Vacancy‐Driven Ni Exsolution in Prussian Blue Analogues Creates Cooperative Defect–Metal Sites for Alkaline Hydrogen Evolution

Alkaline hydrogen evolution reaction (HER) is limited by slow water dissociation and by catalysts that degrade in saline electrolytes. Here we program vacancies to trigger selective Ni exsolution in multimetallic Prussian blue analogues (PBAs), creating cooperative defect–metal interfaces. Low‐temperature annealing of FeMn@CoNi PBAs forms hollow nanocages (PBA‐350) rich in cyanide vacancies and decorated with in situ exsolved Ni nanoparticles. Operando XRD/XAS, operando impedance, and theory reveal a dual‐site mechanism: vacancy‐stabilized Ni lowers the Volmer barrier, adjacent Co facilitates OH* removal, and the vacancy‐modified lattice tunes H* binding toward thermoneutrality. PBA‐350 delivers 28.4 mV at 10 mA cm −2 and a 56 mV dec −1 Tafel slope in 1.0 m KOH with negligible degradation over 100 h at −50 mA cm −2 . An anion‐exchange membrane electrolyzer reaches 1.76 V at 1.0 A cm −2 , and PBA‐350 remains stable in simulated seawater (1.0 m KOH + 0.5 m NaCl) by physically repelling chloride ions via hydration layers, establishing vacancy‐assisted exsolution as a design rule for HER.