Quasi-Phthalocyanine Conjugated Covalent Organic Frameworks with Nitrogen-Coordinated Transition Metal Centers for High-Efficiency Electrocatalytic Ammonia Synthesis

Abstract

Developing high-performance nitrogen reduction reaction (NRR) electrocatalysts is an ongoing challenge. Herein, we report a pyrolysis-free synthetic method for introducing ordered quasi-phthalocyanine N-coordinated transition metal (Ti, Cu, or Co) centers into a conjugated two-dimensional (2D) covalent organic framework (COF) for enhanced NRR performance. Detailed experiments and characterizations revealed that the NRR activity of Ti-COF was clearly better than that of Cu-COF and Co-COF, because of the superior abilities of Ti metal centers in activating inert N₂ molecules and suppressing the hydrogen evolution reaction (HER). The resulting Ti-COF exhibits a high NH₃ yield of 26.89 μg h^-1 mg^-1_cat. and a Faradaic efficiency of 34.62% for NRR. Density functional theory (DFT) calculations verify that Ti-COF can effectively adsorb and activate N₂ molecules and inhibit HER compared with Cu-COF, Co-COF, and pristine COF catalysts. This work opens a new avenue for developing 2D-COF materials that contain abundant coordinated transition metal centers toward electrocatalytic NRR.