Dynamic Kinetics of MgH2 in Solid-State Hydrogen Storage: From the "Dam-Break Effect" to Mechanistic Understanding,...

Gas-solid reaction kinetics describes how gaseous species interact with solid surfaces and bulk regions in energy and materials processes. This behavior is often evaluated using compact descriptors, such as onset temperature, apparent activation energy, equilibrium parameters, or rate constants. These descriptors enable comparison across materials and conditions, but compress a multistage reaction into single-value metrics. The central limitation is that kinetic resistance is treated as an averaged parameter, rather than as a quantity that may emerge at a specific stage, location, or interface and evolve during conversion. This compression becomes problematic when local coordination, bonding, defect structure, heat transfer, mass transport, and interfaces change. MgH2 dehydrogenation provides a representative case. Its sluggish dehydrogenation has been assessed using onset dehydrogenation temperature and apparent activation energy. However, the weak correspondence between these parameters, the gap between theoretical barriers and experimentally fitted activation energy, and the ability of surface catalysts to improve macroscopic kinetics point to a missing kinetic coordinate. This coordinate is reaction progress, which reveals how much reaction has occurred, where dominant resistance is located, and how it is redistributed. In this Account, we discuss the "dam-break effect" (originally proposed as the "burst effect") as a reaction progress dependent kinetic framework for MgH2 dehydrogenation. Unlike conventional single-parameter descriptions, the dam-break effect emphasizes stage-resolved evolution of kinetic resistance from an initially high-resistance surface state to a lower-resistance regime. Microscopic calculations show that hydrogen removal from the first surface layer has the highest barrier, whereas subsequent hydrogen desorption and migration become easier after this layer is breached. Reconstruction of isothermal kinetic curves reveals a rapid decrease in activation energy in the early region, followed by a stable plateau. These observations suggest that the first surface layer behaves as a kinetic dam, and its removal triggers a transition from an initially resistant state to an accelerated regime. We then examine how the dam-break effect connects local environment evolution, surface-controlled dehydrogenation, and inconsistencies among onset temperature, apparent activation energy, and theoretical barriers. This dynamic viewpoint is extended to data-driven design, where databases, machine learning, AI agents, and machine learning potentials transform kinetic observations into computable descriptors. Finally, we discuss application strategies in which materials design targets the surface and near-surface bottleneck, while system operation matches energy input to stage-dependent resistance. Using MgH2 as a model system, this Account aims to promote a broader shift in gas-solid reaction kinetics from static parameter description toward reaction progress resolved dynamic interpretation.