Tailoring hierarchical pore structures in carbon scaffolds for hydrogen storage of nanoconfined magnesium

Various carbon scaffolds have been developed for the nanoconfinement of Mg/MgH2 to achieve the synergistic effects of nanosizing-induced kinetic improvement, Mg-scaffold interaction-induced catalytic enhancement, and mechanical stress-driven thermodynamic stability tuning for advanced hydrogen storage. Yet, systematic studies linking the chemo-structural features of the carbon scaffolds and the hydrogen storage properties of Mg/MgH2 nanoparticles within the scaffolds remain largely unexplored. Here, we investigate the pore-dependent Mg nanoconfinement and its hydrogen storage behavior using hollow carbon nanospheres (HCNs) as scaffolds, dictated by their distinct pore properties. Contrary to the conventional wisdom of nanoconfinement, the Mg nanoparticles confined by the HCN with large mesopores exhibit faster hydrogen sorption kinetics and improved thermodynamic characteristics than those with small mesopores. The detailed analyses reveal that the interplay between reversible mechanical stress upon hydrogenation and nanosizing-induced facile kinetics leads to the observed experimental results. Ab initio computations further show that compressive stress retards bulk diffusion while at the same time enhances the surface hydrogen desorption, suggesting the need for stress optimization within the composite. Our study provides a design guideline for Mg strain engineering and the subsequent carbon scaffold structures for hydrogen storage, possibly leading to advanced hydrogen storage of confined Mg.