Boosting the Electrocatalytic Conversion of Nitrogen to Ammonia on Metal-Phthalocyanine-based Two-Dimensional Conjugated Covalent Organic Frameworks

Electrochemical N₂ reduction reaction (NRR) under ambient conditions is attractive for the great potential in replacing the current Haber-Bosch process towards sustainable ammonia production. Metal-heteroatom-doped carbon-rich materials have emerged as the most promising electrocatalysts for NRR. However, simultaneously boosting their activity and selectivity toward NRR remains a grand challenge, while the principle for precisely tailoring the active sites has been elusive. Herein, we report the first case of crystalline two-dimensional conjugated covalent organic frameworks (2D c-COFs) incorporated with M-N₄-C centers as novel, defined and effective catalysts, and achieve a simultaneous enhancement in the activity and selectivity towards electrochemical NRR to yield ammonia. Such 2D c-COFs are synthesized based on metal-phthalocyanine (M = Fe, Co, Ni, Mn, Zn and Cu) and pyrene building blocks bonded by pyrazine linkages. Significantly, the 2D c-COF catalysts with Fe-N₄-C center exhibit higher ammonia yield rate (33.6 μg h⁻¹mg⁻¹cat) and Faradaic efficiency (FE, 31.9 %) at -0.1 V vs. reversible hydrogen electrode than those with other M-N₄-C centers, making them among the best NRR electrocatalysts (yield rate >30 μg h⁻¹mg⁻¹cat and FE >30 %). In-situ X-ray absorption spectroscopy, Raman spectroelectrochemistry and theoretical calculations unveil that the Fe-N₄-C center acts as a catalytic site. It shows a unique electronic structure with localized electronic states at the Fermi level, allowing for higher N₂ affinity and stronger binding energy of N₂, enabling faster N₂ activation and NRR kinetics than other M-N₄-C centers. Our work opens the possibility of developing metal-nitrogen-doped carbon-rich 2D c-COFs as superior NRR electrocatalysts and provides an atomic understanding of the NRR process on M-Nx-C based electrocatalysts for the design of high-performance NRR catalysts