Isomeric bipyridine-based covalent organic frameworks for efficient electrocatalytic nitrate reduction to ammonia

Abstract

Electrocatalytic nitrate reduction reaction (NO3RR) to ammonia presents a promising and green alternative to the energy-intensive Haber-Bosch process. However, its practical application still relies on the development of highly efficient electrocatalysts. In this study, bipyridine-based covalent organic frameworks (COFs) incorporating metal ions within the sidewalls of ordered mesopores were developed to explore their catalytic performances toward NO3RR. The COFs, constructed from 2,2'-bipyridyl-5,5'-dialdehyde (Py-Bpy-COF), featured a single coordination site on each side of the framework. After screening metal ions (Fe, Co, Ni), Py-Bpy-Fe emerged as the most efficient catalyst, achieving a maximum Faradaic efficiency (FE) of 91.2% for NH3 at -1.6 V vs. SCE and a notable NH3 yield rate of 3909.06 μmol h-1 mg-1COF at -2.0 V vs. SCE. Building upon this, by employing an isomeric bipyridine derivative, 3,3'-bipyridyl-5,5'-dialdehyde, a new COF (Py-PyIm-COF) was synthesized, which introduced two coordination sites per side with adjacent imine nitrogen atoms, potentially increasing the number of active sites upon metalation. This modification resulted in the significantly enhanced NH3 yield rate of 4299.61 μmol h-1 mg-1COF at -2.0 V vs. SCE using the Fe ions coordinated Py-PyIm-COF (Py-PyIm-Fe). This study provides critical insights for the rational design of COF-based electrocatalysts for NO3RR, offering a practical route to more efficient ammonia production.