In Situ Reconstructed Core-Shell Fe-MOF@Cu as a Tandem Electrocatalyst toward Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction enables sustainable ammonia production and simultaneous nitrate pollutant abatement. Yet its multi-step proton-coupled electron transfer mechanism is hindered by slow reaction kinetics. Herein, we report a reconstructed tandem catalyst, Fe-MOFs@Cu, generated through the in situ electrochemical transformation of a designed MIL-101(Fe)@HKUST-1 core-shell precursor. Under nitrate reduction conditions, the HKUST-1 shell is selectively converted into Cu species, whereas the MIL-101(Fe) core remains largely preserved, resulting in a reconstructed Cu/Fe heterointerface with spatially integrated active domains. Electrochemical and kinetic analyses reveal a clear functional differentiation between the two catalytic components. Cu-rich domains preferentially promote nitrate activation to nitrite, while Fe-based sites facilitate water activation and the subsequent hydrogenation of nitrite to ammonia. The close interfacial coupling between these domains enables efficient intermediate conversion and hydrogen utilization, thereby suppressing nitrite accumulation and competing hydrogen evolution. Benefiting from this cooperative catalytic architecture, R-MIL-101(Fe)@Cu achieves an NH₃ yield rate of 0.61 mmol h^-1 cm^-2 with a Faradaic efficiency of 94.9% at -0.5 V vs. RHE, together with excellent operational stability. This work demonstrates how precursor architecture can be deliberately engineered to direct electrochemical reconstruction toward catalytically favorable interfaces for complex multistep electrochemical transformations.