Dual-Enzyme-Mimicking Sites in Covalent Organic Frameworks Enable Highly Efficient Relay Electrosynthesis of Ammonia

Electrocatalytic nitrate reduction poses significant importance in green ammonia synthesis and water denitrification. To date, high-performance nitrate reduction still relies on noble-metal-based catalysts, while transition metal compound and framework catalysts generally suffer from low activities (in terms of turnover frequency values), leading to insufficient ability to rapidly process large amounts of nitrate at the industrial level. To this end, enzyme-mimicking catalytic systems that integrate the high TOF efficiency of enzymatic sites (typically with nonprecious metal centers) into the crystalline, orderly assembled, and porous molecular frameworks hold great theoretical promises, yet the search for any single enzyme-site mimics has failed to overcome current limitations. Herein, we demonstrate the rational design of a series of covalent organic frameworks (COFs) to incorporate periodically alternating metalloporphyrin and metal-bis-(dithiolene), respectively mimicking nitrate reductase and nitrite reductase enzymes, as a dual functional assembly. The dual-enzyme-mimicking Ni-TAPP-Cu boosts the electrocatalytic activity of NO₃RR to a comparable level to the noble-metal catalysts via a relay pathway to individually boost the two kinetically significant nitrate-to-nitrite and nitrite-to-ammonia conversion steps, consequently achieving a remarkable TOF value of 235.7 mg·h^-1·mg^-1 _site, 86.13% NH₃ faradaic efficiency, nearly 100% NH₃-selectivity, and excellent durability at typical acidic wastewater conditions (pH = 3). Mechanistic investigations indicate that Cu sites effectively suppress the HER competition, enhance the adsorption of NO₃ ^-, and selectively generate NO₂ ^-, while Ni sites facilitate the subsequent hydrogenation and conversion of *NO₂. The Cu-Ni-COF exemplifies a promising strategy for integrating dual enzyme-mimicking sites within an ordered, porous framework to facilitate relay catalysis, thereby offering valuable insights for the design of high-performance, sustainable catalytic materials.