The electrocatalytic N2 reduction reaction (NRR) is one of the most promising methods for the on-site and on-demand production of NH3. Single-metal-atom-doped covalent organic frameworks (COFs) are expected to function as efficient NRR electrocatalysts because a designed coordination environment of metal centers is available as a consequence of the wide range of possible designs of COFs. Herein, we used density functional theory (DFT) to systematically investigate the theoretical NRR activity of various single-3d-metal atoms doped into COFs with different coordination numbers to attain a general design guideline for the development of efficient NRR catalysts. The adsorption strength of NRR intermediates decreased as either the coordination number or the number of d-electrons of the metal centers increased. The potential-determining step switched between N-N bond activation and NH3 desorption depending on the adsorption strength of the NRR intermediates. Therefore, an optimal NRR catalyst exhibits a moderate binding strength with intermediates. Among the investigated metal-doped COFs, an Fe metal center with a coordination number of three exhibited the highest theoretical onset potential (-0.49 eV vs the computational hydrogen electrode). In this catalyst, the charge-density and density-of-state analyses revealed moderate πback-donation and σ donation between Fe 3d orbitals and the π∗ orbital of N-N bonds, which resulted in the optimal binding strength of intermediates.
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