Efficient ammonia synthesis via electrocatalytic nitrate reduction over a [8 + 2]-connected three-dimensional metal-bipyridine covalent organic framework

Abstract

Covalent organic frameworks (COFs) have recently emerged as promising platforms for electrocatalytic nitrate reduction to ammonia (NO₃RR), yet most reported systems are limited to two-dimensional architectures. Herein, we present TU-82, a structurally distinct 3D COF featuring an intricate [8 + 2]-connected bcu topology derived from the reticulation of an octatopic D_2h-symmetric tetragonal prism node and a C₂-symmetric bipyridyl linker. TU-82 exhibits high crystallinity, permanent porosity, and robust structural integrity, enabling precise postsynthetic metalation at bipyridyl coordination sites to yield catalytically active TU-82-Fe and TU-82-Cu frameworks. Among them, TU-82-Fe demonstrates superior NO₃RR performance, delivering a faradaic efficiency (FE) of 88.1% at −0.6 V (RHE) and an ammonia yield rate of 2.87 mg h⁻¹ cm⁻² at −0.8 V (RHE), together with a turnover frequency of 7.2 h⁻¹ and excellent operational stability. Density functional theory calculations reveal that the enhanced activity of TU-82-Fe originates from a lower energy barrier (0.354 eV) for the rate-determining NO* → NHO* step along the NHO-mediated reaction pathway. This work pioneers a structural blueprint for deploying 3D COFs in electrocatalysis, fostering deeper insights into framework-controlled reactivity and offering new routes to sustainable nitrate management.