Coupled multishell architecture and vacancy engineering directs nickel ferrite surface reconstruction for water electrolysis

Efficient water electrolysis requires catalysts that reconcile rapid mass transport, abundant accessible sites, and favorable reaction energetics. Here, architectural and defect engineering are combined to construct oxygen-vacancy-rich, triple-shelled NiFe₂O_4-x hollow multishelled structures (HoMSs). The nested porous shells provide a surface area of 165 m²/g, improve electrolyte wetting, and preserve structural integrity, whereas oxygen vacancies (OVs) redistribute charge, reduce metal-oxygen coordination, and promote the formation of catalytically active Ni/Fe oxyhydroxides. In alkaline electrolyte, the optimized NiFe₂O_4-x requires overpotentials of 245, 277, and 301 mV to deliver 20, 50, and 100 mA cm^-2, respectively, with a Tafel slope of 43.4 mV dec^-1. It also delivers 10 mA cm^-2 for the hydrogen evolution reaction (HER) at an overpotential of 93 mV and retains its phase and multishelled architecture after 100 h of operation. In situ spectroscopy and oxygen-isotope-labeling experiments support Adsorbate Evolution Mechanism (AEM) as the predominant oxygen evolution reaction (OER) pathway under the investigated conditions. Within the modeled AEM pathway, Density Functional Theory (DFT) show that OVs lower the free-energy change of the potential-limiting *O-to-*OOH step from 0.738 to 0.594 eV and decrease the work function from 4.42 to 4.28 eV. These results establish a coupled architecture-defect-reconstruction strategy for directing interfacial reaction kinetics in earth-abundant electrocatalysts.