Oxygen vacancy junctions and hydrogen-transfer pathway control in Mg-based solid-state hydrogen storage.

Oxygen-vacancy engineering has become a common strategy for improving Mg-based solid-state hydrogen storage, but oxygen deficiency is often described more broadly than the observed catalytic behaviour justifies. A spectroscopically detectable vacancy is not necessarily a catalytically relevant one, and vacancy concentration alone cannot explain why defect-rich catalysts with apparently similar signatures diverge in sorption behaviour and cycling stability. Here, a catalytically operative vacancy-bearing junction is defined as a defect-containing catalyst-hydride, oxide-metal-hydride, or oxide-carbon-hydride boundary that remains accessible to hydrogen, electronically coupled to neighbouring phases, and structurally traceable during cycling. A junction is pathway-relevant only when it can be linked to a measurable consequence in hydrogen activation, bond lability, local relay, phase-mediated transport, internal transfer continuity, or cycling stability. On this basis, oxygen vacancies are treated not as isolated defect labels, but as local elements of junction chemistry. Representative OV-enabled systems are compared according to the junctions they form and the catalytic regimes they establish, including preserved interfacial access, relay-mediated transfer, heterointerface coupling, internal transport continuity, local activation enhancement, and bond-selective catalysis. This shift redirects catalyst design from defect enumeration toward junction operability and hydrogen-transfer pathway control.