ProgrammableVacancy Topology in High-Entropy-Alloy-InspiredMultiprincipal Alloys Directs Ru-Centered Alkaline Hydrogen Evolution

Abstract Vacancy topology in multiprincipal alloys provides a controllable handle to tune interfacial reaction pathways, yet practical routes to program vacancies in ultrasmall alloys remain scarce. Here, we report an H2-free, volatility-assisted MOF-to-alloy synthesis in an inert atmosphere that yields sub-4 nm MnFeRuCoNi-based multiprincipal alloy nanoparticles with a Ru-proximal vacancy environment. A closed set of mutually reinforcing measurements─PALS/EPR (defect generation), EXAFS/XPS/XANES (coordination/electronic states), multipressure NAP-XPS (Ru→O(H2O) interfacial charge transfer), REELS/TOF-SIMS (H* and H3O+ intermediates), and in situ Raman (Ru–H vibration)─establishes a causal sequence from programmed vacancy topology to local electron redistribution at Ru, accelerated water activation, and step-specific alkaline-HER kinetics. Density-functional theory with Bader analysis shows reduced electron density at Ru and enhanced Ru→O charge transfer upon H2O adsorption in vacancy-rich environments, consistent with experimental observables. In 1.0 M KOH, the catalyst delivers an overpotential of 13 mV at 10 mA cm–2, a 47 mV dec–1 Tafel slope, and ∼2600 h stability at 100 mA cm–2, rivaling Ru benchmarks and approaching Pt/C under matched protocols. The volatility-assisted MOF-to-alloy route may be extendable to other multiprincipal alloy families containing a sacrificial volatile component, thereby offering a promising strategy for encoding vacancy topology in complex alloys.