MOFs-on-MOFs Derived Ternary Sulfide Heterojunctions: Dynamic Ni Site Switching via Interfacial Charge Redistribution...

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 Ni₃S₂/Co₃S₄/FeNi₂S₄/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-Ni³⁺-O sites, while urea preferentially activates on Ni²⁺ 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.