH2. The MgH2 composites show excellent cycling stability, with a hydrogen release retention rate of 97.9% after 10 cycles.