Efficient Ammonia Electrosynthesis from Pure Nitrate Reduction via Tuning Bimetallic Sites in Redox-Active Covalent Organic Frameworks

Abstract

Electrocatalytic nitrate reduction reaction (NITRR) represents a promising approach for ammonia synthesis, but existing application has been constrained by the complex proton-coupled electron transfer and the sluggish kinetics induced by various intermediates. Herein, we synthesized a series of metalized covalent organic frameworks: NiTP-MTAPP MCOFs (M = 2H, Co, Cu, and Fe), based on dual redox-active centers: thiophene-substituted Ni-bis(dithiolene) ligand-Ni[C₂S₂(C₄H₂SCHO)₂]₂ and metallic porphyrin. Through regulating the adsorption and desorption of species at the catalytic sites, we have identified the optimal NITRR electrocatalyst: NiTP-CoTAPP MCOF, which achieved the highest faradaic efficiency (FE) of approximately 85.6% at −0.8 V (vs. RHE) in pure nitrate solution, with an impressive yield rate of 160.2 mmol h⁻¹ g⁻¹_cat. The generation of active hydrogen at [NiS₄] sites achieved dynamic equilibrium with the timely hydrogenation reaction at CoN₄ sites, effectively suppressing the hydrogen evolution reaction. Moreover, the incorporation of thiophene (TP) groups and metal ions facilitates charge transfer. Density functional theory (DFT) calculations demonstrated the reduction in energy barriers at different catalytic sites. The CoN₄−NiS₄ system exhibited the optimal adsorption-to-desorption capability and the lowest energy barrier (0.58 eV) for the rate-determining step (*NO → *HNO), which is supported by the moderate d-band center and Bader charge value.