Understanding Hydrogenation Chemistry at MgB Reactive Edges from 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 molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB without 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 BH units and key chemical intermediates, involving H dissociation, generation of functionalities and molecular complexes containing BH and BH 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 Mg cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.