MOFs‐on‐MOFs Derived Ternary Sulfide Heterojunctions: Dynamic Ni Site Switching via Interfacial Charge Redistribution for Efficient OER and UOR Catalysis

ABSTRACT Bifunctional electrocatalysts simultaneously applicable for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) remain limited by intrinsic active‐site incompatibility and competitive intermediate adsorption. High‐valence metal centers are required for lattice oxygen activation during OER, whereas low‐valence metallic sites are favorable for urea coordination and C─N bond cleavage in UOR, making dual‐reaction optimization difficult. Herein, a hierarchical Ni 3 S 2 /Co 3 S 4 /FeNi 2 S 4 /NF ternary sulfide heterojunction is rationally fabricated via a MOFs‐on‐MOFs strategy followed by in situ sulfidation. The tightly coupled triphase nanointerface and porous wrinkled nanoflower architecture induce intense interfacial charge redistribution, which precisely tunes the reversible oxidation‐state evolution of Ni active sites. The optimized electronic configuration endows the catalyst with outstanding bifunctional electrocatalytic performance, requiring an overpotential of 219 mV for OER and 1.30 V vs. RHE for UOR at 10 mA cm −2 . In situ spectroscopic characterizations and DFT calculations confirm reveal opposite reversible valence evolution of Ni centers: OER follows the lattice oxygen mechanism on O‐Ni 3+ ‐O sites, while urea preferentially activates on Ni 2+ sites. This interface‐driven dynamic switching of Ni active sites fundamentally resolves active‐site competition and optimizes intermediate adsorption. This work clarifies an atomic‐level dynamic adaptation mechanism of Ni‐based catalysts, offering a reliable interfacial modulation strategy for advanced bifunctional electrocatalysts.