Hydrogen storage in TiFe nanoparticles: void-limited saturation and metal-selective charge transfer during sequential loading

We present an atomistic study of hydrogen uptake in an ultrasmall TiFe nanoparticle, linking H₂ adsorption and dissociation at the surface with sequential interstitial loading, void accessibility, and charge redistribution. Fe-rich surface motifs favor dissociative H₂ activation, and representative dissociation-path calculations show that Fe-associated cleavage is kinetically more accessible than the corresponding Ti-associated pathway. Internal storage is controlled by a limited set of void-like environments rather than generic interstitial insertion. Sequential loading remains predominantly exothermic, although discrete high-loading excursions appear because of site competition and local relaxation. Geometric analysis shows a crowding-driven approach to saturation, evidenced by decreasing minimum H–H separations and a heterogeneous clearance distribution with favorable sites in a near-surface/subsurface shell. Mulliken populations reveal progressive charging of the hydrogen sublattice with an asymmetric Ti/Fe host response, while local PDOS analysis supports electronically non-equivalent Ti-associated and Fe-associated dissociated environments. These results provide mechanistic descriptors for hydrogen uptake in nanostructured TiFe-based environments.