Double exchange interaction promoted nitrate electroreduction to ammonia over heteronuclear metal organic framework

The electrochemical nitrate reduction reaction (NO₃RR) holds promise for mitigating groundwater contamination and represents a key strategy for the sustainable production of ammonia (NH₃). Herein, focusing on activating the inert NO double bond in NO₃^-, we propose that efficient electron transfer between the π and π* orbitals of nitrate and the active site of catalyst is essential for initiating and sustaining the reaction. Accordingly, a Fe₂Ni heteronuclear metal-organic framework (MOF) was synthesized specifically for the electrochemical reduction of nitrate to ammonia. Comprehensive analyses reveal that adjacent NiO₆ and FeO₆ units induce a unique double exchange interaction. This interaction facilitates the rearrangement of d-orbital electrons and optimizes the electron filling state of the e_g orbitals, thereby accelerating NO bond polarization and cleavage in NO₃^-. It further promotes destabilization of the *NO intermediate and enhances subsequent deoxygenation and hydrogenation steps. Compared to homonuclear MOF counterparts, the heteronuclear MOF exhibits a 2-3 fold increase in NH₃ yield, reaching 56.0 mg h^-1 mg_cat^-1 at -0.7 V vs. RHE. This work presents a novel approach for deciphering the NO₃RR mechanism and provides molecular-level insights for catalyst design.