Abstract: Hydrogen storage plays a pivotal role in the hydrogen industry, yet its current status presents a bottle-neck. Diverse strategies have emerged in recent years to address this challenge. MgH₂ has stood out as a promising solid-state hydrogen storage material due to its impressive gravimetric and volumetric hy-drogen density, but its practical application is hampered by elevated thermal stability and sluggish kinet-ics. In this study, we introduce a solution by synthesizing Pd metallene through a one-pot solvothermal method, revealing a distinctive highly curved lamellar structure with a thickness of around 1.6 nm. Incor-porating this Pd metallene into MgH₂ results in a composite system wherein the starting dehydrogenation temperature is significantly lowered to 439 K and complete dehydrogenation occurs at 583 K, releasing 6.14 wt.% hydrogen. The activation energy of dehydrogenation for MgH₂ was reduced from 170.4 kJ mol^-1 to 79.85 kJ mol^-1 after Pd metallene decoration. The enthalpy of dehydrogenation of the MgH₂-10 wt.% Pd sample was calculated to be 73 kJ mol^-1 H₂ ^-1 and decreased by 4.4 kJ mol^-1 H₂^-1 from that of dehy-drogenation of pure MgH₂ (77.4 kJ mol^-1 H₂^-1). Theoretical calculations show that the average formation energy and average adsorption energy of hydrogen vacancies can be significantly reduced in the presence of both Pd clusters and Pd single atoms on the surface of MgH₂ /Mg, respectively. It suggests that the synergistic effect of in situ formed Pd single atoms and clusters significantly improves the hydrogenation and dehydrogenation kinetics. The identified active sites in this study hold potential as references for forthcoming multi-sized active site catalysts, underscoring a significant advancement toward resolving hydrogen storage limitations.

Key words: MgH₂, Metallene, Single atoms, Clusters, Hydrogen storage