Two-dimensional
(2D) covalent–organic frameworks (COFs)
offer abundant hollow sites for stably anchoring transition-metal
(TM) atoms to promote single-atom catalysis (SACs), which is expected
to overcome the poor stability of SACs on conventional substrate materials.
Using first-principles calculations within density-functional theory,
a number of TM atoms embedded on a 2D COF Pc-TFPN (TMPc-TFPN) as SACs
for ammonia synthesis under ambient conditions are investigated. Through
a “five-step” screening strategy, WPc-TFPN is highlighted
from 26 TMPc-TFPNs as the best SACs for nitrogen reduction reaction
(NRR) with a low limiting potential of −0.19 V. Meanwhile,
multiple-level descriptors are developed to uncover the origins of
NRR activity, among which a simple descriptor φ that involves
the electronegativity and number of d electrons of TM atoms shows
volcano plot trends of limiting potential of NRR. This work provides
a rational strategy for fast screening SACs for the electrochemical
N2 fixation using 2D COFs containing TM-N4 units
as host materials, which could also be applied to other electrochemical
reactions.