Effective Porosity and Pore–Ligament Architecture in Mg‐Based Solid‐State Hydrogen Storage

Magnesium (Mg)‐based hydrides offer high gravimetric capacity, high volumetric hydrogen density, elemental abundance, and low material cost, but their practical performance is constrained by coupled hydrogen transport, interfacial reaction, heat transfer, and structural evolution during repeated magnesium/magnesium hydride conversion. This mini‐review reframes porosity as a functional rather than purely morphological attribute. We critically synthesize representative precursor‐derived, scaffold‐free, and layered porous Mg architectures, and organize the available structural, transport, conversion, thermal, and cycling evidence around an operational definition of effective porosity: the gas‐accessible pore fraction that, at a specified structural and reaction state, satisfies a stated molecular‐hydrogen transport criterion within a specified timescale and is spatially coupled to internally converting regions of a percolating Mg‐rich solid domain. Strict three‐dimensional determination is distinguished from intermediate spatial validation and routine laboratory proxies to clarify what can and cannot be inferred experimentally. The analysis shows that high initial porosity, high surface area, or rapid powder kinetics alone do not establish storage‐relevant functionality. Secondary phases must be assigned process‐specific roles rather than inferred from coexistence, and apparent activation energies in shaped media may reflect coupled reaction, transport, and thermal effects. The resulting design principle is to preserve gas access, Mg‐rich continuity, internal conversion, thermal contact, and structural integrity simultaneously through shaping and cycling.