Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine

The electrocatalytic reduction of nitrate (NO₃^-) to ammonia (NH₃) not only offers an effective solution to environmental problems caused by the accumulation of NO₃^- but also provides a sustainable alternative to the Haber-Bosch process. However, the conversion of NO₃^- to NH₃ is a complicated process involving multiple steps, leading to a low Faradaic efficiency (FE) for NH₃ production. The structural designability of covalent organic frameworks (COFs) renders feasible and precise modulation at the molecular level, facilitating the incorporation of multiple well-defined catalytic sites with different reactivities into a cohesive entity. This promotes the efficiency of the overall reaction through the coupling of multistep reactions. Herein, heterobimetallic CuP-CoBpy was prepared by postmodification, involving the anchoring of cobalt ions to the CuP-Bpy structure. As a result of the cascade effect of the bimetallic sites, CuP-CoBpy achieved an outstanding NH₃ yield of 13.9 mg h^-1 mg_cat.^-1 with a high FE of 96.7% at -0.70 V versus the reversible hydrogen electrode and exhibited excellent stability during catalysis. A series of experimental and theoretical studies revealed that the CuP unit facilitates the conversion of NO₃^- to NO₂^-, while the CoBpy moiety significantly prompts the reduction of NO₂^- to NH₃. This study demonstrates that tailoring the structural units for the construction of COFs based on each step in the multistep reaction can enhance both the catalytic activity and product selectivity of the overall process.