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

Abstract

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.

Keywords: ammonia electrosynthesis; covalent organic frameworks; dual-enzyme-mimicking; nitrate reduction; non-noble-metal.

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(a) Cu-TAPP center in enzyme-mimic nitrate reductase. (b) Ni­(bded)₂ center in an enzyme mimicking nitrite reductase. (c) Schematic representation of the synthesis of Ni-TAPP-Cu. (d) Catalytic concept of proximate sites in diatomic/atomic cluster and metal doping/alloy systems and spatially distant sites in orderly arranged dual-site frameworks.

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(a) Experimental (purple dots) and Pawley refinement (blue line) PXRD patterns of Ni-TAPP-Cu. (b) Top and side views of Ni-TAPP-Cu. (c) N₂ adsorption–desorption isotherms of Ni-TAPP-Cu. (d) SEM spectra of Ni-TAPP-Cu, scale bar = 500 nm. (e) IR spectra of Ni-TAPP-Cu, TAPP-Cu and Ni­(bded). (f) Thermal gravimetric analysis (TGA) curve of Ni-TAPP-Cu under air atmosphere.

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Electrocatalytic nitrate reduction performance in 0.5 mM (pH = 3) H₂SO₄ with 0.5 M KNO₃ and 0.05 M K₂SO₄. (a) Partial current densities of the NO₃RR at selected potentials. (b) NH₃/NO₂ ^– yield rates and the corresponding FEs-NH₃/NO₂ ^– of Ni-TAPP-Cu at selected potentials. (c) NH₃ yield rates of Ni-TAPP-M and TTF-TAPP-M at selected potentials. (d) FEs-NO₃RR of Ni-TAPP-M and TTF-TAPP-M at selected potentials. (e) Comparison of the electrocatalytic NO₃RR performance of Ni-TAPP-Cu with those of extensively reported electrocatalysts. (f) ¹H NMR spectra of the products after 1 h of electrolysis with electrolytes containing 0.1 M ¹⁴NO₃ ^– or ¹⁵NO₃ ^– as the nitrate source using Ni-TAPP-Cu; the corresponding standard spectra are also given for comparison. (g) NH₃ yield rates and corresponding FEs-NH₃ of 18 electrolysis cycles (−1.3 V_Ag/AgCl).

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Catalytic mechanisms and electrochemical characterization. (a) NH₃ yield rates and the corresponding FEs-NH₃ of TTF-TAPP-Cu in the NO₂RR and NO₃RR. (b) NH₃ yield rates and the corresponding FEs-NH₃ of Ni-TAPP-2H in the NO₂RR and NO₃RR. (c) Ratio of the NH₃ yield rate in the NO₂RR/NO₃RR. (d) Capacitive current densities derived from CV curves versus scan rates for M-TAPP-M. (e) Schematic diagram of the enzyme-mimicking relay catalytic mechanism.

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Free energy diagram of the electrochemical conversion of NO₃ ^– to NH₃ at the Ni and Cu sites in the Ni-TAPP-Cu system.