Understanding Hydrogenation Chemistry at MgB₂ Reactive Edges from Ab Initio Molecular Dynamics

Solid-state hydrogen storage materials often operate via transient, multistep chemical reactions at complex interfaces that are difficult to capture. Here, we use direct ab initio molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB₂ without a priori assumption of reaction pathways. Focusing on highly reactive (101̅0) edge planes where initial hydrogen attack is likely to occur, we track mechanistic steps toward the formation of hydrogen-saturated BH₄^- units and key chemical intermediates, involving H₂ dissociation, generation of functionalities and molecular complexes containing BH₂ and BH₃ motifs, and B-B bond breaking. The genesis of higher-order boron clustering is also observed. Different charge states and chemical environments at the B-rich and Mg-rich edge planes are found to produce different chemical pathways and preferred speciation, with implications for overall hydrogenation kinetics. The reaction processes rely on B-H bond polarization and fluctuations between ionic and covalent character, which are critically enabled by the presence of Mg²⁺ cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB₂-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.